U-PB, O AND HF ISOTOPE EVIDENCE OF DURATION, SOURCES AND CRUSTAL SIGNATURES DURING GRANITE BATHOLITH MAGMATISM IN THE ROSS OROGEN, ANTARCTICA

Granitic batholiths of the ~500 Ma Ross Orogen in Antarctica are voluminous in scale, yet the pace of magmatism, along-strike temporal and geochemical trends, sources of melt, and tectonic setting are poorly known. Isotopic study of the Ross batholith can address these questions and also inform us about the age and composition of the largely ice-covered East Antarctic craton, which the batholiths penetrate near the Ross Sea in the Transantarctic Mountains. New U-Pb age data, coupled with whole-rock geochemical compositions and zircon O- and Hf-isotope analyses, from a large suite of ~40 samples taken between Byrd Glacier and the Thiel Mountains provide a wealth of new geochronologic, tracer and inheritance information. SHRIMP zircon U-Pb ages from these samples range from 476-549 Ma. Several give ages <490 Ma, which is generally considered to be the minimum age of Ross magmatism, and the oldest ages of ~550 Ma are similar to previous ages from outcrop samples but are substantially younger than constraints on early arc magmatism obtained from detrital zircon and glacial clast ages. Geochemically the Ross magmatic suite consists mostly of granites, with lesser granodiorites and tonalites, that show major and trace element characteristics of Cordilleran-type, calc-alkaline volcanic-arc magmas. They have uniform peraluminous compositions of both ferroan and magnesian affinity. Trace-element patterns and oxide mineralogy show that both HFSE-enriched I-type compositions and S-type compositions occur along the length of the belt. SHRIMP zircon δ¹⁸O values range from +5.4 to +11.5‰ and are generally uniform within respective samples. The lower values are equivalent to those of mantle zircon and range to crustally-derived magmatic values. Initial εHf compositions determined by LA-MC-ICP-MS range from -16 to +7, and most samples are uniform composition. Zircon xenocrysts indicate inheritance of older components of about 1.2, 1.6, 2.0 and 2.5 Ga. There is no apparent relationship between age and O- or Hf-isotope compositions, but a strong inverse correlation between εHf and δ¹⁸O indicates that more crustal-like O-isotope compositions correlate with less radiogenic Hf. Four samples of S-type granites that intrude Archean basement lie off this trend and indicate greater contributions of older, unradiogenic crust.