SEPM Strata

amphibolite

Description

Lineated Schistose Foliation produced by the lining up of elongate amphibole (var. hornblend) crystals; seen trending left and right in this image. Other minerals such as quartz and feldspar are completely intermixed among the amphibole.

The dark, shiny appearance of this rock is typical, although it may range to lighter colors too.

It might be possible to confuse this with an Mafic Igneous Rock, since many of the minerals are the same, but the presence of foliation will identify it as igneous.

Type of Metamorphism

amphibolites are part of a metamorphic sequence that begins with magic igneous rocks and progresses through greenschist, to amphibolite, to granulite.

Composition

NORMAL
amphibole
var. hornblend
Plagioclase
quartz
Micas

ACCESSORY
In red if present
garnet

amphiboles often elongage producing Schistose Texture, although they may be randomly oriented also.

Tectonic Association

amphibolite is associated with major mountain building events when mafic igneous rocks (basalts or gabbros) are metamorphosed through depth of burial, and proximity to batholiths (click picture for larger version).

Detail

Description

In this close up we can see the Lineated Schistose Foliation produced by the lining up of elongate amphibole crystals. The other plagiioclase and quartz minerals are completely intermixed among the amphibole crystals.

When fresh the rock is typically shiney with light reflecting off the crystals.