华北克拉通东南缘蚌埠-五河地区TTG片麻岩的岩石学、年代学和地球化学研究

TTG gneiss is a major rock type of the Archaean cratons, and it is widely investigated through petrological, geochronological and geochemical analyses in combination with experimental observations since 1970s. However, it is highly controversial that TTG can be well compared with adakite or not. Like the other cratons, the mainly exposed metamorphic basement rock of the North China Craton (NCC) is also dominated by TTG gneiss, which is intensively studied by authors from both home and abroad in the past few years, while there is no consensus about the petrogenesis and geodynamic implications of the TTG gneiss. The southeastern margin of the NCC, i.e., the target region of the present project, has been documented to find TTG gneiss in the year of 1994. However, its formation and metamorphic ages, and elemental and isotopic geochemistry features are still unclear now. In the present project, we will investigate the petrological, geochemical and geochronological features of the TTG gneiss with the aim to explore its emplacement and metamorphic ages, and to constrain its formation depth and petrogenesis, and further to discuss the feature of crustal growth as revealed by the TTG gneiss. Our results will give new insights on the formation and evolution history of the North China Craton and on the relatioship between TTG and adakite.

TTG片麻岩是太古宙克拉通地壳的主要组成部分，长期以来人们对其进行了大量的研究并开展了实验岩石学观察，但是关于它们是否可以类比adakite以及能否指示板片俯冲作用还尚未达成共识。华北克拉通变质基底广泛发育TTG片麻岩，本项目选择研究基础较为薄弱的华北东南缘蚌埠-五河地区的TTG片麻岩作为研究对象，拟对它们开展系统的岩石学、地球化学和同位素年代学研究，查明TTG片麻岩的形成与变质时代，主、微量元素和Sr-Nd-Hf同位素地球化学特征，限定它们的形成深度和岩石成因以及所具有的地球动力学意义，探讨TTG片麻岩所代表的地壳生长特征。并将获取的TTG片麻岩特征与华北克拉通其它区域乃至全球其它克拉通的TTG进行对比，进一步限定TTG的形成深度、岩石成因和地球动力学意义，以及和adakite之间的成因联系。

本项目选择研究基础较为薄弱的华北克拉通东南缘蚌埠-五河地区的TTG片麻岩作为研究对象，在野外地质调查的基础上，对其开展了系统的岩石学、矿物学、年代学和地球化学研究。结果显示它们具有宽的SiO2含量变化范围（63.15-76.86%），低Al（Al2O3含量<15.0%）、高Na/K比(Na2O/K2O>1)和Sr/Y比、轻重稀土元素分异显著，在元素图解上投点于TTG范围内。锆石LA-ICP-MS定年结果显示TTG片麻岩形成于2510±18Ma和2625±12Ma，并经历了～1.90Ga的变质作用。TTG片麻岩的(87Sr/86Sr)i变化范围为0.675940~0.703821（平均值为0.694494），指示其可能由于遭受到后期变质作用的影响而发生重置；全岩εNd(t)=-5.1~-2.1（平均值为-3.78），模式年龄TDM2为2.54~2.78Ga（平均为2.68Ga）；两个定年样品的锆石εHf(t)变化范围分别为+4.4~+9.2和-1.3~+6.4，模式年龄范围分别为2.46~2.65Ga和2.75Ga~2.97Ga。上述结果表明研究区TTG片麻岩形成于岛弧或大陆边缘环境下，由玄武质岩石部分熔融形成，源区残留矿物主要有石榴石和角闪石，并遭受了部分古老地壳物质的混染；～2.7Ga代表研究区的一次重要的地壳生长时代。