Geochronology and geochemistry of Early Silurian felsic volcanic rocks in the Dabaoshan ore district, South China: Implications for the petrogenesis and geodynamic setting

摘要

The Early Paleozoic orogeny in South China has remained controversial in the long run with debate over intracontinental orogeny versus plate subduction. Limited studies have focused on the Early Paleozoic volcanic rocks in South China, which is crucial for us to understand the tectonic evolution of the South China Block. The Dabaoshan Fe-Cu-Pb-Zn-Mo-W orefield, one of the largest polymetallic deposits in South China, is hosted by Paleozoic volcanic-sedimentary rocks and Mesozoic granitic porphyry. In this study, zircon U-Pb dating yielded ages of 439.4 +/- 2.7 Ma, 439.3 +/- 3.2 Ma, and 442.0 +/- 2.2 Ma for the Dabaoshan dacite. These ages are all in good agreement within errors and indicate that the dacite formed during the early Silurian (ca. 440 Ma). The dacites are slightly peraluminous, calc-alkaline with relatively high K2O + Na2O, FeOt, TiO2, CaO/Na2O ratios, and small negative Eu anomalies. They show enrichment in LILEs and LREE and depletion in Ba, Nb, Sr, P, and Ti. It has high initial Sr-87/Sr-86 values (0.70983-0.71140) and negative epsilon(Nd)(t) values ranging from -7.6 to -10 with T-2DM ages of 1.80-1.99 Ga, epsilon(Hf)(t) values from -4.95 to -15.48 with T-2DM ages of 1.72 Ga to 2.38 Ga. The overlapped Nd-Hf two-stage model ages range from 1.89 Ga to 1.99 Ga, which indicates that the Dabaoshan dacite originated from a Paleoproterozoic source. Combining these geochemical and in situ zircon Hf isotopic data, we propose that the Dabaoshan dacite may have originated from the partial melting of mixed source of Paleoproterozoic metasedimentary and metaigneous rocks as a result of underplating of mafic magma during the post-orogenic collapse and extension followed by lithospheric thining and asthenospheric mantle upwelling in an intraplate setting. Based on field observation and the geochronology results, we propose that the stratiform Cu-Pb-Zn mineralization may be related with the early Silurian volcanic rocks.