Metagabbro Identification Guide

What Metagabbro Looks Like

Metagabbro is gabbro that has been metamorphosed, partially or wholly converting its original pyroxene and calcium-plagioclase into metamorphic minerals such as amphibole (hornblende, actinolite), epidote, chlorite, and sodic plagioclase. It usually keeps the coarse, dark, mottled look of gabbro but takes on greenish or patchy alteration, sometimes with relict igneous texture ("coronas" of amphibole rimming former pyroxene) and a weak foliation.

• Color: dark grey, greenish-grey to greenish-black, mottled black-and-white
• Luster: dull to vitreous; amphibole and feldspar grains can glint
• Transparency: opaque
• Texture: coarse-grained, equigranular; relict igneous fabric common; may show incipient foliation or reaction rims

Step-by-Step Field-ID Checklist

1. Confirm coarse grain and mafic composition: dark, plutonic, visibly crystalline.
2. Look for alteration: green amphibole/chlorite/epidote patches replacing original black pyroxene.
3. Check for relict igneous texture: ghost outlines of original crystals, often with amphibole coronas around former pyroxene cores.
4. Assess fabric: weakly foliated to massive; if strongly foliated and recrystallized, it grades into amphibolite.
5. Find plagioclase: white-to-grey feldspar laths, possibly clouded (saussuritized) to a dull greenish mass.

Key Diagnostic Tests

• Hardness: amphibole/pyroxene ~5.5-6, plagioclase ~6, epidote ~6; chlorite soft (~2-2.5).
• Density: high (~2.9-3.1); feels heavy.
• Streak: grey to pale green.
• Acid: inert unless carbonate alteration veins are present.
• Saussurite test: dull, greenish, clouded plagioclase (saussuritization) is a hallmark of metamorphosed gabbro.

Common Look-Alikes and How to Tell Them Apart

• Fresh gabbro: crisp black pyroxene and clean white plagioclase with no green alteration and no foliation. Greenish reaction products and clouded feldspar mark the metamorphic version.
• Amphibolite: the more fully recrystallized, foliated equivalent dominated by hornblende and plagioclase. Metagabbro still preserves relict igneous texture; amphibolite generally does not.
• Eclogite: the high-pressure equivalent, distinctive with red garnet plus green omphacite and no plagioclase. Garnet-omphacite assemblage means eclogite, not metagabbro.
• Diorite/metadiorite: lighter color, more plagioclase, less mafic; metagabbro is darker and more mafic.

Where It Is Typically Found

Metagabbro occurs in ophiolite complexes (metamorphosed oceanic crust), in deep parts of orogenic belts, and within Precambrian shields. It is common where former gabbroic intrusions have been caught in regional metamorphism or hydrothermally altered along the seafloor.