Provenance study for the Paleozoic sedimentary rocks from the west Yangtze Block: Constraint on possible link of South China to the Gondwana supercontinent reconstruction

摘要

The early Paleozoic and Devonian sedimentary rocks in the western part of the Yangtze Block were suggested to be derived from an exotic source that was once connected to South China. This conclusion was mainly based on that detrital zircons from these rocks give U-Pb age populations of 2400-2600 Ma, 1750-1850 Ma, 900-1000 Ma, 750-870 Ma and 500-680 Ma, but corresponding magmatic rocks of such late Archean, Grenvillian and latest Neoproterozoic-Cambrian ages do not outcrop in this area or adjacent regions. Zircons from the Cambrian sedimentary rocks of this study show the youngest age peak at ca. 550 Ma, and these zircons are mostly euhedral with positive epsilon(Hf)(t) values of +0.6 to +7.4. Because the unconformity between the Cambrian and Ordovician is evident in both west and south sides of South China, which is synchronous with its counterpart along the northern margin of the East Gondwana blocks (e.g. Himalaya region, Qiangtang, Western Australia), it is possible that these zircons were likely derived from a late Neoproterozoic-early Cambrian accretionary orogen (e.g. Bhimphedian orogeny) associated with the initial subduction of a proto-Tethys ocean beneath the margins of Gondwana continents. In contrast, the 500-680 Ma zircon population of the Silurian-Devonian sedimentary rocks give an older age peak at ca. 650 Ma and their epsilon(Hf)(t) values for the 540 Ma grains are all negative (-27.8 to -0.3). These detrital zircons are rounded and could be possibly sourced from 'Pan-African' orogenic belts that formed during the amalgamation of the Gondwana, such as the East African and Prydz-Darling orogens between the segments of Gondwana. Early Neoproterozoic ages are dominant in the above sedimentary rocks of this study and correlate well with the large volume of 900-1000 Ma magmatism in Grenville-age provinces (e.g. Rayner-Eastern Ghats belt in India-East Antarctica) formed from ocean closure and collision tectonics during the Rodinia assembly. Our Hf isoto