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岩石学报 2009
Deformation and metamorphism of HP belt in the North Qilian Mountains and their implications for exhumation
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Abstract:
The North Qilian Mountains is a typical Early Paleozoic orogenic belt where the "cold" oceanic subduction belt occurs with rock assemblages characterized by high-pressure low-temperature blueschist and eclogite. According to the structure, three types of eclogites have been recognized. (1) The granular eclogite stands for peak stage metamoprhism with paragenetic mineral assemblage of Grt + Omp + Phn + Rut ± Lws. Lawsonite and its pseudomorph retain as inclusions in garnet, and P-T conditions are P = 2. 30 ～ 2. 60GPa and T = 465 ～508℃. (2) The schistose eclogite consists of mineral assemblage of Grt + Omp + Czo/Ep + Phn + Gin with penetrative foliation S_1, lineation L_1, and macro-/micro isoclinal folds F_1. P-T calculation yields conditions of P = 1. 9 ～2. 2GPa and T = 466 ～ 510℃. (3) The blueschist-overprinted eclogite is characterized by strong retrogression of eclogite facies minerals into blueschist facies minerals Gln + Ep + Phn ± Ab with penetrative foliation S_2, lineation L_2 and isoclinal shearing folds F_2 under P-T conditions of 1. 15 ～ 1. 37GPa and 422 ～ 487℃. The lineation statistics from glaucophane and phengite shows that D_2 deformation displays kinematic character of oblique strike-slip movement. The later overprinting of greenschist metamorphism appears with penetrative foliation S_3 and lineation L_3. Statistics of L_3 show vertical thrusting movement in perpendicular to the trending of high-pressure tectonic slices. Combining macro-and micro-scope structures, P-T conditions and isotopic geochronology data, we concluded that the grade of retrogression is associated with shearing deformation. Eclogites have undergone early oblique strike-slip deformation under epidote-eclogite facies to blueschist-facies conditions, and finally thrust to the surface before Early Devonian.
