Paleoproterozoic subduction within the Yangtze Craton: Constraints from Nb-enriched mafic dikes in the Kongling complex

Abstract

Niobium-enriched mafic rocks constitute a rare suite of rocks formed in the subduction-related settings and provide important insights into mantle sources and geodynamic processes associated with plate subduction. In this contribution, we report newly-defined Nb-enriched mafic rocks from the Archean-Paleoproterozoic Kongling complex within the northern Yangtze Craton, which shed new light on the Paleoproterozoic subduction process of the craton. LA-ICP-MS zircon U-Pb dating reveals two episodes of Paleoproterozoic intermediate-mafic magmatism: (1) the ca. 2.15 Ga dioritic dikes and the younger Nb-enriched mafic dikes formed in 2.05–2.03 Ga. The ca. 2.15 Ga diorites have high Nb contents of 14.7–18.0 ppm but low Nb/LaPM (0.27–0.31), Nb/U (9.0 to 13.5) and Nb/ThPM values (0.24 to 0.31). The enriched zircon Hf isotope (ɛHf (t) = −4.8 ~ −1.1) and whole-rock Nd isotope (ɛNd (t) = −4.2 ~ −3.7) of the older diorites are similar to those of the coeval high-Mg andesites (~2.12 Ga) from the northern Huangling dome, indicate an enriched mantle source. The geochemical and isotopic data collectively suggest that they were derived from partial melting of a metasomatized sub-arc lithospheric mantle at the initial stage of subduction. The younger mafic rocks (ca. 2.05–2.03 Ga) possess high Nb contents (10.9–24.2 ppm) and lower magnitude of negative Nb anomalies with Nb/U (17.2–51.6), Nb/ThPM (0.40–0.88) and Nb/LaPM (0.49–0.97), which is different from those of normal island arc rock suites. The mafic dikes have negative zircon ɛHf (t) values (−12.6 ~ −10.5), dominantly negative ɛNd (t) values (−4.9 to −1.0, except one with +2.5), indicate an enriched mantle source. The Nb-enriched mafic rocks were likely produced by partial melting of mantle wedge that has been modified by the recycled continental sediments along the subduction zone. We infer that a slab window regime likely prevailed in the Kongling complex at ca. 2.05 Ga during the subduction termination. The opening of a slab window and asthenosphere upwelling possibly triggered partial melting of the enriched mantle wedge, and account for the generation of the younger Nb-enriched mafic dikes.