petrography and geochemistry of the trinity peninsula group

PETROGRAPHY AND GEOCHEMISTRY OF THE TRINITY PENINSULA GROUP,
ANTARCTIC PENÍNSULA: IMPLICATIONS FOR PROVENANCE AND TECTONIC
SETTING
Castillo, P.1, Lacassie, J.P.2, Hervé, F.1 and Kraus, S.3
1
2
Departamento de Geología - Universidad de Chile. Plaza Ercilla 803, Santiago, Chile.
Departamento de Geología Económica - Servicio Nacional de Geología y Minería. Av.
Santa María 0104, Providencia, Santiago, Chile.
3
Instituto Antártico Chileno. Plaza Muñoz Gamero 1055, Punta Arenas, Chile.
paucasti@ing.uchile.cl
Keywords: Trinity Peninsula Group, provenance, geochemistry.
Petrographical and Whole-rock major, trace and rare earth elements data are
combined in a provenance study of a Permo-Triassic turbidite in the Antarctic Peninsula: the
Trinity Peninsula Group. Sandstones and mudstones from six localities and clasts of
conglomerate from one locality, have been analyzed to characterize its source and
depositional tectonic regime. Sandstones modal composition is dominated by quartz and, in
smaller proportions, by feldspar, and it is consistent with being the product of erosion of the
plutonic roots of a magmatic arc. Moreover, the clasts of a conglomerate from View Point,
northeast of Antarctic Peninsula, are mainly granitoids. Major elements show that sediments
underwent low chemical alteration in the source area or during transportation (Chemical
Index of Alteration: 52-66), so compositions accurately represent their source areas. The
chondrite-normalized REE distribution patterns show an enrichment of LREE relative to
HREE and have a negative Eu anomaly (average Eu/Eu* of 0,78 and 0,67 in sandstones and
mudstones respectively). Ratios of immobile trace elements suggest a felsic igneous source
(Th/Sc>1 in sandstones) and a concentration of zircons in sandstones. Sandstones and
mudstones show a systematic grain-size-related geochemical contrast which reflects a
higher content of clay minerals in mudstones than in sandstones, and a higher proportions of
quartz, feldspar, and heavy minerals in sandstones than in mudstones. Textural, framework
compositions and geochemical data point towards a relatively proximal felsic volcanic arc
source, which plutonic roots had been exposed by erosion. This arc is likely to have been
located at an active continental margin.