Structure
The Ilgachuz Range is the second largest shield volcano in the Anahim Volcanic Belt which includes other immediately nearby ranges, the Rainbow Range and Itcha Range. It stands at 2,410 metres (7,907 ft) above sea level - slightly shorter than its neighbor, Rainbow Range. It has a diameter of 25 km (16 mi). The Ilgachuz Range was created by two chemically separate magmatic periods; an early complex series of trachyte and rhyolite eruptions, and late extrusion of a sequence of basaltic lava flows. Evacuation of the volcano's magma chamber resulted in the failure of one or more centrally located calderas. It is divided into Precaldera, Dome Forming, Intra Caldera and Shield Forming assembages.
The Precaldera Assemblage is best exposed on the east side of Pipe Organ Mountain where it contains a bedded pile over 300 m thick of weakly consolidated, moderately to extremely different, pyroclastics, flows and deposits of uncertain origin. Colours range from mottled green to grey, yellow, ochre, red and white. A green tuffbreccia composed of pumice fragments, feldspar crystals and minor debris is recognizable in several areas.
The Dome Forming Assemblages include most of the rhyolite domes, related flows and the Ilgachuz Comendite. The northerly domes are subcircular talus mounds of plate sized pieces of light to dark grey, slightly porphyritic, flowbanded rhyolite with minor obsidian. Massive to banded chalcedony blobs and veinlets are related with these domes. The southern domes are somewhat different in nature, comprising intrusive and extrusive phases of cream colored porphyries. The Sax Dome contains an upper portion of cream coloured, aphanitic to fine quartz porphyry felsite with abnormal green glass filled fractures, and a lower unit of microsyenite with red and green glassy zenocrysts.
The Intra Caldera Assemblage is best exposed on the north edge of the caldera. The lower unit, indicative of caldera formation, is an epiclastic boulder-landslide deposit, roughly bedded and dipping into the caldera. Similar material, grading up into finer debris flows and possibly lahars, has been uncertainly known in the gap between Phacelia Peak and Calliope Mountain suggesting this area is the southern edge of the caldera. Alternatively, largely unsorted breccia and debris deposits exist on the ridge north of Saxifraga Peak, possibly indicating the main, or a subsidiary, caldera edge.
The Shield Forming Assemblage contains a series of basalt and minor comendite eruptions, and is best exposed on Far Mountain and Mount Scot. The basalts issued from fissure vents primarily located peripheral to the calderas. Brick red cinder deposits are considered to be a late phase of this assemblage.
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