Construction of an island arc and back-arc basin system in eastern Central Asian Orogenic belt: Insights from contrasting Late Carboniferous intermediate intrusions in Central Inner Mongolia, North China

Abstract

Characterizing island arc and back arc basin system through detailed magmatic profile remains a pivotal task in any regional paleotectonic and supercontinental reconstructions along ancient convergent plate margins, as is of paramount importance for the northern China–southern Mongolia (NCSM) tract along the eastern Central Asian Orogenic belt (CAOB). Here we document episodic Late Carboniferous intermediate to felsic intrusive suites in the West Ujimqin region of central Inner Mongolia, spanning from ca. 315 Ma granodiorites through ca. 310 Ma hornblende diorites to ca. 305–301 Ma diorites. The earlier granodiorites exhibit dominant magnesian-tholeiitic affinity, enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), and possess whole-rock ISr(t) ratios of 0.707387–0.708799, negative ƐNd(t) of −2.3 to −2.4 and zircon ƐHf(t) values of −0.69 to +4.87 and δ18O of +6.33 to +8.40‰. These features are consistent with the magmatic derivation from dehydration melting of tholeiitic mafic to intermediate lower crustal underplates with obvious high δ18O supracrustal inputs. The ca. 310 Ma pod-like hornblende diorites display high-Ca boninite-like elemental traits and MORB-like isotopic compositions (ISr(t) = 0.704190 to 0.704203, ƐNd(t) = +7.9 to +8.3, zircon ƐHf(t) = +14.7 to +17.5 and δ18O = +4.88 to +6.16‰), indicative of a prior melt-depleted then fluid-enriched mantle source. By contrast, subsequent (301–305 Ma) diorites show arc-like elemental hallmarks, with notable enriched LILEs and depleted HFSEs as well as variable isotopic values (ISr(t) = 0.703690–0.704065, ƐNd(t) = +3.5 to +8.4, zircon ƐHf(t) = +7.10 to +16.4 and δ18O = +3.75 to +6.83‰). They are interpreted to represent the evolved products of hornblende-dominated fractional crystallization from depleted basaltic parental magmas. Given their coincidence with regional back-arc-basin type ophiolitic stratigraphy and detrital zircon age spectra, these intermediate to felsic intrusive suites tend to document the timeline of a back arc basin formation from initial rifting to maturation. In conjunction with Early Carboniferous forearc-type ophiolitic complexes and forearc magmatic records at West Ujimqin and the adjacent region, a Carboniferous island arc and back arc system can be reconstructed from Xilinhot to West Ujimqin. The characterization of this northward (present day coordinates) subduction system in terms of its temporal evolution not only provides overwhelming evidence that modern Pacific-like, archipelago-type arc configuration had prevailed in central Inner Mongolia during the Carboniferous, but also offers vivid insights into the diversity of crustal generation mechanism in a subduction factory that ushered in seminal continental crustal construction in the eastern CAOB.