Ghost Granophyre: A New Quartz Cathodoluminescence Texture from the Troodos Ophiolite

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 represent a large volume of silicic material and are ubiquitous at the gabbro-sheeted dike contact. Our sample set consisted of a transect taken from across a plagiogranite pod in the Troodos ophiolite, Cyprus, extending into the gabbro body. We present a new CL texture, termed “ghost granophyre,” found in a heavily greenschist facies altered intermediate rock from near the gabbro-plagiogranite contact. This texture consists of quartz grains containing CL zoning in the characteristic, elongate, polyhedral shapes of granophyre. EPMA analyses indicate Ti is the CL activator causing this texture in quartz. Ti-in-quartz thermometry data and textural evidence suggest that the ghost granophyre occurs through replacement of the albite in primary magmatic granophyre by quartz in a sub-magmatic process, resulting in homogeneous quartz grains with a strongly bimodal distribution of Ti concentrations. The CL-bright primary magmatic quartz within the ghost granophyre records temperatures of 800C-900C, whereas the CL-dark secondary quartz with the ghost granophyre, as well as significant amounts of quartz found in each plagiogranite sample across the transect, contains concentrations of Ti below the EPMA detection limit of 20ppm, recording temperatures of 481C-580C (at a_Ti ^liq = 0.5, P = 0.5kbar, Zhang et al 2020 calibration.) The location of ghost granophyre-containing quartz grains inside of and rimming large chlorite glomerocrysts suggests that its origin is related to the process of chloritization and greenschist facies alteration. This could be the result of traditional hydrothermal processes or it could be related to the thermal gradient-driven processes and low temperature peralkaline melts which Lundstrom 2020 attributed to both greenschist metamorphism and plagiogranite genesis. The latter possibility holds implications for the origins of plagiogranites, which constitute a ubiquitous yet poorly understood lithology in the oceanic crust.