{"id":319,"date":"2019-06-11T14:50:02","date_gmt":"2019-06-11T14:50:02","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/7-3-plate-tectonics-and-metamorphism\/"},"modified":"2021-12-07T23:37:50","modified_gmt":"2021-12-07T23:37:50","slug":"7-3-plate-tectonics-and-metamorphism","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/7-3-plate-tectonics-and-metamorphism\/","title":{"raw":"7.3 Plate Tectonics and Metamorphism","rendered":"7.3 Plate Tectonics and Metamorphism"},"content":{"raw":"All of the important processes of metamorphism that we are familiar with can be understood in the context of geological processes related to plate tectonics. The relationships between plate tectonics and metamorphism are summarized in Figure 7.3.1, and in more detail in Figures 7.3.2, 7.3.3, 7.3.4, and 7.3.6.\r\n\r\n[caption id=\"attachment_312\" align=\"aligncenter\" width=\"800\"]\"\"<\/a> Figure 7.3.1 Environments of metamorphism in the context of plate tectonics: (a) regional metamorphism related to mountain building at a continent-continent convergent boundary, (b) regional metamorphism of oceanic crust in the area on either side of a spreading ridge, (c) regional metamorphism of oceanic crustal rocks within a subduction zone, (d) contact metamorphism adjacent to a magma body at a high level in the crust, and (e) regional metamorphism related to mountain building at a convergent boundary.[\/caption]\r\n\r\nMost regional metamorphism takes place within the continental crust. While rocks can be metamorphosed at depth in most areas, the potential for metamorphism is greatest in the roots of mountain ranges where there is a strong likelihood for burial of relatively young sedimentary rock to great depths, as depicted in Figure 7.3.2. An example would be the Himalayan Range. At this continent-continent convergent boundary, sedimentary rocks have been both thrust up to great heights (nearly 9,000 metres above sea level) and also buried to great depths. Considering that the normal geothermal gradient (the rate of increase in temperature with depth) is around 30\u00b0C per kilometre, rock buried to 9 kilometres below sea level in this situation could be close to 18 kilometres below the surface of the ground, and it is reasonable to expect temperatures up to 500\u00b0C. Metamorphic rocks formed there are likely to be foliated because of the strong directional pressure (compression) of converging plates.<\/a>\r\n\r\n[caption id=\"attachment_313\" align=\"aligncenter\" width=\"670\"]\"\"<\/a> Figure 7.3.2 Regional metamorphism beneath a mountain range related to continent-continent collision (typical geothermal gradient). (Example: Himalayan Range) [Image Description]<\/a>[\/caption]At an oceanic spreading ridge, recently formed oceanic crust of gabbro and basalt is slowly moving away from the plate boundary (Figure 7.3.3). Water within the crust is forced to rise in the area close to the source of volcanic heat, and this draws more water in from farther out, which eventually creates a convective system where cold seawater is drawn into the crust and then out again onto the sea floor near the ridge. The passage of this water through the oceanic crust at 200\u00b0 to 300\u00b0C promotes metamorphic reactions that change the original pyroxene in the rock to chlorite and serpentine. Because this metamorphism takes place at temperatures well below the temperature at which the rock originally formed (~1200\u00b0C), it is known as [pb_glossary id=\"1538\"]retrograde metamorphism[\/pb_glossary]<\/strong>. The rock that forms in this way is known as [pb_glossary id=\"1539\"]greenstone[\/pb_glossary]<\/strong> if it isn\u2019t foliated, or [pb_glossary id=\"1540\"]greenschist[\/pb_glossary]<\/strong> if it is. Chlorite ((Mg5<\/sub>Al)(AlSi3<\/sub>)O10<\/sub>(OH)8<\/sub>) and serpentine ((Mg,\u00a0Fe)3<\/sub>Si2<\/sub>O5<\/sub>(OH)4<\/sub>) are both \u201c[pb_glossary id=\"1541\"]hydrated minerals[\/pb_glossary]<\/strong>\u201d meaning that they have water (as OH) in their chemical formulas. When metamorphosed ocean crust is later subducted, the chlorite and serpentine are converted into new non-hydrous minerals (e.g., garnet and pyroxene) and the water that is released migrates into the overlying mantle, where it contributes to flux melting (Chapter 3, section 3.2).\r\n\r\n[caption id=\"attachment_314\" align=\"aligncenter\" width=\"706\"]\"\"<\/a> Figure 7.3.3 Regional metamorphism of oceanic crustal rock on either side of a spreading ridge. The dotted rectangles are the areas where metamorphism is taking place.\u00a0 (Example: Juan de Fuca spreading ridge)[\/caption]\r\n\r\nAt a subduction zone, oceanic crust is forced down into the hot mantle. But because the oceanic crust is now relatively cool, especially along its sea-floor upper surface, it does not heat up quickly, and the subducting rock remains several hundreds of degrees cooler than the surrounding mantle (Figure 7.3.4). A special type of metamorphism takes place under these very high-pressure but relatively low-temperature conditions, producing an amphibole mineral known as [pb_glossary id=\"1542\"]glaucophane[\/pb_glossary]<\/strong> (Na2<\/sub>(Mg3<\/sub>Al2<\/sub>)Si8<\/sub>O22<\/sub>(OH)2<\/sub>), which is blue in colour, and is an important component of a rock known as [pb_glossary id=\"1543\"]blueschist[\/pb_glossary]<\/strong>.\r\n\r\nYou\u2019ve probably never seen or even heard of blueschist; that's not surprising. What is a little surprising is that anyone<\/span> has seen it! Most blueschist forms in subduction zones, continues to be subducted, turns into [pb_glossary id=\"1544\"]eclogite[\/pb_glossary]<\/strong> at about 35 kilometres depth, and then eventually sinks deep into the mantle\u2014never to be seen again because that 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 is known as the Franciscan Complex (Figure 7.3.5).\r\n\r\n[caption id=\"attachment_315\" align=\"aligncenter\" width=\"607\"]\"\"<\/a> Figure 7.3.4\u00a0 Regional metamorphism of oceanic crust at a subduction zone. (Example: Cascadia subduction zone. Rock of this type is exposed in the San Francisco area.)[\/caption]\r\n\r\n[caption id=\"attachment_316\" align=\"aligncenter\" width=\"750\"]\"\"<\/a> Figure 7.3.5 Franciscan Complex blueschist rock exposed north of San Francisco. The blue colour of rock is due to the presence of the amphibole mineral glaucophane.[\/caption]\r\n\r\nMagma is produced at convergent boundaries and rises toward the surface, where it can form magma bodies in the upper part of the crust. Such magma bodies, at temperatures of around 1000\u00b0C, heat up the surrounding rock, leading to contact metamorphism (Figure 7.3.6). Because this happens at relatively shallow depths, in the absence of directed pressure, the resulting rock does not normally develop foliation. The zone of contact metamorphism around an intrusion is very small (typically metres to tens of metres) compared with the extent of regional metamorphism in other settings (tens of thousands of square kilometres).\r\n\r\n[caption id=\"attachment_317\" align=\"aligncenter\" width=\"700\"]\"\"<\/a> Figure 7.3.6 d: Contact metamorphism around a high-level crustal magma chamber (Example: the magma chamber beneath Mt. St. Helens.)\u00a0 e: Regional metamorphism in a volcanic-arc related mountain range (volcanic-region temperature gradient) (Example: The southern part of the Coast Range, B.C.)[\/caption]\r\n\r\nRegional metamorphism also takes place within volcanic-arc mountain ranges, and because of the extra heat associated with the volcanism, the geothermal gradient is typically a little steeper in these settings (somewhere between 40\u00b0 and 50\u00b0C per kilometre). As a result higher grades of metamorphism can take place closer to surface than is the case in other areas (Figure 7.3.6).\r\n\r\nAnother way to understand metamorphism is by using a diagram that shows temperature on one axis and depth\u2014which is equivalent to pressure\u2014on the other (Figure 7.3.7). The three heavy dotted lines on this diagram represent Earth\u2019s geothermal gradients under different conditions. In most areas, the rate of increase in temperature with depth is 30\u00b0C per kilometre. In other words, if you go 1,000 metres down into a mine, the temperature will be roughly 30\u00b0C warmer than the average temperature at the surface. In most parts of southern Canada, the average surface temperature is about 10\u00b0C, so at a 1,000 metre depth, it will be about 40\u00b0C. That\u2019s uncomfortably hot, so deep mines must have effective ventilation systems. This typical geothermal gradient is shown by the green dotted line in Figure 7.3.7. At a 10 kilometre depth, the temperature is about 300\u00b0C and at 20 kilometres it\u2019s about 600\u00b0C.\r\n\r\nIn volcanic areas, the geothermal gradient is more like 40\u00b0 to 50\u00b0C per kilometre, so the temperature at a 10 kilometre depth is in the 400\u00b0 to 500\u00b0C range. Along subduction zones, as described above, the cold oceanic crust keeps temperatures low, so the gradient is typically less than 10\u00b0C per kilometre. The various types of metamorphism described above are represented in Figure 7.3.7 with the same letters (a through e) used in Figures 7.3.1 to 7.3.4 and 7.3.6.\r\n\r\n[caption id=\"attachment_318\" align=\"aligncenter\" width=\"700\"]\"\"<\/a> Figure 7.3.7 Types of metamorphism shown in the context of depth and temperature under different conditions. The metamorphic rocks formed from mudrock under regional metamorphosis with a typical geothermal gradient are listed. The letters a through e correspond with those shown in Figures 7.3.1 to 7.3.4 and 7.3.6.[\/caption]\r\n\r\nBy way of example, if we look at regional metamorphism in areas with typical geothermal gradients, we can see that burial in the 5 kilometre to 10 kilometre range puts us in the zeolite[footnote]Zeolites are silicate minerals that typically form during low-grade metamorphism of volcanic rocks.[\/footnote]\u00a0and clay mineral zone (see Figure 7.3.7), which is equivalent to the formation of slate. At 10 to 15 kilometres, we are in the greenschist zone (where chlorite would form in mafic volcanic rock) and very fine micas form in mudrock, to produce phyllite. At 15 to 20 kilometres, larger micas form to produce schist, and at 20 to 25 kilometres amphibole, feldspar, and quartz form to produce gneiss. Beyond a depth of 25 kilometres in this setting, we cross the partial melting line for granite (or gneiss) with water present, and so we can expect migmatite to form.\r\n
\r\n

Exercise 7.3 Metamorphic rocks in areas with higher geothermal gradients<\/p>\r\n\r\n<\/header>\r\n

\r\n\r\nFigure 7.3.7 shows the types of rock that might form from mudrock at various points along the curve of the \u201ctypical\u201d geothermal gradient (dotted green line). Looking at the geothermal gradient for volcanic regions (dotted yellow line in Figure 7.3.7), estimate the depths at which you would expect to find the same types of rock forming from a mudrock parent.\r\n
    \r\n \t
  1. Slate<\/li>\r\n \t
  2. Phyllite<\/li>\r\n \t
  3. Schist<\/li>\r\n \t
  4. Gneiss<\/li>\r\n \t
  5. Migmatite<\/li>\r\n<\/ol>\r\nSee Appendix 3 for Exercise 7.3 answers.<\/a>\r\n\r\n<\/div>\r\n<\/div>\r\n

    Image Descriptions<\/h3>\r\n<\/a>Figure 7.3.2 image description:<\/strong> Regional metamorphism occurring beneath a mountain range due to continent-continent collision. The typical geothermal gradient for slate is 100\u00b0C, for phyllite 200\u00b0C, for schist 300\u00b0C, for gneiss \u00b0C, for migmatite 500\u00b0C. [Return to Figure 7.3.2]<\/a>\r\n

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