Two Contrasting Exhumation Scenarios of Deeply Subducted Continental Crust in North Pakistan

In Western Himalayan Syntaxis, the India-Asia continental collision occurred at ca. 50 Ma, while its uplift history and exhumation mechanism are still in dispute despite decades of studies. A new type of eclogite was found in Naran, located ca. 30 km southwest of the Upper Kaghan Valley. Phase equilibrium calculations and thermobarometer performed on the Naran eclogite documented the peak-P metamorphic condition of 720-780 degrees C at 2.4-2.8 GPa. Two further exhumation stages were identified with the first one at high-P granulite-facies conditions of 750-800 degrees C at 1.6-1.9 GPa, and the second at amphibolite-facies conditions of 550-630 degrees C at 0.5-0.8 GPa. SIMS U-Pb dating of metamorphic zircons yielded an age of 46 +/- 2 Ma, which is interpreted to constrain the high-P metamorphism age along the northwestern margin of the Indian plate. SIMS U-Pb dating of rutile yielded a cooling age of 26 +/- 3 Ma, which is interpreted as cooling age in the amphibolite facies. The average speculated exhumation rate of the Naran massif (similar to 3 mm a(-1)) was much lower than that recovered from the Upper Kaghan Valley massif (86-143 mm a(-1)). The tectonic and metamorphic evolution of the whole Western Himalayan Syntaxis shows the difference in temporal and spatial change within the Paleogene era, indicating the inconsistent exhumation histories of the continental slices. Such a multi-slice exhumation process was probably related to the closure of the Neo-Tethys ocean and the break-off of the Indian lithospheric slab.