Garnet Amphibolite. Metamorphosed mafic rock sample. Gore Mountain, NY, USA. Click on the image to see a larger version with more information.
Glaucophane Schist. Metamorphosed mafic rock sample. Syros, Greece. Click on the image to see a larger version with more information.
Metamorphosed Mafic Rock Information
Introduction
Most mafic igneous rocks, such as basalt, contain little H2O, so the addition of water and hydration reactions are commonly involved to form their low grade metamorphosed equivalents. Low grade metamorphosed mafic rocks have one or more green-colored minerals, including chlorite, epidote, and actinolite, leading to the name greenschist. Other minerals at different grades, such as red garnet, blue glaucophane, brown orthopyroxene, and green omphacite, make metamorphosed mafic rocks more colorful than their igneous protoliths.
Mineralogy of Metamorphosed Mafic Rocks
Metamorphosed Mafic Rock Minerals . Click on the image to visit the list of minerals and links to their properties.
Amphibole minerals, noteably hornblende, and plagioclase feldspar are major players in the mineralogy of metamorphosed mafic rocks. Beware that in many samples, plagioclase does not exhibit the polysynthetic twinning that is ubiquitous in igneous plagioclase. Interestingly, quartz is present in many metamorphosed mafic rocks, even though quartz is not in the igneous protolith. A list of minerals commonly observed in metamorphosed mafic rocks and links to the properties of these minerals can be found here.
Samples of Metamorphosed Mafic Rocks
Eclogite, Norway. A sample of a metamorphosed mafic rock. Click on the image to see a list of metmaorphosed mafic rock images in the Rock Library, including this and other eclogite samples.
The Rock Library includes images of hand samples and thin sections of metamorphosed mafic rocks. A lower modal percentage of phyllosilites in these rocks means that their textures are likely to be more granofelsic and gneissic than schistose. Links to images of metamorphosed mafic rocks can be found on this list.
ACF Projection
There are too many chemical components in metamorphosed mafic rocks and too much variety in mineralogy to reduce these rocks by projection to a 3-component system.
ACF Diagram . Click on the image to show compositions of metamorphosed mafic rock minerals or to plot bulk compositions, both on an ACF diagram.
The digram normally used to show mineral assemblages for metamorphosed mafic rocks is the ACF-diagram: Al2O3-CaO-(FeO+MgO) projected from quartz. Assembages plotted on the ACF diagram may involve 4 (or more) major minerals plus quartz at equilibrium, so do not be surprised to see crossing tie lines. You can plot bulk rock compositions from your own chemical data (upload a .csv file) on an AFM diagram here. By clicking buttons on the linked ACF diagram you can see where the compositions of common minerals plot on the diagram.
Facies of Metamorphosed Mafic Rocks
Metamorphic Facies . Click on the image to see generalized ACF mineral assemblages for each metamorphic facies.
Mineral assemblages in metamorphosed mafic rocks have been used to define metamorphic facies. You can see the IUGS facies definitions by mousing over the facies names on this diagram, which shows approximate T-P conditions for each facies. Click on the image on the right to open another version of the facies T-P diagram. Mouseover that diagram to see generalized ACF mineral assemblages for each metamorphic facies. The facies boundaries on a T-P diagram are approximate because the T-P conditions for facies change can depend on the bulk composition of the mafic rock.
