Modification of Rocks by Temperature and Pressure METAMORPHISM: Modification of Rocks by Temperature and Pressure Chapter 6

continental lithosphere Oceanic crust Oceanic lithosphere Continental crust Mantle lithosphere Asthenosphere Volcanic arc (subduction zone) Zone of continental plate extension Ancient stable continental lithosphere Pressure (kilobars) Depth 0 km 50 30 30 1300°C 50 50 150 1300°C 1300°C isotherm

Pressure increases with depth at about the same rate everywhere,... ...but temperature increases at different rates in different regions. Volcanic arc (subduction zone) Zone of continental plate extension Ancient stable continental lithosphere Pressure (kilobars) Depth 0 km 50 30 30 1300°C 50 50 150 1300°C 1300°C isotherm

Regional high-pressure metamorphism Shock metamorphism Regional Depth, km Oceanic crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere

Regional high-pressure metamorphism Shock metamorphism Regional Contact metamorphism Regional metamorphism Depth, km Oceanic crust 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere Water Seafloor metamorphism Burial metamorphism

slate (phyllite) schist gneiss Metamorphism of sedimentary rocks —shales undergo the most spectacular metamorphosis of all the rocks. The reason is that the clay minerals form at Earth surface temperature (T) (0-30 C) and pressure (P) (1 bar) by weathering of pre-existing rocks. Thus, clay minerals are grossly out of equilibrium with the high temperature and pressure conditions found at depth in the Earth’s crust. When shale is buried to deeper crustal levels beneath, for example, mountain belts, the original clay is transformed to chlorite, and then to biotite/muscovite, with increasing T and P. These micaceous minerals are platy; they grow large and become aligned in the metamorphic stress field, giving the rock a foliated appearance. slate (phyllite) schist gneiss

Increasing intensity of metamorphism slate - phyllite schist gneiss Increasing intensity of metamorphism Increasing crystal size Increasing coarseness of foliation Low grade Intermediate grade High grade

Foliation is the result of compressive forces. Mineral crystals elongate perpendicular to the compressive force. Feldspar Quartz Mica Pyrite Staurolite

Staurolite crystal Mica

Foliated rocks are classified by the degree of cleavage, schistosity, and banding. Diagenesis Low grade Intermediate grade High grade Slate Phyllite Schist (abundant micaceous minerals) Gneiss (fewer micaceous minerals) Migmatite Slaty cleavage Schistosity Banding Banding

Rocks without clay minerals do not form micaceous minerals during metamorphism and, thus, tend not to be foliated. Examples are limestone and sandstone, which form marble and quartzite, respectively. In the limestone, the pre-existing grains of calcite and skeletal fossils (bioclasts) are obliterated and new larger crystals of calcite form in the high P and T conditions. In the sandstone, the quartz grains become welded together by quartz cements. Granoblastic texture results.

Schists have the tendency to grow one mineral much larger than the others. The large mineral is called a porphyroblast. The mineralogy of the porphyroblast changes with increasing T and P. A common porphyroblast is garnet. A series of porphyroblast polymorphs denoting increasing temperature is staurolite, kyanite, silliminite. The fact that certain minerals will grow only within a restricted range of temperatures and pressures gives way to the concept of using mineral assemblages to determine metamorphic conditions–--the minerals behave as paleothermometers and paleobarometers.

With increasing metamorphic grade, mineral composition changes. Degree of metamorphism Diagenesis Low Intermediate High Greenschists Amphibolites Granulites Chlorite White mica (mainly muscovite) Biotite Garnet Staurolite Kyanite Sillimanite Albite (sodium plagioclase) With increasing metamorphic grade, mineral composition changes. Mineral suites define metamorphic facies.

A metamorphic facies is a set of metamorphic mineral assemblages that were formed under similar pressures and temperatures.[1] The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure (See diagram at right).[1] Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in geological history of the area.[1] The boundaries between facies (and corresponding areas on the temperature v. pressure graph), are wide, because they are gradational and approximate.[1] The area on the graph corresponding to rock formation at the lowest values of temperature and pressure, is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.[1]

Low Grade Increasing metamorphic grade Slate Intermediate Grade Phyllite High Grade Pressure (kilobars) Depth (km) Schist Gneiss Blueschist Migmatite Temperature (°C)

Metamorphic facies correspond to particular combinations of Hornfels Contact metamorphism Zeolite 5 Conditions beneath mountain belts 5 10 Blueschist 15 Partial melting begins Amphibolite Granulite Pressure (kilobars) Greenschist Depth (km) 10 20 25 13.5 15 Subduction zone 30 Eclogite 35 20 40 200 400 600 800 1000 Temperature (°C) Metamorphic facies correspond to particular combinations of pressure and temperature... … and can be used to indicate specific tectonic environments.

Index minerals define metamorphic zones. Isograds can be used to plot the degree of metamorphism. Canada ME NY Isograds VT NH MA Key: Not metamorphosed CT Chlorite zone Low grade RI Biotite zone Garnet zone Medium grade Staurolite zone High grade Sillimanite zone

Tectonic transport moves rocks through different pressure-temperature zones, … Low P, Low T High P, High T …and then transports them back to the shallow crust or the surface.

Pressure (kilobars) Depth (km) Temperature (°C) Low Grade Slate RETROGRADE PATH Intermediate Grade Phyllite Pressure (kilobars) Depth (km) PROGRADE PATH Schist Gneiss High Grade Temperature (°C)

Pressure-Temperature Paths Click here to view the Flash animation in your web browser To return to this slide after viewing the animation, close the animation window in your web browser application.