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1.
The isotopic-geochemical composition of the Late Paleocene-Early Eocene basalts in the central part of the Kolyuchinskii-Mechigmen graben on the east of the Chukotka Peninsula is studied. The distribution of major and trace elements and the elemental ratios point to the formation of basalts under conditions of marginal-continental rifting. The basalts of Mount Otdel’naya are characterized by a combination of depleted, intraplate, and suprasubduction geochemical features, which differentiates them both from the rocks of suprasubduction volcanic belts and tholeiites and alkaline lavas of the continental rifts and oceanic island arcs. The specific peculiarities of basalts of Mount Otdel’naya are extremely high, relative to similar basalts of spreading zones of the region and the 87Sr/86Sr ratios, which reflect the involvement of carbonate material in the magma generation zone.  相似文献   

2.
青山群火山岩是华北克拉通破坏期间最具代表性的地幔或地壳熔融产物,记录了华北深部地质演化的重要信息。本文对胶东青山群基性火山岩进行了40Ar/39Ar定年和岩石地球化学分析,结合前人报道的胶东青山群酸性火山岩资料,发现:(1)基性火山岩喷发年龄为122～113Ma,早于青山群酸性火山岩(110～98Ma);(2)基性和酸性火山岩显示了不同的元素和同位素地球化学特征。岩石成因分析表明,基性火山岩为交代富集地幔部分熔融作用的产物,而酸性火山岩为古老下地壳和中生代底侵岩浆的熔融产物(Ling et al.,2009)。因此,胶东地区青山群火山岩记录了岩浆熔融源区从地幔向下地壳的转变。这与长时间尺度的岩石圈减薄过程中热能由地幔向地壳传递过程相吻合,而不同于地壳拆沉作用所预测的岩浆演化趋势。  相似文献   

3.
The ∼133 Ma volcanic rocks of Sangxiu Formation are distributed in the eastern part of the central Tethyan Himalaya and belong paleogeographically to the northeastern margin of Greater India. These volcanic rocks include alkaline basalts and felsic volcanic rocks. Major and trace element abundances and whole-rock isotopic data for selected samples of these volcanic rocks are used to infer their petrogenesis. Geochemically, the Sangxiu basalts are closely similar to the Emeishan high-Ti basalts. Major and trace element data and Sr–Nd isotopic compositions suggest that the Sangxiu basalts may have been derived from an OIB-type mantle source, with discernable contributions from subcontinental lithospheric mantle (SCLM). The basaltic magmas may have formed as a result of the infiltration of plume-derived melts into the base of the lithosphere in a continental rift setting. The Sangxiu felsic volcanic rocks share most of the geochemical features of A-type granite, and have Sr–Nd isotopic compositions which differ considerably from the Sangxiu basalts, suggesting that they originated from the anatexis of ensialic continental crust. The Sangxiu volcanic rocks may be considered as the consequence of an interaction between the Kerguelen hotspot and the lithosphere of the northeastern margin of Greater India at ∼133 Ma, and may represent the initial stage of the separation of Greater India from southwestern Australia.  相似文献   

4.
The present-day North Chilean Coastal Cordillera between 18°30′S and 22°S records an important part of the magmatic evolution of the Central Andes during the Jurassic. Calc-alkaline to subordinate tholeiitic members from four rock groups with biostratigraphically constrained age display incompatible element pattern characteristic of convergent plate-margin volcanism, whereas alkaline basalts of one group occurring in the Precordillera show OIB-type trace element signatures. The correlation of biostratigraphic ages, regional distribution, and composition of the volcanic rocks provides a basis for the discussion on geochemical evolution and isotope ratios.Major and trace element distributions of the volcanic rocks indicate their derivation from mantle-derived melts. LILE and LREE enrichments in calc-alkaline basaltic andesites to dacites and some of the tholeiites hint at the involvement of hydrous fluids during melting and mobile element transport processes. A part of the Early Bajocian to ?Lower Jurassic and Oxfordian andesites and dacites are adakite-like rocks with a substantial participation of slab melt and are characterized by high Sr/Y ratios and low HREE contents. The Middle Jurassic tholeiitic and calc-alkaline basalts and basaltic andesites have been transported and partly stored within a system of deep-seated feeder fissures and crustal strike-slip faults before eruption.The isotopic composition of Sr (87Sr/86Sri=0.7032-0.7056) and Nd (εNdi=2.2-7.1) of the Jurassic volcanic rocks mostly fall in the range characteristic for mantle melts although some crustal components may have been involved. A few samples show slightly more radiogenic Sr isotopic composition, which is probably due to interaction with ancient sea-water. The Pb isotopic composition of the arc rocks is uncoupled from the isotopic composition of Sr and Nd and is dominated by the crustal component. Since the Cretaceous and Modern arc volcanic rocks show Pb isotopic compositions that can be largely explained by in situ Pb isotope growth of Jurassic arc volcanic rocks, we argue that the various Andean arc systems between 18°30′S and 22°S formed on the same type of basement.Most of the investigated samples have high Ba, Zr, and Th concentrations compared to island arc mafic volcanic rocks. About 20% of the Jurassic arc volcanics comprise of dacitic to rhyolitic rocks. These characteristics combined with the Pb isotopic composition that shows the influence of a Palaeozoic (or partly older) basement point to a continental margin setting for the North Chilean Jurassic arc. The distribution of the magmatic rocks throughout time, their textures, and the character of intercalated sedimentary rocks reflect westward movement of the magma sources and of the arc/back-arc boundary relative to the current coast line during the Early Bajocian on a broad front between 19°30′ and 21°S.  相似文献   

5.
The paper reports new isotope-geochemical data on Late Paleocene-Early Eocene basalts from the central part of the Kolyuchin-Mechigmen graben, eastern Chukotka Peninsula. The distribution of the major and trace elements and trace-element ratios indicates that the basalts were formed in a marginal-continental rift setting. The peculiar feature of the basalts is a combination of depleted within-plate and suprasubduction geochemical signatures, which make the volcanic rocks from Mt. Otdel’naya different from rocks of suprasubduction volcanic belts and from tholeiites and alkaline lavas of continental rifts and oceanic islands. Extremely high 87Sr/86Sr ratios in the studied basalts as compared to those of similar volcanic rocks from extension zones are probably related to the involvement of carbonate material in the magma generation zone.  相似文献   

6.
祁漫塔格地区十字沟蛇绿岩地质特征   总被引：7，自引：1，他引：6  
十字沟蛇绿岩产出于东昆仑西段祁漫塔格结合带东部,主要由蛇纹岩、辉长岩、辉绿岩、基性火山岩及硅质岩等组成。其中,从超基性岩—基性岩—基性火山岩,具稀土总量逐渐增加,轻稀土弱亏损型特征,基性火山岩为洋脊玄武岩,总体岩石化学成分与大洋中脊低钾拉斑玄武岩相当,三者的配分型式基本一致,说明其继承了源区地幔的特征,微量元素显示基性火山岩具弧后盆地玄武岩特点。Nεd(t)比值表明玄武岩来自亏损地幔源区,总体地球化学特征显示以N-M ORB型为主,兼具有E-M ORB型的特征,形成于与俯冲作用有关的弧后盆地环境,形成时代为晚奥陶世。后期至少经历了3期以上构造变形变质作用的叠加改造,于早志留世运移至地表。  相似文献   

7.
    洋岛玄武岩的元素和同位素地球化学特征可以示踪深部地幔的化学结构和化学演化过程。HIMU(Highμ,μ=238U/204Pbt=0) 型玄武岩是一类元素和同位素组成特殊的洋岛玄武岩,被认为与地幔柱中再循环的洋壳物质直接相关,因此,HIMU型玄 武岩的成因是地幔柱（热点） 研究中长期关注的话题。本文概述了HIMU型玄武岩的地球化学定义,对HIMU洋岛的分布、 火山演化阶段以及岩性变化做了综合阐述,并在对比经典HIMU型玄武岩与其他板内玄武岩元素地球化学特征、放射成因 同位素组成以及惰性气体同位素组成特征的基础上,简要探讨其源区组成和成因上的不同。  相似文献   

8.
Lithium isotope geochemistry is increasingly being used to trace deep-earth processes, reflecting the observed large variation of Li isotope ratios in mantle-derived rocks, including peridotite xenoliths associated with ancient continents. We briefly review the Li isotopic compositions of major geochemical reservoirs, the assumed mechanisms of Li isotopic fractionation, and, in particular, the origins of isotopically light Li in mantle-derived rocks based on the latest developments in Li isotope geochemistry. Comparison of Li isotope data with existing Sr-Nd isotope ratios reflects the subduction-recycling of ancient oceanic crust and the reappearance of Li in volcanic rocks. This circulation may play an important role in generating the isotopically light-Li component in the mantle – perhaps the enriched mantle end member defined by the Sr-Nd isotopic compositions of oceanic basalts.  相似文献   

9.
We present new regional petrologic, geochemical, Sr–Nd isotopic, and U–Pb geochronological data on the Turonian–Campanian mafic igneous rocks of Central Hispaniola that provide important clues on the development of the Caribbean island-arc. Central Hispaniola is made up of three main tectonic blocks—Jicomé, Jarabacoa and Bonao—that include four broad geochemical groups of Late Cretaceous mafic igneous rocks: group I, tholeiitic to calc-alkaline basalts and andesites; group II, low-Ti high-Mg andesites and basalts; group III, tholeiitic basalts and gabbros/dolerites; and group IV, tholeiitic to transitional and alkalic basalts. These igneous rocks show significant differences in time and space, from arc-like to non-arc-like characteristics, suggesting that they were derived from different mantle sources. We interpret these groups as the record of Caribbean arc-rifting and back-arc basin development in the Late Cretaceous. The> 90 Ma group I volcanic rocks and associated cumulate complexes preserved in the Jicomé and Jarabacoa blocks represent the Albian to Cenomanian Caribbean island-arc material. The arc rift stage magmatism in these blocks took place during the deposition of the Restauración Formation from the Turonian–Coniacian transition (~ 90 Ma) to Santonian/Lower Campanian, particularly in its lower part with extrusion at 90–88 Ma of group II low-Ti, high-Mg andesites/basalts. During this time or slightly afterwards adakitic rhyolites erupted in the Jarabacoa block. Group III tholeiitic lavas represent the initiation of Coniacian–Lower Campanian back-arc spreading. In the Bonao block, this stage is represented by back-arc basin-like basalts, gabbros and dolerite/diorite dykes intruded into the Loma Caribe peridotite, as well as the Peralvillo Sur Formation basalts, capped by tuffs, shales and Campanian cherts. This dismembered ophiolitic stratigraphy indicates that the Bonao block is a fragment of an ensimatic back-arc basin. In the Jicomé and Jarabacoa blocks, the mainly Campanian group IV basalts of the Peña Blanca, Siete Cabezas and Pelona–Pico Duarte Formation, represent the subsequent stage of back-arc spreading and off-axis non-arc-like magmatism, caused by migration of the arc toward the northeast. These basalts have geochemical affinities with the mantle domain influenced by the Caribbean plume, suggesting that mantle was flowing toward the NE, beneath the extended Caribbean island-arc, in response to rollback of the subducting proto-Caribbean slab.  相似文献   

10.
The Rhön area as part of the Central European Volcanic Province (CEVP) hosts an unusual suite of Tertiary 24-Ma old hornblende-bearing alkaline basalts that provide insights into melting and fractionation processes within the lithospheric mantle. These chemically primitive to slightly evolved and isotopically (Sr, Nd, Pb) depleted basalts have slightly lower Hf isotopic compositions than respective other CEVP basalts and Os isotope compositions more radiogenic than commonly observed for continental intraplate alkaline basalts. These highly radiogenic initial 187Os/188Os ratios (0.268–0.892) together with their respective Sr–Nd–Pb isotopic compositions are unlikely to result from crustal contamination alone, although a lack of Os data for lower crustal rocks from the area and limited data for CEVP basalts or mantle xenoliths preclude a detailed evaluation. Similarly, melting of the same metasomatized subcontinental lithospheric mantle as inferred for other CEVP basalts alone is also unlikely, based on only moderately radiogenic Os isotope compositions obtained for upper mantle xenoliths from elsewhere in the province. Another explanation for the combined Nd, Sr and Os isotope data is that the lavas gained their highly radiogenic Os isotope composition through a mantle “hybridization”, metasomatism process. This model involves a mafic lithospheric component, such as an intrusion of a sublithospheric primary alkaline melt or a melt derived from subducted oceanic material, sometime in the past into the lithospheric mantle where it metasomatized the ambient mantle. Later at 24 Ma, thermal perturbations during rifting forced the isotopically evolved parts of the mantle together with the peridotitic ambient mantle to melt. This yielded a package of melts with highly correlated Re/Os ratios and radiogenic Os isotope compositions. Subsequent movement through the crust may have further altered the Os isotope composition although this effect is probably minor for the majority of the samples based on radiogenic Nd and unradiogenic Sr isotope composition of the lavas. If the radiogenic Os isotope composition can be explained by a mantle-hybridization and metasomatism model, the isotopic compositions of the hornblende basalts can be satisfied by ca. 5–25% addition of the mafic lithospheric component to an asthenospheric alkaline magma. Although a lack of isotope data for all required endmembers make this model somewhat speculative, the results show that the Re–Os isotope system in continental basalts is able to distinguish between crustal contamination and derivation of continental alkaline lavas from isotopically evolved peridotitic lithosphere that was contaminated by mafic material in the past and later remelted during rifting. The Hf isotopic compositions are slightly less radiogenic than in other alkaline basalts from the province and indicate the derivation of the lavas from low Lu–Hf parts of the lithospheric mantle. The new Os and Hf isotope data constrain a new light of the nature of such metasomatizing agents, at least for these particular rocks, which represent within the particular volcanic complex the first product of the volcanism.  相似文献   

11.
Osmium isotopic compositions, and Re and Os concentrations have been examined in one komatiite unit and two komatiitic basalt units at Dundonald Beach, part of the 2.7 Ga Kidd-Munro volcanic assemblage in the Abitibi greenstone belt, Ontario, Canada. The komatiitic rocks in this locality record at least three episodes of alteration of Re-Os elemental and isotope systematics. First, an average of 40% and as much as 75% Re may have been lost due to shallow degassing during eruption and/or hydrothermal leaching during or immediately after emplacement. Second, the Re-Os isotope systematics of whole rock samples with 187Re/188Os ratios >1 were reset at ∼2.5 Ga, possibly due to a regional metamorphic event. Third, there is evidence for relatively recent gain and loss of Re in some rocks.Despite the open-system behavior, some aspects of the Re-Os systematics of these rocks can be deciphered. The bulk distribution coefficient for Os (DOssolid/liquid) for the Dundonald rocks is ∼3 ± 1 and is well within the estimated D values obtained for komatiites from the nearby Alexo area and stratigraphically-equivalent komatiites from Munro Township. This suggests that Os was moderately compatible during crystal-liquid fractionation of the magmas parental to the Kidd-Munro komatiitic rocks. Whole-rock samples and chromite separates with low 187Re/188Os ratios (<1) yield a precise chondritic average initial 187Os/188Os ratio of 0.1083 ± 0.0006 (γOs = 0.0 ± 0.6) for their well-constrained ∼2715 Ma crystallization age. The chondritic initial Os isotopic composition of the mantle source for the Dundonald rocks is consistent with that determined for komatiites in the Alexo area and in Munro Township, suggesting that the mantle source region for the Kidd-Munro volcanic assemblage had evolved with a long-term chondritic Re/Os before eruption. The chondritic initial Os isotopic composition of the Kidd-Munro komatiites is indistinguishable from that of the projected contemporaneous convective upper mantle. The uniform chondritic Os isotopic composition of the Kidd-Munro komatiites contrasts with the typical large-scale Os isotopic heterogeneity in the mantle sources for ca. 89 Ma komatiites from the Gorgona Island, arc-related rocks and present-day ocean island basalts. This suggests that the Kidd-Munro komatiites sampled a late-Archean mantle source region that was significantly more homogeneous with respect to Re/Os relative to most modern mantle-derived rocks.  相似文献   

12.
国坤  翟世奎  于增慧  蔡宗伟  张侠 《地球科学》2016,41(10):1655-1664
冲绳海槽是一个处于弧后扩张作用早期的年轻的弧后盆地,是研究弧后扩张作用早期盆地演化和壳幔过程的天然实验室.随着调查研究工作的逐步展开和深入,也发现了一些新的、重要的、亟待解决的科学问题,而火山岩岩石系列归属的厘定又是其他研究工作的基础.在系统收集和整理迄今已有冲绳海槽火山岩资料的基础上,结合近期分析测试数据, 对冲绳海槽火山岩的岩石系列归属进行了重新厘定,探讨了火山岩的构造环境指示意义和浮岩与玄武岩之间的成因联系.研究结果表明:冲绳海槽火山岩分布具有以基性玄武岩和酸性(流纹)英安岩为主的双峰式特征,中性火山岩稀少,基性的玄武岩属于亚碱性系列的橄榄拉斑玄武岩,酸性浮岩可归属为亚碱性岩系的流纹英安岩或流纹岩;在构造环境判别上,冲绳海槽玄武岩表现出大洋中脊和岛弧构造环境的特点,既有别于大洋中脊扩张中心,也有别于成熟型弧后盆地,呈现出弧后早期扩张阶段盆地独特的构造环境特征;广泛分布于冲绳海槽的酸性浮岩表现出一定的岛弧环境的特点;酸性浮岩与玄武岩具有同源性,酸性岩是基性的玄武质岩浆经不同程度结晶分异和同化混染作用的产物.   相似文献   

13.
《International Geology Review》2012,54(14):1768-1785
ABSTRACT

We analysed whole-rock major and trace elements and Sr-Nd-Pb-Hf isotopes of the late Cenozoic volcanic rocks in the Leizhou Peninsula, South China to investigate their mantle source characteristics. These volcanic rocks, collected from Jiujiang, Tianyang and Huoju areas of the Leizhou Peninsula, are characterized by incompatible element enrichment but variable isotopic depletion. The volcanic rocks from Jiujiang and Tianyang show prominent primitive-mantle-normalized positive Nb, Ta and Sr anomalies and depleted Sr-Nd-Pb-Hf isotope compositions, whereas those from Huoju show slight positive to negative Nb and Ta anomalies, a prominent positive Pb anomaly, and more enriched Sr-Nd-Pb-Hf isotope compositions. Two types of mantle metasomatism are required to explain the geochemical characteristics of these rocks. The Jiujiang and Tianyang samples were largely derived from a mantle source metasomatized recently by a low-F melt. Such low-F melt is generated within the asthenospheric mantle, which is enriched in volatiles and incompatible elements with positive Sr anomaly and depleted Sr-Nd-Pb-Hf isotope compositions. The Huoju samples were largely derived from a mantle source metasomatized by recycled upper continental crust material. These two types of mantle metasomatism beneath the Leizhou Peninsula are consistent with trace element characteristics of mantle mineralogy (e.g. clinopyroxene vs. amphibole), which reflects source evolution in space and time (e.g. tectonic setting change).  相似文献   

14.
位于中国东南部的三水盆地、珠江口盆地、雷琼半岛和北部湾地区广泛分布新生代火山岩。火山岩的形成时间具有从内陆向沿海变新的特点,早第三纪三水和珠江口盆地火山岩具有由玄武岩与粗面岩-流纹岩构成的双峰式特点。其中玄武岩和粗面岩的微量元素和稀土元素的配分形式相似,富集大离子亲石元素并且有相似的εNd(T)同位素组成(2.34～6.4),说明它们来自相同的地幔源区,为同源岩浆演化的产物。玄武岩和粗面岩经历了不同的结晶分异过程,其中玄武岩在较深部岩浆房中经历橄榄石和单斜辉石为主的分离结晶作用,而粗面岩则是在浅部岩浆房中由玄武岩浆分异形成的过渡性岩浆再经过强烈的钾长石和斜长石、以及磷灰石的结晶分异形成的。晚第三纪珠江口盆地和北部湾火山岩、雷琼半岛第四纪火山岩则由碱性和拉斑玄武岩构成。这些火山岩的形成时间和地球化学和同位素特征表明它们经历了连续的软流圈地幔上涌和部分熔融过程,受控于自晚中生代以来的地幔柱构造。南海的形成是地幔柱活动引起的地幔上涌和大陆裂解作用的结果。  相似文献   

15.
The paper reports newly obtained K-Ar isotopic-geochronological data on the age of three lava flows (Khrami, Mashavera, and Kura), which begin at the Dzhavakheti volcanic highland in southern Georgia. All of the dated rocks, including those from the Kura Flow, which was previously considered as the Pleistocene, are demonstrated to have a Pliocene age. The lavas of the longest Khrami Flow were erupted at 3.25–3.10 Ma, and those of the Kura and Mashavera Flows at 2.20–2.05 Ma, a fact testifying to two pulses of volcanic activity at the Dzhavakheti Highland. The petrogeochemical and isotopic characteristics of the rocks (87Sr/86Sr = 0.7039–0.7042; ∈Nd = 3.4–5.1) indicate that they are subalkaline within-plate basalts formed by the fractional crystallization of a basic mantle melt with the usually discontinuous selective or rarely continuous contamination with material that was not in geochemical equilibrium with the melt. The volcanics of the Khrami Flow are characterized by the less radiogenic Sr isotopic composition and the highest ∈Nd values, while the younger rocks of the Mashavera and Kura Flows have similar and more “crustal” isotopic signatures. The 87Sr/86Sr ratios of the Dzhavakheti subalkaline basalts are close to the initial Sr isotopic ratios of the Quaternary and Middle Pliocene dacite lavas from the same territory. Considered together with petrogeochemical and geological data, this suggests that all young rocks in Southern Georgia were produced in similar tectonic and geodynamic environments.  相似文献   

16.
《Precambrian Research》2005,136(1):51-66
SHRIMP U–Pb zircon age, geochemical and Sm–Nd isotopic results are reported for the Mamianshan volcanic rocks in the Cathaysia Block of southeastern South China. The Mamianshan volcanic rocks are bimodal in composition and are dominantly transitional to mildly alkaline basalts and subordinate alkaline rhyolite, with an eruption age of 818 ± 9 Ma. The basaltic samples are characterized by LREE-enriched and “humped” trace element patterns, similar to many alkali basalts in continental rifts. Variable ɛNd(T) values between +3.33 and −4.35 indicate that the primary magma of these basalts was derived from an OIB-like mantle source and underwent fractional crystallization plus crustal contamination. The rhyolitic rocks are highly enriched in Th, Ta, Nb, REE, Zr, Hf and Y and depleted in Sr, P, Eu and Ti, sharing affinity to A1-type granites. Combined with their slightly positive ɛNd(T) values (+0.22 to +0.92), the Mamianshan felsic rocks were most likely generated by partial melting of the regional Paleoproterozoic Mayuan amphibolites. The Mamianshan bimodal volcanic rocks in the Cathaysia Block are coeval with the widespread intraplate magmatism around the Yangtze Block. Our results support the idea that a coherent South China Craton was formed during the ca. 1.0 Ga Sibao orogeny, and it subsequently underwent extensive continental rifting related to mantle plume or superplume activities beneath Rodinia since ca. 825 Ma.  相似文献   

17.
Late Cenozoic volcanic province in Central and East Asia   总被引：2，自引：0，他引：2  
The paper presents materials on the inner structure of the Late Cenozoic within-plate volcanic province in Central and East Asia, in which two subprovinces are distinguished: Central Asian and Far Eastern, which comprise a number of autonomously evolving volcanic areas. Some of the volcanic areas are proved to have evolved for a long time, starting in the Late Mesozoic. In spite of differences in their age and structural setting, the volcanic areas evolved according to similar scenarios in the Late Cenozoic. Magmatism in the province was related to a mantle source of the within-plate type. The magmatic associations are dominated by mafic alkaline high-K rocks. The rocks are geochemically close to basalts of the OIB type, and their isotopic composition corresponds to a combination of mantle sources of the PREMA, EMI, and EMII types at the predominance of PREMA. Geological, geochemical, and isotopic lines of evidence suggest that magmatism in the province was related to mantle plumes. This is consistent with geophysical data, which testify that the volcanic areas are underlain by upwellings of the asthenospheric mantle or plumes. Seismic tomography data indicate that the “stems” of the plumes can be traced down to the upper and lower mantle. The province is thought to have been produced when the eastern margin of the Asian plate overlapped one of the branches of the Pacific superplume at approximately 160 Ma. This branch of the superplume is pronounced in the modern mantle structure as a cluster of mantle plumes that control (according to seismic tomography data) the interaction zone of the Pacific and Asian lithospheric plates.  相似文献   

18.
Based on the systematic investigation of the geochemical and isotopic (Sr and Nd) characteristics of basic rocks from various volcanic areas of the Devonian Altai-Sayan rift system, the compositions of mantle magma sources were characterized, and the geodynamic scenarios of their entrainment into rifting processes were reconstructed. It was found that the titanium-rich basic rocks (2.5 < TiO2 < 4.2 wt %) of this region are enriched in lithophile trace elements, including the rare earth elements, compared with N-MORB and are similar in composition to intraplate subalkali basalts of the OIB type. In contrast, moderate-titanium basic rocks (1.1 < TiO2 < 2.5 wt %) are mainly depleted in the highly charged incompatible elements Th, U, Nb, and Ta (La/Yb = 1.2−2.2) and, to a lesser extent, in Zr, Hf, and LREE ((La/Yb)N < 7), but are enriched in Ba. With respect to these characteristics, the moderate-titanium basites are similar to rocks formed in subduction environments. The geochemical parameters of the basites are strongly variable, which probably reflects the heterogeneity of the mantle sources that contributed to the formation of the rift system. In particular, the most notable variations in rock composition related to an increase in the contribution of an OIB-type source to magma composition were observed in the eastern direction, i.e., inland from the paleocontinent margin. The isotopic composition of the basites is relatively stable within individual rift zones and significantly variable at a comparison of rocks from different zones of the region. Based on the isotopic characteristics of the rocks, three melt sources were identified. One of them is chemically similar to the PREMA and is a common component in all observed trends of isotopic variations, irrespective of the position of the particular assemblage in the structure of the region. This component dominates the composition of the titaniumrich basalts with geochemical signatures of the derivatives of enriched mantle reservoirs of the E-MORB and OIB types and is considered as a plume source. Two other isotopic melt sources are related to subduction processes, which is indicated by their dominance during the formation of the moderate-titanium basalts showing the geochemical signatures (primarily, Ta-Nb depletion) of typical volcanic-arc rocks. These differences are consistent with the formation of the Altai-Sayan rift system in a complex geodynamic setting, which developed under the influence of intraplate magma sources (mantle plume) on the region of melt generation in an active continental margin (subduction zone).  相似文献   

19.
B. Mocek   《Lithos》2001,57(4):263-289
Blueschists, eclogites, chlorite–actinolite rocks and jadeite-gneisses of the blueschist unit of Siphnos have been investigated for their geochemical composition. Their protolith nature is characterised and a geodynamic model for the pre-metamorphic evolution of these metavolcanic rocks is proposed on the basis of immobile elements, especially trace elements and rare earth elements (REE).

The protoliths of the eclogites are characterised as calc-alkaline basalts, andesites and Fe-rich tholeiites evolving in an island-arc setting. Trace element data indicate that subducted marine sediments were assimilated in the magma chamber, enriching the protoliths in LILE and Pb. Produced in the early stage of back-arc basin opening, a protolith with affinities to both island-arc and MORB formed the precursor of the chlorite–actinolite rocks. They were created by low degrees of partial melting of very primitive magmas, akin to spinel-peridotites and have affinities to boninites, probably through melting of the peridotitic mantle wedge. Tholeiitic basalts and andesites with N-MORB affinity, especially in their REE-patterns, were then produced by partial melting, possibly in an embryonic back-arc basin. These rocks were the protoliths of the blueschists of Siphnos. Their enrichment in some LILE and Pb indicates a N-MORB source contaminated by marine sediments, probably shales or other Pb-rich sediments. Because the jadeite-gneisses show affinities to MOR-granites and volcanic arc granites, intrusion of their protoliths in a back-arc environment is likely. The protoliths of the quartz-jadeite gneisses are rhyodacites/dacites and rhyolites, those of the glaucophane-jadeite gneisses were andesites.

The proposed geodynamic model, solely based on geochemical data, is consistent with geochemical data from neighbouring islands, though those rock units show much higher chemical variability. Consistent with geotectonic models, which are based on structural and geophysical data, the volcanic protoliths of the Siphnos blueschist unit reflect the transition from subduction to spreading environment and record in detail: subduction, formation of an island-arc, and the evolution of a back-arc basin.  相似文献   


20.
Early–Middle Jurassic igneous rocks (190–170 Ma) are distributed in an E–W-trending band within the Nanling Tectonic Belt, and have a wide range of compositions but are only present in limited volumes. This scenario contrasts with the uniform but voluminous Middle–Late Jurassic igneous rocks (165–150 Ma) in this area. The Early–Middle Jurassic rocks include oceanic-island basalt (OIB)-type alkali basalts, tholeiitic basalts and gabbros, bimodal volcanic rocks, syenites, A-type granites, and high-K calc–alkaline granodiorites. Geochemical and isotopic data indicate that alkaline and tholeiitic basalts and syenites were derived from melting of the asthenospheric mantle, with asthenosphere-derived magmas mixing with variable amounts of magmas derived from melting of metasomatized lithospheric mantle. In comparison, A-type granites in the study area were probably generated by shallow dehydration-related melting of hornblende-bearing continental crustal rocks that were heated by contemporaneous intrusion of mantle-derived basaltic magmas, and high-K calc-alkaline granodiorites resulted from the interaction between melts from upwelling asthenospheric mantle and the lower crust. The Early–Middle Jurassic magmatic event is spatially variable in terms of lithology, geochemistry, and isotopic systematics. This indicates that the deep mantle sources of the magmas that formed these igneous rocks were significantly heterogeneous, and magmatism had a gradual decrease in the involvement of the asthenospheric mantle from west to east. These variations in composition and sourcing of magmas, in addition to the spatial distribution and the thermal structure of the crust–mantle boundary during this magmatic event, indicates that these igneous rocks formed during a period of rifting after the Indosinian Orogeny rather than during subduction of the paleo-Pacific oceanic crust.  相似文献   

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