Metamorphic P–T path and tectonic implications of pelitic granulites from the Daqingshan Complex of the Khondalite Belt, North China Craton

Abstract

Pelitic granulites are cropped out in the Daqingshan Complex of the Khondalite Belt, a Paleoproterozoic tectonic belt in the Western Block of the North China Craton. Petrological studies show that these granulites contain four distinct metamorphic assemblages. The core of a garnet porphyroblast, along with fine-grained inclusions of biotite+plagioclase+K-feldspar+quartz±sillimanite±ilmenite, defines the prograde metamorphic (M1) stage. The peak (M2) assemblage consists of garnet (mantle)+biotite+K-feldspar+plagioclase+quartz±sillimanite±orthopyroxene±ilmenite±magnetite±rutile. Peak metamorphism was followed by a near-isothermal decompression (M3) and the development of coronae of garnet+biotite+cordierite+plagioclase+quartz±K-feldspar±sillimanite±spinel±ilmenite±magnetite (M3) in the Crd–Grt–Bt gneisses during the following garnet breakdown reactions: garnet+sillimanite+melt→cordierite+biotite+Fe-oxide and garnet+melt→biotite+quartz+plagioclase. Retrograde cooling (M4) assemblages are represented by biotite+muscovite+sillimanite+quartz+plagioclase+K-feldspar±ilmenite. Quantitative phase equilibria modeling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 was applied to obtain P–T conditions of <780°C and <9kbar for M1, 840–880°C and 9–11kbar for M2, 800–870°C and 5.0–7.5kbar for M3, and <660°C and 4.1–6.9kbar for M4. The combination of the mineral assemblages, mineral compositions, and metamorphic reaction histories in the Daqingshan pelitic granulites defines a clockwise P–T path that involves periods of near-isothermal decompression and late cooling that followed the peak granulite-facies metamorphism. This result is consistent with the tectonic history of the Daqingshan Complex in the Khondalite Belt, which involved continent–continent subduction and collision followed by exhumation and cooling. This suggests a continent–continent collisional event between the Yinshan and Ordos blocks, which became amalgamated to form the Paleoproterozoic Khondalite Belt in the Western Block of the North China Craton.