Composition,sources, and mechanisms of origin of rare-metal granitoids in the Late Paleozoic Eastern Sayan zone of alkaline magmatism: A case study of the Ulaan Tolgoi massif

The Ulaan Tolgoi massif of rare-metal (Ta, Nb, and Zr) granites was formed at approximately 300Ma in the Eastern Sayan zone of rare-metal alkaline magmatism. The massif consists of alkaline salic rocks of various composition (listed in chronologic order of their emplacement): alkaline syenite → alkaline syenite pegmatite → pantellerite → alkaline granite, including ore-bearing alkaline granite, whose Ta and Nb concentrations reach significant values. The evolution of the massif ended with the emplacement of trachybasaltic andesite. The rocks of the massif show systematic enrichment in incompatible elements in the final differentiation products of the alkaline salic magmas. The differentiation processes during the early evolution of the massif occurred in an open system, with influx of melts that contained various proportions of incompatible elements. The magma system was closed during the origin of the ore-bearing granites. Rare-metal granitoids in the Eastern Sayan zone were produced by magmas formed by interaction between mantle melts (which formed the mafic dikes) with crustal material. The mantle melts likely affected the lower parts of the crust and either induced its melting, with later mixing the anatectic and mantle magmas, or assimilated crustal material and generated melts with crustal–mantle characteristics. The origin of the Eastern Sayan zone of rare-metal alkaline magmatism was related to rifting, which was triggered by interaction between the Tarim and Barguzin mantle plumes. The Eastern Sayan zone was formed in the marginal part of the Barguzin magmatic province, and rare-metal magmas in it were likely generated in relation with the activity of the Barguzin plume.     

Paleozoic plume-lithospheric processes in northeastern Fennoscandia: Evaluation of the composition of the parental mantle melts and magma generation conditions

The paper reports quantitative evaluations of the modal and chemical composition of the mantle whose Paleozoic activation gave rise to the Kola alkaline province in the northeastern Baltic Shield. The volume of alkaline magmatism within the province and the volume of the mantle melts that were generated in the course of the Paleozoic activation cycle were evaluated by three-dimensional density modeling on the basis of gravimetric data. Our studies involved, along with the sampling of alkaline magmatic rocks in the region, the examination of the deep (to a depth of 22.5 km) structure of all alkaline intrusions in the province and the development of their three-dimensional density models. Concentrations of trace elements were precisely analyzed by the ICP-MS technique, and these data were used in order to calculate the weighted mean concentrations of trace elements in rocks of the province, to simulate the melting of mantle sources, and to evaluate the geodynamic sequences of these mantle processes. Our simulations indicate that the total volume of the Paleozoic mantle melts in the northeastern part of the Baltic Shield amounted to 15000 ± 2700 km³. The calculated composition of the partial melts that could be produced by the mantle of average composition shows the necessity for the significant introduction of certain elements into the mantle source. It is demonstrated that primitive melts in the Kola province were highly probably derived at low degrees of melting of the source (0.3–0.5%), whose composition corresponded to phlogopite-bearing (±amphibole) garnet lherzolite under the conditions of the mantle garnet depth facies. The calculated degree of enrichment of this sources was three times higher than the average concentrations of incompatible elements in the primitive mantle. It is demonstrated that magma generating processes affected much of the lithosphere beneath northeastern Fennoscandia and reached a depth of 120 km, i.e., the depth of the mantle facies of garnet lherzolite. The area of this region corresponds to the area of regional Paleozoic magmatism, and its depth correlates with the estimated P-T conditions under which the mantle xenoliths found in regional diatremes were formed.     

Petrology of the parental melts and mantle sources of Siberian trap magmatism

Based on the investigation of olivine phenocrysts and melt and spinel inclusions in them from the picrites of the Gudchikhinsky Formation and olivine phenocrysts and the whole-rock geochemistry from the Tuklonsky and Nadezhdinsky formations of the Noril’sk region, the compositions and conditions of formation and evolution of the parental melts and mantle sources of Siberian trap magmatism were evaluated. Olivine phenocrysts from the samples studied are enriched in Ni and depleted in Mn compared with olivines equilibrated with the products of peridotite melting, which suggests a considerable role of a nonperidotitic component (olivine-free pyroxenite) in their mantle source. The onset of Siberian trap magmatism (Gudchikhinsky Formation) was related to the melting of pyroxenite produced by the interaction of ancient recycled oceanic crust with mantle peridotite. During the subsequent evolution of the magmatic system (development of the Tuklonsky and Nadezhdinsky formations), the fraction of the pyroxenite component in the source region decreased rapidly (to 40 and 60%, respectively) owing to the entrainment of peridotite material into the melting zone. The formation of magmas was significantly affected by the contamination by continental crustal material. The primitive magmas of the Gudchikhinsky Formation crystallized under near-surface conditions at temperatures of 1250–1170°C and oxygen fugacities 2.5–3.0 orders of magnitude below the Ni-NiO buffer. Simultaneously, the magmas were contaminated by continental silicic rocks and evaporites. The parental magmas of the Gudchikhinsky rocks corresponded to tholeiitic picrites with 11–14 wt % MgO. They were strongly undersaturated in sulfur, contained less than 0.25 wt % water and carbon dioxide, and were chemically similar to the Hawaiian tholeiites. They were produced by melting of a pyroxenite source at depths of 130–180 km in a mantle plume with a potential temperature of 1500–1580°C. The presence of low melting temperature pyroxenite material in the source of Siberian trap magmas promoted the formation of considerable volumes of melt under the thick continental lithosphere, which could trigger its catastrophic collapse. The contribution of pyroxenite-derived melt to the magmas of the Siberian trap province was no less than 40–50%. This component, whose solid residue was free of sulfides and olivine, played a key role in the origin of high contents of Ni, Cu, and Pt-group elements and low sulfur contents in the parental trap magmas and prevented the early dispersion of these elements at the expense of sulfide melt fractionation. The high contents of Cl in the magmas resulted in considerable HCl emission into the atmosphere and could be responsible for the mass extinction at the Paleozoic-Mesozoic boundary.     

Revisiting Early–Middle Jurassic igneous activity in the Nanling Mountains,South China: Geochemistry and implications for regional geodynamics

Early–Middle Jurassic igneous rocks (190–170 Ma) are distributed in an E–W-trending band within the Nanling Tectonic Belt, and have a wide range of compositions but are only present in limited volumes. This scenario contrasts with the uniform but voluminous Middle–Late Jurassic igneous rocks (165–150 Ma) in this area. The Early–Middle Jurassic rocks include oceanic-island basalt (OIB)-type alkali basalts, tholeiitic basalts and gabbros, bimodal volcanic rocks, syenites, A-type granites, and high-K calc–alkaline granodiorites. Geochemical and isotopic data indicate that alkaline and tholeiitic basalts and syenites were derived from melting of the asthenospheric mantle, with asthenosphere-derived magmas mixing with variable amounts of magmas derived from melting of metasomatized lithospheric mantle. In comparison, A-type granites in the study area were probably generated by shallow dehydration-related melting of hornblende-bearing continental crustal rocks that were heated by contemporaneous intrusion of mantle-derived basaltic magmas, and high-K calc-alkaline granodiorites resulted from the interaction between melts from upwelling asthenospheric mantle and the lower crust. The Early–Middle Jurassic magmatic event is spatially variable in terms of lithology, geochemistry, and isotopic systematics. This indicates that the deep mantle sources of the magmas that formed these igneous rocks were significantly heterogeneous, and magmatism had a gradual decrease in the involvement of the asthenospheric mantle from west to east. These variations in composition and sourcing of magmas, in addition to the spatial distribution and the thermal structure of the crust–mantle boundary during this magmatic event, indicates that these igneous rocks formed during a period of rifting after the Indosinian Orogeny rather than during subduction of the paleo-Pacific oceanic crust.     

Late Paleozoic granitoids in western Transbaikalia: sequence of formation,sources of magmas,and geodynamics

The evolution of Late Paleozoic granitoid magmatism in Transbaikalia shows a general tendency for an increase in the alkalinity of successively forming intrusive complexes: from high-K calc-alkaline granites of the Barguzin complex (Angara–Vitim batholith) at the early stage through transitional from calc-alkaline to alkaline granites and quartz syenites (Zaza complex) at the intermediate stage to peralkaline granitoids (Early Kunalei complex) at the last stage. This evolution trend is complicated by the synchronous development of granitoid complexes with different sets and geochemical compositions of rocks. The compositional changes were accompanied by the decrease in the scales of granitoid magmatism occurrence with time. Crustal metaterrigenous protoliths, possibly of different compositions and ages, were the source of granitoids of the Angara–Vitim batholith. The isotopic composition of all following granitoid complexes points to their mixed mantle–crustal genesis. The mechanisms of granitoid formation are different. Some granitoids formed through the mixing of mantle and crustal magmas; others resulted from the fractional crystallization of hybrid melts; and the rest originated from the fractional crystallization of mantle products or the melting of metabasic sources with the varying but subordinate contribution of crustal protoliths. Synplutonic basic intrusions, combined dikes, and mafic inclusions, specific for the post-Barguzin granitoids, are direct geologic evidence for the synchronous occurrence of crustal and mantle magmatism. The geodynamic setting of the Late Paleozoic magmatism in the Baikal folded area is still debatable. Three possible models are proposed: (1) mantle plume impact, (2) active continental margin, and (3) postcollisional rifting. The latter model agrees with the absence of mafic rocks from the Angara–Vitim batholith structure and with the post-Barguzin age of peralkaline rocks of the Vitim province.     

Early cenozoic magmatism in the continental margin of Kamchatka

The paper presents isotopic-geochemical features of magmatic rocks that were produced at the continental margin of Kamchatka during its various evolutionary stages. Continental-margin magmatism in Kamchatka is demonstrated to have evolved from the Paleocene until the present time. The Paleocene and Middle-Late Eocene magmatic complexes show features of suprasubduction magmatism. The magmatic melts were derived from isotopically heterogeneous (depleted and variably enriched, perhaps, as a consequence of mixing with within-plate melts) mantle sources and were likely contaminated with quartz-feldspathic sialic sediments. The Miocene preaccretion stage differs from the Paleogene-Eocene one in having a different geochemical and isotopic composition of the mantle magma sources: the magmatic sources of the Miocene suprasubduction magmas contained no compositions depleted in radiogenic Nd isotopes, whereas the sources of the within-plate magmas were enriched in HFSE. The Late Pliocene-Quaternary postaccretion magmas of the Eastern Kamchatka Belt are noted for the absence of a within-plate OIB-like component.     

Proterozoic Gremyakha-Vyrmes Polyphase Massif, Kola Peninsula: An example of mixing basic and alkaline mantle melts

The paper presents newly obtained data on the geological structure, age, and composition of the Gremyakha-Vyrmes Massif, which consists of rocks of the ultrabasic, granitoid, and foidolite series. According to the results of the Rb-Sr and Sm-Nd geochronologic research and the U-Pb dating of single zircon grains, the three rock series composing the massif were emplaced within a fairly narrow age interval of 1885 ± 20 Ma, a fact testifying to the spatiotemporal closeness of the normal ultrabasic and alkaline melts. The interaction of these magmas within the crust resulted in the complicated series of derivatives of the Gremyakha-Vyrmes Massif, whose rocks show evidence of the mixing of compositionally diverse mantle melts. Model simulations based on precise geochemical data indicate that the probable parental magmas of the ultrabasic series of this massif were ferropicritic melts, which were formed by endogenic activity in the Pechenga-Varzuga rift zone. According to the simulation data, the granitoids of the massif were produced by the fractional crystallization of melts genetically related to the gabbro-peridotites and by the accompanying assimilation of Archean crustal material with the addition of small portions of alkaline-ultrabasic melts. The isotopic geochemical characteristics of the foidolites notably differ from those of the other rocks of the massif: together with carbonatites, these rocks define a trend implying the predominance of a more depleted mantle source in their genesis. The similarities between the Sm-Nd isotopic characteristics of foidolites from the Gremyakha-Vyrmes Massif and the rocks of the Tiksheozero Massif suggest that the parental alkaline-ultrabasic melts of these rocks were derived from an autonomous mantle source and were only very weakly affected by the crust. The occurrence of ultrabasic foidolites and carbonatites in the Gremyakha-Vyrmes Massif indicates that domains of metasomatized mantle material were produced in the sublithospheric mantle beneath the northeastern part of the Fennoscandian Shield already at 1.88 Ga, and these domains were enriched in incompatible elements and able to produce alkaline and carbonatite melts. The involvement of these domains in plume-lithospheric processes at 0.4–0.36 Ga gave rise to the peralkaline melts that formed the Paleozoic Kola alkaline province.     

Sources and evolution of the Cenozoic suprasubduction magmatism of the Olyutorsky tectonic block,southern Koryak highland

This paper reports the results of an investigation of the geochemical and isotopic compositions of rocks formed during the Eocene suprasubduction magmatism in the Olyutorsky tectonic block. The contribution of various suprasubduction components to the formation of magmatic melts was estimated; the characteristics of the Eocene and Miocene-Quaternary suprasubduction magmatism of the Olyutorsky tectonic block were compared; and relations of the Cenozoic magmatism to the tectonic development of the block were evaluated. The Eocene-early Oligocene suprasubduction magmas were derived from geochemically and isotopically heterogeneous garnet lherzolites in a mantle wedge. The initially depleted lherzolites of the mantle wedge were probably locally and variably enriched by OIB-type mantle melts before the generation of island-arc magmas and then again depleted below the MORB level by the extraction of magmatic materials from them. In the Eocene, a considerable amount of quartz-feldspar sediments enriched in radiogenic Nd was consumed in the subduction zone, which resulted in a strong contamination of magmas derived from the garnet lherzolites of the mantle wedge. The later stages of subduction were accompanied by active generation of adakite magmas with depleted Nd isotope signatures and HFSE-rich melts showing no evidence for their contamination by sialic sediments. It was supposed that the Late Cenozoic subduction zone plunged northward beneath the Olyutorsky tectonic block. It was shown that the established characteristics of the suprasubduction magmatism of the Olyutorsky tectonic block could be related to Cenozoic spreading processes in the proto-Komandorsky basin of the Bering Sea.     

Early Cretaceous magmatism of Mount Hermon, Northern Israel

 Early Cretaceous (146–115 Ma) magmatism in the region of Mt. Hermon, Northern Israel, is part of an extensive Mesozoic igneous province within the Levant associated with the evolution of the Neotethyan passive margin of Gondwana. The initial stages of activity were characterised by the emplacement of tholeiitic dykes (146–140 Ma) which were uplifted and eroded prior to the eruption of a sequence of alkali basalts, basanites and more differentiated alkaline lavas and pyroclastics from 127 to 120 Ma. The latest stages of activity (120–115 Ma) were highly explosive, resulting in the emplacement of diatreme breccias. Trace element and Sr-Nd-Pb isotope data for the most primitive Early Cretaceous mafic igneous rocks sampled suggest that they were derived by mixing of melts derived by variable degrees of partial melting of both garnet- and spinel-peridotite-facies mantle sources. Though isotopically heterogeneous, the source of the magmas has many similarities to that of HIMU oceanic island basalts. Earlier Liassic (200 Ma) transitional basalts and Neogene–Quaternary (15–0 Ma) alkali basalts erupted within northern Israel also have HIMU affinities. The petrogenesis of the Early Cretaceous and Cenozoic basalts is explained by partial melting of a lithospheric mantle protolith metasomatically enriched during the Liassic volcanic phase, which may be plume-related. Received: 23 July 1998 / Accepted: 6 December 1999     

Within-plate (intracontinental) and postorogenic magmatism of the East European Craton as reflection of the evolution of continental lithosphere

A comparative analysis of within-plate (intracontinental) and orogenic magmatic series formed during various evolution stages of the East European Craton (EEC) was performed using geological-petrological, geochemical, and isotopic data. The example of Baltic shield indicates that the compositions and tectonic settings of mantle melts in the Early Precambrian (Archean and Early Paleoproterozoic) significantly differed from those in the Phanerozoic. The Early Precambrian magmas were dominated by high-Mg low-Ti melts of the komatiite-basaltic and boninite-like series; this tectonomagmatic activity was determined by the ascent of mantle superplumes of the first generation, which originated in the depleted mantle. In the interval of 2.3–2.0 Ga, high-Mg mantle melts gradually gave place to the Fe-Ti picrites and basalts that are typical of within-plate Phanerozoic magmatism; at ～2 Ga, plume tectonics of the Early Precambrian gave way to plate tectonics. This is considered to be linked to the activity of mantle superplumes of the second generation (thermochemical), which originated from the liquid metallic core/mantle interface. Owing to the presence of fluid components, these superplumes reached much higher levels, where spreading of their head portions led to the active interaction with overlaying thinned rigid lithosphere. Sm-Nd isotopic studies showed that orogenic Neoarchean and Middle Paleoproterozoic magmatism of the Baltic shield was connected to the melting of the lithospheric mantle and crust; the melting of crustal sources gave rise to felsic members of the considered complexes. The systematic geochemical variations observed in these rocks with time presumably reflect a general trend toward an increase of the thickness of the continental crust serving as the basement for orogens. Beginning at ～2 Ga, the Meso, Neoproterozoic, and Phanerozoic including, no systematic variations were observed in the isotopic-geochemical characteristics of within-plate magmatism. All considered age sections demonstrate that isotopic-geochemical characteristics of parental mantle melts were strongly modified by crustal contamination. Mesoproterozoic magmatism of EEC was unique in the development of giant anorthosite-rapakivi granite complexes. Kimberlites and lamproites were repeatedly formed within EEC in the time interval from 1.8 to 0.36 Ga; their maximal development was noted in the Late Devonian. It was shown that only kimberlites derived from weakly enriched mantle are diamondiferous in the Arkhangelsk province; in the classic diamond provinces (Africa and Yakutia), diamondiferous kimberlites were derived from both depleted and enriched mantle.     

Conditions of Quaternary magmatism at Spitsbergen Island

Petrological and geochemical data obtained on the Quaternary lavas of volcanoes at Spitsbergen Island indicate that the rocks were produced via the deep-seated crystallization of parental alkaline magmas at 8–10 kbar. The character of clinopyroxene enrichment in incompatible elements indicates that the mineral crystallized from more enriched melts than those inferred from the composition of the host lavas. These melts were close to the parental melts previously found as veinlets in mantle hyperbasite xenoliths in the lavas. According to the character of their enrichment in Pb and Sr radiogenic isotopes and depletion in Nd, the basalts from Spitsbergen Island define a single trend with the weakly enriched tholeiites of the Knipovich Ridge, a fact suggesting the closeness of the enriched sources beneath the continental margin of Spitsbergen and beneath the spreading zone. Magmatic activity at Spitsbergen was related to the evolution of the Norwegian-Greenland basin, which evolved in pulses according to the shift of the spreading axes. The most significant of the latter events took place in the Neogene, when the Knipovich Ridge obtained its modern position near the western boundary of Spitsbergen. Early in the course of the evolution, the emplacement of alkaline melts generated at Spitsbergen into the oceanic mantle could form the enriched mantle, which was later involved in the melting process beneath the spreading zone.     

Cycles of alkaline magmatism

Geochronological data (～1800 dates) have been analyzed by the probabilistic statistical analysis of samplings of different subalkaline and alkaline rocks through the whole of geological time. The distribution of five groups of subalkaline and alkaline rocks within the Late Archean-Phanerozoic are strictly controlled by mantle cycles, which were distinguished from data on the upper mantle magmatic rocks. Since high alkali rocks are plume related, their universal participation in each of the revealed mantle cycles emphasizes the importance of this magmatism in the evolution of the crustal-mantle system. The initial Sr and Nd isotope ratios are subdivided into two groups: with mantle and crustal signatures. Mantle isotope ratios are typically observed throughout the entire geological interval of dated rocks, while the role of crustal isotope signatures increases from the Archean to Phanerozoic, reflecting the increasing the role of fluids and crustal rocks in the magmatic processes during the generation of mantle magmas and their consolidation in the crust. Since alkaline magmatic sources are formed during mantle metasomatism, which enriched the magma generation zones in incompatible elements, the repeated occurrence of this process in separate mantle zones may have lead to the anomalous accumulation of these elements, which should be reflected in the alkaline magmas.     

Late paleozoic-Early Mesozoic within-plate magmatism in North Asia: traps, rifts, giant batholiths, and the geodynamics of their origin

A number of large areas of igneous provinces produced in North Asia in the Late Paleozoic and Early Mesozoic include Siberian and Tarim traps and giant rift systems. Among them, the Central Asian Rift System (CARS) has the most complicated structure, evolved during the longest time, and is a large (3000 × 600 km) latitudinally oriented belt of rift zones extending from Transbaikalia and Mongolia to Middle Asia and including the Tarim traps in western China. CARS was produced in the Late Carboniferous, and its further evolution was associated with the lateral migration of rifting zones; it ended in the Early Jurassic and lasted for approximately 110 Ma. CARS was produced on an active continental margin of the Siberian continent and is noted for largest batholiths, which were emplaced simultaneously with rifting. The batholiths are surrounded by rift zones and compose, together with them, concentrically zoned magmatic areas, with crustal (granitoid) magmatism focused within their central portions, whereas mantle (rift-related) magmatism is predominant in troughs and grabens in peripheral zones. The batholiths show geological and isotopic geochemical evidence that their granitoids were produced by the anatexis of the host rocks at active involvement of mantle magmas. Zonal magmatic areas of the type are viewed as analogues of large igneous provinces formed in the environments characteristic of active continental margins. Large within-plate magmatic provinces in North Asia are thought to have been generated in relation to the overlap of at least two mantle plumes by the Siberian continent during its movement above the hot mantle field. In the continental lithosphere, mantle plumes initiated within-plate magmatic activity and facilitated rifting and the generation of traps and alkaline basite and alkali-salic magmatic associations. Because of the stressed states during collision of various type in the continental margin, the mantle melts did not ascend higher than the lowest crustal levels. The thermal effect of these melts on the crustal rocks induced anatexis and eventually predetermined the generation of the batholiths.     

The limited extent of plume-lithosphere interactions during continental flood-basalt genesis: geochemical evidence from Cretaceous magmatism in southern Brazil

It has been suggested that large areas of the Earth's lithospheric mantle undergo pervasive dehydration melting during the impact of mantle plumes and the Early-Cretaceous Paraná-Etendeka continental flood-basalt (CFB) province has repeatedly been cited as evidence of this phenomenon. During the Cretaceous, however, southern Brazil experienced two phases of mafic magmatism. These igneous events occurred ～50?Ma apart and therefore represent distinct episodes of melt genesis in the underlying mantle. The first phase of magmatism, in the Early Cretaceous, included the emplacement of lava flows associated with the Paraná-Etendeka CFB province and also the intrusion of small-volume mafic alkaline magmas (e.g. Anitápolis, Jacupiranga and Juquiá) in the Dom Feliciano and Ribeira mobile belts. During the Late Cretaceous, both sodic and potassic mafic magmas were emplaced on the margin of the adjacent Luis-Alves craton and intrude the flood-basalts at Lages. On the basis of variations in incompatible trace-element concentrations (e.g. Ba?=?1000 to 2000?ppm), initial ⁸⁷Sr/⁸⁶Sr ratios (0.7048–0.7064) and ?_Nd values (?3 to ?12), we suggest that all of the Late-Cretaceous mafic potassic magmas were derived from the subcontinental lithospheric mantle (SCLM) which was metasomatically enriched during the Proterozoic. We propose that these relatively low temperature, volatile-rich, mafic melts provide direct evidence that the underlying SCLM did not melt wholesale during the previous Early-Cretaceous Paraná-Etendeka CFB event. Late-Cretaceous melting of the SCLM beneath southern Brazil may have been caused by heat conduction from either: (1) ponded ～132?Ma Tristan plume-head material; or (2) ～85?Ma Trindade plume-head material channelled southwards between the thick cratonic keels of the Amazonas and São Francisco cratons. The Late-Cretaceous magmatism appears to have been contemporaneous with uplift across southern Brazil and Paraguay; we suggest that both of these phenomena represent the widespread effects of the impact of the Trindade mantle plume on the base of the SCLM. Plate margin stresses and lithospheric extension associated with the opening of the South Atlantic may also have changed the geothermal gradient beneath southern Brazil and contributed to mantle melting.     

The Purimetla gabbros,Prakasam District,Andhra Pradesh,India

Gabbros at Purimetla occur in close association with the alkaline pluton. Petrography and petrochemistry of these gabbros indicate their tholeiitic nature. Chemical variation of these tholeiites suggests that an initial undersaturated tholeiitic magma yielded oversaturated fractions in the final stages of differentiation. Their regional distribution suggests that basic magmatism preceded the emplacement of the alkaline rocks in the Prakasam alkaline province.     

Intrusive magmatism during early evolutionary stages of the Ural epioceanic orogen: U-Pb geochronology (LA ICP MS,NORDSIM, and SHRIMP II), geochemistry,and evolutionary tendencies

In recent years extensive data have been obtained on all geologically important intrusive complexes in the Central and Southern Urals by U-Pb zircon geochronologic high spatial resolution techniques (LA ICP MS, NORDSIM, and SHRIMP II). This made it possible to revise the current concepts for the magmatic activity of the Ural Paleozoic orogen.Intrusive magmatism that occurred early in the evolution of the Ural orogen was focused mostly in the Tagil megazone, was characterized by several common features, and took place nearly simultaneously within both of its zones: the Platinum Belt and the Tagil volcanic zone.The composition of the parental magmas of all complexes of this age corresponded to an ultramafic or mafic source; i.e., the magma was derived from a mantle source. The gabbroids most closely approximating the composition of the parental magmatic melts show geochemical features of suprasubduction melts, such as negative HFSE (Nb, Ti, and Zr) and positive Ba and Sr anomalies. The REE patterns of these rocks display variable La/Lu ratios, which are usually higher than 1. These geochemical features suggest that this magmatic source was a metasomatized mantle wedge, above which (at a depth of 40–25 km) a block of the pre-Ural basement occurred in Ordovician-Silurian time. The Tagil megazone started to develop on this block. By the Devonian, i.e., by the time when the Magnitogorsk zone began to evolve (～400 Ma) and continental-margin gabbro-tonalite-granodiorite magmatism was initiated (360 Ma), this basement had been destroyed by orogenesis. The major phases of Paleozoic magmatism in the Urals likely corresponded to global epochs of tectono-magmatic activity, because they correlate well with known data on the evolution of the ⁸⁷Sr/⁸⁶Sr ratio in Paleozoic seawater.     

Late Neoarchean geodynamic evolution: Evidence from the metavolcanic rocks of the Western Shandong Terrane,North China Craton

Late Neoarchean metavolcanic rocks are widely distributed in the Western Shandong Terrane (WST). They are classified as ~2590–2580 Ma tholeiites (Group MB-1), ~2550–2530 Ma tholeiites (Group MB-2), calc-alkaline basalts (Group MB-3), high-Si adakites (Group MAD) and ~2520–2500 Ma tholeiites (Group MB-4) based on zircon U-Pb chronological and geochemical data. Their parental magmas have complex origins and were derived from a depleted mantle wedge enriched by slab-derived melts or fluids (Group MB-1); an unaltered depleted mantle (Group MB-2); the delaminated lower crustal materials (Group MAD); a strongly melt- and fluid-metasomatized depleted mantle (Group MB-3); and a fluid- and sediment-metasomatized asthenospheric mantle (Group MB-4). The late Neoarchean geodynamic evolution of the WST revealed by these multi-genetic volcanic rocks can be summarized as follows: (1) an ~2.62–2.53 Ga eastward subduction operated along the ancient continental margin, followed by delamination of unstable continental lithosphere in the back-arc region during ~2.60–2.53 Ga; and (2) delamination-derived mantle magmas ascended and caused the regional extension, further inducing the asthenosphere to passively rise and the back-arc basin to open during ~2.52–2.50 Ga. The above intense mantle magmatism and crust-mantle interactions have consumed abundant mantle energy and facilitated the continental stratification and final cratonization of the WST.     

Late Paleozoic gabbroids of western Transbaikalia: U-Pb and Ar-Ar isotopic ages,composition, and petrogenesis

We provide new isotope-geochronological evidence for the synchronous occurrence of Late Paleozoic basic and granitoid magmatism in western Transbaikalia; this is a strong argument for the contribution of mantle magmas to granitoid petrogenesis. The Late Paleozoic basic rocks originated from the phlogopite-garnet-bearing lherzolitic mantle, which melted under “hydration conditions.” The specific features of Late Paleozoic magmatism in western Transbaikalia were determined by the combination of the activity of a low-energy mantle plume with the final stage of the Hercynian orogeny in space and time. At the early stage of magmatism, during the formation of the Barguzin granites,the plume had only a thermal influence on the crustal rocks heated as a result of Hercynian fold-thrust deformations. The mixing of mantle basic and crustal salic magmas at different levels marked the transition from crustal to mixed (mantle-crustal) granites, which include all post-Barguzin complexes (probably, except for alkali granites). In the geologic evolution of Transbaikalia, the Late Paleozoic magmatism was postorogenic, but it was initiated and influenced by the mantle plume.     

Conditions of formations of slightly enriched tholeiites in the northern Knipovich Ridge

New petrological and geochemical data were obtained for basalts recovered during cruise 24 of the R/V “Akademik Nikolay Strakhov” in 2006. These results significantly contributed to the understanding of the formation of tholeiitic magmatism at the northern end of the Knipovich Ridge of the Polar Atlantic. Dredging was performed for the first time both in the rift valley and on the flanks of the ridge. It showed that the conditions of magmatism have not changed since at least 10 Ma. The basalts correspond to slightly enriched tholeiites, whose primary melts were derived at the shallowest levels and were enriched in Na and depleted in Fe (Na-TOR type). The most enriched basalts are typical of the earlier stages of the opening and were found on the flanks of the ridge in its northernmost part. Variations in the ratios of Sr, Nd, and Pb isotopes and lithophile elements allowed us to conclude that the primary melts generated beneath the spreading zone of the Knipovich Ridge were modified by the addition of the enriched component that was present both in the Neogene and Quaternary basalts of Spitsbergen Island. Compared with the primitive mantle, the extruding magmas were characterized by positive Nb and Zr anomalies and a negative Th anomaly. The formation of primary melts involved melting of the metasomatized depleted mantle reservoir that appeared during the early stages of opening of the Norwegian-Greenland Basin and transformation of the paleo-Spitsbergen Fault into the Knipovich spreading ridge, which was accompanied by magmatism in western Spitsbergen during its separation from the northern part of Greenland.     

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.