The Chronology, Source Region And Its Tectonic Significance Of Volcanic Rocks In Rongma Area, Qiangtang, North Tibet

This dissertation focus on the volcanic rocks in East Rongma area, south Qiangtang terrane. These volcanic rocks are Calc-alkaline basalt, for they contain 42.97-49.97% of Si O2, 2.8% of Na2 O in average and having extremely low K2 O. The Rongma basalt erupted overlies late Jurassic granite and Permian limestone. Limestone thrust to the top of basalt in partial area by the affecting of new tectonic movement. The zircon U-Pb isotopic dating shows that these rocks have complicated source, there are lots of caputured zircons in these rocks, U-Pb age varies from Archean to Cenozoic. The youngest age is 27.8±0.71Ma(Oligocene). Part of zircons in basalt have similar characteristics and age with the zircons in granite, shows that basalt may contaminated by granite. Basalt in studying area show same OIB-like trace element signatures, for they are enriched in REE with highly fractionated LREEs and HREEs without or having minor negative Eu anomalies. There are special features about the basalt in Rongma area, like having strong LILE(K, Rb, Sr et al) negative anomalies and no Nb, Ta negative anomalies. Basalt in studying area show relatively low Zr/Y value(7.04 in average), indicate high melting degree in source region. The high Sm/Yb(3.3 in average) and La/Yb(9.32 in average) value of basalt indicate that the source region is garnet phase peridotite, totally different from spinel phase lherzolite that suggested by prior researchers. Excluding crustal contamination, we believe that the primary magma originated from depleted mantle.The age of Cenozoic volcanic rocks in south Qiangtang area gradually becoming younger from south to north, showing that volcanic activities in south Qiangtang may related to intracontinetal subduction triggerd by collision of India-Asia during Cenozoic. Since 40 Ma, massive crustal thickening happened in south Qiangtang terrane. A series of volcanic rocks, like Nadingcuo, Biluocu, Yulinshan, erupted resulting from partial melting of Banggong-Nujiang remnant oceanic crust, mixed with crustal material of Qiangtang terrane. In about 27 Ma, Banggong-n Nujiang remnant oceanic crust subducted into the lithospheric mantle of south Qiangtang terrane, partial melting of oceanic crust and lithospheric mantle produced the primary magma of basalt in sdudying area.