胶莱盆地中部及西南缘中生代火山岩岩石地球化学特征及成因研究

Study on the Geochemical Characteristics and Gnesis of Mesozoic Volcanic Rocks in Central and Southwestern Margin of Jiaolai Basin

胶莱盆地广泛分布中生代青山群火山岩,盆地中部及西南缘岩石类型为玄武质粗安岩、粗安岩、粗面岩、流纹岩等。岩石SiO_2含量在50.48%~78.72%之间,里特曼指数(σ)介于1.76~5.97之间,总体属钙碱性系列。岩石具富钾富铝的特征,在K_2O-SiO_2图解中总体属钾玄岩系列,在A/NK-A/CNK图解中主要落在过铝质区域。岩石富集LREE及大离子亲石元素K、Rb、Th、U、Pb等,亏损HREE及P元素,普遍出现Eu负异常及弱的Ce负异常。研究表明,流纹岩与玄武质粗安岩、粗安岩、粗面岩岩浆来源不同,流纹岩具有相对较低的ΣREE含量及(La/Yb)_N值,明显的Eu负异常及极低的Cr、Ni含量,表明其来源于地壳;玄武质粗安岩、粗安岩、粗面岩的来源则较为复杂,推测来源于岩石圈地幔物质同俯冲扬子下地壳物质的相互混合。

Mesozoic Qingshan Group volcanic rocks are widely distributed in Jiaolai Basin and the main rock types in the central and southwestern of this basin are basaltic trachyandesite,trachyandesite,trachyte,rhyolite and so on.SiO 2 content of the samples is between 50.48%and 78.72%while Rittmann index is between 1.76 and 5.97,which manifests the samples belong to calc-alkaline series as a whole.The samples are rich in K and Al,with the K 2O-SiO 2 diagram showing a shoshonite series trend and the A/NK-A/CNK diagram showing a peraluminous series trend.The samples are rich in LREE and some LILEs(K,Rb,Th,U and Pb),but lacking in HREE and P.Negative Eu anomalies and weak negative Ce anomalies are common.Rhyolites have a different magma source with trachyandesites,trachyandesites and trachytes based on the data.Relatively lowerΣREE content and(La/Yb)N values,obvious negative Eu anomalies,extremely low content of Cr and Ni all indicate that the rhyolites are derived from the crust.The genesis of trachyandesites,trachyandesites and trachytes is quite complex and the data reveals they origin from the mixing of lithospheric mantle melt and lower crustal material of subducted Yangtze Plate.