U-Pb monazite ages from the Pakistan Himalaya record pre-Himalayan Ordovician orogeny and Permian continental breakup
The Greater Himalayan Sequence in India and Nepal records crustal thickening processes that took place during and following the onset of India-Asia collision (ca. 54–50 Ma). These resulted in late Eocene–early Miocene kyanite- and sillimanite-grade regional metamorphism, and Oligocene–Miocene crustal anatexis, which formed migmatites and leucogranites. We present new U-Pb monazite data for kyanite- and sillimanite-grade gneisses of the Neoproterozoic to Cambrian Tanawal Formation in the Pakistan Himalaya, which have metamorphic ages of 482.4 ± 7.9 Ma and 464.5 ± 4.0 Ma, respectively. These ages, together with along-strike equivalent rocks in the Lesser Himalaya of India and Nepal, help to define an Ordovician orogeny (the Bhimpedian orogeny, also called the Kurgiakh orogeny) spanning at least from 490 to 460 Ma, which followed Neoproterozoic–Cambrian sedimentation and preceded Late Ordovician–Silurian postorogenic molasse deposition and development of a stable shelf margin at 460–440 Ma. These new ages for peak metamorphism of Neoproterozoic rocks in the Pakistan Himalaya overlap with those of widespread S-type granites that exist along the length of the Lesser Himalaya (e.g., Mansehra, Mandi, and Kathmandu granites), Greater Himalaya (e.g., Nanga Parbat, Kinnaur Kailas, and Ama Drime), and in the North Himalayan domes (e.g., the Tso Morari, Kangmar, and Kampa domes). Thus, based on stratigraphic, metamorphic, and U-Pb geochronological criteria, we equate rocks of the Tanawal Formation gneisses to the Lesser Himalayan rocks to the east and rocks of the Alpurai Group to the Higher Himalaya. Our data also confirm that the Lesser, Greater, and Tethyan Himalaya are all part of a single contiguous plate. Staurolite-grade schists from a higher structural level of the Tanawal Formation record U-Pb monazite ages of ca. 272–267 Ma, indicating the occurrence of Permian regional metamorphism in the area concomitant with major rifting and extrusion of the Panjal Trap volcanics.