Metamorphism of diverse basement gneisses of the Ordos Basin: Record of multistage Paleoproterozoic orogenesis and constraints on the evolution of the western North China Craton

Although it remains deeply hidden and largely enigmatic, the basement of the Ordos Basin has played a pivotal role in the evolution of the western part of North China Craton. Diverse gneiss samples collected from boreholes advanced into the deep basement of the Ordos Basin are investigated by integrating petrography, phase equilibria modeling and geochronology. The results reveal the basement gneisses have different protoliths with ages extending from the late Archean to the Paleoproterozoic. Medium to low pressure metamorphism from amphibolite to granulite facies (4.7-7.3 kbar and 740-810 degrees C) is constrained for the basement gneisses, using phase equilibrium modeling in the MnNCKFMASHTO system. Evidence is provided of metamorphic monazite growing later than zircon in the same basement gneisses, and multi-stage Paleoproterozoic orogenesis is delineated using metamorphic zircon and monazite ages from the basement gneisses. The basement gneisses from most boreholes across the basin are comparable to those in adjoining Paleoproterozoic belts in terms of metamorphic and geochronological characteristics. The results of the study suggest, in combination with previous data, that a large area of the basement was involved in the Paleoproterozoic orogenis. The northern part of the Ordos Basin basement is closely associated with the Khondalite Belt.