| Abstrakt: |
Cabezon Peak is a 180-m-tall trachybasalt (alkali basalt) volcanic plug (neck) that has been exposed by erosion of an original volcanic edifice and enclosing sedimentary deposits since emplacement at 2.66±0.66 Ma. The main body consists of dark-gray, columnar-jointed intrusive rock overlain by about 40 m of black to gray, massive to scoriaceous lava. Deep erosion has obscured whether the intrusion was formed beneath a scoria cone or maar/diatreme volcano. Samples of the lava are aphyric to fine-grained holocrystalline containing very sparse phenocrysts of olivine and augite and microphenocrysts of plagioclase, olivine, augite, titanomagnetite, and trace apatite. Because Cabezon Peak lava and intrusive rocks are so phenocryst-poor, it is surmised that the original magma was emplaced at a temperature of about 1200℃, well above the liquidus temperature. The lava also contains xenocrysts (a few percent) and sparse (<0.1%) xenoliths of peridotite (lherzolite), most of which are =0.25 cm in diameter. However, a rare lherzolite xenolith 35 cm in diameter was found. We used reasonable parameters for densities and viscosity applied to the Stokes equation for gravitational settling in a liquid to calculate an estimated minimum ascent rate of 3.9 m/sec (14 km/hr) for Cabezon magma, which is in line with published ascent rates of 2-10 m/sec for other trachybasalt magmas. Assuming Cabezon magma originated in the mantle, the time to rise from about 50 km depth to the surface is roughly 3.6 hr. In this scenario, the lag time before initial eruption of magma at the surface and first appearance of large 35 cm xenoliths would be about 35 hr. However, small, disaggregated xenolith fragments (=1 cm) would appear at the onset of the eruption. Columnar jointing of the main intrusive mass formed during initial cooling of the plug. Column width is greater than in most lava deposits but is similar to plugs (vents) and endogenous domes of equivalent dimension to Cabezon. [ABSTRACT FROM AUTHOR] |
