Multiple metamorphic events in the Palaeozoic Mérida Andes basement, Venezuela: insights from U–Pb geochronology and Hf–Nd isotope systematics

Abstract

ABSTRACTThe metamorphic basement of the Mérida Andes in western Venezuela is constituted of paragneiss, orthogneiss, amphibolite, and metagranitoid of the Iglesias Complex. In this paper, U–Pb zircon geochronology, Lu–Hf, and Sm–Nd isotope systematics of these rocks are presented. Biotite-garnet orthogneiss protoliths intruded paragneiss at ~487 Ma, during Barrovian metamorphism (M1) that reached anatexis at ~467 Ma. Paragneiss and orthogneiss were later intruded by mafic to intermediate lower-crust and mantle-derived magmas (now hornblende orthogneiss and amphibolite) in a back-arc setting at ~460–455 Ma. A new compressional setting led to high-grade metamorphism (M2) at ~450 Ma, followed by decompression melting at ~430–420 Ma (M3). Finally, an upper greenschist- to amphibolite-facies overprint in the complex occurred at ~230 Ma (M4). Ordovician and Late Triassic metagranitoids are probably syn-tectonic to metamorphism M2 and M4, respectively. The Iglesias Complex has several similarities to rocks of the same age in other crustal fragments adjacent to northwest Gondwana, such as the Santander Massif in Colombia, and the Maya block and the Acatlán Complex in southern Mexico and Central America. Both, the early Palaeozoic and the early Mesozoic tectonothermal events, are related to active continental arcs broadly recognized as the Famatinian orogeny and the Proto-Andean Permo-Triassic arc, respectively. No exposures of Rodinia-type basement were found, but isotope systematics suggest that such basement is probably buried underneath the Mérida Andes.