Genetic relation of charnockite to fluids derived from high-grade rocks in the Limpopo Belt

根据非洲南部林波波带各种高品位变质岩的矿物学、岩石学和化学特征，认为绿辉岩形成于P_<固体> (=P_s) < P_<全流体> (=P_f)，与地壳隆升过程或逆变质过程中主要由片理石英片麻岩产生的流体相互作用。因此，在炭化作用的情况下，CO_2的来源并不一定来源于地幔或下伏绿岩带，从而导致黑云母分解成正辉石。本文还根据岩石中观察到的脱水反应，从热力学角度考察了氧逸度和X （CO_2）在这一过程中的变化。此外，炭绿岩和石英辉斑片麻岩流体包裹体的显微测温（均一温度、冻结温度和最终熔融温度）数据表明，它们的成分大多为富h2o。另一方面，为了研究裂缝带流体的行为，总结了在P_s>P_f条件下物相分析和岩石成因的热力学处理方法。将P_s bb_0 P_f平衡点应用于林波坡带石英石斑片麻岩的炭化和正角闪洞等梯度的形成。虽然在P_s=P_f时它们是富co_2的（X (H_2O) <0.2），但在P_f<P_s时，它们的组成范围很广，且相当富H_2O。P_f<P_s处的计算结果可以解释沙砾岩中石英流体包裹体的不同组成。在地壳隆升过程中形成的断裂体系中，提出了具有递减和递增循环的“流体路径”对稳定正旋回起着重要作用。流体在断裂带内的运移（渗透或流入）不仅引起重要的脱水和水化反应，而且可能与造山带内进行性、区域性变质岩的形成有关。少

On the basis of mineralogic, petrologic and chemical features of various high-grade metamorphic rocks in the Limpopo Belt, southern Africa, it is concluded that charnockite formation took place at P_<solid> (=P_s) < P_<total fluid> (=P_f) interacting with fluids derived from mainly sheered quartzofeldspathic gneisses in the process of crustal uplift or during retrograde metamorphism. Accordingly, in the case of charnockitization, source of CO_2 not necessarily originate in either mantle or underlain greenstone belt to cause decomposition of biotite into orthopyroxene. This report also thermodynamically examined how the oxygen fugacity and X (CO_2) vary in this process in terms of dehydration reactions observed in the rocks. In addition to this results, microthermometric data (homonization temperature, freezing temperature, and final melting temperature) of fluid inclusions in both charnockite and quartzofeldspathic gneiss represent that most of them are H_2O-rich in composition. On the … More other hand, in order to examine behavior of the fluids in the fracture zones, thermodynamic treatments for phase analysis and petrogenesis were summarized under conditions of P_s>P_f. The equilibria at P_s>P_f were applied to charnockitization of quartzofeldspathic gneisses and formation of the orthoamphibole isograd in the Limpopo belt. The fluids responsible for both charnockitization and formation of the orthoamphibole isograd are considered to have a wide range of composition and rather H_2O-rich at P_f<P_s, although they are CO_2-rich (X (H_2O) <0.2) at P_s=P_f. The calculation results at P_f<P_s may account various compositions of fluid inclusions in quartz in the charnockites. In the fracture system created during the crustal uplift, it is proposed that the "fluid path" with repetitive cycles of decreasing and increasing P_f has playd an important role for stabilization of orthopyroxenc. Generally migration (infiltration or influx) of fluids in the fracture zones gives rise to not only important dehydration and hydration reactions, but also may be related to formation of progressive, regionally metamorphosed rocks in the orogenic belt. Less