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2 Geological Quarterly

2 2011

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Pliocene age of the oldest basaltic rocks of Penguin Island (South Shetland Islands, northern Antarctic Peninsula)

Pańczyk M., Nawrocki J.

Geological Quarterly

2011

Vol. 55, No. 4

335-344

The Penguin Island volcano is located on the southern shelf of King George Island (South Shetland Islands, West Antarctica). Its activity is regarded as connected with the opening of the Bransfield Strait. Penguin Island is dominated by a 180 m high basaltic stratocone (Deacon Peak) with a 350 m wide crater containing a small basaltic plug inside and radial dykes, and it has a second principal vent – the Petrel Crater maar – that was formed during a phreatomagmatic eruption about 100 years ago. A low-potassium, calc-alkaline sequence of basaltic lava flows with intercalations of beach deposits (Marr Point Formation) forms the basement of the stratocone. The Marr Point Formation lava flows have never been dated before. Combined whole rock 40Ar-39Ar isotopic dating and magnetostratigraphy were applied for this purpose. We obtained an isotopic 40Ar-39Ar plateau age of 2.7 š0.2 Ma, and together with the palaeomagnetic data, middle Pliocene age (Piacenzian) is implied for the basaltic plateau of Penguin Island.

Geochronology of selected andesitic lavas from the King George Bay area (SE King George Island)

Pańczyk M., Nawrocki J.

Geological Quarterly

2011

Vol. 55, No. 4

323-334

Volcanic rocks from the Lions Rump area, which are the basement for a sequence of glaciomarine sediments of the Polonez Cove Formation, and lava flows from the Turret Point–Three Sisters Point area were sampled for thermogeochronological and palaeomagnetic investigations. Generally, andesitic lavas from King George Bay area consist mainly of clinopyroxene (Ti-augite) orthopyroxene (hyperstene) and plagioclase phenocrysts. The groundmass comprises mostly plagioclase laths, clinopyroxene, titanomagnetite and rare orthopyroxene crystals. However, the modal content, size, shape and distribution of phenocrysts are variable and specific for each sample. The Ar-Ar plateaus ages calculated for lavas from the Lions Rump area are very homogenous and point to middle Eocene age (Lutetian, ~44.5 Ma). The similar and consistent ages for volcanic basement for that area excluded the thesis about separate tectonic evolution of the Warszawa and Kraków blocks at least since the middle Eocene. The lavas from Turret Point and Three Sister Point are younger and were emplaced during the late Eocene (Bartonian/Priabonian: 37.3 š0.4 Ma and Priabonian: 35.35 š0.15 Ma, respectively). The results of isotopic investigations are consistent with magnetic polarities of the rocks indicating that the samples from the Lions Rump area are coeval with the lower part of the C20 polarity chron whereas the sample from Turret Point can be correlated with the upper part of the C17 polarity chron