THE MAZ SUSPECT TERRANE: A NEW PROTEROZOIC DOMAIN IN THE WESTERN SIERRAS PAMPEANAS.

CASQUET, C.; PANKHURST, R.J.; RAPELA, C.W.; FANNING, C.M.; GALINDO, C.; BALDO, E.; GONZÁLEZ-CASADO, J.M.; DAHLQUIST, J.; SAAVEDRA, J.

Mendoza

Conferencia; GONDWANA 12 CONFERENCE: GEOLOGICAL AND BIOLOGICAL HERITAGE OF GONDWANA; 2005

CONICET, Academia Nacional de Ciencias, ANPCyT

The sierras de Maz and Espinal (Western Sierras Pampeanas) consist of metamorphic rocks, for the most part metasedimentary, which underwent penetrative deformation and low to high-grade metamorphism during the Famatinian orogeny (Ordovician). Isotope compositions (Nd and Pb) and SHRIMP U-Pb zircon dating (detrital zircons, crystallization ages of igneous protoliths and metamorphic ages) show that three parallel Proterozoic isotope domains can be recognized, also differing in stratigraphy, igneous rocks and metamorphic history. These domains underwent juxtaposition by displacement along reverse to left-lateral shear zones of Famatinian age, resulting in westward stacking of the domains and metamorphic inversion, i.e., metamorphic grade increasing structurally upward. The central Maz domain crops out in central Sierra de Maz and western Sierra de Espinal and essentially coincides with the Maz Complex of Porcher et al. (2004). It has high Nd TDM ages (1.9 ? 2.3 Ga) and contains very radiogenic Pb (ì = 10.2 ? 11.1), suggesting provenance from old continental crust. This domain consists of a reworked Grenvillian basement formed by four groups of rocks: 1) metasedimentary (schists, quartzites and minor marbles), with an inferred depositional age between ca. 1.2 Ga and 1.7-1.8 Ga, and zircon evidence of deep crustal metamorphism at ca. 1.2 Ga, and interleaved mafic granulites, 2) a metagabbro?metadiorite complex of ca. 1.2 Ga, 3) massif-type anorthosites of 1.07 ± 41 Ma (Casquet et al., 2005) and 4) widespread younger ortho-amphibolites of unknown age. The two domains to the east and west of the Maz domain consist for the most part of younger metasedimentary sequences (namely low- to high-grade metapelites quartzites and marbles) with Nd TDM ages between 1.3 and 1.6 Ga and less radiogenic Pb (ì = 9.7 ? 10.0), orthogneisses and ortho-amphibolites. One of these metasedimentary sequences to the west of the Maz domain, has a depositional age that can be constrained by the youngest detrital zircons and the age of interleaved orthogneisses to between ca. 1.08 Ga and 845 Ma and provenance from sources with mixed igneous and metamorphic origin at 1.5, 1.2 and 1.08 Ga. This is the oldest post-Grenvillian sedimentary sequence so far recognized in the Western Sierras Pampeanas. The Maz domain is interpreted as a suspect terrane isotopically similar to the Arequipa?Antofalla block that forms the basement of the Central Andes, and which also contains isotopic evidence for an old crustal provenance (Early Proterozoic) and of a still poorly known Grenvillian tectono-thermal event probably related to amalgamation to Amazonia at ca. 1.2 Ga. Massif-type anorthosites represent a late Mesoproterozoic igneous event centred between the Amazonia, Kalahari?Natal?Tamaqua, and Laurentia?Australia cratons (Casquet et al., 2005).