Chemical changes during retrogressive metamorphism of Lewisian granulite facies rocks from Coll and Tiree

Abstract

The chemical compositions of granulite and amphibolite facies representatives of four metamorphic rock types with different bulk compositions and origins, from a Precambrian metamorphic complex in North-west Scotland, are assessed in the light of its tectonic and metamorphic history. Several striking chemical differences exist, which are thought to have been imparted by retrogressive metamorphism of an early granulite facies complex in which all four rock types were present. The main changes incurred in the conversion of granulite facies rocks to amphibolite facies, and common to all four rock types, involved influx of K, Rb, Th and some other elements and loss of Ba, Sr, P and rare earth elements in the whole complex. Some redistribution of elements occurred. Continued retrogression of gneissic rocks to greenschist facies involved further geochemical changes.