Sr, C and O isotope geochemistry and stratigraphy of Precambrian and lower Paleozoic carbonate sequences from the Western Sierras Pampeanas of Argentina: tectonic implications

Abstract

Sr, C and 0 isotope data are presented for carbonate rocks from the Sierra de Pie de Palo and other crystalline outcrops of the Western Sierras Pampeanas. These are used to distinguish three groups of rocks of quite different ages within the nappe pile that constitutes the Sierra de Pie de Palo. Carbonates from the Grenville-age ophiolitic unit at the bottom of the pile probably resulted from the interaction between hot seawater and contemporaneous oceanic crust. These carbonates are tentatively classified as ophicalcites. Carbonates from the Difunta Correa Sedimentary Sequence are part of a cover sequence to a Grenvillian basement, together forming the upper nappes. Isotope stratigraphy suggests a middle to Late Neoproterozoic age (580-720 Ma) for these carbonates. This cover might be correlated with cover sequences to the Archean and Early to Middle Proterozoic cratons that are well preserved over large areas of Brazil. This fact, together with the apparently absence of an equivalent Neoproterozoic carbonate-bearing cover in the Appalachian margin of Laurentia, suggests that the Western Sierras Pampeanas, which are considered part of the exotic Argentine Precordillera terrane of allegedly Laurentian derivation. could be autochthonous or para-autochthonous to Gondwana. Other geochemical and geological arguments reinforce this hypothesis. The Caucete Group carbonates underlie the nappe pile and are separated from it by a first order thrust (the Pirquitas thrust). These latter carbonates are Cambrian in age and isotopic ally similar to the carbonate platform of the Precordillera. We thus conclude that the Pirquitas thrust is the boundary between the exotic Precordillera terrane and the autochthonous or para-autochthonous Western Sierras Pampeanas.

Structural evolution of a composite middle to lower crustal section: The Sierra de Pie de Palo, northwest Argentina

“…This age is distinctly older than the Middle-Late Cambrian age inferred by Galindo et al [2004] on the basis of 87 Sr/ 86 Sr isotopes from metacarbonates and from detrital zircon ages in quartzites of the Caucete Group . The discrepancy implies that either older, unrecognized units exist within the Caucete Group or units exposed within the structural windows are not correlative with the Caucete Group.…”

“…[7] The Caucete Group [e.g., Ramos et al, 1996Ramos et al, , 1998Vujovich and Kay, 1998;Casquet et al, 2001;Galindo et al, 2004;van Staal et al, 2010] is exposed along the western margin of the range in the footwall of the Las Pirquitas thrust (Figures 2 and 3) and consists of low-to medium-grade quartzite, marble, and less common metavolcaniclastic rocks. On the basis of stratigraphic correlations and detrital zircon ages, the Caucete Group is considered to represent the metamorphosed equivalent of Paleozoic rocks within the Precordillera terrane to the west [e.g., Ramos et al, 1998; Geologic map of the Sierra de Pie de Palo displaying the first-order structures within range.…”

The Western Sierras Pampeanas: Protracted Grenville-age history (1330–1030 Ma) of intra-oceanic arcs, subduction–accretion at continental-edge and AMCG intraplate magmatism

“…It over lies the siliciclastic and carbonate seq uences of the Paleozoic Caucete Group along the thrust (Vujovich and Kay,3). An upper imbricate thrust sequence of marbles, cale-silicate rocks, schists, subordinate amphibolites and abundant acidic including and meta-tonalites, dominate the central and eastern part of the sierra Isotope studies and detrital zircon ages obtained from marbles, cale-silicate schists, qu,arztltl�s and schists in the southern part of the sierra, VI<::: VIL'U"IV considered indicate that these rocks in fact constitute a distinct Neoproterozoic sequence, the Difunta Correa sequence (580-620 Ma, Galindo et al, 2004;Rapela et al, 2005), which overlies Mesoproterozoic basement. A-type dated at 870 Ma , the Difunta Correa sequence and the Mesoproterozoic basement are thoroughly imbricated in the cen tral and eastern part of the sierra.…”

“…All the in this are from the Maz Central Domain 2). The Western Domain consists of mE�ta:secllrrlentaIY rocks younger than 1.0 Ga that underwent Famatinian mE�talmo'rplhism marble beds, caleic One sequence of rocks of thick schists and alent to the Neoproterozoic Difunta Correa mE�ta:secllITl enltaIY quence of Sierra de Pie de Palo, and to rocks of Sierra de Umango (Varela et al, 2001;Galindo et al, 2004). Most rocks within this domain are low-to but rocks are found locally.…”

Correlations of some Neoproterozoic carbonate-dominated successions in South America based on high-resolution chemostratigraphy

“…Gaucher et al 2008a;Poiré & Gaucher, 2009), carbonates occurring at the western side were strongly deformed and metamorphosed during the Early Cambrian Pampean Orogeny. Due to this fact there is a paucity of chemostratigraphic studies at the western boundary of the RPC; of interest are those of the Puncoviscana Formation (Toselli et al 2012) and carbonates of the Difunta Correa Sequence (Galindo et al 2004). The latter unit may be unrelated to the RPC and only later tectonically accreted (e.g.…”