华南晚古生代末超高温变质作用:以十万大山花岗岩中麻粒岩包体为例

摘要

The Darongshan-Shiwandashan Indo-Sinian S-type granites from SE Guangxi Province contain abundant metasedimentaiy granulite enclaves. The mineral assemblages of spinel + quartz and orthopyroxene + sillimanite ± quartz in some granulites indicated ultrahigh-temperature metamorphism in the region. Using garnet-orthopyroxene geothermobarometry, the estimated P-T conditions for peak metamorphism are: P =7. 5 - 8. 0kbar and T = 950 ～ 1000℃, and P = 3. 2 ～ 3. 7kbar and T - 790 ～ 820℃ for retrograde stage. Combined mineral assemblages and melamorphic reaction textures with P-T calculation, the granulite enclaves in the Shiwandashan area experienced a three-stage metamorphic history. The early prograde metamorphism extended into the stable field of sillimanite;The peak stage of the ultrahigh-temperature metamorphism was characterized by the mineral assemblages of spinel + quartz and orthopyroxene + sillimanite ± quartz, and the retrograde metamorphism was marked by formation of orthopyroxene + cordierite and/or orthopyroxene + plagioclase symplectites and coronas around garnet, suggesting a near-isothermal decompression path. This clockwise P-T path in granulites was consistent with the Late Paleozoic to Early Mesozoic crustal evolution, including initial crustal thickening, subsequent rapid exhumation and cooling and remelting.%广西东南部大容山-十万大山地区过铝质(S型)花岗岩中包含丰富的副变质麻粒岩包体.部分麻粒岩包体保留了尖晶石+石英、斜方辉石+夕线石±石英等超高温变质矿物组合,指示这些麻粒岩包体曾经历了超高温变质作用.矿物温压估算结果显示:峰期温压条件为7.5～8.0kbar,950～1000℃,峰后退变质温压条件为3.2～3.7kbar,790～820℃.矿物共生组合、变质反应结构以及温压条件估算表明,大容山-十万大山花岗岩中的麻粒岩包体经历了从单相夕线石的稳定城,到尖晶石+石英和紫苏辉石+夕线石±石英的矿物共生组合的稳定域,最后发生了近等温降压退变质作用,形成了紫苏辉石+斜长石和紫苏辉石+堇青石冠状体后成合晶,构成了典型的顺时针P-T演化轨迹,反映区域当时经历了造山过程中地壳增厚,而后折返快速抬升到地表的动力学过程.