Lab 7: Metamorphic Rocks, Processes, Resources, & Environments

Transcription

1 Name(s): & Section (please circle one): 002 Mon 001 Thurs Lab 7: Metamorphic Rocks, Processes, Resources, & Environments Winter 2015 This exercise is based on questions & materials in the 10 th 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, CO 2 ). 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 1

2 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 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 and graphite. Refer to the introductory pages 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 2

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. 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 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. 3

4 Rossland, B.C. (7.2.A.5a) Willsboro, N.Y. (7.2.A.5.b) Wollastonite s formation is described by the following expression: Si0 2 (aq) + CaC0 3 (s) CaSiO 3 (s) + C0 2 (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. They all contain micas as their principal mineral but often contain lesser amounts of quartz and iron oxides, graphite and traces of pyrite. These all have flattening or shear caused by the directed pressure of regional metamorphism. 4

5 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 line 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 Non-foliated (1) 2. What is the correct textural name for the large garnet crystals in this schist? (1) 5

6 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 (1) 2. What is the correct name for this metamorphic rock from a former subduction zone? a. Jadeite schist b. Omphacite gneiss c. Eclogite d. MacGregor Tartan (1) 3. The mineralogy (garnet & pyroxene) and chemical composition ~50% SiO 2 content of this rock suggests its protolith was (choose one): Mafic Felsic (1) 4. What was the original geological setting for this rock? Seafloor Continental Margin (1) F. Which of the last 2 rocks demonstrates a higher metamorphic grade D or E & Why? (2) 6

7 Activity 7-3: Hand Sample Analyses of Metamorphic Rocks use: A-Amphibolite, B-Blueschist, C-Black and White Marble, D-Andalusite Schist, E-Quartzite, F- Phyllite, 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 A. Amphibolite B. Blueschist C. Marble D. And.schist E. Quartzite F. Phyllite G. Gar.schist H. Skarn I. Serpentinite J. Zeollite rock Textures & Fol or Non Minerals Composition Protolith Parent Rock Geological Setting Use 7

8 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 (Al 2 Si 2 O 5 )(OH) 4, tend to recrystallize to Al 2 SiO 5 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 1 st appearance of an index mineral is an isograd. On the regional 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 8

9 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 either get backed 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. 9

10 Figure to supplement 7.4.B p 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 (Al 2 SiO 5 ) 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 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) 10

11 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) 0 C 0 kb 500 C 1000 C 0 km 6 kb 20 km 12kb 42 km 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) 11

12 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) 12