Ridge-hotspot interaction: the Pacific-Antarctic Ridge and the foundation seamounts

The study of different magmatic provinces between the Resolution fracture z one (33 degrees S-131 degrees W) and the Pacific-Antarctic Ridge (PAR) axis (37 degrees S-111 degrees W) suggests that similar processes of interactio n between hotspot and spreading axial magmatism occurred 20-25 Ma and 0-5 M a ago. There is evidence of this process from the changes in composition ob served in the lavas erupted near 400-300 km between the present day PAR axi s (37 degrees S-111'W) and the eastern tip of the Foundation Seamount (FS) hotspot near 36 degrees 20'S-114 degrees W where the last alkali enriched v olcanics [K/Ti > 0.30, Zr/Y > 6 and (Ce/Yb)(N) > 4] have erupted. This tran sitional province between the PAR and the FS consists of volcanic cones bui lt on several volcanic ridges (< 200 km in length) which have erupted less enriched volcanics such as E-[K/Ti = 0.25-0.33, Zr/Y = 5-6 and (Ce/Yb)(N) = 3-4] and T-[K/Ti = 0.11-0.25, Zr/Y = 2-4 and (Ce/Yb)(N) = 1-2] MORBs than those from the FS. It is also noticed that there is a general decrease in t he degree of the basalt alkalinity (more T-MORBs) towards the PAR axis. The limit of the FS hotspot influence corresponds to the area where the VR int ersect the PAR axis for a distance of about 100 km along its strike between 37 degrees 10'S and 38 degrees 20'S. Indeed, the lava erupted further to t he north and to the south of these latitudes contains N-MORBs [K/Ti = 0.05- 0.11, Zr/Y < 3 and (Ce/Yb)(N) = < 2]. Many Old Pacific Seamounts (OPS, > 20 Ma) also built on volcanic ridges are identified west (> 1200 km from the PAR axis) of a Failed Rift Propagator (FRP) forming the eastern boundary of the ancient Selkirk microplate. Some of these seamounts made of alkali bas alts were built during the initiation of the FS hotspot 20-23 Ma ago. The i nteraction and the influence (thermal) of mantle plume magmatism with the a ncient spreading ridge of the Farallon-Pacific plates was responsible for t he eruptions of the T-MORBs and andesitic lavas. This situation is comparab le to that presently observed on the PAR axis where silicic lavas are also erupted in association with T-MORBs. (C) 1999 Elsevier Science B.V. All rig hts reserved.