苏鲁高压-超高压变质带中蛇纹岩成因和作用研究

Study of serpentinites in various orogenic belts and subduction zones has been one of the major subjects in the field of Earth sciences in the world, as it involves key issues, such as interaction between mantle and crust, mass balance, arc magmatism, and exhumation of ultrahigh pressure (UHP) metamorphic terranes. The Dabie-Sulu belt is the world longest UHP metamorphic terrane formed during the collision of the two major continents; north China and Yangtze cratons. Previous studies of mafic-ultramafic rocks focused mainly on origin, metamorphic evolution (P-T path), crust-mantle interaction, and timing framework of garnet-facies peridotite (pryoxenite) and ecologite, but very few researches focused on origin and role of those serpentinites in the subduction zone. In this study, by drawing on research methods of serpentinites in the global major orogenic belts and using advanced analytical techniques, we plan to study the serpentinites in the Sulu belt by starting with fundamental petrographic observation, followed by identifying their protoliths, tectonic settings, origins and forming mechanism. We would also discuss the roles of serpentinites played during the recycling of elements and water in the subduction zone and interior of mantle wedge, and their relationships with arc magmatic activities. Based on these information and with serpentinite factors incorporated,we suppose to establish a geodynamic model of Sulu orogenic belt involving processes of continental subduction, collision, and exhumation, which would be a valuable information for serpentinite study of the global orogenic belts.

大陆造山带和大陆边缘俯冲带中蛇纹岩的研究是近年来世界地学领域的一个重要课题，因为它涉及俯冲带和地幔楔内的壳-幔相互作用、质量平衡、岛弧岩浆形成机制以及超高压变质岩折返等地球科学的重大问题。大别-苏鲁造山带是世界上最长的超高压变质地体，早中生代形成于华北和扬子两个古克拉通的碰撞拼合。 以往对苏鲁造山带镁铁-超镁铁质岩的研究主要是针对经历过深俯冲的石榴石相橄榄岩（辉石岩）和榴辉岩的成因、变质演化（P-T轨迹）、壳-幔相互作用和年代格架等，但对蛇纹岩（蛇纹石化的尖晶石相橄榄岩）成因和在俯冲带中作用的研究却罕见。本项研究拟借鉴全球主要大陆造山带蛇纹岩的研究思路，重点研究蛇纹岩的原岩类型、构造环境、成因类型和形成机制，探讨蛇纹岩在俯冲带和地幔楔内元素和水循环过程中的作用及其与岛弧岩浆的联系，建立一个有蛇纹岩因素在内的大陆俯冲、碰撞和折返的地球动力学模型，并为全球造山带蛇纹岩研究提供可供对比的信息。

俯冲带中蛇纹岩研究涉及俯冲带和地幔楔内的壳-幔相互作用、元素和水循环、质量平衡、岛弧岩浆形成机制以及超高压变质岩折返等地球科学领域的重大问题。. 研究表明，苏鲁地区蛇纹岩的原岩属亏损地幔橄榄岩，有两种类型：其一是由方辉橄榄岩蚀变而成，原岩部分熔融程度<15%，以青岛仰口地区蛇纹岩为代表；其二是由纯橄岩（包括辉橄岩）蚀变而成，原岩部分熔融程度20-25%，以日照梭罗树和胡家林地区的蛇纹岩和蛇纹岩化纯橄岩为代表。对比研究证实，苏鲁地区蛇纹岩原岩形成于华北克拉通（NCC）南缘弧前地幔环境。我们认为青岛仰口和日照胡家林两个地区的含石榴石变质岩的原岩性质、俯冲深度和变质强度都有差别。仰口榴辉岩可能代表经历了深俯冲的洋壳混入了一些陆壳物质，俯冲深度>200Km（含柯石英）；而胡家林的石榴辉石岩可能代表前期侵入到NCC南缘地幔楔的基性岩浆岩，被扬子板块俯冲过程中形成的 “地幔拖曳牵引流” 带入到俯冲通道中形成石榴辉石岩（深度不及仰口榴辉岩的俯冲深度，不含柯石英）。. 蛇纹岩和蛇纹岩化橄榄岩富集流体迁移元素（FME）。这些元素从俯冲的古特提斯大洋板片（继而是扬子克拉通板片的前端）释放出来，被富H2O流体携带至上覆地幔楔中。梭罗树蛇纹岩和胡家林纯橄岩的俯冲深度浅，橄榄石蚀变为利蛇纹石，主要富集Sb和As；而仰口蛇纹岩俯冲深度深，橄榄石和斜方辉石蚀变为叶蛇纹石，不仅富集Sb和As，也富集Pb、LREE（La、Ce）和U。导致蛇纹岩化的介质是俯冲大洋板片释放的含H2O流体，而非地幔熔体和源自花岗质片麻岩的熔体。不同地区（仰口或梭罗树-胡家林）的蛇纹岩FME含量差异较大，这种不均匀性可能取决于俯冲板片在不同地区差异性释放FME以及蛇纹岩化过程中温度的变化。低密度的俯冲陆壳物质与其它俯冲岩石圈间的脱耦作用致使其裹挟大量的“镁铁-超镁铁质岩透镜体”（包括蛇纹岩、蛇纹岩化橄榄岩、石榴辉石岩）一同折返至地壳浅部。