1. G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 20 Introduction to Metamorphic Petrology March 23, 2009

2. D EFINITION “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. E FFECTS OF I NCREASING T EMPERATURE T HE P RIMARY F ACTOR IN M ETAMORPHISM Increasing temperature has several effects: 1) Promotes recrystallization  increased grain size 2) Drive reactions (endothermic) 3) Overcomes kinetic barriers

4. E FFECTS OF P RESSURE S ECONDARY E FFECT ON M INERALOGY, M AJOR E FFECT ON T EXTURE 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. L ITHOSTATIC VS. D IRECTIONAL P RESSURE 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) Lithostatic P  Uniform Stress Directional P  Deviatoric Stress Foliation Cleavage Schistosity Gneissic Banding Simple Shear Pure Shear Strain Ellipsoid

6. D EVIATORIC S TRESS G ENERATION OF F OLIATION AND L INEATION T EXTURES  1 >  2 =  3  foliation and no lineation (Flattening Strain)  1 =  2 >  3  lineation and no foliation  1 >  2 >  3  both foliation and lineation  1 – Maximum Stress  2 – Medial Stress  3 – Minimal Stress 11 22 22 33 33 11 22 22 33 33

7. I MPORTANCE OF F LUIDS IN M ETAMORPHIC R EACTIONS Evidence for the existence of a metamorphic fluid: F Fluid inclusions F Fluids are required for hydrous or carbonate phases F Volatile-involving reactions occur at temperatures and pressures that require finite fluid pressures

8. I MPORTANCE OF THE P ARENT R OCK P ROTOLITH

9. M INERALOGICAL R ESPONSE TO M ETAMORPHISM Minerals that form depend on: T and P conditions Bulk composition of the source rock Progressive metamorphism of a graywacke (dirty sandstone)

10. T EXTURAL R ESPONSE TO M ETAMORPHISM 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 schistosity to 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. O THER T EXTURAL R ESPONSES P ORPHYROBLASTS Some metamorphic minerals have strong growth habits, meaning they will develop as large well-formed crystals called porphyroblasts Alkali Feldspar StauroliteGarnet

12. P ROGRESSIVE M ETAMOPHISM 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. T YPES OF M ETAMORPHISM 1. Based on principal process or agent  Dynamic Metamorphism  Thermal Metamorphism  Dynamo-thermal Metamorphism 2. Based on setting  Contact Metamorphism  Regional Metamorphism o Orogenic Metamorphism o Burial Metamorphism o Ocean Floor Metamorphism  Hydrothermal Metamorphism  Fault-Zone Metamorphism  Impact or Shock Metamorphism

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

15. T YPES OF M ETAMORPHISM R EGIONAL M ETAMORPHISM Burial Metamorphism Follows normal geothermal gradient with lithostatic pressure Orogenic Metamorphism Broad range of P-T paths with differential pressure dominant

16. T YPES OF M ETAMORPHISM INVOLVING F LUIDS O CEAN - FLOOR M ETAMORPHISM, H YDROTHERMAL M ETAMORPHISM, AND M ETASOMATISM 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. T YPES OF M ETAMORPHISM 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