Paleoproterozoic basaltoids in the North Baikal volcanoplutonic belt of the Siberian craton: age and petrogenesis

The oldest igneous rocks in the Paleoproterozoic (~1.88–1.85 Ga) North Baikal postcollisional volcanoplutonic belt of the Siberian craton are the basaltoids of the Malaya Kosa Formation (Akitkan Group). The youngest are the composite (dolerite–rhyolite) and doleritic dikes cutting the granitoids of the Irel’ complex and the felsic volcanic rocks of the Khibelen Formation (Akitkan Group). The position of Malaya Kosa basaltoids in the Akitkan Group section and published geochronological data on the felsic volcanic rocks overlying Malaya Kosa rocks suggest that their age is ~1878 Ma. The rhyolites from the center of a composite dike were dated by the U–Pb zircon method at 1844 ± 11 Ma, and the dolerites in the dikes are assumed to be coeval with them. Malaya Kosa basaltoids correspond to high-Mg tholeiites and calc-alkaline andesites, whereas the dolerites in the dikes correspond to high-Fe tholeiites. Geochemically, these basaltoids and dolerites are both similar and different. As compared with the dolerites, the basaltoids are poorer in TiO₂ (an average of 0.89 vs. 1.94 wt.%), Fe₂O₃¹ (9.54 vs. 14.71 wt.%), and P₂O₅ (0.25 vs. 0.41 wt.%). However, these rocks are both poor in Nb but rich in Th and LREE, ε_Nd(T) being negative. According to petrographic and geochemical data, they derived from compositionally different sources. It is assumed that the basaltoids originated from subduction-enriched lithospheric mantle, whereas the dolerites originated from refractory lithospheric mantle metasomatized by subduction fluids. The isotopic and geochemical features of mafic rocks in the North Baikal belt are well explained by their formation during crustal extension which followed subduction and collision in the region. The early stages of postcollisional extension evidenced the melting of subduction-enriched lithospheric mantle with the formation of parent melts for Malaya Kosa basaltoids. At the final stages of the formation of the North Baikal belt, during the maximum crustal extension, Fe-enriched melts rose to the surface and generated the dolerites of the dikes.     

Age,Mineralogical and Geochemical Features,and Tectonic Position of Gabbroids of the Dzhigdinskii Massif,Southeastern Environ of the North Asian Craton

Complex mineralogical, geochemical, and geochronological studies of the gabbroids from the Dzhigdinskii Massif located in the western part of the Dzhugdzhur–Stanovoy Superterrane are performed. It is established that the age of the rocks from the Dzhigdinskii Massif is Middle Triassic (244 ± 5 Ma), rather than Early Archean, as was previously assumed. The age of the Dzhigdinskii Massif is close to the age of the formation of the other Triassic gabbroid massifs, such as the Amnunaktinskii (~240 Ma), Lukindinskii (~250 Ma), and Luchinskii (~248 Ma) in the southeastern environ of the North Asian Craton. One of the stages in the formation of the Selenga–Vitim volcanoplutonic belt falls in this period as well. This indicates that the Selenga–Vitim volcanoplutonic belt, along with the granitoids and volcanic rocks, is composed of ultrabasic–basic and basic massifs and that this belt is superposed on the structures of the Selenga–Stanovoy Superterrane, as well as on the western part of the Dzhugdzhur–Stanovoy Superterrane. The gabbro, gabbro–diorite, and series of gabbro and gabbro–diorite with high sodic alkalinity from the Dzhigdinskii Massif show obvious geochemical features of duality, including combination of intraplate and super-subduction origin. In this relation, we can assume that the origin of the gabbroids of the Dzhigdinskii Massif is related to the detachment of the oceanic lithosphere and its subduction into the mantle with the formation of an “asthenospheric window.”     

Field and geochemical characteristics of Mesoarchean to Neoarchean volcanic rocks in the Storø greenstone belt,SW Greenland: Evidence for accretion of intra-oceanic volcanic arcs

The Storø greenstone belt, southern West Greenland, consists of thrust-imbricated slices of Mesoarchean (>3060 Ma) and Neoarchean (ca. 2800 Ma) mafic to ultramafic volcanic rocks, volcaniclastic sediments, and gabbro–anorthosite associations. The belt underwent polyphase metamorphism at upper amphibolite facies conditions between 2650 and 2600 Ma. The contacts between the Mesoarchean and Neoarchean volcanic rocks, and surrounding Eoarchean to Neoarchean tonalite–trondhjemite–granodiorite (TTG) gneisses are tectonic and typically bounded by high-grade mylonites. Regardless of age, the volcanic rocks are dominated by mafic amphibolites with a tholeiitic basalt composition, near-flat to slightly enriched light rare earth element (LREE) patterns (La/Sm_cn = 0.91–1.48), relatively flat to slightly depleted heavy-REE (HREE) (Gd/Yb_cn = 1.0–1.28), and pronounced negative Nb–Ta anomalies (Nb/Nb* = 0.34–0.73) on chondrite- and primitive mantle-normalized diagrams. These geochemical characteristics are consistent with subduction zone geochemical signatures and partial melting of a shallow (<80 km) mantle source free of residual garnet. There is no geochemical evidence for contamination by older continental crust. The overall field and geochemical characteristics suggest that the thrust-imbricated basaltic rocks were erupted in intra-oceanic subduction zone settings. Sedimentary rocks are represented by garnet–biotite and quartzitic gneisses. They are characterized by relatively high contents of transition metal (Ni = 10–154 ppm; Cr = 7–166 ppm) and enriched LREE patterns (La/Sm_cn = 1.38–3.79). These geochemical characteristics suggest that the sedimentary rocks were derived from erosion of felsic to mafic igneous source rocks. Collectively, the structural and lithogeochemical characteristics of the Storø greenstone belt are consistent with collision (accretion) of unrelated Archean volcanic rocks formed in supra-subduction zone geodynamic settings. Accordingly, the Mesoarchean and Neoarchean rock record of the Storø greenstone belt may well be explained in terms of modern-style plate tectonic processes.     

Meso–Neoarchaean crustal evolution of the Bundelkhand Craton,Indian Shield: new data from greenstone belts

ABSTRACT

The Mauranipur and Babina greenstone belts of the Bundelkhand Craton are formed of the Central Bundelkhand greenstone complex (CBGC). This complex represents tectonic collage which has not been previously studied in depth. The purpose of this study is to contribute to the understanding of the main features of the Archaean crustal evolution of the Bundelkhand Craton. The CBGC consists of two assemblages: (1) the early assemblage, which is composed of basic-ultramafic, rhyolitic–dacitic, and banded iron formation units, and (2) the late assemblage, which is a felsic volcanic unit. The units and assemblages are tectonically unified with epidote–quartz–plagioclase metasomatic rocks formed locally in these tectonic zones.

The early assemblage of the Mauranipur greenstone belt is estimated at 2810 ± 13 Ma, from the U–Pb dating (SHRIMP, zircon) of the felsic volcanics. Also, there are inherited 3242 ± 65 Ma zircons in this rock. It is deduced that this assemblage is related to early felsic subduction volcanism during the Mesoarchaean that occurred in the Bundelkhand Craton.

Zircons extracted from metasomatic rocks in the early assemblage’s high-Mg basalts show a concordant age of 2687 ± 11 Ma. This age is interpreted as a time of metamorphism that occurred simultaneously with an early accretion stage in the evolution of the Mauranipur greenstone belt.

The felsic volcanism, appearing as subvolcanic bodies in the late assemblage of the Mauranipur greenstone belt, is estimated to be 2557 ± 33 Ma from the U–Pb dating (SHRIMP, zircon) of the felsic volcanic rocks. This rock also contains inherited 2864 ± 46 Ma zircons. The late assemblage of the Mauranipur greenstone belt corresponds with a geodynamic setting of active subduction along the continental margin during Neoarchaean.

The late assemblage Neoarchaean felsic volcanic rocks from the Mauranipur and Babina greenstone belts are comparable in age and geochemical characteristics. The Neoarchaean rocks are more enriched in Sr and Ba and are more depleted in Cr and Ni than the Mesoarchaean felsic volcanic rocks of the early assemblage.

Through isotopic dating and the geochemical analysis of the volcanic and metasomatic rocks of the CBGC, this study has revealed two subduction–accretion events, the Meso–Neoarchaean (2.81–2.7 Ga) and Neoarchaean (2.56–2.53 Ga), during the crustal evolution of the Bundelkhand Craton (Indian Shield).     

Porphyry Cu-Mo-(Au) mineralization: the age and relationship with igneous rock complexes of the Borgulikan ore field (upper-Amur region)

The Borgulikan ore field is localized in the west of the Umlekan-Ogodzha volcanoplutonic belt made up of various igneous (upper-Amur granite-granodiorite (140–134 Ma), Burunda monzodiorite-granodiorite (130–127 Ma), and Taldan andesite (127–123 Ma)) and superposed (Early Cretaceous Gal’ka trachybasalt-rhyolite (119–115 Ma) and Late Cretaceous trachybasalt-trachyandesite (97–94 Ma)) complexes. ⁴⁰Ar/³⁹Ar dating of porphyry intrusions breaking through the Taldan volcanic complex and associated with Cu-Mo-(Au) mineralization yielded the following ages: early (dark) “pre-ore” quartz monzodiorite porphyrites — 125.8±0.7 Ma (groundmass) and 125.2±1.8 Ma (biotite phenocrysts); late (cream) “syn-ore” quartz monzodiorite porphyrites — 122.6±0.7 Ma (biotite phenocrysts). In age and many geochemical features the quartz monzodiorite porphyrites are close to the Taldan complex volcanics. Both of these rocks seem to belong to the same volcanoplutonic association.     

Volcanoplutonic association of the early-stage evolution of the Imandra-Varzuga rift zone,Kola Peninsula,Russia: Geological,petrogeochemical, and isotope-geochronological data

The western flank of the Paleoproterozoic Imandra-Varzuga rift zone consists of three volcanogenic-sedimentary series and layered mafic-ultramafic intrusions of different age (2.50–2.45 Ga). The earliest Monchegorsk and Monche Tundra layered massifs were formed about 2.50 Ga during the prerift stage of the evolution of the Imandra-Varzuga zone. The early rift stage (～2.45 Ga) produced layered intrusions of the Imandra complex and volcanic rocks of the Strelna Group, consisting of the Kuksha and Seidorechka formations. In terms of chemical composition, the volcanic rocks of the Seidorechka Formation belong to a single basalt-rhyolite series, mostly of normal alkalinity and both tholeiitic and calc-alkaline affinity. The rocks of the Imandra Complex are characterized by moderate LREE enrichment, relatively flat HREE patterns, and a positive Eu anomaly. Similar REE distribution patterns were observed in the volcanic rocks of the Seidorechka Formation, which show a gradual increase in REE content with increasing SiO₂. The upper part of the Seidorechka Formation in the southern Khibiny region is composed of metarhyodacites. They terminate the sequence of the Strelna Group and have a U-Pb zircon age of 2448 ± 8 Ma. This age presumably reflects the upper age boundary of the rocks of the Seidorechka Formation and the end of the early stage of the evolution of the Imandra-Varzuga zone. Xenogenic zircon from the same sample yielded a U-Pb zircon age of 2715 ± 42 Ma. A U-Pb age of 2202 ± 17 Ma was obtained for titanite and rutile and interpreted as the metamorphic age of the Seidorechka Formation. The metavolcanic rocks of the Seidorechka Formation have negative ?_Nd (T) varying from ?2.84 to ?2.32, and I_Sr values of 0.7041–0.7038, which are higher than those of the depleted mantle and suggest their derivation from an enriched mantle reservoir (EM1). The spatial association of the volcanic rocks of the Seidorechka Formation and the rocks of the Imandra Complex, similarity in the behavior of most major elements, similar REE distribution patterns, and close formation ages and isotope signatures give grounds to combine them in a single volcanoplutonic association.     

Stratotype of the Chaya Formation of the Akitkan Group in the North Baikal volcanoplutonic belt: age and time of sedimentation

With U-Pb zircon dating, we determined the age of rhyodacites composing sedimentary covers among coarse-terrigenous rocks of the lowermost Chaya Formation of the Akitkan Group (North Baikal volcanoplutonic belt). These rocks are considered to have formed during the sedimentation. The dates (1863±9 Ma) permitted estimation of the age of basal beds of the Chaya Formation and substantiate the age boundary between the Khibelen and Chaya Formations of the Akitkan Group. The determined age and earlier dates of igneous rocks intruding the Chaya Formation deposits suggest that the latter accumulated for 10 Myr.     

Petrogenesis and tectonic implications of Early Cretaceous andesitic-dacitic rocks,western Qinling(Central China):Geochronological and geochemical constraints

~(40)Ar/~(39)Ar and zircon U-Pb geochronological and whole-rock geochemical analyses for the Laozanggou intermediate-acidic volcanic rocks from the western Qinling orogenic belt,Central China,constrain their petrogenesis and the nature of the Late Mesozoic lithospheric mantle.These volcanic rocks yield hornblende or whole-rock ~(40)Ar/~(39)Ar plateau ages of 128.3-129.7 Ma and zircon U-Pb age of131.3±1.3 Ma.They exhibit Si02 of 56.86-66.86 wt.%,K_2 O of 0.99-2.46 wt.% and MgO of 1.03-4.47 wt.%,with Mg# of 42-56.They are characterized by arc-like geochemical signatures with significant enrichment in LILE and LREE and depletion in HFSE.All the samples have enriched Sr-Nd isotopic compositions with initial ~(87)Sr/~(86)Sr ratios ranging from 0.7112 to 0.7149 and ε_(Nd)(t) values from 10.2 to 6.3.Such geochemical signatures suggest that these volcanic rocks were derived from enriched lithospherederived magma followed by the assimilation and fractional crystallization(AFC)process.The generation of the enriched lithospheric mantle is likely related to the modification of sediment-derived fluid in response to the Triassic subduction/collision event in Qinling orogenic belt.The early Cretaceous detachment of the lithospheric root provides a reasonable mechanism for understanding the petrogenesis of the Laozanggou volcanic sequence in the western Qinling orogenic belt.     

Kalikorva structure and its position in the system of the northern Karelian greenstone belts: Geochemical and geochronological data

New geochemical data on volcanic rocks and the first U-Pb zircon ages for the Kalikorva structure made it possible to determine the time and conditions of their formation and constrain geodynamic models. The lower sequences of the Kalikorva structure is dominated by metatholeiites with high MgO, Cr, and Ni contents, high Mg#, and REE distribution patterns close to the mantle level. They contain rare komatiite interlayers and lenses of pyroxenites and peridotites and can be considered as products of the deep melting of mantle material. At the same time, the tholeiitic metabasalts bear island-arc signatures and are intercalated with metagraywackes and metadacites (adakites). This rock association could be formed under spreading conditions at the beginning of an island-arc regime. The upper sequence is dominated by metagraywackes and contains diverse rocks with both MORB (tholeiitic and komatiitic basalts) and island-arc (calc-alkaline andesite and dacites, subalkaline basalts, and picritic basalts) affinity, which is typical of back-arc basins. The U-Pb dating of zircons from the metadacites and detrital zircons from the metagraywackes of the Kalikorva structure yielded similar ages of 2785 ± 13 and 2766 ± 21 Ma, respectively. They coincide with the age of the late volcanic complex of the Hisovaara Group of the Hisovaara structure (2780 Ma). Both complexes include island-arc associations with subduction signatures and contain adakites, Nb-Ti basalts, and basaltic andesites. The metagraywackes and metadacites of the Chupa sequence of the Belomorian mobile belt are older than the similar rocks of the Kalikorva complex and have an age of 2870 ± 30 Ma. Ages of 2735 ± 20 Ma and 2720 ± 4 Ma were previously obtained for the metaandesites of the Kichany volcanogenic complex, which could be an even younger volcanic arc.     

The Late Triassic Kataev volcanoplutonic association in western Transbaikalia,a fragment of the active continental margin of the Mongol-Okhotsk Ocean

The Kataev volcanoplutonic association has been recognized in western Transbaikalia. It unites the volcanosedimentary rocks of the Kataev Formation and associated granites localized within the lower plates of the Buteel-Nuur and Zagan metamorphic-core complexes. The rocks of the Kataev association are dynamometamorphosed to different degrees, which is due to the tectonic exposure of metamorphic-core complexes in the Early Cretaceous. The U-Pb zircon dating of the Kataev Formation rhyolites yielded an age of 226 ± 3 Ma. The U-Pb zircon age of the granites intruding the Kataev Formation rocks is 223.4 ± 5.0 Ma. The volcanics of the Kataev Formation belong to the subalkalic basalt-andesite-dacite-rhyolite series. The trachybasalts and trachyandesite-basalts of the Kataev Formation have geochemical characteristics of igneous rocks formed as a result of subduction, e.g., they show distinct negative Nb and Ti and positive Ba and Sr anomalies on multielemental patterns. The specific composition of mafic volcanics points to their formation through the melting of a mantle source resulted from the mixing of depleted mantle and subduction components. Trachyandesites have higher Th and U contents than basaltoids. They can result from the contamination of a mantle source, similar in composition to the Kataev Formation basaltoids, with crustal material. The felsic volcanics of the Kataev Formation and granites intruding them show nearly identical geochemical characteristics corresponding to both A-and I-type granites. These rocks might have formed through the melting of a moderately water-saturated magmatic source of diorite-tonalite composition at 742–833°C. We have established that the rocks of the Kataev volcanoplutonic association in western Transbaikalia and Northern Mongolia formed in the Late Triassic synchronously with the calc-alkaline granitoids of the Henteyn–Daurian batholith and the alkali granites and bimodal volcanic associations of the Kharitonovo and Tsagaan-Hurtey volcanoplutonic associations. The synchronous formation of volcanoplutonic associations of normal and high alkalinity agrees with the geodynamic setting of the Andean-type active continental margin existing in the area of present-day western Transbaikalia and Northern Mongolia in the Early Mesozoic. This setting was the result of the subduction of the Mongol-Okhotsk oceanic plate beneath the Siberian continent.     

Geochemical features of felsic magmatism on the northwestern flank of the Khingan-Okhotsk volcanoplutonic belt: Ezop and Yam-Alin zones

The Ezop-Yam-Alin zone was studied with emphasis on the geology, composition, and age of its felsic volcanic and intrusive rocks. All these rocks are moderately enriched in Rb (84–268 ppm), Ba (240–881 ppm), Th (10.5–17.9 ppm), and REE and depleted in Nb (5–12 ppm), Ta (0.5–1.2 ppm), and Zr (92–175 ppm). The age dates obtained by U-Pb (94.8 ± 2.2 Ma) and Rb-Sr (95.2 ± 0.69 Ma) methods for the granites of the Ezop Complex correspond to the Cenomanian.     

华北陆块南缘汝阳群次火山岩SHRIMP锆石U-Pb年龄及其地质意义

华北陆块南缘汝阳群下部的次火山岩一直被看作是与汝阳群同时代的火山喷发夹层。通过研究测得玄武岩SHRIMP锆石U-Pb年龄为213.5±2.4Ma,并在邻近该次火山岩层上部的紫红色泥岩中发现了褪色的角岩化现象,说明该火山岩是晚三叠世沿汝阳群下部顺层侵入的次火山岩岩床。通过对该次火山岩岩石及其地球化学特征的研究,认为该次火山岩起源于有早期俯冲洋壳或陆壳参与再循环的大陆富集型地幔的部分熔融,它与秦岭造山带几乎同时代形成的超高压榴辉岩、埃达克质岩石、高钾钙碱性花岗岩、环斑花岗岩等共同揭示了秦岭造山带自中三叠世全面碰撞造山之后,由碰撞挤压逐渐转变为伸展拉张的深部动力学过程。     

Magmatic and geodynamic evolution of Urumieh–Dokhtar basic volcanism,Central Iran: major,trace element,isotopic, and geochronologic implications

Basic volcanic rocks from the West Nain area of the Urumieh–Dokhtar Magmatic Assemblage demonstrate significant subduction-related geochemical characteristics; these along with the new age data obtained for the volcanic rocks shed new light on the geodynamic evolution of the Iranian segment of Alpine–Himalayan orogeny. The late Oligocene (26.5 Ma) high-Nb basic volcanic rocks are likely to represent a transient rather enriched asthenospheric mantle underlying the otherwise dominantly Eocene–early Oligocene West Nain island arc. Lithospheric mantle geochemical signatures of the low-Zr volcanic rocks (20.6 Ma) and high-Th volcanic rocks (19.7 Ma) imply replacement of the underlying mantle. The substitution of asthenospheric mantle by a lithospheric mantle wedge might have been associated with – or perhaps caused by – an increase in the subduction rate. Culmination of the West Nain magmatism into slab melting that produced the early Miocene (18.7 Ma) adakitic rocks is compatible with subsequent ascent that triggered slab decompression melting.     

Naturaliste Plateau: constraints on the timing and evolution of the Kerguelen Large Igneous Province and its role in Gondwana breakup

Volcanism associated with the Kerguelen Large Igneous Province is found scattered in southwestern Australia (the ca 136 to ca 130 Ma Bunbury Basalts, and ca 124 Ma Wallaby Plateau), India (ca 118 Ma Rajmahal Traps and Cona Basalts), and Tibet (the ca 132 Ma Comei Basalts), but apart from the ～70 000 km² Wallaby Plateau, these examples are spatially and volumetrically minor. Here, we report dredge, geochronological and geochemical results from the ～90 000 km² Naturaliste Plateau, located ～170 to ～500 km southwest of Australia. Dredged lavas and intrusive rocks range from mafic to felsic compositions, and prior geophysical analyses indicate these units comprise much of the plateau substrate. ⁴⁰Ar/³⁹Ar plagioclase ages from mafic units and U–Pb zircon ages from silicic rocks indicate magmatic emplacement from 130.6 ± 1.2 to 129.4 ± 1.3 Ma for mafic rocks and 131.8 ± 3.9 to 128.2 ± 2.3 Ma for silicic rocks (2σ). These Cretaceous Naturaliste magmas incorporated a significant component of continental crust, with relatively high ⁸⁷Sr/⁸⁶Sr (up to 0.78), high ²⁰⁷Pb/²⁰⁴ Pb ratios (15.5–15.6), low ¹⁴³Nd/¹⁴⁴Nd (0.511–0.512) and primitive-mantle normalised Th/Nb of 11.3 and La/Nb of 3.97. These geochemical results are consistent with the plateau being underlain by continental basement, as indicated by prior interpretations of seismic and gravity data, corroborated by dredging of Mesoproterozoic granites and gneisses on the southern plateau flank. The Cretaceous Naturaliste Plateau igneous rocks have signatures indicative of extraction from a depleted mantle, with trace-element and isotopic values that overlap with Kerguelen Plateau lavas reflect crustal contamination. Our chemical and geochronological results therefore show the Naturaliste Plateau contains evidence of an extensive igneous event representing some of the earliest voluminous Kerguelen hotspot magmas. Prior work reports that contemporaneous correlative volcanic sequences underlie the nearby Mentelle Basin, and the Enderby Basin and Princess Elizabeth Trough in the Antarctic. When combined, the igneous rocks in the Naturaliste, Mentelle, Wallaby, Enderby, Princess Elizabeth, Bunbury and Comei-Cona areas form a 136–124 Ma Large Igneous Province covering >244 000 km².     

Early Proterozoic central-type volcano in the Pechenga structure and its relation to the ore-bearing gabbro-wehrlite complex of the Kola Peninsula

The Zapolyarnyi volcanic center is confined to the boundary between the oldest volcanic formations (I and II) of the Pechenga complex. Its structure and rock association are significantly different from those of numerous eruptive centers of areal basaltic volcanism in the Pechenga structure. It is an oval-shaped body, 700 × 300 m in size, composed of volcanic eruptive lava breccia. The clastic material of the breccia includes angular and partially molten fragments of granites, pegmatoid granites, epidosites, quartz, and feldspars embedded in basaltic lava. The basalts are titanium-rich and iron-rich varieties enriched in large-ion lithophile elements (Rb, Ba, and Sr); they are similar in composition, including Rb-Sr and Sm-Nd isotopic characteristics, to the ferropicrites of the youngest volcanic formation (IV) and their differentiation products. The basalts of the volcanic center show ε_Nd(T) values from ?3.13 to ?1.17. In general, these rocks definitely represent the vent facies of an Early Proterozoic central-type volcano. The age of the basalt of the volcanic center is 1918 ± 3 Ma (U-Pb method on zircon) and is similar to the previously determined age of volcanics of volcanic formation IV (1990 ± 40 Ma, Sm-Nd method). The rocks of this formation participated 2000–1900 Ma ago in the formation of the volcanoplutonic ore-bearing ferropicrite-gabbro-wehrlite association of the Pechenga structure. The age of the ore-bearing Pilguyarvi gabbro-wehrlite intrusion was constrained between 1987 ± 5 Ma (U-Pb method on zircon) and 1980 ± 10 Ma (U-Pb method on baddeleyite). In addition, the first data were obtained for the age of comagmatic olivine norites of the Nyasyucka dike complex in the northeastern flank of the Pechenga structure (1941 ± 3 Ma, U-Pb method on baddeleyite) and the peridotites of the Allarechka ore field in the southern framing of the Pechenga structure (1918 ± 29 Ma, U-Pb method on zircon), which were previously considered Archean. Taking into account the geological and geochemical characteristics of the rocks of the Zapolyarnyi paleovolcano and the identical age of the Ludikovian intrusions, it can be concluded that the basalts of the paleovolcano were formed during late stages of the evolution of Early Proterozoic basic-ultrabasic magmatism, which was characterized by extensive explosive activity and strong magmatic differentiation responsible for the generation of the ore-bearing intrusions of the ferropicrite-gabbro-wehrlite association.     

Late Archean magmatic complexes of the Azov terrane,Ukrainian Shield: Geological setting,isotopic age,and sources of material

Geochemical, isotopic-geochemical, and geochronological information was obtained on magmatic rocks from the Saltychan anticlinorium in the Azov domain of the Ukrainian Shield. The rocks affiliate with the calc-alkaline series and a high-Mg series. The rocks of these series notably differ in concentrations of trace elements and REE and range from gabbro to granodiorite-quartz diorite in composition. The NORDSIM ionprobe U-Pb zircons ages of rocks belonging to the Obitochnen Complex and having both elevated and normal mg# correspond to 2908–2940 Ma. The Osipenkovskaya intrusion has an age of 2855 ± 19 Ma. The most alkaline North Obitochnen intrusion was emplaced in the Proterozoic, at 2074 ± 11 Ma. The age of the amphibolite metamorphism of the host gneisses is reliably dated at 3120–3000 Ma. The model Sm-Nd ages of the intrusive rocks do not exceed 3150 Ma. According to geochemical evidence, the parental melts of the magmatic rocks were derived from mantle domains variably enriched in lithophile elements. The results obtained by studying the Sm-Nd isotopic system corroborate the conclusion drawn from geochemical evidence that most of the melts were derived from the mildly enriched mantle, practically without involvement of ancient crustal material. The mantle became enriched in LREE at approximately 3000 Ma, which corresponds to the age of metamorphism of the supracrustal rocks. This process was separated from the derivation of the melts by a time span of 70–80 Ma. The relative age of the intrusive rocks and their variable composition can be most adequately explained by a contribution of heat and material from a plume to the derivation of the parental melts of these rocks.     

Crust-derived felsic magmatism in the Emeishan large igneous Province: New evidence from zircon U-Pb-Hf-O isotope from the Yangtze Block,China

Numerous intrusive bodies of mafic–ultramafic to felsic compositions are exposed in association with volcanic rocks in the Late Permian Emeishan large igneous province (ELIP), southwestern China. Most of the granitic rocks in the ELIP were derived by differentiation of basaltic magmas with a mantle connection, and crustal magmas have rarely been studied. Here we investigate a suite of mafic dykes and I-type granites that yield zircon U-Pb emplacement ages of 259.9 ± 1.2 Ma and 259.3 ± 1.3 Ma, respectively. The ε_Hf(t) values of zircon from the DZ mafic dyke are –0.3 to 9.4, and their corresponding T_DM1 values are in the range of 919–523 Ma. The ε_Hf(t) values of zircon from the DSC I-type granite are between –1 and 3, with T_DM1 values showing a range of 938–782 Ma. We also present zircon O isotope data on crust-derived felsic intrusions from the ELIP for the first time. The δ¹⁸O values of zircon from the DSC I-type granite ranges from 4.87‰ to 7.5‰. The field, petrologic, geochemical and isotopic data from our study lead to the following salient findings. (i) The geochronological study of mafic and felsic intrusive rocks in the ELIP shows that the ages of mafic and felsic magmatism are similar. (ii) The DZ mafic dyke and high-Ti basalts have the same source, i.e., the Emeishan mantle plume. The mafic dyke formed from magmas sourced at the transitional depth between from garnet-lherzolite and spinel-lherzolite, with low degree partial melting (<10%). (iii) The Hf-O isotope data suggest that the DSC I-type granite was formed by partial melting of Neoproterozoic juvenile crust and was contaminated by minor volumes of chemically weathered ancient crustal material. (iv) The heat source leading to the formation of the crust-derived felsic rocks in of the ELIP is considered to be mafic–ultramafic magmas generated by a mantle plume, which partially melted the overlying crust, generating the felsic magma.     

Petrology of volcanic and plutonic rocks from the Uimen-Lebed’ terrain,Gorny Altai

The Uimen-Lebed’ volcanoplutonic terrane is located at the junction of the Gorny Altai, Gornaya Shoriya, and western Sayan structures and is part of the Devonian-Early Carbonaceous Salair-Altai volcanoplutonic belt. The volcanic facies of the terrane composes the contrasting Nyrnin-Sagan Group, which includes basalt-basaltic andesite and basalt-rhyolite associations. The plutonic facies makes up the multiplet Elekmonar Group, which includes two independent complexes: monzogabbro-monzodiorite-granodiorite-granite and granodiorite-granite-leucogranite. The volcanic and plutonic rocks are asymmetrically distributed: volcanic sequences fill inherited depressions in the eastern part of the terrane, whereas plutonic complexes are located in its western part at the fault system branching from the transregional Kuznetsk-Teletsk-Kurai fault zone. The basalts of the Nyrnin-Sagan Group show geochemical signatures of both suprasubduction and rift-related rocks. The evolution of basaltoid magmatism reflects the formation and development of a suprasubduction mantle wedge in the inner part of an active continental margin accompanied by the influence of an intraplate mantle source. The silicic volcanism was generated under lower crustal conditions (P > 10 kbar) at the expense of metabasic materials and was accompanied by the influx of potassium into the anatectic zones. The gabbroids of the Elekmonar Group show suprasubduction geochemical features and no signatures of rift-related structures. The composition of the Elekmonar granitoids indicates significantly shallower (compared with the silicic volcanics) depths of their generation. The Uimen-Lebed’ volcanoplutonic terrane in the northeastern part of Gorny Altai was formed in the inner part of an active continental margin of the Andean type. Its magmatic complexes were formed over a considerable time range, from the early Emsian, when the formation of the active continental margin began, to the end of the Eifelian or, more likely, the beginning of the Givetian stage.     

Age relationships in supracrustal sequences of the northern part of the Murchison Terrane,Archaean Yilgarn Craton,Western Australia: A combined field and zircon U–Pb study

Mapping carried out in the northern Murchison Terrane of the Archaean Yilgarn Craton, Western Australia, shows that correlation of units between isolated greenstone belts is very difficult and an informal stratigraphic subdivision is proposed where the greenstone sequences have been divided into a number of assemblages. The assemblages may not necessarily be time equivalent throughout the region. The lower units (Assemblages 1–3) consist of ultramafic, mafic and intermediate volcanic rocks deposited without significant breaks in volcanism. Felsic volcanic packages (Assemblage 4) are conformable with underlying units, but are spatially restricted. Discordant units of graphitic sedimentary rocks are developed along major crustal structures (Assemblage 5). SHRIMP and conventional U–Pb study of zircons reveal that felsic volcanic rocks of Assemblage 4 in the Dalgaranga Greenstone Belt were emplaced at 2747 ± 5 Ma, whereas those in the adjacent Meekatharra — Mt Magnet Greenstone Belt range in age from 2762 ± 6 to 2716 ± 4 Ma. The age of emplacement of a differentiated gabbro sill in the Dalgaranga Greenstone Belt at 2719 ± 6 Ma places a maximum age on major folding in the belt. The presence of 2.9–3.0 Ga inherited zircons in some of the felsic volcanic rocks indicates contamination with, or reworking of, underlying 3 Ga sialic crust. This distinguishes the Murchison Terrane from the central parts of the Eastern Goldfields terranes to the south, where there is no evidence for a 3 Ga imprint in zircons from volcanic or granitic rocks, and also from the Narryer Gneiss Terrane to the north and west, which is composed of older gneisses and granitoids. The ca 2.76–2.71 Ga felsic volcanism in the Murchison Terrane is significantly older than 2.71–2.67 Ga felsic volcanism in the Eastern Goldfields lending support to models advocating assemblage of the craton by terrane accretion.     

Quaternary bimodal volcanism in the Niğde Volcanic Complex (Cappadocia,central Anatolia,Turkey): age,petrogenesis and geodynamic implications

The late Neogene to Quaternary Cappadocian Volcanic Province (CVP) in central Anatolia is one of the most impressive volcanic fields of Turkey because of its extent and spectacular erosionally sculptured landscape. The late Neogene evolution of the CVP started with the eruption of extensive andesitic-dacitic lavas and ignimbrites with minor basaltic lavas. This stage was followed by Quaternary bimodal volcanism. Here, we present geochemical, isotopic (Sr–Nd–Pb and δ¹⁸O isotopes) and geochronological (U–Pb zircon and Ar–Ar amphibole and whole-rock ages) data for bimodal volcanic rocks of the Ni?de Volcanic Complex (NVC) in the western part of the CVP to determine mantle melting dynamics and magmatic processes within the overlying continental crust during the Quaternary. Geochronological data suggest that the bimodal volcanic activity in the study area occurred between ca. 1.1 and ca. 0.2 Ma (Pleistocene) and comprises (1) mafic lavas consisting of basalts, trachybasalts, basaltic andesites and scoria lapilli fallout deposits with mainly basaltic composition, (2) felsic lavas consisting of mostly rhyolites and pumice lapilli fall-out and surge deposits with dacitic to rhyolitic composition. The most mafic sample is basalt from a monogenetic cone, which is characterized by ⁸⁷Sr/⁸⁶Sr = 0.7038, ¹⁴³Nd/¹⁴⁴Nd = 0.5128, ²⁰⁶Pb/²⁰⁴Pb = 18.80, ²⁰⁷Pb/²⁰⁴Pb = 15.60 and ²⁰⁸Pb/²⁰⁴Pb = 38.68, suggesting a moderately depleted signature of the mantle source. Felsic volcanic rocks define a narrow range of ¹⁴³Nd/¹⁴⁴Nd isotope ratios (0.5126–0.5128) and are homogeneous in Pb isotope composition (²⁰⁶Pb/²⁰⁴Pb = 18.84–18.87, ²⁰⁷Pb/²⁰⁴Pb = 15.64–15.67 and ²⁰⁸Pb/²⁰⁴Pb = 38.93–38.99). ⁸⁷Sr/⁸⁶Sr isotopic compositions of mafic (0.7038–0.7053) and felsic (0.7040–0.7052) samples are similar, reflecting a common mantle source. The felsic rocks have relatively low zircon δ¹⁸O values (5.6 ± 0.6 ‰) overlapping mantle values (5.3 ± 0.3 %), consistent with an origin by fractional crystallization from a mafic melt with very minor continental crustal contamination. The geochronological and geochemical data suggest that mafic and felsic volcanic rocks of the NVC are genetically closely related to each other. Mafic rocks show a positive trend between ⁸⁷Sr/⁸⁶Sr and Th, suggesting simultaneous assimilation and fractional crystallization, whereas the felsic rocks are characterized by a flat or slightly negative variation. High ⁸⁷Sr/⁸⁶Sr gneisses are a potential crustal contaminant of the mafic magmas, but the comparatively low and invariant ⁸⁷Sr/⁸⁶Sr in the felsic volcanics suggests that these evolved dominantly by fractional crystallization. Mantle-derived basaltic melts, which experienced low degree of crustal assimilation, are proposed to be the parent melt of the felsic volcanics. Geochronological and geochemical results combined with regional geological and geophysical data suggest that bimodal volcanism of the NVC and the CVP, in general, developed in a post-collisional extensional tectonic regime that is caused by ascending asthenosphere, which played a key role during magma genesis.