P–T–t evolution of pelitic gneiss from the basement underlying the Northwestern Ordos Basin, North China Craton, and the tectonic implications

Abstract

Using petrography, phase equilibria modeling and in situ (U–Th)–Pb monazite geochronology, we show that pelitic gneiss from close to the bottom of the Qitan1 borehole in the northwest of the Ordos Basin records a four stage metamorphic evolution. The M1–M3 stages, which were suprasolidus, are represented by: rutile relicts in the matrix and biotite+K-feldspar+plagioclase+sillimanite+quartz inclusions in the cores of garnet porphyroblasts (M1); inclusion-free mantles on garnet with ilmenite (partially replacing rutile) and biotite+K-feldspar+plagioclase+sillimanite+quartz in the matrix (M2); and, the appearance of cordierite in the assemblage biotite+K-feldspar+plagioclase+sillimanite+ilmenite+quartz (M3). The final subsolidus mineral assemblage is garnet+biotite+K-feldspar+plagioclase+sillimanite+cordierite+ilmenite+quartz (M4). The P–T conditions for the mineral assemblage evolution (M1→M4) are constrained by a P–T pseudosection constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 chemical system. The P–T–t path is clockwise involving slight heating during near isothermal decompression from ∼8.8kbar at ∼769°C to 5.5kbar at 785°C followed by close-to-isobaric cooling across the solidus. Two cores of monazite and three unzoned grains that have similar populations of dates yield robust ages of 1.96–1.94Ga, interpreted to date the timing of late prograde-to-peak metamorphism (M1–M2). In addition, one rim and two unzoned monazite grains with similar populations of dates yield robust ages of 1.90–1.88Ga, interpreted to date the timing of post decompression slow cooling across the solidus (M3→M4). On the basis of these results, the pelitic gneiss is interpreted to have formed by crustal thickening (M1) during collision between the Yinshan and Ordos Terranes, the latter with the Khondalite Belt protoliths along its northern margin (present co-ordinates). This collision culminated at ca. 1.95Ga and was followed by decompression (M2–M3) and slow cooling (M4) to ca. 1.89Ga during post-collisional extension. The similarity between the P–T–t path determined in this study and those from HP/MP granulites in the Khondalite belt confirms that they experienced a common tectono-metamorphic history. The P–T–t path records an evolution from an apparent thermal gradient of ∼75°C/kbar at peak pressure to ∼150°C/kbar after decompression. Together with the occurrence of mid-Paleoproterozoic magmatism in the basement of the Ordos Basin, these features indicate that the Ordos Terrane was the upper plate at a convergent margin with the Khondalite Belt representing the forearc sediments along its northern edge. Collision with the Yinshan Terrane, part of the subducting plate, led to shallow slab breakoff and the development of a two-sided hot collision and formation of a wide plateau-like hot orogen, similar to those in the numerical models of Sizova et al. (2014).