广西龙胜蛇绿岩带的地质特征及构造环境

GEOLOGICAL CHARACTERSTICS AND TECTONIC ENVIRONMENTS OF THE OPHIOLITE ZONE AT LONGSHENG DISTRICT,GUANGXI

一、地质概况,广西龙胜三门地区位于江南地背斜西南端的东南缘。区内分布的地层主要有前震旦系上板溪群的合桐组(Ptbh)与拱洞组(Ptb-g)、震旦系(Z)、寒武系()(图1)。拱洞组(Ptbg):具复理石建造特征。下部是一套绢云千枚岩、板岩夹变质的砂岩、长石石英砂岩、粉砂岩和变粒岩;上部为一套绢云板岩、石英绢云板岩夹变质砂岩。复理石韵律清晰,自下而上由变质砂岩、变质粉砂岩、石英绢云板岩及绢云板岩组成。并具各种形态的层理。厚384—793米。

The ophiolite zone in Sanmen area of Longsheng district,Guangxi is tec-tonically located at the south-west terminal of the Jiangnan Geoanticline.The stratigraphic sequence in the area includes Hetong Formation and Gong-dong Formation of Upper Banxi Group of Pre-Sinian System,and also Sinian and Cambrian Systems.The spilites occur in the upper member of Hetong Formation.Some deep sea silicolites,shales,and a minor amount of carbonatites are involved in the spilites.In the area where spilites are distributed,there are a large amounts of diabases,and a minor amounts of basic-ultrabasic and ultrabasic complexes.The variation diagram of the chemical compositions of the volcanic rocks-(FeO)/MgO to SiO2 indicates that the volcanic rocks in this area are of calc-alkali series(CA)and tholeiite series(TH);thus the ophiolites belong to the I type which was divided by A.Miyashiro.In the Ti-Zr variation diagram,the dots marking the compositions of the volcanic rocks also lie in the field of island arcs.Furthermore,the contents of trace elements in the volcanic rocks are very similar to the TH series and the CA series of the island arcs.It is suggested that the ophiolites in this area were formed in ancient island arc environments.