UNIVERSITY OF SOUTH ALABAMA GY 111: Physical Geology Lecture 27 Contact Metamorphism Last Time Regional Metamorphism A) Foliated rocks B) Non-foliated rocks Web notes: 26 Regional Metamorphism Involves both heat and pressure Regional Metamorphism The most stable configuration for platy minerals like clays and micas is perpendicular to the stress direction Regional Metamorphism The most stable configuration for platy minerals like clays and micas is perpendicular to the stress direction Foliated Rocks But it can develop rock cleavage Foliated Rocks Higher P/T causes minerals to grow large enough to be visible. The highest pressure can cause mineral separation (distinct bands e.g., quartz, biotite, feldspars). Low Medium slate phyllite Med-High schist High gneiss Foliated Rocks Slate Foliated Rocks Phyllite Foliated Rocks Garnet-Mica schist Foliated Rocks Kyanite schist Foliated Rocks Gneiss Non-foliated Rocks Foliations can only develop during metamorphism if there was mica and/or clay in the original parent rocks. Marble (non-foliated) Non-foliated Rocks Warning: marble can look exactly the same as quartzite Marble (non-foliated) Quartzite (non-foliated) Today’s Agenda Contact Metamorphism A) Metamorphic Aureoles B) Isograds C) Index minerals and metamorphic grade Web notes: 27 Contact Metamorphism Heat only Induced by hot intrusions Metamorphic aureoles http://academic.brooklyn.cuny.edu/geology/grocha/monument/images/metheat.gif Contact Metamorphism Metamorphic grade decreases from the intrusion into the country rock Contact Metamorphism Mixed parent lithologies give rise to varied metamorphic rocks http://csmres.jmu.edu/geollab/Fichter/MetaRx/Images/contact.gif Contact Metamorphism Mixed parent lithologies give rise to varied metamorphic rocks Hornfels http://csmres.jmu.edu/geollab/Fichter/MetaRx/Images/contact.gif Contact Metamorphism Mixed parent lithologies give rise to varied metamorphic rocks Hornfels Metaconglomerate http://csmres.jmu.edu/geollab/Fichter/MetaRx/Images/contact.gif Contact Metamorphism And different metamorphic minerals (more on this shortly) Contact Metamorphism Contact metamorphism is commonly very obvious in outcrop http://z.about.com/d/geology/1/0/A/z/contactmet500.jpg Contact Metamorphism Contact metamorphism is Aureole commonly very obvious in outcrop Limestone Sill Aureole Shale http://z.about.com/d/geology/1/0/A/z/contactmet500.jpg Contact Metamorphism Limestone Marble Sill Hornfels Shale http://z.about.com/d/geology/1/0/A/z/contactmet500.jpg Contact Metamorphism Metamorphic Isograds Isograds Isograds around the Kangmar Dome of Tibet Contact Metamorphism http://www.geol.ucsb.edu/faculty/hacker/geo102C/lectures/KangmarIsograds.jpg Isograds Isograds in Scotland Regional Metamorphism http://www.geol.lsu.edu/henry/Geology3041 Isograds •Index Minerals: minerals that give you a hint of metamorphic grade Pluton Index Minerals (Felsic rocks) Index Minerals (Mafic rocks) Index Minerals (Mafic rocks) Important observation: Amphibole only forms from parent rocks rich in Fe and Mg. Therefore the parent rock of most amphbolites is basalt Today’s Homework 1. Quiz (fill in the blanks; foliated and non foliated rocks) 2. Photo assignment due via email Friday (5:00 PM) Next Time 1. Metamorphic Facies/Retrograd Metamorphism GY 302 Poster Session. Next Tuesday 1:00-3:00PM (Bonus) Rock and Gem Show Thanksgiving weekend (Bonus) proof of attendance required for both bonuses GY 111: Physical Geology Lecture 27: Contact Metamorphism Instructor: Dr. Doug Haywick dhaywick@southalabama.edu This is a free open access lecture, but not for commercial purposes. For personal use only.