Structural setting and detrital zircon U–Pb geochronology of Triassic–Cenozoic strata in the eastern Central Pamir,Tajikistan

Integration of new geological mapping, detrital zircon geochronology, and sedimentary and metamorphic petrography south of the Muskol metamorphic dome in the Central Pamir terrane provides new constraints on the evolution of the Pamir orogen from Triassic to Late Oligocene time. Zircon U–Pb data show that the eastern Central Pamir includes Triassic strata and mélange that are of Karakul–Mazar/ Songpan–Ganzi affinity and comprise the hanging wall of a thrust sheet that may root into the Tanymas Fault c. 35 km to the north. The Triassic rocks are unconformably overlain by Cretaceous strata that bear similarities to coeval units in the southern Qiangtang terrane and the Bangong Suture Zone of central Tibet. Finally, Oligocene or younger conglomerate and interbedded siltstone, the youngest documented strata in the Pamir Plateau proper, record an episode of juvenile magmatism at c. 32 Ma, which is absent in the extant rock record and other detrital compilations from the Pamir but overlaps in age with ultrapotassic volcanic rocks in central Tibet. Zircon Hf isotopic data from the Oligocene grains (εHf(t) ≈ +9.6) suggest a primary mantle contribution, consistent with the hypothesis of Late Eocene lithospheric removal beneath the Pamir Plateau. Supplementary material: Coordinates of field stations, maximum deposition ages from detrital U–Pb analyses, zircon Lu–Hf andU–Pb isotopic data, selected cathodoluminescence images, electronmicroprobe analyses, and mineral formula calculations are available at https://doi.org/10.6084/m9.figshare. c.4299518 Efforts to relate geological units between the Pamir and Tibetan Plateau are hindered by right-lateral offset along the Karakoram Fault, and the development of the northwards-convex Pamir salient (Fig. 1a). Schwab et al. (2004) correlated the terranes of Pamir and Tibet on the basis of the spatial distribution of Mesozoic magmatic belts (the Karakul– Mazar/North Pamir, Central Pamir and South Pamir with the Songpan–Ganzi, Qiangtang, and Lhasa terranes, respectively). Robinson (2009) suggested that offset along the Karakoram Fault in the South Pamir is <170 km, based on the displacement of a Triassic–Jurassic carbonate unit interpreted from satellite imagery. This, along with the similarity of Permian fauna in various terranes (Angiolini et al. 2013), led to the conclusion that both the Central Pamir and South Pamir/Karakoram are correlative to the Qiangtang terrane, and that the North Pamir is correlative to the Songpan–Ganzi terrane (Robinson 2015). Despite these advances, inadequate documentation of rock units in the Central Pamir presents a significant obstacle in evaluating how Pamir geology relates to that in Tibet, and thus in understanding the evolution of the Tibet–Pamir orogenic system as a whole. In particular, many sutures between terranes are inferred, ophiolites mentioned in early literature have not been confirmed by later authors (e.g. Leven 1995), and new geochronological data from Rutte et al. (2017a) have shown that many age assignments of earlier mapping, often assigned ‘by convention’, are incorrect. This study was undertaken to better document the lithology, age, provenance and structure of the upper crust in the eastern Central Pamir. We present new geological mapping, detrital zircon U–Pb data (n = 2940), detrital zircon Lu–Hf isotopic data (n = 36), and sedimentary and metamorphic petrography from the Akbaital–Kalaktash region, south of the Muskol Dome (Fig. 1b). We document the presence of Triassic Karakul–Mazar or From: TRELOAR, P. J., & SEARLE, M. P. (eds) Himalayan Tectonics: A Modern Synthesis. Geological Society, London, Special Publications, 483, https://doi.org/10.1144/SP483.11 © 2018 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http://www.geolsoc. org.uk/permissions. Publishing disclaimer: www.geolsoc. org.uk/pub_ethics by guest on November 27, 2018 http://sp.lyellcollection. org/ Downloaded from