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1.
《International Geology Review》2012,54(5):435-451
Cenozoic basaltic volcanism in southeastern China was related to the lithospheric extension and asthenospheric upwelling at the eastern Eurasian continental margin. The cenozoic basaltic rocks from this region can be grouped into three different series: tholeiitic basalts, alkali basalts, and picritic-nephelinitic basalts. Each basalt series has distinctive geochemical features and is not derived from a common source rock by different degrees of partial melting or from a common parental magma by fractional crystallization. The mineralogy, petrography, and major and trace-element geochemistry of the tholeiites are similar to oceanic island basalts, implying that the mantle source for these Chinese continental tholeiites was similar to that of the oceanic island basalts—an asthenospheric mantle. The alkali basalts and picritic-nephelinitic basalts are enriched in incompatible trace elements, and their geochemical features can be interpreted as a result of partial melting of an enriched lithospheric mantle, or the mixing products of an asthenospheric magma with a component derived from an enriched lithospheric mantle through thermal erosion at the base of the lithosphere. But the lack of a transitional rock type and continuous variational trends among these basalts suggests that the mixing between asthenospheric magmas and lithospheric magmas probably was not significant in the petrogenesis of the basalts from SE China. Low-degree partial melting of enriched lithospheric mantle alone can account for the observed geochemical data from these basalts. 相似文献
2.
V. P. Simanenko S. V. Rasskazov T. A. Yasnygina L. F. Simanenko A. A. Chashchin 《Russian Journal of Pacific Geology》2011,5(1):26-46
The variations of petrogenic oxides and trace elements have been studied in the Cretaceous volcanic rocks recovered by a deep
borehole from the depth interval of 1253–4011 m on Moneron Island. The volcanic section is subdivided into two complexes:
the Early Cretaceous and Late Cretaceous. The rocks of the Early Cretaceous Complex occur below 1500 m. Chemically, they belong
to low-potassium island arc tholeiites, and their trace element distribution suggests their formation in a suprasubduction
mantle wedge under the influence of water fluids that were subsequently released from subducted sediments and oceanic plate
during the dehydration of subducted sedimentary rocks and oceanic basalts and, finally, mainly from basalts. The Early Cretaceous
basalts from the borehole are interpreted as ascribing to the frontal part of the Moneron-Samarga island arc system. The volcanic
rocks of the Late Cretaceous Complex are situated at depths above 1500 m. They also were formed in a suprasubduction setting,
but already within the East Sikhote-Alin continental-margin volcanic belt that was initiated after the accretion of the Moneron-Samarga
island arc system to the Asian continent. The island-arc section of the Moneron borehole contains basaltic andesite dikes,
which are geochemically comparable with the Early-Middle Miocene volcanic rocks of Southwestern Sakhalin. 相似文献
3.
胶东变质超镁铁—镁铁质岩石的地球化学特征及地幔源区性质 总被引：1，自引：1，他引：0
本文以地球化学方法为研究手段,结合野外宏观地质特征,论述了胶东地区前寒武纪变质建造中超镁铁—镁铁质岩石的地球化学性质及其成因。研究结果表明,该区超镁铁—镁铁质岩石为地幔不同程度熔融的产物,超镁铁质岩石相当于科马提岩类;镁铁质岩石为拉斑玄武岩类,并按橄榄质科马提岩→玄武质科马提岩→拉斑玄武岩演化系列的化学组成表现出连续的变化趋势。 拉斑玄武岩中的微量元素(主要是稀土元素)地球化学特征表明,其形成环境与现代板块构造的典型火山(岛)孤或孤后盆地环境相类似,并反映出地幔源区具有“富集型”地幔的特点,且存在着地幔不均一性。 相似文献
4.
《Journal of Asian Earth Sciences》1999,17(3):319-332
The Kermanshah ophiolite is a highly dismembered ophiolite complex that is located in western Iran and belongs to the Zagros orogenic system. The igneous rocks of this complex consist of both mantle and crustal suites and include peridotites (dunite and harzburgite), cumulate gabbros, diorites, and a volcanic sequence that exhibits a wide range in composition from subalkaline basalts to alkaline basalts to trachytes. The associated sedimentary rocks include a variety of Upper Triassic to Lower Cretaceous deep- and shallow-water sedimentary rocks (e.g., dolomite, limestone, and pelagic sediments, including umber). Also present are extensive units of radiolarian chert. The geochemical data clearly identifies some of the volcanic rocks to have formed from two distinct types of basaltic melts: (i) those of the subalkaline suite, which formed from an initial melt with a light rare earth elements (LREE) enriched signature and incompatible trace element patterns that suggest an island arc affinity; and (ii) those of the alkaline suite with LREE-enriched signature and incompatible trace element patterns that are virtually identical to typical oceanic island basalt (OIB) pattern. The data also suggests that the trachytes were derived from the alkaline source, with fractionation controlled by extensive removal of plagioclase and to a lesser extent clinopyroxene. The presence of compositionally diverse volcanics together with the occurrence of a variety of Triassic–Cretaceous sedimentary rocks and radiolarian chert indicate that the studied volcanic rocks from the Kermanshah ophiolite represent off-axis volcanic units that were formed in intraplate oceanic island and island arc environments in an oceanic basin. They were located on the eastern and northern flanks of one of the spreading centers of a ridge-transform fault system that connected Troodos to Oman prior to its subduction under the Eurasian plate. 相似文献
5.
Petrology and chemistry of recent lavas in the northern Marianas: Implications for the origin of island arc basalts 总被引：3，自引：0，他引：3
Petrologic and chemical data are presented for samples from five volcanically active islands in the northern Marianas group, an intra-oceanic island arc. The data include microprobe analyses of phenocryst and xenolith assemblages, whole rock major and trace element chemistry including REE, and Sr isotope determinations (87Sr/86Sr=0.7034±0.0001). Quartz-normative basalt and basaltic andesite are the most abundant lava types. These are mineralogically and chemically similar to the mafic products of other intra-oceanic islands arcs. It is suggested, however, that they are not typical of the ‘island arc tholeiitic’ series, having Fe enrichment trends and K/Rb, for example, more typical of calc-alkaline suits. Major and trace element characteristics, and the presence of cumulate xenoliths, indicate that extensive near surface (< 3 Kb) fractionation has occurred. Thus, even least fractionated basalts have low abundances of Mg, Ni and Cr, and high abundances of K and other large cation, imcompatible elements, relative to ocean ridge tholeiites. However, abundances of REE and small cation lithophile elements, such as Ti, Zr, Nb, and Hf are lower than typical ocean ridge tholeiites. The REE data and Sr isotope compositions suggest a purely mantle origin for the Marianas island arc basalts, with negligible input from subducted crustal material. Thus, subduction of oceanic lithosphere may not be a sufficient condition for initiation of island arc magmatism. Intersection of the Benioff zone with an asthenosphere under appropriate conditions may be requisite. Element ratios and abundances, combined with isotopic data, suggest that the source for the Marianas island arc basalts is more chondritic in some respects, and less depleted in large cations than the shallow (?) mantle source for ocean ridge tholeiites. 相似文献
6.
M.P. Gorton 《Geochimica et cosmochimica acta》1977,41(9):1257-1270
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. 相似文献
7.
Gudmundur E. Sigvaldason Níels Óskarsson 《Contributions to Mineralogy and Petrology》1986,94(3):263-271
The fluorine content of Icelandic tholeiitic and alkaline basalts matches values found in similar rocks from other areas. Covariation between fluorine and incompatible minor elements such as potassium or phosphorus is found in evolved tholeiites and alkali basalts. Lack of such covariation in primitive olivine tholeiites indicates that fluorine and other incompatible minor and trace elements are not controlled by minerals such as amphibole, mica or apatite in the mantle residue, and that the covariation between these elements in the evolved basalts cannot be inherited from the mantle. Model calculations on rocks from the Langjökull area show that olivine tholeiite suites are, if derived by simple fractional crystallization, enriched in incompatible elements much in excess of the increase due to crystal removal. These observations are taken to indicate that the well documented covariation between fluorine and other incompatible elements is not established until evolution of the basaltic magma has started in crustal holding chambers. The constancy of element ratios and enrichment in excess of what can be accounted for by crystal fractionation or incremental addition of new batches of primitive magmas does indicate (1) mineral control involving amphibole, mica or apatite and (2) addition of fluorine, potassium and phosphorous from an external source. It is argued that this source is the crustal envelope of the holding chamber. 相似文献
8.
《Journal of Asian Earth Sciences》1999,17(4):423-442
The Hegenshan ophiolite in Inner Mongolia is a remnant of oceanic lithosphere of probable Devonian age. The ophiolite consists of several blocks composed chiefly of serpentinized ultramafic rocks with lesser amounts of troctolite and gabbro, and sparse lavas and dikes. The ultramafic rocks consist chiefly of depleted harzburgite and minor dunite and are interpreted as mantle tectonites. In the Hegenshan block dunite is relatively abundant and is typically associated with podiform chromitite. Both the chromite ore and the residual chromites in this body are relatively aluminous with average Cr numbers of 44–54. A few small chromite bodies and some of the residual chromites have much higher Cr numbers (72–76). Several blocks have well-layered cumulate sequences of gabbro and troctolite. Sheeted dikes are absent but small mafic dikes are common in some of the ultramafic sections. Most of the mafic dikes have flat chondrite-normalized REE patterns and are strongly depleted in incompatible elements, similar to depleted tholeiites from immature island arcs. The basaltic lavas of the Hegenshan ophiolite have two distinctly different chemical signatures—one similar to the mafic dikes and one similar to ocean island basalts. The entire complex was probably formed within an island arc–marginal basin system that was later accreted to the southern margin of the Siberian Altaids. 相似文献
9.
《Chemical Geology》2003,193(1-2):137-154
The composition of Kuerti mafic rocks in the Altay Mountains in northwest China ranges from highly geochemically depleted, with very low La, Ta and Nb and high εNd(t) values, to slightly enriched, arc lava-like composition. They display flat to light rare earth element (REE)-depleted patterns and have variable depletions in high field-strength elements (HFSE). These mafic rocks were most probably derived from a variably depleted mantle source containing a subduction component beneath an ancient intra-oceanic backarc basin. Together with the slightly older arc volcanic rocks in the Altay region, the Kuerti mafic rocks display generally positive correlations of their key elemental ratios (e.g., Th/Nb, La/Yb and Th/Yb). These indicate that the more mid-ocean ridge basalt (MORB) component was contained in these magmas, the less arc component was present in their mantle source. Therefore, we propose a two-stage melting evolution model to interpret the compositional evolution of the Kuerti mafic rocks and associated arc volcanic rocks. First, arc basaltic melts were extracted from the hydrated arc mantle wedge beneath Kuerti, leaving behind a mantle source that is variably depleted in incompatible trace elements. Then, mafic rocks were erupted during seafloor spreading in the Kuerti backarc basin from the upwelling asthenospheric mantle. The variably depleted mantle source produced mafic rocks with composition ranging from arc lava-like to more geochemically depleted than MORB. The recognition of Kuerti mafic rocks as backarc basin basalts (BABB) is consistent with the proposed tectonic model that an active backarc basin–island arc system along the paleo-Asian ocean margin was formed in the Altay region during Devonian–Early Carboniferous. New data further indicate that the final orogenic event in the Altay Mountains, i.e. the collision of the north and south continental plates in the region, most probably took place in Late Carboniferous and Permian. 相似文献
10.
J. M. Liotard H. G. Barsczus C. Dupuy J. Dostal 《Contributions to Mineralogy and Petrology》1986,92(2):260-268
The Marquesas Archipelago, a volcanic chain in French Polynesia (south-central Pacific Ocean), is predominantly composed of alkalic, transitional and tholeiitic basalts. The variation trends in these intraplate basaltic rocks imply that the magmas were derived from different upper mantle sources. Model calculations using the total inverse method show that the peridotite source of most Marquesas basalts was enriched in incompatible elements compared to a primordial mantle and had higher than chondritic ratios of several elements such as La/Yb, Ti/V and P/Ce. A metasomatic enrichment event is suggested by the sequence of element enrichment in the source relative to the primordial mantle (Ba>Nb>La>Ce>Sr>Sm>Eu> Zr>Hf>Ti>Y>Yb). On the other hand, some lavas including tholeiites of Ua Pou and alkalic basalts of Hiva Oa, were probably derived from relatively depleted upper mantle. In some islands such as Hatutu, the different types of basalts were generated from sources with rather similar compositions. The residual phases of the Marquesas magmas included garnet. The sources of these magmas were similar in trace element chemistry to the oceanic mantle below Hawaii. 相似文献
11.
江西周潭群斜长角闪岩的地球化学特征及其成因 总被引：2，自引：0，他引：2
周潭群斜长角闪岩的基本组成矿物是斜长石和角闪石，不同样品中还可以出现黑云母、石英和碱性长石等。所有斜长角闪岩显示石英拉斑系列特征，其稀土元素配分曲线呈平坦型，显示轻微的轻稀土亏损和Eu亏损，与由岛弧及洋拉斑玄武岩形成的斜长角闪岩的稀土分布型式及稀土特征一致。根据微量元素蛛网图的斜率和Nb、Zr异常情况以及低的Zr/Nb比值、V-Ti、Y－Cr、Zr-Ti-Sr、Zr-Y相关性，认为斜长角闪岩之原岩为岛弧型玄武岩，周潭群的原岩形成于岛弧环境。元素地球化学和Nd同位素特征指示它们的母岩浆起源于亏损程度低的地幔或来源于亏损地幔的岩浆受到地壳物质的混染。 相似文献
12.
13.
The Cenozoic volcanic rock of Shandong Province are mainly alkalic and strongly alkalic basaltic rocks.The Contents of major and trace elements including transitional,incompatible and rare-earth elements were determined.The chemical characterisitics of major and trace elements indicate that these basaltic rocks were derived from a mantle source and probably represent a primary magma,I,e.,unmodifiecd partical melts of mantle peridotite in terms of Mg values,correlatione between P2O5 and Ce,Sr,Ni and Rb concentrations,mantle xenoliths,etc.The abundances of trace elements vary systematically from west to east.The compatible transition elements such as Co,Ni,and Cr show a remarkable depletion,whereas the incompatible and rare-earth elements are abundant as viewed from the chondrite-nor-malized patterns.The chemical composition and correlation are consistent with the tectonic setting.According to the batch and fractional partial melting theory,the trace element contents of Shandong volcanic rocks can be calculated from the two-component mixing model. 相似文献
14.
V. Rama Murthy 《Geochimica et cosmochimica acta》1974,38(4):611-627
K, Rb, Sr, Ba and rare earth elements of some Archean volcanic rocks from the Vermilion greenstone belt, northeast Minnesota, were determined by the isotopic dilution method. The characteristics of trace element abundances, supported by the field occurrences and major element chemistry, suggest that these volcanic rocks were formed in an ancient island arc system. A felsic rock is suggested to be derived by partial melting of a basaltic source, presumably in an ancient subduction zone.It is well known that the distribution coefficients (liquid/source) for the above trace elements are almost invariably greater than one. Continuous extraction of volcanic liquids from the upper mantle through geologic time would result in depletion of these elements in the upper mantle. However, all trace element abundances in many Archean volcanic rocks are almost identical to their modern equivalents. This gross constancy of trace element concentration in rocks of different geologic age raises some important questions as to the evolution of the upper mantle. It is proposed that the trace elements have been repeatedly and fully recycled in a restrictive and closed system of crust and upper mantle during the last three billion years (recycled mantle), or the trace elements have been replenished from the lower part of the mantle by some undefined process (replenished mantle). It is believed that interplay of both recycling and replenishment have been responsible for crust-mantle evolution in geological history. 相似文献
15.
《Russian Geology and Geophysics》2014,55(8):929-940
Petrological and geochemical studies performed with invoking data on the compositions of clinopyroxenes have clarified the conditions of formation of Vendian-Cambrian basaltic complexes in the Dzhida zone of the Paleoasian Ocean (northern Mongolia and southwestern Transbaikalia). The research was based on a comparative analysis with reference igneous basaltic associations. Of special importance are our microprobe data on trace and rare-earth elements in clinopyroxenes from igneous rocks of different present-day geodynamic settings, namely, N-MORB (Mid-Atlantic Ridge, Central Atlantic), OIB (Bouvet Island, South Atlantic), WPB (within-plate tholeiitic plateau basalts of the Siberian Platform), and boninites of ensimatic arcs (Izu-Bonin island arc, Pacific). The studies have shown that the paleo-oceanic structures in the district of the Urgol guyot formed during geodynamic processes under the impact of mantle plumes on oceanic spreading crust, which resulted in oceanic basaltic plateaus and within-plate oceanic islands. All these structures were later superposed by typical island-arc structure-lithologic associations. Formation of basalt complexes in the Dzhidot guyot district proceeded with a stronger effect of enriched plume melts of within-plate oceanic islands as compared with the Urgol guyot. This is evidenced from petrochemical and geochemical data showing the development of OIB-type magmatic systems on the oceanic basement. Data on clinopyroxenes confirm the participation of mantle plume in this process, which led to the evolution of magmas from typical oceanic basalts (MORB) to plateau basalts and OIB. 相似文献
16.
西南三江造山带火山岩—构造组合及其意义 总被引：67，自引：0，他引：67
岩石构造组合是指表示板块边界或特定的板块内部环境特征的岩石结合。中国西南“三江”造山带的火山岩可划分为五种火山岩－构造组合：洋脊型／准洋脊型组合，岛弧及陆缘弧组合，碰撞型组合，碰撞后组合及陆内拉张型组合。阐述了各种火山岩－构造组合的特点及构造含义。对在造山带火山岩岩石－构造组合分析中经常遇到的一些问题，如“构造岩片”研究方法、地球化学判别图解的使用条件、准洋脊型火山型组合的构造含义、蛇绿岩带－火山弧的成对性、岩浆作用的同步性和滞后性、以及火山岩的深部“探针”作用等问题进行了讨论。 相似文献
17.
A mafic–ultramafic intrusive belt comprising Silurian arc gabbroic rocks and Early Permian mafic–ultramafic intrusions was recently identified in the western part of the East Tianshan, NW China. This paper discusses the petrogenesis of the mafic–ultramafic rocks in this belt and intends to understand Phanerozoic crust growth through basaltic magmatism occurring in an island arc and intraplate extensional tectonic setting in the Chinese Tianshan Orogenic Belt (CTOB). The Silurian gabbroic rocks comprise troctolite, olivine gabbro, and leucogabbro enclosed by Early Permian diorites. SHRIMP II U-Pb zircon dating yields a 427 ± 7.3 Ma age for the Silurian gabbroic rocks and a 280.9 ± 3.1 Ma age for the surrounding diorite. These gabbroic rocks are direct products of mantle basaltic magmas generated by flux melting of the hydrous mantle wedge over subduction zone during Silurian subduction in the CTOB. The arc signature of the basaltic magmas receives support from incompatible trace elements in olivine gabbro and leucogabbro, which display enrichment in large ion lithophile elements and prominent depletion in Nb and Ta with higher U/Th and lower Ce/Pb and Nb/Ta ratios than MORBs and OIBs. The hydrous nature of the arc magmas are corroborated by the Silurian gabbroic rocks with a cumulate texture comprising hornblende cumulates and extremely calcic plagioclase (An up to 99 mol%). Troctolite is a hybrid rock, and its formation is related to the reaction of the hydrous basaltic magmas with a former arc olivine-diallage matrix which suggests multiple arc basaltic magmatism in the Early Paleozoic. The Early Permian mafic–ultramafic intrusions in this belt comprise ultramafic rocks and evolved hornblende gabbro resulting from differentiation of a basaltic magma underplated in an intraplate extensional tectonic setting, and this model would apply to coeval mafic–ultramafic intrusions in the CTOB. Presence of Silurian gabbroic rocks as well as pervasively distributed arc felsic plutons in the CTOB suggest active crust-mantle magmatism in the Silurian, which has contributed to crustal growth by (1) serving as heat sources that remelted former arc crust to generate arc plutons, (2) addition of a mantle component to the arc plutons by magma mixing, and (3) transport of mantle materials to form new lower or middle crust. Mafic–ultramafic intrusions and their spatiotemporal A-type granites during Early Permian to Triassic intraplate extension are intrusive counterparts of the contemporaneous bimodal volcanic rocks in the CTOB. Basaltic underplating in this temporal interval contributed to crustal growth in a vertical form, including adding mantle materials to lower or middle crust by intracrustal differentiation and remelting Early-Paleozoic formed arc crust in the CTOB. 相似文献
18.
仲岗洋岛位于班公湖-怒江板块缝合带中西段,前人对仲岗洋岛的研究主要集中在东段,中段洞错一带尚未有人研究。对仲岗洋岛的岩石组合及岩石地球化学进行研究,并对洋岛火山岩的源区及构造环境进行分析。在仲岗洋岛中段洞错北部的一条剖面上采集了8个玄武岩和8个玄武安山岩地球化学样品,主量元素特征表明,二者皆为具有富Ti特征的碱性玄武岩,微量元素特征显示二者富集Nb、Ta元素。稀土元素配分曲线和微量元素蛛网图与典型洋岛玄武岩曲线相似。化学成分指示,仲岗洋岛玄武岩与玄武安山岩可能来自于同一岩浆源区,且二者来源于具有洋岛玄武岩特征的地幔,相容元素Cr、Ni的亏损表明,成岩过程中发生了橄榄石和辉石的分离结晶作用。洋岛形成于大洋板块内以洋壳为基底的洋岛环境。 相似文献
19.
The petrology and geochemistry of the Azores Islands 总被引：7，自引：0，他引：7
William M. White Maria D. M. Tapia Jean -Guy Schilling 《Contributions to Mineralogy and Petrology》1979,69(3):201-213
Forty lavas from the Azores Islands have been analyzed for 87Sr/86Sr ratios, major elements, first transition series metals, and LIL elements. The samples belong to the alkali basalt magma series but range from transitional hy-normative basalts from Terceira to basanitoids from Santa Maria. Differentiated lavas include both typical trachytes and comenditic trachytes and comendites. Major and trace element concentrations define smooth trends on variation diagrams, and these trends can be related to phases crystallizing in the rocks. Systematic interisland differences are also apparent in these variation diagrams. LIL element concentrations in island basalts are roughly twice as high as those in tholeiites from the adjacent Mid-Atlantic Ridge which transects the Azores Plateau. 87Sr/86Sr ratios in lavas from 6 of the 9 islands range from 0.70332 to 0.70354, a range similar to that found in tholeiites from the Mid-Atlantic Ridge transect of the Azores Plateau. This suggests that lavas from these islands and this portion of the Mid-Atlantic Ridge may be derived from a similar source. However, lavas from the islands of Faial and Pico have 87Sr/86Sr ratios up to 0.70394 and ratios in Sao Miguel lavas range up to 0.70525, suggesting basalts from these islands are derived from a chemically distinct source. Differences in the average LIL element concentrations of the least fractionated ridge tholeiites from the Azores Plateau and alkali basalts from the islands result from differences in extent of partial melting and residual mineralogy. The alkali basalts are derived by roughly half as much melting as are the tholeiites. Trace element concentrations in Azores peralkaline lavas preclude their derivation by partial melting of peridotitic mantle or basaltic crust; rather the data suggest they are produced by fractional crystallization of a basaltic parent. 相似文献
20.
The Shortland Islands lie in a northeast-southwest line across the western end of Solomon Islands, immediately adjacent to Bougainville. Three major islands dominate the group.Fauro and surrounding islands, in the northeast, have an altered basement suite comprising tholeiite, icelandite and tholeiitic dacite. This is intruded by a high-level calc-alkaline assemblage of microdiorite, hornblende andesite and rhyodacite and overlain by volcanogenic sandstones derived from an andesitic to dacitic volcano. Pyroclastics comprising high-alumina basalt and pyroxene andesite overlie the volcanogenic sandstones. The tholeiitic basement lavas may be of Late Oligocene to Early Miocene age, and the calc-alkaline rocks are probably also pre-Pliocene in age.Alu, in the centre of the group, also has an altered tholeiitic lava basement, which is intruded by a quartz diorite body and overlain by hypersthene-augite basaltic andesite. Pliocene siltstone and Quaternary shallow marine carbonates cover these igneous rocks over much of the island.Mono, in the southwest, has a small basement exposure of altered pillowed hawaiite, overlain by Miocene pelagic limestone, Pliocene siltstone and Quaternary reef limestone. Isolated clasts of pyroxene andesite and ?benmoreite occur in streams and on beaches.The younger, calc-alkaline suites on all islands were formed in an island arc environment, possibly related to subduction from the southwest beneath the New Britain Trench. The basement lavas on Alu are probably early island arc tholeiites, and both these lavas and the calc-alkaline rocks of Alu share a common trend on variation diagrams. The two igneous suites of Fauro, however, have distinctly different trends. The basement lavas have some chemical similarities with oceanic tholeiites, but an early island arc origin for these lavas cannot be ruled out. The altered hawaiite and benmoreite on Mono probably originated in an oceanic island environment. 相似文献