江南造山带新元古代中期(830～750Ma)岩浆活动及对构造演化的制约

连接扬子地块和华夏地块的江南造山带是华南前寒武纪最重要的构造单元,其形成和演化长期以来备受关注。在江南造山带的范围内广泛发育了新元古代岩浆岩,它们是探讨江南造山带构造演化的重要对象,但其成因和形成的构造背景却备受争论。本文系统收集和分析了江南造山带830~820 Ma花岗岩、800~780 Ma酸性岩和800~750 Ma基性岩的地球化学数据。研究表明,不同时间段的岩石成因类型存在系统差异,830~820 Ma的花岗岩主要为S型花岗岩,800~780 Ma的酸性岩主要为A型酸性岩,而800~750 Ma的基性岩以拉斑系列和碱性系列为主,并在构造判别图中显示了板内玄武岩(WPB)和洋中脊玄武岩(MORB)的特征。综合同位素年代学、岩石地球化学和沉积学等学科领域的研究成果我们认为:扬子北缘和西缘应先于东南缘在1000~900 Ma期间发生碰撞,而此时的东南缘仍为活动大陆边缘;直到~830 Ma,扬子地块与华夏地块沿江南造山带发生拼贴,但只是陆-弧-(微)陆之间的"软碰撞",而无山脉隆升和高级变质作用,各个块体之间处于"联而不合"的状态,大洋岩石圈拆沉之后的软流圈上涌和由拆沉所引起的拉张作用导致了上覆岩石圈和陆壳发生部分熔融,产生了江南造山带830~820 Ma的S型花岗岩;随着全球Rodinia超大陆的裂解,~820 Ma,华南裂谷盆地开启,并在随后的裂解过程中发育了大量与伸展有关的800~780 Ma A型酸性岩和基性岩脉/墙,而其明显高于同时代MORB源区的地幔潜能温度显示,导致Rodina超大陆裂解的地幔柱可能对该时期岩浆岩的地幔源区有一定影响;随着拉张作用的不断加强,出现了760~750 Ma碱性系列和具MORB特征的基性岩,此时的软流圈地幔既提供热量又有物质供应。

Middle Neoproterozoic Magmatic Activities and Their Constraints on Tectonic Evolution of the Jiangnan Orogen

The Jiangnan Orogen, which connects the Yangtze and Cathaysia Blocks, is the most important Precambrian tectonic unit in the South China. Due to its special tectonic location, the formation and evolution of the orogen has long been the focus of active researching and debating. Neoproterozoic magmatic rocks extensively developed within the Jiangnan Orogen. Although they are of crucial importance to discuss the tectonic evolution of the Jiangnan Orogen, their petrogenesis and tectonic setting remains ambiguous. The geochemical data from the Jiangnan Orogen, including the 830~820 Ma granites, 800~780 Ma acid rocks and 800~750 Ma basic rocks, are here systematically collected and analyzed. The results show that the petrogenesis is very distinct at given time intervals. The granites of 830~820 Ma mainly belong to S-type granites, and the acid rocks with age of 800~780 Ma are mainly A-type acidic rocks, while the basic rocks with age of 800~750 Ma display typical tholeiitic and alkaline affinities, and plot in the domains of intraplate basalts and ocean ridge basalts (MORB) in the tectonic discrimination diagrams. Synthesized with results of geochronological, geochemical and sedimentary researches in recent years, we suggest that the collision occurred between 1000~900 Ma on the northern and western margin of the Yangtze Block prior to that of the southeastern margin. Meanwhile, the southeastern margin of the Yangtze Block was still an active continental margin in the subduction zone. At ~830 Ma, the amalgamation between the Yangtze and Cathaysia Blocks took place along the Jiangnan Orogen, and the continent-arc-(micro)continent “soft collision” developed rather than mountain uplift and high-grade metamorphism, and the masses were in a state of “connection rather than integration”; the asthenosphere upwelling and extension due to the delamination of the oceanic lithosphere might cause the partial melting of the overlying lithosphere and crust to generate the 830~820 Ma S-type granites. Accompanied with the global rifting of the Rodinia supercontinent, the South China rift opened at ca. 820 Ma, thus developing a large number of extension-related A-type acidic rocks and basic dykes during 800~780 Ma in the subsequent cracking process. During the following strengthening extension, the basic rocks with alkaline and MORB affinities were formed in the period 760~750 Ma, and during which the asthenosphere mantle provided both heat and material.