Crystallization conditions of dunites in the Konder platiniferous alkaline-ultramafic massif of the southeastern Aldan Shield

The investigation of melt inclusions in Cr spinels yielded direct information on the physicochemical parameters of the magmatic processes responsible for the formation of the Konder platiniferous alkaline-ultrabasic massif (southeastern Aldan Shield). The comparative analysis of the composition of the chromites containing the inclusions revealed that the Cr spinels from the Konder dunites differ significantly from their counterparts in the ultrabasic complexes of ophiolites and the modern oceanic crust. In terms of their composition and REE distribution, the clinopyroxene microcrystals from the Konder chromites are significantly different from the pyroxenes in the basic-ultrabasic ophiolite complexes of associations and identical to the minerals from the Kytlym platiniferous massif (the Urals). With respect to the distribution of the major components, the high-magnesian inclusions are identical to alkaline biotite-pyroxene picrites, testifying to the active participation of ultrabasic (picritic) alkaline magmatic systems in the formation of the dunites in the Konder Massif. The results of the ion probe investigation of the inclusions indicate a high water content (up to 0.54 wt %) in the melts. The data on the distribution of the rare and rare-earth elements in the inclusions suggest that the dunites of the Konder Massif crystallized with the involvement of water-saturated magmas at minimal temperatures of about 1230°C. Such temperatures are consistent with the earlier estimates for the melt inclusions in the olivine of the Konder Massif.     

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.     

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.     

Meimechites, porphyritic alkaline picrites, and melanephelinites of Siberia: Conditions of crystallization, parental magmas, and sources

The investigation of rocks, minerals, and melt inclusions showed that porphyritic alkaline picrites and meimechites crystallized from different parental magmas. At a similar ultrabasic composition, the alkaline picrite melts were enriched in K₂O relative to Na₂O, and contained up to 0.12–0.13 wt % F and less Cr, Ni, and H₂O (only 0.01–0.16 wt % H₂O, versus 0.6–1.6 wt % in the meimechite melts) compared with the meimechite magmas. The crystallization of alkaline picrite melts occurred under stable conditions at relatively low temperatures without abrupt changes: olivine and clinopyroxene crystallized at 1340–1285 and 1230–1200°C, respectively, as compared with 1600–1450 and 1230–1200°C in the meimechites. The alkaline picrite melts evolved toward melanephelinite, nephelinite, tephrite, and trachydolerite; whereas the meimechite magmas gave rise to subalkaline picritic rocks. The partitioning of vanadium between olivine and melt suggests that the meimechite magma crystallized under more oxidizing conditions compared with the alkaline picrite melts: the K_DV values for the meimechite melts (0.011–0.016) were three times lower than those for the alkaline picrite melts (0.045–0.052). The parental magmas of the alkaline picrites and meimechites were enriched in trace elements relative to mantle levels by factors of tens to hundreds. The alkaline picrite magma showed lower LILE and LREE contents compared with the meimechite magma. The magmas had also different indicator ratios of incompatible elements, including those immobile in aqueous fluids. It was concluded that the meimechite and alkaline picrite melts were derived from different mantle sources. The former were generated at lower degrees of melting of an undepleted mantle source, and the meimechite melts were produced by high-degree melting of a probably lherzolite-harzburgite source.     

Refractory chromitites recovered from the Eretria mine,East Othris massif (Greece): Implications for metallogeny and deformation of chromitites within the lithospheric mantle portion of a forearc-type ophiolite

The chrome ores of the abandoned Eretria mine of the East Othris ophiolite occur within a pervasively serpentinized and sheared harzburgite body. They consist of massive chromitites with mylonitic fabric in imbricate shaped pods. Modal analyses of these ores average at about 90–95% chromian spinel (Cr-spinel) and 5–10% secondary silicates. Chromian spinel compositions vary in Cr# [Cr/(Cr + Al) × 100] and Mg# [Mg/(Mg + Fe²⁺) × 100] from 44 to 62 and from 59 to 81, respectively. Trace element (Ti, Ni, V, Mn, Zn, Sc, Co and Ga) contents in Cr-spinel do not show significant variations from grain cores to grain boundaries. However, Cr-spinel compositions show depletions in Ti, Zn and Sc when compared to the composition of accessory Cr-spinel from typical mid-ocean ridge basalts (MORB). Mineral inclusions hosted in Cr-spinel comprise a range of (hydrous and anhydrous) silicate and base metal (BM) minerals occasionally intergrown with phosphate minerals and rare intermetallic compounds. A number of these inclusions have Cr-spinel rims with higher Cr# (63–68) than those of the enclosing Cr-spinel grains.The absence of dunite sheaths around chromitites is interpreted as an artifact of dunite structural obliteration during prolonged ductile shearing within harzburgite. The microtextural characteristics of a number of inclusions in Cr-spinel imply that they were initially fully molten. Furthermore, primary hydrosilicate (amphibole, phlogopite) inclusions in Cr-spinel indicate that chromitites crystallized from a water-bearing melt. Chromian spinel rims around silicate inclusions probably represent early crystals generated from a primitive magma produced by melting of a depleted mantle source.Geochemical calculations demonstrate that the parental melts of chromitites had intermediate affinity between MORB and arc-related magmas. Our preferred hypothesis for the genesis of the Eretria chromitites is that they were formed from a melt originated within the hydrated mantle wedge beneath a nascent forearc basin during subduction initiation.     

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.     

Age and Sources of Dunite from the Konder Massif (Aldan Shield)

The Sm–Nd and Rb–Sr isotope characteristics were studied in clinopyroxenes (Cpx) of ultrabasic rocks (dunite, wehrlite, pyroxenite, and kosvite) from the Konder massif, which is a source of a unique placer platinum deposit. The chemical composition of the clinopyroxenes studied provides evidence for their crystallization from a single melt in the course of magmatic differentiation. The Sm–Nd isotope characteristics of Cpx in dunite from the Konder massif correspond to the regression with an age of 128 ± 40 Ma, which provides evidence for the same age of rocks of the “dunite core,” wehrlite, pyroxenite, kosvite, and alkaline rocks of the subsequent intrusive stage in the Konder massif. Variations in the Sr and Nd isotope characteristics in dunite, wehrlite, pyroxenite, and kosvite result from contamination of the picritic melt with rocks of the continental crust in the course of its cumulative evolution, which allows us to exclude the model of diapiric intrusion of mantle dunite.     

Petrogenesis of late cenozoic basaltic complexes in the southern Baikal and southern Khangai volcanic areas in central Asia: Evidence from melt inclusions

Data on melt inclusions in minerals provide direct information on the physicochemical petrogenetic parameters of Late Cenozoic basaltic complexes in the Southern Baikal and Southern Khangai Volcanic Areas (SBVA and SKVA, respectively) in Central Asia. Newly obtained data on inclusions in olivine reveal differences between the temperatures of the magmatic systems that produced basalts in SBVA and SKVA. The comparison of the experimentally determined homogenization temperatures and parameters calculated from data on the composition of glasses in the melt inclusions allowed us to realistically evaluate the temperatures of the petrogenetic processes that generated Late Cenozoic basaltic complexes in SBVA (1130–1160°C and 1175–1250°C) and SKVA (1145–1185°C, 1210–1270, and about 1300–1310°C). The analysis of fluid phases in the inclusions testifies that basaltic melts in SBVA were rich in carbon dioxide, which ensured elevated pressures (up to 5–6.6 kbar) during the crystallization of the minerals. Data on the composition of inclusions in the olivine highlight differences between the chemistries of magmatic systems in the two territories: elevated TiO₂, Al₂O₃, and CaO concentrations at relatively low FeO and MgO contents in the SBVA melts as compared to analogous concentrations in the SKVA basaltic magmas. The petrochemical and geochemical parameters of the primary melt inclusions and the composition of the olivine generally testify that deep plume magmatic processes were actively involved in the generation of basalts in both SBVA and SKVA. Data on melt inclusions in olivine and the composition of the clinopyroxene reveal similarities between the geochemistry, mineralogy, and crystallization parameters of Late Cenozoic basalts in both SBVA and SKVA and Cretaceous-Paleogene basalts in the Tien Shan and their certain differences from the plume-related systems of the OIB type. These data suggest that the geodynamic environment of the Cenozoic and Late Mesozoic intraplate plume magmatism in Central Asia were different from the geodynamic environment of typical long-lived mantle plumes like that at Hawaii.     

The origin of dunites and olivinites in the alkali-ultrabasic intrusive complexes of the Siberian Craton

The factual evidence and the evidence of experimental studies support a cumulative origin of dunites from alkali-ultrabasic complexes of the Siberian Craton (Guli, Kondyor, Inagli, and others). These rocks formed during deep igneous evolution of melts that are close to biotite-pyroxene alkali picrite (meimechite). Olivinites that sometimes compose completely or in part the particular massifs (Bor-Yuryakh, Odikhincha, Kugda, and others) formed, in turn, due to recrystallization of dunites under the effect of alkali melts and fluids. This insight into the origin of dunites and olivines from these intrusive complexes requires an objective assessment of their ore-bearing potential.     

Comendite Melts of the Early Mesozoic Bimodal Sant Association (Central Mongolia) and Mechanisms of Their Formation

Based on the investigation of melt inclusions using electron and ion microprobe analysis, we estimated the composition, evolution, and formation conditions of magmas producing the the comendites of the Sant bimodal volcanic association (Central Mongolia). The mechanisms of the formation of melts were determined. The primary melt and coexisting crystalline inclusions in quartz from three samples of comendites collected from different parts of the volcanic section were studied. Among the crystalline inclusions, sanidine, zircon, and the REE diortosilicate–chevkinite were identified. The phenocrysts of the comendites were determined to crystallize at temperatures of 880–960°C. The homogeneous glasses of melt inclusions have both trachydacite and rhyolite compositions. They are characterized by high concentrations of Zr, Nb, Rb, Y, Th and REE. Significant differences were determined in concentrations of Li and volatile component (H₂O and F) in the glasses: some of the melts are enriched in these components, whereas other are depleted in them.Analysis of the composition of the glasses of the homogenized melt inclusions in quartz of comendites from the Sant bimodal association allowed us to recognize magmatic processes responsible for formation of the comendite melts. The dominant role among them belongs to crystallization differentiation of the magma, accompanied by a process of liquid immiscibility with participation of fluoride melts.     

Magmatic Evolution of Changbaishan Tianchi Volcano,China–North Korea: Evidence from Mineral-Hosted Melt and Fluid Inclusions

Data on mineral-hosted melt, fluid, and crystalline inclusions were used to study the composition and evolution of melts that produced rocks of Changbaishan Tianchi volcano, China–North Korea, and estimate their crystallization parameters. The melts crystallized within broad ranges of temperature (1220–700°C) and pressure (3100–1000 bar), at a drastic change in the redox potential: Δ log \(f_{O_2}\) from NNO + 0.92 to +1.42 for the basalt melts, NNO –1.61 to –2.09 for the trachybasaltic andesite melts, NNO –2.63 to –1.89 for the comendite melts, and NNO –1.55 to –3.15 for the pantellerite melts. The paper reports estimates of the compositions of melts that produced the continuous rock series from trachybasalt to comendite and pantellerite. In terms of trace-element concentrations, all of the mafic melts are comparable with OIB magmas. The silicic melts are strongly enriched in trace elements and REE. The most strongly enriched melts contain concentrations of certain elements almost as high as in ores of these elements. The paper reports data on H2O concentrations in melts of different composition. It is demonstrated that the variations in the H₂O concentrations were controlled by magma degassing. Data are reported on the Sr and Nd composition of the rocks. The deviations in the Sr isotopic composition are proportional to the ⁸⁷Sr/⁸⁶Sr ratio and could be produced in a melt with a high enough ⁸⁷Sr/⁸⁶Sr ratio during a geologically fairly brief time period. The evolution of melts that produced rocks of the volcano was controlled by crystallization differentiation of the parental basalt magmas at insignificant involvement of melt mixing and liquid immiscibility of silicate and sulfide melts. The alkaline salic rocks were generated in shallow-sitting (13–3.5 km) magmatic chambers in which the melts underwent profound differentiation that gave rise to pantellerites and comendites strongly enriched in trace elements (Th, Nb, Ta, Zr, and REE). Data on the composition of the magmas and parameters of their derivation are used to develop a generalized petrologic–geodynamic model for the origin of Changbaishan Tianchi volcano.     

Silica-rich Melts in Quartz Xenoliths from Vulcano Island and their Bearing on Processes of Crustal Anatexis and Crust-Magma Interaction beneath the Aeolian Arc, Southern Italy

Quartz-rich xenoliths in lavas and pyroclastic rocks from VulcanoIsland, part of the Aeolian arc, Italy, contain silicic meltinclusions with high SiO₂ (73–80 wt %) and K₂O (3–6wt %) contents. Two types of inclusions can be distinguishedbased on their time of entrapment and incompatible trace element(ITE) concentrations. One type (late, ITE-enriched inclusions)has trace element characteristics that resemble those of themetamorphic rocks of the Calabro-Peloritano basement of theadjacent mainland. Other inclusions (early, ITE-depleted) havevariable Ba, Rb, Sr and Cs, and low Nb, Zr and rare earth element(REE) contents. Their REE patterns are unfractionated, witha marked positive Eu anomaly. Geochemical modelling suggeststhat the ITE-depleted inclusions cannot be derived from equilibriummelting of Calabro-Peloritano metamorphic rocks. ITE-enrichedinclusions can be modelled by large degrees (>80%) of meltingof basement gneisses and schists, leaving a quartz-rich residuerepresented by the quartz-rich xenoliths. Glass inclusions inquartz-rich xenoliths represent potential contaminants of Aeolianarc magmas. Interaction between calc-alkaline magmas and crustalanatectic melts with a composition similar to the analysed inclusionsmay generate significant enrichment in potassium in the magmas.However, ITE contents of the melt inclusions are comparablewith or lower than those of Vulcano calc-alkaline and potassicrocks. This precludes the possibility that potassic magmas inthe Aeolian arc may originate from calc-alkaline parents throughdifferent degrees of incorporation of crustal melts. KEY WORDS: melt inclusions; crustal anatexis; magma assimilation; xenoliths; Vulcano Island     

Physico-chemical parameters of Neoproterozoic and Early Cambrian plume magmatism in the Paleo-Asian ocean (data on melt inclusions)

The paper presents new data on physico-chemical parameters of the Neoproterozoic–Early Cambrian plume magmatism in the Paleo-Asian Ocean. The data on clinopyroxenes show the plume-related plateaubasalt magmatic systems of the Katun’ paleoseamounts, which interacted with mid-ocean ridge (MOR) magmas. The Kurai paleoseamount consists mainly of plateaubasalt systems, and the Agardag ophiolites represent products of OIB–type “hot-spot” within-plate magmatism. Our study of inclusions showed that the melts of the Katun’ and Kurai paleoseamounts crystallized at lower temperatures (1130–1190 °C) compared to the Agardag ophiolites (1210–1255 °C). The petrochemical analysis of the melt inclusions showed that the Katun’ and Kurai magmatic systems are different from the Mg- and Ti-richer melts of the Agardag ophiolites: the former are similar to the magmas of the Nauru Basin and Ontong Java Plateau (western Pacific), whereas the latter possess geochemical affinities to OIB-type magmatism. The rare-element compositions of the melts of the Katun’ and Kurai paleoseamounts correspond to those of the Ontong Java Plateau and Nauru Basin lavas. The numerically simulated parameters of the Katun’ paleoseamount primary magmas agree with the data on the magmatic systems of the Siberian Platform and Ontong Java Plateau. For the Kurai paleoseamount, the simulated results suggest interaction of deep-seated OIB-type magmatic systems with MOR ones. The Agardag ophiolites were formed in relation to mantle plume activity at the initial stages of paleo-oceanic complexes formation.     

REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite, Southern Tibet

The Luobusa ophiolite, Southern Tibet, lies in the Indus–YarlungZangbo suture zone that separates Eurasia to the north fromthe Indian continent to the south. The ophiolite contains awell-preserved mantle sequence consisting of harzburgite, clinopyroxene(cpx)-bearing harzburgite and dunite. The harzburgite containsabundant pods of chromitite, most of which have dunite envelopes,and the cpx-bearing harzburgites host numerous dunite dykes.Dunite also exists as a massive unit similar to those of themantle–crust transition zones in other ophiolites. Allof the dunites in the ophiolite have a similar mineralogy, comprisingmainly olivine with minor orthopyroxene and chromite and tracesof clinopyroxene. They also display similar chemical compositions,including U-shaped chondrite-normalized REE patterns. Mantle-normalizedPGE patterns show variable negative Pt anomalies. Detailed analysisof a chromite-bearing dunite dyke, which grades into the hostcpx-bearing harzburgite, indicates that LREE and Ir decrease,whereas HREE, Pd and Pt increase away from the dunite. Thesefeatures are consistent with formation of the dunite dykes byinteraction of MORB peridotites with boninitic melts from whichthe chromitites were formed. Because the transition-zone dunitesare mineralogically and chemically identical to those formedby such melt–rock reaction, we infer that they are ofsimilar origin. The Luobusa ultramafic rocks originally formedas MORB-source upper mantle, which was subsequently trappedas part of a mantle wedge above a subduction zone. Hydrous meltsgenerated under the influence of the subducted slab at depthmigrated upward and reacted with the cpx-bearing harzburgitesto form the dunite dykes. The modified melts ponded in smallpockets higher in the section, where they produced podiformchromitites with dunite envelopes. At the top of the mantlesection, pervasive reaction between melts and harzburgite producedthe transition-zone dunites. KEY WORDS: melt–rock interaction; REE; PGE; hydrous melt; mantle; ophiolite; Tibet     

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.     

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.     

Evidence of melt/rock interaction in the Cr-spinel bearing wehrlite rocks of Bangriposi,India: Implications for nature of the metasomatic agent

Cr-spinel bearing wehrlite rocks of Bangriposi are found within the multiply deformed metasedimentary rocks of Singhbhum Group belonging to North Singhbhum Mobile Belt of eastern India. Detailed mineralogical and geochemical studies reveal that these rocks have suffered a two-stage alteration involving a deeper level modal and cryptic metasomatism and a subsequent shallower depth pervasive hydrothermal alteration. Cryptic metasomatism is defined by elevated LREE contents of the wehrlite and its clinopyroxne grains. Metasomatism induced changes in the modal mineralogy of the rocks include the absence of primary orthopyroxene grains, presence of secondary diopside-phlogopite(now present as vermiculite) defining disequilibrium reaction textures and secondary orthopyroxene rims around serpentinized olivine. The mineralogical and geochemical changes due to the metasomatic event present a contrasting picture in regard to the metasomatic history of the rocks. Possible scenarios involving a single stage or multiple stage metasomatism events have been discussed while explaining the metasomatic reactions that took place. An attempt has been made to estimate the REE concentrations of the final equilibrating melt from REE contents of clinopyroxene grains of the wehrlite. The possibility of the LREE-enriched equilibrating melt of the wehrlite rocks(the deeper level metasomatic agent) being similar to residual melts from the E-MORB type parental melts of nearby gabbro suite has been ruled out by geochemical modeling. REE abundance patterns of several natural enriched melts have been compared with REE pattern of calculated LREE-enriched equilibrating melt of the wehrlite and most resemblance has been observed with calcic and potassic melts. It is therefore suggested that the Cr-spinel bearing wehrlite rocks of Bangriposi has been affected by a calcio-potassic melt in deeper level, prior to the shallow level serpentinization event.     

Trace element geochemistry of Amba Dongar carbonatite complex,India: Evidence for fractional crystallization and silicate-carbonate melt immiscibility

Carbonatites are believed to have crystallized either from mantle-derived primary carbonate magmas or from secondary melts derived from carbonated silicate magmas through liquid immiscibility or from residual melts of fractional crystallization of silicate magmas. Although the observed coexistence of carbonatites and alkaline silicate rocks in most complexes, their coeval emplacement in many, and overlapping initial⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios are supportive of their cogenesis; there have been few efforts to devise a quantitative method to identify the magmatic processes. In the present study we have made an attempt to accomplish this by modeling the trace element contents of carbonatites and coeval alkaline silicate rocks of Amba Dongar complex, India. Trace element data suggest that the carbonatites and alkaline silicate rocks of this complex are products of fractional crystallization of two separate parental melts. Using the available silicate melt-carbonate melt partition coefficients for various trace elements, and the observed data from carbonatites, we have tried to simulate trace element distribution pattern for the parental silicate melt. The results of the modeling not only support the hypothesis of silicate-carbonate melt immiscibility for the evolution of Amba Dongar but also establish a procedure to test the above hypothesis in such complexes.     

青藏高原羌塘南部晚古生代地幔柱？——来自基性—超基性岩的地球化学证据

羌塘南部地区大面积出露基性—超基性火成岩。基性岩墙和玄武岩沿龙木错-双湖板块缝合带的南侧呈近东西向分布,大地构造位置位于冈瓦纳大陆北缘,分布范围西起国境线,东到双湖地区,总长约800km,形成时代集中在晚石炭世—早二叠世（284～318Ma）,反映出岩浆的快速侵位-喷溢的过程。基性岩墙和玄武岩主要为碱性系列岩石,少数为亚碱性系列,Ti和Fe的含量均比较高,Mg#比较低,表明它们都是原始岩浆经不同程度分异结晶后的残余熔体。基性岩墙和玄武岩的稀土元素、微量元素、形成环境等均与板内玄武岩或典型的地幔柱成因玄武岩具有相似的特点。另外,苦橄质岩石多与基性岩墙和玄武岩一起产出,主要为单斜辉石橄榄岩和二辉橄榄岩,矿物成分贫挥发性组分,主量、稀土和微量元素的特点反映出其很可能代表了地幔柱原始岩浆结晶的产物而非岩浆捕获体,而且其源区很可能为石榴子石二辉橄榄岩。由此,初步认为羌塘南部基性岩墙和玄武岩为地幔柱的头部减压熔融的产物,而苦橄质岩石为地幔柱尾柱通道熔融的产物。     

西藏罗布莎蛇绿岩中不同产出的纯橄岩及成因探讨

罗布莎蛇绿岩中的纯橄岩有三种产出情况,除了与豆荚状铬铁矿伴生的薄壳状纯橄岩外,还有产在方辉橄榄岩底部被认为是堆晶岩的厚层状纯橄岩和方辉橄榄岩中的透镜状纯橄岩。厚层状纯橄岩约700～1000m厚,以橄榄石富镁(Fo93～95),单斜辉石低铝富镁(Al2O30.47%～0.85%,Mg#95～97),铬尖晶石高铬低镁(Cr#值平均77,Mg#平均51)为特征。该纯橄岩中的浸染状铬铁矿也是高铬低镁型,但Mg#值(平均59)高于厚层状纯橄岩的副矿物铬尖晶石。薄壳状纯橄岩与厚层状纯橄岩成分相近,其橄榄石Fo92～94,单斜辉石Al2O3<1%和Mg#95～97;铬尖晶石的Cr#值平均71,Mg#值平均52。与薄壳状纯橄岩伴生的块状铬铁矿为高镁高铬型,但Mg#值(平均68)相对更高些,Cr#值平均79。透镜状纯橄岩的特征是橄榄石Fo(91～92)和铬尖晶石Cr#(60左右)均低于前两类纯橄岩,但单斜辉石的Al2O3(1.41%～1.71%)则高于前两者。透镜状纯橄岩的矿物成分与方辉橄榄岩重叠,两者为渐变过渡关系。研究对比表明,罗布莎厚层状纯橄岩不同于经典的蛇绿岩的超镁铁质堆晶岩,认为将其成因解释为拉斑玄武质熔体与地幔橄榄岩的反应较为合理。透镜状纯橄岩与方辉橄榄岩存在成生联系,可能是地幔橄榄岩高度部分熔融的产物,或熔体和方辉橄榄岩在原位发生反应的产物;薄壳状纯橄岩成因与厚层状纯橄岩相同,但与其相伴的块状铬铁矿是否由拉斑玄武质熔体与方辉橄榄岩反应形成,值得商榷。