The interplay between matrix mineralogy and garnet nucleation and growth, in a polymetamorphosed multilayered biotite schist from the northern boundary of the Western Dharwar Craton, India

Abstract

Garnet zoning patterns are fundamental to reconstructing metamorphic pressure-temperature-time (P-T-t) paths, traditionally interpreted under the assumption of local thermodynamic equilibrium. However, distinguishing between the effects of regional P-T evolution and local kinetic controls, such as matrix mineralogy and transport efficiency, remains a significant challenge. This study examines a centimetre-scale, polymetamorphosed, multilayered garnetiferous biotite schist (GTQ) from within the Paleoproterozoic South Maharashtra Shear Zone (SMSZ), India, comprising four layers (L1-L4) with broadly similar mineral assemblages but contrasting mineral proportions, grain sizes, and textures. Because these layers, within a single hand specimen, experienced an identical P-T-deformation history, the sample serves as a natural laboratory to isolate the influence of layer-scale chemical and mineralogical heterogeneity on garnet nucleation and growth. Integrating petrography, electron probe microanalysis, X-ray element mapping, and geothermobarometry, we demonstrate that matrix mineralogy exerts a first-order control on garnet kinetics. In the Fe-rich L2 layer, garnet nucleation was triggered by the pseudomorphic replacement of a metastable Fe-rich amphibole, recording strong reaction overstepping. In contrast, garnet grains in the L1, L3, and L4 layers nucleated heterogeneously on biotite in phyllosilicate-rich microdomains, exhibiting higher nucleation densities and interface-controlled growth. Despite these divergent nucleation histories, garnet across all layers records a shared prograde P-T growth trajectory (approx. 490- 530 degrees C, 6.1-6.4 kbar) followed by a distinct late-stage isothermal loading event defined by Ca-rich rims. The subsequent localized fluid infiltration and retrograde dissolution, controlled by layer-specific rheology and bulk chemistry, further demonstrate the influence of microscale heterogeneity on fluid-rock interaction during deformation. Our results reveal a partial decoupling in garnet growth: while zoning architecture is governed by regional P-T evolution, nucleation density and the volumetric growth of garnet are primarily dictated by matrix-mediated kinetic controls. This study therefore underscores the importance of integrating microstructural archives with P-T constraints when interpreting garnet zoning and reconstructing metamorphic histories.