Tectono-magmatic Processes Research Articles

About age of the lunar surface

Most part of the lunar surface relief was formed during the last 5 Ma. This conclusion was received on the basis of detail analysis of large craters of the Moon, Earth, Mars and Mercury. Falling of the galactic comets in the period 5-0.6 Ma, and the tectonomagmatic processes induced by the comets falling played major role in shaping of the Moon topography. Processes of tectonics and volcanism are occurring on the Moon today also. We found volcano in the Tsiolkovsky crater on the reverse side of the Moon that can serve as good example of that. The volcano has a height of 102 m and is located almost in the bottom center of the crater with a diameter of 180 km on a low oval elevation of plume nature 24-26 km in size. Introduction. It is believed that the relief of the lunar surface, as well as Mercury and Mars have formed more than 3000 Ma as a result of falls on these celestial bodies planetesimals that remained in the interplanetary space after the Solar system formation (Hiesinger et al., 2010). To substantiate this view are used data on the craters, as well as measurements of the isotopic age samples of lunar rocks delivered to Earth and testifying to their formation more than 3000 Ma (Hayes, Walker, 1975). The planetary exploration by spacecraft made in recent years, however, cast doubt on such an ancient age of the surface topography, in particular, the Moon and Mars. Large masses of frozen water were discovered at the poles this celestial bodies. Moreover recently dry riverbeds can be seen on the Mars (Wikipedia). These and many other facts do not find a convincing explanation within the framework of existing concepts. We suggest another interpretation of the observed facts. It is based on attracting galaxycentric paradigm (Barenbaum, 2010) to analyze the distribution a cometary craters on surface planets as well as on the discovery in one of them (in the Tsiolkovsky crater on the reverse side of the Moon) modern volcano (Shpekin, 2009). Our studies show that most of the surface of the Moon, Mars and Mercury are completely saturated by such craters. Owing to that formation the craters is associated with ejection of rocks from depths of ~ 3 km or more large age of the lunar rocks samples says in the first place about the time of solidification of their material but not about the actual age of the formation of the lunar surface. Below we present arguments and evidence that the age of lunar surface is hardly older 5 Ma and the process formation of surface the Moon continues today. General characteristics of the Moon relief. The main topographical features of the topography of the Moon, Mars and Mercury are uplifted the areas of surface - and the more lower areas of the surface -seas. Continents uninterruptedly dotted with large craters, while the seas covered by craters to a much lesser degree. It is significant that the continents tend to be located the southern hemisphere of the celestial bodies whereas marine surfaces is mainly located in its northern hemisphere. There is also an important specificity in the morphology and distribution of craters. According to (Pike, 1977) on the moon we can distinguish two different populations of craters - with a diameter D 15 km. The first are the most numerous in the seas, and the second - on the continents. The depth H of the first is approximately equal to 1/5 of their diameter, while depth craters of the second type is smaller. The first type of craters has a simple structure and is best described by the dependence H = 0.196 D 1.01 , whereas the second type is more complicated. The second type craters has central hills and gentle slopes. In the diameter range from 11 to 400 km these craters are followed the depending H = 1.044D 0.301 . Thus in the process of impact origin craters on the surface may be ejected rocks from a depth

Chemical geodynamics of Western Anatolia

We report major and trace element concentrations and Nd–Sr–Pb isotopic data of 10 post-collisional volcanic domains in Western Anatolia, a seismically active part of the Alpine–Himalayan belt in the Aegean extensional province. Our objective is to provide geochemical constraints for tectono-magmatic processes shaping the late Cenozoic geodynamic evolution of Western Anatolia. Calc-alkaline volcanic rocks occurring to the north of the Izmir–Ankara–Erzincan suture zone show arc-like trace elements and isotopes and were formed by the melting of the metasomatized Neotethyan mantle-wedge; this process was facilitated by asthenospheric upwelling resulting from slab delamination. Calc-alkaline and alkaline volcanic rocks from within the Izmir–Ankara–Erzincan suture zone also show the imprint of subduction fluids in their major and trace elements, but their isotopic compositions indicate derivation from a metasomatized lithospheric mantle followed by assimilation of ancient crust. Volcanics along the N–S-oriented Kirka–Afyon–Isparta trend were derived from the lithospheric mantle that was metasomatized by fluids from the older subduction of the African plate. Golcuk–Isparta volcanic rocks show an asthenospheric imprint; the latter was a consequence of upwelling following a tear in the subducting African lithosphere. Shoshonitic Kula volcanic rocks show very high trace element concentrations, OIB mantle-like trace elements, and Nd–Sr–Pb isotopic signatures, and were formed by partial melting of the upwelling asthenospheric mantle; this event was synchronous with the Aegean extension and possibly also with slab window formation due to ruptures in the African plate. Inherent in the above chemical geodynamic models are the high ϵNd(0) values (+6.4) of the end-member volcanic rocks, implying the presence of an asthenospheric source beneath Western Anatolia that is responsible for the currently observed high heat flow, low Pn wave velocities, high seismicity, and tectonic activity.

Fingerprinting orogenic delamination

A signifi cant portion of the Earth’s lithosphere is recycled into the deeper mantle, as required by mass balance considerations in orogenic environments. The two principal mechanisms for recycling are subduction at plate margins and delamination. Subduction is a well-understood process that is essential to the plate tectonic engine of planet Earth. Delamination, on the other hand, requires recycling via convective removal of the lower parts of the lithosphere, and is more diffi cult to detect. One chief argument for delamination comes from extreme shortening at continental convergent margins, which requires far thicker mantle lithospheres than observed (DeCelles et al., 2009). The second argument comes from the intermediate average composition of the continental crust (Rudnick, 1995), which requires a large ultramafi c complementary residue at the bottom of the continental crust; such a reservoir has not been identifi ed over large portions of continental areas. Delamination (Bird, 1979), convective removal, foundering, and lithospheric dripping are terms used for the process of detachment and sinking of the lower parts of the continental lithosphere other than those that may have been buried into the mantle via continental subduction. Most researchers using the term “delamination” refer to a density-driven process of foundering, and do not imply its original “peeling-off” signifi cance as defi ned by Bird (1979), which is closer to tectonic underplating in shallow subduction environments. Delamination is a form of vertical and spatially localized tectonics often generating amoeba-like or circular surface effects that are regional results of tectonomagmatic processes at convergent plate margins. Mantle delamination is the process of foundering of dense, unstable

Stages of Late Proterozoic magmatism and periods of Au mineralization in the Yenisei Ridge

Geological, isotopic, and geochemical data permitted distinguishing the Mesoproterozoic (1.6–1.05 Ga), Early Neoproterozoic (1.05–0.8 Ga), and Late Neoproterozoic (0.8–0.6 Ga) stages of magmatism and crustal evolution in the Yenisei Ridge. Each of them contributed to the regional Au metallogeny. In the Early Mesoproterozoic, crustal destruction and stretching in the southwestern Siberian craton (Yenisei Ridge) led to the initiation of a pericratonic trough, the formation of rift mafic associations (Rybnaya–Panimba volcanic belt), and the accumulation of fine-grained terrigenous sediments (Sukhoi Pit Group). Black carbonaceous shales and the picrite basalt–basalt association were enriched in Au. In the early Neoproterozoic, the terrigenous strata of the Sukhoi Pit Group were deformed, metamorphosed, and granitized as a result of the Grenville orogeny. Granite-gneiss domes formed in the earlier, syncollisional, period (1.05–0.95 Ga) of this stage, and K–Na granitoid plutons formed in the late collisional one (0.88–0.86 Ga). Premineral metasomatites formed in the zone where these plutons influenced enclosing black shale–terrigenous strata. They host Au deposits, which formed later. The formation of quartz-reef zones correlates with that of thrust nappes (0.85–0.82 Ga). The latter is genetically related to the final stage of the evolution of a collisional orogen. In the Late Neoproterozoic, rift and intraplate magmatism was most intense and frequent (780, 750, 700, 670–650 Ma) in the Tatarka–Ishimba fault system. Manifestations of Mesoproterozoic volcanism and all the Au deposits of the Central metallogenic belt in the Yenisei Ridge are concentrated here. The three periods of gold–arsenopyrite–quartz, gold–sulfide, and Au–Sb mineralization correlate well with the initiation and evolution of rift structures and the manifestations of intraplate magmatism at 800–770, 720–700, and 670–650 Ma. The tectonomagmatic processes which took place in these periods might have been crucial in Au ore accumulation.

Termination of backarc spreading: Zircon dating of a giant oceanic core complex

The Godzilla megamullion is the largest oceanic core complex (OCC) currently known, and is adjacent to the spreading center of the Parece Vela Basin (PVB), an extinct backarc basin in the Philippine Sea. The duration and termination of tectonomagmatic processes during OCC formation are poorly constrained, due to the weak geomagnetic anomalies in the region. Zircon U-Pb dating of gabbroic and leucocratic rocks from the Godzilla megamullion reveals that fault-induced spreading over the ∼125 km length of the OCC lasted for ∼4 m.y., with continuous magmatic accretion at the spreading axis. The latest magmatism constrains the cessation of PVB spreading to ca. 7.9 Ma or later, significantly younger than a previous estimate of ca. 12 Ma. The new ages show that backarc basin formation migrated to the present-day Mariana Trough soon after the cessation of spreading in the PVB.

Geochemistry and petrogenesis of volcanic rocks in the Kuril island arc

Newly obtained precise analytical data on trace elements and radiogenic Sr, Nd, and Pb isotopes testify to anomalous geochemical characteristics of mafic and intermediate Quaternary lavas in Paramushir (in the north of the Kuril arc), Kunashir and Iturup (in the south) islands, which are the largest three islands of the Kuril island arc. The high K and LREE concentrations in the volcanic products in Paramushir Island resulted from the southward expansion of the mantle thermal anomaly of the Kamchatka Peninsula and the involvement of melts related to the melting of oceanic sediments in magma generation. The depleted characteristics of the mafic volcanics are explained by the relatively young tectono-magmatic events during the opening of the Kuril backarc basin. The Kuril island-arc system developed on a heterogeneous basement. The northern islands are a continuation of the volcanic structures of southern Kamchatka, which were formed above an isotopically depleted and hot lithospheric mantle domain of composition close to that of the Pacific MORB type. The southern islands were produced above an isotopically enriched and cold lithospheric domain of the Indian-Ocean MORB type, which was modified in relation to relatively young backarc tectono-magmatic processes. Although issues related to the genesis of the transverse geochemical zoning were beyond the originally formulated scope of our research, the homogeneous enough isotopic composition of the rear-arc lavas in the absence of any mineralogical and geochemical lines of evidence of crustal contamination suggests an independent magmatic source.

Tin deposits of the Sikhote–Alin and adjacent areas (Russian Far East) and their magmatic association

The Sikhote–Alin accretionary belt along the northwestern Pacific Plate hosts the most important tin province of Russia. Here, more than 500 ore deposits were formed between 105 and 55 Ma at transform and active subduction margins. Petrological models suggest an active role of the mantle in the mineralisation processes. The deposits can be divided into three groups according to their mineral content and associated magmatism. The first group, a cassiterite–quartz group is defined by tin-bearing greisens as well as quartz–cassiterite and quartz–cassiterite–feldspar veins and stockworks. The mineralisation shows distinct genetic relationships with S- and A-type granites. The deposits are located mainly in Jurassic accretionary prisms adjacent to the Bureya–Khanka Paleozoic continental terrane margin. The second group is represented by the economically important cassiterite–silicate–sulfide deposits, which produce about 80% of Russian tin. Mineralisation in this group is represented by metasomatic zones or veins related to I-type granitoids. The orebodies consist of cassiterite–tourmaline–quartz or cassiterite–chlorite–quartz associations and contain variable amounts of sulfides. The third group comprises tin deposits containing cassiterite and sulfides with the most complicated ore composition with abundant sulfides and sulfostannates accounting for 60–80% of the total ore mass. In some deposits, zinc, lead and silver dominate, whereas tin is sub-economic. The deposits of this group are generally associated with magmatic rocks of the Sikhote–Alin volcano-plutonic belt. The different associations are found together in the same districts and, locally, also in individual deposits. These are characterised by polychronous and polygenetic mineral systems, formed during long periods of time and in different tectonic settings. This testifies to changes in the many physico-chemical parameters of ore formation and, probably, of ore sources. We suggest that the complex mineral and element compositions of some of the ores were caused by the long-lasting composite tectono-magmatic processes.

Tectonomagmatic evolution of the Earth and Moon

The Earth and Moon evolved following a similar scenario. The formation of their protocrusts started with upward crystallization of global magmatic oceans. As a result of this process, easily fusible components accumulated in the course of fractional crystallization of melt migrating toward the surface. The protocrusts (granitic in the Earth and anorthositic in the Moon) are retained in ancient continents. The tectonomagmatic activity at the early stage of planet evolution was related to the ascent of mantle plume of the first generation composed of mantle material depleted due to the formation of protocrusts. The regions of extension, rise, and denudation were formed in the Earth above the diffluent heads of such superplumes (Archean granite-greenstone domains and Paleoproterozoic cratons), whereas granulite belts as regions of compression, subsidence, and sedimentation arose above descending mantle flows. The situation may be described in terms of plume tectonics. Gentle uplifts and basins (thalassoids) in lunar continents are probable analogues of these structural elements in the Moon. The period of 2.3–2.0 Ga ago was a turning point in the tectonomagmatic evolution of the Earth, when geochemically enriched Fe-Ti picrites and basalts typical of Phanerozoic within-plate magmatism became widespread. The environmental setting on the Earth’s surface changed at that time, as well. Plate tectonics, currently operating on a global scale, started to develop about ∼2 Ga ago. This turn was related to the origination of thermochemical mantle plumes of the second generation at the interface of the liquid Fe-Ni core and silicate mantle. A similar turning point in the lunar evolution probably occurred 4.2–3.9 Ga ago and completed with the formation of large depressions (seas) with thinned crust and vigorous basaltic magmatism. Such a sequence of events suggests that qualitatively new material previously retained in the planets’ cores was involved in tectonomagmatic processes at the middle stage of planetary evolution. This implies that the considered bodies initially were heterogeneous and were then heated from above to the bottom by propagation of a thermal wave accompanied by cooling of outer shells. Going through the depleted mantle, this wave generated thermal superplumes of the first generation. Cores close to the Fe + FeS eutectics in composition were affected by this wave in the last turn. The melting of the cores resulted in the appearance of thermochemical superplumes and corresponding irreversible rearrangement of geotectonic processes.

Do terrestrial planets evolve according to the same scenario? Geological and petrological evidence

The evolution of terrestrial planets (the Earth, Venus, Mars, Mercury, and Moon) was proved to have proceeded according to similar scenarios. The primordial crusts of the Earth, Moon, and, perhaps, other terrestrial planets started to develop during the solidification of their global magmatic “oceans”, a process that propagated from below upward due to the difference in the adiabatic gradient and the melting point gradient. Consequently, the lowest melting components were “forced” toward the surfaces of the planets in the process of crystallization differentiation. These primordial crusts are preserved within ancient continents and have largely predetermined their inner structure and composition. Early tectono-magmatic activity at terrestrial planets was related to the ascent of mantle plumes of the first generation, which consisted of mantle material depleted during the development of the primordial crusts. Intermediate evolutionary stages of the Earth, Moon, and other terrestrial planets were marked by an irreversible change related to the origin of the liquid essentially iron cores of these planets. This process induced the ascent of mantle superplumes of the second generation (thermochemical), whose material was enriched in Fe, Ti, incompatible elements, and fluid components. The heads of these superplumes spread laterally at shallower depths and triggered significant transformations of the upper shells of the planets and the gradual replacement of their primordial crusts of continental type by secondary basaltic crusts. The change in the character of the tectono-magmatic activity was associated with modifications in the environment at the surface of the Earth, Mars, and Venus. The origin of thermochemical mantle plumes testifies that the tectono-magmatic process involved then material of principally different type, which had been previously “conserved” at deep portions of the planets. This was possible only if (1) the planetary bodies initially had a heterogeneous inner structure (with an iron core and silicate mantle made up of chondritic material); and (2) the planetary bodies were heated from their peripheral toward central portions due to the passage of a “thermal wave”, with the simultaneous cooling of the outer shells. The examples of the Earth and Moon demonstrate that the passage of such a “wave” through the silicate mantles of the planets was associated with the generation of mantle plumes of the first generation. When the “wave” reached the cores, whose composition was close to the low-temperature Fe + FeS eutectic, these cores started to melt and gave rise to superplumes of the second generation. The “waves” are thought to have been induced by the acceleration of the rotation of these newly formed planets due to the decrease of their radii because of the compaction of their material. When this process was completed, the rotation of the planets stabilized, and the planets entered their second evolutionary stage. It is demonstrated that terrestrial planets are spontaneously evolving systems, whose evolution was accompanied by the irreversible changes in their tectono-magmatic processes. The evolution of most of these planets (except the Earth) is now completed, so that they “dead” planetary bodies.

Kinematic setting and structural control of arc volcanism

Abstract The evaluation of the kinematic setting and the structural control of volcanic arcs are important in defining the tectono-magmatic processes along convergent plate boundaries. However, our knowledge is fragmented and the available data highlight different behaviours. This study analyzes the kinematic setting, the structural control and the volcanic productivity of 16 arcs. These arcs are characterized by predominant extensional, compressional, strike–slip or oblique motions. There is an overall coupling between the normal vs. parallel motions along the arc and those of the underlying slab. Therefore, the higher the trench-normal (or parallel) component of the subduction rate, the higher is the amount of arc-normal (or parallel) motion. This relation confirms that strain partitioning is, in general, feasible at many convergent settings, involving also the volcanic arc portion. The arc-normal motion may be characterized by extension or compression, as a function of the increase in the trench-normal convergence (or subduction) velocity. The lack of an evident relation between the subduction rate (or convergence rate) and the volcanic productivity of the arc is partly inconsistent with previous studies, which highlight a relation between the subduction rate and melt production below arcs. This discrepancy mainly suggests that different processes control the generation, rise and eruption of magma at different depths, varying the intrusive/extrusive ratio along arcs. The structure of the arc does not control the distribution of the volcanoes; however, it does control the volcanic output through different processes, even though regional or local extension (associated with strike–slip or compressive structures) is the ultimate requisite, in any setting. In general, the higher the amount of extension, the higher is the output rate along the arc.

Irreversible evolution of tectono-magmatic processes at the Earth and Moon: Petrological data

The tectono-magmatic evolution of the Earth and Moon started after the solidification of their magmatic “oceans”, whose in-situ crystallization produced the primordial crusts of the planets, with the composition of these crusts depending on the depths of the “oceans”. A principally important feature of the irreversible evolution of the planetary bodies, regardless of their sizes and proportions of their metallic cores and silicate shells, was a fundamental change in the course of their tectono-magmatic processes during intermediate evolutionary stages. Early in the geological evolution of the Earth and Moon, their magmatic melts were highly magnesian and were derived from mantle sources depleted during the solidification of the magmatic “oceans”; this situation can be described in terms of plume tectonics. Later, geochemically enriched basalts with high concentrations of Fe, Ti, and incompatible elements became widespread. These rocks were typical of Phanerozoic within-plate magmatism. The style of tectonic activity has also changed: plate tectonics became widespread at the Earth, and large depressions (maria) started to develop at the Moon. The latter were characterized by a significantly thinned crust and basaltic magmatism. These events are thought to have been related to mantle superplumes of the second generation (thermochemical), which are produced (Dobretsov et al., 2001) at the boundary between the liquid core and silicate mantle owing to the accumulation of fluid at this interface. Because of their lower density, these superplumes ascended higher than their precursors did, and the spreading of their head parts resulted in active interaction with the superjacent thinned lithosphere and a change in the tectonic regime, with the replacement of the primordial crust by the secondary basaltic one. This change took place at 2.3–2.0 Ga on the Earth and at 4.2–3.9 Ga on the Moon. Analogous scenarios (with small differences) were also likely typical of Mars and Venus, whose vast basaltic plains developed during their second evolutionary stages. The change in the style of tectonic-magmatic activity was associated with important environmental changes on the surfaces of the planets, which gave rise to their secondary atmospheres. The occurrence of a fundamental change in the tectono-magmatic evolution of the planetary bodies with the transition from depleted to geochemically enriched melts implies that these planets were originally heterogeneous and had metal cores and silicate shells enriched in the material of carbonaceous chondrites. The involvement of principally different material (that had never before participated in these processes) in tectono-magmatic processes was possible only if these bodies were heated from their outer to inner levels via the passage of a heating wave (zone) with the associated cooling of the outermost shells. The early evolutionary stages of the planets, when the waves passed through their silicate mantles, were characterized by the of development of super-plumes of the first generation. The metallic cores were the last to melt, and this processes brought about the development of thermochemical super-plumes.

Geophysical insights and early spreading history in the vicinity of the Jan Mayen Fracture Zone, Norwegian–Greenland Sea

Using a new high-resolution aeromagnetic survey (JAS-05) that was acquired along the trend of the Jan Mayen Fracture Zone (JMFZ), west of the Vøring volcanic margin, we investigated the geodynamic framework of the early spreading evolution of the Norwegian–Greenland Sea. The tectonic structure, main faults and magnetic chrons have been reinterpreted based on new magnetic gridded data and integrated with bathymetry, gravity and seismic data. The new interpretation reveals more details about the early spreading history of the Norwegian–Greenland Sea in the vicinity of the JMFZ. Although anomalous melt production (seaward-dipping reflectors, underplating) associated with the breakup of the Mid-Norwegian margin has been described in many studies, we present data that suggest that significant magmatism continued episodically during the opening of the Norwegian–Greenland Sea along the trend of the JMFZ. The Vøring Spur (VS), an anomalous oceanic high, lying north of the eastern segment of the JMFZ exhibits a contrasting Bouguer gravity low and a complex magnetic signature. The gravity signature of the VS can be modelled and explained as an abnormal thick oceanic crust, which locally can reach up to 15 km. We propose that the thick oceanic crust (overcrusting) was syn-rift and formed during Mid- to Late Eocene. A plate reconstruction at Eocene time suggests that the VS could be part of a triple junction initiated during the breakup between the Vøring Marginal High and the Greenland part of the Traill–Vøring igneous complex, now located offshore East Greenland. Mantle upwelling beneath the early spreading ridge and/or local stress reorganisation could have induced transtension and lithospheric thinning along the JMFZ and magmatic activity would have increased locally along this ‘leaky transform’. We suggest that the Early Tertiary tectono-magmatic processes that operated in the Norwegian–Greenland Sea are similar to the processes involved in the modern triple junction evolution of the Azores Plateau region.

The Othris Ophiolite, Greece: A snapshot of subduction initiation at a mid-ocean ridge

The mantle section of the Tethyan-type Othris Ophiolite, Greece, records tectono-magmatic processes characteristic of both mid-ocean ridges and supra-subduction zones. The Othris Ophiolite is a remnant of the Jurassic Neotethys Ocean, which existed between Eurasia and Gondwanaland. Othris peridotites range from fertile plagioclase lherzolites to depleted harzburgites. Abundances of Al 2O 3 and CaO show well-defined inverse linear correlations with MgO, suggesting that the Othris peridotites formed as residua from variable degrees of partial melting. Peridotites from the Fournos Kaïtsa and western Katáchloron sub-massifs are similar to abyssal peridotites and can be explained by a multistage model with some melting in the garnet stability field followed by moderate degrees of anhydrous near-fractional melting in the spinel stability field. In contrast, the peridotites from the Metalleio, Eretria, and eastern Katáchloron sub-massifs, and the Vourinos ophiolite are highly depleted and have extremely low concentrations of Al 2O 3 and heavy rare earth elements. These peridotites have enriched light REE contents compared to the middle REE. These residua are best modelled by hydrous melting due to a flux of slab-derived fluid to the mantle wedge during melting. The occurrence of both styles of melting regimes within close spatial and temporal association in the same ophiolite is explained by intra-oceanic thrusting and forced subduction initiation at (or near) a mid-ocean ridge. Thus, the Othris Ophiolite, and probably Tethyan-type ophiolites in general, represent a transient phase of plate tectonic reorganisation rather than quasi-steady state plate tectonics.

Mantle plumes, associated intraplate tectonomagmatic processes and ore systems

Mantle plumes, associated intraplate tectonomagmatic processes and ore systems

LIPs on Venus

Venus, a planet similar to Earth in heat budget lacks plate tectonics, yet displays ample evidence of extensive volcanic and tectonomagmatic processes, including regions large enough to be considered LIPs. Thus Venus provides an excellent opportunity to examine large-scale magmatic processes outside a plate tectonic framework. I discuss four groups of Venus’ largest tectonomagmatic provinces: volcanic rises, large coronae (Artemis, Heng-O, Quetzalpetlatl, Atahensik), crustal plateaus, and ‘plains with wrinkle ridges’, unit pwr. Unit pwr, widely interpreted as representing catastrophic resurfacing, has been suggested to be the solar system's largest LIP. I argue herein that these features, each covering ∼ 1 million km 2 or more, and recording extensive volcanic activity, record different evolutionary processes, including both endogenic and exogenic processes. Volcanic rises represent surface manifestations of deep-mantle plumes on thick lithosphere. Large coronae may record different evolutionary paths. Artemis represents a plume signature on thin lithosphere. More mapping is required to determine if Heng-O formed by endogenic or exogenic processes. Preliminary mapping suggests Quetzalpetlatl represents a young mantle plume; Atahensik marks a diapiric structure, though buoyancy mode is unconstrained. Crustal plateaus record solidification of huge lava ponds formed by massive partial mantle melting caused by large bolide impact on thin lithosphere. The status of unit pwr as the largest LIP within the solar system is challenged based on geological considerations, and results of a growing body of geologic mapping.

Oblique rifting along the central and southern Kermadec Arc front (30°–36°S), SW Pacific

We present a series of eleven structural maps of the central Kermadec arc front between 30°S and 35°50S interpreted from multibeam bathymetric data. These maps show the structural morphology of eighteen submarine arc volcanoes proximal to the Havre backarc, providing a critical window into the tectonomagmatic processes that operate at divergent arcs transitional to seafloor spreading. Each arc volcano, unaffected by subaerial processes, shows the structural imprint of oblique backarc rifting, providing an exceptional opportunity to evaluate tectonic influences on edifice construction and destruction. Mapped seafloor structures include faults, volcanic ridges, aligned vents and cones, cones aligned with faults, elongate calderas, and structurally controlled sector collapse. In addition to the structural maps, we present the results of quantitative analyses of fault orientation and growth behaviors. We demonstrate that >50% of identified faults strike NE‐SW between 030° and 060°, with a modal value of 047°, evidence in support of an extension axis dextrally oblique to the NNE trend of the rift. Further to this, minimum and maximum values for the extension axis are calculated as 137° and 150°, perpendicular to the modal strikes for all identified faults and for faults within the most oblique surveyed backarc rift domain, respectively. These values confirm moderate obliquity of opening along the Havre rift, with the SHT opening slightly less obliquely than the NHT, consistent with accommodation of a lesser partitioned component of dextral Pacific‐Australian plate convergence.

Evolution of paleoproterozoic magmatism: Geology, geochemistry, and isotopic constraints

Evolution of tectono-magmatic processes in the Paleoproterozoic is divisible into four stages each controlled by its own type of endogenic activity. The critical moment in geological history was at the third stage onset 2.05 Ga ago, when the Archean style of evolution gave way to subsequent one. Magmas of siliceous high- Mg (boninite-like) series (SHMS) dominant during the first stage (2.5-2.3 Ga) had the mixed mantle-crust gen- esis. They originated in highly depleted ultramafic reservoir of asthenospheric mantle being contaminated by the Archean crustal material during ascent to the surface. Typical of the SHMS magmas were the high content of silica, Al, Mg, Cr and elevated concentrations of Ni, Co, Cu, V, PGE and incompatible elements, LREE included, while concentrations of Fe, Ti, Nb and alkalies, especially K, were at a relatively low level. Magma- tism of K-granitoid type was of a limited significance. The second stage, especially its second substage (2.3- 2.05 Ga), was distinct because of mass eruptions of picritic and basaltic magmas enriched in Fe, Ti, Mn, P and incompatible elements, LREE in particular, which also had elevated Cr, Ni, Co, Cu and Ba concentration being relatively depleted in Mg and Al. This change in geochemistry of magmatism was independent of tectonic pro- cesses, which retained the former style. The third stage (2.05-1.8 Ga) was marked by opening of first oceans (Jormua, Purtuniq and other ophiolites) and by formation of orogens comparable with Phanerozoic orogens that was associated with development of subduction zones and back-arc basins with relevant magmatism. High Mg, Fe, Ti, Cr, Ni, Cu, V but low Th concentrations were typical of intense picrite-basaltic volcanism marking onset of the third stage. Magmas of suprasubduction genesis had considerably higher Si, Al, P, Zn, Th concentrations and very low e‡e/Al 2 O 3 ratios. Development of orogens came to the end 1.82-1.80 Ga ago. Appearance of giant belts of intraplate, usually silicic high-K volcanism juxtaposed on stabilized Paleoproterozoic orogens with abnormally thick crust was confined to the fourth stage spanning besides the initial Mesoproterozoic (1.8- 1.5 Ga). Anorthosite-rapakivi granitoid batholiths, which originated above mantle plume heads, corresponded to intermediate magma chambers of relevant magmatic systems. As is suggested, juvenile basaltic melts of this stage retained within the crust provoking a large-scale melting of sialic material. The high K, Ti, Zn, Pb, Zr and elevated Be, Sn, Y, Nb, Rb, F, W, Mo, Li and U contents characterized geochemistry of magmas, which origi- nated at the fourth stage

Petrogenesis of Neoarchaean volcanic rocks of the MacQuoid supracrustal belt: A back-arc setting for the northwestern Hearne subdomain, western Churchill Province, Canada

Rocks exposed in the MacQuoid-Gibson Lakes region, northwest Hearne subdomain, western Churchill Province, Canada comprise three major lithotectonic assemblages: the Principal volcanic belt; the metasedimentary MacQuoid homocline and; the Cross Bay plutonic complex. Neoarchaean supracrustal rocks of the belt range in age from <2745 to <2672 Ma and were intruded during the interval <2689 to 2655 Ma by diverse plutonic units ranging from gabbro through syenogranite, but greatly dominated by tonalite. Volcanic rocks occur only in the Principal volcanic belt and the MacQuoid homocline, are metamorphosed to amphibolite facies and vary from rare pillowed to common massive basalt and andesite, intercalated with less abundant, thin, dacitic to rhyolitic tuffs, lavas and volcaniclastic rocks. Basalt and andesite are dominated by subalkaline, FeO T-rich tholeiites with less common calc-alkaline rocks with higher SiO 2 contents and variable trace element contents. Felsic volcanic rocks exhibit calc-alkaline affinities and similarly diverse trace element abundances. The diverse trace element chemistry of the basalt and andesite supports their derivation from a heterogeneous mantle source(s) capable of generating MORB-, Arc-, BABB- and boninite-like rocks. Two geochemically distinct, arc-like suites were generated through contamination of the primary mantle-derived magmas either via assimilation of lower or middle tonalitic crust, or through contamination of their mantle source through subduction. Geochemical features of the felsic volcanic rocks indicate that these formed via both anatexis of crust in the amphibolite ± garnet stability field and via fractionation of more primitive progenitors in mid-upper crustal magma chambers. ɛNd t = 2680 Ma isotopic compositions cluster near depleted mantle, indicating that significant incorporation of older, >2700 Ma crust likely did not occur. ɛNd t = 2680 Ma values for three specimens, one from each of the Arc-like suites and one BABB-like basalt are slightly lower than the remainder, suggesting very minor incorporation of slightly older crust. These features imply that the processes that generated the MacQuoid supracrustal belt required simultaneous tapping of geochemically distinct mantle reservoirs with concomitant anatexis of sialic crust (garnet stability field) and fractionation of felsic magmas in upper crustal magma chambers. Shallow water deposition of abundant volcaniclastic rocks and semipelite along with minor conglomerate and quartzite was broadly contemporaneous with this magmatism. We envisage a geodynamic setting characterized by tectonomagmatic processes similar to those of modern supra-subduction zone back-arc marginal basins such as the Sea of Japan. Therein, an extensional, back-arc setting, likely proximal to continental crust, provides an explanation for a broad swath of diverse mantle-derived rocks intercalated with less common felsic rocks as well as an abundance of immature clastic metasedimentary rocks.

Geochemical secular variation of magma source during Early to Middle Miocene time in the Niigata area, NE Japan: Asthenospheric mantle upwelling during back-arc basin opening

In the Niigata region of the NE Japan arc, basaltic and intermediate to felsic volcanic rocks (silicic andesite, dacite and rhyolite) have been produced during the Early Miocene, Middle Miocene and Pliocene ages, spanning the pre-Japan Sea opening to post-opening stages. Early Miocene basaltic rocks are characterized by enriched Sr and Nd isotopic signatures (initial 87Sr / 86Sr (SrI) = 0.70557 to 0.70592 and initial 143Nd / 144Nd (NdI) = 0.51255 to 0.51262), whereas Middle Miocene and Pliocene basaltic rocks are slightly enriched than MORB in terms of SrI (0.70314–0.70416) and NdI (0.51286–0.51310). Early Miocene basaltic rocks are also characterized by higher abundances of HFSE and LREE, and higher Zr / Y compared to Middle Miocene and Pliocene basaltic rocks. The geochemical features of Early Miocene basaltic rocks are almost identical to those of basaltic rocks found in continental rift zones, such as the Rio Grande rift. The different geochemical signatures of Early and Middle Miocene basaltic rocks can be ascribed to the geochemical differences in the mantle source. Apart from andesitic rocks of the Kakuda area, most of Early and Middle Miocene intermediate to felsic volcanic rocks from the Niigata region show significantly higher SrI values (0.70673–0.70773) and lower NdI values (0.51234–0.51254) than Early Miocene basaltic rocks, indicating a lower crustal origin for these more felsic volcanic rocks. A possible tectono-magmatic model for the production of post-Early Miocene volcanic rocks from the Niigata region includes: (1) at the Early Miocene (22–20 Ma); commencement of asthenospheric mantle upwelling followed by both partial melting of the overlying lithospheric mantle and crustal rifting, resulting in the production of continental rift zone-type basaltic magmas which have either erupted on the surface or initiated partial melting of the lower crust to produce more felsic magmas, and (2) at the Middle Miocene (after 15 Ma); thinning of the overlying lithosphere due to the opening of the Japan Sea. This was associated with asthenospheric upwelling followed by partial melting of the asthenosphere to generate extensive basaltic magmas. These were either erupted or melted the lower crust to generate more felsic magmas. Such tectono-magmatic processes may also be applicable to the generation of Early to Middle Miocene basaltic to felsic volcanic rocks in many other parts of the back-arc margin of the NE Japan arc.

Magmatic and tectonic history of Iceland's western rift zone at Lake Thingvallavatn

High rates of Holocene sedimentation in a lake spanning the Thingvellir rift zone of western Iceland provide an unusual opportunity to study the interaction of tectonic and magmatic processes on timescales of thousands of years. Lake Thingvallavatn is oriented SW-NE, parallel to the trend of normal faults and fissures which extend northeast from Hengill, a central volcano system. Thingvallavatn’s lake sediments provide a continuous high-fidelity record of tectono-magmatic processes. “Chirp” sub-bottom profiler and sidescan records, together with sediment core information, enabled the lake stratigraphy to be constrained well since the emplacement of a post-glacial lava at 9.1 ± 0.3 ka. This lava, together with three younger horizons, enabled detailed study of the main tectonic and magmatic events. A major Hengill volcanic event which controlled the development of the present-day lake morphology occurred at 1.9 ka with the eruption of a scoria cone within the lake (Sandey). During this event, the Nesjahraun lava was erupted into the southern part of the lake, the Sandey scoria cone was formed, and major faulting and subsidence occurred in the northern part of the lake, resulting in the formation of an asymmetrical rift. Within the southern part of the lake, a deformed sequence of sediments of age 2.9 – 1.5 ka, between undeformed younger and older sediments, indicate that liquefaction phenomena are associated with the emplacement of the Nesjahraun lava. Analysis of fault displacement reveals that the total throw summed over all faults across the width of the rift zone is approximately constant (110 - 130 m) along the long axis of the rift. We estimate an extension rate on the faults of 3.3 – 8.2 mm yr-1 since 9.1 ka, assuming fault dips of 60 - 75°, which represents 17 – 43 % of the total plate boundary extension estimated from global plate motion inversion. We speculate that the remaining extension must either be taken up elsewhere in Iceland, for example in the Eastern Rift Zone or along the South Island Seismic Zone, or that extension estimated over the last 9kyr underestimates the long-term extension rate due to incomplete sampling of the episodic magmatic component.