Arjan H. Dijkstra

On the role of melt-rock reaction in mantle shear zone formation in the Othris Peridotite Massif (Greece)

A 1-km-wide peridotite mylonite shear zone is exposed in the Othris peridotite massif in central Greece. The mylonites contain lenses of relatively coarse olivine crystals, which are interpreted as remnants of the tectonite microstructure in the adjacent wall rocks. Microstructure and texture analysis using light and SEM microscopy suggests that the dominant deformation mechanism in the tectonites was dislocation creep, whereas the deformation in the mylonites was probably controlled by grain-size sensitive (GSS) creep in fine-grained (<50 μm) bands consisting of a mixture of olivine and orthopyroxene. The development of the fine-grained material in the mylonites can be explained by a melt-present reaction taking place in the tectonite protolith. This reaction led to the replacement of orthopyroxene porphyroclasts by fine-grained olivine and orthopyroxene. Tectonites adjacent to the mylonite zone preserve evidence for this reaction in the form of rims of fine-grained olivine and orthopyroxene around orthopyroxene porphyroclasts. This study illustrates the significance of rheological weakening of oceanic mantle lithosphere as a result of a change from dislocation to GSS creep.

Late Neoproterozoic proto-arc ocean crust in the Dariv Range, Western Mongolia: a supra-subduction zone end-member ophiolite

An unusual late Neoproterozoic (c. 572 Ma) ophiolite is exposed in the Dariv Range (western Mongolia), which contains intermediate to acidic lavas and sheeted dykes, and an igneous layered complex consisting of gabbro–norites, websterites, orthopyroxenites and dunites underlain by serpentinized mantle harzburgites. Based on the compositions of the crustal units and the crystallization sequences in the mafic and ultramafic cumulates we conclude that the entire oceanic crust, including the cumulates, was made from arc magmas with boninitic characteristics. The Dariv rocks bear a strong resemblance to rocks recovered from the modern Izu–Bonin–Mariana fore-arc, a fragment of proto-arc oceanic basement, and we propose that the Dariv Ophiolite originated in a similar tectonic setting. A metamorphic complex consisting of amphibolite- to granulite-facies metasedimentary and meta-igneous rocks was thrust over the ophiolite. This metamorphic complex probably represents a Cambrian arc. Thrusting started before 514.7 ± 7.6 Ma as constrained by new sensitive high-resolution ion microprobe U–Pb zircon analyses from a syn- to post-tectonic diorite. The Dariv Ophiolite is a type-example of a proto-arc ophiolite, a special class of supra-subduction zone ophiolites.

Shear zones in the upper mantle: evidence from alpine- and ophiolite-type peridotite massifs

Abstract There is abundant field and microstructural evidence for localization of deformation in alpine- and ophiolite-type mantle massifs. On the basis of field relationships and microstructures we recognize two types of tectonite shear zones (medium- to coarse- and fine-grained), as well as two types of mylonitic shear zones (anhydrous and hydrous peridotite mylonites). In tectonite shear zones, softening processes responsible for localization are probably melt-related weakening in the medium to coarse tectonites and a change in limiting slip system in the fine-grained tectonites. In peridotite mylonites, the most likely cause for softening and localization is a change in dominant deformation mechanism from dislocation to grain size sensitive creep. Microstructural and petrological study of mylonite rocks reveals that reactions, either continuous net-transfer reactions (anhydrous and hydrous) or melt-rock reactions, play a key role in the formation of fine-grained material that promotes grain size sensitive creep. These reactions occur over a broad range of pressure-temperature conditions encompassing a large part of the lithospheric upper mantle. We conclude that mantle shear zones are widespread and that they reduce the (bulk) strength of the lithosphere significantly.

Active intraplate strike-slip faulting and transpressional uplift in the Mongolian Altai

Abstract The Mongolian Altai is a Late Cenozoic intraplate strike-slip deformation belt which formed as a distant strain response to the Indo-Eurasian collision over 2000 km to the south. We report results from 5 weeks of detailed fieldwork carried out during summer 2000 in northwestern Mongolia investigating the crustal architecture of the Altai at latitude 48°N. The region can be divided into discrete Cenozoic structural domains each dominated by a major dextral strike-slip fault system or range-bounding thrust fault. Gentle bends along the major strike-slip faults are marked by transpressional uplifts including asymmetric thrust ridges, restraining bends, and triangular thrust-bounded massifs. These transpressional uplifts (Tsambagarav Massif, Altun Huhey Uul, Sair Uul, Hoh Serhiyn Nuruu, Omno Hayrhan Uula, Mengildyk Nuruu) comprise the highest mountains in the Mongolian Altai and are structural and metamorphic culminations exposing polydeformed greenschist-amphibolite grade basement recording at least two phases of Palaeozoic ductile deformation overprinted by Cenozoic brittle structures. Cenozoic thrust faults with the greatest amounts of displacement bound the W and SW sides of ranges throughout the region and consistently verge WSW. Each major range is essentially a NE-tilted block and this is reflected by asymmetric internal drainage patterns. Many faults are considered active because they deform surficial deposits, form prominent scarps, and define range fronts with low sinuosity where active alluvial fan deposition takes place. Reactivation of the prevailing NW-striking, NE-dipping Palaeozoic basement anisotropy is a regionally important control on the orientation and kinematics of Cenozoic faults. At first order, the Altai is spatially partitioned into a low-angle thrust belt that overthrusts the Junggar Basin on the Chinese side and a high-angle SW-vergent dextral transpressional belt on the Mongolian side. The mechanically rigid Hangay craton and Junggar basement block which bound the Altai on either side have played a major role in focusing Late Cenozoic deformation along their boundaries and within the Altai. The geometric relationship between rigid block boundaries, Palaeozoic basement structural anisotropy, and the dominantly NE SHmax (derived from India’s continued NE indentation) has dictated the kinematics of Late Cenozoic deformation in the Altai, Gobi Altai, and Sayan regions.

Crustal architecture of the Donets Basin: tectonic implications for diamond and mercury-antimony mineralization

Abstract Kimberlite-like rocks and minor diamond finds are reported in the Precambrian Ukrainian Shield south of the Donets Basin. Prolific mercury-antimony mineralization occurs in Carboniferous quartz arenites within the Basin. The tectonic setting is examined on the basis of recent data compilations and ongoing research in the Ukraine and Voronezh shield areas and the Pripyat-Dnieper-Donets palaeorift. In the Donets region, a straightforward analogy of any diamond district with the Archangelsk province is not likely in the absence of a Proterozoic shear comparable with the White Sea-Belomorian Mobile Belt. A deep-reaching, NNW-striking lithosphere lineament is identified here as the Kharkov-Donetsk lineament. It transects the rift between the Donets and Dnieper basins. The structures involved in this lineament have controlled Palaeozoic sedimentation and the extent of Late Permian inversion of the Donets basin. During the inversion, the lineament and associated deep-reaching longitudinal structures provided pathways for the migration of mineralizing fluids from deep levels in the lower crust and upper mantle. The intersection, in the Kharkov area, of this lineament with a northeasterly striking lithosphere root should focus diamond exploration towards the northern shoulders of the rift. The extreme attenuation of the crust beneath the Donets Basin, relative to the western basins of the rift, is associated with crustal detachment and subsidence during and possibly after inversion, concomitant with emplacement of asthenospheric materials at higher levels. Together with the continued subsidence in the western Donets Basin, during the Late Permian inversion, this invokes a tectonic setting for the Hg Sb mineralization not unlike the orogenic-collapse-associated settings of Hg Sb deposits in western Europe. Further investigation of the geodynamics of the Donets Basin would benefit from deep reflection seismics, petrogenetic studies of magmatic products and their xenoliths, and satellite remote sensing analysis.

Base-up growth of ocean crust by multiple phases of magmatism: field evidence from Macquarie Island

Macquarie Island preserves largely in-situ Miocene oceanic crust and mantle formed at a slow-spreading ridge. The crustal section on the island does not conform to a simple ‘layer cake” pseudo-stratigraphy’, but is the result of multiple magmatic episodes. Macquarie Island crust did not grow by top-down cooling, but rather from the base up. Peridotites cooled first and formed the basement into which gabbro plutons were intruded. This was followed by cooling and deformation, and by intrusion of dykes that fed a sheeted dyke–basalt complex. Finally, lava-filled grabens were formed. These relative age relations rule out simple co-genetic relations between rock units.

The geology, landforms and topography of sub-Antarctic Macquarie Island, Australia, as revealed by AIRSAR

Enhancements of AIRSAR data are revealing an extraordinary amount of land-surface and geological information relevant to the islands genesis at a midocean ridge, ongoing uplift and subsequent shaping of the surface by extensive faulting, wave action and active processes including freeze/thawing, wind and rain. Overlaying these enhancements on the TOPSAR DEM enables this active landscape to be analyzed in three dimensions. The level of detail indicates that the radar signals are penetrating the snow cover and being backscattered by the underlying surface of rock, lag gravels and vegetation. Drainage patterns, dammed lakes, levels of incision, fault structures, raised beach deposits and terraces, and possible evidence of glacial action can all be recognized in various enhancements of the AIRSAR data.

Mapping a hidden terrane boundary in the mantle lithosphere with lamprophyres

Lamprophyres represent hydrous alkaline mantle melts that are a unique source of information about the composition of continental lithosphere. Throughout southwest Britain, post-Variscan lamprophyres are (ultra)potassic with strong incompatible element enrichments. Here we show that they form two distinct groups in terms of their Sr and Nd isotopic compositions, occurring on either side of a postulated, hitherto unrecognized terrane boundary. Lamprophyres emplaced north of the boundary fall on the mantle array with εNd −1 to +1.6. Those south of the boundary are enriched in radiogenic Sr, have initial εNd values of −0.3 to −3.5, and are isotopically indistinguishable from similar-aged lamprophyres in Armorican massifs in Europe. We conclude that an Armorican terrane was juxtaposed against Avalonia well before the closure of the Variscan oceans and the formation of Pangea. The giant Cornubian Tin-Tungsten Ore Province and associated batholith can be accounted for by the fertility of Armorican lower crust and mantle lithosphere.Lamprophyres represent hydrous alkaline mantle melts that are a unique source of information about the composition of continental lithosphere. Here the authors use isotopic compositions of lamprophyres to map a hidden terrain boundary and an unknown fragment of Armorica in the mantle lithosphere of southwest Britain.

Mineral assemblages and textures of granite-hosted veins in the Hemerdon W–Sn deposit: constraints from scanning electron microscope chemical X-ray mapping

rare earth production, for example, is two orders of magnitude lower than that of copper. Critical metals are defined as eco-nomically important but produced from just a few mines or countries such that they are particularly vulnerable to supply disruption (European Union 2014; British Geological Survey 2015). These critical metals are essential for new ‘green’ and ‘digital’ technologies such as renewable energy, state-of-the-art medical technologies, computers and smartphones. Considerable effort has been made in the last few years to increase recycling, diversify supply or find alternatives to these critical metals. Less attention has been paid to responsible sourcing. Yet, it seems common sense that raw materials needed for environmentally-friendly technologies should come from environmentally-friendly – and ‘people friendly’ – sources. How easy is this to achieve for the critical metals? Is it a case of ‘beggars can’t be choosers’ or is responsible sourcing an important factor in determining critical metal supply chains? The answer is that responsible sourcing of specialist metals has only really been widely considered in the case of conflict minerals. The USA has legislation that requires the source of Ta (‘coltan’) and of W, Sn and Au from the Great Lakes area of Africa to be certified as conflict-free, and Europe is following suit. The ITRI Tin Supply Chain Initiative assures a chain of custody to prevent conflict minerals entering the supply chain. However, the conflict mineral rules are a ‘single issue’ measure. Although the Ta capacitors in smartphones are now more likely to be conflict-free, there is no assurance about factors such as environmental protection, workforce well-being, community relations, or mine closure planning. The main drivers for responsible mining are not yet responsible sourcing initiatives from consumers but the need for mining companies to (1) satisfy investment banks in order to raise capital, (2) gain their social licence to operate from their host communities, and (3) comply with legislation. These drivers and controls apply to critical metals mines (outside China) just as well as to the mainstream commodities. However, there are so many different management and reporting systems that it is still difficult to identify any clear ‘responsible mined’ mark that could penetrate and influence supply chains. So is there a role for geology in responsible sourcing? There may well be an opportunity here alongside development of new sources of critical metals. For example, there are a wide range of potential rare earth deposits under development and each has different characteristics. Some are hard rock, high grade; some low grade but with higher proportions of the more sought after, and rarer, heavy REE; others can be easily leached without any need for the usual minerals processing. Life cycle analysis-type approaches can compare environmental characteristics right from the first stages of exploration. Several studies have now been done but results vary widely depending on how far along the supply chain the analysis is taken, how co-products are valued and how the data are collected.

The petrology, mineralogy and geochemistry of the large Fen Carbonatite Complex (Norway) REE-Nb resource

The c.580 Ma Fen Carbonatite Complex in Southern Norway is a circular, c.2.5 km diameter pipe-like composite intrusion (Figure 1) consisting of highly alkaline rocks (ijolite), carbonatites and ultramafic lamprophyres (damtjernites). The Fen Complex is also the type area for the pervasive Na–K metasomatic alteration of country-rock gneisses referred to as ‘fenitisation’ and it is the location where the magmatic origin of carbonatites was first proposed by Brögger in 1920s. The Fen Complex is almost certainly the largest REE resource on the European continent with a resource size estimate of >100 MT at c.1 wt-% REEoxide. We will present results from our on-going research project which looks at the key geological stages in the formation of this REE resource, from mantle metasomatism and melting, to late/post-magmatic alteration. Our research has shown that primary magmatic carbonatite in the Fen Complex is calcite-dolomite carbonatite with relatively low grades of REE (typically <2000 ppm Σ-REE), hosted in REE-fluorocarbonate and fluorapatite. These rocks locally contain abundant metasomatic pyrochlore and columbite (up to 5000 ppm Nb) in magnetite-rich zones. In many areas, especially in the eastern part of the intrusion, the primary calcite-dolomite carbonatite is transformed into a red, secondary hematite-carbonatite known locally as Rødbergite. In Rødbergite, REE are greatly enriched (4000–15000 ppm Σ-REE). The transformation of calcite-dolomite carbonatite to Rødbergite is associated with the breakdown of REEfluorocarbonate and fluorapatite to form secondary REE-monazite. There is geochemical evidence that LREE and HREE are partly decoupled during the transformation, and transitional, partly transformed carbonatite contains relatively low HREE/LREE ratios. Apatite aggregates (probably originally of a cumulate origin) acted as a trap for oxidising REE-bearing fluids. Sr and Nd isotope analysis shows that while primary carbonatites have mantle signatures (relatively depleted with respect to CHUR at 580 Ma, εNd(580 Ma) +4), the transformation to Rødbergite is associated with a strong relative increase in radiogenic Sr, clearly pointing to a key role for ‘external’ fluids with a crustal isotopic signature in the formation of secondary Rødbergite. We will outline models for the nature of the Rødbergite formation processes, and discuss the possible geological context for this event.