Paleoproterozoic high-pressure granulite facies metamorphism in the Yinshan Block, North China craton

The Yinshan Block located in the northern margin of the North China Craton (NCC) has been regarded as a typical Archean cratonic block, but it is argued whether the block had been affected by Paleoproterozoic tectonic and metamorphic events. This paper reports the late Paleoproterozoic high-pressure (HP) granulites for the first time from the Xiwulanbulang area in the Yinshan Block, and presents a systematic study on their petrography, mineral chemistry, phase equilibria modelling and zircon dating to confirm their metamorphic evolution and tectonic implications. Two representative samples contain the typical HP basic granulite assemblages involving garnet, clinopyroxene, plagioclase, amphibole and quartz, with garnet commonly occurring as coronae around clinopyroxene and plagioclase. Both samples show clockwise P-T paths with their peak conditions of similar to 930 degrees C/similar to 15kbar (20NM01) and 970-980 degrees C/similar to 13.5 kbar (21NM05) respectively, constrained using the maximum Ti contents in amphibole and minimum anorthite (X-An) in the mantle of plagioclase on P-T pseudosections. Zircon dating yields a magmatic age of similar to 2.3 Ga and metamorphic ages of similar to 2.5 Ga and similar to 1.8 Ga. The similar to 2.3 Ga magmatic age represents the time of protolith gabbro crystallization, which can be comparative to the extension-related bimodal magmatism in the Khondalite Belt. The similar to 1.8 Ga age was determined from metamorphic zircons that have flat HREE patterns with the presence of garnet, interpreted as the time of the post-peak cooling and uplifting of the HP granulite facies metamorphism. The similar to 2.5 Ga is defined from the captured zircon grains by gabbroic intrusions from the country Neoarchean rocks. The HP granulite facies metamorphism registers a geothermal gradient of 19-20 degrees C/km, indicating a collision-related orogenic event that may have occurred at similar to 1.85 Ga along the northern margin of the NCC. Therefore, the Yinshan Block may represent a weak deformation domain in the orogenic belt on the northern margin of the NCC.