Early Paleozoic Subduction in Cathaysia (Ⅱ):New Evidence from the Dashuang High Magnesian-Magnesian Andesite

Peng Songbai; Liu Songfeng; Lin Musen; Wu Changfeng; Han Qingsen

Abstract：The nature of early Paleozoic Caledonian orogeny (Guangxi movement),as one of the key geolocial problems of tec-tonic evolution in South China,has been controversial for a long time.In this study we report a suite of early Paleozoic basaltic andesite-andesite volcanic rocks located at the Dashuang in the Cenxi area of southeastern Guangxi in South China.Petrogeo-chemical data of the basaltic andesite-andesite show the contents of SiO2 =47.8%-58.8%,Al2 O3 = 13.35%- 14.25%, CaO=3.79%-5.61% and TiO2 =0.46%-0.56%.It also shows high contents of MgO=4.39%-9.96%,Ni= 141 × 10-6 -210×10-6 and Cr=427× 10-6 - 750× 10-6 ,indicating that they belong to high magnesian (HM)-magnesian (M) basaltic andesite-andesite from calc-alkaline to transitional series.The volcanic rocks exhibit enriched-LREE,weak negativeδEu abnormally (0.73-0.92),(La/Yb)N=3.4-7.9,(La/Sm)N=3.15-4.95.Basaltic andesite-andesite is characterized by pronounced enrichment of LILEs (e.g.Rb,Ba,Th,U,K,Pb),relative depletion of HFSEs (e.g.Nb,Ta,Ti),and low Ti/V=14.7-21.8.All these show typical characteristics of high magnesian (HM)-magnesian (M)basaltic andesite-andesite (sanukite).The LA-ICP-MS zircon U-Pb age of the magnesian andesite is 443±4 Ma,showing that the formation ages of the volcanic rock series should be between the Late Ordovician and Early Silurian epoch.In conclusion,we suggest that the Dashuang volcanic rocks are the suite of high magnesian (HM)-magnesian (M)basaltic andesite-andesite series formed in the forearc tectonic environment in Late Ordovician-Early Silurian epoch,which provides key evidence for Early Paleozoic oceanic crust subduction between Yangtze Craton and Cathaysia block along Qinzhou-Hangzhou tectonic suture belt in South China.