Scotish Regional Metamorphism - Barrovian

The Dalradian Supergroup of the Grampian Highlands is a thick sequence of deformed and metamorphosed sediments (mainly marine) and volcanics deposited between approximately 800 Ma and 500 Ma. The Dalradian block is structurally bounded to the north by the Great Glen Fault and to the south by the Highland Boundary Fault (Fig. 1). The rocks were affected by the complex Grampian Orogeny, the result of the early stages of closure of the Iapetus Ocean during the early to mid- Ordovician.

George Barrow (1853 - 1932) was a British geologist and is one of the all-time superstars of metamorphic geology. In one of the most important metamorphic studies in history (published in 1893), he was the first to map a metamorphic field gradient by determining a sequence of metamorphic zones in the Scottish Highlands (see Fig. 2). Every first appearance of an index mineral was taken by Barrow as the beginning of a new metamorphic zone. The lines connecting the first appearance of a mineral and separating the zones are isograds. The age of peak metamorphism is now constrained to 465-470 Ma (i.e., Ordovician).

The underlying principles of metamorphic zones were later clarified by the Finnish geologist Pentti Eskola, who introduced the concept of the metamorphic facies. The zones as mapped by Barrow with increasing grade:

Zone of digested clastic mica (now the Chlorite Zone)
          quartz-chlorite-muscovite-plagioclase

   Biotite Zone
quartz-chlorite-biotite-muscovite-plagioclase

   Garnet Zone
quartz-muscovite-biotite-garnet-plagioclase

   Staurolite Zone
 quartz-muscovite-biotite-garnet-staurolite-plagioclase

   Kyanite Zone
 quartz-biotite-muscovite-garnet-kyanite-plagioclase

   Sillimanite Zone
      quartz-biotite-muscovite-garnet-sillimanite-plagioclase

Barrow believed the zones resulted from the heat from the small granitic intrusions found in the high-grade zones. Another geologist, C.E. Tilley, working on the same rocks in a different area suggested that the temperature of each zone was largely determined by the depth of burial. Tilley (1924) suggested that the isograds mark rocks originating under closely similar P-T conditions, essentially what we believe today.

We will spend a day retracing the steps of Barrow and examining the different textural and mineral assemblage evolution of the zones.