Metamorphic P-T-t paths of high-pressure felsic and pelitic from the Qianlishan Complex and tectonic implications for the Khondalite Belt in the North China Craton

Abstract

<p>The Khondalite Belt is an east-west-trending Paleoproterozoic continental-continental collisional belt, separating the Western Block of the North China Craton into the Yinshan Block and the Ordos Block from north to south. In the past years, extensive metamorphic and geochronological investigations for pelitic granulites have been carried out in the Khondalite Belt. However, felsic granulites attract just a little attention although they are widely exposed in the field and potentially preserve key high-pressure information, thus hindering better understanding of the tectonic processes and settings of this critical area. In this study, a link between ‘inter-layered’ felsic and pelitic granulites from the Qianlishan Complex of the Khondalite Belt was established based on comprehensive metamorphic analysis. Three distinct metamorphic stages including peak pressure (M<sub>1</sub>), post-peak decompression (M<sub>2</sub>) and late retrograde cooling (M<sub>3</sub>) stages have been identified in the felsic and pelitic granulites. Felsic granulites experienced high-pressure metamorphism up to ~12 kbar, while estimated peak pressure for pelitic granulites is 11-15 kbar. The decompression stage (M<sub>2</sub>) is represented by cordierite + sillimanite symplectite and/or cordierite coronae with conditions of 5.7-6.5 kbar/800-835 °C in pelitic granulites, and by garnet-sillimanite assemblages formed at conditions of >6.5 kbar/810-865 °C in felsic granulites. The later cooling stage (M<sub>3</sub>) is indicated by sub-solidus biotite-quartz-plagioclase symplectite and later melt crystallization. Clockwise P-T paths involving near-isothermal decompression and near-isobaric cooling were defined by these mineral assemblages and approximate P-T conditions, which suggest a continent-continent collisional event. SIMS zircon U-Pb dating yields a consistent metamorphic age of ~1.95 Ga from felsic granulites, interpreted as the timing of peak metamorphism. The results, combined with previously reported data, suggest that the Khondalite Belt formed by collision between the Yinshan and Ordos blocks at ~1.95 Ga.</p>