Porphyritic anorthoclase phonolite (6.6 cm across) - this Mt. Kenya rock has the same chemistry and texture as kenyte, but the dark-colored matrix of this porphyritic phonolite is more coarsely-crystalline than in kenyte, and also lacks flow-banding.
This rock is Pleistocene in age, and was collected from an outcrop on the southern slopes of Mt. Kenya Volcano.
Large pale-colored crystals = anorthoclase feldspar phenocrysts.
Dark, finely-crystalline groundmass = dominated by sanidine & nepheline.
--------------
Mt. Kenya is an eroded, extinct volcano in south-central Kenya, just next to the East African Rift Valley system. The volcano was active principally during the Pliocene and Pleistocene. The summit area of Mt. Kenya consists of majestic-looking, high spires of rock - these rocks are shallow intrusives that filled up the throat of the old volcano. The eroded slopes of the volcano consist of a variety of extrusive rocks, including trachytes, basalts, phonolites, and kenytes.
---------------
Mostly synthesized from:
Baker (1967) - Geology of the Mount Kenya area. Geological Survey of Kenya Report 79. 78 pp. 4 pls. 1 map.
Price et al. (1985) - Geochemistry of phonolites and trachytes from the summit region of Mt. Kenya. Contributions to Mineralogy and Petrology 89: 394-409.
Loading contexts...
