吉林红旗岭地区造山后镁铁-超镁铁岩体的年代测定及其意义

吉林省中部红旗岭地区镁铁-超镁铁杂岩广泛发育,年代学资料表明,红旗岭-漂河川岩体形成于晚三叠纪(距今约216Ma),与该区A型花岗岩的年龄相当,但略微年轻一些。岩石学和地球化学特征表明,镁铁-超镁铁岩普遍可见包橄结构,橄榄石晶体平衡岩浆属于高MgO玄武质岩浆,微量元素和同位素模拟结果均显示岩浆源区主要为亏损地幔或新生岩石圈地幔。根据这些结果,同时结合相关研究,提出这些镁铁-超镁铁杂岩体的形成与该区大量造山后A型花岗岩带的形成相吻合,其形成主要是由于造山后期岩石圈板块的拆沉减薄效应,软流圈地幔上隆及大量幔源岩浆上侵,底垫于地壳底部,并经历了强烈的结晶分异作用     

Postcollisional mantle-derived magmatism, underplating and implications for basement of the Junggar Basin

The late Paleozoic postcollisional granitoids, mafic-ultramafic complexes, and volcanic rocks are extensively distributed around the Junggar Basin; they are generally characterized by positive ε_Nd(t) values, implying that the magmas were mantle-derived and contaminated with crustal materials to some extents. The emplacement of mantle-derived magmas and their differentiates in the upper crust is the expression of deep geological processes at shallow level, while much more mantle-derived magmas were underplated in the lower crust and the region near the crust-mantle boundary, being component part of basement of the Junggar Basin. The postcollisional mafic-ultramafic complexes would not be generated by re-melting of residual oceanic crust, which was considered as the basement of the Junggar Basin, unless very high degrees of partial melting occurred. Even if old continental crust had been present before collision, it would have been strongly modified by the mantle-derived magma underplating. This interpretation is compatible with the existing geophysical data. Project supported by the National Natural Science Foundation of China (Grants Nos. 4900031 and 49272103)     

Role of mantle-derived magma in genesis of early Yanshanian granites in the Nanling Range,South China: <Emphasis Type="Italic">in situ</Emphasis> zircon Hf-O isotopic constraints

Although a number of petrographic observations and isotopic data suggest that magma mixing is common in genesis of many granite plutons, it is still controversial whether the mantle-derived magmas were involved in granites. We carried out in this study a systematic analysis of in situ zircon Hf-O isotopes for three early Yanshanian intrusions dated at ca. 160 Ma from the Nanling Range of Southeast China. The Qinghu monzonite has very homogeneous zircon Hf-O isotopic compositions, εHf(t) =11.6±0.3 and δ18O=5...     

磁异常揭示的峨眉山大火成岩省的深部结构

峨眉山大火成岩省位于中国西南部,在晚二叠纪约260 Ma喷发出巨量的大陆溢流型玄武岩.对于大火成岩省的岩浆喷发,在地下必定有一个相应的大规模岩浆聚集和运移系统.地球物理方法是探测岩石圈内部的有效方式.峨眉山大火成岩省为镁铁质岩浆喷发,由于镁铁质-超镁铁质岩石一般具有强磁性,因此,在喷发结束之后,地下岩浆系统如果被镁铁质岩浆填充,冷却固化成为岩石圈的一部分,很有可能会引起磁异常.本文使用区域磁异常数据来对峨眉山大火成岩省的深部构造进行研究.该区域的磁异常由一系列离散的异常组成,通过3D磁化率反演可以得到磁性体的空间分布.由于磁异常中具有明显的剩磁,直接使用经典的反演方法会有较大误差,我们首先将磁异常转换为弱敏感于磁化方向的磁异常模量,再使用模量数据进行3D反演,得到地下空间内磁异常源的分布.经过分析认为这些离散分布的磁异常源反映了岩石圈内部的镁铁质-超镁铁质侵入体.侵入体的位置可能反映了底侵和内侵的镁铁质岩浆固化形成的侵入体,代表镁铁质岩浆房位置或者岩浆运移的主要通道.     

The influence of crustal structure on compositions of subduction-related magmas

A survey of Sr isotopic ratios and other compositional features of subduction-related magma suites reveals significant correlations between these averaged parameters and characteristics of the underlying crust (i.e., thickness, composition, and age). These observations lead to the conclusion that crust and(or) mantle rocks in the hanging walls of subduction zones are involved in modification of primary mafic magmas (typically basalt or basaltic andesite). It is proposed that mafic magmas will stagnate within the crust or uppermost mantle where they may differentiate and react with wall rocks. The extent to which such processes manifest themselves will depend upon details of the local crustal structure. In particular, the composition and age of the crust will strongly influence such parameters as Sr, Nd and Pb isotopic compositions. Such data strongly indicate the involvement of crustal rocks in locales underlain by old sialic crust (e.g., central Andes). Depending upon the level of magma stagnation and evolution within the crust, different trends in isotopic composition may result. These isotopic trends may be enhanced by partial melting of the wall rocks to produce relatively silicic anatectic magmas, and locally they may reflect subduction of continental sediments. Interpretation of the isotopic data may be more ambiguous in locales underlain by younger and more mafic continental crust (Cascades, E Eleutians) and those underlain by oceanic crust owing to the similarity in isotopic composition of primary magmas and the latter crustal materials. Yet some degree of crustal involvement in magmatic evolution seems highly probable even in these more primitive terranes. Consequently, most island arc magmas, and especially those more evolved than basalt, are probably not primary in the sense that they do not represent direct melts of the upper mantle. Studies of arc volcanic rocks may yield misleading conclusions concerning processes of magma generation related to subduction unless evolutionary processes are defined and their effects considered. It appears that modern volcanic arcs provide a poor analog for models of early crustal development because the modern mantle-derived magmatic components are more mafic in composition than average continental crust.     

Miocene silicic volcanism in southwestern Idaho: geochronology,geochemistry, and evolution of the central Snake River Plain

New ⁴⁰Ar-³⁹Ar geochronology, bulk rock geochemical data, and physical characteristics for representative stratigraphic sections of rhyolite ignimbrites and lavas from the west-central Snake River Plain (SRP) are combined to develop a coherent stratigraphic framework for Miocene silicic magmatism in this part of the Yellowstone ‘hotspot track’. The magmatic record differs from that in areas to the west and east with regard to its unusually large extrusive volume, broad lateral scale, and extended duration. We infer that the magmatic systems developed in response to large-scale and repeated injections of basaltic magma into the crust, resulting in significant reconstitution of large volumes of the crust, wide distribution of crustal melt zones, and complex feeder systems for individual eruptive events. Some eruptive episodes or ‘events’ appear to be contemporaneous with major normal faulting, and perhaps catastrophic crustal foundering, that may have triggered concurrent evacuations of separate silicic magma reservoirs. This behavior and cumulative time-composition relations are difficult to relate to simple caldera-style single-source feeder systems and imply complex temporal-spatial development of the silicic magma systems. Inferred volumes and timing of mafic magma inputs, as the driving energy source, require a significant component of lithospheric extension on NNW-trending Basin and Range style faults (i.e., roughly parallel to the SW–NE orientation of the eastern SRP). This is needed to accommodate basaltic inputs at crustal levels, and is likely to play a role in generation of those magmas. Anomalously high magma production in the SRP compared to that in adjacent areas (e.g., northern Basin and Range Province) may require additional sub-lithospheric processes. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.     

Constraints on melting and magma production in the crust

Major intrusions of granitic rocks are found in several tectonic settings and, in all cases, crustal melts may contribute to the volumes of granitic magma. High-grade metamorphism and partial melting of the crust take place predominantly under fluid-absent conditions. We present a model for calculating the amounts of melt that may be formed by fluid-absent breakdown of micas and amphiboles in common crustal rock types (pelitic, quartzofeldspathic, intermediate and mafic). Melt proportions depend mainly on the kind of source rock, the pressure at which melting takes place, the temperature and the hydrous mineral (H₂O) content of the source. As a consequence of the pressure dependence of water solubility in silicate melts, any given source rock will produce more melt, by a given fluid-absent reaction, at lower pressure. At a given pressure, higher-temperature reactions can produce more melt from a given source rock. Based on a survey of the compositions of common rock types, we show that the amounts of melt can vary from < 10to> 50vol.%. Thus, crustal rocks vary widely in their “fertility” as magma sources, depending on the types and amounts of hydrous phases they contain. In general, muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation. Biotite breakdown in pelites occurs at higher temperature and, because most high-grade pelites (below granulite grade) are biotite-rich, can yield up to about 50 vol.% of granitoid melt. Rocks of intermediate composition and hornblende-rich mafic rocks are potentially highly fertile magma sources also, provided that the high temperatures necessary for biotite and hornblende breakdown are realized. Pyroxene-rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile. Data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with < 30and probably< 20vol.% melt. The pressure-temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria.     

大别造山带地壳结构反演及其动力学意义

大别造山带是全球最大的碰撞造山带之一,三叠纪时期,扬子板块深俯冲至地幔的200km处,经历了超高压变质作用。白垩纪早期,该造山带发生了强烈的伸展和垮塌,以及大规模的后造山地幔源岩浆侵入和火山活动。本研究收集了大别造山带及其邻区(29°~34°N、114°~119°E)的震相资料,采用双差层析成像技术,对大别造山带地壳结构进行反演,研究地壳结构与后造山地幔源岩浆侵入和火山活动之间的关系。结果显示,大别造山带中上地壳存在低速结构,该低速结构可能是熔融的幔源侵入物质,由于俯冲板片断裂,或下地壳/岩石圈发生拆沉,导致软流圈物质上涌至地壳底部、侵入地壳中,形成大别造山带地壳中的低速结构;同时,合肥盆地显示为低速区,可能是受浅部沉积层影响。研究中横切大别山的4条剖面显示,该地区下方存在北向倾斜高速结构,该高速结构可能是襄樊-广济断层,或者是扬子板块向华北板块下方俯冲的遗迹。     

Intrusion of ultramafic magmatic bodies into the continental crust: Numerical simulation

Intrusions of ultramafic bodies into the lower density continental crust are documented for a large variety of tectonic settings spanning continental shields, rift systems, collision orogens and magmatic arcs. The intriguing point is that these intrusive bodies have a density higher by 300-500 kg m⁻³ than host rocks. Resolving this paradox requires an understanding of the emplacement mechanism. We have employed finite differences and marker-in-cell techniques to carry out a 2D modeling study of intrusion of partly crystallized ultramafic magma from sublithospheric depth to the crust through a pre-existing magmatic channel. By systematically varying the model parameters we document variations in intrusion dynamics and geometry that range from funnel- and finger-shaped bodies (pipes, dikes) to deep seated balloon-shaped intrusions and flattened shallow magmatic sills. Emplacement of ultramafic bodies in the crust lasts from a few kyr to several hundreds kyr depending mainly on the viscosity of the intruding, partly crystallized magma. The positive buoyancy of the sublithospheric magma compared to the overriding, colder mantle lithosphere drives intrusion while the crustal rheology controls the final location and the shape of the ultramafic body. Relatively cold elasto-plastic crust (T_Moho = 400 °C) promotes a strong upward propagation of magma due to the significant decrease of plastic strength of the crust with decreasing confining pressure. Emplacement in this case is controlled by crustal faulting and subsequent block displacements. Warmer crust (T_Moho = 600 °C) triggers lateral spreading of magma above the Moho, with emplacement being accommodated by coeval viscous deformation of the lower crust and fault tectonics in the upper crust. Strong effects of magma emplacement on surface topography are also documented. Emplacement of high-density, ultramafic magma into low-density rocks is a stable mechanism for a wide range of model parameters that match geological settings in which partially molten mafic-ultramafic rocks are generated below the lithosphere. We expect this process to be particularly active beneath subduction-related magmatic arcs where huge volumes of partially molten rocks produced from hydrous cold plume activity accumulate below the overriding lithosphere.     

Continental basalts in the accretionary complexes of the South-west Japan Arc: Constraints from geochemical and Sr and Nd isotopic data of metadiabase

Abstract The Ryoke Belt is one of the important terranes in the South‐west Japan Arc (SJA). It consists mainly of late Cretaceous granitoid rocks, meta‐sedimentary rocks (Jurassic accretionary complexes) and mafic rocks (gabbros, metadiabases; late Permian–early Jurassic). Initial ?_Sr (+ 25– + 59) and ?_Nd (? 2.1–?5.9) values of the metadiabases cannot be explained by crustal contamination but reflect the values of the source material. These values coincide with those of island arc basalt (IAB), active continental margin basalt (ACMB) and continental flood basalt (CFB). Spiderdiagrams and trace element chemistries of the metadiabases have CFB‐signature, rather than those of either IAB or ACMB. The Sr–Nd isotope data, trace element and rare earth element chemistries of the metadiabases indicate that they result from partial melting of continental‐type lithospheric mantle. Mafic granulite xenoliths in middle Miocene volcanic rocks distributed throughout the Ryoke Belt were probably derived from relatively deep crust. Their geochemical and Sr–Nd isotopic characteristics are similar to the metadiabases. This suggests that rocks, equivalent geochemically to the metadiabases, must be widely distributed at relatively deep crustal levels beneath a part of the Ryoke Belt. The geochemical and isotopic features of the metadiabases and mafic granulites from the Ryoke Belt are quite different from those of mafic rocks from other terranes in the SJA. These results imply that the Ryoke mafic rocks (metadiabase, mafic granulite) were not transported from other terranes by crustal movement but formed in situ. Sr–Nd isotopic features of late Cretaceous granitoid rocks occurring in the western part of the Japanese Islands are coincident with those of the Ryoke mafic rocks. Such an isotopic relation between these two rocks suggests that a continental‐type lithosphere is widely represented beneath the western part of the Japanese Islands.     

贺根山镁铁-超镁铁岩与缝合带的关系——来自地球物理的证据

长期以来,对内蒙古贺根山缝合带中的镁铁-超镁铁岩,有着"蛇绿岩"、"岩浆岩"、"幔源熔-流体"等不同认识.近年来在铬铁矿中发现了金刚石等深部地幔矿物,如何认识携带这些物质的幔源熔-流体的上涌?缝合带在镁铁-超镁铁岩的形成过程中起到什么作用?解决问题的关键之一是弄清镁铁-超镁铁岩的深部产状,这需要来自地球物理观测数据的依据.本文基于航磁和重力数据的研究表明,贺根山地区的蛇绿岩块均呈现高磁异常特征,其中贺根山岩块埋深明显大于其他岩块,表现为高磁异常与低重力异常,与地表出露的蛇纹石化的镁铁-超镁铁岩带对应.对航磁化极异常与布格重力异常数据做了向上延拓处理,进行了磁源形态及底界深度的估算,并利用基于相异度算子的边缘增强方法辅助识别断裂.结果表明,贺根山岩块贯穿地壳,且附近存在超壳断裂.地表至中地壳主要由蛇纹石化的镁铁-超镁铁岩组成,下地壳主要为超镁铁质岩组成,它们充填在一组宽约30km的NEE向断裂带中,大地电磁测深剖面揭示的壳-幔电性结构进一步证实控制缝合带的是岩石圈断裂,贺根山缝合带具有明显的根部.由此推测,在地幔底辟上涌的背景下,幔源熔-流体沿着岩石圈断裂持续上升到达浅表,暗示该区板块的拼合可能是通过深部幔源物质的侵入而成的.     

U-Pb isotopic geochemistry of the post-collisional mafic-ultramafic rocks from the Dabie Mountains

The U-Pb isotope geochemical study of the pyroxenite-gabbro intrusion in the Dabie Mountains shows that the post-collisional mafic-ultramafic rocks of the Dabie Mountains are characterized by relative high Pb contents, low U contents and low U/Pb ratios. These characters may be results of interaction between lithosphere or depleted asthenospheric mantle (DMM) and lower crust, but have nothing to do with mantle plume and subducted continental crust. It was first observed that some samples with lower ²⁰⁶Pb/²⁰⁴Pb and higher ²⁰⁷Pb/²⁰⁴Pb ratios show typical characters of the LOMU component. The Pb, Sr, and Nd isotopic tracing shows that three components are needed in the source of the Zhujiapu pyroxenite-gabbro intrusion. They could be old enriched sub-continental lithospheric mantle (LOMU component), lower crust and depleted asthenospheric mantle. The crust-mantle interaction process producing primitive magma of post-collisional mafic-ultramafic rocks in the Dabie Mountains could be described by a lithospheric delamination and magma underplating model. After continent-continent collision, delamination of the thickened lithosphere induced the upwelling of depleted asthenospheric mantle, which caused partial melting of asthenospheric mantle and residual sub-continental lithospheric mantle. The basaltic magma produced in this process underplated in the boundary between the crust and mantle and interacted with lower crust resulting in the geochemical characters of both enriched lithospheric mantle and lower crust.     

Bundelkhand mafic dykes, Central Indian Shield: Implications for the role of sediment subduction in Proterozoic crustal evolution

Abstract The Archean to Paleo–Proterozoic Bundelkhand massif basement of the central Indian shield has been dissected by numerous mafic dykes of Proterozoic age. These dykes are low‐Ti tholeiites, ranging in composition from subalkaline basalt through basaltic‐andesite to dacite. They are enriched in light rare earth elements (LREE), large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE: Nb, P and Ti). Negative Sr anomaly is conspicuous. Nb/La ratios of the dykes are much lower compared with the primitive mantle, not much different from the average crustal values, but quite similar to those of continental and subduction related basaltic rocks. Bulk contamination of the mantle derived magma by crustal material is inadequate to explain the observed geochemical characteristics; instead contamination of the mantle/lithospheric source(s) via subduction of sediment is a better proposition. Thus, in addition to generating juvenile crust along the former island arcs, subduction processes appear to have influence on the development of enriched mantle/lithospheric source(s). The Bundelkhand massif basement is inferred to represent subduction related juvenile crust, that experienced lithospheric extension and rifting possibly in response to mantle plume activities. The latter probably supplied the required heat, material (fluids) and extensional environment to trigger melting in the refractory lithospheric source(s) and emplacement of the mafic dykes. Proterozoic mafic magmatic rocks from Bundelkhand, Aravalli, Singhbhum and Bastar regions of the Indian shield and those from the Garhwal region of the Lesser Himalaya display remarkably similar enriched incompatible trace elements characteristics, although limited chemical variations are observed in all these rocks. This may indicate the existence of a large magmatic province, different parts of which might have experienced similar petrogenetic processes and were probably derived from mantle/lithospheric source(s) with similar trace element characteristics. The minor, less enriched to depleted components of the Jharol Group of the Aravalli terrane and those from the Singhbhum terrane may represent protracted phases of rifting, that probably caused thinning and mobilization of the lithosphere, facilitating the eruption/emplacement of the asthenospheric melts (with N‐ to T‐types mid‐oceanic ridge basalts signatures) and deposition of deep water facies sediments in the younger developing oceanic basins. In contrast, Bundelkhand region did not experience such protracted rifting, although dyke swarms were emplaced and shallow water Bijawar Group and Vindhyan Supergroup sediments were deposited in continental rift basins. All these discrete Proterozoic terranes appear to have experienced similar petrogenetic processes, tectonomagmatic and possibly temporal evolution involving subduction processes, influencing the lithospheric source characteristics, followed by probably mantle plume induced ensialic rifting through to the development of oceanic basins in the Indian shield regions and their extension in the Lesser Himalaya.     

A petrologic model for the constitution of the upper mantle and crust of the Koolau shield,Oahu, Hawaii,and Hawaiian magmatism

A petrological model for the uppermost upper mantle and crust under the Koolau shield to a depth of about 60 km has been derived on the basis of petrology of the upper mantle and crustal xenoliths in nephelinites of the Honolulu Volcanic Series. Three main xenolith suites exist in the Koolau shield: dunites, spinel lherzolites, and garnet-bearing pyroxenites. On the basis of mineralogy, it is inferred that the dunites represent cumulates in shallow crustal tholeiitic magma chambers, the spinel lherzolites form a thick (～ 40 km) layer in the upper mantle, and the garnet pyroxenite suite occurs as veins and stringers in the spinel lherzolites at about 60 km depth.The eruption sequence in a Hawaiian volcano, i.e., tholeiite → transitional basalt → alkali basalt, is generated by partial melting of a volatile-bearing garnet-lherzolite part of a lithospheric plate as it rides over a hot spot. If the tholeiite, transitional, and alkali basalts of Hawaiian volcanoes are generated at the same depth, then the observed sequence of lavas requires replenishment of the source area with volatiles and gradual decrease of the degree of partial melting with time. Post-erosional olivine nephelinites are produced from isotopically distinct, deeper source area, which may be the asthenosphere.     

Crystal fractionation of the basalt comendite series of the mount Edziza volcanic complex, British Columbia: Major and trace elements

The Mount Edziza Volcanic Complex in north-central British Columbia includes a group of overlapping basaltic shields, salic composite volcanoes, domes and small calderas that range in age from 7.5 Ma to less than 2000 years B.P. The volcanic assemblage is chemically bimodal, comprising voluminous alkali olivine basalt and hawaiite, a salic suite of mainly peralkaline trachyte and comendite, plus a relatively small volume of intermediate rocks (trachybasalt, tristanite, mugearite, benmoreite). The complex is the product of five cycles of magmatic activity, each of which began with alkali olivine basalt and culminated with the eruption of salic magma. The regular chemical variation shown by almost 100 major- and trace-element analyses suggests a genetic lineage between the basic and salic members of each cycle. Least-squares mathematical modelling, indicates that the salic rocks (trachyte and comendite) have formed by fractionation of observed phenocryst and cumulate nodule mineral phases from a common alkali olivine basalt parent magma.Hawaiite is thought to be a cumulate rock, formed by partial fractionation and feldspar accumulation within rising columns of primary alkali olivine basalt. Fractionation leading from alkali olivine basalt through trachybasalt and trachyte to comendite is believed to have taken place where primary basalt became trapped in large crustal reservoirs. The early removal of olivine, clinopyroxene and plagioclase, leading to a trachytic residuum, and subsequent fractionation of mainly alkali feldspar, leading to the peralkaline end members, is consistent with major- and trace-element variation and with isotopic and REE data.The chemical diversity of the complex is attributed to its location over a zone of crustal extension where mantle-derived basalt, trapped in large high-level reservoirs, underwent prolonged fractionation.     

Geochemistry and petrology of meta-igneous granulitic xenoliths in Neogene volcanic rocks of the Massif Central,France — implications for the lower crust

Meta-igneous mafic and ultramafic rocks, which constitute about 60% of the granulitic xenoliths enclosed in the Neogene alkali basalts of the Bournac pipe (French Massif Central) have well preserved magmatic trends of element variations. The meta-igneous suite was probably derived from at least two different parental magmas and it may be a part of a gabbroic complex which resembles mafic bodies associated with anorthosites. The xenoliths are also very similar to many other granulitic xenoliths and to meta-igneous mafic granulitic massifs. This indicates that the gabbroic intrusions may be widespread in the lower crust and the close association of gabbroic rocks with meta-sedimentary granulites suggests a model for the composition of the lower continental crust.     

Petrogenesis and significance of the Mesozoic North Taihang complex: Major and trace element evidence

The intensive Mesozoic magmatism in the North China Craton (NCC) has drawn great attention for its particular geochemical signatures (e.g. high-K), petrogenesis and tectonic setting. The North Taihang complex represents the westernmost magmatic belt of th…     

Tectono-magmatic evolution of the 1.9-Ga great bear magmatic zone, Wopmay orogen, northwestern Canada

The 1875-1840-Ma Great Bear magmatic zone is a 100-km wide by at least 900-km-long belt of predominantly subgreenschist facies volcanic and plutonic rocks that unconformably overlie and intrude an older sialic basement complex. The basement complex comprises older arc and back-arc rocks metamorphosed and deformed during the Calderian orogeny, 5–15 Ma before the onset of Great Bear magmatism. The Great Bear magmatic zone contains the products of two magmatic episodes, separated temporally by an oblique folding event caused by dextral transpression of the zone: (1) a 1875-1860-Ma pre-folding suite of mainly calc-alkaline rocks ranging continuously in composition from basalt to rhyolite, cut by allied biotite-hornblende-bearing epizonal plutons; and (2) a 1.85-1.84-Ga post-folding suite of discordant, epizonal, biotite syenogranitic plutons, associated dikes, and hornblende-diorites, quartz diorites, and monzodiorites. The pre-folding suite of volcanic and plutonic rocks is interpreted as a continental magmatic arc generated by eastward subduction of oceanic lithosphere. Cessation of arc magmatism and subsequent dextral transpression may have resulted from ridge subduction and resultant change in relative plate motion. Increased heat flux due to ridge subduction coupled with crustal thickening during transpression may have caused crustal melting as evidenced by the late syenogranite suite. Final closure of the western ocean by collision with a substantial continental fragment, now forming the neoautochthonous basement of the northern Canadian Cordillera, is manifested by a major swarm of transcurrent faults found throughout the Great Bear zone and the Wopmay orogen.Although there is probably no single evolutionary template for magmatism at convergent plate margins, the main Andean phase of magmatism, exemplified by the pre-folding Great Bear magmatic suite, evolves as larger quantities of subduction-related mafic magma rise into and heat the crust. This results in magmas that are more homogeneous, siliceous, and explosive with time, ultimately leading to overturn and fractionation of the continental crust.     

Origin and ascent history of unusually crystal-rich alkaline basaltic magmas from the western Pannonian Basin

The last eruptions of the monogenetic Bakony-Balaton Highland Volcanic Field (western Pannonian Basin, Hungary) produced unusually crystal- and xenolith-rich alkaline basalts which are unique among the alkaline basalts of the Carpathian–Pannonian Region. Similar alkaline basalts are only rarely known in other volcanic fields of the world. These special basaltic magmas fed the eruptions of two closely located volcanic centres: the Bondoró-hegy and the Füzes-tó scoria cone. Their uncommon enrichment in diverse crystals produced unique rock textures and modified original magma compositions (13.1–14.2 wt.% MgO, 459–657 ppm Cr, and 455–564 ppm Ni contents). Detailed mineral-scale textural and chemical analyses revealed that the Bondoró-hegy and Füzes-tó alkaline basaltic magmas have a complex ascent history, and that most of their minerals (～30 vol.% of the rocks) represent foreign crystals derived from different levels of the underlying lithosphere. The most abundant xenocrysts, olivine, orthopyroxene, clinopyroxene, and spinel, were incorporated from different regions and rock types of the subcontinental lithospheric mantle. Megacrysts of clinopyroxene and spinel could have originated from pegmatitic veins/sills which probably represent magmas crystallized near the crust–mantle boundary. Green clinopyroxene xenocrysts could have been derived from lower crustal mafic granulites. Minerals that crystallized in situ from the alkaline basaltic melts (olivine with Cr-spinel inclusions, clinopyroxene, plagioclase, and Fe–Ti oxides) are only represented by microphenocrysts and overgrowths on the foreign crystals. The vast amount of peridotitic (most common) and mafic granulitic materials indicates a highly effective interaction between the ascending magmas and wall rocks at lithospheric mantle and lower crustal levels. However, fragments from the middle and upper crust are absent from the studied basalts, suggesting a change in the style (and possibly rate) of magma ascent in the crust. These xenocryst- and xenolith-rich basalts yield divers tools for estimating magma ascent rate that is important for hazard forecasting in monogenetic volcanic fields. According to the estimated ascent rates, the Bondoró-hegy and Füzes-tó alkaline basaltic magmas could have reached the surface within hours to few days, similarly to the estimates for other eruptive centres in the Pannonian Basin which were fed by “normal” (crystal and xenoliths poor) alkaline basalts.     

Interaction between subducted continental crust and the mantle

The syncollisional mafic-ultramafic intrusions in the North Dabie terrane are characterized by enriched LREE, Rb, Ba, and depletion of high field strength elements, such as Nb, Zr, Ti. The high Ti/& and Ti/Y ratios of their parent magma suggest that crust contamination of magma is not important to the above trace elements characters. They mainly reflect the features of their mantle source which has been modified by metasomatism in subduction environment.