Helanshan high-pressure pelitic granulites: petrological evidence for collision event in the Western Block of the North China Craton

Abstract

The khondalite series in Helanshan region is a main part of the khondalite belt in the western block of the North China Craton,the metamorphic evolution of which is still ambiguous.Recently,high pressure pelitic granulite with typical mineral assemblage of garnet+kyanite+perthite were discovered in the Helanshan khondalite series.Petrographic observation and phase modeling indicate that the pelitic granulite experienced four stages of metamorphism.An early prograde metamorphic stage(M1)was recognized from inclusions of biotite and muscovite in garnet.The peak metamorphic stage(M2)is characterized by the presence of garnet+kyanite+perthite,with a probable PT condition of 1.41.5GPa at ca.850870℃.The post-peak decompression(M3)is indicative of an isothermally overprinting of sillimanite and cordierite from kyanite and garnet with pressures of 0.60.8 GPa at ca.840860℃.Development of most biotite suggested a late-stage retrogression(M4)under PT conditions of 780810℃,0.450.55GPa.This clock-wise P-T path,characteristic of a post-peak isothermal decompression(ITD)and followed by cooling,is consistent with a thinning process of thickened continental crust which may be a result of the collision between the Yinshan block and Ordos block.

賀蘭山孔茲巖系作為華北克拉通西部孔茲巖帶的重要組成部分,其成因問題一直存在爭議。近來,在對賀蘭山富鋁巖系研究過程中,發現了具有'藍晶石+條紋長石+石榴石'特征組合的高壓泥質麻粒巖。通過巖相觀測與相平衡定量模擬,確定其曾經歷四個階段的變質演化過程,即早期進變質階段(M1)、變質峰期階段(M2)、峰期后降壓階段(M3)和晚期退變階段(M4)。早期進變質階段,以石榴石中包裹黑云母和白云母為特征。變質峰期階段,以出現'藍晶石+條紋長石+石榴石'組合為特征,變質溫、壓條件為850～870℃、1.4～1.5GPa。峰期后降壓階段以峰期石榴石和藍晶石轉變為堇青石和夕線石為特征,溫壓條件大致為840～860℃、0.6～0.8GPa。晚期退變階段以出現新生黑云母為標志,溫、壓條件為780～810℃、0.45～0.55GPa。PT軌跡具有順時針演化樣式,峰期后表現為先發生近等溫降壓(ITD)、后發生近等壓冷卻(IBC)的退變過程,反映陸-陸碰撞發生高壓麻粒巖相變質作用,后又快速折返到正常地殼深度的變質動力學過程,進而從巖石學角度證明華北克拉通西部孔茲巖帶是由北部的陰山陸塊與南部的鄂爾多斯陸塊發生碰撞所致。