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Fusion of granodiorite by basalt, central Sierra Nevada

Journal of Research of the U.S. Geological Survey
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Abstract

A trachybasalt plug, 100 m in diameter, has partially fused inclusions and wallrock of porphyritic granodiorite near Tuolumne Meadows, Yosemite National Park, Calif. Granodiorite surrounding the plug was altered within distances of about 3.5 m. Within this distance, (1) trace amounts of glass occur along fractures and grain boundaries, (2) biotite has been altered from dark olive to dark reddish brown and contains bands of fine reddish iron oxide grains, and (3) optic axial angles of potassium feldspar decrease toward the contact.

As much as 20 volume percent of glass occurs in the granodiorite in a reentrant and in inclusions within the plug. Detailed study of two partially fused samples shows that, relative to unfused granodiorite, SiO2 is clearly depleted in both samples, whereas K2O is depleted in one but not the other. Total iron, Al2O3, MgO, Na2O, and H2O show apparent increases in both samples; other constituents show no significant changes. Chemical analyses suggest that much original material was lost from the partially fused rocks, probably by mass migration of melt rather than by chemical diffusion. The composition of glasses now present in the partially fused rocks was dominated by melting of quartz and feldspars, whereas the composition of the early-formed "lost" melt was strongly influenced by subsolidus reaction and subsequent melting of biotite.

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Publication type Article
Publication Subtype Journal Article
Title Fusion of granodiorite by basalt, central Sierra Nevada
Series title Journal of Research of the U.S. Geological Survey
Volume 6
Issue 4
Year Published 1978
Language English
Publisher U.S. Geological Survey
Description 7 p.
First page 459
Last page 465
Country United States
State California
Other Geospatial central Sierra Nevada, Yosemite National Park
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