Enrichment In Incompatible Elements Research Articles

Kinetic factors control trace element and isotope zoning in Archean pyrite corona nodules

Pyrite corona nodules from the ∼2.7 Ga Kapai Slate, a thin, sulfidic carbonaceous shale horizon interbedded with basaltic lava flows in the Yilgarn Craton, Western Australia, have concentric compositional and isotopic zoning with distinctive textural differences between cores and mantles. The sieved-textured cores are enriched in highly compatible trace elements, incl. Ni, As, Ag, Te, Sb, Bi and Pb, and depleted in incompatible Mo and Tl, whereas the radiating-textured mantles are strongly depleted in compatible elements and enriched in incompatible elements, relative to the cores. A striking feature of the data is that both the compatible and incompatible elements are linearly correlated, with correlation coefficients as high as 0.99. A marked drop in the concentration of compatible elements and an increase in incompatible elements at the core-mantle boundary is attributed to a sudden change in the rate of growth of the nodules produced by eruption of the voluminous overlying Paringa Basalt. The weight of the basalt produced sudden compaction of the unconsolidated clays below resulting in upward advection of pore fluid, which thinned the boundary layer around the growing nodules, leading to a marked increase in the rate of pyrite growth. Rapid pyrite growth led to a dramatic depletion in highly compatible elements, and to a build-up in incompatible elements, in the boundary layers around the growing nodule mantles, which resulted in extreme depletion of compatible elements, and enrichment in incompatible elements in the nodule mantles, relative to the cores. The corona nodules are also isotopically zoned with cores that have higher δ34S, with small positive Δ33S values, and mantles that have lower δ34S and higher Δ33S. The increase in Δ33S towards the rims is attributed to S8 being advected to the growing mantles by upward fluid movement during sudden compaction, and the decrease in δ34S to the lighter S isotope, with its higher reactivity and diffusivity, being preferentially incorporated into the fast growing pyrite mantle.The extreme changes in the growth rates of the Kapai Slate corona pyrite nodules provide a new constraint on the partition coefficients of the trace elements between Archean ocean water and sedimentary pyrite. The compatibility of the analysed trace elements decreases in the order Bi > Te > Sb > Ag > Cu > Pb > Ni ≈ As > (Co, Zn, Se, Cd, Mn, W) > Tl > Mo, which is consistent with the order obtained from modern sedimentary pyrites by Large et al. (2014), except for the redox-sensitive elements Mn, Tl and Mo. These differences are attributed to the lower oxygen content of the Archean atmosphere and oceans.

Mineral Chemistry and Whole-rock Geochemistry of Pillow lava from the Arangue Complex, Southeastern Hormozgan, Iran

The Upper Cretaceous Arangue complex is located in the Makran zone at the SE of Iran. The complex consists of ultra-mafic rocks, microgabbro dykes, pillow lavas and lime stones that pillow lavas are mainly exposed to the northwest and southeast part of study area. There are oval and tubular basalt lavas with cracked bread crust surface. They predominantly have plagioclase, clinopyroxene with minor olivine and opaque minerals in a fine-grained groundmass along with glass. Mineral chemistry data show that plagioclases and clinopyroxene composition varies from An68.27-81.73 Ab18.27-31.57 Or0-0.41 and Wo38.1- 47.8Fs8.2-19.3 En38.6-48.7 respectively. In the geochemical diagrams, the Arangue complex pillow lavas fall in the basalt and sub-alkaline fields. Geochemical data indicate that the Arangue complex pillow lavas are tholeiitic. The absence of a distinct Eu anomaly (Eu/Eu*= 0.8-1.2), indicates that plagioclase fractionation is not notable, or that the magma is a little oxidized. The Arangue complex pillow lavas show properties similar to transitional basalts between enriched MORB and OIB and some BABB. However, their enrichment in incompatible elements and low Nb and La / Nb ratios (0.8-2.1) display that these have affinity of the BABB. These were produced by approximately 15-25% partial melting of plagioclase lherzolite where fractionation was controlled by removal of clinopyroxene, spinel, and olivine. Petrogenetic study indicates that the source of mantle lherzolite is subjected to enrichment variables in subduction components consisting of fluids for the Arangue Complex pillow lavas.

Pyroxenite in the mantle source of basanites at the Youkou maar, Adamawa Volcanic Massif (Cameroon Volcanic Line, West Africa)

The Adamawa Volcanic Massif of the Cameroon Volcanic Line (CVL) is dominated by silica-undersaturated basaltic lavas, which have been attributed to plume volcanism. However, the relative contributions of peridotite and pyroxenite sources have not been assessed. Here, we present mineral and whole-rock data for the lavas to consider this issue and to better understand their petrogenesis and source mineralogy. The lavas are porphyritic, with phenocrysts and micro-phenocrysts of olivine, and to a lesser extent, clinopyroxene, in a groundmass of plagioclase, oxides, and apatite. The lava compositions are all basanites. They are silica-undersaturated and are primitive, mantle-derived melts. The basanites display lower CaO and CaO/Al2O3 ratio but higher Fe/Mn values than expected for melts of peridotite, consistent with an appreciable contribution from melting of pyroxenite in the source.Olivine phenocrysts are characterized by low CaO and NiO contents but higher Fe/Mn ratios than those of olivine in melts of peridotitic sources, supporting the contribution of pyroxenite to the primary melts, and that the main mantle source lithology for the basanites was a mixture of spinel-facies peridotite and silica-deficient pyroxenite. The presence of both pyroxenite and peridotite xenoliths in this area attests to the contribution of these two mantle lithologies to the primary melts, and their relative contribution varies on a very local scale. Significant contamination by crustal materials during magma ascent is ruled out. The Youkou basanites provide a key for improving the general understanding of melt production in lithologically heterogeneous mantle beneath the CVL. Beneath the Youkou volcano, partial melting of pyroxenite by decompression formed silica-deficient melts at 60-80 km depth, which modified the peridotite mantle source by a low degree melt enrichment process indicated by the enrichment in incompatible elements. This modified mantle source then underwent low-degree melting to produce the basanitic melts, which are concentrated in a linear zone focused by the combination of the northern edge of the Congo craton and the Central African Shear Zone.

The upper Oligocene San Rafael intrusive complex (Eastern Cordillera, southeast Peru), host of the largest-known high-grade tin deposit

The upper Oligocene San Rafael intrusive complex (SRIC), located at the southwest margin of the Eastern Cordillera of southeast Peru, is an exceptional case study to investigate the petrogenesis of a large composite granitic pluton associated with the largest known high-grade Sn deposit (>1 Mt. Sn). The granitic rocks are quasi-continuously exposed over more than 1.4 km vertically, from the cupola at surface to deeper levels in the underground mine workings, thus providing access to the upper part of a large pluton. We describe new field relationships and petrographic observations of the main granite varieties of the SRIC, and we report quantitative mineral analyses, whole rock geochemistry (major and trace elements, radiogenic Sr, Nd, and Pb isotopes), and in situ UPb dating and trace element analysis of zircons. The SRIC is dominantly composed of a biotite-cordierite -bearing K-feldspar megacrystic granite, which contains comagmatic enclaves and dismembered dikes of fine-grained and medium-grained granites, and subordinate lamprophyres. While the granitic rocks are widely affected by variable degrees of pervasive hydrothermal alteration, the least altered samples are characterized by moderately fractionated peraluminous S-type compositions, with similar enrichments in incompatible elements (Li, B, Ga, Rb, Sn, Cs, Th, and U), relatively high Zr/Hf and Nb/Ta ratios, and strongly crustal Sr, Nd, and Pb isotopic signatures. In situ UPb dating and trace element analysis of zircons indicate that the granitic magmas crystallized contemporaneously within age uncertainty at 24.02 ± 0.25 Ma (megacrystic granite), 24.17 ± 0.28 Ma (fine-grained granite), and 24.26 ± 0.26 Ma (medium-grained granite) at Ti-in-zircon temperatures from 638° to 840 °C (avg = 759 ± 49 °C). The lamprophyres have high contents of lithophile elements (Li, Rb, Cs, Sn, W) while showing more primitive SrNd isotopic compositions, indicating a metasomatized subcontinental lithospheric mantle source contaminated by crustal assimilation and/or mixing with granitic magmas. Tin contents of the main San Rafael granites are moderate (3–40 ppm; avg. = 13 ppm Sn) and fall in the same range of values as reported for “tin granites” from the Central Andean tin belt. Based on our petrological and geochemical data, we conclude that the SRIC results from multiple injection and mingling/mixing of several types of coexisting felsic and mafic magmas in a shallow-crustal reservoir. The granitic magmas were largely derived from partial melting of metasedimentary protoliths and hybridization with mantle-derived mafic melts, in a regional geodynamic setting dominated by anomalous asthenospheric heat flow beneath the southern Peruvian Altiplano during the Oligocene to Miocene. Based on the moderately fractionated peraluminous character of the SRIC, we conclude that the granitic pluton was a passive host for Sn mineralization and only provided the structural focusing for metal-rich magmatic fluids derived from a deeper part of the granitic magma reservoir. We propose a model in which the injection of high-temperature mafic melts into a more reduced, fractionated granitic magma reservoir triggered late-magmatic oxidation, crystal mush reactivation, and exsolution (second boiling) of metal-rich magmatic fluids. The latter ascended toward the upper part of the crystallized granitic pluton and were focused along subvertical dilational structures, where they cooled down and mixed with downwelling meteoric waters, thus generating the world-class San Rafael Sn (Cu) deposit.

Enrichment Nature of Ultrapotassic Rocks in Southern Tibet Inherited from their Mantle Source

Abstract Post-collisional ultrapotassic rocks (UPRs) in the Tibetan Plateau exhibit extreme enrichment in incompatible elements and radiogenic isotopes. Such enrichment is considered to be either inherited from a mantle source or developed during crustal evolution. In this study, to solve this debate we combined mineral textures and in situ geochemical composition of clinopyroxene phenocrysts in UPRs from southern Tibet to reveal their crustal evolution, enrichment cause and constrain metasomatism in their mantle source. Results show that the UPRs experienced an array of crustal processes, i.e., fractional crystallization, mixing, and assimilation. Fractional crystallization is indicated by decreases in Mg# and Ni and enrichment in incompatible elements (e.g. rare earth element (REE), Sr, Zr) toward the rims of normally zoned clinopyroxene phenocrysts (type-I). Magma mixing is evidenced by the presence of some clinopyroxene phenocrysts (type-II, -III) showing disequilibrium textures (e.g. reversed and overgrowth zoning), but in situ Sr isotope and trace element analysis of those disequilibrium zones indicate that late-stage recharged mafic magmas are depleted (87Sr/86Sr: 0.70659–0.71977) compared with the primitive ultrapotassic magmas (87Sr/86Sr: 0.70929–0.72553). Assimilation is revealed by the common presence of crustal xenoliths in southern Tibetan UPRs. Considering the much lower 87Sr/86Sr values (0.707759–0.709718) and incompatible element contents of these crustal xenoliths relative to their host UPRs, assimilation should have resulted in geochemical depletion of southern Tibetan UPRs rather than enrichment. The diluting impact of both assimilation and mixing is also supported by the modeling results based on the EC-E′RAχFC model combining the growth history of clinopyroxene. Trace elements ratios in clinopyroxenes also imply that the mantle source of southern Tibetan UPRs suffered an enriched and carbonatite-dominated metasomatism. Thus, we conclude that enrichment of southern Tibetan UPRs was inherited from the mantle source.

Late-Orogenic Juvenile Magmatism of the Mesoproterozoic Namaqua Metamorphic Province, South Africa, and Relationships to Granulite-Facies REE-Th and Iron Oxide Mineralizations

Abstract The Bushmanland Subprovince of the Mesoproterozoic Namaqua-Natal orogenic belt in southern Africa hosts numerous occurrences of monazite-magnetite-(biotite-apatite-sulfide)-bearing veins and granitoid dykes, including the Steenkampskraal vein system, which is one of the highest-grade REE-Th ore deposits in the world. Here, we provide whole-rock geochemical data along with zircon and monazite U-Pb dates and monazite Sm-Nd isotope analyses of these REE-Th-rich veins and granitoid dykes. The U-Pb geochronology indicates that the monazite-rich veins and granitoid dykes formed between 1050 and 1010 Ma, contemporaneously with late-Namaqua granulite-facies metamorphism. They are also coeval with the Koperberg Suite mafic magmas at 1060–1020 Ma and the late stage of a major event of A-type granitoid magmatism that occurred from 1100 to 1030 Ma (i.e. the Spektakel Suite). Similar to the mafic intrusive rocks from the Koperberg Suite, monazite-rich veins and granitoid dykes, located in the southern part of the Bushmanland Subprovince, have more radiogenic Nd isotopic compositions (ɛNd(t) ∼ −1 to zero) than equivalent dykes and veins to the north (ɛNd(t) ∼ −12 to −6). Mafic rocks of the Koperberg Suite reach Th and La concentrations of >400 ppm that significantly exceed those of other rock types from the region, except for the monazite-rich veins and granitoid dykes, which suggests a genetic link between these rocks. Within veins and granitoid dykes, monazite, biotite and magnetite are commonly anhedral and occur interstitially between the felsic minerals; they are, thus, late crystallizing phases. The whole-rock REE-Th concentrations of the granitoid dykes increase with Fe-Mg contents. Therefore, their incompatible element enrichment is not linked to assimilation-fractional crystallization processes. The Nd isotopic signature as well as Fe-Mg- and REE-Th-rich character of the Koperberg Suite and monazite-rich granitoid dykes might reflect partial melting of lithospheric mantle domains, metasomatized during previous Namaqua subduction events, and the mixing of mantle-derived melts with REE-Th-rich metamorphic fluids during their ascent through the crust. We propose that the monazite-magnetite vein mineralizations represent Fe-P-rich immiscible liquids that exsolved from mantle-derived magmas with compositions similar to the most mafic and monazite-rich granitoid dykes. Within this petrogenetic model, conjugate silicate-rich immiscible liquids formed the more felsic granitoid dykes characterized by lower modal abundances of biotite, magnetite and monazite. Although they do not reach similarly high REE-Th concentrations, other A-type granitoids from the region, represented by the Spektakel Suite, also share geochemical affinities with mafic igneous rocks from the Koperberg Suite; they may have originated by melting of underplated equivalents of these late-orogenic mafic rocks.

Origin of the Middle Paleoproterozoic Tiksheozero Ultramafic-Alkaline-Carbonatite Complex, NE Fennoscandian Shield: Evidence from Geochemical and Isotope Sr-Nd-Hf-Pb-Os Data

This article reports new geochemical, Sr-Nd-Hf-Pb and Re-Os data on the rocks of the Middle Paleoproterozoic (1.99 Ga) Tiksheozero ultramafic-alkaline-carbonatite complex confined to the northeastern margin of the Karelian Craton. We focus on the poorly studied silicate rocks. Based on petrographic and geochemical research, the silicate rocks are subdivided into two groups: an ultramafic-mafic series depleted in REE, and other incompatible elements and an alkaline series enriched in these elements. Isotope studies showed that all rocks have juvenile isotope signatures and were likely derived from a primitive OIB-type mantle source with possible contributions of the subcontinental lithospheric mantle (SCLM). Insignificant crustal contamination is recorded by Pb and Os isotopic compositions. The incompatible element enrichment in the alkaline rocks and depletion in ultramafic-mafic rocks of the mildly alkaline series with allowance for insignificant crustal contamination confirm their derivation from different primary melts. However, a narrow range of Sr, Nd, Hf, and Pb isotope compositions and compact clusters in 207Pb/204Pb-206Pb/204Pb, Nd-87Sr/86Sr and Hf-Nd isotope diagrams indicate their origination from a common mantle source. A model of subsequent two-stage melting is being most consistent with the geochemical data for this complex.

Cr-poor and Cr-rich clinopyroxene and garnet megacrysts from southern African Group 1 and Group 2 kimberlites: Clues to megacryst origins and their relationship to kimberlites

Controversies surround the origin of kimberlite megacrysts, including whether and how they are genetically related to their host kimberlites, the relationship between the Cr-poor and Cr-rich suites and the dominant processes responsible for elemental and isotopic variations of megacrysts from a given kimberlite. We present new in-situ major and trace element and Sr isotopic results for clinoyroxene and garnet megacrysts from four southern African kimberlites: Colossus and Orapa (Group 1 kimberlites on the Zimbabwe craton), and Kalkput and Bellsbank (Group 2 kimberlites on the western Kaapvaal craton), that include both Cr-poor and Cr-rich megacryst varieties. Cr-poor megacrysts are present at Colossus, Orapa and Kalkput and the data exhibit tight, well-defined trends on major element diagrams as well as incompatible and rare earth element abundances similar to those previously reported for Cr-poor megacrysts. Cr-rich megacrysts, which are also present at Orapa and are the only variety present at Bellsbank, generally have higher Mg# values, lack well-defined major element trends and show stronger incompatible element enrichments as well as more radiogenic 87Sr/86Sri ratios than Cr-poor megacrysts from the same kimberlite group. Thermobarometry indicates that the Cr-poor megacrysts equilibrated at temperatures of ≈1200 to 1450 °C and pressures of 4.5 to 7.5 GPa. Cr-rich megacrysts, in contrast, extend to temperatures and pressures as low as 700 °C and 3 GPa, respectively. This indicates that, in the studied suites, Cr-poor megacrysts equilibrated at high temperatures in the lower lithosphere (>135 km), whereas Cr-rich megacrysts typically equilibrated at lower temperatures and pressures. Within the Cr-poor megacrysts from Group 1 and Group 2 kimberlites, there is a clear correspondence between kimberlite group, diagnostic incompatible element ratios (e.g., Nb/La) and Sr isotope ratios that parallel the differences noted between whole-rock Group 1 and Group 2 kimberlites. In the case of Cr-poor megacrysts, similar calculated melt compositions in equilibrium with garnet and clinopyroxene from the same kimberlite were obtained using recent high-pressure mineral‑carbonated melt partition coefficients. This suggests formation in conditions close to trace element equilibrium, and is consistent with crystallization from primitive melts with kimberlite-like trace element compositions. In the case of Cr-rich megacrysts, differences in the compositions of melts in equilibrium with clinopyroxene and garnet tend to be larger, and melts in equilibrium with Cr-rich clinopyroxene tend to show significantly greater incompatible element enrichments than those of estimated near-primary kimberlite melts. This could be due to the different behaviour of clinopyroxene and garnet during metasomatic melt-rock interaction, but the apparent disequilibrium between clinopyroxene and garnet could also be due to some of the Cr-rich megacrysts actually being peridotitic xenocrysts. We propose a model for the origin of southern African megacrysts in which carbonated protokimberlite melts formed stockwork-like bodies of variable size in the deep lithosphere (>130 km), which fed networks of melt-filled veins extending into the surrounding and overlying mantle. Crystallization of larger melt bodies resulted in megacryst assemblages dominated by Cr-poor megacrysts, and the incompatible element and isotopic characteristics of these dominantly reflect those of the protokimberlite melt. In contrast, crystallization of smaller melt bodies and their vein networks resulted in megacryst assemblages dominated by Cr-rich megacrysts, which formed as a result of extensive assimilation and metasomatic melt-rock interaction between protokimberlite and peridotite wallrock at low melt/rock ratios, particularly in the middle to shallow lithosphere where pre-existing potassic metasomatic heterogeneities are prevalent. The Cr-rich nature and enrichments in incompatible elements and radiogenic Sr in the Cr-rich megacrysts reflect extensive interaction of their parental magmas with this metasomatized peridotite.

Neoproterozoic (740-680 Ma) arc-back-arc magmatism in the Sergipano Belt, southern Borborema Province, Brazil

The Novo Gosto amphibolites represent the oldest unit of the Canindé domain, a Neoproterozoic sequence located in the NW Sergipano Belt, southern Borborema Province, Brazil. The domain comprises an association of metavolcano-sedimentary rocks, gabbroic intrusions, bimodal magmatic rocks, and syn-to post-tectonic granitoids. Due to the lithostratigraphic and geochemical complexity of this domain, several controversial interpretations of the tectonic setting have been proposed over the years. The protholits of the Novo Gosto amphibolites are here classified as high Fe tholeiitic basalt type rocks. Three geochemical groups were identified: Group 1 is more depleted in incompatible elements and has a pattern similar to island arc basalts with HFSE depletion (Nb–Ta, Zr–Hf, weak Ti); Group 2 displays a pattern similar to Group 1, but with more evident anomalies (Nb–Ta, Ti) and varied enrichment in incompatible elements; Group 3 is more enriched in incompatible elements than Group 1 rocks, with the pattern being flatter with milder HFSE depletion (Nb–Ta, Zr–Hf, weak Ti, Y) and resembling a geochemical pattern slightly more enriched than E-MORB. Our results suggest an environment similar to the Ryukyu-Okinawa (Japan) and Kamchatka arc-back-arc (Russia) setting. LA-ICP-MS U–Pb zircon dating of one amphibolite sample resulted in a date of 743 ± 3 Ma and confirms the geological field relationships that indicate this unit to be the oldest in the domain. The new petrographic, geochemical, and isotopic data for the Novo Gosto amphibolites lead to the recognition of an ancient Neoproterozoic arc-back-arc system, a rare setting in the earlier phases of the Brasiliano Orogeny within the Borborema Province, which ended with the closure of the restricted Canindé sea and the associated Sergipano ocean at around 630 Ma.

Petrogenesis of coeval shoshonitic and high-K calc-alkaline igneous suites in the Eopyeong granitoids, Taebaeksan Basin, South Korea: Lithospheric thinning-related Early Cretaceous magmatism in the Korean Peninsula

The Early Cretaceous was the initial period of prolonged Cretaceous-Tertiary magmatic episode in the Korean Peninsula, during which a tectonic transition from compression to extension occurred. The Early Cretaceous Eopyeong granitoids in the Taebaeksan Basin consist of two distinct series of magmas: shoshonitic and high-K calc-alkaline series. We investigated the petrology, whole-rock major and trace element geochemistry, and Sr-Nd-Pb isotope compositions of the Eopyeong granitoids to understand their magma evolution processes and source characteristics. Sensitive high-resolution ion microprobe zircon U-Pb analysis was performed on the Eopyeong granitoids to constrain their intrusion ages. Zircon U-Pb ages of the shoshonitic and high-K calc-alkaline series are 110.63 ± 0.52 and 110.65 ± 0.66 Ma, respectively, indicating that the two series formed during the same magmatic event. The mafic mineral assemblages of the shoshonitic series change from two-pyroxene+biotite to clinopyroxene+amphibole+biotite through magma evolution, whereas those of the high-K calc-alkaline series consist of more hydrous assemblages of amphibole+biotite. The shoshonitic series is also distinguished from the high-K calc-alkaline series by its higher La/Yb ratio and negative anomalies of Rb, Ba, and Eu. Mineral geothermobarometry and hygrometry data and geochemical mass balance modeling results show that the two series evolved mainly by fractional crystallization from two distinct parental magmas containing different water contents. Primitive rocks from each series have high compatible element contents such as MgO, Cr, and Ni with high Mg# compared to crustal melts in a similar range of SiO2, suggesting their mantle origin. They also have abundant incompatible elements and ‘crust-like’ isotopic compositions [(87Sr/86Sr)i = 0.7093–0.7098; εNd(t) = −7.1 to −8.8; (206Pb/204Pb)i = 18.133–18.327; (207Pb/204Pb)i = 15.621–15.665; (208Pb/204Pb)i = 38.691–38.862] with negative Nb-Ta anomalies in the primitive mantle-normalized patterns. The enrichment in both compatible and incompatible elements can be best explained by partial melting of phlogopite-bearing lithospheric mantle metasomatized by isotopically enriched upper crustal sediment-derived melt or fluid. The two magma series may have been generated by different degrees of partial melting of the mantle sources with varying water contents. Changes in subduction angle and direction of the paleo-Pacific plate during the Early Cretaceous may have led to lithospheric thinning and upwelling of asthenospheric mantle, which triggered the partial melting of fusible domains of metasomatized lithospheric mantle. The small igneous bodies of the Early Cretaceous shoshonitic to alkaline mafic rocks occur sporadically throughout the southern part of the Korean Peninsula in close spatial association with the Cretaceous basins. They also show crust-like geochemistry and isotopic compositions, similar to the Eopyeong granitoids, implying that the lithospheric thinning-related melting of the metasomatized lithospheric mantle may have been widespread in the southern part of the Korean Peninsula.

Nature of the pre-collisional lithospheric mantle in Central Tibet: Insights to Tibetan Plateau uplift

The properties of accreted terranes in a large collisional orogen determine, in large part, the nature and subsequent development of the orogen. In this paper we present a detailed study of Late Mesozoic sodic mafic magmatic rocks from Central Tibet. These sodic mafic rocks from Bandatso provide an excellent opportunity to assess the nature and evolution of pre-collisional lithospheric mantle in the Himalayan–Tibetan region, and will thus aid our understanding of the evolution of the orogeny. The Bandatso sodic mafic rocks are characterised by marked oceanic island basalt (OIB)-like enrichment in incompatible elements, such as Nb (up to 139 ppm) and light rare-earth elements. This Late Mesozoic (ca. 148 Ma and 90 Ma) mafic magmatism is likely to be the result of two-stage mantle evolution in the northern Qiangtang. The late Jurassic rocks were derived from carbonated lithosphere metasomatized by subducted oceanic sediments during flat subduction. Whereas the late Cretaceous basanites were likely derived from interaction of lithosphere with upwelling asthenosphere induced by removal of lithosphere that had previously been thickened by relamination. Our study suggests the presence of an inherited carbonated lithospheric mantle beneath the northern Qiangtang prior to continental collision. The conclusions from this work suggest that the enrichment of the mantle source of Cenozoic potassic rock closely related to plateau uplift, more likely result from continental subduction after the late Cretaceous (ca. 90 Ma). These conclusions will help our understanding of the Cenozoic geology of this multi-terrane collision system and uplift mechanism of the Tibetan Plateau.

Mineralogical and geochemical constraints on the origin of the Sohland-Rožany Ni-Cu-(PGE) sulfide mineralization (Lausitz Block, Bohemian Massif, Germany/Czech Republic)

The Sohland-Rožany Ni-Cu-(PGE) sulfide mineralization is hosted by a small gabbroic dike that is one of several Ni-Cu sulfide-bearing gabbroic intrusions in the northern Bohemian Massif. The intrusions are associated with a Middle Devonian to Early Carboniferous mafic–ultramafic plumbing system that emplaced within Cadomian granodiorites of the Lausitz Block. The Sohland-Rožany dike comprises varitextured olivine- and hornblende-bearing gabbro and hornblende gabbro which originated from differently fractionated magma batches. Fractionation likely took place in deeper parts of the magmatic systems, e.g. within staging magma chambers. Mineralogical features like Ti-rich primary brown hornblende and diopsidic clinopyroxene as well as overall enriched incompatible element contents suggest moderate alkaline basaltic parental magmas with relatively high volatile contents.The magmatic sulfide mineralization is exclusively hosted by the olivine- and hornblende-bearing gabbro and include disseminated, net-textured and massive sulfides. The ore types are predominated by pyrrhotite-pentlandite-chalcopyrite assemblages accompanied by secondary violarite and pyrite as well as by accessory Pt, Pd, Au and Ag phases. The disseminated and net-textured sulfides have highly variable modal proportions of base-metal sulfides with pyrrhotite-dominated beside chalcopyrite-dominated aggregates on the hand specimen scale. The platinum-group mineral (PGM) assemblage includes Pd-Pt-Ni-bearing bismuthotellurides of the melonite–merenskyite and michenerite–testibiopalladite solid solution series as well as sperrylite. The sulfide ores feature predominantly elevated Ni and highly variable Cu and Pt, Pd and Au tenors (3.5–6.4 wt.% Ni, 0.1–16.6 wt.% Cu, 80–3080 ppb Pt, 50–5320 ppb Pd and 20–3770 ppb Au). The magmatic sulfides are locally characterized by a late-stage hydrothermal overprint where sulfides were replaced by hydrous secondary phases (e.g. actinolite–tremolite and chlorite). A significant redistribution of the PGE due to hydrothermal alteration is unlikely as the majority of PGM show a close spatial association to the base-metal sulfides.Textures and chemical compositions of the sulfides suggest that repeated magma batches eroded already fractionated sulfide pools and entrained early crystalized cumulates of monosulfide solid solution (MSS) together with variable fractionated sulfide melts. Sulfide saturation in the Sohland-Rožany magmas was locally controlled by assimilation of the granodioritic wall-rock in the course of thermomechanical erosion. This process was amplified by high mica contents of the granodiorites. Sulfur isotope signatures (–2.0 to –1.2‰ VCDT) and the S-poor nature of the hosting granodiorites exclude a significant contribution of S from the country rocks.

Intra-continental boninite-series volcanic rocks from the Bangong-Nujiang Suture Zone, Central Tibet

The boninitic volcanic rocks from Rutog, western Bangong-Nujiang Suture Zone (BNSZ), are characterized by high MgO, SiO2 but low TiO2 contents and interpreted as the products of intra-oceanic subduction in earlier studies. Recently, we discovered new breccia-like boninitic volcanic rocks (mainly trachyandesite and trachyte) that overlie the Rutog ophiolitic peridotites in BNSZ. These volcanic rocks not only show typical boninitic petrographical features of porphyritic texture in which euhedral augite phenocrysts (ca. 5 vol%) are embedded in fine-grained groundmass composed of pyroxene, plagioclase, quartz and minor glass, but also display strong boninitic geochemical affinities with high Mg# (61.59–67.90), Al2O3/TiO2 (40.31–55.97) ratios and low TiO2 (0.26–0.35 wt%) contents. In addition, their rare earth elemental patterns show a slight right-dip pattern with (La/Gd)N values of ~1.28 and (Gd/Yb)N values of ~1.31. Given the evolutionary trend from magnesium-rich intermediate to acid series, the boninite-series are therefore used to describe the vlocanic rocks. Zircon U-Pb isotopic dating on these boninite-series volcanic rocks yielded an age of ca. 79.3 ± 2.0 Ma, after the closure of the Bangong-Nujiang Tethyan Ocean (BNTO) that occurred before 96 Ma. The newly recognized boninite-series volcanic rocks probably formed more than 20 Ma after the latest regional magmatism related to subduction process and thus the subduction setting is not suitable for these boninite-series volcanic rocks. The enrichment in compatible elements and depletion in high field elements indicate the Rutog boninite-series originated from a severely depleted mantle while several lines of evidence (e.g., the enrichment in light rare earth elements, large iron lithophile elements) imply that the mantle source has been modified by continental crust. Given the widespread Cretaceous high Sr/Y magmatic rocks within BNSZ, the Rutog boninite-series rocks may provide an example of the crust-mantle interaction as a result of the delamination in a partially thickened and cratonized convergent margin during the late Cretaceous within BNSZ, central Tibetan Plateau.

Geochemical Distinction Between Altered Oceanic Basalt- and Seafloor Sediment-Derived Fluids in the Mantle Source of Mafic Igneous Rocks in Southwestern Tianshan, Western China

AbstractThe role of subducting oceanic crust-derived fluids in generating mafic arc magmatism has been widely documented. However, the subducting oceanic crust is generally composed of basaltic igneous crust and seafloor sediment, which may give rise to different compositions of liquid phases causing metasomatism of the mantle wedge. Because of the similarity in enrichment of fluid-mobile incompatible elements in the two sources of subduction zone fluids, it has been a challenge to distinguish between them when studying the products of mafic arc magmatism. This difficulty is overcome by a combined study of whole-rock Li isotopes and zircon O isotopes in addition to whole-rock major-trace elements and Sr–Nd–Hf isotopes in Late Paleozoic mafic igneous rocks from southwestern Tianshan in western China. Zircon U–Pb dating yields consistent ages of 313 ± 3 Ma to 305 ± 1 Ma for magma crystallization. The mafic igneous rocks exhibit arc-like trace element distribution patterns and depleted whole-rock Nd–Hf isotopes but slightly high (87Sr/86Sr)i ratios of 0·7039– 0·7057. They also show positive zircon εHf(t) values and slightly higher zircon δ18O values of 5·2–7·6‰. There are covariations of whole-rock Sr isotopes with Th/La and Rb/Nb ratios, indicating a contribution from terrigenous sediment-derived fluids to their mantle source in addition to basaltic igneous crust-derived fluids. Based on the slightly higher zircon δ18O values but variably lower whole-rock δ7Li values of -0·8 to 3·5‰ for the target rocks than those of mantle respectively, both altered oceanic basalt- and terrigenous sediment-derived fluids are identified in the mantle source of these mafic igneous rocks. Model calculations for trace elements and Sr–Nd–Li isotopes further confirm that the geochemical compositions of these mafic igneous rocks can be explained by chemical reaction of depleted MORB mantle peridotite with the mixed fluids to generate ultramafic metasomatites at subarc depths. Therefore, chemical metasomatism of the mantle wedge is a key mechanism for the incorporation of crustal components into the source of arc-like mafic igneous rocks above oceanic subduction zones.

Geochemistry of mid- upper eocene intra-continental alkaline volcanic range, south part of central alborz mountains, north of Iran.

The Ziaran volcanic Belt (ZVB), North of Iran contains a number of intra-continental alkaline volcanic range situated on South part of central Alborz Mountains, formed along the localized extensional basins developed in relation with the compressional regime of Eocene. The mid-upper Eocene volcanic suite comprises the extracted melt products of adiabatic decompression melting of the mantle that are represented by small volume intra-continental plate volcanic rocks of alkaline volcanism and their evaluated Rocks with compositions representative of mantle-derived, primary (or near-primary) melts. Trace element patterns with significant enrichment in LILE, HFSE and REEs, relative to Primitive Mantle. Chondrite-normalized of rare earth elements and enrichment in incompatible elements and their element ratios (e. g. LREE/HREE, MREE/HREE, LREE/MREE) shown these element modelling indicates that the magmas were generated by comparably variable degrees of partial melting of garnet lherzolite and a heterogeneous asthenospheric, OIB mantle sources.

The role of celadonite on mantle metasomatism in subduction settings

Celadonite is the most abundant K-rich hydrated mineral formed during basalt alteration. In convergent margins, celadonite from the altered oceanic crust sinks into the mantle and contributes to the mantle wedge metasomatism, recorded by incompatible element enrichment in lavas and mantle xenoliths samples from these settings. However, experimental data on the role of celadonite in subduction zones as a function of P-T are still scarce. Therefore, celadonite stability under “hot” and “cold” subduction settings is investigated experimentally in this study. Results show an early silica loss, with quartz/coesite replacing celadonite from 400 to 600 °C and 2.5–7.7 GPa. At 700 °C and 2.5 GPa, celadonite is partially converted to phlogopite-annite, and at 800 °C and 4.0 GPa celadonite is absent. At temperatures above 800 °C, enstatite and phlogopite are the stable silicate phases from 2.5 to 7.7 GPa. Mössbauer spectroscopy and X-ray diffraction analyses show that the reaction of celadonite to phlogopite-annite is progressive and controlled by the Fe3+ to Fe2+ reduction in the octahedral site. Coupled to this, there is a transition of pyrite to pyrrhotite and the release of reducing fluids enriched in H2S, SiO2, OH− and K+ in the mantle wedge. These fluids have the potential to mobilize chalcophile elements (e.g. Cu, Zn) and act as mineralizing agents in shallower and more oxidizing crustal environments. Data in this study demonstrate that celadonite-phlogopite-annite series minerals are stable down to at least 230 km in the mantle, contributing to the sources of mantle metasomatism, arc-related magma genesis and possibly chalcophile mineralization.

Distribution of trace elements in sulfides from Deyin hydrothermal field, Mid-Atlantic Ridge – Implications for its mineralizing processes

Knowledge about the trace element distribution in sulfides is the key for understanding the trace metal inventory during the formation of the volcanic massive sulfide deposits on the seafloor. The distribution of trace metals in sulfides reflects changes in the physicochemical conditions and precipitation processes during precipitation. Hydrothermal black smoker samples of this study originate from the southern Mid-Atlantic (TVG02, TVG06) and can be classified into two types: (1) (Fe-Zn) sulfides consisting predominantly of pyrite and sphalerite, with minor chalcopyrite, isocubanite, and galena and (2) (Fe-Cu) sulfides, which contain mainly pyrite and chalcopyrite, with rare isocubanite and sphalerite. The sulfide samples were analyzed by electron microprobe for major elements, and by laser ablation inductively coupled plasma mass spectrometry, focused ion beam technique and transmission electron microscopy for in situ trace elements. The data reveal complex hydrothermal processes in high- and low- temperature fluids. Colloform and dendritic pyrite from (Fe-Zn) sulfides are enriched in Mn, Tl, As, V, Pb, and Zn, indicating precipitating from a rapid mixture of low temperature fluids (100–250 °C) with seawater. A continuous enrichment of incompatible elements (Pb and Zn) at the interface of growing pyrite with fluid finally leads to the nucleation and precipitation of sphalerite and galena, and results in the enrichment of As, Hg, Pb, Au, Ag, and Cd in sphalerite micro-inclusions which are hosted in colloform pyrite at low temperature. Galena inclusions occur in the pores and the interstices of sphalerite grains and formed by the enrichment of Pb in the hydrothermal fluid at low-temperature (<250 °C). High Cu, As, Ag, Au, and Sb concentrations in colloform sphalerite indicate a medium to low formation temperature (350–200 °C). Copper, Sb, Se, and Sn enrichments in coarse-grained sphalerite indicate medium temperature hydrothermal fluids. Idiomorphic pyrite and chalcopyrite have high Se and Co contents and indicate precipitaion from high-temperature (>300 °C) fluids.

Origin of geochemically heterogeneous mid-ocean ridge basalts from the Macquarie Ridge Complex, SW Pacific

The Macquarie Ridge Complex (MRC), located at the Australian–Pacific plate boundary south of New Zealand, is a rugged bathymetric ridge comprising a series of submarine seamounts and Macquarie Island, the only subaerial portion of the complex. Mid-ocean ridge basalts (MORBs) from Macquarie Island show various enrichments in incompatible elements with compositions ranging from typical normal MORB to enriched MORB. However, these basalts have isotopic compositions trending towards a high μ-like (μ = 238U/204Pb; HIMU) mantle component, which is unusual for MORB-type rocks. The origin of this mantle signature is not understood, and it is unclear whether this isotopic signature is characteristic of the entire MRC or unique to Macquarie Island. Here we report new major and trace element abundances, and Sr, Nd, and Pb isotopes for samples from the MRC seamounts and from new sampling sites on Macquarie Island. The geochemical and isotopic data show that the entire MRC comprises normal to enriched MORB. Mixing modelling indicates that the heterogeneous isotopic signatures of the MRC basalts are not derived from contamination of the nearby Balleny mantle plume but have affinities with that of the Cenozoic Zealandia intraplate HIMU-like basalts. We propose that the heterogeneous geochemical signatures of the MRC basalts are derived from amphibole-bearing garnet pyroxenite veins, which is supported by the rare earth element partial melting modelling and strong correlations between Nd and Pb isotopic ratios vs La/Sm. We posit that the pyroxenite veins were generated in the oceanic lithospheric mantle, which was metasomatised by hydrous and carbonatitic fluids/melts derived either from delaminated, metasomatised Zealandia subcontinental lithosphere mantle, or from subducted material in the asthenosphere. The subducted material could be derived from ancient and/or recent subduction along the former east Gondwana margin.

Diamond-forming HDFs tracking episodic mantle metasomatism beneath Nyurbinskaya kimberlite pipe (Siberian craton)

We present a new dataset on the composition of high-density fluids (HDFs) in cloudy (n = 25), coated (n = 10) and cuboid (n = 10) diamonds from the Nyurbinskaya kimberlite pipe. These diamonds represent different populations each showing distinct growth histories. The cores of coated diamonds display multiple growth stages and contrasting sources of carbon. Fibrous coats and cuboid diamonds have similar carbon isotopes and nitrogen systematics, suggesting their formation in the last metasomatic events related to kimberlite magmatism, as is common for most such diamonds worldwide. The HDFs in most of these diamonds span a wide range from low-Mg carbonatitic to hydrous silicic compositions. The major- and trace-element variations suggest that the sources for such HDFs range in composition between the depleted mantle and more fertile mantle reservoirs. Hydrous-silicic HDFs could originate from a 13C-enriched source, which originates through subduction of crustal metasedimentary material. Percolation of such HDFs through carbonated eclogites and peridotites facilitates the formation of cuboid diamonds and fibrous coats in the mantle section beneath the corresponding area of the Siberian craton. Cloudy diamonds represent an apparently older population, reflecting continuous diamond formation predominantly from high-Mg carbonatitic HDFs that caused discrete episodes of diamond precipitation. Their high Mg# and enrichment in incompatible elements support a metasomatized peridotitic source for these HDFs.

Immiscibility and the origin of ladder structures, mafic layering, and schlieren in plutons

Abstract Granitic plutons worldwide contain ladder structures (LSs) that consist of nested trough-shaped layers alternating between mafic and felsic compositions. LSs and other forms of modal layering have been attributed to crystal accumulation, but their chemical trends differ greatly from those of cumulates and are discordant with chemical variations of their granitic hosts. Mafic layers reach extreme enrichments in transition metals, high-field-strength elements, and incompatible elements, and are extremely depleted in Si and Al. These geochemical characteristics are difficult to explain by crystal accumulation and conflict with sequences of phase appearance during crystallization. They are characteristic of liquid immiscibility, which is an accepted process in the genesis of tholeiitic and alkalic rocks. We propose that ladder structures and other forms of modal layering are markers of immiscibility in calc-alkaline granitic rocks.