Biotite- and garnet-forming reactions in inverted metamorphic zones in the Balquhidder region of the Scottish Dalradian

Abstract

Synopsis Biotite and garnet zones in the west-central Scottish Dalradian are inverted; higher grade rocks overlying lower grade rocks. Structural and textural information indicates that inversion is not due to overfolding of metamorphic zones. Peak metamorphic conditions were attained in a negative thermal gradient. Biotite forms when phengite becomes sufficiently aluminous, by means of the grade-controlled tschermak substitution, for the chlorite-phengite-biotite three-phase field to enclose the composition of the host rock on an AKF diagram. The reaction is controlled by whole-rock Al-content and metamorphic grade and has the form: chlorite + phengite → muscovite + biotite + quartz + fluid. Garnet may form by any of three continuous reactions, each dependent on rock composition; the dominant reaction (incorporating the biotite-forming reaction) has the form: chlorite(1) + phengite(1) + plagioclase(1) + calcite(1) → chlorite(2) + phengite(2) + plagioclase(2) + calcite(2) + biotite + garnet + fluid. The reactions form garnet principally at the expense of chlorite but involve all phases in the rock. The amount of garnet formed depends critically on whole-rock Mn-content. Trends in rock and mineral compositions across the biotite and garnet zones reveal nothing unusual to illuminate the problem of metamorphic inversion. Although the absence of steps, or changes of gradient, in mineral-chemical trends renders syn- or post-metamorphic thrusting an unlikely solution.