INTRA-FLOW VARIATIONS WITHIN PRIMITIVE BASALTS OF THE POISON LAKE CHAIN, LASSEN VOLCANIC CENTER,CA

The Poison Lake chain within the Lassen Volcanic Center, California, consists of 39 vents and flows, which have been subdivided into eight groups based on their mineralogy, compositions and field characteristics. This project is mainly focused on intra-flow compositional variations - from the vent to the toe of flow – within one flow of the Bogard Buttes group (100 ka). The flow bb5 was chosen for its large surface area (just over 2.2km²) and good exposure of the vent and flow (approximately 4 km long). There are several other Bogard Buttes flows in close vicinity to bb5, which are used to compare the geochemistry of bb5 samples.

Preliminary petrographic data suggests that bb5 is monogenetic but has experienced a complex history. Distinct petrogenetic variations from the vent to the toe of flow are observed in the mineralogy and chemistry of samples collected from the vent and along the length of the flow. The majority of plagioclase phenocrysts have sieve textures and/or oscillatory zoning. Sieve textures indicate that the crystals were not in equilibrium with the melt. Although fractional crystallization can explain, to some degree, the differences in mineralogy from sample to sample, preliminary geochemical variations within single crystals of olivine and the lack of correlation between MgO and Zr between the bb5 samples help rule out simple fractionation as the main process by which these magmas were created. The combination of sieve textures and zoned rims suggests that the magma may have experienced magma mixing. Analysis of new mapping, petrographic and geochemical flow data confirm the idea that this cinder cone has a complex origin.