When did the final closure occur of the eastern Paleo-Asian Ocean: Constraints from the latest Early–Middle Triassic adakitic granites in the southeastern Central Asian Orogenic Belt

Abstract

There is a broad consensus that the Solonker suture zone marked the final closure of the eastern Paleo-Asian Ocean, which led to the formation of the eastern segment of the Central Asian Orogenic Belt (CAOB). However, when and how the final closure occurred still remained controversial. Located in a key position in the southeastern CAOB, the Early Mesozoic magmatic rocks along the northern margin of the North China Craton provide a crucial window to evaluate the subduction and final closure processes of the eastern Paleo-Asian Ocean. To address these issues, data on geochronological, whole-rock geochemical, whole-rock Sr-Nd and in situ zircon Hf isotopic study of the Early Mesozoic granites, as well as major-trace elements compositions of the apatites are reported here. Zircon U-Pb dating results showed that our studied granites were emplaced in the Middle Triassic (243.1–242.6 Ma). These granites have high Sr (250–652 ppm) contents, low heavy rare earth elements (HREEs), Yb (0.46–0.77 ppm) and Y (4.48–8.15 ppm) contents, and show high Sr/Y and (La/Yb)N signatures (36–108 and 21–35, respectively), with negligibly negative to moderately positive Eu anomalies (δEu = 0.80–1.36), indicating adakite-like affinities. Most of these granite samples are characterized by high K2O contents (3.30–4.83 wt%), and belong to the high-K calc-alkaline and shoshonitic series, with peraluminous affinities (A/CNK = 1.02–1.28). Besides, their low MgO (0.10–0.34 wt%), Cr (0.99–3.50 ppm), and Ni (0.96–2.40 ppm) contents indicate a thickened continent crustal origin, rather than subducting slab-derived or delaminated lower crust-derived melts. Analyzed granite samples yielded in situ zircon εHf (t) values (–8.3 to + 5.8) and whole-rock εNd (t) values (–4.38 to –5.69), with predominately negative values, indicating mixture sources of major recycled old components and limited juvenile crustal materials. Their parental magma was originated by partial melting of thickened lower crust, under garnet amphibolite facies conditions. Thickened crust in the southeastern CAOB yielded crustal thickness of ~54–64 km estimated from (La/Yb)N ratios of these lower crust-derived granites. These constraints, in conjunction with the E-W trending collision-related adakitic granitoids (250–243 Ma) along the southeastern CAOB, and other geological evidence, recorded a tectonic transition from this prolonged subduction to final closure of the eastern Paleo-Asian Ocean.