Prolonged anatexis of Paleoproterozoic metasedimentary basement: First evidence from the Yinchuan Basin and new constraints on the evolution of the Khondalite Belt, North China Craton

Abstract

Discerning the history of burial, residence at depth and exhumation of high grade metamorphic rocks from an orogen is essential to understanding crustal differentiation and orogenesis. Such high grade rocks are inferred to exist beneath the Yinchuan Basin, which is overlain by a thick covering of younger sediments, and is located at the inferred southwest boundary of the Paleoproterozoic Khondalite Belt in the northwestern North China Craton. The metamorphic basement sampled from a recently completed deep borehole in the Yinchuan Basin is migmatitic and contains an assemblage of garnet, biotite, sillimanite, plagioclase, quartz, minor K-feldspar and local occurrences of muscovite. Phase equilibrium modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system constrains the peak P–T conditions of anatexis to 6–10kbar at 760–810°C, followed by final crystallization at 4.5–5.5kbar and 690–710°C. Ti-in-zircon thermometry for the narrow metamorphic zircon rims provides a temperature range of 687–753°C, demonstrating formation during retrogression to the solidus. NanoSIMS U–Pb analyses on the metamorphic overgrowths of zircon grains yields an upper intercept age of 1895±36Ma. Chronological and chemical analyses of monazite indicate irregular bright gray domains with relatively high Th and low Y and HREE contents developed during prograde metamorphism (1962±8Ma). Conversely, dark domains with low Th and high Y and HREE overgrew during retrograde cooling (1892±14Ma) that was synchronous with the development of metamorphic zircon rims. LA-ICP-MS U–Pb analyses of detrital zircons reveal that the metamorphic basement beneath the Yinchuan Basin has a sedimentary protolith of Paleoproterozoic age. The results directly confirm the existence of Paleoproterozoic basement beneath the Yinchuan Basin, and the well-constrained P–T–t path supports the involvement of the basement (as part of the western Khondalite Belt) in burial and exhumation processes during the 1.96–1.89Ga period, during which anatexis occurred for ∼70Ma.