Granulite facies metamorphism and crust melting in the Huai’an terrane at ∼1.95 Ga, North China Craton: New constraints from geology, zircon U–Pb, Lu–Hf isotope and metamorphic conditions of granulites

Abstract

Geological implications of ∼1.95Ga and ∼1.85Ga metamorphic ages for HP granulites in the Huai’an terrane are still controversial, which have affected the crustal evolution models for the North China Craton. This study reports two geological sections in which distinct geological relationships of a quartz-dioritic and a tonalitic dike cutting through mafic HP granulite and granulite facies meta-sedimentary rocks are described. In these two sections, magmatic dikes, mafic HP granulites and meta-sedimentary rocks preserve conformable foliation to indicate their similar deformation history. The U–Pb dating and Lu–Hf isotopic analysis of zircons from metamorphic and meta-igneous rocks provide new constraints on the timing and mechanism of thermal events associated with metamorphic and deformation events. Both magmatic dikes were formed around ∼1.95Ga and overprinted by later metamorphic event during ∼1.85Ga. The S-type granite, at geological section (GS) 1, revealed its formation age was formed around 1959±34Ma (n=3, MSWD=1.5), and overprinted by later metamorphic event at 1840±8Ma (n=10, MSWD=1.2). Additionally, zircons from the mafic HP granulite in the GS2 are dominated by metamorphic ages of 1972±22Ma (n=6; MSWD=0.03) with few grains recording an age of 1856±29Ma (n=3; MSWD=0.17). In situ Lu–Hf analyses on zircons from the quartz-dioritic dike, tonalitic dike, mafic HP granulite and S-type granite, reveal that the magmatic dikes were derived from partial melting of ∼2.0–2.2Ga meta-basic rocks and the S-type granite was generated by melting of meta-sediments beyond 840±50°C. It suggested that those meta-sediments were taken into the lower crust in a certain way at granulite facies conditions ca. 1.95Ga ago. A clockwise P-T path acquired from mafic HP granulite revealed peak metamorphic conditions of 880–890°C and 11–13kbar. The temperature results recorded in the ∼1.95Ga zircons of mafic HP granulite, similar to those of high-pressure mineral assemblage in matrix, suggest that the HP granulite metamorphism was approximately ∼1.95Ga ago. According to the results above, this contribution concludes that mafic and meta-sedimentary rocks were metamorphosed together under high-pressure granulite metamorphic conditions ∼1.95Ga ago, followed by high-temperature granulite facies melting event that gave rise to granite and S-type granite at ∼1.95Ga. The granulite facies metamorphic rocks and magmatic dikes in the Huangtuyao graphite mine experienced the same evolution history after ∼1.95Ga. The mafic and pelitic HP granulites in the Huai’an terrane are interpreted to be collisional products of the Paleoproterozoic cratonization of the NCC.