Rb-Sr dating and geochemistry of andesitic-rhyolitic volcanics in the Zengcheng Geopark, Guangzhou, Guangdong Province

A large number of Yanshanian andesite and rhyolite rocks are developed in the Zengcheng Geopark in Guangzhou. Due to the lack of detailed petrogeochemical studies, their genesis and the tectonic significance have not been clarified. Therefore, a more systematic analysis was conducted for the whole-rock geochemistry and isotopic geochemistry of these volcanic rocks. The results show that the andesitic rocks are somewhat analogous to the Andes-type volcanic ones, with obviously depleted Nb, Ta, Sr and Ti and low I_sr values (0.70332-0.7144, averaging 0.7092), lower rare earth ΣREE(ΣREE=158.9×10^-6-215.010^-6, averaging 186.8×10^-6), with obvious LREE and HREE differentiation((La/Yb)_N5.06-9.87, averaging7.01)and negative Eu anomalies (δEu=0.80-1.38, averaging 0.94) in the chondrite-normalized REE distribution patterns. The rhyolitic rocks have high potassium contents with significant negative Ba, Sr, P, Eu, Ti anomalies and positive Pb and Yb anomalies. They are characterized by the I_sr values ranging from 0.71393 to 0.73650(averaging 0.72615), lower rare earth ΣREE(ΣREE=93.410^-6-481.510^-6, averaging 285.710^-6)with obvious LREE and HREE fractionation((La/Yb)_N 0.65~9.51, averaging 4.35) and negative Eu anomalies (δEu=0.01-0.03, averaging 0.02) in the chondrite-normalized REE distribution patterns. The whole rock Rb-Sr Isotopic age is 112 ±12 ma. The integrated geochemical studies demonstrate that both the andesitic and rhyolitic magmas are of crust-mantle mixing origin, of which the andesitic magma was mainly originated from the mantle and the rhyolitic one mostly from the crust. The andesite rocks and rhyolitic volcanics were formed under the tectonic background of Pacific plate subduction and collision in early Jurassic and early Cretaceous respectively.