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- Title
Partial Melting of Mantle and Crustal Sources beneath South Karakorum, Pakistan: Implications for the Miocene Geodynamic Evolution of the India-Asia Convergence Zone.
- Authors
G. Mahéo; J. Blichert-Toft; C. Pin; S. Guillot; A. Pêcher
- Abstract
In south Karakorum, the western prolongation of southern Tibet, three distinct types of magmatic rocks were emplaced during the Neogene: (1) 22â24 Myr old lamprophyres, characterized by strong enrichment in large ion lithophile (LILE) and light rare earth elements (LREE), 87Sr/86Sr(i) = 0·7096, εNd(i) = â7, and εHf = â9, interpreted to reflect partial melting of a previously metasomatized spinel-lherzolite mantle source; (2) the 21â26 Myr old Baltoro high BaâSr granitoids, likewise strongly enriched in LILE and LREE, with 87Sr/86Sr(i) = 0·7034â0·7183, εNd(i) = â6·5 to â11·0, and εHf = â1·8 to â8·0, produced by partial melting of amphibole-bearing rocks in the lower crust, possibly the root of south Karakorum Cretaceous magmatic arc; (3) the 8â9 Myr old Hemasil syenite and its associated lamprophyre, also both enriched in incompatible elements but with isotopic compositions closer to those of depleted mantle (87Sr/86Sr(i) = 0·7043â0·7055, εNd(i) = ·5 â 4·3, and εHf = 10·4 â 11·2). The Hemasil syenite is interpreted as the product of partial melting of a time-integrated depleted spinel-lherzolite source that was enriched in K and LREE during a recent metasomatic event. We propose that the lamprophyres were formed during partial melting of the South Asian mantle previously metasomatized by fluids derived from the subducted Indian continental crust. This melting episode is interpreted to be related to a break-off event that occurred within the subducting Indian continental lithosphere. Intrusion of the resulting lamprophyric melts into the previously thickened south Karakorum crust caused partial melting of calc-alkaline igneous protoliths and generation of the Baltoro granitoids. Late-stage syenitic magmas were produced by low-degree partial melting during upwelling and adiabatic decompression of depleted mantle along the Shigar strike-slip fault.
- Subjects
PAKISTAN; INDIA; MIOCENE stratigraphic geology; FUSION (Phase transformation); GEODYNAMICS; CONVERGENCE (Meteorology); EARTH'S mantle; CRUST of the earth; KARAKORAM Highway (China &; Pakistan); EARTH (Planet)
- Publication
Journal of Petrology, 2009, Vol 50, Issue 3, p427
- ISSN
0022-3530
- Publication type
Article