2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 174-4
Presentation Time: 3:45 PM


WINDLEY, Brian1, WAN, Bo2 and XIAO, Wenjiao2, (1)Department of Geology, University of Leicester, Leicester, LE1 7RH, United Kingdom, (2)State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China, brian.windley@btinternet.com

In order to assist reconstruction of the Paleoproterozoic Nuna supercontinent, we present new data that suggest that the northern margin of the North China Craton (NCC) was attached to the southern margin of Siberia (present coordinates); this is against earlier proposed connections of the NCC with India, Baltica, North America, or Amazonia. We focus on the Alxa block on the northern margin of the NCC where Paleoproterozoic gneisses contain >12 lenses up to a 300 m long of garnet websterite. The main lens at Bayan Hala consists of garnet websterite with minor olivine and spinal, plus retrograde hornblende and orthopyroxene-plagioclase symplectites. Peak metamorphic/subduction conditions were 2.37 GPa at 1000°C. A metagabbro that intrudes garnet websterite has a new zircon metamorphic age of 1.9 Ga. Farther east in the Sanggan area granulite facies gneisses contain >10 lenses of HP garnet meta-gabbros that have peak metamorphic/subduction conditions of 16.8-14.5 kbar and 870-750°C and metamorphic zircon ages of 1.9-1.85 Ga. These rocks were exhumed by wedge extrusion in a classic isoclinal anticline within a subduction-accretion complex. In SE Pre-Sayan in South Siberia a comparable belt of HP granulite facies gneisses contains >20 HP mafic-UM lenses including the Sharyzhalgai Complex with spinel lherzolites, harzburgites, and garnet websterites with peak metamorphism at 3.0-2.3 Ga at 1030-920°C at 1.9 Ga. Notably the Alxa and SE Pre-Sayan blocks have similar lithologies and their garnet websterites at Bayan Hala and Sharyzhalgai have identical isothermal PT trajectories. Conclusion: the northern margin of the NCC and the southern margin of Siberia consist of comparable coeval high-grade accretionary complexes that contain lenses of similar HP ultramafic rocks that both indicate subduction to eclogite facies depths. Paleomagnetic data indicate that the two current margins have the same polar wander paths. Also both margins contain similar 1.8-1.7 Ga rapakivi granites. anorthosites-gabbros and 1.8 Ga basic dyke swarms. Our new configuration of Nuna places together the two active HP Paleoproterozoic accretionary margins of NCC and Siberia, and allows continuity of the coeval Trans-North China orogen and the Central Indian Tectonic zone.