点苍山—哀牢山变质杂岩带变沉积岩的变质演化

Metamorphic evolution of meta-sedimentary rocks within the Diancang Shan-Ailao Shan metamorphic complex belt.

点苍山——哀牢山变质杂岩带位于青藏高原东南缘大理——元江——元阳——河口一带,出露规模达数百千米,是扬子板块和印支陆块之间的一条重要构造带。该变质杂岩带主要由各类正片麻岩、副片麻岩、大理岩所组成,夹有斜长角闪岩、石榴辉石岩和超镁铁质岩石的透镜体或团块。其中,变沉积岩如含夕线石和蓝晶石的片麻岩类岩石保存了多阶段的矿物组合及异常复杂的矿物相转变关系。详细的岩相学、成因矿物学以及矿物相转变关系分析表明,变沉积岩系经历了早期进变质阶段(M1)、峰期角闪——麻粒岩相变质阶段(M2)、峰后近等温减压(脱水熔融)阶段(M3)以及晚期退变质阶段(M4)的变质演化。其中,M1阶段的稳定矿物组合为石榴石+斜长石+白云母+石英+十字石±蓝晶石±黑云母±钾长石,M2阶段的稳定矿物组合为石榴石+黑云母+蓝晶石/夕线石+斜长石+石英、石榴石+黑云母+斜长石+石英±钾长石±夕线石,M3阶段的共生矿物组合为石榴石+黑云母+夕线石+斜长石+石英,M4阶段的矿物组合为黑云母+白云母+斜长石+石英±钾长石±石榴石等。通过传统GB——GASP温压计和二云母温度计的估算结果,配合P-T视剖面定量计算,确定早期进变质阶段(M1)的温压条件为T=560～590℃,P=5.5～6.3kb,峰期角闪——麻粒岩相阶段(M2)的温压条件为T=720～760℃、P=8.0～9.3kb,峰后近等温减压阶段(M3)的温度压力条件为T=640～760℃,P=5.0～7.3kb,晚期退变阶段(M4)的温压条件为T=521～648℃,P=4.0～5.0kb。上述研究结果表明,点苍山——哀牢山变沉积岩记录了典型碰撞造山带型式的顺时针P-T演化轨迹,表明点苍山——哀牢山变质杂岩带的形成与印度板块和欧亚板块之间的俯冲——碰撞存在密切的成因关系。

Meta-sedimentary rocks, characterized by intensive migmatization and/or mylonization, are widely distributed within the Diancang Shan-Ailao Shan metamorphic complex belt. On the basis of detailed mineralogical and petrographic observations, four metamorphic stages have been recognized, including early prograde metamorphic stage （M1）, peak amphibolite-granulite facies metamorphic stage （M2）, near isothermal decompression retrograde stage （M3）, and late amphibolite facies retrograde stage （M4）. Metamorphic P-T conditions for each stage were calculated by P-T pseudosection, and geothemobarometer （GB-GASP） and two-mica thermometer. The mineral assemblage for the early prograde assemblage （M1） is characterized by Pl＋Ms＋Qtz＋St±Ky±Bt±Kfs, which as inclusions are preserved in the core of garnet, and formed at 560～590℃ and 5.5～6.3kb. The typical mineral assemblage for the peak amphibolite-granulite facies metamorphic stage （M2） assemblage is represented by Grt＋Bt＋Ky/Sil＋Pl＋Qtz or Grt＋Bt＋Pl＋Qtz±Kfs±Sil, recording P-T conditions of 720～760℃ and 8.0～9.3kb. The near isothermal decompression retrograde stage （M3） is indicated by a mineral assemblage of Grt＋Bt＋Sil＋Pl＋Qtz, which were formed at P-T conditions of 640～760℃ and 5.0～7.3kb. Abundant retrogressive minerals, including fine-grained biotite and muscovite, together Pl＋Qtz±Kfs±Grt were formed at the late amphibolite facies retrograde stage （M4）, which occurred at P-T conditions of 521～648℃ and 4.0～5.0kb. The integrated clockwise P-T path revealed by the meta-sedimentary rocks indicates that the Diancang Shan-Ailao Shan metamorphic complex belt experienced a tectonic evolution of subduction-collision-exhumation, which was related to the amalgamation between the Indian and Eurasian plates.