Elsevier

Gondwana Research

Volume 9, Issue 4, June 2006, Pages 524-529
Gondwana Research

GR Letter
U–Pb SHRIMP zircon dating of Grenvillian metamorphism in Western Sierras Pampeanas (Argentina): Correlation with the Arequipa-Antofalla craton and constraints on the extent of the Precordillera Terrane

https://doi.org/10.1016/j.gr.2005.12.004Get rights and content

Abstract

Metamorphism of Grenvillian age (ca. 1.2 Ga; U–Pb zircon dating) is recognized for the first time in the Western Sierras Pampeanas (Sierra de Maz). Conditions reached granulite facies (ca. 780 °C and ca. 780 MPa). Comparing geochronological and petrological characteristics with other outcrops of Mesoproterozoic basement, particularly in the northern and central Arequipa-Antofalla craton, we suggest that these regions were part of a single continental crustal block from Mesoproterozoic times, and thus autochthonous or parautochthonous to Gondwana.

Introduction

The Sierras Pampeanas of Argentina, the largest outcrop of pre-Andean crystalline basement in southern South America, resulted from plate interactions along the proto-Andean margin of Gondwana, from as early as Mesoproterozoic to Late Paleozoic times (e.g., Ramos, 2004, and references therein). Two discrete Paleozoic orogenic belts have been recognized: the Early Cambrian Pampean belt in the eastern sierras, and the Ordovician Famatinian belt, which partially overprints it to the west (e.g., Rapela et al., 1998). In the Western Sierras Pampeanas, Mesoproterozoic igneous rocks (ca. 1.0–1.2 Ga) have been recognized in the Sierra de Pie de Palo (Fig. 1) (McDonough et al., 1993, Pankhurst and Rapela, 1998, Vujovich et al., 2004) that are time-coincident with the Grenvillian orogeny of eastern and northeastern North America (e.g., Rivers, 1997, Corrievau and van Breemen, 2000). These Grenvillian-age rocks have been considered to be the easternmost exposure of basement to the Precordillera Terrane, a supposed Laurentian continental block accreted to Gondwana during the Famatinian orogeny (Thomas and Astini, 2003, and references therein). However, the boundaries of this Grenvillian belt are still poorly defined, and its alleged allochthoneity has been challenged (Galindo et al., 2004). Moreover, most of the Grenvillian ages so far determined relate to igneous protoliths, and there is no conclusive evidence for a Grenvillian orogenic belt, other than inferred from petrographic evidence alone (Casquet et al., 2001). We provide here the first evidence, based on U–Pb SHRIMP zircon dating at Sierra de Maz, for a Grenville-age granulite facies metamorphism, leading to the conclusion that a continuous mobile belt existed throughout the proto-Andean margin of Gondwana in Grenvillian times.

Section snippets

Geological setting

The Sierra de Maz, along with the nearby sierras of Espinal, Ramaditas and Asperecitos (Fig. 1), defines a NNW–SSE trending belt of metamorphic rocks ranging from high-grade in the east to low-grade in the west. Three parallel domains can be discriminated in the field, separated by first-order shear zones (Fig. 1). The eastern domain consists of garnet–sillimanite migmatitic paragneisses, few marble outcrops and amphibolites. The central domain consists for the most part of medium-grade

Samples description and conditions of metamorphism

MAZ-6063 is a garnet schist consisting of quartz, biotite, garnet and plagioclase with accessory rutile, ilmenite, zircon, monazite and apatite. Biotite is abundant and defines a foliation that wraps around garnets. Garnet porphyroblasts are subhedral to anhedral and are partially replaced by matrix biotite; they have large cores rich in inclusions of matrix minerals, particularly rutile, and inclusion-free mantles. The garnet is chemically quite homogeneous (Alm55.7–64.3, Prp26.0–34.4, Grs

U–Pb SHRIMP geochronology

A zircon concentrate was obtained from MAZ-6063. The zircons are round to sub-round in shape, mostly ca. 100 μm in diameter. Cathodoluminescence (CL) images reveal cores (simply or complexly zoned, often quite small) mantled by complete overgrowths that often represent > 50% of the grain volume (Fig. 3). The latter are of two types: thick patchily sector-zoned (‘soccer-ball’) zircon, thought to signify high-grade metamorphism at a deep crustal level (e.g., Vavra et al., 1999), and clearer,

Interpretation of zircon ages

The simplest interpretation of the U–Pb data is that the original sediment had a late Palaeoproterozoic provenance and underwent high-grade metamorphism in the lower crust at about 1200 Ma. Slight radiogenic Pb-loss could have occurred as the rock underwent partial exhumation at 1000–1100 Ma. The depositional age is poorly constrained since the number of grains dated is less than that usually considered necessary for a full provenance analysis, but must be between the age of metamorphism and

Discussion

The age of 1208 ± 28 Ma of the M1 metamorphic event is broadly coincident with the age of the Elzevirian accretionary orogenic event recorded in the Grenvillian Province of eastern Canada (∼1250–1190 Ma; Rivers, 1997). Corrievau and van Breemen (2000) provided evidence from metamorphic PT conditions and geochronology that the Grenvillian continent–continent collisional orogeny was initiated at the end of the Elzevirian at ca. 1.2 Ga. In consequence we consider that the M1 metamorphic recognized

Acknowledgements

This work was supported by Spanish (BTE2001-1486) and Argentine public grants. R.J.P. acknowledges a NERC Small Research Grant.

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