Age and origin of post collision Baltoro granites, south Karakoram, north Pakistan

New in-situ U-Pb, Hf and oxygen data indicate that the Baltoro plutonic unit in the southern part of the Karakoram axial batholith was formed by partial melting of pre existing igneous sources with some input from supra-crustal rocks. This is consistent with the previous interpretations emphasizing partial melting of mafic lower crust beneath the Karakoram during the early Miocene 26-21Ma. The Baltoro batholith is mainly composed of biotite monzogranites and garnet two-mica leucogranites. Our zircon U-Pb ages corroborate the previous geochronology ranging from 26.3-17.3Ma., Cretaceous ages (97-72Ma) have also been observed in some zircons. Inherited ages are ranging from 553-2605Ma similar to those reported for the Lhasa block, confirming the westward continuation of south Tibet. The weighted mean ɛHf(0) for each sample varies from -8.7 to -4.0 and ranges from -17.1 to +4.4. These values are similar to the whole rock Hf data from the same rocks and are indistinguishable from those reported for the Cretaceous calc-alkaline magmatic rocks along the east-west transect of the Karakoram Terrane. Also, Miocene two-mica granites in the Darbuk area, eastern Karakoram show the comparable Hf composition. These values are significantly more evolved than the southward juvenile oceanic Cretaceous-Paleogene Kohistan-Ladakh Arc (+5to+16). The initial ɛHf(t) values of the Cretaceous zircons (+0.9 to +4.7) are more correlated to the Cretaceous calc-alkaline basement of Karakoram Batholith than the Kohistan-Ladakh Arc. The Hf composition of inherited cores is significantly different than the magmatic zircons with ɛHf(0) ranging from -58.6 to -14.3 comparable to the mid-crustal migmatitic gneisses southward in the Karakoram Metamorphic Complex. Both Miocene granites and Cretaceous zircons share similar and homogenous igneous type δ18O with mean δ18O range from 7.2 to 9.4‰ (2σ). The mean δ18O of the core domain has a mixed signature ranging from 6.9 ‰ to 9.2 ‰. We suggest that the granites were formed by partial melting of pre existing igneous sources with some input from supracrustal rocks. The intermediate Hf composition of the magmatic zircons between highly evolved inherited core (similar to the mid- upper crustal Dassu migmatite) and calc-alkaline Cretaceous granodiorites suggests possible mixing between the two sources. Possible involvement of upper crust is also evident by the few more evolved Hf composition and slight increase in the observed oxygen composition than typical igneous zircons (> 8‰). This may have resulted by the interaction of the magma with the supracrustal material during the final ascent and emplacement at shallow depths beneath the Karakoram. Our zircon oxygen data did not show evidence of a juvenile (< 6‰) component. Moreover, the Hf compositions observed in Kohistan-Ladakh Arc are too radiogenic ((+6 to +16) than what we observe in the Baltoro granites. Therefore, mixing of such magma with the adjacent juvenile Kohistan-Ladakh batholiths or any direct contribution from juvenile mantle derived magma is unlikely. However this does not exclude the role of mantle melts in providing the additional heat to melt lower crust beneath the Karakoram. Our Hf data is significantly evolved than the Karakoram shear zone leucogranites (+1 to +9) where unlike to Baltoro region, magmas were strongly influenced by the juvenile Ladakh batholith.