ABSTRACT

Late Neoproterozoic gabbroic-diabasic sills are widely distributed in the eastern part of the South Qinling block. Here, a comprehensive geochronological and geochemical study of these mafic sills has been conducted. LA-ICP-MS zircon U-Pb results indicate that the gabbroic-diabasic sills were formed at 632 ± 2 Ma. The sills consist mainly of tholeiitic basaltic rocks with relatively low TiO₂ contents (0.81–1.07 wt.%) and high Fe₂O₃^T/MgO ratios. These rocks display relatively flat to moderate light rare earth elements (LREE) enrichment patterns and positive Eu anomalies (Eu/Eu* = 1.15–1.41). On the primitive mantle normalized spider diagram, the samples show E-MORB-like geochemical features. All of the studied samples display similar Sm-Nd isotopic compositions with negative ε_Nd (t) values ranging from −6.2 to −5.5. Most clinopyroxenes from the mafic sills have relatively low Al/Ti ratios similar to those of typical rift-related cumulates. Therefore, Whole-rock and mineral geochemical features of the studied samples suggest that the mafic sills might have been derived from partial melting of a metasomatised subcontinental lithospheric mantle source in a continental rift setting. It can be advocated that the break-up of the Rodinia supercontinent in the northern part of the Yangtze craton might have been a long-lasting process. By comparison of the coherent Neoproterozoic magmatic events in the northern Yangtze craton and the Tarim block, it can be suggested that the Yangtze craton might have been adjacent to the Tarim block in the reconstruction model of the Rodinia supercontinent.

摘要

在南秦岭地块的东部广泛分布着新元古代晚期辉辉岩-基岩基岩。在这里，已经对这些铁镁质基岩进行了全面的地质年代和地球化学研究。LA-ICP-MS锆石U-Pb结果表明，辉辉岩-辉绿基岩形成于632±2 Ma。窗台主要由TiO ₂含量相对较低（0.81–1.07 wt。％）和Fe ₂ O ₃ ^T含量较高的辉硬性玄武岩组成/ MgO比。这些岩石显示出相对平坦至中等程度的轻稀土元素（LREE）富集模式和正Eu异常（Eu / Eu * = 1.15–1.41）。在原始地幔归一化蜘蛛图上，样品显示出类似E-MORB的地球化学特征。所有研究的样品显示类似的Sm-Nd同位素组成具有负ε_钕（t）值介于-6.2至-5.5之间。镁铁矿基岩中的大多数斜辉石具有相对较低的Al / Ti比，与典型的裂谷相关堆积物相似。因此，所研究样品的全岩石和矿物地球化学特征表明，镁铁质基岩可能是由大陆裂谷中交代的次大陆岩石圈地幔源的部分熔融而产生的。可以提倡的是，罗丹尼亚超大陆在扬子克拉通的北部破裂可能是一个长期的过程。通过比较北部扬子克拉通和塔里木地块的相干新元古代岩浆事件，可以认为，在罗丹尼亚超大陆重建模型中，扬子克拉通可能与塔里木地块相邻。