苏鲁造山带超高压变质作用及其P-T-t轨迹

Ultrahigh-pressure metamorphic P-T-t path of the Sulu orogenic belt, eastern central China

基于超高压变质岩的岩石学,特别是超高压矿物生长成分环带、扩散环带和蚀变作用研究,综合前人的岩石学和年代学研究成果,提出苏鲁造山带超高压变质作用峰期发生在1000-1100℃和6-7GPa条件下,俯冲深度相当于200km,形成年代为240-250Ma。在此基础上,重塑了一个包括八期变质作用的P-T-t轨迹,揭示出超高压变质岩经历了三个不同的折返阶段,即从200km到100km深度的快速折返阶段,抬升速率为5km/Ma,冷却速率为10℃/Ma;从100km到30km的快速折返,抬升速率为4km/Ma,或为近等温降压,或为缓慢降温的快速降压过程;从下地壳到近地表的缓慢折返阶段,抬升速率为1km/Ma,但为快速降温过程,冷却速率可达20℃/Ma。

On the basis of the studies on petrology of ultrahigh-pressure metamorphic rocks, especially the compositional growth zonations, the diffusion zonations and the alterations of the ultrahigh pressure minerals combined with the petrological and geochronological researches published by previous authors, the present paper proposes that the peak stage metamorphic conditions of the Sulu orogenic belt have been estimated to be 1000 -1100℃ and 6-7 GPa, the depth of subduction equivalents to 200 km, and the age of ultrahigh pressure metamorphism is determined as 240-250 Ma. Based on these data the UHP metamorphic P-T-t path has been reconstructed which includes 8 stages of metamorphic process. The UHP metamorphic rocks had surpassed 3 stages of exhumation, i. e. , the very rapid exhumation stage from 200 km to 100 km, with an uplift rate of 5 km/Ma, a cooling rate of 10℃/km; the rapid exhumation stage from 100 km to 30 km with uplift rate 4 km/Ma involving nearly isothermal decompression or slow-lowering of temperature with rapid decompressional process; and the stage of slow exhumation from lower crust to near Earth's surface with exhumation rate of 1 km/Ma and a rapid cooling process, the cooling rate was up to 20℃/Ma.