Magma mixing and exsolution phenomena in peralkaline rhyolites: insights from the Gold Flat Tuff, Nevada

Abstract

The distribution and compositions of chevkinite-group minerals (CGMs) in the pantelleritic Gold Flat Tuff, Nevada, USA, are used to examine three aspects of the evolution of the tuff, which we feel are of general significance in peralkaline magmatism. First, both chevkinite-(Ce) and perrierite-(Ce) occur in certain facies, although normally these phases almost invariably occur in different igneous lithologies. Their co-occurrence in the tuff is due to the mixing of pantelleritic and intermediate magmas. Second, the tuff is the first recorded occurrence of a CGM in a pantellerite eruptive, with possible implications for the crystallization conditions. In particular, low values of aSiO 2 may have stabilized ilmenite + chevkinite rather than aenigmatite, although the unusually high LREE contents ( La-Sm ≤ 1517 ppm) in the pantellerite may have played a role. Third, an unusual lamellar texture in the CGM is revealed by Atomic Force Microscopy to be formed by a rutile-like phase. The lamellae may have formed by exsolution from a rutile-like layer in the crystal structure. An electron backscattered diffraction study of a single crystal showed a structural dislocation not apparent optically or by electron back-scattered imaging. This may have wider implications in mineralogical studies.

Hydrothermal alteration of chevkinite: insights from natural rocks and experiments

“…Material for these studies comes mainly from two sources. First, as part of our own field and petrological studies, e.g., UK Palaeogene granites (Macdonald et al 2013), high-K trachyandesites from Moravia (Macdonald et al 2017), the Kłodzko-Złoty Stok intrusion, Sudetes (Nejbert et al 2020), the Joe Lott Tuff, Utah (Galanciak et al 2020), the Gold Flat Tuff, Nevada (Macdonald et al 2021) and the Krasnopol intrusion, Mazury Complex (Domańska-Siuda et al 2022). Second, through ongoing collaborations with other organizations, e.g., the U.S. Geological Survey, GFZ Potsdam, the Russian Academy of Sciences (Mos cow and Apatity), the Polish Academy of Sciences (Kraków), and the Universities of Edin burgh and Milan.…”

Perspectives on alkaline magmas

“…But why it is so? Peralkaline rhyolites, termed comendites or pantellerites, and possibly many of their undersaturated counterparts (phonolites sensu latto), are largely believed to be the by-products of extensive (>80%) basalt fractionation [Macdonald et al, 2021a], carrying with them all elements not easily incorporated in the structure of the main rock-forming minerals. The excess of alkalies over alumina (i.e., peralkaline) that typifies oversaturated and most undersaturated alkaline felsic magmas, is a key contributing factor that boosts the potential of the silicate melt to accommodate many elements (ions having high field strength or large radius), which are otherwise excluded from its structure (such as it happens in metaluminous liquids).…”

“…In this special issue, some strands of the different aspects listed above are addressed or reviewed. Macdonald et al [2021a] report a detailed state of the art of the occurrence and petrogenesis of the quartz-saturated variety of the alkaline clan (comendite, pantellerite), reviewing inter alia the geodynamic context, mode of production and evolution till eruption of such magmas, using for that purpose all information available. Sautter and Payre [2021], provide an informative review of our current knowledge about the magmatic alkali rocks which have been revealed by orbiting spacecrafts or those scrutinized so far by the rovers/landers active on the surface of Mars, and the implications of these findings for magma production on Mars, as well as for the petrogenesis of Martian meteorites which were until recently the only accessible rocks of that planet.…”