Comparative Study And Significance Of Two Types Of Rhyodacite In The Xiangshan Uranium Ore Field

In this paper,rhyodacite and rhyodacite porphyry of Daguding Formation in Xiangshan are selected as the research objects by comparative analysis of field geological survey,petrography,mineral chemistry(feldspar,biotite)and geochemistry(major elements,trace elements,rare earth elements)to discuss the source characteristics,magmatic evolution,genetic attribution,and tectonic background of the two types of rhyodacite.The rhyodacite of the Daguding Formation in Xiangshan can be divided into two types:one type is the rhyodacite of the spouting facies(K₁d^2a);the other type is the rhyodacite of the intrusive facies(K₁d^2b).The types of feldspar from rhyodacite are more than those of rhyodacite porphyry.The feldspar types of rhyodacite include albite,K-feldspar,potassium-sanidine,oligoclase,and andesine.The types of feldspar from rhyodacite porphyry mainly include albite,potash feldspar and potash-sanidine.The biotite in rhyodacite and rhyodacite porphyry belongs to magnesian biotite;both are formed at 750?±20?with high oxygen fugacity.In addition,they belong to calc-alkaline rocks with mix of mantle material,showing that they originate from the mixed source of crust and mantle.The characteristics of major elements between the two types of rhyodacites in Xiangshan are rich in silicon(Si O₂>68%)and alkali(Na₂O+K₂O>7.5%),belonging to calc-alkaline rocks,kashosite series and high-K calcium-alkali rocks.Two types of rhyodacite rock have similar alkalinity rate index AR(>2.3);rock saturation index A/CNK ratio(>1.1);Ritman index(?)(>2.3);differentiation index DI(>83)).The rhyodacite is a metaluminous-peraluminous rock,while the rhyodacite porphyry is a peraluminous rock.The trace elements in the two types of rhyodacites in Xiangshan have the following characteristics:Th,U and other large ion lithophile elements and La,Rb,Nd and other rare earth elements show positive anomalies,and Ba,P,Ti,Sr and other elements are present.Obviously negative anomaly,Th/U ratio is about 3.8,the two are close,both have crustal material involved in magmatism in the process of rock formation.The two types of rhyodacites in Xiangshan have similar characteristics of rare earth elements:those rhyodacites are rich in rare earths with the relatively high contents of total rare earth content(?REE>249×10-6),which are higher than the continental crust value.The ratio between light and heavy rare earths is large(LREE/HREE>9),showing obvious differentiational degree of light and heavy rare earths and abundance of light rare earths.The ratio of(La/Yb)_N is greater than 11.The light and heavy rare earth fractionation of the two is obvious,and the light rare earth is more significant than the heavy rare earth.The?Ce of the two types of rhyodacites range from 0.73 to 1.10,which are comparable with no obvious anomaly;the?Eu changes are between 0.38 and 0.56,showing obvious negative Eu anomalies.In the distribution model diagram,the heavy rare earths slope slowly downwards,and the light rare earths slope upwards steeply,showing a typical"right-tilting",showing the phenomenon of obvious enrichment of light rare earth elements and strong fractionation of light rare earths during the diagenesis process.Compared with light rare earth elements,fractionation of heavy rare earth is weaker.Two types of rhyodacite are formed in stable continental plates and belong to the product of post-orogenic environment.