Contact Metamorphism 2

The thermal gradient and index minerals in a metamorphic aureoleWhen a batholith is intruded into beds of shale, increases in metamorphic grades are marked by the appearance of an index mineral: index minerals are metamorphic minerals, which are stable under specific temperature and pressure conditions. They indicate the metamorphic grade. In contact metamorphism, biotite is the low grade mineral found in spotted rocks. The Al2SiO5 polymorph andalusite indicates medium grade and is found in andalusite rich rocks. Silimanite, another Al2SiO5 polymorph, indicates high grade and is found in hornfels. Because contact metamorphism is caused by temperature only, an increase in grade represents a thermal gradient. Some of the minerals that crystallise at low grades are stable at higher grade, so more than one index mineral can be found in one rock.The Al2SiO5 polymorphs in contact metamorphismThe Al2SiO5 polymorphs andalusite and silimanite are found in contact aureoles - andalusite is the low to medium temperature, low pressure polymorph found in andalusite slate, whereas silimanite is the high temperature polymorph found in hornfels. With increasing metamorphic grade, contact metamorphism follows a path from andalusite to silimanite on the Al2SiO5 polymorph phase diagram. Kyanite, the high pressure, low temperature polymorph, is not found in contact metamorphic rocks due to the lack of pressure.Formation quartzite and marbleWhen orthoquartzite, a sandstone composed entirely of quartz, is affected by contact metamorphism, all sedimentary structures including cross bedding and graded bedding are destroyed. The quartz grains in the sandstone recrystallise to form an interlocking mosaic of crystals giving it a granoblastic texture. Near to the contact with the igneous intrusion, in the zone of high grade metamorphism, the crystals are larger than they are further away from the contact where the temperatures are not as high. The resulting rock is white or pale grey in colour and known as metaquartzite. Where limestones are affected by contact metamorphism, all sedimentary structures and fossils are destroyed. The grains and cement are composed of calcite will recrystallise to form an interlocking mosaic of crystals giving it a granoblastic or sugary texture. This metamorphic rock is marble. Crystals are larger near to the contact with the igneous intrusion and smaller futher away, due to the thermal gradient. If the parent limestone is composed purely of calcite, the resulting metamorphic rock is white in colour. Impurites in the limestone may give streaks of different colours in the marble.

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