西伯利亚克拉通南缘奥里洪地块麻粒相变质作用及构造意义

Granulite facies metamorphism of the Olkhon terrane in southern Siberian Craton and tectonic significance.

早古生代西伯利亚克拉通南缘发生了大规模的增生-碰撞造山运动,本文研究的地区——奥里洪地块记录了巴尔古津微板块与西伯利亚克拉通碰撞造山的事件。对奥里洪地块出露的两种典型的高级变质岩——石榴辉石岩和石榴黑云片麻岩的矿物成分分析和变质温压计算,表明它们都经历了麻粒岩相的峰期变质作用,峰期变质温度达到770～800℃,而压力曾达到1.0GPa左右;峰后的退变质作用仍具有较高的温度,但压力明显降低(700～730℃,0.65GPa和710～766℃,0.50GPa),显示了一个近等温降压(ITD)的顺时针P-T轨迹特征。石榴黑云片麻岩中变质锆石的原位LA-ICP-MS U-Pb定年表明,麻粒岩相峰期变质年龄为479±2Ma,而峰前变质可能在500Ma就已经开始。峰后的退变质作用很可能发生在475～460Ma之后。整个造山作用持续了至少35Ma。对比蒙古-图瓦地块及中国东北佳木斯-额尔古纳地块已厘定出的变质作用及岩浆活动年龄可以发现,西伯利亚克拉通南缘不同地区增生-碰撞造山作用发生的时间是不同的,奥里洪地区造山作用相对年轻。

A series of accretion-collision events occurred along the southern margin of the Siberian Craton in Early Paleozoic. The Olkhon terrane records the history of the accretion and collision between the Barguzin microcontinent and southern Siberian Craton. Mineral composition analyses and P-T calculations of two typical high-grade metamorphic rocks, the garnet pyroxenite and garnet-biotite gneiss in the Olkhon area indicate that both of them experienced granulite facies metamorphism with peak conditions of 770 similar to 800 degrees C, similar to 1.0GPa. The retrograde metamorphism also experienced relatively high temperature, but significantly deceased pressure (700 similar to 730 degrees C, 0. 65GPa and 710 similar to 766 degrees C, 0. 50GPa), showing approximately an isothermal decompression clockwise P-T-t path. In-situ LA-ICPMS U-Pb dating results of metamorphic zircons from the garnet-biotite gneiss indicate that peak metamorphism probably took place at 479 +/- 2Ma, and pre-peak metamorphism may occurred before 500Ma. The post-peak retrograde metamorphism possibly happened after 475 similar to 460Ma associated with post-orogenic magmatism. Therefore, the accretion-collision events might have lasted for at least 35Ma. Based on the difference in the time of metamorphism and magmatic activities among Mongolian-Tuva terrane, Jiamusi-Erguna terrane and Olkhon terrane, it is suggested that the accretion-collision events along the southern margin of the Siberian Craton occurred in different periods, and the collision orogeny between the Barguzin microcontinent and the Siberian Craton is slightly later than other terranes.