南秦岭增生杂岩带马道地区变泥质岩的变质作用

Metamorphism of Metasedimentary Rocks in Madao Area,South Qinling Accretionary Complex Belt

南秦岭构造带出露于勉略断裂和虞关—留坝断裂之间,是一条复杂的增生杂岩带,也是秦岭造山带的重要组成部分。增生杂岩带内马道地区发育一套由黑云母片麻岩、片岩组成的变泥质岩,内部包含有石英岩、大理岩及超基性岩等岩块,构成了典型的“block-in-matrix”结构。选取了含石榴子石黑云母片麻岩样品进行详细的岩石学研究。结果显示,北部变质岩样品中的石榴子石具有弱退变质成分环带,利用岩石矿物组合中的石榴子石-黑云母温度计、石榴子石-黑云母-斜长石-石英组合温度-压力计,估算峰期压力为0.78~0.79 GPa,温度为705~707℃,退变质时期压力为0.64~0.76 GPa,温度为602~650℃,揭示出岩石峰期高角闪岩相变质后,经历降温减压过程。南部岩石样品中含有特征的十字石＋蓝晶石组合,样品中的石榴子石具有进变质成分环带,其峰期压力为0.49~0.57 GPa,温度为553~562℃,相当于低角闪岩相。通过与其他典型增生杂岩带变质岩的剥露机制对比,认为马道变泥质岩的变质作用演化与南秦岭地区碰撞作用有关,而其剥露过程则主要受到双重逆冲构造控制。

As an important part of the Qinling Orogenic Belt,the South Qinling tectonic belt is an accretionary complex belt occurring between the Mianlüe fault and the Yuguan-Liuba fault. The Madao metasedimentary rocks within this accretionary complex belt mainly comprise biotite gneisses or schists,which holds masses of quartzite,marble and ultramafic rock blocks,showing typical characteristic of block-in-matrix structure. In this contribution,garnet-bearing biotite gneisses are selected for petrological studies,including sillimanite-bearing samples that were collected from the northern part and staurolite-kyanite-bearing samples from the southern part.The texture analyses reveal that the garnets in the biotite gneisses preserve weak compositional zoning that may be resulted from retrograde metamorphism. Meanwhile,through the analyses of garnet-biotite thermometer and garnet-biotite-plagioclase-quartz thermobarometer,the temperature and pressure of peak and retrograde metamorphic stages are obtained as 705- 707 ℃,0. 78- 0. 79 GPa and 602- 650 ℃,0. 64- 0. 76 GPa,respectively,indicating that the metamorphism occurred under higher amphibolite facies conditions. The peak and ret-rograde metamorphic conditions shows decreasing temperature and decreasing pressure features after the peak conditions. However,the garnets in the samples from southern part exhibit compositional zoning indicating a progressive metamorphic process,and the peak metamorphic temperature and pressure of 553- 562 ℃ and0. 49- 0. 57 GPa also are obtained,being indicative of lower amphibolite facies metamorphic condition. Compared with the exhumation processes of metamorphic rocks in typical accretionary complexes in the world,the metamorphic evolution in the Madao area may be related to collisional process in the South Qinling,while its exhumation mechanism is related to duplex thrust structures.