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The Pleiades Volcanic Field is made up of some 20 monogenetic, partly overlapping scoria and spatter cones, erupted in the last 900 ka, cropping out from the ice close to the head of the Mariner Glacier in northern Victoria Land, Antarctica. Erupted products vary from hawaiite to trachyte, defining a complete mild Na‐alkaline differentiation trend. Mafic samples are characterized by multi‐elemental patterns typical of OIB magmas, moderately low 87Sr/86Sr (0.7037) and high 143Nd/144Nd (0.51284), with a clear within‐plate affinity, indicating a subcontinental lithospheric source. With increasing SiO2, 87Sr/86Sr ratios increase up to 0.7052 and 143Nd/144Nd decrease to 0.51277, supporting the hypothesis of open‐system evolution, with significant crustal assimilation during fractional crystallization. The erupted volume of most evolved products (∼7 km3), according to fractionation models, suggests that primitive magmas should have been more than 10 times larger, indicating the occurrence of a large magma plumbing system, unexpected for a volcanic field of monogenetic scoria cones. The occurrence of a complete fractionation trend with large magma chambers and large assimilation rate is unusual, if not unique, among the alkali basaltic volcanic fields and it is matched by a climax of activity during the last glacial maximum (30 ka), as indicated by new 40Ar‐39Ar ages (30 ± 3 ka and 25 ± 2 ka) for samples from the two most prominent edifices. Therefore, we hypothesize a role of a thick ice cap in suppressing eruptions and ultimately leading to prolonged magma residence time in the subsurface, favoring significant fractionation coupled with unusual high rates of crustal assimilation. Plain Language Summary: The Pleiades volcanic field is made up of some 20 monogenetic scoria and spatter cones, which erupted in the last 900 ka close to the head of the Mariner Glacier in northern Victoria Land, Antarctica. The erupted products are very unusual for alkali basaltic volcanism: indeed, whereas few samples show clear within‐plate subcontinental lithospheric characteristics and were directly derived from the mantle source, most of the products formed after extensive fractional crystallization matched by significant crustal assimilation, implying that primitive magma volumes are 10 times larger than outcropping products in an unusually large magma plumbing system. These peculiar features coincided with a climax of activity during the last glacial maximum (30–25 ka). Therefore, we speculate that a thick ice cap favored high rates of crystal fractionation coupled with crustal assimilation and was responsible for increasing magma residence times in chambers at crustal depths and suppressing the eruptive potential of magmas. Key Points: The Pleiades complex (NVL, Antarctica) is made up of some 20 monogenetic cones aged 900–0 ka, defining a complete Na‐alkaline trendFractionation models show much larger volumes of primitive magmas, indicating the occurrence of an unusually large magma plumbing systemA climax of activity occurred during the last glacial maximum (30 ka). Thickness variation of the ice cap may have influenced volcanic activity [ABSTRACT FROM AUTHOR]

The northern Antarctic Peninsula region has undergone ~10 m.y. of eruptive activity by basaltic volcanoes, mainly in subglacial settings. Spectacular exposures of lava-fed deltas, capped by basalt flows and commonly underlain by glacigenic sediments on top of a Cretaceous sedimentary 'basement,' characterize James Ross, Vega, and other islands and promontories in the region. Neogene strata are collectively known as the James Ross Island Volcanic Group and record a cryptic history of glaciation, with the timing of events determinable by argon-isotope dating. Focusing especially on the glacigenic sediments themselves, and their relationships with overlying or bounding volcanic rocks, we define facies associations related to (1) eruptions beneath thick ice (>200 m) that produced lava-fed deltas resting on, and intermingling with, diamictite; and (2) eruptions under marine conditions that typically culminated in the development of several tuff-cone successions, some on top of presumably relict glacially striated surfaces. A combination of provenance studies on clasts in the glacigenic sediments, some of which are derived from the Antarctic Peninsula, and geochronology, leads to the conclusion that an Antarctic Peninsula Ice Sheet extended over James Ross and Vega Islands at about the time that the main volcanic edifices began to grow, i.e., prior to ca. 6.2 Ma at least. Much of the subsequent development of the succession is attributed to the interaction between the growing volcanoes and local ice caps. Full resolution of glacial-interglacial events in this region promises to inform the debate about the stability of the most climatically sensitive part of the Antarctic Ice Sheet during the Neogene Period. Keywords: Neogene glacial fluctuations, James Ross Island, Antarctic Peninsula Ice Sheet, glacier-volcano interaction, glacigenic sediments, lava-fed deltas, argon dating, strontium dating.