Petrogenesis of Early Paleozoic Trachytic Volcanic Rocks in Tudiling Area, South Qinling Orogen: Evidence from Zircon U-Pb Age, Elemental and Sr-Nd Isotope Geochemistry

Volcanic rocks in the Tudiling area of the South Qinling orogen have attracted wide attention due to their enrichments of Nb and Ta. However, the research on the origin of these volcanic rocks is still poor. To delineate their petrogenesis and tectonic setting, zircon U-Pb geochronology, major trace elemental and Sr-Nd isotopic geochemistry of typical trachytic volcanic rocks in Tudiling were studied. The main rock types of the Tudiling Nb-Ta-rich volcanic rocks are aegirine trachyte, trachytic crystal tuffaceous lava, trachytic crystal ignimbrite, trachytic crystal tuff and tuffite. LA-ICP-MS zircon U-Pb dating yielded an age of 445±10 Ma, indicating the volcanic rock was formed in the Early Silurian. The volcanic rocks are characterized by high silica (55.97% to 66.71%) and high alkali (8.64% to 16.74% K2O+Na2O) contents, and belong to alkaline volcanic rocks. The light rare earth elements (LREE) are relatively enriched with (La/Yb)N ratios varying between 11.2 and 93.6. The (87Sr/86Sr)i and εNd(t) values of the volcanic rocks range from 0.7034 to 0.7057 and 2.2 to 2.7 respectively, with corresponding Nd model ages of 0.77 to 0.89 Ga. The Sr-Nd isotopic compositions of the Tudiling volcanic rocks are similar to that of oceanic island basalt (OIB), indicating that the source region may be dominated by the mantle component of HIMU, mixed with EMI and EMII components. Combined with the high Nb/U ratio (10.8 to 106.1) and the negative correlation between CaO, MgO, Fe2O3T, TiO2 and SiO2, the Tudiling ore-bearing volcanic rocks should be the products of crystallization and differentiation of enriched mantle derived magma. The Tudiling Nb-Ta-rich volcanic rocks may have formed in a passive continental margin environment of the northern margin of the South China Block in the Early Paleozoic. © 2023 Science Press. All rights reserved.