大别造山带白鸭山A型花岗岩中镁铁质微粒包体成因及其地质意义

PETROGENESIS OF MAFIC MICROGRANULAR ENCLAVES FROM BAIYASHAN A-TYPE GRANITES IN DABIE OROGENIC BELT AND ITS GEOLOGICAL IMPLICATIONS

大别造山带早白垩世白鸭山A型花岗岩体中广泛分布着暗色微粒包体。对该包体及其寄主岩进行岩相学、矿物学和地球化学研究发现:镁铁质微粒包体(MME)与其寄主岩具有相同的矿物组合,相似的主量元素、微量元素和稀土元素特征,指示MME与寄主岩具有明显的亲缘关系;包体中岩浆成因褐帘石的发现,否定了富云包体的残余体成因,包体中含有碱性长石捕虏晶及大量针状磷灰石,指示岩浆混合作用的影响。结合共分母和不共分母图解判别,揭示MME与寄主岩明显具有岩浆混合作用特征,FeO*-MgO判别图解也指示包体为岩浆混合成因。Nd,Sr同位素组成指示MME的主体成分来源于富集岩石圈地幔,寄主岩体的物质来源为北大别杂岩。综合研究表明,大别地区在早白垩世存在一次壳幔混合作用,可能与岩石圈的伸展减薄有关,岩石圈的伸展导致富集地幔上涌,底侵并诱发古老下地壳熔融,同时发生岩浆混合作用。

The early Cretaceous Baiyashan A type granites in Dabie orogenic belt was characterized by wide distribution of mafic microgranular enclaves. Study of petrography, mineralogy and geochemistry on the enclaves and their host rocks showed that the enclaves and their host rocks had similar mineral assemblages, similar characteristic of major and trace elements, implying that there exited obvious genetic relationship between the enclaves and their host rocks; the discovery of magmatic allanites in enclaves negate the rest model, the presence of alkali feldspar xenoliths and needle-like apatite indicated the effect of magma mixing. MME and their host rocks displayed obvious features of magma mixing in the Covariant diagrams, also the FeO＊ -MgO discrimination diagrams suggested the magma mixing origin of the enclaves. Isotopic compositions of Nd-Sr re- vealed that the main ingredients of MME came from the enriched lithospheric mantle,and the material sources of host rocks were Northern Dabie complex rocks. The research demonstrated that there was a crust-mantle mixing event in Dabie orogenic belt at the early Cretaceous. The event was related to the extension and thinning of lithosphere, which leaded to the upwelling of enriched mantle,brought about underplating of magma and resulted in melting of lower crust as well as magma mixing.