Virtual Field Trip—Group 1 Porphyritic Dikes of the Sierra Blanca Igneous Complex
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Geologic Background
The Sierra Blanca Igneous Complex formed as a cluster of stratovolcanoes during Eocene to Oligocene time, ~38–33 m.y. ago. The volcanic pile is formed mainly from porphyritic andesite lavas and breccias. The andesitic lavas contain sparse to abundant phenocrysts of plagioclase feldspar; pyroxene or amphibole phenocrysts are less common. The lava flows were cut by porphyritic dikes that are texturally and compositionally similar to the lavas, this suggests that the dikes are not much different in age from the lava flows they cut through.
To download Dr. Chamberlin's detailed geologic information about Field Stop 1 in pdf format click here.
Bonnie Dodge, Katherine Livingston, Carla Burns, Rita Kujawa, Zenaida Ahumada,
Kala Scarafiotti, Michael Holsten, and Dr. Richard Chamberlin. Group 1 is leaning parallel to apparent inclination of the dike. Dike is nearly vertical.
Geologic profile of road cut at Stop 1—three miles west of Alto
Microdiorite dike with chilled andesite margins, approx. 7m thick.
Igneous Complex-A road-cut at the foot of the Sierra Blanca volcanic pile, three miles west of Alta.
A thick coarsely porphyritic andesite, non-horizontal lava flow, sloping to the east.
~38–33 m.y. old.
Polygonal columnar joints indicate the direction of cooling (nearly horizontal).
Spheroidal weathering visible towards the top of the dike.
The dike has a definite elbow just above the second bench.
Edges of the dike lack visible crystals in matrix, from rapid cooling in shallow subsurface environment. The matrix in the middle contains very fine crystals. Textures indicate two stages of cooling, one at depth where the larger phenocrysts formed and second stage at or near the surface where the matrix grains crystallized.
The 7m dike shows similarities with the small dikelet next to it, but has obvious differences from the smaller 1m-thick dike to the west. The 7m dike has medium-sized crystals while the 1m dike has small to medium-sized crystals, which gives the impression of being fed by two different magma bodies (with different cooling histories).
The Sierra Blanca Igneous Complex formed as a cluster of stratovolcanoes during Eocene to Oligocene time, ˜38–33 m.y. ago. The volcanic pile is formed mainly from porphyritic andesite lavas and breccias. The andesitic lavas contain sparse to abundant phenocrysts of plagioclase feldspar; pyroxene or amphibole phenocrysts are less common. The lava flows were cut by porphyritic dikes that are texturally and compositionally simiar to the lavas, this suggests that the dikes are not much different in age from the lava flows they cut through.
The Sierra Blanca Igneous Complex formed as a cluster of stratovolcanoes during Eocene to Oligocene time, ˜38–33 m.y. ago. The volcanic pile is formed mainly from porphyritic andesite lavas and breccias. The andesitic lavas contain sparse to abundant phenocrysts of plagioclase feldspar; pyroxene or amphibole phenocrysts are less common. The lava flows were cut by porphyritic dikes that are texturally and compositionally simiar to the lavas, this suggests that the dikes are not much different in age from the lava flows they cut through.