Name(s): _____________________________ & _____________________________
Section (please circle one):
001 Tues
002 Thurs
Lab 7: Metamorphic Rocks, Processes, Resources, & Environments
Winter 2016 This exercise is based on questions & materials in the 10th ed. AGI lab manual.
Read Chapter 7 from your manual covering Metamorphic rocks, textures & minerals. Use
materials as provided to answer the questions of the same number in the lab manual. Answer questions
from Lab 7 in the spaces provided on this sheet.
Introduction:
Metamorphic rocks are recrystallized entirely in the solid state and usually in the presence of fluids
(water, CO2). Their parent materials protoliths may be any rock type or even metamorphic fluids which
carried new solutes such as vein infillings in hydrothermal settings. Metamorphism involves a mineral
response to new conditions different from the original setting such as: 1.) directed stresses (compaction,
flattening, rotation, shear), 2.) a change in heat causing dehydration, decarbonation and thermal
expansion or the reverse such as hydration upon cooling and 3.) a change in overall pressure favouring
minerals of greater density or the reverse. The places where rocks encounter changing conditions most
easily or at the fastest rate generally occur along plate margins. Wherever there is a high geothermal
gradient such as near intrusions, in the crust along an arc or with rapid burial rocks encounter new
thermal conditions. This is also the most likely place to experience rapid pressure increase or directed
stresses. While rocks also uplift, cool off and decompress; by this time fluids have been driven off and
there is little permeability to bring new ones in. As a result regional metamorphic rocks tend to record
their maximum conditions of metamorphism. They also tend to cover vast map areas along present or
former mountain belts. Thermal metamorphism occurs very close to hot intrusions, generally within a
few metres. Contact metamorphism occurs there and is generally thin and spotty in its outcrop as rock
types can vary locally and the aureole is narrow. In this type of setting rocks like shales or fine grained
volcanic rocks are baked to dark coloured tough hard hornfels sometimes with a few large spotted
porphyroblasts. Also hydrothermal veins are emplaced and coarse grained skarns of unusual mineralogy
can form including many with economically valuable metal sulfides. In areas of very high strain like
along deep fault zones or at impact or blast sites, rocks can become sheared, brecciated or pulverized.
These settings are termed dynamic and the strain rate dominates the textures rather than changes to heat
or pressure.
We recognize metamorphic rocks by new textures: brecciation, foliation, lineation and the growth of
new mineral assemblages. While many familiar minerals persist in metamorphic settings: feldspars,
quartz, micas, hornblende, pyroxenes, calcite, magnetite, pyrite etc. there are also many new minerals.
Some of these include: aluminosilicates (andalusite, kyanite, sillimanite), staurolite, cordierite, garnet,
chlorite, zeolites, epidote, wollastonite, serpentine and many new unusual amphiboles (tremolite,
actinolite, riebeckite). As it turns out many of these new minerals are platy or elongate giving the rocks
special textures. They are also often sensitive indicators or temperature, pressure or fluids and in this
way act like environmental indicators of peak metamorphic conditions.
Examine the photos and terms for rock textures, new minerals and rock names. Some of these are pretty
logical compared to some of the “wacke” sedimentary names. Rocks tend to have a mineral grade
indicator and a textural name. A garnet biotite schist is a foliated shiny flattened or folded micaceous
rock with some garnets and essential biotite. A hornblende gneiss has alternate dark hornblende and
light layers of coarse grained quartzo-feldspathic silicate minerals on a scale up to hundreds of meters. A
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mylonite (milled in Greek) is a fine grained rock from inside a fault zone. Marble (from limestone or
dolostone) and quartzite (from quartz rich sandstone) are special compositions and can only become
coarser grained with metamorphic recrystallization.
Activity 7-1: Metamorphic Rock Inquiry from hand specimens/photos
Questions
A. Examine the photos provided in the manual on p.199 and find rocks like them (7.1.A.1 to A.6) and in
the Wards kits. There is a lot of natural variation in mineralogy, crystal sizes, foliation, schistosity,
gneissosity, porphyroblasts, folding and veining in metamorphic rocks. For example if the table below
held a line for the rock type “black slate”, its protolith would be shale or mudrock, its texture would be
laminated or having slaty rock cleavage and its composition would likely include microscopic clays,
micas, quartz and graphite. The volumetrically common protoliths are: pelites (mudrocks), psammites
(sandstones), carbonates (limestone or dolostone), volcanics (pyroclastics, flows) and plutonics
(granites, diorites, gabbros). Shists generally contain 3 or more aluminum bearing minerals and come
from muddy sediments. Gneisses may come from pelites or plutonic rocks so it is necessary to
determine their mineralogy to tell meta-igneous orthogneisses from meta-sedimentary paragneisses.
Refer to the introductory pages 187-198 and especially the metamorphic minerals on p.193 and the
classification table on p.197. Fill out the table below._____________________________________ (18)
Rock name
Composition: min’s/grains Textures ____ Protolith
1. Gneiss
2. Folded Schist
3. Marble
4. Garnet Schist
5. Phyllite
6. Mica Schist
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Activity 7-2: Metamorphic Rock Analysis & Interpretation from Specimens
Carbonate Rocks and Calc-Silicates
A. From our Lab Samples: obtain a piece of layered fossiliferous limestone (7.2.A.1), the photo shows
beddling plane from above and cross section below, and a pink and grey marble (7.2.A.2). Compare the
2 rocks. Do not be destructive of the fossils! Use hand lenses or binocular microscopes only on these
specimens please. If you do acid tests streak a corner of the rock and test the acid on the powdered
streak.
1. Do a simple test to determine what mineral makes up the majority of both of these rocks. What is your
test _____________________________and what is the mineral? ____________________________ (2)
2. The limestone has 2 textures or structures which are no longer present in the marble. a.) What are
these? __________________________________ & ______________________________________ (2)
3. Examine your marble and the photo above and in the lab manual of one in Figure 7.1.A.3. Describe
the new texture using the correct term from table 7.16 on p 197. This rock is: Foliated Non-foliated (1)
4. Impure marbles with some clays, quartz, iron oxides in the protolith can make a small proportion of
other indicator minerals like micas from clay minerals, Ca-amphiboles (actinolite, tremolite) from
Calcite and Quartz, or Ca-pyroxenes (Wollastonite, Diopside) or Ca-Garnet (Grossularite) along with
the coarser grained non-foliated Calcite or Dolomite to indicate the grade or temperature conditions of
metamorphism. Look at the grade minerals in the chart 7.6 on p 193. Examine your pink and grey
marble specimen and check closely for another mineral.
This “grade” indicator mineral is __________________ & the marble’s grade is: low intermediate (2)
5. Find a piece of Wollastonite Skarn from Rossland, B.C. (7.2.A.5a). This contact metamorphic rock
was formed from a pure Calcite limestone of Permo-Pennsylvanian age Mount Roberts Formation was
cut by intrusions of the Eocene Coryell suite. This sent hot (> 375°C, wet, silica bearing fluids to
recrystallize the sedimentary limestones into a coarse grained contact metamorphic skarn. Note the
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blades of Wollastonite have cleavages at 84°/96° similar to other pyroxenes. We also have a piece of
Wollastonite skarn from an abandoned mine in the Adirondacks near Willsboro, New York (7.2.A.5.b).
This deposit is 1.1 Ga in age (Grenville Orogeny) where Anorthositic Gneiss cuts metasediments. Find
both rocks from our collection like the images shown below. In some places the protolith is a muddy
limestone (marl) and the metamorphic derivatives are called calc-silicates from having a mixture of
calcium and silicate bearing metamorphic minerals such as diopside, tremolite/actinolite, grossular
garnet and Calcic-scapolite.
Rossland, B.C. (7.2.A.5a)
Willsboro, N.Y. (7.2.A.5.b)
Wollastonite’s formation is described by the following expression:
Si02 (aq) + CaC03 (s)  CaSiO3 (s) + C02 (g)
Aqueous Silica + Solid Calcite  Solid Wollastonite + Carbon Dioxide lost to crust/atmosphere. This is
not an ”equation”, although things balance, because the gas is lost to the system. Most prograde
metamorphism is characterized by dehydration or decarbonation reactions. Since the volatiles are lost
during metamorphism, the rocks tend to recrystallize only once and get stuck in this higher grade
mineral assemblage. Once a rock metamorphoses, there is no going back. In addition to the calcium and
silica, the Willsboro rock has some magnesium and iron as seen with its extra minerals.
Describe the texture of these contact metamorphic skarn rocks:
Foliated
Non-foliated
(1)
Test the hardness of wollastonite _____________ and explain why this is a useful mineral for clutch
pads and brake linings? _______________________________________________________________
________________________________________________________________________________ (2)
B. The most common sediments are mudrocks (shales, argillites, siltstones, wackes). When these are
regionally metamorphosed on convergent margins the make a series of different metamorphic rocks
from slate to phyllite to schist. Obtain and examine specimens of these three different regional
metamorphic mudrocks: 7.2.B.1 Slate,
7.2.B.2 Phyllite and
7.2.B.3 Schist.
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They all contain micas as their principal mineral but often contain lesser amounts of quartz, microcline
(K-Feldspar), iron oxides, graphite and traces of pyrite. These all have flattening or shear caused by the
directed pressure of regional metamorphism.
1. Describe the grain size in each of these rocks: Slate: _______, Phyllite ______ and Schist ______ (3)
2. What is distinctive about the change in texture from Phyllite to Schist ________________________
________________________________________________________________________________ (1)
3. Explain what happened to make the micas which grew in these three metamorphosed mudrocks are all
parallel or sub parallel to each other. What was responsible for this? ___________________________
_________________________________________________________________________________(2)
4. Examine Figure 7.1 p.188 and the rocks above. Explain how we can tell that rocks with these textures
originated either at shallow depths under: continental mountain belts on the upper plate of convergent
margins or within major crustal fault zones? ______________________________________________
__________________________________________________________________________________
_________________________________________________________________________________ (2)
C. Examine Figure 7.4 p.201 and obtain a piece of 7.2.C.1 coarse grained folded gneiss or schist from
our lab collection.
1. Describe a process which could make a fold in a coarse grained brittle rock like this without
shattering it. ______________________________________________________________________ (2)
2. Where is a likely tectonic or geological environment and depth for this process to occur, or where
might something like this be taking place today? _________________________________________
________________________________________________________________________________ (2)
D. Examine the regionally metamorphosed rock next below and in figure 7.1.A.4 above and find one
like this in the Ward’s kit of a coarse grained mica schist with larger garnet crystals.
1. The texture of this rock is (choose one):
Foliated
5
Non-foliated
(1)
2. What is the correct textural name for the large garnet crystals in this schist? __________________ (1)
3. What is the correct name for this regional metamorphic rock including the mineral which tells its
grade and its dominant texture : ______________________ _____________________________ (2)
4. Check back to your mineral compositions to confirm that both the abundant micas and the garnet in
this rock are dominantly aluminosilicate minerals with small amounts of other elements (Fe, Mg, Ca).
From this bulk composition, assuming the elements just rearranged into new metamorphic minerals,
what was the protolith for this metamorphic rock?
(Choose one):
Quartz-sandstone
Shale
Dolostone
Basalt
(1)
E. When Basalt and gabbro of the ocean crust (dominantly plagioclase feldspar and pyroxene) are
subducted into the Mantle, they experience extreme increase of pressure before they have much time to
heat up. They produce an extremely pretty coarse grained rock with red garnet (Omphacite) and green
pyroxene (Jadeite) like that pictured next to E on p 168. We have a sample & thin section of this rock
set up on the side bench for you to examine. To obtain a rock like this, a whole subduction zone needs to
be uplifted and eroded by more than 80 kilometres. Needless to say there aren’t many places on Earth
where this has happened. Southern Oregon since Jurassic and Norway since Devonian are 2 of them!
Needless to say even if we find this in the center of a continent, we can tell it used to be a subduction
zone on a convergent margin.
1. This rock as pictured above is (choose one):
Foliated
Non-foliated
2. What is the correct name for this metamorphic rock from a former subduction zone?
a. Pyroxene schist
b. Garnet gneiss
c. Eclogite
d. MacGregor Tartan
(1)
(1)
3. The mineralogy (garnet & pyroxene) and chemical composition ~50% SiO2 content of this rock
suggests its protolith was a volcanic rock of this general type (choose one):
Mafic Felsic (1)
4. What was the original geological setting for this rock? Seafloor
Continental Margin Arc
(1)
F. Which of the last 2 rocks demonstrates a higher metamorphic grade D or E & Why? ___________
_______________________________________________________________________________ (2)
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Activity 7-3: Hand Sample Analyses of Metamorphic Rocks use: A-Amphibolite,
B-Blueschist, C-Black and White Marble, D-Andalusite Schist, E-Quartzite, FPhyllite, G-Garnet Schist & H-Apatite-Magnetite-Tremolite Skarn, I-Serpentinite,
J. Zeolite facies volcanic, from our collection, in order, on the metamorphic
worksheets below.
(60)
On the table below note: 1) the general texture such as gneissose, lineated, schistose, veined, brecciated,
porphyroblastic, foliated etc and whether the rock is foliated or non foliated. Minerals means mineral
names such as glaucophane, epidote, pyrite in the blueschist, the names for the mica types in a schist,
types of porphyroblasts such as garnet, andalusite, staurolite. Protolith refers to the starting composition
or parent rock such as: pelitic for mudstones, volcanic for blueschist or amphibolites, granitic for gneiss.
Geological settings refer to site of origin: accretionary wedge, hydrothermal volcanic, shallow or deep
subduction zone for blueschist versus eclogite, contact metamorphic for skarns, hornfels or
hydrothermal rocks. Use refers to our own industrial applications: dimension stone (building) for marble
or gneiss, insulators, ceramics, roofing slates etc.
#Rock
name
Textures &
Minerals
Protolith
Fol or Non
Composition Parent Rock Setting
A.
Amphibolite
B.
Blueschist
C.
Marble
D.
And.schist
E.
Quartzite
F.
Phyllite
G.
Gar.schist
H.
Skarn
I.
Serpentinite
J.
Zeollite rock
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Geological Use
Activity 7.4: Metamorphic Grades, Facies (mineral indicators of Temperature &
Pressure) and Geological Maps.
A. Mudrocks are common protoliths especially in trenches and accretionary wedges along convergent
Continental margins. Clay minerals like Kaolinite (Al2Si2O5)(OH)4 , tend to recrystallize to Al2SiO5
aluminosilicate polymorphs like: Andalusite (white blocky low pressure form, common around shallow
intrusions), Kyanite (blue bladed high pressure dense polymorph formed deep under mountain belts) or
Sillimanite (red-brown fibrous form near intrusions or in high temperature granulites from the base of
the crust). Identifying these minerals and finding rocks where both occur nearby are the best
geobarometers and geothermometers for measuring apparent geothermal gradients across metamorphic
terrains developed in mudrocks.
1. Minerals react and form successively with increasing metamorphic grade. Due to dehydration
reactions, most metamorphic rocks record the highest grade metamorphism they experienced. Map
zones on the ground with the same mineral are said to be the same grade and the 1st appearance of an
index mineral is an isograd. On the cartoon map below, colour in the rocks with the highest grade. (2)
2. Go back to the Plate exercise on p. 35 figure 2.8.b. Note the location of the oceanic and continental
geothermal gradients. Transfer them to this diagram below. Pay attention that where the pressure axis
says 10,000 atmospheres that is about 10 kilobars. Hint 2: this figure below fits like a postage stamp in
the shallow low temperature corner of that phase diagram (1 cm by 3 cm on that figure), so all of
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metamorphic conditions are way colder than any melting conditions. Metamorphic rocks may be coarse
grained and sparkly like igneous rocks but they came from much lower temperature and are the result of
solid state recrystallization with or without fluids, with or without strain and not from magmas!
Plot the steeper oceanic and shallower continental geothermal gradients on the figure below. Label them
OG and CG respectively! ____________________________________________________________ (6)
Metamorphic Facies, P-T conditions & Geological Environments of Formation
Facies and Index minerals: Zeolites = Burial Metamorphism (Zeolites: natrolite, chabazite, analcite)
and hydrothermal alteration of volcanic and immature sediments. Shallow rocks heated close to igneous
intrusions get baked to fine grained tough Hornfels (various minerals: Cordierite, Chlorite, Biotite,
Hornblende, Pyroxene). Hydrothermal systems close to intrusions can generate veins (Quartz,
Chlorite, Epidote) or coarse grained Skarns of many different unusual mineralogies but Garnet,
Epidote, Magnetite and Pyroxenes are common. Subduction Zone Metamorphism is cold but ultra high
pressure as for Blueschists (Glaucophane, Riebeckite, Arfedsonite all Na-Fe-Amphiboles) and Eclogites
(Jadeite, Omphacite). Most mudrocks under Continental Mountain Belts and deeper regions of Island
Arcs experience Greenschist (chlorite, muscovite, biotite + garnet) or Greenstones (chlorite, epidote,
albite) for volcanics. Deep mountain belts experience Amphibolite facies (Hornblende + Garnet).
Deep, dry, hot mountain belts experience Granulite metamorphism (Pyroxene, Sillimanite). Migmites
are a mixed texture of high grade metamorphic rocks and cross cutting granitic veins, dykes and
pegmatites. This marks the beginning of wet melting and notice the backwards curve on the wet solidus
line that marks the onset of melting. These rocks seem to be limited to the deepest parts of continentcontinent collisional mountain belts and they always contain a hydrous ferromagnesian mineral like
biotite or hornblende. 2-mica granite migmatites (biotite and muscovite) form from the wet melting of
sediments from the accretionary wedge as the slab pulls away.
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Figure to supplement 7.4.B p.205
1 GPa = 10 Kilobars so this diagram represents the upper 30 km of the crust if heated to different T°C.
(Pyrophyllite is an aluminous clay mineral with a structure identical to the magnesian mineral talc.)
B. George Barrow, while mapping regional metamorphic rocks in the Scottish Highlands, encountered a
particular repetitive sequence of minerals as he approached the contact with a granite intrusion. This
reflects cooler, wetter minerals further away and hotter drier minerals closer to the igneous contact. The
sequence he found was: Chlorite  Biotite  Garnet  Staurolite  Kyanite  Sillimanite. It also
reflected a general coarsening of grain size and a progression from greenschist through amphibolite
facies rocks. The diagram below puts the aluminosilicate boundaries and Barrovian minerals on a
general metamorphic facies diagram. The aluminosilicate minerals are polymorphs with identical
chemical formulae (Al2SiO5) but different structures.
0. Examine the rocks in our lab provided labeled: 7.4.B.1 Kyanite, 7.4.B.2 Andalusite and 7.4.B.3
Sillimanite and make 2 observations about each for the appearance of the aluminosilicate mineral and
the rock texture.
7.4.B.1 Kyanite ___________________________________________________________________
_______________________________________________________________________________ (2)
7.4.B.2 Andalusite ________________________________________________________________
_______________________________________________________________________________ (2)
7.4.B.3 Sillimanite ________________________________________________________________
_______________________________________________________________________________ (2)
1.a Study the mineral isograds and stability fields on the above diagram and that on p.205 in your
manual. What is the Pressure __________ kbar and temperature _________ °C of the triple point? (2)
1.b Compare the mineral isograds in 7.4.A and the stability diagram above. Note that only Kyanite and
Sillimanite occur in that map. Starting at 0-depth and 0-temperature, draw a possible smoothe
burial curve that could represent the map in Part A. (Hint: Don’t intersect the Andalusite field). (2)
2.a look at the 2 isograd maps on the bottom of p. 205. Which map experienced the higher pressure
metamorphic conditions (circle one):
Map A
Map B
(1)
2.b What was the minimum temperature of metamorphism for the rocks on map B? _________ °C (1)
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C.1 Pentti Eskola in Finland noted that for the same starting composition of basalt, a variety of different
metamorphic facies could result: greenstone (greenschist), amphibolite and granulite. Ikuo Kuno,
mapping in the metamorphic rocks of Japan found that similar basaltic protoliths gave rise to blueschists
or eclogites. We now realize that these 2 regions had very different geothermal gradients. Eskola’s rocks
came from a steep geotherm in the core of an arc. Kuno’s rocks came from the Japanese forearc region
of what was once a subduction zone. Place the names of those meta-basalt facies in the correct regions
on the graph below.
(10)
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C 2. On the Convergent margin block diagram below for a region like the modern day Western Pacific
margin with Austral-Asia, plot the same five metamorphic facies showing where they likely formed with
respect to one another. You may assume that the top of the mantle asthenosphere is approximately
950°C. Plot the same 4 meta-basalt facies on the diagram at the top of the next page. Not that it dips the
other way to the one in the book but the 4 facies should occur in equivalent positions. ________ (4)
C.3 Examine the legend of the Victoria Map Sheet on the wall in the lab. Find: the Metchosin
Volcanics, The Leech River Schist and the Wark and Colquitz gneiss. The Metchosin Volcanics are 56
Ma seafloor basalt from the subducted Farallon Plate that have only been slightly metamorphosed to
Zeolite facies or lower Greenschist grade. Plot a TM-Z on the section above for where this happened.
The Leech River Schist was originally Jurassic and Early Cretaceous Sediments accreted to the margin
of North America (Wrangellia) and metamorphosed to greenschist to amphibolite facies. Plot ML-G on
the section above for where this likely occurred. The Wark and Colquitz Gneiss (Pacific Rim Complex)
are upper Amphibolite grade gneisses that were probably metamorphosed under a Jurassic Island Arc.
Plot PWC-A on the section to show where this likely occurred. Notice that none of the places you chose
quite match the current location of these geological map units where the currently occur quite close
together. Study your 2 cross sections, the one above and the Victoria Map then explain what happened
to put these 3 groups of rocks so close together as we find them now.
(6)
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