Sedimentology, geochronology and geochemistry of the proterozoic sedimentary rocks in the Yangtze Block, South China

Abstract

The South China Craton comprises the Yangtze Block in the northwest and Cathaysia Block in the southeast. Located in the southeastern Yangtze Block, the Jiangnan Orogen formed through the amalgamation between the Yangtze and Cathaysia Blocks. The Yangtze Block has sporadically exposed Archean rocks in the north, Paleoproterozoic to Mesoproterozoic volcano-sedimentary sequences in the southwest and widespread Neoproterozoic sedimentary sequences accompanied by syn-sedimentary igneous rocks on the western and southeastern margins. The late Paleoproterozoic to early Mesoproterozoic Dongchuan, Dahongshan and Hekou groups in the southwestern Yangtze Block formed in a series of fault-controlled, rift-related basins associated with the fragmentation of the supercontinent Columbia. These sedimentary sequences were deposited between 1742 and 1503 Ma, and recorded continuous deposition from alluvial fan and fluvial sedimentation during the initial rifting to deep marine sedimentation in a passive margin setting. Sedimentation during initial rifting received felsic detritus mainly from adjacent continents, whereas sedimentation in a passive margin basin received detritus from felsic to intermediate rocks of the Yangtze Block. Paleoproterozoic to Mesoproterozoic rift basins in the southwestern Yangtze Block are remarkably similar to those of north Australia and northwestern Laurentia in their lower part (1742-1600 Ma), but significantly different after ca. 1600 Ma. The southwestern Yangtze Block was likely connected with the north Australia and northwestern Laurentia in Columbia but drifted away from these continents after ca. 1600 Ma. Traditionally thought Mesoproterozoic sedimentary sequences in the southeastern Yangtze Block are now confirmed to be Neoproterozoic in age and include the 835-830 Ma Sibao, Fanjingshan and Lengjiaxi groups, and 831-815 Ma Shuangqiaoshan and Xikou groups. These sequences are unconformably overlain by the ~810-730 Ma Danzhou, Xiajiang, Banxi, Heshangzheng, Luokedong and Likou groups. The regional unconformity likely marked the amalgamation between the Yangtze and Cathaysia Blocks and thus occurred at ~815-810 Ma. The lower sequences (835-815 Ma) received dominant Neoproterozoic (~980-820) felsic to intermediate materials in an active tectonic setting related to continental arc and orogenic collision, whereas the upper sequences represent sedimentation in an extensional setting with input of dominant Neoproterozoic granitic to dioritic materials (~740-900 Ma). The upper parts of the Shuangqiaoshan and Xikou groups, uncomfortably underlain by lower units, are molasse-type assemblages with additional input of pre-Neoproterozoic detritus, representing accumulation of sediments in a retro-arc foreland basin associated with the formation of the Jiangnan Orogen. Stratigraphic correlation, similarly low-δ18O and tectonic affinity of igneous rocks from different continents suggest that the Yangtze Block should be placed in the periphery of Rodinia probably adjacent to northern India. Paleoproterozoic (~2480 Ma and ~2000 Ma) and Early Neoproterozoic (711-997 Ma) were the most important periods of crustal and magmatic events of the southeastern Yangtze Block, but there is a lack of significant Grenvillian magmatism. Early Neoproterozoic magmatism highlights the contribution from both juvenile materials and pre-existing old crust, whereas ~2480 Ma and ~2000 Ma events are marked by reworking of pre-existing continental crust. Magmatism at 1600-1900 Ma was dominated by reworking of pre-existing crust, whereas the 1400-1600 Ma magmatic event recorded some addition of juvenile materials.