Zircon U–Pb geochronology of the Nyainqentanglha Group from the Lhasa terrane: New constraints on the Triassic orogeny of the south Tibet

Abstract

The studied amphibolite-facies metamorphic rocks, typical of the Nyainqentanglha Group, occur as a tectonic block in the central Lhasa terrane, south Tibet, and consist mainly of gneiss, amphibolite, quartz schist and marble. The amphibolite has a similar mineral paragenesis of plagioclase + garnet + biotite + amphibole + quartz ± muscovite, and were metamorphosed under P–T conditions of 0.63–0.66 GPa and 588–622 °C. Zircons from three dated samples include two types. First type of zircons has an inherited magmatic core and a metamorphic rim; whereas the other type of zircons is of metamorphic origin. LA-ICP-MS in situ U–Pb analysis indicates that the inherited magmatic cores of zircons yield widely variable ²⁰⁶Pb/²³⁸U ages ranging from 1857 Ma to 743 Ma, recording the magmatic-thermal events of the material source of the metamorphic rock protoliths. In contrast, the metamorphic zircons yield concordant ²⁰⁶Pb/²³⁸U ages of 225–213 Ma. The results indicate that the Nyainqentanglha Group has been subjected to Triassic metamorphism, instead of the Precambrian metamorphic basement of the Lhasa terrane as considered traditionally. These rocks, together with the coeval Sumdo eclogite and Pana blueschist, form a Permian to Triassic metamorphic belt with an E–W extension of up to 300 km. Therefore, this work provides new constraints on the late Paleozoic to early Mesozoic orogeny in the central Lhasa terrane, south Tibet.

Introduction

The Lhasa terrane located at the south Tibet is bounded by the Yarlung-Zangbo Suture Zone (YZSZ) and the Bangong-Nujiang Suture Zone (BNSZ) (Yin and Harrison, 2000 and references therein). Previous studies identified that the Lhasa terrane was composed of Precambrian metamorphic basement, Paleozoic–Mesozoic sedimentary rocks and Mesozoic and Cenozoic igneous rocks (e.g., Yin and Harrison, 2000, Pan et al., 2006). Most of the previous works focused on the igneous rocks of the Lhasa terrane, which provided important constraints on the Mesozoic Andean-type and Cenozoic Himalayan-type orogenies in south Tibet (Pan et al., 2002, Pan et al., 2006, Mo et al., 2003, Mo et al., 2005, Mo et al., 2006, Mo et al., 2007a, Mo et al., 2007b, Mo et al., 2008, Chung et al., 2003, Chung et al., 2005, Liao et al., 2007, Chu et al., 2006, Zhu et al., 2008, Zhu et al., 2009a, Zhu et al., 2009b, Wen et al., 2008a, Wen et al., 2008b, Ji et al., 2009, Xu et al., 2010a, Zhao et al., 2009, Zhang et al., 2010a, Zhang et al., 2010b). However, only a few studies involving metamorphism of the Lhasa terrane has been carried out (Wang et al., 2008, Wang et al., 2009, Booth et al., 2009, Dong et al., 2010).

The Lhasa terrane was divided into three segments, i.e. the Northern Lhasa (NL), Central Lhasa (CL) and Southern Lhasa (SL) (Zhu et al., 2009a), which are separated by the Zhari Nam Tso-Nam Tso-Jiali fault (ZNJF) in the north and the Luobadui-Milashan fault (LMF) in the south (Fig. 1). The high-grade metamorphic rocks formed under the amphibolite-facies, and even granulite-facies conditions occur in the whole Lhasa terrane, including Naguo, Nyainqentanglha, Linzhi, Bomi and Ando areas (Fig. 1). These rocks were called generally as the Nyainqentanglha Group, although the Linzhi Group and Gangdese Group were also used for the metamorphic rocks from the southern part of the Lhasa terrane (Geng et al., 2006, Dong et al., 2010, Zhang et al., 2010c). Because some Proterozoic ages were obtained by the traditional dating technologies, the previous works concluded generally that the Nyainqentanglha Group has been metamorphosed at the Precambrian, representing the Precambrian metamorphic basement of the Lhasa terrane (e.g., Xu et al., 1985, Dewey et al., 1988, Harris et al., 1988, Hu et al., 2003). However, with the application of zircon U–Pb SHRIMP and LA-ICP-MS in situ dating technologies, recent studies have shown that the protolith of the Linzhi Group was formed at the early Paleozoic, and has experienced the multi-stages of metamorphism during Mesozoic and Cenozoic (Wang et al., 2008, Wang et al., 2009, Dong et al., 2010, Zhang et al., 2010b, Zhang et al., 2010c). These new achievements have resulted in the debates about the formation and evolution of high-grade metamorphic rocks from other areas of the Lhasa terrane. In this paper, we present the zircon U–Pb dating results from the amphibolite and schist of the Nyainqentanglha Group in the central Lhasa terrane. The obtained metamorphic zircon ages demonstrate that these rocks have only experienced the Triassic metamorphism, which provides new constraints on the late Paleozoic to early Mesozoic orogeny of the Lhasa terrane.