PRELIMINARY GEOCHEMICAL CONSTRAINTS ON THE EVOLUTION OF PILLOW RIDGE TINDAR, EDZIZA VOLCANIC COMPLEX, NCVP

Pillow Ridge is a ~4 km long, ~600 m high glaciovolcanic tindar at the northwestern end of the Mount Edziza Volcanic Complex (MEVC) in northwestern British Columbia, Canada. Edwards et al. (2009) described 15 distinct lithofacies, including five distinct lithostratigraphic packages of pillow lavas (Lpw1-5). The uppermost units, Lpw4 and Lpw5, are separated by two units of tuff-breccia (TB): the lowermost has fluidal, vesiculated, vitric clasts, and the upper has distinct fragments of pillow lava. The TB is cut by dikes that feed the overlying Lpw5. The observed association of pillow lavas with feeder dikes that intrude tuff-breccia has been referred to as the TDP lithofacies association, and based on field relationships was interpreted by Edwards et al. (2009) as having formed during an initial explosive eruptive phase followed by an effusive phase. Another package of pillow lavas (Lpw) is exposed at the northwestern end of Pillow Ridge in ‘Pillow Canyon’, a narrow canyon that exposes pillow and marginal sedimentary units. Edwards et al. (2009) did not include Lpw in the Lpw1-5 lithostratigraphic sequence because their relationship to units at higher elevations is not clearly exposed.

Initial geochemical analyses show that the lithofacies units can be separated into distinctive geochemical groups. Most of the units (Lpw1-3, Lpw5, feeder dike) are incompatible element-enriched (Zr, Y). A less-enriched group comprises Lpw4, some dikes, and pillows from the southern end of the ridge. Preliminary analyses of clasts in the TB suggest that they are similar to the underlying pillows (Lpw4) rather than the overlying pillows (Lpw5); further analyses of clasts in the two tuff-breccia units will clarify the interpretation of the TDP lithofacies association.

Geochemically, samples from Lpw fall between the two compositional groups. To better understand their emplacement, we are analyzing H2O abundances in samples of fresh glassy pillow rinds. Previous FTIR analysis on Lpw1-5 units yielded H2O concentrations from 0.54-0.81 wt.%, which were interpreted as recording hydrostatic eruption pressures from 2.9-6.2 MPa and paleo-lake depths varying between 300-1200 m. If Lpw formed during emplacement of one of the other pillow lava units (Lpw1-5), we anticipate that it should record higher pressures than the overlying units.