Zircon U-Pb Dating and Hf Isotopic Compositions of Porphyrites from the Ningwu Basin and Their Geological Implications

Ningwu volcanic basin is one of the most important basins related to mineral deposits in middle-lower reaches of the Yangtze River belt. Four volcanic units occur in the basin, in an ascending order, the Longwangshan Formation, the Dawangshan Formation, the Gushan Foramtion and the Niangniangshan Formation, with some high-level sub-volcanic igneous rocks intruding them. Detailed zircon U-Pb dating and Hf isotopic composition measurement were performed on the host rocks (gabbro-diorite porphyrites) of the Washan iron deposit. The zircon U-Pb ages of three samples are (123±1) Ma, (127±3) Ma and (125±2) Ma, respectively.Together with previously published precise zircon U-Pb dating results for volcanic rocks (127 to 131Ma), these results have placed a better constraint on the magmatic and mineralization activity duration of the Ningwu volcanic basin. The zircons from the Ningwu porphyrites have eHf(t) values from -6.3 to -8.6. Together with other available geochemical data, the Hf isotope data indicate that the Ningwu porphyrites may have derived from upwelling depleted asthenospheric mantle mixed with enriched mantle and crust, which was controlled by the extensive lithosphere extension at around 125 Ma. The lower eHf(t) values and slightly younger ages of the gabbro-diorite porphyrites than their hosting volcanic rocks may indicate more crust material involvement in the porphyrite petrogenesis.