Chapter 7 Metamorphism and Metamorphic Rocks Introduction Metamorphism - The transformation of rocks, usually beneath Earth's surface, as the result of heat, pressure, and/or fluid activity, produces metamorphic rocks During, rocks are subjected to sufficient heat, pressure and fluid activity to change mineral composition or texture, or both. This occurs below the melting point in the solid state. Introduction Metamorphism is responsible for producing a number of economically valuable materials, like marble, a favorite of sculptors throughout history. Metamorphism: Modification of Rocks by Temperature and Pressure Metamorphism is an important process that is closely related to plate tectonics, the growth of continents, and even climate change. Geo-inSight, pgs. 18-181 Introduction Distribution of Metamorphic Rocks 1. Shields oldest part of the continental 2. Cores of large mountain ranges Metamorphism and the Earth system Important Points: Metamorphism is driven by Earth s s internal heat closely related to plate tectonics (used to determine past geohistory) Fig. 7.1, p. 169 1
The Agents of Metamorphism The three principal agents of are heat, pressure and fluid activity. Intrusive magmas or deep burial provide heat which causes. Pressure is produced by overlying rocks (lithostatic) or is differential pressure produced by various stresses. Fluid activity increases the rate of (hydrothermal fluids movement through the rock) The Agents of Metamorphism Heat Heat is an important agent Heat Increases the rate of reactions Sources of heat include: Extrusive lava Intrusive magma Deep burial Temperature increase with depth. The geothermal gradient ave. about 25 o C/ (varies 2 to 6 o C/ at 15~45 o C). The Agents of Metamorphism Pressure What are lithostatic and differential pressures, and why are they important? Lithostatic pressure is a uniform field of pressure experienced by most rocks beneath Earth s surface. Like the hydrostatic pressure experienced by divers underwater, the pressure acting on a rock embedded in the feels the same from all directions. The Agents Pressure Lithostatic pressure is a uniform field of pressure Fig. 7.2, p. 17 Fig. 7.2, p. 17 The Agents of Metamorphism Pressure What are lithostatic and differential pressures, and why are they important? Differential pressure is a nonuniform field of pressure; the pressure acting on a rock in some directions is stronger than it is in others. Many metamorphic rocks form under conditions of differential pressure, which influences the development of metamorphic structures and textures in significant ways. The Agents of Metamorphism Fluid Activity Fluids within sedimentary rocks or issuing from magmas can accelerate chemical changes during & cause new minerals to form. Seawater moving through hot basaltic rock can change the olivine minerals to metamorphic serpentine minerals 2Mg 2 SiO 4 + 2H 2 O Mg 3 Si 2 O 5 (OH)+MgO Olivine + water Serpentine + ion Fig. 7.2, p. 17 2
Agents of pressure & temperature increase with depth Agents of pressure & temperature increase with depth Large variation in temperature by plate tectonic location The Agents of Metamorphism The Three Types of Metamorphism The type of that results largely depends on which of the three agents was dominant. Contact Dynamic Fig. 7.4, p. 171 The Three Types of Metamorphism Contact Metamorphism Contact metamorphic rocks form under conditions of high temperature and low pressure. They are arrayed in aureoles, or metamorphosed zones, around plutons and other intrusive igneous bodies. The Three Types of Metamorphism Dynamic Dynamic is associated with faults and areas where high pressure builds up in the, but the temperature is low, such as in the accretionary wedges at convergent plate boundaries. Fig. 7.5, p. 171 Fig. 7.6, p. 174 Fig. 7.7, p. 174 3
The Three Types of Metamorphism is the most common type of. Types of As the name implies, regional has a broad range. Temperature and pressure both act as driving forces for metamorphic reactions in regional. Continental Fig. 7.4, p. 171 shock, contact, regional, seafloor, & burial Shock Shock Continental Continental Meteor impacts country rock to tektites, like glass droplets at low temperatures 1-2C and moderate depths 1-2 (pressure 3-8 kbars) Shock high-pressure Shock high-pressure Contact Continental Continental High pressure : ocean-continent subduction zone, pressures of 8 to 12 kbars (high) and >28 kbars (ultra) Contact surrounding magma melts rising above subducting plate 4
Shock high-pressure Contact Shock high-pressure Contact Continental Continental Burial Bural at low temperatures 1-2C and moderate depths with pressure <3 kbars Burial Water Types of Seafloor Metasomatism around the Ophlite sequences at mid-ocean ridges Grades of Metamorphism Grades of Metamorphism Grades of (depend on temp & press). Vary from low Intermediate high low (lower grade ) high (higher pressure and/or higher temperature => more change) The Three Types of Metamorphism Grade (or degree) of often determined by Index minerals, which form under specific temperatures and pressure. How are Metamorphic Rocks Classified? Metamorphic rocks are classified principally according to texture. Foliated - Foliated texture is produced by the preferred orientation of platy minerals. Nonfoliated - Nonfoliated textures do not exhibit preferred orientation of minerals. Fig. 7.8 p. 1 Fig. 7.9a, p. 177 5
How are Metamorphic Rocks Classified? Classification of Common Metamorphic Rocks Horizontal Beds: bedding planes bedding planes Table 7.1, p. 176 Metamorphism changes sedimentary rocks, thus forming slaty cleavage planes. slaty cleavage planes Bedding vs Foliation Foliation depends on stress direction and can be at 9 angle to bedding shale bedding planes sandstone layers 5 cm The original bedding can be seen in the thin sandy layers. The original bedding can be seen in the thin sandy layers. shale sandstone layers shale sandstone layers foliation plane original bedding 5 cm 5 cm 6
The original bedding can be seen in the thin sandy layers. causes cleavage planes to develop. shale sandstone layers foliation plane original bedding Mica Feldspar Quartz 5 cm Pyrite Staurolite Foliation is the result of compressive forces. Foliation is the result of compressive forces. Mineral crystals become elongated perpendicular to the compressive force. Feldspar Feldspar Quartz Quartz Mica Mica Pyrite Staurolite Pyrite Staurolite Foliated rocks contain platy minerals that are aligned along a preferred orientation. Staurolite crystal Staurolite crystal Mica Mica 7
Comparison of intensity to crystal size and coarseness of foliation Increasing intensity of Low grade Intermediate grade High grade Increasing crystal size Increasing coarseness of foliation How are Metamorphic Rocks Classified? Foliated Metamorphic Rocks Foliated texture is produced by the preferred orientation of platy minerals. Foliated metamorphic rocks form a graded series of grain size and/or development of foliation, from fine grained slate, to phyllite and coarser grained schist, to gneiss, with segregated bands of minerals. Amphibolite is another fairly common coarse grained foliated metamorphic rock. Fig. 7.9d, p. 177 7.1a, p. 177 How are Metamorphic Rocks Classified? Foliated Metamorphic Rocks Schist and Gneiss How are Metamorphic Rocks Classified? Foliated Metamorphic Rocks Migmatite contain streaks of granite Fig. 7.13, p. 178 Fig. 7.14, p. 179 Diagenesis Shale Low grade Intermediate grade Slate Phyllite Schist (abundant micaceous minerals) Gneiss (fewer micaceous minerals) High grade Migmatite Foliated rocks are classified by the degree of cleavage, schistosity, and banding. Shale Diagenesis Low grade Intermediate grade Slate Phyllite Schist (abundant micaceous minerals) Gneiss (fewer micaceous minerals) High grade Migmatite Slaty cleavage Schistosity Banding Banding Typical sequence of increasing of the sedimentary rock shale from slate (low grade) to migmatite (high grade) Slaty cleavage Schistosity Banding Banding Typical sequence of increasing of the sedimentary rock shale from slate (low grade) to migmatite (high grade) 8
causes a preferred orientation of crystals slaty cleavage - (lowest grade foliation) ability to split into thin sheet along smooth parallel surface foliation - a set of flat or wavy parallel planes produced by deformation (often visible doesn t t always split along foliation) classification of foliated rocks size of crystals nature of foliation degree to which minerals are segregated into bands metamorphic grade nomenclature of foliated rocks Changes with metamorphic grade slate (low grade) phyllite schist gneiss migmatite (high grade) How are Metamorphic Rocks Classified? Nonfoliated Metamorphic Rocks Nonfoliated textures do not exhibit preferred orientation of minerals. Common nonfoliated metamorphic rocks are marble, quartzite, greenstone, and hornfels. Fig. 7.16, p. 179 Fig. 7.17, p. 182 Non-foliated (granoblastic)) textures nomenclature of granoblastic (non-foliated) metamorphic rocks hornfels grainy, elongate pyroxene crystals quartzite white, hard quartz marble metamorphosed limestone greenstone low grade meta. mafics amphibolite amphibole & plagioclase feldspar granulite high grade, granofels 9
foliated with non- foliated (grano- blastic) texture : Schist with garnet porphyro- blasts Metamorphic Zones and Facies Metamorphic zone a belt of rocks showing roughly the same degree of Fig. 7.18, p. 182 Meta Zones via Index Minerals Grade (or degree) of often determined by Index minerals, which form under specific temperatures and pressure. Index minerals define metamorphic zones. Canada ME NY Isograds VT NH MA Key: Not metamorphosed CT RI Low grade Chlorite zone Biotite zone Medium grade Garnet zone Staurolite zone High grade Sillimanite zone Fig. 7.8 p. 1 Index minerals define metamorphic zones. NY Canada VT NH MA CT RI ME Isograds Key: Low grade Not metamorphosed Chlorite zone Biotite zone Isograds can be used to plot the level or degree of. Pressure (kilobars) Slate Low Grade Phyllite Blueschist Increasing metamorphic grade Intermediate Grade Schist High Grade Gneiss Migmatite Depth () Medium grade Garnet zone Staurolite zone High grade Sillimanite zone Temperature ( C) 1
Metamorphic Zones and Facies Metamorphic facies rock groups with distinctive mineral assemblage formed under similar temperature & pressure Metamorphic Zones and Facies How do metamorphic zones and metamorphic facies differ? Metamorphic zones show the gradational metamorphic change within a single rock composition. Metamorphic facies are groups of many different rock compositions whose mineral contents all indicate common temperature and pressure conditions during. Fig. 7.19, p. 183 Plate Tectonics and Metamorphism Metamorphism can occur along all types of plate boundaries, but is most common and extensive along convergent boundaries. Plate Tectonics and Metamorphism Metamorphic rocks formed near the surface and within an oceanic-continental convergent plate boundary zone result from low temperature and high pressure conditions. Blueschist facies Fig. 7.2, p. 183 Fig. 7.2, p. 183 Plate Tectonics and Metamorphism Higher temperatures and pressures existing at depth within such plate boundaries produce higher grades of in a subducting oceanic plate. Metamorphism and Natural Resources Mineral resources which are metamorphic rocks include marble and slate. Slate Fig. 7.2, p. 183 Fig. 7.21, p. 184 Fig. 7.1c, p. 177 11
Metamorphism and Natural Resources Mineral resources which are metamorphic rocks include marble and slate. Marble Metamorphism and Natural Resources Mineral resources which are metamorphic minerals include graphite, talc, asbestos and garnet. Metamorphic Minerals Geo-inSight, pgs. 18-181 GeoFocus Fig. 1, p. 172 Fig. 7.13 a, p. 178 End of Chapter 7 12