华北克拉通太古宙构造热事件时代及演化

Timing of Tectonothermal Events in Archean Basement of the North China Craton

华北克拉通存在3.8 Ga以上的形成演化历史,存在4.0~4.1 Ga锆石年龄记录.本文对华北克拉通太古宙变质基底构造热事件进行了综述,重点是事件时限.最古老变质锆石年龄记录为3.71~3.75 Ga和4.0 Ga,为北秦岭造山带西段古生代变质火山-沉积岩中的碎屑或外来锆石.古太古代(~3.3 Ga)构造热事件在鞍山地区广泛存在,导致条带状奥长花岗岩形成.新太古代早期-中太古代晚期变质锆石年龄可进一步划分为两期:2.65~2.85 Ga和~2.6 Ga. 2.65~2.85 Ga变质锆石年龄记录存在于胶东、鲁西、鲁山地区.由于后期构造热事件影响,在很多情况下难以确定变质锆石的准确年龄,但>2.65 Ga构造热事件在华北克拉通无疑存在.与2.65~2.85 Ga构造热事件相比,~2.6 Ga构造热事件更为发育,除在鲁西地区广泛存在外,在胶东、鲁山等地也存在.可把2.6 Ga作为华北克拉通新太古代早期和晚期的年龄界线.华北克拉通最重要的太古宙构造热事件出现在新太古代晚期(2.49~2.53 Ga).所有太古宙岩石分布区,都遭受了这一构造热事件影响.总体上,与华北克拉通南部地区相比,华北克拉通北部地区普遍记录了更高级别变质作用,可能与地壳剥蚀深度不同有关.华北克拉通太古宙变质作用强度和范围随时间演化不断增大,在新太古代晚期达到高潮.与太古宙大陆壳厚度和规模随时间演化不断增大的演化趋势一致.一些地区存在古元古代最早期(2.40~2.47 Ga)甚至更年轻的变质锆石年龄记录,并不意味着构造热事件的真实存在,而是古元古代晚期构造热事件强烈叠加改造的缘故.

The North China Craton(NCC) has a long-term formation and evolution history up to 3.8 Ga rock records, with 4.0-4.1 Ga zircons. This paper summarizes the Archean tectonothermal events of the Craton, mainly focusing on the timing of the events. The oldest metamorphic zircon ages are 3.71-3.75 Ga and 4.0 Ga identified for detrital or xenocrystic zircons from Paleozoic meta-volcano-sedimentary rock in western segment of the North Qinling Belt near the southern margin of the NCC.Paleoarchean(~3.3 Ga) tectonothermal event widely occurs in the Anshan area, resulting in formation of banded trondhjemite gneisses. Early Neoarchean-late Mesoarchean metamorphic zircon ages can be subdivided into two groups: 2.65-2.85 Ga and ~2.6 Ga. 2.65-2.85 Ga metamorphic zircon ages have been obtained in western Shandong, eastern Shandong and Lushan. It is difficult to determine the exact age of tectonothermal event because of strong overprinting of the late Neoarchean. However, late Mesoarchean to early Neoarchean tectonothermal event must have occurred in the NCC. Compared with the 2.65-2.85 Ga tectonothermal events, the ~2.6 Ga event was more clearly developed in the NCC, such as in eastern Shandong and Lushan,besides being widely identified in western Shandong. 2.6 Ga is a convenient time to set as the early Neoarchean-late Neoarchean boundary. The most important Archean tectonothermal events occurred at the late Neoarchean(2.49-2.53 Ga), which have been identified in every Archean basement area of the NCC. In general, its northern domain recorded higher-grade metamorpism than its southern domain probably due to the former exposuring deeper crustal level. In the NCC, metamorphic intensity and scope became stronger and larger with time going on, peaking at the end of the Noearchean, consistent with the Archean continental crust becoming larger in thickness and scale with time. In some areas where the late Paleoproterozoic tectonothermal event was well developed, some metamorphic zircons recorded the earlest Paleoproterozic and even younger ages(2.40-2.47 Ga). This may not mean the existence of these events, but is a result of strong overprinting of the late Paleoproterozoic events.