124 Microscopic Petrography – Lavas of the Union Peak Volcano – Intrusions near Bald Top

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

Microscopic Petrography


Lavas of the Union Peak Volcano

     Intrusions near Bald Top

At the southeast base of Bald Top, the Union Peak lavas are intruded by irregular dikes, parts of which are pale buff or gray, and the bulk dark green, weathering to a deep-brown crust. Compared with the surrounding lavas, these dikes are much coarser in grain and are typified by their ophitic texture. Essentially, they consist of olivine crystals up to 2 mm. in length, intergrown with zoned labradorite. Augite, though, forming occasional large ophitic plates, is far subordinate to olivine, and hypersthene is even less abundant.

But the most distinctive feature of the dikes is the difference in the degree of alteration of the pale and dark parts. The dark-green parts owe their color to widespread change of the olivine to antigorite and bowlingite, and to the presence, in triangular interspaces, of yellowish-green chlorophaeite. This wholesale serpentinization of the olivine is all the more striking in view of the rarity of such alteration in the adjacent lavas.

Here and there the dark-green, serpentinized rocks are cut by branching joints, bordering which the lava is distinctly paler. Seen in the field, the pale lava appears as “veins” 2 to 3 inches wide, sharply delimited from the dark-green rocks on either side by thin bands stained with limonite. The textures and primary minerals in the light and dark types are identical. But contrary to expectation, for one might suppose alteration to be most advanced along joints, the pale lava is perfectly fresh and contains no chlorophaeite.

To this apparent anomaly, Fuller’s studies elsewhere3 provide a clue. According to his interpretation, the pale rocks bordering the joints remained fresh because of the escape of volatiles, and this accounts also for their more open, diktytaxitic texture. Farther from the joints which served as avenues of escape for volatiles, the rocks suffered autometamorphism, chiefly hydration, though the feldspars were not affected. What the significance of the limonite-stained bands separating the fresh from the hydrated rocks may be, is not understood.


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