Zircon U-Pb dating and geochemistry of the volcanic rocks at Huachanggou area, Mian-Lue suture, South Qinling
南秦岭勉略带铧厂沟火山岩锆石U-Pb年代学及地球化学研究

The Huachanggou volcanic rocks in Southwest Shaanxi are mainly dacite (~90vol%) structurally interlayered with basalt tectonic lenses (~10vol%). The basalts have SiO2 contents of 43.6%~54.7%, characterized by low contents of K and Ti, and high contents of Na and Mg. Their REE concentrations range 24×10-6~29×10-6 with slightly enriched LREE and MREE, and slightly positive Eu and Sr anomalies. They show positive Rb and Ba anomalies and negative Nb and Zr anomalies, and yield (La/Yb)N, Th/Yb, Th/Nb, Nb/La and Hf/Th ratios of 1.81~2.87, 0.19~0.23, 0.11~0.2, 0.26~0.7 and 0.50~0.67, respectively, resembling the characteristics of sub-alkaline arc basalts. The dacites have SiO2 contents of 59.5%~72.3%, REE concentrations of 116×10-6~187×10-6 and negative Eu anomalies, and are rich in LILE (e.g. Rb, Ba, Th and K), but depleted in HFSE (e.g. Nb, P, Ti and Ta), similar to the volcanic rocks developed in volcanic arcs. The basalt sample yields zircon SHRIMP U-Pb age of 801.7±4.7Ma (MSWD=1.18; n=12); which is same to the zircon LA-ICP-MS U-Pb age of 802.1±5.3Ma (MSWD=1.02; n=19) obtained from the dacite. Hence, the volcanic rocks at Huachanggou area constitute a sub-alkaline basalt-to-dacite suite which suggests that there existed a southward subduction of an oceanic plate beneath the Yangtze craton in Neoproterozoic; and the Devonian sedimentary strata, together with their structurally sandwiched slices of the Neoproterozic volcanic rocks, form a tectonic mélange resulted from Late Paleozoic-Triassic suturing of the Mian-Lue Ocean.