Zircon U‐Pb geochronology and geochemistry of the metabasite and gabbro: Implications for the Neoproterozoic and Paleozoic tectonic settings of the Qinzhou Bay‐Hangzhou Bay suture zone, South China

Abstract

The Qinzhou Bay‐Hangzhou Bay Suture Zone (QHSZ) lies between the northwestern Yangtze Block and the southeastern Cathaysia Block in South China. An integrated zircon U‐Pb age, whole‐rock and mineral geochemical analyses have been enabled for understanding the Neoproterozoic and early Paleozoic geological evolution of the QHSZ. Zircon U‐Pb dating reveals that the Guizi metabasite and Zhuya gabbro were formed at 948 ± 11 Ma and 450.8 ± 2.9 Ma, respectively. The Guizi metabasites are characterized by Nb enrichments (8.18–16.11 ppm) with high Nb/La (0.76–0.98) and Nb/U (17.86–25.65) ratios. The samples are also LREE/HREE ([La/Yb]N = 2.66–3.29) and LILEs (Rb, Th, U and Pb) enriched with weakly depleted HFSEs (Nb, P and Ti), resembling the Nb‐enriched basalts. These geochemical signatures were likely to be originated from a newly subducted slab. Elemental contrasts illustrate that the Guizi metabasites are geochemically consistent with the Neoproterozoic Nb‐enriched basalts along the Wuyi‐Yunkai Domain, South China. Age and elemental characteristics of the Guizi samples suggest an earliest Neoproterozoic subduction within the QHSZ. The Zhuya gabbro samples have high contents of MgO (11.23–14.48%), Cr (520.8–782.7 ppm) and Ni (126.1–203.7 ppm), (La/Yb)N = 1.89–2.41, Eu/Eu* = 0.84–0.90, positive LILEs (Rb, Th, Sr and Pb) anomalies and negative HFSEs (Nb, P and Ti) anomalies. Clinopyroxene geochemical data show a significant arc cumulates trend. They were considered to be derived from an E‐MORB‐like source modified by subduction‐related fluids. Geochemical modelling indicates that a mixture of the Neoproterozoic mafic magma and SCB sediments (20–40%) can match the elemental and Sm‐Nd isotopic compositions of the Zhuya gabbros, which suggests the involvement of Neoproterozoic subduction‐modified mantle. Combined with regional geological and geochemical data, it is concluded that the Yangtze and Cathaysia blocks were likely sutured along the QHSZ forming an earliest Neoproterozoic arc‐back‐arc system, and the early Paleozoic mafic rocks are dominantly remelting products of the Neoproterozoic subduction modified mantle.