R.P. Esser, P.R. Kyle and N.W. Dunbar

Dept. of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801-4796, USA.

FAX: (575) 835-6436, e-mail: esser@mailhost.nmt.edu

⁴⁰Ar/³⁹Ar results from The Pleiades volcanic center indicate episodic eruptive activity between 830 ka and the present. The Pleiades (72°40’S 165°30’E) is a 13 kilometer long sequence of volcanic cones and domes, lava flows and associated pyroclastic rocks situated on the crest of the Transantarctic Mountains (TAM) at the head of the Mariner Glacier. The Pleiades is composed of silica-undersaturated alkalic lavas that range from primitive basanites/tephrites to highly evolved peralkaline trachytes. Four previously published conventional K/Ar ages for lavas from The Pleiades were young (<50 ka) but too imprecise (±125-465%) to be useful.

Fifteen ⁴⁰Ar/³⁹Ar incremental heating ages suggest that at least four episodes of volcanism occurred at The Pleiades center between 830 ka and present. Three trachytic cones erupted at ~830 ka mark the earliest activity at The Pleiades. The oldest dated lava flow (832±56 ka) is from a small cone ~100 meters north of Alcyone Cone. Two separate cones of approximately the same age, 832±56 and 826±4 ka, erupted ~2 km northeast and northwest of Taygete Cone, respectively. At 627±10 ka, phonolitic lavas erupted from a cone ~4 km northwest of Taygete Cone, marking the only occurrence of phonolite at The Pleiades. A ~300 ky year hiatus was followed by the eruption of less evolved lavas. A phonotephritic cone erupted at the northern edge of The Pleiades at 337±37 ka. A tephriphonolitic dike and associated lava flows erupted two kilometers to the east of Taygete Cone at 312±6 ka.

The most intense period of volcanic activity at The Pleiades center began ~100 ka. At 93±4 ka, a trachytic flow erupted ~3 km east of Taygete Cone at the eastern edge of The Pleiades. A significant phase of cone building occurred at Mt. Atlas, the largest volcanic cone at The Pleiades, at approximately 65 ka, as evidenced by two benmoreite lava flows (61±4 and 68±4 ka). At approximately 45 ka, lava flows were erupted on the western flank of Mt. Pleione (42±4 and 38±8 ka) and near the summit of Alcyone Cone (48±2 ka). On the west side of Mt. Pleiones, a benmoreitic lava from a series of interlayered benmoreitic and trachytic flows yields an age of 20±7 ka. The youngest activity (6±6 ka) at The Pleiades occurred at Taygete Cone, an endogenous dome of peralkaline trachyte. This near-zero age for Taygete Cone is consistent with evidence of recent volcanism, including fresh hydrothermal activity and compositionally similar pumice lapilli scattered over surfaces in the northern Pleiades.

Ash layers from explosive eruptions associated with dated Pleiades lavas are potentially useful as regional marker horizons. Preliminary geochemical data suggest that some englacial tephra from blue ice areas were erupted from The Pleiades. Although direct dating of these englacial layers has been problematic, further geochemical work may yield one-to-one correlations with ⁴⁰Ar/³⁹Ar dated Pleiades lavas, allowing them to be used as precise, reliable time-stratigraphic markers within the Antarctic Ice Sheet.