Tectonic evolution of the early Mesozoic blueschist‐bearing Qiangtang metamorphic belt, central Tibet

Abstract

[1] A >500-km-long east-west trending metamorphic belt in the Qiangtang terrane of central Tibet consists of tectonic melange that occurs in the footwalls of Late Triassic -Early Jurassic domal low-angle normal faults. The melange is comprised of a strongly deformed matrix of metasedimentary and mafic schists that encloses lesser-deformed blocks of metabasites, Carboniferous-Triassic metasedimentary rocks, and early Paleozoic gneiss. Both the blocks and melange matrix exhibit greenschist, epidoteblueschist, and locally, epidote-amphibolite facies mineral assemblages. Thermobarometry reveals that the metamorphic belt experienced pressures of >10 kbar. Maximum equilibration temperatures for mafic schists in the melange matrix decrease from east to west, from $660°C near Shuang Hu (33°N, 89°E), $500°C near Rongma (33°N, 87°E), to $425°C near Gangma Co (34°N, 84°E). Equilibration at consistently high pressures over a large range of temperatures is compatible with metamorphism of Qiangtang melange within a low-angle subduction zone beneath a continental margin. Coupled structural, thermobarometric, and 40 Ar/ 39 Ar studies suggest that Qiangtang melange was exhumed in an intracontinental setting from depths of >35 km to upper crustal levels in <12 Myr by Late TriassicEarly Jurassic crustal-scale normal faulting. Detrital zircons from metasandstones within the melange matrix yield U-Pb ion-microprobe ages that range from early Paleozoic to Early Archean, and could have been sourced from terranes to the north of the Jinsha suture. Our results support a model in which Qiangtang melange was underthrust $200 km beneath the Qiangtang terrane during early Mesozoic flat-slab southward subduction of Paleo-Tethyan oceanic lithosphere along the Jinsha suture. This model predicts that significant portions of the central Tibetan continental mantle lithosphere were removed during early Mesozoic low-angle oceanic subduction and that the present-day central Tibetan deeper crust includes large volumes of underthrust early Mesozoic melange.

Late Triassic high-Mg andesite/dacite suites from northern Hohxil, North Tibet: Geochronology, geochemical characteristics, petrogenetic processes and tectonic implications

“…10), as suggested by some previous workers based on tectonic, sedimentary, metamorphic and paleomagnetic analyses (e.g., Kapp et al, 2003;Nie et al, 1994;Reid et al, 2007;Roger et al, 2003;Yin and Harrison, 2000). With the ongoing double-sided subduction, the eastern Songpan-Ganzi oceanic basin diminished in size or even disappeared, and collisions between different blocks or arc fragments (e.g., the South China, Kunlun-Qaidam, North China, South Qinling, Qilian, Yidun, and Qiangtang Blocks; Fig.…”

Silurian high-pressure granulites from Central Qiangtang, Tibet: Constraints on early Paleozoic collision along the northeastern margin of Gondwana

“…1a, b). It is a tectonic complex composed of blueschist, eclogite, ophiolite, OIB-type basalt, metasedimentary rocks, and minor chert (BGMR, 1993;Li et al, 1995Li et al, , 2006Li et al, , 2008Kapp et al, 2000Kapp et al, , 2003Zhang et al, 2010aZhang et al, , 2010bZhang et al, , 2010cZhang et al, , 2014Zhai et al, 2007Zhai et al, , 2011aZhai et al, , 2011b. The ophiolitic mélange in the LSSZ consists mainly of ultramafic rocks, gabbro, basalt, pillow lava, and minor chert (Zhai et al, 2013).…”

Middle Jurassic MORB-type gabbro, high-Mg diorite, calc-alkaline diorite and granodiorite in the Ando area, central Tibet: Evidence for a slab roll-back of the Bangong-Nujiang Ocean

“…The ocean was suggested to have closed during the Triassic based on the presence of the Triassic high-pressure metamorphic rocks belt in the central Qiangtang (Zhai et al, 2011a(Zhai et al, , 2011b(Zhai et al, , 2013. However, Kapp et al (2000Kapp et al ( , 2003 suggested that the Longmu-Shuanghu metamorphic rocks belt was only a metamorphic core complex produced by underthrusting of Jinshajiang ocean lithosphere beneath the Qiangtang Terrane during the Early Mesozoic, then to be exhumed in an intracontinental setting by Late Triassic-Early Jurassic normal faulting. Recently, Zeng et al (2015) proposed that the exhumation of Triassic Qiangtang metamorphic event was due to the northward subduction of Bangong-Nujiang Ocean beneath the Qiangtang terrane during late Triassic.…”

“…The suture zone is made up of a discontinuous belt of flysch, mélange and ophiolitic fragments (Kapp et al, 2003;B.D. Wang et al, 2016;K.J.…”

“…Recently, more and more evidence indicates that the preCenozoic tectonics and magmatism probably contributed to the accretion and formation of the central and northern Tibetan Plateau before the final India and Eurasian collision (Guynn et al, 2012;Kapp et al, 2003Kapp et al, , 2007Long et al, 2015;Murphy et al, 1997;Pullen et al, 2011). For example, an active continental margin is proposed to have developed in the Southern Qiangtang caused by the subduction of the Bangong-Nujiang Ocean (BNO), which is predated to the collision between the Lhasa and Qiangtang terranes Liu et al, 2014).…”