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Differentiated magmas in a nascent island arc: the Nidar Ophiolite case in Ladakh, Indian Himalayas.

Olivier Reubi 1, Othmar Müntener1, Jean-Luc Epard1

Abstract

The production of silicic magmas in intra-oceanic arcs is thought to play a crucial role in the evolution from basaltic oceanic crust to andesitic continental crust.  The processes enabling the genesis of magmatic sequences with bulk andesitic compositions in these settings remain however elusive. The Nidar ophiolite (eastern Ladakh, Indian Himalayas) represents an early Cretaceous juvenile suprasubduction crustal sequence. Upper crustal plutonics, subvolcanic dykes and pillow lavas form two distinct differentiation series extending from basalts to respectively andesites and rhyolites. Basaltic to quartz-bearing rhyolitic dykes and lavas are geochemically and petrographically related to a suite of clinopyroxene-rich olivine gabbros, gabbros and gabbronorite. This series is characterized by subdued increase in incompatible trace elements. Later km-sized intrusions of hornblende-gabbro to quartz diorite form a second differentiation series characterized by marked increase in incompatible elements, except for LILE. Mafic magmas from both series have diagnostic geochemical features of arc melts but are more depleted than typical primitive arc basalts. Although variably enriched in LILE and HFSE, all differentiated magmas are significantly less enriched in these elements than the bulk continental crust. This work shows that substantial volumes of silicic magmas are produced through contrasted differentiation pathways in upper crustal magmatic systems during the earliest stages of intra-oceanic arc development. Despite their volcanic importance, these magmas appear to have limited influence on the geochemical maturation of the continental crust, as far as trace elements are concerned.