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1.
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   

2.
Active spreading ridges in the North Fiji Basin range from well-developed stable ridges where largescale mantle upwelling is in progress to proto-ridges where spreading is incipient. South of 17°S, where the central ridge of the North Fiji Basin has a bathymetric profile normally expected of a fast-spreading, steadystate mid-ocean ridge, basalts are evolved N-type MORBs. North of 17°S, where the central ridge is propagating northward into old North Fiji Basin crust and spreading is in the initial stages, two types of basalt have been recovered: N-type MORBs from this northern arm of the central ridge are believed to be samples of older North Fiji Basin crust; basalts with transitional alkalic chemistry (up to 0.5% Ne in the Norm) and characterized by strong relative enrichments in Rb, Ba, K, Nb, La, Ce, Sr, P, Zr, and Ti are believed to be associated with incipient rifting. Among the latter group are compositions that are intermediate between transitional alkalic types and MORBs and these are geochemically similar to the back-arc basin (BABB) magma type defined by Sinton and Fryer (1987) from a study of Mariana back arc basin basalts. Dredges along the South Pandora Ridge, a transform zone characterized by short spreading segments, are dominated by basalts that are enriched in large-ion lithophile and high field strength minor and trace elements and compositions range from types resembling ocean island tholeiites to transitional alkalic varieties. Basalts from Rotuma are regarded as alkalic end-members of the South Pandora Ridge magmatic spectrum. In areas of the North Fiji Basin where relatively fast spreading must be accompanied by largescale asthenospheric upwelling, depleted (N-type) MORBs dominate, whereas in areas of slow mantle upwelling, or where some other tectonic effect (e.g. a transform fault) causes a transient thermal disturbance within the lithosphere or upper asthenosphere, enriched (alkalic) magmas either dominate or make a significant and noticeable contribution to the overall chemical characteristics of basalts being erupted. The MORBs have a depleted asthenospheric source, and the alkalic component is believed to derive from an enriched lithospheric or shallow asthenospheric source. The BABB magma type may simply be part of the spectrum of mixed magmas that can occur in the transitional tectonic settings represented by the early development of most back-arc basins.  相似文献   

3.
We combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central Andean plateau between 18° and 20° S from Late Cretaceous to present. By integrating geophysical results of upper mantle structure, crustal thickness, and composition with recently published structural, stratigraphic, and thermochronologic data, we emphasize the importance of both the crust and upper mantle in the evolution of the central Andean plateau. Four key steps in the evolution of the Andean plateau are as follows. 1) Initiation of mountain building by 70 Ma suggested by the associated foreland basin depositional history. 2) Eastward jump of a narrow, early fold–thrust belt at 40 Ma through the eastward propagation of a 200–400-km-long basement thrust sheet. 3) Continued shortening within the Eastern Cordillera from 40 to 15 Ma, which thickened the crust and mantle and established the eastern boundary of the modern central Andean plateau. Removal of excess mantle through lithospheric delamination at the Eastern Cordillera–Altiplano boundary during the early Miocene appears necessary to accommodate underthrusting of the Brazilian shield. Replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the Eastern Cordillera and Altiplano at 24–23 Ma, and further volcanism recorded by 12–7 Ma crustal ignimbrites. 4) After 20 Ma, deformation waned in the Eastern Cordillera and Interandean zone and began to be transferred into the Subandean zone. Long-term rates of shortening in the fold–thrust belt indicate that the average shortening rate has remained fairly constant (8–10 mm/year) through time with possible slowing (5–7 mm/year) in the last 15–20 myr. We suggest that Cenozoic deformation within the mantle lithosphere has been focused at the Eastern Cordillera–Altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination.  相似文献   

4.
The Indo–Asian continental collision is known to have had a great impact on crustal deformation in south-central Asia, but its effects on the sublithospheric mantle remain uncertain. Studies of seismic anisotropy and volcanism have suggested that the collision may have driven significant lateral mantle flow under the Asian continent, similar to the observed lateral extrusion of Asian crustal blocks. Here we present supporting evidence from P-wave travel time seismic tomography and numerical modeling. The tomography shows continuous low-velocity asthenospheric mantle structures extending from the Tibetan plateau to eastern China, consistent with the notion of a collision-driven lateral mantle extrusion. Numerical simulations suggest that, at the presence of a low-viscosity asthenosphere, continued mass injection under the Indo–Asian collision zone over the past 50 My could have driven significant lateral extrusion of the asthenospheric mantle, leading to diffuse asthenospheric upwelling, rifting, and widespread Cenozoic volcanism in eastern China.  相似文献   

5.
Our knowledge of the lithosphere beneath the Carpathian–Pannonian Region (CPR) has been greatly improved through petrologic, geochemical and isotopic studies of upper mantle xenoliths hosted by Neogene–Quaternary alkali basalts. These basalts occur at the edge of the Intra-Carpathian Basin System (Styrian Basin, Nógrád-Gömör and Eastern Transylvanian Basin) and its central portion (Little Hungarian Plain, Bakony-Balaton Highland).The xenoliths are mostly spinel lherzolites, accompanied by subordinate pyroxenites, websterites, wehrlites, harzburgites and dunites. The peridotites represent residual mantle material showing textural and geochemical evidence for a complex history of melting and recrystallization, irrespective of location within the region. The lithospheric mantle is more deformed in the center of the studied area than towards the edges. The deformation may be attributed to a combination of extension and asthenospheric upwelling in the late Tertiary, which strongly affected the central part of CPR subcontinental lithosphere.The peridotite xenoliths studied show bulk compositions in the following range: 35–48 wt.% MgO, 0.5–4.0 wt.% CaO and 0.2–4.5 wt.% Al2O3 with no significant differences in regard to their geographical location. On the other hand, mineral compositions, particularly of clinopyroxene, vary according to xenolith texture. Clinopyroxenes from less deformed xenoliths show higher contents of ‘basaltic’ major elements compared to the more deformed xenoliths. However, clinopyroxenes in more deformed xenoliths are relatively enriched in strongly incompatible trace elements such as light rare earth elements (LREE).Modal metasomatic products occur as both hydrous phases, including pargasitic and kearsutitic amphiboles and minor phlogopitic micas, and anhydrous phases — mostly clinopyroxene and orthopyroxene. Vein material is dominated by the two latter phases but may also include amphibole. Amphibole mostly occurs as interstitial phases, however, and is more common than phlogopite. Most metasomatized peridotites show chemical and (sometimes) textural evidence for re-equilibration between metasomatic and non-metasomatic phases. However, amphiboles in pyroxenites are sometimes enriched in K, Fe and LREE. The presence of partially crystallized melt pockets (related to amphiboles and clinopyroxenes) in both peridotites and pyroxenites is an indication of decompression melting and, rarely, incipient partial melting triggered by migrating hydrous melts or fluids. Metasomatic contaminants may be ascribed to contemporaneous subduction beneath the Carpathian–Pannonian Region between the Eocene and Miocene.Sulfide inclusions are more abundant in protogranular and porphyroclastic xenoliths relative to equigranular types. In mantle lithologies, sulfide bleb compositions vary between pentlandite and pyrrhotite correlating with the chemistry and texture of the host xenoliths. While sulfides in peridotites are relatively rich in Ni, those in clinopyroxene-rich xenoliths are notably Fe-rich.  相似文献   

6.
西准噶尔地区出露多条蛇绿混杂岩带,对其进行精确的锆石U-Pb年代学及岩石地球化学研究可以为揭示西准噶尔地区古大洋形成与演化过程、恢复古构造格局及追溯岩浆源区物质来源提供线索.本文对玛依勒蛇绿混杂岩中的辉长岩及玄武岩进行了LA-ICP-MS锆石U-Pb年代学及全岩地球化学研究,获得辉长岩中锆石的加权平均206Pb/238U年龄为572.2±9.2Ma,属于早震旦纪,该年龄是准噶尔乃至北疆地区报道的最古老的蛇绿混杂岩年龄.玛依勒蛇绿混杂岩中的枕状玄武岩为碱性玄武岩,岩石具有高Ti(TiO2=1.65％ ～3.13％)、高Fe(FeOT=8.93％ ～ 18.11％)、高Mg(MgO=3.95％ ～ 5.27％)及高P(P2O5 =0.17％～0.51％),Th/Ta比值相对较高(=1.1～1.9),LREE和HREE分异较为明显((La/Yb)N =2.5 ～7.4)等特征,这些特征与洋岛玄武岩类似,可能形成于大洋板内的洋岛或海山环境.其中的辉长岩地球化学特征不同于玄武岩,可能形成与俯冲有关的环境.玛依勒蛇绿混杂岩中玄武岩与EMI型洋岛玄武岩具有相似的地球化学特征,表明其岩浆源区可能为EMI型富集地幔.岩石成因与软流圈地幔关系密切,软流圈的上涌导致尖晶石相二辉橄榄岩地幔源区大比例部分熔融,是岩石圈-软流圈地幔相互作用的产物.  相似文献   

7.
Combined analyses of Nd isotopes from a wide range of Neoarchaean–Cretaceous igneous rocks provides a proxy to study magmatic processes and the evolution of the lithosphere. The main igneous associations include the Neoproterozoic granitoids from the southern Brazilian shield, which were formed during two tectonothermal events of the Brasiliano cycle: the São Gabriel accretionary orogeny (900–700 Ma) and the Dom Feliciano collisional orogeny (660–550 Ma). Rocks related to the formation of the São Gabriel arc (900–700 Ma) mainly have a depleted juvenile signature. For the Neoproterozoic collisional event, the petrogenetic discussion focuses on two old crustal segments and three types of mantle components. However, no depleted juvenile material was involved in the formation of the Dom Feliciano collisional belt (800–550 Ma), which implies an ensialic environment for the Dom Feliciano orogeny. In the western Neoproterozoic foreland, records of a Neoarchaean lower crust predominate, whereas a Paleoproterozoic crust does in the eastern Dom Feliciano belt. The western foreland includes two amalgamated geotectonic domains, the São Gabriel arc and Taquarembó block. In the collisional belt, the old crust was intensely reworked during the São Gabriel event. In addition to the Neoproterozoic subduction-processed subcontinental lithosphere (São Gariel arc), we recognize two old enriched mantle components, which also are identified in the Paleoproterozoic intraplate tholeiites from Uruguay and the Cretaceous potassic suites from eastern Paraguay. One end member displays the prominent influence of Trans-Amazonian (2.3–2.0 Ga) or older subduction events, whereas the other can be interpreted as a reenrichment of the first during the latest Trans-Amazonian collisional or younger events. This reenriched mantle is documented in late Neoproterozoic suites from the western foreland (605–550 Ma) and younger suites from the eastern collisional belt (600–580 Ma). The other enriched mantle component with an old subduction signature, however, appears only in older rocks of the collisional belt (800–600 Ma). The participation of the subduction-related Brasiliano mantle as an end member of binary mixing occurred in some early Neoproterozoic suites (605–580 Ma) from the western foreland, but the contribution of the Neoarchaean lower crust increased near the late igneous event (575–550 Ma).  相似文献   

8.
The Baikal rift zone: the effect of mantle plumes on older structure   总被引：8，自引：0，他引：8  
The main chain of SW–NE-striking Cenozoic half-grabens of the Baikal rift zone (BRZ) follows the frontal parts of Early Paleozoic thrusts, which have northwestern and northern vergency. Most of the large rift half-grabens are bounded by normal faults at the northwestern and northern sides. We suggest that the rift basins were formed as a result of transformation of ancient thrusts into normal listric faults during Cenozoic extension.Seismic velocities in the uppermost mantle beneath the whole rift zone are less than those in the mantle beneath the platform. This suggests thinning of the lithosphere under the rift zone by asthenosphere upwarp. The geometry of this upwarp and the southeastward spread of its material control the crustal extension in the rift zone. This NW–SE extension cannot be blocked by SW–NE compression generated by pressure from the Indian lithospheric block against Central Asia.The geochemical and isotopic data from Late Cenozoic volcanics suggest that the hot material in the asthenospheric upwarp is probably provided by mantle plumes. To distinguish and locate these plumes, we use regional isostatic gravity anomalies, calculated under the assumption that topography is only partially compensated by Moho depth variations. Variations of the lithosphere–asthenosphere discontinuity depth play a significant role in isostatic compensation. We construct three-dimensional gravity models of the plume tails. The results of this analysis of the gravity field are in agreement with the seismic data: the group velocities of long-period Rayleigh waves are reduced in the areas where most of the recognized plumes are located, and azimuthal seismic anisotropy shows that these plumes influence the flow directions in the mantle above their tails.The Baikal rift formation, like the Kenya, Rio Grande, and Rhine continental rifts [Achauer, U., Granet, M., 1997. Complexity of continental rifts as revealed by seismic tomography and gravity modeling. In: Jacob, A.W.B., Delvaux, D., Khan, M.A. (Eds.), Lithosphere Structure, Evolution and Sedimentation in Continental Rifts. Proceedings of the IGCP 400 Meeting, Dublin, March 20–22, 1997. Institute of Advanced Studies, Dublin, pp. 161–171], is controlled by the three following factors: (i) mantle plumes, (ii) older (prerift) linear lithosphere structures favorably positioned relative to the plumes, and (iii) favorable orientation of the far-field forces.  相似文献   

9.
The Sr, Nd and Pb isotopic compositions for the Kovdor phoscorite–carbonatite complex (PCC), Kola Peninsula, NW Russia, have been determined to characterize the mantle sources involved and to evaluate the relative contributions of a plume and subcontinental lithospheric mantle in the formation of the complex. The Kovdor PCC is a part of the Kovdor ultramafic–alkaline–carbonatite massif, and consists of six intrusions. The initial isotopic ratios of the analyzed samples, calculated at 380 Ma, display limited variations: εNd, + 2.0 to + 4.7; 87Sr/86Sr, 0.70319 to 0.70361 (εSr, − 12.2 to − 6.2); 206Pb/204Pb, 18.38 to 18.74; 207Pb/204Pb, 15.45 to 15.50; 208Pb/204Pb, 37.98 to 39.28. The Nd and Sr isotope data of the Kovdor PCC generally fit the patterns of the other phoscorites and carbonatites from the Kola Alkaline Province (KAP), but some data are slightly shifted from the mixing line defined as the Kola Carbonatite Line, having more radiogenic 87Sr/86Sr ratios. However, the less radiogenic Nd isotopic compositions and negative Δ7/4 values of Pb isotopes of the analyzed samples exclude crustal contamination, but imply the involvement of a metasomatized lithospheric mantle source. Isotopic variations indicate mixing of at least three distinct mantle components: FOZO-like primitive plume component, EMI-like enriched component and DMM-like depleted component. The isotopic nature of the EMI- and DMM-like mantle component observed in the Kovdor samples is considered to be inherited from metasomatized subcontinental lithospheric mantle. This supports the previous models invoking plume–lithosphere interaction to explain the origin of the Devonian alkaline carbonatite magmatism in the KAP.  相似文献   

10.
Late Neoarchean metavolcanic rocks are widely distributed in the Western Shandong Terrane (WST). They are classified as ~2590–2580 Ma tholeiites (Group MB-1), ~2550–2530 Ma tholeiites (Group MB-2), calc-alkaline basalts (Group MB-3), high-Si adakites (Group MAD) and ~2520–2500 Ma tholeiites (Group MB-4) based on zircon U-Pb chronological and geochemical data. Their parental magmas have complex origins and were derived from a depleted mantle wedge enriched by slab-derived melts or fluids (Group MB-1); an unaltered depleted mantle (Group MB-2); the delaminated lower crustal materials (Group MAD); a strongly melt- and fluid-metasomatized depleted mantle (Group MB-3); and a fluid- and sediment-metasomatized asthenospheric mantle (Group MB-4). The late Neoarchean geodynamic evolution of the WST revealed by these multi-genetic volcanic rocks can be summarized as follows: (1) an ~2.62–2.53 Ga eastward subduction operated along the ancient continental margin, followed by delamination of unstable continental lithosphere in the back-arc region during ~2.60–2.53 Ga; and (2) delamination-derived mantle magmas ascended and caused the regional extension, further inducing the asthenosphere to passively rise and the back-arc basin to open during ~2.52–2.50 Ga. The above intense mantle magmatism and crust-mantle interactions have consumed abundant mantle energy and facilitated the continental stratification and final cratonization of the WST.  相似文献   

11.
This review considers the magmatic processes in the Carpathian–Pannonian Region (CPR) from Early Miocene to Recent times, as well as the contemporaneous magmatism at its southern boundary in the Dinaride and Balkans regions. This geodynamic system was controlled by the Cretaceous to Neogene subduction and collision of Africa with Eurasia, especially by Adria that generated the Alps to the north, the Dinaride–Hellenide belt to the east and caused extrusion, collision and inversion tectonics in the CPR. This long-lived subduction system supplied the mantle lithosphere with various subduction components. The CPR contains magmatic rocks of highly diverse compositions (calc-alkaline, K-alkalic, ultrapotassic and Na-alkalic), all generated in response to complex post-collisional tectonic processes. These processes formed extensional basins in response to an interplay of compression and extension within two microplates: ALCAPA and Tisza–Dacia. Competition between the different tectonic processes at both local and regional scales caused variations in the associated magmatism, mainly as a result of extension and differences in the rheological properties and composition of the lithosphere. Extension led to disintegration of the microplates that finally developed into two basin systems: the Pannonian and Transylvanian basins. The southern border of the CPR is edged by the Adria microplate via Sava and Vardar zones that acted as regional transcurrent tectonic areas during Miocene–Recent times.Major, trace element and isotopic data of post-Early Miocene magmatic rocks from the CPR suggest that subduction components were preserved in the lithospheric mantle after the Cretaceous–Miocene subduction and were reactivated especially by extensional tectonic processes that allowed uprise of the asthenosphere. Changes in the composition of the mantle through time support geodynamic scenarios of post-collision and extension processes linked to the evolution of the main blocks and their boundary relations. Weak lithospheric blocks (i.e. ALCAPA and western Tisza) generated the Pannonian basin and the adjacent Styrian, Transdanubian and Z?rand basins which show high rates of vertical movement accompanied by a range of magmatic compositions. Strong lithospheric blocks (i.e. Dacia) were only marginally deformed, where strike–slip faulting was associated with magmatism and extension. At the boundary of Adria and Tisza–Dacia strike–slip tectonics and core complex extension were associated with small volume Miocene magmatism in narrow extensional sedimentary basins or granitoids in core-complex detachment systems along older suture zones (Sava and Vardar) accommodating the extension in the Pannonian basin and afterward Pliocene–Quaternary inversion. Magmas of various compositions appear to have acted as lubricants in a range of tectonic processes.  相似文献   

12.
The Bayan Gol ophiolite fragment is a portion of the North Tianshan Early Carboniferous ophiolite belt. This ophiolite belt represents a geological record of an Early Carboniferous “Red Sea type” ocean basin that was developed on the northern margin of the Tianshan Carboniferous-Permian rift system in northwestern China. The late Early Carboniferous Bayan Gol ophiolite suite was emplaced in an Early Carboniferous rift volcanosedimentary succession of shallow-marine to continental facies (Volcanics Unit). Ophiolitic rocks in the Bayan Gol area comprise ultramafic rocks, gabbros with associated plagiogranite veins, diorite, diabase, pillow basalts and massive lavas. The Early Carboniferous tiffing and the opening process of the North Tianshan ocean basin produced mafic magmas in composition of tholeiite and minor amounts of evolved magmas. Compositions of trace elements and Nd, Sr and Pb isotopes reveal the presence of two distinct mantle sources: (1) the Early Carboniferous rift mafic lavas from the Volcanics Unit were generated by a relatively low degree of partial melting of an asthenospheric OIB-type intraplate source; (2)younger (late Early Carboniferous, -324.8 Ma ago) mafic lavas from the Ophiolite Unit were formed in a relatively depleted MORB-like mantle source, located in the uppermost asthenosphere and then gradually mixed with melts from the asthenospheric OIB-like mantle. A slight interaction between asthenosphere-derived magmas and lithospheric mantle took place during ascent to the surface. Subsequently, the most depleted mafic lavas of the ophiolite assemblage were contaminated by upper-crustal components (seawater or carbonate crust).  相似文献   

13.
Kent C. Condie   《Lithos》2005,79(3-4):491-504
In terms of high field strength element ratios Nb/Th, Zr/Nb, Nb/Y and Zr/Y, most basalts from non-arc type Archean greenstones are similar to oceanic plateau basalts, suggestive of mantle plume sources. A large number of these basalts have ratios similar to primitive mantle composition. Perhaps the Archean mantle was less fractionated than at present and “primitive mantle” comprised much of the deep mantle and made a significant contribution to mantle plumes. The near absence of Archean greenstone basalts similar to NMORB in composition is also consistent with a relatively unfractionated mantle in which a shallow depleted source (DM) was volumetrically insignificant. The element ratios in basalts also indicate the existence of recycled components (HIMU, EM1, EM2) in the mantle by the Late Archean. This suggests that oceanic lithosphere was recycled into the deep mantle and became incorporated in some mantle plumes by the Late Archean. High field strength element ratios also indicate an important contribution of continental crust or/and subcontinental lithosphere to some non-arc Archean greenstone basalts. This implies that at least thin continental lithosphere was relatively widespread in the Archean.  相似文献   

14.
W.G. Ernst   《Gondwana Research》2007,11(1-2):38
In the early Earth, accretionary impact heating, including collision with a large, Mars-sized object, decay of short-lived radioisotopes, and (after an initial thermal run-up) continuous segregation of the liquid Fe–Ni core resulted in extensive the melting of the silicate mantle and in the formation of a near-surface magma mush ocean. Progressive, continuous degassing and chemical–gravitational differentiation of the crust–mantle system accompanied this Hadean stage, and has gradually lessened during the subsequent cooling of the planet. Mantle and core overturn was vigorous in the Hadean Earth, reflecting deep-seated chemical heterogeneities and concentrations of primordial heat. Hot, bottom-up mantle convection, including voluminous plume ascent, efficiently rid the planet of much thermal energy, but gradually decreased in importance with the passage of time. Formation of lithospheric scum began when planetary surface temperatures fell below those of basalt and peridotite solidi. Thickening and broadening of lithospheric plates are inferred from the post-Hadean rock record. Developmental stages of mantle circulation included: (a) 4.5–4.4 Ga, early, chaotic magma ocean circulation involving an incipient or pre-plate regime; (b) 4.4–2.7 Ga, growth of small micro-oceanic and microcontinental platelets, all returned to the mantle prior to 4.0 Ga, but increasing in size and progressively suturing sialic crust-capped lithospheric amalgams at and near the surface over time; (c) 2.7–1.0 Ga, assembly of cratons surmounting larger, supercontinental plates; and (d) 1.0 Ga–present, modern, laminar-flowing asthenospheric cells capped by gigantic, Wilson-cycle lithospheric plates. Restriction of komatiitic lavas to the Archean, and of ophiolite complexes ± alkaline igneous rocks, high-pressure and ultrahigh-pressure metamorphic terranes to progressively younger Proterozoic–Phanerozoic orogenic belts supports the idea that planetary thermal relaxation promoted the increasingly negative buoyancy of cooler oceanic lithosphere. The Thickening of oceanic plates enhanced the gravitational instability and the consequent overturn of the outer Earth as cold, top-down oceanic mantle convection. The scales and dynamics of deep-seated asthenospheric circulation, and of lithospheric foundering + shallow asthenospheric return flow evidently have evolved gradually over geologic time in response to the progressive cooling of the Earth.  相似文献   

15.
贺娟  王启宇  闫国川 《地球科学》2018,43(8):2586-2599
前人对金沙江-红河构造带上的煌斑岩研究工作主要集中在南段哀牢山地区.对构造带中段鲁甸地区新发现的煌斑岩脉进行了锆石U-Pb年代学和全岩地球化学研究.结果表明,煌斑岩形成时代为始新世末期,与滇西新生代富碱斑岩高峰期一致.鲁甸煌斑岩具有高钾、富碱、高Mg#,富集大离子亲石元素（LILE）和轻稀土元素（LREE）,亏损高场强元素（HFSE,尤其是Ta-Nb-Ti）的特征.其岩浆源区为受俯冲流体和熔体交代的岩石圈地幔,源区组分为含金云母的尖晶石相方辉橄榄岩.结合同期的镁铁质火山岩和富碱斑岩研究成果,滇西区域的岩石圈地幔富集过程可能为元古宙时期与罗迪尼亚超大陆聚合相关的俯冲作用.始新世时期,在印度和亚洲大陆碰撞过程中,金沙江-红河构造带的富集岩石圈地幔发生拆沉或对流减薄,软流圈物质上涌,引发富集的岩石圈地幔部分熔融,形成本期煌斑岩岩浆作用.   相似文献   

16.
秦岭显生宙地幔组成及其演化   总被引：2，自引：1，他引：2  
通过对秦岭造山带及扬子克拉通北缘显生宙时期 3个含地幔捕虏体的煌斑岩、钾镁煌斑岩、碱性玄武岩以及 11个不含捕虏体的辉石岩、辉长岩、玄武岩出露点的岩石地球化学对比研究 ,揭示出研究区地幔演化经历了自古生代的OIB亏损地幔到中生代的高度富集地幔再到中生代末期 -新生代的OIB MORB的亏损地幔的两次明显变更。制约这种变更的主要因素是熔融岩浆时源区发生的层圈相互作用类型。鉴于大陆岩石圈软流层体系的特征 ,有必要划分出岩石圈 /软流层相互作用带(过渡带 ) ,它是大陆岩浆作用的重要源区。  相似文献   

17.
Temperature estimates and chemical composition of mantle xenoliths from the Cretaceous rift system of NW Argentina (26°S) constrain the rift evolution and chemical and physical properties of the lithospheric mantle at the eastern edge of the Cenozoic Andean plateau. The xenolith suite comprises mainly spinel lherzolite and subordinate pyroxenite and carbonatized lherzolite. The spinel lherzolite xenoliths equilibrated at high-T (most samples >1000 °C) and P below garnet-in. The Sm–Nd systematics of compositionally unzoned clino- and orthopyroxene indicate a Cretaceous minimum age for the high-T regime, i.e., the asthenosphere/lithosphere thermal boundary was at ca. 70 km depth in the Cretaceous rift. Major elements and Cr, Ni, Co and V contents of the xenoliths range between values of primitive and depleted mantle. Calculated densities based on the bulk composition of the xenoliths are <3280 kg/m3 for the estimated P–T conditions and indicate a buoyant, stable upper mantle lithosphere. The well-equilibrated metamorphic fabric and mineral paragenesis with the general lack of high-T hydrous phases did not preserve traces of metasomatism in the mantle xenoliths. Late Mesozoic metasomatism, however, is obvious in the gradual enrichment of Sr, U, Th and light to medium REE and changes in the radiogenic isotope composition of an originally depleted mantle. These changes are independent of the degree of depletion evidenced by major element composition. 143Nd/144Ndi ratios of clinopyroxene from the main group of xenoliths decrease with increasing Nd content from >0.5130 (depleted samples) to ca. 0.5127 (enriched samples). 87Sr/86Sri ratios (0.7127–0.7131, depleted samples; 0.7130–0.7134, enriched samples) show no variation with variable Sr contents. Pbi isotope ratios of the enriched samples are rather radiogenic (206Pb/204Pbi 18.8–20.6, 207Pb/204Pbi 15.6–15.7, 208Pb/204Pbi 38.6–47) compared with the Pb isotope signature of the depleted samples. The large scatter and high values of 208Pb/204Pbi ratios of many xenoliths indicates at least two Pb sources that are characterized by similar U/Pb but by different Th/Pb ratios. The dominant mantle type in the investigated system is depleted mantle according to its Sr and Nd isotopic composition with relatively radiogenic Pb isotope ratios. This mantle is different from the Pacific MORB source and old subcontinental mantle from the adjacent Brazilian Shield. Its composition probably reflects material influx into the mantle wedge during various episodes of subduction that commenced in early Paleozoic or even earlier. Old subcontinental mantle was already replaced in the Paleozoic, but some inheritance from old mantle lithosphere is represented by rare xenoliths with isotope signatures indicating a Proterozoic origin.  相似文献   

18.
The Late Cenozoic geodynamics of the Alpine-Himalayan belt comprised the collision between continental-lithosphere plates and blocks and the effect of the Neo-Tethyan active residual asthenosphere, which reached the northern margin of the belt after the ocean had closed. From the late Eocene to the early Pliocene, strong deformation, lateral migrations of flaked plates, metamorphism, and magmatism (they all consolidated the crust) took place in the lithosphere with the participation of mobile asthenospheric components. In the Pliocene–Quaternary, the asthenosphere beneath the consolidated crust partly replaced the dense mantle lithosphere with remaining paleoocean mafic rocks, which subducted into the mantle. Phase transformations and deformations in the subducting metamafic slabs caused mantle earthquakes. The less compact metamafic rocks experienced metamorphic weakening under the effect of the asthenosphere and incorporated into the Earth’s crust. The upper-mantle and lower-crust weakening led to a drastic intensification of uplifting and the formation of mountain ranges. Recent volcanism is also attributed to the activity of the Neo-Tethyan asthenosphere.  相似文献   

19.
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   

20.
《Lithos》2007,93(1-2):149-174
Strong compositional variations are observed in the late-Miocene to Quaternary volcanic rocks of the eastern Trans-Mexican Volcanic Belt. Geochemical and isotopic analyses of samples well constrained in age indicate an abrupt change in magma composition in the late-Miocene (∼ 7.5 Ma), when calc-alkaline, subduction-related magmatism was replaced by mafic, alkaline, OIB-like volcanism. Afterwards, volcanism migrated toward the trench and the erupted lavas showed increasing contributions of subduction components reflected in higher Th/Nb, La/Sm(n), Ba/Nb, and Ba/Th ratios. Lavas from volcanic fields located closer to the trench show clearer, although strongly variable, arc signatures as well as evidence of subducted sediment contributions. Farther from the trench, only lavas emplaced in late-Pliocene time appear to be slightly modified by subduction components, whereas the youngest Quaternary lavas can be regarded as intraplate lavas modified by crustal assimilation.The sudden change in magma composition in the late-Miocene is related to detachment of the subducting slab, which allowed the infiltration of enriched asthenospheric mantle into the mantle wedge. After detachment, the subducting plate started to increase its dip because of the loss of slab pull. This caused (1) the migration of the arc toward the trench, (2) convection of enriched asthenosphere into the mantle wedge, and (3) an increasing contribution of slab components to the melts, in a process that resulted in a highly heterogeneous source mantle. The variable contribution of subduction-related components to the magmas is controlled by the heterogeneous character of the source, the depth of the subducting plate, and the previous magmatic history of the areas.  相似文献   

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