Interactions between hotspots and the Southwest Indian Ridge during the last 90 Ma:Implications on the formation of oceanic plateaus and intra-plate seamounts

This study investigates the relationship between the hotspot-ridge interaction and the formation of oceanic plateaus and seamounts in the Southwest Indian Ocean.We first calculated the relative distance between the Southwest Indian Ridge (SWIR) and relevant hotspots on the basis of models of plate reconstruction,and then calculated the corresponding excess magmatic anomalies of the hotspots on the basis of residual bathymetry and Airy isostasy.The results reveal that the activities of the Marion hotspot can be divided into three main phases:interaction with the paleo-Rodrigues triple junction (73.6-68.5 Ma),interaction with the SWIR (68.5-42.7 Ma),and intra-plate volcanism (42.7-0 Ma).These three phases correspond to the formation of the eastern,central,and western parts of the Del Cano Rise,respectively.The magnitude and apparent periodicity of the magmatic volume flux of the Marion hotspot appear to be dominated by the hotspot-ridge distance.The periodicity of the Marion hotspot is about 25 Ma,which is much longer than that of the Hawaii and Iceland hotspots (about 15 Ma).

Oceanic Plateau Formation Implied by Ontong Java Plateau, Kerguelen Plateau and Shatsky Rise

Oceanic plateaus are a significant type of large igneous provinces in the oceans,providing insights to regional tectonic events and mantle behavior.The three world's largest oceanic plateaus,the Ontong Java Plateau,the Kerguelen Plateau and the Shatsky Rise,are representatives in displaying extraordinary fluxes of magma from mantle to lithosphere.Detailed description incorporating transdisciplinary observations on marine geology,geophysics and geochemistry allow us to test the two lively-debated oceanic plateau formation hypotheses(mantle plume and plate boundary models).Predictions from either hypothesis merely obtain partial support.It is therefore unclear to differentiate one model from another one regarding the oceanic plateau formation.Careful comparisons of the three oceanic plateaus show many commonalities and even more differences in their formation and evolution.This diversity signifies one may not be typical of all.Notably,several key common features,i.e.,massive and rapid eruption and near-ridge formation setting,imply that the lithospheric volcanic emplacement of oceanic plateaus was controlled by seafloor spreading despite a mantle plume exists peripherally.If a coincidence of mantle plume and spreading ridge occurs,it may indicate a plume-ridge interaction.One possible mechanism is that spreading ridge is dragged by a plume and migrates to the location of the plume.Another possibility is that the asthenosphere is fed by a plume nearby and generates melting anomalies along the spreading ridge.

Aseismic ridge subduction and flat subduction:Insights from three-dimensional numerical models

Flat subduction can significantly influence the distribution of volcanism,stress state,and surface topography of the overriding plate.However,the mechanisms for inducing flat subduction remain controversial.Previous two-dimensional(2-D)numerical models and laboratory analogue models suggested that a buoyant impactor(aseismic ridge,oceanic plateau,or the like)may induce flat subduction.However,three-dimensional(3-D)systematic studies on the relationship between flat subduction and buoyant blocks are still lacking.Here,we use a 3-D numerical model to investigate the influence of the aseismic ridge,especially its width(which is difficult to consider in 2-D numerical models),on the formation of flat subduction.Our model results suggest that the aseismic ridge needs to be wide and thick enough to induce flat subduction,a condition that is difficult to satisfy on the Earth.We also find that the subduction of an aseismic ridge parallel to the trench or a double aseismic ridge normal to the trench has a similar effect on super-wide aseismic ridge subduction in terms of causing flat subduction,which can explain the flat subduction observed beneath regions such as Chile and Peru.

A primitive mantle source for the Neoarchean mafic rocks from the Tanzania Craton

Mafic rocks comprising tholeiitic pillow basalt, dolerite and minor gabbro form the basal stratigraphic unit in the ca. 2.8 to 2.6 Ga Geita Greenstone Belt situated in the NW Tanzania Craton. They outcrop mainly along the southern margin of the belt, and are at least 50 million years older than the supracrustal assemblages against which they have been juxtaposed. Geochemical analyses indicate that parts of the assemblage approach high Mg-tholeiite （more than 8 wt.% MgO）. This suite of samples has a restricted compositional range suggesting derivation from a chemically homogenous reservoir. Trace element modeling suggests that the mafic rocks were derived by partial melting within the spinel peridotite field from a source rock with a primitive mantle composition. That is, trace elements maintain primitive mantle ratios （Zr/Hf = 32-35, Ti/Zr - 107-147）, producing flat REE and HFSE profles [（La/Yb）pm = 0.9 -1.3], with abundances of 3-10 times primitive mantle and with minor negative anomalies of Nb [（Nb/ La）pm - 0.6-0.8] and Th [（Th/La）pm = 0.6-0.9]. Initial isotope compositions （εNd） range from 1.6 to 2.9 at 2.8 Ga and plot below the depleted mantle line suggesting derivation from a more enriched source compared to present day MORB mantle. The trace element composition and Nd isotopic ratios are similar to the mafic rocks outcropping -50 km south. The mafic rocks outcropping in the Geita area were erupted through oceanic crust over a short time period, between -2830 and-2820 Ma; are compositionally homogenous, contain little to no associated terrigenous sediments, and their trace element composition and short emplacement time resemble oceanic plateau basalts. They have been interpreted to be derived from a plume head with a primitive mantle composition.

A Dynamic Study of Ekman Characteristics by Using 1998 SCSMEX and TIPEX Boundary Layer Data

A dynamic study on Ekman characteristics by using 1998 SCSMEX and TIPEX boundary layer data is made. The results are as follows: (1) Similar dynamical Ekman characteristics are observed in the Tibetan Plateau and in the South China Sea and its surrounding area. (2) The thickness of the boundary layer is about 2250 m over the Tibetan Plateau, and considering its variation, the thickness could be up to 2250–2750 m. In the tropical southwest Pacific, the thickness of the boundary layer is about 2000 m, and the variation is smaller; a smaller thickness of the boundary layer is in the plain area of the Bohai Sea. (3) Because of the difference in elevation between the Tibetan Plateau and the tropical ocean area, the influence of the boundary layer on the atmosphere is quite different although the two areas have almost the same thickness for the boundary layer, the height where the friction forcing occurs is quite different. (4) The vertical structure of turbulence friction is quite different in the Plateau and in the tropical ocean area. Calculations by 1998 SCSMEX and TIPEX boundary layer data indicate that even in the lowest levels, eddy viscosity in the Tibetan Plateauan can be 2.3 times than in the tropical ocean area.

Implication of Paleoproterozoic Basalt Fertility Related to Mantle Plume Activity in Nassara Gold Mineralization (Burkina Faso, West Africa)

The Birimian Nassara volcanic formations are located south of Gaoua in the southern part of the Boromo belt. Within these formations is the Nassara gold deposit where mineralization is hosted at the contact between basaltic volcanic rocks and sedimentary rocks. It is with the aim of understanding the geodynamic context of the basaltic rocks and the implication of their primary gold potential in the Nassara gold deposit that this work is carried out. To achieve our objectives, 28 samples of fresh basaltic rocks were geochemically analyzed for their major and trace element compositions. These analyses show that the Nassara basalts are Fe-rich tholeiitic basalts. Rare earth profiles (La/SmN = 0.75 - 1.50;La/YbN = 0.65 - 2.18) are fairly flat and without europium anomaly (Eu/Eu* = 0.90 - 1.09), nor niobium. In the Zr/Nb vs. Nb/Th and Nb/Y vs. Zr/Y binary diagrams, the Fe-rich tholeiitic basalts of Nassara, as well as those of the Houndé and Boromo belts, are placed in the field of oceanic plateau basalts related to a mantle plume system. A gold fertility test carried out on these basalts was positive. As other studies have already shown, the genetic link between gold deposits and mantle plumes appears to be a general rule. The scenario for the Nassara gold deposit is that it is the source magma that was already more or less enriched in gold and other related elements on its way up. The remobilization of this gold would have occurred during the Eburnean orogeny with the help of metamorphic, hydrothermal and deformation phenomena to be redeposited at the level of shear zones with economic grades. Through this analysis, we show that the fertility of the initial lithologies is very important for the formation of economic size deposits in the proximal shear zones. Exploration work should now integrate this dimension to define the best targets.

A greenstone belt in southeast Tibet:An accreted middle–late Permian oceanic plateau

Studies of accreted oceanic plateau sections provide crucial information on their structures,compositions,and origins.We investigate the petrogenesis of ultramafic–mafic rocks in the Tangjia–Sumdo greenstone belt of southeast Tibet using petrography,whole-rock geochemistry,and U-Pb zircon geochronology.These rocks are divided into four groups based on geochemical characteristics that include depleted and tholeiitic mafic rocks,transitional mafic rocks,enriched and alkaline mafic rocks,and picritic ultramafic rocks.Depleted and tholeiitic mafic rocks have the oldest crystallization ages(-272 Ma),followed by picritic ultramafic rocks(-270 Ma),transitional mafic rocks(267–254 Ma),and enriched and alkaline mafic rocks(252–250 Ma).Hafnium and neodymium isotope ratios of depleted and tholeiitic mafic rocks(ε_(Hf)(t)=+13.1–+16.9;ε_(Nd)(t)=+6.9–+7.1),transitional mafic rocks(ε_(Hf)(t)=+1.8–+16.9;ε_(Nd)(t)=+0.8–+5.5),enriched and alkaline mafic rocks(ε_(Hf)(t)=+0.5–+5.4;ε_(Nd)(t)=1.5 to+1.9)and picritic ultramafic rocks(ε_(Hf)(t)=+14.9–+17.2;ε_(Nd)(t)=+7.8–+9.0)are similar to those of N-MORB,E-MORB,OIB and depleted-type picritic mafic rocks in other oceanic plateaus,respectively.The geochemical characteristics of the depleted and tholeiitic mafic rocks suggest that they formed by partial melting of depleted spinel lherzolite in a mid-ocean ridge setting,whereas the picritic ultramafic rocks suggest a high degree of partial melting of depleted lherzolite in a hot mantle plume head.The transitional mafic rocks formed by partial melting of moderately enriched garnet lherzolite.The youngest rocks(enriched and alkaline mafic rocks)formed by partial melting of a more enriched garnet lherzolite(compared to transitional mafic rocks)at relatively low temperatures.We propose that the depleted and tholeiitic mafic rocks represent normal oceanic crust of the Sumdo Paleo-Tethys Ocean and the transitional mafic rocks,enriched and alkaline mafic rocks and picritic ultramafic rocks are the fragments of the oceanic plateau,which were related to middle–late Permian mantle plume activity in the Sumdo Paleo-Tethys Ocean.We further suggest that the majority of the Tangjia–Sumdo greenstone belt represents a middle–late Permian oceanic plateau that reflects a previously unrecognized middle–late Permian mantle plume.

Ridge subduction, magmatism, and metallogenesis

Modern oceans contain large bathymetric highs(spreading oceanic ridges,aseismic ridges or oceanic plateaus and inactive arc ridges)that,in total,constitute more than 20–30%of the total area of the world’s ocean floor.These bathymetric highs may be subducted,and such processes are commonly referred to as ridge subduction.Such ridge subduction events are not only very common and important geodynamic processes in modern oceanic plate tectonics,they also play an important role in the generation of arc magmatism,material recycling,the growth and evolution of continental crust,the deformation and modification of the overlying plates,and metallogenesis at convergent plate boundaries.Therefore,these events have attracted widespread attention.The perpendicular or high-angle subduction of mid-ocean spreading ridges is commonly characterized by the occurrence of a slab window,and the formation of a distinctive adakite–high-Mg andesite–Nb-enriched basalt-oceanic island basalt(OIB)or a mid-oceanic ridge basalt(MORB)-type rock suite,and is closely associated with Au mineralization.Aseismic ridges or oceanic plateaus are traditionally considered to be difficult to subduct,to typically collide with arcs or continents or to induce flat subduction(low angle of less than 10°)due to the thickness of their underlying normal oceanic crust(>6–7 km)and high topography.However,the subduction of aseismic ridges and oceanic plateaus occurred on both the western and eastern sides of the Pacific Ocean during the Cenozoic.On the eastern side of the Pacific Ocean,aseismic ridges or oceanic plateaus are being subducted flatly or at low angles beneath South and Central American continents,which may cause a magmatic gap.But slab melting can occur and adakites,or an adakite–high-Mg andesite–adakitic andesite–Nb-enriched basalt suite may be formed during the slab rollback or tearing.Cu-Au mineralization is commonly associated with such flat subduction events.On the western side of the Pacific Ocean,however,aseismic ridges and oceanic plateaus are subducted at relatively high angles(>30°).These subduction processes can generate large scale eruptions of basalts,basaltic andesites and andesites,which may be derived from fractional crystallization of magmas originating from the subduction zone fluid-metasomatized mantle wedge.In addition,some inactive arc ridges are subducted beneath Southwest Japan,and these subduction processes are commonly associated with the production of basalts,high-Mg andesites and adakites and Au mineralization.Besides magmatism and Cu-Au mineralization,ridge subduction may also trigger subduction erosion in subduction zones.Future frontiers of research will include characterizing the spatial and temporal patterns of ridge subduction events,clarifying the associated geodynamic mechanisms,quantifying subduction zone material recycling,establishing the associated deep crustal and mantle events that generate or influence magmatism and Cu-Au mineralization,establishing criteria to recognize pre-Cenozoic ridge subduction,the onset of modernstyle plate tectonics and the growth mechanisms for Archean continental crust.