Francis, Petrology 212, 2014
Petrology Lab. 10: Meta-Igneous and Meta-Carbonate
Rocks
I. Metabasic Volcanic Rocks:
Station A:
Zeolite Facies to Prehnite-Pumpellyite sub-Facies
These specimens are sub-greenschist facies meta-volcanic rocks.
They are
characterized in hand specimen by the presence of quartz, calcite, and/or zeolite
filling amygdules and veins that represent former gas vesicles and fractures in the
volcanic rock. If the amygdules are filled with calcite, Ca-bearing zeolites are not
stable and clay minerals are present in their place. The volcanic matrix commonly
has an altered “look”, either because it is oxidized to a reddish or yellow brown
colour, or displays a bleached chalky appearance due to the development of clay
minerals.
Prehnite and pumpellyite develop at slightly higher grade, Prehnite can commonly be
identified in amygdules, but the identification of both in the fine-grained volcanic
groundmass typically requires examination with a microscope. With advanced
development the rocks begin to take on a green colour, marking the beginning of the
greenschist facies
Volcanic rocks in the Zeolite and Prehnite-Pumpellite facies typically show little sign
of physical deformation or penetrative strain, and primary volcanic textures are
commonly preserved. Massive samples that are free of fractures and gas vesicules
may retain much of their primary igneous mineralogy.
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Station B: Blueschist, Greenschist, and Amphibolite Facies
These specimens are low to medium grade meta-volcanic rocks. Examine each
specimen and determine to which of the following metamorphic facies it belongs.
Place it in the P-T diagram for metabasites.

Blueschist facies meta-volcanic rocks are characterized by the presence of the
blue amphibole glaucophane (Na2Mg3Al2Si8O22(OH)2) with lawsonite and
aragonite rather than calcite. Amphibole-rich metavolcanic rocks in the
blueschist facies are not really schists, but have pronounced fabrics produced by
penetrative strain. They form at relatively low temperatures, but high pressures,
and are thus commonly fine grained and poorly equilibrated (“dirty”). The
blushchist facies is thought reflect the P-0T conditions in former subduction
zones.

Greenschist facies meta-volcanic rocks are characterized by the green minerals;
epidote, actinolite, and chlorite, plus albite. Basaltic compositions are dominated
by these minerals, but more siliceous bulk compositions such as rhyolites and
dacites develop significant muscovite and as a result have whiter colours. If
recrystallization is extensive during deformation-induced strain, these rocks
become schists because of the abundance of phyllosilicates. In the absence of
extreme deformation, however, primary magmatic textures may be preserved
even where the mineralogy has been completely replaced by greenschist facies
minerals. In such cases, the rocks are typically too fine-grained to identify the
metamorphic mineralogy in hand specimen, but a distinct green colour to the finegrained groundmass is telltale. Sequences of basalts and andesites in the
greenschist facies are commonly referred to as greenstones or greenstone belts.

Amphibolite facies meta-volcanic rocks are typically dark grey to black in colour
because of the presence of hornblende rather than actinolitic amphibole, and the
absence of actinolite and chlorite. Ca-bearing plagioclase is stable and garnet will
appear in Fe-rich compositions with increasing metamorphic grade. Amphibolites
are typically coarser grained and better crystallized than greenschist facies rocks,
and commonly exhibit a pronounced lineation defined by the preferential
alignment of prismatic amphibole. Fine scale volcanic structures are commonly
obliterated by recrystallization and deformation, but large structures such as
pillows may still be recognizable. With increasing metamorphic grade,
amphibolites often develop gneissic textures, with the preferential concentration
of hornblende and plagioclase into alternating light and dark bands, and the
progressive destruction of primary magmatic textures, making distinction between
volcanic and plutonic protoliths difficult. Reflects higher grade regional
metamorphism, or more local thermal aureoles around granite intrusions in
greenstone sequences.
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Station C: Granulite and Eclogite Facies
The transition from the amphibolite to the granulite facies is marked by the
breakdown of phyllosillicates, and then amphiboles, by dehydration to form
pyroxenes.

Granulites are high-grade granular gneisses characterized by the
anhydrous assemblage orthopyroxene, plagioclase,  clinopyroxene 
sillimanite. Amphibole persists in lower grade granulites, but breaks down to
pyroxene with increasing grade in the pyroxene granulite facies. Garnet is
present in Fe-rich bulk compositions, but is commonly absent. In many
granulites, plagioclase has a slightly yellowish colour, and is characteristically
granular rather than the lath shape typical of igneous plagioclases.

Eclogites and Garnet Pyroxenites
Eclogites are the high-pressure equivalents of basalt and/or gabbro. They are
coarse-grained equigranular rocks characterized by the mineral assemblage:
purplish-red pyropic (Mg3Al2(SiO4)3) garnet, a green jadeitic (NaAlSi2O6)
clinopyroxene called omphacite,  kyanite,  quartz. Feldspar is absent.
Eclogite nodules are associated with diamonds in kimberlite pipes, and
isolated eclogite blocks (knockers) are found within blueschist facies rocks in
old subduction zones.
Garnet pyroxenites are similar to eclogites, in that they consist largely of
garnet and clinopyroxene. Unlike eclogites, however, they have ultramafic
compositions that are low in Al. In these rocks, feldspar is absent because
their bulk composition is too low in Al. Their clinopyroxene is less sodic than
that of eclogite, and is commonly darker black-green in colour in contrast to
the lighter green of the jadeitic clinopyroxene in true eclogites. Garnet
pyroxenites are distinguished from eclogites because they form at
significantly lower pressures, in the granulite metamorphic facies.
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II. Meta-Plutonic Rocks:
Station D: Retrograde Metamorphism
Plutonic rocks more rarely exhibit the effects of prograde metamorphism because their
igneous mineralogy already reflects equilibration at elevated temperatures and they do
not have the water that promotes reactions. However, deep-seated plutonic rocks
frequently exhibit the effects of retrograde metamorphism associated with either cooling
following magmatic crystallization or later recrystallization and hydration associated with
deformation.

Cooling: The grey rock (BP-21) at this station crystallized as troctolite from magmas
at elevated pressure. It was originally composed of brownish equant olivine and
subhedral grey plagioclase laths. Examine the contacts between olivine grains and
plagioclase. They are lined with a resistant green material that is a very fine-grained
symplectite of amphibole – garnet that developed by a reaction between the olivine
and plagioclase as cooling occurred. What do these reaction coronas tell us about the
trajectory of this rock in pressure - temperature space?

Hydration: The lighter coloured specimen with granular white feldspar (BP-15) is
found along the margins of the troctolite intrusion, where there has been hydration
during deformation and recrystallization. The magmatic olivine is gone, replaced by
knots of amphibole and garnet, and the original subhedral grey feldspar laths have
become granular and white in colour. The only chemical change that has occurred,
however, is the addition of water, which, along with deformation-induced
recrystallization, has converted the troctolite into garnet-hornblende gneiss, the very
mineralogy of the early symplectites.
All the samples at this station (BP-8, BP-15, BP-16, & BP-21) come from the same
intrusion and represent various stages of the hydration - recrystallization process. Try to
arrange them from the "most igneous" to the most "metamorphic" in terms of textural
appearance.
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III. Meta-Carbonate Rocks:
Station E:
This station contains a variety of meta-carbonate and “skarn” rocks. The original
sedimentary carbonate rocks have developed “calc-silicate” mineral assemblages because
of decarbonation reactions between carbonate minerals and original clastic quartz grains,
or silica introduced by hydrothermal solutions emanating from the magmatic intrusion
supplying the heat. Examine each specimen, determine its mineralogy, and place it in the
T-XCO2 diagram.
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