ERSC 3P21. Metamorphic Petrology

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1 ERSC 3P21 Metamorphic Petrology, and adjustments in solid rocks in response to and conditions which have been imposed due to changes in (_) and (_) The conditions of metamorphism differ from the conditions under which the rocks in question e.g. basalt solidifies on the Earth s surface at T= ~1,200 C and 1 atm pressure Original rock is referred to as the 1

2 Metamorphic Rocks To recognize a metamorphic rock and distinguish it from an igneous or sedimentary rock two criteria are used: Metamorphic the group of minerals that form in a rock as a result of metamorphism, and; Metamorphic parallel surfaces or layers that develop in a rock as a result of metamorphism. hornblende Foliation Copyright 2002 G.C. Finn plagioclase garnet resulting from the in temperature and pressure, i.e. by burial of a rock within the Earth gradient C/km gradient 3.5 kbar/km The progressive change from to to to to is an example of prograde metamorphism 2

3 Prograde In general, Prograde is characterized by a series of, which release water Mineral(OH) 2 ===> Mineral + H 2 O Muscovite + Quartz ===> Al 2 SiO 5 + Kspar + H 2 O Muscovite ===> Kspar + Corundum + H 2 O can also take place as a result of temperature and pressure conditions This type is referred to as Retrograde For retrograde metamorphism to occur water must be, to hydrate the minerals, and thus it takes much longer to occur than prograde metamorphism Retrograde Retrograde metamorphism is usually associated with a (hot water) Retrograde metamorphism does not happen automatically when a highgrade rock is uplifted to the Earth s surface under cold dry conditions is a very, very slow process, taking billions of years to complete 3

4 Prograde vs Retrograde Temperature Retrograde Pressure Prograde Peak of Limits of Lower limit Marks the boundary between metamorphism and Lower T = C Excludes any changes Generally defined as the first appearance of a mineral which does not occur in rocks Limits of Upper Limit generally considered to end when large scale of character are produced Upper T, will vary over a wide pressure range, with excess water, depending on the of the rock In Mudstones C In mafic lithologies C 4

5 Assemblage vs Association Within an individual thin section of a metamorphic rock and are observed Mineral Association Consists of all minerals in a single thin section Minerals may or may not be in with each other Assemblage vs Association Within an individual thin section of a metamorphic rock Mineral Assemblages and Mineral Associations are observed Mineral Assemblage ( ) Consists of those minerals which grew at the same time, i.e. in. They exhibit a stable or meta-stable coexistence during the metamorphic event of interest. These minerals are in with each other. A given mineral assemblage forms under P and T Grade is used to signify the, which corresponds approximately to a progressive increase in temperature Four Divisions of Grade 5

6 Boundaries between the four divisions of grade are marked by significant changes in the in common rocks Changes correspond to specific reactions that occur within the lithology Diagenesis Water saturated granite solidus Calculated Geothermal Gradient Pressure (kbar) Depth (km) Temperature (C) Pressure (kbar) Diagenesis VERY LOW GRADE Water saturated granite solidus Calculated Geothermal Gradient Look at this boundary HIGH GRADE LOW 4 GRADE MEDIUM 10 GRADE Depth (km) Temperature (C) 6

7 Boundary between medium and high grade Based on two reactions 1. At P < 3.0 kbars Musc + qtz ===> or + Al 2 SiO 5 +H 2 O T = 580 C, P = 1 kbar, T = 600 C, P = 3 kbar 2. At P > 3.0 kbars Ab + qtz + or ===> Granitic Liquid Corresponds to the water saturated granite solidus T = 660 C, P = 3 kbar, T < 600 C, P = 6 kbar 7