TIMING AND ORIGIN OF CRUSTAL MELTING IN SOUTH KARAKORAM: IMPLICATION FOR THE TIBET-HIMALAYA GEODYNAMICS

We present the in-situ isotopic record of zircons (U-Pb, O and Hf) from the mid-crustal migmatitic gneisses exposed in the core of the Dassu dome in south Karakoram north of the India-Asia suture zone. Our U-Pb zircon ages provide evidence for Proterozoic inherited cores (1.8 - 1.9 Ga and 2.3 - 2.5 Ga) surrounded by a migmatitic overgrowth dated at 6 - 20 Ma. Igneous type δ¹⁸O values from inherited cores (1.8 - 1.9Ga) varies from 8 ‰ to 9.5 ‰, however, few low U/Th magmatic inherited cores (2.3-2.5 Ga) appeared to be juvenile with mantle like values of 5.5±2.7 ‰. The Miocene migmatitic overgrowths rendered indistinguishable δ¹⁸O values from their inherited core. The igneous type δ¹⁸O values from the migmatitic rims indicate that the melt was generated by anatexis of infracrustal sources. The initial εHf(t) from Proterozoic inherited cores is slightly non-radiogenic and ranges at weighted mean from -5.3±1.0 to -3.0±1.3, however, inherited cores of 2.3 – 2.5 Ga yielded juvenile εHf(t) values ranging from +0.1 to +4.6. The present day εHf(0) of Proterozoic inherited cores ranges from -47.2±1.0 to -44.3±1.2, oldest core (2.3 - 2.5 Ga) yielded the most non-radiogenic value of -50±1.3. In contrast, the present day weighted mean εHf(0) for the Neogene migmatitic part is -30.6±0.9. Considerably non-radiogenic character of migmatitic rims implies that the Hf composition of the migmatitic zircons is not controlled exclusively by the dissolution of the inherited cores and that contamination by external melts is likely. However our data do not allow distinguishing between mantle or crustal origin. The uniform Hf-O isotopic structure from the Proterozoic inherited cores suggest their derivation from a single infra crustal source or minimal input from other sources. The older inherited zircons (2.3 - 2.5 Ga) were precipitated from juvenile mantle magma. The inherited ages are similar to those obtained in the Lhasa block and suggest a similar origin. Our new ages imply that the migmatization lasted from >20 Ma to 6 Ma. During this time period no horizontal mid-crustal flow is observed as required by the channel flow models, but rather limited vertical motion. This suggests that the partially melted middle crust south of the Karakoram-Tibet area does not flow southward and consequently is not related with High Himalayan Crystalline extrusion.