TO THE 90TH ANNIVERSARY OF IVAR MURDMAA

On August 6, 2021, the chief researcher of the IO RAS, Doctor of Geological and Mineralogical Sciences, Professor Ivar Oskarovich Murdmaa turned 90 years old. The main focus of I.O. Murdmaa is the study of bottom sediments of seas and oceans, their lithology, mineralogy, deposition processes, facies and formations, the theory of oceanic sedimentogenesis. He first distinguished marine volcanoterrigenous sediments and described the facies variability of modern sediments of island arcs. Ivar Murdmaa is known for his studies in mineralogy of oceanic sediments, processes of pelagic sedimentogenesis and associated iron-manganese nodules formation. Studying sediment formation in rift zones of mid-ocean ridges, he identified a new genetic type of sediments named edaphogeonus sediments, elaborated mineralogical criteria for their recognition and formation processes. In recent years I.O. Murdmaa is actively developing the theoretical concept of "sedimentosphere", paying special attention to a new direction – the study of the erosion-accumulative activity of bottom currents and the formation of contourites.

Geodynamic conditions of massive sulfide formation in the Magnitogorsk megazone

Research subject. Volcanism, rock geochemistry, geodynamics, and massive sulfide formation in the Magnitogorsk megazone (MMZ) of the Southern Urals in the Middle Paleozoic.Materials and Methods. Across the largest part of the massive sulfide deposits under investigation, the authors conducted route studies, including geological surveys of individual ore fields and quarries of deposits, core samples of deep wells and transparent sections. Representative analyses of petrogenic and microelements were performed using wet chemistry and ICP-MS in analytical centers in Russia and Europe. Along with the authors’ data, analytical materials published by Russian and foreign researchers were used. Geodynamic reconstructions were carried out taking into account regional data on gravics, thermal field, magnetometry, and seismic stu dies, including «Urseis-95».Results. The geodynamic reconstructions established that the main elements of the paleostructure of the Southern Urals in the Devonian were the subduction zone of the eastern dip and asthenospheric diapirs that penetrated into the «slab-window», which determined the type of volcanic belts, the composition and volume of volcanic rocks of pyrite-bearing complexes, and ore matter of pyrite deposits. The following geodynamic zones in the MMZ were identified: 1 – polychronous accretion prism; 2 – frontal and developed island arcs (D1e2–D2ef1); 3 – zone of back-arc spreading (D1e2); 4 – rear island arc (D2ef1).Conclusions. All investigated zones and ore areas are characterized by an autonomous development of volcanism, a special deep structure and a different composition, as well as by a different volume of massive sulfide deposits that vary in the Cu and Zn ratios and Pb, Ba, Au amounts. In the MMZ volcanic complexes, three groups of plume source basalts are distinguished. The results can be used in predictive-estimation and search operations for massive sulfide mineralization.

The Ludicovian of the Raahe–Ladoga Zone of the Fennoscandian Shield (Isotope-Geochemical Composition and Geodynamic Nature)

—In the northern Ladoga area, the age of the Sortavala Group rocks in the southeast of the Raahe–Ladoga zone of junction of the epi-Archean Fenno-Karelian Craton and the Paleoproterozoic Svecofennian province, their relationship with dome granitoids, the age of the provenances, and the time of metamorphic processes were estimated. The study was focused on the Nd isotope composition of rocks, the geochemical and isotope-geochronological parameters of zircon from the granite-gneisses of the Kirjavalakhti dome, the basal graywackes of the lower unit and the trachytes of the middle unit of the Sortavala Group, and the plagio- and diorite-porphyry dikes cutting the volcanosedimentary units of this group. The new isotope-geochemical data show a Neoarchean age of the granitoids of the Kirjavalakhti dome (2695 ± 13 Ma) and their juvenile nature (εNd(T) = +1.5). The granitoids underwent tectonometamorphic transformations (rheomorphism) in the Paleoproterozoic (Sumian) (2.50–2.45 Ga), which are recorded in the U–Th–Pb isotope system of the rims of the ancient cores of zircon crystals. The volcanosedimentary complex of the Sortavala Group formed on the heterogeneous polychronous (3.10–2.46 Ga) continental crust of the epi-Archean Fenno-Karelian Craton. With regard to the errors in determination of the age of clastic zircon, the minimum concordant U–Th–Pb ages of 1940–1990 Ma of detrital zircon from volcanomictic graywackes of the Pitkyaranta Formation can be taken as the upper age bound of terrigenous rocks, which agrees with the maximum age of the Sortavala Group rocks estimated from the U–Th–Pb (SIMS) age of 1922 ± 11 Ma of the Tervaoya diorites (Matrenichev et al., 2006). According to the proposed new tectonic model, the accumulation of the volcanosedimentary complex of the Sortavala Group, its metamorphism, erosion, and overlapping by the Ladoga Group turbidites had already occurred in the pericratonic part of the epi-Archean Fenno-Karelian Craton by the time of the Svecofennian continent–island arc collision, subduction, and formation of bimodal volcanoplutonic complexes of the young Pyhäsalmi island arcs and felsic volcanics of the Savo schist belt (1920–1890 Ma).

Plate Tectonics

Plate tectonics, the grand unifying theory of geology, and its relation to the Earth is explained in this chapter. The planet transforms through time by means of the movement of rigid plates carrying the continents riding on the plastic material in the Earth’s upper mantle. Three major plate boundaries are divergent margins, where new ocean floor is being created along mid-ocean ridges and plates separate from one another; convergent margins, where the material is subducted and consumed as different types of plates collide, creating trenches, island arcs or mountain ranges, and transform boundaries; and where plates slide past one another. Besides the three predominant boundaries, hot spots caused by mantle plumes and diffuse boundaries make up additional dynamic forces in tectonics. Beyond these categories, geologists still are learning about tectonics; some boundaries are unknown or speculative. Plate tectonics explains why many of the Earth’s hazards are found where there are. Earthquakes trace many plate margins, as do volcanoes. The area around the Pacific Ocean is called the “Ring of Fire” because of the many volcanoes related to subducting plates. Tectonics accounts for why certain rocks are located where they are; for example, all rock types are found at convergent margins. The theory also predicts where valuable mineral and economic deposits are located.

P-velocity of the upper mantle

The authors have constructed models featuring seismic P-wave velocity distribution in the upper mantle beneath oceanic, continental and transition regions, such as mid-ocean ridges, basins, trenches, island arcs, and back-arc troughs, Atlantic transitional zones, flanking plateaus of mid-ocean ridges, platforms, geosynclines, rifts, recent activation zones. The models are in agreement with the deep-seated processes in the tectonosphere as predicted in terms of the advection-polymorphism hypothesis. The models for areas of island arcs and coastal ridges are similar to those for alpine geosynclines disturbed by recent activation. The models for areas of mid-ocean ridges and back-arc troughs are identical. They fit the pattern of recent heat-and-mass transfer in the case of rifting, which, given the basic crust with continental thickness, leads to oceanization. The model for the basin reflects the effect of thermal anomalies smoothing beneath mid-ocean ridges or back-arc troughs about 60 million years later. The model for the trench and flanking plateau reflects the result of lateral heating of the mantle’s upper layers beneath the quiescent block from the direction of the island arc and basin (trench) and mid-ocean ridge and basin (flanking plateau). A detailed bibliography on regions covered by studies was presented in the authors’ earlier publications over past eight years. There are quite significant differences between models for regions of the same type that are described in publications of other authors. This is largely due to the fact that individual authors adopt a priori concepts on the velocity structure of the upper mantle. High variability of seismic P-wave velocities within the subsurface depth interval has been detected as a result of all sufficiently detailed studies. This variability is responsible for the sharp increase in the scatter of arrival times of waves from earthquakes at small angular distances. The corresponding segments of travel-time graphs were simply ignored, and the graphs started from about 3° after which the scatter of arrival time acquired a stable character. Accordingly, velocity profiles were constructed, as a rule, starting from depths of about 50 km. The constructed velocity profiles vary little from region to region with the same type of endogenous regimes. This enables us to maintain that the models represent standard (typical) VP distributions in the mantle beneath the regions, just as presumed in terms of the theory.

Supplemental Material: Forced subduction initiation within the Neotethys: An example from the mid-Cretaceous Wuntho-Popa arc in Myanmar

Age information on ophiolites and arcs from Myanmar and Andaman, geochronological and geochemical data for the mid-Cretaceous Mawgyi Volcanics in the Wuntho-Popa arc of central Myanmar, and published ages associated with the Neotethyan ophiolites and island arcs, with which the reader can replicate our analyses.

Supplemental Material: Forced subduction initiation within the Neotethys: An example from the mid-Cretaceous Wuntho-Popa arc in Myanmar

Age information on ophiolites and arcs from Myanmar and Andaman, geochronological and geochemical data for the mid-Cretaceous Mawgyi Volcanics in the Wuntho-Popa arc of central Myanmar, and published ages associated with the Neotethyan ophiolites and island arcs, with which the reader can replicate our analyses.

Mid-Ordovician subduction, obduction, and eduction in western Newfoundland; an eclogite problem

The narrow, short-lived Taconic-Grampian Orogen occurs along the north-western margin of the Appalachian-Caledonian Belt from, at least, Alabama to Scotland, a result of the collision of a series of early Ordovician oceanic island arcs with the rifted margin of Laurentia. The present distribution of Taconian-Grampian ophiolites is unlikely to represent a single fore-arc from Alabama to Scotland colliding at the same time with the continental margin along its whole length; more likely is that there were several Ordovician arcs with separate ophiolites. The collision suture is at the thrust base of obducted fore-arc ophiolite complexes, and obduction distance was about two hundred kilometres. Footwalls to the ophiolites are, sequentially towards the continent, continental margin rift sediments and volcanics and overlying rise sediments, continental shelf slope carbonates, and sediments of foreland flexural basins. The regionally-flat obduction thrust complex between the ophiolite and the rifted Laurentian margin is the collision suture between arc and continent. A particular problem in drawing tectonic profiles across the Taconic-Grampian Zone is several orogen-parallel major strike-slip faults, both sinistral and dextral, of unknown displacements, which may juxtapose portions of different segments. In western Newfoundland, most of the Grenville basement beneath the Fleur-de-Lys metamorphic complex (Neoproterozoic to early Ordovician meta-sediments) was eclogitised during the Taconic Orogeny and separated by a massive shear zone from the overlying Fleur-de-Lys, which was metamorphosed at the same time but in the amphibolite facies. The shear zone continued either to a distal intracontinental “subduction zone” or to the main, sub-fore-arc, subduction zone beneath which the basement slipped down to depths of up to seventy kilometres at the same time as the ophiolite sheet and its previously-subcreted metamorphic sole were being obducted above. Subsequently, the eclogitised basement was returned to contact with the amphibolite-facies cover by extensional detachment eduction, possibly enhanced by subduction channel flow, which may have been caused by slab break-off and extension during subduction polarity flip. Although the basal ophiolite obduction thrust complex and the Fleur-de-Lys-basement subduction-eduction surfaces must have been initially gently-dipping to sub-horizontal, they were folded and broken by thrusts during late Taconian, late Ordovician Salinic-Mayoian, and Acadian shortening.

Trace Element Signatures in Pyrite and Marcasite From Shallow Marine Island Arc-Related Hydrothermal Vents, Calypso Vents, New Zealand, and Paleochori Bay, Greece

Fluid conditions of shallow marine hydrothermal vent sites (<200 mbsl) in island arcs resemble those of subaerial epithermal systems. This leads to a distinct mineralization-style compared to deeper arc/back-arc (>200 mbsl) and mid-ocean ridge-related environments (>2000 mbsl). At Calypso Vents in the Bay of Plenty and Paleochori Bay at the coast of Milos Island, fluids with temperatures <200°C are emitted through volcaniclastic sediments in water depths <200 mbsl. The hydrothermal mineralization from these fluids is dominated by pyrite and marcasite showing diverse textures, including colloform alternations, semi-massive occurrences surrounding detrital grains, vein-type pyrite, and disseminated fine-grained assemblages. Pyrite and marcasite from Calypso SE show elevated concentrations of volatile elements (e.g., As, Sb, Tl, Hg) implying a vapor-rich fluid phase. By contrast, elements like Zn, Ag, and Pb are enriched in hydrothermal pyrite and marcasite from Calypso SW, indicating a high-Cl liquid-dominated fluid discharge. Hence, vapor-liquid element fractionation induced by fluid boiling is preserved in the seafloor mineralization at Calypso Vents. Hydrothermal mineralization at very shallow vent sites (<10 mbsl), like Paleochori Bay, are affected by wave action causing a seasonal migration of the seawater-fluid interface in the sediment cover. The δ34S composition of native S crusts and crystalline S (0.7–6.7‰) is indicative for host rock leaching and thermochemical reduction of seawater sulphate. By contrast, the highly negative δ34S signature of native S globules in sediments (−7.6 to −9.1‰) is related to microbial sulphate reduction or a subordinate magmatic fluid influx. Alunite-jarosite alteration (Paleochori Bay) and a mineral assemblage consisting of orpiment, realgar, and native S (Calypso Vents) may also suggest a contribution by an oxidised (sulphate-rich) low pH fluid of potential magmatic origin. However, fluid boiling is pervasive at Calypso Vents and Paleochori Bay, and the condensation of vapor-rich fluids in a steam-heated environment may produce a similar alteration and mineralization assemblage without a significant magmatic fluid influx, as known from some subaerial epithermal systems.