COEVAL ALKALINE AND SUBALKALINE MAGMATISM IN A CONTINENTAL ARC: GEOCHEMICAL AND ISOTOPIC INSIGHTS ON CONTRASTING MAGMA SOURCES IN THE ROSS OROGEN, ANTARCTICA

The Ross orogen is a 3500 km-long continental arc that formed during subduction of paleo-Pacific oceanic lithosphere beneath East Antarctica in the late Neoproterozoic− early Paleozoic. Plutonic rocks with strong geochemical and isotopic contrasts occur in the southern Victoria Land segment of the arc. Alkaline rocks, including nepheline syenites, carbonatites, and A-type granites, of the Koettlitz Glacier Alkaline Suite (KGAS) are bound to the north and south, respectively, by calc-alkaline granitoids in the Dry Valleys (DV) and Byrd Glacier (BG) areas. Geochronology reveals that magmatism initiated at different times in each region but confirms that alkaline magmatism in the KGAS was partially synchronous with typical calc-alkaline magmatism in the adjacent DV and BG areas.

A combination of whole-rock major- and trace-element geochemistry, zircon U-Pb and trace-element composition (measured simultaneously by laser-ablation split-stream analysis), zircon Hf isotopes, and whole-rock Sr and Nd isotopes (for select samples) was used to thoroughly classify ~75 igneous samples spanning ~450 km along the orogen. This "campaign-style" geochemical, geochronologic, and isotopic approach allows assessment of spatial and temporal variations in the source and style of magmatism along a major segment of the arc. Most magmatism in the DV area spanned a short period, from ca. 515−492 Ma, and records a restricted range of ɛHf from 0 to −6.8. Igneous rocks in the KGAS were emplaced between ca. 565 and 500 Ma and have a much wider range in ɛHf from +2 to −12. Most magmatism in the BG area also occurred over a restricted period of time, from ca. 550−530 Ma, followed by minor late-stage magmatism. A 13 ɛHf-unit range in primitive samples (<60 wt% SiO₂) reveals gross source heterogeneity. The most primitive rocks show light REE enrichment and high LILE/HFSE ratios and may have been sourced from variably-enriched sub-continental lithospheric mantle. Evolved rocks of large granitoid plutons have a similar range of Hf isotope compositions and may be differentiates (through large-scale fractional crystallization and remelting/remobilization processes) of mafic magmas sourced from the subarc lithospheric mantle, therefore representing addition and differentiation of large volumes of juvenile crust through arc magmatism.