CONSTRUCTION OF THE OCEANIC LITHOSPHERE BY MAGMATIC INTRUSIONS - PETROLOGICAL EVIDENCE FROM PLUTONIC ROCKS FORMED ALONG THE FAST-SPREADING EAST PACIFIC RISE

We report textural and compositional characteristics of plutonic rocks found in four sites exposed 5000 m below sea level along the southern East Pacific Rise system. Studied outcrops of plutonic rocks are loca ted along escarpments in two transform faults (Garrett and Terevaka) a nd in two deep regions exposed by the action of propagating rifts (Pit o Deep and Hess Deep). The transform faults have a large variety of he terogeneous plutonic rocks with mineral compositional trends extending from relatively primitive values in the olivine-bearing gabbros to ve ry fractionated values in the ferrogabbros. An abundance of ferrogabbr os in these transform faults suggests small and ephemeral magma chambe rs in which liquid and crystal mush evolved by crystal fractionation i n small pockets and intrusive sills. Conversely, Pito Deep and Hess De ep have mainly homogeneous cumulates without the fractionated end memb ers; instead, mineral compositions range to highly primitive values. T he ubiquity of homogeneous olivine-bearing gabbros at Pito Deep and He ss Deep suggests a more robust magma chamber with frequent injection a nd mixing, involving larger volumes of crystal-liquid mush, which buff ers compositions. We infer that the lithosphere created near ridge-tra nsform intersections of fast-spreading centers is controlled by a cold er mantle regime and will be lithologically and compositionally differ ent from lithosphere created near hotter segment centers and away from such discontinuities. Our observations show that magmatic intrusion i s a fundamental process in the construction of the lower oceanic crust .