There is a strong genetic relationship between the petrogenesis of I-type granitoids and the evolution of continental crust in orogenic belts. This study of I-type granitoids in the East Kunlun orogen, Northern Tibetan Plateau, shows that reworking of old continental lithosphere is an important key to this genetic relationship. The East Kunlun has numerous Triassic granitic plutons that are related to subduction of the Palaeo-Tethyan ocean and terrane collision in the early Mesozoic. U–Pb analysis of zircons from these Triassic granitoids indicates that the granitic magmatism lasted from 249 to 223 Ma. Based on elemental and isotopic compositions and their petrogenesis, the magmatism can be divided into three groups. (1) Group 1 consists of quartz diorites and granodiorites (241–249 Ma), which are metaluminous high-K calc-alkaline I-type granitoids and exhibit typical subduction-related chemical characteristics. They were derived from lower crust mainly composed of Precambrian metabasaltic basement rocks with different degrees of involvement of mantle material. (2) Group 2 consists of granitic porphyries and syenogranites (231–238 Ma), which are high Rb/Sr, metaluminous to weakly peraluminous high-K alkali-calcic I-type granitoids, showing characteristics of typical pure crustal-derived granitoids. They were derived from partial melting of a Mesoproterozoic metagreywacke source in the lower crust. (3) Group 3 consists of porphyry granodiorites ( c . 223 Ma), which are metaluminous high-K calc-alkaline I-type granitoids and exhibit the typical geochemical characteristics of adakites (e.g. high La/Yb and Sr/Y ratios and low Y and Yb contents). Their high K ₂ O and low Mg# with evolved Sr–Nd–Hf isotopic compositions indicate that they were most probably derived from thickened mafic lower continental crust, which underwent partial melting induced by underplated hot mafic magma. Combining the present work with previous studies, we propose that the subduction of the Palaeo-Tethyan ocean lasted from 278 to 241 Ma, and the collision between the Bayan Har terrane and the East Kunlun occurred at 231–238 Ma, whereas the group 3 granitoids most probably formed in a post-collisional setting. Overall, all the studied I-type granitoids were derived from partial melting of old continental lower crust with minor addition of lithospheric mantle material; thus reworking of old continental lithosphere is an important mechanism for the evolution of orogenic crust. Supplementary materials: Analytical methods, zircon U–Pb data, geochemical data, and Sr–Nd–Hf isotope data for the granitoids are available at www.geolsoc.org.uk/SUP18758 .

Numerous calc-alkaline granitoid intrusions in the eastern Kunlun Orogen provide a valuable opportunity to constrain the evolution of the orogen. The age and genesis of these intrusions, however, remain poorly understood. The granitoid intrusions near the Balong region, eastern Kunlun Orogen, consist of granodiorite, diorite and syenogranite. The granodiorite contains crystallized segregations, abundant mafic microgranular enclaves (MMEs) and small quartz diorite stocks. In situ zircon U–Pb dating reveals that the granodiorites and quartz diorites were emplaced between 263 and 241 Ma, whereas the syenogranite was produced at c. 231 Ma. The granodiorite and quartz diorite have a calc-alkaline affinity and are metaluminous and Na-rich, with slightly enriched Sr–Nd isotope compositions. The granodiorite is characterized by fractionated REE patterns, whereas the quartz diorite displays a relatively flat REE pattern. The MMEs are consistent with the granodiorite in terms of incompatible elements and Sr–Nd isotope composition. Compared to the granodiorite and diorite, the syenogranite has higher SiO₂, K, Rb, Th and Sr contents and a lower Rb/Sr ratio. The results presented here, when combined with regional geological data, indicate that the granodiorite and quartz diorite were derived from dehydration melting of mafic lower crustal rocks during the N-directed subduction of the Anyemaqen ocean lithosphere in Late Permian–Middle Triassic times, whereas the syenogranite was produced at a higher crustal level in a syn-collisional setting compared to the granodiorite.

The early Permian Xiaomiao mafic dike swarm in the East Kunlun orogenic belt (EKOB) provides an excellent opportunity to study the petrogenesis of such swarms developed in supra-subduction zone environments, and to investigate the early plate tectonic history of the Palaeo-Tethyan Ocean. Hornblende ⁴⁰Ar–³⁹Ar dating results indicate that the mafic dikes formed in the early Permian (277.76 ± 2.72 Ma). The Xiaomiao mafic hypabyssals have the following compositional range: SiO₂ = 46.55–55.75%, MgO = 2.80–7.38%, Mg^# = 36–61, and (Na₂O + K₂O) = 2.87–4.95%. Chemically, they display calc-alkali affinities, ranging in composition from gabbro to gabbroic diorite. All analysed dikes are enriched in light rare earth elements and large-ion lithophile elements (e.g. Rb and Ba), but are depleted in heavy rare earth elements and high field strength elements (e.g. Nb, Ta, and Ti). Their I_Sr and ϵ_Nd(t) values range from 0.707 to 0.715 and –2.60 to +2.91, respectively. They are geochemically similar to subduction-related basaltic rocks (e.g. island arc basalt), but differ from E-MORB and N-MORB. Petrographic and major element data reveal that fractional crystallizations of clinopyroxene, olivine, hornblende, and Fe–Ti oxides may have occurred during magma evolution, but that crustal contamination was minor. Based on geochemical and Sr–Nd isotopic bulk-rock compositions, we suggest that the mafic dikes were likely generated by 10–20% partial melting of a spinel + minor garnet lherzolite mantle source metasomatized by subducted, slab-derived fluids, and minor sediments. Based on our results, we propose that the early evolution of the Palaeo-Tethyan Ocean involved the spreading and initial subduction of the Carboniferous to early Permian ocean basin followed by late Permian subduction, which generated the magmatic arc.