北大别灰色片麻岩的岩石化学特征及大地构造背景

Petrochemistry and tectonic background of the gray gneisses in north Dabie terrane

北大别变质杂岩的基本组分为一套成分相当于 Ｔ Ｔ Ｇ 岩套的灰色片麻岩。岩石化学成分中 Ｓｉ Ｏ２含量约为４７％ ～７９％ 。而且随着 Ｓｉ Ｏ２ 含量的增加， Ｔｉ Ｏ２ 、 Ａｌ２ Ｏ３ 、 Ｍｇ Ｏ、 Ｃａ Ｏ 和 Ｆｅ Ｏ 的含量相应下降， 这表明从英云闪长岩质片麻岩到奥长花岗岩质或花岗岩质片麻岩具岩浆成因， 是角闪石和斜长石的分离作用控制下岩浆分异的产物。岩石的稀土元素总量较低，轻重稀土分馏较弱， 近于平行的稀土元素配分曲线表明所有岩石具有同成因的亲缘关系。岩石富集大离子亲石元素， 亏损高场强元素和 Ｎｂ。偏基性的片麻岩较花岗质片麻岩更富集 Ｃｓ、 Ｂａ，、 Ｒｂ、 Ｓｒ和较少亏损 Ｎｂ 和 Ｔｉ。随着 Ｓｉ Ｏ２ 含量增加， Ｎｂ 亏损加大， Ｓｒ的富集程度减小， 过渡族金属元素 Ｔｉ强烈亏损， 表明岩浆中发生了镁铁矿物和斜长石的分离。花岗质片麻岩表现出岛弧花岗岩所具有的 Ｆｅ Ｏ、 Ｍｇ Ｏ、 Ｃａ Ｏ 含量低和 Ｆｅ Ｏ ／ （ Ｆｅ Ｏ＋ Ｍｇ Ｏ） 比值低的特征。亏损 Ｎｂ 和 Ｙ 的性质同样证明了他们属于火山弧花岗岩。在 Ｈｆ／３ Ｔｈ Ｔａ （ Ｎｂ） 判别图上所有岩石的投影点均落到岛弧钙碱性岩系的范围。主元素和微量元素的性质证明北大别的灰?

The principle component of the north Dabie metamorphic complex (NDC) is a suite of gray gneisses, which is compositionally comparable to the tonalite trondjemite granodiorite association Gray gneisses, including amphibolite gneiss, amphibolite plagioclase gneiss, quartzo feldspar gneiss, have the SiO 2 contents ranging from 50% to 75% or more On the SiO 2 variation diagram most of major oxide display a smooth trend from tonalitic gneiss to trondhjemitic or granitic rocks, implying a magmatic differentiation model for the origin of the gray gneisses The differentiation was probably controlled by hornblende and plagioclase fractionation The gray gneisses have lower ΣREE content with low grade fractionation of light REE/ heavy REE On primary mantle normalized REE fractional diagrams, the subparallel patterns indicate a cogenetic relation for all samples All rocks are enriched in large ion lithophile (LIL) elements and depleted in high field strength (HFS) elements, as well as Nb at the level of primary mantle The increasing of Nb depletion, the decreasing of Sr enrichment, and the transitional metal element Ti becoming greatly depleted with the increase of the SiO 2 in the rocks imply the fraction of mafic minerals and plagioclase The granitoids have lower FeO, MgO, CaO contents, low FeO */ (FeO *+MgO) ratios, and Nb and Y depletion On Hf/3 Th Ta (Nb) diagram, all plots set near the Hf Th join and cumulate to Th corner, providing a calc alkaline series of arc setting All of these imply that the gray gneiss is of the calc alkaline association formed in a volcanic arc