Characteristics And Origin Of The Tongguanshan Pluton, Tongling, Anhui Province

The formation of intermediate-acidic intrusions with xenoliths in Tongling area, Anhui Province, has a close relationship with the crust-mantle interaction and Cu-Au-Fe-S mineralization in the Middle-Lower Yangtze River area. Quartz monzodiorite intrusion in the Tongguanshan mine is large in size and wide in distribution. Diorite xenoliths are very considerable and typical. The quartz monzodiorite intrusion and diorite xenoliths are the ideal objects for the research of crust-mantle interaction. Field investigation, microscopic observation, major and trace elements and Hf isotope analyses have been carried out on the Tongguanshan quartz monzonite diorite and diorite xenoliths. On the basis of the obtained data, their origin has been discussed.We find that there are a large number of diorite xenoliths in quartz monzonite diorite intrusion. They occur in crowds to form a group of xenoliths. The diorite xenoliths are characterized by plastic deformation and often contain host rock clots. A series of disequilibrium magma crystallization textures are observed under microscopy in the quartz monzonite diorite and diorite xenoliths, including plagioclase phenocrysts with core-mantle-rim zoning, mantled rim, poikilitic Kf, sieve texture and quartz phenocrysts with a harbor or rounded shape. These disequilibrium textures provide an important evidence for the existence of magma mixing in their formation process.Geochemical studies show that the quartz monzonite diorite intrusion and diorite xenoliths of the Tongguanshan mine are metaluminous high potassium calc-alkaline series. Their trace element distribution pattern is very similar, belong to right type. They are enriched in large ion lithophile elements and light rare earth elements(129.07~179.37μg/g), but depleted in high field strength elements and heavy rare earth elements(9.10~16.40μg/g), with similar distribution patterns and weakly negative Eu anomalies(δEu=0.84~0.97), showing the formation of a homologous magma evolution. The ratios of(176Hf/177Hf)s of the quartz monzonite diorite and diorite xenoliths are defined in the range of 0.282142~0.282388, and εHf(t) in the range of-19.89~-11.59, indicating very low Hf isotope composition and a wide range of variation. The above results show that the magma responsible for the formation of them may be derived from a mixed source of crust with mantle.The geological, petrographical and geochemical data support the occurrence of a complex magmatic processes in the formation of the Tongguanshan quartz monzonite diorite intrusion and diorite xenoliths. Mantle-derived magma underplating occurred in the tectonic setting of Yanshanian continent extension. Assimilation existed between the underplated magma and the lower crust in the deep magma chamber to form an intermediate-basic magma. Then Assimilation also took place between the upwelling intermediate-basic magma and the middle crust in the shallow magma chamber to form an intermediate-acidic magma. Magma mixing generally occurred during replenishment of the follow-up intermediate-basic magma from the deep magma chamber into the shallow magma chamber of intermediate-acidic composition. The emplacement, cooling and crystallization of the mixed magma are responsible for the formation of the intrusion and xenoliths.