Isotope-Geochemical Features of Apoultrabasic Metasomatites of the Sayan–Baikal Folded Area

—The paper is concerned with a geochemical study of apoultrabasic metasomatites of the Ospa–Kitoi, Parama, and Ust’-Kelyana ophiolite massifs located in the southern folded framing of the Siberian craton. The isotope (O, C, H, Sr, and Rb) systems of dunites, serpentinites, nephrites, listvenites, and talc–carbonate rocks are studied. The isotopic composition of oxygen in olivines from dunites is characterized by δ18O = 4.6–5.5‰. The δ18O values of serpentinites (4.67–7.35‰) point to the mantle genesis of fluids and might have been inherited from ultrabasic rocks. Nephrites are slightly enriched in heavy oxygen isotope (δ18O = 6.13–9.54‰). This indicates that their fluid phase was transported from serpentinites and captured a small portion of the crustal component. The widest variations in δ18O values, from 8.12 to 17.46‰, are observed in minerals from listvenites. Carbonates from these rocks show a highly heterogeneous isotopic composition of oxygen (δ18O = 12.9–18.8‰) and carbon δ13C = –2.8 to +2.8‰). These rocks formed with the contribution of metamorphogenic fluids. According to the isotopic composition of hydrogen, the examined serpentinites are divided into two groups: with δD values specific to “magmatic water” (δD = –73.50 to –85.00‰) and those typical of meteoric fluids (δD = –151.90 to –167.20‰). The listvenites are characterized by low Rb and high Sr contents. Their 87Sr/86Sr values (0.70702–0.70971) indicate the contribution of a crustal source. The study of fluid inclusions in minerals from listvenites has shown that the rocks formed under relatively low-temperature conditions. The homogenization temperatures of fluid inclusions in quartz and magnesite from listvenites of the Ospa–Kitoi massif are 184–290 ºC and 122–182 ºC, respectively. In the Parama massif, the homogenization temperature of fluid inclusions in quartz is 130–170 ºC. The solutions that formed listvenites of the Ospa–Kitoi massif were slightly saline (TDS = 2.9–8.4 wt.% NaCl eq.), with NaCl and Na2CO3 being the main salt components.

Geochemical and paleomagnetic characteristics of Vestfold Hills mafic dykes in Prydz Bay region: implications on the Paleoproterozoic connection between East Antarctica and proto-India

The Archean age granite gneiss basement along the Prydz Bay coastline in East Antarctica hosts N-S, E-W, NE-SW, and NW-SE trending mafic dyke swarms in the Vestfold Hills region that intruded between 2420 and 1250 Ma. The dyke trends do not show a direct correlation with the dyke geochemistry but can be broadly discriminated into high-Mg and Fe-rich tholeiites. The former type being more siliceous, LILE, HFSE, and LREE enriched, crystallized from a fractionated melt with a notable crustal component or fluid enrichment through the previous subduction process. The Fe-rich tholeiites are less siliceous, have lower abundances of LILE and REE, and were derived from an undifferentiated, primitive melt. The geochemical characteristics of both types underline a shallow level and a high degree of melting in the majority of cases, and a broadly Island Arc Basalt (IAB) affinity. Paleomagnetic analysis of hand samples shows directional groups consistent with geochemical groupings. The Vestfold Hills dykes show a possible linkage with the coeval mafic dykes in Eastern Dharwar and Bastar cratons of the South Indian Block, based on the similarity in the Paleoproterozoic paleolatitudes.

Tracing Pre-Mesozoic Tectonic Sutures in the Crystalline Basement of the Protocarpathians: Evidence from the Exotic Blocks from Subsilesian Nappe, Outer Western Carpathians, Poland

Pre-Mesozoic exotic crystalline blocks within the Outer Carpathian flysch have potential to unravel the nature of their eroded basement source(s) and to reconstruct the Paleozoic–Precambrian history of the Protocarpathians. Strongly tectonized Campanian–Maastrichtian grey marls in the Subsilesian Nappe of the Outer Western Carpathians in Poland contain a variety of different lithology types, including granitoids and andesites. Petrological investigations coupled with zircon and apatite U-Pb dating were performed on crystalline (subvolcanic) exotic blocks from a locality in the Subsilesian Nappe. U-Pb zircon dating yields magmatic crystallization ages of c. 293 Ma for the microgranitoid and c. 310 Ma for the andesite block, with inherited zircon cores yielding Archean, Paleoproterozoic, Mesoproterozoic and Cadomian ages. Whole rock trace element and Nd isotope data imply that the melt source was composed of a significant Neoproterozoic crustal component in both the microgranite and andesite. The Late Carboniferous–Permian magmatic activity likely continues outside the Carpathian Belt and can be linked to a Late Paleozoic transtensional zone, which is a continuation of the Lubliniec–Kraków Zone that extends under the Carpathians to Moesia. This Late Paleozoic transtensional zone was probably reactivated during the Late Cretaceous under a transpressional regime within the Żegocina tectonic zone, which caused the uplift of the Subsilesian Ridge and intensive erosion.

New zircon radiometric U-Pb ages and Lu-Hf isotopic data from the ultramafic-mafic sequences of Ranau and Telupid (Sabah, eastern Malaysia): Time to reconsider the geological evolution of Southeast Asia?

New zircon U-Pb geochronology from a peridotite suite near Ranau and the Telupid ophiolite in Sabah, eastern Malaysia, contradict previous studies, which assumed that the Sabah mafic-ultramafic rocks are largely ophiolitic and Jurassic–Cretaceous in age. We show that these rocks formed during a magmatic episode in the Miocene (9.2–10.5 Ma), which is interpreted to reflect infiltration of melts and melt-rock reaction in the Ranau subcontinental peridotites during extension, and concurrent seafloor spreading forming the Telupid ophiolite further south. Older zircons from the Ranau peridotites have Cretaceous, Devonian, and Neoproterozoic ages. Zircon Lu-Hf isotopic data suggest their derivation from a depleted mantle. However, significant proportions of crustal components have been incorporated in their genesis, as evidenced by their less-radiogenic Hf signature compared to a pristine mantle reservoir. The involvement of a crustal component is consistent with our interpreted continental setting for the Ranau peridotite and formation in a narrow backarc basin for the Telupid ophiolite. We infer that the Sulu Sea, which was expanding throughout much of the Miocene, may have extended to the southwest into central Sabah. The Telupid oceanic strand formed during the split, collapse, and rollback of the Sulu arc due to the subduction of the Celebes Sea beneath Sabah. Incorporation of the Sulu arc in the evolving Miocene oceanic basin is a potential source to explain the involvement of crustal material in the zircon evolution of the Telupid ophiolite.

Origin of Ru-Os Sulfides from the Verkh-Neivinsk Ophiolite Massif (Middle Urals, Russia): Compositional and S-Os Isotope Evidence

This study presents new compositional and S-Os isotope data for primary Ru-Os sulfides within a platinum-group mineral (PGM) assemblage from placer deposits associated with the Verkh-Neivinsk massif, which is part of the mantle ophiolite association of Middle Urals (Russia). The primary nature of Ru-Os sulfides represented by laurite (RuS2)–erlichmanite (OsS2) series is supported by occurrence of euhedral inclusions of high-Mg olivine (Fo92–94) that fall within the compositional range of mantle (primitive) olivine (Fo 88–93). The sulfur isotope signatures of Ru-Os sulfides reveal a range of δ34S values from 0.3 to 3.3‰, with a mean of 2.05‰ and a standard deviation of 0.86‰ (n = 18), implying that the sulfur derived from a subchondritic source. A range of sub-chondritic initial 187Os/188Os values defined for Ru-Os sulfides (0.1173–0.1278) are clearly indicative of derivation from a sub-chondritic source. Re-depletion (TRD) ages of the Verkh-Neivinsk Ru-Os sulfides are consistent with prolonged melt-extraction processes and likely multi-stage evolution of highly siderophile elements (HSE) within the upper mantle. A single radiogenic 187Os/188Os value of 0.13459 ± 0.00002 determined in the erlichmanite is indicative of a supra-chondritic source of HSE. This feature can be interpreted as evidence of a radiogenic crustal component associated with a subduction event or as an indication of an enriched mantle source. The mineralogical and Os-isotope data point to a high-temperature origin of the studied PGM and two contrasting sources for HSE in Ru-Os sulfides of the Verkh-Neivinsk massif.

Chapter 5.2b Erebus Volcanic Province: petrology

Igneous rocks of the Erebus Volcanic Province have been investigated for more than a century but many aspects of petrogenesis remain problematic. Current interpretations are assessed and summarized using a comprehensive dataset of previously published and new geochemical and geochronological data. Igneous rocks, ranging in age from 25 Ma to the present day, are mainly nepheline normative. Compositional variation is largely controlled by fractionation of olivine + clinopyroxene + magnetite/ilmenite + titanite ± kaersutite ± feldspar, with relatively undifferentiated melts being generated by <10% partial melting of a mixed spinel + garnet lherzolite source. Equilibration of radiogenic Sr, Nd, Pb and Hf is consistent with a high time-integrated HIMU sensu stricto source component and this is unlikely to be related to subduction of the palaeo-Pacific Plate around 0.5 Ga. Relatively undifferentiated whole-rock chemistry can be modelled to infer complex sources comprising depleted and enriched peridotite, HIMU, eclogite-like and carbonatite-like components. Spatial (west–east) variations in Sr, Nd and Pb isotopic compositions and Ba/Rb and Nb/Ta ratios can be interpreted to indicate increasing involvement of an eclogitic crustal component eastwards. Melting in the region is related to decompression, possibly from edge-driven mantle convection or a mantle plume.

Geochemical characteristics of the Late Devonian basaltoids of the Kanin Peninsula and the Middle Timan

The petrochemistry and geochemistry of dolerites and basalts of the Late Devonian Kanin-Timan complex of the Kanin Peninsula and the Middle Timan are considered. Petrochemically, the rocks of the Kanin-Timan complex of the Kanin Peninsula and the Tsilma river area of the Middle Timan correspond to basaltoids of the normal range of alkalinity and partially to moderately alkaline varieties, and belong to the tholeiitic series. The least differentiated varieties are dolerites of the southeastern Kanin Peninsula, the most differentiated are the basalts of the river Tsilma of the Middle Timan. The lowest REE concentrations were found in the rocks of the central part of the Kanin Peninsula (36.5-56.8 g/t); in the same samples, the lowest LaN/YbN values were recorded (1.85 and 2.4, respectively), which indicates an increased degree of melting of the source. The highest REE concentrations were found in basalts from the river Tsilma (77.13-88.33 g/t), LaN/YbN values (2.49-2.7, respectively). The influence of the crustal component in the formation of melts from which rocks of the Kanin-Timan complex were formed, was established. The source of the melt was spinel-garnet lherzolite, the degree of melting varied from 10 to 30%. The maximum degree of melting was 30%, at which melts were formed, that gave rise to the least differentiated rocks of the Northern Timan and the central part of the Kanin Peninsula. The mantle source, that gave rise to the melts from which the rocks of the Kanin-Timan complex were formed, was enriched with subduction and crustal components, a similar type of source is characteristic of the basaltoids of the No-rilsk trough.

Oxygen-Hafnium-Neodymium Isotope Constraints on the Origin of the Talnakh Ultramafic-Mafic Intrusion (Norilsk Province, Russia)

The ultramafic-mafic Talnakh intrusion in the Norilsk province (Russia) hosts one of the world’s major platinum group element (PGE)-Cu-Ni sulfide deposits. This study employed a multitechnique approach, including in situ Hf-O isotope analyses of zircon combined with whole-rock Nd isotope data, in order to gain new insights into genesis of the Talnakh economic intrusion. Zircons from gabbrodiorite, gabbroic rocks of the layered series, and ultramafic rocks have similar mantle-like mean δ18O values (5.39 ± 0.49‰, n = 27; 5.64 ± 0.48‰, n = 34; and 5.28 ± 0.34‰, n = 7, respectively), consistent with a mantle-derived origin for the primary magma(s) parental to the Talnakh intrusion. In contrast, a sulfide-bearing taxitic-textured troctolite from the basal part of intrusion has high δ18O (mean of 6.50‰, n = 3), indicating the possible involvement of a crustal component during the formation of sulfide-bearing taxitic-textured rocks. The Hf isotope compositions of zircon from different rocks of the Talnakh intrusion show significant variations, with ɛHf(t) values ranging from –3.2 to 9.8 for gabbrodiorite, from –4.3 to 11.6 for unmineralized layered-sequence gabbroic rocks, from 2.3 to 12 for mineralized ultramafic rocks, and from –3.5 to 8.8 for mineralized taxitic-textured rocks at the base of the intrusion. The significant range in the initial 176Hf/177Hf values is ascribed to interaction of distinct magma sources during formation of the Talnakh intrusion. These include (1) a juvenile source equivalent to the depleted mantle, (2) a subcontinental lithospheric source, and (3) a minor crustal component. Initial whole-rock Nd isotope compositions of the mineralized taxitic-textured rocks from the base of the intrusion (mean ɛNd(t) = –1.5 ± 1.8) differ from the other rocks, which have relatively restricted ranges in initial ɛNd (mean ɛNd = 0.9 ± 0.2). The major set of ɛNd values around 1.0 at Talnakh is attributed to limited crustal contamination, presumably in deep magma chambers, whereas the smaller set of negative ɛNd values in taxitic-textured rocks is consistent with greater involvement of a crustal component and reflects an interaction with the wall rocks during emplacement.

Thermal maturation of a complete magmatic plumbing system at the Sierra de Velasco, Northwestern Argentina

The formation of magmatic plumbing systems in the crust involves mass and heat transfer from deep to shallow levels. This process modifies the local geotherm and increases the thermal maturation of the crust, affecting the rheological state of the host rock and the composition of magma. Here, we report a petrological, geochemical, isotopic and geochronological integrated study of the Huaco (~354 Ma) and Sanagasta (~353 Ma, from a new U–Pb zircon age) units from the Carboniferous (Lower Mississippian) Huaco Intrusive Complex, NW Argentina. Similar values of ϵNd t and δ18O, of −3.2 ± 0.7 and +11.2‰ ± 0.3‰ (V-SMOW), respectively, for both units indicate that they shared the same source, as a result of mixing and later homogenization of a crustal component at the Late Devonian (~378 to 366 Ma), with metasomatized mantle-derived melts. Slightly higher contents of TiO2, FeO, MgO, CaO and rare earth elements for the Sanagasta unit in comparison with the Huaco unit suggest an increase in the degree of partial melting, which may have been caused by a higher temperature at the lower crust. In addition, the previous structural model of the Huaco Intrusive Complex points to an increase in thermal maturation in the upper crust, which drives a change in the emplacement style from tabular subhorizontal (Huaco) to vertically elongated (Sanagasta) bodies. Therefore, the evolution of the intrusive complex may reflect a generalized thermal maturation of the complete magmatic column, at both upper and lower crustal levels.