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The Variscan Central-Sudetic Ophiolites (CSO) are located in the Sudetes, the NE segment of the Bohemian Massif. The Devonian CSO display highly depleted, harzburgite mantle sections containing gabbroic dykes and local occurrences of mostly isotropic, large gabbroic bodies as well as volcanic rocks. The ultramafic rocks locally show melt percolation-derived clinopyroxene-olivine aggregates and chromitites. The low REE composition and depletion in LREE relative to HREE of the clinopyroxene as well as the chromite Cr# and Mg# values are typical for phases formed from refractory melts occurring in the supra-subduction zone environment. The gabbroic bodies consist of differently evolved, mostly cumulate rocks, while the volcanic rocks form a relatively monotonous basalt sequence. The trace element compositions of both the plutonic and volcanic rocks display depleted N-MORB affinities, their derivation from a refractory mantle source is further reflected by previously published depleted mantle-like Sr-Nd isotopic compositions. The ultramafic and mafic members of the CSO show greenschist- to lower amphibolite-facies metamorphic overprints. The occurrence of depleted harzburgites and isotropic gabbros forming the mantle and the crustal sections of the CSO, respectively, indicate intermediate- to slow-spreading conditions and a heterogeneous structure of the oceanic lithosphere, untypical for layered ophiolites. The geochemical characteristics of the melt-percolation phases of the ultramafic rocks, the bulk-rock compositions of the mafic rocks and the homogeneous volcanic sequences suggest a formation of the CSO in a mature back-arc basin setting, lacking subduction-derived contamination. Spreading rate conditions and geochemical affinities of the CSO are similar to those of other Devonian ophiolites localized along the Variscan Belt, for instance the Upper Ophiolitic Units in the NW Iberian Massif (Spain), thereby confirming the general absence of N-MORB type lithosphere in the European Variscan Orogen.

Collision-induced subduction transference may have triggered subduction of the Neo-Tethys oceanic lithosphere. To verify this proposition, we present an integrated geochemical and geochronological studies of new and published data of the Permian-Triassic magmatism in the Lhasa Terrane. The newly identified Chongni mafic intrusions (gabbros and diorites) in the North Lhasa Terrane (NLT) were emplaced during the Early Permian (ca. 275 Ma). The mafic rocks have (87Sr/86Sr)t ratios between 0.7037 and 0.7042, and eNd(t) values between + 5.2 and + 5.8. The zircons of the Chongni mafic rock samples have eHf(t) values from + 9.1 to + 12.9. The isotopic data in conjunction with trace element features suggest that the Chongni mafic intrusions have been derived mainly by partial melting of a metasomatized mantle wedge source. The newly identified Quxu granodiorites and hornblende gabbros in the South Lhasa Terrane (SLT) were emplaced in the Late Triassic (ca. 228–203 Ma). The granodiorites are characterized by (87Sr/86Sr)t ratios of 0.7036–0.7037 and eNd(t) values of +6.6 to +6.7, while the hornblende gabbro samples have (87Sr/86Sr)t ratios of 0.7033–0.7035 and eNd(t) values of +7.3 to +7.4. The zircons of the granodiorite and hornblende gabbro samples revealed eHf(t) values of +10.3 to +14.0 and +10.8 to +16.4, respectively. The isotopic data in combination with trace element signatures suggest that the granodiorites were derived from a juvenile basaltic lower crust with a minor input of mantle components, while the hornblende gabbros are interpreted to have formed during partial melting of a formerly metasomatized depleted mantle wedge source. The Early Permian and Late Triassic magmatic rocks consistently show arc-related signatures, which probably record the northward subduction of the Sumdo Paleo-Tethys and the Neo-Tethys oceanic slabs, respectively. This evidence, in combination with that of other studies on the Permian-Triassic magmatism across the Lhasa Terrane, indicates that arc magmatism within the NLT switched to collisional magmatism at ca. 260 Ma due to the collision of oceanic islands/plateaus and, further on, the collision of the SLT with the NLT. Furthermore, the change from ca. 265–255 Ma alkaline magmatism to ca. 240–200 Ma calc‐alkaline magmatism within the SLT may reflect the geodynamic transition from lithospheric extension to Andean-style orogenesis. Therefore, we propose that subduction initiation of the Neo-Tethys oceanic lithosphere in the Tibetan Plateau resulted from the southward transference of the Sumdo Paleo-Tethys subduction zone, involving the collision of oceanic islands/plateaus and, subsequently, the collision of the SLT with the NLT. Additionally, the geological records of the Tethyan orogenic system suggest that the subduction initiation of the Neo-Tethys oceanic lithosphere is a uniform response to the collision between Cimmeria and Eurasia.

The Central-Sudetic Ophiolites (CSO) are located in the Sudetes constituting the NE fragment of the Bohemian Massif, one of Variscan basement outcrops in Central Europe. The CSO involve the Ślęża, Braszowice-Brzeźnica, Szklary and Nowa Ruda massifs that are dated at 404.8 ± 0.3 – 401.2 ± 0.3 Ma (Awdankiewicz et al., 2020). These massifs display highly depleted, harzburgite mantle sections containing gabbroic dykes and local occurrences of mostly isotropic, large gabbroic bodies as well as volcanic rocks. The ultramafic rocks locally show melt percolation-derived clinopyroxene-olivine aggregates and chromitites. The low REE composition and depletion in LREE relative to HREE of the clinopyroxene as well as the chromite Cr# and Mg# values point to phases formed from refractory melts occurring in the supra-subduction zone environment. The gabbroic bodies consist of differently evolved, mostly cumulate rocks, while the volcanic rocks form a relatively monotonous basalt sequence. The trace element compositions of both the plutonic and volcanic rocks display depleted N-MORB affinities, their derivation from a refractory mantle source is further reflected by depleted mantle-like Sr-Nd isotopic compositions. The ultramafic and mafic members of the CSO show greenschist- to lower amphibolite facies metamorphic overprints.The CSO represent an ancient supra-subduction-type oceanic lithosphere that formed in a slow- to intermediate spreading regime. The lithosphere of the CSO is heterogeneous and lacks the structure of a typical layered ophiolitic complex, but rather resembles that of slow spreading oceanic complexes with gabbroic bodies formed due to local magma injections. Melt percolation phases in ultramafic member as well as plutonic and volcanic rocks of the CSO display geochemical signatures accounting for their derivation from a refractory mantle source, typical of N-MORB-type melts depleted in supra-subduction zone settings but lacking subduction-related enrichment. These rocks of the CSO are believed to have formed in a mature, intra-oceanic back-arc basin. Chemical affinities between the CSO and other Devonian ophiolites belonging to the Middle Allochthon (for instance Careón ophiolite in the NW Iberian Massif, Spain or Tisoviţa Iuţi ophiolitic massif in Romania) confirm that a typical MORB-type lithosphere is absent in the European Variscides. Therefore, these ophiolites are thought to constitute fragments of lithosphere that were generated in supra-subduction-zone domains during the amalgamation of Pangea.Reference:Awdankiewicz, M., Kryza, R., Turniak, K., Ovtcharova, M., Schaltegger, U., 2020. The Central Sudetic Ophiolite (European Variscan Belt): precise U-Pb zircon dating and geotectonic implications. Geological Magazine 158, 555–556.

Immiscible sulphide liquids, preserved as magmatic sulphide droplets, are believed to strongly control the partitioning behaviour of chalcophile trace elements [1-2]. Hence, the chemical composition of sulphide droplets can be used to understand the fractionation processes of chalcophile elements in magmatic systems that reached sulphide saturation. We carried out LA-ICP-MS analysis of sulphide droplets from gabbros of the lower oceanic crust recovered by deep ocean drilling from mid-ocean ridge spreading centres in the Pacific (ODP147), Indian (ODP176, ODP179, IODP360) and the Atlantic (OPD209 and IODP305) Oceans. For comparison, sulphide droplets from mid-ocean ridge basalts from the East Pacific Rise, Mid-Atlantic Ridge and Southwest Indian Ridge were analysed. Our results show that most gabbros host abundant large magmatic sulphide droplets (mostly above 100 µm up to 1 mm) significantly exceeding those from the related lava units [2-4]. The droplets are commonly associated with or incorporated in olivine or clinopyroxene suggesting an early-stage sulphide saturation but are locally also incorporated in Fe-oxides indicating a later-stage formation during magma cooling [4-5]. The Ni contents of sulphide droplets hosted in gabbros from Hess Deep (Pacific Ocean) are highly variable ranging from ~10 µg/g to weight % levels. Nickel is also strongly controlled by olivine fractionation, and thus can be seen as a parameter indicating whether sulphide saturation was reached before or after the onset of olivine crystallisation. Due to the highly variable Ni contents and in combination with petrographic observations, we suggest that the magma reached early sulphide saturation at Hess Deep, as typically seen in mid-ocean ridge magmatic systems. However, the variable Ni contents in the sulphide droplets indicate that the magma was sulphide-saturated over a longer time span. Alternatively, the magma may frequently switch between being sulphide undersaturated and saturated, due to decreasing pressure during magma ascent accompanied by crystal fractionation at different levels in the crust. Generally, the trace element composition of the sulphide droplets hosted by gabbros from the different drill sites overlap. However, there are significant differences in the compositions of sulphide droplets from lava samples and from associated gabbroic xenoliths [4]. Thus, analysis of droplets from lavas alone provide an incomplete picture of the chalcophile element evolution of the magmatic system. We find no clear differences in sulphide composition with spreading rate or degree of melting as suggested for the silicate melt portion. Instead, the composition of sulphide droplets indicates that fractionation during magma ascent in the crust is the main driver that causes the observed chemical variations, which is part of ongoing investigations.[1] Wood, B. J. and Kiseeva, E. S. (2015), Earth and Planetary Science Letters, 424, 280-294. [2] Patten, C. et al. (2013), Chemical Geology, 358, 170–188. [3] Peach et al. (1990), Geochimica et Cosmochimica Acta, 12, 3379-3389. [4] Keith, M. et al. (2017), Chemical Geology, 451, 67–77. [5] Jenner, F. E. et al. (2010), Journal of Petrology, 51, 2445-2464.

Drill core samples from the Profitis Ilias Pb-Zn-Cu-Ag-Au vein mineralization on Milos Island, Greece provide new insights into (i) the metal sources, (ii) the primary vertical metal(loid) distribution, and (iii) the supergene enrichment processes in a transitional shallow-marine to subaerial hydrothermal environment. Metal contents of unaltered and altered host rocks combined with Pb isotope analyses of hydrothermal sulfides suggest that most metal(loid)s were derived by leaching of basement rocks, whereas the distinct enrichment of Te is related to the addition of Te by a magmatic fluid. The trace element contents of base metal sulfides record decreasing Au, Te, Se, and Co, but increasing Ag, Sb, and Tl concentrations with increasing elevation that can be related to progressive cooling and fluid boiling during the hypogene stage. The formation of base metal veins with porous pyrite hosting hessite inclusions at ~ 400 m below the surface was triggered by vigorous fluid boiling. By contrast, the enrichment of native Au associated with oxidized Fe and Cu phases in the shallower part of the hydrothermal system resulted from supergene remobilization of trace Au by oxidizing meteoric water after tectonic exhumation to subaerial levels. Disseminated pyrite with higher Tl/Pb ratios and locally elevated Hg concentrations relative to vein pyrite reflects infiltration of the host rocks by boiled liquids and condensed vapor fluids. The vertical and temporal evolution of the Profitis Ilias mineralization, therefore, provides unique insights into the transport and precipitation of Au, Ag, Te, and related metal(loid)s by multiple fluid processes.

Eclogite-hosted metamorphic veins mark former fluid migration pathways during a subduction-exhumation cycle, and allow to trace fluid-mediated element transfer across lithologies, to ultimately metasomatize the mantle wedge. Fluids can be generated by dehydration reactions at different P-T conditions in various lithologies, all influencing how different chemical constituents are dissolved and re-precipitated. Here we present a study of eclogite-hosted quartz-rich metamorphic veins in the Variscan Münchberg Massif. The eclogites probably represent subducted continental crust that was variably hydrated and subjected to amphibolite facies conditions before reaching eclogite facies peak conditions of ca. 3 GPa and 700°C.Isolated, mm-sized quartz pockets with euhedral high-pressure minerals are common in the Münchberg eclogites, but continuous veins that may have allowed focused fluid flow beyond specimen scale are rare. Nevertheless, where such veins occur, they can contain high-pressure minerals such as garnet and omphacite, but also rutile, zircon, and allanite, indicating high field strength-element (HFSE) mobility at least on the specimen scale. Garnet- and omphacite-bearing veins are typically 1-10 mm thick with average crystal sizes of 1 mm and less. A different vein type is mostly similar in thickness, but consists of quartz + phenocrysts (sometimes >1 cm long) of kyanite, phengite, and/or rutile. Symplectite-rich selvages surrounded by mostly fresh host eclogite are common.Oxygen isotope thermometry of quartz-garnet, quartz-phengite, and quartz-kyanite pairs yield temperatures around 700°C, interpreted to represent vein crystallization. δ18O values of vein quartz (+6.1 to +10.5‰) from all vein types are identical to δ18O values of host rock quartz (the latter were predicted from mass balance modelling at 700°C based on host rock δ18O values from +4.0 to +7.9‰). While it is evident that the garnet- and omphacite-bearing veins were formed under eclogite facies conditions, pressures are uncertain for the quartz-rutile, quartz-phengite, and quartz-kyanite veins. Still, vein formation at relatively high pressures seems probable, as solubilities of chemical components tend to increase with pressure, facilitating HFSE mobilization from the source rock. We propose that internal fluids were generated by dehydration of phengite and/or zoisite and/or amphibole from the eclogites. Isolated quartz-rich pockets formed in eclogites that may have released only small amounts of fluid, whereas continuous metamorphic veins were formed in eclogites that produced more fluid, probably reflecting a more intense hydration before eclogite facies metamorphism. The internal origin of the fluids supported by oxygen isotope evidence argues against fluid transport over large distances. The fluids may have largely remained in place before being consumed for symplectite formation upon retrogression to amphibolite facies conditions.