In any given metamorphic setting there can be a variety of parent-rock types exposed to metamorphism. While these rocks will be exposed to the same range of pressures and temperatures, the metamorphic rock that results will depend on the parent rock. A convenient way to indicate the range of possible metamorphic rocks in a particular setting is to group those possibilities into metamorphic facies. In other words, a given metamorphic facies groups together metamorphic rocks that form under the same pressure and temperature conditions, but which have different protoliths.
Figure 10.26 shows the different metamorphic facies as patches of different colours. The axes on the diagram are temperature and depth; the depth within the Earth will determine how much pressure a rock is under, so the vertical depth axis is also a pressure axis. Therefore, each patch of colour represents a range of temperatures and pressures where particular types of metamorphic rocks will form. The metamorphic facies are named after rocks which form under those particular conditions (e.g., eclogite facies, greenschist facies, amphibolite facies etc.), but those names don’t mean that the facies is limited to that one rock type.
Another feature to notice in the diagram are the many dashed lines. The yellow, green, and blue dashed lines represent the geothermal gradients in different environments. Recall that the geothermal gradient describes how rapidly the temperature increases as you go deeper into the Earth. In most areas (green dashed line), the rate of increase in temperature with depth is 30°C/km. In other words, if you go 1,000 m down into a mine, the temperature will be roughly 30°C warmer than the average temperature at the surface. In volcanic areas (yellow dashed line), the geothermal gradient is more like 40° to 50°C/km, so the temperature rises much faster as you go down. Along subduction zones (blue dashed line), the cold oceanic crust keeps temperatures low, so the gradient is typically less than 10°C/km.
In the context of these diagrams, the yellow, green, and blue dashed lines tell you what metamorphic facies you will encounter for rocks from a given depth in that particular environment. A depth of 15 km in a volcanic region falls in the amphibolite facies. Under more typical conditions, this is the greenschist facies, and in a subduction zone it is the blueschist facies. You can make the connection more directly between the metamorphic facies and the types of metamorphism discussed in the previous section. Notice the letters a through e in Figure 10.26. These match up with the labels in Figure 10.27 below, and can also be found in Figures 10.18, 10.19, 10.21, and 10.24 in the previous section.
One other line to notice in Figure 10.26 is the red dashed line on the right-hand side of the figure. This line represents temperatures and pressures where granite will begin to melt if there is water present. Migmatite is to the right of the line because it forms when some of the minerals in a metamorphic rock begin to melt, and then cool again.
Exercise 10.3 Metamorphic Rocks in Areas with Higher Geothermal Gradients
Figure 10.26 shows the types of rock that might form from mudrock at various points along the curve of the “typical” geothermal gradient (dotted green line). Looking at the geothermal gradient for volcanic regions (dotted yellow line in Figure 10.26), estimate the depths at which you would expect to find the same types of rock forming from a mudrock parent.
|Metamorphic Rock Type||Depth (km)|
Some common minerals in metamorphic rocks are shown in Figure 10.28, arranged in order of the temperature ranges within which they tend to be stable. The upper and lower limits of the ranges are intentionally vague because these limits depend on a number of different factors, such as the pressure, the amount of water present, and the overall composition of the rock. Even though the limits of the stability ranges are vague, the stability range of each mineral is still small enough that the minerals can be used as markers for those metamorphic conditions. Minerals which make good markers of specific ranges of metamorphic conditions are called index minerals. When geologists are examining the metamorphic rocks in a region, they can use the index minerals to map out zones that experienced different pressures and temperatures.
The Meguma Terrane of Nova Scotia: An Example of How to Use Index Minerals
The southern and southwestern parts of Nova Scotia were regionally metamorphosed during the Devonian Acadian Orogeny (around 400 Ma), when a relatively small continental block (the Meguma Terrane) was pushed up against the existing eastern margin of North America. The clastic sedimentary rocks within this terrane were variably metamorphosed. Figure 10.29 is a map of zones where different index minerals can be found. This allows us to see where metamorphism was stronger or weaker.
The strongest metamorphism (the sillimanite zone) is in the southwest. Progressively weaker metamorphism exists toward the east and north. The rocks of the sillimanite zone were likely heated to over 700°C, and therefore must have buried to depths between 20 km and 25 km. The surrounding lower-grade rocks were not buried as deep, and the rocks within the peripheral chlorite zone were likely not buried to more than about 5 km.
A probable explanation for this pattern is that the area with the highest-grade rocks was buried beneath the central part of a mountain range formed by the collision of the Meguma Terrane with North America. As is the case with all mountain ranges, the crust became thickened as the mountains grew, and it was pushed farther down into the mantle than the surrounding crust. This happens because Earth’s crust is floating on the underlying mantle. As the formation of mountains adds weight, the crust in that area sinks farther down into the mantle to compensate for the added weight. The likely pattern of metamorphism in this situation is shown in cross-section in Figure 10.30a. The mountains were eventually eroded (over tens of millions of years), allowing the crust to rebound upward and exposing the metamorphic rock (Figure 10.30b).
Building a narrative for the metamorphism in Nova Scotia’s Meguma Terrane is just one example of how index minerals can be used.
Exercise 10.4 Scottish Metamorphic Zones
The map shown here represents the part of western Scotland between the Great Glen Fault and the Highland Boundary Fault. The shaded areas are metamorphic rock, and the three metamorphic zones represented are garnet, chlorite, and biotite.
Label the three coloured areas of the map with the appropriate zone names (garnet, chlorite, and biotite).
Indicate which part of the region was likely to have been buried the deepest during metamorphism.
British Geologist George Barrow studied this area in the 1890s and was the first person anywhere to map metamorphic zones based on their mineral assemblages. This pattern of metamorphism is sometimes referred to as “Barrovian.”
- Facies is a Latin word meaning form, appearance, or face, and in geology it is used to specify a group of rocks. Sedimentary facies, for example, group all the sedimentary rocks that form in a particular depositional environment. ↵
- No, it’s not a spelling mistake! A terrane is a distinctive block of crust that is now part of a continent, but is thought to have come from elsewhere, and was added on by plate-tectonic processes. ↵