Genesis and metallogenic significance of the Indosinian intermediate-acidic intrusive rocks in the west porphyry belt, Zhongdian island arc, Yunnan

Abstract：The west porphyry belt in the Zhongdian island arc is mainly composed of four Indosinian intermediate-acidic intrusive complex bodies, i.e., A’re, Chundu, Xuejiping and Lannitang, related somewhat to Cu mineralization. Geochemical characteristics of the A’re porphyry are similar to those of Adakite: SiO2>58.06% (averagely 58.64%)，Al2O3 (14.77%-17.29%), Sr (490×10-6-1409×10-6) , Sr/Y (37.44-83.79) and La/Yb (4.62-31.48，averagely 28.51) are high, Y (13.1×10-6-16.82×10-6) and Yb (1.20×10-6-1.61×10-6) are low, δEu is not distinct (0.90-1.02), Mg (0.91%-3.38%) and Mg# (37.84-70.00) are high, chondrite-normalized REE patterns are rightly-oblique, and there exist enrichment of LREE and heavy depletion of HREE. All these features are similar to characteristics of the other three porphyries. Based on the studies of major oxides, REE and Sr, the authors hold that the intermediate-acidic intrusive complex bodies in the west porphyry belt characterized by island-arc rocks to adakitic rocks were most probably formed by the rifting of the subducting Ganzi-Litang oceanic crust, which caused the successive partial melting of the mantle wedge and oceanic crust, during which the partially melted primitive magma exerted multi-staged upward intrusion along the present Geza fault on the eastern side and was contaminated by the crust in varying degrees. The garnet+clinopyroxene+amphibole association remained in the magma source area. There is no distinct difference between the four porphyries in magmatic evolution. Various degrees of mantle-crustal contamination might have played a certain role in the diversities of mineralization.