Growth of coronal garnet in anhydrous high-Fe ultrabasic rocks: The interplay between metamorphic reaction progress, local strain heterogeneity and grain boundary diffusion

Abstract

Coronal garnets at pyroxene-plagioclase interfaces are traditionally used to estimate P-T conditions and reconstruct P-T paths. However, the mechanism by which the coronal garnet grows is poorly constrained. We address the issue based on the analyses of textures, mineral compositions, and crystallographic orientations of coronal garnets and associated minerals. In the Bolangir anorthosite massif (Eastern India), coronal garnets are common at the interfaces of plagioclase phenocrysts and Fe-rich pyroxenes within ferrodiorites, and at ferrodiorite-anorthosite interfaces. The plagioclase phenocrysts comprise weakly-recrystallized and chemically homogenous An-rich cores (Pl sub(1)) that are partly and/or completely replaced by mantles of smaller, chemically-zoned, dynamically recrystallized An-poorer plagioclase (Pl sub(2)) grains. Chemically similar beads and coalescent grains of garnet occur inward within the recrystallized Pl sub(2) mosaic lacking ferromagnesian minerals/Fe-Ti oxides. Aggregates of elongate garnet grains oriented orthogonal to ferrodiorite-anorthosite interfaces also form continuous corona layers. The coronal garnets formed at peak P-T conditions (900 degrees C, less than or equal to 7 kb) due to the NC(FM)AS continuous reaction Pl sub(1) + Opx + Cpx → Pl sub(2) + Grt, and progressed from rim inwards into the phenocrysts. Well-faceted garnet corona associated with Pl sub(2) exhibits a random crystal preferred orientation (CPO). However, low-angle subgrain boundaries, cellular microstructure, internal lattice distortion and orientation dispersion with rational crystallographic rotation axis in the coronal garnets indicate locally developed growth-induced stress field affecting the garnets. The growth of coronal garnets hosted within Pl sub(2) well inside the plagioclase necessitates the transport of Fe, Mg from the melanocratic matrix along distances comparable to the radii (approx. 350 micrometre) of the phenocrysts. We argue that grain boundary diffusion along networks of grain boundaries of Pl sub(2) provided pathways for Fe, Mg transport to the zones where coronal garnets formed via reactions that consumed Pl sub(1) and produced Pl sub(2). The formation of garnet induced heterogeneous strain development within the plagioclase phenocrysts, intense at the plagioclase margin and neighboring coronal garnets. The lack of CPO in the garnets attests to the absence of far-field stress; but dislocations and lattice distortion indicate that local stresses developed to accommodate garnet growth within plagioclase. In other words, garnet grew by a feedback mechanism that linked chemical potential gradient with the elastic contribution induced by the time-space varying local stresses generated by the growing garnet grains within the confines of the plagioclase phenocrysts.