Metamorphic Petrology

Midterm 2003 Name:

1. (20) The following 4 sketches show porpyroblasts. The dashed line on the top two shows the orientation of the foliation in the matrix of the rock. On the bottom two sketches, the dashed lines indicate oriented inclusions within the porphyroblast.

A. Show the sense of shear on each of the four porphyblasts with a pair of arrows.

B. Is the porphyroblast on the bottom-left pre-, syn-, or post-tectonic?

C. Is the porphyroblast on the bottom-right pre-, syn-, or post-tectonic?

2. (20) For each of the of the following rocks, state the igneous or sedimentary rock type that is the likely protolith. If it is impossible to accurately determine the protolith, simply state ≥Impossible≤:

D. Forsterite-talc-tremolite hornfels:

E. Garnet-amphibole-plagioclase schist:

F. Kyanite-garnet-biotite schist:

G. Calcite-wollastonite-diopside marble:

H. Quartzite:

I. Quartz-feldspar-biotite gneiss:

J. Andalusite-garnet-biotite hornfels:

K. Kyanite-garnet-biotite gneiss:

L. Quartz-feldspar-mica mylonite:

M. Diopside-Quartz-Dolomite gneiss

3. (40) While working on rocks next to an intrusion in central Idaho, you discover that there is a sequence of metamorphic assemblages in an impure dolomite. Far away from the intrusion, you observe the assemblage calcite (CaCO₃) - dolomite (CaMg(CO₃)₂ - quartz (SiO₂). Next to the intrusion, where the contact metamorphism was at the highest grade, is the assemblage calcite ≠ quartz ≠ tremolite (Ca₂Mg₅Si₈O₂₂(OH)₂. In between is the assemblage quartz - calcite ≠ talc (Mg₃Si₄O₁₀(OH)₂.

A. What is the best chemical system with which you can describe these three assemblages?

B. Assume that water and carbon dioxide are always present, so we can project from them. What three-component system can you use (projected from water and carbon dioxide) to describe the system?

C. Consider the three component system you describe above. How many phases constitute a divariant assemblage, the number of phases you would find in most rocks?

D. On the diagram below, graph the position of each of the five minerals:

E. Now draw the chemographic diagrams of the three assemblages. Start with the assemblage farthest from the intrusion, and finish with the one closest to the intrusion:

F. State the two reactions that relate the assemblages sequentially towards the intrusion:

Reaction 1:

Reaction 2:

4. (20) The ≥enthalpy≤ of a reaction is the heat it consumed by the reaction. As you might imagine, most reactions that take place spontaneously at any given temperature and pressure produce heat; in other words, the reaction releases heat to the surroundings.

However, not all reactions do this. Some reactions absorb heat (input of heat is required to make them happen at a given T and P). Is it possible for such a reaction to take place spontaneously (with no input of energy)? How is it possible?