Metamorphic P-T-t paths of the Zanhuang metamorphic complex: Implications for the Paleoproterozoic evolution of the Trans-North China Orogen

摘要

Garnet-bearing metapelites are cropped out in the Zanhuang metamorphic complex located in the central section of the N-S trending Trans-North China Orogen. Petrological studies indicate that these garnet-bearing metapelites preserve three distinct metamorphic assemblages (M-1-M-3). The cores of garnet porphyroblasts and fine-grained inclusions of plagioclase+quartz +/- biotite +/- apatite +/- magnetite +/- rutile +/- ilmenite define the prograde metamorphic stage (M-1). The assemblage of the peak metamorphic stage (M-2) consists of biotite + plagioclase + quartz +/- kyanite +/- hornblende +/- magnetite +/- ilmenite in equilibrium with the lowest-X-Mn garnet rims. The peak metamorphism was followed by a near-isothermal decompressional stage (M-3) and the development of symplectitic coronas of biotite + plagioclase + quartz +/- hornblende +/- magnetite surrounding garnet porphyroblasts. Quantitative phase equilibria modeling and traditional thermobarometric estimations were applied to yield P-T conditions of 4.5-5.9 kbar and 551-596 degrees C for stage M-1, 9.6-12.3 kbar and 770-830 degrees C for stage M-2, and similar to 4.7 kbar and similar to 635 degrees C for stage M-3. The combination of the mineral inclusions, mineral compositions, and metamorphic reactions in the Zanhuang metapelites defines a clockwise P-T path with nearly isothermal decompression, which is considered to have been related to the amalgamation of the Eastern and Western Blocks to form the North China Craton. Abundant U-Pb spot analyses of metamorphic zircons from the gneiss and amphibolite reveal two discrete, meaningful ages of metamorphism within the Zanhuang complex: the first age dates to -2507 Ma, and the second age dates to similar to 1839 Ma. The first age group (similar to 2507 Ma) was most likely associated with a regional Neoarchean granulite-facies metamorphic event within the North China Craton, whereas the second age group (similar to 1839 Ma) is interpreted as the result of a Paleoproterozoic orogenic event that occurred in the Trans-North China Orogen (TNCO). (C) 2014 Elsevier B.V. All rights reserved.