Deformation And Metamorphic History Of The Chinese Altai Orogenic Belt

Based on the detailed field geological observations and petrographical analyses on the garnet-bearing paragneisses exposed at the Wuqiagou area of Fuyun County in the sorthern margin of the Chinese Altai orogen,at least three-phase tectonic deformation and metamorphic events have been determined:1)The first phase of metamorphism and deformation D₁ can be divided into the early burial metamorphism and the later extension.At the initial stage of burial metamorphism,M₁ metamorphic mineral assemblage of Bi+Pl+Q formed,indicating P-T conditions about⁵90°C/4.6kbar.Subsequently,it reached peak P-T conditions of⁶30°C/4.5 kbar with the growth of M₂ metamorphic mineral assemblage of Sil+Grt+Bi+Pl+Q.Then,relating to the horizontal extension during 400-380 Ma,lower crust experienced extensive decompression melting,and formed the main foliation?S₁?in the area based on migmatitization strip;2)The second phase of deformation D₂ is a folding deformation activating at³80 Ma.It formed a series of NE-SW trending upright folds F₁ based on foliation S₁.The metamorphic grade presents a symmetric distribution at the core of folds F₂ along the direction of F₁ axial because of this deformation.This stage is the end time of the growth of garnet porphyroblast,and the P-T conditions are⁶30°C/4.5 kbar;3)The third phase of deformation D₃ is a compression-shear deformation in NE-SW direction,which was activating at²80 Ma.This deformation is shown as a series of NE-SW trending folds and composes regional antiform F₂ based on foliation S₁ which was reworked by D₂ and followed by a sinistral strike-slip deformation.This deformation corresponds to M₄ metamorphism,which is represented by an assemblage of Spl+Crd+Bi+Pl+Q with P-T conditions of 615°C/².1 kbar.The above P-T estimates define a clockwise P-T path.This clockwise P-T trajectory reflects that the rocks underwent an extensional decompression after burial,and this process was accompanied by the input of heat.The sorthern margin of the Chinese Altai orogen underwent an eontaction-extension-contraction tectonic cycle in Early-Middle Devonian.It was caused by ocean plateau entered subduction zone with subducted oceanic crust.Synchronous high-grade metamorphism probable caused by ridge subduction.It is also in compressive environment in Permian.Synchronous HT-UHT metamorphic rocks was brought into shallow crust by pressing-twisting structural deformation.