PETROGENESIS OF LATE-STAGE, HIGH-K MAGMAS WITHIN A CONTINENTAL ARC: AN EXAMPLE FROM THE ROSS OROGEN, ANTARCTICA

Field, geochemical, and geochronological data from the Mt. Loke region of Antarctica provide insight into the youngest magmatic and tectonic events within the Ross orogeny of Antarctica. Bimodal high-K to ultrapotassic dikes and plutons crosscut earlier calc-alkaline plutons intruded during the main phase of subduction-related Ross orogeny magmatism, and are among the youngest intrusive suites in southern Victoria Land. Bulk rock major and trace element data indicate that mafic dikes form a high-K differentiation trend with high Mg# and LREE and LILE concentrations suggestive of an enriched mantle source. Low Nb/U, Th/U, and Ce/Pb ratios are consistent with introduction of LREE and LILE into the mantle source region via subduction of terrigenous sediments. U/Pb zircon geochronology and field relationships indicate that felsic porphyry dikes are younger than high-K mafic rocks. Major and trace element data indicate that the porphyry dikes likely represent differentiates of a common parental magma that evolved dominantly by plagioclase fractionation. These new data suggest that the Ross orogeny transitioned from dominantly calc-alkaline to high-K magmatism immediately prior to cessation of subduction, and that this was accompanied by melting of subcontinental lithospheric mantle enriched by previous subduction of terrigenous sediments.