Chapter 4 - The singular St. Peter and St. Paul Archipelago, equatorial Atlantic, Brazil

The St. Peter and St. Paul archipelago (SPSPA) is composed of abyssal mantle rocks. It consists of a small group of islets (five) and rocks (five), located near the axis of the Mid-Atlantic Ridge, in an inside corner position on the southern edge of the northern transform limit of the St. Paul Transform Fault System. The archipelago forms the summit of the Atoba Ridge, lying about 100km north of the Equator at 0 degrees 55′N and 29 degrees 20′43″W The Atoba Ridge is a pressure ridge of the St. Paul Transform Fault System that was uplifted by local compression along the boundary between the African and South American Plates. The almost NNW–SSE compressional stress has been active for the last 6600years before present, providing an uplift rate of approximately 1.5mm/year, as suggested by teleseismic evidence. The SPSPA is thus seismically active; predictably, most of its earthquakes show predominantly east–west right-lateral strike-slip displacement along the St. Paul Fracture (33 focal mechanisms of earthquakes Mb≥5.4), coherent with the offset sense of the transform fault. However, in the south of the SPSPA, there were a significant number of compressive earthquakes (four focal mechanisms of earthquakes Mb≥5.8) that show a secondary stress component of NNW–SSE compression, revealing differential movement and independent behavior between the islets. Thus, the SPSPA is an upper mantle block that was uplifted tectonically during strike-slip movement as part of an extensively sheared and thrust-faulted transverse ridge. During ascent and emplacement at high mantle temperatures in a transitional plastic-brittle regime, intense mylonitization took place, producing progressive grain size reduction. Anhydrous fluids could not penetrate the rock during the plastic regime, but only after their uplift, it transferred them into a brittle regime; therefore, the peridotite mylonites were serpentinized from the joints. The rocks chemically correspond to harzburgite, and more rarely dunite, along with rare kaersutite-rich alkaline ultramafic dikes. The latter rock type is millimetrically interlayered with peridotite rock. These rocks are ultramylonitized and pervasively reduced to a very fine grain size. Mass-balance calculations of whole-rock geochemical data indicate that the early mylonitization did not significantly change the major peridotite composition. The emerged spinel- and pargasite-peridotites of St. Peter and St. Paul are residual mantle rocks after 3%–13% of partial melting, and the major, trace rare earth element patterns of whole-rock and nonzoned minerals suggest that these residual peridotites of SPSPA resulted from mixing of peridotite and kaersutite mafic end-members. The alkaline fluids derived from low melting mantle (about 1.0%) that percolated, reacted with, and refertilized the residual mantle of the SPSPA by chromatographic effects. The kaersutite layers of the layered peridotite-kaersutite ultramylonite probably formed by freezing of the trapped fluids that percolated the peridotite before the mylonitization of fluid conduits.