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 thus its properties are used as an indicator for specific conditions of formation and for the reconstruction of geological processes [1
]. In particular, cathodoluminescence (CL) properties are useful, since numerous studies of the luminescence behavior of quartz have shown highly variable characteristics depending on the specific P,T,X conditions during quartz formation [3
Investigations of natural and synthetic quartz specimens showed various luminescence emission bands, which cause the visible luminescence colors under the electron beam [6
]. The visible CL of natural quartz mainly consists of two broad emission bands centered at ~450 nm (blue emission) and 620–650 nm (red emission). The blue band is usually very broad and consists of up to four overlapping component bands centered at 390, 420, 450, and 500 nm [12
]. The most frequent 450 nm emission is due to the recombination of self-trapped excitons, which involves an irradiation-induced oxygen Frenkel pair consisting of an oxygen vacancy and a peroxy linkage (≡Si–O–O–Si≡) [6
]. The orange to red emission band at about 620–650 nm has been detected in almost all synthetic and natural quartz types. This emission is attributed to the recombination of electrons in the non-bridging oxygen band-gap state with holes in the valence band [13
] established one of the first classification schemes of quartz based on its CL colors, which he used for the provenance analysis of detrital quartz in sandstones. Over the past decades a considerable number of such methodological provenance studies have been published based both on CL colors and the spectral characteristics of quartz [13
]. The recent state of a general classification of quartz from different environments based on investigations of a wide spectrum of quartz-bearing rocks showed that quartz of igneous, volcanic, hydrothermal, pegmatitic, and sedimentary origin can mostly be recognized due to their specific CL colors, spectra, and textures, e.g., [4
]. In contrast, the knowledge about the CL of metamorphic quartz, in particular the development and possible transformation of CL characteristics during ongoing metamorphic processes, is still incomplete.
] distinguished brown luminescent quartz (major peak around 620 nm and minor peak near 450 nm) of low-grade metamorphic rocks or slowly cooled high-grade metamorphic rocks from violet or blue luminescent quartz (major peaks at 450 and 620 nm) of high-grade metamorphic rocks, which have undergone relatively fast cooling. Similar observations were made by Sprunt et al. [23
], who found a relationship between CL color of quartz and metamorphic grade of naturally deformed quartzite. The originally different CL colors in the undeformed parent rock material change toward a uniform brownish-red color through high-grade levels of metamorphism. Owen [24
] suggested that quartz CL turns to a uniform reddish-brown color above the garnet zone during high-grade metamorphism. Augustsson and Reker [21
] indicated that the change from quartz with brown/dark blue CL to brighter blue CL takes place at ca. 500–600 °C in amphibolite facies. So, metamorphic quartz that recrystallizes at high temperatures (e.g., granulite) reverts to a blue CL color comparable to that of plutonic quartz [4
]. In addition, diffusion during metamorphic processes as well as fluid-controlled recovery seems to influence the CL characteristics of quartz in high-grade metamorphic rocks. For instance, heterogeneous internal textures in quartz and a correlation of the blue CL with the Ti content have been observed [25
The literature data show that CL of quartz in metamorphic rocks is relatively complex and not all factors influencing the CL behaviour are known. The present study aimed to enhance knowledge about the CL characteristics of quartz in different metamorphic rocks and possible variations’ dependence on the metamorphic conditions. The precondition for the detection of changes in CL properties of metamorphic quartz is the systematic investigation of a metamorphic profile covering distinct metamorphic grades in a defined geological frame. Therefore, the study was based on samples from a profile along the Gomatum and Hoarusib valleys within the Kaoko belt (Namibia), made up of rocks of greenshist to granulite facies. The mineralogical composition and thermobarometric development of these rocks are well known [27
], so the CL data can be discussed in the established petrogenetic context. Combined analyses by polarizing microscopy and CL microscopy and spectroscopy provided new data concerning the CL characteristics of quartz in different grades of metamorphic rocks and revealed possible changes in the development of the CL properties through ongoing metamorphism.
2. Geological Background and Sample Material
The Kaoko belt is located in northwestern Namibia and represents a Neoproterozoic orogen, which was generated during the Pan-African-Brasiliano orogenic cycle in West Gondwana [28
]. Simultaneously to this orogenesis, a collision between the Kongo, Kalahari, Rio-de-la-Plata, and São-Francisco cratons took place, forming these belt systems with the Kaoko, Damara and Gariep belt on the African side and the Dom Feliciano and Ribeira belts on the South American side [29
The Kaoko belt has been subdivided by Miller [30
] into three tectono-stratigraphic zones, Eastern, Central and Western. The NNW to SSE trending Sesfontein-Thrust and the NNW to SSE trending Puros Mylonite Zone represent the boundaries between these tectono-stratigraphic zones (Figure 1
). The Eastern Kaoko Zone is characterized by low-grade metamorphic to unmetamorphosed sedimentary and carbonate rocks of the Otavi and Mulden groups. The Central Kaoko Zone includes Archean to Palaeoproterozoic orthogneiss and metasedimentary rocks. The metamorphic grade in the Central Zone increases from greenschist facies in the east to amphibolite facies in the west [27
]. Mesoproterozoic migmatite and Palaeoproterozoic gneiss are dominant in the eastern part of the Western Kaoko Zone, whereas the western part is mostly composed of Neoproterozoic Pan-African granitoid and metagranitoid [31
]. The metamorphic grade of the Western Kaoko Zone is characterized by a high-temperature/low-pressure Buchan-type metamorphism [27
The selected samples are mainly gneiss and mica schist of 500–750 °C metamorphic temperatures from a profile along the Gomatum and the Hoarusib valleys in the Central and Western Kaoko Zone (Figure 1
). Different metamorphic zones with specific mineral associations can be described along this profile with increasing temperatures from east to west (Figure 2
Garnet zone: 500 ± 30 °C, 9 ± 1 kbar
Staurolite zone: 580 ± 30 °C, 7–8 kbar
Kyanite zone: 590 ± 30 °C, 6.5–8 kbar
Kyanite-sillimanite-muscovite zone (ky-sill-mu zone): 650 ± 20 °C, 9 ± 1.5 kbar
Sillimanite-K-feldspar zone (sill-ksp zone): 690 ± 40 °C, 4.5 ± 1 kbar
Garnet-cordierite-sillimanite-K-feldspar zone (grt-cd-sill-ksp zone): 750 ± 30 °C, 4.0–5.5 kbar
The garnet, staurolite, kyanite, and kyanite-sillimanite-muscovite zones are located in the Central Kaoko Zone along the Gomatum Valley (Table 1
, Figure 2
). The samples from these metamorphic zones are mostly composed of biotite, muscovite, plagioclase, quartz, and some characteristic minerals from those specific zones such as garnet, staurolite, kyanite, and sillimanite.
The garnet-cordierite-sillimanite-K-feldspar zone is situated in the east of the Western Kaoko Zone and the rocks also consist of biotite, muscovite, plagioclase, and quartz, whereas the characteristic minerals are K-feldspar, garnet, sillimanite, and cordierite (Table 1
, Figure 2
3. Analytical Methods
Polished thin sections were prepared for microscopic and cathodoluminescence (CL) investigations from all samples listed in Table 1
. Polarizing microscopy was carried out using a Zeiss Axio Imager A1m (ZEISS Microscopy, Jena, Germany) to document the mineral composition and micro-texture of the different rock types. Micrographs were obtained with a digital camera (MRc5) coupled with Axiovision software (ZEISS Microscopy, Jena, Germany).
CL microscopy and spectroscopy were performed on carbon-coated thin sections using a hot-cathode CL microscope HC1-LM (LUMIC, Bochum, Germany) [33
]. The system was operated at 14 kV and 0.2 mA (current density ca. 10 µA/mm2
) with a defocused electron beam. Luminescence images were captured during CL operations using a peltier cooled digital video-camera (OLYMPUS DP72, OLYMPUS Deutschland GmbH, Hamburg, Germany). CL spectra in the wavelength range 370 to 920 nm were recorded with an Acton Research SP-2356 digital triple-grating spectrograph with a Princeton Spec-10 CCD detector (OLYMPUS Deutschland GmbH, Hamburg, Germany) that was attached to the CL microscope by a silica-glass fiber guide. CL spectra were measured under standardized conditions (wavelength calibration by an Hg-halogen lamp, spot width 30 µm, measuring time 5 s). Irradiation experiments were performed to document the behaviour of the quartz crystals under electron bombardment. Samples were irradiated 5 min under constant conditions (14 kV, 0.2 mA) and spectra were measured initially and after every 1 min. The evaluation of time-dependent spectral CL measurements provided further information about the stable or transient behavior under the electron beam and was indispensable for the identification of luminescence-active defect centres.
4.1. Garnet Zone
Sample GK-97-124, from the garnet zone in the eastern Gomatum Valley, is composed of quartz (10 vol % of the groundmass), biotite, muscovite, plagioclase, and distributed idiomorphic garnet porphyroblasts. Quartz is commonly xenomorphic, sometimes arranged in more or less monomineralic layers parallel to the foliation and frequently displays an undulatory extinction (Figure 3
a,b). It has weak blue luminescence stable under the electron beam (Figure 3
c) and the CL spectrum predominantly shows a weak emission band with a maximum around 450 nm and another band at about 650 nm (Figure 3
Sample GK 96-82, also from the garnet zone (in the western Gomatum Valley), which corresponds to the kyanite-sillimanite-muscovite zone (in the western part of the Central Kaoko Zone; Figure 2
), is dominantly composed of quartz (40 vol %; Figure 3
e), with similar properties to those of quartz described in GK 97-124. This quartz shows an intense short-lived blue-green luminescence, which is often heterogeneously distributed within the quartz crystals (Figure 3
f) and decreases under electron irradiation turning into a weak purple CL (Figure 3
g,h). The spectrum shows a broad emission band with a maximum around 490 nm and another weak emission band around 620–650 nm (Figure 3
4.2. Staurolite Zone
GK 96-67 from the staurolite zone is composed of staurolite and garnet prophyroblasts in a quartz, biotite, plagioclase and muscovite groundmass. Quartz (up to 15 vol % of the rock) also forms layers parallel to the foliation direction and usually shows undulatory extinction. Its properties are similar to that of quartz characterized in GK 96-82, with an intense short-lived blue-green luminescence, a dominating 490 nm and a weak 620–650 nm emission band in the CL spectrum.
4.3. Kyanite Zone
Quartz is up to 15 vol % in rock samples from the kyanite zone (GK 97-47 and GK 97-48) and forms the groundmass together with biotite, muscovite and plagioclase. It appears in almost all monomineralic layers across the samples and shows undulatory extinction (Figure 4
a). The is characterised by a strong short-lived blue-green luminescence, similar to that of sample GK 96-82 and GK 96-67 (Figure 4
c,d). The CL spectrum shows a strong emission band around 490 nm and the additional weak 620–650 nm emission (Figure 4
b), also similar to quartz in sample GK 96-82 (garnet zone) and GK 96-67 (staurolite zone).
4.4. Kyanite-Sillimanite-Muscovite Zone
The kyanite-sillimanite-muscovite zone (GK 97-127) is dominantly made up of quartz (30 vol %) together with kyanite, sillimanite, muscovite, biotite, and plagioclase. Quartz is anhedral with common undulatory extinction, and CL properties (an intense short-lived blue-green luminescence and emission band at ca. 490 nm) are similar to those of samples from the Central Kaoko zone.
The slight shoulder around 700–710 nm appearing in the CL spectra of GK 97-127 is probably not related to quartz but due to a CL signal from plagioclase, which is intimately intergrown with quartz (see bright areas in Figure 4
4.5. Garnet-Cordierite-Sillimanite-K-Feldspar Zone
Samples GK 97-06B, GK 97-14, GK 96-116 and GK 96-110 from the garnet-cordierite-sillimanite-K-feldspar zone show more or less similar mineralogical and spectroscopic properties. Rocks from this zone are made up of cordierite, sillimanite, K-feldspar, and garnet in the foliated matrix of biotite, plagioclase and quartz. Quartz (25 vol % of each sample) is mostly xenomorphic with undulatory extinction (Figure 4
e). Its CL properties are characterized by an intense blue luminescence (Figure 4
g,h). The spectrum shows an intense 450 nm emission band that slightly decreases during electron irradiation, whereas the intensity of the 650 nm emission increases (Figure 4
4.6. Summary of Results
The measured CL images for all studied quartz show exclusively bluish luminescence colors. Differences were detected in the intensity and homogeneity of the CL between different quartz grains and also within the same quartz crystals. In many samples, portions with short-lived bluish-green CL were observed, the intensity of which strongly decreased under the electron beam. The initial emission spectra of all studied quartz obtained by spectral measurements are made up of three main emission bands: a strong blue band at 450 nm, a second band at ca. 490 nm (a component band of a strong 500 nm emission overlapping with a weak 450 nm emission), and an emission band in the orange-red region at 620–650 nm. The presence or absence and relative intensities of these bands cause the visible CL colors.
The CL properties of quartz in metamorphic rocks from Kaoko belt (Namibia) are variable and mainly depend on the regional geological conditions than the metamorphic grade. 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. However, CL spectra reveal differences in the luminescence characteristics, which enable to distinguish different quartz types.
Mineral recrystallization, fluid mobility and incorporation of trace elements during quartz neoformation resulted in CL properties similar to those of quartz from igneous rocks (dominant blue band at 450 nm) and from hydrothermal/pegmatitic origin (short-lived bluish-green CL emission at 500 nm). Often these quartz grains show heterogeneous internal textures, sometimes with patchy bluish-green CL indicating a heterogeneous distribution of related luminescence centres.
The presented results have also a significant relevance for the use and interpretation of quartz CL for provenance studies in sedimentary petrology. Up to now, in particular the occurrence of quartz with brown CL (low-T) and blue CL (high-T) was used as indicator for metamorphic source rocks of detrital quartz in sediments. Because of the fact that bright blue quartz commonly indicates a plutonic origin, only in the case that little plutonic material is expected in the potential source areas, blue quartz can be assumed to be derived from high-T metamorphic rocks. In addition, the detection of transient bluish-green CL in high-T metamorphic quartz similar to that of hydrothermal/pegmatitic quartz limits its application in provenance studies of quartz-rich sediments.
One limitation of the present investigation is that the sample material represents exclusively polymineralic rock samples. Therefore, the data have to be proved by further investigations of monomineralic metamorphic rocks consisting only of quartz.