The sub-continental lithospheric mantle (SCLM) beneath the Cameroon Volcanic Line (CVL) is vertically and laterally heterogeneous, consisting of a complex mixing of DMM, HIMU, and EM1, affected by modal or cryptic metasomatism, depending on the area. The petrography, whole-rock geochemical data, and minerals’ chemical composition of lavas and mantle xenoliths from the Bini Warack area, combined with Sr isotope compositions, provide constraints on the origin and thermochemical evolution of the SCLM beneath this sector of the CVL. The host lavas are basanite, basalt, and latite with OIB affinity, characterized by moderate to high silica and alkali contents (SiO2 = 42.35–56.56 wt%, K2O+Na2O = 2.34–7.07 wt%), a high Ba/Rb ranging from 12.2 to 26.1, a low Rb/Sr from 0.03 to 0.08, strong enrichment in LREE relative to HREE (LaN/YbN: 9.3–30), and moderate enrichment in radiogenic isotopes (e.g., 0.702987 < 87Sr/86Srinitial < 0.703206; 0.512854 <143Nd/144Ndinitial < 0.512918) with positive εNdinitial (+4.84 to +6.09). These features are consistent with an origin of the lavas by a low degree of partial melting (<2%) of a lherzolitic mantle source containing 2% to 6% garnet. These lavas have then evolved by fractional crystallization without any evidence of crustal contamination. The studied mantle xenoliths are spinel-bearing lherzolites, characterized by U/Th ratios typically lower than 1, a slight enrichment in LILE relative to HFSE, and mainly consist of minerals with fertile composition (Fo84-91; spinel Cr#: 0.1–0.22; Al-rich pyroxenes). They are consistent with refractory mantle peridotite that evidences low partial-melting degrees. Trace element concentrations of host lavas (high Ba/Rb: 12.2–26.1 and low Rb/Sr: 0.03–0.08), together with olivine’s crystals chemical features (high Ca/Fe and 100*Mn/Fe ratios; low 100*Ni/Mg ratios) and low Ca/Al ratios (<5) of clinopyroxenes in spinel-bearing lherzolite xenoliths suggest that the SCLM beneath the Bini Warack area is likely a juvenile lithospheric mantle which that undergone a carbonate-rich metasomatism.

The Castillon massif, in the northern Pyrenees, features a complex of decimetric to metric ultramafic and mafic layers emplaced within metasedimentary series (from the bottom to the top: garnet, sillimanite, and kyanite-bearing gneisses and sillimanite + cordierite-bearing gneisses). Ultramafic and mafic layers and metasediments have been deformed and metamorphosed under granulitic facies conditions during the Hercynian orogenesis. The mineralogical, petrologic, and geochemical characteristics of the studied samples allow us to define two distinct series: 1) a pyroxene-bearing magmatic series (UM-M1) consisting of ultramafic (UM: dunites, harzburgites, and orthopyroxenites) and mafic (M1: norites, gabbro–norites, and gabbros) rocks; and 2) a pyroxene-free and hornblende-bearing series (M2; mela-, meso-, and leucogabbros). The leucogabbros exhibit some characteristics of anorthosites, including the high Al2O3 whole-rock content (31 wt%), high An content (An84–96) in plagioclase, weak rare earth element enrichment, and very positive Eu anomalies. We propose that the rocks of the ultramafic and pyroxene-bearing rock-series (UM-M1 series) are all associated with the same magmatic event and that the M2 series rocks are associated with a distinct separate event. Isotopic data suggest that these formations are Ordovician. The M2 rocks have juvenile Nd isotopic signatures (εNd(460) from +4.59 to +8.11), suggesting that they are derived from superheated alumina-rich basaltic or basaltic–andesite melts extracted from a relatively depleted mantle source. In contrast, most of the M1 rocks derived from parental basaltic melts show partially crustal contamination, with only a few clearly juvenile samples (εNd(460) from +0.45 to +6.59). We propose a geodynamic evolution for the Castillon massif involving a two-stage genesis and the emplacement of the two series. First, the emplacement of olivine-saturated basaltic melts in a deep metasedimentary crust resulted in the M1 series. The second step involves the emplacement of alumina-rich basaltic or basaltic–andesite melts to produce the M2 hornblende-bearing series (mela-, meso-, and leucogabbros) devoid of pyroxenes. The leucogabbros show strong similarities with common anorthosites although they have not been previously observed in the Variscan Pyrenees.

The gold-rich Kett & eacute; formations belong to the Adamawa-Yad & eacute; block of the Neoproterozoic Central Africa Orogenic Belt. They consist of a complex association of tonalite, granodiorite, and heterogeneous granite including rafts of metapyroxenite, amphibolite, paragneiss and migmatite. Metapyroxenite and amphibolite are metaluminous (A/CNK: 0.65-0.95), calc-alkaline, enriched in LILE and LREE, with epsilon Nd-610 < 0 and T-DM ages of 1.7 Ga, highlighting their enriched mantle source. Paragneiss are rich in LREE, Cr, V, Sc and Ni and display A/CNK (1.06-1.08) and A/NK (1.5-3.49) ratios consistent with a sedimentary protolith made of poorly sorted greywackes and litharenites with a contribution from mafic rocks of the juvenile crust. Granitoids display a variety of signatures, ranging from metaluminous to peraluminous (A/CNK: 0.94-1.04), calc-alkaline to alkaline. The heterogeneous granite in diffuse contact with migmatite have epsilon Nd-654 values of -9.61 and -8.11, and are interpreted to reflect local magma collection within the zone of dominant partial melting of the paragneiss. The tonalite and granodiorite, characterized by magmatic textures, display Mg# of 39.9-48.8, low HFSE contents, and enrichment in LILE and LREE. They were likely formed by the fractional crystallisation of a dioritic magma derived from the partial melting of amphibolite. The main structure of the migmatite is a shallow- to moderately dipping NE-SW trending syn-migmatitic foliation S-mgm, marked by alternating leucosome and mesosome layers, with relics of an S-0/S-1 foliation locally preserved within the mesosome. Granulite-facies relics with a peak at 8-10 kbar/>= 800 degrees C are preserved in metapyroxenite. Networks of texturally continuous leucosome veins concordant to discordant to the syn-migmatitic foliation, localization of granitic veins in shear zones and fold axial planes, attest for deformation in the presence of melt. Granitoids form kilometre-scale plutons with gently dipping magmatic fabrics (S-m-S-m/C-2) concordant to the syn-migmatitic foliation. These fabrics are transposed into a steeply dipping (47 degrees-74 degrees) E-W to NW-SE trending mylonitic fabric (S-myl-S-3/C-3), bearing a moderately plunging (15 degrees-49 degrees) stretching lineation L-3, marking the folded Mama Shear Zone (MaSZ). These data indicate that the Kett & eacute; region has recorded magmatic accretion of a mafic crust originating from an enriched mantle source. This crust and its associated sediments were subsequently tectonically thickened during the Pan-African orogeny and affected by partial melting. Migration of dioritic to granitic magmas from the partially molten orogenic root led to syntectonic crustal differentiation.

The Ziver volcanism, located in northern Cameroon within the Central African Rift, forms an integral part of the Cameroon-Chad Volcanic Line (CCVL). The new mineralogical, geochemical, and isotopic data presented here provide fresh insights into the sources and petrogenesis of lavas from this little-studied area. The primary mineral assemblage consists of olivine, clinopyroxene, Fe-Ti oxides, and feldspars. Clinopyroxenes are predominantly calcic, with compositions ranging from diopside to clinoenstatite. Feldspars occur as andesine in mafic rocks, whereas in felsic lavas they are represented by K-albite and Na-sanidine. The volcanic suite defines a bimodal alkaline series composed of mafic (basanite, basalt, hawaiite) and felsic (trachyte, rhyolite) lavas, characterized by moderate to high alkali contents (K2O + Na2O = 4.10-12.25 wt%). These lavas display moderately enriched radiogenic isotope signatures, with Sr-87/Sr-86 ratios of 0.70311-0.71856 and Nd-143/Nd-144 ratios of 0.51276-0.51295. Geochemical and isotopic data (0.7036 < (Sr-87/Sr-86)(initial) < 0.7203; -10.65 < epsilon Ndi < 6.44) indicate an intraplate OIB affinity, derived from low-degree (1-3%) partial melting of an enriched garnet lherzolite mantle plume source. Magmatic differentiation is dominated by fractional crystallization with minimal crustal contamination, consistent with the CCVL as a whole.

Unraveling the paleogeography is crucial to understand the changes in Earth Systems at the onset of the Cryogenian Period around similar to 720 Ma. The Cryogenian Period is marked by widespread glaciations notably preserved in the Arabian Nubian Shield, and is a turning point in the supercontinent cycle between the dispersal of the Rodinia supercontinent and the assembly of Gondwana. Filling gaps from the paleomagnetic database remains an indispensable tool to solve the Proterozoic paleogeographic and paleogeodynamic puzzle, especially during challenging periods of continental dispersion. Through a detailed coupled paleomagnetic, structural and geochronologic study, the Shaat dyke swarm (similar to 720 Ma), intruding the Mirbat basement on the south-western coast of the Oman sultanate, has proven to be reliable witnesses of the evolution of the ancient Earth. A characteristic remanent magnetization (ChRM) was isolated in stable single to pseudo-single domain (SD/PSD) magnetite. Well constrained site mean directions obtained for 20 dykes lead to a mean direction for the Shaat dyke swarm of Dm = 249.9 degrees, Im = -61.3 degrees (N = 20, alpha 95 = 5 degrees, k = 43.1), yielding a paleomagnetic pole at 104.9 degrees E, 25.9 degrees N, (A95 = 7.2 degrees, K = 21.3). The reliability of the paleomagnetic pole is assessed by 5 of the 7 Criteria of Meert et al. (2020). This new key paleomagnetic Mirbat pole represents the only reliable paleomagnetic pole for the Omani Neoproterozoic block/terrane at similar to 720 Ma. It confirms the connection between the Omani terrane and the Indian Shield (already link to the Seychelles and Madagascar blocks) and defines a medium paleolatitude of similar to 42.4 degrees for the Omani block at similar to 720 Ma. The confirmed connection also implies a moderate latitude (similar to 40 degrees) for the Indian Shield at the beginning of the major Sturtian glaciation.

Mafic volcanic rocks from the Cenozoic Urumieh–Dokhtar magmatic arc (UDMA) of Iran, a segment of the Alpine–Himalayan orogenic belt, provide rather restricted ranges of trace-element abundances and patterns as well as Sr–Nd–Pb isotopic signatures. However, they are distinct enough to help characterize the geochemical signatures inherited from their arc system components. The volcanic rocks are classified into three series: the LILE-rich, LILE-poor, and incompatible trace-element-rich series (ITE-rich series, which includes samples with OIB-like – oceanic-island basalt – patterns). The LILE-rich series is derived from a mantle source metasomatized by fluid-rich slab partial melts, whereas the LILE-poor series, high in immobile and highly incompatible elements that include La and Ce, is derived from a mantle source metasomatized by fluid-poor slab partial melts. Slab melting is favored by the young, hot slab subduction of a then narrow, contracting Neotethyan oceanic plate. The ITE-rich series bear the signatures of mantle metasomatized by slab partial melts that were induced by, and reacted with, asthenospheric mantle that ascended through a slab window or rupture. Given almost primitive geochemical signatures of the mafic rocks, the Sr–Nd isotopic modeling indicates mantle wedge : slab melt : sediment melt contributions of 45:27.5:27.5 and 55:09:36 for the LILE-rich and LILE-poor series respectively. The mafic volcanic rocks, which extend from the mantle array (i.e., the NHRL; Northern Hemisphere reference line) toward an enriched mantle, on the Pb–Pb isotopic plots, further support this finding. Eocene to Early Miocene ages for these three series favor intermittent volcanism of each rock series over an extended period of time, rather than single episodic magmatism for each geochemically distinct magma series. Dominance of LILE-rich series rocks in the northern part of the study area (the Kahak area) points to a more hydrous, more altered slab compared to the slab beneath the central part, where the LILE-poor series is dominant.

Prominent exposures of mantle rocks in the form of peridotites are observed extending in the E-W direction having a maximum width of 1.5km at the Dras village, Ladakh, India. The Dras peridotites are mainly dunites bearing chromite mineralization with minor harzburgites and wehrlites. The peridotites are emplaced within the Dras volcanics along with gabbros and radiolarian cherts. They display protogranular textures grading into equigranular mosaic textures typical of mantle peridotites. In dunites, olivines exhibit straight boundaries meeting at 120⁰ indicating recrystallization. Spinels associated with dunites are disseminated as tiny inclusions and at places they are lodged on olivine triple junctions. Porphyroclastic olivine in the harzburgites display kink bands whereas orthopyroxenes (enstatite) in harzburgites are subhedral with exsolution lamellae of clinopyroxene (dioside). Overall textures suggest that the peridotites have undergone progressive deep-seated deformation and solid-state recrystallization. The chromite mineralization associated with dunites displays a variety of structures viz. banded, lenticular, pull-apart, schlieren, massive, disseminated etc. Magnesite veins forming an intricate network in dunites are observed. Geochemically, the peridotites are relatively fresh (LOI - 0.1 wt % to 5 wt %) with Mg# between 89 and 91, comparable with residual oceanic peridotites. Major element chemistry of the peridotites indicates they are abyssal peridotites, however, depleted REEs, trace elemental concentrations along with enriched LILEs, especially Cs and Nb-Ta and Zr-Hf anomalies indicate formation in a subduction setting. Olivines contain Mg-Cr-rich and Fe-poor rims compared to the cores. Spinels in dunites are chromites with Cr# 68-82 and Mg# 34-48 whereas, in harzburgites, spinels are magnesio-chromites with Cr# 44-55 and Mg# 56-62. Spinel and olivine data suggest that dunites have undergone very high degrees of partial melting about 35% possibly in the supra-subduction zone (SSZ) setting and may have interacted with boninite-like melts. Harzburgites, on the contrary, are formed by lower degrees of partial melting ranging from 20-25%. However, when remodelled using the clinopyroxene trace element concentrations, the clinopyroxenes from harzburgites suggest 15% to 23% degrees of partial melting. Temperature estimates calculated on select mineral pairs yield temperatures of 816⁰C to 1046⁰C for the peridotites. Distinct petrological and geochemical signatures displayed by the rocks in the present study indicate that the Dras samples show mixed affinities with harzburgites formed at MOR setting whereas dunites being ultra-depleted and refractory owing to higher degrees of partial melting were modified in an SSZ environment.Keywords: Dras, peridotites, mantle, dunite, chromite mineralization, Ladakh.

Abstract. Geochemical and isotopic data from the Paleoproterozoic greenstone belt of Mako (Eastern Senegal) provide evidence for the existence of two tholeiitic and one calc-alkaline series. Tholeiitic series are composed of ultramafic and mafic rocks. Tholeiites 1 are characterized by flat Light Rare Earth Elements (LREE) and Middle Rare Earth Elements (MREE) patterns and by initial isotopic compositions of 87Sr/86Sr (2.1 Ga) from 0.70083 to 0.70368 and εNd (2.1 Ga) from +1.55 to +5.90 (except sample IL49). Tholeiites 2 are characterized by a more enriched LREE and MREE pattern, and by initial isotopic compositions 87Sr/86Sr (2.1 Ga) from 0.7009 to 0.70263 and εNd (2.1 Ga) from +2.14 to +7.35. In addition tholeiites 2 show a slightly negative Nb anomaly, while the tholeiites 1 have no Nb anomaly. Calc-alkaline rocks are characterized by more enriched LREE and MREE patterns and more depleted Heavy Rare Earth Elements (HREE), indicating more pronounced general LREE/HREE fractionation; they have a more pronounced negative Nb and Ti anomalies and initial isotopic signatures of 87Sr/86Sr (2.1 Ga) from 0.7009 to 0.70618 and εNd (2.1 Ga) from +3.12 to +6.40. Comparison of geochemical and isotopic data of the three magmatic series with those of other Paleoproterozoic domains of the West African Craton and juvenile volcanic rocks leads us to propose an evolutive model for the Mako series comprising an initial emplacement of an oceanic plateau, in which the first tholeiitic series was generated, followed by a subduction process that triggers the contamination of the mantle plume source and the generation of the second tholeiitic series at the earliest stages of the arc built; and ending with the maturation of an island arc and the calc-alkaline series emplacement, which has a chemical signature indicating that the source region was likely a metasomatized mantle probably mixed with some plume mantle source.

Download This Paper Open PDF in Browser Add Paper to My Library Share: Permalink Using these links will ensure access to this page indefinitely Copy URL Copy DOI

Migmatite formation is dependent of several and inter-related factors, such as P-T conditions, protolith composition and the abundance of volatiles, which usually hinders the full understanding of anatectic progression and how it relates to orogenic evolution. We have constrained the petrological evolution of the anatectic complex (mostly metatexites and diatexites) of the Porto-Viseu Metamorphic Belt in the Central Iberian Zone (CIZ) using petrography, geochemical and isotopic data.Major and trace element geochemistry allowed us to conclude that the studied metatexites were formed through water-present melting of muscovite from metapelitic schists, whereas diatexites were mostly formed by dehydration-melting of muscovite and, occasionally, by the dehydration-melting of biotite. Additionally, melting evolution from incipient (metatexites) to pervasive (diatexites and anatectic granitoids) included SiO2 enrichment and elemental depletion (except for Ca, P, Na and K). The geochemical evolution of these elements during melting progression was essentially controlled by fractionation/melting of plagioclase, K-feldspar, mica and apatite, whereas REE abundance was constrained by accessory minerals, such as monazite, apatite and zircon. In both cases, the geochemical control was variable and dependent on the fate of these mineral phases during the progression of melting, as well as the variable degree of melt loss or gain, with diatexites and granites being the result of substantial melt gain.The 87Sr/86Sr320 values have a large dispersion (0.708–0.766), which is common for rocks that experienced these metamorphic conditions, whereas the εNd320 values are less variable, roughly from −3 to −9 (diatexites: −8.24 to −2.96; metatexites: −8.79 to −5.15; staurolite-bearing schists: −7.57 to −5.19; granite: −8.21), being both isotopes independent of lithological type or degree of melting. Therefore, we interpret their Sr isotopic variation as mostly related to secondary alteration processes, whereas Nd isotopes, which are usually more immobile, should correlate to the initial protolith. In fact, their isotopic results are broadly consistent to the values of other CIZ metasediments that did not experience such high metamorphic conditions and melting (Douro-Beiras Supergroup), being most likely derived from these rocks, namely the Douro Group.

This paper presents and discusses new geochronological and petrological data on a suite of calc-alkaline plutons composed predominantly of diorites and tonalites from the West Massif Central. Their petrochemical fingerprints are compatible with partial melting of a hydrous mantle wedge followed by fractional crystallization of amphibole and plagioclase before final emplacement between 5 and 8 kbar within the continental upper plate of a subduction system. In situ U-Pb zircon dating on tonalites yields a fairly narrow age range of 365−354 Ma (including uncertainties) for igneous crystallization. These calc-alkaline plutons imply active margin magmatism near the Devonian-Carboniferous boundary and are contemporaneous with the back-arc magmatism and HP metamorphism as dated by recent studies. However, such isolated igneous bodies do not form a transcrustal magmatic arc but rather represent dispersed plutons emplaced within less than 30 Myr when all data from the Variscan belt of France are considered. In Limousin, they intrude migmatitic paragneisses and retrogressed eclogites from the Upper Gneiss Unit (UGU), suggesting that the high pressure rocks were already exhumed at 19−30 km depth before 365 Ma. Moreover, the diorites and tonalites are never found within units below the UGU. It therefore suggests that these tectono-metamorphic units of the Western French Massif Central were piled up after 354 Ma. Altogether these results support the monocyclic model for Variscan geodynamics in the French Massif Central, with the transition between oceanic subduction and continental collision taking place between Upper Devonian and Lower Carboniferous.

The Antarctic Circumpolar Current plays a pivotal role in global climate through its strong influence on the global overturning circulation, ocean heat and CO 2 uptake. However, when and how the Antarctic Circumpolar Current reached its modern-like characteristics remains disputed. Here we present neodymium isotope and sortable silt records from sediment cores in the Southwest Pacific and South Indian oceans spanning the past 31 million years. Our data indicate that a circumpolar current like that of today did not exist before the late Miocene cooling. These findings suggest that the emergence of a homogeneous and deep-reaching strong Antarctic Circumpolar Current was not linked solely to the opening and deepening of Southern Ocean Gateways triggering continental-scale Antarctic Ice Sheet expansion during the Eocene–Oligocene Transition (∼34 Ma). Instead, we find that besides tectonic pre-conditioning, the expansion of the Antarctic Ice Sheet and sea ice since the middle Miocene Climate Transition (∼14 Ma) played a crucial role. This led to stronger density contrast and intensified Southern Westerly Winds across the Southern Ocean, establishing a vigorous deep-reaching circumpolar flow and an enhanced global overturning circulation, which amplified the late Cenozoic global cooling.

The footprints of mafic melts travelling from the depths to the surface are abundant in the mantle section of ophiolites. They constitute an important source of information about the melt migration mechanisms and related petrological processes in the shallowest part of the mantle beneath former oceanic spreading centres. In the field, these so-called ‘melt migration structures’ attract attention when they consist of mineral assemblages contrasting with that of their host peridotite. They therefore record a particular moment in the migration history: when the melt becomes out of equilibrium with the peridotite and causes a reaction impacting its modal composition, and/or when a temperature drop initiates the crystallization of the melt. The existence of cryptic effects of migration revealed by geochemical data shows that melts do not always leave a trail visible in the field. Although incomplete and patchy, the melt migration structures preserved in ophiolites are witnesses of processes that do actually occur in nature, which constitutes an invaluable support to the interpretation of geophysical data and inescapable constraints for numerical simulations and models of chemical geodynamics. Here we show how field observations and related petrological and geochemical studies allow us to propose answers to fundamental questions such as these: At which temperature is porous flow superseded by dyking? What are the factors governing melt trajectories? What is the nature of the ‘universal solvent’ initiating infiltration melting and making channelized porous flow the most common mode of transport of magmas through a peridotite matrix regardless the tectonic setting? A fundamental message delivered by ophiolites is that the shallow mantle behaves as a particularly efficient reactive filter between the depths and the surface of the Earth. Unexpectedly, the reactions occurring there are enhanced by the hybridization between mafic melts and a hydrous component, whatever its origin (i.e. magmatic vs. hydrothermal). This hybridization triggers out of equilibrium reactions, leading to the formation of exotic lithologies, including metallic ores, and impacting the global geochemical cycle of a whole range of chemical elements.

Despite the substantial amount of research on the Cretaceous Samail ophiolite in Oman, the factors controlling the size and metal endowment of the mafic-hosted, Cu-Au(-Zn-Ag) volcanogenic massive sulfide (VMS) deposits remain elusive. This work shows that the volcanostratigraphic position, hydrothermal venting style, and oxidation processes are critical factors controlling the distinct features of the Shinas and Mandoos deposits. Mandoos is a large (8 Mt, 1.8 wt % Cu, 0.18 g/ton Au) orebody preserving abundant primary vent-related features formed via mound growth and collapse within a wide hydrothermal field, overlying a poorly developed stockwork. The smaller Shinas deposit (0.8 Mt, 2.6 wt % Cu, 0.63 g/t Au) represents a higher-temperature system evolving from low & INT;S2/& INT;O2 conditions, locally sealed by jaspers, to a mound growth stage with widespread subseafloor brecciation/replacement with associated zone refining. Mandoos formed at the onset of the postaxial stage (Geotimes-Tholeiitic Alley transition), and Shinas is hosted within the Alley units. Volcanism in Samail was seemingly continuous, and the low ENd and Nb/Ta of the Shinas hanging-wall lavas record the onset of significant modifications of the mantle source during the postaxial stage. Mandoos is enriched in Te + As + Se & PLUSMN; Zn & PLUSMN; Ga & PLUSMN; Sb relative to Shinas, where higher Cu + Au + Tl & PLUSMN; Mo grades possibly reflect leaching of protoarc-like lavas. Rare earth element patterns in the ores mimicking the deposit footwall can be employed to constrain volcanostratigraphic positioning and indicate that the footwall lavas may also represent a source of metals. Formation of metal-rich ochres at the sea floor likely led to Cu + Au upgrading in the ores during seawater-induced oxidation, which was enhanced during subaerial gossan formation.

The Samail ophiolite in Oman, represents one of the most well-preserved remnants of the Tethyan oceanic crust, and hosts numerous mafic-hosted volcanogenic massive sulfide (VMS) deposits. In this work we present Pb isotopic data from 13 VMS deposits in Oman, in different volcanostratigraphic positions, as well as pristine volcanic glasses from the ophiolite main volcanic units. Our data shows that the volcanic units and VMS deposits in Oman exhibit a wide range of Pb isotopic compositions. The volcanic glasses show an increasingly radiogenic Pb isotopic composition from the Geotimes lava unit (V1) into the uppermost Boninitic Alley unit (V2), supporting significant isotopic modification of the mantle source trough time. The VMS deposits isotopic composition generally reflects that of their host volcanic units, indicating that the footwall represents the major source of Pb for each deposit. Some deposits hosted in the uppermost Tholeiitic and Boninitic Alley units, however, show highly variable isotopic signatures, possibly due to leaching of Pb from different volcanic units at depth. These results demonstrate shallower metal sourcing than is typically considered for VMS deposits and highlights the importance of the footwall volcanic architecture in controlling the metal endowment of the VMS deposits.

The Samail ophiolite in Oman was sampled by scientific drilling targeting crucial sections of the oceanic crust and mantle during the Oman Drilling Project- OmanDP [1]. Drillhole CM1A aimed at characterizing the transition from the lower crust to the mantle Moho Transition Zone (MTZ), where both magmatic and hydrothermal exchanges took place. Four magmatic sequences were defined: SI- Layered Gabbro, with thin wehrlite and dunite layers (1.5-160.2 m); SII- fully serpentinized Dunite (160.2-250.0 m); SIII- Dunite with rodingitized gabbro (250.0-311.0 m) and; SIV- Mantle, harzburgite with opx-dunite levels (311.0-404.2 m). We present a sulfur and Sr isotope profile to characterize the sulfur cycling during hydrothermal alteration within the MTZ (SI-SIII). Acid Volatile Sulfides (AVS), Cr-Reducible Sulfur (CRS) and acid-soluble sulfate (SO4) were sequentially extracted and analyzed for δ34S on the same whole-rock powders analyzed for Sr isotopes. The crust-mantle transition records extreme and often decoupled variations in sulfur (δ34S=-25.8 to +56.9‰) and 87Sr/86Sr (0.703088-0.711688) signatures. Total extracted sulfur from sulfide (TS=AVS+CRS) contents increase gradually from the top to the bottom of SI from ca ~65-2820 ppm, to maximum of 5043 ppm in a Cpx-Pl-dunite layer ca. 16 m above SII. Sulfide assemblages comprises magmatic pyrrhotite+pentlandite+chalcopyrite and secondary pyrrhotite (in Fe-serpentine pseudomorphs)+bornite+cubanite+millerite+sphalerite±haezlewoodite. Excluding one dunite layer with δ34SAVS=+11.4‰, the δ34SAVS,CRS (-0.6 to +3.3‰) for SI are close to slightly elevated relative to mantle values. Scarce sulfates have identical δ34S relative to coexisting sulfides implying formation via abiotic oxidation of precursor sulfides. Despite widespread background alteration, olivine gabbros preserve primitive 87Sr/86Sr ratios (0.703088-0.703332) whereas serpentinised ultramafic layers have significantly more radiogenic signatures (0.707817-0.711688), close to or above Cretaceous seawater (87Sr/86Sr=0.70745). Gradual enrichment in sulfides by magmatic processes in SI, towards the MTZ, was followed by hydrothermal alteration with minor incorporation of seawater sulfate, leading to highly decoupled Sr-34S enrichment in the ultramafic layers due to their Sr-depleted nature. Narrow pegmatoid dikelets (amphibole+zoisite+prehnite+titanite) within SI have low TS (<80 ppm), mildly radiogenic 87Sr/86Sr (<0.704923) and a fracture-hosted, higher fS2sulfide assemblage (pyrite+Co-pentlandite+siegenite) with δ34SCRS down to -25.8‰ implying low-T (<110 C), open-system bacterial sulfate reduction (BSR) processes. The Dunite Sequence-SII has decreasing TS towards its interior (2-1253 ppm), consistent with extensive desulfurization producing an assemblage (awaruite+pentlandite+Co-pentlandite+magnetite, coexisting with brucite), during extremely low oxygen and sulfur fugacities typical of early serpentinization stages. SIII is highly heterogenous and S-depleted (3-623 ppm), with a heazlewoodite-bearing assemblage and lower 87Sr/86Sr (0.703952) relative to SII dunites (0.707065). The MTZ upper limit (SII) marks the onset of large shifts in S-isotopic composition, tendentially increasing downward throughout SII (δ34SCRS=-2.5, +15.6‰; δ34SSO4=+19.2, +32.4‰) and SIII (δ34SCRS=+1.4, +56.9‰; δ34SSO4=+19.4, +36.5‰). The occurrence of both sulfides and sulfates with δ34S above Cretaceous seawater sulfate (~18‰) can be explained by input of fluids at the top of SII which composition progressed towards extreme heavy values via closed system BSR during multi-staged serpentinization events. AJ acknowledges WWU International Visiting Scholars and EU-H2020 Marie Sklodowska-Curie #894599 Fellowships, FCT-project UIDB/GEO/50019/2020 [1] Kelemen PB, Matter JM, Teagle DAH, Coggon JA, OmanDP Science Team (2020) Proceedings of the OmanDP: College Station, TX (IODP).

The transition from the gabbroic oceanic crust to the residual mantle harzburgites of the Oman ophiolite has been drilled at Holes CM1A and CM2B (Wadi Tayin massif) during Phase 2 of the International Continental Scientific Drilling Program Oman Drilling Project (November 2017–January 2018). In order to unravel the formation processes of ultramafic rocks in the Wadi Tayin massif crust‐mantle transition zone and deeper in the mantle sections beneath oceanic spreading centers, our study focuses on the whole rock major and trace element compositions (together with CO2 and H2O concentrations) of these ultramafic rocks (56 dunites and 49 harzburgites). Despite extensive serpentinization and some carbonation, most of the trace element contents (REE, HFSE, Ti, Th, U) record high temperature, magmatic process‐related signatures. Two major trends are observed, with good correlations between (a) Th and U, Nb and LREE on one hand, and between (b) heavy REE, Ti and Hf on the other hand. We interpret the first trend as the signature of late melt/peridotite interactions as LREE are known to be mobilized by such processes (‘‘lithospheric process’’) and the second trend as the signature of the initial mantle partial melting (‘‘asthenospheric process’’), with little or no overprint from melt/rock reaction events.

Well-developed corona structures are observed and described in detail in the cumulate troctolites from Chainigund village, Kargil. The gabbro-troctolite unit is situated 5 km NW of Kargil city and consists of gabbros, troctolites, and anorthosites with doleritic dykes cross-cutting the unit at places. The host gabbros are fresh and display both fine and coarse-grained varieties. Troctolites occur as pods and veins within the gabbro and are composed of plagioclase (77-80 vol%), olivine (10-16 vol%), pleonaste spinel (6-8 vol%), amphiboles (2 -3 vol%) and opaques (0.5-2vol %). Both olivines and plagioclases are unzoned with spectacular coronas around the olivines (Fo 74.9-76.7) at the contact with plagioclase feldspar (An90.6-95.2). From center outwards, the discontinuous reaction series consists of the following members: Olivine, enstatitic orthopyroxene, magnesio-hornblende (Amph1) enclosed by a symplectitic rim of pargasite (Amph2) and pleonaste spinel and concludes at the plagioclase interface i.e. Ol-Opx-Amph1-Amph2-Spl-Plg. The mineral textures of the corona structure indicate formation in the presence of an interstitial fluid trapped between cumulus olivine and plagioclase. The reaction of this fluid with the olivine resulted in a rim of peritectic orthopyroxene around olivine which was subsequently replaced to form Amph 1 between the orthopyroxene and plagioclase. This is evident by the horse-shoe shaped outline and intermingling boundary shared by orthopyroxene and Amph 1. The formation of outer Amph 2 and spinel symplectite layers could be attributed to the replacement of precursor clinopyroxene and plagioclase at high temperatures (1050-1150° C ± 40° C). The Amph-Spl symplectites, presence of oxidizing conditions (magnetite and ilmenite), discontinuous reactions and local or short-range diffusion phenomena thus indicate that the corona structures are a result of metasomatic interaction of cooling magma with the previously formed minerals. Keywords: Corona structures; troctolite gabbro; olivine- plagioclase contact; Kargil; Ladakh; India.