Metamorphic rocks in central Tibet: Lateral variations and implications for crustal structure

Abstract

Insights about lateral variations in the age, composition, and structure of the central Tibetan crust are provided by geologic investigations of metamorphic rocks in the Qiangtang terrane. Previous studies have shown that a tectonic mélange of Triassic age with blueschist-and eclogite-bearing blocks within a greenschist-facies matrix is exposed over an E-W distance of ~600 km in the central Qiangtang terrane. New mapping shows that the mélange extends over a N-S distance of ~150 km, nearly to the trace of the early Mesozoic Jinsha suture in the north. The mélange , exposed structurally beneath Upper Paleozoic to Mesozoic strata in the footwalls of early Mesozoic normal faults, is composed mostly of Paleozoic metasedimentary and crystalline rocks. These fi ndings support the hypothesis that a large part of the central and northern Qiangtang terrane crust is composed of supracrustal rocks. The Duguer Range, <25 km south of the southernmost mélange, exposes high-temperature, lowpressure metasedimentary rocks and orthogneisses with 476-471 Ma U-Pb crystallization ages that represent continental basement of the Qiangtang terrane. The boundary at the surface between mélange and Gondwanan basement corresponds closely with abrupt changes in crustal geophysical properties, and allows us to extrapolate this relationship to depth. The resultant crustal structure can be explained by development of major Mesozoic structures documented in surface geology. Antiformal geophysical features imaged in the midcrust are likely inherited from Mesozoic tectonics, suggesting that the crust may not have been as mobile as that required by models invoking large-scale lower-crustal fl ow beneath the central Tibet Plateau during the Cenozoic India-Asian collision.

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

“…In addition to rocks related to the Paleo-Tethys Ocean, geologic evidence of Caledonian tectonism has been reported along the northern margin of South Qiangtang near the LSSZ, including granitic gneiss (ca. 476-471 Ma; Pullen et al, 2011;Hu et al, 2010), amphibolite (± garnet) (ca. 411 Ma, Pullen et al, 2011;ca.…”

“…476-471 Ma; Pullen et al, 2011;Hu et al, 2010), amphibolite (± garnet) (ca. 411 Ma, Pullen et al, 2011;ca. 432 Ma, Li et al, 2008), and ophiolites (ca.…”

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

“…J.X. Li et al, 2014;Liu et al, 2014Liu et al, , 2015Pullen et al, 2011). Based on geochemical studies, these Jurassic magmatic rocks were interpreted to be the products of the northward subduction of the Bangong-Nujiang Ocean Liu et al, 2011;Pullen et al, 2011).…”

“…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).…”

“…; J.X. Pullen et al, 2011). Recently, early Jurassic granitoids were discovered in the Ando microcontinent along the margin of the Southern Qiangtang terrane (Liu et al, 2011.…”