Andean Geology 38 (2): 284-318. Julio, 2011
Andean Geology
formerly Revista Geológica de Chile
www.andeangeology.cl
Low-grade metamorphism of Cambro-Ordovician successions in the
Famatina belt, Southern-Central Andes: Burial-inversion history
linked to the evolution of the proto-Andean Gondwana margin
Gilda Collo1, Margarita Do Campo2, Fernando Nieto3
1
2
3
Laboratorio de Análisis de Cuencas, CICTERRA-Universidad Nacional de Córdoba, Av. Vélez Sarsfield 1611, 2º piso, of. 7, X5016GCA
Córdoba, Argentina.
gcollo@efn.uncor.edu
Instituto de Geocronología y Geología Isotópica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellón
INGEIS, Ciudad Universitaria, 1428 Buenos Aires, Argentina.
marga@ingeis.uba.ar
Departamento de Mineralogía y Petrología, Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada-CSIC, Avda.
Fuentenueva s/n, 18002 Granada, Spain.
nieto@ugr.es
ABSTRACT. The metamorphic P-T conditions of low-grade units from the Famatina belt, Central Andes of Argentina, were estimated through petrography, X-ray diffraction, and electron microscopy. For the Middle-Upper
Cambrian Negro Peinado Formation a tectono-metamorphic event associated with intense intrafoliar folding, with
estimated temperatures between 290 and 400°C (KICIS: 0.16-0.27Δº2θ, biotite blastesis and compositional homogeneity in dioctahedral micas) and intermediate pressure conditions (white mica b parameter: 9.010Å-9.035Å),
was recognized. The Achavil Formation (Middle-Upper Cambrian) presents a main metamorphic event associated
with temperatures between 200 and 290°C (KICIS: 0.26-0.41Δº2θ) and intermediate- to low-pressure conditions
(white mica b parameter values: 8.972Å-9.017Å). Some illitic substitution in dioctahedral micas also indicates
lower metamorphic grade than the Negro Peinado Formation. For Upper Cambrian to Middle Ordovician sequences
a burial metamorphic pattern, with a progressive decrease in metamorphic grade from Volcancito Formation to
Cerro Morado Group (ca. 490-465 Ma; KICIS: 0.31-0.69Δº2θ) and absence of tendency changes linked to stratigraphic discontinuities was proposed. Mica and chlorite are the main phyllosilicates in the oldest units, while Ilt/
Sme (R3) mixed-layer is almost the only one in the youngest. White mica b parameter indicates intermediate- to
low-pressure conditions for all these sequences. This burial metamorphic pattern presents a marked break as the
youngest Ordovician unit (La Aguadita Formation, after ca. 452 Ma) records higher metamorphic conditions
(IKCIS: 0.28-0.19Δº2θ) than units from the Ordovician arc, with estimated temperatures between 270 and 330ºC
and intermediate-pressure conditions. Our results indicate that basin contraction and inversion processes related
to the Ordovician Ocloyic Orogeny involved at least two well-discriminated and not superposed metamorphic
episodes in this region.
Keywords: Western Gondwana margin, Ocloyic orogeny, Famatina belt, Low-grade metamorphism.
307 Collo et al.indd 284
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285
RESUMEN. Metamorfismo de bajo grado de sucesiones cambro-ordovícicas en el cinturón del Famatina, Andes
Centrales de Argentina: Historia de enterramiento-exhumación ligada a la evolución del margen proto-andino
de Gondwana. Las condiciones P-T del metamorfismo de las unidades de bajo grado del cinturón del Famatina, Andes
Centrales de Argentina, fueron estimadas mediante petrografía, difracción de rayos-X y microscopía electrónica. La
Formación Negro Peinado (Cámbrico Medio-Superior) fue afectada por un evento tectono-metamórfico asociado con un
plegamiento intrafoliar intenso, para el cual se estimaron temperaturas entre 290 y 400°C (IKCIS: 0,16-0,27 Δº2θ, blástesis
de biotita y homogeneidad composicional en micas dioctaédricas) y presiones intermedias (parámetro b de la mica blanca:
9,010 Å y 9,035 Å). La Formación Achavil (Cámbrico Medio-Superior) presenta un evento metamórfico asociado con
temperaturas entre 200 y 290°C (KICIS: 0,26-0,41Δº2θ) y condiciones de presión intermedias-bajas (parámetro b de la
mica blanca: 8,972Å-9,017Å). Para las sucesiones comprendidas entre el Cámbrico Superior y el Ordovícico Medio, se
identificó un patrón metamórfico de soterramiento, con una disminución progresiva en el grado metamórfico desde la
Formación Volcancito hasta el Grupo Cerro Morado (ca. 490-465 Ma; IKCIS: 0,31-0,69 Δº2θ) y ausencia de cambios de
tendencia ligados a las discontinuidades estratigráficas. La mica blanca y la clorita son los principales filosilicatos en
las unidades más antiguas, mientras los interestratificados Ilt/Sme (R3) son casi exclusivos en las más jóvenes. El parámetro b de la mica blanca indica condiciones de presión intermedia-baja para estas unidades. Este patrón metamórfico
de soterramiento presenta un marcado quiebre hacia la unidad ordovícica más joven (Formación La Aguadita, posterior
a los ca. 452 Ma) con temperaturas estimadas entre 270 y 330°C (IKCIS: 0,28-0,19Δº2θ) y condiciones de presión intermedias. Nuestros resultados indican que los procesos relacionados con la contracción e inversión de la cuenca durante la
orogenia Oclóyica involucran en esta región al menos dos episodios metamórficos bien diferenciados y no superpuestos.
Palabras clave: Margen occidental de Gondwana, Orogenia Oclóyica, Cinturón de Famatina, Metamorfismo de bajo grado.
1. Introduction
FIG. 1. Location map with distribution of main mountain
ranges of the Central Southern Andes (Western
Argentina): LP: Lules-Puncoviscana belt; Cho:
Choromoro belt; Q: Sierra de Quilmes; C: Capillitas; A: Ancasti; V: Velazco; E: El Espinal; U:
Umango; M: Sierra de Maz; VF: Valle Fértil;
PP: Pie de Palo; Ch: Chepes and Llanos de La
Rioja; SL: Sierras de San Luis; Co: Sierras de
Córdoba; SN: Sierra Norte. Bold dashed lines
show boundaries between main geological
provinces. The black box indicates the location
of the detailed area in figure 3.
307 Collo et al.indd 285
64ºO
BOLIVIA
N
Cho
24ºS
Jujuy
LP
Salta
ARGENTINA
Q
Tucumán
C
CHILE
28ºS
A
U
E
M
Catamarca
V
tina
ma
Fa belt
The Famatina belt, in the Central Andean
Region of Argentina (Fig. 1), is part of an ancient subduction-collisional margin that formed
along western Gondwana during the Early
Palaeozoic (Vaughan and Pankhurst, 2008).
Within this belt, Middle-Upper Cambrian to
Middle-Upper Ordovician deposits constitute
an almost continuous succession affected by
low-grade metamorphism. The well-constrained
and mostly preserved Cambro-Ordovician stratigraphy in this region allows the reconstruction of the sedimentary history related to the
evolution of the Ocloyic arc (Astini, 2003;
Astini and Dávila, 2004). Three major stages
of sedimentation record the period preceding
the initiation of subduction up to the uplift and
basin inversion in the final stages of the Ocloyic
68ºO
VF
La Rioja
SN
Ch
PP
Co
PRECORDILLERA
Western
Sierras
Pampeanas
Santiago
Mendoza
0
32ºS
SL
San Luis
100
Córdoba
Eatern
Sierras
Pampeanas
200 km
13-07-2011 12:58:56
286
Low-grade metamorphism of Cambro-ordoviCian suCCessions in the famatina beLt...
orogeny, which has been commonly related with the
collision of the Precordillera terrane from the west (ca.
460 Ma). The earliest stage started with the deposition
of the Middle-Upper Cambrian synorogenic units,
the Negro Peinado and Achavil Formations, deposited in a peripheral foreland that developed during
the final stages of the Pampean orogeny (Pampean
synorogenic suite, ca. 520-500 Ma; Fig. 2; Collo
and Astini, 2008; Collo et al., 2009). A second stage
comprises the sequences associated with the initiation
of an east-dipping subduction regime to the west,
and the concomitant development of the Ordovician
Ocloyic magmatic arc (Astini, 2003), including volcanic units representing two main volcanic episodes
(Fig. 2). The latest depositional stage is recorded by
the Upper Ordovician La Aguadita Formation (after
ca. 452 Ma), that would correspond to a foreland
basin, synorogenic with the Ocloyic orogeny, and
mainly sourced by the Ocloyic granitoids (Ocloyic
synorogenic suite, ca. 480-460 Ma, Fig. 2; Astini et
al., 2005; Rapela et al., 1998; Dahlquist et al., 2005).
U-Pb geochronology from the region (Rapela
et al., 2007; Collo et al., 2009), together with K-Ar
analyses (Collo et al., 2008), indicate that, contrary
to long-held belief, the post-depositional history and
the related low-grade metamorphism of the CambroOrdovician units are entirely associated with the
Ordovician Ocloyic orogeny. Medium- to high-grade
metamorphism linked to this orogeny is well described in rocks cropping out east-southeast of the study
region (cf. Steenken et al., 2004; 2006; Delpino et al.,
2007; Verdecchia, 2009), suggesting the existence of
low-pressure metamorphism coupled with back-arc
extension and subsequent closure of the basins.
With the aim of understanding the post-depositional
history of the diverse low-grade metamorphic units
within the Famatina belt, their thermobarometric
conditions were first constrained through petrographic and mineralogical analyses. Subsequent detailed
SEM analysis identified metamorphic reactions not
previously observed through optical microscopy and
XRD analyses, and also determined mineral chemistry at the micron scale. The results showed that the
youngest Ordovician unit of the easternmost back-arc
region records higher metamorphic conditions than
the older units within the Ordovician arc (Collo et
al., 2005; Collo and Astini, 2008). Although lowgrade patterns evidencing metamorphic inversion
are common, they are mainly recorded within forearc
regions between trenches and accretionary prisms,
307 Collo et al.indd 286
where the younging-upward increase in metamorphic
grade is the result of progressively older overriding
strata. This seems not to be the case of the studied
successions genetically linked to a continental arc
setting (Astini, 2003).
In this contribution, we present the set of data
obtained, together with a discussion of the geodynamic evolution of the subduction-collisional
margin of western Gondwana during the CambroOrdovician at these latitudes, as it emerges from
the assemblage of petrographic, geochronological,
microtectonic, and structural information. The
uniqueness of the volcano-sedimentary CambroOrdovician record at Famatina belt also makes
this a key region for understanding complex
subsidence-burial mechanisms that occurred at the
higher structural levels within the Ocloyic Orogeny, whose post-depositional history has mainly
been established through the study of middle- to
high-grade metamorphic units.
2. Low-grade rocks from Famatina belt
The low-grade units from Famatina belt crop
out in different tectonic sheets associated with the
Andean uplift and affected by intensive shortening.
The Negro Peinado Formation (Middle-Upper
Cambrian) crops out as a relatively continuous
central sheet (Las Trancas sheet) without stratigraphic
contacts with Lower Palaeozoic units (Fig. 3a). Its
low-grade metamorphism is associated with tight
isoclinal folding, with the S1 foliation parallel to the
S0 and sandstone levels boudinated within pelitic
layers. Most of the remaining Cambro-Ordovician
rocks crop out in two central-western sheets (Los
Damascos and Los Colorados sheets; Fig. 3a). Within
these sheets, the Achavil Formation (Middle-Upper
Cambrian; Collo, 2006; Collo and Astini, 2008) is
unconformably overlain by the Cambro-Ordovician
Volcancito Formation and the subsequent wellknown Ordovician volcano-sedimentary successions
(over 4,000 m in thickness) comprising the Bordo
Atravesado Formation, the Cerro Tocino Volcanics,
and the Famatina and Cerro Morado groups (Fig. 3b).
Although this volcaniclastic sequence clearly preserves
its sedimentary attributes, it is also affected by lowgrade metamorphism (Toselli and Weber, 1982; Collo,
2008). The Achavil Formation records complex folding
associated with the oldest strain events identified in
the region. It is the result of the superposition of two
13-07-2011 12:58:56
Collo et al./ Andean Geology 38 (2): 284-318, 2011
287
(520-510 Ma) Post-orogenic Pampean stage: early synorogenic
exhumation and sedimentation with west polarity
s.l.
Exhumed Pampean arc
Brasilian arc
Negro Peinado Formation
MB
PB
Stage I
MB
(510-500 Ma) Late synorogenic sedimentation and peneplanization of the
Pampean Orogeny
Achavil Formation
s.l.
Pampean arc
Brasilian arc
PB
MB
MB
(500-495 Ma) Late Cambrian onset of subduction
Post Pampean deformation related
to onset of subduction
s.l.
MB
(495-480) Late Cambrian - Early Tremadocian forearc
Volcancito Formation
s.l.
Stage II
MB
(~480-475) Late Tremadocian - Early Arenigian Intra-interarc: beginning of
volcanic activity
Famatina Group
Famatinian
K-bentonites
Famatinian
volcanic arc
s.l.
MB
Onset of granitoids
emplacement
(~475-460) Late Arenigian - Early Llanvirnian volcanic stage and tectonism
Cerro Morado Group
Precordillera K-bentonites
Black shales
s.l.
Angular discordance
U1
MB
Ocloyic deformation, main burial episode and
ending of granitoid emplacement
Stage III
(~460-435 Ma) Partial exhumation of arc and Middle to Early Ordovician foreland
synorogenic deposits
Precordillera
Peripheral foreland Pie de Palo
s.l.
Trapiche
Group
intermediate block
Exhumed Famatinian arc
Las Chacras
melange
Backarc foreland
La Aguadita Formation
MB
Grenvillian basement
Suture zone
FIG. 2. Evolution of Famatinian Cambro-Ordovician units within the Ocloyic Orogeny
as proposed by Astini et al. (2007), Astini and Dávila (2004), and Collo et al.
(2009).
307 Collo et al.indd 287
different deformational stages, a N-S
compressive phase and a further
E-W com-pressive phase (P1ACH
and P2ACH), probably linked to the
onset of subduction in the western
margin of Gondwana during the
Upper Cambrian (Collo et al.,
2006). The discordance between the
Achavil and Volcancito Formations
could also be correlated with the
Iruyic discordance identified in NW
Argentina between the Mesón and
Santa Victoria groups (Astini et al.,
2005). An east-verging fold-thrust
belt associated with the Ocloyic
orogeny itself is evidenced by the
angular unconformity between the
Early Ordovician Famatina Group
and the overlying Ordovician Cerro
Morado Group, both part of the
anticline of Los Colorados in the
central Famatina belt (Dávila et
al., 2003; Astini and Dávila, 2002).
The youngest La Aguadita
Formation crops out in a small,
independent eastern sheet (Valle de
la Aguadita sheet; Fig. 3a) and, as
with the Negro Peinado Formation,
there is no stratigraphic relation
between this unit and the other
Cambro-Ordovician sequences.
Petrologic, mineralogical (XRD),
and textural analyses (Collo et
al., 2005) of metasandstones
and metapelites from the Loma
de las Damas outcrop indicate
that the rocks were affected by a
regional low-grade metamorphicdeformational episode whose main
blastesis was contemporary with
the development of the S1 foliation.
3. Analytical methods
3.1. XRD analysis
Samples for clay mineral
X-ray analysis were prepared
following the recommendations
of Moore and Reynolds (1997).
13-07-2011 12:58:57
28º28’36’’ LS
452
Middle Ordovician
N
Campanas
B
La Aguadita Fm.
A
10 km
-
-
n
11
Quaternary
-
-
Cerro Tocino
volcanics
Neogene
-
Bordo
Atravesado Fm.
479
Late Paleozoic
La Aguadita Fm.
El Portillo Fm.
Molles Fm.
Suri Fm.
LDS
LTS
Cerro
Faltriquera
Cerro Faltriquera
Famatina
Volcancito Fm.
Middle-Late Ordovician
Cerro Tocino Volcanites
488
Ñuñorco Granitoids
Early Ordovician
Middle-Upper Cambrian
Middle Cambrian
Volcancito Fm.
Filo Los Arenales
Achavil Fm.
Negro Peinado Fm.
505
-
Upper Cambrian
Paimán Granitoids
-
Upper Member
(Peña Negra)
Lower Member
(Filo Azul)
Achavil Fm.
La Escondida Fm.
250 m
Lineaments
LCS
Molles Fm.
imá
Synclines
-
-
28º44’53’’ LS
es
Interpreted fault
pointing out
escarpment
SPS
lon
Anticlines
mb
Thrust fault
-
Early Ordovician
Río
Bla
Pa
Town
Ra
Vinchina - Bermejo Valley
de
Dam
rra
Streams
rra
11 Road
RS
Sie
Los Colorados
Anticline
-
Suri Fm.
nc
o
Sie
REFERENCES
-
-
Famatina Group
Río El Durazno
-
13-07-2011 12:58:58
FIG. 3. A. Geological map of the central region of the Famatina belt (modified from Dávila and Astini, 2007). VAS: Valle La Aguadita thrust sheet; SPS: Sierra de Paimán thrust sheet; RS:
Ramblones thrust sheet; LTS: Las Trancas thrust sheet; LDS: Los Damascos thrust sheet; LCS: Los Colorados thrust sheet; B. Cambro-Ordovician stratigraphy within the Famatina
belt (modified from Astini and Dávila, 2004).
Low-grade metamorphism of Cambro-ordoviCian suCCessions in the famatina beLt...
Antinaco Valley
VAS
-
Carboniferous
472
LDS
-
La Escondida
Fm.
El Portillo Fm.
5
Cerro Morado Group
0
Late Ordovician
67º41’42’’ LW
288
307 Collo et al.indd 288
67º52’58’’ LW
Collo et al./ Andean Geology 38 (2): 284-318, 2011
The <2 µm fraction was separated for 17 pelitic
samples. Clay-mineral composition was established
by the comparison of orientated aggregates that
were air-dried (AO), ethylene-glycol solvated (EG),
and heated at 500ºC (HO). X-ray analyses were
determined with Philips PW1050 (INGEIS) and
X-Pert Pro (Departamento de Físico-Química-UNC)
diffractometers, employing Cu radiation from 4
to 30º2θ with a step size of 0.03º2θ and a count
time of 0.5 s per step. Clay-mineral phases were
semi-quantified using MIF factors and the recommendations of Moore and Reynolds (1997). The
Kübler Index (KI) was measured in both AO and
EG orientated aggregates, and CIS values (Crystallinity Index Standard, Warr and Rice, 1994) were
established from the regression equation for the
Philips PW1050 diffractometer: y=1.2175x+0.0833
(R2=0.975). In samples with evidence of smectite
or illite/smectite mixed-layer (hereafter Ilt/Sme),
KI(CIS) measurements were carried out on the 5Å
illite reflection in EG-treated aggregates in order
to avoid the influence of expandable layers. KI(CIS)
values were only measured in rocks without blastic
biotite. The white mica b parameter (Guidotti and
Sassi, 1986) was measured in rock slices orientated
perpendicular to the main foliation, and the quartz
(211) reflection positioned at 1.541Å was used as
internal standard. Mineral abbreviations proposed
by Whitney and Evans (2010) were used.
3.2. Scanning and transmission electron microscope
Fifteen metapelitic levels belonging to the
Negro Peinado, Achavil, and La Aguadita formations were analysed by scanning electron microscopy
(Zeiss DSM950 SEM and Variable Pressure SEM)
and a Philips CM-20 scanning transmission electron
microscope (STEM) equipped with an ultra-thin
window EDX detector (Centro de Instrumentación
Científica, Granada University). The main goal
was to understand the mineralogical and textural
evolution of low-grade rocks in Famatina belt, first
constrained through petrography and XRD (Kübler
index, white mica b parameter). Further goals were
to characterize the chemical composition of
phyllosilicate minerals and to fix geobarometric
constraints (Massone and Szpurka, 1997). These
three units were chosen as they recorded the highest metamorphic grades. For textural analysis,
backscattered electron images were used, where-
307 Collo et al.indd 289
289
as the chemical composition of phyllosilicates
was established through EDX on carbon-coated
polished samples studied by SEM and through
analytical electron microscopy (AEM) in Au grids
under TEM. Quantitative AEM analyses were
obtained from thin edges using a 1000x200Å
scanning area. Counting times of 15 s and 100 s
minimized alkali-loss problems as short counting
times improve reproducibility for K + and Na +
(Nieto et al., 1996). Standards used to obtain the
K-factors for the transformation of intensity ratios
to concentration were albite, biotite, spessartine,
muscovite, olivine, and titanite (following the methods of Cliff and Lorimer, 1975 and Champness
et al., 1981). Mica compositions were established
from samples with and without biotite blastesis.
The structural formulae of dioctahedral micas
were calculated considering 22 negative charges
and 0.25 to 0.50 Fe2+/Fe3+ ratios depending on the
iron-bearing phases present in individual samples
(or in each unit) (cf. Guidotti et al., 1994). Mica
formulae showing Mg++ contents higher than (Si-3)
were considered to be contaminated with chlorite,
whereas data showing anomalously low interlayer
charges and high Si contents were interpreted
as being contaminated with quartz. Both groups
of data were discarded. Chlorite formulae were
calculated considering 28 negative charges and
ferrous iron (Fe2+). Small amounts of K+, Na+, and
Ca++ identified in some chlorites from both units
were interpreted as indicative of mica contamination; in such cases, compositions were corrected
following the recommendations of Nieto (1997).
4. Results
4.1. Petrography, clay mineralogy, and crystalchemical indices
4.1.1. Negro Peinado Formation (Middle-Upper
Cambrian)
This unit mainly comprises blackish-green
phyllites alternating with centimetric levels of
metasandstones (metawackes and subordinate metaarkoses; Fig 4a). Fine-grained white mica, chlorite,
quartz, and feldspars are the principal phases in all
phyllites from this sequence (Fig. 4b and c), with
associated biotite blastesis in some localities. These
minerals define a zonal to continuous spaced me-
13-07-2011 12:58:58
290
Low-grade metamorphism of Cambro-ordoviCian suCCessions in the famatina beLt...
FIG. 4. The Negro Peinado Formation. a. Outcrop view of the sequence showing millimetric alternance of silty-sandy and shale
layers; b. and c. Metapelites showing development of P and Q domains coincident with the metamorphic S1 foliation (c: BSE
image). Mica, chlorite, and blastic biotite grains are commonly larger than 50 mm and appear as orientated crystals (0.01 to
0.02 mm long) surrounding quartz grains (0.05 to 0.1 mm). Quartz generally appears as elongated grains, with sutured-serrated
to straight edges, occasionally with undulatory extinction; d. and e. Metasandstones, see the sutured to serrated contacts and
the beard quartz-feldspar grains surrounded by a phyllosilicate matrix defining the metamorphic foliation S1 (e: BSE images).
White mica is very thin (0.02 mm) and occasionally forms interleaved phyllosilicate grains with chlorite and biotite, elongated
parallel to the metamorphic foliation, but with individual crystal (001) planes perpendicular to S1. Chlorite is wider than mica
(0.05 to 0.1 mm), tabular, and intensely green; it presents variable interference colours, from intense blue to brown, indicating
compositional variations within individual samples, from magnesian-chamosite to ferrous-clinochlore, respectively (cf. Nieto,
1997). Sometimes quartz grains are broken and displaced, with the fractures filled by chlorite crystals with the (001) planes
oblique to the metamorphic foliation. The Achavil Formation; f. outcrop view of the sequence showing the alternance of shales
and sandy levels and folding; g. and h. Metapelites with oblique sedimentary S0 and metamorphic S1 foliations (h: BSE image).
Subrounded opaque minerals, occasionally as aggregates, and biotite and mica detrital grains were also observed. Chlorite
frequently replaces detrital biotite grains; i. and j. Quartzose metasandstones with sutured contact between quartz grains and
undulatory extinction (j: BSE image).
tamorphic foliation (S1) mainly subparallel to the
sedimentary layering (S0), with a lepidoblastic to
lepidogranoblastic texture depending on the millimetric compositional-granulometric layering. P and
Q domains are well developed. Occasionally very
thin sandy layers are folded within the fine-grained
domains. Intergrown muscovite-biotite grains are
also common. Metawackes are mainly composed
307 Collo et al.indd 290
of quartz and plagioclase with minor amounts of
K-feldspar (Fig. 4d and e). Lepidogranoblastic
domains are mainly formed by orientated phyllosilicates, although quartz grains are abundant. Blastic
biotite, when present, appears as thin flakes and is
commonly intergrown with chlorite. In granoblastic
domains, quartz (0.05 to 0.6 mm) is sigmoidal or
eye-shaped, with undulatory extinction; it also
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Collo et al./ Andean Geology 38 (2): 284-318, 2011
presents subgrains and sutured to serrated contacts.
In these domains, the phyllosilicates surround
quartz and feldspar grains that frequently develop
beards and pressure shadows associated to the S1
foliation. Meta-arkoses are mainly composed of
quartz-feldspathic grains (84% quartz and 16%
plagioclase, according to XRD analyses) with
scarce matrix.
The clay fraction from fine-grained levels mainly
consists of mica (~50-90%) and chlorite (~10-50%),
with scarce smectite and the sporadic appearance
of interstratified illite-smectite. In one sample, a
7Å phase (identified by its reflections at 7.15Å
and 3.57Å) was interpreted as kaolinite (Table 1).
Changes in chlorite composition between samples
were infered through XRD analyses, as variable
height ratios of odd and even basal reflections
suggest variable total Fe contents.
KI(CIS) values measured in five biotite-free samples
range between 0.28 and 0.16ºΔ2θ, belonging to the
upper anchizone-epizone fields transition (Table 1).
KI values, together with the biotite blastesis registered
in several samples, indicates temperatures between
290 and 400°C. The white mica b parameter gave a
mean value of 9.021Å (n: 13, s: 0.009), consistent
with intermediate-pressure facies metamorphism
according to Guidotti and Sassi (1986). Most of the
values range between 9.010Å and 9.035Å, with the
two lower values (9.004Å and 9.005Å) corresponding to samples with biotite blastesis (see samples
CH64 and CH73 in Appendix A), reflecting the
lower phengitic substitution in white micas from
biotite-muscovite associations (Ernst, 1963) related
with the reaction: phengite+chlorite→muscovite+
biotite+quartz+water.
4.1.2. The Achavil Formation (Middle-Upper
Cambrian)
This unit constitutes a relatively homogeneous
to highly rhythmic stratified sequence of green
to dark grey dominantly shales with millimetric
compositional-granulometric layering (Fig. 4f). The
pelite/sandstone thickness ratio is generally over 10,
and exceptionally some micro-conglomeratic levels
are preserved. Primary features are generally obliterated by neoformation and transformation processes
that give rise to blastopelitic to lepidogranoblastic
textures. In metapelites (Fig. 4g and h), neoformed
fine-grained white mica and chlorite (<0.01 mm)
307 Collo et al.indd 291
291
surround thin quartz-feldspathic grains defining
smooth metamorphic foliation (cf. Passchier and
Trouw, 1998) subparallel to oblique to the sedimentary
layering. SEM images depict muscovite and chlorite
neoformed grains, seldom larger than 10 mm. Sandy
levels (Fig. 4i and h) are lighter in colour than pelites,
they correspond to quartzose sandstones to quartzose
metawackes generally having millimetric parallel to
cross-layering. They have a blastopsamitic texture
with quartz grains up to 0.13 mm in size, exhibiting
undulatory extinction and sutured to serrated contacts,
and subordinate plagioclase and lithic fragments.
Scarce neoformed phyllosilicates (fine-grained white
mica and chlorite) are orientated parallel to S0. The
development of metamorphic foliation is not clear
within these levels. Common Fe-bearing phases are
pyrite, as ehuedral crystals and framboidal aggregates, and ilmenite; sometimes they are replaced by
oxidized secondary Fe phases. The XRD analyses
of clay fractions indicate higher mica abundances
in Achavil (~68-98%) than in the Negro Peinado
Formation metapelites (Table 1), whereas chlorite
represents a subordinate phase (2-32%). Despite the
mineralogical homogeneity of most of the samples,
two diagrams present a group of low-intensity reflections in the heat-treated aggregates (500ºC) at
~15.8, 7.8, 4.2, and 3.8Å, probably corresponding
to interstratified chlorite/vermiculite (Chl/Vrm).
Moreover, one sample presents traces of smectite
(4%). KI(CIS) values range between 0.41 and 0.26ºΔ2θ
(n: 11), indicating that these rocks were buried under
low- to upper-anchizone conditions (Merriman and
Peacor, 1999), with temperatures of around 200 to
290°C. The white mica b parameter, measured in
eight samples, shows a mean value of 9.009 Å with
a standard deviation of 0.005 (Table 1).
4.1.3. Volcaniclastic units (Upper Cambrian-Middle
Ordovician)
4.1.3.1. The Volcancito Formation
This unit is formed by two contrasting members.
The lower calcarenitic member (Lower Member, cf.
Astini, 2003) is characterized by the presence of
carbonate intraclasts (mudstones) with inclusions
of siliceous spicules and bioclasts (Nuia, Girvanella, pelmatozoa, trilobites, brachiopods, ostracods,
gastropods; Astini, 2001, 2003), all surrounded by
sparitic cement. It also contains abundant quartz
13-07-2011 12:58:59
Chl/Vrm
Bt
CIS(AD)
IK
CIS(EG)
33
-
14
-
X
-
X
X
0.32
0.40
0.29
0.30
0.29
0.34
-
0.31
0.40
0.28
0.29
0.34
-
X
X
-
100
98
96
-
-
-
0.83
0.69
0.68
0.69
0.64
0.50
-
-
-
0.84
1.43
0.55
0.46
0.54
1.32
-
18
-
X
X
-
-
0.52
0.48
0.51
0.51
0.59
0.53
0.48
0.53
0.51
0.53
0.52
-
Berthierine
Lag
LD1p
LD2p
LD3p
LD6p
LD6a
LD7p
LD8p
LD9p
61
92
88
89
75
90
94
4
5
7
9
8
8
-
3
3
5
3
3
2
6
X
X
-
X
-
X
X
-
La Escondida
Formation
LE1
LE7
CM5
-
-
-
-
2
4
Molles Formation
Sme
CM-32
CM-40
LMA2
LMA115
M2
99
92
89
90
1
8
11
10
-
-
-
-
100
-
S1
S2
S4
S5
S6
S7
S8
S10
S12
S13
S18
89
80
76
82
80
76
80
97
95
95
97
11
20
24
18
20
24
20
3
5
5
3
-
-
-
-
-
-
X
X
-
CIS range
0.28-0.40
n:6
0.5 -0.69
n:3
0.46-1.32
n:5
0.41-0.59
n: 15
b
b parameter
b mean
9.028
9.026
9.022
9.026
9.020
9.023
9.024
n:4
s:0.004
-
9.011
n:4
s: 0.006
Low-grade metamorphism of Cambro-ordoviCian suCCessions in the famatina beLt...
13-07-2011 12:59:00
Ilt/Chl
Chl
La Aguadita Formation
MINERALOGY
Kln Vrm Ilt/Sme
Ilt
Suri Formation
Sample
292
307 Collo et al.indd 292
TABLE 1. INTEGRATED TABLE WITH DATA FROM ALL THE ANALYSED UNITS. KÜBLER INDEXES AND WHITE MICA b PARAMETER VALUES CONSIDERED IN
EACH UNIT ARE SHOWN IN BOLD. SEE TEXT FOR FURTHER EXPLANATIONS.
Suri Formation
Chl
10
8
-
Sme
-
Berthierine
-
MINERALOGY
Kln Vrm Ilt/Sme
-
Ilt/Chl
-
Chl/Vrm
-
Bt
-
CIS(AD)
0.46
0.41
0.55
0.42
-
IK
CIS(EG)
-
95
5
-
-
X
-
-
X
-
0.35
-
90
10
-
-
X
-
-
X
-
0.31
-
AC-V
PS1
PS2
QV1a
QV1b
QV1c
QV2
QV3
QV4
QV5
QV6a
QV6b
QV6c
QV7
QV9
77
90
93
93
89
91
78
92
80
79
85
95
77
88
-
5
7
11
9
4
8
20
21
15
5
23
12
-
2
10
2
-
AA1
AA2
V5m
V10
V11
85
72
92
99
87
15
28
8
1
13
-
21
X
-
-
X
-
-
18
-
X
-
-
0.41
0.44
0.37
0.34
0.36
0.38
0.39
0.37
0.38
0.35
0.39
0.37
0.34
0.35
0.33
0.38
0.34
0.38
-
-
-
-
-
-
X
X
-
0.41
0.37
0.34
0.27
-
CIS range
0.41-0.59
n: 15
0.31-0.35
n:2
b parameter
b
b mean
9.011
n:4
9.015
s: 0.006
9.003
9.015
9.012
-
0.33-0.39
n:15
9.026
9.017
9.020
9.011
-
9.018
n:4
s:0.006
0.26-0.41
n:11
9.011
-
9.009
n:8
s:0.005
293
13-07-2011 12:59:00
BA
BA1D
Volcancito Formation
Ilt
90
100
92
-
Achavil
Formation
Bordo
Atravesado
Formation
Sample
S22
S23
QS5
SC
QS1
QS33
QSp8
Sch
Collo et al./ Andean Geology 38 (2): 284-318, 2011
307 Collo et al.indd 293
Table 1 continued.
294
Chl
Sme
Berthierine
V12
V13
V14
V15
V15a
V16
ACH
546
547
Q Ach
84
96
83
75
79
-
13
4
17
25
21
-
4
-
-
-
-
NP2
NP3
NP4
NP5
NP11
NP12
CH73
CH64
Pl
QP
QP3a
QP4
QP6a
QP6p
QP7p
CNP
CNP2
CH55
517
49
54
67
63
-
42
33
36
-
10
4
1
-
44
-
-
-
Ilt/Chl
Chl/Vrm
Bt
CIS(AD)
IK
CIS(EG)
-
-
-
-
0.41
0.39
0.38
0.40
0.39
0.26
0.27
-
0.34
-
X
-
-
-
X
X
X
-
0.24
0.24
0.27
0.30
-
0.20
0.23
0.16
-
b parameter
b mean
CIS range
b
0.26-0.41
n:11
9.013
9.006
9.011
9.017
9.006
8.972
9.01
9.009
n:8
s:0.005
0.16-0.28
n:5
9.027
9.026
9.030
9.024
9.016
9.023
9.005
9.010
9.020
9.026
9.035
9.026
9.021
9.014
9.004
-
9.021
n:13
s:0.009
13-07-2011 12:59:00
Low-grade metamorphism of Cambro-ordoviCian suCCessions in the famatina beLt...
Ilt
Achavil Formation
MINERALOGY
Kln Vrm Ilt/Sme
Sample
Negro Peinado Formation
307 Collo et al.indd 294
Table 1 continued.
Collo et al./ Andean Geology 38 (2): 284-318, 2011
and plagioclase grains, quartzose metamorphic
lithoclasts, opaque minerals, zircon, chlorite, and
subordinate muscovite. The intraclasts and some
bioclasts (predominantly pelmatozoa) are broken,
crushed, and rotated, with fractures filled by calcite and, occasionally, folded. Stylolites depicting
concentrations of less soluble phases (clay, iron
oxides, and organic matter), and orientated parallel
to the S0 primary foliation, were also identified. In
levels where the carbonate cement is not abundant,
quartz grains present sutured to plane contacts, and
some concave-convex contacts involving clay-silt
intraclasts. Pyrite is the most abundant opaque
mineral, and appears as framboidal aggregates, but
also as irregular and euhedral crystals (nano- and
micropyrite; Brand, 2004; Schoonen, 2004). Clay
minerals associated with the carbonate cement
include white mica and chlorite. Sometimes radial chlorite surrounds pyrite grains and replaces
some framboidal aggregates. In addition, there are
295
multiple fractures, post-dating most of the aforedescribed diagenetic features, filled with calcite,
quartz, and opaque minerals. In this member, finegrained rocks show millimetric S0 compositional
layering, with alternating light quartz-rich bands
and dark bands with higher organic matter and
phyllosilicatic contents.
The upper pelitic member (Upper Member, cf.
Astini, 2003; Fig. 5a and b) is mainly formed of
shales with distinct parallel lamination and composed predominantly of quartz, plagioclase, organic
matter, opaque minerals, fine-grained white mica,
chlorite, and isolated intraclasts. The neoformed
phyllosilicates are orientated parallel to the S0.
Interlayered with these dominant pelitic levels are
fine to medium (0.07 to 0.1 mm) quartz-sandstone
levels and scarce intraclastic mudstone and conglomerate levels. The sandstones have quartz grains with
plane, sutured to serrated contacts, lithic fragments,
feldspar, opaque minerals (predominantly euhuedral
FIG. 5. The Volcancito Formation:
a. Metapelite with development of secondary foliation
parallel to the sedimentary
layering; b. Metasandstone
with quartz overgrowths
in optical continuity and
plane to sutured contacts.
The Suri Formation: c. Metapelites with argillaceous
mineral growth parallel to
sedimentary layering; d.
Metasandstone with abundant quartz grains showing
overgrowths in optical
continuity and plane to
concave-convex contacts.
The Molles Formation: e.
Volcaniclastic sandstone
with abundant subrounded
and embayed quartz grains;
f. Detail showing carbonate
cement surrounding quartz
overgrowths.
307 Collo et al.indd 295
13-07-2011 12:59:00
296
Low-grade metamorphism of Cambro-ordoviCian suCCessions in the famatina beLt...
and framboidal pyrite), and intergrown chlorite
and mica grains in a quartz-micaceous matrix with
abundant chlorite and scarce carbonate cement
(micrite). Some levels within this member have
haematitic aggregates, probably generated at the
expense of the dissolution and transformation of
existing pyrite. Haematite is also present filling
fractures that crosscut the unit.
XRD analyses performed on the clay fraction
of pelitic levels of both members indicate it is composed of mica (77 to 95%) and chlorite (4 to 23%),
with subordinate quartz and plagioclase (Table 1).
However, lesser amounts of interstratified Chl/Vrm
and Ilt/Chl, kaolinite, and vermiculite were identified
in some samples. Particularly, the two samples (PS1,
PS2; see Table 1) collected in the Peña Negra locality
have subordinate smectite (2 to 10%), whereas the
sample collected immediately below the unconformity
that separates the Carboniferous from the Volcancito
Formation is composed of mica (77%), kaolinite
(21%), and smectite (2%). KI(CIS) values in the Río
Volcancito section range from 0.33 to 0.39 Δº2q
(n=15), indicating low-anchizone metamorphism.
White mica b parameter ranges between 9.011 and
9.026Å (n: 4; s: 0.006).
4.1.3.2. Bordo Atravesado Formation
The first record of volcanism in the Ordovician
Famatina belt is represented by silicified tuffs that
appear in the Bordo Atravesado Formation. This
unit consists of finely laminated mudstones to
siltstones, interbedded with massive fine-grained
silicified sandstones commonly showing parallel
to hummocky cross-stratification (Zimmerman and
Esteban, 2002; Astini, 2003; Albanesi et al., 2005).
Silicified episodic tuffs levels have thicknesses
between 5 and 35 cm and commonly develop internal layering and syn-sedimentary deformation,
indicating abrupt deposition. Microscopically,
they constitute homogeneous masses of micro- to
cryptocrystalline quartz (chert) locally intergrown
with fine-grained white mica and dispersed pyrite
cubes. The mud-siltstone levels present clear parallel
granulometric-compositional lamination and consist
predominantly of very small quartz crystals (up to
0.04 mm), mica, chlorite, organic matter, and opaque
mineral aggregates (up to 3 mm).
The sandstones are dominated by quartz grains,
mudstone, siltstone, and sandy lithoclasts, calcite,
307 Collo et al.indd 296
abundant pyrite, zircon (up to 0.1 mm), folded
detrital white-mica grains, and scarce feldspar and
bioclasts, all in a fine-grained matrix mainly composed of white mica, chlorite (in aggregates of up
to 0.2 mm), quartz, and opaque minerals, none of
which have any clear orientation. The quartz grains
have undulatory extinction, bands, and deformation
lamellae, subgrains, and hairy edges in the contact
with the clay matrix. In sectors where the matrix is
less abundant, contacts range from plane to sutured.
Silica overgrowths in optical continuity within quartz
grains (probably associated with early cementation)
were also observed. Pyrite occurs predominantly as
framboidal aggregates and euhedral crystals clearly
identifiable in hand sample with edges sometimes
altered to chlorite.
The mineralogy of the clay fraction was established in 2 pelitic levels of this unit. The samples
are predominantly composed of mica (90-95%),
subordinate chlorite (5-10%), and scarce amounts
of interstratified chlorite/vermiculite (Chl/Vrm) and
vermiculite (Table 1). Chl/Vrm was identified by
the (002) reflection at 12Å in the heated diagram,
corresponding to a basal spacing of ~24Å, resulting
from the sum of the chlorite 14Å and the vermiculite
10Å spacings after collapse due to heating. Vermiculite was identified by decreasing intensity of the
reflections at 14Å, 7Å, and 3.58Å in the heated
diagram, accompanied by an increase in the intensity
of the 10Å reflection. KI(CIS) values for this unit are
0.31 and 0.35 Δº2q.
4.1.3.3. Famatina Group
4.1.3.3.1. Suri Formation. This unit includes
various epiclastic deposits interbedded with volcaniclastic, volcanogenic, and volcanic rocks (Fig. 5c
and d; Mángano and Buatois, 1994; Astini, 2003).
The pelites from this sequence, mainly located at the
base of the unit, range in colour from bluish-grey to
black. They have S0 layering defined by alternating
bands with different proportions of phyllosilicates
and changes in organic matter contents, and are
mainly composed of phyllosilicates, fine-grained
quartz, and subordinate opaque minerals. The
siltstone levels that alternate with the pelites have
sporadic subrounded to subangular quartz grains (up
to 0.05 mm), often elongated parallel to S0 and with
undulatory extinction. Some elongated intraclasts
were also observed. The sandstones of this unit are
13-07-2011 12:59:00
Collo et al./ Andean Geology 38 (2): 284-318, 2011
297
immature and have alternating bands composed
of quartz grains and plagioclase, with white mica,
chlorite, and biotite, and subordinate opaque minerals, sporadic siltstone intraclasts, and fragmented
bioclasts in a silty-clay dominated matrix. Quartz
grains have some plane contacts, products of silica overgrowth in optical continuity, but they also
show sutured contacts related to pressure-solution
processes and, sometimes, subgrains. White mica
appears with fringed edges and as thin fibres surrounding larger quartz grains. Biotite frequently
has chloritized edges. Detrital chlorite has sticky
and, occasionally, fan habits. The anomalous brown
interference colour of detrital chlorites indicates
a composition with similar proportions of Fe and
Mg, corresponding to ferrous clinochlore (Nieto,
1997). Mica grains with intergrown chlorite were
also identified. Pyrite is the most abundant opaque
mineral, appearing as framboidal aggregates, and
irregular to euhedral crystals, but haematite was
also identified. The sandstone matrix is composed
predominantly of very fine quartz (up to 0.02 mm)
and subordinate phyllosilicates. Towards the top
of the unit, sandstone levels become thicker and
contain abundant volcanic lithics and volcaniclastic
grains (varieties of siliceous tuffs and ignimbrites).
The clay mineral fraction (< 2 μm) of the pelites
is mainly composed of mica (76-97%), chlorite
(3-24%), and quartz, with subordinate plagioclase
(Table 1). In the air-dried diagram of some samples,
the 14Å reflection is more intense than the 7Å reflection, and it decreases in intensity after heating,
together with an increase in intensity of the 10Å,
5Å, and 3.3Å reflections. This behaviour is likely
due to the presence of minor amounts of vermiculite,
characterized by an intense reflection at 14.2Å that
collapsed to 10Å after heating at 500ºC. In another
group of samples, the heated diagram shows a
reflection around 12Å that probably corresponds
to interstratified Chl/Vrm. In one of the samples,
an Ilt/Chl mixed-layer (~18%) was also identified.
KI(CIS) values in this unit range between 0.41 and 0.59
Δº2q (n: 15), and the mean white mica b parameter
is 9.011 (n: 4; s: 0.006).
alternating with a clastic succession represented by
sandstone layers up to 0.2 m thick. Among the volcanic
components, Astini (1998) mentioned subvolcanic
intrusives with columnar jointing, volcaniclastic
deposits, ignimbritic bodies, siliceous tuffs, and accretionary lapilly levels. Muddy interstratified layers
are scarce, with local heterolithic lenticular facies
where pelites were intensely bioturbated (Dávila,
2003). In pelitic levels, a fracture pencil cleavage S1
developed subperpendicular to the S0. The coarser
clastic levels (Fig. 5e and f) have abundant rounded
or embayed quartz grains, mostly with straight extinction, and abundant grains of potassium feldspar,
with a smaller amount of plagioclase, volcanic lithic
fragments with porphyritic texture, rhyolitic glass
fragments with flux textures (probably ignimbrites),
massive to microcrystalline vitreous aggregates,
fragmented bioclasts and, less frequently, abundant
microcrystalline glauconitic grains. Volcanic glass
grains commonly have edges completely transformed to microcrystalline quartz and are covered by a
thin layer of clay minerals with high birefringence.
Pores are filled with carbonate cement and chlorite
aggregates. Detrital grains usually present abundant
plane and concave-convex contacts as a result of
pressure-solution processes. Between these contacts
are found relicts of thin layers of clays covering
the detrital grains. Quartz overgrowths in optical
continuity with detrital grains were only observed
in some levels.
The clay mineralogy of the scarce pelitic layers
cropping out in the Molles River locality consists
of illite (89-92%), chlorite (8-11%) and quartz
(Table 1). In the Cachiyuyo River section, two
samples corresponding to layers located a few
metres below the discordance that separates the
Molles Formation from the overlying Cerro Morado
Group mainly comprise interstratified illite/smectite
(Ilt/Sme;with 95% illitic layers) and subordinate
quartz. In this sequence, KI(CIS) values range from
0.46 to 1.32 Δº2q (n: 5), corresponding to the early
to late diagenetic zones.
4.1.3.3.2. Molles Formation. A detailed sedimentological-stratigraphic analysis of the Molles
Formation was carried out by Astini and Benedetto
(1996), Astini (1998, 2003), and Dávila (2003). The
sequence (~500 m in thickness) has volcanic levels
4.1.3.4.1. La Escondida Formation. The Cerro
Morado Group is separated from the Famatina
Group by an angular discordance. It begins with
the volcanic-volcaniclastic sequence of El Portillo
Formation and continues with the deposition of La
307 Collo et al.indd 297
4.1.3.4. Cerro Morado Group
13-07-2011 12:59:00
298
Low-grade metamorphism of Cambro-ordoviCian suCCessions in the famatina beLt...
Escondida Formation, which contains some pelitic
levels. Most of the psamitic and micro-conglomeratic
levels of this sequence are partially silicified. Astini
and Dávila (2002) interpreted both the vitreous
tuffs and the accretionary lapilly occurring within
these levels as falling deposits associated with
violent eruptions, linked to subaerial explosions of
phreatomagmatic origin, often associated with acid
volcanism. Microscopic features evidence viscose
fluxes with plastic deformation that contribute to
the elimination of porosity.
One of the analysed pelitic levels is located at
around 10 m from the base of the unit, whereas the
remaining two are in the middle part of the sequence and correspond to yellowish-green bioturbated
marine pelites interbedded with yellow sandstone
and limestone levels with high brachiopod contents.
The pelitic sample at the bottom of the sequence
shows a clay-mineral fraction mainly composed of
interstratified Ilt/Sme (90-95% illitic layers) with
traces of vermiculite. The two remaining samples
contain interstratified I/S, similar to that from the
previous sample, and also show traces of kaolinite,
vermiculite, and chlorite. In all three samples, subordinate quartz and potassium feldspar were also
identified (Table 1). KI(CIS) values range from 0.50
to 0.69 Δº2q (n: 3).
4.1.4. La Aguadita Formation (Upper Ordovician)
This unit can be divided into two informal members,
the lower mainly composed of metarkoses (150 m
thick) and the upper consisting of alternating metapelite and thin metasandstone levels (Fig. 6). S0 can
be recognized and is overprinted by a subparallel to
oblique secondary S1 foliation (350°N/63°E). In the
upper member, foliation is microscopically defined
by the orientation of the (001) planes of neoformed
and transformed phyllosilicates and the direction of
the long axis of the quartz and feldspar grains. The
S1 plane is interpreted as the product of the mechanical rotation and deformation of detrital minerals
by pressure-solution and recrystallisation processes,
which generate a fine-grained matrix parallel to the
foliation (P domains; Knipe, 1981) and modified
detrital grains parallel to the S1 foliation that is not as
strong (Q domains). Metapelites are very fine-grained
with centimetric bedding generated by alternating
clay and silt-clay layers, which reflect the sedimentary S0. The neoformed phyllosilicates are arranged
around the larger grains so that their (001) planes are
orientated defining metamorphic S1 foliation oblique
to subparallel to S0. Back-scattered electron images
indicate that pelites from La Aguadita Formation
are composed of quartz, K-white mica, chlorite, K-
FIG. 6. La Aguadita Formation. a. Outcrop view of the Upper Member with alternance of metasandstones and shales; b. Metaarkose
from the Lower Member; c. Metasandstone showing quartz-feldspathic grains surrounded by a quartz-mica matrix defining
a blastopsamitic to granolepodoblastic S1 foliation. These crystals have clear grain boundaries parallel to the S1 foliation,
whereas the perpendicular edges are diffuse and develop pressure shadows with beards of mica, quartz, and chlorite, typical
of dissolution-recrystallization processes. Potassium feldspar is usually altered to fine-grained white mica; d. Metapelite with
granulometric-compositional layering belonging to the S0 foliation and the orientation of neoformed phyllosilicates defining
the S1 secondary foliation; e. Typical representative texture of metasandstone at backscattered electron scale.
307 Collo et al.indd 298
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Collo et al./ Andean Geology 38 (2): 284-318, 2011
feldspar, and scarce epidote, which together define
a soft cleavage (cf. Passchier and Trouw, 1998).
In addition, blastic biotite up to 2 μm in thickness
and mainly associated with pressure shadows was
identified in two of the samples. K-white mica and
K-feldspar constitute large domains (>50 μm) mostly
orientated according to the well-developed slaty
cleavage; they present diffuse contacts indicative
of their concomitant development.
Metagreywackes have a blastopsamitic to granolepidoblastic texture producing rough cleavage.
The primary compositional layering (S0) is evidenced by alternating millimetric bands composed of
quartz, potassium feldspar, and plagioclase grains
surrounded by an epidote-quartz-mica matrix. XRD
whole-rock analyses show that metagreywackes from
the upper member mainly comprise quartz, chlorite,
mica, epidote, plagioclase, potassium feldspar, and
smectite (Table 1).
In the lower member, metarkoses have a clastic
to blastopsamitic texture with millimetric quartzfeldpar grains surrounded by a scarce matrix, predominantly composed of fine-grained white mica and
abundant opaque minerals. XRD analyses performed
on representative samples indicate that metarkoses
are composed of quartz, chlorite, mica, epidote, potassium feldspar, and plagioclase. In some samples,
dioctahedral mica is present in low proportions, and
in others it is totally absent.
Epidote blasts (as tabular grains or 1.5 mm
aggregates) identified in metarkoses and metasandstones replace detrital plagioclase and are mainly
associated with anhedral to radial chlorite, calcite
(0.15 mm.), and pyrite; they have been interpreted
as the product of pervasive hydrothermal propylitic
alteration (Collo et al., 2005).
The clay fraction from La Aguadita Formation
metapelites mainly consists of mica (61-94%) with
subordinate chlorite (4-9%). All these rocks have
minor amounts of smectite (2-6%) and associated
vermiculite, kaolinite, and interstratified Ilt/Sme
(R0), Ilt/Chl, and Chl/Vrm (Table 1). Given the
presence of smectite in all the analysed samples,
KI(CIS) values were measured both in air-dried and
glycolated aggregates with similar results. KI(CIS)
values from this unit range between 0.40 and
0.26ºΔ2θ (n: 6), indicating low to high anchizone
conditions (Table 1). Moreover, SEM analyses
identified blastic biotite in two samples, demonstrating that this unit attained green-schist facies
307 Collo et al.indd 299
299
metamorphism (~400ºC). Consequently, smectite
vermiculite, kaolinite, as well as interstratified Ilt/
Sme (R0), Ilt/Chl, and Chl/Vrm phases were interpreted as retrograde diagenesis products (Collo et
al., 2005). No relationship was identified between
the KI values and the stratigraphic position of the
samples in this section.
The white mica b parameter measured for finegrained rocks from this unit has a mean value of
9.024 (n: 4; s: 0.004, Table 1) corresponding to the
intermediate-pressure facies series of Guidotti and
Sassi (1986). Therefore, according to the temperature
deduced from mineralogy and KI values, a medium
palaeogeothermal gradient (~25°C/km) can be inferred, similar to those obtained for the oldest Negro
Peinado Formation.
4.2. Chemical composition of phyllosilicates from
Negro Peinado, Achavil and La Aguadita
Formations
4.2.1. Pampean synorogenic suites: The Negro
Peinado and Achavil formations
Neoformed white micas within the Negro
Peinado Formation show Fe+Mg contents of 0.120.59 apfu, silica contents of 3.06-3.39 apfu, and
interlayer charges of 0.82-1.03 apfu. In samples
containing blastic biotite, dioctahedral mica compositions are close to the muscovite end member
(Appendix A). Fe 3+/FeTot (XFe 3+)=0.75 was used
in the calculations of structural formulae given
the presence of haematite associated with the
mica blastesis (Guidotti et al., 1994). Although
moderate dispersion could be observed in the
white mica composition from this unit, analyses
carried out in individual samples show little internal variations (Fig. 7), which is in accordance
with advanced compositional homogenization. It
is worth emphasizing that, with the exception of
a few grains, these micas do not show significant
illitic substitution (SiAl-1□K -1).
In the Achavil Formation, XFe 3+=0.50 was
employed in the calculations of dioctahedral mica
structural formulae due to the presence of pyrite
and ilmenite in these samples. White micas from
this unit show a greater dispersion in Fe+Mg contents (0.11-0.70 apfu), and similar silica contents
(3.05-3.43 apfu) and interlayer charge (0.82-1.02
apfu) than the ones from the Negro Peinado For-
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300
307 Collo et al.indd 300
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FIG. 7. Diagrams showing chemical compositions of dioctahedral phyllosilicates in Negro Peinado, Achavil, and La Aguadita formations; solid circles and lines indicate theoretical muscovite
compositions and arrows represent different exchange vectors.
Collo et al./ Andean Geology 38 (2): 284-318, 2011
mation (Appendix A; Fig. 7). Moreover, micas
from this unit show greater internal variation in
individual samples than the ones from the Negro
Peinado Formation, indicating a lack of chemical
equilibrium between neoformed phases.
Dioctahedral micas from both units show clear
phengitic substitution and a few analyses depict
scarce paragonitic substitution (Na + contents
generally up to 0.11 apfu in the Negro Peinado
Formation and up to 0.17 apfu in the Achavil
Formation). Na + contents reaching 0.78 apfu
in one sample from the last unit (see ACH in
Appendix A) may be attributable to intermediate
meta-stable Na-K micas or to paragonite and
muscovite intergrowths at a very small scale (Livi
et al., 2008). Some ferrimuscovitic substitution
can also be inferred for both units considering the
difference between (Si-3)-Mg and Fe contents as
reflecting the presence of Fe3+.
Chlorite from the Negro Peinado Formation
presents Fe/Fe+Mg ratios between 0.43 and 0.63,
corresponding to ferrous clinochlore to magnesian
chamosite (Appendix B). Fe and Mg show an inverse tendency compatible with the FM substitution
(FeMg-1, Fig. 8; Laird, 1988). AlIV(Fe, Mg)-1Si-1
substitution seems to be somewhat unimportant,
as indicated by the total Al values (Fig. 8). The
same tendencies were observed for the Achavil
Formation, although in this unit chlorite has a
wider compositional range. In fact, chlorites from
the three analysed samples outline three different
groups in the Fe versus Mg diagram. Chlorites
from the Achavil Formation show Fe/Fe+Mg
ratios of 0.36 to 0.65 (Clinochlore to magnesic
chamosite), with a narrow range (0.45-0.49) in
the 546 sample. Chlorite from both units plots
close to the full octahedral occupancy line in the
diagram proposed by Hillier and Velde (1991), what
is characteristic of metamorphic chlorites (Fig.
8), although some grains with higher Si contents
in the Achavil Formation are in accordance with
its lower grade. The lower dispersion in chlorite
compositions in the Negro Peinado Formation is
consistent with the narrower range in Fe/Fe+Mg
ratios depicted by white micas in this unit.
In the Negro Peinado Formation, subordinate
di-trioctahedral substitution was also identified
(Fig. 8). These grains are characterized by their
lower octahedral cation sums and Fe+Mg contents
and higher Si and Al contents than other chlorites.
307 Collo et al.indd 301
301
4.2.2. Ocloyic synorogenic suite: La Aguadita
Formation
In this unit, XFe3+=0.75 was used in the calculations of dioctahedral mica formulae given the
presence of haematite associated with the mica
blastesis. White mica has Fe+Mg contents of
0.11-0.51 apfu, silica contents of 3.08-3.46 apfu,
and interlayer cation contents of 0.80-0.98 apfu
(Appendix A; Fig. 7). A direct correlation between
Si and Fe+Mg contents is consistent with phengitic
substitution. Paragonitic substitution is very low in
general, but one grain with Na contents up to 0.12
apfu is also present. Some ferrimuscovitic substitution can also be inferred based on the difference
between (Si-3)-Mg and Fe.
Chlorites from La Aguadita Formation have Fe/
Fe+Mg ratios of 0.51-0.65 (Appendix B; Fig. 8),
corresponding to magnesian chamosite. Most of the
analyses plot near the fully octahedral occupancy line
consistent with metamorphic chlorites. Some grains
depicting slightly higher Si contents could correspond
to diagenetic chlorite (Hillier and Velde, 1991).
4.3. Phengite geobarometry
Since the Si content of white mica is affected
by phengitic and illitic substitutions, the illitic
component should be taken into account for a
correct use of the phengite geobarometer (Agard et
al., 2001; Abad et al., 2006). Therefore, phengites
from the three analysed units that evidenced illitic
substitution (interlayer charge <0.85 and (Si-3)
higher than Mg+Fe content) were excluded from
the geobarometric calculation. Omitting grains
with illitic substitution, maximum Si contents in
individual samples ranges from 3.12 to 3.39 apfu
in the Negro Peinado Formation and from 3.20
to 3.42 apfu in the Achavil Formation (Appendix
A). Pelites from the two units do not contain
the limiting paragenesis Ms-KFsp-Qz-Phl, and
consequently the estimated pressures should be
considered minimum values (see Massone and
Schreyer, 1987). Using the recalculated Si isopleths grid published by Massone and Szpurka
(1997), and considering the estimated maximum
temperatures according to KI(CIS) and mineralogy ,
values between ~1 and ~4.5 kbar can be estimated
for the Negro Peinado Formation and between ~1
and 4 kbar for the Achavil Formation. In turn, two
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FIG. 8. Chemical composition diagrams of chlorites from Negro Peinado, Achavil, and La Aguadita formations.
302
307 Collo et al.indd 302
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Collo et al./ Andean Geology 38 (2): 284-318, 2011
303
5. Discussion
5.1. Characterization of the metamorphic-deformational episodes affecting the low-grade units
5.1.1. The Middle-Upper Cambrian Negro Peinado
Formation
The recrystallization and pressure-solution
processes identified in the rocks of the Negro Peinado Formation, the development of S1 foliation in
phyllite and meta-sandstone levels, the presence of
final members of the di- and tri-octahedral reaction
series (namely phengite and chlorite, respectively),
as well as the local biotite blastesis are clear indi-
The Achavil Formation also shows clear petrographic evidence of metamorphic transformations
such as reorientation of detrital grains, intracrystalline deformation, pressure-solution, and mineral
recrystallization, although in most locations it preserves its sedimentary attributes. High anchizonal
KI(CIS) values from this unit are consistent with the
clay mineral association recognized in metapelites,
bag
aw
a
Otago
N. New H
st
Boso
60
5.1.2. The Middle-Upper Cambrian Achavil Formation
uén
San
40
Vichu
q
Cumulative frequency
80
Negro Peinado Fm.
Achavil Fm.
La Aguadita Fm.
Volcancito and Suri Fms.
ampshire
100
cators of the succession having been affected by
low-grade metamorphism. These results, along
with measured crystallographic parameters, point
to a metamorphic-deformational event, M1NP-D1NP,
associated with intense intrafoliar folding P1NP in
greenschist facies, ranging from high anchizone to
epizone to the biotite zone (~290-400°C), and under
intermediate pressure (Fig. 9). No metamorphic trend
was observed in the sequence.
Ryoke
samples from La Aguadita Formation presenting
the limiting paragenesis (LD2p and LD9p) show
silica contents of up to 3.23 and 3.35 apfu, respectively, suggesting maximum pressure conditions
of around ~2.5 and ~5 Kbar.
20
0
8.96
8.98
9.00
9.02
9.04
9.06
White mica b parameter (A)
FIG. 9. Cumulative frequency versus b-parameter values. As reference low-pressure (white), intermediate-pressure (light grey), and
high-pressure (dark grey) fields from Guidotti and Sassi (1986) are shown, and curves from other well-characterized low-grade
metamorphic terranes are added (cf. Sassi and Scolari, 1974).
307 Collo et al.indd 303
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Low-grade metamorphism of Cambro-ordoviCian suCCessions in the famatina beLt...
For the Cambro-Ordovician volcaniclastic
sequence overlying the Achavil Formation,
metamorphic conditions from anchizone to late
diagenesis are interpreted on the basis of KI(CIS)
values (Fig. 10), that show a clear tendency to
more intensive post-depositional changes in
the more deeply buried rocks. A clear prograde
trend can also be established considering the
distribution of the neoformed clay minerals
throughout the sequence, with illite and chlorite
as the main phyllosilicates in the oldest units
and almost solely Ilt/Sme (R3) in the youngest.
This decrease in grade from the Volcancito
Formation to La Escondida Formation follows
a trend consistent with burial metamorphism.
Changes in clay mineral assemblages of
pelitic levels within each unit, such as the
total absence of chlorite in the upper levels of
the Molles Formation and in pelites from La
Escondida Formation (Cerro Morado Group),
contrasting with more than 10% in other levels,
could be associated to changes in the type of
source material in the basin. The predominance
of mica in these units is probably due to an
increase in the supply of rhyolitic components.
This contribution is clear in the coarser-grained
rocks, as mentioned by Astini and Dávila (2002).
From a textural point of view, it can be seen
that the processes of mineral recrystallization
are more intense in the Volcancito Formation
rocks, whereas in younger successions they are
rare or absent. At the same time, a decrease in
intracrystalline deformation as well as in pressuresolution processes is evident towards the top of
the sedimentary column. The relative abundance
of serrated contacts in rocks of the older units
with respect to the younger ones evidences the
differential strength of these processes.
307 Collo et al.indd 304
0
Epiz.
Anquizone
0,25
0,42
1
Diagenesis
La Aguadita
Formation
460
Carboniferous
La Escondida
Formation
475
El Portillo
Formation
Angular discordance
Molles
Formation
Suri Formation
5.1.3. The Cambro-Ordovician volcaniclastic
units
Kübler index (CIS Dº2q)
Cerro Morado Group
and define a main metamorphic event M1ACH
with estimated temperatures reaching ~290°C
and intermediate- to low-pressure conditions.
Although textural relations between M1ACH and
the deformational episodes P1ACH and P2ACH
could not be clearly established, the oblique
cleavage probably developed in association
with the second folding episode.
Famatina Group
304
Cerro Tocino
volcanics
480
Bordo
Atravesado
Formation
Volcancito
Formation
488
500
505
Achavil Fm.
N.Peinado Fm.
FIG. 10. Kübler index value distribution along the sequences. The decrease in metamorphic grade towards the younger sequences
can be clearly observed, as well as the visible break between
the Ordovician volcaniclastic units and the middle Ordovician
La Aguadita formation.
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Collo et al./ Andean Geology 38 (2): 284-318, 2011
Considering the Cambro-Ordovician succession
cropping out in the central-western sheets (from
the Volcancito to La Escondida formations), it
can be observed that neoformed phyllosilicates
are predominantly orientated parallel to S0. The
marked parallelism between the sedimentary S0
and the secondary cleavage defines a compaction
fabric (compaction cleavage) in most of the units,
suggesting that the post-depositional changes would
have been associated with the sequence burial,
suggesting that the main burial episode would not
have been linked to a major deformational episode.
The disposition of neoformed minerals alongside
the sedimentary layering established for the finegrained rocks indicates that the maximum P-T
conditions, and thus the diagenetic-metamorphic
grade reached by the sequence, were controlled by
basin subsidence processes.
The white mica b parameter measured in Volcancito and Suri formations suggests conditions
close to the boundary between intermediate and
intermediate-low pressure facies series (Guidotti
and Sassi, 1986). The cumulative frequency curve
for the white mica b parameter of all the rocks corresponding to this structutal sheet resembles the
curve for the eastern sector of the orogenic belt of
Royke (Fig. 9), assigned to low to intermediate-low
pressure facies series, with an average b parameter
of b 9.015Å (Sassi and Scolari, 1974).
Based on KI(CIS) values, temperatures between
~ 240-255°C can be established for the bottom of
the Cambro-Ordovician column (about 4,000 m
preserved), with temperatures between ~125-145ºC
estimated for the upper levels. This increase in
temperature with depth (~130ºC in 4,000 m) with a
relatively constant slope (see Fig. 9), is consistent
with a gradient of approximately 33ºC/km, similar
to that suggested from the b parameter.
Colour alteration indices (CAI) for conodonts
recovered from the Suri Formation (2-3; Albanesi
and Astini, 2000) are consistent with the temperatures
estimated for this unit (~165-200°C). CAI values
between 2 and 5 obtained for the Lower Member
of the Volcancito Formation (Albanesi et al., 1999)
suggest a not well constrained temperature range
(60-140ºC and 300-480ºC for minimum and maximum temperatures, respectively) in comparison
to that set by the petrographic and mineralogical
analysis (~210-240°C). Moreover, textural features
(e.g., sutured contacts and undulatory extinction
307 Collo et al.indd 305
305
in quartz grains, stylolites), indicates that the sequence was affected by pressure-solution processes
and intracrystalline deformation consistent with
the conditions established through KI(CIS) and clay
mineralogy.
5.1.3.1. Significance of Chl/Vrm through the
Cambro-Ordovician succession
Subordinate clay-mineral phases such as Chl/
Vrm, kaolinite, smectite, vermiculite, and Ilt/Chl
appear throughout the Cambro-Ordovician sequence.
Interstratified Chl/Vrm present in some samples
could correspond to a metaestable phase from which
chlorite formed, being part of the prograde reaction
series corrensite→corrensite-chlorite→chlorite. The
presence of intermediate phases belonging to this
series, in which the vermiculite would be generated
by detrital biotite alteration (Bozkaya and Yalçin,
2004), is consistent with late diagenetic conditions
(Jiang and Peacor, 1994; Merriman and Peacor,
1999). However, some of the samples containing
interstratified Chl/Vrm show KI(CIS) values corresponding to high-anchizone conditions (e.g., Achavil
Formation), under which Chl/Vrm is generally not
stable, and it seems more likely they are the product
of retrograde reactions. It should be noted that some
millimetric veins of chlorite+opaque minerals were
identified in the metapelites. Within this setting,
vermiculite and Chl/Vrm could be the product of
chlorite alteration under oxidizing conditions (Gaudin et al., 2005), as evidenced in the coarse-grained
rocks, likely associated with localized interaction
with post-metamorphic fluids. Moreover, kaolinite
and smectite would be the product of local alteration
(hydrolysis) of feldspar and illite in the rocks during
or after exhumation of the sequence. This seems to
be the case for sample AC-V, taken near the top of
the Volcancito Formation immediately below the
discordance with the Carboniferous, in which kaolinite (21%) probably results from the predominant
transformation of feldspars interacting with meteoric
water following the uplift of the unit.
5.1.4. The youngest La Aguadita Formation
The upward transition from anchizonal to diagenetic grade is clear in the Cambro-Ordovician sequence
from the central-western sheets within the Famatina
belt. However, the eastern and youngest La Aguadita
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Low-grade metamorphism of Cambro-ordoviCian suCCessions in the famatina beLt...
Formation (deposition after ca. 452 Ma, Astini et al.,
2003) depicts a sharp increase in metamorphic conditions in comparison with the upper levels croping out
in the central-western sheets (Fig. 10). The existence
of a tectonic fabric (S1), oblique to S0, shows that
metamorphic processes here were concomitant with
a deformational episode during which the detrital and
neoformed grains were orientated perpendicular to
the main strain; suggesting an intense metamorphicdeformational (M1LA-D1LA) event (Collo et al., 2005).
This main metamorphic-deformational episode attained anchizone to greenschist facies (temperatures
reaching ~400°C) and intermediate-pressure facies
series (see Fig. 9).
A post-M1LA propylitic hydrothermal event can
be inferred for this unit taking into account the compositional variation observed between coarse-grained
rocks (with epidote, chlorite, calcite, and pyrite)
and metapelites. In several systems with propylitc
alterations, the first appearance of epidote has been
recorded over the 200°C isotherm (Bird and Spieler, 2004). Moreover, the presence of smectite and
interstratified Ilt/Sme (R0) in the <2μm fraction of
La Aguadita Formation metapelites is incompatible
with both the medium-anchizonal degree and the
development of metamorphic foliation. These phases
could be the result of the interaction of fluids with
the rocks during the cooling of the hydrothermal
system that generated the propylitic association in
coarse-grained rocks, or to the influence of meteoric
water after exhumation (Collo et al., 2005). Both
processes can be regarded as retrograde diagenetic
events (Nieto and Peacor, 1993; Nieto et al., 1994;
Merriman and Peacor, 1999) as this term, according
to its most recent definition (Nieto et al., 2005),
covers all very low-temperature processes occurring
during the ascent of the rocks to the surface after
the metamorphic peak, including hydrothermal
alterations. It is clear that the genesis of these retrograde phyllosilicates in La Aguadita Formation
metapelites took place after the prograde mica and
chlorite formation (270-330ºC).
5.2. Mica chemical compositions and metamorphic conditions
Although textural and compositional changes
taking place within diagenetic to low epizone conditions are gradual and difficult to quantify, a trend
towards chemical and textural homogenization,
307 Collo et al.indd 306
including an increase in grain size of phyllosilicates,
is clear in rocks above the epizone (Nieto and Abad,
2007). This trend can be clearly observed in both the
Famatinian Cambrian Negro Peinado and Achavil
formations, but also in the Upper Ordovician La
Aguadita Formation, cropping out in the eastern
belt and affected by the strongest metamorphic
conditions. The Negro Peinado Formation shows
compositional homogeneity in dioctahedral micas
consistent with epizonal metamorphism (Abad et
al., 2006). The Achavil Formation has compositional features, especially in dioctahedral micas,
indicating a lower metamorphic grade, as was
previously established through the Kübler index.
In spite of its younger age, La Aguadita Formation
reveals an intermediate behaviour between the
Negro Peinado and Achavil Formations in terms
of phyllosilicate evolution.
The large dispersion in the pressures estimated through the phengite geobarometer for the
Achavil, Negro Peinado and La Aguadita formations could indicate continuous crystallization of
micas over the entire decompression path (Abad
et al., 2003). Moreover, a careful treatment of
this geobarometer is required given the lack of
chemical and thermodynamic equilibrium even at
individual sample scale in anchizonal to low-grade
metamorphic rocks, frequently resulting in the
coexistence of metastable phases. Moreover, it is
worth to emphasize that modifications of ~50°C
in temperature estimations change substantially
the obtained pressure values.
5.3. Regional implications of the low-grade metamorphism in the Famatina belt
Although the post-depositional history in sedimentary basins associated with arc and back-arc
settings is frequently complex, two contrasting
situations can be considered depending predominantly on the relationship between heat flow and
deformation. In cases where gradients are high and
deformation is not very strong, a direct relationship
between the increase in metamorphic grade and the
age of the affected sequences is expected, what is
known as the burial metamorphic pattern. On the
other hand, when tectonic stress predominates, the
above-mentioned relationship does not occur, and
then the metamorphic grade does not necessarily
increase from younger to older units. The post-
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Collo et al./ Andean Geology 38 (2): 284-318, 2011
depositional evolution of the Cambro-Ordovician
Famatinian units cannot be fully explained by only
one of these extreme models.
The Negro Peinado Formation presents a tight
isoclinal folding synchronous with metamorphism
(phyllosilicates oriented non-parallel to S0) likely
associated with contraction at a convergent plate
margin (Merriman and Peacor, 1999). This unit
is intruded in several localities by Early-Middle
Ordovician granitoids (ca. 484-463 Ma, Pankhurst
et al., 2000; Dahlquist et al., 2008) associated
with crustal recycling, that developed contact
metamorphic aureoles whose textures indicate that
the intrusion was coeval with the folding affecting
Negro Peinado rocks. Thus, the age of the main
metamorphic episode affecting these rocks could be
constrained between ca. 485-460 Ma. The few K-Ar
ages available for the Negro Peinado Formation are
roughly consistent with an Ordovician syntectonic
metamorphic event (ca. 463-457 Ma, Collo et al.,
2008), and compatible with the crustal thickening
events proposed for the region immediately after
granitoids intrusions (Dalhquist et al., 2008).
The post-depositional evolution of the MiddleUpper Cambrian Achavil Formation is more difficult
to unravel. Two possible interpretations could be
made for the metamorphism affecting the Achavil
Formation: 1. it could have taken place prior to the
deposition of the Volcancito Formation (ca. 490 Ma),
and contemporary to one of the deformational phases
affecting the unit; or alternatively; 2. maximum
burial conditions could have been reached after the
deposition of the entire volcaniclastic column (ca.
460 Ma). The first hypothesis is supported by the
development of the main secondary microscopic
foliation oblique to the sedimentary layering; the
second one is based on the fact that no significant
break is recorded between maximum metamorphic
conditions and pressure facies series between the
Achavil and Volcancito formations, in spite of the
existing discordance between them (~490; Collo
and Astini, 2008). In this context, the oblique metamorphic foliation in the Achavil Formation could
be interpreted as developed before the deposition
of the Volcancito Formation (ca. 490 Ma), whereas
maximum burial conditions, recorded by phengitic
substitution in white micas, would be reached after
the deposition of the complete Cambro-Ordovician
succession (after ca. 460 Ma). A coeval metamorphism and a prograde transition between the Achavil
307 Collo et al.indd 307
307
and the Negro Peinado formations previous to ca.
490 Ma is difficult to conciliate with ages of the
granitoids and the textural relations between them
and the last unit. Similar orientations between folding in Negro Peinado Formation and P2 folding
in Achavil Formation are not enough to argue an
equivalent folding episode as, since the Ordovician,
compressive events in the region have involved
predominantly W-E shortening. Evidences of earlier
deformational phases in Negro Peinado Formation
could have been obliterated by the following and
main tectonic episode. On the other side, for the
younger units we can clearly establish a correlation
between sedimentary age and metamorphic grade
from the Volcancito Formation to the Cerro Morado
Group throughout the sequence in the central-western
sheets (Fig. 10). This relationship occurs despite
the existing discordances between the Famatina
and Cerro Morado groups (~470 Ma; Dávila et al.,
2003) as there are no breaks in KI(CIS) trends between
these units. Moreover, other burial indicators, such
as the cleavage development and textural features
mentioned above, also point to a lack of any break
in metamorphic grade in these Cambro-Ordovician
sequences. This relative continuity indicates that
for these units the burial was not episodic, with
significant basin inversion events and discrete metamorphic cycles. On the contrary, it could be inferred
that it took place in a single event, affecting the
whole Cambro-Ordovician volcaniclastic sequence,
and post-dating the afore-mentioned discordance.
Overall, the increase in metamorphic grade, from
deep diagenesis to high anchizone, from younger to
older units, is consistent with a burial metamorphic
pattern (cf. Merriman and Peacor, 1999). In such a
pattern, the diagenetic-metamorphic grade reached
by the sequence is controlled by basin subsidence
regimes, probably over a thin and hot lithosphere,
typical of arc regions within subduction zones
(Hyndman et al., 2005; Currie and Hyndman, 2006),
as is supported by the estimated P/T conditions for
central-western sheets. However, a marked break
in this burial metamorphic pattern occurs between
the younger units of central-western sheets and the
upper Ordovician levels of La Aguadita Formation,
as the last one depicts strongest metamorphicdeformation conditions than the older Ordovician
sequences cropping out to the west (Fig. 10).
The burial metamorphic pattern, as well as the
intermediate- to low-pressure conditions inferred
13-07-2011 12:59:04
308
Low-grade metamorphism of Cambro-ordoviCian suCCessions in the famatina beLt...
for the Cambrian-Ordovician volcaniclastic successions within the Famatina belt, are broadly consistent
with the proposed back-arc extension between ca.
480-460 Ma in this region. Moreover, a subsequent
crustal thickening and orogenic exhumation at a
hinterland position and later local deposition and
tectonic burial in a retro-foreland basin to the east
are supported by the data. The differential metamorphism of La Aguadita Formation is likely the result
of orogenic shortening and coeval tectonic stacking
associated with the progressive advancement of the
fold-thrust belt from the west towards the foreland
after ca. 452 Ma. The preservation of the burial
metamorphic pattern to the west could be accounted
by considering a dominant tectonic inversion within
the arc region during the final stages of the Ocloyic
orogeny (Fig. 2; 460-435 Ma stage). Differences
between intermediate-pressure metamorphism in
La Aguadita Formation and low-pressure metamorphism associated to extension described for the other
Ordovician units croping out to the east and north
(Steenken et al., 2006; Delpino et al., 2007; Fernández
et al., 2008; Verdecchia, 2009) could be attributed
to their diachronic development. The characteristics
of the tectono-metamorphic event that affected La
Aguadita Formation indicate a compressive regime that is compatible with the termination of the
subduction and with the beginning of the terrane
collision and lithosphere cooling and thickening (cf.
Hyndman et al., 2005). Further thermobarometric
and geochronologic analyses seem to be necessary
to better understand the post-depositional evolution
of the Negro Peinado and Achavil formations.
Acknowledgements
We are grateful to the Consejo Nacional de Investigaciones Científicas y Técnicas, the Agencia Nacional
de Promoción de Ciencia y Tecnología, the Secretaría
de Ciencia y Tecnología of the Universidad Nacional de
Córdoba and Research Project CGL2007-66744-C02-01
(Spanish Ministry of Science and Technology), which all
supported our Research Projects in western Argentina. The
stay of G. Collo and M. Do Campo at the University of
Granada and the field work of F. Nieto in Argentina were
supported by AECI projects A/5120/06 and A/7712/07.
We acknowledge Dr. R. Astini for his constructive review
of the manuscript, I. Guerra Tschuschke for help with the
SEM analyses, which were performed at the Centro de
Instrumentación Científica, Universidad de Granada, and
J. Santamarina for the measurement of b parameter values.
307 Collo et al.indd 308
C. Laurin helped with the English in the manuscript. The
authors also thank thorough reviews by Dr. C. Casquet and
an anonymous reviewer and the editorial recommendations
that allowed improving this work.
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312
APPENDIX A. CHEMICAL COMPOSITIONS FOR DIOCTAHEDRAL MICAS.
Si
AlIV
AlVI
Fe
Mg
Mn
Ti
∑ oct
K
Na
Ca
∑ int
La Aguadita Formation
LAG-1
3.19
0.81
1.73
0.14
0.18
0.00
0.00
2.04
0.89
0.00
0.00
0.89
LAG-2
3.20
0.80
1.70
0.13
0.17
0.00
0.01
2.02
0.91
0.03
0.00
0.94
LD2p-1
3.20
0.80
1.58
0.24
0.20
0.00
0.01
2.04
0.93
0.00
0.01
0.94
LD2p-2
3.20
0.80
1.65
0.16
0.20
0.01
0.01
2.03
0.91
0.01
0.01
0.93
LD2p-3
3.24
0.76
1.63
0.20
0.18
-0.01
0.01
2.02
0.89
0.02
0.00
0.91
LD2p-4
3.35
0.65
1.60
0.17
0.21
0.01
0.01
2.00
0.87
0.02
0.00
0.89
LD2p-5
3.21
0.79
1.65
0.17
0.20
0.00
0.01
2.03
0.92
0.01
0.00
0.93
LD2p-6
3.22
0.78
1.65
0.15
0.19
0.01
0.01
2.02
0.93
0.02
0.00
0.95
LD2p-7
3.30
0.70
1.60
0.20
0.21
0.00
0.01
2.02
0.87
0.01
0.00
0.88
LD2p-8
3.22
0.78
1.62
0.19
0.21
0.00
0.01
2.04
0.90
0.01
0.00
0.91
LD2p-9
3.22
0.78
1.59
0.21
0.22
0.01
0.02
2.04
0.89
0.01
0.00
0.91
LD2p-10
3.22
0.78
1.60
0.21
0.21
0.00
0.01
2.03
0.88
0.02
0.01
0.92
LD9p-1*
3.35
0.65
1.64
0.18
0.22
0.00
0.00
2.04
0.80
0.00
0.00
0.80
LD9p-2
3.09
0.91
1.86
0.06
0.08
0.00
0.00
2.01
0.90
0.07
0.00
0.98
LD9p-3*
3.46
0.54
1.49
0.24
0.26
0.00
0.02
2.01
0.78
0.02
0.01
0.81
LD9p-4*
3.38
0.62
1.58
0.20
0.20
0.00
0.02
2.00
0.83
0.02
0.01
0.85
LD9p-5*
3.30
0.70
1.59
0.20
0.23
0.00
0.01
2.04
0.81
0.03
0.01
0.85
LD9p-7
3.12
0.88
1.88
0.05
0.05
0.00
0.02
2.01
0.78
0.12
0.00
0.90
LD9p-8*
3.29
0.71
1.59
0.19
0.24
0.01
0.03
2.05
0.81
0.01
0.00
0.82
LD9p-9
3.11
0.89
1.61
0.25
0.12
0.00
0.06
2.04
0.87
0.03
0.00
0.90
LD9p-10
3.21
0.79
1.60
0.21
0.20
0.00
0.03
2.04
0.86
0.02
0.00
0.89
LD9p-11
3.08
0.92
1.89
0.06
0.06
0.00
0.01
2.02
0.88
0.03
0.00
0.91
LD9p-12
3.21
0.79
1.69
0.17
0.16
0.01
0.01
2.04
0.85
0.01
0.00
0.86
LD9p-13
3.15
0.85
1.84
0.13
0.04
0.00
0.00
2.00
0.90
0.01
-0.01
0.91
LD9p-14
3.23
0.77
1.71
0.16
0.14
0.00
0.01
2.02
0.87
0.03
0.00
0.89
Achavil Formation
V5m-1
3.34
0.66
1.48
0.26
0.25
0.01
0.03
2.03
0.94
0.00
0.00
0.94
V5m-2
3.35
0.65
1.43
0.33
0.26
0.00
0.03
2.04
0.85
0.05
0.01
0.91
V5m-3
3.41
0.59
1.48
0.24
0.24
0.00
0.03
2.00
0.90
0.01
0.01
0.91
V5m-4
3.27
0.73
1.45
0.36
0.20
0.01
0.02
2.04
1.00
0.00
0.00
1.00
V5m-5
3.26
0.74
1.38
0.40
0.23
0.01
0.02
2.05
1.00
0.01
0.00
1.01
V5m-6
3.27
0.73
1.42
0.38
0.21
0.01
0.02
2.03
1.00
0.01
0.00
1.01
V5m-7
3.26
0.74
1.44
0.37
0.21
0.00
0.02
2.05
0.98
0.00
0.00
0.98
V5m-8
3.23
0.77
1.38
0.42
0.23
0.01
0.03
2.06
0.97
0.03
0.00
0.99
V5m-9
3.16
0.84
1.77
0.13
0.08
0.00
0.02
2.00
0.88
0.09
0.00
0.96
V5m-10
3.28
0.72
1.63
0.20
0.20
0.01
0.02
2.06
0.81
0.03
0.01
0.84
V5m-11*
3.28
0.72
1.70
0.15
0.17
0.00
0.02
2.04
0.80
0.03
0.00
0.83
V12-1
3.23
0.77
1.50
0.26
0.21
0.01
0.08
2.05
0.87
0.01
0.00
0.88
V12-2
3.17
0.83
1.69
0.18
0.11
0.01
0.03
2.01
0.94
0.02
0.00
0.96
307 Collo et al.indd 312
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Collo et al./ Andean Geology 38 (2): 284-318, 2011
313
Appendix A continued.
Si
AlIV
AlVI
Fe
Mg
Mn
Ti
∑ oct
K
Na
Ca
∑ int
Achavil Formation
V12-3
3.19
0.81
1.52
0.29
0.17
0.01
0.05
2.04
0.94
0.03
0.00
0.96
V12-4
3.26
0.74
1.50
0.28
0.23
0.00
0.03
2.04
0.92
0.04
0.00
0.96
V12-5
3.25
0.75
1.49
0.29
0.24
0.01
0.02
2.05
0.95
0.03
0.00
0.98
V12-6
3.25
0.75
1.51
0.27
0.23
0.00
0.02
2.04
0.96
0.03
0.00
0.99
V12-7
3.35
0.65
1.59
0.19
0.26
0.00
0.01
2.04
0.85
0.00
0.01
0.86
V12-8
3.08
0.92
1.85
0.08
0.06
0.00
0.02
2.02
0.87
0.08
0.00
0.95
V12-9*
3.41
0.59
1.61
0.15
0.23
0.00
0.03
2.01
0.80
0.02
0.01
0.82
V12-10
3.17
0.83
1.69
0.15
0.16
0.00
0.03
2.03
0.92
0.03
0.00
0.95
V12-11
3.09
0.91
1.67
0.22
0.09
0.00
0.05
2.04
0.90
0.05
0.00
0.95
V15-1
3.18
0.82
1.60
0.23
0.12
0.00
0.04
2.00
1.01
0.01
0.00
1.02
V15-2
3.23
0.77
1.59
0.22
0.16
0.00
0.03
2.00
0.99
0.03
0.00
1.01
V15-3
3.14
0.86
1.76
0.12
0.13
0.00
0.01
2.03
0.91
0.06
0.00
0.97
V15-4
3.26
0.74
1.39
0.36
0.27
0.01
0.03
2.06
0.96
0.03
0.00
0.99
V15-5
3.05
0.95
1.84
0.06
0.06
0.00
0.04
2.01
0.89
0.09
0.00
0.97
V15-6
3.19
0.81
1.71
0.18
0.10
0.01
0.03
2.04
0.75
0.12
0.00
0.86
V15-7
3.19
0.81
1.73
0.14
0.13
0.00
0.01
2.01
0.90
0.05
0.00
0.95
V15-8
3.19
0.81
1.76
0.13
0.12
0.00
0.01
2.01
0.91
0.03
0.00
0.94
V15-9
3.16
0.84
1.75
0.14
0.11
0.00
0.01
2.01
0.92
0.06
0.00
0.98
V15-10
3.19
0.81
1.80
0.11
0.08
0.01
0.01
2.01
0.91
0.01
0.00
0.92
V15-11
3.30
0.70
1.35
0.50
0.18
0.01
0.02
2.05
0.96
0.00
0.00
0.97
V15-12
3.25
0.75
1.70
0.15
0.14
0.00
0.01
2.01
0.90
0.02
0.00
0.92
V15-13
3.16
0.84
1.74
0.11
0.12
0.00
0.03
2.01
0.92
0.03
0.01
0.95
V15-14*
3.24
0.76
1.82
0.09
0.10
0.00
0.01
2.02
0.82
0.01
0.01
0.83
V15-15
3.28
0.72
1.57
0.22
0.21
0.00
0.02
2.02
0.87
0.03
0.03
0.93
V15-16
3.24
0.76
1.67
0.17
0.17
0.00
0.02
2.03
0.90
0.01
0.00
0.91
V15-17*
3.36
0.64
1.55
0.24
0.22
0.00
0.02
2.03
0.85
0.00
0.00
0.85
V15-18*
3.43
0.57
1.55
0.09
0.38
0.00
0.02
2.04
0.84
0.00
0.00
0.84
V15-19
3.42
0.58
1.46
0.22
0.33
0.00
0.02
2.03
0.80
0.11
0.00
0.91
V15-20*
3.37
0.63
1.61
0.23
0.16
0.00
0.02
2.01
0.85
0.00
0.00
0.85
ACH-1
3.37
0.63
1.31
0.36
0.34
0.00
0.02
2.03
0.18
0.78
0.00
0.96
ACH-2
3.17
0.83
1.84
0.09
0.11
0.00
0.00
2.03
0.86
0.00
0.00
0.86
ACH-3
3.16
0.84
1.74
0.12
0.18
0.00
0.00
2.04
0.92
0.00
0.00
0.92
ACH-5
3.20
0.80
1.68
0.14
0.15
0.00
0.03
2.01
0.81
0.12
0.00
0.93
ACH-6
3.16
0.84
1.62
0.18
0.21
0.00
0.04
2.04
0.55
0.39
0.00
0.94
ACH-7*
3.37
0.63
1.50
0.17
0.28
0.00
0.00
1.95
0.61
0.21
0.00
0.82
ACH-8*
3.33
0.67
1.51
0.18
0.26
0.00
0.00
1.94
0.68
0.15
0.00
0.83
546-1
3.18
0.82
1.60
0.25
0.14
0.00
0.04
2.04
0.89
0.05
0.00
0.94
546-3
3.14
0.86
1.81
0.10
0.09
0.00
0.02
2.02
0.88
0.05
0.00
0.93
546-4
3.17
0.83
1.66
0.23
0.15
0.00
0.02
2.06
0.87
0.03
0.01
0.90
546-6
3.24
0.76
1.61
0.20
0.20
0.00
0.03
2.05
0.72
0.17
0.00
0.89
307 Collo et al.indd 313
13-07-2011 12:59:05
Low-grade metamorphism of Cambro-ordoviCian suCCessions in the famatina beLt...
314
Appendix A continued.
Si
AlIV
AlVI
Fe
Mg
Mn
Ti
∑ oct
K
Na
Ca
∑ int
Achavil Formation
546-7
3.14
0.86
1.83
0.10
0.07
0.00
0.01
2.00
0.86
0.08
0.00
0.95
546-11
3.11
0.89
1.79
0.10
0.16
0.00
0.02
2.06
0.85
0.04
0.00
0.88
546-12
3.24
0.76
1.72
0.11
0.17
0.00
0.02
2.02
0.85
0.05
0.00
0.89
Negro Peinado Formation
NP2-3
3.20
0.80
1.57
0.25
0.16
0.00
0.02
2.00
1.00
0.02
0.00
1.01
NP2-4
3.12
0.88
1.62
0.22
0.11
0.00
0.04
2.00
1.01
0.02
0.00
1.03
NP2-5
3.14
0.86
1.66
0.21
0.12
0.00
0.03
2.02
0.92
0.04
0.00
0.96
NP2-6
3.17
0.83
1.62
0.21
0.17
0.00
0.01
2.02
0.93
0.04
0.00
0.98
NP3-1*
3.17
0.83
1.74
0.16
0.12
0.00
0.02
2.04
0.82
0.02
0.00
0.84
NP3-2
3.25
0.75
1.58
0.23
0.19
0.00
0.02
2.02
0.90
0.01
0.00
0.91
NP3-3
3.27
0.73
1.59
0.22
0.18
0.00
0.01
2.01
0.89
0.05
0.00
0.94
NP3-4
3.33
0.67
1.55
0.25
0.18
0.00
0.02
2.00
0.86
0.03
0.00
0.89
NP3-5
3.19
0.81
1.61
0.23
0.17
0.01
0.02
2.03
0.89
0.04
0.00
0.92
NP3-6
3.14
0.86
1.82
0.06
0.11
0.00
0.03
2.03
0.84
0.04
0.00
0.88
NP3-7
3.15
0.85
1.65
0.21
0.13
0.00
0.03
2.02
0.93
0.03
0.00
0.96
NP3-8
3.21
0.79
1.56
0.26
0.19
0.00
0.02
2.04
0.89
0.03
0.00
0.92
NP3-9
3.22
0.78
1.60
0.24
0.17
0.00
0.02
2.03
0.88
0.03
0.00
0.91
CH73-1
3.09
0.91
1.79
0.11
0.09
0.01
0.03
2.02
0.87
0.07
0.00
0.94
CH73-3
3.18
0.82
1.77
0.11
0.09
0.00
0.03
2.00
0.84
0.07
0.00
0.91
CH73-4
3.06
0.89
1.83
0.11
0.05
0.00
0.02
2.01
0.90
0.08
0.00
0.98
CH73-5
3.11
0.89
1.76
0.12
0.11
0.00
0.03
2.02
0.87
0.07
0.00
0.95
CH73-9
3.11
0.89
1.77
0.12
0.10
0.00
0.02
2.01
0.91
0.06
0.00
0.97
CH64-1
3.15
0.85
1.75
0.11
0.13
0.00
0.02
2.01
0.92
0.05
0.00
0.97
CH64-2
3.16
0.84
1.73
0.14
0.13
0.00
0.02
2.03
0.86
0.06
0.00
0.91
CH64-3
3.29
0.71
1.73
0.12
0.14
0.00
0.01
2.00
0.81
0.05
0.00
0.87
CH64-4
3.12
0.88
1.84
0.05
0.10
0.00
0.02
2.00
0.88
0.07
0.00
0.95
CH64-5
3.14
0.86
1.76
0.10
0.14
0.00
0.01
2.02
0.89
0.04
0.00
0.94
QP3a-3
3.18
0.82
1.64
0.18
0.17
0.00
0.01
2.01
1.01
0.00
0.00
1.01
QP3a-8
3.09
0.91
1.87
0.07
0.05
0.01
0.02
2.00
0.95
0.03
0.00
0.98
QP3a-11
3.09
0.91
1.77
0.12
0.09
0.00
0.02
2.00
0.98
0.03
0.00
1.01
QP3a-13
3.23
0.77
1.61
0.19
0.18
0.01
0.02
2.01
0.90
0.02
0.01
0.93
QP3a-14
3.27
0.73
1.67
0.10
0.20
0.00
0.02
2.00
0.90
0.05
0.00
0.95
QP3a-17
3.33
0.67
1.50
0.21
0.30
0.00
0.01
2.03
0.90
0.03
0.00
0.93
QP3a-18
3.32
0.68
1.42
0.28
0.31
0.00
0.02
2.03
0.91
0.04
0.00
0.95
QP3a-19
3.33
0.67
1.44
0.25
0.32
0.00
0.02
2.03
0.91
0.04
0.00
0.94
QP3a-20
3.26
0.74
1.57
0.19
0.26
0.00
0.01
2.04
0.88
0.03
0.00
0.91
QP3a-21
3.24
0.76
1.66
0.15
0.20
0.00
0.02
2.02
0.88
0.03
0.00
0.91
QP3a-22
3.15
0.85
1.75
0.13
0.14
0.00
0.01
2.03
0.90
0.03
0.00
0.93
307 Collo et al.indd 314
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Collo et al./ Andean Geology 38 (2): 284-318, 2011
315
Appendix A continued.
Si
AlIV
AlVI
Fe
Mg
Mn
Ti
∑ oct
K
Na
Ca
∑ int
Negro Peinado Formation
QP3a-23
3.27
0.73
1.55
0.17
0.23
0.01
0.06
2.01
0.91
0.02
0.00
0.93
QP3a-24
3.21
0.79
1.67
0.13
0.18
0.00
0.02
2.01
0.92
0.03
0.00
0.95
QP3a-25
3.32
0.68
1.62
0.16
0.20
0.01
0.02
2.00
0.89
0.01
0.00
0.90
QP3a-26
3.27
0.73
1.59
0.21
0.18
0.00
0.03
2.01
0.77
0.11
0.01
0.89
0.87
QP3a-28
3.25
0.75
1.74
0.13
0.11
0.01
0.02
2.00
0.85
0.02
0.00
QP3a-29*
3.23
0.77
1.59
0.25
0.20
0.01
0.02
2.06
0.74
0.07
0.01
0.82
QP3a-30
3.39
0.61
1.57
0.18
0.22
0.00
0.02
2.00
0.85
0.03
0.00
0.87
QP3a-31
3.32
0.68
1.64
0.16
0.18
0.00
0.02
2.01
0.84
0.03
0.00
0.87
QP3a-32
3.26
0.74
1.78
0.09
0.11
0.00
0.01
2.00
0.81
0.05
0.00
0.86
CNP-1
3.12
0.88
1.75
0.14
0.11
0.00
0.02
2.03
0.85
0.06
0.00
0.91
CNP-2
3.09
0.91
1.80
0.11
0.07
0.00
0.02
2.01
0.91
0.06
0.00
0.97
CNP-3
3.07
0.93
1.81
0.11
0.07
0.00
0.03
2.02
0.89
0.06
0.00
0.95
CNP-4
3.13
0.87
1.78
0.12
0.08
0.00
0.02
2.01
0.90
0.03
0.00
0.93
CH55-2
3.25
0.75
1.68
0.15
0.15
0.00
0.02
2.00
0.87
0.03
0.00
0.91
517-1
3.22
0.78
1.61
0.23
0.13
0.00
0.02
2.00
0.88
0.07
0.00
0.95
517-2
3.23
0.77
1.62
0.19
0.16
0.01
0.02
2.00
0.94
0.04
0.00
0.97
517-3
3.10
0.90
1.71
0.19
0.10
0.00
0.02
2.02
0.93
0.03
0.00
0.96
517-4
3.09
0.91
1.71
0.19
0.09
0.00
0.02
2.01
0.94
0.04
0.00
0.98
517-5
3.21
0.79
1.71
0.18
0.10
0.00
0.01
2.01
0.82
0.09
0.00
0.90
*: analyses showing illitic substitution.
307 Collo et al.indd 315
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APPENDIX B. CHEMICAL COMPOSITIONS FOR TRIOCTAHEDRAL MICAS.
Si
AlIV
AlVI
Fe
Mg
Mn
Ti
∑ oct.
F/FM
La Aguadita Formation
Lag-1
Lag-2
Lag-3
Lag-4
Lag-5
Lag-6
Lag-7
Lag-8
Lag-9
Lag-10
Lag-11
2.82
2.92
3.27
2.79
2.72
2.62
2.73
2.73
2.64
2.69
2.79
1.14
1.06
0.71
1.20
1.23
1.38
1.27
1.27
1.36
1.31
1.21
1.36
1.29
1.50
1.29
1.28
1.42
1.35
1.35
1.35
1.38
1.33
2.62
2.71
2.30
2.64
2.73
2.54
2.45
2.57
2.65
2.59
2.76
1.88
1.84
1.74
1.94
1.95
1.91
2.03
1.94
1.91
1.89
1.73
0.10
0.09
0.09
0.10
0.10
0.11
0.12
0.09
0.09
0.11
0.11
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
5.96
5.94
5.63
5.97
6.06
5.98
5.96
5.96
6.01
5.97
5.94
0.58
0.60
0.57
0.58
0.58
0.57
0.55
0.57
0.58
0.58
0.61
LD9a-1
LD9a-2
LD9p-1
LD9p-2
2.81
2.71
3.21
3.08
1.17
1.24
0.76
0.88
1.32
1.32
1.51
1.39
2.35
2.47
2.47
2.82
2.22
2.20
1.64
1.55
0.07
0.06
0.06
0.07
0.00
0.00
0.01
0.00
5.96
6.06
5.69
5.83
0.51
0.53
0.60
0.65
V12-1
2.71
1.28
1.54
2.81
1.49
0.05
0.00
5.88
0.65
V12-2
2.68
1.27
1.20
2.84
2.06
0.03
0.01
6.12
0.58
V12-3
2.62
1.35
1.24
2.83
2.00
0.03
0.01
6.11
0.59
V12-4
2.69
1.26
1.29
2.78
1.97
0.02
0.01
6.07
0.59
V15-1
2.95
1.03
1.20
2.66
1.98
0.06
0.03
5.92
0.57
V15-2
2.95
1.02
1.16
2.68
2.02
0.05
0.04
5.95
0.57
V15-3
3.14
0.82
1.58
2.58
1.47
0.05
0.00
5.70
0.64
V15-4
2.90
1.12
1.46
2.63
1.65
0.06
0.00
5.80
0.62
V15-5
2.82
1.19
1.16
2.72
2.06
0.03
0.01
5.98
0.57
V15-6
2.76
1.23
1.07
2.81
1.55
0.06
0.31
5.80
0.65
V15-7
2.98
0.99
1.08
2.61
2.03
0.06
0.12
5.90
0.56
V15-8
2.89
1.09
1.43
2.33
1.97
0.05
0.05
5.83
0.54
V15-9
2.86
1.14
1.38
2.35
2.09
0.05
0.00
5.88
0.53
V15-10
2.80
1.15
1.57
2.46
1.81
0.05
0.00
5.89
0.58
V15-11
3.29
0.68
1.12
1.67
2.97
0.03
0.03
5.82
0.36
V15-14
2.98
1.00
1.56
2.28
1.88
0.05
0.00
5.76
0.55
V15-15
3.26
0.72
1.60
2.19
1.77
0.04
0.00
5.60
0.55
V15-16
2.88
1.12
1.23
2.24
1.68
0.04
0.38
5.57
0.57
V15-17
2.91
1.06
1.56
2.41
1.79
0.04
0.00
5.80
0.57
546-1
2.73
1.23
1.43
2.20
2.31
0.03
0.00
5.97
0.49
546-2
2.62
1.33
1.34
2.29
2.42
0.04
0.02
6.09
0.49
546-3
2.80
1.15
1.45
2.15
2.29
0.03
0.02
5.94
0.48
546-4
2.67
1.28
1.43
2.25
2.30
0.03
0.00
6.02
0.49
546-5
2.65
1.33
1.41
2.22
2.34
0.03
0.00
6.01
0.49
546-6
2.61
1.38
1.37
2.14
2.49
0.03
0.00
6.04
0.46
546-7
2.67
1.32
1.37
2.10
2.51
0.03
0.00
6.01
0.45
546-8
2.63
1.37
1.22
2.09
2.40
0.03
0.16
5.91
0.47
Achavil Formation
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317
Appendix B continued.
Si
AlIV
AlVI
Fe
Mg
Mn
Ti
∑ oct.
F/FM
Negro Peinado Formation
NP2-1
2.67
1.28
1.22
2.10
2.47
0.07
0.13
5.99
0.46
NP2-2
2.65
1.31
1.29
2.05
2.65
0.07
0.00
6.07
0.44
NP2-3
2.81
1.15
1.40
1.93
2.54
0.07
0.00
5.95
0.43
NP2-4
2.77
1.20
1.49
1.91
2.43
0.07
0.00
5.90
0.44
NP2-5
3.03
0.92
2.03
1.53
1.93
0.04
0.00
5.53
0.44
NP2-6
2.75
1.21
1.48
2.18
2.21
0.06
0.01
5.94
0.50
NP2-7
2.75
1.22
1.37
2.20
2.30
0.07
0.01
5.96
0.49
NP2-8
2.99
0.97
1.49
2.06
2.19
0.07
0.00
5.82
0.48
NP2-9
2.96
1.01
1.47
2.06
2.22
0.07
0.01
5.83
0.48
NP2-10
2.74
1.22
1.40
2.16
2.34
0.09
0.00
5.99
0.48
NP3-1
2.68
1.28
1.34
2.18
2.47
0.05
0.00
6.04
0.47
NP3-2
2.67
1.28
1.26
2.34
2.44
0.06
0.00
6.10
0.49
CH73-1
2.60
1.39
1.42
2.54
2.00
0.05
0.00
6.01
0.56
CH73-2
2.63
1.38
1.51
2.40
1.95
0.05
0.01
5.91
0.55
CH73-3
2.63
1.37
1.49
2.31
2.09
0.05
0.00
5.94
0.53
CH73-4
2.63
1.36
1.58
2.30
1.96
0.07
0.00
5.91
0.54
CH73-5
2.60
1.37
1.49
2.38
2.08
0.04
0.00
5.99
0.53
CH73-6
2.53
1.47
1.48
2.38
2.10
0.04
0.00
6.00
0.53
CH73-7
2.55
1.45
1.40
2.46
2.11
0.06
0.00
6.03
0.54
CH73-8
2.62
1.36
1.40
2.41
2.17
0.05
0.00
6.02
0.53
CH73-9
2.56
1.42
1.48
2.46
2.03
0.04
0.00
6.01
0.55
CH73-10
2.50
1.50
1.46
2.40
2.09
0.05
0.00
6.02
0.53
CH73-11
2.56
1.44
1.45
2.42
2.07
0.05
0.00
5.99
0.54
CH73-12
2.61
1.39
1.48
2.40
2.02
0.05
0.00
5.95
0.54
CH73-13
2.58
1.39
1.46
2.47
2.04
0.06
0.00
6.02
0.55
CH73-14
2.59
1.41
1.45
2.35
2.13
0.05
0.00
5.98
0.52
CH73-15
2.54
1.46
1.38
2.49
2.14
0.04
0.00
6.04
0.54
CH73-16
2.58
1.42
1.44
2.45
2.10
0.00
0.00
5.99
0.54
CH73-17
2.55
1.45
1.45
2.41
2.10
0.04
0.00
6.00
0.53
CH73-18
2.56
1.42
1.44
2.42
2.12
0.04
0.00
6.03
0.53
CH73-19
2.55
1.44
1.43
2.50
2.07
0.03
0.00
6.04
0.55
CH64-1
2.72
1.27
1.42
2.42
2.07
0.02
0.00
5.94
0.54
CH64-2
2.79
1.20
1.40
2.35
2.13
0.03
0.01
5.92
0.52
CH64-3
2.94
1.02
2.01
1.91
1.65
0.02
0.00
5.59
0.54
CH64-4
2.68
1.28
1.37
2.51
2.13
0.03
0.00
6.03
0.54
CH64-5
2.69
1.28
1.41
2.39
2.12
0.03
0.01
5.97
0.53
CH64-6
2.84
1.16
1.65
2.10
1.97
0.03
0.00
5.75
0.52
CH64-7
2.86
1.12
1.38
2.46
2.07
0.00
0.00
5.91
0.54
CH64-9
2.97
1.00
1.48
2.37
1.94
0.03
0.00
5.81
0.55
CH64-10
2.72
1.23
1.34
2.30
2.37
0.02
0.01
6.03
0.49
CH64-11
2.65
1.34
1.37
2.32
2.28
0.02
0.00
6.00
0.50
CH64-12
2.67
1.32
1.32
2.35
2.32
0.03
0.00
6.02
0.50
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Appendix B continued.
Si
AlIV
AlVI
Fe
Mg
Mn
Ti
∑ oct.
F/FM
Negro Peinado Formation
CH64-13
2.75
1.23
1.49
2.18
2.18
0.03
0.01
5.89
0.50
CH64-14
2.70
1.28
1.34
2.36
2.28
0.02
0.00
6.01
0.51
CH64-15
2.71
1.27
1.36
2.29
2.30
0.03
0.00
5.99
0.50
CH64-16
2.89
1.10
1.46
2.25
2.07
0.03
0.00
5.82
0.52
CH64-17
2.80
1.15
1.55
2.25
2.03
0.02
0.01
5.87
0.53
CH64-18
2.68
1.30
1.36
2.34
2.27
0.03
0.00
6.00
0.51
CH64-19
2.61
1.39
1.39
2.34
2.25
0.03
0.00
6.00
0.51
CH64-20
2.87
1.10
1.73
2.08
1.91
0.02
0.00
5.75
0.52
CH64-22
2.70
1.27
1.35
2.30
2.33
0.03
0.00
6.01
0.50
CH64-23
2.73
1.25
1.47
2.28
2.15
0.03
0.00
5.93
0.52
QP3a-1
2.70
1.26
1.35
2.66
1.93
0.07
0.01
6.02
0.58
QP3a-2
2.72
1.24
1.46
2.58
1.85
0.07
0.00
5.96
0.58
QP3a-3
2.67
1.31
1.38
2.62
1.92
0.07
0.01
6.00
0.58
QP3a-5
2.68
1.32
1.45
2.50
1.93
0.06
0.00
5.94
0.57
QP3a-6
2.98
0.97
1.43
2.72
1.63
0.07
0.00
5.86
0.63
QP3a-7
2.72
1.29
1.28
2.91
1.71
0.06
0.00
5.97
0.63
QP3a-8
2.63
1.34
1.24
3.03
1.78
0.07
0.00
6.12
0.63
QP3a-9
2.62
1.37
1.27
2.88
1.82
0.07
0.01
6.06
0.61
QP3a-10
2.74
1.27
1.33
2.80
1.77
0.07
0.00
5.97
0.61
QP3a-11
2.84
1.15
1.38
2.75
1.69
0.06
0.01
5.90
0.62
QP3a-12
2.73
1.25
1.32
2.86
1.73
0.08
0.00
5.99
0.62
QP3a-13
2.69
1.30
1.30
2.87
1.76
0.08
0.00
6.02
0.62
QP3a-14
2.76
1.22
1.37
2.77
1.75
0.06
0.01
5.96
0.61
QP3a-15
2.65
1.32
1.28
2.91
1.80
0.08
0.01
6.07
0.62
QP3a-16
2.65
1.35
1.42
2.49
1.98
0.07
0.01
5.96
0.56
QP3a-18
2.66
1.33
1.40
2.53
1.97
0.07
0.01
5.97
0.56
QP3a-19
2.88
1.12
1.50
2.37
1.88
0.06
0.00
5.81
0.56
QP3a-21
2.84
1.15
1.44
2.39
1.98
0.06
0.00
5.87
0.55
QP3a-22
2.67
1.33
1.36
2.49
2.05
0.07
0.01
5.99
0.55
CNP-1
2.63
1.35
1.53
2.41
1.96
0.04
0.01
5.95
0.55
CNP-2
3.18
0.82
1.64
2.09
1.85
0.00
0.00
5.59
0.53
CH55-1
2.71
1.26
1.40
2.66
1.87
0.04
0.01
5.98
0.59
CH55-2
2.64
1.33
1.39
2.74
1.85
0.04
0.01
6.03
0.60
CH55-3
2.65
1.31
1.36
2.76
1.89
0.04
0.00
6.05
0.59
CH55-4
2.70
1.28
1.44
2.62
1.85
0.04
0.00
5.96
0.59
517-1
2.67
1.31
1.41
2.38
2.12
0.07
0.00
5.98
0.53
517-2
2.59
1.36
1.35
2.47
2.22
0.08
0.00
6.11
0.53
517-3
2.61
1.36
1.35
2.44
2.17
0.09
0.01
6.06
0.53
517-4
2.64
1.36
1.48
2.05
2.33
0.08
0.00
5.94
0.47
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