{"id":392,"date":"2018-08-24T15:02:34","date_gmt":"2018-08-24T19:02:34","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/10-4-metamorphic-facies-and-index-minerals-2\/"},"modified":"2023-07-04T12:57:55","modified_gmt":"2023-07-04T16:57:55","slug":"metamorphic-facies-and-index-minerals","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/metamorphic-facies-and-index-minerals\/","title":{"raw":"10.5 Metamorphic Facies and Index Minerals","rendered":"10.5 Metamorphic Facies and Index Minerals"},"content":{"raw":"

Metamorphic Facies<\/h1>\r\nIn any given metamorphic setting there can be a variety of protolith types exposed to metamorphism.\u00a0 While these rocks will be exposed to the same range of pressure and temperatures conditions within that setting, the metamorphic rock that results will depend on the protolith. A convenient way to indicate the range of possible metamorphic rocks in a particular setting is to group those possibilities into metamorphic facies<\/strong>. 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.\r\n\r\nFigure 10.34 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. Each patch of colour represents a range of temperature and pressure conditions where particular types of metamorphic rocks will form. Metamorphic facies are named for rocks that form under specific conditions (e.g., eclogite facies, amphibolite facies etc.), but those names don't mean that the facies is limited to that one rock type.\r\n\r\n[caption id=\"attachment_387\" align=\"aligncenter\" width=\"650\"]\"\" Figure 10.34<\/strong> Metamorphic facies and types of metamorphism shown in the context of depth and temperature. The metamorphic rocks formed from a mudrock protolith under regional metamorphism with a typical geothermal gradient are listed. Letters correspond to the types of metamorphism shown in Figure 10.36. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2016), CC BY 4.0. Image source.<\/a>[\/caption]\r\n\r\n
\r\n\r\n<\/a>Concept Check: Metamorphic Facies<\/strong>\r\n\r\n
\r\n\r\nFill in the missing words to complete the summary.\r\n\r\nMetamorphic \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 <\/span>\u00a0group together metamorphic rocks that form under \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 <\/span>\u00a0(hint:<\/strong> the same or different?) pressure and temperature conditions, but which have \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 <\/span>\u00a0(hint:<\/strong> the same or different?) parent rocks.\r\n\r\nThe groups are named for a single metamorphic rock that forms under those specific conditions. They can include \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 <\/span>\u00a0(hint:<\/strong> Many or no?) other metamorphic rocks.\r\n\r\nTo check your answers, navigate to the below link to view the interactive version of this activity.<\/strong>\r\n\r\n<\/div>\r\n[h5p id=\"122\"]\r\n<\/div>\r\nAnother 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 with depth in Earth. In most areas (green dashed line), the rate of increase in temperature with depth is 30 \u00b0C\/km. In other words, if you go 1,000 m down into a mine, the temperature will be roughly 30 \u00b0C warmer than the average temperature at the surface.\u00a0 In volcanic areas (yellow dashed line), the geothermal gradient is more like 40 to 50 \u00b0C\/km, so the temperature increases much faster as you go down. Along subduction zones (blue dashed line), the cold ocean lithosphere keeps temperatures low, so the gradient is typically less than 10 \u00b0C\/km.\r\n\r\nThe yellow, green, and blue dashed lines in Figure 10.34 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.\u00a0 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 by matching up the letters a<\/em> through e<\/em> in Figure 10.34 with the labels in Figure 10.35.\r\n\r\n[caption id=\"attachment_388\" align=\"aligncenter\" width=\"650\"]\"\" Figure 10.35<\/strong> Environments of metamorphism in the context of plate tectonics: (a) regional metamorphism related to mountain building at a continent-continent convergent boundary, (b) seafloor (hydrothermal) metamorphism of oceanic crust in the area on either side of a spreading ridge, (c) 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. Source: Karla Panchuk (2018) CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.<\/a>[\/caption]\r\n\r\nOne other line to notice in Figure 10.34 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 water is 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 and crystallize again.\r\n
\r\n\r\n<\/a>Practice with Metamorphic Facies and Geothermal Gradients<\/strong>\r\n\r\nNote: It's okay to peek at the metamorphic facies diagram (Figure 10.34) if you need to.<\/em>\r\n\r\n
\r\n\r\nMatch the words into the correct boxes.<\/strong>\r\n\r\nThe \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> geothermal gradient is in subduction zones, because temperatures are \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> at depth than in other locations. Most subduction zone conditions fall within the \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> facies, named for a uniquely coloured foliated rock.\r\n\r\nThe \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> geothermal gradient is in volcanic regions because temperatures get \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> at shallower depths than in other locations. At the highest pressures and temperatures, the volcanic region geothermal gradient falls within the \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> facies.\r\n\r\nContact metamorphism falls within relatively \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> pressure conditions in the \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> facies. This is because as you go deeper, temperatures get too \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> for there to be a big contrast between magma and other rocks.\r\n\r\nFill-in-the-blank options:\r\n