Cooling, exhumation, and deformation in the Hindu Kush, NW Pakistan: New constraints from preliminary ~(40)Ar/~(39)Ar and fission track analyses

摘要

Asian crust in the Hindu Kush region in northern Pakistan records a protracted history of rifting, subduction and collision not commonly preserved within the Himalaya. Because of this, it is key to understanding the development of the southern Eurasian margin both prior to and after collision with India. New mica Ar-40/Ar-39 and apatite fission track geochronologic data from this region provide constraints on the kinematics of the Hindu Kush. Ar-40/Ar-39 muscovite and biotite ages from the late Cambrian Kafiristan pluton are 379.7 +/- 1.7 Ma and 47.2 +/- 0.3 Ma, respectively. The muscovite age may record cooling or partial resetting, while the biotite age is interpreted to record a thermal disruption associated with the early stages of continental collision in the Himalayan system. A 111.0 +/- 0.6 Ma muscovite age from the northern part of the Tirich Mir pluton (similar to 123 Ma old; U-Pb) is interpreted to indicate a recrystallization event similar to 12 Myrs after its intrusion. In addition, a younger muscovite age of 47.5 +/- 0.2 Ma was derived from the opposite side of the same pluton in the immediate hanging wall of the Tirich Mir fault. This Eocene age is interpreted to represent the time of recrystallization during fault (re)activation in the early stages of India-Asia continent-continent collision. Ar-40/Ar-39 biotite analysis from the Buni-Zom pluton yields an age of 61.6 +/- 1.1 Ma and is interpreted to reflect cooling at mid upper crustal levels subsequent to the pluton's emplacement in the middle Cretaceous. Finally, 17.1-21.3 Ma Ar-40/Ar-39 ages from the Garam Chasma pluton and surrounding metapelites indicate cooling immediately following crystallization of the leucogranite body in the earliest Miocene/latest Oligocene.