Download provenance study on neoproterozoic rocks of nw argentina

UNIVER SIDAD DE CONCEPCIÓN
DEPARTAMENTO DE CIENCIAS DE LA TIERRA
10° CONGRESO GEOLÓGICO CHILENO 2003
PROVENANCE STUDY ON NEOPROTEROZOIC ROCKS OF NW
ARGENTINA: PUNCOVISCANA FORMATION – FIRST RESULTS
ZIMMERMANN, U.1
1
Dep.of Geol., RAU University, Auckland Park 2092, South Africa [email protected]
INTRODUCTION
Since more than 20 years the western border of Gondwana is object of controversies related to
the basic question if crustal growth is related to terrane accretion or to “recycling” of the same
crustal rocks during the Vendian and Lower Paleozoic. Different hypotheses were developed
regarding the evolution of that margin. One of the key element to understand the crustal
evolution, is the Vendian to Lower Cambrian so-called PVF. Turner (1960) described rock
successions in northwestern Argentina (Fig. 1) of Pre-Ordovician age comprising greywackes
and sand- and siltstones, but dominated by pelites as the PVF. Afterwards, it was established that
mostly all Vendian to Lower Cambrian very-low to low grade metasedimentary rocks in the
region are classified, such as Suncho, Negro Peinado or La Cébila Formation (e.g. comp. in
Aceñolaza et al., 1988), are equivalents of the PVF. Widely distributed medium- to high-grade
metasedimentary rocks (Fig. 1), those rocks were interpreted as exhumed deeper crustal levels of
the PVF (Willner, 1990). Other authors deny this opinion and interpret the different metamorphic
rocks related to different events, consequently of different ages (Mon and Hongn, 1990). Based
on only punctual petrographic work, the depositional area was defined as a passive margin based
on petrography and mainly major element geochemistry (Jezek, 1990, Willner et al., 1985, Rossi
Toselli et al., 1997), or on preliminary trace element data (Do Campo and Ribeiro Guevara,
2002). Only few publications interpret the entire formation as a product of an evolution from
passive margin to back-arc deposits (Omarini et al., 1999) or as foreland deposits (Kraemer et al.,
1995; Keppie and Bahlburg, 1999). This contribution likes to review new and published
petrographic and geochemical data based on a modern approach in provenance studies, including
the modelling and quantification of alteration, rock composition and tectonic setting (e.g.
McLennan et al., 1990, 1993; Bahlburg and Floyd, 1999). Finally, a working hypothesis is
established to give a productive input in the current discussion.
PROBLEMS
The difficult situation of understanding the metasedimentary deposits of the Puncoviscana
Foramtion and equivalents is based on mainly four problem complexes:
1. A complete lithostratigraphic column is lacking: The PVF is composed of mainly shales,
siltstones, rare coarse-grained sandstones, greywackes, conglomerates and few carbonates.
However, it is not clear if these lithofacies are repetitive or not.
Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad.
2. The depositional and diagenetic age of the formation is controversial: Intrusive ages of
mainly felsic plutonites and subordinated mafic magmatites are determined with different dating
techniques, pre-dates the PVF to Lower Cambrian to Uppermost Vendian 500-530 Ma (comp. in
Rapela et al., 1992, 1998; Pankhurst and Rapela, 1998). K/Ar data on whole rock samples of the
sedimentary successions (Adams et al., 1990) coincide with trace fossil interpretations in some
outcrops (e.g. Durand and Aceñolaza, 1990) and point to a similar depositional age. However, Do
Campo et al. (1999) argue, their K-Ar age dating on authigenic single grains (K/Ar on mica)
reflect an older age for deposition (630 Ma) and diagenesis (580 Ma).
3. Relation between medium- to high-grade and low grade metasedimentary rocks: The
Sierras Pampeanas s.l. contains a high amount of medium- to high grade metamorphic rocks,
associated are gneisses and migmatites. Willner (1990) shows arguments to interpret those rocks
as deeper crustal levels of the PVF, where Mon and Hongn (1991) find reasons to state a different
tectonic evolution. However, an unresolved problem, are the occurrences of low-grade (PVF and
equivalents) metasedimentary rocks in the higher grade terrains.
4. Unresolved geodynamic and paleotectonic setting of the Puncoviscana basin: The rocks of
the PVF were interpreted based on petrological and mainly major element data (e.g. Willner et
al., 1985; Rossi Toselli, 1997) and trace element data for the region in the Puna (Bock et al.,
2000; Do Campo and Ribeiro Guevara, 2002; Fig. 1 nr. 1) as passive margin deposits, probably
related to a rifting process. Subsequentely the units were folded during the Pampean Orogeny.
Kraemer et al. (1995) and Keppie and Bahlburg (1999) interpret the same deposits as a foreland
basin infill, evolved syntectonically during the collision of Pampia with the western border of
Gondwana, represented by the (Sierra) Córdoba magmatic arc (Rapela et al., 1998). However, the
petrological, geochemical and isotopegeochemical data-set is too sparse to model a provenance
for the whole formation. For quantitative petrography in a sense of the point-counting method of
Gazzi-Dickinson a scarcity of representative distribution of suitable rocks has to be stated.
SAMPLING LOCALITIES AND DESCRIPTION
Samples were taken from several localities in the southern region (Fig. 1) and combined with
published geochemical data of outcrops in the central part and northern part (Willner et al., 1985,
1990, Bock et al., 2000) as well with data from Precambrian formations of the Famatina Range
(Rossi et al., 1997, Zimmermann et al., in press.)
Area 1: Puna and Cordillera Oriental: Campo Volcán (S25°39’ W67°47’2; Fig. 1; 7 samples;
VOL): quartz-rich and plagioclase-poor red pelites, siltstones. The rocks are preliminarily
described as Volcán Formation (Zimmermann and van Staden, 2002). - Northern Puna and
Cordillera Oriental (16 samples): Purmamarca, El Muñano, Rio Taique and Quebrada del Toro
(Fig. 1): sandstones, pelites, greywackes, litharenites (Jezek, 1990; Do Campo et al., 1999; Bock
et al., 2000). Willner et al. (1985) presented mostly major element analyses, Bock et al. (2000)
mainly trace elements geochemistry. Newer data from Do Campo and Ribeiro Guevera (2002)
could not incorporated, because data sets were not available during the writing of the manuscript.
Area 2: Sierra Ambato: Siján (35 samples, ((±S28°15’ W66°08’; Fig. 1)): pelites, siltstones are
overlain by conglomerates and greywackes, and are named preliminarily Rincón Formation
(Zimmermann and van Staden, 2002). The red pelites were compared with those of Volcán (see
above) and it could be shown that they show similarities, in mineralogy, deformation and geo-
chemistry (Zimmermann and van Staden, 2002). - Concepción (S28°38’ W66°03’; 7 samples;
Fig. 1): bluish feldspathic metaarenites, -greywackes (matrix: 15-20%), Qt70-80 F15-25 L5-10.
The rocks are named El Quemadito Formation (van Staden and Zimmermann, 2002).
Area 3: Sierra Famatina: Negro Peinado and La Aguadita Formation (19 samples) were
sampled in the same locations to those of Rossi et al. (1997). The relation ship between the two
formations is not clear. Dark fine-grained rocks (pelite to siltstones), dominated by sub-rounded
quartz (60%) with scarce feldspar (10%) and metamorphic lithoclasts (< 2%) (Rossi et al., 1997).
The new data are combined with the most representative samples from Rossi et al. (1997).
Area 4: Cachi (Valles Calchaquies): Cuesta de Obispo, Sierra de Amblayo, Cachi, El Escoipe,
Quebrada Don Bartolo (Fig. 1) and are composed mostly of quartz and high amounts of plagioclase
(P/F 0.68-0.98), as well as sedimentary lithoclasts (Jezek, 1990).
Area 5: Tucumán: Sierra San Javier, Rio Choromoro, Rio Gonzalo and Sierra de Nogalito close
to the city of Tucumán (Fig. 1). The petrographic composititon is similar to Area 4 (Jezek, 1990).
For both outcrops mainly major element geochemistry and few trace elements were carried
through (Willner et al., 1985).
The metasedimentary rocks in all introduced outcrops are typically polyphase deformed to
recognize them as pre-Ordovician deposits. Other outcrops which will not be discussed in the
following chapters, but which were sampled are San Antonio de los Cobres, Rio Taique,
Quebrada Randolfo, La Pedreda, El Corralito (Puna and Cordillera Oriental), around Siján and
Pomán, La Cébila (Sierra Ambato), Suncho (Fig. 1, n°. 6), Molinos, Seclantes (Valles
Calchaquies) and Choromoro (Tucumán).
PETROGRAPHY
The framework mineral composition was quantified using the point-counting method of Gazzi
and Dickinson as described by Ingersoll et al. (1984). Representative datasets for the PVF are
difficult to arise because of the scarcity of applicable sandstones. In most of the outcrops the
rocks are strongly altered what render more difficult the distinction (f.e. feldspars). The data
presented by Jezek (1990) vary for quartz between 50-89 %, for feldspar between 4-38%) and
lithoclasts between 8-49%. The rocks comprise a very high amount of plagioclase (P) in relation
to alkali feldspar (AFS; P/AFS 0,68-1,0!). The partly absence of AFS and extremely low
abundance of volcanic lithoclasts are a combination which is difficult to classify and interpret. It
differs in general from anlyses by XRD of the rocks in the Puna, Siján and point-counting of
rocks from Concepción, as shown above (e.g. Zimmermann and van Staden, 2002). The results
could lead to model a depositional area similar to a rifted or passive margin. The immaturity of
the coarse grained rocks, which points to a short transportation, would be an expected
characteristic for a rift environment. However, thick packages of sandy to silty turbidites
associated with different shales indicate more likely a shelf position. However, a typical rift
sedimentation succession (Einsele, 1992: 437ff) cannot be observed. In contrary, conglomerates
are rare and with small thicknesses, an exception is the outcrop at Suncho (Fig. 1, n°. 6). Basic
volcanic activity is stated during the deposition of the PVF (comp. in Coira et al., 1990).
However, only relicts of rhyolitic and dacitic volcanism are preserved in conglomerates of the
Suncho Formation (Durand, 1990, van Staden and Zimmermann, subm.). In other coarse grained
rocks no trace of volcanic activity is stated (e.g. van Staden and Zimmermann, subm.).
GEOCHEMISTRY
In contrast to petrography geochemical analysis could be applied to fine and coarse grained
clastic sediments for provenance purposes. Certain elements (like REEs, Th, and Sc) in
terrigeneous sediments that are less easily mobilized provide information about the composition
of the source (e.g. McLennan et al., 1990). Geochemistry can also give insights in identification
of constituents that might be important to tectonic interpretations. REE are employed as reliable
provenance indicators because they tend to be transferred unfractionated into sediment and
therefore reflect the average REE composition of the source (McLennan, 1989). If geochemical
techniques are applied to mineralogically altered samples, like in this case, they can aid to
quantify the occurrence and/or the extent of some secondary processes (e.g. Fedo et al., 1995).
Major elements – composition and alteration
Moderate SiO2-contents between 58 and 82% characterize the sedimentary rocks. Three samples
from the La Aguadita Formation (LA2) are different from all samples as they show lower silica
and elevated Fe2O3 and MgO abundances. Both samples are very fine grained and interpreted
here as basic to intermediate ashes interlayered in siliciclastic rocks of the same outcrop, and
should be seen separately in the whole discussion. The chemical index of alteration (CIA; Nesbitt
and Young, 1984) ranges from about 70 to 80 (Fig. 2). Chemical weathering strongly affects the
major-element geochemistry and mineralogy of siliciclastic sediments (e.g. Nesbitt and Young,
1982). High CIA values, as here observed, reflect the removal of labile cations (e.g. Ca2+, Na+,
K+) relative to stable constituents (e.g. Ti4+) during weathering. In Fig. 2 the CIA is combined
with the A-CN-K diagram, mole percent Al2O3 plotted versus CaO* + Na2O versus K2O where
CaO* includes only calcium associated with silicate minerals (Nesbitt and Young, 1984). Typical
unweathered igneous rocks, representing upper continental crust, fall in a field around 50%
Al2O3, average shale is plotted for reference. The estimated weathering trend of a homogeneous
source, based on the removal of alkali and earth alkali elements during weathering, (stippled
arrow in Fig. 20, the interpreted weathering path (black arrow), the trend of metasomatic K
addition (after Fedo et al., 1995) towards the K-apex (Fig. 2). The samples of all localities show a
homogeneous and pronounced deviation from the expected trend, interpreted as a K-addition
during K-metasomatism of kaolinite in weathered residues to illite, and as a substitution of K for
Ca and Na in feldspar. The strong mobilization of alkali elements is reflected in all attempts to
use major-elements for provenance aspects after Bhatia (1983) and Roser and Korsch (1986,
1988). The samples of each formation plot with a broad spread in provenances. Consequently, the
use of major-elements characterizing provenance or tectonic setting of the PVF should be done
carefully. It is suggested, according to results of Bahlburg (1998) and Zimmermann and Bahlburg
(in press), that mobility of major-elements distorts provenance information for Lower Paleozoic
sedimentary rocks, to interpret major-element geochemistry only as a measurement of alteration.
TRACE ELEMENTS
The whole sample suite has homogeneously a rhyodacitic to dacitic compositions based on Zr/Ti
and Nb/Y ratios (according to Winchester and Floyd, 1977). Exception is one of the two ashes,
which has characteristics of trachyandesitic rocks. None of the sedimentary samples shows an
enrichment in Cr, V, Ni or Sc that would be an indicator for a mafic (ophiolitic) precursor (Floyd
and Leveridge, 1987). REE spider diagrams for all the formations (Fig. 3) show relatively
uniform patterns comparable to PAAS (Post-archaen Average Australian Shale composite, Nance
and Taylor, 1976). Comparing these values to the PAAS, the Negro Peinado Formation is
enriched in all REE, Volcán and Siján Formations show similar pattern, the samples of the
northern Puna and Cordillera Oriental are slightly depleted in REE. The other formations show
depleted LREE and enriched HREE concentrations. LaN/YbN (the subscript “N” indicates
chondrite normalized values) ratios are generally close or slightly depleted related to average
Upper continental crust (UCC, after McLennan, 2001) value of 9.2. Depleted in REE are the
ashes of La Aguadita Formation (LA2), and point to pattern similar as basalts. Eu/Eu* ratios are
constant between 0.45 and 0.71 for all, but of LA2, which show values of about 0.9-1.0.
A good tracer for provenance is the ratio Th/Sc, as Th is highly incompatible and enriched in
felsic rocks, and Sc traces a mafic component (e.g. Taylor and McLennan, 1985). Most all
siliciclastic samples plot above the UCC of 0.79 (McLennan, 2001) and show a narrow scatter.
Samples from Concepción, Sierra Famatina and the northern Puna and Cordillera Oriental show
values close to 1,0 for Th/Sc (Fig. 4). The values for the samples from Siján and Volcán are
higher (1.22 and 1.52). However, mainly all samples show only a slight trend towards a recycling
component: Zr/Sc ratios vary between 8.73 for the non-volcanic samples of La Aguadita
Formation to 27.87 for the Volcán Formation. There is no geographical trend to observe.
Elevated Zr/Sc ratios do not coincide with grain sizes, except in the Negro Peinado Formation.
Interpretation of the trace element geochemistry
Most of the trace element ratios pointing to UCC with few exceptions, underlining the scarcity of
a mafic or ultramafic component. However, the volcanic samples of the La Aguadita Formation
(LA2) show trace element composition related to a volcanic arc environment. For all other
samples the trace element ratios like Th/Sc, Zr/Sc, Th/U, REE patterns, normalized ratios of
LaN/YbN, LaN/SmN and GdN/YbN as well as Eu/Eu* values point to UCC. However, the
formations show a spread, especially in Zr/Sc values (Fig. 4), which could reflect a variable
recycling or different fractionated source areas. Source areas like medium- to high-grade
metamorphic rocks including para- and orthogneisses of mainly felsic to intermediate
composition combined with the influence of igneous rocks are likely. According to GdN/TbN and
Eu/Eu* values no Archean component can be detected (McLennan, 1989).
TECTONIC SETTING
Trace element ratios such as La/Th, La/Sc, and Ti/Zr have been used to discriminate sandstones
of different tectonic settings (Bhatia and Crook, 1986; Floyd and Leveridge, 1987; McLennan et
al., 1990, 1993). Such an approach has to be used with caution because it has been shown that
specific tectonic settings do not necessarily produce sedimentary rocks with unique geochemical
signatures (McLennan, et al., 1990; Bahlburg, 1998), however the use for coarse- and finegrained rocks could be shown (e.g. McLennan et al., 1990; Zimmermann and Bahlburg, in press).
In Fig. 5 most of the samples plot in the fields continental island arc and active continental
margin. However, the volcanic samples of La Aguadita Formation points to an oceanic island arc
reflecting a strong mafic influence. The interpretation leads to an arc-related or active continental
margin setting, which cannot be confirmed by other trace element compositions. Nearly all
samples of all formations have normalized to average upper continental crust (after McLennan,
2001), depleted Pb, enriched Nb and Ta (Fig. 6) and enriched Ti concentrations. After Hofmann
(1988, 1997), arc derived rocks show exactly contrary characteristics. The three volcanic rocks of
LA2 accomplish an arc related signature. 4 samples of the Siján Formation show negative Nb and
Ta anomalies, but they are high in silica (77-81%) and diluted in all trace elements including
REE. The sum of the latter is ≤300 ppm, whereas other samples of the same formation range
between 400-500 ppm. A difference of about 30%, comparable with the difference in silica
composition, can be stated, and interpreted as a dilution effect. The restricted recycling
component, suggested by the low Zr/Sc ratios (Fig. 4) and low Zr concentrations, shifts the
samples away from the upper crust composition (from the Zr/10 apex in Fig. 5). As Hf and Zr are
usually concentrated in heavy minerals, especially in zircon, this effect can be interpreted as a
result of restricted recycling. The UCC nature of the PVF was shown in Figs. 3 and 4, and is
obvious in the selected element ratios of Fig. 6. Combining Eu/Eu* values, as an indicator for
provenance, with the mentioned trace element ratios, a trailing edge setting shows mainly
comparable values in the characteristics after McLennan et al. (1990, 1993), with Th/Sc ≥ 1,
Th/U > 3.8, Eu/Eu* 0.6–0.7, and LaN/YbN between 4.4 and 13.6. It is possible to interpret the
metasedimentary rocks of the PVF as first or second cycle deposits, which were not well mixed.
However, a significant amount of sedimentary lithoclasts could be observed, which points a to
cannibalistic reworking or an older sedimentary source in the region. Nd-isotopes of
Tremadocian deposits in the Puna showed older Nd-model ages (2.1-2.2 Ga; Zimmermann and
Bahlburg, in press), than rocks from the Puncoviscana Foamtion (1.5-1.8 Ga; Bock et al., 2000).
CONCLUSIONS
Different outcrops of very-low to low-grade metamorphic rocks of the PVF were sampled to
model the provenance of and leads to following preliminary conclusions:
Representative data for the whole PVF from sandstones using the quantifying petrographic
method after Gazzi-Dickinson are not possible to carry through, because (i) of the low abundance
of coarse grained sandstones and (ii) the unmaturity of the rocks. The few petrographic data
points to a mixed provenance, related to a collisional (?) orogen, and Jezek (1990) interpret the
signature as a “fold-thrusted foreland deposits”, according to the here presented model.
Major element geochemistry shows a high chemical index of alteration (CIA) between 70 and 82
related to a substantial K-metasomatism. The pronounced mobility of alkali and earth-alkali
elements yield provenance discrimination diagrams based on major elements not useful.
Trace element geochemistry could define the rocks of all formations as mostly upper continental
crust related with only a slight recycling component, and no significant geochemical trend in N-S
or E-W directions. Three samples of the La Aguadita Formation are interpreted as probable retroarc volcanics or rift basalts in an arc related tectonic setting. This does not coincide with data of
the siliciclastic rocks. Trace element geochemistry combined with petrography suggests a
depositional area not far away from the sources, supported by the immaturity of the rocks, the
low recycling component and the notable spread in the compositions of the mainly fine-grained
rocks, which points to not well mixed detrital material. This is expressed in a shifting of the
samples away from a rifted margin or an active continental margin to a more arc-like
composition. This signature is not supported by geochemical characteristics like low Ti, Nb, Ta
and high Pb anomalies, Th/Sc or Zr/Sc ratios. In contrary, mostly all samples show enriched
values in Nb and Ta, UCC values of Th/Sc. The siliciclastic rocks of the different formations
were not deposited in a volcanic arc. The mafic ashes of the La Aguadita Formation could be
transported sub-areal from an adjacent volcanic arc, probably the Sierra Córdoba. However, as no
further data on the ashes are available, this is a first hypothesis to combine the data at this point.
The upper crustal composition combined with their low recycled component and the immature
mineralogy denies a passive margin setting and suggests depositional areas like continental rifts
or foreland basins. A strong argument against a rift interpretation is the absence of a typical rift
sedimentation sequence. The rocks are characterized mainly by monotonous turbiditic sequences
of different, but mainly fine, grain-sizes, more typical for deeper shelf regions. The introduced
foreland basin model would favor the collision of Pampia and Western Gondwana during the
Uppermost Neoproterozoic and the syntectonic evolution of the Puncoviscana basin.
This is a contribution to the IGCP 436 Pacific Gondwana margin.
Acknowledgments: The author thanks RAU for financial support and especially Mr. R. Lucero (Mina Rodohuasi in
Catamarca) for logistic support, Fernando Hongn and Heinrich Bahlburg for inspiring discussions.
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Figure 1: Outcrop locations of the Puncoviscana Formation and equivalents, as well as medium- to high grade
metamorphic rocks of Pre-Ordovicican age. Outcrops: 1=Puna; 2=Sierra Ambato; 3=Sierra Famatina; 4=Valles
Calchaquies; 5=Tucumán; 6=Suncho (Sierra de la Ovejería) (after Grissom et al., 1998).
PVF (Jezek, 1990)
PVF-this work
passive margin
Guinea
South Korea
Puna-Tremadoc
continental collision
Ganges
continental arc
Middle America
Java
Peru-Chile
back-arc
South China
Puna-Tremadoc
Puna-Arenig
Puna-Llanvirn
Puna-Llandeilo
fore-arc
Marianes
Japan
Qt
64.8 (55-89)
70-80
F
11.3 (4-38)
15-25
L
23.9 (8-50)
5-10
P/F
0.68-1 (P/K)
0.2
Lv/L
0
0
73.1
47.7
89
26.1
49.8
7
0.7
2.5
4
0.44
0.34
0.36
0.06
45
32.3
22.6
0.45
0.01
10.1
54.8
33.3
74.6
35.4
49
15.3
9.8
17.7
0.78
0.47
0.79
0.99
19.2
33
49
78
67
37.8
42
17
12
8
43
25
34
9
25
0.8
0.4
0.81
0.22
0.72
1
0.3
0.98
0
0.62
0
3.8
5.9
18.3
94.1
77.9
1
0.9
1
0.99
1
Qt=quartz total; f=feldspar; l=lithoclasts; p=plagioclase; k=alkalifeldspar; lv=volcanic lithoclasts
Table 1: Petrographic data of the Puncoviscana Formation from Jezek (1990), Zimmermann and van Staden (2002),
data for comparison from compilation in Bahlburg (1998) and Zimmermann and Bahlburg (in press).
Figure 2: Cia and A-CN-K diagram (after Fedo et al., 1005) combined. Ka0= kaolinite; ill=illite; plag=plagioclase;
ksp=alkalifeldspar; LAII= mafic ashes from La Aguadita Formation.
Figure 3: Average REE-patterns of the rocks from the different outcrops. LA1= siliciclastics from the La Aguadita
Fm.; LA2= mafic ashes from La Aguadita Fm.; PAAS (post-Archean average Australian shale after Nance and
Taylor, 1976; chondrite normalization after Taylor and McLennan, 1985).
Figure 4: Th/Sc//Zr/Sc relation (after McLennan et al., 1993). LA2= mafic ashes from La Aguadita Fm.
Figure 5: Provenance plots after Bhatia & Crook (1986). LA1= siliciclastics from the La Aguadita Fm.; LA2= mafic
ashes from La Aguadita Fm.
Figure 6: Ta-Nb-Th/Sc-Zr/Sc concentrations and values normalized to UCC (McLennan, 2001) and silica
concentrations. LA1= siliciclastics from the La Aguadita Fm.; LA2= mafic ashes from La Aguadita Fm.