Characteristics And Petrogenesis Of The Tianebaodanshan Intrusion In The Tongling Area, Anhui Province, China

Tongling area in Anhui Province is an important part of the Middle-Lower Yangtze River Fe-Cu-Au polymetallic metallogenic belt. In the area occur a great number of Mesozoic felsic intrusions closely associated with Cu-Au deposits. The Tianebaodanshan quartz diorite intrusion in the area has a genetic relationship with the Tianmashan Au deposit. A detailed petrographical, mineralogical and geochemical study has been carried out on the Tianebaodanshan quartz diorite. Petrographical observation and mineralogical analysis indicate that light mineral is dominated by plagioclase, and dark mineral by biotite and amphibole. Plagioclase consists of oligoclase with An23-30and andesine with An31-38. Amphibole include early edenite and late magnesiohornblende. The temperature, pressure and depth during formation of the quartz diorite were estimated using the geobarometer of Al in hornblende and the amphibole-plagioclase geothermometer. The temperature of mineral crystallization at shallow levels, as estimated from the composition of early hornblendes and plagioclases, falls within the range721-761℃, with a corresponding pressure and depth of crystallization of301-505Mpa and9.9-16.6km, respectively. Using the compositions of late hornblendes and plagioclases, the conditions of mineral crystallisation at very shallow levels (representing the conditions of final magma emplacement) are estimated to be632-692℃and128-167Mpa, corresponding to a depth of emplacement of4.2-5.5km. Biotite belongs to Mg-biotite and its ratios of Mg/(Mg+Fe2++Mn) range from0.45to0.70, indicating derivation of magma from mixing source of mantle with crust. The Tianebaodanshan quartz diorite is poor in Si and Mg, but rich in Na, Ca and Al, and belongs to a metaaluminous calc-alkaline rock series. This rock has higher contents of LILE such as Rb, Ba, Th, U, K and Sr, and lower contents of HFSE such as Zr, Hf, Ti, Nb, Ta, P, with lower contents of REE and higher ratios of LREE with HREE. A complex magmatic process occurred during the formation of the Tianebaodanshan intrusion. Mantle-derived basaltic magma underplating followed by assimilation of underplated basaltic magma with crustal materials in middle crust (about14.7km) led to the formation of quartz dioritic magma in a shallow magma chamber. Crystallization of this magma after its emplacement up to very shallow level (about4.8km) would responsible for the formation of the Tianebaodanshan quartz diorite.