Physicochemical conditions of crystallization of dunites of the Nizhnii Tagil Pt-bearing massif (Middle Urals)

Studies of primary multiphase silicate inclusions in accessory Cr-spinels from the fine-grained dunites of the Nizhnii Tagil Pt-bearing massif reveal their similarity to melt inclusions trapped by chromite during its growth. The analyzed Cr-spinels with multiphase silicate inclusions differ in composition from ore chromites of the same massif and from chromites (with melt inclusions) from ultramafic oceanic complexes but are similar to Cr-spinels in dunites from Pt-bearing alkaline ultramafic massifs (Konder and Inagli). According to petro- and geochemical data on heated multiphase silicate inclusions, the studied Cr-spinels crystallized with the participation of subalkalic picrobasaltic melts similar to the magmas of the Konder Pt-bearing massif and having almost the same chemical composition as tylaites. The differences between the compositions of olivines formed within the multiphase silicate inclusions and of the rock-forming minerals show that the studied Cr-spinels formed from an intercumulus liquid melt in the olivine crystal interstices during the cumulate crystallization of most of the Nizhnii Tagil massif dunites in the intrusive chamber. Numerical modeling based on the compositions of heated multiphase silicate inclusions in accessory Cr-spinels demonstrates that olivines and Cr-spinels from the studied dunites crystallized at 1430 to 1310 °C and then olivine formation continued to 1280 °C during the evolution of melts.     

Dunites of the Inagli massif (Central Aldan), cumulates of lamproitic magma

We consider a hypothesis for the origin of PGE-bearing ultramafic rocks of the Inagli massif (Central Aldan) through fractional crystallization from ultrabasic high-potassium magma. We studied dunites and wehrlites of the Inagli massif and olivine lamproites of the Ryabinovy massif, which is also included into the Central Aldan high-potassium magmatic area. The research is focused on the chemistry of Cr-spinels and the phase composition of Cr-spinel-hosted crystallized melt inclusions and their daughter phases. Mainly two methods were used: SEM-EDS (Tescan Mira-3), to establish different phases and their relationships, and EPMA, to obtain precise chemical data on small (2-100 μm) phases. The obtained results show similarity in chromite composition and its evolutionary trends for the Inagli massif ultramafites and Ryabinovy massif lamproites. The same has been established for phlogopite and diopside from crystallized melt inclusions from the rocks of both objects. Based on the results of the study, the conclusion is drawn that the ultramafic core of the Inagli massif resulted from fractional crystallization of high-potassium melt with corresponding in composition to low-titanium lamproite. This conclusion is consistent with the previous hypotheses suggesting an ultrabasic high-potassium composition of primary melt for the Inagli ultramafites.     

Cr-spinel assemblage from the Upper Triassic gritstones of the northeastern Siberian Platform

An assemblage of Cr-spinels widespread in Carnian (Upper Triassic) diamondiferous deposits in the northeastern Siberian Platform is studied. Analysis of their morphology and chemical composition has revealed two dominant varieties of Cr-spinels and has demonstrated certain regularities in their distribution in the study area. Correlations have been established between the areal distribution of the recognized types of Cr-spinels and diamond varieties typical of kimberlite sources and between the distribution of Cr-spinels and rounded diamond dodecahedrons. The phase and chemical compositions of polyphase inclusions in the Cr-spinels are studied. The spatial arrangement of inclusions along the crystal growth zones indicates their primary genesis and trapping from the melt during crystallization. Compositional features of some minerals in the inclusions—SiO₂ impurity in apatite and high CaO contents (0.2-0.8 wt.%) in olivines—point to a nonkimberlite source of these Cr-spinels. The presence of K- and Na-containing phases and calcite in the inclusions indicates saturation of the initial melt with alkalies, Ca, and CO₂. The data obtained suggest that the numerous Late Vendian diatremes in K-rich alkaline basites of the Olenek Uplift area are the source of the dominant Cr-spinel variety.     

Genesis of melilitolite from Colle Fabbri: inferences from melt inclusions

Melilite and wollastonite from the Colle Fabbri stock contain silicate melt and silicate-carbonate inclusions. The homogenization temperatures of silicate inclusions are within the magmatic temperature range of mantle ultrabasic melts: about 1,320?±?15 °С. Their composition is melilititic and evolves to the composition of leucite tephrite and phonolite. The composition of silicate-carbonate inclusions are high SiO₂, Ca-rich, enriched in alkalies and are similar to that of inclusions of carbonatite melts in the minerals of melilitolites of other intrusive ultramafic complexes. They are also similar to the compositions of metasomatized travertine covering the melilitolite stock. The presence of primary silicate and silicate-carbonate inclusions evidences that the melilitite magma from which melilitolites of Colle Fabbri crystallized was associated with carbonatite liquid. This liquid was highly fluidized, mobile and aggressive. Actively interacting with overlying travertine, the liquid enriched them with alkalies, aluminosilicates and incompatible elements, which resulted in the equalization of their compositions. Heterogeneous compositional dominions were formed at the contact between melilitolite and wall pelites. In the minerals of these contact facies high-Si melt inclusions of varying composition have been observed. Their occurrence is related to the local assimilation by the high-temperature melilitite magma of pelitic country rocks. The content of incompatible elements in melilitite melts and melilitolites is higher than the mantle norm and they have peculiar indicator ratios, spectra, Eu/Eu^* ratio, which suggest a peculiar mantle source.     

Petrogenesis of the Fe-Ti intrusive complexes in the Sierra Leone region, Central Atlantic

The study of melt inclusions in Cr-spinels from melanocratic troctolites provided the first direct information on the physicochemical parameters of enriched magmatic systems that produced high-Fe and high-Ti intrusive complexes in the Sierra-Leone region (Central Atlantic, 6°N). These complexes are made up of predominating hornblende Fe-Ti oxide gabbronorites and gabbrodiorites with subordinate amount of ultramafics, diorites, quartz diorites, and trondhjemites. The study of melt inclusions and rocks showed that the majority of gabbroids of the Central Atlantic (Sierra Leone area and 15°20′ Fracture Zone) were derived from N-MORB-type melts, whereas differentiated Fe-Ti-oxide rocks were crystallized from other melts, which were preserved as inclusions in the Cr-spinels from the melanocratic troctolites of the Sierra Leone region. The ion-microprobe study of these inclusions yield direct evidence on the elevated water content (up to 1.24–1.77 wt %) in the parental melts of Fe-Ti oxide rocks. Data on trace and rare-earth element distribution together with high (La/Sm)_N and (Ce/Yb)_N ratios in the inclusions indicate the possible influence of deep plume source on the generation of these magmas. Simulation based on melt inclusion data testifies that high-Fe intrusions of the Sierra Leone area were crystallized from the water-saturated magmas at relatively low temperatures (1020–1240°C). It was shown that the geochemically enriched Fe-Ti melts were presumably formed regardless of N-MORB-type magmatism predominant in Central Atlantic, under the influence of new mantle plume that caused melting of hydrated oceanic lithosphere.     

Petrographic-geochemical types of Triassic alkaline ultramafic rocks in the Northern Anabar province,Yakutia, Russia

A classification suggested for alkaline ultramafic rocks of the Ary-Mastakh and Staraya Rechka fields, Northern Anabar Shield, is based on the modal mineralogical composition of the rocks and the chemical compositions of their rock-forming and accessory minerals. Within the framework of this classification, the rocks are indentified as orangeite and alkaline ultramafic lamprophyres: aillikite and damtjernite. To estimate how much contamination with the host rocks has modified their composition when the diatremes were formed, the pyroclastic rocks were studied that abound in xenogenic material (which is rich in SiO₂, Al₂O₃, K₂O, Rb, Pb, and occasionally also Ba) at relatively low (La/Yb)_PM, (La/Sm)_PM, and not as much also (Sm/Zr)_PM and (La/Nb)_PM ratios. The isotopic composition of the rocks suggests that the very first melt portions were of asthenospheric nature. The distribution of trace elements and REE indicates that one of the leading factors that controlled the diversity of the mineralogical composition of the rocks and the broad variations in their isotopic–geochemical and geochemical characteristics was asthenosphere–lithosphere interaction when the melts of the alkaline ultramafic rocks were derived. The melting processes involved metasomatic vein-hosted assemblages of carbonate and potassic hydrous composition (of the MARID type). The alkaline ultramafic rocks whose geochemistry reflects the contributions of enriched vein assemblages to the lithospheric source material, occur in the northern Anabar Shield closer to the boundary between the Khapchan and Daldyn terranes. The evolution of the aillikite melts during their ascent through the lithospheric mantle could give rise to damtjernite generation and was associated with the separation of a C–H–O fluid phase. Our data allowed us to distinguish the evolutionary episodes of the magma-generating zone during the origin of the Triassic alkaline ultramafic rocks in the northern Anabar Shield.     

Subduction of lithospheric upper mantle recorded by solid phase inclusions and compositional zoning in garnet: Example from the Bohemian Massif

This paper presents monomineral and multiphase inclusions in garnet from eclogites and clinopyroxenites, which form layers and boudins in garnet peridotites from two areas in the Moldanubian zone of the Bohemian Massif. The garnet peridotites occur in felsic granulites and reached UHP conditions prior to their granulite facies overprint. In addition to complex compositional zoning, garnets from hosting eclogites and clinopyroxenites preserve inclusions of hydrous phases and alkali silicate minerals including: amphiboles, chlorites, micas and feldspars. Amphibole, biotite and apatite inclusions in garnet have a high concentration of halogens; CO₂ and sulfur are involved in carbonates and sulfide inclusions, respectively. The inclusion patterns and compositional zoning in garnet in combination with textural relations among minerals, suggest that the ultramafic and mafic bodies are derived from lithospheric mantle above the subduction zone and were transformed into garnet pyroxenites and eclogites in the subduction zone. Based on compositional, mineral and textural relations, all of these rocks along with the surrounding crustal material were overprinted by granulite facies metamorphism during their exhumation.     

Genesis of the Khaluta alkaline-basic Ba-Sr carbonatite complex (West Transbaikala, Russia)

The Khaluta carbonatite complex comprizes fenites, alkaline syenites and shonkinites, and calcite and dolomite carbonatites. Textural and compositional criteria, melt inclusions, geochemical and isotopic data, and comparisons with relevant experimental systems show that the complex formed by liquid immiscibility of a carbonate-saturated parental silicate melt. Mineral and stable isotope geothermometers and melt inclusion measurements for the silicate rocks and carbonatite all give temperatures of crystallization of 915–1,000°C and 890–470°C, respectively. Melt inclusions containing sulphate minerals, and sulphate-rich minerals, most notably apatite and monazite, occur in all of the lithologies in the Khaluta complex. All lithologies, from fenites through shonkinites and syenites to calcite and dolomite carbonatites, and to hydrothermal mineralisation are further characterized by high Ba and Sr activity, as well as that of SO3 with formation of the sulphate minerals baryte, celestine and baryte-celestine. Thus, the characteristic features of the Khaluta parental melt were elevated concentrations of SO₃, Ba and Sr. In addition to the presence of SO₃, calculated fO₂ for magnetites indicate a high oxygen fugacity and that Fe⁺³>Fe⁺² in the Khaluta parental melt. Our findings suggest that the mantle source for Khaluta carbonatite and associated rocks, as well as for other carbonatites of the West Transbaikalia carbonatite province, were SO₃-rich and characterized by high oxygen fugacity.     

Lamprophyres of the Tomtor Massif: A result of mixing between potassic and sodic alkaline mafic magmas

The paper presents data on inclusions in minerals of the least modified potassic lamprophyres in a series of strongly carbonatized potassic alkaline ultramafic porphyritic rocks. The rocks consist of diopside, kaersutite, analcime, apatite, and rare phlogopite and titanite phenocrysts and a groundmass, which is made up, along with these minerals, of potassic feldspar and calcite. The diopside and kaersutite phenocrysts display unsystematic multiple zoning. Chemically and mineralogically, the rock is ultramafic foidite and most likely corresponds to monchiquite. Primary and secondary melt inclusions were found in diopside, kaersutite, apatite, and titanite phenocrysts and are classified into three types: sodic silicate inclusions with analcime, potassic silicate inclusions with potassic feldspar, and carbonate inclusions, which are dominated by calcite. Heating and homogenization of the inclusions show that the potassic lamprophyres crystallized from a heterogeneous magma, with consisted of mixing mafic sodic and potassic alkaline magmas enriched in a carbonatite component. The composition of the magmas was close to nepheline and leucite melanephelinite. The minerals crystallized at 1150–1090°C from the sodic melts and at 1200–1250°C from the potassic ones. The sodic mafic melts were richer in Fe than the potassic ones, were the richest in Al, Mn, SO₃, Cl, and H₂O and poorer in Ti and P. The potassic mafic melts were not lamproitic, as follows from the presence of albite in the crystallized primary potassic melt inclusions. The diopside, the first mineral to crystallize in the rock, started to crystallize in the magmatic chamber from sodic mafic melt and ended to crystallize from mixed sodic–potassic melts. The potassic mafic melts were multiply replenished in the chamber in relation to tectonic motions. The ascent of the melts to the surface and rapidly varying P–T parameters of the magma were favorable for multiple separations of carbonatite melts from the alkaline mafic ones and their mixing and mingling.     

The Olivinite of the Krestovskaya Intrusion—the Product of Larnite-Normative Alkali Ultramafic Magma: Melt Inclusion Data

Olivinites of the Krestovskaya Intrusion consist of predominant amount of olivine, and minor Ti-magnetite, perovskite, and clinopyroxene (from single grain to a few vol %). Primary crystallized melt inclusions were found and studied in olivine, perovskite, and diopside of the olivinites. Daughter phases in olivine-hosted melt inclusions are monticellite, perovskite, kalsilite, phlogopite, magnetite, apatite, and garnet andradite. Perovskite-hosted melt inclusions contain such daughter phases as kalsilite, pectolite, clinopyroxene, biotite, magnetite, and apatite, while daughter phases in clinopyroxene-hosted melt inclusions are represented by kalsilite, phlogopite, magnetite, and apatite. According to melt inclusion heating experiments, olivine crystallized from above 1230°C to 1180°C. It was followed by perovskite crystallizing at ≥1200°C and clinopyroxene, at 1170°C. According to analysis of quenched glass of the melt inclusions, the chemical composition of melts hosted in the minerals corresponds to the larnite-normative alkali ultramafic (kamafugite) magma significantly enriched in incompatible elements. The high incompatible element concentrations, its distribution, and geochemical indicator ratios evidenced that the magma was derived by the partial melting of garnet-bearing undepleted mantle.     

Formation conditions of allivalites, olivine-anorthite crystal enclaves, in the volcanics of the Kuril-Kamchatka arc

In this paper we address allivalites, coarse-and giant-textured olivine-anorthite rocks occurring as separate blocks in the eruption products of many volcanoes from the frontal part of the Kuril-Kamchatka arc. New data are reported on the petrography, mineralogy, and composition of melt inclusions in minerals from ten allivalite samples from Ksudach, Il’inskii, Zavaritskii, Kudryavyi, and Golovnin volcanoes. The crystallization temperatures of allivalite minerals were estimated as 970–1080°C at a melt water content of 3.0–3.5 wt % and oxygen fugacity NNO = 1–2. A genetic link was established between the compositions of melt inclusions and interstitial glasses in allivalites and volcano rocks. The cumulate nature of allivalites was demonstrated. Using mass balance calculations, the degree of fractionation of primary melts during the formation of cumulate layers was estimated for various volcanoes as 22–46%.     

Physicochemical Conditions of Crystallization of Rocks from Ultrabasic Massifs of the Siberian Platform

A great volume of original information on the formation of the ultrabasic rocks of the Siberian Platform has been accumulated owing to the study of melt inclusions in Cr-spinels. The inclusions show the general tendencies in the behavior of the magmatic systems during the formation of the ultrabasic massifs of the Siberian Platform, tracing the main evolution trend of decreasing Mg number with SiO₂ increase in the melts with subsequent transition from picrites through picrobasalts to basalts. The compositions of the melt inclusions indicate that the crystallization conditions of the rocks of the concentrically zoned massifs (Konder, Inagli, Chad) sharply differ from those of the Guli massif. Numerical modeling using the PETROLOG and PLUTON softwares and data on the composition of inclusions in Cr-spinels yielded maximum crystallization temperatures of the olivines from the dunites of the Konder (1545–1430°C), Inagli (1530–1430°C), Chad (1460–1420°C), and Guli (1520–1420°C) massifs, and those of Cr-spinels from the Konder (1420–1380°C), Inagli (up to 1430°C), Chad (1430–1330°C), and Guli (1410–1370°C) massifs. Modeling of the Guli massif with the PLUTON software using the compositions of the melt inclusions revealed the possible formation of the alkaline rocks at the final reverse stage of the evolution of the picritic magmas (with decrease of SiO₂ and alkali accumulation) after termination of olivine crystallization with temperature decrease from 1240–1230°C to 1200–1090°C. Modeling with the PLUTON software showed that the dunites of the Guli massif coexisted with Fe-rich (with moderate TiO₂ contents) melts, the crystallization of which led (beginning from 1210°C) to the formation of pyroxenes between cumulate olivine. Further temperature decrease (from 1125°C) with decreasing FeO and TiO₂ contents provided the formation of clinopyroxenes of pyroxenites. For the Konder massif, modeling with the PLUTON software indicates the possible formation of kosvites from picrobasaltic magmas beginning from 1350°C and the formation of clinopyroxenites and olivine–diopside rocks from olivine basaltic melts from 1250°C.     

First Finds of Platinum and Palladium Minerals in Sulfide Ores of the Khudolaz Intrusive Complex (Southern Urals)

Forms of occurrence of platinum (sperrilite, moncheite) and palladium (Sb-michenerite, Pd–Bi phase) minerals in intrusive rocks of the Khudolaz differentiated complex have been studied. Platinum minerals were identified in disseminated Cu–Ni sulfide ores from ultramafic olivine–hornblende rocks of the Khudolaz complex, whereas palladium minerals were found in ores from olivine–hornblende gabbroids. The structural arrangement of grains as inclusions in sulfides of the primary magmatic association testifies that they were formed as a result of segregation of platinum group elements, which partitioned into the composition of sulfides during low-temperature mineral formation process at the late-magmatic stage.     

The types and genesis of ophicalcites in Lower Devonian olistostromes at cobalt-bearing massive sulfide deposits in the West Magnitogorsk paleoisland arc (South Urals)

Ophicalcites were earlier found in the Lower Devonian olistostromes overlapping cobalt-bearing massive sulfide deposits in the ultramafic rocks of the West Magnitogorsk paleoisland arc. They are composed of angular clastics of serpentinites and carbonates few millimeters to several centimeters in size, which are cemented with hematite-calcite and quartz-hematite-calcite matrix with aragonite, magnesite, and siderite admixtures. In chemical composition Cr-spinels from serpentinites of the ophicalcites are similar to those from the underlying serpentinites and are suprasubduction products of active continental margins. The ¹³C/¹²C and 18O/16O ratios of calcite from the breccia matrix are typical of hydrothermal deposits and are close to those of carbonate in sulfide ores and talc-carbonate metasomatites. Study of fluid inclusions from the calcite cement has shown that the ophicalcites formed from low- to moderate-temperature (100–280 °C) hydrothermal fluids as a result of postore hydrothermal emanations on ultramafic seafloor rocks similar to modern hydrothermal fields in MORs and island arcs. Hydrothermal and tectonosedimentation processes in the roof of ultramafic massifs at the vents of hydrothermal fluids led to erosion, redeposition, and cementation of ophicalcites of four types. The subsequent tectonic and gravitational processes resulted in their denudation and accumulation in olistostromes.     

Conditions of crystallization of olivine shonkinites in the Inagli massif (Central Aldan)

The olivine shonkinites localized among dunites and alkali gabbroids in the northern part of the alkaline ultrabasic Inagli massif (northwestern part of Central Aldan) have been studied. The obtained data on the chemical and trace-element compositions of the rocks and minerals and the results of melt inclusion study showed that the olivine shonkinites crystallized from alkaline basanite melt enriched in Cl, S, CO2, and trace elements. Clinopyroxene crystallized at 1180-1200 °C from a homogeneous silicate-salt melt, which was probably separated into immiscible silicate and carbonate-salt fractions with temperature decreasing. The composition of the silicate fraction evolved from alkaline basanite to alkaline trachyte. The carbonate-salt fraction had an alkaline carbonate composition and was enriched in S and Cl. The same trend of evolution of clinopyroxene-hosted melts and the igneous rocks of the Inagli massif suggests that the alkali gabbroids, melanocratic alkali syenites, and pulaskites formed from the same magma, which had a near-alkaline basanite composition during its crystallization differentiation. The geochemical studies showed that the olivine shonkinites and glasses of homogenized melt inclusions in clinopyroxene grains have similar contents of trace elements, one or two orders of magnitude higher than those in the primitive mantle. The high contents of LILE (K, Rb, and Sr) and LREE in the olivine shoshonites and homogenized inclusions suggest the enriched mantle source, and the negative anomalies of HFSE and Ti are a specific feature of igneous rocks formed with the participation of crustal material. The slight depletion in HREE relative to LREE and the high (La/Yb)_n ratios in the rocks and inclusion glasses (10.0-11.4 and 4.7-6.2, respectively) suggest the presence of garnet in the mantle source.     

The Ruiga intrusion: A typical example of a shallow-facies paleoproterozoic peridotite-gabbro-komatiite-basaltic association of the Vetreny Belt,Southeastern Fennoscandia

The Ruiga differentiated mafic-ultramafic intrusion in the northwestern part of the Vetreny Belt paleorift was described for the first time based on geological, petrological, geochronological, and geochemical data. The massif (20 km² in exposed area) is a typical example of shallow-facies peridotite-gabbro-komatiite-basalt associations and consists of three zones up to 810 m in total thickness (from bottom to top): melanogab-bronorite, peridotite, and gabbro. In spite of pervasive greenschist metamorphism, the rocks contain locally preserved primary minerals: olivine (Fo _75–86), bronzite, augite of variable composition, labradorite, and Cr-spinels. A mineral Sm-Nd isochron on olivine melanogabbronorite from the Ruiga Massif defines an age of 2.39 ± 0.05 Ga, while komatiitic basalts of the Vetreny Belt Formation were dated at 2.40–2.41 Ga (Puchtel et al., 1997). The rocks of the Ruiga intrusion and lava flows of Mt. Golets have similar major, rare-earth, and trace element composition, which suggests their derivation from a single deep-seated source. Their parent magma was presumably a high-Mg komatiitic basalt. In transitional crustal chambers, its composition was modified by olivine-controlled fractionation and crustal contamination, with the most contaminated first portions of the ejected melt. In terms of geology and geochemistry, the considered magmatic rocks of the Vetreny Belt are comparable with the Raglan Ni-PGE komatiite gabbro-peridotite complex in Canada (Naldrett, 2003).     

萨尔托海铬铁矿中的矿物包体研究

铬铁矿中矿物包体的研究对于探索铬铁矿的成因具有重要的指示意义。通过薄片扫描电镜背散射结合电子探针分析,在萨尔托海高铝铬铁矿中发现了大量矿物包体,包括:单斜辉石、斜方辉石、橄榄石、钠-金云母、钠-铬韭闪石、出溶的单斜辉石等,这些矿物呈单相或多相产出,在铬尖晶石结晶时被包裹。含水矿物钠-金云母,钠-铬韭闪石的大量出现指示含水环境,表明铬铁矿形成于含水的母岩浆中,可能形成于受俯冲流体/熔体影响的弧后扩张背景。结合之前在萨尔托海铬铁矿中发现的超高压矿物金刚石,认为萨尔托海铬铁矿的形成可能经历了深部地幔和浅部含水大洋岩石圈两个阶段。     

Alkaline gabbroids of the Chirii outcrop (North Minusa depression): mineralogy and melt evolution

We studied the chemical composition of rock-forming minerals in gabbroids from the Chirii outcrop and the evolutionary features of parental basic melt during the crystallization of these rocks. Results were compared with data for basanites from pipes of the North Minusa depression. The mineralogical composition and thermobarogeochemical data of the gabbroids were examined in detail, and chemical analyses of rock-forming minerals (clinopyroxene, plagioclase, amphibole, biotite, titanomagnetite, and apatite) were carried out. Based on the homogenization temperatures of primary melt inclusions, we established the minimum temperatures and sequence of mineral crystallization in the gabbroids: clinopyroxene (>1160 °C), plagioclase, magnetite → amphibole (>950 °C) → biotite. The rock crystallization proceeded at shallow depths. Thermometric data are confirmed by results of modeling of equilibrium gabbroid crystallization. The crystallization of parental basic melt was accompanied by the accumulation of SiO₂, Al₂O₃, alkalies, and Cl and depletion in femic components. The melt evolved to granodiorite and alkali-syenite compositions. Compared with basanites from pipes, the parental melt had a longer evolution. The geochemical features of the gabbroids indicate that they, like basanites, crystallized from intraplate alkali-basaltoid magmas. But in petrochemistry and mineralogy the Chirii gabbroids differ considerably from the pipe basanites.     

Evolution of the sublayer of the Sudbury Igneous Complex: geochemical, Sm–Nd isotopic and petrologic evidence

The mineralized sublayer at the base of the Sudbury Igneous Complex (SIC) consists of two variants, the noritic contact sublayer and radial and concentric quartz dioritic offset dykes. Both are characterized by the presence of significant quantities of Ni–Cu–PGE sulphides and by a prominent population of recrystallized diabasic-textured and melanocratic to ultramafic fragments. The two variants of the sublayer contain compositionally distinct inclusion populations and inclusion-bearing matrices. Contact sublayer and offset dykes hosted by north range granitoid footwall can be distinguished from those hosted by south range basaltic and metasedimentary footwall environments. The compositional variation in SIC rocks can be described in terms of contributions from exposed crustal rocks and differentiation of the resultant melt(s).

The basaltic inclusion population is characterized by hornfels recrystallization of the plagioclase, and is geochemically and isotopically identified with Huronian basalts which comprise the south range footwall, with (Ce/Yb)_N ratios of around 2.5 and _Nd¹⁸⁵⁰ between −2 to −5. The melanocratic inclusions in the sublayer are typically coarse-grained and undeformed, with incompatible element contents and radiogenic isotopic compositions intermediate between those of the basaltic inclusions and those of the melt sheet, which has (Ce/Yb)_N ratios of around 10 and _Nd¹⁸⁵⁰ around −9. Calculated crystallization models are consistent with derivation of the ultramafic inclusions by crystallization from a magma produced by mixing of molten basaltic footwall with basal melt sheet. It is proposed that the sublayer appeared as the marginal facies of a meteorite impact melt sheet as a result of footwall melting following the impact. This basal layer was progressively enriched in sulphides and mafic cumulates from above through differentiation during cooling. Offset dykes were emplaced and the magmas effectively removed from the system. Subsequently, continued evolution of the marginal facies produced the more mafic inclusions in the contact sublayer. No extracrustal (e.g., mantle) component is envisioned in this model to explain the silicate compositional distributions in the SIC, and mafic crustal rocks in the target zone are implicated as the metal source for the SIC deposits.     

Geochemical behaviours of chemical elements during subduction-zone metamorphism and geodynamic significance

ABSTRACT

Seafloor subduction and subduction-zone metamorphism (SZM) are understood to be the very cause of both subduction-zone magmatism and mantle compositional heterogeneity. In this article, we compile geochemical data for blueschist and eclogite facies rocks from global palaeo-subduction-zones in the literature, including those from the Chinese Western Tianshan ultrahigh pressure (UHP) metamorphic belt. We synthesize our up-to-date understanding on how chemical elements behave and their controls during subduction-zone metamorphism. Although the compositional heterogeneity of metamorphic minerals from subducted rocks has been recently reported, we emphasize that the mineral compositional heterogeneity is controlled by elemental availability during mineral growth, which is affected by the protolith composition, the inherited composition of precursor minerals, and the competition with neighbouring growing minerals. In addition, given the likely effects of varying protolith compositions and metamorphic conditions on elemental behaviours, we classify meta-mafic rocks from global palaeo-subduction-zones with varying metamorphic conditions into groups in terms of their protolith compositions (i.e. ocean island basalt (OIB)-like, enriched mid-ocean ridge basalt (MORB)-like, normal [N]-MORB-like), and discuss geochemical behaviours of chemical elements within these co-genetic groups rather than simply accepting the conclusions in the literature. We also discuss the geochemical consequences of SZM with implications for chemical geodynamics, and propose with emphasis that: (1) the traditionally accepted ‘fluid flux induced-melting’ model for arc magmatism requires revision; and (2) the residual subducted ocean crust cannot be the major source material for OIB, although it can contribute to the deep mantle compositional heterogeneity. We also highlight some important questions and problems that need further investigations, e.g. complex subduction-zone geochemical processes, different contributions of seafloor subduction and resultant subduction of continental materials, and the representativeness of studied HP–UHP metamorphic rocks.