Petrogenesis of the Permian Intermediate-Mafic Dikes in the Chinese Altai, Northwest China: Implication for a Postaccretion Extensional Scenario

The Central Asian Orogenic Belt is a long-lived accretionary orogen, and the late Paleozoic has been considered to be a critical period for the terminal amalgamation of its three tectonic collage systems. However, the exact timing of amalgamation and the geological process of such a huge accretionary orogenic belt are poorly understood. This study presents new geochronological and geochemical data for Permian intermediate-mafic dikes in the Chinese Altai, a key region between the Mongolian and the Kazakhstan collage systems. According to mineral assemblages and petrographic textures, the intermediate-mafic dikes can be categorized as gabbronorite and quartz diorite. The gabbronoritic and quartz dioritic dikes have zircon U-Pb ages of 276.7 ± 2.9 and 273.2 ± 4.3 Ma, respectively. The gabbronorites are characterized by low SiO2 (47.1–51.3 wt%) and high MgO (5.33–8.46 wt%), together with medium Cr (71.2–95.7 ppm) and Ni (80.6–192 ppm) contents. Geochemical modeling indicates that the parental magma was possibly contaminated by 4%–12% crustal materials. Zircon εHf(t) (+13.2 to +16.7) and whole-rock εNd(t) (+4.9 to +6.1) values as well as moderate Sm/Yb ratios (1.75–1.89) imply that the parental magma might have originated from a depleted mantle source dominated by spine lherzolite. In contrast, the quartz diorites exhibit higher SiO2 (57.3–58.3 wt%) and lower whole-rock εNd(t) (∼+2.5) and zircon εHf(t) (+9.1 to +14.4) values, implying that they have a magma source unlike the depleted mantle of the gabbronorites. The parental magma may be derived from mafic lower crust. The quartz diorites have high Y (>39.8 ppm) and heavy rare earth element (e.g., Yb >3.64 ppm) concentrations as well as low Sr/Y (<12) ratios, consistent with geochemical fingerprints of a magma reservoir at shallow depths (<10 kb). Major element compositions of the quartz diorites are comparable to those of intermediate liquids generated by ∼40% partial melting of alkali-enriched basaltic rocks at conditions of T = 1050°–1100°C and P = 8 kbar. Such a high geothermal gradient is inferred to be a consequence of intraplating and/or underplating of hot basaltic magmas in an extensional setting, which may shed light on the ubiquitous tectonic scenario after amalgamation of tectonic collages.