CHARACTERISTICS AND EVOLUTION OF VARIOUS GENETIC TYPES OF GRANITE IN GUANGXI

The granitoids in Guangxi may be classified into four genetic types. 1. Mantle-derived differentiated type.Examples are given by the Bentong granite mass and other masses of Sibao age. The granite masses, small in scale, are spatially associated with spilite-keratophyre and basic-ultrabasic intrusions and are intruded into these rocks. Plagioclase granite predominates, in which SiO₂ constitutes 61-68％ and the Na₂O/K₂O ratio is between 0.69-1.30 and in which the biotite is high-Mg and the abundances of the trace elements Cr, Ni, Co and Cu are high. Part of the masses contain chromite. 2. Crustal metasomatic type. Rocks of this type are formed by partial melting of autochthonous and hypautochthonous granites due to elevated temperatures and their ascent and crystallization. Examples include the Xuefeng-age Motianling granite mass and the Caledonian Beijie granite rock mass. Being spatially in harmony with the country rocks, the rock masses display abrupt intrusive contacts or metasomatic intrusive contacts with the country rocks. There are a lot of residual country rocks at the edges of the masses, which are highly variable in size and are aligned parallel to the strike of the country rocks. Biotite（or two-mica）granite predominates, with K-feldspar predominating over plagioclase. Part of the rocks show a gneissic structure. SiO₂ accounts for more than 74％ and Na₂O/K₂O is between 0.47-0.68.The accessoryminerals include a zircon, monazite and garnet suite and rounded zircon iscommon.No non-ferrous metal or rare metal deposits are formed. 3. Crustal anatectic type. Anatectic granite in Guangxi covers 91％ of the area of granitoids, including Caledonian, Variscan, Indosinian and Yanshanian granite masses. The rock masses are large in scale and of regional zonal or areal distribution. They are not closely related to regional volcanic activity. The rocks are dominated by biotite granite, with K-feldspar predominating over plagioclase. SiO₂ constitutes over 70％,Na₂O/K₂O is mostly less than 0.7 and Al₂O₃/（Na₂O+K₂O+CaO）is mostly greater than 1.10.Biotite is low-Mg and High-Fe and the accessory minerals consist of an ilmenite, zircon, monazite and garnet suite. The abundances of the trace elements Sn, W, Li, Bi and Pb are high, forming deposits of tungsten,tin, niobium and tantalum. 4.Mixed syntectic type. Syntectic granite in Guangxi is poorly exposed, making up 7.0 of the granitoids. Part of Caledonian, Indosinian and Yanshanian granite masses are represented. The rock masses are small in scale, mostly covering an area of less than 20 sq km. They are controlled by the Bobai-Cenxi and Pingxiang-Dali deep fractures. Part of the masses are closely associated with basic and intermediate-acid volcanic rocks in time and space. The lithologies are complex, including quartz diorite, plagioclase granite, granodiorite, granite,syenite and quartz monzonite. In the rocks, plagioclase（andesine）predominates over K-feldspar, the quartz content is low, SiO₂ constitutes 55-70％,Na₂O/K₂O is between 0.70-1.65, and Al₂O₃/（Na₂O+K₂O+CaO） is mostly, less than 1.1. Biotite in the masses is high-Mg, and the accessory minerals are represented by a magnetite, zircon,apatite,rutile,sphene and orthite suite. The magnetite/ilmenite ratio is above 50∶1. The abundances of the trace elements Cu, Mo, W, Pb, Zn, Cr, Ni, Co and V in part of the masses are high, forming deposits of tungsten-molybdenum, copper-molybdenum, lead, zinc,gold and silver.