Early Jurassic granitoids from deep drill holes in the East China Sea Basin: implications for the initiation of Palaeo-Pacific tectono-magmatic cycle

The timing of the Palaeo-Pacific Plate (PPP) subduction in East Asia, following the amalgamation of the North and South China Blocks (NCB and SCB), remains equivocal despite several investigations on the widespread subduction-related Mesozoic magmatism in this region. Here we report newly discovered granodiorites in the East China Sea Basin (ECSB) from deep boreholes (2945–2983 m), which yield SHRIMP zircon U–Pb age of 174 ± 1.1 Ma. The rock shows relatively high Sr/Y and La/Yb ratios, low contents of Ni, Cr, and MgO, and markedly negative values of <i>ε</i>Hf(<i>t</i>) (21.0 to −27.0). These geochemical and isotopic features are similar to those of the tonalite-trondhjemite-granodiorite (TTG) suites produced by the partial melting of the thickened lower continental crust. Our data, together with the ca. 190–180 Ma I-type granites reported from the eastern Zhejiang–Fujian and Taiwan areas, lead us to conclude that the continental crust in the coastal areas of South China underwent some degree of thickening during the Early Jurassic. A comparison with the coeval magmatic rocks in South Korea and Japan suggests that a large NE-trending continental arc (ca. 190–170 Ma) might have been constructed along the eastern margin of Asia at this time. Our study provides insights into the initiation of the Palaeo-Pacific tectono-magmatic cycle immediately following the consolidation of the SCB and NCB.

华北克拉通（North China Block, NCB）与华南克拉通（South China Block, SCB）拼合后，东亚区域古太平洋板块（Palaeo-Pacific Plate, PPP）的俯冲时限虽经学界针对该区域广泛分布的俯冲相关中生代岩浆作用开展的诸多研究，但其具体时间仍尚无定论。本文报道了东海盆地（East China Sea Basin, ECSB）深钻孔（2945–2983 m）中新发现的花岗闪长岩，其SHRIMP锆石U-Pb年龄为174 ± 1.1 Ma。该岩石具有较高的Sr/Y与La/Yb比值，较低的Ni、Cr及MgO含量，并呈现显著负偏的εHf(t)值（21.0至−27.0）。上述地球化学与同位素特征，与由加厚下陆壳部分熔融形成的英云闪长岩-奥长花岗岩-花岗闪长岩（tonalite-trondhjemite-granodiorite, TTG）组合的特征高度相似。结合浙闽东部及台湾地区已报道的约190–180 Ma的I型花岗岩，我们认为华南沿海地区的陆壳在早侏罗世期间经历了一定程度的加厚作用。通过与韩国、日本同期岩浆岩的对比分析，我们提出：该时期亚洲大陆东缘可能发育了一条延伸规模可观的北东向大陆弧（约190–170 Ma）。本研究为厘清华北与华南克拉通拼合后古太平洋构造-岩浆旋回的启动时限提供了新的认知。