Publication details
Genesis of spherules in Permian rhyolites from the Zamtyn Nuruu area
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| Year of publication | 2024 |
| Type | Conference abstract |
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| Description | Dikes of peralkaline rhyolite cut across Permian and older rock beds in the Zamtyn Nuruu area of western Mongolia. During mapping expeditions conducted by the Czech Geological Survey between the years 2005 and 2007, it was discovered that these dikes contain high-temperature crystallization domains, including spherulites. Studied spherulites and host rhyolites were characterized from thin section samples with polarized light microscopy and electron microprobe. Both spherulites and the host rock were composed mainly of quartz and feldspar. Sampled plagioclase was identified as albite (Ab93–100 Or0–7). All of K-feldspar had the composition of orthoclase (Ab2–5 Or95–98). Pyroxene grains were identified as aegirine (Aeg61–94 Jd5–39), amphiboles as ferro-eckermannite (Na 1.78–1.94 apfu). The spherulites crystalized from supercooled rhyolitic melt, often around subautomorphic crystals of quartz, feldspar or around vesicles, and grew as fibrous crystals of quartz or feldspar from the centre outward at the same rate in all directions. Crystals of amphibole or pyroxene formed concurrently were incorporated into the structure. The spherulites ranged in diameter from a few mm to about 3 cm and were subdivided into three types based on observed textures and mineral composition: Type one contained a crystallization centre and was composed of intergrown fibrous crystals of quartz and feldspar and included amphibole and pyroxene crystals. Larger samples of the type one spherulites were distinguished by visible zoning. The second type contained no visible crystallization centre and was mainly composed of albite and included pyroxene, amphibole, or hematite crystals. Type three also did not contain a visible crystallization centre was composed entirely of albite. The rock exhibited signs of later hydrothermal alteration. Primary magmatic minerals were replaced by albite ± chlorite ± quartz pseudomorphs, chalcedony, limonite, kaolinite, smectite and muscovite. |