In south Karakorum, the western prolongation of southern Tibet, three distinct types of magmatic rocks were emplaced during the Neogene: (1) 22^24 Myr old lamprophyres, characterized by strong enrichment in large ion lithophile (LILE) and light rare earth ele- ments (LREE), 87Sr/86Sr(i)¼07096, "Nd(i)¼^7, and "Hf¼^9, interpreted to reflect partial melting of a previously metasomatized spinel-lherzolite mantle source; (2) the 21^26Myr old Baltoro high Ba^Sr granitoids, likewise strongly enriched in LILE and LREE, with 87Sr/86Sr(i)¼07034^07183, "Nd(i)¼^65to^110, and "Hf¼^18to ^80, produced by partial melting of amphibole-bear- ing rocks in the lower crust, possibly the root of south Karakorum Cretaceous magmatic arc; (3) the 8^9 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)¼07043^07055, "Nd(i)¼þ35^þ43, and "Hf¼þ104^þ112).The Hemasil syenite is interpreted as the product of partial melting of a time-integrated depleted spinel- lherzolite source that was enriched in Kand LREE during a recent metasomatic event.We propose that the lamprophyres were formed during partial melting of the South Asian mantle previously metaso- matized 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 litho- sphere. Intrusion of the resulting lamprophyric melts into the pre- viously thickened south Karakorum crust caused partial melting of calc-alkaline igneous protoliths and generation of the Baltoro grani- toids. Late-stage syenitic magmas were produced by low-degree par- tial melting during upwelling and adiabatic decompression of depleted mantle along the Shigar strike-slip fault.
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