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1.
Mafic alkalic volcanism was widespread in the Carpathian–Pannonian region (CPR) between 11 and 0.2 Ma. It followed the Miocene continental collision of the Alcapa and Tisia blocks with the European plate, as subduction-related calc-alkaline magmatism was waning. Several groups of mafic alkalic rocks from different regions within the CPR have been distinguished on the basis of ages and/or trace-element compositions. Their trace element and Sr–Nd–Pb isotope systematics are consistent with derivation from complex mantle-source regions, which included both depleted asthenosphere and metasomatized lithosphere. The mixing of DMM-HIMU-EMII mantle components within asthenosphere-derived magmas indicates variable contamination of the shallow asthenosphere and/or thermal boundary layer of the lithosphere by a HIMU-like component prior to and following the introduction of subduction components.Various mantle sources have been identified: Lower lithospheric mantle modified by several ancient asthenospheric enrichments (source A); Young asthenospheric plumes with OIB-like trace element signatures that are either isotopically enriched (source B) or variably depleted (source C); Old upper asthenosphere heterogeneously contaminated by DM-HIMU-EMII-EMI components and slightly influenced by Miocene subduction-related enrichment (source D); Old upper asthenosphere heterogeneously contaminated by DM-HIMU-EMII components and significantly influenced by Miocene subduction-related enrichment (source E). Melt generation was initiated either by: (i) finger-like young asthenospheric plumes rising to and heating up the base of the lithosphere (below the Alcapa block), or (ii) decompressional melting of old asthenosphere upwelling to replace any lower lithosphere or heating and melting former subducted slabs (the Tisia block).  相似文献   

2.
This paper reviews the Mesozoic continental flood basalts (CFBs) associated with the break-up and dispersal of Gondwana from 185-60 Ma, the conditions for melt generation in mantle plumes and within the continental mantle lithosphere, and possible causes for lithospheric extension. The number of CFB provinces within Gondwana is much less than the number of mantle plumes that are likely to have been emplaced beneath it in the 300 Ma prior to its initial break-up. Also, the difference between the age of the peak of CFB volcanism and that of the oldest adjacent ocean crust decreases with the age of volcanism during the break-up and dispersal of Gondwana. The older CFBs of Karoo and Ferrar appear to have been derived largely from source regions within the mantle lithosphere. It is only in the younger Paranâ-Etendeka and Deccan CFBs that there are igneous rocks with major, trace element and radiogenic isotope ratios indicative of melting within a mantle plume. These younger CFBs are also clearly associated with hot spot traces on the adjacent ocean floor. The widespread 180 Ma magmatic event is attributed to partial melting within the lithosphere in response to thermal incubation over 300 Ma. In the case of the Ferrar (Antarctica) this was focussed by regional plate margin forces. The implication is that supercontinents effectively self-destruct in response to the build up of heat and resultant magmatism, since these effects significantly weaken the lithosphere and make it more susceptible to break-up in response to regional tectonics. The younger CFB of Paranâ-Etendeka was generated, at least in part, because the continental lithosphere had been thinned in response to regional tectonics. While magmatism in the Deccan was triggered by the emplacement of the plume, that too may have been beneath slightly thinned lithosphere.  相似文献   

3.
杨文采 《地质论评》2014,60(5):945-961
本篇讨论大陆岩石圈拆沉、伸展与裂解作用过程。由于大陆岩石圈厚度大而且很不均匀,产生裂谷的机制比较复杂。大陆碰撞远程效应的触发,岩石圈拆沉,以及板块运动的不规则性和地球应力场方向转折,都可能产生岩石圈断裂和大陆裂谷。岩石圈拆沉为在重力作用下"去陆根"的作用过程,演化过程可分为大陆根拆离、地壳伸展和岩石圈地幔整体破裂三个阶段。大陆碰撞带、俯冲的大陆和大洋板块、克拉通区域岩石圈,都可能产生岩石圈拆沉。大陆岩石圈调查表明,拉张区可见地壳伸展、岩石圈拆离、软流圈上拱和热沉降;它们是大陆岩石圈伸展与裂解早期的主要表现。从初始拉张的盆岭省到成熟的张裂省,拆离后地壳伸展成复式地堑,下地壳幔源玄武岩浆侵位,断裂带贯通并切穿整个岩石圈,表明地壳伸展进入成熟阶段。中国东北松辽盆地和西欧北海盆地曾处于成熟的张裂省。岩石圈破裂为岩浆侵位提供了阻力很小的通道网。岩浆侵位作用伴随岩石圈破裂和热流体上涌,成熟的张裂省可发展成大陆裂谷。多数的大陆裂谷带并没有发展成威尔逊裂谷带和洋中脊,普通的大陆裂谷要演化为威尔逊裂谷带,必须有来自软流圈的长期和持续的热流和玄武质岩浆的供应。威尔逊裂谷带岩石圈地幔和软流圈为地震低速带,其根源可能与来自地幔底部的地幔热羽流有关。  相似文献   

4.
Continental rift systems and anorogenic magmatism   总被引：1，自引：0，他引：1  
Precambrian Laurentia and Mesozoic Gondwana both rifted along geometric patterns that closely approximate truncated-icosahedral tessellations of the lithosphere. These large-scale, quasi-hexagonal rift patterns manifest a least-work configuration. For both Laurentia and Gondwana, continental rifting coincided with drift stagnation, and may have been driven by lithospheric extension above an insulated and thermally expanded mantle. Anorogenic magmatism, including flood basalts, dike swarms, anorthosite massifs and granite-rhyolite provinces, originated along the Laurentian and Gondwanan rift tessellations. Long-lived volcanic regions of the Atlantic and Indian Oceans, sometimes called hotspots, originated near triple junctions of the Gondwanan tessellation as the supercontinent broke apart. We suggest that some anorogenic magmatism results from decompression melting of asthenosphere beneath opening fractures, rather than from random impingement of hypothetical deep-mantle plumes.  相似文献   

5.
The Carpathian–Pannonian Region contains Neogene to Quaternary magmatic rocks of highly diverse composition (calc-alkaline, shoshonitic and mafic alkalic) that were generated in response to complex microplate tectonics including subduction followed by roll-back, collision, subducted slab break-off, rotations and extension. Major element, trace element and isotopic geochemical data of representative parental lavas and mantle xenoliths suggests that subduction components were preserved in the mantle following the cessation of subduction, and were reactivated by asthenosphere uprise via subduction roll-back, slab detachment, slab-break-off or slab-tearing. Changes in the composition of the mantle through time are evident in the geochemistry, supporting established geodynamic models.Magmatism occurred in a back-arc setting in the Western Carpathians and Pannonian Basin (Western Segment), producing felsic volcaniclastic rocks between 21 to 18 Ma ago, followed by younger felsic and intermediate calc-alkaline lavas (18–8 Ma) and finished with alkalic-mafic basaltic volcanism (10–0.1 Ma). Volcanic rocks become younger in this segment towards the north. Geochemical data for the felsic and calc-alkaline rocks suggest a decrease in the subduction component through time and a change in source from a crustal one, through a mixed crustal/mantle source to a mantle source. Block rotation, subducted roll-back and continental collision triggered partial melting by either delamination and/or asthenosphere upwelling that also generated the younger alkalic-mafic magmatism.In the westernmost East Carpathians (Central Segment) calc-alkaline volcanism was simultaneously spread across ca. 100 km in several lineaments, parallel or perpendicular to the plane of continental collision, from 15 to 9 Ma. Geochemical studies indicate a heterogeneous mantle toward the back-arc with a larger degree of fluid-induced metasomatism, source enrichment and assimilation on moving north-eastward toward the presumed trench. Subduction-related roll-back may have triggered melting, although there may have been a role for back-arc extension and asthenosphere uprise related to slab break-off.Calc-alkaline and adakite-like magmas were erupted in the Apuseni Mountains volcanic area (Interior Segment) from15–9 Ma, without any apparent relationship with the coeval roll-back processes in the front of the orogen. Magmatic activity ended with OIB-like alkali basaltic (2.5 Ma) and shoshonitic magmatism (1.6 Ma). Lithosphere breakup may have been an important process during extreme block rotations (60°) between 14 and 12 Ma, leading to decompressional melting of the lithospheric and asthenospheric sources. Eruption of alkali basalts suggests decompressional melting of an OIB-source asthenosphere. Mixing of asthenospheric melts with melts from the metasomatized lithosphere along an east–west reactivated fault-system could be responsible for the generation of shoshonitic magmas during transtension and attenuation of the lithosphere.Voluminous calc-alkaline magmatism occurred in the Cãlimani-Gurghiu-Harghita volcanic area (South-eastern Segment) between 10 and 3.5 Ma. Activity continued south-eastwards into the South Harghita area, in which activity started (ca. 3.0–0.03 Ma, with contemporaneous eruption of calc-alkaline (some with adakite-like characteristics), shoshonitic and alkali basaltic magmas from 2 to 0.3 Ma. Along arc magma generation was related to progressive break-off of the subducted slab and asthenosphere uprise. For South Harghita, decompressional melting of an OIB-like asthenospheric mantle (producing alkali basalt magmas) coupled with fluid-dominated melting close to the subducted slab (generating adakite-like magmas) and mixing between slab-derived melts and asthenospheric melts (generating shoshonites) is suggested. Break-off and tearing of the subducted slab at shallow levels required explaining this situation.  相似文献   

6.
The source of hotspot volcanism lies in metasomatized regions of the continental mantle proximal to ancient sutures and failed rifts. Such regions are prone to melting under hotcell conditions on continental rifting, and to erosion into the deeper mantle by asthenospheric flow. In opening basins, rifting parallel to such sutures or failed rifts delaminates and cycles continental mantle into the MORB source. Rifting at some angle to a suture or failed rift generates a hotspot track by preferential melting of the metasomatized mantle as it is cycled toward the rift axis. Continental mantle eroded into the asthenosphere becomes displaced from the continent by net westward drift of the lithosphere relative to the deep mantle to give rise to hotspot volcanism in long-lived ocean basins.  相似文献   

7.
甘肃龙首山岩带西井镁铁质岩体成因及其构造意义   总被引：2，自引：0，他引：2  
西井岩体位于北祁连造山带以北,阿拉善地块西南缘的龙首山隆起带。以往的研究多以沿龙首山断裂分布的镁铁-超镁铁质岩带作为和金川岩体相关的岩浆事件进行,而本次选择西井镁铁质岩体进行了精确的地质年代学和地球化学研究,确定了西井岩体岩性主要为橄榄辉石岩和辉长岩,成岩时代为 (421.0±9.0) Ma,可以和北祁连高压变质带榴辉岩年龄相对应;εNd(t)为4.06~5.52,(87Sr/86Sr)i为0.704 548~0.707 575,具有地幔岩石圈特征;微量元素及其同位素计算表明西井岩体经历了约10%的下地壳物质混染。据此得出西井岩体及其龙首山岩带早志留世镁铁质侵入岩体成因模式为:祁连洋壳连续俯冲过程中洋壳与陆壳分离,热的软流圈物质持续冲击地幔岩石圈的底部;由于热传导效应,大地热流沿着地幔岩石圈上升,使得80 km深度的湿的橄榄岩层发生熔融,产生玄武质岩浆作用,玄武质岩浆上升过程中与下地壳物质发生约10%混染,形成西井岩体及其龙首山镁铁超镁铁质岩带。  相似文献   

8.
The subduction of spreading ridges creates a special geodynamic setting distinguished by the interference of convergent and divergent boundaries between lithospheric plates and their long-term interaction accompanied by the formation of characteristic geological complexes and structures. The available data on subduction of the contemporary Chile Ridge make it possible to reconstruct such settings in the geological past. The subduction of the spreading ridge leads to uplift of the continental margin, cut off the accretionary wedge by means of tectonic erosion, emplacement of a fold-thrust structure and longitudinal strike-slip faults, and creates settings favorable for obduction of the young oceanic lithosphere. A lithospheric window expressed in geological and geophysical features opens beneath the continental margin at the continuation of the ridge axis. The subduction-related volcanic activity ceases above this window, giving way to specific proximal magmatism close to the boundary with the ocean and distal magmatism at a distance from this boundary. The proximal bimodal magmatism was related to the sources of tholeiitic basalts characteristic of the ridge involved in subduction and to the partial melting of its oceanic crust and sediments. The distal basaltic magmatism is a product of melting of the fertile oceanic asthenosphere ascending through the lithospheric window with subsequent transformation of magma in the mantle wedge and the continental crust. The use of the Chilean tectonotype for paleoreconstructions is limited by the diverse settings of ridge subduction. The Paleogene magmatism at the Pacific margin of Alaska, where the kinematics of subduction was close to the Chilean subduction, is similar to the proximal igneous rocks of Chile in composition and zoning, retaining some geological differences. Another aspect of the paleoreconstruction is discussed on the basis of Jurassic and Cretaceous granitoids of the Ekonai Terrane of the Anadyr-Koryak System and terranes of southern Alaska. These localities are known for a special, accretionary type of granitoids in the forearc region related to anatectic magma formation without participation of the plunging ridge. Proceeding from comparison with the Chilean tectonotype, the criteria for the identification of granitoids varying in their origin are considered. The effect of subducting ridges on continental margins changed over geologic time and was subject to the rhythm of supercontinental cycles.  相似文献   

9.
Zvi Garfunkel   《Lithos》2008,100(1-4):49-65
Models of continental flood basalt (CFB) formation are evaluated by examining their implications for the setting, mainly temperature and depth, of melting which is assumed to result from adiabatic decompression. Most attractive is the model of melting in upwelling bodies (probably plume heads) rising to the base of the continental lithosphere. This constrains the melting to 120–150 km or deeper (continental lithospheric thickness) and thus the plume potential temperatures to ≥ 300 °C higher than ambient mantle. The primary melts should be hot, MgO-rich, modified during ascent, and assimilate components of the lithosphere, which can provide the continental-like geochemical signature of many CFB. Circulation within the upwellings and presence of eclogite patches also influence magma generation and composition. Dehydration melting when plumes heat the lowermost lithosphere can generate CFB only if the source region contains ca. 15% hydrous minerals beneath the entire area covered by flood volcanics, which is difficult to justify. On the other hand, assimilation of “continental” chemical components from large parts of the lithosphere does not require such extreme metasomatism. Decompression melting under strongly thinned rifted lithosphere requires lower potential temperatures of the rising material and lesser modification of the primary magmas than the plume head model of CFB formation. Available observations do not support the contemporaneity of flood volcanism with rifts having the required sizes and histories, but more information is needed to further test this model. On the other hand, magma production can assist rift initiation and lithospheric rupture, so subsequent thinning can explain the common formation of volcanic rifted margins immediately following CFB emplacement. Ancient LIP should record the same processes as seen in young CFB.  相似文献   

10.
Timing and source of several Fe-mineralized mafic-ultramaficintrusions in the Damiao area are investigated here by coupling new geochronological and Hf–Fe isotopic data with previous results. Although regarded as a Late Paleoproterozoic assemblage previously, two ～140 Ma intrusions are recognized by zircon U–Pb dating, indicating emplacement of these intrusions from Middle Devonian to Early Cretaceous times. Both Hf and Fe isotopic features lead to the conclusion that distinct mantle components contributed to their magma generation. As the first magmatic phase, the ～395 Ma intrusions were mainly derived from the slightly-enriched SCLM that was prevalent during the Paleozoic. However, asthenospheric material was strongly involved in the formation of the ～215 Ma Gaositai intrusion. Therefore, the initiation of lithospheric destruction in the northern NCC is inferred to have occurred in Late Triassic time, triggered by post-orogenic extension following the ～250 Ma collision between the Siberian Craton and the NCC. The ～140 Ma intrusions originated from a significantly-enriched mantle component probably resided in the predominant slightly-enriched SCLM. This mantle source would have melted in the Late Mesozoic, when the thin lithosphere enabled enhanced heat transfer from the asthenosphere. In summary, these distinct mantle sources of mafic–ultramafic magmatism provide a record of mantle heterogeneity and the gradual upward migration of the lithosphere–asthenosphere boundary during lithospheric destruction.  相似文献   

11.
Geochemical data are presented for primitive alkaline lavasfrom the Chyulu Hills Volcanic Province of southern Kenya, situatedsome 100 km east of the Kenya Rift Valley. In addition to theirprimitive compositions, a striking and ubiquitous feature isa strong but variable depletion in K relative to other highlyincompatible elements when normalized to primitive mantle values.Semi-quantitative models are developed that best explain thepetrogenesis of these lavas in terms of partial melting of asource that contained residual amphibole (but not phlogopite).The presence of amphibole implies a source in the subcontinentallithosphere rather than the asthenosphere. It is suggested thatthe amphibole is of metasomatic origin and was precipitatedin the lithospheric mantle by infiltrating fluids and/or meltsderived from rising mantle plume material. A raised geothermas a consequence of the continued ascent of the plume materialled to dehydration melting of the metasomatized mantle and generationof the Chyulu Hills lavas. It is proposed that the Chyulu HillsVolcanic Province represents an analogue for the earliest stagesof continental rift initiation, during which interaction betweena plume and initially refractory lithosphere may lead to thegeneration of lithospheric melts. KEY WORDS: rift-related alkaline volcanism; residual amphibole; subcontinental lithosphere  相似文献   

12.
Mantle xenoliths from Hainan and Qilin, South China have been studied to constrain the nature of the upper mantle and mantle processes beneath a continental margin. The extremely low Ti (160–245 ppm) contents in clinopyroxenes from some spinel lherzolites, indicative of high degrees of partial melting are inconsistent with the relatively high clinopyroxene modes (7.4–13%) in these samples. This inconsistency could be due to polybaric melting that started in the garnet stability field, then, after the breakdown of garnet to pyroxene and spinel, continued in the spinel stability field. Polybaric melting, due to adiabatic decompression of upwelling mantle, would leave a residual mantle in which the degree of depletion decreases with depth. The predicted stratified lithospheric mantle is evidenced by the negative correlation between the forsterite content in olivine and the equilibration temperature, proportional to the depth in the lithosphere from which the xenolith was derived. The lower part of the lithospheric mantle beneath South China consists predominantly of fertile and moderately depleted peridotites, which are either devoid of LREE enrichment, or show the trace element signature of incipient metasomatism, and plot within the Phanerozoic mantle domain. In contrast, the upper part of the mantle contains harzburgite and cpx-poor lherzolite, which are strongly affected by metasomatism of melt/fluid of highly variable composition. The anomalously high orthopyroxene mode (up to 47%) makes some of these refractory samples compositionally similar to the Proterozoic/Archean mantle. Their low equilibrium temperature (800–900 °C) points to the presence of old lithospheric relicts in the uppermost mantle beneath South China. Such lithosphere architecture may have resulted from partial replacement of Archean–Proterozoic lithosphere by asthenosphere that rose adiabatically subsequent to lithospheric thinning during the Cenozoic.  相似文献   

13.
Mantle plumes and plate motions   总被引：3，自引：0，他引：3  
This paper elaborates the hypothesis that convection plumes may be rising from the lower mantle to spread out in the asthenosphere and drive lithospheric plates about and thus possibly provide the primary mechanism which governs the behaviour of the earth's surface. The paper notes some characteristics of plumes and identifies more than thirty by the hot spots which overlie them. Most lie close to mid-ocean ridges and have produced aseismic ridges trending away from them on either plate. A few have been overridden by plates to produce single, isolated chains of seamounts and islands. One plume may have uplifted the Colorado Plateau. Such distinctions serve to identify five types of hot spots.Most plates are in motion over the lower mantle. They are considered to be driven by the plumes, but their paths are influenced by interactions with other plates. Some temporarily become more or less stationary relative to the lower mantle. It is held that stationary plates, of which Africa and Southeast Asia may be present examples, develop special characteristics among which much volcanism, epeirogenic uplift, rifting and the development of basins and swells are diagnostic.It is well-known that if two plates approach one another at a subduction zone that a continental plate generally overrides an oceanic one. It is here suggested that the question of which plate is more nearly stationary over the mantle is important and determines the character of the continental margin. It is held that, if a continental plate advances over an oceanic one which is fixed over the mantle, a migrating marginal trench and mountains of Andean type with huge batholiths will form on the leading edge of the continent. On the other hand, if a continental plate is fixed and one or more oceanic plates are advancing and sliding under it, island arcs (and, when a collision with another continent occurs, mountains of Appalachian type) will form along each coast towards which a plate is advancing.  相似文献   

14.
K. Vijaya Kumar  K. Rathna 《Lithos》2008,104(1-4):306-326
Mesoproterozoic rift-zone magmatism in the Prakasam Alkaline Province of Eastern Ghats Belt, India is represented by three geochemically distinct primary mafic magmas and their plutonic differentiates. The three mafic magmas correspond to the alkali basaltic dykes, gabbroic dykes and lamprophyric dykes. The dyke activity is synchronous with the host plutons and belongs to the 1350–1250 Ma period Mesoproterozoic magmatism. Geochemical signatures suggest that the alkali basaltic dykes have a source in the thermal boundary layer, which has a history of prior melt extraction followed by enrichment. Both the gabbroic and lamprophyric dykes are derived from lithospheric sources and their geochemical variation can be explained by “vein-plus-wall-rock melting model”. Vein/wall-rock ratio is low for the sources of gabbroic dykes, whereas it is high for the lamprophyric dykes. Geochemistry of the gabbro dykes further indicates preservation of previous arc-signals by the lithosphere beneath the Prakasam Alkaline Province during the Mesoproterozoic. Geochemical signatures of lamproite, which could be a cratonic expression of the rift-triggered magmatism in the Prakasam Province, suggest a general increase in the metasomatic imprint with increasing lithosphere thickness from cratonic margin towards interior. It is found that geochemistry of continental rift-zone magmatism of the Prakasam rift is remarkably similar to that of the Gardar rift of South Greenland. It appears that the geodynamic conditions under which melting occurred in the Prakasam Alkaline Province are similar to that of a propagating rift with variable contributions from the convective mantle and subcontinental lithosphere mantle to the rift-zone magmas. The present study illustrates how fertility and chemical heterogeneity of the lithosphere play significant roles in the creation of enormous geochemical diversity characteristic of continental rift-zone magmatism.  相似文献   

15.
The paper reports results of the analysis of the spatial distribution of modern (younger than 2 Ma) volcanism in the Earth’s northern hemisphere and relations between this volcanism and the evolution of the North Pangaea modern supercontinent and with the spatial distribution of hotspots of the Earth’s mantle. Products of modern volcanism occur in the Earth’s northern hemisphere in Eurasia, North America, Greenland, in the Atlantic Ocean, Arctic, Africa, and the Pacific Ocean. As anywhere worldwide, volcanism in the northern hemisphere of the Earth occurs as (a) volcanism of mid-oceanic ridges (MOR), (b) subduction-related volcanism in island arcs and active continental margins (IA and ACM), (c) volcanism in continental collision (CC) zones, and (d) within-plate (WP) volcanism, which is related to mantle hotspots, continental rifts, and intercontinental belts. These types of volcanic areas are fairly often neighboring, and then mixed volcanic areas occur with the persistent participation of WP volcanism. Correspondingly, modern volcanism in the Earth’s northern hemisphere is of both oceanic and continental nature. The latter is obviously related to the evolution of the North Pangaea modern supercontinent, because it results from the Meso-Cenozoic evolution of Wegener’s Late Paleozoic Pangaea. North Pangaea in the Cenozoic comprises Eurasia, North and South America, India, and Africa and has, similar to other supercontinents, large sizes and a predominantly continental crust. The geodynamic setting and modern volcanism of North Pangaea are controlled by two differently acting processes: the subduction of lithospheric slabs from the Pacific Ocean, India, and the Arabia, a process leading to the consolidation of North Pangaea, and the spreading of oceanic plates on the side of the Atlantic Ocean, a process that “wedges” the supercontinent, modifies its morphology (compared to that of Wegener’s Pangaea), and results in the intervention of the Atlantic geodynamic regime into the Arctic. The long-lasting (for >200 Ma) preservation of tectonic stability and the supercontinental status of North Pangaea are controlled by subduction processes along its boundaries according to the predominant global compression environment. The long-lasting and stable subduction of lithospheric slabs beneath Eurasia and North America not only facilitated active IA + ACM volcanism but also resulted in the accumulation of cold lithospheric material in the deep mantle of the region. The latter replaced the hot mantle and forced this material toward the margins of the supercontinent; this material then ascended in the form of mantle plumes (which served as sources of WP basite magmas), which are diverging branches of global mantle convection, and ascending flows of subordinate convective systems at the convergent boundaries of plates. Subduction processes (compressional environments) likely suppressed the activity of mantle plumes, which acted in the northern polar region of the Earth (including the Siberian trap magmatism) starting at the latest Triassic until nowadays and periodically ascended to the Earth’s surface and gave rise to WP volcanism. Starting at the breakup time of Wegener’s Pangaea, which began with the opening of the central Atlantic and systematically propagated toward the Arctic, marine basins were formed in the place of the Arctic Ocean. However, the development of the oceanic crust (Eurasian basin) took place in the latter as late as the Cenozoic. Before the appearance of the Gakkel Ridge and, perhaps, also the oceanic portion of the Amerasian basin, this young ocean is thought to have been a typical basin developing in the central part of supercontinents. Wegener’s Pangaea broke up under the effect of mantle plumes that developed during their systematic propagation to the north and south of the Central Atlantic toward the North Pole. These mantle plumes were formed in relation with the development of global and local mantle convection systems, when hot deep mantle material was forced upward by cold subducted slabs, which descended down to the core-mantle boundary. The plume (WP) magmatism of Eurasia and North America was associated with surface collision- or subduction-related magmatism and, in the Atlantic and Arctic, also with surface spreading-related magmatism (tholeiite basalts).  相似文献   

16.
This paper reports a new 1° × 1° global thermal model for the continental lithosphere (TC1). Geotherms for continental terranes of different ages (> 3.6 Ga to present) constrained by reliable data on borehole heat flow measurements (Artemieva, I.M., Mooney, W.D. 2001. Thermal structure and evolution of Precambrian lithosphere: a global study. J. Geophys. Res 106, 16387–16414.), are statistically analyzed as a function of age and are used to estimate lithospheric temperatures in continental regions with no or low-quality heat flow data (ca. 60% of the continents). These data are supplemented by cratonic geotherms based on electromagnetic and xenolith data; the latter indicate the existence of Archean cratons with two characteristic thicknesses, ca. 200 and > 250 km. A map of tectono-thermal ages of lithospheric terranes complied for the continents on a 1° × 1° grid and combined with the statistical age relationship of continental geotherms (z = 0.04  t + 93.6, where z is lithospheric thermal thickness in km and t is age in Ma) formed the basis for a new global thermal model of the continental lithosphere (TC1). The TC1 model is presented by a set of maps, which show significant thermal heterogeneity within continental upper mantle, with the strongest lateral temperature variations (as large as 800 °C) in the shallow mantle. A map of the depth to a 550 °C isotherm (Curie isotherm for magnetite) in continental upper mantle is presented as a proxy to the thickness of the magnetic crust; the same map provides a rough estimate of elastic thickness of old (> 200 Ma) continental lithosphere, in which flexural rigidity is dominated by olivine rheology of the mantle.Statistical analysis of continental geotherms reveals that thick (> 250 km) lithosphere is restricted solely to young Archean terranes (3.0–2.6 Ga), while in old Archean cratons (3.6–3.0 Ga) lithospheric roots do not extend deeper than 200–220 km. It is proposed that the former were formed by tectonic stacking and underplating during paleocollision of continental nuclei; it is likely that such exceptionally thick lithospheric roots have a limited lateral extent and are restricted to paleoterrane boundaries. This conclusion is supported by an analysis of the growth rate of the lithosphere since the Archean, which does not reveal a peak in lithospheric volume at 2.7–2.6 Ga as expected from growth curves for juvenile crust.A pronounced peak in the rate of lithospheric growth (10–18 km3/year) at 2.1–1.7 Ga (as compared to 5–8 km3/year in the Archean) well correlates with a peak in the growth of juvenile crust and with a consequent global extraction of massif-type anorthosites. It is proposed that large-scale variations in lithospheric thickness at cratonic margins and at paleoterrane boundaries controlled anorogenic magmatism. In particular, mid-Proterozoic anorogenic magmatism at the cratonic margins was caused by edge-driven convection triggered by a fast growth of the lithospheric mantle at 2.1–1.7 Ga. Belts of anorogenic magmatism within cratonic interiors can be caused by a deflection of mantle heat by a locally thickened lithosphere at paleosutures and, thus, can be surface manifestations of exceptionally thick lithospheric roots. The present volume of continental lithosphere as estimated from the new global map of lithospheric thermal thickness is 27.8 (± 7.0) × 109 km3 (excluding submerged terranes with continental crust); preserved continental crust comprises ca. 7.7 × 109 km3. About 50% of the present continental lithosphere existed by 1.8 Ga.  相似文献   

17.
The paper considers the role of the lithospheric mantle and asthenosphere during the Late Cenozoic collision volcanism of the Lesser Caucasus. The results of petrogeochemical studies show that the products of volcanism of the West Volcanic Zone of Armenia and the calc-alkaline andesite–dacite–rhyodacite complex of the Neogene Kelbadzhar and Karabakh plateaus were formed from an enriched source in a suprasubduction setting. Late Pliocene–Quaternary moderately alkaline and alkaline volcanic rocks of the Lesser Caucasus differ in petrogeochemistry from suprasubduction volcanic rocks. In trace element contents and patterns, they are similar to rocks formed from an enriched mantle source. Comparative analysis of the geological and geophysical data suggests the model of lithospheric slab break-off of the thickened lithosphere as the triggering mechanism for Late Cenozoic magmatism of the Lesser Caucasus.  相似文献   

18.
The lower plate is the dominant agent in modern convergent margins characterized by active subduction,as negatively buoyant oceanic lithosphere sinks into the asthenosphere under its own weight.This is a strong plate-driving force because the slab-pull force is transmitted through the stiff sub-oceanic lithospheric mantle.As geological and geochemical data seem inconsistent with the existence of modernstyle ridges and arcs in the Archaean,a periodically-destabilized stagnant-lid crust system is proposed instead.Stagnant-lid intervals may correspond to periods of layered mantle convection where efficient cooling was restricted to the upper mantle,perturbing Earth's heat generation/loss balance,eventually triggering mantle overturns.Archaean basalts were derived from fertile mantle in overturn upwelling zones(OUZOs),which were larger and longer-lived than post-Archaean plumes.Early cratons/continents probably formed above OUZOs as large volumes of basalt and komatiite were delivered for protracted periods,allowing basal crustal cannibalism,garnetiferous crustal restite delamination,and coupled development of continental crust and sub-continental lithospheric mantle.Periodic mixing and rehomogenization during overturns retarded development of isotopically depleted MORB(mid-ocean ridge basalt)mantle.Only after the start of true subduction did sequestration of subducted slabs at the coremantle boundary lead to the development of the depleted MORB mantle source.During Archaean mantle overturns,pre-existing continents located above OUZOs would be strongly reworked;whereas OUZOdistal continents would drift in response to mantle currents.The leading edge of drifting Archaean continents would be convergent margins characterized by terrane accretion,imbrication,subcretion and anatexis of unsubductable oceanic lithosphere.As Earth cooled and the background oceanic lithosphere became denser and stiffer,there would be an increasing probability that oceanic crustal segments could founder in an organized way,producing a gradual evolution of pre-subduction convergent margins into modern-style active subduction systems around 2.5 Ga.Plate tectonics today is constituted of:(1)a continental drift system that started in the Early Archaean,driven by deep mantle currents pressing against the Archaean-age sub-continental lithospheric mantle keels that underlie Archaean cratons;(2)a subduction-driven system that started near the end of the Archaean.  相似文献   

19.
板块俯冲起始与大陆地壳演化   总被引：1，自引：0，他引：1  
组成大陆地壳的物质主要来自两个地质过程:地幔柱活动和板块俯冲。目前大多数研究认为板块俯冲起始于30多亿年前。在板块俯冲起始之前,基性的初始地壳物质受热重熔是大陆地壳生长的主要方式,其中,地幔柱活动是关键。地幔柱不仅向地壳输送玄武质岩浆,同时导致已有玄武质岩石和沉积岩通过部分熔融向中酸性岩石转化。当原始岩石圈强度足够大时,地幔柱会导致岩石圈倾斜、破裂,产生下滑力,诱发板块俯冲。板块俯冲引发岩浆活动,产生大量的岩浆岩,如岛弧安山岩、弧后盆玄武岩等。这些岩浆岩通过喷发、侵位,再经由块体拼贴、增生等过程加入到大陆地壳,是大陆地壳生长的主要途径。同时,板内岩浆活动乃至地幔柱活动等也与板块俯冲有直接或者间接的联系。俯冲再循环物质促进地幔柱发育,也为大陆地壳的生长提供物源和热能。与此同时,大陆地壳不断风化剥蚀,其中一部分沉积物随俯冲板块再循环到地幔,而板块俯冲过程也通过俯冲剥蚀等过程,将仰冲盘岩石圈物质刮削带入地幔。这些是大陆地壳消减的主要途径。目前大陆地壳增生和消减基本处于动态平衡。  相似文献   

20.
拆沉作用(delamination)及其壳—幔演化动力学意义   总被引：45，自引：0，他引：45  
拆沉作用导致下地壳和岩石圈地幔下沉，相应软流圈上涌至壳—幔边界，使下地壳、岩石圈地幔和软流圈三者发生物质交换，引起岩浆作用、山脉隆升、伸展、垮塌，形成坳陷盆地，并最终使大陆地壳向长英质方向演化，产生与其它行星不同的、独一无二的中性安山质或英云闪长质成分。拆沉作用是对经典板块构造理论的重要补充与完善  相似文献   

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