dacite - teaching hand specimen of gray porphyritic dacite
Dacite was a common rock in the Archean continents, forming when hot young oceanic crust subducted beneath thick continental plates. As the hot oceanic plate subducted into the mantle, it partially melted, dewatered and became hydothermally altered, with the melt gaining quartz and sodium and forming a magma that differentiated and became even more sodium and silica rich as it rose. As it cooled near the surface, quartz (sodium silicate), plagioclase (sodium feldspar) and hornblende crystallized.
Today, subducting oceanic crust is colder and does not melt before dewatering, inhibiting the formation of dacite. Dacite is the volcanic equivalent of granodiorite and can be thought of as a quartz rich andesite. This dacite was intruded during the Tertiary. The phenocrysts are plagioclase and quartz, with smaller biotite.
Secondary students should be able to identify this as a porphyry because of the phenocrysts - the larger crystals of quartz and feldspar that are surrounded by a finer gray groundmass - and because of the color, they should see that it is intermediate between oceanic basalts (dark and dense) and granitic continental rocks (lighter and less dense).
This dacite was collected on Mt. General, near Hinkley in San Bernardino County, California.
Notice, in the QAPF diagrams, how dacite overlaps granodiorite and tonalite in composition. Select one diagram, click to enlarge, and hover over it to get left and right image arrows. Toggle back and forth to compare.
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