Barrow believed the zones resulted from the heat from the small granitic intrusives found in the high grade zones - Contact Metamorphism
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 (geothermal gradient), modified at depth by heat from the intruded granites.
P - T Grid for Dalradian Sequence
Tilley (1924) suggested that the isograds mark rocks originating under closely similar physical conditions of temperature and pressure - not greatly different from what we believe today.
The first appearance of an index or zone mineral indicates a definite metamorphic grade, as long as the rocks were of an appropriate composition for that mineral to grow.
The appearance of a particular mineral depends on the following variables:
The last variable is the most important factor in the production of a given mineral under different metamorphic conditions.
An isograd mapped within a single, homogeneous layer closely approaches the idealized concept of a line of equal grade.
Correlation of an isograd from one area with one mapped in a widely separated area, can in rocks of similar composition be hazardous as each rock may have formed under different conditions at different times.
A Finnish geologist, Eskola, was the first to attempt to correlate metamorphic zones between widely separated areas.
Working in Finland, Eskola found that contact metamorphic assemblages were related to grade and composition.
When he compared his results to others he found that some mineral assemblages were the same between areas, while in other areas in rocks of similar composition the assemblages were quite different.
Led Eskola to calculate that rocks from the two different areas were metamorphosed under different conditions, e.g., P & T and prompted him to propose a classification of metamorphic rocks - an association of metamorphic rocks, each consisting of a mineral assemblage consistently related to the composition of the rock - Facies.
Handout shows the relationship between metamorphic zones and metamorphic facies. Not only for pelitic rocks as mapped by Barrow, but also for Basic and Calcerous rocks.
If all the rock types in the table were interbedded correlation of the mineral assemblages within each, rock type would be very simple.
Such a correlation, for several rock types within a single metamorphic grade, defines the metamorphic facies for that grade.