Tremolite-hornblende relationships are reported for high grade and relatively low grade zones within the Barrovian type Skagit Suite, using rocks that range widely enough in composition to qualify as potential hosts of any calcic amphibole from practically A1-free to A1-rich types. Out of over 100 samples analysed by microprobe, 22 representative analyses are listed, with end-member calculations.
In the sillimanite grade core of the Skagit Suite, calcic amphiboles from various, commonly metasomatized metamorphic ultramafics and genetically related hornblendites, from amphibolites, schists and gneisses, and from some metamorphosed impure dolomites show continuous solid solution between tremolite and highly aluminous hornblendes ranging from almost Fe-free to moderately Fe-rich types. A1total/A1IV is nearly constant and approximates 1·44. The second group of calcic amphiboles studied is from the lower-medium range of the epidote amphibolite facies (comprehensively defined), that is, from near and above the oligoclase isograd. Host rocks are variously metasomatized meta-peridotites, and amphibolites and schists. There is a large compositional gap between analyses of tremolites and of moderately to highly aluminous hornblendes. A1total/A1IV approximates 1·73 both in the tremolite and hornblende fields, as against 1·44 at high grade. Fe-poor hornblendes, such as are stable at high grade, were not found at the lower grade. At both grades, Al shows good overall correlation with Na+K, with A occupancy, and with Ti (with more Ti at high grade).
The compositional gap between tremolite and hornblende analyses from the lower grade rocks does not necessarily define a solvus because no tremolite-hornblende pairs were found. Rather, the analyses provide outer limits on the possible width of a solvus at this grade (T). However, certain data suggest that a true miscibility gap not only exists but probably is not a great deal narrower than the gap between the actual analyses. Besides, the proposed restriction on A1/Fe ratios at this grade would reduce the probability of finding tremolite-hornblende pairs.