Research On The Geological Characteristics Of Suanjingzi Rock Mass In Beishan,Gansu Province

The Suanjingzi massif is located in the Beishan area of Gansu Province.The basement structure is dominated by nearly east-west striking faults.Intense magmatic activity in the Mesozoic endowed this pluton with mainly felsic rocks.Qingbaikou dolomitic marbles,Silurian andesites,Carboniferous and Cretaceous sandstones comprises the folded strata framework in the Suanjingzi area.With Suanjingzi massif as the main research object,this paper presents the petrological,geochemical and geochronological characteristics.Based on these data,we investigate the petrogenesis and emplacement mechanism of this massif,especially the relationship between the faulting and magmatism.Moreover,regional structural framework and tectonic evolution of the research area are also discussed.The conclusions are listed as below:1.As a granitoid batholith,the Suanjingzi massif can be divided into three independent units:gneissic granodiorite,medium-coarse grained granodiorite and medium-fine grained granodiorite.With massive structure,this batholith has textures of medium-coarse grained and medium-fine grained.The three units hold different outcrop characteristics,while they share similar compositions of rock-forming minerals and similar species of accessory and altered minerals.2.The Suanjingzi massif can be categorized as the felsic rocks based on the SiO₂content of 67.81%-70.93%.The index?is between 1.31 and 1.70,suggesting calc-alkaline rock series.The saturation of Al₂O₃is greater than 1,indicating peraluminous rock series.These parameters indicate that the three units of Suanjingzi massif belong to differentiated I-type granite.The spider diagram of minor elements show a trend of right inclination,with obvious positive anomalies of Th,La,Sr and Sm,and obvious negative anomalies of Nb,Ta and Zr.These features are similar to the chemical characteristics of granitoids generated in the island arcs.However,the Nb/Ta ratio ranges between 8.86-14.36,indicating that the magma is mainly derived from the crust.The enriched LREE and depleted HREE suggest that the magma may be originated from the partial melting of the lower crust.The absence of obvious Eu anomaly implies the feature of I-type granites.3.The emplacement ages of the Suanjingzi massif are 352-359 Ma,350-359 Ma and 346-355Ma for the units of No.1,No.2 and No.3,respectively.All of them were emplaced in the Early Carboniferous,which were the products of magmatic activity in Middle Warrisian.The magmas were generated in the volcanic arc environment,a consequence of orogenic uplift in the active continental margin.Belonging to the syn-collisional magmatic arc type granite,this massif were emplaced passively during the dike expansion.The piedmont faults in the area control the emplacement and distribution of this massif,with the massif mainly trending NE-SW.The massif hosts NE-NW and NE-NW striking faults,which divide it into irregular blocks.