Early Cretaceous gabbro–granite complex from central Inner Mongolia: Insights into initial rifting and crust–mantle interaction in the northern China–Mongolia basin–range tract

Abstract Coeval high-K calc-alkaline to shoshonitic bimodal igneous suites hold a key to monitoring important geodynamic processes and crust–mantle interactions within post-collisional to within-plate extensional settings. Here we report a unique early Cretaceous gabbro–granite complex from the poorly exposed Erenhot basin region of central Inner Mongolia, eastern Central Asian Orogenic Belt (CAOB). The mafic suite includes small syenogabbro to monzodiorite stocks, while the felsic suite mainly comprises syenogranites with minor mafic to intermediate microgranular enclaves. The former exhibits a transitional character from alkali-calcic to alkalic, notable enrichment in large ion lithophile elements (LILEs) and weak to moderate depletion in high field strength elements (HFSEs), and possesses low initial 87 Sr/ 86 Sr ratios (0.70486–0.70537), positive e Nd (t) (+2.1 − +2.6) and zircon e Hf (t) (+2.3 − +9.0) values. These features suggest its derivation from low-degree melting of amphibole-bearing lherzolites in the lithospheric mantle that had been metasomatized by subducting slab-derived fluids. The host granites are magnesian, alkali-calcic and peraluminous. They show typical trace elemental features of arc-related felsic melts, with initial 87 Sr/ 86 Sr ratios of 0.70499 to 0.70593, e Nd (t) of −0.1 to +1.4, zircon e Hf (t) of +4.8 to +8.3 and δ 18 O values from +4.53 to +6.60‰. These elemental and isotopic traits are consistent with partial melts of a mixed protolith composed of newly-underplated basaltic materials and extant older lower crust. The monzonitic enclaves are alkalic, highly enriched in LILEs and depleted in HFSEs, with variable whole-rock Sr Nd (I Sr (t) = 0.70216–0.704678, e Nd (t) = +0.2 − +1.8) and zircon Hf (e Hf (t) = +0.9 − +7.1) isotopic values. These transitional compositions are compatible with hybridization processes between mantle-derived mafic magma and crust-derived felsic magma. Given its temporal coincidence with initial rifting in the regional basin formation and the closure of Mongol–Okhotsk ocean, Early Cretaceous Erenhot gabbro–granite complex could not only serve as spatial markers for relating magmatic underplating with basin formation, but also catch a unique snapshot into crust–mantle interactions within a post-orogenic extensional regime probably underpinned by lithospheric dripping and gravitational collapse after the Mongol–Okhotsk suturing.