内蒙古土贵乌拉孔兹岩带超高温变质作用：变质反应结构及P-T指示

Ultrahigh temperature metamorphism of Tuguiwula khondalite belt,Inner Mongolia: Metamorphic reaction texture and P-T indication.

内蒙古土贵乌拉地区分布了含石榴石花岗岩、夕线石榴黑云片麻岩以及二者以不同比例互层状组成的岩石,超高温变质岩以暗色含尖晶石堇青石夕线石榴黑云片麻岩条带或透镜体产于石榴石花岗岩和夕线石榴片麻岩中。基于[K]FMAS[H]系统以及岩石成因格子,认为超高温变质岩经历了三个阶段的变质:早期变质作用阶段,以同一石榴石颗粒中含有夕线石、尖晶石、石英等单相矿物包体为特征,表明变质作用进入到尖晶石+石英组合稳定域;峰期变质作用阶段,以尖晶石+石英、假蓝宝石+石英、斜方辉石+夕线石+石英三种超高温矿物组合为特征,表明峰期变质条件稳定在这三种矿物组合稳定域,指示变质温度高于1000℃;退变质阶段过程中,粗颗粒斜方辉石边部和核部 Al 含量的重新平衡,指示温度降低到950～970℃左右,表明了近等压冷却的退变质作用,随后由于快速抬升,发生了近等温降压的退变质作用,这个过程以各种反应边和后成合晶结构为标志,例如尖晶石与石英、假蓝宝石与石英之间有堇青石的反应边,石榴石外围有堇青石和斜方辉石的后成合晶。最后黑云母和斜方辉石分解为黑云母标志着高级变质作用的结束。内蒙古土贵乌拉地区超高温变质岩经历了逆时针的 P-T 演化特点。

The rock types of garnet-bearing granites, sillimanite-garnet-biotite-bearing gneisses （khondalites） and interlayered rocks with the granite and gneisses in various ratio mainly distribute in Tuguiwula, Inner Mongolia. UHT rocks occurs as dark colored sillimanite-garnet-biotite-spinel-corderite-bearing gneiss belts or lens in both garnet-bearing granites and khondalites. Based on the [ K]FMAS[ H]system and petrogenetic grid, the metamorphic history could be classified into three stages according to the mineral assemblage and reaction textures. The early stage is marked by the monophase mineral inclusions of quartz, spinel and sillimanite in one single grain of garnet, indicating the metamorphism prograded into the stable field of spinel ＋ quartz. The peak stage is marked by the mineral assemblages of spinel ＋ quartz, sapphirine ＋ quartz, othropyroxene ＋ sillimanite ＋ quartz, which indicated the temperature exceeding 1000℃. After the peak stage, the UHT rocks underwent initial isobaric cooling retrogressive metamorphis when the temperature dropped to ca. 970℃, indicated by the rebalanced A1 solution in othropyroxene, and followed by the isothermal decompression retrogressive metamorphism due to fast exhumation. The isothermal decompression metamorphism is marked by various rims, corona and symplectite, such as corderite rim between spinel and quartz, sapphirine and quartz, corderite and othropyroxene symplectite around garnet. The anticlockwise P-T path of the UHT metamorphism was inferred.