Granulite xenoliths and their zircons, Tuoyun, NW China: Insights into southwestern Tianshan lower crust

Granulite xenoliths from the Cretaceous-Paleogene Tuoyun basalts in the southwestern Tianshan area (NW China) are mainly pyroxene granulite and olivine granulite, with minor garnet granulite and quartz granulite. They can be divided into two types by geochemistry and petrography; both are interpreted as magmatic rocks that have undergone multistage metamorphism. Type I granulites are foliated, mafic to intermediate in composition and have negative Zr and Hf anomalies. Type II xenoliths are massive mafic rocks with high Ni contents, and little or no Zr and Hf anomaly. P–T estimates for garnet granulites suggest the xenoliths were derived from depths of ca. 40 km, near the base of the crust. LAM-ICPMS U–Pb data for zircons from four xenoliths lie on discordia with upper intercepts of 690–770 Ma and lower intercepts of 80–125 Ma. Oscillatory zoning in zircon cores suggests that the upper-intercept ages reflect magmatic crystallization and subsequent granulite-facies metamorphism, and the lower intercepts reflect a major thermal event in the lower crust, related to the eruption of the host basalts. Hf-isotope compositions show little variation with age, indicating that most zircons with young ages were thermally reset. Mean Hf model ages for the zircons from each xenolith range from 1.3 to 1.7 Ga. These data and whole-rock chemistry suggest that the protoliths of the granulites were generated by underplating of mafic magmas that assimilated variable proportions of pre-existing crust. Simple modeling of the Hf-isotope data indicates that this lower crust contained components ≥2.5 Ga, and probably >3.4 Ga, in age in Neoproterozoic time. These observations suggest that the southwestern Tianshan (e.g. in Tuoyun area) may represent the presence of a microcontinental block within the Paleozoic East Central Asian Orogenic Belt.