GEOCHEMICAL CHARACTERISTICS OF THE EARLY JURASSIC DACITES OF THE XINGLONGGOU FORMATION IN BEIPIAO AREA, WEST LIAONING PROVINCE

Abstract

The early Jurassic dacites of the Xinglonggou Formation in Beipiao area of West Liaoning Province have similar geochemical characteristics of adakite, which is generated from partial melting of young oceanic crust. Dacites are high SiO2, (≥63.93% ) high Al2O3, (≥15.40%) high Na2O (≥3.65% ) high MgO (≥2.32%, 0.48 ~ 0.61 in Mg#) lavas characterizes by high trace element contents Cr>119ppm, Ni>75ppm, Sr >463ppm) and low heavry rare earth element contents (Yb <1.70ppm, Y <17ppm), and high La/Yb (>18) high Sr/Y (>34) low Rb/Sr (≤0.31) ratios, and by strongly fractionated LREE and negligible negative Eu anomalies (Eu/Eu* =0.80 ~ 0.87). Their Nd-Sr-Pb isotopic compositions (143Nd/144 Nd = 0.512414 ~ 0.512502, εND,(t) = -2.10 ~ -0.38, TDM = 0.89 ~ 1.02Ga, 87Sr/86 Sr = 0.7073 ~ 0.7075, εSr(t) = 11.16 ~ 13.78, 206Pb/204Pb = 18.28 ~ 18.40, 207Pb/204Pb = 15.42 ~ 15.53, 208Pb/ 204Pb = 38.08 ~ 38.27) differ from the Archeozoic lithosphere mantle and the middle Jurassic, later Jurassic and the early Cretaceous volcanic rocks of the North China Craton. The Author thinks that the magmas were generated from partial melting of subducted oceanic slab of the Paleo-Asian Ocean and interacted with the Archeozoic lithosphere mantle during the ascending magmas, implying that the subduction of the Paleo-Asian Ocean beneath the North China Craton in Paleozoic might took placed.