江南造山带西段桂北新元古代基性-超基性岩岩石成因研究及其对构造演化的约束

The mafic-ultramafic rocks are regarded as products of mantle melting. The high-Mg ones can even represent the primary magma by direct partial melting of the upper mantle, thus, the systematic geologic, geochemical, geochronologic study on these rocks can provide constraints on the mantle nature and tectonic settings. The tectonic setting of the Neoproterozoic mafic-ultramafic rocks in the western Jiangnan Orogen remains a long-term debated issue, which restricted the understanding of the tectonic evolution process in this region in Neoproterzoic. In this program, we will undertake comprehensive field investigation and precise geochronologic dating on the widespread mafic-ultramafic rocks in north Guangxi in Jiangnan Orogen, to construct the spatial and temporal framework of the mafic-ultramafic intrusions. We would also focus on the geologic, petrologic, element and isotopic geochemical features of these rocks, to determine the magmatic series, restore the original magma composition, estimate the lithosphere thickness, and trace the magma sources. Therefore, we can further reveal the geodynamic process in deep, and provide important constraints on the tectonic evolution of the western Jiangnan Orogen.

基性-超基性岩是地幔熔融的产物，尤其是高镁基性－超基性岩能够直接代表上地幔部分熔融形成的原始岩浆，对此类岩石的岩石性质及成因研究可以约束地幔性质及其构造背景。长期以来，江南造山带西段新元古代基性-超基性岩石成因尚存在争议，这制约了我们对其构造演化机制的认识。本项目以江南造山带西段桂北地区内广泛分布的基性-超基性侵入岩为研究对象，通过野外地质调查和对基性-超基性岩的测年工作，构建该区基性-超基性侵入体的时空分布格架。通过对这些岩体进行系统的地质特征、岩石学、元素和同位素地球化学研究，厘定岩浆系列，恢复原始岩浆主要组分，估算岩浆形成时的岩石圈厚度，示踪岩浆物质来源。通过探讨其岩浆性质与岩石成因，揭示江南造山带西段深部地球动力学过程，从而为解决江南造山带西段演化机制的争议提供重要约束和关键依据。

本项目对江南造山带西段桂北地区金车辉长岩和花桥橄榄单斜辉石岩体分别开展了矿物化学、SHRIMP 锆石U-Pb测年、锆石原位Hf同位素、岩石主微量元素和Sr-Nd-Pb同位素分析。研究内容和取得的主要成果包括：（1）橄榄石均已蚀变为蛇纹石，单斜辉石均为普通辉石，Wo27.81‒29.1En50.2‒54.9Fs17.2‒21.1，斜长石基本已经蚀变为钠长石，Ab91.8‒96.2An3.0‒7.6Or0.5‒1.1，利用单斜辉石估算的结晶温度约为1223-1355℃；（2）获得了桂北金车辉长岩SHRIMP锆石U-Pb年龄为806.4±4.6 Ma（MSWD=1.6，n=20），锆石具有正的εHf(t)值（7.4‒12.0）;（3）金车辉长岩和花桥橄榄单斜辉石岩烧失量较高，为3.35-11.40wt.%，平均值为7.29wt.%，整体上看，这些岩石具有与“弧”岩浆相似的地球化学特征，均属于亚碱性拉斑系列岩石，稀土元素总含量较低19.15‒35.21 ppm，具有富集的轻稀土元素和大离子亲石元素的特征，亏损高场强元素，如具有Nb-Ta-Ti负异常；（4）Sr同位素变化较大，0.706326‒0.708526，正的εNd(t)值，0.7‒3.4，结合Pb同位素特征，识别出花桥地区有EM II型富集地幔；（4）结合区域地质特征，申请者推测在860-805 Ma时期，扬子板块西南缘处于活动大陆边缘环境，即在该时期江南造山带西段处于弧后盆地俯冲构造背景。