Fluid-induced redistribution of REEs within alumino-silicate veins and peraluminous host rock in the Central Indian Tectonic Zone

Geological fluids play crucial roles in the stabilisation of minerals and in the mobilization and redistribution of elements during mid- to lower-crustal metamorphism, thereby influencing the chemical evolution of continental crust. The current study demonstrates extensive fluid-induced alterations and formation of a highly peraluminous (A/CNK~30), ferromagnesian yet silica-calcium and alkali-poor rock from the Makrohar granulite belt, part of the Proterozoic mobile belt, Central Indian Tectonic Zone. The study demonstrates a black, garnet-rich, massive rock composed of garnet, cordierite, sillimanite, quartz, and ilmenite, lacking gneissic banding and intruded by multiple veins in the outcrop. Thin microscopic veinlets consist of biotite (80% modal volume) with smaller proportions of quartz±cordierite and fibrolites. Late-stage veins of variable thickness, evident from the outcrop scale, contain coarse-grained sillimanite-quartz-garnet, with large sillimanite grains growing at high angles to the vein boundary, indicating a syntaxial growth. Vein garnets frequently grow inward from the vein wall, often growing on older garnet in the host. Phase equilibrium modelling, coupled with conventional thermobarometry, constrains the P-T conditions of the stabilization of the host rock at approximately 600ºC and 5 kbar. Slightly magnesian cores of the host garnet (X_Mg) yield marginally higher temperatures (~680ºC) than the rim (garnet isopleth yielding ~580ºC).

Garnet grains in the host rock display a distinct positive europium anomaly (Eu/Eu*), likely resulting from garnet growth in the absence of plagioclase. A moderate Gd/Dy ratio in the host garnet indicates stabilization at approximately 4 kbar, supporting low-pressure estimates from conventional barometry and phase-equilibria modelling. Rim-to-rim trace element profiles along host garnet grains show a uniform distribution of Sc, Y, and HREEs in the core, with oscillations and a sharp increase near the rim, suggesting that reverse zoning in HREEs was likely caused by homogenization by intragranular diffusion in the core but remained largely unaffected towards the rim. Whole-rock chemistry of the host, feldspar-free high-variance mineralogy, absence of leucosome and reverse zoning of Y-HREE, positive Eu/Eu* within garnet indicate potential metasomatic alteration of the host itself.

Garnets within the quartz-sillimanite veins exhibit distinct oscillations in trace element concentrations along wall-to-wall line scans, indicating minimal effects of diffusion and grain growth in the presence of vein-fluid. Ca and Mn zoning within vein garnet exactly replicate each other with gradual increase from vein-wall to vein-axis regions of the grains. Y and HREEs show resonating patterns with sharp central peaks in the mid-axis and oscillatory zoning within the vein-wall garnet portions. Sc and MREEs, i.e., Sm, Eu, Gd, Tb still show central peaks along with an annular maxima added with the oscillation within the vein-wall garnets. REE mobilization, at least in micro-scale, is further evident from large monazite clusters observed in sillimanite-quartz-garnet veins. The presence of large sillimanite grains further demonstrates the fluid's capacity to transport aluminium. The absence of any hydrous phases in the vein supports the prevalence of low-H₂O-bearing fluid. XCO2-µK2O and µK2O-µFeO topology further confirms that the intrusion of low-H₂O fluid presumably destabilized the host biotite, producing garnet and quartz in the vein.