Growth and deformation of the Ladakh batholith, northwest Himalayas: Implications for timing of continental collision and origin of calc-alkaline batholiths

The calc-alkaline Ladakh batholith (NW'Himalayas) was dated to constrain th e timing of continental collision and subsequent deformation. Batholith gro wth ended when collision disrupted subduction of the Tethyan oceanic lithos phere, and thus the youngest magmatic pulse indirectly dates the collision. Both U-Pb ages on zircons from three samples of the Ladakh batholith and K -Ar from one subvolcanic dike sample were determined. Magmatic activity nea r Lch (the capital of Ladakh) occurred between 70 and 50 Ma, with the last major magmatic pulse crystallizing at ca. 49.8 +/- 0.8 Ma (2 sigma). This w as followed by rapid and generalized cooling to lower greenschist facies te mperatures within a few million years, and minor dike intrusion took place at 46 +/- l Ma. Field observations, the lack. of inherited prebatholith zir cons, and other isotopic evidence suggest that the batholith is mantle deri ved with negligible crustal influence, that it evolved through input of fre sh magma from the mantle and remelting of previously emplaced mantle magmat ic rocks. The sedmimentary record indicates that collision in NW Himalaya o ccurred around 52-50 Ma. Ii this is so, the magmatic system driven by subdu ction of Tethys ended immediately on collision. The thermal history of one sample from within the Thanglasgo Shear Zone (TSZ) was determined by Ar-Ar method to constrain timing of batholith. internal deformation. This is a wi de dextral shear zone within the batholith, parallel to the dextral, N 30 d egrees W-striking crustal-scale Karakoram Fault. Internal deformation of th e batholith, taken up partly by this shear zone, has caused it to deviate f rom it regional WNW-ESE trend to parallel the Karakoram Fault. Microstructu res and cooling history of a sample from the TSZ indicate that shearing too k place before 22 Ma, implying that (1) the history of dextral shearing on NW-striking planes in northern Ladakh started at least 7 m.yr. before the < 15 Ma Karakoram Fault, (2) shearing was responsible for deviation of the re gional trend of the Ladakh batholith, and (3) dextral shearing occured with in a zone approximately 100 km wide that includes the Ladakh batholith and portions of the younger Karakoram batholith.