CONSTRAINING CONTINENTAL BASALTIC MAGMA CHAMBER PROCESSES: TEXTURAL AND IN SITU GEOCHEMICAL INVESTIGATION OF PLAGIOCLASE FROM PISGAH CRATER, CALIFORNIA

Pisgah Crater, a cinder cone located in the Mojave Desert, California, erupted three lava flow units at ~24 ka (Wood and Kienle, 1990). Alkali basalt to hawaiite flows show geochemical variations that indicate progressive crustal wallrock assimilation (e.g., Ramos and Reid, 2005). In order to test this hypothesis, crystal size distribution data, Nomarski textural imaging, and in situ major element and Sr isotope data were collected on plagioclase crystals from each lava flow unit.

Two crystal populations were identified based on crystal size distribution and anorthite (An) contents, and average crystal residence times are calculated using assumed growth rates of 10^-9 and 10^-10 mm/s (Marsh, 1988). Phenocrysts and microphenocrysts are > 0.5 mm, have similar An contents (An₇₃–An₈₁), and have average residence times of decades to centuries. Microlites are < 0.5 mm, have slightly lower An contents (An₇₁–An₇₈), and have average residence times of < 2 years. Nomarski analysis indicates three distinct textures (oscillatory zones, dissolution zones, and spongy cored crystals) are present in all flow units. Of these, spongy-cored crystals exhibit the most variation in An contents (An₇₃–An₈₈). Phenocryst and microphenocryst Sr isotope data clearly identify distinctions between the late flow (0.70466–0.70563) and the earlier two flow units (0.70398–0.70431).

These and published results (Glazner et al., 1991; Ramos, 2000) indicate that the magmatic plumbing system associated with Pisgah Crater may have been constructed as a plexus of sills and dikes undergoing progressive crustal contamination by relatively homogeneous-composition wallrock. Based mostly on in situ Sr isotopes, we hypothesize that the early and middle flow units erupted from one magma chamber, whereas the late flow erupted from a separate chamber. To further document the magma plumbing system, additional in situ data, combined with quantitative modeling of geochemical data, are required.