The Northern Cordilleran Volcanic Province (NCVP) is the most active volcanic belt in Canada, yet there are few detailed petrologic studies of the individual volcanoes. Tseax volcano is the southernmost volcanic centre of the NCVP, the second youngest (~1700 CE) eruption in Canada, and led to one of Canada's worst natural disasters killing up to ~2000 Nisga'a First Nation people. We present a conceptual model for the origins, evolution and pre-eruptive storage of the Tseax magma which erupted in ~0.5 km³ of volcanic material, mainly in the form of valley-filling lava flows. All Tseax products are alkali Fe- Ti- rich basanite-to-tephrite having trace element distributions similar to the other mafic NCVP lavas. Phenocrysts consist of plagioclase, olivine and titanomagnetite, often forming glomerocrysts suggesting co-crystallisation of these 3 mineral phases. Clinopyroxene is never observed as a phenocryst phase. The lavas have low Mg^#’s implying they are fractionated relative to a mantle-derived ‘parent’ magma. Trace elements indicate the magma derives from melting of a fertile mantle source (i.e., (Nb/K)_n ~ 1), most likely the upper asthenosphere. There is no evidence in the Tseax volcanic rocks for magma mixing or lithospheric contamination during ascent. The phenocryst assemblage suggests rapid ascent of a low viscosity magma to <5.5 km where titanomagnetite becomes the first phase on the liquidus (~ 1133 °C). There the magma stalled for a very short period of time under PT conditions where clinopyroxene did not reach saturation. Based on the size of the plagioclase phenocrysts, the magma stalled for less than a year and cooled down to 1094–1087 °C prior to eruption.

北科迪勒拉火山省 (NCVP) 是加拿大最活跃的火山带，但对单个火山的详细岩石学研究很少。Tseax 火山是 NCVP 最南端的火山中心，是加拿大第二年轻（约公元 1700 年）的喷发，并导致了加拿大最严重的自然灾害之一，导致约 2000 名 Nisga'a 原住民死亡。我们提出了一个关于在约 0.5 km ^{3 内}喷发的 Tseax 岩浆的起源、演化和喷发前储存的概念模型火山物质，主要以填谷熔岩流的形式。所有 Tseax 产品都是富含铁钛的碱金属玄武岩到软玉，其微量元素分布与其他镁铁质 NCVP 熔岩相似。斑晶由斜长石、橄榄石和钛磁铁矿组成，通常形成球晶，表明这三种矿物相共结晶。单斜辉石从未被观察为斑晶相。熔岩具有低 Mg ^#，这意味着它们相对于地幔衍生的“母体”岩浆是分馏的。微量元素表明岩浆来源于肥沃的地幔源的熔化（即，（Nb/K）_n ~ 1)，很可能是上软流圈。在 Tseax 火山岩中没有证据表明上升过程中岩浆混合或岩石圈污染。斑晶组合表明低粘度岩浆快速上升到< 5.5 km，在那里钛磁铁矿成为液相线上的第一相（~ 1133 °C）。在单斜辉石未达到饱和的 PT 条件下，岩浆在那里停滞了很短的时间。根据斜长石斑晶的大小，岩浆停滞不到一年，在喷发前冷却到 1094-1087°C。