CONDITIONS OF THE ORIGIN OF THE HYPERBASITE COMPLEX OF GIBBS ISLAND, SOUTH SHETLAND ISLANDS, WEST ANTARCTICA

On the South Shetland island are, ultrabasic rocks were detected only on Gibbs Island, on which they are accompanied by green and gray schis ts. The hyperbasites are classified into two groups: metadunite and ma ssive and schistose antigorite serpentinite. The compositions of the p rimary chrome spinellid from each of the rock groups overlap and are c haracterized by high chrome mole fractions and relatively low iron oxi dation ratios and titanium contents. These features suggest that the p rimary pyroxene of the rocks was orthopyroxene. The specific geochemic al and mineralogical features of the rocks are indicative of their aff iliation with the dunite-harzburgite-orthopyroxenite low-Ti magmatic s eries related to island-are boninites. Most of the Gibbs hyperbasites, including all pyroxene-bearing varieties, should be assigned to ultra basic cumulates, whereas the protolith of some of the metadunites is c omparable with dunites from the dunite-harzburgite mantle complexes an d can be interpreted as restitic. The metamorphism of the Gibbs hyperb asite complex is characterized by a retrograde P-T trend, which reflec ts the tectonic displacement of the rocks to the upper structural floo rs in the modem sequence of the western South Shetland island are. The movements occurred above a ''sealed'' subduction zone. Evidently, the Gibbs hyperbasite Massif is part of a tectonic collage that comprises fragments of island-are complexes of different ages and is not geneti cally related to the basement of the recently active volcanic are of t he South Shetland Islands.