The geochemistry and origin of quaternary volcanism in the New Hebrides

Analyses of young volcanic rocks from the New Hebrides reveal the existence of two geochemical groups which may be identified on the basis of their contents of K₂O and related trace elements. Low K₂O rocks are believed to be comparable with conventional island arc volcanism, whereas the high K₂O rocks are believed to be related to volcanism associated with tensional rifting. By comparison with high pressure experimental data it is concluded that these rocks can be derived by partial melting of hydrous mantle above the Benioff zone. However this mantle must have different concentrations of incompatible elements from the source of ocean ridge tholeiites, and residual minerals must have RE partition coefficients which differ from those of phenocrysts in volcanic rocks.     

Identifications of products of geographical origin based on geochemical zoning

The protection system of products of geographical origin has become one hot subject with which various countries throughout the world are concerned. Results of element-Pb isotope geochemical mapping indicate that there emerge a number of famous-brand high-quality product groups of geographical origin in China. The environment of production of China’s famous-brand white spirits of geographical origin at the Sichuan-Guizhou border is still a mystery to be solved. China has two main communities of white spirits of geographical origin, i.e., the most famous are Moutai and Wuliangye spirits. By element-Pb isotope geochemical tracing, we can establish the natural background geological, geographical and geochemical scientific data mark ers in close relation with source areas. Research on the product of geographical origin, named Kaempferia galangal L. in the Shuangjiao district, Yangchun, Guangdong Province, showed that the trace elements have an excellent heritance between Kaempferia galangal L. and soil in this area, and the trace elements are highly effective for tracing the origin.     

A study on the geochemical characteristics of Upper Permian continental marginal arc volcanic rocks in the northern segment of South Lancangjiang Belt

1IntroductionThe northern segment of the South LancangjiangBelt refers to the terrain about200km east of theYunxian-Lingcang granite in the South LancangjiangBelt(Fig.1).During the seventh Five-Year Plan peri-od,Mo Xuanxue et al.(1993)undertook the resear…     

The northern New Hebrides back-arc troughs: history and relation with the North Fiji basin

The New Hebrides back-arc troughs (southwest Pacific) are located between the New Hebrides trench-arc system and the active North Fiji marginal basin. They are restricted to the southern and northern segments of the arc and were generally related to effects of the Indo-Australian subducting plate (rolling-back and/or subduction of the d'Entrecasteaux ridge). A detailed bathymetric and magnetic survey over the northern back-arc troughs is used to propose a new model for the origin of the New Hebrides back-arc troughs. The northern troughs extend over a width of 60 km and are composed of N-S trending grabens and horsts, discontinuous along strike and associated with volcanism. The troughs are disrupted southward at 13° 30′S, where the Hazel Holme fracture zone intercepts the New Hebrides island arc. The E-W trending Hazel Holme fracture zone is an extensional feature bisecting the North Fiji basin. In its western end, the Hazel Hohne fracture zone is composed of a succession of horsts and grabens striking N90 ° –N100 ° E. Geometrical and structural relationships between the back-arc troughs and the Hazel Holme fracture zone suggest that both these extensional features result from the same process and are closely linked. The northern troughs-western end of the Hazel Holme fracture zone region is dominated by N130°–135°E trending magnetic lineations typical of oceanic crust. These lineations are oblique to the horsts and grabens systems, and are characteristic of the old North Fiji basin oceanic crust. Consequently we conclude that the northern back-arc troughs are partly developed on the North Fiji basin oceanic basement and that extensional tectonic processes postdate the oldest North Fiji basin oceanic crust. Morphological and structural evidence suggests that both the back-arc troughs and the Hazel Holme fracture zone are recent, still active and result from NE-SW extensional tectonics. Because other tectonic features throughout the North Fiji basin are related to the same stress field, it is inferred that such a NE-SW extension could be a large-scale deformation affecting the North Fiji basin. It is proposed that the back-arc troughs are primarily related to this recent extension within the North Fiji basin, but their locations along the arc are also influenced by the subduction of the d'Entrecasteaux ridge which produces, south of 13°30′S, nearly E-W trending compression and prevents the formation of troughs. Possibly, these recent extensional tectonic processes result from a major reorganization in the spreading process of the North Fiji basin, and could be as young as 0.6–0.7 Ma.     

望天鹅火山岩石地球化学特征及其成因

东北吉林望天鹅新生代火山岩（7．04—1．86Ma）主要由玄武粗安岩和碱性流纹岩组成。玄武粗安岩的SiO2介于49．38％-53．31％之间;K2O＋Na2O为4．27％-7．72％;∑REE为163．69×10^-6-258．55×10^-6,Eu异常不明显,δEu为0．72～1．17;碱性流纹岩具高SiO2（70．39％～71．49％）含量,高碱（K2O＋Na2O为9．28％～9．49％）,高稀土总量（∑REE介于309．30×10^-6～465．03×10^-6之间）,明显的Eu负异常,3Eu为0．52～0．71。碱性流纹岩的微量元素含量较玄武粗安岩高,且具有明显的Sr、P、Ti负异常。望天鹅火山岩的^87Sr／^86Sr比值在0．705156-0．709029之间,而^144Nd／^143Nd比值变化较小,介于0．512295～0．512602之间;放射性成因Pb同位素变化范围也较小,^206Ph／^204Pb为17．254～18．090,^207Ph／^204Pb为15．460～15．507,^208Pb／^204Ph为37．278—38．048。综合分析火山岩的主量元素、微量元素和Sr-Nd—Pb同位素特点,初步认为望天鹅火山的玄武粗安岩和碱性流纹岩具有相同的地幔源区。玄武粗安岩起源于微弱富集上地幔,在上升过程中结晶分异形成了碱性流纹岩。     

Geological evidence bearing on the Miocene to Recent structural evolution of the New Hebrides arc

The New Hebrides archipelago is a complex reversed-arc system that can be divided into four major volcanic provinces. The Western Belt is an Early to Middle Miocene extinct volcanic arc that, as a result of polarity reversal, is now incorporated into the frontal arc of the present-day configuration. The Eastern Belt initially received detritus in the early Middle Miocene from a tholeiitic arc complex but in the Mio-Pliocene became the locus of a more calc-alkaline arc volcanism. Volcanic activity then ceased in the Eastern Belt but is well-represented as a third and largely submerged Marginal Province through the Pliocene into the Early Pleistocene. The present volcanic line, the Central Chain, is essentially a continuation of the Marginal Province volcanism into Recent times.Initial tectonic events in the New Hebrides arc were associated with the regional disruption in the Middle Miocene of an east-facing system, with consequent termination of Western Belt arc volcanism. The Western Belt remained as a landmass during the lowermost Late Miocene but subsided following a Late Miocene renewal of island arc volcanism to the east. This latest phase was coeval with initial expansion of the North Fiji Basin and marked the advent of the New Hebrides as a westwards-migrating reversed-arc system. During arc migration there were apparent hiatuses in island arc volcanism, the most notable being a Middle Pliocene to Late Pleistocene period of quiescence in the central sector.Tectonism in the Early Pleistocene-Recent raised the fore-arc, brought about rifting and extension to the rear and concentrated volcanism along the presently-active Central Chain.     

The origin and geochemical evolution of the Woodlark Rift of Papua New Guinea

Geochemical, isotopic, and geochronologic data for exhumed rocks in the Woodlark Rift of Papua New Guinea (PNG) allow a tectonic link to be established with the Late Cretaceous Whitsunday Volcanic Province (WVP) of northeastern Australia. Most of the metamorphic rocks in the Woodlark Rift have Nd isotopic compositions (ε_Nd = + 1.7 to + 6.2) similar to the Nd isotopic compositions of rocks in the WVP (ε_Nd = + 1.3 to + 6.6; Ewart et al., 1992), and contain inherited zircons with 90 to 100 Ma U–Pb ages that overlap the timing of magmatism in the WVP. None of the metamorphic rocks in the Woodlark Rift have the highly evolved Hf and Nd isotopic compositions expected of ancient continental crust. Magmas were erupted in the WVP during the middle Cretaceous as eastern Gondwana was rifted apart. The protoliths of felsic and intermediate metamorphic rocks in the Woodlark Rift are interpreted to be related to the magmatic products produced during this Cretaceous rifting event. Some mafic metamorphic rocks exposed in the western Woodlark Rift (eclogites and amphibolites) are not related to the WVP and instead could have originated as basaltic lavas crystallized from mantle melts at (U)HP depths in the Late Cenozoic, or as fragments of Mesozoic aged oceanic lithosphere.Isotopic and elemental comparisons between basement gneisses and Quaternary felsic volcanic rocks demonstrate that felsic lavas in the D'Entrecasteaux Islands did not form solely from partial melting of metamorphic rocks during exhumation. Instead, the isotopic compositions and geochemistry of Quaternary felsic volcanic rocks indicate a significant contribution from the partial melting of the mantle in this region. When combined with geophysical data for the western Woodlark Rift, this suggests that future seafloor spreading will commence south of Fergusson Island, and west of the present-day active seafloor spreading rift tip.     

Role of backarc processes in the origin of across-arc geochemical zoning in volcanics of early evolutionary stages in Kunashir Island

Geochemical data obtained on volcanic rocks produced during the early evolutionary stages in Kunashir Island provide insight into certain important aspect of the evolution of the subduction system. The mafic lavas of all age intervals exhibit clearly pronounced across-arc geochemical zoning, which implies that these rocks were produced in the environment of a subducted oceanic slab. The high Ba/Th and U/Th ratios of basalts from the frontal zones suggest that an important role in magma generation was played by a low-temperature aqueous fluid. The arc lavas of the Early Miocene, Pliocene, and Pliocene-Pleistocene episodes in the evolution of the island arc system provide evidence of the melting of subducted sediments, which testifies, when considered together with the calculated P-T conditions under which the high-Mg basalts were derived, that backarc tectono-magmatic processes affected subduction-related magmatic generation. Active mantle diapirism and volcanic activity in the opening Kurile Basin resulted in the heating of the suprasubduction mantle in the rear zone, the involvement of the upper sedimentary layer of the oceanic slab in the process of melting, and the eventual generation of basaltic magmas with unusual geochemical characteristics.     

Study on the geochemical characteristics of ocean-ridge and oceanic-island volcanic rocks in the Nan-Uttaradit zone,northern Thailand

Field investigations and laboratory integrated research as indicated that ophiolite mélange in the Nan-Uttaradit zone, northern Thailand, consists of fragments of tectonites such as metamorphic peridotite (extremely silicified serpentinite), cumulates (pyroxenolite, gabbro, and gabbro-diorite), ocean-ridge basalt, oceanic-island ba-salt and radiolarian silicalite, and it was formed during D3-P. The rock series, rock types and petrogeochemical characteristics of metamorphic tholeiites in the Nan area of the Nan-Uttaradit zone are similar to those of ocean-ridge basalts (C1) in China's Ailaoshan zone. As for the Hawaiites in the Nan area of the Nan-Uttaradit zone, their major elements, REEs and trace elements are similar to those of oceanic-island basalts in China's Jinshanjiang zone (P11). In the Uttaradit area of this zone the metamorphic alkaline basalts show transitional petrogeochemical characteristics between ocean-ridge basalts and oceanic-island basalts, which were still formed in oceanic-island environments. The above-described basalts are all oceanic volcanic rocks and they are the most important part of the Paleo-Tethys oceanic crust in the Nan-Uttaradit zone.     

An Early Devonian to Early Carboniferous volcanic arc in North Tianshan,NW China: Geochronological and geochemical evidence from volcanic rocks

The Late Paleozoic volcanic and sedimentary rocks are widespread in the North Tianshan along the north margin of the Yili block. They consist of basalt, basaltic andesite, andesite, trachyandesite, dacite, rhyolite, tuff, and tuffaceous sandstone. According to zircon sensitive high-resolution ion microprobe (SHRIMP) dating, the age of the Late Paleozoic volcanic rocks in Tulasu basin in western part of North Tianshan is constrained to be Early Devonian to Early Carboniferous (417–356 Ma), rather than Early Carboniferous as accepted previously. Geochemical characteristics of the Early Devonian to Early Carboniferous volcanic rocks are similar to those of arc volcanic rocks, which suggest that these volcanic rocks could be the major constituents of a continental arc formed by the southward subduction of North Tianshan Oceanic lithosphere. Geochemical studies indicate that the magma source of the volcanic rocks might be the mantle wedge mixed with subduction fluid, which is geochemically enriched than primitive mantle but depleted than E-MORB. The calculation shows that the basalt could be formed by ∼10% partial melting of subduction fluid modified mantle wedge. Andesites with high initial ⁸⁷Sr/⁸⁶Sr (0.7094–0.7104) and negative εNd(t) (−4.45 to −4.79) values reveal the contribution of continental crust to its source. The calculation of assimilation–fractional crystallization (AFC) shows that the fractional crystallization process of the basaltic magma, which was accompanied with assimilation by different degree of continental crust, produced andesite (7–9%), dacite (∼12%) and rhyolite (>20%).     

辽北德仁组火山岩LA-ICP-MS锆石U-Pb年龄及岩石地球化学特征

通过对辽北平岗盆地中德仁组火山岩的岩石学、地球化学和年代学的研究,探讨了其岩石成因、形成时代、构造环境,以及华北克拉通东部岩石圈减薄时间和减薄机制。用LA-ICP-MS技术测定了平岗盆地中德仁组中部英安质角砾晶屑岩屑凝灰岩和上部流纹岩中的锆石U-Th-Pb同位素组成。获得前者的锆石~(206)Pb/~(238)U年龄为246.8±3.5Ma和169.6±4.6Ma,代表了2期岩浆活动的年龄,是平岗盆地基底早三叠世和中侏罗世岩浆活动的表现;后者的锆石~(206)Pb/~(238)U年龄为117.0±0.9Ma,表明流纹岩形成于早白垩世。德仁组火山岩富集大离子亲石元素K、Rb、Ba,亏损高场强元素Nb、Ta、Zr、Hf、Y、Yb;富集轻稀土元素,亏损重稀土元素,稀土元素配分曲线呈右倾型,表现出壳源特征。德仁组火山岩形成于活动大陆边缘火山弧环境,原始岩浆来源于盆地基底岩浆岩重熔。研究区在117~135Ma期间,发生了由正地形向负地形的重大转变,区域上发育伸展盆地和变质核杂岩,并在基底岩石重熔过程中受到地幔混染作用,表明研究区在早白垩世发生了岩石圈减薄,且减薄的起始时间早于117.0±0.9Ma,减薄机制为拆沉作用。     

不同构造环境中双峰式火山岩的主要特征

总结了双峰式火山岩研究的最新进展。研究表明,双峰式火山岩可以出现在多种大地构造背景中,如大陆裂谷、洋岛、大陆拉张减薄、弧后扩张、造山后、洋内岛弧和成熟岛弧／活动陆缘等。流纹岩主要有两种成因,一种流纹岩与玄武岩来自同一源区,流纹岩通常是玄武岩浆演化的产物,出露面积相对很少,并与玄武岩具有的地球化学特征;另一种流纹岩与玄武岩不同源,流纹岩是地壳深熔作用形成的,流纹岩相比玄武岩常常是占优势的中双峰式并且     

The volcanoes of an oceanic arc from origin to destruction: A case from the northern Luzon Arc

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.     

Marginal basin—volcanic arc origin of metabasic rocks of the Circum-Rhodope Belt,Thrace, Greece

Summary In the upper stratigraphic levels of the Thracian Circum-Rhodope Belt, pillowed or massive metavolcanics and metapyroclastic rocks occur. In the deeper part of the stratigraphic column a composite suite of greenschists, cumulate and noncumulate gabbros, metagabbros, serpentinites, chlorite and talc schists are found. Detailed petrographical study revealed that the metavolcanics consist of four lava types. From the basic to the more evolved types, these lavas are: pyroxeno-phyric lavas, aphyric oligophyric lavas, albite-rich lavas and porphyric felsites. Based on geochemical criteria the metavolcanics are classified as tholeiitic basalts and andesites, to dacites-rhyodacites. The content of MgO, Cr, Ni, TiO₂, Zr and REE, and some petrographic features of the pyroxeno-phyric lavas suggest boninitic affinities.The projection of chemical data on several discrimination diagrams, the REE patterns, the occurrence of lavas with boninitic affinity, the chemistry of clinopyroxenes, the crystallization sequence of the primary minerals, the presence of both basic and more evolved volcanic rocks, as well as the high ratio LIL/HFS indicate that the protoliths of the metavolcanics were formed in an immature island arc setting. The greenschists present both weak MORB and strong VAB characters suggesting that their protoliths developed in a short-lived back-arc basin. As the whole sequence of the metabasic and meta-ultrabasic rocks of the Thracian Circum-Rhodope Belt would be considered as an incomplete and dismembered ophiolite, the geodynamical environment of its formation is assumed to be a system of volcanic arc-marginal basin. Both subducted and obducted slabs were parts of the Palaeotethys oceanic realm, while the system was situated along the continental margin of the Rhodope Massif.

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Randbecken- vulkanbogenherkunft metabasischer gesteine des Zirkum-Rhodope-Gürtels, Thrakien, Griechenland

Zusammenfassung In den oberen stratigraphischen Horizonten des thrakischen Zirkum-Rhodope-Gürtels kommen metavulkanische und metapyroklastische Gesteine mit Pillow-oder massigem Gefüge vor. In den tieferen Teilen der stratigraphischen Säule wurde eine komplexe Abfolge aus Grünschiefern, Gabbros, Metagabbros, Serpentiniten, Chlorit-und Talkschiefern festgestellt. Gründliche petrographische Untersuchungen zeigten, daß die Metavulkanite aus pyroxeno-phyrischen, aphyrisch-oligophyrischen und albitreichen Laven sowie aus porphyrischen Felsiten bestehen. Aufgrund geochemischer Kriterien werden die Metavulkanite als tholeiitische Andesite oder Dacite bis Basalte eingestuft. Der Gehalt an Mg0, Cr, Ni, TiO₂, Zr und REE sowie petrographische Merkmale der pyroxeno-phyrischen Laven weisen auf einen boninitischen Charakter hin.Die Protolithe der Metavulkanite wurden im Bereich eines unreifen Inselbogens gebildet. Hierfür sprechen: (a) die Projektionslage der chemischen Daten in mehreren Diskriminierungsdiagrammen; (b) die REE-Häufigkeitskurven; (c) das Vorkommen von boninitischen Laven; (d) der Chemismus der Klinopyroxene; (e) die Kristallisationsabfolge der primären Mineralphasen; (f) die Vergesellschaftung basischer mit mehr sauren vulkanischen Gesteinen; (g) das hohe LIL/HFS-Verhältnis. Die Grünschiefer zeigen Übergangsmerkmale zwischen MORB und VAB.Die gesamte Folge der metabasischen und -ultrabasischen Gesteine des thrakischen Zirkum-Rhodope-Gürtels könnte als ein unvollständiger und zergliederter Ophiolithkomplex angesehen werden, der in einem Vulkanbogen-Randbecken war. Sowohl sub duzierte als auch obduzierte Anteile waren am Kontinentalrand des Rodope-Massifs angeordnet und stellen Teile des ozeanischen Bereiches der Paläotethys dar.

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With 8 Figures     

The northern Ossa-Morena Cadomian batholith (Iberian Massif): magmatic arc origin and early evolution

Diorites and related rocks in the Mérida area of northern Ossa-Morena (SW Iberia) are intrusive into Precambrian metavolcanic and metasedimentary sequences. Cumulate products from the H₂O-rich magmas are amphibole-rich gabbros to hornblendites. Major and trace element compositions, including Sr and Nd isotope data, allow the definition of a calc-alkaline series likely formed in relation to an immature arc setting. Crystallization of the intrusives has been established between ca. 570 and 580 Ma by U-Pb dating of constituent zircons. Garnet growth in dioritic rocks reflects a tectono–thermal overprint dated by Sm-Nd internal isochrons at around 555 Ma. Older Sm-Nd and Lu-Hf results between ca. 593 and 637 Ma on the same rocks suggest an earlier stage of regional metamorphism within the arc environment. The northern Ossa-Morena composite batholith and related metamorphic units have been tectonized and dismembered in the course of subsequent low-grade events during final stages of the Cadomian orogeny and the Variscan cycle. The units studied represent a well preserved segment of the arc region that evolved in Neoproterozoic times along the western border of Gondwana to conform the Cadomian–Avalonian basement of the Hercynian realm.This revised version was published in October 2004 with corrections to the names and order of the authors.     

祁连地块中元古代火山岩地球化学特征

分布于祁连造山带中祁连地块东端北缘的中元古代皋兰岩群火山岩为一套以基性-中基性-中酸性火山岩组合的浅变质岩,通过岩石组合、岩石化学、稀土及微量元素地球化学的综合分析研究,认为该套火山岩产于陆缘弧及弧后活动盆地的构造环境.     

柴北缘早古生代岛弧火山岩中埃达克质英安岩的发现及其地质意义

在柴达木北缘吉绿素滩间山群火山岩中发现埃达克质 (adakitic)英安岩 ,其主要地球化学特征如下 :SiO26 3.7%～ 6 4.3%,Al2 O3 15 .0 7%～ 15 .2 7%,MgO 3.47%～ 3.72 %,K2 O/Na2 O 0 .14～ 0 .2 5 ,Y和Yb含量较低 ,Y为17.82× 10 -6～ 18.10× 10 -6,平均 17.96× 10 -6,Yb为 1.47× 10 -6～ 1.5 5× 10 -6,Sr含量较高 ,为 5 2 7× 10 -6～ 5 37× 10 -6,亏损HREE ,具弱负Eu和Sr异常。除MgO高于正常的埃达克岩外 ,与世界上典型埃达克岩极为相似。据其地球化学研究结果并结合该区埃达克质英安岩 (5 14.2± 8.5Ma)和榴辉岩 (494.6± 6 .5Ma)的年龄数据 ,推测柴北缘板片在晚寒武世开始俯冲 ,俯冲板片在 75～ 85km深处开始部分熔融形成埃达克质岩浆 ,而熔融残留部分继续俯冲 ,在早奥陶世形成含石榴石残留相的榴辉岩。柴北缘埃达克质英安岩的发现进一步证实了早古生代柴北缘发生过洋陆俯冲作用。     

The origin of island arc high-alumina basalts

A detailed examination of the hypothesis that high-alumina basalts (HAB) in island arcs are primary magmas derived by 50–60% partial melting of subducted ocean crust eclogite shows that this model is unlikely to be viable. Evidence suggests that the overwhelming majority of arc HAB are porphyritic lavas, enriched in Al₂O₃ either by protracted prior crystallization of olivine and clinopyroxene, or by plagioclase phenocryst accumulation in magmas of basaltic andesite to dacite composition. Experimentally-determined phase relationships of such plagioclase-enriched (non-liquid) compositions have little bearing on the petrogenesis of arc magmas, and do not rule out the possibility that arc HAB can be derived by fractionation of more primitive arc lavas. Although models invoking eclogite-melting can match typical arc HAB REE patterns, calculations indicate that the Ni and Cr contents of proposed Aleutian primary HAB are many times lower than such models predict. In contrast, Ni vs Sc and Cr vs Sc trends for arc HAB are readily explained by olivine (+Cr-sp) and clinopyroxene-dominated fractionation from more primitive arc magmas. GENMIX major element modelling of several HAB compositions as partial melts of MORB eclogite, using appropriate experimentally (26–34 kb)-determined garnet and omphacite compositions yields exceptionally poor matches, especially for CaO, Na₂O, MgO and Al₂O₃. These mismatches are easily explained if the HAB are plagioclase-accumulative. Groundmasses of arc HAB are shown to vary from basaltic andesite to dacite in composition. Crystal fractionation driving liquid compositions toward dacite involves important plagioclase separation, resulting in development of significant negative Eu anomalies in more evolved lavas. Plagioclase accumulation in such evolved liquids tends to diminish or eliminate negative Eu anomalies. Therefore, the absence of positive Eu anomaly in a plagioclase-phyric HAB does not indicate that plagioclase has not accumulated in that lava. In addition, we show that plagioclase phenocrysts in arc HAB are not in equilibrium with the liquids in which they were carried (groundmass). Exceptional volumes of picrite and olivine basalt occur in the Solomons and Vanuatu arcs; the presence in lavas from these and other arcs (Aleutian, Tonga) of olivine phenocrysts to Fo₉₄, finds no ready explanation in the primary eclogite-derived HAB model. We suggest that most lavas in intra-oceanic arcs are derived from parental magmas with >10% MgO; fractionation of olivine (+Cr-sp) and clinopyroxene drives liquids to basalt compositions with <7% MgO, but plagioclase nucleation is delayed by their low but significant (<1%?) H₂O contents. Thus evolved liquid compositions in the basaltic andesite—andesite range may achieve relatively high Al₂O₃ contents (<17.5%). The majority of arc basalts, however, have Al₂O₃ contents in excess of 18%, reflecting plagioclase accumulation. We give new experimental data to show that HAB liquids may be generated by anhydrous, low-degree (<10%) partial melting of peridotite at P<18 kb. Relative to arc HAB, these experimental melts have notably higher Mg#(69–72) and are in equilibrium with olivine Fo_87–89. Only further detailed trace element modelling will show if they might be parental magmas for some arc HAB.