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

The Easter microplate-Crough Seamount region located between 25 degrees S-116 degrees W and 25 degrees S-122 degrees W consists of a chain of seamounts forming isolated volcanoes and elongated (100-200 km in length) en echelon volcanic ridges oriented obliquely NE (N 065 degrees), to the present day general spreading direction (N 100 degrees) of the Pacific-Nazca plates. The extension of this seamount chain into the southwestern edge of the Easter microplate near 26 degrees 30' S-115 degrees 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 pillow 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), transitional (K/Ti=0.11-0.25) and enriched (K/Ti>0.25) MORBs which are similar in composition to other more recent basalts from the Southwest and East Rifts spreading axes of the Easter microplate. Incompatible element 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 similar to present day volcanism found in the Easter microplate. The volcanics 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 ratios is comparable to the submarine basalts from the recently investigated Ahu and Umu volcanic field (Easter hotspot) (Scientific Party SO80, 1993) and centered at about 80 km west of Easter Island. The oblique ridges and their associated seamounts are likely to represent ancient leaky transform faults created during the initial stage of the Easter microplate formation (approximate to 5 Ma), It appears that volcanic activity on seamounts overlying the oblique volcanic ridges has continued 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 Crough seamount (<800 m depth) was formed by earlier (7-10 Ma) hotspot magmatic activity which also created Easter Island.