AN INTEGRATED PROVENANCE AND TECTONIC SETTING STUDY OF THE NEOPROTEROZOIC TONIAN SEDIMENTARY SEQUENCES IN EAST GUIZHOU PROVINCE, CHINA

The detrital zircons in the Tonian sandstones from east Guizhou Province record the direct information of the magmation, provenances and tectonic settings of Jiangnan Orogen during Tonian period. These informaiton can be further used to understand the integrated Neoproterozoic geological evolution of the South China. In this study, 1673 analyses of detrital zircon U-Pb ages derived from 12 formations of Tonian strata representing two districts (east Guizhou Provence and north Guangxi Provence) in the western Jiangnan Orogen are combined for the sedimentary study.

The zircons from the lowermost part of the basement sequence are 120–220 μm in length, subround in shape and incomplete in morphology. These zircons display two Paleoproterozoic ages peak (ca. 1845 Ma and ca. 2325 Ma) which are consistent with the Paleoproterozoic strata in the Yangtze Block, implying the sedimentary basin was initially an inner block basin and mainly sourced from the Yangtze Block.

However, the zircon age population of the lower part of the basement sequence largely from a ca. 870 Ma arc in the south and little the old detritus from Yangtze Block in the north, this bidirectional sources is similar with the back arc basin setting.

Afterward, the zircons from the upper part of the basement sequence are 100–240 μm in length, mostly euhedral and subangular morphology and present a unimodal age population peak at ca. 835 Ma. This zircon age population suggest it may deposited in a convergent setting. The sediments mainly come from the locally distributed 835 Ma syn-collisional igneous rocks.

The lower part of the cover sequence presents major zircon age peaks at ca. 815–809 Ma and minor peaks at 2018 Ma and 2485 Ma, implying the basin had gradually transited into an inner craton basin and received the detritus from the locally distributed ca. 809 Ma bimodal magmatic rocks and a different Paleoproterozoic detritus from the Yangtze Block.

Our research suggest that the eruption of the ca. 840 Ma basalt was attributed to the development of the back arc basin during 870–835 Ma and the angular unconformity was the product of the amalgamation of the Yangtze and Cathaysia Blocks during 835–820 Ma. Our tectonic model further indicates that the rifting of South China Craton was not started before ca. 815 Ma.