Metamorphic Rocks

Metamorphic rocks Subjected to heat (and stress) Results in changes in the appearance Mineralogical Textural Any rock can be metamorphosed But rocks that show metamorphic change best are: fine grained clastic sedimentary rocks mafic volcanic rocks

Metamorphic textures Foliated Non-foliated Rocks have some sort of alignment of minerals Physical alignment of platy minerals gives a rock that tends to split more easily in one direction Foliation may also be just layering of light and dark minerals Non-foliated Rocks are “massive”

Foliated rocks Foliated rocks result from heat and directed stress. Heat causes minerals to grow – either get larger or react to form new minerals Stress causes mineral to become oriented Because heat and stress cannot be imposed in small areas, foliated rocks, where they occur, occupy large “regions’ Hence, the term “Regional Metamorphism” for foliated rocks The term dynamothermal is also used (dynamo meaning pushing or stress, thermal meaning heating)

platy mineral rock cleavage

Foliated rocks Various different kinds of “foliated” rocks Slate Phyllite Schist Gneiss These rocks are of different “grade” Major differences are in grain size, but there are associated changes in mineralogy and the nature of the foliation

Foliated rocks Slate (low grade) Very fine grained (cannot see individual grains or identify the minerals) Have “slaty cleavage” – a very smooth surface that is not the same as the bedding Slaty cleavage is the result of aligned microscopic grains of clay, and possibly chlorite or muscovite

Pool table tops made of slate

Foliated rocks Phyllite (low to medium grade) Coarser grained than slate The cleavage is shiny and results from coarser grained (than slate) muscovite or chlorite (and possibly biotite) grains Cleavage is commonly not as smooth as a slaty cleavage

Note the glossy sheen typical of phyllite.

Foliated rocks Schist (medium to high grade) The foliation is clearly the result of aligned mineral grains (muscovite, biotite, chlorite, talc, some amphibole) Grains are clearly visible The parting (schistosity) is rough due to the grain size

Foliated rocks Gneiss (high grade) Layering of minerals (light and dark layers) Dark layers usually of biotite or amphibole and grains are also usually aligned The highest grade rocks undergo partial melting and the rock usually has lenses of igneous rock (granite) and is called a “migmatite”

Gneissic layering; layers of identifiable minerals

Dynamic metamorphism Change of texture but (often) without significant mineral growth Faults/shear zones Foliated but othewise appear unchanged E.g., sheared granite At most extreme: mylonites, v. fine grained (can’t identify minerals) and strongly foliated (though lacking the easy parting of slaty cleavage)

Foliated granite

Mylonite So sheared it’s hard to tell what the orignal rock was

Non-foliated rocks Metamorphism by heat alone produces non-foliated rock (rocks that lack a foliation) Grade of metamorphism is determined by grain size and mineralogy Heat alone can be imposed in small areas as long as there is a heat source (commonly an igneous body) nearby Hence non-foliated rocks are usually the result of “Contact Metamorphism”. But there are some exceptions…where non-foliated rocks are “regional” in most senses of the word

Non-foliated rocks In some cases stress (in regional metamorphism) may not produce much of a foliation. This commonly occurs in rocks consisting of a single mineral particularly if that mineral does not typically grow as inequant grains. quartz sandstone becomes a quartzite limestone becomes a marble basalt becomes an amphibolite (though amphibolite is usually weakly foliated)

Doing the lab Fill in the six columns F/N Other textures/structures Identifiable minerals Rock name Grade Original rock type

F/N Foliated or non-foliated Does the sample show: Tendency to split Layering of minerals

Other textures/structures Relict bedding (in most slates): usually visible as colour banding that is different from the tendency to split (cleavage) Porphyroblasts: larger crystals in a finer groundmass (i.e., the metamorphic equivalent of the igneous term “phenocrysts”).

Relict bedding in slate; colour banding, v. f. g Relict bedding in slate; colour banding, v.f.g., no identifiable minerals

Identifiable minerals Write down the names of the minerals you can identify (visually or by specific properties) Do not use the term “mica”; be specific and write muscovite, or biotite There are a few new minerals that you have not encountered in the minerals lab (lab 1). Andalusite, kyanite, sillimanite (all different forms of Al2SiO5, just like graphite and diamond are forms of carbon) Staurolite, cordierite All are found in metamorphosed clay rich rocks and are on display at the back of the lab

Andalusite and staurolite

Rock name Use the handout table NOTE: The handout is a modified version of Table 8.1, p. 190 of your textbook. Identify the rock on the basis of the grain size and mineralogy.

Grade Table 8.1 (and modified) gives approximate grade ranges for the common metamorphic rocks The second table (back of handout) gives slightly more detailed estimates of grade depending on mineralogy

Original rock (protolith) Tables also give likely “protoliths” for the common metamorphic rocks. However several different rock types have a number of possible protoliths. It is possible to distinguish which protolith is involved from the mineralogy of the rock Gneiss rich in quartz and feldspar (little biotite/muscovite) is likely derived from felsic igneous rock or arkosic sandstone. Gneiss with abundant biotite/muscovite is likely derived from a clay rich sediment

Some points Use the hand-lens Write neatly Use a PENCIL (and eraser if necessary) Starting this week, marks will be deducted for sloppy work. If we can’t read your work we can’t grade it.

Questions?

Reminder Field trip next week. Keep and eye on the weather Often cold (may be rainy) But on and off the bus Dress warmly (layers) Bus leaves promptly at 1:30 pm