Discovery of modern (post-1850 CE) lavas in south-central British Columbia, Canada: Origin from coal fires or intraplate volcanism?

Abstract

We describe three unusual lavas in the Northern Cordillera in south-central British 15 Columbia, Canada, occurring as spatter, scoria and blocks over small ~ 400 m 2 areas. The 16 lavas coat and weld cobbles and pebbles in glacial till and are vesicular and glassy with 17 microlites of clinopyroxene and plagioclase, and xenocrysts of quartz, feldspar or 18 clinopyroxene. Chemically the lavas are basaltic trachyandesite (55-61 wt% SiO 2) with 19 trace element patterns similar to average British Columbia upper crust, except for having 20 higher V and lower Zr, Hf, Nb, Th and U. Melting experiments and plagioclase-melt 21 thermometry on the glasses, and phase equilibrium in simple systems, require liquidus 22 temperatures of 1150-1300°C. Interaction of the liquids with carbonaceous matter at 23 2 low pressure formed Fe metal spherules and SiC. Radiocarbon ages of charcoal and 24 dendrochronology show the lavas are modern, emplaced in the last ~120 years. The 25 similar bulk composition of these lavas to several other Quaternary-aged volcanic centers 26 in the North American Cordillera, some of which show recent seismic activity, could 27 suggest a possible tectonic origin, but the deposits are unusually small and show no 28 central vent for emplacement. Conversely, the balance of evidence would suggest an 29 origin from coal fires or hot gas venting, but is less consistent with the observed calc-and 30 per-alkaline lava compositions, and the lack of known local coal-bearing strata as a heat 31 source. Other anthropogenic origins for the lavas are considered less plausible. 32 33 transpressive tectonic regime, with thin lithosphere and high heat flow (Hyndman et al, 36 2007). Several Neogene and younger lavas related to transpression occur throughout the 37 orogen with the youngest known volcanism dated at ~300 yrs B.P. (Edwards and 38 Russell, 2000; Higgins, 2009). Recent seismicity (in 2007) beneath a Quaternary volcanic 39 edifice (Nazko-Fig. 1) in central British Columbia have been interpreted as modern 40 magma movement at depth (Cassidy et al., 2011). A similar interpretation was made 41 previously for other occurrences of intraplate seismicity in the region (Rogers, 1981). 42 Two young lavas , near Cache Creek, and in the southeast corner of British 43 Columbia (Fig. 1) are not of tectonic origin, but rather are paralava related to coal fires 44 (Church et al, 1979; Bustin et al, 1982). The natural burning of coal seams, induced by 45 lightning strikes, wild fires or spontaneous combustion, can fuse the surrounding country 46 3 rock to form paralava that may intrude or escape upward in chimneys (Cosca et al., 1989; 47

Non-Magmatic Glasses

“…Well documented examples in the USA occur in the Grimes Canyon area, Monterey Formation, California (Bentor et al 1981); the Powder River Basin, Wyoming (Cosca et al 1989;Herring & Modreski 1986); and various coal-bearing areas of Montana and Colorado (Rogers 1918). The phenomenon of combustion metamorphism has been found to be quite common (Bentor et al 1981;Grapes 2010), including examples in China (de Boer et al 2001), Mongolia (Peretyazhko et al 2018), India, Russia (e.g., Chelyabinsk brown coal basin) (Sokol et al 1998), Iran, Jordan (Khoury et al 2015), Israel (Ron & Kolodny 1992;Sokol et al 2014), Czechia, Germany, England, Italy (Melluso et al 2004), Mali (Svensen et al 2003), Canada (Canil et al 2018;Mathews & Bustin 1984), New Zealand, and Australia. Some terms are commonly used for pyrometamorphic rocks as follows.…”

“…In summary, pyrometamorphic glasses and rocks are highly variable due to the varying temperatures, mineral protoliths, and local environmental conditions, resulting in equally variable disequilibrium melting, volatilization, mixing, and crystallization (Canil et al 2018;Peretyazhko et al 2018;Sokol et al 1998).…”