Studies of amphibolites and constituent hornblendes from an area of progressive metamorphism near Lead, South Dakota

Abstract

The progressive metamorphism and origin of the amphibolites of the Lead-Deadwood area, S. Dakota, are considered in detail. Eighty specimens of amphibolite were collected along a belt 3 miles long extending across the entire garnet zone and part of the staurolite zone of the enclosing meta-sediments. These amphibolites were studied petrographically and from them thirty-four were used for modal analysis and chemical analyses for total iron (as Fe2O3), CaO, TiO2, K2O, MnO and trace elements. Twelve of the constituent hornblendes were analysed for all major and minor elements and trace elements. The other 22 constituent hornblendes were analysed for total iron (as Fe2O3), CaO, TiO2, K2O, MnO and trace elements. Seven of the constituent horn-blendes and six constituent plagioclases were used for optical determinations and x-ray diffractometric studies. On the basis of relict texture some of the amphibolites are concluded to be definitely ortho-amphibolites. Field relations of a few hornblende-bearing rocks suggest that they have meta-sedimentary origin. The origin of the remaining majority of the amphibolites is inconclusive. The metamorphism of the amphibolites is essentially iso-chemical throughout the range of conditions represented by the appearance of successively the garnet and the staurolite isophases in the enclosing meta-sediments. What fluctuations in composition are observed appear to be random in character, attributable to minor variations in original composition or to retrograde alteration. The compositions of the constituent hornblendes do not show any significant correlation with intensity of metamorphism and the average composition remains roughly constant throughout the sequence. The concentrations of the various components in the horn-blendes seem to be closely related to the concentrations of the same components in the corresponding amphibolites. The ratios of the concentrations of the various components between horn-blende and amphibolite do not show any significant trend with metamorphic rank and are reasonably constant over the range of metamorphic conditions represented here. The composition of the hornblendes is closely related to specific gravity and broadly related to dimensions of unit cells but there is no close correlation between composition and optics. Size of unit cells in hornblendes appear to be unrelated to metamorphic rank.