Composition and formation settings of the siliceous-volcanogenic complexes of the Nizhneussuriisk segment,Kiselevka-Manoma terrane,West Sikhote Alin

This paper reports new data on the mineralogical-petrographical composition, genetic types of sedimentary rocks, and geochemical features of the volcanic rocks of the Snarsky area of the Nizhneussuriisk segment of the Kiselevka-Manoma accretionary complex, which is developed on the right bank of the lower reaches of the Ussuri River. The Middle Jurassic-Aptian sedimentary rocks of the area are represented by pelagic radiolarian cherts, semipelagic siliceous-clayey deposits, subordinate shallow limestones, volcanomictic conglomerates, and turbidite sandstones. The basalts of the area are represented by alkaline basalts resembling within-plate ocean-island basalts and N-MORB-type tholeiitic basalts. Rock associations of two physicogeographical and geodynamic settings are distinguished: the pelagic setting with within-plate alkaline volcanic rocks and the hemipelagic one with volcanic edifices of spreading basalts.     

Subaqueous early eruptive phase of the late Aptian Rajmahal volcanism,India: Evidence from volcaniclastic rocks,bentonite, black shales,and oolite

The late Aptian(118-115 Ma) continental flood basalts of the Rajmahal Volcanic Province(RVP) are part of the Kerguelen Large Igneous Province,and constitute the uppermost part of the Gondwana Supergroup on the eastern Indian shield margin.The lower one-third of the Rajmahal volcanic succession contains thin layers of plant fossil-rich inter-trappean sedimentary rocks with pyroclasts,bentonite,grey and black shale/mudstone and oolite,whereas the upper two-thirds consist of sub-aerial fine-grained aphyric basalts with no inter-trappean material.At the eastern margin and the north-central sector of the RVP,the volcanics in the lower part include rhyolites and dacites overlain by enstatite-bearing basalts and enstatite-andesites.The pyroclastic rocks are largely felsic in composition,and comprise ignimbrite as well as coarse-grained tuff with lithic clasts,and tuff breccia with bombs,lapilli and ash that indicate explosive eruption of viscous rhyolitic magma.The rhyolites/dacites(68 wt.%) are separated from the andesites( 60 wt.%) by a gap in silica content indicating their formation through upper crustal anatexis with only heat supplied by the basaltic magma.On the other hand,partially melted siltstone xenoliths in enstatite-bearing basalts suggest that the enstatite-andesites originated through mixing of the upper crust with basaltic magma,crystallizing orthopyroxene at a pressure-temperature of ~3 kb/1150℃.In contrast,the northwestern sector of the RVP is devoid of felsic-intermediate rocks,and the volcaniclastic rocks are predominantly mafic(basaltic) in composition.Here,the presence of fine-grained tuffs,tuff breccia containing sideromelane shards and quenched texture,welded tuff breccia,peperite,shale/mudstone and oolite substantiates a subaqueous environment.Based on these observations,we conclude that the early phase of Rajmahal volcanism occurred under predominantly subaqueous conditions.The presence of grey and black shale/mudstone in the lower one-third of the succession across the entire Rajmahal basin provides unequivocal evidence of a shallow-marine continental shelf-type environment.Alignment of the Rajmahal eruptive centers with a major N—S mid-Neoproterozoic lineament and the presence of a gravity high on the RVP suggest a tectonic control for the eruption of melts associated with the Kerguelen plume that was active in a post-Gondwana rift between India and Australia-Antarctica.     

Geochemistry of the Boil Mountain ophiolitic complex,northwest Maine,and tectonic implications

A coherent ophiolitic complex of pyroxenite, serpentinite, metagabbro, mafic volcanics, felsic volcanics and sediments crops out in NW Maine, adjacent to the Chain Lakes massif. The complex (here informally referred to as the Boil Mountain ophiolitic complex) is about 500 m.y. old. The volcanic sequence is not typical of ophiolites in that it contains a large proportion of felsic volcanics. The mafic volcanics are divided into two geochemical groups. A stratigraphically lower group is depleted in Ti, Zr, Y, Cr and REE contents similar to basalts from supra-subduction zone ophiolites. An upper mafic group has trace element contents similar to normal mid-ocean ridge basalts. The felsic volcanics are mostly rhyolitic and similar to low-K rhyolites found in the forearc of the Marianas trench and in an island arc sequence in the Klamath Mountains, California. The flat REE patterns of the felsic volcanic rocks are similar to those found in siliceous rocks in the Oman ophiolite. The presence of thick sequences of felsic volcanics, the abundance of pyroxenite, the low Ti, Zr and REE contents of some mafic rocks, the flat REE patterns of the felsic volcanics, and the composition of clinopyroxene all suggest the complex was formed in the vicinity of a subduction zone. The complex may be correlated with ophiolitic fragments in the eastern part of the Dunnage Zone in Newfoundland, rather than the main ophiolite belt of the western Appalachians.     

“三江”义敦岛弧带玄武岩喷发序列与裂谷—岛弧转化

岩石-构造组合是恢复古板块构造历史的最有效手段之一,同时是表征古板块边界与板内环境的最重要的地质证据。本文拟从岩石-构造组合角度,通过对义敦岛弧带玄武岩,特别是前岛弧期玄武岩喷发序列、岩石组合、地球化学特征和其形成背景的研究,试图从较深层次上揭示岩浆-构造内在联系,探索义敦古岛弧的形成与发展。     

Garnet-bearing basalts: an example from Mt. Hasan, Central Anatolia, Turkey

Summary Mt. Hasan is a complex stratovolcano located in Central Anatolia, Turkey. It exhibits four evolutionary stages: (1) Kecikalesi volcano, (2) Palaeovolcano, (3) Mesovolcano and (4) Neovolcano. Each stage is characterized by differentiated magmas varying from basaltic andesite to rhyolite, but basaltic activities are recorded on the volcano flanks. Garnets occur in rhyodacitic and rhyolitic pumices of the Palaeo-, Meso- and Neovolcanoe, as well as in the basaltic andesite lava flows of Palaeovolcano. In addition, garnets are recognised also in recent basalts which were erupted contemporaneously with Neovolcano during the late Quaternary. Garnets appear as euhedral grains in a basaltic groundmass. Mineralogy and geochemistry of older volcanics (Kecikalesi, Palaeovolcano) exhibit a tholeitic trend, while Mesovolcano and Neovolcano are calc-alkaline in affinity. However, the recent basalts exhibit alkaline chemistry and mineralogy. Garnets of Mt. Hasan volcanic rocks are pyrope- and almandine-rich and display rather uniform composition throughout the volcanic evolution. Trace element data suggest that while subduction components in magma composition decreased from Miocene to late Quaternary, the within plate character increased. We propose that the generation of garnet-bearing volcanics reflects an enriched source inherited from ancient subduction processes. Received November 1, 1999; revised version accepted June 30, 2001     

High-Field-Strength Elements in Mafic Volcanics from North China Craton: Implications for Archean-Proterozoic Boundary and Source Composition

ＩＮＴＲＯＤＵＣＴＩＯＮＡｌａｒｇｅｃｈａｎｇｅｉｎｃｈｅｍｉｃａｌｃｏｍｐｏｓｉｔｉｏｎｏｆｍａｎｔｌｅａｔｔｈｅＡｒｃｈｅａｎ－Ｐｒｏｔｅｒｏｚｏｉｃｔｒａｎｓｉｔｉｏｎ（Ａｒ－Ｐｔｂｏｕｎｄａｒｙ）ｏｆ２．５Ｇａｗａｓｓｕｇｇｅｓｔｅｄｂｙｍａ...     

Lower Proterozoic orthorocks in the Svecofennides of the Savo belt (Western Ladoga Region): Geochemical properties

On the basis of petrochemical data, orthorocks are defined among highly metamorphosed sequences of the Landenpokh’ya Group attributed to the Kalevian and Svecofennian systems in the Ladoga region. The contents of major and trace elements in orthorocks, which constitute some volcanogenic sedimentary complexes in the western Ladoga region, are discussed. The volcanics of the first complex (Kukhka Island area) belong to the calc-alkaline series and are characterized by the presence of OIB-type basaltic andesites. The basites of the second complex (Kil’pola Island area) are attributed to the tuffaceous rocks of the WPB-type tholeiitic series. The volcanics of the third complex (Kuznechnoe-Khiitola area) are largely represented by dacites referred to the Svecofennian mature island arc. The presented geological and geochemical data imply that the first and second complexes, which include intraplate volcanics, could be formed on the basite Jatulian-Ludikovian basement (protolith). These Kalevian rocks of the western Ladoga region are correlated with the volcanogenic sedimentary complexes in the southern part of the Savo belt in Finland and belong to the Karelian province.     

Petrographic and Geochemical Evidence for Magma Mixing and Crustal Contamination in the Post-Collisional Calc-Alkaline Yozgat Volcanics,Central Anatolia,Turkey

Petrographic, major-oxide, and trace-element data are presented for the Yozgat volcanics. These rocks range in composition from basalts through basaltic andesites and andesites to dacites. Major-oxide variations are largely explicable in terms of fractional crystallization, involving removal of observed phenocrysts and microphenocrysts. However, complex zoning patterns and resorbtion phenomena shown by phenocrysts in these lavas, and observed epitaxitic pyroxene growth around quartz xenocrysts imply that they are hybrids formed by a mixing process. In addition, observed enrichments in crustal elements such as K, Rb, Ba, Sr, and P provide clear evidence for the crustal assimilation of granitoid and metasedimentary xenoliths. The following model is suggested for the evolution of the Yozgat volcanics. The primitive magma underwent fractionation in an intracrustal magma chamber to yield more evolved liquids. Influx of hot, primitive magma into the magma chamber promoted vigorous convection-crustal assimilation and eruption of the volcanic rocks in the study area.     

Chemostratigraphy of lava sequences from the Rio Itapicuru Greenstone Belt,Bahia State,Brazil

The Rio Itapicuru greenstone terrain of north-central Bahia State consists of belts of supracrustal rocks surrounding granitic plutons and domes. The basal supracrustal rocks are predominantly massive metabasalts with minor amounts of intercalated chemical sedimentary rocks and mafic tuffs. They are overlain by a middle unit of intermediate to acid pyroclastic rocks, lavas, and volcaniclastic sediments, and an upper unit of greywackes, sandstones and conglomerates.A geochemical study of major and trace elements of the volcanic rocks indicates the existence of a chemical discontinuity between the basaltic and the acid to intermediate members. The basalts are typical tholeiites with Ti, Zr, Sr, Y and Nb contents analogous to those of modern ocean-floor tholeiites or, alternatively, low-K tholeiites of primitive island arcs. In contrast, compositional variations of the hornblende-bearing andesites and dacites fall along indisputably calc-alkaline trends of low FeO and TiO₂ contents which decrease with increasing differentiation. The lithostratigraphic and chemical variations within lavas of the Rio Itapicuru greenstone are comparable to those described from the Western Australian greenstone belts. Only in greenstone belts of the Canadian type do thick calc-alkaline sequences containing abudant basaltic andesites overlie conformably and transitionally the underlying tholeiitic basalts. Elsewhere the calc-alkaline sequences, if present, do not contain basaltic andesites and are chemically unrelated to the underlying basalts.     

Geochemistry of subalkaline and alkaline extrusives from the Kermanshah ophiolite,Zagros Suture Zone,Western Iran: implications for Tethyan plate tectonics

The Kermanshah ophiolite is a highly dismembered ophiolite complex that is located in western Iran and belongs to the Zagros orogenic system. The igneous rocks of this complex consist of both mantle and crustal suites and include peridotites (dunite and harzburgite), cumulate gabbros, diorites, and a volcanic sequence that exhibits a wide range in composition from subalkaline basalts to alkaline basalts to trachytes. The associated sedimentary rocks include a variety of Upper Triassic to Lower Cretaceous deep- and shallow-water sedimentary rocks (e.g., dolomite, limestone, and pelagic sediments, including umber). Also present are extensive units of radiolarian chert. The geochemical data clearly identifies some of the volcanic rocks to have formed from two distinct types of basaltic melts: (i) those of the subalkaline suite, which formed from an initial melt with a light rare earth elements (LREE) enriched signature and incompatible trace element patterns that suggest an island arc affinity; and (ii) those of the alkaline suite with LREE-enriched signature and incompatible trace element patterns that are virtually identical to typical oceanic island basalt (OIB) pattern. The data also suggests that the trachytes were derived from the alkaline source, with fractionation controlled by extensive removal of plagioclase and to a lesser extent clinopyroxene. The presence of compositionally diverse volcanics together with the occurrence of a variety of Triassic–Cretaceous sedimentary rocks and radiolarian chert indicate that the studied volcanic rocks from the Kermanshah ophiolite represent off-axis volcanic units that were formed in intraplate oceanic island and island arc environments in an oceanic basin. They were located on the eastern and northern flanks of one of the spreading centers of a ridge-transform fault system that connected Troodos to Oman prior to its subduction under the Eurasian plate.     

Geochemistry and petrology of the lower Miocene bimodal volcanic units in the Tunçbilek–Domaniç basin,western Anatolia

ABSTRACT

The lower Miocene (~22–19 Ma) volcanic units in the NE–SW-trending Tunçbilek–Domaniç basin, located in the northeastern-most part of the Neogene successions in western Anatolia, are composed of (1) high-K, calc-alkaline dacitic to rhyolitic volcanic rocks of the Oklukda?? volcanics; (2) calc-alkaline low-MgO (evolved) basalts; and (3) high-MgO mildly alkaline basalts of the Karaköy volcanics. Sr isotopic ratios of the volcanic units increase from high-MgO (~0.7055–0.7057) to low-MgO basaltic rocks (~0.7066–0.7072) and then to dacitic-rhyolitic rocks (0.7081–0.7086). Geochemical features of the volcanic rocks reveal that the calc-alkaline evolved basalts were formed by mixing of basic and acidic magmas.

Geochemical studies in the last decade show that the Miocene mafic volcanic rocks in western Anatolia are mainly composed of high-MgO shoshonitic-ultrapotassic rocks (SHO-UK), of which mantle sources were variably, but also intensely metasomatized with crustally derived materials during collisional processes in the region. However, geochemical comparison of the high-MgO basalts of the Karaköy volcanics with the SHO-UK rocks in this region reveal that that the former has too low ⁸⁷Sr/⁸⁶Sr_(i) and high ¹⁴³Nd/¹⁴⁴Nd_(i) ratios, with lower LILE and LREE abundances, which are firstly described here. These features are interpreted to be derived from more slightly enriched lithospheric mantle sources than that of the SHO-UK. Accepting the SHO-UK rocks in the region were derived from mantle sources that had been metasomatized by northward subduction of crustal slices during Alpine collisional processes, it is proposed that the imbrication and direct subduction of crustal slices were not reached to, and were limited in the mantle domains beneath the basin. The dacites of the Oklukda?? volcanics might be formed either by high-degree melting of the same sources with the SHO-UK, or by melting of the lower crustal mafic sources as previously proposed, and then evolved into the rhyolites via fractional crystallization with limited crustal contribution.     

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.     

GEOCHEMISTRY OF VOLCANIC ROCKS IN WESTERN PART OF JINSHAJIANG STRUCTURAL BELT AND ITS TECTONIC SETTING

GEOCHEMISTRY OF VOLCANIC ROCKS IN WESTERN PART OF JINSHAJIANG STRUCTURAL BELT AND ITS TECTONIC SETTING     

Petrogenesis and geodynamic implications of the Late Carboniferous felsic volcanics in the Bogda belt,Chinese Northern Tianshan

An integrated study on petrology and geochemistry has been carried out on the Late Carboniferous I-type felsic volcanics of the Liushugou Formation in the Bogda belt to constrain the late Paleozoic tectonic evolution of the Bogda belt. The felsic volcanics were dated to be 315 to 319 Ma and are composed of trachy-andesite–trachyte ignimbrites and rhyolite lavas. They are in conformable contact with high-Al basalt. The eruption of the felsic volcanics and high-Al basalt is not bimodal volcanism, but is related to bimodal magma (basaltic and rhyolitic magmas). MELTS modeling and comparison with previous basaltic melting experiments indicate that the felsic volcanics are likely produced by partial melting of hydrated mafic crust rather than fractional crystallization of high-Al basalt. It is also supported by relatively large amounts of felsic volcanics to high-Al basalts and remarkably different incompatible element ratios (e.g., Th/Zr, Nb/Zr and U/Zr) of the rocks. The Bogda felsic volcanics have positive ε_Nd(t) values (6.2–7.4), low Pb isotopes and low zircon saturation temperatures, consistent with a derivation from a juvenile crust in an arc setting. The intermediate ignimbrites display melting–mingling textures and abundant feldspar aggregates and have various δEu ratios, indicating that magma mingling and feldspar fractionation processes may have played an important role in the genesis of the ignimbrites. In contrast, the Early Permian felsic rocks in this region are of post-collisional A-type. We therefore propose that the Bogda belt was an island arc in the Late Carboniferous and then switched to a post-collisional setting in the Early Permian due to the arc–arc collision at the end of the Late Carboniferous.     

Geology, Geochemistry and Tectonic Significance of Mafic-ultramafic Rocks of Mesoproterozoic Phulad Ophiolite Suite of South Delhi Fold Belt, NW Indian Shield

A thick sequence of mafic-ultramafic rocks, occurs along a major shear zone (Phulad lineament), running across the length of Aravalli Mountain Range for about 300 kms. It has been suggested, that this sequence may represent a fragment of ophiolite or a rift related metavolcanic suite made up of basalts and fractionated ultramafics. The geological and tectonic significance of the complex is assessed using field relationships, petrography and geochemistry. Structurally, the lowest part of the complex comprises a discontinuous band of plastically deformed harzburgite (mantle component) followed by layered cumulus gabbroic rocks (crustal component). A complex of non-cumulus rocks comprising hornblende schists, gabbros, sheeted dykes and pillowed basalts structurally overlies layered gabbros. Huge bodies of diorite intrude volcanics.

Geochemical classification suggests that all non-cumulus mafic rocks are sub-alkaline basalts except one variety of dykes which shows mildly alkaline character. The sub-alkaline rocks are tholeiite to calc-alkaline with boninite affinity. Tectono-magmatic variation diagrams and MORB normalised patterns suggest a fore arc tectonic regime for the eruption of these rocks.

The mafic rocks of Phulad Ophiolite Suite are zoned across the strike in terms of their distribution from west to east. The hornblende schists and basalts are exposed at the westernmost margin followed by gabbros and dykes. The alkaline dyke occurs at the easternmost part. The rocks of Phulad suite are juxtaposed with shallow water sediments in the east followed by platformal sediments and then continental slope sediments in the further east indicating gradual thickening of the crust from west to east and an eastward subduction. The geochemical interpretation presented in this study, together with discussion of lithological association is used to decipher the tectonic evolution of the Mesoproterozoics of NW Indian shield.     

陕西商州地区丹凤变质火山岩的地球化学特征

陕西商州地区丹凤变质火山岩具有洋内岛弧火山岩地球化学特征,它们是分别来自不同源区的拉斑玄武和钙碱性2个系列共存的一套变质火山岩。其Th/Ta比值高及Ni、Ta、Ti、Y和Yb含量低,表明岩石受到消减带组分的影响。种种证据表明,丹凤变质火山岩是早古生代华北地块南缘消减带之上洋内岛弧环境的产物。     

东天山自然铜矿化带玄武岩的起源、演化及成岩构造背景

新疆东天山地区与玄武岩有关的自然铜矿化带位于东天山觉罗塔格构造带内,自西向东有十里坡、黑龙峰、长城山、东尖峰等主要矿(化)点,自然铜矿化主要发育在玄武岩、杏仁状玄武岩及凝灰岩夹层中。本文基于玄武岩的地球化学特征,研究东天山自然铜矿化带玄武岩是否与地幔柱有关、岩浆源区性质、岩浆演化、成岩构造背景等问题。研究显示,东天山自然铜矿化带玄武岩与地幔柱岩浆活动无直接关系,整个天山地区是否存在石炭-二叠纪地幔柱岩浆活动也需要进一步的研究; 玄武岩起源于亏损岩石圈地幔,是演化岩浆的产物; 演化的玄武质岩浆形成后,在岩浆房中或上升至地表的过程中没有发生明显的分离结晶作用,也没有受到明显的地壳物质混染; 与玄武岩对应的地幔橄榄岩平衡原生岩浆演化的高镁岩浆的产物,可能为东天山地区与铜镍硫化物成矿有关的基性-超基性岩,指示这些铜镍硫化物矿床可能与地幔柱岩浆活动也没有关系; 玄武岩形成于新疆北部后碰撞构造阶段的伸展期,是在拉张应力体制下,由于软流圈上涌导致岩石圈地幔部分熔融而形成。     

西准噶尔早泥盆世马拉苏组火山岩岩石成因研究

新疆西准噶尔北部地区的早泥盆世马拉苏组出露有少量富钠低钾的拉斑质中基性熔岩,这些分布于谢米斯台断裂北侧的玄武安山岩和玄武岩多呈夹层状断续产出于火山碎屑岩之中。马拉苏中基性熔岩的Mg#与主、微量元素协变关系及Th-Th/Nd图反映了其并非同源岩浆演化的结果。马拉苏火山岩中的玄武安山岩富集LILE、亏损HFSE,具有较高的Th含量及较低的Hf/Th和(Nb/Th)PM比值,显示出弧火山岩的地球化学特征。其中的玄武岩则具有略为平坦的稀土元素分配样式,较低的Th含量及较高的Hf/Th和(Nb/Th)PM比值,此同MORB地球化学特征极为相似;虽然其也显示有轻微的LILE富集、HFSE亏损,但是较高的La/Nb比值则暗示这同地壳或俯冲物质组分的卷入有关,且一系列构造环境判别图解也进一步印证了马拉苏组内的玄武岩应属似MORB基性熔岩。此外,两类岩石的高场强元素比值Zr/Nb、Hf/Ta同全球平均大洋中脊玄武岩的相应比值极为接近,反映了马拉苏组中基性火山岩的物质源区主体均为MORB地幔物质源区。La/Yb-Gd/Yb原始地幔标准化比值的模拟计算进一步显示了马拉苏组玄武安山岩与受改造(俯冲沉积物或地壳物质的混染)的石榴子石或尖晶石-石榴子石地幔橄榄岩物质源区的部分熔融作用有关,而似MORB型玄武岩则源自尖晶石地幔橄榄岩源区的部分熔融。结合区内同期的蛇绿岩、火山岩和碱性花岗岩的地球化学研究,我们可以进一步推断此类兼具有似MORB和弧火山岩地球化学特征的早泥盆世马拉苏火山岩应当是西准噶尔地块北部在早古生代受后期俯冲作用影响下经历弧后扩张形成的火山-岩浆地质记录。     

Ar/Ar age and geochemistry of the post-collisional Miocene Yamadağ volcanics in the Arapkir area (Malatya Province), eastern Anatolia, Turkey

The Neogene Yamadağ volcanics occupy a vast area between Sivas and Malatya in eastern Anatolia, Turkey. These volcanic rocks are characterized by pyroclastics comprising agglomerates, tuffs and some small outcrops of basaltic–andesitic–dacitic rocks, overlain upward by basaltic and dacitic rocks, and finally by basaltic lava flows in the Arapkir area, northern Malatya Province. The basaltic lava flows in the Arapkir area yield a ⁴⁰Ar/³⁹Ar age of 15.8 ± 0.2 Ma, whereas the dacitic lava flows give ⁴⁰Ar/³⁹Ar ages ranging from 17.6 through 14.7 ± 0.1 to 12.2 ± 0.2 Ma, corresponding to the Middle Miocene. These volcanic rocks have subalkaline basaltic, basaltic andesitic; alkaline basaltic trachyandesitic and dacitic chemical compositions. Some special textures, such as spongy-cellular, sieve and embayed textures; oscillatory zoning and glass inclusions in plagioclase phenocrysts; ghost amphiboles and fresh biotite flakes are attributable to disequilibrium crystallization related to magma mixing between coeval magmas. The main solidification processes consist of fractional crystallization and magma mixing which were operative during the soldification of these volcanic rocks. The dacitic rocks are enriched in LILE, LREE and Th, U type HFSE relative to the basaltic rocks. The basaltic rocks also show some marked differences in terms of trace-element and REE geochemistry; namely, the alkaline basaltic trachyandesites have pronounced higher HFSE, MREE and HREE contents relative to the subalkaline basalts. Trace and REE geochemical data reveal the existence of three distinct magma sources – one subalkaline basaltic trachyandesitic, one alkaline basaltic and one dacitic – in the genesis of the Yamadağ volcanics in the Arapkir region. The subalkaline basaltic and alkaline basaltic trachyandesitic magmas were derived from an E-MORB type enriched mantle source with a relatively high- and low-degree partial melting, respectively. The magmatic melt of dacitic rocks seem to be derived from an OIB-type enriched lithospheric mantle with a low proportion of partial melting. The enriched lithospheric mantle source reflect the metasomatism induced by earlier subduction-derived fluids. All these coeval magmas were generated in a post-collisional extensional geodynamic setting in Eastern Anatolia, Turkey.     

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.