钦-杭结合带南段新元古代俯冲作用:来自粤西贵子混杂岩变基性岩年代学和地球化学的证据

新元古代以来,扬子地块和华夏地块拼贴形成钦-杭结合带。粤西贵子混杂岩为揭示钦-杭结合带南段地质演化提供了一个重要的窗口。贵子混杂岩构造岩块为强烈变形-变质的变基性岩、深海硅质岩,基质为石英岩、云母片岩以及具鲍马序列富锰质硅泥质岩等。对混杂岩中变基性岩年代学和元素地球化学分析显示,锆石U-Pb年龄为948±11Ma,除少部分属低钾(拉斑)外,其余均为钙碱性系列,CIPW标准矿物成分属石英拉斑玄武岩-橄榄拉斑玄武岩组合。变基性岩具有高TiO_2(平均1.85%)、P_2O_5(平均0.21%)的特点;无Eu异常或轻微负异常,配分曲线右倾(球粒陨石标准化(La/Yb)N=2.65~3.89),大离子亲石元素(Rb、Ba、Th、U)富集,高场强元素(Nb、Zr、Hf)轻微亏损,表现为洋壳消减作用下岛弧岩浆岩特征。高的Nb(7.56×10~(-6)~16.11×10~(-6),均大于7×10~(-6))、Nb/U(18.39~25.65)、(Nb/La)N(0.76~0.94,原始地幔标准化)显示为岛弧环境富Nb玄武岩。本研究认为,云开地块出露的这套构造混杂岩,是新元古代古华南洋俯冲在钦-杭结合带南段的记录,可为该带的南东边界提供线索。

Neoproterozoic subduction of the south section of Qin-Hang orogenic junction belt: Evidence from the geochronology and geochemistry for the metabasite in Guizi mélange, western Guangdong Province, South China

The Yangtze and Cathaysia blocks had integrated and formed the Qin-Hang orogenic juncture belt (QHOJB) along Jiangshan-Shaoxing fault since Neoproterozoic. The complex tectonic evolution of the QHOJB have attracted considerable attentions. The Guizi mélange is located in the southern section of QHOJB. According to the field geological observations, the tectonic blocks are strongly deformed metamorphic metabasite and deep-sea chert, matrix rocks are quartzite, mica-schist and manganese enriched siliconargilaceous rock with Bouma sequence. A set of U-Pb zircon geochronological and elemental data for the metabasite from Guizi mélange are present in this paper. The metabasite yields a weighed mean ²⁰⁶Pb/²³⁸U age of 948±11Ma. Based on the SiO₂, Na₂O, K₂O, these samples belong to calc-alkaline series except for a small portion of tholeiitic series. CIPW standard mineral compositions are similar to quartz tholeiite-olive tholeiite. They also show high TiO₂ (average 1.85%), P₂O₅ (average 1.85%), no or slightly negative Eu anomaly, moderate LREE/HREE fractionation (presented by chondrite normalized (La/Yb)_N=2.65~3.89, LILE (Rb, Ba, Th, U) enriched and HFSE (Nb, Zr, Hf) slightly depleted characteristics. Such geochemical signatures indicate that they are typical subduction-related arc volcanic rocks. In addition, Nb (7.56×10^-6~16.11×10^-6), Nb/U (18.39~25.65), (Nb/La)_N (0.76~0.94, primary mantle normalized) are similar to those Nb-enriched basalts instead of typical island arc basalts. Combining with geological and geochemical study, this paper suggests the Guizi tectonic mélange is the record of Neoproterozoic paleo South China Ocean subduction in the southern section, providing an important clue for the southeast boundary of QHOJB.