摘要
在苏鲁-大别山造山带内,黑云斜长片麻岩习惯被称为副片麻岩,它也是中国大陆科学钻探工程主孔岩芯中一种重要岩石类型,不仅构成了孔深738-1113m之间斜长片麻岩岩性段的主体,在其它岩性段也有少量以“夹层”的形式产出。其中除少数斜长片麻岩可能是由榴辉岩经斜长角闪岩退变而来或由含泥质的沉积岩变质而来外,绝大多数斜长片麻岩的地球化学特征与现残存的扬子地台北缘晚元古代双峰式火山岩的酸性端元非常相似,指示它们的原岩可能类似于这些酸性火山碎屑岩-凝灰岩。斜长片麻岩中不同元素之间的相关性差别很大,这主要与超高压变质及随后的退变质过程中元素间活性的差异有关。其中SiO2、TiO2、Al2O3、FeO、CaO、Cr、Ni、V、Co、Fe、Mn、Cu、Zn、Sc等与MgO具明显相关性,它们与MgO一样属不活泼元素。重稀土元素及Y也属于不活泼元素。而K2O、Pb、Rb、Ba、U、Th等大离子亲石元素与MgO之间没有任何相关性,这些元素属活泼元素。Na2O、Sr、Nb、Ta、Zr、Hf等则介于两类之间,可视为准不活泼元素。LREE虽然有一定的活性,但即使是超高压变质作用,也没有从根本上改变稀土元素的分配模式,仍可有效地应用于对原岩性质的恢复。对比研究表明,斜长片麻岩与花岗片麻岩在地球化学性质上存在着明显的差异,其中前者的MgO、TiO2、CaO以及过渡元素的含量明显偏高,尤以低的K2O/Na2O和Rb/Sr比而明显不同于花岗片麻岩。此外,与花岗片麻岩强烈的负Eu异常(δEu=0.21-0.26)和Ba负异常不同,斜长片麻岩中Eu负异常的程度相对较弱(8Eu大于0.5),且总体表现为Ba正异常。绝大多数“副片麻岩”和花岗片麻岩(>90%)可以根据全岩的地球化学特征加以有效区分。
The Plagiogneiss, which was used to regard as paragneiss in the Sulu-Dabie orogen, is also one of the important rock types in the main hole of the Chinese Continental Scientific Drilling Project. It not only comprises the principal part of the plagiogneiss lithological segment (738-1113m) , but also exists as "interlayer" in other lithological segments. Almost all of the plagiogneisses are similar in geochemical characteristics with those of the acidic members of Late Proterozoic bimodal volcanics remained in the north margin of the Yangtze craton except minority of them which may formed by retrograde metamorphism of the eclogites or by metamorphism of pelitic sedimentary rocks, that means the protolith of the most paragneisses may be acidic ignimbrite. Obviously, some elements are more mobile than others during UHP metamorphism. The contents of SiO2, TiO2, Al2O3, FeO* , CaO, Cr, Ni, V, Co, Fe, Mn, Cu, Zn and Sc in the plagiogneisses show well correlation with MgO, these elements and Mg belong to immobile elements. The heavy rare earth and Y also belong to immobile elements. No or weak correlations with MgO have been seen for K2O, Pb, Rb, Ba, U, Th (i. e. , LILE) , so these elements belong to mobile elements during UHP metamorphism. The mobilities of Na2O, Sr, Nb, Ta, Zr and Hf are between immobile and mobile. Although with some degree mobility for LREE, the REE patterns had not been inflected radically during UHP metamorphism, therefore they can be used for protolith reconstruction. The plagiogneisses obviously differ from granitic gneisses in the drilling hole in geochemistry, the contents of MgO, TiO2, CaO and transition elements in the plagiogneisses are much higher than in the orthogneisses. Particularly the lower K2O/Na2O and Rb/Sr ratios in the plagiogneisses are greatly different from the granitic gneisses. There are also great differences in REE patterns and trace element distributions between the two types of gneisses in the hole, the plagiogneisses are characterized by medium negative Eu anomalies (δEu >0.5) and positive Ba anomalies as a whole, but the granitic gneisses with strong negative Eu anomalies (δEu = 0. 21 -0. 26) and negative Ba anomalies. So most of the granitic gneisses and the plagiogneisses ( > 90% ) can be discriminated on the basis of their whole rock geochemical characteristics.