Name(s): _______________________________________________
Section (please circle one):
1wed
or
2thur
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.) directed stresses (compaction, flattening,
rotation, shear), 2.) a change in heat causing dehydration, decarbonation and thermal
expansion or the reversesuch 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,
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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 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 2 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 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 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.
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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.
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
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)
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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 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 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 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 is (choose one):
Foliated
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 Felsic
4. What was the original setting for this rock ?
Seafloor
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Continental Margin
(1)
(1)
Activity 7-2: Hand Sample Analyses of Metamorphic Rocks using
Photos: 7.# as: 2-Hydrothermal Chromite Skarn, 3-Banded
Gneiss, 5-Slate, 6-Phyllite, 7-Schist, 8-Augen (Feldspathic)
Gneiss, 10-Hornfels, 11- Quartzite, 12-Anthracite Coal, 13bMeta Conglomerate, 14-Pyrite Schist, 16-Serpentinite from the
Lab Manual and “Lettered Rocks” A-Amphibolite, B-Blueschist,
C-Black and White Marble, D-Andalusite Schist, E-Quartzite, FPhyllite, G-Garnet Schist & H-Columbite Bearing Skarn from
our collection, 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 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.
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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.
Examine specimens of each of these 3 aluminosilicate minerals from our collection
then do the questions.
Questions
B.
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 Map B
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b.) What was the minimum pressure in kbar _______ and minimum depth in km _____
for this metamorphism?
(2)
2. 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)
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. __________________________________________(10)
0°C
_________________________ 500°C _________________________ 1000°C
0 kb
0 km
6 kb
20 km
12kb
42 km
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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.
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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