Download EPSc 201 Lab 3: Sedimentary and Metamorphic Rock Identification

Name ____________________
EPSc 201: Earth and the Environment
Lab #8 – Metamorphism and Metamorphic Rocks
The laboratory this week will introduce students to the study of metamorphic rocks
through a series of directed exercises, some of which involve “real” rock samples.
Please familiarize yourself with Chapter 7 in the Lab Manual (“Metamorphic Rocks,
Processes, and Resources”).
Metamorphism and Metamorphic Rocks
Metamorphism is the process by which physical and chemical changes in a rock
are brought about by changes in geologic pressures and temperatures, often in
combination with chemically active fluids. Many of the minerals in metamorphic (or
changed) rocks are the same silicates, oxides, carbonates, etc. that are found in igneous
and sedimentary rocks. However, there are many, many other minerals found mainly,
or exclusively, in metamorphic rocks. Often these characteristic metamorphic minerals
make up only a relatively minor component of a typical metamorphic rock.
Of the three principal types of rocks, metamorphic rocks show the greatest
variety in their texture and mineralogy. In terms of texture, metamorphic rocks range
from massive (or structureless) to intensely foliated and/or lineated. In terms of
mineralogy, the chemical composition of the protolith leads to a wide variety of mineral
groupings (called assemblages) characterized by a specific mineral (called an index
mineral). For example (as noted below), if a shale is metamorphosed at progressively
higher Temperatures and Pressures, a sequence of diagnostic minerals form that are
different from those produced during progressive metamorphism of a basalt under the
same conditions.
Shale: Chlorite Biotite Garnet  Staurolite Kyanite Sillimanite
Basalt: Chlorite Epidote Actinolite Hornblende Pyroxene
Types of metamorphism
Contact metamorphism occurs in rocks adjacent to magmatic intrusions,
predominantly due to the high temperatures there.
Regional metamorphism is the result of large segments of the earth's crust
being deformed during periods of major mountain building. In some cases, this
1
deformation occurs over areas hundreds of km in width and thousands of km in length.
Rocks in these deformation belts are subjected to stretching and squeezing stresses that
cause drastic physical changes in the rock. Rock masses buried to such great depth
deform or “flow” as a plastic rather than a brittle solid. Because of this, many rocks
formed under these conditions have a texture that is characterized by the parallel
arrangement of platy minerals such as the micas (i.e., a metamorphic foliation).
Characterization of Metamorphic Rocks
Metamorphic rocks are described on the basis of their mineralogy, their texture, and
their structure.
Mineral Composition. All the constituent minerals and their relative percentage of a
metamorphic rock should be noted. Some of these minerals are included in the name of
a metamorphic rock (e.g., garnet-biotite schist)
Texture and Structure. Metamorphic textures comprise several main types: nonfoliated, foliated, lineated, and foliated-lineated. Non-foliated metamorphic rocks have
no preferred orientation to their mineral grains. Foliated metamorphic rocks exhibit any
of several possible types of planar features that are the result of the mineral
constituents within the rock having a parallel or subparallel arrangement (like a deck of
cards). Lineated metamorphic rocks are those in which individual minerals (or mineral
aggregates) are elongated in a particular direction, but lack a planar arrangement (like a
pile of pencils).
------------------------------------------------------------------------------------------------------------
Exercise #1.
Examine samples #71 and #76. One is a quartzite and the other is a marble: Which is
which, and how can you tell? What were their protoliths?
-----------------------------------------------------------------------------------------------------------Exercise #2.
In the field, geologists have documented the gradual transition from sedimentary shale
to metamorphic slate, from slate to phyllite, and from phyllite to schist. You are given a
sample of shale (#145), slate (#78), phyllite (#179), and schist (#80). Compare the
mineralogical and textural changes in these rocks and record them in the table below:
2
Shale to slate
Slate to Phyllite
Phyllite to schist
Mineralogical
changes
Textural
changes
------------------------------------------------------------------------------------------------------------
Exercise #3.
Sample #140 is a greenstone (consisting of Albite+Chlorite+Actinolite+Epidote) whereas
sample #84 is an amphibolite (consisting of Plagioclase+Hornblende). Both are the
products of the metamorphism of basalt (consisting of Plagioclase+Pyroxene). To
understand better how these mineralogical changes reflect rock chemistry, fill out the
table below by listing the minerals in which these elements reside. (You’ll have to look
up a few of the mineral compositions to complete this exercise.)
Element
Basalt
Greenstone
Na
Mg
Al
Si
Ca
Fe
3
Amphibolite
Exercise #4.
New York City Geology
One of the reasons why New York City can sustain such an extensive network of
underground subways, waterways, and utility conduits is the nature of the crystalline
bedrock. Shown below is a geologic map of this region, on which several formations are
indicated. For our purposes, the most important rock bodies are the Proterozoic
“Fordham Gneiss” (shown in yellow) and the Cambrian “Manhattan Schist” (shown in
red). Samples of each are available as part of this lab exercise (#91 and #96,
respectively). Please compare and contrast these two rocks. What do you think their
protoliths were?
------------------------------------------------------------------------------------------------------------
4
Exercise #5.
Deformed Rocks
Regional metamorphism is nearly always accompanied by rock deformation of
some sort that imparts a new fabric and/or new structures onto pre-existing ones. In
this exercise, you will examine two contrasting examples of metamorphic rock fabric.
Sample ID14-1C. This is a sample of a matrix-supported, chert-pebble conglomerate
(what does this mean in terms of the protolith?), collected from the 3.8 billion-year-old
Isua region of West Greenland. Yes folks, this is one of the oldest rocks known to exist
on Earth!! The dark matrix consists mainly of biotite, hornblende, and quartz, together
with the odd garnet. The large white objects are deformed pebbles of what were once
chert but are now microcrystalline quartz.
Pebbles such as those seen here are useful as strain markers, meaning that they
reveal the type of deformation that a rock body has experienced (other strain markers
would include fossils and oolites). In general, two broad types of deformation are
recognized: (1) Flattening, in which originally spherical objects are squashed into
pancake-shaped bodies; and (2) Constriction, in which originally spherical objects are
stretched into cigar-shaped bodies.
Examine the deformed pebbles in sample ID14-1C, and determine whether the
quartz pebbles have been flattened or stretched as a result of deformation. Discuss
briefly how you arrived at your interpretation. Note that the sample is shaped in such a
way that you can view the pebbles in three dimensions.
Sample FT-H. This is a sample of a metamorphosed, graywacke turbidite from the Black
Hills, South Dakota. If you examine the sawn surface you will note a variety of layers,
which actually come in pairs with the pale parts being sandy and the dark parts being
muddy. So this sample contains about a half-dozen turbidite layers, but don’t be fooled
by their present thickness! This sample was collected from the limb of a large anticline
(several 10s of meters across). In general, layers become thinned by plastic flow along
the fold limbs. So, the layers may have been much thicker than they appear now.
Notice the aligned dark ovoids that are arranged at an angle to the sedimentary
layers. These ovoids are clots of quartz+biotite, and their alignment imparts a new,
planar metamorphic fabric to the sample known as axial-plane cleavage. You may also
notice the dark specks scattered throughout the sample surface: These are crystals of
the aluminum-rich metamorphic mineral chlorioid [formula = (Fe,Mg)2Al4Si2O10(OH)4].
5
So, for this exercise, draw an anticline with about a half-dozen layers in it to
mimic the turbidite sample. Then draw a series of dashed lines parallel to the axial
plane of the fold. Finally, draw the position of the sample based on your drawing and
the appearance of the rock.
------------------------------------------------------------------------------------------------------------
Exercise #6.
Garnets are Great
Samples WS89 and CLA08-1 both contain prominent crystals of garnet, but the
rocks have very different mineral assemblages. In addition to garnet, WS89 contains
Mucovite + Quartz + Biotite + Staurolite [(Fe,Mg)2Al9Si4O23(OH)], whereas CLA08-1
contains Plagioclase + Hornblende (in addition to garnet). Examine the samples closely,
paying particular attention to the garnet crystals and their surroundings.
What was the protolith of these two samples?
Garnet has the chemical formula [(Fe,Mg,Ca)3Al2Si3O12], in which Fe-Mg-Ca show
extensive solid solution. In which sample do you expect the garnet to be Ca-rich? Capoor?
HINT: Once you know the protolith, you have an idea of what the rock composition is
like. Mineral compositions generally are controlled by rock compositions.
------------------------------------------------------------------------------------------------------------
Exercise #7.
Complete Activity 7.3 on pages 173 and 174 of the Lab Manual.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
6