Metamorphic Rocks Most figures and tables contained here are from course text: Understanding Earth Fourth Edition by Frank Press, Raymond Siever, John Grotzinger, and Thomas H. Jordan Metamorphic rocks Formed by heat, pressure and fluid activity Fluid activity is necessary to move ions around but fluid is almost always available at depth so metamorphism is more dependent on the necessary heat and pressure being available. The two types of metamorphism are: Contact metamo rphism: which is due to heat from a near by magma body Regional metamorphism is due to tectonic activity related to collision at subduction zones and burial in mountain building events and related to the subducting slab. 1
Heat: Sources for heat include: Volcanism: rock near magma bodies will be heated by the magma Depth of burial the average thermal gradient for the crust is 25 degree Celsius. This is an average so it varies with tectonic settings. The thermal gradient is generally lowest is deep sedimentary basins on old passive margins and in stable craton areas. It is highest in rift areas or other extensional environments. 2
Pressure The main source of pressure is depth of burial. At great depth this will result in hydrostatic pressure (stress the same in all directions. Tectonic stress: Plate motion and collisions will cause large stresses in the rock. This is generally deviatoric or direction stress. 3
Types of metamorphism Shock meta morphism Regional meta morphism High pressure Contact meta morphism Sea floor meta morphism Burial meta morphism Types of metamorphism Regional includes: Burial metamorphism (low temperature high pressure) Pressure metamorphism (due to collision at subduction zone) Both (high pressure and temperature due to extreme depth of burial during mountain building and associated heat) Contact is due to contact with a magma chambe r Special cases: Shock metamorphism due to meteorite impact. Seafloor meta morphism is the alteration of mid oceanic ridge basalt by circulation of hot ocean water. Grade of metamorphism is directly proportional to the amount of heat and pressure involved in the process 4
Metamorphic texture Foliated texture- banded or a flowing like texture, can resemble cleavage: Foliated texture is caused when platy mineral orient themselves perpendicular to regional principle stress direction. Foliated metamorphic rocks listed from lowest grade metamorphic rock to highest grade foliated metamorphic rock. Low grade High grade Slate, Phyllite, Schist, Gneiss, Migmatite Parent rock clay or volcanic ash, grain to o small to see, cle aves well Parent rock clay or volcanic ash glossy due to larg er mica and ch lo rite Platy (mica) min erals larg e enough to see. Quartz and/or feldspar starts to appear, course, wavy fo liatio n Courser more p oo rly fo liated, due to fewer micas and more quartz, feldspar and mafic minerals forming Th is is an igneous metamorphi rockhybrid 5
Metamorphism also increases grain size The higher the grade of metamo rphism the larger the grain size 6
Granoblastic texture Minerals grow in the same dimension in all directions, fewer platy minerals. These usually form in areas where the stress is not directional like in contact metamo rphism. The identity of these rocks often reflect the parent rock. Hornfels high temperature contact metamorphism Quartzite: metamorphosed quartz sandstone Marble: metamorphosed limestone or dolostone Greenstones: metamorphosed mafic volcanic rock altered by seawate r Amphibolites: medium to high grade foliated or non-foliates rocks typically from mafic volcanic rocks 7
Porphyroblastic texture: large grains in fine matrix 8
Index mineral are minerals that indicate a degree of metamorphism Index mineral can be mapped to identify regions that were influenced by varying degrees of metamorphism Index minerals and grade of metamorphism 9
Metamorphism of mafic rocks Mineral types in ordered according to grade of metamorphism Low Grade of metamorphism high Chlorite, muscovite, biotite, garnet, staurolite kyanite Albite (Na plagioclase) Sillimanite Mafic parent rock Zeolite Epidote garnet pyroxene Na plagioclase Ca Greenschist 10
Foliated rock types in different grade of metamo rphic facies dependent on which in index minerals they contain Low grade Medium grade High grade 0 3 6 9 12 pressure (killobars) No natural occurring rocks slate Phyll ite Blues chist schist Gn eiss Migmatite 0 200 400 600 800 1000 temperature (degrees Celsius) 40 30 20 10 0 Depth (km) Granoblastic rock types in different grades of metamorphic facies dependent on which in index minerals they contain Granoblastic rock types in different grades of metamorphic facies dependent on which in index minerals they contain 0 3 6 9 12 pressure (killobars) No natural occurring rocks Low grade Medium grade High grade Hornfels Zeolite Bluesch ist Sudbuction zone Contact metamo rphism Beneath mountain belts Greenschist Eclogite Amphibolite Granulite 0 200 400 600 800 1000 temperature (degrees Celsius) 40 30 20 10 0 Depth (km) 11
A D B C A D B C 12
A D B C A D B C 13