Metamorphic Rocks, Processes, Resources, & Environments

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 in fillings 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 & 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 larger porphyroblasts or sieve like or spotted poikiloblasts. 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” for you are platy or elongate giving the rocks special foliated textures. These minerals 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
2 mylonite (meaning “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. To tell their grade requires finding sensitive non-carbonate accessory minerals from “dirty” silicate rich bands in the original limestone.
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 texture: 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 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 hematite and graphite.
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. Slate
  3. Marble
  4. Garnet Schist
  5. Phyllite
  6. Mica Schist
    3
    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.
  7. 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)
  8. The limestone has 2 textures or structures which are no longer present in the marble. a.) What are these? ___________________________ & _______________________________ (2)
  9. 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)
  10. 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)
  11. 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 Syenite suite. This sent hot (> 375°C, wet, silica bearing fluids to recrystallize the sedimentary limestones into a coarse grained contact metamorphic skarn. Note the
    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 Orthogneiss cut calcareous metasediments. Find both rocks from our collection like the images shown below.

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