BASALTIC MAGMA GENESIS AND MANTLE SOURCES BENEATH TWO CENOZOIC VOLCANOES, MT. EARLY AND SHERIDAN BLUFF, EAST ANTARCTICA

Mt. Early and Sheridan Bluff are two Early Miocene (20-19 Ma) volcanoes located near 87° South latitude in the southern Transantarctic Mountains. These two volcanoes are geospatially isolated from all other exposed Cenozoic volcanism by ~1000 km and lie ~175 km inland from the shoulder of the West Antarctic Rift System (WARS). The basaltic lavas are unique in comparison with WARS, such that subalkaline (tholeiite) to alkaline (hawaiite and mugearite) magmas were nearly simultaneously generated while WARS volcanism is almost exclusively alkaline.

The major objective of this study is to unravel the petrogenesis of basaltic magmas at Mt. Early and Sheridan Bluff. Isotopic data (Sr, Nd, Pb, O) added to the previous studies provide insight into mantle sources and the cause of melting beneath the East Antarctica craton. The δ¹⁸O of olivine is mildly elevated (average = 5.56 ± 0.14 ‰) relative to olivine in WARS basalts (average=5.12‰). The samples have consistently more radiogenic Sr, lower Nd and a narrower range of Pb isotopic values that fall outside of the range of the isotopically defined PREMA/FOZO mantle source relative to WARS basalts. There are no systematic variations between isotopes and whole rock geochemistry expected for models of combined assimilation and fractional crystallization (AFC). Therefore, it is unlikely that magmas were contaminated within the crust during ascent. Alternatively, the enriched component must be from crustal materials lying within mantle lithosphere or within the sub-lithosphere. Models of mixing between mantle endmembers indicate that the EMII source could be a significant contributor to the enriched isotopic signature. A possible mechanism for mantle melting is lithospheric delamination which occurred beneath this portion of the Transantarctic Mountains and could have recently (c. 20 Ma) added crustal materials into the mantle or alternatively these materials could have been added much earlier via subduction. The tholeiitic magmas, which contain a greater proportion of EMII, erupted nearly simultaneously with the alkaline magmas at Sheridan Bluff. Models indicate that alkaline magmas were produced by lower degrees of melting relative to tholeiite. The conditions required for variable melt production and tapping of different mantle melt sources are being investigated.