Greenschist facies mylonite documenting the tectonic evolution of the South China Sea continental margin since Mesozoic

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.

南海（South China Sea, SCS）活动大陆边缘向被动大陆边缘的转变通常被推断发生于晚中生代至早新生代，但由于缺乏直接证据，这一转变的具体时间与过程仍不明确。国际大洋发现计划（International Ocean Discovery Program, IODP）U1504站位在南海北部大陆边缘外边缘隆起的基底中采集到绿片岩相糜棱岩，该糜棱岩包含南海区域中生代与新生代构造演化的重要信息。我们的前期研究已在该糜棱岩中识别出两期伸展变形，即早期韧性变形与晚期脆性变形，但尚未对其进行年代约束。本文基于脉体与糜棱岩片理的交切关系，在绿片岩相糜棱岩中进一步识别出三期碳酸盐脉，即糜棱岩前脉、糜棱岩期脉与糜棱岩后脉。通过对这些碳酸盐脉开展年代学研究，可有效约束上述伸展变形的时限，从而为南海区域基底演化提供新认识。

选取10件碳酸盐样品进行激光剥蚀电感耦合等离子体质谱（laser ablation inductively coupled plasma mass spectrometry, LA-ICP-MS）U-Pb定年，但仅3件样品（糜棱岩前脉：U1504B 16R 1W 58-62与U1504B 19R 2W 26-29；糜棱岩期脉：U1504B 15R 2W 45-48）具有足够高的U/Pb比值及238U/206Pb-207Pb/206Pb变化范围，可用于确定特拉-瓦森伯格（Tera-Wassenburg）Concordia图的下交点年龄。其余7件样品因普通Pb含量较高，导致238U/206Pb及U/Pb比值极低，故不适用于U/Pb定年。选取1条代表性糜棱岩后碳酸盐脉进行稳定碳氧同位素测试，获得4组理想数据。选取2条代表性糜棱岩后碳酸盐脉进行锶同位素测试，获得7组理想数据。