西秦岭岗察岩体地球化学特征及意义

Geochemical Characteristics and Significance of the Gangcha Complex in the West Qinling

岗察岩体位于西秦岭造山带西端,该岩体形成于三叠纪中晚期,具有从中基性-酸性演化趋势。ω(SiO2)为55.15%~74.84%,Al2O3为13.12%~17.98%,K2O为1.23%~5.24%,K2O/Na2O为0.43~1.49,A/CNK为0.86~1.06,样品σ值为1.37~2.40,岩体属准铝质-弱过铝质钙碱性系列。微量元素球粒陨石标准化蜘蛛图解呈明显的右倾谱型,总体表现为Y、Yb、Zr、P、Nb、Ti(HFS)相对亏损,Rb、Th、K(LIL)富集的特征,指示岩体具有活动大陆边缘环境的特征。稀土元素(La/Yb)N为3.51~18.89,在稀土元素分配曲线上轻稀土元素富集且分异明显,重稀土元素相对亏损,Eu元素"谷"形明显,δEu为0.31~0.83,表明岩浆演化过程中有长石的结晶分异。微量与稀土元素蛛网图解形态及其组成特征还表明了岗察岩体岩浆源物质主要是由上地壳重熔演化而成。Rb-(Y+Nb)判别图解、R1-R2因子判别图解说明岩体处于西秦岭大陆活动边缘,经历了中三叠世中期西秦岭扬子地块与祁连地块之间板块碰撞造山运动。

The Gangcha complex,which is located in the western part of West Qinling,was formed in the middle-late stage of the Triassic,and shows an evolutional trend from basic-intermediate to acid rocks. Its content of SiO2ranges from 55. 15% to 74. 84%,Al2O3from 13. 12% to 17. 98%,K2O / Na2O from 0. 43 to 1. 49,A / CNK from 0. 86 to 1. 06,and σ from 1. 37 to 2. 40. It belongs to calc-alkaline series from quasi-aluminous to weak peraluminous. The trace elements are characterized by enrichment in LIL such as K,Rb and Th and depletion in HFS such as Y,Yb,Zr,Nb,P,and Ti. The( La / Yb)Nvalues range from 3. 51 to 18. 89,and the REE of the rocks is enriched in LREE and depleted in HREE and with medium europium anomalies( δEu: 0. 31 ~0. 83). The characteristics of the REE and trace elements indicate the rocks are derived from crustal materials and formed in active continental margin. The Rb-( Y + Nb) and R1- R2 diagrams illustrate the rock-body is an continental margin of western Qinling,and has experienced plate collision orogeny of the West Qinling between Yangtze block and the Qilian block in the Middle Triassic.