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, 1 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. 2 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. 3 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) 4 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 5 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) 6 7 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. 8 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 9 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 10 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) 11