胶东文登长山南花岗闪长岩体的岩浆混合成因:闪长质包体及寄主岩石的地球化学、Sr-Nd同位素和锆石Hf同位素证据

Magma mixing for the origin of granodiorite: Geochemical, Sr-Nd isotopic and zircon Hf isotopic evidence of dioritic enclaves and host rocks from Changshannan granodiorite in the Jiaodong Peninsula, eastern China.

胶东昆嵛山杂岩文登长山南花岗闪长岩体中广泛分布具有火成结构的闪长质包体,这些包体主要为椭圆形或纺锤形,定向排列,大小不等(几cm至几m),颜色较寄主岩深,粒度较细。包体具有与寄主岩石相似的矿物组合,但铁镁质矿物及斜长石含量明显比寄主岩石高,而石英和钾长石含量低于寄主岩石;镜下包体具有明显的不平衡反应结构,广泛发育针状磷灰石。在地球化学特征上,包体和寄主岩石部富集大离子亲石元素和轻稀土元素,亏损高场强元素,并具有相近的Sr、Nd同位素组成,ISr为0．70784-0．70818,εNd为-15．0--15．5。然而,包体和寄主岩石的主量元素在相关图上呈明显的线性关系, 并且岩石学和锆石Hf同位素特征也明显表明文登长山南岩体在成岩过程中发生了镁铁质岩浆和长英质岩浆的混合作用。在岩浆混合作用过程中,全岩Sr、Nd同位素和晚期生成的矿物组成比较快速地达到了均一化,而主量元素和高温矿物锆石Hf同位素组成则残留了原始岩浆的部分特征。研究表明,锆石Hf同位素在岩浆混合作用过程中不容易达到同位素平衡,其同位素组成比全岩Sr、Nd同位素更能有效地示踪混合岩浆的源区特征和岩浆混合过程。

A large number of directional microgranular dioritic enclaves occur in Changshannan granodiorite from Kunyushan granitoids in the Jiaodong Peninsula, eastern China. Mierogranular dioritic enclaves, commonly showing ellipsoidal and spindled shapes, display igneous textures and range from meter- to centimeter-sized fragments. The enclaves are darker-colored and finer-grained than their hosting granodiorlte, and exhibit a very similar mineralogy to those of their granitic hosts. The enclaves have higher contents of ferro-magnesian phases and plagioclase, and lower contents of quartz and K-feldspar than those found in the hosting granodiorite.The chemical composition of biotite and hornblende within the dioritic enclaves is mostly similar to those of the same phases found in the host granodiorite, but the mineralogy display obvious disequilibrium structure. Aeicular apatite is common in the enclaves. Those observations suggest that the enclaves are derived from a hybrid-magma formed as a result the intrusion of a mafic magma into the base of a felsic magma chamber. Geochemical data show dioritic enclaves and hosting granodiorite are enriched in LILEs and LREEs and depleted in HFSEs. Enclaves and hosting granodiorite, similar in Sr- and Nd- isotopic compositions, have low initial ^87Sr/^86Sr ratios（0.70784-0.70818） and negative εNd （t） values （ - 15. 0 - - 15. 5 ） , and display a trend characterized by a linear correlation between major elements. Petrological and zircon Hf isotopic features indicate the occurrence of magma mixing between mafic magma and felsic magma in the original processes of Changshannan granodiorite. During the magma mixing, the Sr and Nd isotope and later crystallized minerals can much quickly homogenize, but major element and Hf isotope of high temperature mineral such as zircon have preserved some information of the primary magmas. The results indicate that although magma mixing has produced similar Sr and Nd isotopic compositions in the enclaves and the hosting granodiorite, the zircon Hf isotope has not easily reached equilibrium and has more effectively traced the hybrid-magma sources and the magma mixing processes.