MAGMATIC EVOLUTION OF THE EASTER MICROPLATE-CROUGH SEAMOUNT REGION (SOUTH EAST PACIFIC)

The Easter microplate-Crough Seamount region located between 25 degree s S-116 degrees W and 25 degrees S-122 degrees W consists of a chain o f seamounts forming isolated volcanoes and elongated (100-200 km in le ngth) en echelon volcanic ridges oriented obliquely NE (N 065 degrees) , to the present day general spreading direction (N 100 degrees) of th e Pacific-Nazca plates. The extension of this seamount chain into the southwestern edge of the Easter microplate near 26 degrees 30' S-115 d egrees W was surveyed and sampled. The southern boundary including the Orongo fracture zone and other shallow ridges (<2000 m high) bounding the Southwest Rift of the microplate consists of fault scarps where p illow lava, dolerite, and metabasalts are exposed. The degree of rock alternation inferred from palagonitization of glassy margins suggests that the volcanic ridges are as old as the shallow ridges bounding the Southwest Rift of the microplate. The volcanics found on the various structures west of the microplate consist of depleted (K/Ti<0.1), tran sitional (K/Ti=0.11-0.25) and enriched (K/Ti>0.25) MORBs which are sim ilar in composition to other more recent basalts from the Southwest an d East Rifts spreading axes of the Easter microplate. Incompatible ele ment ratios normalized to chondrite values {(Ce/Yb)(N) = 1-2.5}, {(La/ Sm)(N) = 0.4-1.2} and {(Zr/Y)(N) = 0.7-2.5} of the basalts are also si milar to present day volcanism found in the Easter microplate. The vol canics from the Easter microplate-Crough region are unrelated to other known South Pacific intraplate magmatism (i.e, Society, Pitcairn, and Salas y Gomez Islands). Instead their range in incompatible element r atios is comparable to the submarine basalts from the recently investi gated Ahu and Umu volcanic field (Easter hotspot) (Scientific Party SO 80, 1993) and centered at about 80 km west of Easter Island. The obliq ue ridges and their associated seamounts are likely to represent ancie nt leaky transform faults created during the initial stage of the East er microplate formation (approximate to 5 Ma), It appears that volcani c activity on seamounts overlying the oblique volcanic ridges has cont inued during their westward drift from the microplate as shown by the presence of relatively fresh lava observed on one of these structures, namely the first Oblique Volcanic Ridge near 25 degrees S-118 degrees W at about 160 km west of the Easter microplate West Rift. Based on a reconstruction of the Easter microplate, it is suggested that the Cro ugh seamount (<800 m depth) was formed by earlier (7-10 Ma) hotspot ma gmatic activity which also created Easter Island.