Petrbgenesis of the Mesozoic granites and Mo mineralization of the Luanchuan ore field in the East Qinling Mo mineralization belt, Central China

摘要

Numerous Mo deposits associated with the Late Jurassic to Early Cretaceous granite porphyries in the southern margin of the North China Craton make up the East Qinling Mo mineralization belt, one of the most economically important Mo mineralization belt worldwide. Two of the largest porphyry- and skarn-type Mo deposits in the belt are hosted in two granite porphyries in the Luanchuan ore field which emplaced at -150 Ma and -135 Ma, respectively. The granite porphyries are calcic-alkalic to alkalic, and metaluminous to peraluminous. They are strongly depleted in Eu, Sr, Ba, P and Ti, indicating that they underwent intensive fractionation of plagioclase, apatite and Fe-Ti oxides. The granite porphyries in the Luanchuan ore field are likely to be connected with nearly coeval Heyu batholith to the east as indicated by the regional geophysical data. The Heyu granite batholith has bulk compositions similar to the granite porphyries, and is possibly a precursor of the granite porphyries. The Heyu granite batholith and granite porphyries have ε_(Nd)(t) values varying from -11.3 to -17.5 and zircon ε_(Hf)(t) values from -5.8 to -35.6. They have two-stage Nd modal ages [T_(DM2)(Nd)] ranging from 1.68. to 2.47 Ga and Hf model ages [T_(DM2)(Hf)] from 1.32 to 2.86 Ga, much younger than those for Mesozoic granitoids elsewhere in the eastern part of the North China Craton which are believed to have formed from remelting of the Archean basement. Mesozoic granites in the southern margin of the North China Craton overall have Pb isotope compositions similar to the basement of the Yangtze Block rather than the North China Craton. Therefore, we argue that the granite porphyries and related Mo deposits in the Luanchuan ore field were unlikely sourced from the Taihua Group, instead they may have formed from remelting of the subducted continental crust of the Yangtze Block with T_(DM2)(Nd) ages of ～1.8 to ～2.2 Ga Partial melts of the subducted continental crust of the Yangtze Block interacted with melts and/or fluids derived from the enriched mantle wedge, which experienced metasomatism due to the dehydration of subducted continental crust of the Yangtze Block, consequently resulting in the formation of the granite porphyries and porphyry- and skarn-type Mo deposits in the Luanchuan ore field.