PILLOWS FORMED BY THE INJECTION OF LAVA INTO THE CRYSTAL MUSH OF A THIN STEENS BASALT FLOW, DISASTER PEAK, NEVADA

An unusual 2.8-meter thick lava flow with coarsely plagioclase-phyric layers alternating with aphyric layers and pillow-like structures occurs within the a ~1500 m section of Miocene Steens Basalt exposed at Disaster Peak, Nevada. This internal stratigraphy consists of a basal, aphyric layer (25 cm thick) overlain in succession by a lower porphyritic layer (55 cm), an aphyric layer (40 cm), a middle porphyritic layer (90 cm) with aphyric pillow structures near its base, an upper porphyritic “layer” (36 cm), and an aphyric top layer (30 cm). The porphyritic layers contain ~70%-80% plagioclase phenocrysts up to at least 3 cm in diameter. The lower two layers are megascopically identical but the upper “layer” has much whiter phenocrysts, a darker groundmass and appears to represent a zone of alteration rather than a discrete layer.

The field data suggests that the porphyritic zones were initially a single layer of crystal mush that was subsequently dilated and intruded by a nearly aphyric liquid to form the discrete aphyric layers and pillows within, and parallel to, the porphyritic layers. The aphyric layers and aphyric pillows, containing < 5% plagioclase phenocrysts, are moderately vesicular, and have thin chilled margins. The pillows are up to at least 40 cm thick, extending 1.2 m horizontally. The aphyric lava intruded the crystal mush after it was about 70-80% crystallized. The chilled pillow margins indicate this liquid was hotter than the crystal mush. Locally, phenocrysts lie parallel to the pillow margins and in one case a thin layer of porphyritic liquid became trapped between a pillow and an underlying aphyric layer.

The magma source of the aphyric liquids is unknown; however, the field data is consistent with it having been derived from the same source as the porphyritic lava. We suggest that the initially erupted liquid was charged with phenocrysts that accumulated at the top of a magma chamber due to floatation, leaving a lower zone depleted in phenocrysts which was later tapped to produce the aphyric layers and pillows. Nearly aphyric interiors of otherwise highly porphyritic Steens lavas have been observed higher in the section at Disaster Peak and at Abert Rim, Oregon, therefore supporting the idea of producing initial phenocryst-rich liquid followed by aphyric liquid during the brief eruption of a thin flow.