. Pumiceous levels in the shoreline of the South Shetland Islands.

CASELLI, A.T., REMESAL, M.B. AND PARICA, C.A

Santiago de Chile

Congreso; GEOSUR; 2007

Asociacion Geologica Chile

The recent sub aerial volcanism in the Scotia Arc is represented by two main centers, Deception Island and the South Sandwich Islands. Both centers are characterized by explosive volcanism and emissions of pumiceous blocks. The presence of pumiceous blocks was registered in the shoreline of some of the South Shetland Islands. Recent investigations (Risso et al., 2002) interpreted these deposits in the beaches from a single source in the 1962 sub marine eruption of the South Sandwich Islands. In this paper the source and spread of pumiceous levels sited in the paleobeaches of Half Moon Island and Byers Peninsula in the South Shetland Islands are discussed. New researches in such places and the outcrops from Deception Island suggest that the real origin of these pumiceous blocks implies a more complex provenance. Pumiceous samples in beaches levels of Half Moon and Livingston Island and outcrops at Deception Island were analyzed. These data were compared against data from the South Sandwich published by Pearce et al. (1995). The petrography and the geochemistry let to find out two different pumiceous groups, the first one related to Deception Island eruptions and the second one related to the South Sandwich rocks. Deception Island rocks belong to a transitional series, with extreme members in basalts and rhyolites after Le Maitre et al (1989). On the opposite side, the South Sandwich Islands rocks show three different trends within the subalkaline field: tholeitic, low K tholeitic and calcalkaline. Meanwhile pumiceous from Deception define a high LILE (Rb) and HFSE (Zr) trend; the pumiceous from South Sandwich are depleted in Zr and Rb. The normalized abundance patterns of REE in rocks with SiO2 ≥ 60% remarks the differences in between both emission centres. Pumiceous from Deception Island show enrichment in LREE and weak Eu anomaly. Instead of the samples from South Sandwich Islands where there are not enrichment and Eu anomaly in the REE patterns. Samples from Half Moon Island (trachyte, < 20% Q) are characterized by the dark brown colour and medium size. These megascopic features and the geochemistry signature (i.e. alkalis enrichment, LILE, HFSE and LREE) are in agreement with the samples from the outcrops Deception Island. Pumiceous from Byers Peninsula are bigger, lighter, with more major proportion of vesicles than Deception and Half Moon Islands. The geochemistry of these samples (rhyolite, >65% silica) shows high affinity with the South Sandwich trends: subalkaline, low LILE, low HFSE and flat REE pattern. The differences established let propose the hypothesis about the wide spread of volcanic products with a double origin instead of the former proposal of a single origin. In this way, the pumiceous levels in the beaches of the South Shetland Islands proof that the eruptions of Deception Island have a very important incidence.