New Insight From the First Application of Ti-in-Quartz (TitaniQ) Thermometry Mapping in the Eastern Khondalite Belt, North China Craton

The thermal regime of the lower crust is a critical factor that controls crustal anatexis, high-grade metamorphism, and granite formation, which finally results in crustal differentiation. However, the large-scale thermal regime in the Precambrian continental crust is generally not well established. In this study, we first applied Ti-in-quartz (TitaniQ) thermometer to map the temperature variation in the lower crust within an area of similar to 10,000 km(2) in the Paleoproterozoic eastern Khondalite Belt, North China Craton. The studied rocks are aluminous gneisses/granulites, which contain abundant quartz that generally coexists with rutile. The results show that matrix-type quartz with substantial rutile exsolution generally contains the maximum Ti concentration, which is <300 ppm higher than that of inclusion-type quartz. This result suggests that two quartz types probably formed at the prograde and near-peak to early cooling metamorphic stages, respectively. Therefore, the temperature mapping result based on the maximum Ti concentrations of the matrix-type quartz can better represent the thermal regime than inclusion-type quartz. Our regime shows that the hottest Paleoproterozoic lower crust is underneath the Liangcheng-Heling'er-Zhuozi area, where ultrahigh-temperature (UHT) metamorphism is closely associated with abundant charnockite. The hottest region may represent the root of an ancient large hot orogeny. Our study provides a new insight into the formation of UHT metamorphism.