Volcanic sequences at Yankee Harbour, Greenwich Island, South Shetland Islands, Antarctica.

PARICA, C. A.; REMESAL, M. B.; SALANI, F. M.; RINALDI, C. A.

Alemania

Otro; 21th Colloquim on Latein American Earth Sciences; 2009

Greenwich Island is one of the islands of the South Shetland Islands Archipelago, like in most of them magmatic sequences belonging to Cretaceous orogenic register had been described. In Greenwich Island there are descriptions at Hardy Point and adjacent areas on the western and south western coasts. De Almeida et al. (2000) pointed out that the volcanic and intrusive rocks in the area are lava flows of basaltic andesites and epizonal  intrusions of granitic to granodioritic compositions, and their geochemistry supports calcalkaline affinities. The rocks exposed on the eastern and south eastern coasts, at Yankee Harbour on the Mc Farlane Strait have no previous references. This sequences are represented by pyroclastic and lava facies intruded by dykes. The pyroclastic facies is represented by purple pyroclastic flows highly altered and minor textural variations. There are pyroclastic flow deposits with an important green clastic fraction. These are classified as lapillitic tuffs with lithoclasts of andesites and basic vulcanites. There are denaturalized vitreous patches into chlorite and clays and shards replaced by zeolites. The crystalline fraction is represented by fractured plagioclase crystals and low percentages of pyroxene. There are interbedded levels with acrecional lapilli into the matrix. A discontinuous level of stained strombolian flows covers the pyroclastic deposits. The lava facies is represented by basaltic andesites with porphyritic textures in thick flows with minor textural variations. The best outcrops are located in the cliffs of Yankee Harbour. There are basic rocks composed by a phenocrystalline fraction with high corroded plagioclase. The groundmass is composed of plagioclase and pyroxene of intergrained texture. The subvolcanic facies is represented by basic andesitic dykes with different attitudes (E-W and NE-SW) which cross all the sequences described before. They are 3 metres thick and show chilled borders. Porphyry to vitreous textures and filled vesicles were observed. There are rocks highly altered with phantoms of plagioclase crystals and mafic minerals (clinopyroxene?). These minerals are completely replaced by albite, carbonates, zeolites. The groundmass has a relictic ophitic texture with opaque minerals. All the sequence shows hydrothermal alterations. Sequences described by De Almeida et al (2000) for Hardy Point expose similar features to Half Moon Island (Parica y Remesal, 2004, 2007, Parica et al., 2002) meanwhile the sequences observed in Yankee Harbour are close similar to those described for Byers Peninsula (Parica et al, 2007).