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Isotope evidence for the origin of Andean granites

Published online by Cambridge University Press:  03 November 2011

R. J. Pankhurst
Affiliation:
British Antarctic Survey, NERC, High Cross, Madingley Road, Cambridge CB3 OET, U.K.
M. J. Hole
Affiliation:
British Antarctic Survey, NERC, High Cross, Madingley Road, Cambridge CB3 OET, U.K.
M. Brook
Affiliation:
Nerc Isotope Geology Centre, c/o British Geological Survey, Geochemical Division, 64 Gray's Inn Road, London WC2X 8NG, U.K.

Abstract

The genesis of subduction-related magmas in the Andean region of South America and the Antarctic Peninsula is considered in relation to the Palaeozoic to Cenozoic granitoids belts which are thought to parallel palaeo-coastlines. Their Sr-Nd isotope systematics show a wide range of initial compositions (87Sr/86Sr0 0·7038 to >0·710; εNd, +4 to –10) requiring material input from both depleted mantle and continental crust. In local transects there are consistent trends with time of emplacement, from enriched (crustal) to depleted (mantle) sources, regardless of the sense of migration of magmatism (towards or away from the continent). These trends represent mixing between mantle-derived material and anatectic melts of the lower crust: in each case the crustal end-member reflects the age and isotopic composition of the local deep crustal basement (Precambrian in the easternmost Andes, Palaeozoic in the W and in the Antarctic Peninsula). The depleted end-member could be derived by melting within the subducted oceanic crust, the overlying mantle or previously crystallised mafic underplating. One of the most important factors controlling the mixing process is the angle of subduction, resulting in magma generation under variable tectonic conditions.

Type
Research Article
Copyright
Copyright © Royal Society of Edinburgh 1988

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