THOLEIITIC-ALKALIC TRANSITION AT SUBGLACIAL VOLCANOS, TUYA REGION, BRITISH-COLUMBIA, CANADA

Ash Mountain, South Tuya, and Tuya Butte are three small basaltic volc anoes in the Stikine volcanic belt of northern British Columbia. The v olcanoes rise 700, 500, and 400 m above their bases and are about 3.2, 1.6, and 2.6 km(3) in volume, respectively. They began eruptive activ ity under several hundred meters of overlying glacial ice, or water in an ice-impounded lake, and undegassed pillow lava was erupted and for ms the bases of all three. Later, as the vents grew into shallow water , explosive phreatomagmatic activity erupted partly degassed glassy tu ffs. Finally, when the volcano emerged through the surface of the ice or water (or the water was drained), degassed subaerial lava flows wer e erupted and were converted to assemblages of foreset-bedded pillow b reccia and pillow lava when subaerial flows crossed a shoreline and fl owed into meltwater lakes. The undegassed subglacial pillow base of As h Mountain is overlain by partly degassed pillows and hyaloclastite tu ff cut by dikes; at South Tuya the pillow base is overlain by hyalocla stite tuffs and lenses of pillow lava; at Tuya Butte the pillow base i s overlain by foreset-bedded pillow lava, pillow breccias, and hyalocl astite tuffs, which in turn are overlain by subaerial lava flows compo sing a small shield volcano. The undegassed basal subglacial pillow la va of the three volcanoes contain 0.10 +/- 0.01 wt % sulfur and simila r to 0.5 wt % H2O. The overlying partly degassed assemblages contain 0 .06 +/- 0.02% sulfur and similar to 0.2% H2O at Ash Mountain, 0.07 +/- 0.01% sulfur at South Tuya, and 0.03 +/- 0.01% sulfur at Tuya Butte. The differences in the degree of degassing can be related to the natur e of eruption and quenching and the distance of flow of the subaerial lava. When the volcanoes switched from subglacial to shallow water or subaerial eruptions, as shown by change to more explosive activity and then to subaerial lava flows (and by a marked reduction of sulfur in volcanic glass), the magma shifted from tholeiitic to alkalic composit ion. This transition occurs at each of the three volcanoes. The tholei itic and alkalic magmas cannot be related by shallow crystal fractiona tion and apparently originated by differing degrees of deep melting at a mantle source. Prior to eruption the tholeiitic melts overlay alkal ic melts in shallow chambers underlying each of the volcanoes because of their lower density and were, therefore, the first to erupt under s ubglacial conditions. As the volcano grew through the ice (or ice-impo unded water), the volcanic conduit vented to the atmosphere, producing a partial depressurization of the conduit and the subsurface chamber. This sudden reduction in confining pressure caused enhanced vesiculat ion of volatile saturated melts, particularly of the more volatile-ric h alkalic melts, causing them to rise to the top of the chamber and er upt.