Textural heterogeneities and vent area structures in the 600-year-old lavas of the Inyo volcanic chain, eastern California

The most recent eruption in the Inyo volcanic chain (600 y.B.P.) produced two chemically and mineralogically distinct lava types. The two are here distinguished by their textural characteristics and are termed coarsely porphyritic (CP) and finely porphyritic (FP). The vesicular CP lava is texturally relatively homogeneous, whereas the FP lava is markedly variable in texture. The two lava types mixed during or shortly before eruption, resulting in the formation of a small volume of hybrid lava and dramatic mixing textures. The FP magma was extruded as air-fall pumice and lava from all three vents while the CP magma primarily extruded as lava during the later stages of eruption at the southern two of the three 600-B.P. magmatic vents. Previous workers have interpreted the 600-B.P. eruption as the result of intrusion of a rhyolitic dike along north-trending fractures. The eruptive sequence of the two lava types, combined with the along-chain and within-flow distribution of textural and chemical variations within the FP lava, indicate the following details regarding stratifications within the conduits and preeruptive dike: (1) the FP magma overlaid the CP magma with an intervening mechanically mixed zone; (2) the FP magma was itself chemically zoned both vertically and laterally, but SiO 2generally decreased southward; and (3) the FP magma probably wedged out to the south. The orientation of flow foliations and extrusive fractures in the vent areas of two of the three lava flows from this eruption reveal the geometry of conduits which fed the flows. The conduits for the last-erupted lava on the Deadman Creek and Obsidian flows were pipelike in shape rather than fissures, but Obsidian flow displays evidence for an originally elongate vent that probably evolved to a more pipelike shape toward the end of eruption.