Metamorphic evolution and Zircon ages of Garnet-orthoamphibole rocks in southern Hengshan, North China Craton: Insights into the regional Paleoproterozoic P-T-t history

摘要

Garnet-orthoamphibole rock (GOR) from southern Hengshan of the North China Craton (NCC) shows a prominent porphyroblastic texture involving garnet, orthoamphibole, kyanite, sillimanite, chlorite, rutile and quartz, with or without talc, hornblende and staurolite. Garnet develops coronae or symplectite assemblages of cordierite, anthophyllite, ilmenite and quartz. The P-T and P-H2O evolution for three representative samples were determined by using pseudosections calculated with THERMOCALC. Generalized prograde paths are inferred from subtle garnet growth zoning, chlorite compositions and inclusion assemblages in garnet. The peak mineral assemblages are predicted to be characteristic of presence of garnet, gedrite, chlorite, kyanite and rutile under P-T conditions estimated to be 9-12 kbar and 650-670 degrees C. The post-peak history is characterized by isothermal decompression, developing coronae and symplectite assemblages under a mix of fluid-present and fluid-absent conditions. An idealized P-X-Mg [=Mg/(Mg + Fe) in mole] pseudosection drawn at T=655 degrees C and a P-M(H2O) pseudosection at T=630 degrees C suggest that the garnet-free cordierite-orthoamphibole rock (COR) can form from decompression of GOR assemblages when the rock is high-in X-Mg value or the decompression temperature is low. Zircon U-Pb dating yields a metamorphic age of 1964 +/- 25 Ma (similar to 1.95 Ga) and a protolith age of 2533 +/- 14 Ma, where the former was interpreted to represent the prograde- or peak stages of metamorphism. This paper validates an argument that the amalgamation of the NCC via a crust thickening event occurred at similar to 1.95 Ga, followed by an isothermal decompression and a prolonged nearly isobaric cooling process from 1.90 to 1.78 Ga. (C) 2014 Elsevier B.V. All rights reserved.