Stages Of Metasomatism Research Articles

Petrological and Geochemical Comparison of Rodingites from Kimi -Evia island with outcrops from adjacent regions

The present study is focused on the geochemical and petrological comparison between rodingites from the Kimi region in Central Evia (Greece) with those from East and West Othris, as well as with rodingites from Skyros island. Based upon their whole rock geochemical and petrographical data it is suggested that rocks from Skyros island and Kimi-Evia display similar features. Kimi-Evia and Skyros rodingites are characterized by their highly comparable REE patterns with significant LREE enrichments. This is most likely attributed to the effects of carbonic rich fluids, enhancing the transfer of REE via CO3-2 ligands. The West Othris rodingites display similar MREE and HREE patterns with the least fractionated rodingites from Kimi-Evia and Skyros, whereas the LREE are significantly depleted. The East Othris rodingites differ from all other rodingite occurrences, since they exhibit much lower ΣREE contents. Rodingitization in all studied areas was associated with serpentinization processes and was evolved in several metasomatic stages, in which vesuvianite was formed during the last episodes at relatively low temperatures.

Mineral chemistry and formation conditions of calc-silicate minerals of Qozlou Fe skarn deposit, Zanjan Province, NW Iran

The Qozlou Fe skarn deposit is located at the Abhar–Mahneshan belt of the Central Iranian Zone. It is associated with Upper Eocene porphyritic granite that intruded into the Upper Cretaceous impure carbonaceous rocks. The Qozlou granite has high-K calc-alkaline affinity and is classified as subduction-related metaluminous I-type granitoids. Skarn aureole in the Qozlou is composed of endoskarn and exoskarn zones, with the exoskarn zone being the main skarn and mineralized zone. It includes garnet skarn, garnet-pyroxene skarn, pyroxene skarn, epidote skarn, and pyroxene-bearing marble sub-zones. The Qozlou Fe deposit is 300 m long and 5–30 m wide. Magnetite is the main ore mineral associated to pyrite, chalcopyrite, and pyrrhotite. Garnet, clinopyroxene, actinolite, epidote, calcite, and quartz occur as gangue minerals. Covellite, hematite, and goethite were formed during the supergene processes. The ore and gangue minerals have massive, banded, disseminated, brecciated, vein–veinlets, replacement, and relict textures. EPMA data indicate that garnets have andradite–grossularite compositions (Ad39.97–100–Gr0–49.62) and clinopyroxenes have diopsidic composition (En29.43–42.5–Fs14.31–20.99–Wo43.08–50.17). Based on mineralogical and textural criteria, skarnification processes in the Qozlou skarn can be categorized into three discrete stages: (1) isochemical (metamorphic–bimetasomatic), (2) metasomatic prograde, and (3) metasomatic retrograde. Anhydrous calc-silicate minerals (garnet and clinopyroxene) were formed during the prograde metasomatic stage, while ore minerals and hydrous calc-silicate minerals were formed during the retrograde ore-forming sub-stage. Temperature and ƒO2 conditions range between 430 and 550 °C and 10−26 and 10−23, respectively, for the metasomatic prograde stage. The retrograde metasomatizing fluids had likely ƒS2 = 10−6.5 and temperatures < 430 °C at the beginning of the ore-forming sub-stage.

Metasomatic Processes in the Lithospheric Mantle Beneath the No. 30 Kimberlite (Wafangdian Region, North China Craton)

AbstractThis paper presents the first major and trace element compositions of mantle-derived garnet xenocrysts from the diamondiferous No. 30 kimberlite pipe in the Wafangdian region, and these are used to constrain the nature and evolution of mantle metasomatism beneath the North China Craton (NCC). The major element data were acquired using an electron probe micro-analyzer and the trace element data were obtained using laser ablation inductively coupled plasma-mass spectrometry. Based on Ni-in-garnet thermometry, equilibrium temperatures of 1107–1365 °C were estimated for peridotitic garnets xenocrysts from the No. 30 kimberlite, with an average temperature of 1258 °C, and pressures calculated to be between 5.0 and 7.4 GPa. In a CaO versus Cr2O3 diagram, 52% of the garnets fall in the lherzolite field and 28% in the harzburgite field; a few of the garnets are eclogitic. Based on rare earth element patterns, the lherzolitic garnets are further divided into three groups. The compositional variations in garnet xenocrysts reflect two stages of metasomatism: early carbonatite melt/fluid metasomatism and late kimberlite metasomatism. The carbonatite melt/fluids are effective at introducing Sr and the light rare earth elements, but ineffective at transporting much Zr, Ti, Y, or heavy rare earth elements. The kimberlite metasomatic agent is highly effective at element transport, introducing, e.g., Ti, Zr, Y, and the rare earth elements. Combined with compositional data for garnet inclusions in diamonds and megacrysts from the Mengyin and Wafangdian kimberlites, we suggest that these signatures reflect a two-stage evolution of the sub-continental lithospheric mantle (SCLM) beneath the NCC: (1) early-stage carbonatite melt/fluid metasomatism resulting in metasomatic modification of the SCLM and likely associated with diamond crystallization; (2) late-stage kimberlite metasomatism related to the eruption of the 465 Ma kimberlite.

New isotope constraints on the time of formation of the Nersinskii dolerite complex from the Biryusa-Sayan area

New data on the Sm-Nd isotopic composition of the dolerites of the Nersinskii complex that are consistent with the petrographic study and the U-Pb dating of detrital zircon from the overlying sedimentary sequences of the Biryusa-Sayan area are presented. The data obtained indicate the absence of the supposed magmatic event 1.6 billion years ago and clearly indicate the Neoproterozoic age of the intrusions. Their source could have been the metasomatized mantle; after formation, dolerites went through several stages of metasomatism, which was reflected in the evolution of its isotopic systems.

Origin of trondhjemite and albitite at the expense of A-type granite, Aravalli orogen, India: Evidence from new metasomatic replacement fronts

The A-type Harsora-Dadikar granites in the Alwar complex of northern Aravalli orogen, NW India provide evidence for subsolidus-requilibration of feldspars. They record three new discrete stages of albitisation, producing trondhjemite and albitite sequentially at the expense of original granite. Stage-I metasomatism deanorthised the magmatic oligoclase and transformed the grey least-albitised granite to pinkish grey microcline-oligoclase granite. Stage-II converted the latter to trondhjemite by replacement of microcline to oligoclase. Stage-III metasomatism led to the formation of albitite/albite granite from trondhjemite, where the metasomatically formed oligoclase was replaced by albite. This stage of metasomatism resulted in nearly complete disappearance of amphibole and biotite, producing a monomineralic rock (albitite), which is consistent with Korzhinskii theory of infiltration metasomatism. The reaction fronts delineating the Stage-II and Stage-III are sharp and easily discernible by their prominent color differences in Harsora on the outcrop scale. Chemically, the mineral transformations during three stages are manifested by the differential gains/losses in Na, K, Ca, Rb, Ba, Sr, Fe and Mg. The formation of albite, Cl-rich marialitic scapolite and Cl-rich amphibole in the albitised granites are suggestive of Na- and Cl-brines as the metasomatising fluids. The fluid-rock interactions, which can significantly transform the pristine mineralogy of granitoids, should be carefully considered to avoid any misinterpretations about their petrological history.

Oxides of the Pyrochlore Supergroup from a Nonsulfide Endogenic Assemblage of Pb–Zn–Sb–As Minerals in the Pelagonian Massif, Macedonia

The specific features of the chemical composition, isomorphism, and zoning have been studied for pyrochlore supergroup minerals (PSM) from metasomatic rocks of ore occurrence no. 9 (Nezilovo, Pelagonian Massif, Republic of Macedonia). Ore occurrence no. 9 has an unusual nonsulfide association of chalcophile elements: Zn, As, Sb and Pb. Pyrochlore supergroup minerals crystallized at all stages of metasomatism. Zoning of pyrochlore supergroup minerals and their relationships with associated minerals (Zn-bearing silicates, tilasite, barite, carbonates, various accessory Pb-, Sb- and Zn-oxides, etc.) suggest a complex geochemical evolution of ore occurrence no. 9. The possible mechanism for the formation of nonsulfide parageneses of chalcophile elements is discussed.

Two metasomatic events recorded in apatite from the ore-hosting dolomite marble and implications for genesis of the giant Bayan Obo REE deposit, Inner Mongolia, Northern China

In the Bayan Obo REE deposit in Inner Mongolia, Northern China, three major orebodies are hosted in dolomite marble of the Bayan Obo Group. There are carbonatite dikes in the ore district. Apatite is a common accessary mineral in the ore-hosting dolomite marble (DM apatite) and in carbonatite dikes (IC apatite). These two types of apatite are both fluorapatite, and have low SiO2, uniform P2O5, and variable CaO contents. Total REY (REEs + Y) contents are correlated with Na2O contents, indicating that REY of both types of apatite enter lattice via the substitution reaction: Na+ + (REY)3+ = 2Ca2+. These features, combined with high REY (6230–18,906 ppm) and Sr (9653–17,200 ppm) contents of DM apatite, indicate that DM apatite likely had a carbonatite origin. Some DM apatite grains are partially replaced by albite and quartz. Fluid inclusions crosscutting both apatite and albite or quartz indicate that they formed later than quartz and albite replacement. The back-scattered electron images show that DM apatite grains contain many micro-pores (fluid inclusions), and monazite inclusions formed from the fluid inclusions. However, no monazite inclusions are observed within quartz and albite, excluding the possibility that the monazite inclusions were precipitated directly from the fluids. The monazite inclusions were therefore formed during fluid-induced dissolution–reprecipitation processes, where DM apatite served as the source of LREEs. This also explains the depletion of some LREEs in DM apatite. The formation of monazite inclusions in apatite requires fluids with relatively low Na and Si concentrations, different from the fluids responsible for quartz and albite replacement. DM apatite was affected by two stages of fluid activities: the first stage of metasomatism by alkaline fluids that were likely derived from carbonatite magmas when the deposit first formed (represented by quartz and albite replacement), followed by a second stage of modification that caused LREEs depletion and the formation of new REE minerals. Thus, the Bayan Obo REE ore deposit was modified by a significant thermal event after the formation, which provided negligible or only small amounts of REEs.

Origin of the high-temperature Olserum-Djupedal REE-phosphate mineralisation, SE Sweden: A unique contact metamorphic-hydrothermal system

The Swedish part of the Fennoscandian Shield hosts a variety of rare earth element (REE) deposits, including magmatic to magmatic-hydrothermal types. This paper focuses on the origin of the Olserum-Djupedal REE-phosphate mineralisation located in the sparsely studied Västervik region, SE Sweden. Here, mineralisation occurs in three main areas, Olserum, Djupedal and Bersummen. Primary hydrothermal REE mineralisation formed at high temperatures (about 600 °C), leading to precipitation of monazite-(Ce), xenotime-(Y), fluorapatite and minor (Y,REE,U,Fe)-(Nb,Ta)-oxides in veins and vein zones dominated by biotite, amphibole, magnetite and quartz. The veins are hosted primarily by metasedimentary rocks present close to, or within, the contact aureole of a local 1.8 Ga ferroan alkali feldspar granite pluton, but also occur within in the chemically most primitive granite in the outermost part of that pluton. In the Djupedal area, REE-mineralised metasedimentary bodies are extensively migmatised, with migmatisation post-dating the main stage of mineralisation. In the Olserum and Bersummen areas, the REE-bearing veins are cross-cut by abundant pegmatitic to granitic dykes. The field relationships demonstrate a protracted magmatic evolution of the granitic pluton and a clear spatial and temporal relationship of the REE mineralisation to the granite.The major and trace element chemistry of ore-associated biotite and magnetite support genetic links between all mineralised areas. Biotite mineral chemistry data further demonstrate a distinct chemical trend from metasediment-hosted ore-associated biotite distal to the major contact of the granite to the biotite in the granite-hosted veins. This trend is characterised by a systematic decrease in Mg and Na and a coupled increase in Fe and Ti with proximity to the granite-hosted veins. The halogen compositions of ore-associated biotite indicate elevated contents of HCl and HF in the primary REE mineralising fluid. Calculated log(fHF/fHCl) values in the Olserum area suggest a constant ratio of about −1 at temperatures of 650–550 °C during the evolution of the primary hydrothermal system. In the Djupedal and Bersummen areas, the fluid locally equilibrated at lower log(fHF/fHCl) values down to −2. High Na contents in ore-associated biotite and amphibole, and the abundance of primary ore-associated biotite indicate a K- and Na-rich character of the primary REE mineralising fluid and suggest initial high-temperature K-Na metasomatism. With subsequent cooling of the system, the fluid evolved locally to more Ca-rich compositions as indicated by the presence of the Ca-rich minerals allanite-(Ce) and uvitic tourmaline and by the significant calcic alteration of monazite-(Ce). The later Ca-rich stages were probably coeval with low to medium-high temperature (200–500 °C) Na-Ca metasomatism variably affecting the granite and the wall rocks, producing distinct white quartz-plagioclase rocks.All observations and data lead us to discard the prevailing model that the REE mineralisation in the Olserum-Djupedal district represents assimilated and remobilised former heavy mineral-rich beds. Instead, we propose that the primary REE mineralisation formed by granite-derived fluids enriched in REE and P that were expelled early during the evolution of a local granitic pluton. The REE mineralisation developed primarily in the contact aureole of this granite and represents the product of a high temperature contact metamorphic-hydrothermal mineralising system. The REE mineralisation probably formed synchronously with K-Na and subsequent Na-Ca metasomatism affecting the granite and the wall rocks. The later Na-Ca metasomatic stage is probably related to a regional Na ± Ca metasomatic and associated U ± REE mineralising system operating concurrently with granitic magmatism at c. 1.8 Ga in the Västervik region. This highlights the potential for discovering hitherto unknown REE deposits and for the reappraisal of already known deposits in this part of the Fennoscandian Shield.

On the relative timing of listwaenite formation and chromian spinel equilibration in serpentinites

Ultramafic rocks exposed at the Earth's surface generally record multiple stages of evolution that may include melt extraction, serpentinization, carbonatization, and metamorphism. When quantitative thermometry based on mineral chemistry is applied to such rocks, it is often unclear what stage of their evolution is being observed. Here, in peridotites with extensive replacement of silicate minerals by carbonates (listwaenites), we present a case study that addresses the timing of carbonate formation relative to closure of exchange reactions among relict primary minerals. Massive and schistose serpentinized peridotites of Neoproterozoic age outcrop at Gabal Sirsir, South Eastern Desert, Egypt (northwestern corner of the Arabian-Nubian Shield or ANS). Petrography, bulk composition, and mineral chemistry are all consistent with a strongly depleted mantle harzburgite protolith for the serpentinites. Bulk compositions are low in Al_2O_3 and CaO and high in Mg# [molar Mg/(Mg+Fe) = 0.89–0.93]. Relict spinel has high Cr# [molar Cr/(Cr+Al)] and low Ti, while relict olivine has high Mg# and NiO contents. Based on compositions of coexisting relict olivine and chromian spinel, the protolith experienced 19 to 21% partial melt extraction. Such high degrees of partial melting indicate a supra-subduction zone environment, possibly a forearc setting. Along thrust faults and shear zones, serpentinites are highly altered to form talc-carbonate rocks and weathering-resistant listwaenites that can be distinguished petrographically into Types I and II. The listwaenitization process took place through two metasomatic stages, associated first with formation of the oceanic crustal section and near-ridge processes (∼750–700 Ma) and subsequently during obduction associated with the collision of East and West Gondwana and escape tectonics (∼650–600 Ma). In the first stage, Mg# of chromian spinel in the serpentinites continuously changed due to subsolidus Mg–Fe^(2+) redistribution, while the Mg# of chromian spinel in the Type I listwaenites was frozen due to the absence of coexisting mafic silicates. During the second stage, the Type II listwaenites formed along shear zones accompanied by oxidation of relict chromian spinel to form ferritchromite and Cr-bearing magnetite in both serpentinites and listwaenites. The high Cr# of chromian spinel relics in both serpentinites and listwaenites preserves primary evidence of protolith melt extraction, but divalent cations are more easily mobilized at low temperature. Hence, relict chromian spinel in listwaenites shows significantly higher Mg# and lower MnO than that in serpentinite, suggesting that nearly complete alteration of ultramafic rocks to form listwaenite took place prior to re-equilibration between chromian spinel and the surrounding mafic minerals in serpentinites. Furthermore, the ferritchromite in the serpentinites has higher Mn content (1.1–2.1 wt%) than that in the listwaenites (0.6–0.9 wt%), indicating its formation after carbonatization since carbonate minerals are a favorable sink for Mn.

A Late Mesozoic short-lived shift from fluid-dominated to sediment-dominated mantle metasomatism in the northeast South China Block and its tectonic implications

Early Cretaceous northwest (NW)-trending dolerite and amphibole lamprophyre dykes exposed in NW Zhejiang Province provide a number of new insights into the nature of the subcontinental mantle on the northeast (NE) South China Block (SCB). These dykes have a high Al2O3 (14.04–17.89 wt%) and K2O (0.66–2.69 wt%) contents but relatively low Na2O (2.48–4.61 wt%) and TiO2 (1.33–2.79 wt%) makeup alongside moderate K2O/Na2O ratios between 0.26 and 1.04. These amphibole lamprophyre dykes also have higher MgO, Cr, and Ni contents than those of comparable dolerites that have SiO2 content ranging from 46.32 to 49.87 wt%. The most striking feature of these intrusions is that they contain higher contents of Rb, Th, U, Nb, Ta, and LREE compared to their dolerite counterparts, although both amphibole lamprophyres and dolerites do exhibit similar geochemical patterns that are indicative of subduction-related origins. These features imply that an ambient peridotitic mantle that acted as the source for the amphibole lamprophyre magma source may have reacted with silicate-rich melts leading to olivine consumption while maintaining orthopyroxene. The geochemical composition of these dolerites are likely influenced to a variable extent by the fractionation of olivine, orthopyroxene, clinopyroxene, Fe–Ti oxides, and apatite, while their amphibole lamprophyre counterparts have been modified to a minor degree by amphibole fractionation. Measured Sr–Nd isotopic compositions suggest relatively constant Nd isotopic compositions (−0.36 to +1.52) with more variable Sr isotopic compositions (0.7071 to 0.7306). We hence propose that both the dolerite and amphibole lamprophyre dykes in this region are the products of mantle source metasomatism by the subducted Paleo-Pacific slab. The dolerite dykes are mainly associated with slab-derived fluids, while the lamprophyre dykes are related to both slab-derived fluids and sediment melts. Evidence in support of metasomatism comprising distinct two-stage processes including a fluid-dominated phase followed by a sediment melts-dominated metasomatism stage, further suggests that these mafic dykes most likely formed in a back-arc setting.

Implication of Apatite and Anhydrite for Formation of an Iron‐Oxide‐Apatite (IOA) Rare Earth Element Prospect, Benjamin River, Canada

AbstractThe Benjamin River apatite prospect in northern New Brunswick, Canada, is hosted by the Late Silurian Dickie Brook plutonic complex, which is made up of intrusive units represented by monzogranite, diorite and gabbro. The IOA ores, composed mainly of apatite, augite, and magnetite at Benjamin River form pegmatitic pods and lenses in the host igneous rocks, the largest of which is 100 m long and 10–20 m wide in the diorite and gabbro units. In this study, 28 IOA ore and rock samples were collected from the diorite and gabbro units. Mineralogical observations show that the apatite–augite–magnetite ores are variable in the amounts of apatite, augite, and magnetite and are associated with minor amounts of epidote‐group minerals (allanite, REE‐rich epidote and epidte) and trace amounts of albite, titanite, ilmenite, titanomagnetite, pyrite, chlorite, calcite, and quartz. Apatite and augite grains contain small anhydrite inclusions. This suggests that the magma that crystallized apatite and augite had high oxygen fugacity. In back scattered electron (BSE) images, apatite grains in the ores have two zones of different appearance: (i) primary REE‐rich zone; and (ii) porous REE‐poor zone. The porous REE‐poor zones mainly appear in rims and/or inside of the apatite grains, in addition to the presence of apatite grains which totally consist of a porous REE‐poor apatite. This porous REE‐poor apatite is characterized by low REE (&lt;0.84 wt%), Si (&lt;0.28 wt%), and Cl (&lt;0.17 wt%) contents. Epidote‐group minerals mainly occur in grain boundary between the porous REE‐poor apatite and augite. These indicate that REE leached from primary REE‐rich apatite crystallized as allanite and REE‐rich epidote. Magnetite in the ores often occurs as veinlets that cut apatite grains or as anhedral grains that replace a part of augite. These textures suggest that magnetite crystallized in the late stage. Pyrite veins occur in the ores, including a large amount of quartz and calcite veins. Pyrite veins mainly occur with quartz veins in augite. These textures indicate pyrite veins are the latest phase.Apatite–augite–magnetite ore, gabbro–quartz diorite and feldspar dike collected from the Benjamin River prospect contain dirty pure albite (Ab98Or2–Ab100) under the microscope. The feldspar dikes mainly consist of dirty pure albite. Occurrences of the dirty pure albite suggest remarkable albitization (sodic alteration) of original plagioclase (An25.3–An60 in Pilote et al., 2012) associating with intrusion of monzogranite into gabbro and diorite.SO42− bearing magma crystallized primary REE‐rich apatite, augite and anhydrite reacted with Fe in the sodic fluids, which result in oxidation of Fe2+ and release of S2− into the sodic fluids. REE, Ca and Fe from primary REE‐rich apatite, augite and plagioclase altered by the sodic fluids were released into the fluids. Then Fe3+ in the sodic fluids precipitated as Fe oxides and epidote‐group minerals in apatite–augite–magnetite ores. Finally, residual S2− in sodic fluids crystallized as latest pyrite veins.In conclusion, mineralization in Benjamin River IOA prospect are divided into four stages: (1) oxidized magmatic stage that crystallized apatite, augite and anhydrite; (2) sodic metasomatic stage accompanying alteration of magmatic minerals; (3) oxidized fluid stage (magnetite–epidote group minerals mineralization); and (4) reduced fluid stage (pyrite mineralization).

Sapphire Megacrysts In Syenite Pegmatites From the Ilmen Mountains, South Urals, Russia: New Mineralogical Data

Abstract Sapphires in the Ilmen Mountains of Russia9s South Urals are found in syenite pegmatites where the mineral forms blue transparent to translucent megacrysts up to 6 cm in length. The corundum crystallized during the magmatic stage in equilibrium with columbite-(Fe), K-Na-feldspar, and ± perthite. There was a later metasomatic stage in which muscovite formed by the reaction corundum + orthoclase + H 2 O ↔ muscovite and diaspore precipitated. Highly distributed syn- and post-tectonic brittle deformation of the host rocks is responsible for the polysynthetic twinning and parting of blue sapphires along the r face, which decreases the gem quality of the raw material. The formation of the corundum syenite pegmatites is associated with the intrusion of miascites in the Ilmenogorsky complex of alkaline rocks. Columbite-(Fe), zircon, and minerals from the alkali feldspar group were identified as syngenetic inclusions within the blue sapphire. Monazite-(Ce), sub-micron grains of uraninite, muscovite and diaspore, and exsolved micron-sized needles of ilmenite were also found. The chemical composition of the sapphires fits well with those of magmatic sapphires from the literature with 10,000Ga/Al above 2.7, Ga/Mg > 29, Fe/Mg > 429, Cr/Ga 19. Solid inclusions within the sapphires of the Ilmen syenite pegmatites, as well as their chemistry, provide possible genetic linkages with sapphires from alluvial and eluvial deposits of alkali basaltic terrains in Houai Sai (northern Laos) and several mines in Chantaburi province (Thailand).

Petrogenesis of ultramafic rocks from the eastern Orhaneli ophiolite, NW Turkey: Hints on the initiation and evolution of melt-peridotite interaction processes within a heterogeneously depleted mantle section

The eastern Orhaneli ophiolite in NW Anatolia (Turkey) consists of voluminous dunite and minor harzburgite intruded by clinopyroxenite veins. Harzburgite contains spinel of low Cr# [100×Cr/(Cr+Al)=40–45] and diopside of low Al2O3 and TiO2 contents, whereas dunite contains spinel of higher Cr# (62–82) and diopside (blebs) (even more) depleted in Al2O3 and TiO2 (than harzburgite). The concentrations of Heavy Rare Earth Elements (HREE) in harzburgite are consistent with derivation of this type of peridotite from 19% dry melting of a fertile mantle protolith at a MOR regime. Dunites have lower concentrations of HREE than harzburgite implying that dunites were generated by higher degrees (>30%) of (cumulative) melting of the same protolith. Furthermore, the characteristic U-shapes of the chondrite-normalized REE-patterns of dunites indicate the involvement of hydrous melt-peridotite interaction processes in their genesis. Nevertheless, a set of mosaic-in-texture dunite samples contain olivine that has lower Fo# [100×Mg/(Mg+Fe2+)] and NiO contents than olivine in harzburgite. These dunites are probably of cumulate origin as it is also indicated by their enrichment in Pt and Pd (≤17.92ppb). Micro-textural and Re-Os isotopic data support that clinopyroxenite intrusions do not have a pure magmatic origin and their formation was partly controlled by metasomatic processes. Overall data indicate that the eastern Orhaneli ultramafic rocks have a complex petrological history including various stages of partial melting, metasomatism and magmatism in an evolving from MOR to SSZ geotectonic setting.

Carbonated sediment recycling and its contribution to lithospheric refertilization under the northern North China Craton

Subduction of carbonated sediments is the principal mechanism for the replenishment of mantle carbon reservoirs, but the processes of transport of carbonated sediments into the mantle and their incorporation into mantle chemical and isotopic heterogeneities are poorly constrained. Here we use in-situ major and trace element and Sr isotopic compositions and mineral Sr-Nd isotopes of the peridotite xenoliths (lherzolites with minor harzburgites) in the Oligocene Fanshi basalts from the northern North China Craton (NCC) to characterize and decode two metasomatic events caused by subduction of carbonated sediments. Clinopyroxenes (Cpx) in the harzburgites are characterized by high (La/Yb)N (8.4–66), Ca/Al (4.7–6.4) and Zr/Hf (30–66) ratios but low Ti/Eu ratios (478–1268) coupled with strongly enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) but depleted high field-strength elements (HFSE), suggesting that they have been metasomatized by carbonatitic melt. Highly enriched Sr-Nd isotopic compositions of these Cpx (87Sr/86Sr=0.70640–0.70716, 143Nd/144Nd=0.512304) imply that metasomatic carbonatitic melt was derived from carbonated sediments. Compositional zonation of Cpx in a harzburgite shows decreasing 87Sr/86Sr ratio, increasing Sr content and decreasing Zr/Hf ratio from cores to the rims, documenting a later stage of metasomatism. In situ Sr isotopes in Cpx from both harzburgites and lherzolites converge in diagrams of 87Sr/86Sr vs Sr, Zr/Hf, Ti/Eu and (La/Yb)N, indicating that these peridotites could have been extensively affected by the second metasomatic event. Cpx in the lherzolites display lower (La/Yb)N (0.16–10.6) and Zr/Hf ratios (25–39), suggesting that the later metasomatic agent is probably a carbonate-rich silicate melt and not carbonatite. Sr and Nd isotopes of these Cpx (87Sr/86Sr=0.702075–0.706148, 143Nd/144Nd=0.512410–0.513286) exhibit negative correlation along a simple mixing line between depleted mantle and carbonated sediments. The low Ce/Pb ratio (<12) of the metasomatic agent suggests that the carbonate-rich silicate melt also originated from carbonated sediments. The timing of the two metasomatic events and the regional tectonic setting are consistent with derivation of the carbonated sediments from the Paleo-Asian Oceanic slab: low degree melting formed early carbonatite melts, which caused decreasing carbonate content of carbonated sediments, whereas subsequent higher degree melting of residual carbonated sediments produced the later carbonate-rich silicate melt. Both contributed significantly to lithospheric refertilization under the northern NCC.

Mineralogy and metasomatic evolution of the Mianeh iron skarn deposit, Norduz-Agarak border, NW Iran

The Mianeh iron skarn deposit lies in the Arasbaran region within the Qaradagh metallogenic district, NW Iran. This high-grade massive magnetite skarn originated by the interaction of Upper Cretaceous limestone with metasomatic ore-bearing fluids associated with hypabyssal Oligo-Miocene quartz diorite. Mineral chemistry of the primary clinopyroxenes demonstrates the sub-alkaline, volcanic arc setting of magmatism. Two general stages of skarnification are recognized: (1) silicate skarn (stage I) is composed essentially of grossular and low-Fe diopside formed before the main mineralization and (2) magnetite-garnet skarn (stage II) composed of strongly anisotropic coarse-grained garnets with a narrow compositional zoning radially formed by addictive infiltrating of silica and iron-rich metasomatic fluids which overprint and/or crosscut the early stage silicate skarn. Anhydrous prograde calc-silicate assemblages were replaced by a series of hydrous calc-silicates (epidote, tremolite-actinolite) and/or quartz, calcite, magnetite, hematite, and pyrite. Magnetite (±hematite) is the dominant hypogene ore mineral that initially precipitated coincident with the late prograde to the early retrograde metasomatic stages. Mineralogical studies suggest that silicate skarn formation commenced at temperatures about 560 °C, X(CO2)fluid ≤ 0.15, αSiO2∼−1.0, and fluid pressure 1.0 kbar. The magnetite-garnet skarn formed from H2O-rich fluids [X(CO2)fluid < 0.1] at a temperature of 525 to 450 °C and maximum log ƒO2 between −20.2 and −23. During the late stages of prograde skarn development, the stability field of andradite shifted to low ƒO2 and ƒS2 conditions resulting in main iron ore deposition (as magnetite). The andradite replacement temperature and presence of pyrite (instead of pyrrhotite) suggest that logƒS2 remained constant at about −6 to −7 during cooling of the system.

Evolution of borate minerals from contact metamorphic to hydrothermal stages: Ludwigite-group minerals and szaibélyite from the Vysoká – Zlatno skarn, Slovakia

Borate minerals of the ludwigite group (LGM) and szaibelyite in association with hydroxylclinohumite, clinochlore, a serpentine mineral, magnesian magnetite, spinel, magnesite, dolomite and sulphide minerals, occur in a magnesian exoskarn in the R-20 borehole located in the Vysoka – Zlatno Cu-Au porphyry-skarn deposit, located within the Stiavnica Neogene stratovolcano, Western Carpathians, central Slovakia. The skarn is developed along the contact of Miocene granodiorite to quartz-diorite porphyry and a Middle-Upper Triassic dolomite-shale-psammite-anhydrite sedimentary sequence. The boron minerals were investigated by electron probe micro-analyser (EPMA) and micro-Raman techniques. The source of boron could have been from the granodiorite/quartz diorite intrusion; however some supply of B from adjacent evaporite-bearing sediments is also possible. Based on textural and compositional data, the minerals originated during two stages. (1) An early high-temperature, contact-metamorphic and metasomatic stage comprises coarse-crystalline aggregate of LGM (types 1 to 3) in association with hydroxylclinohumite, magnetite, and rarely spinel inclusions in LGM. Compositional variations of LGM show a crystallization sequence from early azoproite [≤17 wt% TiO2; ~0.40 atoms pre formula unit (apfu) Ti, which correspond to ≤79 mol% of the Mg2(Mg0.5Ti0.5)O2(BO3) end-member], Ti-Al-rich members of LGM, “aluminoludwigite “[≤14 wt% Al2O3; ≤0.53 apfu, ≤53 mol% of Mg2AlO2(BO3) end-member] and Al-rich ludwigite in the central zone of crystals, to Ti-Al-poor ludwigite in outer parts of crystals. (2) Minerals of the late retrograde serpentinization and hydrothermal stage form irregular veinlets and aggregates, including partial alteration of hydroxylclinohumite to the serpentine-group mineral and clinochlore, replacement of LGM by szaibelyite, formation of the latest generation of Fe-rich, Ti-Al poor ludwigite in veinlets (type 4), and precipitation of dolomite, magnesite and sulphide minerals (valleriite, sphalerite, chalcopyrite). The distinct compositional zoning of the LGM documents a complex evolution of the skarn beginning with a high-temperature stage 1 and ending with a low-temperature overprint, stage 2.

Textural and Chemical Constraints on the Formation of Disseminated Granite-hosted W-Ta-Nb Mineralization at the Dajishan Deposit, Nanling Range, Southeastern China

The Dajishan deposit, located in the Nanling Range of southeastern China, is the largest wolframite deposit in the world. Approximately two-thirds of the tungsten resource is contained in large quartz-wolframite-K-feldspar-muscovite-scheelite-sulfide-(beryl) veins, but the remainder occurs as wolframite and lesser scheelite, disseminated through the Nb-Ta mineralized No. 69 granite, which is characterized by abundant snowball quartz crystals. Disseminated granite-hosted wolframite mostly occurs as a replacement after lath-shaped muscovite, although an interstitial variety is present in some samples. The major element chemistry of the disseminated and vein-hosted wolframite is similar, however, the former has one to two orders of magnitude higher concentrations of high field strength elements (HFSE), including Nb and Ta. Disseminated muscovite in the No. 69 granite is Fe and Mg rich. Replacement rims on these muscovite crystals are enriched in Ta, Sn, and Cs, but more depleted in Nb, related to the original phase. Vein-hosted muscovite is characterized by lower Nb, Ta, Rb, and Cs contents than the granite-hosted muscovite, although Sn and W concentrations are comparable. Columbite-(Mn) is the principal Nb-Ta ore mineral and occurs disseminated through the No. 69 granite. Where it is hosted by snowball quartz crystals, some of the columbite has been altered and overgrown by wodginite. The restriction of these altered columbite crystals to within snowball quartz, along with the Ta and Sn enrichment of the alteration rims on muscovite, tie the formation of snowball quartz to a Ta-(Sn) metasomatic event. High Ge/Ti and Al/Ti ratios in the snowball quartz relative to interstitial quartz are also consistent with a late stage of formation for the former variety. The disseminated, granite-hosted W mineralization formed through fluid-rock interaction. The fluids that precipitated the disseminated wolframite were most likely those that formed the veins, but were modified through reaction with the HFSE-enriched No. 69 granite. Nb-Ta mineralization, however, is represented by magmatic columbite and by a later metasomatic stage of wodginite. The low solubility of Ta in aqueous fluids, combined with the evidence for Ta-(Sn) metasomatism, suggests that the Ta-(Sn) metasomatic event was caused by hydrosilicate liquids, a transitional fluid that lies compositionally between silicate magmas and hydrothermal fluids. Whole-rock and mineral chemistry both suggest that deep-level fractional crystallization of the No. 69 granite was required to elevate Ta and Nb concentrations to allow saturation of columbite.

Paleozoic tholeiitic magmatism of the Kola province: Spatial distribution, age, and relation to alkaline magmatism

This paper focuses on the occurrences of tholeiitic magmatism in the northeastern Fennoscandian shield. It was found that numerous dolerite dikes of the Pechenga, Barents Sea, and Eastern Kola swarms were formed 380–390 Ma ago, i.e., directly before the main stage of the Paleozoic alkaline magmatism of the Kola province. The isotope geochemical characteristics of the dolerites suggest that their primary melts were derived from the mantle under the conditions of the spinel lherzolite facies. The depleted mantle material from which the tholeiites were derived shows no evidence for metasomatism and enrichment in high fieldstrength and rare earth elements, whereas melanephelinite melts postdating the tholeiites were generated in an enriched source. It was shown that the relatively short stage of mantle metasomatism directly after the emplacement of tholeiitic magmas was accompanied by significant mantle fertilization. In contrast to other large igneous provinces, where pulsed intrusion of large volumes of tholeiitic magmas coinciding or alternating with phases of alkaline magmatism was documented, the Kola province is characterized by systematic evolution of the Paleozoic plume–lithosphere process with monotonous deepening of the level of magma generation, development of mantle metasomatism and accompanying fertilization of mantle materials, and systematic changes in the composition of melts reaching the surface.

Trace element and Sr isotope records of multi‐episode carbonatite metasomatism on the eastern margin of the North China Craton

AbstractLherzolite xenoliths entrained in the Changle Cenozoic basalts were analyzed to infer mantle process beneath the eastern block of the North China Craton. These xenoliths were classified into two types. Clinopyroxenes (Cpx) in the type‐1 xenoliths are strongly enriched in large ion lithophile elements and light rare earth elements (LREE) but depleted in high field‐strength elements and heavy rare earth elements, and show high Ca/Al, Zr/Hf, and (La/Yb)N ratios but low Ti/Eu ratios. These features indicate that they were crystallized from a carbonatitic melt. Cpx in the type‐2 xenoliths are mostly characterized by a chemical zonation, that is, LREE and Sr contents and (La/Yb)N and Eu/Ti ratios gradually increase from the cores to the rims. Some fresh cores preserve the original signatures of the depleted mantle. These observations indicate partial modification of pre‐existing Cpx by carbonatite metasomatism. Two episodes of metasomatism were identified based on Sr isotopic compositions of Cpx and carbonate inclusions within olivines. Both the carbonate inclusions and Cpx cores in the type‐2 xenoliths have relatively high 87Sr/86Sr ratios (&gt;0.7033), suggesting metasomatism due to CO2‐rich silicate melt derived from the recycled oceanic crust. However, low 87Sr/86Sr ratios of Cpx rims in the type‐2 xenoliths suggest a late stage of metasomatism by a low‐87Sr/86Sr carbonatitic melt (&lt;0.7024), which might have been caused by upwelling of the asthenosphere mantle. Such multiple metasomatism could have played an important role in changing the chemical composition of mantle from refractory and depleted to fertile and enriched.

Metasomatic origin of garnet xenocrysts from the V. Grib kimberlite pipe, Arkhangelsk region, NW Russia

This paper presents new major and trace element data from 150 garnet xenocrysts from the V. Grib kimberlite pipe located in the central part of the Arkhangelsk diamondiferous province (ADP). Based on the concentrations of Cr2O3, CaO, TiO2 and rare earth elements (REE) the garnets were divided into seven groups: (1) lherzolitic “depleted” garnets (“Lz 1”), (2) lherzolitic garnets with normal REE patterns (“Lz 2”), (3) lherzolitic garnets with weakly sinusoidal REE patterns (“Lz 3”), (4) lherzolitic garnets with strongly sinusoidal REE patterns (“Lz 4”), (5) harzburgitic garnets with sinusoidal REE patterns (“Hz”), (6) wehrlitic garnets with weakly sinusoidal REE patterns (“W”), (7) garnets of megacryst paragenesis with normal REE patterns (“Meg”). Detailed mineralogical and geochemical garnet studies and modeling results suggest several stages of mantle metasomatism influenced by carbonatite and silicate melts. Carbonatitic metasomatism at the first stage resulted in refertilization of the lithospheric mantle, which is evidenced by a nearly vertical CaO-Cr2O3 trend from harzburgitic (“Hz”) to lherzolitic (“Lz 4”) garnet composition. Harzburgitic garnets (“Hz”) have probably been formed by interactions between carbonatite melts and exsolved garnets in high-degree melt extraction residues. At the second stage of metasomatism, garnets with weakly sinusoidal REE patterns (“Lz 3”, “W”) were affected by a silicate melt possessing a REE composition similar to that of ADP alkaline mica-poor picrites. At the last stage, the garnets interacted with basaltic melts, which resulted in the decrease CaO-Cr2O3 trend of “Lz 2” garnet composition. Cr-poor garnets of megacryst paragenesis (“Meg”) could crystallize directly from the silicate melt which has a REE composition close to that of ADP alkaline mica-poor picrites. P-T estimates of the garnet xenocrysts indicate that the interval of ∼60–110 km of the lithospheric mantle beneath the V. Grib pipe was predominantly affected by the silicate melts, whereas the lithospheric mantle deeper than 150 km was influenced by the carbonatite melts.