Geological And Geochemical Characteristics And Genesis Of Hemushan Diorite In Anhui Province

Ningwu basin is a Mesozoic volcano rock basin, Anhui Hemushan diorite is located in the southern section of the Ningwu basin and it is the metallogenic geological body of the Hemushan iron deposit.Based on the previous data, systematic field geological investigation as well as indoor analysis, this thesis finds out the characteristics of petrography, geochemistry and geochronology of the Hemushan diorite. Further on, to a certain extent, this thesis analyses the petrogenesis and the process of diagenetic dynamics.Hemushan diorite with a exposed area about 1 km2, occuring as a pophysis and cupola shows intrusive contact with the overlying huangmaqing limestone. Petrographic studies indicate that the feldspars of Hemushan diorite are albite, labradorite, andesine and orthoclase, and hornblendes of that are edenite, tremolite, cummingtonite and actinolite.Major elements analysis shows that Hemushan diorite is with the characteristics of Sodium-rich(4.90~8.25%), Potassium–poor(0.62~2.59%) and metaluminous(A/CNK 0.70~0.87).Its Rittmann index Sigma varies from 3.46 to 4.59, which means it belongs to the calc alkaline series. The differentitation index is 53.28~72.96.Trace elements analysis shows the enrichment of large ion lithophile element and the deficiency high field strength element. The whole rock REE chondrite distribution pattern showes a right-deviation smooth curve, meaning relative enrichment of LREE and deficency of HREE. Besides, there is still an unconspicuous Eu negative anomaly in this curve.The ISr of the samples varies from 0.70537 to 0.70633. The INd and εNd(t) of the samples change from 0.51211 to 0.52131 and from-5.33 to-7.06 respectively. Zircon U-Pb age is 134.9±2.1Ma, which indicates Hemushan diorite formed in the early Cretaceous of Yanshan. Using aluminum amphibole piezometer, the magmatic crystallization pressure is 72~165MPa,with a corresponding depth of 2.36~5.44 km.Petrography, geochemistry and geochronology indicate that a complex magmatic process occurred during the formation of the Hemushan diorite. Mantle-derived basaltic magma from partial melting of fertile mantle underplated into the lower crust, during which the magma had a certain degree assimilation with crustal material.Then the magma reached to shallow crust,forming shallow magma chamber. Afterwards, differerntiation, fractional crystallization, condensation as well as the asthenosphere upwelling and lithospheric thinning environment of the Mesozoic geodynamic background in the eastern part of Chinese would be responsible for the formation of Hemushan diorite.