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

摘要

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/La-PM (0.27-0.31), Nb/U (9.0 to 13.5) and Nb/Th-PM values (0.24 to 0.31). The enriched zircon Hf isotope (epsilon(Hf) (t) =-4.8 similar to -1.1) and whole-rock Nd isotope (epsilon(Nd) (t) = -4.2 similar to - 3.7) of the older diorites are similar to those of the coeval high-Mg andesites (similar to 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/Th-PM (0.40-0.88) and Nb/La-PM (0.49-0.97), which is different from those of normal island arc rock suites. The mafic dikes have negative zircon epsilon(Hf) (t) values (-12.6 similar to -10.5), dominantly negative epsilon(Nd) (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.