科研之友
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摘要
Many Cu-Mo-Au deposits are considered to be related to adakitic porphyries formed in non-arc settings, e.g., in collisional orogenic zones and intra-plate environments, but their genesis is still under discussion. The Aolunhua porphyry complex and its related Mo-Cu deposit from the eastern Central Asian Orogenic Belt (CAOB) provide important insights into this issue. The porphyries are characterized by high Sr (496-705 ppm) and Sr/Y and La/Yb ratios similar to those of typical adakitic rocks, and low 'Sr ratios (0.7049-0.7052) and positive eNd(t) (+0.5 to + 1.4) and &NKr) (+3.5 to + 9.8) values. These features, along with the occurrence of mafic microgranular enclaves (MMEs), compositional and textural disequilibrium of plagioclase phenocrysts and relatively high Mg# values (45-52), indicate that they were derived from mixing of felsic magma from partial melting of a juvenile arc-type lower crust and mafic magma from a lithospheric mantle previously metasomatized by subduction zone fluids/melts. High Sr/Y and La/Yb ratios are indicative of contribution from enriched mantlederived materials (with high LILEs; e.g., Sr, La), which were strengthened by subsequent fractionation of ferromagnesian phases such as pyroxene and hornblende. MMEs hosted by the ore-bearing porphyry have zircon U-Pb ages of ca. 132 Ma, similar to those of the host rocks. The enclaves have elevated Mg (56-63), LILEs (e.g., Sr = 660-891 ppm), LREE (LaN = 68-150, (La/Sm)N = 3.0-4.0, (La/Yb)N = 12.0-19.6) and ratios of radiogenic isotopes of Nd- and Hf (eNd = +0.7 to +1.6; alif = +33 to +10.9), suggesting that their parental magmas were derived from the metasomatized mantle source. The Mo-Cu mineralization was probably related to the high water content, high oxygen and sulfur fugacity of hybrid magma. Formation of the adakitic porphyries and related Mo-Cu deposits of the eastern CAOB could be related to the Early Cretaceous lithospheric extension, caused by the subdu
- 出版日期2013-11
- 单位北京大学
