Ghost granophyre: A new texture with implications for plagiogranite differentiation and Ti-in-Quartz thermometry

Abstract

Understanding the origins of the Earth and its crust is a fundamental goal of the geosciences and the construction of the oceanic crust comprises half the story of our planet’s unique bimodal crustal distribution. The oceanic crust is overwhelmingly mafic with the exception of plagiogranites, which can represent relatively extensive volumes of silicic material locally and are common at the gabbro-sheeted dike contact. The origin of plagiogranites is poorly understood. Granophyre, which is also poorly understood, is a common texture in plagiogranites as well as other granitoids. I present observations of “Ghost Granophyre,” a new cathodoluminescence texture of granophyre in Troodos quartz crystals which I argue results from pseudomorphic replacement of the albite in granophyre by quartz. In combination these observations point toward both a new model for plagiogranite differentiation via reactive flow of recently described low temperature silicate liquids which span the magmatic-hydrothermal transition, as well as a way to make granophyre without the involvement of a silicate melt. These findings expand the definition of the term granophyre, which still sees heavy usage in the petrology literature as being synonymous with rapid crystallization of a silicate melt, to include granophyre formed through fluid-crystal interactions that are hundreds of degrees subsolidus with respect to traditional silicate melts. This work also brings into question the applicability of the Ti-in-Quartz geothermometer—a powerful and widely used tool for estimating crystallization temperatures in silicic igneous lithologies generally—to situations like Troodos where pervasive mineralogical reactions occur and drive bulk compositional differentiation to silicic lithologies. If a near-ubiquitous “igneous” texture like granophyre can be created without a silicate melt, then long-held assumptions regarding other “igneous” textures should be reexamined.