Structural and thermal evolution of the Karakoram metamorphic complex, Hunza and Baltoro regions, North Pakistan

M.P. Searle, D.J. Waters, J.E. Fraser, A.V. Thow in collaboration with R.R. Parrish (NIGL)

The southern margin of the Asian plate prior to the collision of the Kohistan arc and Indian plate comprised the Hindu Kush and Karakoram ranges of north Pakistan and the Qiangtang and Lhasa blocks of south Tibet. Whereas Tibet is a region of high elevation (ca 5000 meters) but low relief and exposes mainly upper crustal sedimentary and volcanic rocks, the Hindu Kush and Karakoram ranges are extremely high (up to 8700 meters), deeply incised and expose middle and deep crustal metamorphic rocks, gneiss domes and granite batholiths. Pre-collision granites include, the K2, Muztagh Tower and Hushe gneiss units. Previous U-Pb geochronology has revealed at least 4 major periods of staurolite - kyanite - sillimanite grade metamorphism (ranging from 65 Ma to ca 3 Ma), several phases of leucogranite dyke injection (Hunza dykes) and intrusion of the massive post-collision granite batholith the Baltoro granites during the Early Miocene. Miocene granites formed by crustal melting contemporaneously with upper mantle melting which resulted in the intrusion of lamprophyric dykes. This project covers regional structural mapping, geochemistry, thermobarometry, metamorphic modelling and U-Th-Pb geochronology of the entire Hunza, Baltoro glacier and Hushe valley regions of the Karakoram ranges in North Pakistan.

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Crustal melting in the Karakoram: Baltoro granite batholith

M.P. Searle, D.J. Waters, A.V. Thow in collaboration with R.R. Parrish (NIGL)

The Baltoro granite batholith in North Pakistan comprises biotite monzogranites and garnet two-mica leucogranites formed as a result of crustal melting probably with extra heat input from upper mantle melting during the Miocene. Initial U-Pb ages of 25-20 Ma were reported from samples along the Baltoro glacier. New U-Pb monazite ages from biotite + muscovite + garnet leucogranites from the Trango Towers, Cathedral and Paiyu peaks and the K7 massif are Early Miocene spanning 19.8 - 17. 5 Ma. The youngest ages come from the Masherbrum complex granites at the highest structural levels that have been fed from a massive sheeted sill complex. This project includes regional scale mapping along all the major glacial valleys of the Karakoram Range combined with U-Th-Pb dating of major granite phases.

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Structural evolution of the Karakoram Fault, Ladakh

M.P. Searle, R. J. Phillips, M.J. Streule, D.J. Waters in collaboration with R.R. Parrish (NIGL) and E. Rutter (Manchester)

The dextral Karakoram Fault has been widely regarded a major lithospheric scale strike-slip fault that has accommodated large-scale eastward extrusion of the thickened crust of Tibet. The deepest levels of the fault are well exposed in northern Ladakh, where the fault splays into two branches, the major Tangtse strand along the SW and Pangong Fault to the NE. Numerous older metamorphic rocks granitoids are preserved within the transpressional uplifted Pangong Range in between the two strands of the fault. These include early hornblende-bearing granodiorites and orthogneisses intruded by post-collision leucogranites. Spectacular mylonites occur along both strands of the fault which clearly truncate earlier fabrics away from the shear zone margins. Ductile shearing has been demonstrated to have occurred between 15.68 ± 0.52 Ma and 13.73 ± 0.28 Ma the U-Pb ages of early parallel set of sills and a post-kinematic set of cross-cutting leucogranite dykes. This project is now aiming to date the age of metamorphism along the fault, determine whether leucogranite sheets and dykes are pre- syn- or post-kinematic with respect to strike-slip movement and further constrain timing of fault slip and amounts of right-lateral offset.

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Structural and thermal evolution of the Hindu Kush, NW Pakistan

M.P. Searle, P.R. Hildebrand in collaboration with R.R. Parrish, S.R. Noble (NIGL)

The Hindu Kush Ranges along the NW frontier of Pakistan with Afghanistan were part of the southern margin of Asia prior to collision and accretion of the Kohistan Arc during Late Cretaceous-Palaeocene time and the Indian Plate during Early Eocene. Pre-collision metamorphism along the Hindu Kush has been dated using U-Pb on monazites as Jurassic - Cretaceous and pre-collision magmatism includes the Tirich Mir granodiorites. Post-collision crustal melt granites are also present for example the two-mica + garnet + tourmaline Gharam Chasma leucogranite which has U-Pb monazite, xenotime and uraninite ages of 24 Ma. This project is aimed at unraveling the pre- and post-collision evolution of the Hindu Kush as well as interpreting the deep seismic zone present today beneath the range.

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Subduction-related metamorphism along the Main Mantle Thrust, Kohistan arc, North Pakistan

M.P. Searle, D.J. Waters, S. Gough in collaboration with R.R. Parrish (NIGL)

The Kohistan arc is a complete section through a Cretaceous island arc accreted onto the southern margin of Asia (Karakoram margin) prior to the collision of India approximately 50 million years ago. From base to top the Kohistan island arc includes a series of ultramafic rocks including dunite, harzburgite and websterite (Sapat complex), garnet granulites (Jijal complex), massive amphibolites (Kamila complex) intruded by multiple intrusions of a huge gabbro-norite complex (Chilas complex). The upper levels of the arc comprise a series of intra-oceanic arc rocks (Chalt, Dras Groups) erupted on a foundation of dominantly MORB related meta-volcanics. The Kohistan arc represents a Mariana-type intra-oceanic arc that collided with Asia during the Late Cretaceous and was subsequently deformed and tilted after the accretion and collision of India during the Eocene.

The northern margin of the Indo-Pakistan plate collided with the Kohistan arc and closed the Tethyan Ocean which separated them during the late Mesozoic and early Tertiary. The leading margin of the Indian plate was subducted to depths over 100 km forming coesite-bearing eclogites in the leading continental margin rocks at 37.5-25.5 kbar and 770-720oC. ID-TIMS U-Pb dating of allanite, zircon, titanite and rutile has constrained the prograde and retrograde P-T paths of the Kaghan valley eclogites. Peak UHP metamorphism occurred at 46.3 Ma in NW Pakistan.

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