Mineralogic variations across a thick amphibolite,Bighorn Mountains,Wyoming, USA

Studies of serial suites of samples collected along six sections measured across the width of an amphibolite body, some 750 meters thick, reveal (1) the presence of a hornblende concentration near the presumed base of the body, (2) an upward sodium-enrichment trend reflected by plagioclase, and (3) an upward iron-enrichment trend reflected by variation in 100Mg/Mg+Fe²+Fe³+Mn values for hornblende. Consistent with field evidence, the data indicate formation of the amphibolite by metamorphism of a tabular mafic igneous body. The mineralogic variations are suggestive of those expected during the differentiation of a mafic sill in which the trends are controlled by early crystallization and subsequent gravitational accumulation of pyroxene. In addition to indicating igneous parentage, these variations establish the stratigraphic sequence.