Metamorphic index minerals: Some metamorphic minerals FIGURE 7.13 Estimated temperature-pressure can survive only under a specific range of temperature-pressure conditions in regionally metamorphosed shales (T-P) conditions. They are called index minerals because their from northern Scotland, superimposed on the presence in a rock indicates what the T-P range was during meta aluminosilicate phase diagram morphism 10 Index minerals provide a relative intensity scale; that is we know which of these minerals represents the highest Kyanite 3? metamorphic grade, but we can’t determine the precise inten- sity of the metamorphism experienced in terms of degrees Cel sius or kilobars of pressure. Estimates of absolute (numerical) T-P conditions are based on experimental work in which mate- rials with compositions similar to particular protoliths have been heated and squeezed to replicate conditions at which index minerals form. Estimates of metamorphic conditions for regionally metamorphosed shales in northern Scotland are 20 Sillimanite 10 2 Andalusite shown in FIGURE 7.13. S00 Index minerals were discovered when field geologists noted the change in mineralogy in wall rocks adjacent to plutons. Exer- cise 7.5 will guide you through the reasoning used by those nmetaNote that the garnet zone could be followed by either a ??? 400 600 500 700 Temperature (C morphic pioncers kyanite or a sillimanite zone, depending on the precise T-P conditions Index Minerals and Metamorphic Zones EXERCISE 7.5 Name Course: Section Date: It wasn’t until the early 20th century that geologists recognized that contact metamorphic minerals were not randomly distributed throughout the contact aureole (the area around a pluton affected by contact metamorphism). The figure below shows the distribution of minerals in the contact aureole around the Hartland pluton, a small granite batholith in central Maine. The field mapper has noted that the minerals chlorite, biotite, and andalusite appeared in metamorphosed shales at different distances from the pluton. Examine the figure on the right and answer the following questions C
(a) Chlorite and biotite are platy minerals. Do you think they are foliated in this contact aureole? Explain. Miles
(b) Where in the contact aureole would the highest temperatures have occurred? Explain your reasoning. Metamorphic zones | AndalusiteBiotiteChlorite Hartland granite (continued) (un) dag apog q) aanssaid Index Minerals and Metamorphic Zones (continued) EXERCISE 7.5 Section: Name: Course: Date:
(c) What does the distribution of minerals in the contact aureole suggest about the temperatures at which these mapped minerals are stable?
(d) Based on your answers above, list the three minerals mapped in this figure in order of increasing metamorphic grade. 1 2 3. Temperatures in the area containing biotite, but not chlorite or andalusite, must have been appropriate to form biotite, but either too hot or too cool to form chlorite and andalusite. Biotite is therefore an index mineral for the T-P conditions in that part of the aureole, and this area is called the biotite zone. Similarly, the chlorite and andalu- site zones represent areas in which those minerals were stable. Chlorite, biotite, and andalusite are therefore index minerals for metamorphosed shales. If limestones and dolostones had also been present in the aureole, they would have produced different index minerals because they don’t have the chemical composition needed to make these three minerals Line B in the figure separates the biotite and chlorite zones, which means it separates the areas that were under conditions at which chlorite was stable from areas that were under conditions at which biotite was stable. This line is called the biotite isograd (iso equal, grad = grade) because the metamorphic temperatures were exactly the same everywhere along it: just hot enough to make biotite. Similarly, dashed line A in the figure is the andalusite isograd. Metamorphic zones and isograds are also mapped in areas of regional metamorphism and may involve other meta- morphic index minerals.
What Do You Think Geologists in Africa have found economically valuable concentrations of chromite (the principal ore of chromium) that formed by magmatic differentiation in thick mafic sills. In the same area, there are also mafic lava flows that look very much like the sills, but which contain none of the valu- able minerals. An investment firm is considering purchasing mineral rights for a mountain near the chromite deposits that has numerous tabular bodies of mafic igneous rock. You have been hired as a consultant to determine whether these bodies are intrusive sills, and therefore potential chromite sources worth the investment, or extrusive flows that would not be worth the money.
What metamorphic evidence would you look for that would distinguish the two possibilities? On a separate sheet of paper, write a brief report to the firm explain- ing your reasoning and what you would expect to find in either of the two cases.