Name(s): ______________________________________________________
Lab Day (please circle one):
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LAB 7: METAMORPHIC ROCKS, PROCESSES, RESOURCES, & ENVIRONMENTS
This exercise is based on questions & materials in the 9th edition AGI lab manual.
Read Chapter 7 from your manual covering sediments. Use materials as provided and answer the
questions of the same number in the lab manual.
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.
2.
3.
a directed stress or stresses (compaction, flattening, rotation, shear)
a change in heat causing; dehydration, de-carbonation, thermal expansion or the reverse
such as hydration upon cooling
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 increases or directed stresses. While rocks uplift, cool off and
decompress, by the time fluids have been driven off, 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 also occurs there. It
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 which sometimes contains a few large spotted porphyroblasts. Hydrothermal veins are also
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 such as; 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
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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
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.
Answer questions from Lab 7 in the manual in the spaces provided on this sheet.
ACTIVITY 7-1: METAMORPHIC ROCK ANALYSIS & INTERPRETATION FROM PHOTOS AND
HAND SPECIMENS.
QUESTIONS
A. Obtain a piece of pink and grey marble and compare it to a layered fossiliferous limestone. Do not
be destructive of the fossils! Use hand lenses or binocular microscopes only on these please. If you
do acid tests, streak a corner of the rock and test the acid on the powdered streak.
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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 two textures or structures which are no longer present in the marble.
a.
What are these? ___________________________________________________ (2)
b.
Examine your marble and the photo of one in Figure 7.9. Describe the new texture using the
correct term from table 7.15. This rock is: Foliated or Non-foliated (1)
3.
Impure marbles with some clays, quartz, iron oxides in the protolith can make a small proportion of
other indicator minerals like micas, amphiboles or pyroxenes to indicate the grade or temperature
conditions of metamorphism. Examine your pink and grey marble specimen and check closely for
another mineral: _______ & grade: Low or Intermediate (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: 7.5 Slate, 7.6 Phyllite and 7.7 Schist. They all contain micas as their principal
minerals +/- some quartz and iron oxides.
1.
Describe the grain size in each of these rocks: Slate: _________________________, Phyllite
_____________________ and Schist ________________________________ (3)
2.
What happens to the mica grain size through this series from slate 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)
C.
Examine Figure 7.3 on p 157 and obtain a piece of coarse grained folded gneiss or schist from our lab
collection.
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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 for this process to occur, or where might
something like this be taking place today? _________________________________________________ (2)
D. Examine the regionally metamorphosed rock next to D on p 168 and obtain a lab specimen of a
coarse grained schist with large garnet crystals from our collection.
1.
This rock is (choose one)? Foliated or Non-foliated (1)
2.
What is the correct textural name for the large garnet crystals? ______________ (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 tnew metamorphic minerals,
what was the protolith for this metamorphic rock? (choose one): Quartz sandstone or Shale or
Dolostone or 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 a pretty coarse grained rock with red garnet (Omphacite) and green pyroxene
(Jadeite) like that pictured next to E on p 168. We have a 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 two of them!
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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 is? (choose one) Foliated or Non-foliated (1)
2.
What is the correct name for this metamorphic rock from a former subduction zone?
__________________________________________________________________________ (1)
3.
What is the protolith for this regional metamorphic rock? (choose one) Mafic or Felsic (1)
4.
What was the original setting for this rock? (choose one) Seafloor or Continental Margin (1)
ACTIVITY 7-2: HAND SAMPLE ANALYSES OF METAMORPHIC ROCKS
Use photos from the Lab Manual: as
#7-2
#7-3
#7-5
#7-6
#7-7
#7-8
#7-10
#7-11
#7-12
#7-13 B
#7-14
#7-16
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Hydrothermal Chromite Skarn
Banded Gneiss
Slate
Phyllite
Schist
Augen (Feldspathic) Gneiss
Hornfels
Quartzite
Anthracite Coal
Meta Conglomerate
Pyrite Schist
Serpentinite
and the sample “Lettered Rocks” from our
collection
A. Amphibolite
B. Blueschist
C. Black and White Marble
D. Andalusite Schist
E. Quartzite
F. Phyllite
G. Garnet Schist
H. Columbite Bearing Skarn
in order, on the metamorphic worksheets below. (100)
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ACTIVITY 7.3: METAMORPHIC GRADES, FACIES (MINERAL INDICATORS OF TEMPERATURE &
PRESSURE AND GEOLOGICAL MAPS .
A. Mudrocks are common protoliths and clay minerals tend to recrystallize to Al 2SiO5 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.
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.
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B.
Examine specimens of each of these 3 aluminosilicate minerals from our collection then do the
questions.
QUESTIONS
1.
Compare the 2 metamorphic facies maps on the bottom of p 173.
a)
Which one of the map areas was metamorphosed at higher pressure: Map A or Map B
b) What was the minimum pressure in kbar _______ and minimum depth in km _____ for this
metamorphism? (2)
2.
C.
From the aluminosilicate phase boundaries = isograds, what is the maximum temperature of
metamorphism encountered by rocks in Map B where the aureole steps from Sillimanite to Kyanite?
________________________ °C (2)
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. _____________________________ (10)
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0°C
_________________________
500°C
_________________________
1000°C
0 kb
0 km
6 kb
20 km
12kb
42 km
D. 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.
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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 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 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 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. ______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________ (8)
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