The gabbro to amphibolite transition along a hydration front

Abstract

In most metamorphic terrains the transition to the preserved mineral assemblage can be established to have occurred during progressive metamorphism through lower-grade facies involving fluid saturated conditions. However, in some circumstances metamorphism can occur in a non-progressive way as a distinct overprint of either igneous protoliths such as gabbro, or of earlier higher-grade metamorphic rocks such as granulites. In this contribution we use thermodynamic modelling, together with petrographic and mineral chemistry analysis, to study a suite of amphibolite facies metamafic rocks that were variably affected by metamorphism, with the objective of explaining the textures and mineral assemblages observed. The study samples form a continuum in terms of textural relationships and mineral compositions, but they can be divided into three main types: metagabbro, lineated metagabbro and amphibolite. Metagabbro samples preserve both relict igneous and metamorphic textures and a combination of igneous and metamorphic mineralogy. They preserve a variety of reaction textures, such as coronas of garnet around ilmenite, double coronas of low-Al (actinolitic) and high-Al (pargasitic) hornblende around clinopyroxene and limited degrees of recrystallization of igneous plagioclase. Lineated metagabbro samples preserve similar reaction textures as the metagabbro but show a strong lineation and have higher proportions of pargasitic hornblende and polygonal plagioclase combined with lower proportions of clinopyroxene, igneous plagioclase and actinolitic hornblende. Amphibolite samples are composed of pargasitic hornblende and polygonal plagioclase, presenting little igneous plagioclase and practically no garnet and clinopyroxene, and they preserve none of the reaction textures of the other two types. These samples are interpreted to be near chemically equilibrated on a hand sample scale, though with some mineral zoning preserved. P-XH2O and T-XH2O pseudosections show that the main mineral assemblage differences between the amphibolites and the other two types are primarily controlled by the amount of H2O that equilibrated with the rocks. While all rocks experienced an influx of fluid, only the amphibolites represent thermodynamically fluid-saturated conditions, the metagabbro and lineated metagabbro having equilibrated under H2O undersaturated conditions, allowing the presence of garnet. In parallel with the variation in fluid influx, the variation in microstructure appears to be the result of varying length scales of diffusion. The complex corona structures in the metagabbros are interpreted to reflect the pre-existing igneous texture and the apparent low mobility of Al during metamorphism. Here, igneous clinopyroxene is largely pseudomorphed by the actinolitic hornblende whereas the higher-Al pargasitic hornblende and garnet occur adjacent to plagioclase or replacing it. Zoning patterns in amphibole crystals in the amphibolite are similar to those in metagabbro samples, which is consistent with earlier stages of the evolution of the amphibolite actually being similar to those preserved in the metagabbro.