Geochronology and geochemistry of the late Neoproterozoic A-type granitic clasts in the southwestern Tarim Craton: petrogenesis and tectonic implications

Due to sparse data for deciphering the late Neoproterozoic tectonic history, there is still considerable debate on whether long-lasting superplume-related or long-duration subduction-related dynamics dominated the Tarim Craton. In this contribution, our field investigations detail the late Neoproterozoic siliciclastic successions, and we report the first granitic conglomerates with zircon U–Pb ages of 753.9 ± 3.7 Ma in the SW Tarim Craton. Importantly, detrital zircons from the thick Cryogenian sedimentary basin also contain a major zircon population at ca. 750 Ma. Together with seismic data, this suggests a large ca. 750 Ma magmatic event in the SW Tarim Craton. Geochemically, the granitic clasts exhibit A-type granite features with high SiO₂, high alkali but extremely low K, high FeO^T/MgO and Ga/Al, and high high-field strength elements (HFSEs) (i.e. Nb, Hf, and Ta) with significant depletion in Rb, K, Sr, P, Eu, and Ti, and significant negative Eu anomalies (Eu* = 0.13–0.36), showing ferroan granite affinities. Including the detrital zircons, the ca. 750 Ma zircons have a large range of negative εHf(t) values (−19.46 to −1.16). Elemental and zircon Hf isotope data suggest that the granites were derived from Palaeoproterozoic reworked continental crust and are probably related to crustal thinning and extension. By comparison with previous studies, we conclude that Rodinia breakup was diachronous in the outer parts of the supercontinent.

由于新元古代晚期构造演化历史的解译数据较为匮乏，当前学界对塔里木克拉通（Tarim Craton）究竟以长期与超级地幔柱（superplume）相关的动力学过程为主，还是长期俯冲（subduction）相关的动力学过程为主，仍存在较大争议。本研究通过野外调查详细刻画了新元古代晚期的硅质碎屑沉积序列（siliciclastic successions），并首次在塔里木克拉通西南部报道了锆石U-Pb年龄（zircon U–Pb ages）为753.9±3.7 Ma的花岗质砾岩（granitic conglomerates）。尤为重要的是，该厚层成冰纪（Cryogenian）沉积盆地所产碎屑锆石（detrital zircons）同样存在一个约750 Ma的主要峰值群体。结合地震数据，该结果指示塔里木克拉通西南部曾发生一期大规模的约750 Ma岩浆事件。地球化学特征显示，这些花岗质砾石具有A型花岗岩（A-type granite）的典型特征：高二氧化硅（SiO₂）、高碱质但钾含量极低，全铁氧化物与氧化镁比值（FeO^T/MgO）及镓铝比（Ga/Al）较高，高场强元素（high-field strength elements, HFSEs，即铌Nb、铪Hf和钽Ta）含量富集，同时显著亏损铷（Rb）、钾（K）、锶（Sr）、磷（P）、铕（Eu）与钛（Ti），并表现出明显的负铕异常（negative Eu anomalies，Eu*=0.13~0.36），整体具备亚铁质花岗岩的地球化学亲和性。连同碎屑锆石在内，这批约750 Ma的锆石具有宽泛的负εHf(t)值范围（-19.46~-1.16）。主量元素与锆石Hf同位素数据表明，该花岗岩起源于古元古代（Palaeoproterozoic）再造大陆地壳，可能与地壳减薄及伸展作用相关。通过与前人研究对比，我们认为罗迪尼亚超大陆（Rodinia）的裂解在超大陆外缘区域具有穿时性。