Magma Activity And Genesis Of Shujiadian Area In Tongling Ore District

The Shujiadian area,located in the central part of the nearly E-W direction Tongling-Daijiahui tectonic-magmatic-metallogenic belt,is a favorable section for ore deposits.The area has experienced strong magmatic events,and formed several intermediate-acid intrusions.We apply systematic zircon U-Pb chronology,electron probe analysis of rock-forming minerals,petrochemistry,Lu-Hf isotope geochemistry and fluid inclusion on intrusive rocks from drilling cores of Shujiadian area,to discuss the magmatic activity sequence,tectonic settings of petrogenesis,igneous origin and evolution,relationship between intrusive rocks and metallogenesis,etc.The ages of the Mesozioc intrusions in Shujiadian area are obtainedby zircon LA-ICP-MS U-Pb dating method.The age of granodiorite-porphyry is 146.9±1.8?46.7±2.1 Ma,pyroxene monzodiorite144.8±5 Ma,quartz monzobiorite 141.0±2.1 Ma,granodiorite138.0±2.4 Ma,granite porphyry 130.5±2.3 Ma,felsic vein 129.4± 1.5 Ma.Thus,in consideration with precedent researches,we consider the magmatic activity sequence of the Shujiadian area is:granodiorite-porphyry?pyroxene monzodiorite? quartz monzobiorite?granodiorite?granite porphyry vein/felsic vein.Petrochemistry of the intrusive rocks in Shujiadian area indicates thattheycan be divided into shoshonite series and high-K calc-alkaline series.Major element characteristics indicate that the material source and evolution of the two series bear a certain genetic relationship,though there hasdistinct difference between them.The rocks of the two series have different trace element enrichment or depletion features.They have approximately the same REE patterns,which are all distinct LREE enrichment type.The covariant diagrams of the REE indicate that the assimilation contamination-fractional crystallization may be themain process of the intermediate-acid magma evolution in the area.Electron probe analysis results show that the alkali feldspar of the shoshonite series of the area is mainly orthoclase,plagioclase comprises of andesine in majority,oligoclase and labradorite in minor,pyroxene is mainly sahlite.The alkali feldspars of the high-K calc-alcaline series are mainly albite an orthoclase,plagioclase is mainly andesine and oligoclase,pyroxene is mainly augite.The biotite of the rocks of the two series is magnesian type.The fluid inclusions of the granodiorite-porphyry,the quartz monzobiorite and the granite porphyry in the area are mainlytwo-phases(gas-liquid),there are a few of three-phase inclusions that containcrystals.The Raman analysis results show that the liquid in the inclusionsis predominantlywater,the gas content is very low.The homogenization temperature of fluid inclusions is relatively high,with high salinity,indicating that the fluids in the three rocks also play roles in metallogenesis of the area.In general,the high potassium calc-alkaline series rocks containold inherited zircons,while it is not the case with shothonite series up to present.Lu-Hf isotope study indicates that the two series originate from the enrichment lithospheric mantle.The high-K calc-alkaline series contains inherited zircons with>0(?Hf(t)=3.2 and 6.5,respectively,with two stage model ages 2650 Ma and 2664 Ma respectively),indicating that there has been crustal accretion events during this period,i.e.the mantle-derived material joined to the ancient crust.Based on precedent research,we consider the formation and evolution of the two series rocks are in close relationship with the subduction of the ancient pacific plate,and in controls within the uniform tectonic framework of the Tongling region.The deep process of the magmatic evolution can be discribed as follow:the mantle-derived alkaline basaltic magma firstly formed the deep magma reservoir near Moho,then the fractional crystallizationinitiated to form the accumulative enclaves;deep magma thermal energy and crystallizationheat caused partial melting of the low-melting-point components in the middle-upper crust and fomed shallow magma reservoir;the granodioritic magma in shallow reservoir and the pyroxene monzodiorite from the deepreservoir intruded in order.;during the process abovementioned,part of the pyroxene monzodioritic magma mingled with part of the granodioritic magma to form the quartz monzobiorite and then intruded.