Abstract
Quartz of metamorphic rocks from the Kaoko belt (Namibia) representing metamorphic zones from greenshist to granulite facies were investigated by cathodoluminescence (CL) microscopy and spectroscopy to characterize their CL properties. The samples cover P-T conditions from the garnet zone (500 ± 30 °C, 9 ± 1 kbar) up to the garnet-cordierite-sillimanite-K-feldspar zone (750 ± 30 °C, 4.0–5.5 kbar). Quartz from 10 different localities and metamorphic environments exclusively exhibits blue CL. The observed CL colors and spectra seem to be more or less independent of the metamorphic grade of the host rocks, but are determined by the regional geological conditions. Quartz from different localities of the garnet-cordierite-sillimanite-K-feldspar zone shows a dominant 450 nm emission band similar to quartz from igneous rocks, which might be related to recrystallization processes. In contrast, quartz from different metamorphic zones in the western part of the central Kaoko zone (garnet, staurolite, kyanite, and kyanite-sillimanite-muscovite zone) is characterized by a heterogeneous blue-green CL and a dominant 500 nm emission band that strongly decreases in intensity under electron irradiation. Such CL characteristics are typical for quartz of pegmatitic and/or hydrothermal origin and indicate the participation of fluids during neoformation of quartz during metamorphism.
Highlights
Quartz is one of the most important constituents of the Earth’s crust and the most frequent silica mineral
Quartz deriving from rocks covering P-T conditions from the garnet zone (500 ± 30 ◦ C, 9 ± 1 kbar) up to the garnet-cordierite-sillimanite-K-feldspar zone (750 ± 30 ◦ C, 4.0–5.5 kbar) exclusively exhibits visible blue CL
Fluid mobility and incorporation of trace elements during quartz neoformation resulted in CL properties similar to those of quartz from igneous rocks and from hydrothermal/pegmatitic origin
Summary
Quartz (trigonal alpha-quartz) is one of the most important constituents of the Earth’s crust and the most frequent silica mineral. It occurs in magmatic, metamorphic, and sedimentary rocks and its properties are used as an indicator for specific conditions of formation and for the reconstruction of geological processes [1,2]. The visible CL of natural quartz mainly consists of two broad emission bands centered at ~450 nm (blue emission) and. The orange to red emission band at about 620–650 nm has been detected in almost all synthetic and natural quartz types
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