Late Paleoproterozoic medium-P high grade metamorphism of basement rocks beneath the northern margin of the Ordos Basin, NW China: Petrology, phase equilibrium modelling and U–Pb geochronology

Abstract

Basement rocks from drill cores at the northern margin of the Ordos Basin were investigated by integrating petrography, geochronological analysis and mineral equilibria modelling in the Na2OCaOK2OFeOMgOAl2O3SiO2H2OTiO2O system. The P–T conditions constrained by mineral compositional isopleths on calculated pseudosections indicate that the rocks underwent medium-pressure high grade metamorphism. The pelitic rocks reached granulite facies with peak P–T conditions of 780–810°C at c. 6.3–7.8kbar. SHRIMP U–Pb analyses of detrital zircons indicate Paleoproterozoic protoliths for the basement rocks. NanoSIMS U–Pb analyses of metamorphic overgrowths on zircon grains yield upper intercept ages of 1964±21 and 1960±21Ma for pelitic (QT1-2) and psammitic (LT1-2) samples, respectively. The basement rocks are petrologically, geochronologically and metamorphically comparable with rocks from the Khondalite Belt, which posits the existence of khondalite series rocks beneath the northern margin of the Ordos Basin. Therefore, the Paleoproterozoic mobile belt between the Yinshan and Ordos blocks is likely to be wider than previously thought.