Petrogenesis of volcanic and intrusive rocks of the Zhuanqiao stage, Luzong Basin, Yangtze metallogenic belt, east China: implications for ore deposition

Abstract

The Mesozoic Luzong volcanic basin is located in the Lower Yangtze River fault-depression, along the northern margin of the Yangtze Block. Volcanic and plutonic rocks are widely distributed in the basin and are spatially related to Cu, Fe, Au, Pb, and Zn ore deposits. The magmatic rocks are dominated by an alkali-rich shoshonitic suite of volcanics and intrusives, including both a trachybasalt-basaltic trachyandesite-trachyte series and a diorite-monzonite-syenite-alkali feldspar granite series. Volcanic sequences within the Luzong Basin are subdivided into four stages, namely the Longmenyuan, Zhuanqiao, Shuangmiao, and Fushan stages with magmatism occurring between 136 and 124 Ma. Most mineral deposits were formed during the Zhuanqiao stage from about 134 to 131 Ma, during a longer (140–130 Ma) period of transition from compression to extension within the Luzong Basin. Major element concentrations in the Zhuanqiao volcanic rocks range from 53.97 to 62.59 wt.% SiO2, 0.57–0.94 wt.% TiO2, 3.52–6.07 wt.% Na2O, and 3.60–8.66 wt.% K2O. Combined K2O + Na2O totals lie between 7.87 and 12.49 wt.%, and K2O + Na2O/Al2O3 ranges between 0.47 and 0.71. In comparison, the Zhuanqiao intrusions have SiO2 concentrations between 52.19 and 67.70 wt.%, TiO2 between 0.26 and 1.13 wt.%, Na2O between 1.33 and 6.55 wt.%, and K2O between 2.15 and 8.25 wt.%, with K2O + Na2O values ranging from 5.92 to 12.39 wt.% and K2O + Na2O/Al2O3 ratios between 0.37 and 0.75. The Zhuanqiao volcanic rocks have an average ∑REE (rare earth element) content of 223.17 ppm, whereas the average ∑REE of the Zhuanqiao intrusives is 250.06 ppm. Weak negative Eu anomalies (Eu/Eu* between 0.69 and 1.00) characterize the volcanics, whereas a larger range with more significant negative Eu anomalies (Eu/Eu* values of 0.35–0.94) is present in the plutonics. The Zhuanqiao volcanic rocks and plutons have (Nb/Th)PM ratios of 0.05–0.14 and 0.02–0.14 and (La/Sm)PM of 3.33–4.95 and 3.36–7.29, respectively. Initial 87Sr/86Sr and 143Nd/144Nd ratios and ϵSr(t) and ϵNd(t) of the Zhuanqiao volcanic rocks are 0.7059–0.7067, 0.51194–0.51218, 22.0–33.3, and −10.2 to −5.5, respectively; those of the intrusives are 0.7060–0.7082 and 0.51203–0.51214 and 23.3–54.7 and −8.6 to −6.4, respectively. The geochemistry presented here suggests that both volcanic and intrusive Zhuanqiao stage magmatic rocks were cogenetic and were probably derived from an enriched mantle (EM) source, most probably metasomatized mantle relating to the EM type-I (EM-I). The Zhuanqiao stage magmatism apparently was controlled by differentiation and assimilation of crustal material in a high-level magma chamber. Formation of the Zhuanqiao stage magmatic rocks coincided with the formation of much of the Cu and Fe mineralization within the Luzong Basin. This coeval magmatism and mineralization during the Zhuanqiao stage can be subdivided into three differing mineral deposit affinities: magmatism associated with magmato-hydrothermal vein Cu (e.g. the Jingbian, Shimenan, and Chuanshandong deposits), porphyry-hosted Fe (e.g. the Luohe and Nihe deposits), and skarn Fe (e.g. the Longqiao deposit). Combining geological characteristics with variations in SiO2, K2O + Na2O, and MgO enabled the division of the Zhuanqiao stage magmatism in the Luzong Basin into geochemically distinct units associated with these differing styles of mineralization.