Petrogeochemical aspects of the Late Cretaceous and Paleogene ignimbrite volcanism of East Sikhote-Alin

The chemical and trace-element features of the Late Cretaceous and Early Paleogene ignimbrite complexes of East Sikhote Alin are discussed. The Turonian-Campanian volcanic rocks of the Primorsky Complex compose linear structure of the Eastern Sikhote Alin volcanic belt. They are represented by crystalrich rhyolitic, rhyodacitic, and dacitic S-type plateau ignimbrites produced by fissure eruptions of acid magmas. The Maastrichtian-Paleocene volcanic rocks occur as isolated volcanic depression and caldera structures, which have no structural and spatial relations with the volcanic belt. This period is characterized by bimodal volcanism. The Samarginsky, Dorofeevsky, and Severyansky volcanic complexes are made up of basalt-andesite-dacite lavas and pyroclastic rocks, while the Levosobolevsky and Siyanovsky complexes are comprised of rhyolitic and dacitic tuffs and ignimbrites. Petrogeochemically, the felsic volcanic rocks are close to the S-type plateau ignimbrites of the Primorsky Complex. The Paleocene-Early Eocene silicic volcanics of the Bogopolsky Complex are represented by S- and A-type dacitic and rhyolitic tuffs and ignimbrites filling collapsed calderas. The eruption of A-type ferroan hyaloignimbrites occurred at the final stage of the Paleogene volcanism (Bogopolsky Complex). The magmatic rocks show well expressed mineralogical and geochemical evidence for the interaction between the crustal magmas and enriched sublithospheric mantle. It was shown that the revealed differences in the mineralogical and geochemical composition of the ignimbrite complexes are indicative of a change in the geodynamic regime of the Asian active continental margin at the Mesozoic-Cenozoic transition.     

Cenozoic volcanism of the eastern Sikhote Alin: Petrological studies and outlooks

Based on geological and isotope geochemical data obtained during the past decade, the eastern Sikhote Alin volcanic belt can be considered as a polygenic structure with spatially superimposed magmatic complexes of different geodynamic stages. Only Late Cretaceous intermediate and silicic volcanics enriched in LILE and depleted in HFSE can be interpreted as typical subduction complexes. Cenozoic lavas of mainly basic composition were formed after the termination of active subduction under complex dynamic conditions of the rearrangement of eastern Eurasia owing to the collision with the Indian plate. The eruption of Eocene-Oligocene-early Miocene basalts corresponded to the transform continental margin environment, rupture of an ancient subducted slab, and upwelling of hot depleted oceanic asthenosphere of the Pacific MORB-type into the Asian subcontinental lithosphere with EMII-like isotopic characteristics. The late Miocene-Pliocene magmatic activity of the eastern Sikhote Alin showed an intraplate character, but the composition of erupted magmas was strongly affected by previous tectonomagmatic events: subduction of different ages and opening of the Sea of Japan Basin. The distinct EMI isotopic signature of low-potassium plateau basalts, which is not observed in the lavas of earlier stages of volcanic belt evolution, suggests that the continental asthenosphere contributed to magma formation, and the direction of mantle flows changed owing to the formation of a new subduction zone.     

Crustal Contamination of Early Cretaceous Magmatic Rocks of the Western Transbaikal Rifting Zone

Results of isotope Sr, Ns, and O analyses of volcanic rocks from the Uda sector of the West Transbaikal Rift Zone have allowed estimation of the character of interaction of their parental mantle melts with crustal rocks. The smallest magnitude of this interaction has been found in the compositions of Late Cretaceous (83–70 Ma) volcanics, the geochemical and isotope markers of which suggest their derivation from a moderately enriched mantle compositionally resembling OIB sources. The Early Cretaceous volcanics were derived from mantle sources that included a mantle enriched by subduction. While ascending through the crust, the parental melts of the Uda Complex (130–111 Ma) were contaminated by the lower crust matter. The Zazin Complex magmas (143–135 Ma) have features suggesting their interaction with upper crustal granitoids of the Angara–Vitim Batholith.

     

Composition and Depositional Settings of Lower Cretaceous Terrigenous Rocks of the Kema River Basin,Eastern Sikhote Alin

Based on structure, mineralogy, petrography, and geodynamic setting of sedimentation, Barremian(?)-Albian terrigenous rocks in the Kema terrane (Eastern Sikhote Alin) are interpreted as back-arc rocks of the Moneron-Samarga island-arc system. The composition of terrigenous rocks indicates that an ensialic volcanic island arc, the basement of which was composed of the oceanward-advancing continental crust fragment, served as the main source of clastic material. Genetic features of rocks suggest their formation in the lower zone and near the foothill of submarine slope, as well as in adjacent areas of the basin plain. Accumulation of the thick gravitational sequence in the rear zone of the island arc was accompanied by active volcanic processes.     

Lithological composition of island-arc complexes in the Russian Far East

The results of study of geochemistry of terrigenous rocks from the contrast (in structure) Cretaceous-Paleogene complexes of Sikhote Alin and Kamchatka are summarized. The data obtained were interpreted based on comparison with the geochemical composition of recent and ancient sediments accumulated in the well-known geodynamic settings. It is shown that the chemical composition of terrigenous rocks and some petrochemical ratios can serve as reliable indicators of various island-arc settings. These indicators make it possible to discriminate sufficiently reliably these settings in paleobasins of orogenic zones.     

Oxygen isotopes in the Cretaceous-Paleogene granites of Primorye and some problems of their genesis

The Cretaceous-Paleogene granites of the Eastern Sikhote Alin volcanic belt (ESAVB) and Late Cretaceous granitoids of the Tatibin Series (Central Sikhote Alin) are subdivided into three groups according to their oxygen isotope composition: group I with δ¹⁸O from +5.5 to +6.5‰, group II with δ¹⁸O from +7.6 to +10.2‰, and group III with less than +4.5‰. Group I rocks are similar in oxygen isotope composition to that of oceanic basalts and can be derived by melting of basaltic crust. Group II (rocks of the Tatibin Series) have higher δ¹⁸O, which suggests that their parental melts were contaminated by sedimentary material. The low ¹⁸O composition of group III rocks can be explained by their derivation from ¹⁸O-depleted rocks or by subsolidus isotopic exchange with low-¹⁸O fluid or meteoric waters. The relatively low δ¹⁸O and ⁸⁷Sr/⁸⁶Sr in the granitoids of Primorye suggest their derivation from rocks with a short-lived crustal history and can result from the following: (1) melting of sedimentary rocks enriched in young volcanic material that was accumulated in the trench along the transform continental margin (granites of the Tatibin Series) and (2) melting of a mixture of abyssal sediments, ocean floor basalts, and upper mantle in the lithospheric plate that subsided beneath the continent in the subduction zone (granites of the ESAVB).     

Geochemical constraints on the origin of mafic and silicic magmas at Cordón El Guadal, Tatara-San Pedro Complex, central Chile

The aim of this study is to quantify the crustal differentiation processes and sources responsible for the origin of basaltic to dacitic volcanic rocks present on Cordón El Guadal in the Tatara-San Pedro Complex (TSPC). This suite is important for understanding the origin of evolved magmas in the southern Andes because it exhibits the widest compositional range of any unconformity-bound sequence of lavas in the TSPC. Major element, trace element, and Sr-isotopic data for the Guadal volcanic rocks provide evidence for complex crustal magmatic histories involving up to six differentiation mechanisms. The petrogenetic processes for andesitic and dacitic lavas containing undercooled inclusions of basaltic andesitic and andesitic magma include: (1) assimilation of garnet-bearing, possibly mafic lower continental crust by primary mantle-derived basaltic magmas; (2) fractionation of olivine + clinopyroxene + Ca-rich plagioclase + Fe-oxides in present non-modal proportions from basaltic magmas at ∼4–8 kbar to produce high-Al basalt and basaltic andesitic magmas; (3) vapor-undersaturated (i.e., P _H2O<P _TOTAL) partial melting of gabbroic crustal rocks at ∼3–7 kbar to produce dacitic magmas; (4) crystallization of plagioclase-rich phenocryst assemblages from dacitic magmas in shallow reservoirs; (5) intrusion of basaltic andesitic magmas into shallow reservoirs containing crystal-rich dacitic magmas and subsequent mixing to produce hybrid basaltic andesitic and andesitic magmas; and (6)␣formation and disaggregation of undercooled basaltic andesitic and andesitic inclusions during eruption from shallow chambers to form commingled, mafic inclusion-bearing andesitic and dacitic lavas flows. Collectively, the geochemical and petrographic features of the Guadal volcanic rocks are interpreted to reflect the development of shallow silicic reservoirs within a region characterized by high crustal temperatures due to focused basaltic activity and high magma supply rates. On the periphery of the silicic system where magma supply rates and crustal temperatures were lower, cooling and crystallization were more important than bulk crustal melting or assimilation. Received: 2 July 1997 / Accepted: 25 November 1997     

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.     

Collision-related slab break-off volcanism in the Eastern Anatolia,Kepez volcanic complex (TURKEY)

The Neogene–Quaternary volcanic products, related to Arabian and Anatolian Plate collision along the Bitlis Suture Zone, cover wide areas on both plates. One of these volcanic exposures on the Arabian Plate is the Kepez volcanic complex (KVC). This study aims explain to petrogenesis of KVC. Although some examples display alkaline affinities, the majority of the volcanic rock is calc-alkaline and can be defined in three main groups. ⁴⁰Ar/³⁹Ar data obtained from dacite, basalt and andesite rock groups within the KVC yield ages of between 13.5 and 15.5 Ma. Geochemical and petrographical data show that the andesitic rocks are products of homogeneous mixing between basic end-member magmas and dacitic magmas which are the products of partial melting of lower crustal compositions. Basaltic products of KVC are asthenospheric mantle derived, while dacitic and andesitic volcanic rocks are crustal origin. High Sr and Nd isotope ratios may indicate that andesitic and dacitic rocks originated from continental crust. The lithospheric mantle, which is subducting underneath the Anatolian plate, must have experienced slab break-off processes 13–15 million years ago and sunk into the asthenosphere. KVC were produced with the collision between Arabian and Anatolian Plates and related uplift of the East Anatolia region.     

Petrology and Ar/Ar chronology of Erdembaba and Kuyucak volcanics exposed along the North Anatolian Fault Zone (Eastern Pontides,NE Turkey): Implications for the late Cenozoic geodynamic evolution of Eastern Mediterranean region

The Cenozoic geodynamic evolution of the Eastern Pontides Orogenic Belt (EPOB), which geographically corresponds to the northeastern part of Turkey, is still controversial due to lack of systematic geological, geochemical and chronological data. This paper provides new geochemical and chronological data from the Kuyucak and Erdembaba volcanics exposed along the North Anatolian Fault Zone, which is one of the most seismically-active intracontinental strike-slip fault systems on the globe, in the Reºadiye (Tokat) area that is situated in the southern part of the EPOB. Kuyucak volcanics consist predominantly of basaltic rocks including mainly olivine, augite and plagioclase. Erdembaba volcanics comprise mainly dacite and minor trachydacite-trachyandesite that are composed mainly of augite, plagioclase, hornblende and rarely biotite. The Geochemical properties of the Erdembaba volcanic define small amounts of magma mixing events and it follow a linear trend from a primitive end member through to upper crustal evolution. The ⁴⁰Ar/³⁹Ar age determinations of whole rock of volcanic rock samples yielded the ages ranging from 1.33 to 6.31 Ma (late Miocene- Pleistocene). Both groups display an enrichment in LIL and LREE elements relative to HFS and HREE elements, strong negative Nb and Ti anomalies. All geochemical and Ar/Ar chronological data indicate that the late Miocene Kuyucak volcanic rocks, which are characterized by low concentrations of SiO₂ (&lt;50 wt.%), were generated by mantlederived magmas that underwent contamination processes during their transfer to the surface. Whereas, Erdembaba volcanic rocks including high concentrations of SiO₂ were derived from partial melting of the plunging continental crust beneath the Eurasian plate in Pliocene, after closure of Neotethys ocean that was situated in the southern part of the EPOB.     

内蒙古乌兰盖盆地中生代火山岩年代学、地球化学特征及其地质意义

笔者对大兴安岭西部乌兰盖盆地南、北缘出露的中生代火山岩进行了详细的岩石学、激光全熔⁴⁰Ar/³⁹Ar测年及地球化学研究,探讨了中生代火山岩成因与地质意义。乌兰盖盆地南、北缘的中生代火山岩主要为中性岩(安山岩)与酸性岩(流纹岩)类,各类岩石总体激光全熔⁴⁰Ar/³⁹Ar定年结果为(151.8±1.5)~(120.2±1.8) Ma,说明其形成时代总体为晚侏罗世晚期至早白垩世;岩石总体为一套钙碱性系列到高钾钙碱性系列的中性至酸性岩石组合,各类岩石地球化学特征与壳源岩石的地球化学特征基本一致,表明它们应来自于地壳物质局部熔融形成的壳源岩浆系列。该套火山岩形成于蒙古鄂霍茨克洋(古太平洋)闭合碰撞造山构造背景,在早白垩世151.8 Ma左右区内曾发生地壳加厚的造山过程,其岩浆深部动力学背景与岩浆源区的性质主要归因于增厚的造山带下地壳发生的部分熔融作用。     

Cenozoic forearc gabbros from the northern zone of the Eastern Pontides Orogenic Belt,NE Turkey: Implications for slab window magmatism and convergent margin tectonics

The Eastern Pontides Orogenic Belt represents one of the best examples of fossil convergent margins in the eastern Mediterranean region. However, the origin and geodynamic setting of the late Mesozoic–Cenozoic magmatism in this belt remain controversial due to lack of systematic geological, geochemical and chronological data. The general consensus is that the late Mesozoic–Cenozoic igneous activity is related to northward subduction of oceanic lithosphere in the late Mesozoic and following collision between Tauride and Pontide blocks in the early Cenozoic. Here we present a comprehensive study focusing on the origin and geodynamic setting of gabbro bodies exposed along a narrow zone, parallel to the southeastern coast of the eastern Black Sea basin, in the Northern Zone of the Eastern Pontides Orogenic Belt.The studied gabbro bodies are hosted within late Cretaceous basaltic, andesitic, and dacitic volcanics including pyroclastic rocks and interbedded sedimentary rocks. The gabbro bodies range in size from 0.1 km² to 1.5 km², and outcrop patterns vary from round or elliptical to markedly elongate with sharp and discordant contact with the host rocks. Their mineral assemblage includes mainly clinopyroxene, plagioclase, minor olivine, amphibole, magnetite and rarely orthopyroxene, biotite, zircon and titanite. The occurrence of sutured grain boundaries on clinopyroxene and plagioclase, and the presence of reverse compositional zoning in clinopyroxene and olivine suggest mixing between magmas of contrasting compositions during mineral growth. Thermobarometric computations indicate that the temperature at the beginning of crystallization was ~ 1250 °C and crystallization was polybaric. Zircon and titanite U–Pb ages indicate that these small intrusions were emplaced into crustal rocks of the Eastern Pontides Orogenic Belt during Lutetian (45 ± 2 Ma). The depletion of HFSE is consistent with the involvement of an arc-related source in the petrogenesis of these rocks, and low to moderate enrichment Ce, Rb, Ba, K, Pb, Sr and Th suggests that involvement of subducted oceanic sediment was modest. The low Th content and low Th/Yb indicate that the role of sediment addition was nevertheless limited. The Nd, Sr and Pb isotopic data are consistent with the interpretation that the dominant source component in these gabbros is a depleted, peridotitic mantle, and that crustal contamination is relatively unimportant. We suggest that mafic magmas that produced the gabbroic intrusions were derived from melting of a depleted mantle source under the forearc region of the Eastern Pontides Orogenic Belt during southward subduction of two oceanic plates separated by a mid-ocean ridge, leading to the formation of a slab window. We also infer fractional crystallization and assimilation during both magma storage in the crust–mantle transition zone and transfer into the overlying arc crust.     

Nature of voluminous meimechite–picrite associations in Siberia and other regions

Analysis of petrochemical and geochemical information of the same levels, which characterize rocks and primary melt inclusions in olivines of heterochronic meimechite–picrite associations in Siberia (Maimecha–Kotui province), Primorye (Sikhote–Alin), and Kamchatka demonstrated that, besides the similar appearance and identical structural patterns, they are considerably discrepant in the concentration and distribution of incompatible and rare earth elements. Those differences are also observed for the compositions and evolutionary trends of parental high-temperature magnesium-rich melts. This, in turn, was assumed to be a consequence of a variable degree of melting of the mantle protoliths in the mentioned regions, which is supported by geochemical modeling.     

富闪深成岩的成因及其地球动力学意义

富闪深成岩是一套以角闪石为标志矿物的侵入杂岩,可由角闪石岩、角闪辉长岩、角闪闪长岩、(英云)闪长岩、黑云母花岗岩等不同酸度系列的岩石组成。在元素地球化学方面,富闪深成岩可呈低钾拉斑质、钙碱性或钾玄质,富集大离子亲石元素和轻稀土元素。起源于经俯冲作用改造的岩石圈地幔或软流圈地幔部分熔融的基性母岩浆通过分离结晶、同化-混染,或与壳源岩浆混合,形成富闪深成岩系列中不同的岩性端元。高水含量作为富闪岩浆体系的基本特征不仅塑造了杂岩体的标志性角闪石矿物学特征,也主导了岩浆的钙碱性演化趋势。富闪杂岩形成环境相对特殊,通常产于板块会聚终末期的洋脊-海沟交互场景和后碰撞阶段,并受到同时期区域构造作用控制。基于野外产出上的共生性和地球化学的相似性,富闪深成岩与高Ba-Sr花岗岩、晚太古代赞岐岩类存在密切的成因关联,对其进行类比研究,不仅有助于富闪深成岩成因解析,也可为探讨大陆地壳生长-分异机制及地球早期构造体制提供崭新视角。最后,作为显生宙克拉通早期破坏过程的重要岩浆记录,华北克拉通北缘发育的多期晚古生代富闪深成杂岩有望为揭开富闪深成岩成因和地球动力学奥秘提供重要的野外实验室。     

大陆板片-地幔相互作用:来自大别造山带碰撞后安山质火山岩的地球化学证据

俯冲到地幔深度的地壳物质不可避免地在板片-地幔界面与地幔楔发生相互作用,由此形成的超镁铁质交代岩就是造山带镁铁质火成岩的地幔源区.因此,造山带镁铁质火成岩为研究俯冲地壳物质再循环和壳-幔相互作用提供了重要研究对象.为了揭示俯冲陆壳物质再循环的机制和过程,对大别造山带碰撞后安山质火山岩开展了元素和同位素地球化学研究.这些安山质火山岩的SIMS锆石U-Pb年龄为124±3~130±2 Ma,表明其形成于早白垩世.此外,残留锆石的U-Pb年龄为中新元古代和三叠纪,分别对应于大别-苏鲁造山带超高压变火成岩的原岩年龄和变质年龄.它们具有岛弧型微量元素特征、富集的Sr-Nd-Hf同位素组成,以及变化的且大多不同于正常地幔的锆石δ18O值.这些元素和同位素特征指示,这些安山质火山岩是交代富集的造山带岩石圈地幔部分熔融的产物.在三叠纪华南陆块俯冲于华北陆块之下的过程中,俯冲华南陆壳来源的长英质熔体交代了上覆华北岩石圈地幔楔橄榄岩,大陆俯冲隧道内的熔体-橄榄岩反应产生了富沃、富集的镁铁质地幔交代岩.这种地幔交代岩在早白垩世发生部分熔融,就形成了所观察到的安山质火山岩.因此,碰撞造山带镁铁质岩浆岩的地幔源区是通过大陆俯冲隧道内板片-地幔相互作用形成的,而加入地幔楔中长英质熔体的比例决定了这些镁铁质岩浆岩的岩石化学和地球化学成分.      

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.     

辽西北票蓝旗组火山岩锆石U-Pb年龄和Hf同位素组成

辽西北票常河营子地区有中生代蓝旗组火山岩分布,其中上部安山质角砾熔岩的锆石LA-ICPMS U-Pb年龄分析结果表明,其结晶年龄为159.4±3.4Ma,属晚侏罗世.锆石~(176)Hf/~(177)Hf比值介于0.282098～0.282789之间,ε_(Hf)(t)值为-20.4～+4.1,主体分布在华北克拉通地壳演化线之上,位于古元古代地壳演化范围内,所给出的亏损地幔年龄(t_(DM))和平均地壳模式年龄(t_(crust))分别为0.7～1.6Ga和0.9～2.5Ga.结合已发表蓝旗组中酸性火山岩的岩石地球化学及Sr-Nd同位素组成特征,我们认为安山质火山岩源于古老(如晚太古代)下地壳玄武质岩石的部分熔融,其形成过程可能与中生代幔源岩浆底侵作用有关.     

Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling

In the Northern Andes of Ecuador, a broad Quaternary volcanic arc with significant across-arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes. Quaternary volcanic centers occur, from west to east, along the Western Cordillera (frontal arc), in the Inter-Andean Depression and along the Eastern Cordillera (main arc), and in the Sub-Andean Zone (back-arc). The adakite-like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt–mantle interactions or of lower crustal melting, fractional crystallization, and assimilation processes. In this paper, we present petrographic, geochemical, and isotopic (Sr, Nd, Pb) data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers (Pululagua, Pichincha, Ilalo, Chacana, Sumaco) forming a NW–SE transect at about 0° latitude and encompassing the frontal (Pululagua, Pichincha), main (Ilalo, Chacana), and back-arc (Sumaco) chains. All rocks display typical subduction-related geochemical signatures, such as Nb and Ta negative anomalies and LILE enrichment. They show a relative depletion of fluid-mobile elements and a general increase in incompatible elements from the front to the back-arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab. We observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene- and/or amphibole-bearing cumulates and of magma mixing at all frontal and main arc volcanic centers. Within each volcanic center, rocks display correlations between evolution indices and radiogenic isotopes, although absolute variations of radiogenic isotopes are small and their values are overall rather primitive (e.g., ε_Nd = +1.5 to +6, ⁸⁷Sr/⁸⁶Sr = 0.7040–0.70435). Rare earth element patterns are characterized by variably fractionated light to heavy REE (La/Yb_N = 5.7–34) and by the absence of Eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation. We interpret the petrographic, geochemical, and isotopic data as indicating open-system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower- to mid-crustal levels as well as by assimilation of mafic lower crust and/or its partial melts. Thus, we propose that the adakite-like signatures of Ecuadorian rocks (e.g., high Sr/Y and La/Yb values) are primarily the result of lower- to mid-crustal processing of mantle-derived melts, rather than of slab melts and slab melt–mantle interactions. The isotopic signatures of the least evolved adakite-like rocks of the active and recent volcanoes are the same as those of Tertiary ”normal” calc-alkaline magmatic rocks of Ecuador suggesting that the source of the magma did not change through time. What changed was the depth of magmatic evolution, probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic Carnegie Ridge.     

Volcanic rocks of the Khabarovsk accretionary complex,southern Far East Russia

The results of study of the volcanic rocks of the Khabarovsk accretionary complex, a fragment of the Jurassic accretionary prism of the Sikhote Alin orogenic belt (the southern part of the Russian Far East), are presented. The volcanic rocks are associated with the Lower Permian limestones in the mélange blocks and Triassic layered cherts. The petrography, petrochemistry, and geochemistry of the rocks are characterized and their geodynamic formation conditions are deduced. The volcanic rocks include oceanic plume basalts of two types: (i) OIB-like intraplate basalts formed on the oceanic islands and guyots in the Permian and Triassic and (ii) T(transitional)-MORBs (the least enriched basalts of the E-MORB type) formed on the midoceanic ridge in the Permian. In addition to basalts, the mélange hosts suprasubduction dacitic tuff lavas.     

Zircon evaporation ages and geochemistry of metamorphosed volcanic rocks from the Vinjamuru domain,Krishna Province: evidence for 1.78 Ga convergent tectonics along the southeastern margin of the Eastern Dharwar Craton

Palaeoproterozoic intermediate to potassic felsic volcanism in volcano‐sedimentary sequences could either have occurred in continental rift or at convergent magmatic arc tectonic settings. The Vinjamuru domain of the Krishna Province in Andhra Pradesh, SE India, contains such felsic and intermediate metavolcanic rocks, whose geochemistry constrains their probable tectonic setting and which were dated by the zircon Pb evaporation method in order to constrain their time of formation. These rocks consist of interlayered quartz–garnet–biotite schist, quartz–hematite–baryte–sericite schist as well as cherty quartzite, and represent a calc‐alkaline volcanic sequence of andesitic to rhyolitic rocks that underwent amphibolite‐facies metamorphism at ~1.61 Ga. Zircons from four felsic metavolcanic rock samples yielded youngest mean ²⁰⁷Pb/²⁰⁶Pb ages between 1771 and 1791 Ma, whereas the youngest zircon age for a meta‐andesite is 1868 Ma. A ~2.43 Ga zircon xenocryst reflects incorporation of Neoarchaean basement gneisses. Their calc‐alkaline trends, higher LILE, enriched chondrite‐normalized LREE pattern and negative Nb and Ti anomalies on primitive mantle‐normalized diagrams, suggest formation in a continental magmatic arc tectonic setting. Whereas the intermediate rocks may have been derived from mantle‐source parental arc magmas by fractionation and crustal contamination, the rhyolitic rocks had crustal parental magmas. The Vinjamuru Palaeoproterozoic volcanic eruption implies an event of convergent tectonism at the southeastern margin of the Eastern Dharwar Craton at ~1.78 Ga forming one of the major crustal domains of the Krishna Province. Copyright © 2012 John Wiley & Sons, Ltd.