安徽铜陵地区幔源镁铁质团块研究及其地质意义

摘要

It is generally accepted that mantle input under crust-mantle interaction plays an important role in sustainable magma fractionation and eventually leads to release of magmatic fluids and formation of world class W, Sn and other types of ore deposits. Many models dealing with the evolution of mantle-derived magmas are derived from numerical simulations based on isotopic geochemistry, temperature and pressure or physical geography data. Few models use detailed chemical analysis of mantle-derived rock enclaves. The authors found direct evidence for crust-mantle interaction in Tongling area, East China, in the form of mafic clots (MFC), which are interpreted to represent fragments of altered alkaline basaltic magma in magma chambers at the base of the continental crust (BCC). These MFC are unevenly distributed within the microgranular dioritic enclaves (MME) which, in turn, are included in the host quartz monzodiorite. The authors have combined all available data with new element distribution maps of the MFC in order to present a general framework model for the evolution of mafic to siliceous magmas. Observations and researches indicate that the MFC were formed from the fractional crystallization and evolution alkaline basaltic magma in magma chambers at the BCC. The basaltic magma resulting from previous partial melting of the upper mantle was eventually brought to the magma chambers at the BCC by magmatic underplating and crust-mantle interaction at 140 Ma. And the MME were formed from the fractional crystallization and evolution dioritic magma by the assimilation of the derived magma from the base of the continental crust with the metamorphic rocks in a crust magma chamber at the depth of 12~16 km. The discovery of the MFC and MME effectively prove that the dioritic magma in the middle crust originated from the mixing basaltic magma of mantle-crust at the BCC.%幔源岩石包体研究，是认识上地幔岩石圈物质组成、幔源岩浆演化及壳幔动力学过程的重要手段。铜陵地区小铜官山石英二长闪长岩中发育有微粒闪长质包体，并且这些微粒闪长质包体中不均匀地分布着镁铁质团块，三者的形成过程可视为铜陵地区岩浆演化的缩影，为了解本区深部岩浆作用过程提供了有力的证据。在前人研究的基础上，笔者借助电子探针、扫描电镜、电镜能谱和二次飞行时间离子探针(Tof-SIMS)对产于铜陵地区微粒闪长质包体中的镁铁质团块进行了详细的研究，首次获得了一套精确的矿物化学资料和元素分布图，总结了镁铁质团块的特征，并讨论了本区的深部岩浆作用过程。矿物学研究表明，镁铁质团块中的角闪石和辉石均已发生了不同程度的透闪石化和阳起石化蚀变，蚀变过程中，从镁钙闪石到镁角闪石，再到透闪石，随着Si的增加，角闪石呈现出Mg的富集和Ti、Al贫化的特点。团块中的富Cr磁铁矿、Ti磁铁矿和少量的铝直闪石指示了其具有深源性。Tof-SIMS元素分布图显示，透闪石主要由Al、Si、Ca、Sc、V、Cr、Mn、Cu和Sr元素组成，透辉石主要由Si、Mg、Ca、Cu和Rb组成。在铜陵地区，上地幔部分熔融形成一套玄武岩浆，受岩浆底侵作用影响，玄武岩浆上侵，进入下地壳深位岩浆房，与下地壳硅镁层发生同化混染作用，形成一套轻度演化的中基性(辉长质)玄武岩浆，镁铁质团块就是这类中基性玄武岩浆直接结晶形成的。后受构造作用影响，这类中基性玄武岩浆上侵到中地壳岩浆房(12~16 km)，与中地壳的变质岩系发生同化混染和结晶分异作用形成一套中性闪长质岩浆，微粒闪长质包体就是这套闪长质岩浆发生结晶分异作用而形成的。镁铁质团块和微粒闪长质包体清楚地解释了铜陵地区深部岩浆作用过程，并有力地证明了铜陵地区中地壳的闪长质岩浆来源于下地壳的壳幔混源岩浆。