Layered Gabbros Research Articles

Formation of the Weiya magmatic Fe–Ti oxide deposit and its ore-hosting layered gabbro intrusion, Eastern Tianshan (Xinjiang, NW China)

The Weiya is the largest magmatic Fe–Ti oxide deposit in the Chinese Eastern Tianshan orogen, and is divided into an eastern, central and western section by younger granitoid and diabase intrusions. The Weiya eastern section contain Fe-Ti oxide ores and ore-bearing apatite-poor layered gabbro, which display significant difference in mineral assemblage from the layered gabbro in the central section. We presented new data on the geology, mineralogy, petrology, geochemistry, geochronology, and Sr-Nd isotopes of the gabbroic rocks and the Fe-Ti oxide ores from the eastern section. Igneous zircons from the Weiya anorthosite yielded a SIMS U–Pb concordia age of 237.9 ± 0.71 Ma, coeval with the apatite-rich gabbro in the central section. The gabbroic rocks and Fe-Ti oxide ores in the eastern section and the central section have parallel chondrite-normalized REE patterns and N-MORB-normalized multi-element patterns, together with highly-similar Sr-Nd isotope features, suggesting that the two are comagmatic. Olivine Fo value (62.0–66.7), clinopyroxene Mg# number (68.1–76.2) and plagioclase An contents (39.9–55.4) suggest that the Weiya parental magma was highly evolved after extensive fractional crystallization in the deeper magma chamber, and the residual magma became increasingly oxidized and metals- and volatiles-rich. During the magma emplacement, plagioclase accumulated extensively in the magma chamber roof and further enriched Fe–Ti in the residual melts, leading to Fe–Ti oxide mineralization.

Formation of Igneous Layering in the Lower Oceanic Crust From the Samail Ophiolite, Sultanate of Oman

AbstractAs the largest and best exposed example of paleo fast‐spreading oceanic crust on land, the Samail ophiolite in the Sultanate of Oman represents an ideal natural laboratory for investigating deep crustal processes at fast‐spreading mid‐ocean ridges. We studied two layered gabbro sequences from different stratigraphic depths: one from the middle of the plutonic crust showing decimeter‐scale modal layering (i.e., varying phase proportions) with olivine abundances gradually decreasing from layer bases to tops (Wadi Somerah, Sumail block) and one located near the crust‐mantle boundary showing millimeter‐scale olivine‐rich layers (Wadi Wariyah, Wadi Tayin block). Our multimethod approach of field, petrographic, geochemical, and microstructural observations focuses on documenting layered textures that are widely observed within the lower oceanic crust as well as understanding their formation mechanisms within the context of small scale crustal accretion processes beneath fast‐spreading mid‐ocean ridges. Results from the mid‐crustal sequence indicate moderate cooling rates (Ca‐in‐olivine: log[dT/dt; °C yr−1] = −2.21 ± 0.7) and correlated variations in mineral compositions and microstructures. We infer that decimeter‐scale layers in Wadi Somerah were deposited by density currents of crystal‐laden magma within a sill environment that potentially experienced occasional magma replenishment. The millimeter layering in Wadi Wariyah is best explained by Ostwald ripening emphasizing initial heterogeneities possibly being provoked by cyclical nucleation of olivine through the competing effects of element diffusion and rapid cooling. Fast cooling is recorded for the crustal base (Ca‐in‐olivine: log[dT/dt; °C yr−1] = −1.19 ± 0.5, Mg‐in‐plagioclase: log[dT/dt; °C yr−1] = −1.35°C ± 0.6) demonstrating that heat locally can be lost very efficiently from the lowermost crust.

Fluid Infiltration Through Oceanic Lower Crust in Response to Reaction‐Induced Fracturing: Insights From Serpentinized Troctolite and Numerical Models

AbstractThe mechanisms of fluid penetration through the gabbroic lower crust are important for the hydration of oceanic lithosphere. In the Oman ophiolite, which preserves an entire sequence of oceanic lithosphere formed at a fast‐spreading ridge, the layered gabbros and dunites are extensively serpentinized. In this paper, we describe the characteristic textures of serpentinized troctolite and olivine gabbros recovered from the CM1A site of the Oman Drilling Project. In the troctolite, an olivine mesh texture is pervasively developed and is characterized by two types of veins: early lizardite + brucite + magnetite and late Al‐rich lizardite + magnetite. These veins suggest the initiation of serpentinization at <350°C and a supply of Si and Al from plagioclase during the later stages of serpentinization. Plagioclase surrounding serpentinized olivine grains commonly shows radial fracturing. Numerical simulations using the discrete element method applied to coupled fluid flow, reaction, and fracturing reveal that volume expansion of olivine grains during serpentinization results in the simultaneous fracturing of olivine and surrounding plagioclase, and that the thermal stress during cooling of oceanic lithosphere might also cause preferential olivine fracturing prior to serpentinization. The simulations also predict a self‐organizing fracture network that connects the olivine grains and passes through both olivine‐rich and olivine‐poor layers, resulting in permeability enhancement during serpentinization. Our results suggest that reaction‐induced fracturing plays an essential role in the infiltration of seawater through the lower crust and into the mantle within oceanic lithosphere.

Radiolarian Biochronology, Detrital Zircon Geochronological and Geochemical Constraints on Provenance and Depositional Environment of Cherts in the Southern Belt of the Western Yarlung Zangbo Suture Zone, Tibet

AbstractOphiolites in the southern belt (SB) occur as much larger peridotite massifs compared with those of the northern belt (NB), sporadically overlain by a thin layer of isotropic gabbro in the ...

Magmatic Processes at the Oman Ophiolite Paleoridge – Perspectives on the Role of Water

AbstractThe Oman ophiolite is regarded as best proxy for accreted oceanic crust from typical fast‐spreading ridge systems on land. However, the Oman ophiolite is influenced by initial subduction zone initiation, and the nature of the details of the subduction zone setting is still under controversial debate. While a first magmatic phase shows features of magmatic accretion very similar to those known from the East Pacific Rise, except that the primary melts were slightly water‐enriched, a second type of magmatism is characterized by an apparent subduction‐zone related imprint, producing rocks like FAB basalts and boninites in the upper crust, as well as cross‐cutting gabbronorites and wehrlites in the deeper crust. In this paper, we apply diverse experimental studies in wet tholeiitic and peridotitic systems performed at lower pressures (100 to 500 MPa) in the experimental lab of the University Hannover, in order to constrain the details of the magmatic processes proceeded at the Oman ophiolite paleoridge during the Cretaceous, with special focus on the influence of water on the phase stabilities and phase relations. The experiments were performed in vertically oriented internally heated pressure vessels (IHPV) (see Berndt et al., 2002; Fig. 1). This facility uses as pressure medium mixtures of Ar and H2 in order to adjust the required fH2 in the vessel, enabling us to control the redox conditions. The fH2 prevailing in the IHPV at high P and T was measured with a Shaw‐membrane made of platinum. The overall variation in fO2 in all experimental series was in the range between ∼FMQ‐1 and ∼FMQ+3.2, thus covering the range of oxygen fugacities prevailing in natural MORB magmas (Bezos and Humler, 2005).For understanding the magmatic processes during the Oman ophiolite paleoridge accretion, transects through the lower (GT1) and middle (GT2) crust have been drilled in the frame of ICDP (International Continental Scientific Drilling Program). Drill sites have been selected in the Wadi Tayin massif, which is known that the influence of magmatic phase 2 characterized by subduction‐related primary melts is minimal. Details and progress obtained in the Oman Drilling Project (OmanDP) can be found here: (https://www.omandrilling.ac.uk/). Regarding the first magmatic phase of the processes at the Oman ophiolite paleoridge, a characteristic observation made during the description of the drilled cores GT1 (lower crust) and GT2 (mid‐curst) was that quite often layers in the layered gabbro series occur which show the presence of clinopyroxene joining olivine instead of plagioclase (under near liquidus conditions). In terms of lithologies this could be interpreted as presence of wehrlitic crystal mushes as early cumulates instead of troctolitic, which are the typical ones for primary magmatism at typical fast‐spreading ridges. This situation could be experimentally simulated by adding a moderate to high water activity to primitive MORB at pressures ≥ 200 MPa, resulting in a shift of the clinopyroxene‐in curve to higher temperatures above the plagioclase‐in curve (Feig et al. 2006; see Fig. 2).Regarding the second, late‐stage magmatic phase, the formation of typical Oman high‐Ca‐boninites could be experimentally simulated by water‐saturated partial melting of Oman harzburgite at 200 MPa and relatively low temperatures between 1100 and 1200°C. Depleted gabbronorites crosscutting layered gabbros of phase 1 magmatism can be regarded as cumulates formed in these boninitic melts. Late wehrlites crosscutting layered gabbro could be produced by accumulation of olivine and clinopyroxene at temperatures between 1040 and 1080°C in a hydrous gabbroic system at pressures > 100 MPa with bulk water content of 2–3 wt%.

Composition and Seismic Properties of the Oceanic Lithosphere: A Synthesis of Ophiolites and Core Samples of the IODP

AbstractOur knowledge of the oceanic lithosphere largely comes from analogy with ophiolite complexes and the direct scientific drilling of the present‐day oceanic crust (e.g., Christensen and Salisbury, 1975, 1989; Smith and Vine, 1989; Dilek and Furnes, 2011, 2014). In this study, we summarized previous experimental results on seismic properties of oceanic lower crust and upper mantle according to different tectonic settings. The results are used to highlight the compositional heterogeneity and the nature of the oceanic Moho. Observation in different ophiolites reveal an ideal oceanic lithosphere profile with ideal petrologic units and seismic units (Dilek and Furnes, 2011, 2014). The lithospheric mantle beneath ocean basins is composed of tectonized peridotites, which include layered lherzolites and harzburgites and lenses of dunites with chromitites and nearly correspond to the seismic Layer 4. The overlying layered gabbros and mafic sheeted dike complex equal to the seismic Layer 3, as a result of crystallization from a magma chamber. The transitional unit between the former two petrologic units consists of layered ultramafic and mafic rocks, corresponds to the petrological Moho. The seismic Layer 2 and 1 are well defined by pillow lavas and massive flows, and the overlying abyssal sediments, respectively. Compared these results with the refraction seismic profiles, the oceanic crust and upper mantle show different composition and structure. The P‐wave velocities of the Layer 3 gabbros varies from 6.7 to 7.0 km s‐1 and have low velocity gradients of <0.1 km s‐1. Although the gradual increase of P‐ and S‐wave velocities with depth can be attributed to the increasing proportion of mafic minerals from the top to the bottom, prehnite‐pumpellyite facies alteration of basalts, greenschist‐faces metamorphism to epidote‐amphibolite facies metamorphism of gabbros will decrease the velocities of the Layer 2 and Layer 3 (Christensen and Salisbury, 1975, 1989), because the P‐wave velocities of chlorite and hornblende are 6.00 and 7.00 km s‐1, respectively, lower than those of plagioclase and pyroxene, respectively (Carlson, 2004). In addition, local velocity anomalies near the petrologic Moho can be related serpentinization of ultramafic rocks (Salisbury and Christensen, 1978;Carlson et al., 2009). In the Layer 4, the characteristic P‐wave velocities of the upper mantle should fall in the range of 7.8 to 8.2 km s‐1. Poisson's ratios of chrysotile and lizardite, which are stable in oceanic crustal environments according to the phase diagram, is 0.267 and 0.359, respectively, higher than those of olivine and pyroxene (Wang et al., 2013). Serpentinization will significantly decreased velocities and densities of peridotites and is the main reason for the variation of the Moho reflectivity beneath oceans.

The timing of sulfide segregation in a Variscan synorogenic gabbroic layered intrusion (Beja, Portugal): Implications for Ni-Cu-PGE exploration in orogenic settings

A growing number of magmatic Ni-Cu sulfide deposits discovered in orogenic environments has renewed the interest on this category and reinforced that most orogenic mafic magmas are PGE depleted. The Beja Layered Gabbroic Sequence (LGS) was emplaced during the same Variscan tectono-magmatic event as the Aguablanca Ni-Cu-PGE deposit in Spain, however, no economic mineralization was yet found in LGS. Here we analyze three sulfide ore-showings developed within different environments of LGS that provide a unique frame to assess sulfide segregation in orogenic mafic magmas: the Internal Sulfides hosted at the core of the layered suite, the Marginal Sulfides located at a complex domain where the LGS is intruded by late magmatic anorthosite-tonalite-trondhjemite (ATT) suite and the Sulfide Veins, associated to late magmatic-hydrothermal activity. For this we performed detailed mineralogical (electron probe micro-analysis, enabling to assess ƒS2-T for sulfide equilibrium), whole-rock and isotopic (sulfur and lead) geochemistry for the sulfide occurrences which are further supported by a wide data-set available for LGS and related mesocratic suites.The Internal Sulfides developed under low sulfidation, non-dynamic magmatic conditions, similar to those of the accessory sulfides in LGS gabbros. They segregated from chalcophile depleted melts at the end of a crystal fractionation cycle, without contribution from external sulfur.The Marginal Sulfides have a pyrite-bearing, magmatic sulfide assemblage hosted in exceptionally coarse-grained clinopyroxenites at the LGS SW border. The metal-poor, Sulfide Veins occur within a metasomatic halo adjoining amphibole-albite pegmatoid dykes. The latter two sulfide types share geochemical affinities and represent late-magmatic systems developed during uplift and intrusion by evolved, lower-crustal contaminated magmas. The Marginal Sulfides are mafic precursors of the coeval magmatic-hydrothermal system to which pegmatoid dykes and Sulfide Veins are related. Although estimated primary PGE abundances for LGS magmas are within the range of MORBs, all sulfide types derived from PGE ± Ni depleted melts due to sulfide segregation during underplating. The presence of deep crustal faults that prevent magma stalling and sulfide saturation at depth is paramount to generate orogenic magmatic sulfide deposits of economic value.

Layered gabbros and peridotites from Piton des Neiges volcano, La Réunion Island

We conducted a petrological study of the layered gabbros and peridotites outcropping in the cirque of Salazie, combining field investigations, geochemistry, petrography and, mineralogy to constrain the magmatic evolution of Piton des Neiges volcano (La Réunion). Three new hectometer-long outcrops are reported here (Sites 1, 2 and 3) in addition to the gabbroic series (hereafter named Site 4) previously described in the literature. The Sites 1 and 2 are located in the upper parts of the Mât River and show dunite and wehrlite units with no apparent foliation, lineation, or layering. The Sites 3 and 4 are located in the lower parts of the river and are made of olivine-gabbro, gabbro and ferrogabbro units showing marked foliation and lineation, and abundant layering. The mean composition of primary phases varies between Sites 1–2 and Sites 3–4: Fo89.5 to Fo69.8 in olivine, Mg# of 90 to 69 in clinopyroxene, and Mg# of 44 to 0 in Cr-spinel, titanomagnetite and, ilmenite, respectively. Here we show that the layered gabbros and peridotites exposed at the four sites represent multiple levels of magma storage. Based on the results of previous gravimetric studies and a drill hole related to geothermal exploration, we propose that the gabbroic series belongs to a magmatic system predating the building of Piton des Neiges and centered underneath the cirque of Salazie. The high volume of erupted materials in the NE quadrant of the volcanic edifice, and the southward magmatic flow direction in the layered gabbros suggest that the initial eruptive center of Proto-Piton des Neiges was above the cirque of Salazie, ~3 km north of the present-day summit. The landscape of Piton des Neiges may have changed when its NE flank experienced a major lateral collapse, after complete solidification of the gabbroic bodies. The magmatic activity, then, shifted southward after the flank collapse. Finally, we suggest that peridotites outcrops are associated with the present-day Piton des Neiges.

Testing final closure time of the Paleo-Asian Ocean along the Solonker suture by a transition of compressional and extensional setting

The giant Central Asian Orogenic Belt is an extensive accretionary orogen, of which the Solonker suture, as a major regional suture, coincides closely with an early Permian paleobiogeographical boundary. This suture is considered to mark the location of the final closure of the Paleo-Asian Ocean between the North China Craton and the Mongolian Terrane. Although the closure time of the Paleo-Asian Ocean along the Solonker suture has generally been regarded as Late Permian–Early Triassic, uncertainty remains because of a lack of typical collision-related features (e.g., high-grade regional metamorphism and well-developed fold–thrust structures) and a scarcity of outcrops.The present study reports Early Permian foliated gabbros and dikes (288–275 Ma) and Middle-Late Permian undeformed layered gabbros, strongly peraluminous granites, and I-type granites (265–254 Ma) in the Xinhure area along the northern margin of the North China Craton. The Early Permian foliated intrusions have a subduction-related geochemical signature and were derived from partial melting of lithospheric mantle modified by subduction-related melts or fluids at the active margin of the North China Craton. In contrast, the Late Permian undeformed layered gabbros and strongly peraluminous granites were derived from partial melting of lithospheric mantle and middle–upper crust, respectively, triggered by asthenospheric upwelling. Therefore, a transition from an end-compressional to an extensional environment according to a transition from collision termination to post-collision of the North China Craton and Mongolian Terrane may have occurred between 275 Ma and 262 Ma. This time span can be considered as the final closure time of the Paleo-Asian Ocean along the Solonker suture.

Magnetic anisotropy reveals Acadian transpressional fabrics in an Appalachian ophiolite (Thetford Mines, Canada)

SUMMARY Magnetic anisotropy has proved effective in characterizing primary, spreading-related magmatic fabrics in Mesozoic (Tethyan) ophiolites, for example in documenting lower oceanic crustal flow. The potential for preservation of primary magnetic fabrics has not been tested, however, in older Palaeozoic ophiolites, where anisotropy may record regional strain during polyphase deformation. Here, we present anisotropy of magnetic susceptibility results from the Ordovician Thetford Mines ophiolite (Canada) that experienced two major phases of post-accretion deformation, during the Taconian and Acadian orogenic events. Magnetic fabrics consistent with modal layering in gabbros are observed at one locality, suggesting that primary fabrics may survive deformation locally in low strain zones. However, at remaining sites rocks with different magmatic origins have consistent magnetic fabrics, reflecting structurally controlled shape preferred orientations of iron-rich phases. Subhorizontal NW-SE-oriented minimum principal susceptibility axes correlate with poles to cleavage observed in overlying post-obduction, pre-Acadian sedimentary formations, indicating that the magnetic foliation in the ophiolite formed during regional NW-SE Acadian shortening. Maximum principal susceptibility axes plunging steeply to the NE are orthogonal to the orientation of regional Acadian fold axes, and are consistent with subvertical tectonic stretching. This magnetic lineation is parallel to the shape preferred orientation of secondary amphibole crystals and is interpreted to reflect grain growth during Acadian dextral transpression. This structural style has been widely reported along the Appalachian orogen, but the magnetic fabric data presented here provide the first evidence for transpression recorded in an Appalachian ophiolite.

Devolatilization of Subducting Slabs, Part II: Volatile Fluxes and Storage

AbstractSubduction is a crucial part of the long‐term water and carbon cycling between Earth's exosphere and interior. However, there is broad disagreement over how much water and carbon is liberated from subducting slabs to the mantle wedge and transported to island‐arc volcanoes. In the Tian et al. (2019) Part I, we parameterize the metamorphic reactions involving H2O and CO2 for representative subducting lithologies. On this basis, a two‐dimensional reactive transport model is constructed in this Part II. We assess the various controlling factors of CO2 and H2O release from subducting slabs. Model results show that up‐slab fluid flow directions produce a flux peak of CO2 and H2O at subarc depths. Moreover, infiltration of H2O‐rich fluids sourced from hydrated slab mantle enhances decarbonation or carbonation at lithological interfaces, increases slab surface fluxes, and redistributes CO2 from basalt and gabbro layers to the overlying sedimentary layer. As a result, removal of the cap sediments (by diapirism or off‐scraping) leads to elevated slab surface CO2 and H2O fluxes. The modeled subduction efficiency (the percentage of initially subducted volatiles retained until ∼200 km deep) of H2O and CO2 is increased by open‐system effects due to fractionation within the interior of lithological layers.

Bulk Liquid for the Skaergaard Intrusion and Its PGE-Au Mineralization: Composition, Correlation, Liquid Line of Descent, and Timing of Sulphide Saturation and Silicate–Silicate Immiscibility

AbstractPreferred and modelled bulk composition of the Skaergaard intrusion are compared to coeval basaltic compositions in East Greenland and found to relate to the second evolved cycle of Geikie Plateau Formation lavas and coeval Skaergaard-like dikes in major and trace element (Mg# ∼45, Ce/Nb ∼2·5, (Dy/Yb)N ∼1·35), and precious metal composition (Pd/Pt ∼3, Au/Pt ∼2) as well as in age (∼56 Ma). Successful comparisons of precious metal compositions only occur with Skaergaard models based on mass balance. The bulk liquid of the intrusion evolved along the liquid line of descent to immiscibility between Si- and Fe-rich silicate liquids after ∼90% of crystallization (F = ∼0·10) in agreement with experimental constraints. Immiscibility led to accumulation and fractionation of the Fe-rich silicate melt in the mushy floor of the intrusion and continued accumulation of granophyre component in the remaining bulk liquid. The composition of plagioclase in the precious metal mineralized gabbro and modelling of Pd/Pt and Au/Pt in first formed droplets of sulphide melt suggest that sulphide saturation was reached in interstitial melts in crystal mushes in the floor and roof and in bulk liquid with a composition equivalent to that of the bulk liquid at lower UZa times and after crystallization of 82–85% of the bulk liquid (F = 0·19–0·16). Prior to sulphide saturation in UZa type melt, the precious metals ratios of the bulk liquid were controlled by the loss of Pt relative to Pd and Au in agreement with the low empirical and experimental solubility of Pt of ∼9ppb compared to a much higher value for Pd and Au. The relative timing between sulphide saturation (F = ∼0·18) and immiscibility between silicate melts (F = ∼0·10) and modelled precious metal ratios underpin the proposed multi-stage model for the mineralization, advocating initial accumulation in the mushy floor of the magma chamber controlled by sulphide saturation in mush melts rather than bulk melt, followed by redistribution of precious metals in a macro-rhythmic succession of gabbroic layers of the upward migrating crystallization zone.

Relative moment tensors and deep Yakutat seismicity

SUMMARYWe introduce a new relative moment tensor (MT) inversion method for clusters of nearby earthquakes. The method extends previous work by introducing constraints from S-waves that do not require modal decomposition and by employing principal component analysis to produce robust estimates of excitation. At each receiver, P and S waves from each event are independently aligned and decomposed into principal components. P-wave constraints on MTs are obtained from a ratio of coefficients corresponding to the first principal component, equivalent to a relative amplitude. For S waves we produce constraints on MTs involving three events, where one event is described as a linear combination of the other two, and coefficients are derived from the first two principal components. Nonlinear optimization is applied to efficiently find best-fitting tensile-earthquake and double-couple solutions for relative MT systems. Using synthetic data, we demonstrate the effectiveness of the P and S constraints both individually and in combination. We then apply the relative MT inversion to a set of 16 earthquakes from southern Alaska, at ∼125 km depth within the subducted Yakutat terrane. Most events are compatible with a stress tensor dominated by downdip tension, however, we observe several pairs of earthquakes with nearly antiparallel slip implying that the stress regime is heterogeneous and/or faults are extremely weak. The location of these events near the abrupt downdip termination of seismicity and the low-velocity zone suggest that they are caused by weakening via grain-size and volume reduction associated with eclogitization of the lower crustal gabbro layer.

Syncollisional gabbro in the Irtysh shear zone, Eastern Kazakhstan: Compositions, geochronology, and geodynamic implications

New data of petrography, mineralogy, and geochemistry from the Irtysh shear zone in Eastern Kazakhstan reveal three types of gabbro: (1) dolerite and gabbrodolerite, (2) biotite-bearing gabbronorite and gabbrodiorite, and (3) layered peridotite-gabbro intrusions. Rocks of types 1 and 2 occur mainly as small bodies and share similarity in major- and trace-element compositions. Layered gabbro (type 3) forms the largest intrusion in the area and has a different composition: higher CaO and Al2O3 but lower K2O, TiO2, P2O5, Zr, Nb, REE. Correspondingly, the gabbro of two first types may result from differentiation of the same tholeiite magma, whereas the layered gabbro rather derived from a magma intermediate between tholeiite and calc-alkaline series. According to geochemical and isotopic signatures, both types of primary magma originated from the same sublithospheric depleted mantle by low- and high-degree partial melting of spinel lherzolite. The obtained 317–313 Ma U-Pb and Ar-Ar ages of the gabbro fall within the time of collisional events in the area. The suggested model explains the origin of syncollisional gabbro as intrusion of ascending mafic melts from a mantle wedge at the transform boundary of colliding plates along the Irtysh shear zone where strike-slip faulting produced slab windows open to inputs of hot asthenospheric material.

Distinct sulfur saturation histories within the Palaeogene Magilligan Sill, Northern Ireland: implications for Ni – Cu – platinum group element mineralisation in the North Atlantic Igneous Province

The ∼60 m thick Magilligan Sill is part of the British Palaeogene Igneous Province in the North Atlantic. The sill comprises layers of dolerite and olivine gabbro, and it intrudes a thick sequence of Mesozoic mudstones and marls, which are locally baked at the sill margins. Since 2014, the sill has been an exploration target for orthomagmatic Ni – Cu – platinum group element (PGE) sulfide mineralisation analogous to the Noril’sk-Talnakh intrusion in Russia. We present new petrological, geochemical, and S isotope data to assess the prospectivity of the sill and the underlying magmatic plumbing system. Most sulfides in the dolerite portions of the sill are <50 μm in size and comprise only pyrite with PGE abundances below the detection limit. In the olivine gabbros, >150 μm size pentlandite, chalcopyrite, and pyrrhotite grains contain <4 ppm total PGE, 1460 ppm Co, and 88 ppm Ag. Pyrite from the dolerites have δ34S ranging from −10.0‰ to +3.4‰ and olivine gabbro sulfides range from −2.5‰ to −1.1‰, suggesting widespread crustal contamination. The S/Se ratios of sulfides in the dolerites and olivine gabbros range from 3500 to 19 500 and from 1970 to 3710, respectively, indicating that the latter may have come from upstream in the magma plumbing system. The Magilligan Sill records multiple injections of mafic magma into an inflating sill package, each with distinct mechanisms towards S saturation. Whilst the sulfide minerals in the sill do not constitute significant mineralisation themselves, detailed in situ studies highlight a divergence in S saturation histories and suggest that a larger volume of olivine gabbro sulfides at depth may be prospective.

What do variable magnetic fabrics in gabbros of the Oman ophiolite reveal about lower oceanic crustal magmatism at fast spreading ridges?

The magmatic processes responsible for accretion of the lower oceanic crust remain one of the least-constrained components of the global seafloor spreading system. Samples of gabbroic rocks recovered by scientific ocean drilling are too limited to allow effective assessment of spatial variations in magmatic flow within in situ lower crust. Extensive exposures of gabbros in ophiolites, on the other hand, provide opportunities to study accretion processes in three dimensions across wide areas and at a resolution that allows variations in magmatic fabrics through the crust to be quantified. Here, we show that magnetic anisotropy provides a reliable proxy for lower-crustal magmatic fabrics in the world’s largest ophiolite in Oman. Important differences in magnetic fabrics are detected that reflect variations in magmatic processes on a range of scales. Fabrics in layered gabbros are aligned with modal layering and display a consistency in the orientation of maximum principal axes of anisotropy between localities at a regional scale. These fabrics are compatible with subhorizontal preferred alignment of crystals orthogonal to the inferred orientation of the Oman spreading axis, resulting from magmatic flow or deformation of melt-rich crystal mushes during spreading. In contrast, magnetic anisotropy in foliated gabbros at the top of the lower crust reveals for the first time distinctly different linear and anastomosing fabric styles between localities sampled at the same pseudostratigraphic level. These differences reflect spatial variations in the style and trajectory of flow in the crystal mush beneath the axial melt lens during upward melt migration at the spreading axis.

Subduction of the Mesoarchaean spreading ridge and related metamorphism, magmatism and deformation by the example of the Gridino eclogitized mafic dyke swarm, the Belomorian Eclogite Province, eastern Fennoscandian Shield

A model of Mesoarchaean spreading ridge subduction and related metamorphism, magmatism and deformation in an active continental margin is presented here. The eclogites exposed along northeastern boundary of the Belomorian orogen in the eastern Fennoscandian Shield were formed as a result of Mesoarchaean–Neoarchaean subduction and collision. The Belomorian eclogite province contains two eclogite associations have been recognized within TTG gneisses: (1) the subduction-type Salma association and (2) the Gridino mafic dykes. Protoliths of the Salma eclogites represent a sequence comprising oceanic layered gabbro, formed at ∼2.9 Ga in a slow-spreading ridge. The subduction began in a time span 2.9–2.87 Ga, and eclogite-facies events occurred between ca.2.87 and ca.2.78 Ga. Injection of a mafic magma into an active continental margin, recorded by the Gridino dyke swarm, is attributed to subduction of a mid-ocean ridge, commencing at 2.87 Ga. The dykes represent typical fissure intrusions and underwent metamorphism to eclogite, granulite and amphibolite facies. The dykes divide into two compositions (Fe-Ti gabbro and Mg-Cr gabbronorite) which represent alternating episodes of arc and slab window magmatism. The Fe-Ti gabbros have high Ti and low abundances of fluid-mobile and other LIL elements compared to HFSE and plot in the MORB and within-plate oceanic basalt fields. The Fe-Ti gabbros formed from low-degree of partial melting from a depleted mantle source. This petrogenesis is consistent with slab window model in which magmas were derived by decompression melting of suboceanic mantle that upwelled into the opening beneath continental margin following spreading ridge subduction. The Mg-Cr gabbronorites have low Ti along with high abundances of fluid-mobile elements and other LILE compared to HFSE and plot in arc basalt fields that imply melting of subduction-modified hydrous subcontinental mantle.As the mid-ocean spreading ridge entered a trench, continental rocks above subduction zone underwent deformations with pseudotachylyte-bearing strike-slip faults development that is an evidence of a fossile earthquake. Paleoseismic event was limited to crustal conditions above the brittle-ductile transition, corresponding to temperatures below 450 °C and depths above 15 km. The slab window opening induced upwelling of anhydrous mantle and formation of the Fe-Ti gabbro dykes. The Mg-Cr gabbronorite was formed at slab window edges from metasomatized mantle.Summarizing evidences, such as ophiolites, eclogite, arc magmatism, we suggest that the modern style of subduction and plate tectonics began already in Mezoarchaean time 2.9-2.8 Ga ago. Special aspect of the paper is an argumentation of Meso-Neoarchaean age of eclogite facies metamorphism in the Belomorian eclogite province.

Chlorine-rich amphibole in deep layered gabbros as evidence for brine/rock interaction in the lower oceanic crust: A case study from the Wadi Wariyah, Samail Ophiolite, Sultanate of Oman

Hydrothermal veins and dykelets that cross-cut layered olivine gabbros deep in the plutonic section of the Samail Ophiolite, Sultanate of Oman, point towards the occurrence of hydrothermal circulation in the deep oceanic crust, and these features record interactions between rock and high temperature seawater-derived fluids or brines. Deep penetration of seawater-derived fluids down to 100 m above the Moho transition zone and the consequent interactions with the host rock lead to hydrothermal alteration from granulite facies grading down to greenschist facies conditions. Here we present a study of veins and dykelets formed by hydrothermal interaction cutting layered gabbro in the Wadi Wariyah, using petrographic, microanalytical, isotopic, and structural methods. We focus on amphiboles, which show a conspicuous compositional variation from high-Ti magnesiohastingsite and pargasite via magnesiohornblende and edenite, to Cl-rich ferropargasite and hastingsite (up to 1.5 a.p.f.u. Cl) and actinolite. These minerals record a wide range of formation conditions from magmatic to hydrothermal, and reveal a complex history of interactions between rock and hydrothermal fluid or brine in a lower oceanic crustal setting. Large variations in Cl content and cation configurations in amphibole suggest formation in equilibrium with fluids of different salinities at variable fluid/rock ratios. The presence of subsolidus amphibole extremely enriched in chlorine implies phase separation and brine/rock interactions. 87Sr/86Sr values of 0.7031 to 0.7039 and stable δ18O isotopic compositions of 4.1 to 5.6‰ of the different amphibole types suggest a rock-dominated environment, i.e. with low fluid/rock ratios. However, the slight departure from mean Oman isotope values may indicate there was some influence of seawater in the aforementioned fluid-rock interactions. Our study provides new petrological data for the subsolidus evolution of gabbro-hosted amphibole-rich veins in the presence of a seawater-derived fluid.

Hydrothermal fault zones in the lower oceanic crust: An example from Wadi Gideah, Samail ophiolite, Oman

Hydrothermal circulation is a key process for chemical and isotopic exchange between the solid Earth and oceans, and for the extraction of heat from newly accreted crust at mid-ocean ridges. However, due to a dearth of samples from intact oceanic crust, or continuous sample suites from ophiolites, there remain major shortcomings in our understanding of hydrothermal circulation in the oceanic crust, especially in the lower plutonic crust. In particular, it remains unknown whether fluid recharge and discharge occurs pervasively or if it is mainly channelled within discrete zones such as faults.Here, we describe a hydrothermally-altered fault zone that crops out in the Wadi Gideah in the layered gabbro section of the Samail ophiolite of Oman. A one metre thick normal fault comprising deformed chlorite ± epidote fault rock with disseminated chalcopyrite and pyrite, offsets gently dipping layered olivine gabbros. The chlorite-rich fault rocks surround strongly altered clasts of layered gabbro, 50 to 80 cm in size. Layered gabbros in the hanging wall and the footwall are partially altered and abundantly veined by epidote, prehnite, laumontite and calcite veins. In the wall rocks, igneous plagioclase (An82±2%) is partially altered towards more albitic compositions (An75-81), and chlorite + tremolite partially replaces plagioclase and clinopyroxene. Clinopyroxene is moderately overgrown by Mg-hornblende.Whole rock mass change calculations show that the chlorite-rich fault rocks are enriched in Fe, Mn, H2O + CO2, Co, Cu, Zn, Ba, and U, but have lost significant amounts of Si, Ca, Na, Cr, Ni, Rb, Sr, Cs, light rare earth elements (LREE), Eu, and Pb. Gabbro clasts within the fault zone as well as altered rock from the immediate hanging wall show enrichments in Na, volatiles, Sr, Ba and U and depletions of Si, Ti, Al, Fe, Mn, Mg, Ca, Cu, Zn, Rb, Cs, LREE, and Pb. Chlorite thermometry suggests a formation temperature of 300–350 °C for the fault rock and based on the Si loss and solubility of silica in hydrothermal fluids the intensity of alteration requires a fluid to rock ratio of up to 900:1. Strontium isotope whole rock data of the fault rock yield 87Sr/86Sr ratios of 0.7043–0.7048, which is considerably more radiogenic than fresh layered gabbro from this locality (87Sr/86Sr = 0.7030–0.7033), and similar to black smoker hydrothermal signatures based on epidote, measured from epidote veins in the footwall and elsewhere in the ophiolite (87Sr/86Sr = 0.7043–0.7051). Altered gabbro clasts within the fault zone show similar values with 87Sr/86Sr ratios of ~0.7045–0.7050. In contrast, the hanging and footwall gabbros display values only slightly more radiogenic than fresh layered gabbro. The elevated strontium isotope composition of the fault rock and clasts together with the observed secondary mineral assemblages and calculated mass changes strongly supports the intense interaction with seawater-derived up-welling hydrothermal fluids, active during oceanic spreading. Assuming that such a fault zone is globally representative of faulting in the lower crust, an extrapolation of our results from mass change calculations to elemental fluxes, shows a significant contribution to the global hydrothermal budgets of Si, Ti, Fe, Mn, Mg, Ca, H2O, Cu, Zn, Sr and Cs.

Inclusions in an isoferroplatinum nugget from the Freetown Layered Complex, Sierra Leone

ABSTRACTInclusions of platinum-group minerals (PGM) within alluvial isoferroplatinum nuggets from the Freetown Peninsula, Sierra Leone, are aligned with their shape determined by the structure of their host. The edges of the majority of the inclusions lie at 0°, 45° or 90° to external crystal edges of the nugget which shows that the inclusions are not randomly oriented earlier minerals incorporated within their host. The inclusions are later infills, probably formed at the surface of the nugget during growth and subsequently enclosed by the growing nugget. PGM on the present surface of the nugget represent the last stage of this partnership. A single nugget containing abundant inclusions is described here but similar features are observed in other nuggets from the same area. The inclusions contain laurite (RuS2), irarsite–hollingworthite (IrAsS–RhAsS), Pd–Te–Bi–Sb phases, Ir-alloy, Os-alloy, Pd-bearing Au, an Rh–Te phase, Pd–Au alloy and Pd–Pt–Cu alloy. PGM found on the nugget surface include laurite, irarsite and cuprorhodsite (CuRh2S4). The Pd–Te–Bi–Sb phases may include Sb-rich keithconnite (Pd20S7) and compositions close to the kotulskite–sobolevskite solid-solution series (PdTe–BiTe). Textural evidence suggests that formation of the nuggets began with the isoferroplatinum host and the voids were filled starting intergrowths of laurite and irarsite–hollingworthite with both laurite and irarsite–hollingworthite often showing compositional zonation and each of them replacing the other. Filling of the voids probably continued with Pd-Cu-bearing gold, Sb-rich keithconnite (Pd,Pt)20.06(Te,Sb,Bi)6.94, keithconnite, telluropalladinite Pd9(Te,Bi)4, RhTe and finally Ir-alloy and then Os-alloy. The nuggets are thought to be neoform growths in the organic- and bacterial-rich soils of the tropical rain forest cover of the Freetown intrusion. The mineralogical assemblage in the layered gabbros of the intrusion has been previously shown to differ from the alluvial assemblage in the rivers and these inclusions, not seen in Pt3Fe in the unaltered rocks, add a further item to the catalogue of differences.