Lab Activity - Metamorphic Rocks

PHYSICAL GEOLOGY Metamorphic Rocks The dynamics of plate tectonics change the surface of the Earth; metamorphic rocks record the history of these changes. The textures, mineral assemblages, and compositions of the rocks give insight into what kind of changes have occurred. When a rock is exposed to conditions unlike those in which it had formed the minerals within the rock become unstable. As this occurs the rock will undergo metamorphism (change in form) as the minerals come into equilibrium with the new environment. These changes include growth of new minerals, the alteration of existing minerals, changes in mineral orientation, mineral segregation, and even partial melting. Types of metamorphism Regional metamorphism occurs with the increase of temperature and pressure. This environment occurs in regions of mountain building commonly associated with convergent tectonic boundaries. If there is an increase in temperature with little to no increase in pressure, like a body of magma or a lava flow coming in contact with the surrounding country rock, it is referred to as contact metamorphism. Burial metamorphism occurs as deep sedimentary basins are filled, burying sediments successively deeper under several kilometers of overburden. Relatively low temperatures and pressures aided by fluids act as the agents of alteration. Moderate to high shear stress and low temperature conditions, which occur commonly at transform boundaries and along faults, causes cataclastic metamorphism. Shock metamorphism occurs as an instantaneous high pressure and temperature event due to a meteor impact. Types of alterations Mineral regrowth occurs as some minerals dissolve (not melt) and others grow— this leads to a rock with a texture with larger mineral crystals than that of the protolith (original rock before metamorphism). Mineral alteration may occur due to a change from a less stable to a more stable atomic structure without a change in composition (polymorphs). Mineral alteration may also occur due to a compositional change with the addition of elements due to reaction with infiltrating hydrothermal fluids. Through an increase of heat and directed pressure ( stress ) minerals may reorient themselves perpendicular to the maximum pressure direction. Elongated, platy, sheet-like and prismatic crystals are more prone to this alignment ( foliation ). Elongate, bladed, and needle-like crystals may reorient to define a linear fabric within the rock ( lineation ). If the rock continues to experience an increase in heat and directed pressure the minerals will begin to segregate into bands of darker and lighter minerals as the rock continues to recrystallize. As the pressure and temperature continues to increase; slight melting will occur giving the rock characteristics of both metamorphic and igneous rock ( migmatite ). The light-colored silicate minerals generally have lower melting temperatures; the dark-colored silicate minerals 1

PHYSICAL GEOLOGY Metamorphic Rocks have higher melting temperatures and are generally unaffected by melting except under extreme temperature environments. Metamorphic rock classification schemes are broken down to two major categories: foliated or non-foliated. Non-foliated metamorphic rocks are identified by their composition. Foliated metamorphic rocks are identified not only by their composition but also by the type of foliation. Foliated rocks form in a continuum, grading from lower grade (intensity) to higher grade. Typically, these rocks will start with a sedimentary rock (shale) as their protolith. Shale will first alter to a rock with a slaty cleavage, next develop a schistose foliation and finally show a gneissic foliation. 2

PHYSICAL GEOLOGY Metamorphic Rocks Foliation classifications Slaty cleavage is characterized by a flat foliation consisting of microscopic minerals typically exhibiting a dull surface. Schistose foliations vary from flat to wavy with minerals which are visible. Gneissic foliations are characterized by alternating lighter and darker bands of segregated minerals. Metamorphic Rock Classification Texture Type of Foliation Composition (Mineralogy) Protolith Rock Name Foliated Slaty Commonly microscopic quartz and clay minerals Shale Slate Shale, Slate Phyllite Schistose Visible minerals including but not limited to quartz, micas, amphiboles and garnet* Shale, Slate, Phyllite Schist Gneissic Lighter bands are commonly quartz, micas and feldspars; darker bands are micas and amphiboles* Shale, Slate, Phyllite, Schist, Granitic igneous rocks Gneiss Non-foliated N/A Quartz Quartz sandstone Quartzite Calcite Limestone Marble Rock fragments Breccia, Conglomerate Metaconglomerate Mafic minerals such as plagioclase and augite (a pyroxene) Shale Hornfels Wollastonite , Garnet, Pyroxene Siliceous limestone Skarn Serpentine Basalt Serpentinite Actinolite (light-green amphibole) and/or Chlorite (green mica) Basalt Greenschist Dark coarse-grained minerals (often hornblende +/- epidote or garnet) Basalt Amphibolite Dark coarse-grained mineral (green sodic pyroxene) with garnet Basalt Eclogite Either Talc Peridotite or Dolostone Soapstone 3