Metamorphic Rocks

1. Metamorphic Rocks • Metamorphism – the process by which heat, pressure and chemical reactions change the mineral composition and/or structure of any type of preexisting rock without melting it • Metamorphic process – occurs at temperatures and pressures > those of lithification (~200C) and < those that melt rock into magma (~800C)

2. Metamorphic Process • Heat – > depth > temp = geothermal gradient • (~72 degrees/mi) • Intrusions - temp diff between intrusion and surrounding rock • Pressure - > depth > pressure • Lithostatic or confining = from all directions • Directed – plate boundaries, folding and faulting • Changes texture not mineralogy – the shape and arrangement of minerals in a rock • Foliation – arrangement of minerals perpendicular to directed pressure • Solutions – liquids and/or gases are catalysts for metamorphic processes • Water • Water-rich minerals – clays and amphiboles • Produce the growth of new minerals • Can add or remove ions to change the composition of rocks

3. Types of metamorphism • Contact – Rocks surrounding magmas are altered by heat and circulation of hot fluids • < meta the further you get away from contact • Hi-temp minerals close/Lo temp minerals farther away • Coarse-grained close/finer grained father away • < water close >water farther away • Temperature gradient between intrusion and country rock • Size can vary with size and temp of intrusion

4. Contact metamorphism • Subsurface intrusion of molten material creates zone of metamorphism in the surrounding country rock (Image below) • Surface extrusion of lava flow creates metamorphism of underlying soil horizon (Image at right – red layer)

5. Types of metamorphism (cont’d) • Regional – two types, affect large areas of crust • Burial – deep sedimentary basins • > 6mi depth • Mostly sed. Rx • No foliation • Dynamothermal – plate tectonic boundaries • Foliated • Convergent plate pressure • Thickening

6. Other types of metamorphism • Hydrothermal – chemical alteration by heated water • Mostly at divergent plate boundaries • Fringes of magmatic intrusions • Hi-grade mineral deposits • Fault-zone – mylonites Localized along fault planes • Frictional heat and pressure • Shock – meteorites • Very localized • Hi-pressure and temp • Minerals occur at no other geologic setting (coesite)

7. Impact BrecciasPopigai impact crater, Siberia • Some ejecta "clasts" are as big as a house • Megabreccias, similar to those found on the lunar highlands, are not uncommon at terrestrial impact structures

8. Grade of Metamorphism • Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form. • Low-grade metamorphism takes place at temperatures between about 200 to 320oC, and relatively low pressure.  Low grade metamorphic rocks are characterized by an abundance of hydrous minerals (minerals that contain water, H2O, in their crystal structure). • Clay minerals • Serpentine • Chlorite • High-grade metamorphism takes place at temperatures greater than 320oC and relatively high pressure.  As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H2O and non-hydrous minerals become more common. • Muscovite – hydrous mineral that eventually disappears at high grade • Biotite – a hydrous mineral that is very stable at high grade • Kyanite – a non-hydrous mineral • Garnet – a non-hydrous mineral • Pyroxene – a non-hydrous mineral

9. Metamorphic Grade • Slate – low meta because it appears just like the parent rock • Phyllite – low to moderate grade as platy texture begins to dominate • Schist – moderate to high grade and texture is extremely platy and mineralogy begins to change • Gneiss – high grade because mineralogy and texture have changed completely • Metamorphic zones • Systematic change in mineralogy • Change occurs as zones • Index minerals

10. Regional Metamorphism and Index Mineral Zones

11. Metamorphism and Plate Tectonics

12. Folded Mountain BeltsAppalachians

13. Himalayan vs Appalachian

14. Classifying Metamorphic Rocks • 2 main classes – foliated and non-foliated • Foliated from sedimentary rx • Slate – derived from shales • Red – hematite, green - chlorite, purple – manganese oxides, black – carbon rich • Smooth rock cleavage • Phyllites – thin wavy foliation • Fine grained – micas, chlorites and graphite • Shiny • Schist – • Coarser grained • New minerals • Same overall composition • Gneiss – light and dark bands • Mineral differentiation • Coarse grained • Different composition

15. Metamorphic Textures • Slate – low meta because it appears very similar to the parent rock

16. Metamorphic Textures • Phyllitic Texture - This texture is formed by the parallel arrangement of platy minerals, usually micas, that are barely macroscopic (visible to the naked eye).

17. Metamorphic Textures • Schistose Texture This is a foliated texture resulting from the sub-parallel to parallel orientation of platy minerals such as chlorite or micas.

18. Metamorphic Textures • Gneissic – alignment of minerals and segregation of light and dark minerals (banding) high grade because mineralogy and texture have changed

19. Non-foliated Metamorphic Rocks • Quartzite – formed from sandstones by additional growth of quartz crystals between grains • Marble – formed from limestone • Anthracite – formed from coal • Crystal size related to degree of metamorphism • > meta > crystals • Often related to heat not burial pressure

20. Foliated Rocks From Igneous Rocks • Greenschist – • Chlorite, epidote and amphibole • Basalt and gabbro • Subduction zones • Lo-temp, lo-press • Blueschist – • Hi-press, low temp • Glaucophane • Subduction zones only • Gneiss – • Granites and diorites • Coarse grained • No dramatic change in composition and texture

21. Non-foliated metamorphic rocks from volcanic rocks • Serpentinite • Basic to ultramafic meta • Water-rich environment • Beautiful but weak ornamental rock • Hornfels • Low-hi grade contact meta • Of no economic importance • Eclogite • Low temp & high press

22. Migmatites Border metamorphic & igneous textures Full range between gneiss and granite Lower temp/lighter minerals Begin to flow Quartz & feldspar Higher temp/darker minerals Biotites & pyroxenes