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

Abstract

The present metamorphic crystalline basement of the South China Block was formed largely as a result of the early Paleozoic (∼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 metamorphism at ∼427 Ma, within the time span of Paleozoic orogeny. Most zircon Lu-Hf and O isotopic results display relatively uniform compositions, with δ 18 O values scattering around +9‰ and the calculated ε Hf (t) values of most metamorphic zircons ranging from +9.8 to +15.1. The 143 Nd/ 144 Nd and 147 Sm/ 144 Nd ratios of the three samples are 0.513075-0.513103 and 0.20508-0.205832, respectively. The ε 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 ∼540 Ma, older than the previous result of ∼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.