Provenance Study on Neoproterozoic Rocks of Nw Argentina: Puncoviscana Formation – First Results
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U N I V E R S I D A D D E C O N C E P C I Ó N 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 1Dep.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).