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Metamorphic Rocks Metamorphism and metamorphic rocks GEOL115 Alexander Lusk Outline: • Metamorphic rocks – Defini>on and major types of metamorphism – Rock cycle • Metamorphic processes • Deformaon and development of a foliaon/ schistosity/cleavage • Progressive (prograde) metamorphic series • Environments of metamorphism • Metamorphic facies Metamorphic rocks If pre-exis>ng rocks (e.g. igneous, sedimentary, metamorphic) are buried within in the Earth, new minerals can grow in the solid rock through recrystallizaon and the rock can deform in response to tectonic forces. This rock may have started off as a sediment of mud or clay, which later become a sedimentary rock such as mudstone or shale … What minerals are present? Recall the rock cycle: Recall where different rock types form: Types of metamorphism Regional metamorphism: as rocks are buried deeper and deeper within the earth they are subject to increasingly higher pressures and temperatures, which result in recrystallization. The directed pressures (stresses) produce planar structures called foliation. Contact metamorphism: rocks within the crust that are intruded by magma are recrystallized by the magma’s high temperatures. These rocks are commonly not deformed (foliated). Processes involved: 1. Neomineralizaon – growth of new minerals replacing minerals that become unstable. 2. Grain growth – growth of new or exis>ng mineral grains. 3. Deformaon – creates a schistosity or foliaon defined by the orientaon of platy or elongate mineral grains. Neomineralizaon Porphyroblasts: Porphyroclasts: Grain growth: Exis>ng crystals grow: Quartz overgrowths Original quartz grain Rock deformaon: Cleavage and schistosity Folia-on – repe>>ve layering and parallel alignment of platy (flat) minerals, such as micas, by shearing and/or differen>al pressure Examples of Foliaon Slaty rock cleavage – low grade metamorphism Examples of Foliaon Phyllic texture – intermediate grade metamorphism Examples of Foliaon Schistosity – intermediate to high grade metamorphism Examples of Foliaon Gneissic Banding – high grade metamorphism Metamorphic Textures Environments of metamorphism: 1.Subduc(on zones 2.Connental collision zones 3.Magma(c environments Tectonic sengs of metamorphism Rock cycle in subduc>on zone context contact metamorphism regional metamorphism Subduc>on zones: • Sedimentary and volcanic rocks from the ocean floor carried down subduc>on zones experience substan>al increase in pressure, modest increase in temperature. • Rocks are commonly broken and disrupted: tectonic melange • High pressure / low temperature metamorphism: – blueschist facies – eclogite facies Tectonic melange – California coast ranges Microscopic view of Blueschist – Catalina Islands Collisional environments • Where crust is thickened by con>nental collision rocks become buried and heated; may start to melt. Regional metamorphism • Rocks become duc>le and start to flow • Sedimentary rocks are converted into slate or schist; – abundant well-oriented mica – porphyroblasts of minerals like garnet • If rocks start to melt, produce gneiss. – abundant quartz and feldspar, strong foliaon Magmac environments • Large intrusions (plutons) rise into the crust, heat surrounding rocks. • Temperature increases, but pressure does not. Contact metamorphism • Li^le or no deformaon • Neomineraliza2on, but no fabrics – hornfels • Magmac environments include: – mid-ocean ridges – plutons related to subduc>on zone magmasm – plutons related to collision-related mel>ng. Zone of contact metamorphism Contact metamorphic rocks in the roof of a granite pluton, Sierra Nevada batholith Metamorphic Facies Mineral assemblages indica2ve of a certain range of P-T condi2ons Using minerals to get a rock’s history Garnet as a tape recorder Green “island”: 1 Ga garnet growth. White: 450 Ma garnet growth aer earlier garnet cracked up. Red: 360 Ma garnet growth. Every >me that Africa and North America collided this garnet grew and recorded the P-T condions. Andalusite replaced by kyanite and staurolite. Kyanite is the high P form of Andalusite. Kyanite is being deformed. This rock records burial and deformaon of the rock. .
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