Download metamorphic rocks supplement, geol 1010

METAMORPHIC ROCKS SUPPLEMENT, GEOL 1010 Changes That Occur Within Rocks During Metamorphism 1. Recrystallization - Pre-existing minerals grow larger. Recrystallization produces a rock that has fewer larger crystals than the pre-existing rock. Recrystallization is particularly important in the formation of marble and quartzite. 2. Neomorphism - New minerals form from chemical reactions between pre-existing minerals. During neomorphism, there is not necessarily any change in the chemical composition of rock; no atoms or molecules enter or leave the rock. The atoms present in the rock simply rearrange to form new minerals. This happens if the pre-existing minerals (especially minerals like clay) are chemically unstable at metamorphic pressure and temperature (P&T) conditions. They react to form new, stable minerals. The rock does not melt. The metamorphic minerals that form provide information on the conditions (P & T) of metamorphism. This means that you can estimate the P & T of metamorphism (especially T) of many rocks based on the minerals present in the rock. For example, the presence of biotite & garnet together in a rock indicates metamorphism at ~ 500oC (~850oF). However, some minerals are very resistant to neomorphism; a prime example is quartz. Neomorphism happens in many rocks during metamorphism, and it often occurs in combination with recrystallization. 3. Metasomatism – In metasomatism, material is added to or removed from the rock, and the chemical composition of the rock is changed. In nearly all cases, this is caused by hot water-rich fluids that contain dissolved material. The fluids flow through the rock and chemically react with it, leaving behind some of their dissolved material – and potentially carrying away other chemicals. Hot water-rich fluids are called hydrothermal fluids and metasomatism usually also can be called hydrothermal metamorphism. Many metamorphic rocks have experienced minor amounts of metasomatism, and a few have experienced extreme amounts. Many important ore/mineral deposits are a result of intense hydrothermal metamorphism. Hydrothermal metamorphism is often associated with the intrusion of magma, which provides the heat and causes the hydrothermal fluids to circulate by convection.

Other important changes that occur in rocks during metamorphism a. Foliations and lineations can form from deformation during metamorphism. b. Any pore space that was present in the rock – for example between sand grains in sandstone – is destroyed. The great pressure rocks enjoy causes the grains and crystals the rock to move closer together during recrystallization and neomorphism. In fact, because metamorphism creates a rock that has virtually no pore space and consists of crystals that have grown together and interlock as the crystals in an igneous rock do, they and igneous rocks are sometimes called crystalline rocks.

Metamorphic rocks handout, GEOL 1010, M. Bunds Instructor

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REGIONAL METAMORPHISM AT CONVERGENT PLATE BOUNDARIES

hi T/P

hi P/T hi T&P

hi T&P

Many regionally metamorphosed rocks that we see on the surface of the Earth formed at convergent plate boundaries. At convergent plate boundaries rocks that are susceptible to metamorphism, such as clay-rich sedimentary rocks, get buried deep in the crust during mountain building and then later are exposed as the mountains are eroded. Three main types of regional metamorphism occur at subduction zones (see the illustration above): 1.

2.

3.

Hi P/T. This means that the ratio of pressure to temperature is high; the rocks are metamorphosed at extreme pressure and moderate temperature. Blueschist (a schist made blue by the presence of the mineral glaucophane; we looked at a piece in class) often results. Hi P/T conditions often exist in the subducted oceanic crust because the rocks are subjected to extreme pressures as soon as they reach depths of ~40 or more kilometers (25 mi), but it takes millions of years for them to warm to the temperature that is present at that depth in most parts of the Earth. Hi T/P. This means that the ratio of temperature to pressure is high; the rocks are metamorphosed at moderate to high temperature but relatively low pressure. The rock greenschist can result. The region near the volcanoes and magmatic intrusions, shown as hi T/P in the figure above, tends to be relatively hot at shallow depths. This is due to the igneous activity that can occur there for tens of millions of years while subduction occurs near by. Hi T & P. The rocks deep in the roots of the mountains that form at a convergent plate boundary are metamorphosed at very high temperature and pressure. Many rocks in the Appalachian Mountains, on the East Coast, are examples. They were metamorphosed in the roots of large mountains that formed ~300 million years ago when the N. America and Eurasia collided. Since that time erosion has removed the overlying rocks and exposed the high grade rocks that we find there today.

Paired metamorphic belts: Both the hi P/T and hi T/P rocks at a subduction zone occur in elongate belts parallel to the subduction zone. They are called paired metamorphic belts. There is a great example in northern California (and they are present in many places along the subduction zones that ring the Pacific). The high P/T rocks (blueschists) occur in the Coast Ranges (near the coast), and the high T/P rocks are present along the western foothills of the Sierra Nevada Mountains. They record the fact that prior to the formation of the San Andreas Fault ~30 million years ago, there was a subduction zone along the coast of California for nearly 150 million years. (Recall that the Sierra Nevada batholith also formed over that subduction zone).

Metamorphic rocks handout, GEOL 1010, M. Bunds Instructor

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