PORPHYROBLASTIC MICROSTRUCTURES AND METAMORPHIC GRADE: CONTROLS ON SYSTEMATIC TRENDS FROM A HIMALAYAN INVERTED BARROVIAN SEQUENCE (Invited Presentation)

The degree to which a rock volume is equilibrated during a metamorphic mineral reaction is likely to impact the size, spatial distribution, morphology and/or chemical compositions of product porphyroblasts. Therefore, it has long been recognized that insight into both the petrogenetic processes operating in a rock during metamorphism and the relative influence of microstructure-controlling parameters may be gleaned by augmenting geochemical approaches with comprehensive microstructural characterization. Here, we present results from across the inverted Barrovian sequence in the Sikkim Himalaya. X-ray µ-computed tomography constraints highlight a correlation between the average radii and abundance of garnet populations and grade through continuous portions of the sequence from the garnet to kyanite zones. Incipient garnet nucleation in all zones is predicted to occur at non-systematic temperatures between 495 ˚C and 520 ˚C, and phase equilibria approaches effectively reproduce variations in porphyroblast volume and chemical compositions from low-to-high grade.

Despite the efficacy of these models at constraining the metamorphic evolution of Sikkim, assumptions inherent to this approach do not account for kinetic controls and impedances on crystallization or the development of microstructure. The relative influence of some of these controls can be discerned in anomalously late-crystallized garnet populations, where the distribution of favorable nucleation sites was dictated by the interplay between heterogeneous bulk compositions, epitaxial crystallization, and strain-assisted nucleation. These irregular porphyroblastic microstructures also evince coeval transport-controlled and interface-controlled garnet growth in cases where effective matrix diffusivities of rate-limiting components were domainal. Conversely, systematic microstructure–grade trends across the inverted Barrovian sequence may be linked to progressive ripening of garnet populations in samples characterized by rapid rock-wide intergranular diffusivities and interface-controlled growth. If so, rocks from Sikkim highlight the critical influence interfaces and their associated energies can exert on the evolution of metamorphic microstructures beyond the nucleation phase.