北苏鲁荣成地区超高压变质带的形成与折返动力学

Exhumation kinetics of northern Sulu ultrahigh-pressure metamorphic belt, Rongcheng Area.

山东省荣成地区位于苏鲁高压-超高压变质带的东北部,以花岗质片麻岩、副片麻岩为主,夹有少量的榴辉岩、石榴辉石岩、麻粒岩、超基性岩、石英岩、大理岩和斜长角闪岩等,各类岩石的锆石中普遍含有柯石英包裹体,表明荣成地区岩石曾经历超高压变质作用过程。荣成地区区域构造格架表现为面理产状总体为NNE-SSW走向,向南转为由NE-SW走向,呈弧形展布,倾向SE或SSE,超高压变质岩石由一系列近平行的剪切岩片组成,岩片之间的分界线为一些韧性剪切带,是折返阶段角闪岩相-绿片岩相的产物。结合野外宏观变形现象、显微构造分析以及糜棱岩中石英的优选方位EBSD(eleetron backscatterdiffraction,电子背散射技术)测量结果,可以判断北苏鲁荣成地区韧性剪切带主要形成于中、低温(550～350℃)条件,并具有由NW向SE"斜向正滑"的剪切指向。根据韧性剪切带中所含的榴辉岩透镜体的显微构造和绿辉石与金红石的优选方位的EBSD的研究,重塑超高压变质阶段的流变学特征:绿辉石和金红石都具有高温的滑移系组构模式。横穿苏鲁高压-超高压变质带的地震反射剖面揭示了苏鲁高压-超高压变质带呈厚10km以上的穹形板片,位于苏鲁高压-超高压变质板片下部的荣成及江苏刘山以韧性正断裂系列为主。Ar-Ar测年结果表明,发生这种伸展韧性剪切作用的时间在117～130Ma。位于板片上部的南苏鲁以韧性逆冲性断裂系列为主。基于以上各方面的研究,进一步验证了苏鲁超高压-超高压变质带折返动力学的挤出模式。

Rongcheng of Shandong Province is located at northeast of Sulu UHP (ultrahigh-pressure) metamorphic terrane. It is composed of gneiss, paragneiss, eclogites, garnet pyroxenite, ultramafic rocks, granulites, quartzite, marble and amphibolites. Coesites as a reliable marker of ultra-high pressure environment are found in eclogites, quartzites, gneisses, marbles and amphibolites. The occurrence of mylonites extends NNE-SSW at the north part and NW-SE at the south part. Different parallel slices compositing the Rongcheng UHP metamorphic terranes are divided by some ductile shear zones (about 2km). Based on field work microstructural analyses and EBSD studies of quartz, a conclusion is arrived: These ductile shear zones are approximately SE trending normal faults. Lattice-preferred orientation (LPO) of omphacite and garnet in several eclogite samples of Rongcheng were determined from electron backscatter diffraction (EBSD) technique shows that: LPO of omphacite and rutile are characterized by high temperature type, where as garnet appears to be randomly oriented. From the original seismic reflection profile of Sulu UHP terrane an erosion antiform structure is shown. Rongcheng and Liushan normal faults are situated in the lower part of this antiform, and southern Sulu thrusts are situated in the above part. Ar-Ar dating study results indicates these normal faults were formed at 117 similar to 130Ma. Based on the petrologic, geochemical, structural studies, the extrusion model of Sulu UHP terrane is verified further.