F. Gaidies

Characterisation of a garnet population from the Sikkim Himalaya: insights into the rates and mechanisms of porphyroblast crystallisation

The compositional zoning of a garnet population contained within a garnet-grade metapelitic schist from the Lesser Himalayan Sequence of Sikkim (India) provides insight into the rates and kinetic controls of metamorphism, and the extent of chemical equilibration during porphyroblast crystallisation in the sample. Compositional profiles across centrally sectioned garnet crystals representative of the observed crystal size distribution indicate a strong correlation between garnet crystal size and core composition with respect to major end-member components. Systematic steepening of compositional gradients observed from large to small grains is interpreted to reflect a progressive decrease in the growth rate of relatively late-nucleated garnet as a result of an increase in interfacial energies during progressive crystallisation. Numerical simulation of garnet nucleation and growth using an equilibrium approach accounting for chemical fractionation associated with garnet crystallisation reproduces both the observed crystal size distribution and the chemical zoning of the entire garnet population. Simulation of multicomponent intracrystalline diffusion within the population indicates rapid heating along the pressure–temperature path, in excess of 100

Population-wide garnet growth zoning revealed by LA-ICP-MS mapping: implications for trace element equilibration and syn-kinematic deformation during crystallisation

^{\circ }

Characterization of polymetamorphism in the Austroalpine basement east of the Tauern Window using garnet isopleth thermobarometry

∘C Myr

THERIA_G: a software program to numerically model prograde garnet growth

^{-1}

Constraining the conditions of Barrovian metamorphism in Sikkim, India: P–T–t paths of garnet crystallization in the Lesser Himalayan Belt

-1. Radial garnet growth is correspondingly rapid, with minimum rates of 1.4 mm Myr