Abstract: The Gupo high-Mg andesite(HMA)is newly recognized in the Guanzi town ophiolite located in the Shangdan suture zone, Qinling Orogen.The Gupo high-Mg andestieis mainly composed of basaltic andesite, andesite, and pyroxene quartz diorite, which has relatively high contents of MgO(6.89%~8.99%), Mg^#61.29~67.05), SiO₂(54.27%~58.47%), and low contents of TiO₂(0.27%~0.99%), K₂O(0.09%~2.32%), Al₂O₃(12.09%~18.38%), and TFeO/MgO(0.88~1.13).The contents of REE are also low(16.59×10^-6~74.5×10^-6).The distribution pattern of REE is flat and slightly right type(La_N/Yb_N=1.03~4.93)without obvious abnormity of Eu.In addition, the rocks are enriched with large ion lithophile element such as Cs, Rb, Ba, K, and U, depleted of high field-strength element such as Nb, Ta, and Hf with low value of Ti/V.All these geochemical characteristics are identical to the typical high-Mg andesite, and are similar to the Sanukite from the Setouchi arc volcanic belt in the SW Japan.The subducted oceanic crust and the deep sediments may produce the aquiferous fluid from melted subducted oceanic crust and the deep sediments may react with the overlying mantle peridotite to form the metamorphic basalt-andesite.LA-ICP-MS U-Pb zircon date obtained from the gabbro-diorite coexisting with the Gupo metamorphic basaltic andesite indicates that the high-Mg andesitic magmatism event happened at 514±4.5 Ma.The Guanzi town high-Mg andesites was probably formed from the fore-arc tectonic environment because of the oceanic crust subducted in the Middle Cambrian.This study provides the evidence of the Shangdan oceanic subduction in the Middle Cambrian.