Petrology Final

The geothermal gradient is lower than average in a subducting plate.

TRUE

A facies should contain no more than 6 essential minerals.

FALSE. A mineral assemblage should contain no more than 6 essential minerals.

The effects of contact metamorphism are most evident in shallow, low pressure environments.

TRUE

Relict structures are not preserved in hornfels and granofels.

FALSE

Cordierite has a large molar volume, which indicates it’s stable at low pressures.

TRUE

Ordinary prograde metamorphic reactions are common in high-temperature mafic igneous rocks.

FALSE. Ordinary prograde metamorphic reactions are not possible in these rocks since they cannot be dehyrdated.

What is the agent associated with fault-zone metamorphism?

Shear Stress

Where might a low P/T baric series be found?

Contact metamorphic zone, High heat orogenic zone, rift zone

Metalbasites in the blueschist facies are characterized by the presence of a sodic, blue amphibole. Normally this is:

Glaucophane!

Which of the following minerals is likely to form porphyroblasts?

Garnet!

Eclogites associated with migmatic gneisses are usually associated with which temperature group? 

medium

The Buchan type of metamorphic trajectory is characteristic of which baric series? 

Low P/T

What mineral causes the “green” color in greenschist?

Actinolite, chlorite, epidote

The transition from granulite to eclogite is marked by the disappearance of one mineral and the appearance of another. Those means are, respectively: 

Plagioclase; Garnet

Zonal metamorphic patterns, in which rocks preserve a geographic sequence of increasing grade, suggests that retrograde metamorphism is not terribly significant. Since cooling in large metamorphic complexes should be slow, time should not be a significant factor in precluding retrograde reactions. What might inhibit them? 

Retrograde reactions are exothermic. However, this assumes the fluids (water or carbon dioxide) are available for rehydration or recarbonation reactions. In most cases, the fluids are drive off during prograde reactions, and are not available.

The transition from greenschist to amphibolite is marked by the appearance of two different minerals, either oligoclase or hornblende. In high pressure sequences (Barrovian) hornblende appears before oligoclase. In lower P/T terranes (Buchan), and in contact aureoles, the plagioclase appear first. Explain with this occurs 

High pressure and high temp = calcium rich

sodium and potassium = lower pressure and temperature

Hornblende is appearing because actinolite is disappearing. Lower pressure plagioclase is coming from a potassium rich feldspar, mother is albite.

zeolite phases

lower pressure and temp

Hornfels

attached to aurole metamorphism, close, localized

Granulite

contact metamorphism