MAGMATISM IN THE GARRETT TRANSFORM-FAULT (EAST PACIFIC RISE NEAR 13-DEGREES-27-MINUTES-S)

The Garrett transform is characterized by recent (zero age) volcanic activity located within the active tectonic domain of the transform valley at depths greater than the 3500 m. This intratransform volcanic activity contributed to the formation of constructional edifices forming ridges (>300 m in height) and small mounds (<20 m in height) built near slivers of serpentinized peridotites. The erupted lavas are depleted mid ocean ridge basalts (MORBs) with low ratios of K/Ti (0.02-0.11), Zr (30-100 ppm), Y (18-50 ppm), and (La/Sm)(N) (0.25-0.60). Their more depleted nature and smaller range of variability for the compatible elements (Ni=70-180 ppm, Mg#=0.58-0.71, where Mg# is the magnesium number (=Mg+2/Mg+2+Fe+2)) are the main points of difference between the Garrett intratransform volcanics (GITV) and those from the ultrafast south East Pacific Rise (SEPR). However, ferrobasalts (Mg#=0.41-0.55) were collected from the intratransform walls as well as at the EPR-transform intersection. The GITV are even more depleted in incompatible elements than lava from the north East Pacific Rise (21 degrees N-11 degrees 26'N). The Garrett recent lava is believed to have erupted after the successive, incremental partial melting and discontinuous melt extraction of a composite lherzolitic mantle similar to that of the SEPR. The limited range of incompatible element ratios (Zr/Y=1-2.5, (Ce/Yb)(N)=0.4-1) and K/Ti ratios (<0.14) and the samples more porphyritic nature with respect to other SEPR rocks suggest that the intratransform volcanics from the Garrett are extracted from their source and channeled directly toward the surface without extensive mixing in magma chambers. In order to explain the restricted range of compositional variabilities and the absence of the enriched basalts produced by prior melting, we postulate that even though they were produced, these most enriched end-member lavas did not reach the surface; instead, we propose these melts contribute to the formation of impregnated mantle material in the lithosphere. We suggest that this same petrogenetic style of intratransform volcanism might also characterize other oceanic provinces associated with low magmatic to quasiamagmatic regimes.