浙东燕山晚期花岗岩的地球化学特征

GEOCHEMICAL CHARACTERISTICS OF GRANITES OF LATE YANSHANIAN STAGE IN EAST ZHEJIANG

浙东燕山晚期侵入岩分为I型和A型,本文研究了两个不同成因类型花岗岩的岩石化学、地球化学特征。A型花岗岩与I型花岗岩相比较,富硅、富碱、贫镁、贫钙、低铝,(Na+K)/Al(分子数)比值高,富Nb、Zr、Ga、F,而贫Sr、Ba、Cr、Ni、Co、V,Rb/Sr、Ga/Al比值高,LREE/HREE比值低,稀土元素球粒陨石标准化曲线为“V”型,δEu强烈亏损;是下地壳残留的麻粒岩相较低程度部分熔融或是早期部分熔融形成Ⅰ型岩浆之后残余的亏损物质部分熔融的产物。Ⅰ型花岗岩是由下地壳岩石部分熔融形成母岩浆,在岩浆上升过程中与中上陆壳物质发生混染和分离结晶而形成。

This paper deals with petrochemical and geocnemical characteristics of two different genetic granites, i.g., Ⅰ-type and A-type granites of Late Yanshanian stage in East Zhejiang. In comparison with A-type granites, I-type granites are characterized by lower SiO2, K2O, Na2O and higher A12O3, CaO, MgO, Sr, Ba, Cr, Ni, Co, V, Sc, etc., K2O+ Na2O/ Al2O3(moles) ratio is less than 0.78. Ⅰ-type granites have relatively higher K / Rb, Ba / Rb ratio. Their REE contents range from 162.65 to 199.16 × 10-6 and LREE / HREE from 3.93 to 7.50 being of LREE enrichment. La/Yb, Eu/Yb, Eu/Sm, Ce/Yb, Gd / Yb are high. The curves of chondrite-normalized REE patterns incline to right and show Eu anomaly changing from positive to middle negative (δEu= 1.08-0.54). The parental magma of Ⅰ-type granites was formed by partial melting in different degree of the lower crust when the basaltic magma provided heat source, and was contaminated by some crustal material during its ascending. A-type granites consist of alkaline feldspar granite and alkaline granite and are characterized by enrichmemt in Si, K2O+Na2O and poor in Mg, Ca, Al, K2O +Na2O3/ A12O3 (moles) ratio is 0.90 and 1.00 respectively. They have higher Rb, Nb, Ta, Zr, Ga, F Contents and Rb / Sr, Ba / Sr, Ca / Sr, K / Ba, K / Sr, Nb / Ta, Ga / Al. F / Cl ratios. Their REE contents range from 113.75 to 484.81 × 10-6 and LREE/HREE from 1.18 to 5.08. The curves of chondrite-normalized REE patters show a 'v' mode and have strong negative Eu anomaly (δEu = 0.51-0.14). A-type granites were generated from residual granulites of the lower crust by partial melting of lower degree or from partial melting of residual depleted materials after generating of the I-type magma.