From arc accretion to within-plate extension: Geochronology and geochemistry of the Neoproterozoic magmatism on the northern margin of the Yangtze Block

Abstract

Early Neoproterozoic arc magmatism and arc accretion processes are fundamental issues in reconstructing the Rodinia supercontinent. In this study we present new data of zircon U-Pb geochronology, Hf-O isotopes and whole-rock geochemistry for the magmatic rocks (including 12 felsic volcanic rocks, 3 granodioritic intrusions, 8 basaltic extrusive rocks and 12 gabbroic intrusions), which are spatially independent, in the Dahongshan area on the northern margin of the Yangtze Block. The investigated intermediate-felsic rocks are dated at ca. 970 Ma, ca. 885–865 Ma, ca. 840–830 Ma and ca. 810–790 Ma by in situ ion probe and laser zircon U-Pb dating methods. The mafic rocks are dated at ca. 870 Ma by in situ ion probe. Combined with previous age data, our new geochronology results suggest continuous magmatic activities from ca. 970 Ma to ca. 790 Ma in the Dahongshan area with a magmatic flare-up at ca. 840 Ma. The ca. 970–840 Ma mafic rocks, as well as the contemporaneous intermediate-felsic rocks, show enrichments in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depletions in high-field strength elements (HFSEs), representing arc-related tectonic settings. Zircon grains from these rocks generally have positive εHf(t) values from +7.3 to +14.7 and slightly elevated δ18O values from +5.59 to +6.89, indicating a dominant juvenile source with minor supracrustal incorporation. In contrast, the ca. 830–790 Ma mafic rocks are characterized by E-MORB- (enriched mid-ocean ridge basalt) and OIB- (oceanic island basalt) like and the contemporaneous felsic rocks show characteristics of A2-type granites, implying a within-plate extension environment. Zircon grains from the ca. 830–790 Ma felsic rocks display broad εHf(t) values from −5.7 to +15.3 and low mantle δ18O values from +3.81 to +6.11, implying multiple magma sources for their generation and the imprinting of high-temperature hydrothermal alteration. The relative contributions of the mature crustal component in the Dahongshan magmas decreases from 10–20 % in the ca. 970–840 Ma felsic rocks to 0–10 % in the ca. 830–790 Ma felsic rocks. The Neoproterozoic magmatic activity on the northern margin of the Yangtze Block recorded an integrated evolution from accretion of previous oceanic arc (ca. 970–870 Ma) at ca. 870–840 Ma to within-plate extension (ca. 830–790 Ma) in response to the convergence and divergence of the supercontinent Rodinia.