S. G. Skolotnev

Younger and older zircons from rocks of the oceanic lithosphere in the Central Atlantic and their geotectonic implications

Local U-Pb dating of zircons separated from various rocks in the crest zone of the Mid-Atlantic Ridge (MAR) and Carter Seamount (Sierra Leone Rise) is performed. Younger zircons formed in situ in combination with older xenogenic zircons are revealed in enriched basalts, alkaline volcanic rocks, gabbroic rocks, and plagiogranites. Only older zircons are found in depleted basalts and peridotites. Older zircons are ubiquitous in the young oceanic lithosphere of the Central Atlantic. The age of the younger zircons from the crest zone of the MAR ranges from 0.38 to 11.26 Ma and progressively increases receding from the axial zone of the ridge. This fact provides additional evidence for spreading of the oceanic floor. The rate of half-spreading calculated from the age of the studied zircons is close to the rate of half-spreading estimated from magnetic anomalies. The age of the younger zircons from Carter Seamount (58 Ma) corresponds to the age of the volcanic edifice. Older zircons make up an age series from 53 to 3200 Ma. Clusters of zircons differing in age reveal quasiperiodicity of about 200 Ma, which approximately corresponds to the global tectonic epochs in the geological evolution of the Earth. Several age groups of older zircons combine grains close in morphology and geochemistry: (1) Neoproterozoic and Phanerozoic (53–700 Ma) prismatic grains with slightly resorbed faces, well-preserved or translucent oscillatory zoning, and geochemical features inherent to magmatic zircons; (2) prismatic grains dated at 1811 Ma with resorbed faces and edges, fragmentary or translucent zoning, and geochemical features inherent to magmatic zircons; (3) ovoid and highly resorbed prismatic grains with chaotic internal structure and metamorphic geochemical parameters; the peak of their ages is 1880 Ma. The performed study indicates that older xenogenic zircons from young rocks in the crest zone of the MAR were captured by melt or incorporated into refractory restite probably in the sublithospheric mantle at the level of magma generation in the asthenosphere. It is suggested that zircons could have crystallized from the melts repeatedly migrating through the asthenosphere during geological history or were entrapped by the asthenosphere together with blocks of disintegrated and delaminated continental lithosphere in the process of breakup of the continents older than Gondwana. The variability in the age of older zircons even within individual samples may be regarded as evidence for active stirring of matter as a result of periodically arising and destroyed within-asthenospheric convective flows varying in orientation and scale.

New data on the structure of the Vitoria-Trindade seamount chain (western Brazil basin, South Atlantic)

In 2008, during cruise 24 of the R/V Akademik Vavilov, much of our research work was focused on the central segment (Jaseur and Davis seamounts, Dogaressa Bank) of the Vitoria-Trindade seamount chain (west of the Brazil basin) extending along 20.5° S. Work was conducted to survey the upper part of the sedimentary cover and to perform subbottom profiling. The samples dredged on the seamount slopes are represented by volcanites and Fe-Mn crusts.

First data on the age of rocks from the central part of the Vitoria-Trindade Ridge (Brazil Basin, South Atlantic)

Micropaleontological and isotope-geochronological investigations of calcareous sedimentary rocks and volcanites dredged out from the central portion of the submarine Vitoria-Trindade Ridge during the 24th cruise of R/V Akademik Vavilov have been conducted. It has been established based on micropaleontological analysis, which included determination of the species composition of foraminifera and nannoplankton, that the sequence of sedimentary rocks having a pelagic nature formed on the slopes of the volcanic seamounts in the central portion of the Vitoria-Trindade Ridge from the Early to Mid-Miocene to the Holocene; a good correlation between the degree of lithification of these rocks and their age is observed. It has also been established that the carbonate platforms on the abraded tops of the Davis Seamount and the Dogaressa Bank, which are located in the east-central portion of the Vitoria-Trindade Ridge, started forming in the Early Miocene (19–24 Ma). It has been determined using local U-Pb dating of zircon grains with a SHRIMP-II high resolution secondary ion mass spectrometer that the volcanites forming the upper portion of the volcanic rock sequence of the Jaseur Seamount (29.8 ± 6.6 Ma) located in the west-central portion of the Vitoria-Trindade Ridge date to the Oligocene. The investigations conducted have confirmed the opinion that the Vitoria-Trindade Ridge formed in general because of the activity of the hot spot located under the volcanic Trindade and Martin Vaz Islands. However, separate extended lenticular segments of this ridge existed for a long time as single structures, within which the age of the seamounts was not linearly dependent on the distance from the location of the hot spot. Lenses of hot mantle matter that form at the sublithospheric level as a result of impulses of plume activity and move along with the lithospheric plate play a defining role in the development of individual segments forming the Vitoria-Trindade Ridge.

The structure of the Knipovich-Mohns junction (North Atlantic)

In 2007, the Geological Institute of the Russian Academy of Sciences (GIN RAS) carried out investigations in the North Atlantic, in the southern part of the Knipovich Ridge extending for 600 km from the Mohns spreading ridge to the Molloy fault zone (chief of the cruise A.V. Zaionchek). The investigations were conducted according to the Program of the RAS Presidium entitled “Basic Problems of Oceanology: World Ocean Physics, Geology, Biology, Ecology” (Project “Regularities of the Structure and Formation of the Oceanic Crust in Characteristic Regions of the Atlantic Ocean: Tectonics, Magmatism, Composition and Genesis of Fe‐Mn Deposits,” supervisor Academician Yu.M. Pushcharovskii). The problem facing the expedition was to study the geological structure of the Knipovich‐Mohns junction. With the help of the R/V Akademik Nikolaj Strakhov (Cruise 25), there were conducted complex areal, medium-scale, regional geological study of the selected object, which involved echo-sounding with SeaBat 7150 multibeam sounder, continuous seismic profiling (CSP), high-frequency sounding with the Edgetech 3300 profiler, and bottom dredging (Fig. 1). Within the region of 74 ° N, south of the Greenland Fault Zone (FZ) extending southeastward, the Mohns spreading ridge passes into the north‐south-trending Knipovich Ridge. The peculiarity of both ridges is that these are unified extensive spreading structures not broken into segments by transform faults. The ridges differ in the time and conditions of formation. From the beginning of formation, regular and steady growth of the oceanic crust in the rift zone was characteristic of the Mohns Ridge, which is marked by the symmetrical and natural position of linear magnetic anomalies relative to the rift valley axis [1, 2]. The Knipovich Ridge began forming under unsteady geodynamic conditions, which was reflected in the disordered position and fragmentation of magnetic anomalies. The region of the Mohns and Knipovich junction attracts the attention of researchers in that this is a unique area where one spreading ridge passes into another with rift valley structures gradually bending by 40 ° without apparent transform faults serving as accommodation zones for stresses generated in the course of plate motions. Hence, the geodynamics of structures in this key region has been the subject of investigations.

Alkali volcanism of the Bathymetrists Seamounts chain (Central Atlantic): Description and comparison

were analyzed for major and trace elements at the chemical laboratories of the Geological Institute (Moscow) and the Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences (table). All volcanic rocks dredged from Carter Seamount are strongly altered. The losses of ignition vary from 10 to 14%. In terms of major element composition, the volcanic rocks of group B1 are ascribed to olivine melilitites, while B2 are classed with nephelinites. Olivine melilitites contain 3‐5% clay pseudomorphs (0.5‐ 5 mm in size) after olivine phenocrysts. Microphenocrysts (up to 0.1‐0.3 mm) of clinopyroxene (augite and Ti-augite) and melilite account for 2‐3%. The matrix is made up of lathlike microlites of melilite (20‐30%) and clinopyroxene (augite) (20‐30%), as well as equant grains of Ti-magnetite (about 5%) embedded in a flaky aggregate of secondary minerals (possibly chlorite and zeolite). About 5‐8% is occupied by 0.5‐3 mm pores filled with various zeolites. Nephelinites contain about 2‐3% microphenocrysts of greenish egirine‐augite 0.2‐1 mm in size. The groundmass consists of microlites of egirine‐augite (15‐20%) and nepheline (20‐30%) and grains of Ti-magnetite (about 5%) set in a flaky matrix of secondary minerals. In the oxide‐MgO variation diagrams, the composi

New SHRIMP U-Pb Age Data on Zircons from Plagiogranites in the Ophiolites of the Kamchatsky Mys Peninsula, Eastern Kamchatka

Geological setting. Kamchatsky Mys peninsula has composite fold-and-thrust structure, constructed by Cretaceous and Paleocene-Eocene volcanic and terrigenous-tuffaceous rocks and tectonic slices of serpentinite mélange, gabbroids and ultrabasic rocks [Khotin, 1976; Zinkevich et al., 1985; Rasnitsyn et al., 1985; Fedorchuk, 1989; Shapiro, 1987; Accretional ... , 1993; Boyarinova et al., 2000, 2001; Saveliev, 2004]. Its southern part (Afrika block) according to [Zinkevich et al., 1985] is constructed by four allochtonous complexes. They are composed of: 1) Aptian-Albian and Albian-Cenomanian calcareouscherty-effusive assemblages (Afrika complex), 2) Campanian-Maastichtian cherty-tuffaceous and terrigenous deposits, 3) Paleocene-Lower Eocene chert-volcanic sequence (Kamensk complex), 4) serpentinite mélange and gabbroids. Serpentinite mélange contains unaltered ultrabasic rocks (Mountain Soldatskaya massif) and smaller bodies of gabbroids, Cretaceous and Paleogene tuffaceous and cherty rocks. Ophiolite fragments at Kamchatsky Mys peninsula are represented by gabbroids of Olenegorsk pluton and ultrabasic rocks of Soldatsky massif, by blocks of gabbroids with plagiogranites in serpentinite mélange, different basalts and calcareous-jasper-cherty deposits of Aptian-Cenomanian Afrika complex and tholeiite basalts, mudstones of Paleocene-Eocene Kamensk complex.

Origin of Submarine Volcanism at the Eastern Margin of the Central Atlantic: Investigation of the Alkaline Volcanic Rocks of the Carter Seamount (Grimaldi Seamounts)

This paper addresses the composition, geochemistry, isotopic characteristics, and age of rocks from the Carter Seamount of the Grimaldi seamount group at the eastern margin of the Central Atlantic. The age of the seamount was estimated as 57–58 Ma. Together with other seamounts of the Grimaldi system and the Nadir Seamount, it forms a “hot line” related to the Guinea Fracture Zone, which was formed during the late Paleocene pulse of volcanism. The Carter Seamount is made up of olivine melilitites, ankaramites, and analcime-bearing nepheline tephrites, which are differentiated products of the fractional crystallization of melts similar to an alkaline ultramafic magma. The volcanics contain xenoliths entrained by melt at different depths from the mantle, layer 3 of the oceanic crust, which was formed at 113–115 Ma, and earlier magma chambers. The rocks were altered by low-temperature hydrothermal solutions. The parental melts of the volcanics of the Carter Seamount were derived at very low degrees of mantle melting in the stability field of garnet lherzolite at depths of no less than 105 km. Anomalously high Th, Nb, Ta, and La contents in the volcanics indicate that a metasomatized mantle reservoir contributed to the formation of their primary melts. The Sr, Pb, and Nd isotopic systematics of the rocks show that the composition of the mantle source lies on the mixing line between two mantle components. One of them is a mixture of prevailing HIMU and the depleted mantle, and the other is an enriched EM2-type mantle reservoir. These data suggest that the formation of the Carter Seamount volcanics was caused by extension-related decompression melting in the Guinea Fracture Zone of either (1) hot mantle plume material (HIMU component) affected by carbonate metasomatism or (2) carbonated basic enclaves (eclogites) ubiquitous in the asthenosphere, whose isotopic characteristics corresponded to the HIMU and EM2 components. In the former case, it is assumed that the melt assimilated during ascent the material of the metasomatized subcontinental mantle (EM2 component), which was incorporated into the oceanic lithospheric mantle during rifting and the breakup of Pangea.

New data on composition and structure of the Pernambuco Seamounts, Brazil basin, south Atlantic region

330 Research vessel Akademik Ioffe performed investi gation of the Pernambuco Seamounts in the Brazil basin in the profile of cruise 33 in 2011. The chain of seamounts is oriented in the northwestern direction and consists of individual roughly NS , roughly EW , NE , and NW trending segments. The two highest seamounts in the northern part of the chain were investigated. They characterize roughly NS and EW trending segments conjugated at a right angle. The structure of the upper part of the sedimentary cover was studied with a SES 2000 deep seismoacoustic profilograph, while the bottom relief was investigated with an ELAC echosounder. Judging from the mor phology of slopes, the seamounts are characterized by a three level structure reflecting a few stages of their formation. Formation of swells was followed by devel opment of volcanic edifices emergening above sea level. Extinction of volcanic activity and marine abra sion of volcanic summits were followed by growth of carbonate platforms. On the slopes and at the feet of paleovolcanoes, there are abundant landslide and coarse grained debris deposits overlapped by pelagic stratified sediments. The latter are occasionally bro ken by neotectonic deformations.

New Data on the Geological Structure of the Junction between the Cape Verde Seamount and the Cape Verde Basin, Central Atlantic

The regions of conjugation of continental rise with abyssal oceanic basins at the margins of the Atlantic— a transitional zone between continental and oceanic lithospheres—are still poorly studied in geological terms. In the course of expeditions conducted by the Geological Institute (Moscow), the structure of this zone was studied at the continental slope of Africa, south of the Cape Verde Islands. In this area, the continental rise widens sharply making up a near-latitudinal promontory that divides the abyssal Cape Verde Basin in the south and the Canary Basin in the north. The study area is situated in the pinchout area of the system of transform fracture zones (TFZ) located south of the Fifteen Twenty TFZ [1, 2]. The near-latitudinal linear ridges and troughs on the bottom of the Cape Verde Basin are the eastern flanks of the Vema, Doldrums, Arkhangelsky, and Vernadsky TFZs of the Mid-Atlantic Ridge. Near the continental slope of Africa, these TFZs are cut off by the WNW-trending escarpment (Fig. 1). The bathymetric survey of a local area in the deepwater Cape Verde Basin that adjoins the southern margin of the Cape Verde Seamount (Figs. 1, 3) was carried out during Cruise 22 of the R/V Akademik Nikolai Strakhov in 2000. We have established an azimuthal unconformity between near-latitudinal depressions and ridges that extend from MAR, on the one hand, and the WNW-trending transversal Cabo Verde Escarpment, on the other hand [3]. The seafloor in the studied test area is complicated by volcanic edifices and by the anomalously deep (>6000 m) Strakhov Basin trending in the NW direction discordantly relative to other structural units. The previously unknown Neva deepwater channel was also found (Fig. 3). This paper has been prepared on the basis of the data collected during Cruise 16 of the R/V Akademik Ioffe in 2004. During this cruise, the sedimentary cover was studied with continuous seismic profiling (CSP). The structure of the upper part of the sedimentary cover and the bottom topography was investigated with a Parasound acoustic profilograph along a profile between 11.52 ° N × 22.67 ° W and 10.13 ° N × 24.07 ° W (Fig. 1). The structure of the upper part of the sedimentary cover in the Neva Channel and the Strakhov Basin was studied with the same method. The bedrock samples and cores of bottom sediments were recovered in the same place. Structure of the sedimentary cover (based on CSP data). The CSP profile across the junction of continen

Morphology and impurity elements of zircon in the oceanic lithosphere at the Mid-Atlantic ridge axial zone (6°–13° N): Evidence of specifics of magmatic crystallization and postmagmatic transformations

The paper presents newly obtained original data on the morphology, internal structure (as seen in cathodoluminescence images, CL), and composition of more than 400 zircon grains separated from gabbroids and plagiogranites (OPG) sampled at the axial zone of the Mid-Atlantic Ridge (MAR). The zircons were analyzed for REE by LA-ICP-MS and for Hf, U, Th, Y, and P by EPMA. Magmatic zircon in the gabbroids crystallized from differentiating magmatic melt in a number of episodes, as follows from systematic rimward increase in the Hf concentration, and also often from the simultaneous increase in the (U + Th) and (Y + P) concentrations. These tendencies are also discernible (although much less clearly) in zircons from the OPG. Zircon in the OPG is depleted in REE compared to the least modified zircons in the gabbro, which suggests that the OPG were derived via partial melting of gabbro in the presence of seawater-derived concentrated aqueous salt fluid. Another reason for the REE depletion might be simultaneous crystallization of zircon and apatite. The CL-dark sectors, which are found in practically all of the magmatic zircon grains, have Y/P (a.p.f.u.) ≫ 1 which most likely resulted from OH accommodation in the zircon structure, a fact suggesting that the OPG parental melt contained water. High-temperature hydrothermal processes induced partial to complete recrystallization of zircon (via dissolution-reprecepitation), a process that was associated with ductile and brittle deformations of the zircon-hosting rocks. The morphology of the hydrothermal zircons varies depending on pH and silica activity in the fluid from weakly corroded subhedral crystals with typical vermicular microtopography of the crystal faces to completely modified grains of colloform structure. Geochemically, the earlier hydrothermal transformations of the zircons resulted in their enrichment in La and other LREE, except only Ce, whose concentration, conversely, decreases compared to that of the unmodified magmatic zircons. The hydrothermal zircon displays a reduced Ce anomaly and its most altered domains typically host minute inclusions of xenotime, U and Th oxides and silicates, and occasionally also baddeleyite, which suggests that the hydrothermal fluid was reduced and highly alkaline. These features were acquired by the seawater-derived fluid when it circulated within the axial MAR zone area due to phase separation in the H2O–NaCl system and particularly as a result of fluid interaction with the abyssal peridotites of oceanic core complexes. Our data demonstrate that zircon is a sensitive indicator of tectonic and physicochemical processes in the oceanic crust.