136 Microscopic Petrography – The Younger Dacite Flows – The Cleetwood Dacite

The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

Microscopic Petrography

 

The Younger Dacite Flows

     The Cleetwood Dacite

In general, the top and bottom of the Cleetwood flow are composed of glistening black obsidian. Locally, this glassy part of the flow is crowded with pink spherulites; elsewhere, the jet-black glass is streaked in shades of red and brown owing to oxidation of iron along zones of brecciation and gas concentration. The center of the flow and the feeder, on the other hand, consist mainly of pale-gray, lithoidal, and somewhat more vesicular dacite.

Whether obsidian or holocrystalline, the Cleetwood lava is a hypersthene dacite with accessory augite and hornblende. In other dacite flows erupted from the Northern Arc of Vents, basic inclusions are rare; in the Cleetwood flow, they are almost completely wanting. As in all the younger dacite flows, the content of ferromagnesian crystals is very low as compared with that of porphyritic feldspar. In this respect, the lavas are readily distinguished from the andesites of Mount Mazama.

 

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