U-Pb monazite ages from the Pakistan Himalaya record pre-Himalayan Ordovician orogeny and Permian continental breakup

Abstract

The Greater Himalayan sequence in India and Nepal records crustal thickening processes during and following the onset of India-Asia collision (ca. 54-50 Ma), which resulted in Late 2 Eocene-Early Miocene kyanite-and sillimanite-grade regional metamorphism, and Oligocene-Miocene crustal anatexis forming migmatites and leucogranites. In the Pakistan Himalaya, these events are not recorded in the exposed rocks beneath the obducted Kohistan island arc. Instead, the kyanite-grade gneiss of the Besham Group and sillimanite-grade gneiss of the calcareous Alpurai schist record U-Pb monazite 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, record an Ordovician-age orogeny (the Bhimpedian orogeny) spanning at least from 490 to 460 Ma, following Neoproterozoic-Cambrian sedimentation, and preceding late Ordovician-Silurian post-orogenic molasse deposition and development of a stable shelf margin at 460-440 Ma. These new ages for peak metamorphism in the Pakistan Himalaya overlap with those of widespread S-type granites occurring along 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). We suggest that in Pakistan, the unexposed Tethyan and Greater Himalayan sequences have been overthust by the Kohistan arc, and the Besham-Alpurai gneisses are equivalent to Lesser Himalayan rocks to the east.

Timing and Source of Melting at the Eastern Edge of the Gurla Mandhata Core Complex, Nw Nepal Himalaya

“…This unit has been dated at lower Ordovician, ca. 480 Ma to 470 Ma (Debon et al, 1986;Le Fort and Rai, 1999;Godin et al, 2001;Ogasawara et al, 2018;Palin et al, 2018). This Upper Cambrian-Lower Ordovician magmatism occurred at continental scale and is linked to the Bhimphedian event (e.g.…”

“…Cawood et al, 2007). This event is related to an Andean-type margin that developed at the northern margin of the Indian continent following the assembly of Gondwana Le Fort et al, 1986;DeCelles et al, 2000;Godin et al, 2001;Miller et al, 2001;Gehrels et al, 2003;Cawood et al, 2007;Martin et al, 2007;Gehrels et al, 2011;Stübner et al, 2017;Palin et al, 2018). The augen orthogneiss yields a large range of initial Sr isotopic ratio, from 0.755 to 0.89 (Guillot and Le Fort, 1995).…”

“…The Himalaya also exposes extensive volumes of migmatites, granites and granitic dykes, which are products of crustal melt (e.g. Searle and Fryer, 1986;Brown, 2001;Godin et al, 2001;Harris et al, 2004;Searle et al, 2008;Jain et al, 2013;Weinberg, 2016;Palin et al, 2018;Cottle et al, 2019). These magmatic rocks are now exposed due to exhumation processes such as erosion and normal-sense faults.…”

Rituals and Institutional Maintenance: The Case of the Beating Retreat Ceremony

“…While discontinued in 1965 and 1971, given India–Pakistan hostilities, it has been a popular ritual attracting, on average, 15,000 citizens from various parts of India and non-Indian tourists. This daily ritual at Wagah, on the India–Pakistan border, has been referred to as a demonstration of “carefully choreographed contempt” (Palin, 2004) and as a “spectacular theatre of nationalism,” crucial in sustaining differences and identities of Indian and Pakistani collectives (Menon, 2013, p. 23).…”

Tectonometamorphic evolution of the tip of the Himalayan metamorphic core in the Jajarkot klippe, west Nepal

“…However, sparse preservation of pre-Himalayan metamorphic event(s) implies that metamorphic conditions can only be used to constrain tectonic models for the Himalayan orogeny if they are tied to geochronological constraints (e.g., DeCelles, Gehrels, Quade, LaReau, & Spurlin, 2000;Gehrels et al, 2003;Marquer, Chawla, & Challandes, 2000). In the hinterland of the mountain belt, evidence for pre-Himalayan metamorphism includes Cambro-Ordovician monazite and allanite inclusions in garnet (e.g., Catlos, Sorensen, & Harrison, 2000;Catlos et al, 2002;Kohn, Wieland, Parkinson, & Upreti, 2004), Cambro-Ordovician monazite in the matrix of kyanite and sillimanite schists (Palin et al, 2018), Ordovician monazite in leucosome, interpreted as inherited from the host schist (Godin et al, 2001), early Cambrian garnet porphyroblasts (Argles, Prince, Foster, & Vance, 1999), and foliated and folded greenschist to upper amphibolite facies metamorphic rocks intruded by late Cambrian granite bodies (e.g., Manickavasagam et al, 1999;Marquer et al, 2000). However, pre-Himalayan metamorphism has been strongly overprinted by Cenozoic metamorphism in most cases, such that the metamorphic conditions recorded in the rocks are generally representative of the state of the middle crust during the Himalayan orogeny.…”