Garnet composition as a petrogenetic indicator: An example from a marble - calc-silicate granulite interface at Kondapalle, Eastern Ghats Belt, India

Abstract

The P-T-fluid histories of aluminous garnet+clinopyroxene-bearing calc-silicate granulites are commonly constrained through interpretation of sequential reaction textures based on activity-corrected petrogenetic grids for model systems, for example the widely used CAS(CO₂-H₂O) topologies. Application of these topologies to calc-silicates of diverse bulk composition is limited by the fact that the role of clinopyroxene and hence of garnet composition in the garnet-forming reactions is ignored or not adequately considered. In this contribution the complexities that arise from the involvement of clinopyroxene in the garnet-forming reactions are critically evaluated. As an example, the paragenetic relations and compositional features of clinopyroxene and coronitic garnet across paired scapolite- and wollastonite-dominated reaction zones (scp+cpx± qtz,cal,qtz - wo+cal+cpx± scp) developed between marble layers (cal± cpx,wo,scp) and their host calc-silicate granulite (cpx+pl± qtz,scp) at an exposure in the high-grade Eastern Ghats Belt (India) are examined. The microtextural features and systematic changes of modal and phase compositions across the profile indicate that clinopyroxene, scapolite, plagioclase, quartz and calcite as well as wollastonite participated in the garnet-forming reactions. Using the sub-assemblage qtz+cal+cpx+scp+grt+v in the system CaO-FeO-Al₂O₃-SiO₂-CO₂ it is demonstrated that at the ambient P-T-X_fluid conditions, the reaction coefficients of the participating phases are uniquely defined by the garnet composition. As a result, the P-T slope of a garnet-forming reaction changes with the garnet composition, with sharp changes at "singular points" across which one or more phases change their sides with respect to garnet in a given reaction. To consider the consequences of these complexities on the reaction topology of calc-silicate granulites, a new petrogenetic grid for the system CaO-FeO-Al₂O₃-SiO₂-CO₂ involving clinopyroxene has been constructed. The effects of fluid composition (CO₂-H₂O-O₂), fluid pressure (P_fluid<P_load) and extra components in the participating phases (Fe₂O₃, Na₂O) on the topology are discussed. Application to several well-described calc-silicate occurrences and the studied site shows that the new petrogenetic grid provides an improved tool to constrain the P-T-X_fluid evolutionary history of calc-silicate occurrences of diverse bulk composition. Our analysis emphasizes the great potential of garnet composition not only as a monitor of the P-T-X_fluid regime but also for deciphering the garnet-forming reactions in high-grade rocks.