Pre-Tertiary felsic magmatism of the Nepal Himalaya: recycling of continental crust

At least seven different groups of felsic magmatic rocks have been observed in the Lesser and Higher Himalayan units of Nepal. Six of them are pre-Him alayan. The Ulleri Lower Proterozoic augen gneiss extends along most of the length of the Lesser Himalaya of Nepal and represents a Precambrian felsic volcanism or plutono-volcanism, mainly recycling continental crustal mater ial: this volcanism has contributed sediment to the lower group of formatio ns of the Lesser Himalaya. The Ampipal alkaline gneiss is a small elongated body appearing as a window at the base of the Lesser Himalayan formations of central Nepal; it originated as a Precambrian nepheline syenite pluton, contaminated by lower continental crust. The "Lesser Himalayan" granitic be lt is well represented in Nepal by nine large granitic plutons; these Cambr o-Ordovician peraluminous, generally porphyritic, granites, only occur in t he crystaline nappes; they were probably produced by large-scale melting of the continental crust at the northern tip of the Indian craton, during a g eneral episode of thinning of Gondwana continent with heating and mantle in jection of the crust, The Formation III augen gneisses of the Higher Himala ya, such as the augen gneiss of the Higher Himalayan crystalline nappes (Go sainkund) are coeval to the "Lesser Himalayan" granites, and their more met amorphic (lower amphibolite grade) equivalents. Limited outcrops of Cretace ous trachytic volcanism lie along the southern limb of the Lesser Himalaya and are coeval with spilitic volcanism in the Higher Himalayan sedimentary series. This volcanism foreshadows the general uplift of the Indian margin before the Himalayan collision. The predominance of felsic over basic magma tism in the 2.5 Ga-long evolution of the Himalayan domain constitutes an un ique example of recycling of continental material with very limited additio n of juvenile mantle products. (C) 1999 Elsevier Science Ltd. All rights re served.