PETROCHEMICAL CHARACTERISTICS OF VOLCANIC ROCKS IN GUANGXI AND THEIR RELATION TO GEOLOGICAL STRUCTURE

In this paper an attempt is made to analyse and approach the problem of the structural settings during the formation of volcanic rocks and their control on ore deposition through a preliminary study of the petrochemical composition of volcanic rocks of various ages in Guangxi. According to the magmatic formation and petrochemical characteristics, it is suggested that the Proterozoic ophiolite suite distributed widely in Jinwandashan and Longsheng, northern Guangxi, is the product of an ancient convergent plate. With the geological development and the accretion of continental crust, the subduction zone migrated eastwards gradually. At that time the proposed "Jiangnan ancient island arc" might be only an island chain within Guangxi. The late Paleozoic volcanic rocks in western Guangxi were controlled by rift-type fractures and transformed gradually from continental crust to oceanic crust. Obduction occurred locally in Early Triassic time. The Late Cretaceous volcanic rocks in southeastern Guangxi originated in the inland down-faulted zone, and the magma was contaminated to some extent by sialic materials from the upper layer of the earth’s crust. The Tertiary volcanic rocks on offshore islands belong to oceanic alkaline olivine basalt. They formed in a stable tectonic setting of marginal sea, being the product of the circum-Pacific volcanic activity. The author proposes diagrams of alkalinity vs. sodic coefficient and alkalinity vs. basicity, which are used to discriminate volcanic rocks associated with copper and iron deposits in Guangxi. The diagrams indicate that the Banxi-aged and late Sinian volcanic rocks are favourable for the formation of iron deposits, that the Late Cretaceous volcanic rocks are favourable for the formation of iron, copper and pyrite deposits and that the Triassic volcanic rocks show a good prospect for copper, lead, zinc and polymetallic ore deposits.