13.8: Metamorphic Rocks of California
Metamorphic Rocks of California
Most soapstones Links to an external site. lack foliation and contain the mineral talc with lesser amounts of chlorite, pyroxenes, micas, amphiboles, carbonates, and other minerals (Figure 13.16). Recall that talc feels greasy and is incredibly soft. These properties are retained in the rock, and you will be able to feel the soapy-ness of the sample as well as scratch it with your fingernail. Typically, soapstone forms at subduction zones where heat, hot fluids (water), and directed pressure metamorphose protoliths like peridotites, dunites, and serpentinites. Many of the soapstones in California are found in the Santa Catalina Schist Links to an external site. on Santa Catalina Island and the Sierra Pelona Schist near Palmdale.
Figure 13.16: Soapstone. Notice the distinct pearly luster. Will feel soapy. (CC-BY 4.0; Chloe Branciforte)
Serpentinite Links to an external site. also lacks foliation and is composed mainly of minerals from the serpentine group Links to an external site. (Figure 13.17). Serpentinization Links to an external site. occurs at subduction zones where heat, hot fluids (water), and directed pressure metamorphose peridotite or dunite. The name serpentinite alludes to the rocks mottled green, snake-skin appearance. This rock is significant because it is California’s state rock Links to an external site.. Concentrations of serpentinite exist in the Franciscan Complex Links to an external site., Coast Ranges, the Trinity Ophiolite Links to an external site., portions of the Sierra Nevada and Klamath Mountains.
Figure 13.17: Serpentinite, California’s state rock. Notice the mottled greenish-color. (CC-BY 4.0; Chloe Branciforte)
At subduction zones, the oceanic crust is relatively cool in comparison to the mantle, especially along its sea-floor upper surface. The metamorphism that takes place here is unique, with high-pressure but relatively low-temperature conditions. At these locations a mineral called glaucophane Links to an external site., which is blue in color, forms. This mineral is an important component of the rock known as blueschist Links to an external site. (Figure 13.18).
Figure 13.18: Blueschist from the Franciscan Complex near Jenner, California. (CC-BY 2.0; James St. John via Flickr Links to an external site.)
As blueschist continues to subduct, it will eventually transform into eclogite Links to an external site. at a depth of approximately 35 km (22 miles) (Figure 13.19). The eclogite will continue to sink deep into the mantle, never to be seen again because the rock will eventually melt. In only a few places in the world, where the subduction process has been interrupted by some other tectonic process, has partially subducted blueschist rock returned to the surface. One such place is the area around San Francisco; the rock here is known as the Franciscan Complex Links to an external site..
Figure 13.19: Eclogite from the Franciscan Complex, near Valley Ford, California. This eclogite formed when oceanic crust was subducted and cooked underneath the North American continental lithosphere during the Late Jurassic (~155-158 mya). (CC-BY 2.0; James St. John via Flickr Links to an external site.)
|
Metamorphic Rock |
Composition and Test |
Metamorphic Environment & Grade |
Protolith |
|
Serpentinite |
Serpentine Patchy green, waxy appearance, might scratch glass (H3-6) |
Regional - Subduction zone, Hydrothermal low to intermediate |
Peridotite Dunite Serpentinite |
|
Soapstone |
Talc Soft (H=1), greasy feel; pearly luster |
Regional - Subduction zone, Hydrothermal low to intermediate |
Dunite Peridotite |
|
Blueschist |
Blue color |
Regional - Subduction zone Low temperature, High pressure |
Basalt Gabbro |
|
Eclogite |
Dense; may have garnet present; may be greenish in color |
Regional - Subduction zone Low temperature, High pressure |
Basalt Gabbro |