GEOCHEMISTRY AND GENESIS OF THE HOST ROCK OF DAHEISHAN MOLYBDENUM DEPOSIT IN YONGJI, JILIN PROVINCE

The granitic rock masses in Daheishan porphyry Mo deposit are mainly granodiorite and granodiorite porphyry, occurred in composite massif. The ore-bearing granodiorite porphyry has a high content of Al₂O₃(A/CNK=1.04 -1.25) and low Mg^#(Mg^#=21-44), with distinct LREE and HREE fractionations, without or with slightly negative Eu anomalies(δEu=0.82-1), which indicates that there may be no plagioclase remaining in the source area. It is similar to the C-type adakite in eastern China, which is enriched in large ion lithophile elements(LILEs, such as Rb, Ba and Sr) and depleted in Nb, Ta, Y, Yb, Th, P and Ti, with a high ratio of Sr/Y and Sr/Yb. The ore-free granodiorite (A/CNK=1.01-1.03), with rich LREEs, depleted HREEs and moderately negative Eu anomalies(δEu=0.62-0.63), is enriched in LILEs(such as Rb and Th) and depleted in P, Ti and high field strength elements (HFSEs, such as Nb, Ta and Zr), and has a low ratio of Sr/Y and Sr/Yb. The features of Hf isotopes show that the Daheishan granitic rocks were formed by the Mesozoic melting of the accreted crust from the Meso-Neoproterozoic depleted mantle. The tectonic evolution history of Pacific plate, together with the detrital zircon age of Heilongjiang complex and the age of the blueschist facies metamorphism of ophiolitic mélange in Heilongjiang group indicate that the formation of Daheishan granitic rocks is related to the westward subduction of Pacific plate beginning in the Late Triassic-Early Jurassic Epoch.