Amphibolite of Elk Knob State Park

Amphibolite aka "metamorphosed ocean-floor basalt"

Amphibolite is the dark-colored metamorphic rock found at Elk Knob. Amphibolite forms as a black igneous rock called basalt is buried deep in the earth's crust during mountain building and is metamorphosed by heat and pressure.

The mineral composition of the basalt is altered during metamorphism and original pyroxenes change to amphibole when exposed to medium- to high-grade metamorphism, hence the term "amphibolite".

Lavas that are basaltic in composition often erupt from volcanic centers in the ocean such as mid-ocean ridges, volcanic arcs, and/or hot spot volcanoes like Hawaii. The amphibolite at Elk Knob, therefore, represents metamorphosed oceanic crust.



Triassic sediments
Here's a closer look at some coarse-grained amphibolite. Amphibolite is dominantly composed of black hornblende and light gray to white plagioclase. Garnet does occur locally. The amphibolite at Elk Knob is commonly folded as a result of mountain building processes. Triassic Sediments Chapel Hill WWTP
Green epidote commonly occurs as layers and/or on joint surfaces at Elk Knob. This image shows an epidote boudin, or sausage-shaped layer, which forms by extension of the strong epidote layer in a more ductile groundmass of amphibolite.

The three following images are thin-section microphotographs of garnet-bearing amphibolite from Elk Knob.

To make a thin-section, rock is cut to 1/32-mm thick and apoxied to a glass slide, then polished. The thin-section is studied using a petrographic microscope that allows the geologist to use reflected light microscopy to detail the mineral composition and study textures in rocks. This information is used to determine the rocks origin, metamorphic, and/or deformation history.

These sections show Elk Knob rocks record two, possibly three, periods of metamorphism. Each event is marked by different minearl assemblages and represent different pressure-temperature conditions in the life of rocks at Elk Knob.

This microphotograph shows hornblende (green), plagioclase (transparent), and garnet (large, pink) typical of the upper amphibolite body at Elk. Note the mineral colors may be different in thin-section under plane-polarized light, as compared to their color in hand-sample. The presence of garnet in some amphibolite while absent in other amphibolite, reflects differences in original bulk rock composition (Wilson and Raymond, 2011).  The observed mineral assemblage agrees with peak PT estimates of amphibolite facies conditions at ~7 kb and 700 degrees celsius (Raymond and Abbott, 1984).

Triassic Sandstone

Microphotograph of garnet porphyroblasts (large grains that grow during metamorphism) surrounded by plagioclase coronas and a matrix of hornblende (green)-plagioclase (transparent). The plagioclase coronas around the garnets in this photo (and the photo above) are very common in the garnet-bearing amphibolites and and can even be seen in hand sample.

This texture may indicate that Elk Knob amphibolite experienced eclogite facies (higher pressure and temperature) conditions as suggested by similar textures observed in amphibolite samples near Elk Knob that are associated with retrograde eclogite (Wilson and Raymond, 2011).

Triassic Conglomerate
This microphotograph shows biotite (brown) and chlorite (elongated, light yellow-green) surrounded by plagioclase (transparent) and hornblende (dark green). The presence of biotite and chlorite (seen here), and epidote (not shown) indicates Elk Knob rocks also experienced lower grade, greenschist facies metamorphism.