Rb-Sr provenance dates of feldspar in glacial deposits of the Wisconsin Range, Transantarctic Mountains

Abstract

Glacial deposits in the Wisconsin Range (lat. 85° to 86°30′S, long. 120° to 130°W) of the Transantarctic Mountains include a deposit of till on the summit plateau at an elevation of 2,500 m above sea level and glaciolacustrine sediments along the Reedy Glacier. The plateau till and underlying sediments consist of six units that appear to record the replacement of ice-free, periglacial conditions by ice cap glaciation of pre-Pleistocene age. Alternatively, the plateau till may have been deposited by the East Antarctic ice sheet either when it was thicker than at present or when the Wisconsin Range was lower in elevation. Feldspar size fractions from the plateau till have Rb/Sr ratios that increase with grain size from 1.4 (67 to 125 µm) to 4.24 (500 to 1,000 µm). These size fractions define a straight line on a Rb-Sr isochron diagram and yield a date of 480 ± 21 Ma that is indistinguishable from the age of the granitic basement rocks of the Wisconsin Range dated previously. This result therefore supports the hypothesis that the plateau till was deposited by a local ice cap and suggests that the Wisconsin Range was sufficiently elevated to permit an icecap to form prior to the growth of the East Antarctic ice sheet. The glaciolacustrine sediments along Reedy Glacier probably were deposited in an ice-marginal melt-water pond along the margin of a temperate Reedy Glacier soon after the East Antarctic ice sheet first reached full size. The Rb/Sr ratios of feldspar in this sediment are lower than those of feldspar in the plateau till and range only from 0.46 to 0.66. Nevertheless, three feldspar fractions form a straight line on the Rb-Sr isochron diagram, the slope of which indicates a date of 576 ± 21 Ma. The difference in the date derived from the feldspar of the glaciolacustrine sediment may be caused by the presence of a component of Precambrian feldspar derived from the East Antarctic Shield.