U-Pb, O and Hf isotope evidence of duration, sources and crustal signatures during granite batholith magmatism in the Ross Orogen, Antarctica

Abstract:

Voluminous granitic batholiths of the ~500 Ma Ross Orogen in Antarctica are enigmatic in their pace of magmatism, temporal and geochemical trends, sources of melt, and tectonic setting. Isotopic study can address these questions and also reveal the age and composition of the ice-covered East Antarctic craton, which the batholiths intrude in the Transantarctic Mountains. New U-Pb age data, coupled with whole-rock geochemical compositions and zircon O- and Hf-isotope analyses, from a geographically diverse suite of samples provide a wealth of new geochronologic, tracer and inheritance information. SHRIMP zircon U-Pb ages from these samples range from 476-549 Ma with several <490 Ma, which is typically considered the minimum age of Ross magmatism. 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.