Meta-igneous rocks

Meta-igneous rocks

1. Metamorphic facies

   • Greenschist
     • chlorite, albite, actionlite, epidote, quartz
     • chlorite + green amphibole
   • Amphibolite
     • green amphibole (hornblende = dark green/black amphibole), garnet, plag
   • Hornfels
   • Sanidinite
   • Eclogite
     • high density - transformation of blue schist into something denser and loss of hydrous minerals.
     • green/red colours diagnostic - green pyroxene (jadeite) and garnet
   • Granulite
     • with increasing T, amphibole breaks down to produce pyroxenes: clino and ortho
     • represent unusually hot conditions - greater T than associated with their depth of formation so thought to be formed with crustal thickening and excess heating
     • no hydrous mineral phases - represents deep burial and dehydrated roots of continental crust
     • pyx, garnet/plag
   • Epidote-amphibolite
     • epidote + green amphibole
   • Blueschist
     • Grown under v high pressures - subduction zones
     • characterised by sonic blue amphibole = glaucophane
   • Greenschist --> amphibolite --> granulite corresponds to typical continental geotherm
     • most common facies dries of regional metamorphism - equivalent to classical Barrovian series

2. Hydration of original mafic minerals needed to generate mm mineral assemblages for most facies

   • w/o H2O most igneous rocks will be unaffected whilst surrounding sediments will be metamorphosed
   • coarse-grained intrusions of gabbro are most resistant - low H2O content whilst basalts are most susceptible

3. not much difference between low-P metabolites and med-P

   • distinguish between contact mm facies and medium-pressure regional mm facies if 1) metapelites are present (regional) 2) textures etc

4. metasediments (and barrow zones) tend to give more information that metamorphic facies

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