INTRATELLURIC VERSUS SUBAERIAL GROWTH OF PHENOCRYSTS IN COLUMBIA RIVER BASALT GROUP LAVAS

Lofgren (1974, 1980) demonstrated experimentally that phenocrysts could grow in basaltic lavas cooled subaerially at constant, linear rates. Hooper (1997) suggested this mechanism could explain the correlation of porphyritic textures with unusually thick canyon-filling flows of the Columbia River Basalt Group (CRBG). The distinction is important because the typical basaltic phenocryst paragenesis (plagioclase, followed by olivine, then augite), if produced subaerially, cannot be used to test fractional crystallization models in magma reservoirs of the Columbia River Basalt Group (Hooper, 1997).

Modal and grain size data were analyzed from three CRBG ponded flows and their feeder dikes. Two of the dike-flow pairs are coarsely plagioclase-phyric with phenocrysts up to 20 mm in diameter. Both dikes contain higher average concentrations of phenocrysts as well as microphenocrysts of larger average diameters than do their respective flows. If phenocryst growth occurred subaerially, then the lavas should contain more and larger phenocrysts than their respective feeder dikes (i.e. the sum of intratelluric and subaerial growth) which is clearly not the case for these units.

The third dike-flow pair is aphyric in spite of attaining thicknesses up to 146 m and 57 m for the flow and dike respectively. It is clear in this case that even extraordinary flow thickness was insufficient to promote nucleation and growth of phenocrysts or even microphenocrysts.

The following characteristics of the units are compatible with intratelluric rather than subaerial phenocryst nucleation and growth: abundant synneutic plagioclase phenocrysts; zoned phenocrysts with large, unzoned, euhedral cores; and plagioclase phenocrysts in each dike have an identical average anorthite content to that in its respective flow.

Although phenocryst growth in lavas is theoretically possible, their abundance in thick, ponded lavas should not be assumed as proof of its occurrence. Rather than providing the environment for phenocryst growth, thick, ponded, porphyritic lavas more likely act as repositories for deposition of intratelluric phenocrysts by the sheet and/or intracanyon flows feeding into them. Flow differentiation would concentrate phenocrysts within lava streams that ultimately feed into lava ponds over extended periods of time.