Paper No. 3
Presentation Time: 1:40 PM
THE ORIGIN OF COARSE ANTECRYSTS IN FLOOD BASALT LAVAS: A LA-MC-ICPMS SR ISOTOPE MICROANALYTICAL STUDY OF THE COLUMBIA RIVER BASALTS
Many continental flood basalt lavas contain large (1 - 10 cm) plagioclase phenocrysts. In the case of the Columbia River Flood Basalt Province, they are confined to early-erupted Steens and Imnaha lavas; flows of the later-erupted Grande Ronde and other formations either contain small plagioclase phenocrysts or are completely aphyric. Using laser ablation multicollector ICPMS, we show that coarse plagioclase crystals in the Steens and Imnaha are more radiogenic in Sr than their host matrix and, consequently, than the whole rock (Ramos et al., 2013). Nonetheless, their major element compositions are approximately in equilibrium with the groundmass at crustal pressures as shown by MELTS simulations. They may therefore be considered to be antecrysts rather than true phenocrysts; they did not grow from the melt in which they are hosted at eruption, but from one very similar albeit isotopically distinct. Here, we link these observations to a model in which antecrysts grow as semi-rigid mush during the initial colonization of continental crust by basaltic magma, and are remobilized and ascend to the surface by later passage of magma along the same pathways.
Colonization of continental crust by basalt may begin with early intrusions that form a dike-and-sill network in which individual intrusive sheets cool, crystallize and simultaneously heat the surrounding crust (Dufek and Bergantz, 2005). Eventually, the temperature of the system rises above the basalt solidus and intrusions do not fully crystallize but exist as rigid mush zones with interstitial melt, facilitating diffusional uptake of crustal Sr by plagioclase crystals in the mush. Injection of fresh magmas “cores out” the intrusive pathways and brings these crustally-contaminated plagioclase antecrysts to the surface. In this model, the occurrence of large plagioclase antecrysts is an artefact of the early, waxing stage of continental flood basalt magmatism.
Dufek & Bergantz, J. Petrol. v.46, p.2167-2195 (2005).
Ramos et al., GSA Special Paper v.497, p.231-257 (2013).