Early Carboniferous association of mafic dike and A-type granite in north-central Inner Mongolia, North China: Insights into initial backarc rifting and backarc basin tempo

Abstract Coeval bimodal igneous suites can feature various extensional geodynamic regimes throughout plate tectonic Wilson cycles. Their characterization holds a key to chronicling pivotal orogenic tempos and tracing attendant crust/mantle interactions along deep-time convergent plate margins. Here we aim to document the time cycle of critical backarc basins in the eastern Central Asian Orogenic Belt (CAOB) by unravelling a unique early Carboniferous diabase-granite suite from the Wulagai region of north-central Inner Mongolia. The ca. 354 Ma diabases range in SiO2 from 49.9% to 53.7%, exhibiting magnesian and high-K alkaline affinities, moderate enrichment in light rare earth elements and large ion lithophile elements but variable depletion in high field strength elements. These elemental traits, plus their initial bulk 87Sr/86Sr of 0.70373–0.70402, εNd(t) values from +4.79 to +5.28, zircon εHf(t) from +7.75 to +16.3, and δ18O from 4.69‰ to 5.88‰, argue for a parental magmatic derivation from partial melting of juvenile fluid-metasomatized lithospheric mantle. By contrast, ca. 351 Ma granites vary in SiO2 from 73.5% to 77.0% and display hallmark A-type magmatic affinities, with radiogenic bulk εNd(t) of +5.74 to +6.06, zircon εHf(t) of +7.57 to +16.5, and δ18O of 5.51‰ to 6.29‰. These are typical of partial melts of newly underplated but dehydrated meta-mafic to charnockitic protoliths and a subsequent minor degree of fractional crystallization. Coinciding with the earliest regional late Paleozoic ophiolitic and arc magmatic relics, this rare early Carboniferous bimodal plutonic suite presents a unique deep-time window into crust-mantle interactions and concomitant crustal growth within an incipient backarc basin in response to slab rollback of northwardly subducted Paleo-Asian oceanic plate. Besides bookending pivotal backarc basin initiation and closure in the eastern CAOB, this case study carries further seminal implications for exemplifying the general tempo of deep-time backarc basins and a whole spectrum of crustal formation/differentiation mechanisms during the life cycle of backarc basins.