THE DEVELOPMENT OF CARBONATE.BEARING BIOTITE ISOGRAD ASSEMBI.AGES FROMTETE JAI..]NE CACHE BRITISH COLt.IMBIA, CANADA

ment were atalyzed following a structural study across the biotite isograd northeast of TSte Jaune Cache. Seven pelite and pelitic psammite samples show a restricted range in both their All(Mg*Fe) and Mg/(Mg*Fe) ratios, Somewhat similar values were obtained from the eighth sample, a calcareous pelite. Microprobo analyses show that the muscovites associated with biotite and cblorite have a consistent yet anomalous composition: they are enriched in Al and Na, and depleted in Mg and Fe. Thus they have a low phengite conient, and are similar to staurolite-almandine, or even kyanite-almandine sub-facies muscovites reported from elsewhere. In con-trast, the coexisting biotites and chlorites have com-positions which are normal for the observed meta-morphic grade. Two ferromagnesian carbonates, ankerite and siderite, seem to have equilibrated with the silicate assemblage. Partition equilibria favour depletion of the silicate system in Fe and Mg, and, as the musco-vite structure allows flexibility in the Al/(Mg + Fe) ratio, its composition is affected. The range of rock compositions able to crystallize biotite, above the level of the iwgrad, is thought to have been enlarged by the influence of carbonate on tJle muscovite composition. The preence of ankerite coexisting witJr calcite and siderite in at least 5 of the samples permits tempe-rature estimates to be made using the carbonate geothermometer (Rosenberg 1967). The temperatures of 340-370'C obtained are reasonable but are subject to some uncertainty due to the presence of minor Mn in the carbonates and the possible effects of variations in Pa2o:rmong samples during crystallization. INtnoouctloN In recent years data have become available on the mineralogical and mineral-chemical changes that occur in lower greenschist facie