ON THE PETROLOGY OF THE PORPHYRIES OF MALASONGDUO, EASTERN XIZANG, CHINA, WITH SPECIAL REFERENCE TO THE RELATIONSHIP BETWEEN MAGMATISM AND MINERALIZATION
on the basis of petrological study of Malasongduo plutons as well as others in the same Yulong belt of copper ore of the porphyry type, the following conclusions are derived from the discussion of the relationship between magmatism and copper mineralization. In this area, the copper mineralization is related to several rock types such as monzogranite, granite and alkali-granite porphyries, which are ore-bearing or free from ore, and were emplaced from Eocene to Oli-gocene. These porphyries may belong to mantle granites slightly hybridized by crustal material, and two of them, i.e. plutons A and K in Malasongduo, seem to be transitional granite varieties from andesitc suite of mantle granites to calc-alkali granite suite of crust granites but closer to the former, as compared to the geochemical types of granites classified by Л. B. TaycoH (1974, 1978, and 1979) . It is considered that the copper mineralization in this area might have been controlled by movement of materials in upper mantle and by the global tectonic activities during the Himalayan movement. A kind of primary magma is thought to have been originated by partial melting of astheno-sphere, and after the separation of gabbroial layer of subcrust, the ore-bearing porphyries might have finally been emplaced as a result of evolution of the residual magma. An inference is made that a copper-rich horizon may exist in the depths from 140 to 280 KM in the upper mantle so as to explain why the primary magmas in connection with copper-nickel sulf ides bearing plutons and copper bearing intermediate to acidic ones may be originated from similar depths as determined by K2O/SiO2and Rb/Si02 ratios. The main factors which have influence on copper content of a porphyry pluton are (1) depth of the porphyry magma originated, (2) the chemical compositions of the primary magma and the magmatic fluid (which may decide the amount of copper transported) ,(3) the amount of copper enriched during the evolution of magma and the activity of the fluid, (4) the degree of contamination of the deep source magma by the crustal material and (5) the extent of post - magmatic potassium replacement. The method of multi-variable progressive regression analysis has been used to investigate the relations between copper and the main oxides, and the mathematical equations have been established, by means of which the content of copper in a porphyry pluton may be quantitatively estimated.