Geol 2312 Igneous and Metamorphic Petrology

1. Geol 2312 Igneous and Metamorphic Petrology Lecture 20 Introduction to Metamorphic Petrology March 23, 2009

2. Definition “Metamorphism is a subsolidus process leading to changes in mineralogy and/or texture and often in chemical composition in a rock. These changes are due to physical (P & T) and/or chemical conditions* that differ from those normally occurring at the surface of planets and in zones of cementation and diagenesis below this surface. They may coexist with partial melting.” IUGS-SCMR *Chemical conditions commonly involve a fluid phase Pressure and Temperature conditions of Metamorphic Grades Lower Limit - T >100-200°C, P not constrained; depends or reactability of starting rock; zeolite type an accepted indicator. Upper Limit - onset of “SIGNIFICANT” melting

3. Effects of Increasing TemperatureThe Primary Factor in Metamorphism • Increasing temperature has several effects: 1) Promotes recrystallization  increased grain size 2) Drive reactions (endothermic) 3) Overcomes kinetic barriers

4. Effects of PressureSecondary Effect on Mineralogy, Major Effect on Texture • Pressure usually follows “Normal”gradients, but may be perturbed in several ways, typically: • Low T/P geotherms in subduction zones • High T/P geotherms in areas of plutonic activity or rifting Winter (2001) Figure 21-1. Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill.

5. Lithostatic vs. Directional Pressure Stress is an applied force acting on a rock (over a particular cross-sectional area) Strain is the response of the rock to an applied stress (= yielding or deformation) Simple Shear Pure Shear Foliation Cleavage Schistosity Gneissic Banding Lithostatic P  Uniform Stress Directional P  Deviatoric Stress

6. Deviatoric StressGeneration of Foliation and Lineation Textures s3 s3 s2 s2 s1 s1 s2 s2 s3 s1 – Maximum Stress s2 – Medial Stress s3 – Minimal Stress s3 • s1 > s2 = s3  foliation and no lineation (Flattening Strain) • s1 = s2 > s3  lineation and no foliation • s1 > s2 > s3  both foliation and lineation

7. Importance of Fluids in Metamorphic Reactions • Evidence for the existence of a metamorphic fluid: • Fluid inclusions • Fluids are required for hydrous or carbonate phases • Volatile-involving reactions occur at temperatures and pressures that require finite fluid pressures

8. Importance of the Parent Rock Protolith

9. Mineralogical Response to Metamorphism • Minerals that form depend on: • T and P conditions • Bulk composition of the source rock Progressive metamorphism of a graywacke (dirty sandstone)

10. Textural Response to Metamorphism • Reflects the intensity, directionality and duration of pressure (or stress). • Increased grain size - During prograde metamorphism or at a particular grade that is maintained for a long period of time, minerals will tend to increase in size. • Foliation - As new platy minerals grow, they will align themselves perpendicular to the maximum stress direction. For clay mineral and fine-grained micas, the planar fabric that results is referred to as a slaty cleavage. In higher grade rocks, coarser grained mica minerals are said to impart a schistosityto the rock. • Gneissic Banding - In very high grade rocks, the dark minerals tend to segregate from the lighter colored minerals (feldspar and quartz) resulting in banded rock..

11. Other Textural ResponsesPorphyroblasts Some metamorphic minerals have strong growth habits, meaning they will develop as large well-formed crystals called porphyroblasts Alkali Feldspar Staurolite Garnet

12. Progressive Metamophism • Prograde: increase in metamorphic grade with time as a rock is subjected to gradually more severe conditions • Retrograde: decreasing grade as rock cools and recovers from a metamorphic or igneous event • Prograde reactions are endothermic and easily driven by increasing T • Devolatilization reactions are easier than reintroducing the volatiles • Geothermometry indicates that the mineral compositions commonly preserve the maximum temperature

13. Types of Metamorphism 1. Based on principal process or agent • Dynamic Metamorphism • Thermal Metamorphism • Dynamo-thermal Metamorphism 2. Based on setting • Contact Metamorphism • Regional Metamorphism • Orogenic Metamorphism • Burial Metamorphism • Ocean Floor Metamorphism • Hydrothermal Metamorphism • Fault-Zone Metamorphism • Impact or Shock Metamorphism

14. Types of Metamorphism Contact (or Thermal) metamorphism • Grades up to very high temperatures, low-mod. lithostatic pressures • Forms a metamorphic aureole adjacent to igneous intrusions that is best developed in the cooler upper crust (epizone) • Commonly develops granoblastic texture creating a rock called HORNFELS Granoblastic Texture – equant grains, 120° jcts

15. Types of Metamorphism Regional Metamorphism Burial Metamorphism Follows normal geothermal gradient with lithostatic pressure Orogenic Metamorphism Broad range of P-T paths with differential pressure dominant

16. Types of Metamorphism involving FluidsOcean-floor Metamorphism, Hydrothermal Metamorphism, and Metasomatism Generally low temperatures and pressures Distinguished from other forms of metamorphism by the loss and/or gain of material (usually transported by a fluid). Other types of metamorphism are thought to occur as nearly closed processes (except for water loss) Hydrothermally Altered Basalt – Spilite / Greenstone

17. Types of Metamorphism Fault Zone Metamorphism (Cataclasis) Variable temperature, very high directed P Typically localized to narrow zones of intense mechanical deformation (shear zones). Rock types formed breccia (fragmental), gouge (clay), mylonite (strongly foliated)

18. Summary of Metamorphic Rock Types