North Cascades National Park:
Making Sense of Metamorphic Rocks and Terranes in General

A lot of things can happen to rocks after they have formed- moving about on tectonic plates, folding, and faulting. When geologists are confronted with rocks that have also been heated, squeezed, and recrystallized (metamorphosed), they might well wonder what sense can be made out of them.
An alternative to getting bogged down in the details is to look at the big picture beyond the scrim of geologic events. Most rocks are born in one of three major settings, and even after considerable terrane shuffle and metamorphism, these birthplaces can be recognized in rocks of the North Cascades:

Rocks of the three major birth places: continental, oceanic, and volcanic arc.

Oceanic rocks

Oceanic rocks are born in the deep ocean, mostly well out to sea, far from the debris shed by continents and volcanic arcs. The sediments are muds, silts, fine-grained sands, and siliceous (quartz) oozes. Many are made up of the shells of tiny marine plankton, radiolarians, whose siliceous skeletons accumulate and have been well preserved on the deep ocean floor. They become radiolarian cherts. Associated with these deposits are basalts erupted from the oceanic ridges and oceanic islands like Hawaii. Also, not uncommonly associated with the oceanic rocks are pieces of the underlying mantle, that is, ultramafic rock. After metamorphism, many oceanic sedimentary rocks become mica schists; the chert becomes distinctive mica-quartz schist or quartzite; and oceanic basalt becomes dark amphibolite (hornblende-plagioclase rock). Ultramafic rock may recrystallize to a variety of somewhat esoteric schists, but commonly becomes slippery green serpentinite.

Arc rocks

Arc rocks are rocks derived from volcanic arcs. Arc rocks are both volcanic and sedimentary, because not only do volcanoes grow as the arc develops, but the volcanoes erode, and the eroded debris is deposited along the arc as sediment fans. If the arc is in the ocean or adjacent to it, the erupted and eroded debris spreads out under the sea as submarine fans. These fans include layers of volcanic rocks alternating with sandstones and shales made up of volcanic-rock grains. Other rocks that may be present are limestone from reefs that may form around volcanic islands and crystallized magma, mostly granitic igneous rocks, from the chambers that fed the arc volcanoes. Metamorphosed arc rocks have very little mica-quartz schist (at least from near the flanks of the volcanoes, where siliceous oozes have no time to accumulate) and have a greater variety of minerals than metamorphosed oceanic rocks, reflecting a greater variety of volcanic rocks. The limestone reefs metamorphose to marble. In the North Cascades, some metamorphosed arc rocks include metamorphosed conglomerate (or metaconglomerate), a certain clue that the volcanoes were eroded by streams. Magma chambers beneath the arc volcanoes crystallize to become arc-root plutons. These granitic rocks may be squeezed and recrystallized by metamorphism, but their origins are still recognizable.

Continental-derived

Continental-derived rocks are rocks eroded from old continental landmasses. This group is a little more tricky to recognize. If the continents are highly eroded, then the debris carried into the ocean, to form sandstones, shales, and conglomerates, is rich in fragments of metamorphic and granitic igneous rocks. Naturally some rocks will be derived from arcs and continents and even uplifted oceanic rocks, and such mixtures are not so easily classified. After metamorphism, continental-derived rocks may only be distinguished from arc-rocks, if distinguished at all, by subtle differences in chemical composition.

Geology in the National Parks home | U.S. Department of the Interior | U.S. Geological Survey
Privacy statement / General Disclaimer
USGS Western Earth Surfaces Processes Team Webmaster
http://geology.wr.usgs.gov/docs/usgsnps/index.html
This page was last updated on 01/13/04