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

根据南部非洲林波波带各种高级变质岩的矿物学、岩石学和化学特征，认为橄榄石形成于P_&lt；Solid&gt；(=P_S)&P_&lt；总流体&Gt；(=P_f)，在地壳抬升过程中或退变质过程中，与主要来自剪切石英片麻岩的流体相互作用。因此，在炭黑岩化的情况下，CO_2的来源不一定来自地幔或下伏绿岩带，导致黑云母分解为斜方辉石。这份报告还从热力学上考察了氧逸度和X(CO_2)在这一过程中的变化，即在岩石中观察到的脱水反应。此外，紫苏花岗岩和石英长石片麻岩中流体包裹体的显微测温数据(均一化温度、冻结温度和最终熔融温度)表明，它们的成分大多富含H2O。在…上另一方面，为了考察流体在断裂带中的行为，总结了在P_S和P_f条件下进行物相分析和岩石成因的热力学处理，并将P_S和P_f的平衡应用于石英片麻岩的碳质玄武化和林波波带正角闪石等值线的形成。在P_f&lt；P_S是富CO_2(X(H_2O)&lt；0.2)的流体，但在P_f&lt；P_是富CO_2的流体。P_f&lt；P_S的计算结果可以解释沸石岩中石英中流体包裹体的各种成分。在地壳隆升过程中形成的断裂系统中，P_f递减循环的“流体通道”对斜方辉石的稳定起到了重要作用。通常情况下，流体在断裂带的迁移(渗透或流入)不仅引起重要的脱水和水化反应，而且可能与造山带递进的区域变质岩石的形成有关。较少

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