Neoproterozoic metavolcanic suites in the Micangshan terrane and their implications for the tectonic evolution of the NW Yangtze block, South China

Abstract

The Micangshan terrane is located along the NW margin of the Yangtze block and contains the second oldest Precambrian basement in South China. It comprises the Beiba and the Wangcang-Nanjiang segments, while the oldest suites (∼2.1 Ga Houhe complex) occur in the Beiba. The Hekou and the Wangjiaping metamorphic volcano-sedimentary sequences in the Wangcang are characterized by intensive metamorphism and deformation, long being regarded as the equivalent of the Houhe suite. This contribution presents an integrated study of zircon U-Pb geochronology and Hf isotope, whole rock elemental and Sr-Nd isotope geochemistry on volcanic rocks of the metastrata. It reveals that they formed at ∼ 865–860 Ma and underwent upper amphibolite facies metamorphism at ∼ 815 Ma with lower limits in P-T conditions of ∼ 5.0 kbar and ∼ 650–660 °C. The Hekou metavolcanics comprise dacitic-rhyolitic and alkaline basaltic rocks with minor tholeiitic mafics, whereas those of the Wangjiaping are composed of dacitic and tholeiitic mafic rocks. The Hekou alkaline metamafics show affinity to high-Nb to Nb-enriched basalts (HNB-NEB) with εNd(t) values of + 5.80 to + 6.25, whereas tholeiitic metamafics of the both sequences display characteristics typifying high-Al basalt (HAB) with εNd(t) value ∼+2.66. The Hekou and the Wangjiaping metafelsics have εNd(t) ranges of −2.33 to + 5.60 and −0.82 to + 0.99 (except one + 2.29), respectively, and the dacitic rocks show features akin to the high Mg-number andesites. The ∼ 865–860 Ma magmatism is suggested to have occurred in an extensional basin with a post-subduction affinity. It was induced by upwelling of convective asthenosphere succeed the ceased slab subduction: the Wangjiaping metafelsics were derived from partial melting of juvenile lower crust, the Hekou dacitic rocks were produced by partial melts of the relict of subducted oceanic crust interacted with mantle wedge; the Hekou HNB-BEB were sourced by metasomatized mantle wedge, whereas the HAB lavas were generated by anhydrous partial melts of the upwelling asthenosphere. The ∼ 815 Ma metamorphism was caused by an amalgamation between the South Qinling belt and the proto margin of the northwestern Yangtze block. This event was followed by an expanding rift from the Micangshan successively to the South Qinling.