International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

REACTION CORONAS AROUND QUARTZ XENOCRYSTS IN THE BASALTIC ANDESITE FROM DETUNATA (APUSENI MOUNTAINS, ROMANIA)

Pages: 369 - 378

Authors: NICOLAE HAR

Abstract: Olivine-bearing basaltic andesite from Detunata (Apuseni Mts, Romania) contains xenolithic material of sedimentary origin. Fragments of sandstone as well as isolated xenocrysts of quartz originated from sandstone, were identified. Reaction coronas consisting of pyroxene, tridymite, quartz fragments and glass are developed around some of the sandstone fragments and quartz xenocrysts, as a result of interaction between the xenolithic material and the host basaltic magma. Coronas can be divided into two distinctive parts. The outer one consists of a glassy matrix containing prismatic crystals of augite. The inner part of the coronas consists of isolated crystals of augite, fragments of quartz and tridymite in a glassy groundmass. Tridymite is also present along fractures in the quartz xenocrysts. Electron probe microanalysis and Raman spectroscopy were used to investigate the corona pyroxenes, glass, and silica polymorphs. The corona pyroxenes have chemical compositions typical for augite. The glass is highly siliceous (SiO2 = 72.0–76.8 wt. %) as a result of quartz dissolution, high in alkalis (Na2O = 1.38–3.22 wt. %; K2O = 4.72–6.23 wt. %) and aluminum (Al2O3 = 9.31–12.18 wt. %). Tabular and twinned crystals of tridymite have a high content of alkalis (Na2O+K2O = 0.30–0.39 wt. %) and alumina (Al2O3 = 0.54–0.86 wt. %) as compared to quartz xenocrysts. Raman spectra of tridymite show the most representative peaks at 403 and 422 cm–1. The geochemistry of reaction corona, diffusion processes, and temperature were the controlling factors in the genesis of the newly-formed minerals in the reaction zones. The xenolithic material was partly assimilated, and represented an important source of Si4+, Al3+, Ca2+, Mg2+, Fe2+ and K+. The spatial relationship between reaction pyroxenes and host basaltic andesite suggests that magma could also be the source for cations such as Ca2+, Mg2+ and Fe2+. The relatively higher content in alkalis of the volcanic glass from the basaltic andesite groundmass as compared to the glass of the corona suggests an enrichment in alkalis of the late differentiated magmatic melt which could represent the main source of alkaline cations (Na+ and K+). The genesis of the corona took place at low pressure, during the eruption of the basaltic magma. The presence of tridymite in the corona indicates temperatures higher than 870 °C while the presence of calcite in some xenoliths points to temperatures below 920 °C.

Keywords: basaltic andesite, quartz xenocrysts, coronas of reaction, pyroxene, tridymite

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