SIGEP Geological and Palaeontological Sites of Brazil SIGEP 002 Saint Peter and Saint Paul’s Archipelago Tectonic uplift of infra-crustal rocks in the Atlantic Ocean Thomas Ferreira da Costa Campos1a Joaquim das Virgens Neto1b Narendra Kumar Srivastava 1c Reinaldo Antônio Petta 1d Léo Afraneo Hartmann2e João Francisco Silveira de Moraes3f Liana Mendes4g Susan Roberta Mello Silveira5h a [email protected]; b [email protected]; c [email protected]; d [email protected] ; e [email protected]; f [email protected]; g [email protected]; h [email protected] 1Departamento de Geologia da Universidade Federal do Rio Grande do Norte; Caixa Postal 1639, CAMPUS da UFRN, 59072-970 NATAL-RN; 2Instituto de Geociências da Universidade Federal do Rio Grande do Sul; 3CPRM - Serviço Geológico do Brasil; 4Departamento de Biologia da Universidade Federal do Rio Grande do Norte; 5Departamento de Biologia da Universidade Federal Rural de Pernambuco © Campos,T.F.C.; Virgens Neto,J.; Srivastava,N.K.; Petta,R.A.; Hartmann,L.A.; Moraes,J.F.S.; Mendes,L.; Silveira,S.R.M.. 2005. Saint Peter and Saint Paul’s Archipelago - Tectonic uplift of infra- crustal rocks in the Atlantic Ocean. In: Winge,M.; Schobbenhaus,C.; Berbert-Born,M.; Queiroz,E.T.; Campos,D.A.; Souza,C.R.G.; Fernandes,A.C.S. (Edit.) Geological and Palaeontological Sites of Brazil. Available on line 24/12/2005 at the address http://www.unb.br/ig/sigep/sitio002/sitio002english.pdf [actually http://www.sigep.cprm.gov.br/sitio002/sitio002english.pdf ] (The above bibliographic reference of author copy right is required for any use of this article in any media, being forbidden the use for any commercial purpose) Saint Peter and Saint Paul’s Archipelago Tectonic uplift of infra-crustal rocks in the Atlantic Ocean SIGEP 002 Thomas Ferreira da Costa Campos1a Joaquim das Virgens Neto1b Narendra Kumar Srivastava 1c Reinaldo Antônio Petta 1d Léo Afraneo Hartmann2e João Francisco Silveira de Moraes3f Liana Mendes4g Susan Roberta Mello Silveira5h The Saint Peter and Saint Paul’s Archipelago (SPSPA), also known as “St. Paul’s Rocks”, is the smallest Brazilian oceanic archipelago. It is located in the mid-equatorial North Atlantic Ocean (0º55’02”N; 29º20’42”W), about 821 km from the Brazilian coast and 1,586 km from the coast of Africa. The SPSPA is a distinctive archipelago, because it does not consist of volcanic rocks, as do most oceanic islands. It is composed instead of plutonic rocks originated in the upper mantle of the earth. The origin and age of SPSPA are uncertain. The SPSPA corresponds either to a relic of the sub-continental lithosphere mantle, which is associated with the non- expansive part of the Equatorial Mid-Atlantic basement, or to a protrusion of oceanic mantle uplifted by the action of the St. Paul Fracture, concomitantly with the processes of mylonitization and serpentinization. It is estimated that its formation took place between the periods of 100-35 Ma. The emerged part is composed of serpentinized peridotite and kaesurtite Mylonite of uncertain origin. The mylonitization obliterated the primary textures of these rocks, concomitantly with the serpentinization by pervasive hydrothermal fluids and/or seawater during late tectonic movements that fractured the rocks. The Quaternary Saint Peter and Saint Paul’s Formation (QSPSPF), outcropping in certain parts of the archipelago, is constituted of clastic sediments derived from the basement and biogenic activities, cemented by calcium carbonate. The presence of thin marine terraces, which marks 4 paleo-levels above the current sea level, and the variety of lithofacies (stratifications and fossils in the QSPSPF) suggests that the QSPSPF was deposited under a progressive sub-aerial regime dominated by waves. It also suggests that the SPSPA was situated below the current sea level during the beginning of the Quaternary, and that tectonic uplift occurred episodically rather than at a constant rate. Keywords: Abyssal Peridotite; Mylonite; Serpentinization; Oceanic expansion; Saint Peter and Saint Paul’s Archipelago; Equatorial Atlantic. INTRODUCTION feeding and reproductive zone. Consequently, SPSPA is a unique area for the study of energy transfer The Saint Peter and Saint Paul’s Archipelago processes along the trophic chain. (SPSPA) is the smallest oceanic archipelago in the The SPSPA is also under the influence of the Brazilian territory. It is an ultramafic complex direct zone of intertropical convergence, whose probably originated in the terrestrial mantle. dynamics influence the rain regimes along the coast of Considering its peculiar geological, mineralogical and Northeast Brazil, positioning the archipelago in one of geochemical characteristics, the SPSPA is a subject of the areas with the highest pluviometric indices in the special interest to the scientific community because, whole Atlantic Ocean, and, consequently, with the despite its age uncertainty (100-35 Ma), it can provide smallest levels of superficial salinity. Due to its long information about the nature of the upper mantle of distance from the continental coast, part of the wildlife the Earth, the processes of continental breakage and in the SPSPA has a high degree of endemism, which drift (e.g. Laurentia, Gondwana, and Pangea), and its gives the archipelago an additional importance, the correlation with the opening of the Atlantic Ocean conservation of biodiversity. (Melson et al., 1972). These particular characteristics attribute to SPSPA an importance to the conservation SITE LOCATION of mineral diversity and geologic evolution. Moreover, since the archipelago is located in the Equatorial The SPSPA is located at the farthest point of the system of marine currents, it has a strong influence in Brazilian territory in the North Equatorial Atlantic the life cycle of many migratory species (fishes, Ocean, and is the only group of Brazilian oceanic crustaceans and birds), which utilize the region as a islands in the northern hemisphere. It is localized ____________________________________________________________Geological and Palaeontological Sites of Brazil 1 62.14 nautical miles (100 km) to the north of the archipelago is also approximately 1,210 km from the Equator (Fig. 1). The central point coordinates of city of Recife, at coordinates N 32º 30’E. SPSPA is SPSPA are 0º 55' 02"N E, 29º 20' 42"W. The nearest also situated at almost half way to coast of Guinea- point to the Brazilian coast, in Cabo do Calcanhar, Rio Bissau (Africa), at approximately 985 nautical miles Grande do Norte State, is approximately 510 nautical (1,824 km). miles (1,010 km), at coordinates N 41º 15’ E. The Figure1: a) Location of St. Peter and St. Paul’s Archipelago, Equatorial Atlantic; b) Demarcation of 200 nautical miles that define the Brazilian Economic Exclusive Zone (EEZ). HISTORICAL ASPECTS R.V. Beagle (England), landed on the SPSPA in 1831, as part of his scientific voyage around the world. He The date of the discovery of the SPSPA is was the first researcher to suggest that the rocks of St. uncertain, because the records of exploration were Paul were not volcanic. During the same trip he made considered state secrets. However, Portuguese the first chemical analysis of the rocks (Renard, 1882). historical records suggest that the archipelago was M’Cornick landed on the archipelago with the H.M.S. discovered accidentally in 1511, when the Portuguese Erebus in 1839, and disagreed with Darwin. He navigator Manuel de Castro Alcoforado, captain of the suggested that the SPSPA had a volcanic origin, a São Pedro vessel, was separated from the group statement not confirmed later. In 1873, Tizard et al. commanded by D. Garcia de Noronha and ran performed the first systematic study in the geology of aground upon the St. Paul’s rocks. He was rescued by the crystalline basement of the archipelago, during the another vessel of the same group called São Paulo, exploratory expedition of the H.M.S. Challenger hence the name St. Paul’s rocks. However, Spanish (England), commanded by Sir Charles W. Thomson. historical records indicate that in 1513, the Spanish The S.Y. Scotia, in its trip to Antarctica in 1902, navigator Juan da Nova de Castello made the first dropped anchor on SPSPA and made the first register of the island (the name of the vessel he was photographic register of the rocks. In 1911, the team leading is unknown). The exact location of the of the German ship Deutschland collected geological archipelago is not shown on the world’s nautical and biological samples from the archipelago. The record of 1513, made by the Turkish. However, the expedition commanded by Shackeleton-Rowett in Portuguese nautical records suggest that the 1921, landed with the H.M.S Quest (England) and Portuguese Diego Ribeiro made the first record for conducted the second systematic study in the geology the SPSPA in 1529. That is why the world ‘s nautical of the rocks from the crystalline basement of the record of Mercator, made in 1538, already shows the SPSPA. In 1922, an airplane landed in the region of position of the archipelago. the archipelago for the first and last time. It was the The first landing on SPSPA was made by the hydro airplane Luzitania (Portugal), commanded by the French navigator Beuvet du Losier in 1738, followed Portuguese navigators Gago Coutinho and Sacadura by the American navigator Amasa Delano in 1799, Cabra. who was on board the S.Y. Perseverance. In 1813, the In 1930, the ship Belmonte from the Brazilian Navy, Tenant-Capitan George Crichton, who was on board installed the first lighthouse to help navigation. During the H.M.S. Rhin, elaborated upon the first nautical the Second World War, the ships U.S.S. Atka and record of the SPSPA. Charles Darwin, on board the U.S.S. Omaha did some research and modified the ____________________________________________________________Geological and Palaeontological Sites of Brazil 2 names of some islands previously baptized by the ecosystem) called phenomena of nutrient expedition of H.M.S. Challenger. During the decade of enrichment, due to the resurgence from the 1960’s, the SPSPA was visited by many American interaction between the oceanic currents and the scientific expeditions [e.g.: Wiseman (1960) on board submarine relief. In relation to the meteorological of the H.M.S. Owen; Melson et al. (1964 and 1968) on conditions, the SPSPA is under the direct influence board of the R.V. Chain and H.M.S. Atlantis II]. of the Intertropical Convergence Zone, whose Between the years of 1990 and 1994, Russian and dynamics affects the rain systems of the Brazilian German researchers, on board the R.V. Strakhov and northeast coast. For this reason, the SPSPA is one of R.V. Sonne-84, did a gravimetric and bathymetric study the areas with the largest pluviometric indices in the in the region around SPSPA. In 1996, the Brazilian entire Atlantic Ocean, and consequently, reduced government, through the Interministerial Commission superficial salinity. for the Sea Resources (CIRM), created a group for the occupation and research on the SPSPA, with the goal Biological Aspects of installing a scientific station on the Belmont Island. This research expedition was launched from the ship As a function of its geographical location, which Canopous. is strategically positioned between two hemispheres With the inauguration of the SPSPA Scientific and also between the South American and African Station (ECASPSPA) in 1998, the first scientific continents, the SPSPA presents a strong influence in studies under the program called Pro-Archipelago the life cycle of many migratory species like fishes, began. The program allowed the permanent crustaceans, and birds. These species utilize the occupation of the archipelago, and also allowed Brazil archipelago and its surroundings as a shelter, to add 200 nautical miles to its territory in the feeding, and reproductive zone. Exclusive Economic Zone (EEZ) (Fig.2). Between The low species richness in SPSPA can be December 1997 and January 1998, the French and associated with its small area and consequent local Brazilian researchers on board the submersible Nautille microhabitat scarcity. The marine substrate is and the ship R.V. Nadir, conducted a submarine predominantly rocky, sometimes covered by various geological study of the area between the Romanche algae species, which are not very diverse. However, and São Paulo fractures. It was in one of these there can be small sandy areas in the marine bottom, expeditions that the Nautille’s researchers did the first where there are species of mollusks, crustaceans, dives in the area, at depths greater than 5,100 meters. and other typical organisms of this type of substrate. In 2001, T. F. C. Campos and the 83rd expedition of The fish fauna of the archipelago is known to the Pro-Archipelago program traveled to SPSPA on represent common components of the Brazilian, board the boat Transmar II (Brasil) and did the first Caribbean, and African ichthyofauna as a result of systematic study of the 21st century in the geology of dispersion and establishment of larvae, since SPSPA the emerged rocks of the basement of SPSPA. In 2002, T. F. C. Campos and the 105th expedition of the is under influence of two important marine currents. Pro-Archipelago program traveled to SPSPA on board There is a clear zoning process occurring in the of the Transmar I (Brasil) and began the first systematic distribution of organisms. As an example, study on the emerged sedimentary rocks of the individuals of Entomacrodus vomerinus (monkey- archipelago (Edwards, 1985; Silveira et al., 2003). fish) occur in shallow areas like the Poças de maré (tidelands), while species like the butterfly fish SITE DESCRIPTION (Chaetodon obliquus) and the endemic serranid Anthias salmopunctatus occur at greater depths Climatic and Hydrological Aspects along the rocky walls of the archipelago. About 14 species of algae were registered in the From a hydrological point of view, the SPSPA is SPSPA. At the tides’ oscillation zone there are inserted in the Equatorial system of marine currents, incrusted red algae, and at the following under the direct influence of the South-Equatorial sublitoraneous region there are the zoanthid Pallythoa current that superficially flows EW and the submerged caribeorum (at depths from 3 to 8 m) and numerous Equatorial current flowing in the opposite direction green algae species Caullerpa sp. (at depths from 3 to (WE) at depths between 60 to 100 m, coordinates 1.5º 30 m). The coral species Madracis decactis and Scolymia N and 1.5º S. This submerged current is the fastest of wellsi are concentrated at depths between 30 and 45 m. all equatorial currents, reaching speeds greater than 2 Two species of black coral (Anthipates sp.) were knots (about 3.6 km/h). registered in the archipelago, occurring at depths These two currents acting together generate a above 45 m. To date, 58 species of reef fishes are hydrological pattern of elevated complexity already registered (Sergeants, angelfishes, hogfishes, (together with a large influence due to the island damselfishes, morays, crevalle jacks, basses, butterfly fishes, blennies, etc), and 17 species of pelagic fishes ____________________________________________________________Geological and Palaeontological Sites of Brazil 3 (sharks, tunas, mackerels, remoras, dolphin fishes, present irregular and curvilinear, winding contours, needlefishes, flying fishes, moon fishes), which utilize and its slopes have strong declivities (>80o). the archipelago mainly as a feeding zone (e.g. Thunnus The four biggest islands (Belmonte, Sudeste, albacares), and, during some particular periods of the Nordeste, and Cabral) form a horseshoe shape cove, year, as an area for reproduction (e.g. Cypselurus with average dimensions of 100 m of length by 50 m cyanopterus). of width, 8 m of depth, and its coverage is pointed Among the 58 species of reef fishes, the most towards NW. Narrow channels separate these islands. numerous families, in terms of occurrence of The cove bottom is constituted of sediments species are: Muraenidae (with seven species), originated through biological activities and Carangidae and Pomacentridae (with five species desegregation of rocks that constitute the archipelago. each), and Serranidae and Scaridae (with three The emerged relief of SPSPA is pronounced, and its species each). About 80% of the reef fishes of culminating point (18 m of altitude) is located on the SPSPA occur on the Brazilian marine coast due Nordeste Island. The highest point of the Belmonte to its continental proximity, as well as the Island reaches about 16 m of altitude, while on the suitability of the sub current that moves from Sudeste Island the highest elevations reach 17 m. The continental Brazil to the archipelago. submarine relief of the archipelago exhibits elongated Due to its isolation, the SPSPA presents a elevations like a crest, with a slight declivity at EW considerable degree of endemism. As an example, direction and strong declivity at NS direction. SPSPA consists of the emerged portion of a mid- we can cite four species of damselfishes (Stegastes ocean transversal mountain chain with EW direction, sanctipauli, Enneanectes smith, Chaetodon obliquus, which is placed parallel to the north edge of the of the Anthias salmonpuctatus). Another example of the São Paulo transform fracture zone, close to the effects of isolation is the appearance of the limiting region of the South American and African’s angelfish Holacanthus ciliares of various colors, divergent tectonic plates. The São Paulo Fracture which does not normally occur in other locations, extends for about 580 km between the coordinates and can reflect a high level of crossbreeding 1ºN-30º20’W and 0º30’N-24ºW. The oceanic crust at between species. The fishes present other colors, the opposite side of the active area of the São Paulo such as blue and white individuals. There are also transform fracture zone presents an age difference of non-pigmented samples of the pomacentrid about 40 Ma (Rusby, 1993). Chromis multilineata. In the agitated waters The failure zone of the São Paulo transform surrounding the islands of the archipelago, some fracture zone around the archipelago reaches 120 km rare marine species are found with relative of width, and its valleys can reach an average depth of frequency (e.g. Whale shark). The appearance of –3,600 m. However, in the surroundings of SPSPA, rare species, together with the presence of abyssal depths can be observed greater than –5,000 m, dolphins, rays, barracudas, mackerels, tunas, at the transform faults as well as at the top of the sharks, crevalle jacks, and hundreds of other Atlantic Ridge (Hekinian et al., 2000). Considering the fishes, promote a spectacle of life worth seeing. isobath of –3,800 m, this mountainous chain has a SPSPA is also a landing territory for local sigmoid shape, with about 100 km of length by 40 km populations of marine birds (about 2000 of width, and a general orientation NE-SW. According individuals), the brown boobies (Sula leucogaster) to Hekinian et al. (2000), this sigmoid shape can be and the brown noddies (Anous stolidus). Due to explained by the junction of two rock massifs that scarcity of dry areas for nidification, the form the submarine mountainous chain. occurrence of these species is really high. The brown boobies represent 0.4 birds/m2, which Geological and Petrographical Aspects probably hinders the appearance of other migratory birds on the archipelago (Mendes, 2004 BASEMENT ROCKS – Personal communication). The submarine mountain chain, whose tops Geomorphological Aspects constitute the St. Peter and St. Paul’s Archipelago, is tectonically active and, consequently, subjected to The SPSPA constitutes a small group of islands earthquakes. This mountain chain is situated at the (10) and many rockheads, situated at proximities of the top of the São Paulo transform fracture zone, which Mid-Atlantic Ridge, at about 100 km north of the indicates that its formation was controlled by the Equator at the coordinates 0º55'02"N and 29º20'42"W movement of this zone and its group of fractures (Fig.1 and 2). The total emerged area reaches about 4.2 (Fig.3). acres, and the distance between the extreme points – the Erebus and Pillar rocks- reaches 420 m. The islets ____________________________________________________________Geological and Palaeontological Sites of Brazil 4 Figure 2: Overview of St. Peter and St. Paul’s Archipelago, Equatorial Atlantic: a) From QuickBird® satelite; b) From a helicopter. ____________________________________________________________Geological and Palaeontological Sites of Brazil 5 31oW 30oW 29oW 28oW 27oW 26oW 25oW 1o 50´N 2 Dorsal Oceânica (DOA) rquipélago de São pe3dro e S3ão Paulo 3 2 Alto d3a 2Serra Le 3oa 3 23 3 3 3 3 1oN 3 >4 3OD 3 3 2 3 3 >4 3 3 DO 3 0 > 420 40 630 80 1>004 3 3 Zona3 de Fratu3ra de São Pa3ulo3 3 3 3 3 2 >4 3 3 >4DO 3 3 >4 >4 DO 0o km >4 - Zonas Abissais das falhas transfromantes Figure 3: Sketch of the transverse crest-valley system that constitutes the St. Paul’s transform fracture zone, Equatorial Atlantic (Gorini, 1981 apud Hekinian et al., 2000). The SPSPA is considered sui generis in the Atlantic contributing to the process of serpentinization and due to the occurrence of ultramafic plutonic rocks, diapiric ascent of this flank. The North flank is part of which were mylonitized while being positioned a block of extensive shearing associated with a through the action of tectonic forces related to the transversal thrust fault that was elevated during the breakage of the Pangea continent, and consequent transcurrent tectonism, which suggests a more rigid expansion of the Atlantic Ocean. There are no lithosphere than the one of the South flank during the volcanic rocks on the emerged part of the archipelago, protrusion of the serpentinized peridotite (Fig. 4). only infracrustal plutonic rocks (Bonatti, 1976; Melson Yet, intra-transform volcanism occurred due to et al., 1972). the extensional movements on the top of this massif. The rock massif that constitutes the immersed part In this way, volcanic rocks that rise at the graben around the archipelago is divided into two flanks structure of the South flank could have been extruded (North and South), probably related to an EW- during the tectonic adjustment. Consequently, the trending, active transform fault (Hekinian et al., 2000) positioning of the peridotites that form the SPSPA is (Fig.4). A graben (<- 2,500 m) is responsible for this related to the combined effect of serpentinite division, which presents doleritic intrusions and small diapirism (protrusion) and differential shearing within basaltic flows. These two flanks present distinct the São Paulo transform fracture zone, which resulted geological characteristics: i) The North flank, where in the elevation of a portion of the SPSPA massif SPSPA is located, is a group of scarps of strongly above the current sea level. tectonized rocks, essentially serpentinized peridotite The outcrops of the emerged part of the whole mylonites, serpentinized banded peridotitic mylonites, archipelago are intensely fractured (macro and micro) few gabbroic intrusions, and basaltic flows; ii) The due to movements after mylonitization. During the South flank is less tectonized, and is formed by non- lifting of the North flank of the SPSP massif there was deformed serpentinized peridotite, and can also sediment deposition that originated the SPSP present basaltic flows. This flank is yet covered by formation, with a Quaternary age (Campos et al., 2002, hardened calcareous crust. 2003, 2004). The mechanism responsible for positioning the The Northwest portion of the Belmonte Island SPSP massif inside the São Paulo transform fracture (Fig. 5a) consists of a bulky and homogeneous zone is not yet identified. Wolfe et al. (1993) and aphanitic rock of elevated hardness, presenting a Hekinian et al. (2000) proposed two distinct tectonic grayish-white to grayish-green coloration (Fig. 6a). On processes that originated this sigmoid-shaped massif: the other hand, the Southeast portion of the Belmonte The South flank would have been formed through the Island and the other islands essentially consist of a extensional tectonic movement (pull-apart), along with heterogeneous aphanitic rock of low hardness, diapiric and denudation processes, while the North presenting a very diverse coloration, from grayish- flank was formed through an extensive transversal white and grayish-green to reddish-brown, or a strike-slip and thrust fault. The existence of volcanic, mixture of all these colorations (Fig. 6c). sub-volcanic, and serpentinized non-mylonitized The homogenous rock corresponds to a Peridotite in the South flank suggests that this part of mylonitized peridotite, while the heterogeneous rock the lithosphere was once less rigid, and consequently corresponds to a fabric of interfingering mylonitic and more permeable to the circulation of fluids, serpentinized phases, where the serpentinized phase is ____________________________________________________________Geological and Palaeontological Sites of Brazil 6 developed from the micro-fracturing pattern at the corresponds to an alkaline mylonitized rock rich in cost of the mylonitic phase, up to the total kaersutite. At a small area of the eastern part of the consumption of the last. At the Southeast Island and, Southeast Island, alternate levels occur showing either more rarely, at the South portion of the Belmonte incipient serpentinization of peridotitic mylonites or Island there are shoots and enclaves of a bulky and alkaline kaersutitic mylonite, presenting millimetric to homogeneous aphanitic rock of elevated hardness with centimetric dimensions parallel to each other (Fig. 6d), a gray anthracitic coloration (Fig. 6b), which and sometimes presenting micro-folding. Profundidade Arquipélago de São Pedro e São Paulo (m) Plataforma Carbonática 0 com colônia de espongiários (-1.200 m) Flanco Sul Graben -1000 Vulcânico (0o5’N; 29o29’W) (-2700 m) Flanco Norte -2000 (0o57’N; 29o23’W) -3000 ? -4000 ? ? ? Distância 0 5 10 15 20 25 30 (km) - Peridotito Milonitizado - Dolerito - Sedimentos Consolidados - Peridotito Não Deformado - Gabro - Lavas Vulcânicas Figure 4: Sketched geological profile of North and South flanks of the St. Peter and St. Paul Archipelago transversal ridge separated by EW active transform fault (apud Hekinian et al. 2000) The tectonic processes obliterated almost all the orientation that varies, and its crystallographic axis b is mineral assemblage of the SPSPA’s rocks, which made generally found perpendicular to the mylonitic impracticable until now the determination of its modal foliation (Tilley, 1947; Denisova, 1991). This foliation composition (Melson et al., 1972). The original is reflected in preferential plans of microfracturing plutonic nature of these rocks is evident through the presented by some samples. existence of ellipsoid porphyroclasts of olivine (0.1 – Petrographically, the ultramafic mylonites of 2.0 mm, rarely 4.0 mm), pyroxenes (0.5 – 1.5 mm), SPSPA still have vestiges of an initial deformation at amphiboles (0.5 – 6.0 mm), and spinel (0.1 – 4.0 mm), high temperatures that occurred in the upper mantle, which are strongly fragmented and fractured to the as well as a strong plastic deformation that occurred in point that the plutonic mineral assemblage is the lower crust during the tectonic process that mistakenly identified as matrix (< 0.01 mm) of similar originated the São Paulo transform fracture zone, composition. The porphyroclasts of olivine where the initial peridotites were mylonitized. The correspond to 60% of the total volume of combination of the syntectonic recrystallization and porphyroclasts, and the mylonitic matrix corresponds the plastic deformation gave rise to a typical mylonitic to more than 60% of the rocks, and it is texture, which obliterated the initial optical orientation criptoserpentinized. This suggests that the original of the minerals without producing a new texture on phenocrysts were comminuted and recrystallized the porphyroclasts. The mylonitic orientation of the through tectonic action and sinuously involved in a archipelago’s peridotites is discordant with the fine mylonitic matrix (Fig. 7a, b, c, d). This matrix can direction of the São Paulo transform fault, which appear as alternating clear and dark bands, with indicates that this block was brought to the surface in different granulation with average of 0.01 to 0.02 mm, a series of complex tectonic movements (Melson et al, even though it can reach 0.06 mm (Fig. 7a, b). The 1972). dark color is caused by the concentrations of Later tectonic movements originated more minuscule grains of magnetite originated in the fractures on these rocks, facilitating the infiltration of process of matrix serpentinization. fluids responsible for serpentinization (Fig. 7a, b, c, d). In some cases, it is still possible to identify The serpentinization process was not uniform, mylonitic foliation in the matrix, in parallel association originating rocks with different degrees of with the large axis of the ellipsoid porphyroclasts. serpentinization, from incipient to elevated (Fig. 6c). However, the porphyroclasts of olivine have an optical On diffractometric X-ray studies of the material ____________________________________________________________Geological and Palaeontological Sites of Brazil 7 that was most serpentinized, it was only possible to oxides of iron were also identified. The serpentines are identify lizardite and chrysolite as the minerals of the present in a pseudomorphic shape, as well as very fine serpentine group that predominated as a product of grains in the matrix. alteration of peridotitic mylonites. Magnetite and a 29°20’ 43” W b Rocha Beagle Rocha Pillar Ilha Cabral (Nordeste) Ponto Culminante (17 m) Oceano Ilha Nordeste Atlântico Ilha Belmonte Ponto Culminante (18 m) (Sudoeste) Rocha Cambridge Pier Farol (16 m) Ilha Challenger Base (Sudeste) 0°55’00” N Científica Rocha 0°55’00” N Coutinho Ilha Sul Oceano Atlântico 0 40 m Rocha Erebus Escala 1/500 29°20’ 43” W 10 m - Rocha sedimentar fossilífera Unidade Viuvinhas (conglomerados e arenitos) - Arenito calcífero - Milonito com níveis finos de peridotito - Arenito calcífero conglomerático e rocha alcalina quersutítica Unidade Atobas - Milonito com níveis espessos de rocha alcalina quersutítica e finos de peridotito - Arenito médio a grosseiro - Peridotito milonitizado e serpentinizado - Arenito conglomerático médio a grosseiro - Conglomerado com seixos arredondados a subangulosos - Peridotito milonitizado - Embasamento Figure 5: a) Geological sketch map of the emerged area of St. Peter and St. Paul Archipelago, Equatorial Atlantic; b) Geological sketch map of St. Peter and St. Paul Formation. The rocks of the ultramafic complex of SPSPA have five sequential mineral assemblages. Plutonic relic ROCKS OF THE SEDIMENTARY COVER minerals compose the first sequence; a mylonitic matrix with porphyroclasts of olivine, pyroxenes, The St. Peter and St. Paul Formation sedimentary pargasites, spinels and kaersutite composes the second; rocks that occur in SPSPA (Campos et al., 2002, 2003, the third sequence is composed by serpentine that 2004) are essentially formed by clastic polymitic affects all the crystals; the fourth also contains sediments shed from the rocky substrate, from serpentine formed by fracturing of the rocks, and the consolidated and well-worked biogenic material (Fig. fifth sequence is composed by a white carbonatic mass 5b). This biolithoclastic sedimentary sequence rests that cuts the serpentine. The serpentinization process discordantly over a mylonitized peridotitic basement, most strongly affected the olivine and pyroxenes, and is constituted by two units: i- Atobás unit; ii- principally the enstatite. The amphiboles were not Viuvinhas unit. serpentinized. The origin of this formation is related to the ____________________________________________________________Geological and Palaeontological Sites of Brazil 8 deposition of marine sediments in superficial waters of pebbles that rolled into the bay due to seismic action a paleo-bay. During the process of deposition there and erosion caused by waves. These pebbles remained were at least two seismic-tectonic episodes: one under the action of the waves until they became generated a hiatus between two periods of deposition, rounded. With time, the granulation of the sediments and the other basculated the whole sequence. diminished to the point that they deposited as rough The evolution of the St. Peter and St. Paul sand, although rare angulated pebbles resulted from Formation initiated with the opening of a new barrier, the watersheds through the waves action and/or taking advantage of the weak zone originated by seismic activity. An expressive tectonic event caused fracturing of the basement diaclasamento, with general the inclination of the Atobás unit layers to the SE. direction NW. The invasion of seawater caused the deposition of the Atobás unit, essentially formed by c1 Milonito quersutítico (com veios de carbonato) Figure 6: Representative hand-samples of emerged rocks from SPSPA: a) Peridotitic mylonite; b) Kaersutitic mylonite; c- Serpentinized peridotite mylonite; d- Kaersutite-peridotitic banded mylonite . This tectonic event also caused an opening in the occurrence of decimetric blocks ((cid:31) 1 m) in this unit. fracture system of the islands of SPSPA, whose spaces With the development of the Viuvinhas unit sequence, were filled with gray sandstone containing angulated there was a deposition of the reddish sandstones in a pebbles with length varying between 5 and 15 cm, thus calmer environment. Finally, a second seismic event initiating the deposition of the Viuvinhas unit. caused the inclination of the whole St. Peter and St. The deposition of the Viuvinhas unit covered the Paul Formation, and a new rupture on the opening of limits of the fractures, in a more extensive area. the cove turned the island into its current Probably, the seismic activity was responsible for the configuration, where only parts of this bay remain ____________________________________________________________Geological and Palaeontological Sites of Brazil 9