Abstract

The Kohistan arc terrane was initiated offshore of Asia during the mid-Cretaceous as an intraoceanic island arc above the subducting Tethyan oceanic slab. At ca. 100 Ma it was accreted to the south margin of Asia, and subsequently behaved as a continental margin volcanic arc until being underplated by continental India in the early Tertiary. Three main volcanic and volcano-sedimentary sequences are recognised from the intraoceanic stage of arc evolution. Modelling of geochemical differences from basaltic rocks from these sequences illustrates significant changes in magma source regions during arc evolution. The oldest sequence, the Kamila Amphibolites, are tholeiitic to calc-alkali basalts, erupted on to thickened, plateau-like oceanic crust. Trace element and REE chemistry have clear arc signatures. Modelling shows the basalts were derived by up to 16% melting of spinel lherzolite at between 30 and 80 km. Field and geochemical data demonstrate an apparent upward transition from this sequence into the Jaglot Group. Jaglot Group basalts are also tholeiitic to calc-alkali, but are interbedded with turbiditic volcaniclastic sediments, the proportion of sediment to volcanic rocks increasing eastward. Basalts in the west are chemically similar to those in the Kamila sequence. Those from the eastern part of the sequence are more enriched in the REE than in the west although both have similar patterns. Modelling shows these basalts were derived by high (>80 km) pressure melting of garnet lherzolite, differences in REE chemistries being consistent with lower percent melting in the west (8%) than in the east (12%). Basalts from the Chalt Volcanic Group in the north of the arc, which include tholeiitic, pillowed, high-Mg basalts and high-Mg andesites and boninites, have signicantly different geochemical signatures from basalts of the other sequences. In contrast to those, the Chalt basalts are HREE-enriched relative to the LREE. Modelling suggests derivation through 10% percent melting of a cpx-depleted spinel-rich mantle at shallow depths, probably during back-arc rifting, above a north-dipping subduction zone. On the basis of geochemistry and modelling there is no evidence for significant metasomatism of the mantle wedge in the source region. It is hence likely that differences in basalt chemistry reflect differences in depth of melting and of melt percentage, both of which can be related to either magmatic thickening of the arc or to extensional thinning.