Submersible observations of Equatorial Atlantic mantle: The St. Paul Fracture Zone region

The St. Paul F.Z. is a large structural domain made up of multiple transfor m faults interrupted by several Intra-Transform Ridge (ITR) spreading segme nts. Two regions were studied in details by submersible: (1) The ITR short (< 20 km in length) segment near 0 degrees 37'N-25 degrees 27'W and 1 degre es N-27 degrees 42'W and (2) The St. Peter and St. Paul's Rocks (SPPR) mass if located at 29 degrees 25'W (<3700 m depth). (1) The short ITR segments c onsist of a magma starved rift valley with recent volcanic activities at 47 00 m depth. A geological profile made along the rift valley wall showed loc alized volcanics (basalts and dykes) which are believed to overlay and intr ude the ultramafics. The geological setting and the high ultramafic/volcani c ratio suggest an extremely low magmatic supply and crustal-mantle uplift during lithospheric stretching and denudation. (2) The St. Peter and St. Pa ul's Rocks (SPPR) massif consists of a sigmoidal ridge within the active tr ansform zone. The SPPR is divided into two different geological domains cal led the North and the South Ridges. The North Ridge consists of strongly te ctonized fault scarps composed of banded and mylonitized peridotite, sporad ic gabbros (3900-2500 m) and metabasalts (2700-1700 m). The South Ridge is less tectonized with undeformed, serpentinized spinel lherzolite (2000-1400 m) and basalts. Extensional motion and denudation accompanied by diapirism affected the South Ridge within a transform domain. Instead, the North Rid ge was formed during an important strike-slip and faulting motion resulting in the uplifted portion of the St. Paul F.Z. transverse ridge. There is a regional compositional variation of the volcanics where E-MORBs and alkali basalts are produced on the SPPR massif and are comparable to the adjacent northern segments of the Mid-Atlantic Ridge. On the other hand, N and T- MO RBs collected from the eastern part of the St. Paul F.Z. (25<degrees> 27' W IRT) are similar to the volcanics from the southern segments of the MAR. T he peridotites exposed in these provinces (SPPR and ITR) are similar in the ir REE and trace element distribution. Different degrees (3-15%) of partial melting of a mixed composite mantle consisting of spinel and amphibole bea ring lherzolite veined with 5-40% clinopyroxenite gave rise to the observed MORBs and alkali basalts.