Metamorphic Geology
Here are links to some photographs of metamorphic rocks, arranged geographically rather than in terms of metamorphic theme. As part of your revision, try to relate the pictures and comments to your lecture notes (whatever module you are currently taking).
The Himalayas - a young, still active mountain belt
The Indus gorge region, Pakistan
The Indus gorge - cut into regionally metamorphosed rocks. They were metamorphosed during the Himalayan orogeny perhaps 50 My ago, but were already metamorphic prior to that, in the Precambrian.
The Indus gorge rocks are gneisses showing a strong planar location fabric (banding). How might that have formed?
Nearby gneisses show a linear fabric
In thin section note garnet with reaction rims of biotite and kyanite, and strong fabric (PPL, XPL)
The mountain Malubuting shows red rocks in lower cliffs, green rocks higher up. The red rocks were part of the Indian continent, green rocks probably oceanic.
The Alps - a Tertiary mountain belt
Many mountain belts are formed by continent-continent collision. In the Cretaceous, a small ocean (the Piemonte ocean) lay to the north of peninsular Italy and south of France. The ocean was subducted south-eastwards beneath Italy. The continental margin of France was carried along behind the ocean and eventually impinged on the edge of the Italian continent. Being buoyant, this could not be permanently subducted: it was thrust underneath slivers of the Piemonte ocean but then the whole pile was uplifted. The result is a sandwich of 3 tectonic units each of which was thrust NW over the one below.
TOP Sesia Zone (old Italian continental unit)
MIDDLE Piemonte Zone (relict bits of Piemonte ocean)
BOTTOM Internal Basement Massifs (Monte Rosa, Dora Maira massif)
Warning - grossly oversimplified! Pictures are arranged starting at the tectonically lowest level, but field pictures often include more than one unit.
The Monte Rosa, one of the highest Alpine mountains, made of brown metapelites and metagranites. The lower ground on the right is green Piemonte ophiolitic material at eclogite facies. These are structurally above the Monte Rosa rocks - the contact dips to the right, starting at the hillock just right of the ice.
The Dora Maira massif is the pale brown rocks in the distance (west of Turin). Green rocks in foreground were once part of the Piemonte ocean that lay between France and peninsular Italy. They are now thrust on top of the old French continental margin.
The Dora Maira massif is famous for the occurrence of coesite, a quartz polymorph, with in continental rocks. The picture (PPL) shows a coesite inclusion in garnet. The coesite (high relief) is partly retrogressed to quartz (low relief). What does cosite imply about the rock's burial history?
Eclogites in the lower part of a valley near the Matterhorn, a marble unit (white cliffs) and then greenschist-facies schists above. These different rocks types are all broadly part of the Piemonte zone. What can it mean if greenschist-facies rocks are adjacent to (in this case immediately above) eclogite facies rocks?
Metagabbro - the texture is gabbroic - green pseudomorphs inherited from pyroxene, white from plagioclase, but the minerals now are high-pressure metamorphic. The metagabbro is part of an ophiolite, a relic of the Piemonte ocean between France and peninsular Italy.
Deformed and metamorphosed pillow lavas now at eclogite facies.
PPL thin section of eclogite with large purple glaucophane porphyroblasts. The green mineral is omphacite. What does its texture indicate?
XPL thin section of greenschist facies metabasite, probably once eclogite facies and retrogressed. The aligned mineral showing brownish interference colours is chlorite, the diamond-shaped porphyroblasts are tremolite. What is the relation of the two minerals?
The greenschist facies calcareous schists show shear bands dipping to the right in this photograph. These imply the top part of this outcrop moved to the right (East) relative to the base. This is movement direction is important on a larger scale in this part of the Piemonte zone.
XPL thin section of greenschist facies albite-bearing mylonite with porphyroclasts of pyroxene and hornblende, from the Piemonte zone. What might the protolith have been?
The Matterhorn consists of greenschist facies metagranites of the Sesia zone (the big face) structurally above Piemonte zone eclogites (cliffs and grassy ledges below snowline).
Tight folding in greenschist facies metagranites of the Sesia Zone.
The Sesia Zone consists mainly of metagranites and metapelites, in part at eclogite facies. The photo shows boudins of metabasic material within metagranite.
Sometimes the Sesia zone metagranites are undeformed and look in hand specimen like ordinary granite. In thin section (PPL, XPL) the metagranite shows fine-grained pseudomorphs after plagioclase (right and left of view), pristine quartz (why isn't this altered?), and a border of garnet between plagioclase pseudomorphs and biotite. What does this border signify?
The Lewisian Gneiss of NW Scotland
Once a mountain belt, but started forming at 2800 Ma, last metamorphic events at 1400 Ma, now overlain unconformably by unmetamorphosed sediments.
Metabasic rock with red garnets surrounded by white plagioclase coronas.
Thin section (PPL) of metabasic rock with plagioclase poikiloblast.
Tight fold in quartzofeldspathic gneiss. What is a likely protolith for such a rock?
Quartz mylonite in thin section (XPL).
West Norway
Garnet coronas forming round huge black orthopyroxenes in a meta-anorthosite.
Pyrenees
Metapelite with large kyanite grains
Metapelite with large kyanite grains again
Metapelite with large yellow-brown staurolite grains
Western USA
A reminder that beneath a thin veneer of sediment lurks metamorphic rock forming most of the solid Earth.
Sediments with obvious bedding forming the upper cliffs of the Grand Canyon
Sediments above metamorphic basement in the Grand Canyon
Sediments above metamorphic basement in the Grand Canyon
