Fluid-induced changes in suites of high-grade rocks along the Mahanadi Shear Zone in northern Eastern Ghats Belt, India

The present study focuses on the complex fluid-induced processes involved in the evolution of two suites of high-grade rocks from the northern part of the Eastern Ghats Belt, India. We make a comparative study on the role of fluids in the modification of lower crustal rocks; the felsic gneiss/granite mylonite in the Mahanadi Shear Zone (MSZ) and investigate the origin of several micrometres to meters thick syenite veins hosted in and at the contacts of mafic granulite and charnockite away from the MSZ. The syenite (K-feldspar-hyalophane-clinopyroxene-titanite-fluorapatite-allanite-epidote-calcite/REEcarbonates-actinolite-quartz with or without ilmenite-thorite-zircon) is coarse-grained and bears mineralogical features distinct from either side of the contact. We document features like orthopyroxene changing to clinopyroxene, anorthitic rims on plagioclase, and myrmekite patches at the syenite and mafic granulite interface. K-feldspar and hyalophane occur in coarse-recrystallized pockets with the latter often occurring along grain boundaries of the former. Fluorapatite grains occur as euhedral megacrysts and are marginally replaced by patchy as well as large crystals of allanite (typically zoned and consisting of thorite inclusions), epidote, and actinolite grains. The entire assemblage is infiltrated by calcite veins and patches. We interpret this as a metasomatic transformation of the original charnockite rock (orthopyroxene- K-feldspar- quartz ± ilmenite and titanite) that was driven by late-stage magmatic fluid, charged with CO₂-F-H₂O species. This late fluid could have mobilised Ca from the mafic granulite and formed the syenite veins. An alternative mechanism by syenite magmatism looks like a distant possibility as found in the north-western margin of the belt. The felsic gneiss from the MSZ also hosts evidence of a channelised fluid flow associated with shearing. Textures of K-feldspar micro-veins and patches in and around quartz and plagioclase matrix are likely to be caused by fluid action. Monazites in this rock preserve extensive fluid alteration signatures, indicated by compositional zoning, sub-domains and resetting of ages, suspected to have been caused by the process of coupled dissolution-reprecipitation. The unaltered monazite shows distinct age signatures in the range ca. 1000-900 Ma which presumably implies the timing of a major tectonothermal event that joined the Angul domain in the north and the Phulbani domain in the south. The fluid-mediated monazite domains, on the other hand, show a spectrum of ages in the range ca. 890-810 Ma. Our combined mineralogical-textural and geochronological study thus identifies a channelised fluid event during pervasive shearing associated with the amalgamation of two crustal domains of the Eastern Ghats Belt.