Oxygen isotope evidence for crustal assimilation and magma mixing in the Granite Harbour Intrusives, Northern Victoria Land, Antarctica

The stable isotope composition (O,H) of whole-rock and mineral separates of Cambrian-Ordovician gabbros, diorites, granodiorites and granites forming the Mt. Abbott composite intrusions (Northern Victoria Land, Antarctica) was measured to constrain the origin and evolution of the magmas postdating the Ross Orogen. The 18O values of olivine gabbros plot in the field of slightly evolved mantle-derived melts (18OWR=6.8–7.4‰). The O-isotope character of the mantle source inferred from the 18O values of cumulous olivine in gabbros (5.7–6.8‰) is enriched in18O compared to modern arc-related magmas. Geochemical data and concurrent high 18O values, and initial strontium (87Sr/86Sr=0.7060) and neodymium (143Nd/144Nd=0.5122) isotope ratios indicate that the olivine gabbros formed by crustal contamination of a primary calc-alkaline basaltic melt. The diorites have high 18O values, among the highest ever measured for dioritic rocks (8.7–10.3‰), and Sr-isotope ratios that partially overlap with the adjacent and mingled felsic lithologies (0.708–0.710). The diorites have pyroxene with high, nearly constant 18O values (8.2–8.6‰) that are independent from the silica content of the rocks; thus, they did not increase in response of the chemical evolution of the rocks. The diorites originated from the same primary calc-alkaline basalt experiencing different amounts of crustal contamination, and underwent different degrees of mixing with the adjacent granites, producing granodioritic facies and quartz/feldspar xenocrystic diorites. The 18O, 87Sr/86Sr and 143Nd/144Nd compositions of the granites and granodiorites overlap (10.8–12.1‰, 0.7096–0.7108, 0.5119–0.5120). They are distinct from the values of the mafic rocks and indicate that gabbros and granites were not cogenetic. The granites are a separate melt component likely derived from nonmodal partial melting of fertile meta-igneous protoliths.