Geochronology, geochemistry and tectonic significance of high-pressure metamorphic rocks from Yadan area in Central Qiangtang, Tibet

The collision of the southern and northern Qiangtang massifs along the Longmucuo-Shuanghu suture zone marks the final closure of the Paleo-Tethyan Ocean in the Central Qiangtang. However, the tectonic evolution of the Paleo-Tethyan Ocean between the two massifs, especially the opening time, has always been ambiguous. In this study, we carried out the study about the petrography, chronology and geochemistry of the metamorphic complexes in Rongma area, southern Longmucuo-Shuanghu suture zone, to restrict the opening and closing time of the Paleo-Tethyan Ocean in northern Tibet and its tectonic evolution after closure. The plagioclase amphibolite yields a weighted average age of 717 +/- 7 Ma (n = 61, MSWD = 1.9) by LA-ICP-MS zircon U-Pb dating, which indicates the Precambrian basement exists in the southern Qiangtang massif. In addition, a plagioclase amphibolite also was obtained with yielding a weighted average age of 163 +/- 2Ma (n = 14, MSWD = 1.9), indicating that the protolith was formed in the Middle Jurassic. The LA-ICP-MS zircon U-Pb dating shows that two garnet-polysilicon-muscovite quartz schists yielded concordant ages ranging from 267Ma to 1349 Ma and from 214Ma to 2050Ma, respectively. And two groups of polysilicon-muscovite samples show a similar plateau age of 224.2 +/- 1.5Ma and 223.9 +/- 1.5Ma, through 40Ar/39Ar dating, respectively, implying that regional metamorphism may sustained from similar to 224Ma to 214Ma. The whole-rock geochemical characteristics of Neoproterozoic and Middle Jurassic plagioclase amphibolites show that the former has affinity to E-MORB but the latter is characterized by OIB. Based on the regional rock assemblages and geochemical characteristics, it indicates that both of the two plagioclase amphibolites are formed in extensional environments. The former may be formed in an intracontinental extension environment, responding to the breakup of Rodinia supercontinent and the latter may be formed in a continental rift setting. Combined with previous data, we propose that an unknown continental block might have begun to rift from the Qiangnan-Indian unite block in the Meso-Neoproterozoic (similar to 717Ma), and "the Paleo-Tethys" Ocean might have opened at some time from similar to 717Ma to similar to 517Ma. Furthermore, the opening time of the Paleo-Tethyan Ocean may be earlier than previously thought. The Paleo-Tethyan Ocean might be closed at similar to 224Ma, and the collisional orogeny between the northern and southern Qiangtang massifs might last after 214Ma. Then the region occurred in an intraplate extension setting in the Middle Jurassic, with occurring of alkaline basalts characterized by OIB and bimodal igneous rocks in the region. We proposed the evolution of the ancient ocean might be analogous to the Paleo-Asian Ocean.