Formation of intraarc volcanosedimentary basins i

Authors: Marc Polliand Urs Schaltegger Martin Frank Lluis Fontboté

Publish Date: 2005/02/16

Volume: 94, Issue: 2, Pages: 231-242

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Abstract

During late Early to Late Cretaceous the Peruvian coastal margin underwent fast and oblique subduction and was characterized by important arc plutonism the Peruvian Coastal Batholith and formation of volcanosedimentary basins known as the Western Peruvian Trough WPT We present highprecision U–Pb ages and initial Hf isotopic compositions of zircon from conformable volcanic and crosscutting intrusive rocks within submarine volcanosedimentary strata of the WPT hosting the Perubar massive sulfide deposit Zircons extracted from both the volcanic and intrusive rocks yield concordant U–Pb ages ranging from 6789±018 Ma to 6971±018 Ma indicating that basin subsidence submarine volcanism and plutonic activity occurred in close spatial and temporal relationship within the Andean magmatic arc during the Late Cretaceous Field observations satellite image interpretation and plate reconstructions suggest that dextral wrenching movements along crustal lineaments were related to oblique subduction Wrench tectonics is therefore considered to be the trigger for the formation of the WPT as a series of pullapart basins and for the emplacement of the Coastal Batholith The zircon initial εHf values of the dated magmatic rocks fall between 55 and 74 and indicate only very subordinate influence of a sedimentary or continental component The absence of inherited cores in the zircons suggest a complete lack of old basement below the WPT in agreement with previous U–Pb and Sr isotopic data for batholithic rocks emplaced in the WPT area This is supported by the presence of a most likely continuous block of dense ~30 g/cm3 material observed beneath the WPT area on gravimetric crustal cross sections We suggest that this gravimetric anomaly may correspond to a piece of lithospheric mantle and/or oceanic crust inherited from a possible Late Permian–Triassic rifting Such young and mafic crust was the most probable source for arc magmatism in the WPT areaThis research was supported by the Swiss National Science Foundation grants FN 2000–5415098 and FN 20–6200000 and is part of a PhD thesis of the first author at the University of Geneva We gratefully acknowledge W Mueller and L Oldham who instigated this project and transmitted us their knowledge of the Perubar deposit area the geologists and staff of the Perubar mine for their help during the field work periods especially R Egoavil and C Zumarán and the geologists from Glencore Peru M Steinmann and S Bureau for logistical support during the project

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