华北克拉通新太古代早期构造热事件的响应:来自左权地区ca.2.7Ga TTG片麻岩的证据

新太古代早期是全球地质历史上一个重要的地壳生长时期,世界众多克拉通中广泛存在2. 7Ga左右的岩浆年龄记录。华北克拉通最主要的岩浆事件发生在新太古代晚期,这与世界其他克拉通广泛存在～2. 7Ga的构造热事件明显不同。但全岩Nd和锆石Hf同位素研究表明,华北克拉通～2. 5Ga的岩石主体来自于中太古代晚期-新太古代早期大陆物质的重熔或再造。因此,厘定～2. 7Ga地质事件在华北克拉通的空间分布对深入理解新太古代地壳形成与演化具有重要科学意义。华北克拉通已识别出的～2. 7Ga的花岗质岩石主要分布在胶东、鲁西、武川、赞皇和太华等少数杂岩区,中部带的恒山、阜平和中条杂岩中亦有零星出露。左权变质杂岩位于中部带中南段,赞皇杂岩西南,初步地球化学和锆石年代学研究表明,该地区有多种岩石类型记录了～2. 7Ga的年龄信息,包括副片麻岩、长英质浅色体、磁铁矿角闪片麻岩和TTG片麻岩。其中,TTG片麻岩的原岩为英云闪长岩,锆石发育明显的核边结构,核部具有清晰的岩浆环带,两个不同LA-ICP-MS实验室获得的不一致线上交点年龄分别为2727±14Ma和2731±12Ma,代表了TTG岩浆岩的结晶年龄。同时,左权变质岩石中较好地保存了新太古代晚期的岩浆和变质年龄记录,推测其所代表的构造热事件与华北克拉通～2. 5Ga所经历的大规模幔源岩浆的底侵作用有关。

Response to the Early Neoarchean tectono-thermal events in the North China Craton: Evidence of ca. 2.7Ga TTG gneisses from the Zuoquan metamorphic complex

The Early Neoarchean was an important period of crust formation, and rocks at ca. 2.7Ga are widespread in many cratons globally. The North China Craton (NCC) experienced its main period of crustal growth in the Late Neoarchean, whereas whole-rock Nd and Hf-in-zircon isotopic geochronology revealed that ca. 2.5Ga granitoids mainly formed through reworking of Late Mesoarchean to Early Neoarchean continental material. Thus, the spatial distribution of ca. 2.7Ga rocks is of great scientific value for studying crustal growth and evolution of the NCC. It is identified to be well exposed in a few areas in the NCC, including eastern Shandong, western Shandong, Wuchuan, Zanhuang and Taihua. And it also locally occurres in the Hengshan, Fuping and Zhongtiao complexes in the Trans-North China Orogen (TNCO). The Zuoquan metamorphic complex is to the southwest of the Zanhuang complex, located at the middle-southern segment of the TNCO. Geochemistry and geochronology revealed that ca. 2.7Ga ages were recorded by types of rocks here, for example paragneiss, leucosome, magnetite-bearing amphibole gneiss, and TTG gneiss. TTG gneiss is tonalitic in composition, and contains zircons with metamorphic rims and magmatic cores. Two LA-ICP-MS laboratories gave upper concordia intercept ages of magmatic zircon cores with 2727±14Ma and 2731±12Ma, respectively. It was interpreted to be formation age of the tonalite. The Zuoquan rocks also recorded both metamorphic and magmatic ages of the Late Neoarchean, and the tectono-thermal event was probably related to the intrution and underplating of large amounts of mantle-derived magma occurred at ca. 2.5Ga in the NCC.