The Gold Flat Tuff, Nevada: Insights into the evolution of peralkaline silicic magmas

Abstract

The Gold Flat Tuff is the youngest (9.15 Ma) ash-flow sheet erupted from the Black Mountain Volcanic Centre, southwest Nevada, USA. This paper explores some aspects of the very complex nature of the tuff's plumbing system. The main body of the deposit is a mixed magma product, comprising pantelleritic and comenditic melts derived from independently evolving reservoirs, and antecrysts and enclaves derived from a range of basic to intermediate sources. Metre-scale cognate xenoliths point to the presence of alkali feldspar accumulation zones. The pantellerite contains phenocrysts of fluorite and chevkinite-(Ce). The inferred intermediate magma component contains perrierite-(Ce) phenocrysts. The pantellerite has unusually high contents of F (≤2.2 wt%), F+Cl (≤2.9 wt%) and ZrO2 (≤1.04 wt%). The high halogen contents may have influenced the evolution of the strongly peralkaline magma. The crystallization conditions are poorly constrained but those for the pantelleritic magma may have been close to water-saturation (>4 wt% melt water) at temperatures ~740 °C and fO2 around FMQ.

Experimental and thermodynamic constraints on mineral equilibrium inpantelleritic magmas

“…Aenigmatite occurs in assemblages with ilmenite and olivine (>92 mol% Fa) over a wide range of temperature (<800°C) and oxygen fugacities (NNO-2.1 to NNO-0.9), but only at high silica activities where it frequently occurs in assemblages with quartz. The control of silica activity J o u r n a l P r e -p r o o f on aenigmatite crystallization has also been noted by Macdonald et al (2011Macdonald et al ( , 2019, who described the antipathetic relationship between fayalite and aenigmatite as: This reaction is plotted in Figure 11b. The position of the curve as plotted is based on the activities of ilmenite, and fayalite from the highest-temperature sample with the fayalite-ilmenite-aenigmatite assemblage (the tuhualite-bearing 160541; Bagiński et al, 2018).…”

Relationships between monazite, apatite and chevkinite-group minerals in the rhyolitic Joe Lott Tuff, Utah, USA

“…In their study of the pantelleritic Gold Flat Tuff, Black Mountain volcanic centre, Nevada, Macdonald et al (2019b) described a mixed-magma rock containing both chevkinite-(Ce) and perrierite-(Ce). In this case, and in contrast to the Joe Lott Tuff, the rock hosting both phases was clearly a mixture of rhyolitic and intermediate magmas.…”

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

“…: mol. (Na 2 O + K 2 O/Al 2 O 3 )) in the range 1.41-1.83 [Macdonald et al, 2019b]. SiO 2 ranges from 70.7 to 73.5 wt% and FeO* from 5 to 7 wt%.…”

“…The tuff shows complex vertical and lateral variations in composition and texture, related to a very complex evolutionary history which is still poorly understood. For example, Macdonald et al [2019b] presented mineral chemical evidence, such as a range in feldspar compositions from An 79−0 and in olivine compositions from Fo 54.5−1.9 , indicating that melts ranging from basaltic through trachyandesitic to comenditic mixed with the pantellerite prior to, or during, eruption.…”

“…We know of no other peralkaline rhyolite with such elevated contents of these elements. Macdonald et al [2019b] used trachybasalts (belonging to the Basalt of Thirsty Mountain, geographically associated with the Gold Flat Tuff but ∼4.5 Ma older than it) as proxies for the intermediate component of the tuff. They are hy ± ol ± q-normative, with SiO 2 50-51 wt% and Mg-number 0.35-0.46 (Mg/(Mg + Fe), with all Fe calculated as Fe 2+ ).…”