MANTLE XENOLITHS FROM JAMES ROSS ISLAND, ANTARCTIC PENINSULA: A PRELIMINARY WHOLE-ROCK AND MINERAL CHEMISTRY STUDY

FERNANDO CALABOZO; JORGE STRELIN; RANDALL A. KELLER; JOSÉ AFFONSO BROD; FRANCISCO FUENTES IZA

Córdoba

Congreso; XIX Congreso Geológico Argentino; 2014

James Ross Island Volcanic Group (JRIVG) is one of the largest (>4500 km2) neogene mafic volcanic provinces in the Antarctic Peninsula region (Nelson, 1966). The main lithologies are alkalic olivine basalts, with minor basanites, picrites, and segregations of tephri-phonolite compositions, which together show strong geochemical similarities with OIB-type basalts (Hole et al., 1995, Lawver et al., 1995, Ko?ler et al., 2009). JRIVG magmatism is probably related to back-arc extension following slab roll-back beneath the Antarctic Peninsula volcanic arc (Hole et al., 1995), allowing hot asthenospheric mantle to adiabatically upwell and melt. Several authors reported ultramafic xenolith-localities in the James Ross Island area (fig. 1a), namely Ekelöf Point (Nelson, 1966, Keller and Strelin, 1992), Santa Marta cove (Bastías et al., 2012) and two previously undescribed localities at Rabot Point and Lockyer island. However no systematic studies have been conducted so far in the JRIVG xenolith suite. We present here preliminary results of whole-rock major and trace elements on 18 peridotite xenoliths and 2 host lavas, in addition to major elements mineral chemistry on a subset of 10 selected xenoliths. The analyzed samples belong to Ekelöf and Rabot Points, where xenolith sizes allowed whole-rock analysis. Host rocks are alkali olivine basalts, with MgO between 7.67 and 10.18 %, Mg# between 0.57 and 0.62 and Ni between 161-231 ppm, implying little magma differentiation, controlled mostly by olivine fractionation.Xenolith modes were calculated by least-square fit of the bulk averaged composition of the mineral microprobe analysis and the whole-rock xenoliths composition. In order of abundance, xenoliths population comprises spinel harzburgites and lherzolites, spinel pyroxenites, and one mafic xenolith (metagabbro?) found at the Ekelöf Point locality. The main mineral phases in the peridotite xenoliths are, in order of abundance, olivine, pale-brown orthopyroxene, green clinopyroxene and brown spinel. The predominant texture is coarse protogranular (grain-size up to 1 cm), but transitions to foliated porphyroclastic textures are common, showing abundant grain-boundary recrystallization and strained orthopyroxene and olivine porphyroclasts. One sample shows an equigranular texture accompanied by a reduction in grain-size. Olivines are Fo90.4-91.5, with moderate concentrations of NiO (0.36-0.40%) and low Cr2O3 contents (