40AR/39AR CHRONOLOGY OF THE PLEIADES VOLCANIC CENTER, NORTHERN VICTORIA LAND, ANTARCTICA, A POTENTIAL SOURCE OF LATE-PLEISTOCENE ENGLACIAL TEPHRA LAYERS

 

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 (7240S 16530E) 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 (83256 ka) is from a small cone ~100 meters north of Alcyone Cone. Two separate cones of approximately the same age, 83256 and 8264 ka, erupted ~2 km northeast and northwest of Taygete Cone, respectively. At 62710 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 33737 ka. A tephriphonolitic dike and associated lava flows erupted two kilometers to the east of Taygete Cone at 3126 ka.

The most intense period of volcanic activity at The Pleiades center began ~100 ka. At 934 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 (614 and 684 ka). At approximately 45 ka, lava flows were erupted on the western flank of Mt. Pleione (424 and 388 ka) and near the summit of Alcyone Cone (482 ka). On the west side of Mt. Pleiones, a benmoreitic lava from a series of interlayered benmoreitic and trachytic flows yields an age of 207 ka. The youngest activity (66 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.