Integrated Zircon U-Pb-O-Hf and Whole-Rock Sm-Nd Studies of Paleozoic Amphibolites in the Chencai Area of the Cathaysia Block, South China

摘要

The present metamorphic crystalline basement of the South China Block was formed largely as a result of the early Paleozoic (similar to 460-410 Ma) orogeny, which affected large areas of this continental block. Paleozoic metamafic rocks (garnet amphibolites) with typical normal mid-ocean ridge basalt chemical compositions were recently identified from an uplifted lower-crustal rock assemblage in the Chencai area of the Cathaysia Block. This article focuses on the first integrated studies of secondary-ion mass spectroscopy (SIMS) zircon U-Pb dating and zircon Lu-Hf-O and whole-rock Sm-Nd isotopic analyses on these metamafic rocks, for the purpose of better constraining the ancient geodynamic processes of this orogeny. The SIMS zircon U-Pb dating results show that these mafic rocks underwent high-grade metamorphismat similar to 427 Ma, within the time span of Paleozoic orogeny. Most zircon Lu-Hf and O isotopic results display relatively uniform compositions, with delta O-18 values scattering around +9 parts per thousand and the calculated epsilon(Hf)(t) values of most metamorphic zircons ranging from +9.8 to +15.1. The Nd-143/Nd-144 and Sm-147/Nd-144 ratios of the three samples are 0.513075-0.513103 and 0.20508-0.205832, respectively. The epsilon(Nd)(t) values are high positive, ranging from +8.05 to +8.63. These ratios resemble those of basaltic rocks newly derived from a depleted-mantle source. Zircon Hf model ages are similar to 540 Ma, older than the previous result of similar to 496 Ma, suggesting that these newly formed crustal materials were likely extracted from the depleted-mantle source during the early Paleozoic. It is thus inferred from such isotopic characteristics, as well as previously published data of the metamafic rocks, that the previous notion-that a deep lithospheric fracture reaching asthenospheric mantle was absent from the Early Paleozoic South China Orogen-should be reconsidered.