Introduction to Metamorphic Petrology

See http://plate-tectonic.narod.ru/metpetrographylinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html


I. Definitions

A. Metamorphism

-Sum of all changes in a pre-existing rock: textural, mineralogical, chemical
-These result from changes in: temperature; pressure;composition
-Broadest sense includes all changes between diagenesis and melting
-Protolith: pre-existing rock that is changed.

B. Types of metamorphism

-Regional; affects a large area; iInvolves both T and P (medium-high)
-Burial; affects sediments in a subsiding basin; relatively low T and P
-Contact; affects rocks around the margins of an igneous intrusion; involves mostly T
-Shock ; affects rocks impacted by a bolide; high P

C. Metamorphic petrology

-Use mineralogy, texture, structure to determine conditions under which a rock formed.
-Then deduce the geologic history of a mountain belt, intrusion, etc.

II. Model metamorphic terrane

A. an idealized metamorphic terrane developed on folded and faulted sedimentary rocks

-Two types of boundaries shown: contacts separate rock types; isograds separate metamorphic zones

B. Changes

-Mineralogy => intensity of met. increases from west to east
-Consider pelitic schist:

-West: Musc + And + Q
-Middle: And changes to Sil
-Eastern part: Q disappears, Kfs appears.

-Consider quartzite:

-West: Musc + Kfs + Q.
-Middle: When quartzite reaches second pelitic isograd, a change does occur - Sil gone and Musc appears.
-East: "Stringers" of granite appear, parallel to the foliation.

-Overall, 4 zones (Roman numerals) :

-Zone I and II distinguished by changes in schist mineralogy (And vs. Sil). No changes in quartzite.
-Zone III visible in both: schist gains Kfs, quartzite gains Sil.
-Zone IV shows up only in quartzite (migmatite)

C. Reactions

-Mineral assemblages are related through reactions at isograds.
-Zone I > II = And > Sil
-Zone II > III? Sil and Kfs are produced at the expense of Q and Musc:
KAl2AlSi3O10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O
Musc Q Kfs Sil vapor
-Use a chemical diagram similar to phase diagrams.

D. Representation of mineral assemblages

-Choose the minimum # of components necessary
-AlO3/2, SiO2, HO1/2, KAlSi3O8

-Tetrahedral diagram
-Project from one vertex to opposite side to reduce it to a ternary diagram

III. Metamorphic mineral facies

A.Definition: unique set of mineral assemblages.

-Usually plotted in a diagram showing which assemblage will form for a given rock composition under certain conditions
- one mineral facies diagram for each zone:

-Zone I: schist in Q-And-Musc'' subtriangle, quartzite in Q-Kfs-Musc''
-Zone II: only change is And replaced by Sil
-Zone III: Q-Musc'' tieline is broken, new tieline between Sil-Kfs forms in a tieline-switching reaction.
-Zone IV: liquid field appears

B. Concept

-Different mineral assemblages in rocks with same bulk composition implies different physical conditions.
-Different mineral assemblages in rocks metamorphosed under the same conditions implies different bulk compositions.

D. Petrogenetic grid

-Mineral assemblages depend on T, P, and pH2O: pPhase relations have been experimentally determined for many reactions; thus we can combine experimental data and field observations to deduce metamorphic conditions.

-This presents mineral assemblages for various T and P conditions, where pH2O = lithostatic pressure
-Now pick out diagrams that represent example. This gives T-P pathway of metmaorphism

-What if water isn''t the only fluid component? pH2O = 0.3. Now metamorphic path could range between two dashed lines.
-Interpretation: assemblages represent a "fossil geotherm".

E. ACF diagram

-Useful ternary diagram for common metamorphic minerals in rocks with quartz
-Start with a tetrahedron:

-SiO2
-A = Al2O3 + Fe2O3 - (Na2O + K2O)
-C = CaO
-F = FeO + MgO + MnO

-Project minerals to ACF face: Sil and Opx lie on edges of tetrahedron, so they project to vertices of triangle ; An, Gt, and Cpx lie on faces of tetrahedron, so they project to sides of triangle

E. Petrogenetic grid for facies classification

-Common assemblages of metamorphic minerals are given names
-Names then imply a certain range of T/P conditions
-Note boundaries are zones, not lines