Ig Met Pet Lecture 21: Metamorphism of Mafic Rocks

What are the four groups of metamorphic facies?

High pressure, medium pressure, low pressure, low grade

What is characteristic of high pressure facies?

low molar volume phases under conditions of high pressure

Where do blueschist facies occur?

Areas of low T/P gradients, characteristically developed in subduction zones

What is typical of medium pressure facies?

Conform to the typical geothermal gradient, with most exposed metamorphic rocks belonging to this category

What is the origin of granulites?

Deeply eroded cratons of Precamrbian age, as these rocks are not thought to form today

What is typical of low pressure facies?

Contact metamorphism, with very high geothermal gradient

Where do you find sanidinite, and how often

Xemoliths in basic magmas and the innermost portions of some contact aureoles, it is rare

What is typical of low grade facies?

Rocks often fail to recrystallize thoroughly, and equilibriums is not always attained

Are low grade facies always present?

No, they are not always represented, and greenschist is the lowest grade developed in many terranes

What should a traverse up grade through a metamorphic terrane follow?

One of several possible metamorphic field gradients, though it can cross through a sequence if extensive enough

What are the three metamorphic series?

Franciscan, Barrovian, Bucham/Aubukuma

Fransiscan series:

High P/T, zeolite, P-P, blueschist, eclogite

Barrovian series:

Medium P/T, zeolite, P-P, greenschist, amphibolite, granulite

Bucham/Aubukama series:

Low P/T, zolite to hornfels

What are zone-facies series useful for?

Comparing metamorphic rocks from different areas and categorizing metamorphic belts

What are the different types of basaltic protoliths?

Basalts, gabbros, mafic dikes, mafic greywacke

What is the chemical makeup of a mafic protolith?

High in Fe, Mg, and Ca, with 52-65 wt.% SiO2

What are major minerals in basaltic protoliths?

Plagioclase, amphibole, pyroxene, and olivine

What are methods to choose three components to represent protoliths?

Ignore components, combine components, limit rock type, or use projections

The AFC diagram

Illustrates metamorphic mineral assemblages in mafic and carbonate rocks, on a triangular diagram

What does the AFC diagram do?

Concentrates only on the minerals that appear or disappear during metamorphism, acting as indicators of metamoprhic grade

What do A, C, and F represent for basaltic protoliths?

A is Al2O3 + Fe2O3 - Na2O -K2O, C is moles of CaO -3.3 P2O5, and F is moles of FeO, MgO, and Mno

What happens to the rest of the components in ACF diagrams?

We project from minerals which contain them, eliminating them

Why is Na2O, K2O, and 3.3 P2O5 subtracted?

This accounts for K-feldspar and apatite

Why is water omitted from the ACF diagram?

It is assumed that it is perfectly mobile

What is necessary for a projected phase diagram to be truly valid?

The phases from which it is projected must be present in the mineral assemblages represented

Why are some groups of minerals represented as lines or fields on ACF diagrams?

They are solid solutions with ranges in composition

What do very low pressure facies typically require (to the point where they don’t always occur)?

unstable protoliths

What causes an unstable protolith?

Substantial chemical changes in protolith, due to circulating fluids

What type of metamorphism are very low pressure facies characteristic of for basaltic protoliths?

Burial or hydrothermal metamorphism

What can zeolite facies be considered for basalic protoliths?

Altered igneous rocks, or very low grade metamorphic rocks

What mineral reactions take place in the zeolite facies of basaltic protoliths?

Albite replaces more calcic plagioclase, chlorite appears, laumonite replaces heulandite

What is the typical mineral assemblage of zeolite facies in metabasalts?

Laumonite, calcite, quartz, albite, and chlorite

Prehnite-Pumpellite Facies in metabasalts

marked by the disappearance of laumonite, resulting in the creation of prehnite or prehnite and pumpellyite

What kind of reaction produces prehnite and pumpellyite?

Devolitalization

What is the mineral assemblage for P-P in metabasalts?

Prehnite, pumpellyite, quartz, albite, chlorite, phengite, and titanite

Where does the transition from zeolite to P-P occur?

around 250°C, and a depth of 3-13 km

Where does greenschist occur?

2-10 kb and 400-550°C

What are the two major changes from greenschist to amphibolite facies?

Albite to oligoclase and actinolite to hornblende

Where do amphibolites occur?

2-10 kb, 550-700°C

What occurs at the transition from amphibolite to granulite if aqueous fluid is present?

Migmatites form

Why are quartzo-feldspathic rocks more likely to form migmatites than mafic ones?

Mafic ones melt at higher temperatures, and may be drier

Where does granulite occur?

2-10 kb, >700°C

What are the two necessary conditions for granulite formation?

Unusually hot conditions and dry rocks

The progressive nature of metamorphic rocks says what about mineral assemblage progression?

That a rock at high grade probably progressed through a sequence of mineral assemblages

What does a clockwise PTt path indicate for metabasalts?

Initial phase of crustal thickening followed by isothermal exhumation/uplift

What tectonic settings correspond to a clockwise PTt path?

Subduction zones and collisional tectonic settings

What does a counter clockwise PTt path indicate for metabasalts?

Crustal heating and thickening due to intrusion and underplating of mantle magmas, followed by isobaric cooling

What tectonic environments are diagnostic of a counter clockwise PTt path?

Back arc basins, hotspots, or continental rifting