Experimental studies of greenschist and amphibolite facies metamorphism of basic rocks

The transition from the greenschist to the amphibolite metamorphic facies is essentially defined on the basis of changes in the mineral assemblages found in basic rocks. In broad terms, the change is from a chlorite - actinolite - albite - epidote - quartz - bearing assemblage to one dominated by hornblende and plagioclase. The reactions involved in the transition are complex, and are manifested in cryptic compositional variation in co- existing phases as much as in actual phase changes.

To appreciate the significance of these compositional variations, the nature of the solid solutions in the various minerals must be understood. In this work, the clinozoisite - epidote solid solution series has been explored in an unrealistic divariant reaction and in a naturalistic multivariant reaction, and it is shown that the former yields data with a wider field of application. A preliminary examination of the edenite - tremolite substitution in calcic amphiboles has also been undertaken, using a simple iron -free divariant reaction.

One of the major factors controlling greenschist facies metamorphism, and producing changes in the phase assemblage, is fluid phase composition. A mixed volatile model, simulating basic rocks in the system CAFMS - H2O - CO2 has been derived from field observations, and, in this thesis, is quantified experimentally, at 5 Kb, in an iron -free system. Subsequently, using both established and assumed solid solution properties for the various minerals, and phase chemical data from natural Dalradian metadolerites, several alternative 'real rock' models have been constructed, and are distinguished, and judged on field evidence, and independent experimental work on the various titanium minerals found in the metadolerites.

The field of study is too large for a complete synthesis of the thesis work to be constructed, and so various convergent lines of research are suggested, to link the present isolated elements.