148 Microscopic Petrography – Cinder Cones and Associated Flows – Red Cone

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

Microscopic Petrography


Cinder Cones and Associated Flows

     Red Cone

The most basic rock yet analyzed from the park comes from a flow at the southeast base of Red Cone (no. 9), but whether it issued from the cone itself or is part of the pre-Mazama basement is debatable.

Patton has already devoted much attention to the textural variations of the Red Cone rocks, and his observation that they are almost all olivine-rich and hypersthene-poor is here corroborated. An analyzed specimen from the flow has approximately the following content: microliths of labradorite, 50 per cent; intergranular specks of augite, 30 per cent; brown, interstitial glass, 10 per cent; fresh, porphyritic olivine, 5 per cent; granular ore, 5 per cent. Cristobalite is plentiful. Distinctive of the rock is the absence of porphyritic feldspar.

In most flows, olivine is easily detected even by the unaided eye on account of its pale-green color. Not uncommonly, however, and particularly where the lavas are reddish or brown instead of gray or black, the olivine has a ruddy, iridescent tint. Thin slides of such lavas show that the olivine is partly altered to serpentine and hematite, and the associated grains of ore are also filmed with hematite dust. Tridymite occurs in the dense base of these altered lavas, and its origin, like that of the hematite, may be ascribed to the activity of residual vapors.

Though most of the flows from Red Cone are either devoid of large crystals of feldspar or only sparingly dotted by them, some of the pyroclastic ejecta carry zoned phenocrysts of basic labradorite in large amount. Moreover, though most of these ejecta are as rich in olivine as the lavas, there are some in which the mineral is virtually absent. In such rocks, its place is taken by small prisms of hypersthene.


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