Geochemistry and zircon U-Pb-Hf isotopes of Paleozoic granitoids along the Solonker suture zone in Inner Mongolia, China: Constraints on bidirectional subduction and closure timing of the Paleo-Asian Ocean

The closure of the Paleo-Asian Ocean resulted in formation of the Central Asian Orogenic Belt (CAOB) accompanied by considerable Phanerozoic juvenile crustal growth. As an international frontier topic, whether the Solonker suture zone is the site of final closure of the Paleo-Asian Ocean and records the terminal evolution of the CAOB in Inner Mongolia is still in controversial. This study presents the geochemistry and zircon U-Pb-Hf isotopes of granitoids along the Solonker suture zone to constrain the subduction and closure processes of the Paleo-Asian Ocean. Granitoids from the Xilinhot area in the northern accretionary zone comprise diorites and granodiorites with LA-ICPMS zircon U-Pb ages of 316 ± 2 Ma, 302 ± 2 Ma, 285 ± 2 Ma and 243 ± 4 Ma, positive εHf(t) values of +2.39 to +13.31 and TDM2 model ages of 481–1191 Ma. The ca. 316–285 Ma granitoids were formed by fractional crystallization of regional coeval calc-alkaline basaltic magma in an arc-related setting and the ca. 243 Ma granitoid was generated mainly from partial melting of dominant basaltic source rocks and minor metagraywackes in the northward subduction-related setting. The granitoids from the Linxi area in the southern accretionary zone include muscovite monzogranites and monzogranites with LA-ICPMS zircon U-Pb ages of 525 ± 6 Ma, 274 ± 2 Ma and 261 ± 4 Ma. The ca. 525 Ma muscovite monzogranites have negative zircon εHf(t) values of −19.36 to −13.61 with TDM2 model ages of 2348–2711 Ma. They were generated from partial melting of Neoarchean to Paleoproterozoic supracrustal materials. The ca. 274–261 Ma monzogranites show negative zircon εHf(t) values ranging from −10.48 to −0.66 with TDM2 model ages of 1335–1942 Ma. They were derived from remelting of Paleoproterozoic to Mesoproterozoic supracrustal materials in the southward subduction zone along the northern active margin of the North China Craton. Combining our results with existing data, we infer that the early Paleozoic southward subduction of the Paleo-Asian Ocean started before ca. 525 Ma, and the northward subduction zone lasted to ca. 243 Ma, which implies the bidirectional subduction of the ocean lasted at least until ca. 243 Ma. Our work suggests that along the Solonker suture zone, the regional amalgamation may have occurred after ca. 243 Ma, which closed the Paleo-Asian Ocean.