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1.
《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   

2.
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   

3.
How was Taiwan created?   总被引：4，自引：0，他引：4  
Since the beginning of formation of proto-Taiwan during late Miocene (9 Ma), the subducting Philippine (PH) Sea plate moved continuously through time in the N307° direction at a 5.6 cm/year velocity with respect to Eurasia (EU), tearing the Eurasian plate. Strain states within the EU crust are different on each side of the western PH Sea plate boundary (extensional in the Okinawa Trough and northeastern Taiwan versus contractional for the rest of Taiwan Island). The B feature corresponds to the boundary between the continental and oceanic parts of the subducting Eurasian plate and lies in the prolongation of the ocean–continent boundary of the northern South China Sea. Strain rates in the Philippines to northern Taiwan accretionary prism are similar on each side of B (contractional), though with different strain directions, perhaps in relation with the change of nature of the EU slab across B. Consequently, in the process of Taiwan mountain building, the deformation style was probably not changing continuously from the Manila to the Ryukyu subduction zones. The Luzon intra-oceanic arc only formed south of B, above the subducting Eurasian oceanic lithosphere. North of B, the Luzon arc collided with EU simultaneously with the eastward subduction of a portion of EU continental lithosphere beneath the Luzon arc. In its northern portion, the lower part of the Luzon arc was subducting beneath Eurasia while the upper part accreted against the Ryukyu forearc. Among the consequences of such a simple geodynamic model: (i) The notion of continuum from subduction to collision might be questioned. (ii) Traces of the Miocene volcanic arc were never found in the southwestern Ryukyu arc. We suggest that the portion of EU continental lithosphere, which has subducted beneath the Coastal Range, might include the Miocene Ryukyu arc volcanoes formed west of 126°E longitude and which are missing today. (iii) The 150-km-wide oceanic domain located south of B between the Luzon arc and the Manila trench, above the subducting oceanic EU plate (South China Sea) was progressively incorporated into the EU plate north of B.  相似文献   

4.
Volcanoes were created, grew, uplifted, became dormant or extinct, and were accreted as part of continents during continuous arc–continent collision. Volcanic rocks in Eastern Taiwan’s Coastal Range (CR) are part of the northern Luzon Arc, an oceanic island arc produced by the subduction of the South China Sea Plate beneath the Philippine Sea Plate. Igneous rocks are characterized by intrusive bodies, lava and pyroclastic flows, and volcaniclastic rocks with minor tephra deposits. Based on volcanic facies associations, Sr–Nd isotopic geochemistry, and the geography of the region, four volcanoes were identified in the CR: Yuemei, Chimei, Chengkuangao, and Tuluanshan. Near-vent facies associations show different degrees of erosion in the volcanic edifices for Chimei, Chengkuangao, and Tuluanshan. Yuemei lacks near-vent rocks, implying that Yuemei’s main volcanic body may have been subducted at the Ryukyu Trench with the northward motion of the Philippine Sea Plate. These data suggest a hypothesis for the evolution of volcanism and geomorphology during arc growth and ensuing arc–continent collision in the northern Luzon Arc, which suggests that these volcanoes were formed from the seafloor, emerging as islands during arc volcanism. They then became dormant or extinct during collision, and finally, were uplifted and accreted by additional collision. The oldest volcano, Yuemei, may have already been subducted into the Ryukyu Trench.  相似文献   

5.
The shift of lava geochemistry between volcanic front to rear-arc volcanoes in active subduction zones is a widespread phenomenon. It is somehow linked to an increase of the slab surface depth of the subducting oceanic lithosphere and increasing thickness of the mantle wedge and new constraints for its causes may improve our understanding of magma generation and element recycling in subduction zones in general. As a case study, this paper focuses on the geochemical composition of lavas from two adjacent volcanic centres from the volcanic front (VF) to rear-arc (RA) transition of the Southern Kamchatkan subduction zone, with the aim to examine whether the shift in lava geochemistry is associated with processes in the mantle wedge or in the subducted oceanic lithosphere or both. The trace element and O-Sr-Nd-Hf-Pb (double-spike)-isotopic composition of the mafic Mutnovsky (VF) and Gorely (RA) lavas in conjunction with geochemical modelling provides constraints for the degree of partial melting in the mantle wedge and the nature of their slab components. Degrees of partial melting are inferred to be significantly higher beneath Mutnovsky (∼18%) than Gorely (∼10%). The Mutnovsky (VF) slab component is dominated by hydrous fluids, derived from subducted sediments and altered oceanic crust, eventually containing minor but variable amounts of sediment melts. The composition of the Gorely slab component strongly points to a hydrous silicate melt, most likely mainly stemming from subducted sediments, although additional fluid-contribution from the underlying altered oceanic crust (AOC) is likely. Moreover, the Hf-Nd-isotope data combined with geochemical modelling suggest progressive break-down of accessory zircon in the melting metasediments. Therefore, the drastic VF to RA shift in basalt chemistry mainly arises from the transition of the nature of the slab component (from hydrous fluid to melt) in conjunction with decreasing degrees of partial melting within ∼15 km across-arc. Finally, systematic variations of key inter-element with high-precision Pb-isotope ratios provide geochemical evidence for a pollution of the Mutnovsky mantle source with Gorely melt components but not vice versa, most likely resulting from trench-ward mantle wedge corner flow. We also present a geodynamic model integrating the location of the Mutnovsky and Gorely volcanic centres and their lava geochemistry with the recently proposed thermal structure of the southern Kamchatkan arc and constraints about phase equilibria in subducted sediments and AOC. Herein, the slab surface hosting the subducted sediments suffers a transition from dehydration to melting above a continuously dehydrating layer of AOC. Wider implications of this study are that an onset of (flush-) sediment melting may ultimately be the main trigger for the VF to RA transition of lava geochemistry in subduction zones.  相似文献   

6.
位于青藏高原南部的冈底斯岩浆弧形成于中生代新特提斯大洋岩石圈的长期俯冲过程中,而且在印度与亚洲大陆碰撞过程中叠加了强烈的新生代岩浆作用,是世界上典型的复合型大陆岩浆弧,已经成为研究汇聚板块边缘岩浆作用和大陆地壳生长与再造的天然实验室。基于对现有研究成果的总结,我们将冈底斯岩浆弧的岩浆构造演化划分为5个阶段：第1阶段发生在晚白垩世之前,以新特提斯洋岩石圈长期正常俯冲和钙碱性弧岩浆岩的发育为特征;第2阶段发生在晚白垩世时期,以活动的新特提斯洋中脊发生俯冲和强烈的岩浆作用与显著的新生地壳生长为特征;第3阶段发生在晚白垩世晚期,以残余的新特提斯大洋岩石圈俯冲和正常弧型岩浆作用为特征;第4阶段发生在古新世至中始新世,以印度与亚洲大陆碰撞、俯冲的新特提斯洋岩石圈回转和断离,及其诱发的幔源岩浆作用、新生和古老地壳的强烈再造为特征;第5阶段为发生在晚渐新世到中中新世的后碰撞阶段,深俯冲印度岩石圈的回转和断离,或加厚岩石圈地幔的对流移去导致了加厚下地壳的部分熔融和埃达克质岩石的广泛发育,同时伴随幔源钾质超钾质岩浆作用。冈底斯弧岩浆作用与岩浆成分的系统时空变化很好地记录了从新特提斯洋俯冲到印度亚洲大陆碰撞的完整构造演化过程。  相似文献   

7.
周华伟  林清良 《地学前缘》2002,9(4):285-292
文中介绍有关西藏—喜马拉雅碰撞带的一项地震层析成像研究。根据一个用天然地震数据产生的全球波速模型 ,印度板块有可能以近水平状俯冲于整个西藏高原之下至 16 5～ 2 6 0km深度。西藏岩石圈具有低波速地壳和高波速下岩石圈 (75～ 12 0km深 )。在 12 0～ 16 5km深度范围 ,西藏岩石圈与俯冲的印度板块之间有一层低速软流圈物质。高原中部从地表到 310km深处有一低速体 ,说明地幔物质有可能穿过俯冲板块的脆弱部位上隆。这些结果以及野外实测的地壳缩短值说明高原的抬升得助于印度板块的近水平俯冲。我们推论俯冲印度板块的升温上浮以及上覆软流层的存在是造成西藏高原高海拔抬升以及内部地表仍相对平坦的主要原因。2 0 0 1年 1月 2 6日在印度西部发生的毁灭性大地震有可能是俯冲应力在印度板块后缘薄弱处引发的岩石圈大断裂。  相似文献   

8.
Arc–continent collision is a key process of continental growth through accretion of newly grown magmatic arc crust to older continental margin. We present 2D petrological–thermo-mechanical models of arc–continent collision and investigate geodynamic regimes of this process. The model includes spontaneous slab bending, dehydration of subducted crust, aqueous fluid transport, partial melting of the crustal and mantle rocks and magmatic crustal growth stemming from melt extraction processes. Results point to two end-member types of subsequent arc–continent collisional orogens: (I) orogens with remnants of accretion prism, detached fragments of the overriding plate and magmatic rocks formed from molten subducted sediments; and (II) orogens mainly consisting of the closed back-arc basin suture, detached fragments of the overriding plate with leftovers of the accretion prism and quasi insignificant amount of sediment-derived magmatic rocks. Transitional orogens between these two endmembers include both the suture of the collapsed back-arc basin and variable amounts of magmatic production. The orogenic variability mainly reflects the age of the subducting oceanic plate. Older, therefore colder and denser oceanic plates trigger subduction retreat, which in turn triggers necking of the overriding plate and opening of a backarc basin in which new oceanic lithosphere is formed from voluminous decompression melting of the rising hot asthenosphere. In this case, subducted sediments are not heated enough to melt and generate magmatic plumes. On the other hand, young and less dense slabs do not retreat, which hampers opening of a backarc basin in the overriding plate while subducted sediments may reach their melting temperature and develop trans-lithospheric plumes. We have also investigated the influences of convergence rate and volcanic/plutonic rocks' ratio in newly forming lithosphere. The predicted gross-scale orogenic structures find similarities with some natural orogens, in particular with deeply eroded orogens such as the Variscides in the Bohemian Massif.  相似文献   

9.
Subduction zones of continental, transitional, and oceanic settings, relative to the nature of the overriding plate, are compared in terms of trace element compositions of mafic to intermediate arc rocks, in order to evaluate the relationship between subduction parameters and the presence of subduction fluids. The continental Chilean Southern Volcanic Zone (SVZ) and the transitional to oceanic Central American Volcanic Arc (CAVA) show increasing degrees of melting with increasing involvement of slab fluids, as is typical for hydrous flux melting beneath arc volcanoes. At the SVZ, the central segment with the thinnest continental crust/lithosphere erupted the highest-degree melts from the most depleted sources, similar to the oceanic-like Nicaraguan segment of the CAVA. The northern part of the SVZ, located on the thickest continental crust/lithosphere, exhibits features more similar to Costa Rica situated on the Caribbean Large Igneous Province, with lower degrees of melting from more enriched source materials. The composition of the slab fluids is characteristic for each arc system, with a particularly pronounced enrichment in Pb at the SVZ and in Ba at the CAVA. A direct compositional relationship between the arc rocks and the corresponding marine sediments that are subducted at the trenches clearly shows that the compositional signature of the lavas erupted in the different arcs carries an inherited signal from the subducted sediments.  相似文献   

10.
以全球大地构造为背景讨论了玄武岩浆起源和演化的一些基本概念.这些概念的正确理解有助于合理解释各种环境中火成岩的形成机制,也有助于依据野外岩石组合来判别古构造环境.在此基础上结合已有资料和观察,对中国东部中生代岩石圈减薄及中-新生代基性火山岩成因提出了一些新解释.这些解释与地质观察相吻合,且符合基本的物理学原理.虽然中国东部基性火山活动可称为"板内"火山活动,但它实际上是板块构造的特殊产物.中国东部中生代岩石圈减薄是其下部被改造为软流层的缘故.这种改造是加水"软化"所致.水则源于中国东部地幔过渡带(410～660 km)内古太平洋(或其前身)俯冲板块脱水作用.其将岩石圈底部改造为软流层的过程,实际上就是岩石圈减薄的过程.因为软流层是地幔对流的重要部分,而大陆岩石圈则不直接参与地幔对流.中生代玄武岩具有εNd＜0的特征,说明其源于新近改造而成的软流层,亦即原古老岩石圈之底部.中国大陆北北东-南南西向的海拔梯度突变界线与东-西部重力异常,陆壳厚度变化,以及地幔地震波速变化梯度吻合.因此可将北北东-南南西向梯度线称为"东-西梯度界".该界东-西海拔高差(西部高原与东部丘陵平原),陆壳厚度差异(西部厚而东部薄)和100～150 km的深度范围地幔地震波速差异(西部快而东部慢),均受控于上地幔重力均衡原理.这表明西部高原岩石圈厚度＞150～200 km,而东部丘陵平原岩石圈厚度＜80km."遥远"的西太平洋俯冲带具有自然的地幔楔吸引作用.此吸引作用可引起中国东部"新生"软流层东流.软流层东流必将引起西部高原底部软流层的东向补给(流动).这一过程必然导致东移软流层的减压,即从西部的深源(岩石圈深度＞150～200 km处)到东部的浅源(岩石圈深度～80km处).东移软流层的减压分熔可合理解释具有软流圈地球化学特征(εNd＞0)的新生代中国东部基性火山活动及玄武岩的成因.这些对中国东部中-新生代地质过程的解释,将为更加细致的,以岩石学和地球化学为主的讨论所验证.  相似文献   

11.
A geographic information system (GIS “Volcanic belts”) was used for analyzing the spatial and temporal relationship between tectono-magmatic cycles in the Cenozoic that took place at the convergent plate boundaries, mostly in volcanic arc-back-arc systems. The onset of back-arc basins and subaerial arc volcanism and their main evolutionary stages are shown to have occurred about the same time. These processes are still ongoing, which is indicated by today’s active volcanoes, high heat flows, and high deep-focus seismicity. The crust underlying both tectonic structures undergoes transformation, which results in a significant thinning of the “granite” layer within the volcanic belts, whereas crust within the back-arc basins changes its properties to the transitional (suboceanic) and oceanic type crusts. All processes that occur at the convergent plate boundaries can be described within the arc-back-arc system, the principal dynamic components of which are the asthenospheric plume upwelling above the continent edge and the oceanward-spreading plume head. This was accompanied by a gradual crustal thinning in the back-arc region and the formation of areas with oceanic crust, as well as by involvement of crustal material, together with rocks of the subducting slab, into subduction processes. As a result, the continental crust is removed from the tectonosphere and stored in the “slab cemetery.” Only a minor portion of the crustal materials is returned to the surface as subduction-related magmatism.  相似文献   

12.
Compositions of post-Miocene basalts erupted in the Garibaldi and Central America volcanic arcs exhibit significant correlations with the age of the subducted plate. In general, SiO2, Al2O3, CaO, V, and (Sr/P)N decrease and FeO, MgO, TiO2 and Na2O increase as the age of the subducted plate decreases. Variations in CaO/Al2O3, SiO2, (Sr/P)N, and Ba are compatible with lesser slab input, and hence less hydrous melting conditions in the mantle wedge in segments of the arcs overlying the youngest oceanic lithosphere. This interpretation is supported by comparison with peridotite melting experiments, which suggest higher melt pressures and temperatures in the mantle wedge above very young oceanic lithosphere. These observations point to a model in which dehydration of the downgoing slab occurs at shallow depths in subduction systems involving oceanic lithosphere younger than about 20 Ma. Because young oceanic lithosphere is relatively warm, little post-subduction heating is required to produce metamorphic reactions that release slab volatiles. Geodynamic models indicate most volatile-liberating reactions will occur within the seismogenic zone in oceanic lithosphere younger than 20 Ma, thus limiting the volatile flux beneath the arc and encouraging drier, higher temperature and higher pressure melting conditions in the mantle wedge in comparison to typical arc systems. Liberation of volatiles in the downgoing plate is strongly dependant on the shear stress on the fault, but is predicted to occur within the seismogenic zone for shear stresses greater than 33 MPa. Similarly, early loss of volatiles is predicted over a wide range of convergence rates, plate dips, and convergence angles. These results are shown to be robust for realistic ranges of slab dip, convergence angle, and shear stress, suggesting that volatile-poor melt generation is a characteristic of modern and ancient arc systems that involve subduction of young oceanic lithosphere.  相似文献   

13.
It is proposed that major continental collision normally causes two orogenies. The first is characterized by ophiolite obduction, and the second by widespread deformation, often accompanied by metamorphism and granite intrusion. The two orogenies are separated by a relatively quiescent orogenic pause of 40–60 Ma. The two stages of continental collision are illustrated by examples from the Paleozoic Newfoundland Appalachians, and the Mesozoic-Cenozoic Tethyan collision belts of the Zagros and Himalayas.

The stages of continental collision are explained in terms of the forces driving plate motions, which are dominated by the downward pull of subducting oceanic lithosphere and, to a lesser extent, by the outward push of spreading oceanic ridges.

The Taconic stage marks attempted subduction of continental crust. The buoyancy of continental crust offsets the negative buoyancy of subducting oceanic lithosphere and other driving forces so that plate motion is halted. Orogeny involves vertical buoyancy forces and is concentrated along the narrow belt of plate overlap at the subduction zone.

In a major collision the Taconic stage destroys a substantial proportion of the earth's subducting capacity. It is an event of such magnitude that it has global consequences, reducing sea-floor spreading and the rate of convection. This results in retention of heat within the earth and a consequent increase in the forces driving the plates. The orogenic pause represents the time taken for these forces to become strong enough to overcome the obstruction of buoyant continental crust and renew subduction at the collision zone.

The Acadian stage of collision occurs when renewed subduction is achieved by detachment of continental crust from its underlying lithosphere. As the subcrustal lithosphere is subducted, the crust moves horizontally. The result is crustal shortening with widespread deformation and generation of anatectic granitic magma, as well as subduction related volcanism.

The effects of continental collision on the rate of sea-floor spreading can be related to eustatic changes in sea level, glaciations, and mass extinctions. There may also be connections, through changes in the rate of mantle convection, to the earth's magnetic polarity bias and rotation rate.  相似文献   


14.
詹美珍  孙卫东  凌明星  李贺 《岩石学报》2015,31(7):2101-2114
菲律宾吕宋岛上约5Ma以来的斑岩铜金矿床主要集中在北部的Baguio和Mankayan地区,它们在时空上与黄岩海山链密切相关。1907~2013年间的地震数据表明,在吕宋岛中部(16°N)附近存在地震稀疏带。吕宋岛上的斑岩铜金矿床分布在该地震稀疏带的两侧。收集到的相应时期埃达克岩的Sr/Y-(La/Yb)N、Sr/Y-Y和La/Yb-Yb图解表明,这些埃达克岩几乎都是洋壳部分熔融形成的。与吕宋岛北部侵入型埃达克岩相比,位于16°N附近的埃达克岩具有更高的Sr含量,这可能与南海古扩张脊俯冲撕裂形成的板片窗有关。斜长石是辉长岩的主要矿物之一,因此,撕裂的洋壳边缘的辉长岩层部分熔融,形成具有更高Sr含量的埃达克质岩浆。而位于吕宋岛南部Bataan弧中的埃达克质火山岩,可能是在南海古扩张脊俯冲之前形成的。根据已发表的斑岩铜金矿床数据,Mankayan地区的成矿年龄在约3.5~1.4Ma,Baguio地区的成矿年龄在约3.1~0.5Ma之间,有从北向南变年轻的趋势,这与黄岩海山链沿马尼拉海沟向南迁移一致。此外,吕宋岛北部Mt.Cagua到Baguio之间存在一个延伸了220km的第四纪火山活动的空隙,该区域大部分火山已经在中新世停止活动。这可能是黄岩海山链的俯冲使得俯冲倾角逐渐变缓、挤压加强而导致的。同时期的斑岩铜矿床正好分布在这一火山空隙中,是俯冲洋壳部分熔融的产物。  相似文献   

15.
It is being accepted that earthquakes in subducting slab are caused by dehydration reactions of hydrous minerals. In the context of this “dehydration embrittlement” hypothesis, we propose a new model to explain key features of subduction zone magmatism on the basis of hydrous phase relations in peridotite and basaltic systems determined by thermodynamic calculations and seismic structures of Northeast Japan arc revealed by latest seismic studies. The model predicts that partial melting of basaltic crust in the subducting slab is an inevitable consequence of subduction of hydrated oceanic lithosphere. Aqueous fluids released from the subducting slab also cause partial melting widely in mantle wedge from just above the subducting slab to just below overlying crust at volcanic front. Hydrous minerals in the mantle wedge are stable only in shallow (< 120 km) areas, and are absent in the layer that is dragged into deep mantle by the subducting slab. The position of volcanic front is not restricted by dehydration reactions in the subducting slab but is controlled by dynamics of mantle wedge flow, which governs the thermal structure and partial melting regime in the mantle wedge.  相似文献   

16.
俯冲带部分熔融   总被引：3，自引：3，他引：0  
张泽明  丁慧霞  董昕  田作林 《岩石学报》2020,36(9):2589-2615
俯冲带是地幔对流环的下沉翼,是地球内部的重要物理与化学系统。俯冲带具有比周围地幔更低的温度,因此,一般认为俯冲板片并不会发生部分熔融,而是脱水导致上覆地幔楔发生部分熔融。但是,也有研究认为,在水化的洋壳俯冲过程中可以发生部分熔融。特别是在下列情况下,俯冲洋壳的部分熔融是俯冲带岩浆作用的重要方式。年轻的大洋岩石圈发生低角度缓慢俯冲时,洋壳物质可以发生饱和水或脱水熔融,基性岩部分熔融形成埃达克岩。太古代的俯冲带很可能具有与年轻大洋岩石圈俯冲带类似的热结构,俯冲的洋壳板片部分熔融可以形成英云闪长岩-奥长花岗岩-花岗闪长岩。平俯冲大洋高原中的基性岩可以发生部分熔融产生埃达克岩。扩张洋中脊俯冲可以导致板片窗边缘的洋壳部分熔融形成埃达克岩。与俯冲洋壳相比,俯冲的大陆地壳具有很低的水含量,较难发生部分熔融,但在超高压变质陆壳岩石的折返过程中可以经历广泛的脱水熔融。超高压变质岩在地幔深部熔融形成的熔体与地幔相互作用是碰撞造山带富钾岩浆岩的可能成因机制。碰撞造山带的加厚下地壳可经历长期的高温与高压变质和脱水熔融,形成S型花岗岩和埃达克质岩石。  相似文献   

17.
There are, in principle, direct relations between several important phenomena associated with subduction zones: the depth of oceanic trenches, the magnitude of the net force from trenches acting on subducting plates, the distribution and fault plane orientations of earthquakes, the magnitude of stresses on subduction faults, the bathymetry of back-arc regions, and the magnitudes of gravity and geoid anomalies. These phenomena are related through the stresses transmitted through surface and subducted lithosphere, and are associated with the mass anomaly of the subducted lithosphere. Quantitative estimates suggest that observed trench depths imply a trench pull force on subducting plates which is comparable to the ridge push force but much less than the excess weight of the subducted lithospheric slab. It is further suggested that either the mass anomaly of subducted lithosphere is much less than would be expected on the basis of conventional thermal and compositional models or that (a) a large resistance acts on the upper part of slabs due to high-stress corner flow, and (b) the mass anomaly of the slab is 70–90% compensated either by a broad 1 km-deep back-arc depression or a low density mantle wedge above the slab or both.  相似文献   

18.
Subduction of high bathymetric relief, such as aseismic ridges and magmatic plateaus, is considered to be responsible for dramatic changes in the dynamics and kinematics of the subduction zone. For example, the buoyancy of high bathymetric relief is thought to flatten the dip of the subducting slab, modifying the structural and magmatic evolution of the overriding plate and terminating arc volcanism. In addition, the effect of ridge subduction in retreating plate boundaries can inhibit subduction rollback, a process that could locally pin the subduction hinge and lead to the development of cusps and slab tearing. Here we discuss the tectonic response to subduction of high bathymetric relief using examples from the circum-Pacific subduction systems. We demonstrate that flattening of the subduction dip angle is only significant in the eastern Pacific, where the average slab dip angle is relatively shallow. In the western Pacific, in contrast, the average subduction dip angle is steeper and there is no significant flattening of the dip angle in areas of ridge subduction. Subduction of high bathymetric relief in the circum-Pacific is commonly associated with reduced arc volcanism, and in many cases, the area of ridge subduction coincides with a volcanic gap. In the overriding plate, ridge subduction is associated with pronounced changes in the style of deformation, involving uplift, reactivation of basement thrusts, development of orogen-perpendicular tear faults and block rotations leading to oroclinal bending. The discussed characteristic patterns associated with ridge subduction provide important guidelines for reconstructing past plate tectonic processes, and could help constraining the geodynamics of ancient subduction systems.  相似文献   

19.
Based on petrological and geochemical arguments, it is possible that arc magma is derived from subducted oceanic crust. In this paper, regional thermal models have been constructed to study the feasibility of melting cold subducted oceanic crusts at shallow depth (i.e. at depths of about 100 km) by a dynamic mantle. Calculated results suggest that plate subduction will generate an induced flow in the wedge above the subducting slab. This current continuously feeds hot mantle material into the corner and onto the slab surface. A high temperature thermal environment can be maintained in the vicinity of the wedge corner, immediately beneath the over-riding plate. Our regional models further demonstrate quantitatively that production of local melting is possible just about 30 km down dip from the asthenosphere wedge corner. Additional geological processes such as reasonable amounts of shear heating and minor dehydration (which will lower the local melting temperature) will further increase the probability of melting a cold subducted oceanic crust at shallow depth.  相似文献   

20.
Volcanic gaps due to oblique consumption of aseismic ridges   总被引：1，自引：0，他引：1  
The present-day consumption of oceanic ridges and other buoyant rises and fragments at circum-Pacific subduction zones, and presumably elsewhere, are closely related to existing gaps in volcanism. Examples are the gaps associated with the Nazca, Juan Fernandez, Cocos, Marcus-Necker and Louisville ridges. The buoyancy of these ridges breaks the continuity of the subducted plate, which may lead to reduced water supply required for melting of magma, and therefore create temporary volcanic gaps. The oblique consumption of these ridges causes the gap to migrate with time. This mechanism may be useful in interpreting time-space patterns of past volcanic chains associated with subduction in terms of the consumption of the disruptive oceanic plateaus and ridges.  相似文献   

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