首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 276 毫秒
1.
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   

2.
东天山石炭纪企鹅山群火山岩岩石成因   总被引：13，自引：1，他引：12  
土屋矿区南北大沟企鹅山群火山岩的岩石地球化学研究表明:东天山企鹅山群火山岩主要为拉斑系列,少量为钙碱系列;岩石类型为玄武岩、玄武安山岩、英安岩和流纹岩。稀土、微量元素和Sr、Nd同位素特点揭示:该火山岩系形成于大陆裂谷环境;其源区主要为软流圈地幔,同时有岩石圈地幔源组分卷入,酸性岩浆是玄武质岩浆结晶分异的产物。  相似文献   

3.
ABSTRACT

A Paleogene accretionary complex, the Mineoka–Setogawa belt is distributed adjacent to the northern portion of the collision zone between Honshu and Izu–Bonin–Mariana (IBM) arcs in central Japan, comprising a mélange of ophiolitic fragments of various sizes. The Eocene-Oligocene plutonic rocks in this belt (gabbro, diorite, and tonalite) have been interpreted as fragments brought from the deep crust beneath the IBM arc through tectonic collisions. The geochemical characteristics of the gabbro and associated basaltic dike are similar to those of the Eocene IBM tholeiitic basalt; thus, the gabbro was likely formed via the crystallization of the Eocene tholeiitic basaltic magmas, which was produced by the partial meltings of a depleted mantle wedge. A comparison with experimental results and geochemical modeling indicates that the tonalite was generated by 10–30% dehydration melting of the gabbro. Actually, Eocene–Oligocene felsic veins, which are coeval with the plutonic rocks, occur in the Mineoka–Setogawa gabbro. Plagioclase crystals in the diorite comprise Ca-rich and -poor parts in a single crystal. Their compositional characteristics are consistent with those of plagioclase in the gabbro and tonalite, respectively. The textures and chemical composition of plagioclase indicate that the diorite was formed by the mixing between mafic and silicic magmas. The whole-rock composition of the diorite also indicates the evidence for the mixing between basaltic magmas which were fractionated to variable degrees and homogeneous silicic magma. The mixing model proposed from the first direct observations of the IBM middle crust exposed on the Mineoka–Setogawa belt is applied to the genesis of the Eocene to present intermediate rocks in the IBM arc. If the continental crust were created at intra-oceanic arc settings such as the IBM arc, the magma mixing model would be one of the most likely mechanisms for the genesis of the continental crust.  相似文献   

4.
《International Geology Review》2012,54(15):1842-1863
ABSTRACT

The late Mesozoic magmatic record within the Erguna Block is critical to evaluate the tectonic history and geodynamic evolution of the Great Xing’an Range, NE China. Here, we provide geochronological and geochemical data on Late Jurassic–Early Cretaceous plutonic-volcanic rocks in the northern Erguna Block and discuss their origin within a regional tectonic framework. Late Mesozoic magmatism in the Erguna Block can be divided into two major periods: Late Jurassic (162–150 Ma) and Early Cretaceous (140–125 Ma). Late Jurassic quartz monzonite and dacite show adakite characteristics such as high Al2O3, high Sr, and steeply fractionated REE patterns. Contemporary granitoids and rhyolites are also characterized by strong enrichment of light rare earth elements (LREE) and significant depletion in heavy rare earth elements (HREE), but with more pronounced negative Eu anomalies. Early Cretaceous trachytes and monzoporphyries exhibit moderate LREE enrichment and relatively flat HREE distributions. Coeval granites and rhyolites have transitional signatures between A-type and fractionated I-type felsic rocks. Both Late Jurassic and Early Cretaceous rocks have distinctive negative Nb, Ta, and Ti anomalies, and positive zircon εHf(t) values, suggesting that these magmas were derived from partial melting of Meso-Neoproterozoic accreted lower crust, although melting occurred at a variety of crustal levels. The transition from adakite to non-adakite magmatism reflects continued crustal thinning from Late Jurassic to Early Cretaceous. Our data, together with recently reported isotopic data for plutonic and volcanic rocks, as well as geochemical data, in NE China, suggest that Late Jurassic–Early Cretaceous magmatism in the Erguna Block was possibly induced by post-collisional extension after closure of the Mongol-Okhotsk Ocean.  相似文献   

5.
TAMURA  Y. 《Journal of Petrology》1995,36(2):417-434
The Mio-Pliocene Shirahama Group, Izu Peninsula, Central Japan,a well-exposed submarine volcanic arc complex of lava flows,pyroclastic rocks and associated shallow intrusives, is characterizedby a tholeiitic series (basalt to dacite) and a calc-alkalineseries (andesite to dacite). Chemical variations in the tholeiiticseries and calc-alkaline series are consistent with crystalfractionation from basalt and magnesian andesite (boninite),respectively. Crystal–liquid phase relations of thesemagmas have been investigated by study of sample suites fromthese two series. Compositions of liquids in equilibrium withphenocrysts were determined by microprobe grid analyses, inwhich 49 points were averaged in 03 mm 03 mm groundmassareas. The liquid compositions, coupled with the phenocrystmineralogy of the same samples, define the liquid lines of descentof these volcanic arc magmas. Major findings include the following:(1) Crystallization of the tholeiitic series magma is consistentwith early stage crystallization in the simple system Fo–Di–Silica–H2O,with olivine having a reaction relation to augite and the tholeiiticliquid. (2) The later stage products of the tholeiitic seriesmagma are, however, crystal-poor (<10%) dacites with no maficminerals, suggesting that tholeiitic liquids, hypersthene andaugite were no longer on the cotectic (3) A characteristic ofthe calc-alkaline series magmas is the development of rhyoliticliquids. Hypersthene, augite, plagioclase and Fe–Ti oxideoccur in most calc-alkaline rocks studied, and hornblende andquartz can be found in about half of these. However, their differentiationpaths show that the cotectic relation between quartz and liquidended at a later stage, resulting in the resorption of quartzphenocrysts and ultimately in the formation of quartz-free magmas.(4) The late-stage liquids of both the tholeiitic and calc-alkalineseries have deviated from their cotectics, which cannot be explainedby fractional crystallization alone. The addition of H2O froman outside system is probably required to explain the differentiationpaths. (5) The formation of chilled margins, the in situ crystallizationof a magma chamber in the solidification zone, and/or the migrationof groundwater into the magma chamber are thought to be likelyprocesses affecting magmas during their migration and intrusioninto the crust. An extreme effect of H2O addition would be tolower the liquidus temperatures of all precipitating silicatephases far below their restorable range before eruption, resultingin the production of aphyric magmas. Even when a temperaturedecrease in the magma chamber causes a liquid to intersect theliquidus of a pre-existing phase, the addition of H2O shiftsthe cotectic toward SiO2, resulting in quartz being the lastphase to crystallize. The resorption of quartz is interpretedto be the result of a liquidus boundary shift caused by theaddition of H2O. The genesis of aphyric rhyolites is thereforeinferred to result from fractional crystallization followingaddition of H20. KEY WORDS: Shirahama Group; Japan; island arc; rhyolite; magma series  相似文献   

6.
Tholeiites accompanying a majority of alkali basalts are restricted to the highly productive central part of the CECV plume activity in Vogelsberg and Hessian Depression. They mainly occur as quartz tholeiites which according to experiments of partial melting and material balances are products of olivine tholeiitic primary melts. The differentiation from olivine to quartz tholeiitic melts took place in lower crustal magma chambers where olivine tholeiitic melt intruded due to a density comparable with that of the country rocks. The fractionation due to separation of olivine and some clinopyroxene caused contamination of tholeiite magmas by tonalitic partial melts from the wall rocks of the magma chambers. The latter process is indicated by relatively high Rb, K and Pb and low Nb concentrations and by Nd, Sr and Pb isotopes. Contaminating crustal melts, which roughly attained a proportion of 10%, contained very low 143Nd/144Nd ratios from a Nd/Sm fractionation as old as 2.6 Ga. This is the first evidence from mafic rocks of this high age in the lower crust beneath Central Europe. Modelling with incompatible elements allows to recognize olivine tholeiites as products of about 1% partial melting of plume rocks consisting of 35% primitive and 65% depleted mantle materials. The production of tholeiites other than alkali basalts is restricted to the highest plume activity and the largest fraction of MORB type source rocks. Received: 10 December 1999 / Accepted: 23 June 2000  相似文献   

7.
The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K2O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization.The low abundances of trace elements, together with low K2O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO2-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.  相似文献   

8.
Abstract

The granitoid suites encountered by drilling in the northern South China Sea (SCS) remain important for understanding the evolution of the late Mesozoic Southeast Asian continental margin. They comprise a range of rock types including diorite, tonalite, granodiorite, monzogranite and syenogranite with SiO2 spanning 56.4–76.8%. Newly acquired secondary ion mass spectrometry (SIMS) U–Pb ages of samples from 14 boreholes indicate two key magmatic episodes: Late Jurassic (161.6–148.2 Ma) and Early Cretaceous (136.5–101.7 Ma). Jurassic magmatism probably began in late Middle Jurassic time, documented by the dates of inherited zircons. The granitoids are dominated by metaluminous to weakly peraluminous I-type granites, are transitional between magnesian and ferroan, and encompass calc-alkaline, high-K calc-alkaline, and shoshonitic series. The geochemical signatures suggest that these granitoids were mostly generated in a normal continental arc environment. Notable features of the I-type samples are well-defined negative Nb–Ta–Ti anomalies typical of arc-related magmas. Taken together, the late Mesozoic arc granites of the SCS, the accretionary wedge of the Palawan terrane to the southeast, and the zone of lithospheric extension north of the SCS throughout Southeast China, define a southeast-to-northwest trench-arc-backarc architecture for the late Mesozoic Southeast Asian continental margin whose geodynamic setting is related to subduction of the Palaeo-Pacific slab beneath the Asian continent. Two key subduction episodes are recognized, one in Late Jurassic and the other in Early Cretaceous time.  相似文献   

9.
Chemical interaction between tholeiitic magmas of the East Greenland Tertiary macrodike complex and anatectic melts of the Precambrian basement produced a wide range of hybrid magmas. Field evidence indicates that, although coexisting magmas were stirred, mechanical mixing only occurred to a limited extent before segregation of magmas into a stratified system. The initial 87Sr/86Sr and 143Nd/144Nd isotope ratios for hybrid compositions fall between those of the mafic and felsic end-members. However, the covariation of these isotope ratios differs from that expected of bulk mixing. Major- and trace-element distributions in hybrid magmas are also inconsistent with simple mixing, as well as with fractional crystallization coupled with bulk assimilation (AFC) involving reasonable end-members of the macrodike-crust system. Rather, the chemical and isotopic modification of mafic and felsic magmas of the macrodike complex appears to have been controlled fundamentally by interdiffusion of silicate liquid species during mingling and buoyant roofward segregation of crust-derived granophyres. The relationships among juxtaposed hybrid magmas of the Miki Fjord macrodike are shown to be consistent with expectations of selective diffusional exchange based on available experimental interdiffusion data for silicate liquids. Comparison between these hybrid compositions and rocks from the felsic series of the Vandfaldsdalen macrodike suggest that the latter compositions were affected by a similar opensystem process operating presumably during the transient development of the felsic cap. Once hybrid magmas ponded at the roof of the intrusion they effectively were isolated from further exchange.  相似文献   

10.
The Middle Jurassic plutonism of the Central High Atlas (Morocco) was emplaced in N45° trending anticlinal ridges. It is characterised by various petrographic facies including mafic rocks (troctolites), intermediate rocks (diorites, monzodiorites), and evolved rocks (syenites), together with heterogeneous facies resulting from mixing between acidic and the intermediate magmas. Mineralogical and chemical data show (i) the transitional character of the Jurassic magmatic series of the Central High Atlas and (ii) the implication of continental crust as a contaminant during fractional crystallization. To cite this article: R. Zayane et al., C. R. Geoscience 334 (2002) 97–104.  相似文献   

11.
Abstract

The Guerrero suspect terrane composed of Late Jurassic-Early Cretaceous sequences, extends from Baja California up to Acapulco and is considered to be coeval with the Late Mesozoic igneous and sedimentary arc sequences of the Greater Antilles, Venezuela and Western Cordillera of Colombia. New geological, petrological and geochemical data from central and southern Mexico, led us to propose a new model for the building of the Alisitos-Teloloapan arc. This arc, partly built on the Pacific oceanic lithosphere and partly on continental fragments, could be related to the subduction of an oceanic basin - the Arperos basin - under the Paleo-Pacific plate. This subduction was dipping southwest.

At the beginning of the magmatic activity of the oceanic segment of this arc, depleted tholeiitic basalts were emitted in a submarine environnement below the CCD. While subduction was going on, the arc magmas evolved from LREE depleted tholeiites to slightly LREE enriched tholeiites and then, to calc-alkaline basalts and andesites enriched in LREE and HFSE. Concurrently, the arc sedimentary environment changed from deep oceanic to neritic with the deposition of Aptian-Albian reefal limestones, at the end of the arc building. In the continent-based segment, the arc magmas are exclusively differentiated calc-alkaline suites depleted in HREE and Y, formed of predominantly siliceous lavas and pyroclastic rocks, emitted in a sub-aerial or shallow marine environment.

Thus, taking into account this above mentioned model, the Cretaceous volcanic series, accreted to the margins of cratonal America, in Colombia, Venezuela, Greater Antilles and Mexico, could be related to the same west-south-west dipping subduction of oceanic basins, fringing the North and South American continental cratons and connected directly with the inter-American Tethys. While the subduction was proceeding, this magmatic arc drifted towards the North and South American cratons and finally, collided with the continental margins at different periods during the Cretaceous.  相似文献   

12.
ABSTRACT

The spatial/temporal proximity of Mt. Etna to the Hyblean Plateau and the Aeolian slab makes the discussion on the nature of its mantle source/s extremely controversial. In this study, a detailed geochemical overview of the entire Mt. Etna evolutionary sequence and a comparison with the magmatism of the Hyblean Plateau was proposed to: (i) simulate the composition of Mt. Etna tholeiitic to alkaline primitive magmas in equilibrium with a fertile mantle source; (ii) model the nature, composition and evolution of the mantle source from the tholeiitic stage (600 ka) to present magmatism. According to our simulations, two amphibole + phlogopite-bearing spinel lherzolite sources are able to explain the wide range of Etnean primary magmas. The enrichment in LILE, 87Sr/86Sr, Rb and H2O of the magmas emitted after 1971 (but also discontinuously generated in both historic and prehistoric times) are caused by different melting proportions of amphibole and phlogopite in a modally and compositionally homogeneous mantle domain, with melting degrees analogous to those required to produce magmas erupted prior to 1971. The behaviour of the hydrous phases during melting could be ascribed to a variable H2O/CO2 activity in the mantle source, in turn related to the heat/fluxes supply from the asthenospheric upwelling beneath Mt. Etna. All these considerations, strengthened by numerical models, are then merged to review the complex Pliocene/Lower Pleistocene to present day’s geodynamic evolution of eastern Sicily.  相似文献   

13.
ABSTRACT

The northern zone of the Chon Aike Igneous Province, located in the North Patagonian Massif, exhibits extensive outcrops of Jurassic volcanic rocks of the Marifil Formation. In the Arroyo Verde area, the initial volcanic stage of the Marifil Formation, that we denominate V0 (192.6 ± 2.5 Ma), includes coulées, megabreccias and lapilli tuffs assigned to plinian-type volcanism. This magmatism was generated by cold-wet-oxidized magmas that resemble those produced in active continental margins and volcanic arcs. The second stage, located unconformably over the first, includes welded lava-like ignimbrites, massive lapilli tuff and rhyolitic lava flow that resemble the Snake River-type volcanism. The magma that produces this volcanic stage exhibit the characteristics of hot-dry-reduced magmas emplaced in intraplate continental environments associated with continental rifting. This stage coincides with the V1 volcanic episode early recorded in the Marifil Formation.  相似文献   

14.
Abstract

The Gangdese batholith, Tibet, records the opening and closing of the Neo-Tethyan ocean and the resultant collision between the Indian and Eurasian plates. The Mesozoic magmatic rocks play a crucial role in understanding the formation and evolution of the Neo-Tethyan tectonic realm. This study focuses on Jurassic intrusive rocks in the Xietongmen area of the southern margin of the Lhasa terrane adjacent to the Yarlung–Tsangpo suture. Zircon U–Pb dating yielded Middle Jurassic dates for ca 170?Ma hornblende gabbro and ca 173?Ma granodiorite intrusions. All of the samples are medium- to high-K calc-alkaline, and the majority are metaluminous and enriched in the large ion lithophile elements and depleted in the high-field-strength elements. This indicates the magma was generated in a subduction-related tectonic setting. The intrusive rocks have high and positive εHf(t) values (hornblende gabbro: 13.3–18.7; granodiorite: 14.2–17.6) that yield Hf model ages younger than 312?Ma. These new data, combined with the results of previous research, suggest that the Jurassic igneous rocks were derived from a metasomatised region of an asthenospheric mantle wedge. Extremely depleted Sr–Nd–Pb–Hf isotope compositions are similar to the Yarlung ophiolite and igneous rocks within other intra-oceanic island arcs. Together with the existence of sandstone that is identified as the product of the oceanic island arc environment, we suggest formation in an intra-oceanic island arc.

  1. The new zircon U–Pb dating has yielded Middle Jurassic ages for the ca 170?Ma hornblende gabbro and ca 173?Ma granodiorite phases of the Xietongmen intrusion.

  2. Jurassic igneous rocks formed from a metasomatised asthenospheric mantle wedge by northward subduction of the Neo-Tethys oceanic crust beneath the southern margin of the Lhasa terrane.

  3. Late Triassic–Jurassic igneous rocks, which are characterised by highly depleted isotopic compositions within the Southern Lhasa sub-terrane, record residual intra-oceanic island arcs in the eastern Tethyan belt.

  相似文献   

15.
ABSTRACT

Clinopyroxene megacrysts in volcanic rocks can provide substantial information on the evolution of the magmatic system at depth. Although considerable attention has been paid to these crystals, their origin is not yet completely resolved. The clinopyroxene megacrysts worldwide can be divided into two major types in general: the green Cr-diopside type and the black Al-augite type. There is a consensus view that the Cr-diopside megacrysts are mantle xenocrysts, whereas two contrasting opinions exist regarding the origin of the black Al-augite megacrysts. One favours a cognate origin, viewing them as crystallization products of the host magmas under high-pressure; while the other argues that they are xenocrysts crystallized from previous alkali basalts or fragments of mantle peridotites, pyroxenites or pegmatite veins. A review study on the clinopyroxene megacrysts in Meso-Cenozoic volcanic rocks from the North China Craton (NCC) and their comparison with those worldwide provides new constraints on their origin, namely, the Cr-diopside megacrysts, as previously thought, are all xenocrysts, representing disaggregated clinopyroxene crystals from clinopyroxene-rich mantle rocks. Contrary to the formerly proposed cognate origin, the Al-augite megacrysts are also xenocrysts, having no direct genetic link to their host rocks. They crystallized from melts that have formed earlier than the host magmas, and probably accumulated in a magma chamber or occurred as sheets or veins filling a fracture network surrounding a magma chamber in the upper mantle. During the subsequent eruption of the host lavas, these previously formed crystals were incorporated into the magma and were brought up to the surface.  相似文献   

16.
Archean metavolcanic rocks from three greenstone belts (Suomussalmi,Kuhmo and Tipasjärvi) of eastern Finland have been subjectto a detailed geochemical study which leads to a discussionof their petrogenesis and the problem of compositional heterogeneityin the Archean mantle. Lithostratigraphically, the greenstonebelts are roughly divided into a lower and an upper volcanicsequence. Rocks of komatiitic and tholeiitic compositions arerestricted to the lower sequence, while andesitic tuffs, dacite-rhyodacitelavas and minor basalts of alkaline affinity occur in the uppersequence. All rocks from the greenstone belts have been subjectto regional metamorphism of the upper greenschist facies tothe lower garnet amphibolite facies. Consequently, the geochemicaldistinction of original magma types and the discussion of petrogenesishave relied heavily on the abundances of less mobile elements,such as TiO2, rare earth elements (REE), and some transitionmetals (e.g. Ni and Cr). Using all the possible discriminants of major element compositions,we have concluded that two general magmatic series that existin the lower volcanic sequence might be distinguished by theparameter of TiO2 content: the komatiitic series is characterizedby having TiO2 1.0 per cent and the tholeiitic series by 1.0per cent. The general series do not imply that a cogenetic relationshiplinked only by fractional crystallization exists in each series. Several magmatic types could be distinguished by their characteristicREE distribution patterns. In general, the komatiitic rocksshow flat HREE (heavy REE) and flat or depleted LREE (lightREE) patterns; the tholeiitic rocks show fractionated patternswith some degree of LREE enrichment, whilst the acidic rocksdemonstrate highly fractionated patterns with significant HREEdepletion. Model calculations indicate that: (1) the komatiiticand the tholeiitic series have no clear genetic relationship;(2) some basaltic komatiites (MgO < 12 per cent) could havebeen derived by crystal fractionation from a melt of peridotitickomatiite composition (MgO 30 per cent), but others requirevarious degrees of partial melting from the same or differentsource regions to account for their trace element abundances;(3) both partial melting and fractional crystallization haveinterplayed for the production of various rocks within the tholeiiticseries; (4) three different types of source materials are proposedfor all magmas from the lower volcanic sequence. All three typeshave the same initial HREE (about 2x chondrites) but differentLREE (from very depleted to 2x, flat) abundances; (5) volcanicrocks of the upper volcanic sequence must have originated atgreat depths where garnet remains in the residue after partialmelting and melt segregation. The recognition of the strongly LREE-depleted mantle sources,deduced from the REE patterns of peridotitic komatiites fromFinland, Canada and Rhodesia, may suggest that this depletionis a worldwide phenomenon, and that the Archean upper mantleis as heterogeneous in composition as the modern upper mantle.The causal effect of the depletion might be related to the generationof some contemporaneous LREE-enriched tholeiitic rocks, or morelikely, to contemporaneous or previous continental crust formingevents.  相似文献   

17.
Basaltic magmas emplaced into the root zone of the Slieve Gullion volcano have crystallised to rocks varying in texture from dolerite to gabbro. A mineralogical variation from olivine bearing to quartz bearing varieties has been recorded and with the presence of coexisting Ca-rich and Ca-poor pyroxenes, a tholeiitic assemblage is indicated.Geochemical data confirm this prognosis and a continuous spread of compositions from basalt to tholeiitic andesite are interpreted in terms of fractional crystallisation. Modelling of the fractional crystallisation processes indicate an approach to cotectic conditions with fractionation at low pressures involving olivine, plagioclase, clinopyroxene and Ti-magnetite. Primitive magma compositions, indicated by low values of D.I. and 100 Mg/Mg + Fe2+ (atomic)>61, show low concentrations of the large ion lithophile elements such as Rb, Ba, Zr, Y, and K. The relatively high CaO content (>11 %) of these rocks invite comparison with high-calcium low-alkali tholeiitic liquids recognised in extrusive and intrusive magmas elsewhere in the North Atlantic Tertiary volcanic province and with magmas currently erupted at active spreading ridges.In north west Britain the field and stratigraphic distribution of these high-calcium low-alkali magmas suggests that they occupy a distinct chronological niche towards the top of the Palaeocene-Eocene volcanic succession succeeding eruption of mildly alkali and transitional basalts. As such, the refractory (high CaO, MgO, etc.) and large ion lithophile depleted geochemistry can be explained either by differential partial melting in the upper mantle source region or melting of a depleted and refractory mantle source which has already contributed to basalt genesis.  相似文献   

18.
[研究目的]通过查明大兴安岭乌拉盖地区火山岩的年龄、地球化学特征,探讨火山岩形成的时代、成因及构造背景,为研究大兴安岭南部地区的岩浆活动和构造演化过程提供依据.[研究方法]选取大兴安岭乌拉盖地区满克头鄂博组火山岩,系统开展LA-MC-ICP-MS锆石U-Pb年代学和岩石地球化学研究.[研究结果]研究区满克头鄂博组火山岩...  相似文献   

19.
The Alaska–Aleutian island arc is well known for eruptingboth tholeiitic and calc-alkaline magmas. To investigate therelative roles of chemical and temporal controls in generatingthese contrasting liquid lines of descent we have undertakena detailed study of tholeiitic lavas from Akutan volcano inthe oceanic Aleutian arc and calc-alkaline products from Aniakchakvolcano on the continental Alaskan Peninsula. The differencesdo not appear to be linked to parental magma composition. TheAkutan lavas can be explained by closed-system magmatic evolution,whereas curvilinear trace element trends and a large range in87Sr/86Sr isotope ratios in the Aniakchak data appear to requirethe combined effects of fractional crystallization, assimilationand magma mixing. Both magmatic suites preserve a similar rangein 226Ra–230Th disequilibria, which suggests that thetime scale of crustal residence of magmas beneath both thesevolcanoes was similar, and of the order of several thousandyears. This is consistent with numerical estimates of the timescales for crystallization caused by cooling in convecting crustalmagma chambers. During that time interval the tholeiitic Akutanmagmas underwent restricted, closed-system, compositional evolution.In contrast, the calc-alkaline magmas beneath Aniakchak volcanounderwent significant open-system compositional evolution. Combiningthese results with data from other studies we suggest that differentiationis faster in calc-alkaline and potassic magma series than intholeiitic series, owing to a combination of greater extentsof assimilation, magma mixing and cooling. KEY WORDS: uranium-series; Aleutian arc; magma differentiation; time scales  相似文献   

20.
李艳芳  邱检生  王睿强  徐航  洪宇飞 《地质学报》2019,93(12):3020-3046
本文选取冈底斯带东段加查县东北部丝波绒曲复式岩体为对象,对其进行了岩相学、地质年代学、岩石地球化学以及Sr-Nd-Hf同位素组成的综合研究,据此探讨了该复式岩体的成因及其对构造演化的启示。研究结果表明,该复式岩体由早侏罗世辉长岩-花岗岩杂岩(188～185Ma)和始新世花岗质岩石(～47Ma)构成,两期花岗质岩石中普遍发育塑变形态的镁铁质包体。早侏罗世杂岩由角闪辉长岩和英云闪长岩组成,角闪辉长岩中的主要铁镁矿物为角闪石,它们为一套钙碱性弧岩浆岩组合,具有亏损的Sr-Nd-Hf同位素组成。始新世花岗质岩石主要为二长花岗岩-花岗闪长岩,它们较早侏罗世英云闪长岩更为富碱,属钙碱性-高钾钙碱性I型花岗岩,其同位素组成也较早侏罗世英云闪长岩富集。综合分析表明,该区早侏罗世复合辉长岩-花岗岩的形成受控于新特提斯洋板片北向俯冲的构造背景,角闪辉长岩起源于受俯冲板片脱水交代的上覆地幔楔的部分熔融,共生的英云闪长岩则为同期幔源岩浆底侵诱发初生地壳部分熔融产生的长英质岩浆与幔源岩浆不同程度混合的产物。始新世花岗岩的形成受控于新特提斯洋板片断离的构造背景,是由具"弧"型地球化学特征的初生地壳再造的产物,并有少量印度陆壳富集组分参与成岩。  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号