Regional Metamorphic Zones

Mapping the Dalradian Supergroup In 1893, George Barrow mapped a sequence of highly deformed regionally metamorphosed rocks in the south eastern part of the Scottish Highlands. The metamorphism and deformation occurred during closure of the Iapetus Ocean and the Caledonian orogeny about 400 Ma. These Precambrian rocks are known as the Dalradian Supergroup. He used the first appearance of some of these metamorphic minerals which he termed index minerals, to draw isograds. Some of the minerals that crystallise at low grades are stable at higher grades so more than one index mineral can be found in one rock. He was able to map metamorphic zones using index minerals and isograds, which define the boundaries of the zones. Although he did not do all the mapping personally, the system he devised was named after him and the zones are called Barrovian zones. Index Minerals and Metamorphic ZonesThe chlorite zone represents low grade regional metamorphism. The rock is slate where most of the rock has recrystallised but some clay minerals may still exist. Schists develop as a result of increasing temperatures and pressures and can be found in both the biotite and garnet zone. The grain sizes increase with metamorphic grade. Schists formed at lower temperatures and pressures are composed of quartz, mucovite mica and biotite mica. Medium grade metamorphism results from higher temperatures and pressures and many schists formed at this grade contain garnet, and less commonly, kyanite porphyroblasts.Kyanite is typically found in gneisses and the kyanite zone represents high grade regional metamorphism. The silimanite zone represents high grade regional metamorphism with very high temperatures and pressures. The rocks are gneisses. Estimates based on the silimanite zone indicate a maximum temperature of about 700 degrees and maximum pressure of about 7kb. This pressure exists at a depth of aout 25km below the surface of the continental crust. It gives a geothermal gradient of about 28 degrees km-1.Quartz and plagioclase feldspar are stable throughout the whole range of grades. This makes them no use as index minerals.The Al2SiO5 polymorphs in regional metamorphismThe Al2SiO5 polymorphs kyanite and silimanite are found in regional metamorphic rocks. A rock formed at high pressure and low temperature may contain kyanite. A rock formed at high temperature or at high temperature and high pressure many contain silimanite, which can be found in contact and regional metamorphism, both of which can involve high temperature. With increasing metamorphic grade, regional metamorphism follows a path from kyanite to silimanite on the Al2SiO5 polymorph phase diagram.

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