桐柏-大别造山带剪切带阵列的构造岩研究

TECTONITES IN THE SHEAR ZONE ARRAY OF THE TONGBAI-DABIE OROGENIC BELT

桐柏—大别造山带剪切带阵列经受长期的应变软化、应变局部化和变形分解作用,形成了一系列构造岩,包括糜棱岩、变晶糜棱岩、半塑性糜棱岩和面理化碎裂岩等。糜棱岩普遍具二相糜棱结构或由相连的弱相基质或层分离强相残斑或布丁所形成的IWL(Interconnected weak matrix or layers)结构。构造岩中显微构造各式各样,并具一定的组合形式和演化规律,反映了不同环境下主导变形机制的多样性及变形机制间转换的规律性。代表性剪切带岩石化学成分变异的研究表明,除流体流动外,体积亏损是造成某些组分,尤其是一些不活动组分,如钛、磷、锆、钒、钇等相对富集的主导因素,并由化学成分的变异和质量平衡计算,求出了该剪切带的体积亏损率以及流体/岩石值。较大的体积亏损和大量渗透体的存在,对剪切带的化学行为和流变行为都有着重要的影响作用。

A series of tectonites were formed in the shear zone array of the Tongbai-Dabie Orogenic Belt, including mylonites, blastomylonites, semi-plastic mylonites and foliated cataclasites as a result of multiple strain localization, strain softening and deformation partitioning. The mylonitic rocks are characterized by a two-phase mylonitic structure, or an inteconnected weak matrix or layer (IWL) structure formed by separating clasts or boudins of the stronger phase by interconnected weaker phase matrix or layers. Various kinds of microstructures are found in the tectonites. The assemblage and evolutionary trend of the microstructures are controlled by the variation, alternation and transition of the dominant deformation mechanisms in different environments. The variation in chemical composition of the rocks from a typical shear zone is presented. The enrichment of immobile elements, such as Ti, P, Zr, V and Y, in the mylonites and ul-trumylonites is inteprected as being due to volume losses of the tectonites. The volume losses are estimated by mass balance calculations, based on the variation of elements chemistry. The calculation also indicates the loss of large amounts of SiO2 and infiltrated fluids. The large volume losses and fluid flow play a very important role for the chemical and rheological behaviours of the rocks in the shear zoes.