Strontium and oxygen isotope study of lower crustal xenoliths, Royal Society Range, McMurdo Sound Region, Antarctica

Bibliography: pages [77]-85.Xenolith-bearing volcanic rocks of the Erebus Volcanic Province have been erupted across the Transantarctic Mountains - Ross Embayment boundary. Lower-crustal basic-granulite inclusions have been collected from a total of 11 volcanic centers on both sides of the Transantarctic Mountains - Ross Embayment boundary. Two-pyroxene and clinopyroxene-plagioclase granulites comprise the majority of the xenoliths found in the Transantarctic Mountains. The granulites contain plagioclase, augite, and orthopyroxene, with minor amounts of scapolite, apatite, spinel, potassium feldspar, quartz or garnet. Lower-crustal inclusions from the Ross Embayment are similar to the Transantarctic Mountains inclusions, except that some contain olivine, spinel is mainly found in reaction rims, and garnet is absent. Comparison of these samples with the results of experimental work by A.J. Irving in 1974 on a similar crustal inclusion from Australia indicates that the Transantarctic Mountains inclusions originated at greater depths (15-40 km) than the Ross Embayment inclusions (10-25 km), as described by J.H. Berg, R.A. Hank, D.L. Herz, and J.A. Gamble in 1984. The Sr and O isotopic ratios also vary across the Transantarctic Mountains - Ross Embayment boundary. The Ross Embayment lower-crustal inclusions yield lower values for (δ¹⁸)^O and ⁸⁷Sr/⁸⁶Sr ratios. The ⁸⁷Sr/⁸⁶Sr ratios for the lower-crustal xenoliths of the Transantarctic Mountains range from 0.708 to 0.716 but cluster in the range of 0.712 to 0.714, and the values for the same samples range from +9.3 to +12.6. The ⁸⁷Sr/⁸⁶Sr ratios for lower-crustal xenoliths of the Ross Embayment range from 0.700 to 0.711 but cluster in the range of 0.703 to 0.704, and their (δ¹⁸)^O values range from +5.3 to +9.4. The isotopic ratios for both oxygen and strontium are higher for the Transantarctic Mountains and distinctly different from the ratios for the Ross Embayment, thus indicating that the lower crust of the Ross Embayment is fundamentally different from the lower crust of the Transantarctic Mountains. The lower-crustal xenoliths of the Transantarctic Mountains and the Ross Embayment are isotopically different from the Jurassic Kirkpatrick Basalts. This implies that the lower crust originated either during the early Paleozoic Ross Orogeny or even earlier during the Precambrian.M.S. (Master of Science