EARLY PALEOZOIC DEPOSITIONAL ENVIRONMENT AND INTRAPLATE TECTONO-MAGMATISM IN THE CATHAYSIA BLOCK (SOUTH CHINA): EVIDENCE FROM STRATIGRAPHIC, STRUCTURAL, GEOCHEMICAL AND GEOCHRONOLOGICAL INVESTIGATIONS
Résumé
The early Paleozoic geological evolution of the South China Craton composed of the Yangtze and Cathaysia Blocks has been the focus of long debate. The Cathaysia block has been central to the controversy regarding convergent margin versus intraplate environment in the early Paleozoic. In order to address the early Paleozoic evolution of Cathaysia, we undertook a systematic study of the stratigraphic sequences, deformational features and geochronology of magmatic event. Our results show that (1) during the early Paleozoic, the Jiangnan domain of the SE Yangtze block was characterized by a carbonate platform and the Cathaysia block by a graptolitefacies clastic rock assemblage, (2) in the Cathaysia block, a littoral-neritic depositional environment prevailed in Cambrian whereas a neritic-bathyal setting dominated during the early-middle Ordovician, and (3) the Late Ordovician depositional sequence in Cathaysia witnessed a period of transition from neritic-bathyal to littoral-land environment, marking the initial uplift process. Paleo-current measurements on the crossbeds revealed northwestward and westward transport directions, suggesting a source area to the east-southeast. All samples collected from the Cambrian-Ordovician strata show similar chemical characteristics; they have negative Nd(t) values ( 9.7 to 13.7) and two-stage Nd(t) model ages at ca.2.04 to 2.36 Ga. This suggests that the early Paleozoic rocks were derived from the eroded Paleoproterozoic basement, and little or no mantle component was identified. During the Silurian, the Cathaysia block underwent strong folding, thrusting, weak metamorphism and large-scale anatexis accompanied by granitoid emplacement, building the South China Fold Belt. The maximum shortening is estimated at 67 percent. A kinematic analysis of the ductile sheared rocks revealed a fan-shape thrust pattern, with top-to-the southeast in the southeastern and top-to-the northwest in the northwestern Cathaysia block. Zircon U-Pb dating of four granitic plutons yielded 206Pb/238U ages of 435 4 Ma, 424 5 Ma, 428 3 Ma and 427 2 Ma. All the zircon Hf(t) values are negative ( 6 to 9) and show a peak of two-stage Hf model ages around 1.9 Ga, indicating that the Silurian granitic magma was derived from the recycling of Paleoproterozoic basement. Major features of the early Paleozoic South China Fold Belt include the lack of early Paleozoic ophiolites and volcanic rocks, the absence of coeval HP-type blueschists, and the absence of mantle-derived juvenile magmatic rocks. Consequently, a subductioncollision- type orogeny is excluded. The magmatism most probably took place in an intraplate tectonic setting with little or no input of mantle components. We therefore conclude that the South China Fold Belt was an intraplate orogen, and is possibly related to the global early Paleozoic continental assembly.