Greenschist facies mylonite documenting the tectonic evolution of the South China Sea continental margin since Mesozoic
Description
The transition from active to passive continental margin of the South China Sea (SCS) is usually inferred to occur in the Late Mesozoic to the Early Cenozoic, however, the specific time and processes for the transition are still unclear due to the lack of direct evidence. The International Ocean Discovery Program (IODP) site U1504 has sampled greenschist facies mylonite from the basement in the Outer Margin High of the northern SCS continental margin, which contains significant information regarding the Mesozoic and Cenozoic tectonic evolution of the SCS region. Our previous work has found two episodes of extensional deformation in the mylonite, namely early ductile and later brittle deformation, but without age control. Here, we further identify in the greenschist facies mylonite three episodes of carbonate veins (pre-mylonite, syn-mylonite and post-mylonite) on basis of the intersecting relationship between the veins and the mylonite foliation. Through performing the chronological study of these carbonate veins, the timing of these extensional deformations can be well constrained, which will offer us new insights into the basement evolution of the SCS region. Ten carbonate samples are used for LA-ICP-MS U-Pb dating, but only 3 samples (Pre-mylonite: U1504B 16R 1W 58-62 and U1504B 19R 2W 26-29; syn-mylonite: U1504B 15R 2W 45-48) have sufficiently high U/Pb values and variable ranges of 238U/206Pb-207Pb/206Pb to determine the ages of the lower intersection on the Tera-Wassenburg Concordia diagram. Other 7 samples are characterized by high levels of common Pb and therefore have quite low 238U/206Pb and U/Pb ratios, therefore unsuitable for U/Pb dating. A representative post-mylonite carbonate vein was selected for stable C and O isotope testing, and 4 sets of ideal data were obtained. Two representative post-mylonite carbonate veins were selected for Sr isotope testing, and 7 sets of ideal data were obtained.