西秦岭教场坝岩体岩浆混合成因的新认识

A NEW RECOGNITION OF MAGMA MIXING PROCESS ABOUT JIAOCHANGBA ROCK BODY,WESTERN QINlING

教场坝岩体是一岩浆多次脉动形成的复式深成岩体,由3个单元组成,它是岩浆混合作用形成的混合花岗岩,在混染较强的花岗闪长岩中分布有大量的镁铁质包体。寄主岩石化学成份贫SiO2,富FeO*+MgO,铝饱和度(A/CNK)>1.0,岩石为过铝质类型,K2O/Na2O>1.0;镁铁质包体(MME)富SiO2,贫FeO*+MgO,铝饱和度(A/CNK)<1.0,K2O/Na2O<1.0,二者均为钙碱性系列,两者间有明显的物质交换。包体与寄主岩石微量元素均富集大离子亲石元素Ba,Sr,Rb,Sm,Be及高场强元素Nb,Th等,亲铁元素Co,Cr,Ni也相对富集,但二者演化规律不同;稀土元素总量寄主岩相对较低(REE=254.78×10-6),稀土配分模式发育明显的铕负异常,δEu值较低(平均0.48);镁铁质包体稀土总量相对较高(REE=298.75×10-6),配分模式发育极微弱的铕负异常,δEu值较高(平均0.78)。研究表明:镁铁质包体具幔源型花岗岩类特征,而寄主岩石二长花岗岩具壳源型花岗岩特征,包体与寄主岩石具不同的岩浆来源。氧同位素显示镁铁质包体中有大量地壳物质的加入,教场坝岩体的形成与岩浆混合作用有关,是酸性岩浆与基性岩浆混合的产物。

Composed of three units, the Jiaochangba rock body is a compound rock body forming with pulsant magma activity and formed by process of magma mixing. There are large numbers of MEE in granodiorite of mixing violence. Host rock compositions are characteristic of low SiO2, enrichment in FeO^＊ ＋ MgO, （A/CNK）〉 1.0, high Al2O3 content, K2O/Na2O〉 1.0. Mafic micro-granular enclaves are enriched in SiO2 , deficient in FeO^＊ ＋MgO, （A/CNK）（1.0,K2O/Na2O〈1.0. The two parts exchanged some elements and take the character of calc-alkaline series. As one of common character,trace elements are enriched in large ion lithophile elements Ba,Sr,Rb, Sm,Be,Nb andTh. The elements of Co,Ni,Cr are also enriched in the rocks. Host rock is of lower REE （REE=254.78× 10^- 6） and rare earth elements distribution models are characteristic of Eu abnormity. δEu value is lower （average 0.48）. Rare earth elements of the mafic micro-granular enclave are high （ REE = 298.75 × 10^-6 ） witjh distribution models of unconspicuous or low Eu negative abnormity and with high δEu value （average 0. 78）. Research showed that MME has a character of mantle granite and the host rock has a character of crust granite,they have different material sources. Oxygen isotope shows that crust material is entered in mafic micro-granular enclave. As the production of acid magma compound with bas magma,Jiaochangba rock body have important connection with magma mixing.