北祁连山元古宙末－寒武纪主动大陆裂谷火山作用

北祁连山元古宙末－寒武纪大陆裂谷火山岩系为双峰式火山岩套，主要由基性与酸性火山岩组成。基性火山岩有磁性玄武岩与拉斑玄武岩两个岩浆系列，且富集ＬＲＥＥ与ＬＩＬ，其岩浆源区为与洋岛玄武岩源相似的富集地幔柱源。软流圈地幔柱上涌导致岩石圈地慢部分熔融，其熔体与地幔柱衍生熔浆混合，形成本区具有中等钕，锶同位素比值特点的基性岩浆。基性岩浆上侵至陆壳，引起下部陆壳深熔，产生长英质岩浆。地幔柱上隆促使大陆扩张，及至形成北祁连山元古宙末－寒武纪大陆裂谷。     

Trace element evidence for the origin of ocean island basalts: an example from the Austral Islands (French Polynesia)

The Austral Islands, a volcanic chain in the South-Central Pacific Ocean (French Polynesia) are composed mainly of alkali basalts and basanites with subordinate amounts of olivine tholeiites and strongly undersaturated rocks (phonolite foidites and phonolite tephrites). The basaltic rocks have geochemical features typical of oceanic island suites. The distribution of incompatible trace elements indicate that the lavas were derived from a heterogeneous mantle source. The chondrite-normalized patterns of the incompatible elements in basaltic rocks of the Austral Islands are complementary to those of island arc tholeiites. As supported by isotope data, the observed trace element heterogeneities of the source are probably due to mixing of the upper mantle with subducted oceanic crust from which island arc tholeiitic magma was previously extracted.     

大兴安岭北段中—新生代玄武岩成分变异:对地幔热演化过程意义

近年来,东北地区地幔热演化过程的相关研究相对较少,而揭示东北地区地幔热演化过程的有效手段就是研究东北地区玄武岩的成分变异特征.系统总结并对比了大兴安岭北段早白垩世玄武质岩石和新生代玄武质岩石的化学成分变异,以便揭示研究区中生代晚期-新生代的地幔热演化过程.大兴安岭北段早白垩世玄武岩在化学上属于拉斑玄武岩系列,以亏损Nb、Ta、Ti等高场强元素为特征,它们的La/Nb和La/Ta比值分别介于1.8~5.6和30~87,暗示岩浆起源于岩石圈地幔;它们的初始87Sr/86Sr值、ε_Nd（t）和ε_Hf（t）值分别介于0.704 5~0.706 9、-1.52~+3.60和+1.74~+7.77,表明岩浆源区属于弱亏损-弱富集的岩石圈地幔;早白垩世玄武质岩石的Sr-Nd-Pb同位素成分指示岩浆源区是由DM和EMⅡ型地幔端元混合而成,并经历了俯冲流体的交代.表明大兴安岭北段早白垩世玄武质岩浆源区为受早期俯冲流体交代的岩石圈地幔.新生代超钾质和钾质玄武岩具有Nb-Ta的弱负异常,87Sr/86Sr值为0.704 7~0.705 7、ε_Nd（t）值为-6.3~-0.8,而地幔捕掳体具有Sr-Nd同位素亏损特征;钠质玄武岩具有Nb-Ta的正异常,较超钾质和钾质玄武岩具有低的87Sr/86Sr（0.703 5~0.704 2）以及高的ε_Nd（t）值（+3.4~+6.6）,类似MORB的同位素组成,这些特征说明大兴安岭北段新生代玄武质岩石起源于软流圈地幔.综上所述,大兴安岭北段早白垩世和新生代玄武质岩石成分的差异不仅指示其岩浆源区从岩石圈地幔转变为软流圈地幔,更为重要的是它揭示了研究区地幔的热演化过程——从早白垩世高的地温梯度到新生代低的地温梯度的转变.这一过程也是岩石圈从中生代晚期到新生代逐渐增厚的过程.结合区域构造演化,可以得出大兴安岭北段早白垩世的玄武质岩浆作用与岩石圈伸展、减薄形成的裂陷作用相关,而新生代玄武质岩浆作用则与陆内裂谷作用相关.      

Petrology and geochemistry of post–kinematic mafic rocks from the Paleoproterozoic Ubendian belt, NE Katanga (Democratic Republic of Congo)

The Paleoproterozoic post-kinematic Ubendian mafic rocks from northeastern Katanga (Democratic Republic of Congo) are olivine-and-quartz tholeiites which in many respects resemble Phanerozoic continental tholeiites. The analogies are suggested by the petrographic features and the major element diagrams classically used to infer magmatic affinity. The clinopyroxene compositions straddle the boundary between clinopyroxenes from orogenic and extensional tectonic settings. In addition, the whole-rock compositions are mostly Ti- and P-poor as in low Ti–P continental flood basalts and in subduction-related mafic magmas. The same conclusion is sustained by the trace-element compositions (e.g., occurrence of mafic magmas with high Th/Ta and La/Ta values; low Sr/Ce ratios, etc). These geochemical features indicate involvement of a subduction component at the source of these extensional igneous rocks. Convective mixing of asthenospheric mantle with the overlying lithospheric mantle enriched during the Ubendian subduction or mixing of melts from both mantle components can account for the composition of the post-orogenic Ubendian mafic rocks.     

Origin of some magmas in oceanic and circum-oceanic regions

Partial melting experiments on spinel-lherzolite, a rock which probably occurs in relatively shallow parts of the oceanic upper mantle, demonstrate that alkali basaltic melt is formed at depths of at least 20 kbar whereas tholeiitic melt is formed at lower pressures (< 15 kbar) under anhydrous conditions. The specimen studied was a relatively iron-rich natural spinel-lherzolite (Fe/Mg+Fe=0.15) and the melts produced have ratios comparable to those obtained in basalts. Slight increase of degree of partial melting produces picritic melt over a wide pressure range. Under hydrous (water-excess) conditions, andesitic melt is produced by partial melting of the same natural spinel-lherzolite and a synthetic lherzolite. The melting experiments on two different abyssal tholeiites from the Mid-Atlantic Ridge suggest that the derivation of olivine tholeiite from a more mafic magma or a mantle peridotite (lherzolite) is possible, but is limited to depths shallower than 25 km under essentially anhydrous conditions, whereas plagioclase tholeiite may have been formed by fractional crystallization at depths of about 20 km in the presence of a small amount (～ 2 wt.%) of water.It is suggested that under mid-ocean ridges, partial melting of spinel-lherzolite at depths shallower than 60 km would produce olivine-tholeiitic magma, which differentiates at shallower levels (20–25 km) under either essentially anhydrous or hydrous (but vapor-absent) conditions to produce abyssal tholeiites of olivine-tholeiite type or plagioclase-tholeiite type. It may be also possible that the former olivine-tholeiite is generated by direct partial melting of plagioclase-lherzolite. Alkali basalts in the oceanic region may be generated at depths greater than 50 km by relatively small degree of partial melting. Along island arcs and continental margins, where the subduction zones probably exist, partial melting of lherzolite would take place in the presence of water that may be supplied by breakdown of hydrous minerals in the subducted oceanic crust, thereby producing andesitic magmas. High-alumina basalt magma could be produced by partial melting of the dehydrated oceanic crust in the subduction zone at depths between 40 and 60 km, where garnet is unstable above the solidus.     

The transformation of the lithospheric mantle beneath South China Block (SCB): constraints from petrological and geochemical studies of Daoxian and Ningyuan basalts and their melt inclusions

ABSTRACT

The lithospheric mantle beneath the South China Block (SCB) underwent a dramatic transformation from depleted to enriched during late Mesozoic. With a view to deeply understand this process, here we investigate the Mesozoic basalts and their melt inclusions from the Daoxian and Ningyuan regions within the central SCB. The geochemical features of the melt inclusions in these basalts suggest that these rocks originated from the lithospheric mantle enriched through interaction with K-rich aqueous fluids released from subducted Palaeo-Pacific oceanic sediments, whereas the Ningyuan basalts were mainly derived from the asthenospheric mantle source. Geochemical modelling indicates that the Daoxian basalts were generated from 15%-25% of partial melting of garnet lherzolite, whereas the Ningyuan basalts originated from 10%-20% of partial melting of garnet pyroxenites. Our data, combined with those from other Jurassic basalts suggest a temporal evolution of the SCB mantle sources during the Late Mesozoic. Diverse crust–mantle interactions through mixing of the asthenospheric melts with variable proportions of subducted Palaeo-Pacific oceanic sediments might account for the spatial heterogeneity of mantle sources observed beneath the SCB. The transition from Tethyan tectonic realm to the Palaeo-Pacific tectonic regime might have played a significant role in the transformation of the lithospheric mantle beneath the SCB.     

Revisiting Early–Middle Jurassic igneous activity in the Nanling Mountains,South China: Geochemistry and implications for regional geodynamics

Early–Middle Jurassic igneous rocks (190–170 Ma) are distributed in an E–W-trending band within the Nanling Tectonic Belt, and have a wide range of compositions but are only present in limited volumes. This scenario contrasts with the uniform but voluminous Middle–Late Jurassic igneous rocks (165–150 Ma) in this area. The Early–Middle Jurassic rocks include oceanic-island basalt (OIB)-type alkali basalts, tholeiitic basalts and gabbros, bimodal volcanic rocks, syenites, A-type granites, and high-K calc–alkaline granodiorites. Geochemical and isotopic data indicate that alkaline and tholeiitic basalts and syenites were derived from melting of the asthenospheric mantle, with asthenosphere-derived magmas mixing with variable amounts of magmas derived from melting of metasomatized lithospheric mantle. In comparison, A-type granites in the study area were probably generated by shallow dehydration-related melting of hornblende-bearing continental crustal rocks that were heated by contemporaneous intrusion of mantle-derived basaltic magmas, and high-K calc-alkaline granodiorites resulted from the interaction between melts from upwelling asthenospheric mantle and the lower crust. The Early–Middle Jurassic magmatic event is spatially variable in terms of lithology, geochemistry, and isotopic systematics. This indicates that the deep mantle sources of the magmas that formed these igneous rocks were significantly heterogeneous, and magmatism had a gradual decrease in the involvement of the asthenospheric mantle from west to east. These variations in composition and sourcing of magmas, in addition to the spatial distribution and the thermal structure of the crust–mantle boundary during this magmatic event, indicates that these igneous rocks formed during a period of rifting after the Indosinian Orogeny rather than during subduction of the paleo-Pacific oceanic crust.     

华北晚中生代和新生代玄武岩中单斜辉石巨晶来源及其对寄主岩岩浆过程的制约:以莒南和鹤壁为例

本文对华北克拉通晚中生代和新生代碱性玄武质岩石中的单斜辉石巨晶进行了主、微量元素和Sr-Nd同位素的综合研究,发现晚中生代和新生代单斜辉石巨晶存在明显的主、微量元素和同位素组成上的差异。新生代单斜辉石巨晶有Al-普通辉石和次透辉石两类;而中生代单斜辉石巨晶只有Al-普通辉石。新生代单斜辉石SiO_2含量高、REE配分型式为上凸型、LILE和放射性元素含量高,并具有比寄主碱性玄武岩更亏损的Sr和Nd同位素组成;而中生代单斜辉石SiO_2含量低、REE配分型式为LREE富集型、LILE和部分HFSE以及放射性元素含量低,并具有比寄主碱性玄武岩稍富集的Sr和Nd同位素组成;巨晶的结构、矿物成分和地球化学特征,以及Mg-Fe在熔体与单斜辉石间的分配状况皆说明,新生代碱性玄武岩中单斜辉石巨晶是碱性玄武岩浆在高压下结晶的,因此二者是同源的;而中生代单斜辉石巨晶是被寄主岩浆偶然捕获的捕虏晶,是不同源的。华北新生代单斜辉石巨晶存在于碱性玄武岩和拉斑玄武岩中,它们具有比寄主碱性玄武岩更亏损的Sr和Nd同位素组成,说明即使是碱性玄武岩也不能完全代表软流圈来源的原始岩浆,其在上升过程中或多或少存在同位素组成富集的物质的混入。同时,拉斑玄武岩不是碱性玄武质岩浆直接结晶分异的产物,亦不是完全由部分熔融程度的不同造成的。拉斑玄武岩中存在岩石圈地幔物质的贡献或是岩浆房内碱性玄武质岩浆受地壳混染作用的结果。     

中国东南部晚中生代-新生代玄武岩与壳幔作用

中国东南部的火山活动在早中生代时期仅有很小规模,晚中生代最早的、较大规模的岩浆活动始于中侏罗世早期,至早白垩世是火山岩浆活动的鼎盛期,在近100个Ma的时间内形成了大面积分布的晚中生代火山-侵入岩,而在新生代则以面积较小的玄武岩浆喷出活动为主,局限分布于沿海一带。以晚中生代湘南、赣南和闽西南的近EW向火山岩带和浙、闽沿海地区的近NNE向火山岩带,以及新生代近NNE向火山岩带为研究对象,对这些火山岩的地球化学特征对比研究,结合时空分布,讨论了它们的起源及其与壳幔相互作用的关系,以及它们形成的构造环境,其结果显示,EW向晚中生代火山岩带（180～170Ma）的西段玄武岩独立产出,且明显属碱性系列;而中段和东段玄武岩和流纹岩伴生,其中的玄武岩均为亚碱性系列的拉斑玄武岩。它们形成于板内拉张构造环境,是中国东南部特提斯构造域向太平洋构造域转换、晚中生代大规模岩浆作用的序幕。研究表明,该火山岩带自西向东表现出不同程度的壳幔相互作用,玄武岩在成岩过程中有少量陆壳组分加入。NNE向晚中生代火山岩带（130～90Ma）主要为流纹质岩石,安山岩和玄武岩很少。即使是双峰式火山岩也以酸性岩为主,玄武岩仅占全部火山岩体积的30％以下。其中的玄武岩主要属钙碱性系列,少数属拉斑系列。它们形成于火山弧构造环境,是中国东南部受太平洋构造域影响发生大规模火山岩浆作用的主旋律。其中玄武岩岩浆成分受到了较高程度的陆壳物质混染,同时代的中性火山岩是由底侵的玄武岩岩浆和陆壳物质来源的酸性岩浆发生岩浆混合作用而形成的,反映了强烈的壳幔相互作用。NNE向新生代火山岩带,分布在浙闽沿海,以碱性系列玄武岩为主,均含幔源包体,并受NNE向大陆边缘断裂构造的控制。它们形成于板内裂谷环境,是中国东南沿海由晚中生代火山弧构造环境转换为新生代板内裂谷环境的标志,起源于软流圈地幔,并有EMII岩石圈地幔的混合组分,但基本没有受到陆壳物质的混染。     

Erratum to: Review of post-collisional volcanism in the Central Anatolian Volcanic Province (Turkey), with special reference to the Tepekoy Volcanic Complex

Neogene-Quaternary post-collisional volcanism in Central Anatolian Volcanic Province (CAVP) is mainly characterized by calc-alkaline andesites-dacites, with subordinate tholeiitic-transitional-mildly alkaline basaltic volcanism of the monogenetic cones. Tepekoy Volcanic Complex (TVC) in Nigde area consists of base surge deposits, and medium to high-K andesitic-dacitic lava flows and basaltic andesitic flows associated with monogenetic cones. Tepekoy lava flows petrographically exhibit disequilibrium textures indicative of magma mixing/mingling and a geochemisty characterized by high LILE and low HFSE abundances, negative Nb–Ta, Ba, P and Ti anomalies in mantle-normalized patterns. In this respect, they are similar to the other calc-alkaline volcanics of the CAVP. However, TVC lava flows have higher and variable Ba/Ta, Ba/Nb, Nb/Zr, Ba/TiO₂ ratios, indicating a heterogeneous, variably fluid-rich source. All the geochemical features of the TVC are comparable to orogenic andesites elsewhere and point to a sub-continental lithospheric mantle source enriched in incompatible elements due to previous subduction processes. Basaltic monogenetic volcanoes of CAVP display similar patterns, and HFS anomalies on mantle-normalized diagrams, and have incompatible element ratios intermediate between orogenic andesites and within-plate basalts (e.g. OIB). Accordingly, the calc-alkaline and transitional-mildly alkaline basaltic magmas may have a common source region. Variable degrees of partial melting of a heterogeneous source, enriched in incompatible elements due to previous subduction processes followed by fractionation, crustal contamination, and magma mixing in shallow magma chambers produced the calc-alkaline volcanism in the CAVP. Magma generation in the TVC, and CAVP in general is via decompression melting facilitated by a transtensional tectonic regime. Acceleration of the extensional regime, and transcurrent fault systems extending deep into the lithosphere favoured asthenospheric upwelling at the base of the lithosphere, and as a consequence, an increase in temperature. This created fluid-present melting of a fluid-enriched upper lithospheric mantle or lower crustal source, but also mixing with asthenosphere-derived melts. These magmas with hybrid source characteristics produced the tholeiitic-transitional-mildly alkaline basalts depending on the residence times within the crust. Hybrid magmas transported to the surface rapidly, favored by extensional post-collision regime, and produced mildly alkaline monogenetic volcanoes. Hybrid magmas interacted with the calc-alkaline magma chambers during the ascent to the surface suffered slight fractionation and crustal contamination due to relatively longer residence time compared to rapidly rising magmas. In this way they produced the mildly alkaline, transitional, and tholeiitic basaltic magmas. This model can explain the coexistence of a complete spectrum of q-normative, ol-hy-normative, and ne-normative monogenetic basalts with both subduction and within-plate signatures in the CAVP.     

Review of post-collisional volcanism in the Central Anatolian Volcanic Province (Turkey), with special reference to the Tepekoy Volcanic Complex

Neogene-Quaternary post-collisional volcanism in Central Anatolian Volcanic Province (CAVP) is mainly characterized by calc-alkaline andesites-dacites, with subordinate tholeiitic-transitional-mildly alkaline basaltic volcanism of the monogenetic cones. Tepekoy Volcanic Complex (TVC) in Nigde area consists of base surge deposits, and medium to high-K andesitic-dacitic lava flows and basaltic andesitic flows associated with monogenetic cones. Tepekoy lava flows petrographically exhibit disequilibrium textures indicative of magma mixing/mingling and a geochemisty characterized by high LILE and low HFSE abundances, negative Nb–Ta, Ba, P and Ti anomalies in mantle-normalized patterns. In this respect, they are similar to the other calc-alkaline volcanics of the CAVP. However, TVC lava flows have higher and variable Ba/Ta, Ba/Nb, Nb/Zr, Ba/TiO₂ ratios, indicating a heterogeneous, variably fluid-rich source. All the geochemical features of the TVC are comparable to orogenic andesites elsewhere and point to a sub-continental lithospheric mantle source enriched in incompatible elements due to previous subduction processes. Basaltic monogenetic volcanoes of CAVP display similar patterns, and HFS anomalies on mantle-normalized diagrams, and have incompatible element ratios intermediate between orogenic andesites and within-plate basalts (e.g. OIB). Accordingly, the calc-alkaline and transitional-mildly alkaline basaltic magmas may have a common source region. Variable degrees of partial melting of a heterogeneous source, enriched in incompatible elements due to previous subduction processes followed by fractionation, crustal contamination, and magma mixing in shallow magma chambers produced the calc-alkaline volcanism in the CAVP. Magma generation in the TVC, and CAVP in general is via decompression melting facilitated by a transtensional tectonic regime. Acceleration of the extensional regime, and transcurrent fault systems extending deep into the lithosphere favoured asthenospheric upwelling at the base of the lithosphere, and as a consequence, an increase in temperature. This created fluid-present melting of a fluid-enriched upper lithospheric mantle or lower crustal source, but also mixing with asthenosphere-derived melts. These magmas with hybrid source characteristics produced the tholeiitic-transitional-mildly alkaline basalts depending on the residence times within the crust. Hybrid magmas transported to the surface rapidly, favored by extensional post-collision regime, and produced mildly alkaline monogenetic volcanoes. Hybrid magmas interacted with the calc-alkaline magma chambers during the ascent to the surface suffered slight fractionation and crustal contamination due to relatively longer residence time compared to rapidly rising magmas. In this way they produced the mildly alkaline, transitional, and tholeiitic basaltic magmas. This model can explain the coexistence of a complete spectrum of q-normative, ol-hy-normative, and ne-normative monogenetic basalts with both subduction and within-plate signatures in the CAVP.     

Geochemical response of magmas to Neogene–Quaternary continental collision in the Carpathian–Pannonian region: A review

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.     

The Harrat Al-Birk basalts in southwest Saudi Arabia: characteristic alkali mafic magmatism related to Red Sea rifting

Harrat Al-Birk volcanics are products of the Red Sea rift in southwest Saudi Arabia that started in the Tertiary and reached its climax at ~5 Ma. This volcanic field is almost monotonous and is dominated by basalts that include mafic–ultramafic mantle xenoliths (gabbro, websterite, and garnet-clinopyroxenite). The present work presents the first detailed petrographic and geochemical notes about the basalts. They comprise vesicular basalt, porphyritic basalt, and flow-textured basalt, in addition to red and black scoria. Geochemically, the volcanic rock varieties of the Harrat Al-Birk are low- to medium-Ti, sodic-alkaline olivine basalts with an enriched oceanic island signature but extruded in a within-plate environment. There is evidence of formation by partial melting with a sort of crystal fractionation dominated by clinopyroxene and Fe–Ti oxides. The latter have abundant titanomagnetite and lesser ilmenite. There is a remarkable enrichment of light rare earth elements and depletion in Ba, Th and K, Ta, and Ti. The geochemical data in this work suggest Harrat Al-Birk basalts represent products of water-saturated melt that was silica undersaturated. This melt was brought to the surface through partial melting of asthenospheric upper mantle that produced enriched oceanic island basalts. Such partial melting is the result of subducted continental mantle lithosphere with considerable mantle metasomatism of subducted oceanic lithosphere that might contain hydrous phases in its peridotites. The fractional crystallization process was controlled by significant separation of clinopyroxene followed by amphiboles and Fe–Ti oxides, particularly ilmenite. Accordingly, the Harrat Al-Birk alkali basalts underwent crystal fractionation that is completely absent in the exotic mantle xenoliths (e.g. Nemeth et al. in The Pleistocene Jabal Akwa Al Yamaniah maar/tuff ring-scoria cone complex as an analogy for future phreatomagmatic to magmatic explosive eruption scenarios in the Jizan Region, SW Saudi Arabia 2014).     

西准噶尔克拉玛依岩体的成因:年代学、岩石学和地球化学证据

西准噶尔地区克拉玛依岩体主要由闪长岩和花岗岩组成,锆石SHRIMP法给出的岩体结晶年龄为(315.5±2.8)Ma,为晚石炭世侵入岩.克拉玛依岩体具岛弧岩石的元素地球化学特征:总体上岩石富钠.A/CNK=0.57～0.84,富集LREE,(La/Yb)N=4.7～6.1,Eu显示弱负异常(δEu=0.76～0.91),...     

大别造山带前陆阳新盆地古近纪玄武岩地球化学特征与成因研究

大别造山带前陆阳新盆地古近纪玄武岩的成分从石英拉斑玄武质到橄榄拉斑玄武质,以相对较高的大离子亲石元素(Rb、Ba、K、Th、U等)丰度和弱到中等亏损高场强元素(Nb和Ta)为特征,岩石地球化学的总体特征类似于初始裂谷的拉斑玄武岩,而与大陆拉斑玄武岩所表现出的强烈亏损高场强元素Nb和Ta的地球化学特征明显不同。该拉斑玄武质岩浆喷发前曾经历过较小程度(约4．5％)的以橄榄石为主的矿物分离结晶,玄武岩的母岩浆则是源区物质经过约15％的部分熔融形成的,源区物质可能包含了来自下地幔的FOZO和富集型大陆岩石圈地幔两种组分。大别造山带内及前、后陆地区古近纪拉斑玄武岩的地球化学特征具有可比性,意味着随着大别造山带山根的拆沉,来自下地幔的FOZO岩浆(可能以地幔柱的形式)上涌,对大陆岩石圈的地幔部分发生了大规模的改造,使得原性质明显不同的两大岩石圈地幔在会聚带附近其地球化学分区性变得模糊。所形成的玄武岩总体具有较多的FOZO特征,但(除造山带内部)也不同程度地保留有岩石圈地幔的性质。     

长江中下游中生代安山质火山岩记录的新元古代大洋板片-地幔相互作用

大陆弧安山岩的形成是大洋板片向大陆边缘之下俯冲的结果,但是在具体形成机制上存在很大争议.针对这个问题,对长江中下游地区中生代安山质火山岩及其伴生的玄武质和英安质火山岩进行了系统的岩石地球化学研究,结果对大陆弧安山质火成岩的成因提出了新的机制.分析表明,这些岩石形成于早白垩世,它们不仅表现出典型的岛弧型微量元素分布特征,而且具有高度富集的Sr-Nd-Hf同位素和高的放射成因Pb以及高的氧同位素组成.通过全岩和矿物地球化学成分变化检查发现,地壳混染和岩浆混合作用对其成分的富集特征贡献有限,而其岩浆源区含有丰富的俯冲地壳衍生物质才是其成分富集的根本原因.虽然这些火山岩的喷发年龄为中生代,但是其岩浆源区形成于新元古代早期的华夏洋壳俯冲对扬子克拉通边缘之下地幔楔的交代作用.大陆弧安山岩地幔源区中含有大量俯冲洋壳沉积物部分熔融产生的含水熔体,显著区别于大洋弧玄武岩的地幔源区,其中只含有少量俯冲洋壳来源的富水溶液和含水熔体.正是这些含水熔体交代上覆地幔楔橄榄岩,形成了不同程度富集的超镁铁质-镁铁质地幔源区.在早白垩纪时期,古太平洋俯冲过程的远弧后拉张导致中国东部岩石圈发生部分熔融,其中超镁铁质地幔源区熔融形成玄武质火山岩,镁铁质地幔源区则熔融形成安山质火山岩.因此,大陆弧安山岩成因与大洋弧玄武岩一样,可分为源区形成和源区熔融两个阶段,其中第一阶段对应于俯冲带壳幔相互作用.      

滇西澜沧老厂地区玄武岩岩石成因与构造意义

对滇西澜沧老厂地区玄武岩进行了系统的主微量元素和Nd-Pb同位素地球化学研究,结果表明该玄武岩为典型的洋岛玄武岩(OIB)。Nd-Pb同位素研究表明,玄武岩浆含富集地幔组分。老厂地区玄武岩浆活动可能与地幔热柱有关,玄武岩可能为地幔热柱(软流圈)熔融产生的岩浆与富集的岩石圈地幔岩浆混合的产物。     

Ar/Ar age and geochemistry of the post-collisional Miocene Yamadağ volcanics in the Arapkir area (Malatya Province), eastern Anatolia, Turkey

The Neogene Yamadağ volcanics occupy a vast area between Sivas and Malatya in eastern Anatolia, Turkey. These volcanic rocks are characterized by pyroclastics comprising agglomerates, tuffs and some small outcrops of basaltic–andesitic–dacitic rocks, overlain upward by basaltic and dacitic rocks, and finally by basaltic lava flows in the Arapkir area, northern Malatya Province. The basaltic lava flows in the Arapkir area yield a ⁴⁰Ar/³⁹Ar age of 15.8 ± 0.2 Ma, whereas the dacitic lava flows give ⁴⁰Ar/³⁹Ar ages ranging from 17.6 through 14.7 ± 0.1 to 12.2 ± 0.2 Ma, corresponding to the Middle Miocene. These volcanic rocks have subalkaline basaltic, basaltic andesitic; alkaline basaltic trachyandesitic and dacitic chemical compositions. Some special textures, such as spongy-cellular, sieve and embayed textures; oscillatory zoning and glass inclusions in plagioclase phenocrysts; ghost amphiboles and fresh biotite flakes are attributable to disequilibrium crystallization related to magma mixing between coeval magmas. The main solidification processes consist of fractional crystallization and magma mixing which were operative during the soldification of these volcanic rocks. The dacitic rocks are enriched in LILE, LREE and Th, U type HFSE relative to the basaltic rocks. The basaltic rocks also show some marked differences in terms of trace-element and REE geochemistry; namely, the alkaline basaltic trachyandesites have pronounced higher HFSE, MREE and HREE contents relative to the subalkaline basalts. Trace and REE geochemical data reveal the existence of three distinct magma sources – one subalkaline basaltic trachyandesitic, one alkaline basaltic and one dacitic – in the genesis of the Yamadağ volcanics in the Arapkir region. The subalkaline basaltic and alkaline basaltic trachyandesitic magmas were derived from an E-MORB type enriched mantle source with a relatively high- and low-degree partial melting, respectively. The magmatic melt of dacitic rocks seem to be derived from an OIB-type enriched lithospheric mantle with a low proportion of partial melting. The enriched lithospheric mantle source reflect the metasomatism induced by earlier subduction-derived fluids. All these coeval magmas were generated in a post-collisional extensional geodynamic setting in Eastern Anatolia, Turkey.     

Geochemistry of Two Types of Basalts in the Emeishan Basaltic Province: Evidence for Mantle Plume-Lithosphere Interaction

Based on the temporal-spatial distribution and geochemical characteristics, the Emeishan basalts can be divided into two types: high-P2O-TiO2 basalt (HPT) and low-P2O5-TiO2 basalt (LPT), which differ distinctly in geochemistry: the LPTs are characterized by relatively high abundances of MgO, total FeO and P2O5 and compatible elements (Cr, Ni, Sc), and relatively low contents of moderately compatible elements (V, Y, Yb, Co), LREE and other incompatible elements compared with the HPT. On the diagrams of trace element ratios, they are plotted on an approximately linear mixing line between depleted and enriched mantle sources, suggesting that these two types of basalts resulted from interactions of varying degrees between mantle plume and lithospheric mantle containing such volatile-rich minerals as amphibole and apatite. The source region of the LPT involves a smaller proportion of lithospheric components, while that of the HTP has a larger proportion of lithospheric components. Trachyte is generated by pa     

Generation of calc-alkaline andesite of the Tatun volcanic group (Taiwan) within an extensional environment by crystal fractionation

The Pliocene–Pleistocene northern Taiwan volcanic zone (NTVZ) is located within a trench-arc–back-arc basin and oblique arc–continent collision zone. Consequently the origin and tectonic setting of the andesitic rocks within the NTVZ and their relation to other circum-Pacific volcanic island-arc systems is uncertain. Rocks collected from the Tatun volcanic group (TTVG) include basaltic to andesitic rocks. The basalt is compositionally similar to within-plate continental tholeiites whereas the basaltic andesite and andesite are calc-alkaline; however, all rocks show a distinct depletion of Nb-Ta in their normalized incompatible element diagrams. The Sr-Nd isotope compositions of the TTVG rocks are very similar and have a relatively restricted range (i.e. I_Sr = 0.70417–0.70488; εNd_(T) = +2.2 to +3.1), suggesting that they are derived directly or indirectly from the same mantle source. The basalts are likely derived by mixing between melts from the asthenosphere and a subduction-modified subcontinental lithospheric mantle (SCLM) source, whereas the basaltic andesites may be derived by partial melting of pyroxenitic lenses within the SCLM and mixing with asthenospheric melts. MELTS modelling using a starting composition equal to the most primitive basaltic andesite, shallow-pressure (i.e. ≤1 kbar), oxidizing conditions (i.e. FMQ +1), and near water saturation will produce compositions similar to the andesites observed in this study. Petrological modelling and the Sr-Nd isotope results indicate that the volcanic rocks from TTVG, including the spatially and temporally associated Kuanyinshan volcanic rocks, are derived from the same mantle source and that the andesites are the product of fractional crystallization of a parental magma similar in composition to the basaltic andesites. Furthermore, our results indicate that, in some cases, calc-alkaline andesites may be generated by crystal fractionation of mafic magmas derived in an extensional back-arc setting rather than a subduction zone setting.