帕米尔高原沙克达拉穹窿变质作用及新生代下地壳演化

帕米尔高原从西到东展布的8个新生代变质穹窿构成帕米尔高原变质地壳的主体,沙克达拉穹窿是其中最大的一个。沙克达拉穹窿变质杂岩中石榴矽线石片麻岩峰期组合(Grt+Ky+Bi+Rt+Pl+Qz)变质作用温压条件为T约810 ℃/P约10 kbar, 石榴石单斜辉石基性麻粒岩峰期组合(Grt+Cpx+Rt+Pl+Qz)变质作用温压条件为T约824 ℃/P约16.3 kbar, 榴闪岩退变较强,其残留峰期组合(Grt+Pl+Hbl+ilm+Qz)变质作用温压条件为T约683 ℃～873 ℃/P约8.6～11.7 kbar。基性麻粒岩变质锆石的U-Pb年龄为19～35 Ma,反映了从晚始新世到早中新世帕米尔高原下地壳加热加厚过程。帕米尔穹窿的变质作用可以与高喜马拉雅结晶岩系类比,在新生代印度亚洲大陆碰撞过程中,帕米尔陆内各地体沿前新生代缝合带的陆内俯冲可能是帕米尔下地壳加厚的主要动因。

The Cenozoic metamorphism and tectonic evolution of the lower crust of Shakhdara dome,Pamir Mountains

Pamir metamorphic crust is dominated by eight metamorphic domes exposed across Pamir from east to west,among which Shakhdara dome is the biggest one.Peak T-P condition of Shakhdara garnet-sillimanite gneiss is T～810 ℃/P～10kbar represented by its peak assemblage Grt+Ky+Bi+Rt+Pl+Qz,while the peak T-P condition undergone in Shakhdara's garnet-clinopyroxene granulite is T～824 ℃/P～16.3kbar calculated from the peak assemblage Grt+Cpx+Rt+Pl+Qz.As to garnet-amphibolites in Shakhdara,which had experienced relatively severe retrograde,T～683 ℃～873 ℃/P～8.6～11.7kbar is revealed as the metamorphism condition of its residual peak assemblage Grt+Pl+Hbl+ilm+Qz.Peak metamorphic age 19～35 Ma of the mafic granulites dated by U-Pb method on zircons may reflect the thickening and heating process of Pamir lower crust from Late Eocene to Early Miocene.The metamorphism of Pamir domes could be compared with that of High Himalaya crystalline complex and we suggest that Cenozoic Pamir intra-continental-subduction alone the formerly formed suture zone after India-Asia Collision could be the trigger of Cenozoic thickening and metamorphism of lower crust in Pamir.