Earth and Space Sciences 212

Transcription

1 Earth and Space Sciences 212 METAMORPHIC ROCKS LAB During metamorphism a rock undergoes changes in mineralogy and/or texture. These changes may be brought about by changes in temperature or pressure, by the introduction or removal of chemical components (particularly volatile components such as H 2 O and CO 2 ), or by the application of non-uniform stresses. The relative importance of each of these factors varies according to the type of metamorphism. There are two basic types of metamorphism and each is briefly described below: REGIONAL This is the most common type of metamorphism. It covers wide areas (thousands of km 2 ) and occurs in orogenic regions, i.e., root zones of active mountain building. Pressures and temperatures range from moderate to high (usually ~150 to ~800 C) and pressures (from <1 Kb to >10Kb) but, except for the progressive loss of volatiles, regional metamorphism is, to a first approximation, an isochemical process. Regionally metamorphosed rocks are commonly foliated, suggesting that deviatoric stress is also an important factor. An important variant of regional metamorphism (sometimes referred to as burial metamorphism) is associated with subduction zones at convergent plate margins. In metamorphism of this type, sequences of sediments and mafic volcanics are dragged down to great depth (high pressure) while experiencing only low to moderate temperature increases. Distinct assemblages are developed in these rocks and they may even retain some vestiges of their original pre-metamorphic morphologies. CONTACT Contact metamorphism occurs in the country rock surrounding an igneous intrusion. This is primarily a static thermal event--non-hydrostatic stress is less important than in regional metamorphism. Metasomatism (mass transfer) may also occur if fluids given off by the intrusion react with the country rock. The width of the area metamorphosed by the intrusion (the contact aureole) depends upon the size of the intrusion itself, and in many cases the aureole may be subdivided into several zones, distinguishable by different mineral assemblages. CLASSIFICATION The classification and naming of metamorphic rocks is not as systematic as that of igneous or sedimentary rocks but in many respects the name is more descriptive. Problems arise from the fact that a given metamorphic process does not always yield the same rock--it depends upon the bulk composition of the parent rock, which may have been igneous or sedimentary. Furthermore, since there are different types and grades of metamorphism, two dissimilar looking metamorphic rocks could be produced from the same parent. For these reasons, the root name of metamorphic rocks is largely based on texture and structure (e.g., slate, schist, gneiss, hornfels, granulite) or on composition (e.g., marble, quartzite, serpentinite, amphibolite, or parentage (e.g., metaconglomerate, metabasite). Some of the more important textures and structures developed in metamorphic rocks are discussed below. In most cases the root name is modified to denote the principal mineral constituents (e.g., talc-tremolite schist, diopside marble, lawsonite-glaucophane schist [blueschist], garnet amphibolite, etc.) FOLIATION Foliation is a generic term used to describe planar structures developed during metamorphism by the parallel arrangement or distribution of minerals. It may be irregular, curved, or even folded (if deformed). Schistosity, a foliation produced by the alignment of platy minerals (especially micas), and gneissosity, the foliation resulting from alternating bands of light and dark minerals, are two common examples. A rock may show more than one foliation if it has been subjected to more than one deformation. In this case the foliations are given the labels S1, S2, etc., in order of decreasing age (S1 is the oldest). Older foliations are deformed during the formation of younger ones and this is used to determine the order in which they formed. (see sketch below)

2 LINEATION Lineation is a parallel alignment of linear elements in the rock. These linear elements may be elongated minerals such as amphiboles, linear mineral aggregates, fold axes, or the intersections of two foliations. As with foliations, a rock may have more than one lineation, in which case the lineations are labeled L1, L2, etc. in order of decreasing age. (see sketch below) PORPHYROBLAST Porphyroblasts are large mineral grains which have grown during metamorphism, equivalent texturally to phenocrysts in an igneous rock. The size of the porphyroblast depends primarily upon the growth rate. In many cases it is possible to determine at what stage during metamorphism the grain grew by examining its relationship to the fabric of the rock. Some examples are shown below. foliation wraps around porphyroblast indicating that deformation occurred after crystallization. porphyroblasts are aligned parallel to the foliation - indication of growth during the deformational event randomly oriented porphyroblasts which do not disturb the foliation - indication of growth after deformation has ended. PARENTAGE In most cases, the texture of the parent rock (aka protolith) is obliterated during metamorphism. Metamorphic mineral assemblages, however, reflect the composition of the parent rock, e.g., i. Abundant aluminum-rich and/or potassium-rich minerals (e.g., andalusite/kyanite/sillimanite, staurolite, garnet, muscovite, biotite) = pelitic parent (shale, mudstone) ii. high quartz and feldspar content = psammitic parent (silica-rich igneous rock, sandstone, chert) iii. abundant calc-silicate minerals (garnet, diopside) = calcareous (impure limestone or dolostone) iv. marble (calcite or dolomite) = calcareous (pure limestone or dolomite parent) v. Abundant ferromagnesian minerals plus plagioclase (e.g., amphiboles, chlorite, epidote, pyroxenes, garnet,

3 plagioclase) = Mafic parent (basalt/gabbro) vi. Mg-rich minerals such as serpentine, talc, anthophyllite, tremolite, forsterite, enstatite, diopside = Magnesian parent (ultramafic) COMMON TYPES OF METAMORPHIC ROCKS It is convenient to break down many common metamorphic rocks on the basis of whether they are strongly foliated, weakly foliated, or non-foliated: I. STRONGLY FOLIATED SLATE: a very fine-grained rock which breaks along a set of well-developed fractures, referred to as slaty cleavage. This cleavage results from parallel growth of minute micaceous minerals, during regional metamorphism (and deformation) of shales, mudrocks, or other fine-grained sediments. PHYLLITE: a fine-grained schistose rock with a lustrous sheen due to growth of micaceous minerals (still very fine-grained but now visible with a hand lens). Produced by more advanced regional metamorphism of slate; cleavage is not as perfect. SCHIST: a strongly foliated rock with individual mineral grains visible to the naked eye. Micas (muscovite and biotite) are usually abundant and their orientation defines the foliation. Schists represent regional metamorphism of a higher grade than that required to produce a phyllite. Schists are not restricted to any specific composition and may develop in pelites, mafic protoliths (e.g., greenschists and blueschists) or ultramafic protoliths (e.g., talc schist). II. MODERATELY FOLIATED GNEISS: a medium- to coarse-grained rock in which the foliation is defined primarily by mineral segregations (usually micaceous layers alternating with quartzo-feldspathic layers) which may give the rock a banded look. Generally gneisses form at a higher grade than schists and may be derived from either an igneous parent (orthogneiss) or sedimentary parent (paragneiss). MIGMATITE: a rock which appears to be partially metamorphic and partially igneous, contains pods, veins, or layers of leucocratic granitic rocks (called the leucosome) which may be either sharply bounded or gradational into the darker metamorphic material (called the melanosome). Migmatites commonly form at high grade of regional metamorphism (above the temperature at which granite melts). MYLONITE: a rock deformed by plastic, ductile flow in an environment of moderate temperature and high strain rate. Often looks banded or streaky and may contain eyes or lenses of undeformed material. Mylonites are associated with large faults. III. MASSIVE TO WEAKLY FOLIATED AMPHIBOLITE: medium- to coarse-grained rock containing mainly hornblende and plagioclase. Commonly lineated due to prismatic habit of the amphibole, sometimes schistose. A mafic parent. GRANULITE: a fine- to medium- grained equigranular rock commonly containing quartz, feldspar, pyroxene +/- garnet (note: anhydrous assemblage). Micas and amphiboles are generally absent. Granulites may have a weak foliation defined by either mineral segregations or flattened quartz and feldspar grains. Indicative of very high grade regional metamorphism of mostly mafic parent. GREENSTONE: Metamorphism at low to medium grades of a mafic parent (basalt or gabbro) produces a greenstone, so called because of its abundance of green minerals, e.g. epidote, chlorite, actinolite. Original igneous minerals are generally replaced but the primary textures is commonly preserved.

4 SERPENTINITE: A rock composed almost entirely of serpentine--formed by the low temperature hydration of peridotite. May contain small amount of talc or brucite. Dark green/black in color, dense, commonly weathers orange/brown. ECLOGITE: a medium-grained rock, often greenish, rock consisting of omphacite (a green sodic pyroxene) and garnet--produced by extremely high pressure regional metamorphism of basalt. QUARTZITE: a metamorphic rock consisting of recrystallized and interlocking quartz grains formed from a sandstone parent. The rock breaks through rather than around the grains. MARBLE: a metamorphic rock consisting of recrystallized and interlocking grains of calcite or dolomite. SKARN: a rock produced by contact metamorphism, generally accompanied by metasomatism, of a limestone or dolomite. Generally composed of red and green calcium-rich silicate minerals such as grossular garnet, epidote, diopside, idocrase, etc., often in beautiful crystals. HORNFELS: a fine-grained non-schistose rock produced by contact metamorphism. Mineral grains are not oriented but porphyroblasts are common, in which case the rock may have a spotted appearance. METAMORPHIC GRADE (will be discussed in lecture) Not only the texture but also the mineralogy of a metamorphic rock changes as a function of grade. In a general sense, increasing grade correlates with increasing temperature. Increasing grades of regional metamorphism are characterized by the successive appearance of distinctive minerals, called index minerals. While mapping in metamorphic terrane, it is conventional to draw lines on a map indicating the location where a particular mineral first appears. Such a line on the map is called an ISOGRAD (meaning same grade) and the area in the field over which this distinctive assemblage occurs is called a ZONE. In most metamorphic rocks grain size correlates roughly with grade: higher-grade rocks are coarser-grained than their lower-grade equivalents. The processes that increase grain size (mostly, grain-size coarsening driven by a reduction in surface energy, similar to the "corning" of snow as it ages) are dominant.

5 METAMORPHIC FACIES (will be discussed in lecture) The important concept of metamorphic facies was developed by Eskola early this century as a means of systematizing progressive metamorphism of rocks of a variety of compositions. A metamorphic facies has been defined by Turner (1981) as a set of metamorphic mineral assemblages, repeatedly associated in space and time, such that there is a constant and predictable relation between mineral composition and chemical composition. Mafic rocks provide the assemblage diagnostic of each facies and the facies is named for the typical mafic assemblages but the facies concept not restricted to mafic rocks (this will be explained in more detail in lecture). Assignment of pressure and temperature conditions to the various facies is common but it is important to note that the definition of facies makes no mention of either P or T. The most recent development in metamorphic facies is outlined in a paper by Evans (1990) and his suggestions for facies names and boundaries are shown in the figure below. Table 1. Characteristic mineral assemblages for five common metamorphic facies for pelitic, mafic, and ultramafic compositions. For each facies, the upper row gives the typical mineral assemblage, and the lower row (±) lists possible additional minerals. Minerals in the latter may not necessarily occur throughout the facies and may be restricted to fairly specific bulk compositions. Mafic compositions provide the assemblage diagnostic of each facies (capitalized). Other facies have been proposed but are not include in this table, e.g., the sub-greenschist facies [which is commonly subdivided into a zeolite facies and a prehnite-pumpellyite facies] and the facies of contact metamorphism. PELITIC MAFIC ULTRAMAFIC muscovite + chlorite + albite CHLORITE + EPIDOTE + antigorite + chlorite + + quartz ALBITE diopside + magnetite Greenschist ± chloritoid, biotite, K-feldspar, actinolite, biotite, calcite olivine, brucite, talc stilpnomelane, paragonite muscovite + biotite + HORNBLENDE + olivine + chlorite + plagioclase + quartz PLAGIOCLASE tremolite + Cr-spinel Amphibolite ± almandine, staurolite, kyanite, epidote, clinopyroxene, antigorite, talc, enstatite sillimanite, andalusite, garnet, cummingtonite, anthophyllite, cordierite, Mg-chlorite, biotite cummingtonite, sillimanite + K-feldspar + ORTHOPYROXENE + olivine + diopside + plagioclase + quartz CLINOPYROXENE enstatite + spinel Granulite ± biotite, almandine, cordierite, plagioclase, hornblende, plagioclase, hornblende, hypersthene, kyanite, garnet phengite + chlorite + quartz GLAUCOPHANE/CROSSITE antigorite + olivine + + LAWSONITE or EPIDOTE magnetite + chlorite Blueschist ± albite, jadeite, lawsonite, pumpellyite, chloritoid, chlorite, brucite, talc, diopside almandine, chloritoid, garnet, albite, omphacite, aragonite Eclogite phengite + almandine + OMPHACITE + GARNET olivine quartz + RUTILE + kyanite ± kyanite, jadeite, omphacite glaucophane, barroisite, talc, diopside, antigorite, hornblende, epidote, chlorite, pyrope muscovite, hypersthene

6 PELITIC ROCKS (parent = shale or mudstone) In the classic sequence of progressive regional metamorphism of pelites from the southern Scottish Highlands described by George Barrow in 1894 (known as the Barrovian sequence), the sequence of index minerals and zones is: CHLORITE, BIOTITE, GARNET, STAUROLITE, KYANITE, SILLIMANITE. This is a well-known and important sequence which has been found in other areas of the world, but it is not the only one which may develop. Slightly different sequences can be developed in other pelitic terranes because they (a) may have a slightly different bulk composition or (b) have followed a different P-T path during metamorphism. It is also important to remember that this sequence of minerals would not develop in a non-pelitic rock. In pelites, the aluminosilicate polymorphs (andalusite, kyanite, and sillimanite) are important indicators of pressure and temperature. As you will remember from mineralogy, andalusite is the low pressure form, kyanite is the high pressure form, and sillimanite is the high temperature form. Make sure you can draw this P-T diagram accurately : Black Slate, Stillaquamish River Area, WA This rock displays well-developed slaty cleavage. The black color is due to the presence of very fine graphite. Notice also a faint lineation on the cleavage surface. Question: (1) What changes have taken place in the original shale during metamorphism? (2) What is the source of the graphite? : Darrington Phyllite, North Cascades, WA The protolith of this sample was a mudrock rich in organic material. Note the sheen which distinguishes a phyllite from a slate. Quartz segregations are also visible. Two episodes of deformation are recorded in this rock: the first produced S1, a fine compositional lamination, while the second produced S2, a cleavage almost at right angles to S1. S1 and S2 intersect to give the rock strong crenulation (lineation). Question: (1) What causes the sheen on the foliation surfaces? 202-V7: Chloritoid phyllite, Plymouth, Vermont Original bedding has been transposed into the schistosity (cleavage) which is axial planar to folded quartz-carbonate veins (some samples only). Chloritoid [Fe 2 Al 4 O 2 (SiO 4 ) 2 (OH) 4 ] occurs in phyllites in some Barrovian-type terrains; it is a precursor to staurolite. Questions: (1) What kind of silicate is chloritoid? (2) When did chloritoid grow relative to the deformation? Lab 435: Garnet-Staurolite-Biotite Schist, North of Skykomish, Washington Note the prismatic staurolite porphyroblasts. The rock is dark because of the abundant graphite, Compare this sample with the staurolite-kyanite-mica schist from Gotthard in the mineralogy collection. Questions: (1) At what stage during the metamorphic history of this rock did the porphyroblasts form?

7 (2) Has there been deformation since then? B Kyanite-bearing pelitic gneiss, Esplanade Range, BC Note the coarse grain size and the folded quartz veins. Questions: (1) What minerals are present besides the blue kyanite? : Skagit Gneiss, North Cascades, WA Note the coarse texture, lack of schistosity, and development of light and dark mineral segregations which characterize a gneiss. The protolith was probably a greywacke because there are no Al 2 SiO 5 polymorphs in this rock. Question: (1) List all the minerals can you identify in this sample (Display Sample): Andalusite-Biotite Schist (Chiwaukum Schist), Stevens Pass, WA Observe the huge andalusite porphyroblasts (some with the chiastolite cross) which have grown in response to a contact metamorphic event. Questions: (1) Did the porphyroblasts grow before, during, or after development of the schistosity? (2) what other minerals can you identify in this sample? 202-CM8 Sillimanite-garnet gneiss, Central Massachusetts This is a very high grade pelitic metamorphic rock (700+ C) that probably underwent partial melting as suggested by the quartzo-feldspathic veinlets. Questions: (1) Estimate the modal % of sillimanite (2) What sort of garnet is present in this sample? (3) Is there any muscovite? : Quartzite, Location Unknown The parent rock for this specimen was probably a quartz arenite.

8 Question: (1) What changes have occurred during metamorphism? : Greenschist, North Cascades Question: (1) What mineral gives the rock its green color? MAFIC ROCKS (2) What enables this rock to develop a schistosity? : Amphibolite (in Chiwaukum Schist), Leavenworth, WA This amphibolite is finer grained and less well-foliated than sample Question: (2) What is the green mineral in the veins? Did these veins develop before or after the foliation? (If you're having trouble seeing this, look at more than one sample) : Garnet Amphibolite, Wolverine Complex, Central B. C. Questions: (1) In what ways does this rock differ from the other amphibolites you have looked at? (2) Is it a higher grade rock? If so, why? : Blueschist (Shuksan), North Cascades, WA The distinctive purplish-blue color of this rock is due to the presence of the sodic amphibole glaucophane. Epidote (pistachio green mineral), chlorite, and plagioclase (albitic) are also present. The protolith of this rock was probably very similar to that of sample Question: (1) Why is the mineralogy in this sample different from that in (2) What sort of P-T conditions are required to form blueschists and where are such conditions found? : Blueschist, Bandon, Oregon WRITE A COMPLETE DESCRIPTION OF SAMPLE

9 202-JAV (display sample): Blueschist-eclogite, Sesia Zone, Val d Aosta, Italy This strongly foliated, compositionally heterogeneous rock is transitional between the blueschist and eclogite facies. Questions: (1) Identify the blue, green, red, brown, and silvery minerals (display sample): Garnet Granulite, Kilbourne Hole, NM The presence of garnet in this rock indicates that it was subjected to higher pressures than a granulite containing only pyroxenes. Question: (1) What minerals (other than pyroxene and garnet) can you identify? : Eclogite Knocker, Franciscan Formation, Jenner, CA In the Franciscan melange, scattered knobs of eclogite, which are resistant to weathering, are called knockers. Notice how dense eclogite is--what you'd expect for a high pressure rock. This sample contains a lot of glaucophane -- presumably indicating that the eclogite passed through the blueschist facies during retrograde metamorphism. Questions: (1) What would be a possible P-T path for this rock during the retrograde metamorphism? (2) What minerals can you identify in this rock? ULTRAMAFIC ROCKS and CALCAREOUS ROCKS A. Ultramafic (parent = peridotite) : Serpentinite, Johnson Pass, Whatcom Co., WA The primary igneous minerals of the ultramafic protolith have been completely replaced by serpentine group minerals. Serpentinites deform easily and are often found in fault zones where they may act as a lubricant. Serpentinites commonly show the development of slickensides. Questions: (1) What are the three common serpentine minerals and which one(s) would you expect to find in low-grade serpentinites such as this one? (2) What is the principal compositional change a peridotite undergoes during serpentinization? : Talc-Actinolite Schist, S.E. Alaska This sample comes from the margin of a small ultramafic pod. The protolith was a peridotite. The metamorphism was not isochemical. Question: (1) What can you say about the deformation of this rock?

10 (2) What oxide constituents were added to the protolith during the metamorphism? B. Calcareous (parent = limestone, dolostone) : Marble, Lee Flat Formation, Argus Range, CA Note the moderately coarse-grained equigranular texture of this rock -- it has been completely recrystallized. Question: (1) This rock formed from carbonate sediments. Were they pure or impure? How can you tell? (2) Why is there no schistosity?