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North Cascades National Park
Volcanoes and Their Roots: Making granite and its relatives

The different kinds of plutons-granite, granodiorite, tonalite, gabbro-encountered in the North Cascades and mentioned throughout this website-acquired their different compositions mostly through two very different but related processes: magmatic differentiation and differential melting.

Magmatic differentiation

When magma begins to crystallize, the first-formed crystals are commonly of minerals richer in iron, magnesium, and calcium. These are dark minerals such as hornblende and pyroxene and calcium-rich plagioclase feldspar. The remaining melt is thus depleted in iron, magnesium, etc. and relatively enriched in other elements such as sodium, aluminum, and silicon. If this melt is drained away from the early-formed crystals, it will, when it crystallizes, produce a lighter-colored rock richer in quartz, potassium- and sodium-rich feldspar, and biotite.

Simplified sketch of magmatic differentiation as seen in cross-section of magma chamber.
Simplified sketch of magmatic differentiation as seen in cross-section of magma chamber.

Magmatic differentiation has been demonstrated to work in the laboratory and careful chemical studies of granitic rocks substantiate it as well. Geologists believe that this process of magmatic differentiation can produce all the different kinds of plutons of similar age occurring in a local area. In some plutons, a gradation from dark rocks rich in hornblende and pyroxene to a core of light-colored rock indicates that the liquid remaining after the early crystals formed became concentrated in the center of the mass. By looking at the chemical changes in suites of granitic rocks and volcanic rocks, geologists surmise that the usual beginning melt has the composition of basalt not unlike that of the ocean floor.

If basaltic magma crystallizes slowly but without differentiation, the resulting rock will be a gabbro. We find such gabbros in several places in the Chilliwack batholith. One is on Mount Sefrit, west of Ruth Creek and another is near Copper Mountain above the Chilliwack River. The basaltic magma that produced these dark rocks came up from great depths without pausing to differentiate. However most of the root plutons of the Cascade Volcanic Arc and the volcanic remnants are differentiated somewhat, to produce tonalite and granodiorite and their volcanic equivalent, dacite. Only a few Cascade arc plutons differentiated to granite and its volcanic equivalent, rhyolite. Volcanic andesite, the result of only slight differentiation, is present in the Cascade Arc, but its plutonic equivalent, diorite, is rare, a discrepancy possibly due to lack of data.

rock classification
Left and right classification triangles show the equivalent mineralogical positions of plutonic and volcanic igneous rocks respectively.

Differential melting

Differential melting is the reverse of magmatic differentiation. In a subduction zone, the amount of water rising from the descending oceanic plate; into the overlying mantel and lower crust will vary a great deal depending on how much is in the subducted plate in the first place. Larger amounts of water will cause the hot rocks to melt sooner and at a lower temperature than they would if permeated with lessor amounts of water, . The melted material will then have more silica-the reverse of the magmatic differentiation process described above. If this early-formed melt is separated from the remaining unmelted rock, a magma with composition different from the unmelted material will result. Professor Jeffery Tepper, who has studied North Cascade arc-root plutons more than most, along with his colleagues, believes that the amount of water released from the subducting plate has greatly influenced the composition of the plutons in the batholith.

Volcanoes and Their Roots home

Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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