Petrology of the Western Reykjanes Peninsula, Iceland

The active tholeiitic volcanic zone of the Reykjanes Peninsulaconsists of five volcanic fissure swarms, the two westernmostof which are the subject of this petrological study. The recent(less than 12,000 years) extrusives of the swarms group morphologicallyand petrographically into small picrite basalt lava shields,large olivine tholeiite lava shields and tholeiite fissure lavas;formed in that chronological succession. The picrite basalts exhibit a primitive mineralogy with chromite,olivine (Fo 89) and plagioclase (An 90) as phenocrysts and mayrepresent a primary liquid from the mantle. Simultaneous crystallizationof olivine, plagioclase and augite to form glomerocrysts inthe fissure lavas indicate low pressure cotectic crystallizationconditions. Twenty-eight new major element chemical analyses of the lavasare presented. They are generally characterized by a low contentof alkalies and high CaO. The lavas constitute two main suites,a lava shield suite and a fissure lava suite. There is a positivecorrelation between the volume of individual lavas and the contentof incompatible elements of the lavas within each group. Likewisethere is an overall chemical trend through time demonstrated,for example, by a rise in K₂O from about 0.02 per cent to 0.24per cent during the last, approximately, 12,000 years. There is an apparent chemical zoning within each volcanic swarmsuch that the most evolved and youngest lavas are found in thecentral axial area of the swarm. This central area is also characterizedby graben subsidence, high magnetic anomalies and high temperaturethermal areas, all indicative of shallow magma reservoir(s).In spite of indications of fractional crystallization in theevolution of the olivine tholeiites and tholeiites, some otherprocesses must be sought to explain the volume chemistry relations.Cyclic volcanic activity is tentatively suggested to explainthe observed regular temporal variations within the swarm, eachcycle starting with the formation of picrite basalts.     

Petrochemistry of Cenozoic basalts and associated rocks in the Baikal rift zone

The main episode of Cenozoic volcanic activity occurred simultaneously with formation of the Sayan—Baikal uplift, before the rift depressions were initiated. Volcanism and rifting in this region have developed as independent processes, connected with each other only by an ultimate primary mantle energy source. The volcanic regions do not coincide with the rift depressions, except in the Tunka graben.Chemical features of the volcanics show that during the entire period of volcanic activity there was a complex alternation of basaltic lavas of alkaline, intermediate and tholeiitic composition. Both alkaline and subalkaline lavas are distributed over the entire volcanic region, excepting the Tunka depression where tholeiitic lavas are predominant. However, there is neither mineralogical nor chemical evidence for the existence of two separate magma types within the Baikal rift zone.Judging by the presence of high-pressure, lherzolitic megacrysts of clinopyroxene, and to a lesser extent titaniferous biotite and amphibole in alkaline basalts, variations of lava chemistry are connected with high-pressure fractionation of initial melts, which was more complete for sources outside the rift zone. The predominance of tholeiitic lavas in the Tunka depression is likely to have been caused by a higher degree of partial melting and quick ascent of magma to the surface, facilitated by a high geothermal gradient under the depression where crustal extension is taking place.     

Chemistry and distribution pattern of recent basaltic rocks in Iceland

A survey of Recent basaltic rocks in Iceland is presented. The basalts are classified into three groups: tholeiites, transitional alkali basalts and alkali olivine basalts. The basalts can be divided into petrological regions where the composition of lavas seem to have been fairly constant throughout postglacial and possibly late-Pleistocene time. The tholeiites delineate the crest region of the Mid-Atlantic Ridge as it transects Iceland, and the mildly alkali olivine basalts and the transitional alkali basalts characterize the flank volcanic zones. Tholeiitic and alkalic diffrentiated rocks appear to have a distribution in accordance with the basalt distribution pattern. There is some correlation between the chemistry of the zones and the crustal structure of Iceland. Areal discharge of volcanic rocks varies consistently between the petrological regions being highest in the tholeiite regions. The total output of volcanic rocks along the Mid-Atlantic Ridge in the Iceland area reaches maximum in middle Iceland.     

Basalt geochemistry used to investigate past tectonic environments on Cyprus

Study of the geochemical fingerprints of four geologically distinct suites of volcanic rocks on Cyprus are used to sketch a tectonic history of the island. Lavas from the Mamonia complex resemble alkalic within-plate basalts; lower pillow lavas and diabases of the Troodos Massif have features both of ocean-floor and island-arc tholeiites and could have been erupted in an interarc basin; the upper pillow lavas of the Troodos Massif resemble primitive tholeiitic basalts from island arcs; lavas from the Kyrenia range resemble transitional to alkalic within-plate basalts. The low TiO₂ concentrations from the Troodos Massif may indicate a slow spreading rate. The Sr concentrations in the upper pillow lavas indicate an eruption at a maximum distance of 80 km above a Benioff zone. The results suggest formation of the Troodos Massif in the Campanian by spreading in an interarc basin followed by eruption of island-arc tholeiites. Obduction of continental material and ocean islands may have taken place in the Maestrichtian and Middle Miocene.     

Chemostratigraphy of lava sequences from the Rio Itapicuru Greenstone Belt,Bahia State,Brazil

The Rio Itapicuru greenstone terrain of north-central Bahia State consists of belts of supracrustal rocks surrounding granitic plutons and domes. The basal supracrustal rocks are predominantly massive metabasalts with minor amounts of intercalated chemical sedimentary rocks and mafic tuffs. They are overlain by a middle unit of intermediate to acid pyroclastic rocks, lavas, and volcaniclastic sediments, and an upper unit of greywackes, sandstones and conglomerates.A geochemical study of major and trace elements of the volcanic rocks indicates the existence of a chemical discontinuity between the basaltic and the acid to intermediate members. The basalts are typical tholeiites with Ti, Zr, Sr, Y and Nb contents analogous to those of modern ocean-floor tholeiites or, alternatively, low-K tholeiites of primitive island arcs. In contrast, compositional variations of the hornblende-bearing andesites and dacites fall along indisputably calc-alkaline trends of low FeO and TiO₂ contents which decrease with increasing differentiation. The lithostratigraphic and chemical variations within lavas of the Rio Itapicuru greenstone are comparable to those described from the Western Australian greenstone belts. Only in greenstone belts of the Canadian type do thick calc-alkaline sequences containing abudant basaltic andesites overlie conformably and transitionally the underlying tholeiitic basalts. Elsewhere the calc-alkaline sequences, if present, do not contain basaltic andesites and are chemically unrelated to the underlying basalts.     

First report of Lower Permian basalts in South Tibet: tholeiitic magmatism during break-up and incipient opening of Neotethys

The Neo-Tethys Ocean began to form at Early Permian times, when continental flood basalts were emplaced in various areas of the newly-formed Indian passive margin, exposed today in the so-called Tibetan Sedimentary Zone of the Himalaya. Lower Permian mafic volcanic rocks, which have long been known from various Himalayan localities from Kashmir to Arunachal Pradesh, are here for the first time reported to occur also in South Tibet (Bhote Kosi Basalts of the Gyirong County). The basalts unconformably overlie lowermost Permian diamictites, with locally intervening black shales and debris flow deposits, and are followed in turn by chert-bearing quartzarenites and silty to phosphatic marls yielding brachiopods of Roadian–Wordian age. The age of the lavas can thus be bracketed as late Early Permian (post-Sakmarian and pre-Roadian).The geochemistry of these subalkalic tholeiites, akin to MORBs, testifies to their similarity not only with the adjacent Nar-Tsum Spilites of central Nepal, but also with the Panjal Traps and Abor Volcanics of the western and eastern Himalayas respectively. The geochemical signature of Lower Permian volcanic rocks is in fact uniform all along the Himalayan Range, and markedly different from that of basaltic–rhyolitic alkalic products sporadically emplaced during the previous rifting stage. Rift volcanism in the Tethys Himalaya began in the Early Carboniferous and came to an end in Sakmarian times. In the Early Permian, initial submergence of the rift shoulders and sediment starvation were followed by tholeiitic magmatism, which is therefore interpreted as following break-up and incipient sea-floor spreading in the Neotethys Ocean. Roughly contemporaneous emplacement of continental flood basalts of similar geochemical signature along a 2000 km long rift axis would in fact suggest extensive mantle melting at the transition from continental rifting to break-up and opening of the Neotethys between Northern Gondwana and the Peri-Gondwanian blocks.     

A note on the petrology of some lava types from East New Guinea

Quaternary and upper Tertiary lavas from the volcanic islands along the north coast of east New Guinea consist of pyroxene andesites, andesitic basalts, and basalts; many contain accessory olivine. The lavas of Mts Lamington, Yelia, and Victory, in southeast Papua and New Guinea, are rather more acid hornblende and lamprobolite andesites. Peralkaline obsidian and rhyolite occur at Fergusson and Dobu Islands, east of the Papuan mainland. Chemical analyses show that most of the lavas are of calc‐alkaline type, although some from Karkar and Long Islands appear to have tholeiitic tendencies.     

Origin of Tertiary to Recent EM- and subduction-like chemical and isotopic signatures in Auca Mahuida region (37°–38°S) and other Patagonian plateau lavas

The alkaline volcanic rocks of the 1.8–0.9 Ma Auca Mahuida and post-mid-Pliocene Rio Colorado backarc volcanic fields east of the Andean Southern Volcanic Zone at ~37°–38°S have pronounced intraplate-like chemical signatures with some striking similarities to oceanic DM-EM1-like lavas of the south Atlantic Tristan da Cunha type. These backarc lavas are considered to have formed as a series of mantle batches typified by 4–7 % melting, with decompression melting initiating in a garnet-bearing mantle above a steepening subduction zone, and final equilibration occurring near the base of a ~65- to 70-km-thick lithosphere at temperatures of ~1,350–1,380 °C. Evolved Auca Mahuida mugearite to trachytic samples are best explained by crystal fractionation with limited mixing of partial melts of recently underplated basalts, in line with isotopic signatures that preclude significant radiogenic contamination in a preexisting refractory crust. Higher Ba/La and subtly higher La/Ta ratios than in nearby ~24–20 Ma primitive basalts or oceanic (OIB) lavas are attributed to the residual effects of slab fluids introduced during a shallow subduction episode recorded in the arc-like chemistry of the adjacent 7–4 Ma Chachahuén volcanic complex. Positive Sr, K and Ba spikes on mantle-normalized patterns of both primitive Auca Mahuida and ~24–20 Ma basalts, like those in EM-like OIB basalts, are attributed to mixing of continental lithosphere into the asthenosphere. In Patagonia, this mixing is suggested to have peaked as the South America continent accommodated to major late Oligocene plate convergence changes, as similar Sr, K and Ba spikes and DM-EM1 signatures are absent in ~50–30 Ma backarc lavas north of 51°S, and all of those south of 51°S. Introduction of an EM1-like component associated with lateral mantle flow of a Tristan da Cunha source is largely precluded by its Cretaceous age and distance to Patagonia.     

Petrogenetic Evolution of the Torfaj?kull Volcanic Complex, Iceland I. Relationship Between the Magma Types

The Torfaj?kull volcano, Iceland's largest silicic centre, issituated close to the junction of the active, southerly propagatingEastern Rift Zone and the South Eastern Zone, an older crustalsegment. This paper provides major, trace, and some Sr isotopedata on postglacial (<10000 y) rocks, i.e., tholeiitic magmasof the Eastern Rift Zone and transitional basalts, icelandites,and rhyolites of the Torfaj?kull centre, and assesses the relationshipsbetween the magma types in terms of the development of the Icelandiccrust. Tholeiitic magmas from the Eastern Rift Zone are LILE-enrichedrelative to MORB. They have undergone extensive olivine-plagioclase-clinopyroxenefractionation at low pressures. Compared with the tholeiites,Torfaj?kull transitional basalts show LILE/HFS enrichment andhigher (⁸⁷Sr/⁸⁶Sr)₁ ratios. They define several magmatic lineagesand have equilibrated over a wide range of pressures. Both basalttypes were derived by very small degrees of partial meltingof compositionally similar mantle sources, the main differencebeing that the tholeiites were generated in the spinel Iherzolite,and the transitional basalts in the garnet lherzolite, stabilityfields, a conclusion previously reached by Meyer et al. (1985).The mantle sources may have contained LILE-enriched streaks. Low-pressure differentiation of Torfaj?kull transitional basaltproduced an iceiandite to sub-alkaline rhyolite sequence bycrystal fractionation, the rhyolites representing >90% crystallizationof parental basalts. The rhyolites were emplaced as nine separatelava fields, formed during 11 eruptive episodes. The compositionalrange within each field is limited, and, although similar, theranges define several magmatic lineages. Continued fractionationof plagioclase-alkali feldspar-clinopyroxene-magnetite-apatite-zirconassemblages generated peralkaline rhyolites in certain post-glacialeruptions. Chemical variations in the deposits from the Hrafntinnuskerperalkaline eruption were related predominantly to alkali feldsparfractionation, and the melts were erupted from a zoned magmachamber. All postglacial volcanic rocks at Torfajokull havebeen mantle derived and thus represent new additions to theIcelandic crust.     

Petrogenetic Evolution of the Torfaj?kull Volcanic Complex, Iceland II. The Role of Magma Mixing

In southern Iceland, tholeiitic basalt magmas propagating laterallyfrom the active Eastern Rift Zone into the older cmstal segmentof the South Eastern Zone have been injected into Torfaj?kull,a mature volcanic centre dominated by rhyolites. Eruptions ofcomplex suites of mixed and hybrid rocks have been triggered,involving tholeiites of the rift zone and transitional basaltsand rhyolites of the Torfaj?kull centre. Three-component hybridsare an unusual feature of the activity. The distribution ofvarious magma mixing and hybrid types is related to the periodicinjection of tholeiite into a magma chamber, or chambers, whererhyolite overlies parental transitional basalts. Pre-postglacial rhyolites (>10000 y) at Torfajokull are predominantlyperalkaline, whereas later rhyolites are, with few exceptions,subalkaline. Furthermore, the injection of rift zone magmas,and the consequent abundance of rhyolite-basalt mixing, havebeen important features of magmatism at the centre only in postglacialtimes. Reduced repose times in the magma reservoirs have preventedthe production of peralkaline rhyolites. These trends are interpretedin terms of the southerly migration of the Eastern Rift Zone.     

Temporal evolution of the kerguelen plume: Geochemical evidence from 38 to 82 ma lavas forming the Ninetyeast ridge

Abstract Basaltic basement has been recovered by deep-sea drilling at seven sites on the linear Ninetyeast Ridge in the eastern Indian Ocean. Studies of the recovered lavas show that this ridge formed from ~ 82 to 38 Ma as a series of subaerial volcanoes that were created by the northward migration of the Indian Plate over a fixed magma source in the mantle. The Sr, Nd and Pb isotopic ratios of lavas from the Ninetyeast Ridge range widely, but they largely overlap with those of lavas from the Kerguelen Archipelago, thereby confirming previous inferences that the Kerguelen plume was an important magma source for the Ninetyeast Ridge. Particularly important are the ~ 81 Ma Ninetyeast Ridge lavas from DSDP Site 216 which has an anomalous subsidence history (Coffin 1992). These lavas are FeTi-rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic, Upper Miocene lavas in the Kerguelen Archipelago. The isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the Kerguelen plume (Weis et al. 1993a,b). Despite the overlap in isotopic ratios, there are important compositional differences between lavas erupted on the Ninetyeast Ridge and in the Kerguelen Archipelago. The Ninetyeast Ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios, such as La/Yb and Zr/Nb, which are intermediate between those of Indian Ocean MORB (mid-ocean ridge basalt) and the transitional to alkalic basalts erupted in the Kerguelen Archipelago. These compositional differences reflect a much larger extent of melting for the Ninetyeast Ridge lavas, and the proximity of the plume to a spreading ridge axis. This tectonic setting contrasts with that of the recent alkalic lavas in the Kerguelen Archipelago which formed beneath the thick lithosphere of the Kerguelen Plateau. From ~ 82 to 38 Ma there was no simple, systematic temporal variation of Sr, Nd and Pb isotopic ratios in Ninetyeast Ridge lavas. Therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume. Although Class et al. (1993) propose that some of the isotopic variations reflect such aging, we infer that most of the isotopic heterogeneity in lavas from the Ninetyeast Ridge and Kerguelen Archipelago can be explained by mixing of the Kerguelen plume with a depleted MORB-like mantle component. However, with this interpretation some of the youngest, 42–44 Ma, lavas from the southern Ninetyeast Ridge which have²⁰⁶pb/²⁰⁴Pb ratios exceeding those in Indian Ocean MORB and Kerguelen Archipelago lavas require a component with higher²⁰⁶Pb/²⁰⁴Pb, such as that expressed in lavas from St. Paul Island.     

Late cenozoic alkali-olivine basalts of the Basin-Range Province,USA

Petrographic and chemical analyses demonstrate that late Cenozoic mafic lavas from the Basin-Range Province, western United States, are predominantly alkali-olivine basalts. Associated with these lavas are lesser volumes of basaltic andesite which appear to be differentiates from the more primitive alkali basalts. Late Cenozoic basalts from adjacent regions (Columbia River Plateau, Snake River Plain, Yellowstone area, High Cascades and Sierra Nevada) are predominantly tholeiitic. This apparent petrologic provincialism is supported by complementary variations in heat flow, seismic velocities, crustal thickness, magnetic anomalies and geologic setting.Alkali-olivine basalts from Japan and eastern Australia are analogous to those from the Basin-Range province both in composition and tectonic environment. It is suggested that these lavas are the products of a unique environment characterized by high heat flow and a thin crust.Recent melting experiments on peridotites and basalts and measurements of heat flow allow limits to be placed on the depth of origin of Basin-Range alkali-olivine basalt magmas. It is proposed that these lavas are produced by partial melting (less than 20%) of peridotitic mantle material at depths between 40 and 60 km in response to an elevated geothermal gradient. The basaltic andesites may be derived from hydrous alkali basalt magma by fractionation at depths of 30 to 40 km.     

Low-O18 basalts from Iceland

The volcanic rocks of Iceland are anomalous in their oxygen isotope content. Recent tholeiitic and transitional alkali basalts from Iceland range in (δO¹⁸ from 1·8 to 5δ7%. Most of the tholeiitic basalts and their phenocrysts are at least 1% lower in δO¹⁸ than unaltered basalts from other oceanic islands or oceanic ridges. The Icelandic basalts that resemble ridge basalts in δO¹⁸ also resemble them in major element chemistry. δO¹⁸ values of alkali olivine basalts are closest to those of other oceanic islands. Secondary alteration processes have lowered as well as raised the δO¹⁸ values of some Icelandic rocks, but such surface mechanisms cannot account for the distribution of oxygen isotopes in the Recent basalts of Iceland. Three mechanisms that could give rise to the low-O¹⁸ magmas are (1) exchange of oxygen between magma and low-O¹⁸ hydrothermally altered rock, (2) exchange with low-O¹⁸ meteoric water, or (3) an exceptional mantle under Iceland. None of the above models can satisfactorily account for all the observations.     

Volcanic rocks and processes of the Mid-Atlantic Ridge rift valley near 36 ° 49′ N

Eighty samples of submarine basaltic lava were sampled from an 8 km segment of the floor and walls of the inner rift valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (project Famous). The samples were collected from outcrops and talus slopes by the three submersibles: Alvin, Archimede, and Cyana at water depths of about 2600 meters.The early formed mineral content of the pillow lavas' glassy margins enables classification of the rocks into 5 types: (1) olivine basalt, (2) picritic basalt, (3) plagioclase-olivine-pyroxene basalt, (4) aphyric basalt, and (5) plagioclase-rich basalt. Chemical and mineralogical study indicates that at least 4 types are directly interrelated and that types (1) and (2) are higher-temperature, primitive lavas, and types (3) and (4) are lower-temperature, differentiated lavas derived from the primitive ones by crystal-liquid differentiation. The plagioclase-rich basalts also have a chemical composition of their glass comparable to that of the most differentiated basalts (types 3 and 4) but they differ in their greater amount of early formed plagioclase (12–35%).In general, the mineralogical variation across the rift valley shows an assymetrical distribution of the major basalt types. Despite the mineralogical diversity of the early formed crystals, the chemistry of the basalt glasses indicates a symmetrical and a gradual compositional change across the rift valley. Based primarily on their chemistry, the rock types 1 and 2 occupy an axial zone 1.1 km wide and make up the central volcanic hills. Differentiated lavas (types 3, 4) occupy the margins and walls of the inner rift valley and also occur near the center of the rift valley between the central hills.FeO/MgO ratios of olivine and coexisting melt indicate that the average temperature of eruption was 40 ° C higher for the primitive melts (types 1 and 2). Aside from major elements trends, the higher temperature character of the primitive basalts is shown by their common content of chrome spinel.The thickness of manganese oxide and palagonite on glassy lava provide an estimate of age. In a general fashion the relative age of the various volcanic events follow the compositional zoning observed in the explored area. Most of the youngest samples are olivine basalt of the axial hills. Most older samples occur in the margins of the rift valley (West and N.E. part of explored area) but are significantly younger than the spreading age of the crust on which they are erupted. Intermediate lava types occur mainly east of the rift valley axis and in other areas where plagioclase—olivine—pyroxene basalt and aphyric basalt are present.The above relations indicate that the diverse lava types were erupted from a shallow, zoned magma chamber from fissures distributed over the width of the inner rift valley and elongate parallel to it. Differentiation was accomplished by cooling and crystallization of plagioclase, olivine, and clinopyroxene toward the margins of the chamber. The centrally located hills were built by the piling up of frequent eruption of mainly primitive lavas which also are the youngest flows. In contrast smaller and less frequent eruptions of more differentiated lavas were exposed on both sides of the rift valley axis.Contribution n ° 480 du Départment Scientifique, Centre Océanologique de Bretagne     

The Prince of Wales Formation – post-flood basalt alkali volcanism in the Tertiary of East Greenland

 In the nunataks of the Prince of Wales Mountains the tholeiite flood basalts of the East Greenland Tertiary Province are unconformably overlain by alkaline lavas. The majority of the alkaline lavas are strongly porphyritic picrites, ankaramites and hawaiites. These rocks have lower ¹⁴³Nd/¹⁴⁴Nd and higher ⁸⁷Sr/⁸⁶Sr than the tholeiitic flood basalts and are isotopically akin to ocean island basalts. The alkaline lavas also have high concentrations of incompatible elements which on normalised plots have a pattern which is similar in shape to that of enriched oceanic island basalts. The isotopic and chemical characteristics of these late-stage representatives of the East Greenland volcanic activity are attributed to their derivation from the peripheral regions of the East Greenland plume, the axial region of which was moving progressively eastwards relative to the westwards drift of the Greenland plate. It is proposed that the incompatible element contents of the magmas so produced were dominated by small degree melts formed beneath a cap of continental lithosphere in the marginal regions of the plume. Received: 5 June 1995 / Accepted: 11 December 1995     

Plume dynamics beneath the African plate inferred from the geochemistry of the Tertiary basalts of southern Ethiopia

Southern Ethiopian flood basalts erupted in two episodes: the pre-rift Amaro and Gamo transitional tholeiites (45-35 million years) followed by the syn-extensional Getra-Kele alkali basalts (19-11 million years). These two volcanic episodes are distinct in both trace element and isotope ratios (Zr/Nb ratios in Amaro/Gamo lavas fall between 7 and 14, and 3-4.7 in the Getra-Kele lavas whereas ²⁰⁶Pb/²⁰⁴Pb ratios fall between 18-19 and 18.9-20, respectively). The distinctive chemistries of the two eruptive phases record the tapping of two distinct source regions: a mantle plume source for the Amaro/Gamo phase and an enriched continental mantle lithosphere source for the Getra-Kele phase. Isotope and trace element variations within the Amaro/Gamo lavas reflect polybaric fractional crystallisation initiated at high pressures accompanied by limited crustal contamination. We show that clinopyroxene removal at high (0.5 GPa) crustal pressures provides an explanation for the common occurrence of transitional tholeiites in Ethiopia relative to other, typically tholeiitic flood basalt provinces. The mantle plume signature inferred from the most primitive Amaro basalts is isotopically distinct from that contributing to melt generation in central Ethiopian and Afar. This, combined with Early Tertiary plate reconstructions and similarities with Kenyan basalts farther south, lends credence to derivation of these melts from the Kenyan plume rather than the Afar mantle plume. The break in magmatism between 35 and 19 Ma is consistent with the northward movement away from the Kenya plume predicted from plate tectonic reconstructions. In this model the Getra-Kele magmatism is a response to heating of carbonatitically metasomatised lithosphere by the Afar mantle plume beneath southern Ethiopia at this time.     

Cenozoic lithospheric extension induced magmatism in Southwest Japan

Island chains off western Kyushu are the surface exposure in the northern margin of the Taiwan–Sinzi Folded Zone that spreads along the arc–trench system in the back-arc side from SW Japan to Taiwan. Intermittent igneous activity between the Middle Miocene and Holocene occurred on these islands and widely covered or intruded sedimentary rocks of Early–Middle Miocene. Geochemistry of the volcanic rocks from the Hirado, Ikitsuki and Takushima islands believed to relate to the back-arc opening along the East China and Japan Seas shows a temporal change in source material. Submarine to sub-aerial volcanism occurred on Hirado Island at 15 Ma during the final opening stage of the East China Sea producing tholeiitic basalt and associated andesite–dacite. These eruptives show low incompatible element contents and high FeO*/MgO ratios and reflect a tholeiitic differentiation trend. High Sr and Pb and low Nd isotopic ratios suggest the involvement of EM2-like lithospheric mantle and crustal material in the formation of these syn-opening volcanic rocks. Post-opening alkali basalt volcanism occurred at 9–6 Ma on the islands is characterized by OIB-like higher large ionic lithophile elements (LILE) and high field strength elements (HFSE) compared to 15 Ma basalts in this region and Quaternary basalts along the volcanic front. They have variable range of incompatible element concentrations and ratios along with variable Sr, Pb and Nd isotopic ratios suggesting the involvement of both lithospheric and asthenospheric sources at variable melting degrees (from 4% to less than 15%). The observation that the isotopic compositions of Quaternary alkali basalts south of the studied area are even more depleted suggests an increase in the involvement of asthenospheric source with time.     

滇西墨江西部石炭、二叠纪火山岩

墨江西部出露有石炭纪和晚二叠世两套火山岩,它们呈平行的两条带分布。石炭纪火山岩以拉斑玄武岩为主,少量流纹岩,它们是由上地幔源区10%部分熔融形成的原始岩浆,经橄榄石、辉石和斜长石不同程度结晶分异作用形成。其喷发环境可能是一个发育在大陆边缘的张性海盆地。晚二叠世火山岩较为复杂,熔岩与火山碎屑岩交互发育,熔岩主要属于拉斑系列,还含有少量钙碱性系列岩石;形成于陆缘火山弧环境。     

Petrology and geochemistry of the Banks Peninsula volcanoes,South Island,New Zealand

Within the volcanic sequence of the twin volcanoes of Lyttelton and Akaroa, Banks Peninsula, New Zealand a number of different magma series have been distinguished.An early series of hawaiites (McQueens Valley Formation) was erupted about 32 m.y. ago and is of transitional or mildly tholeiitic chemistry. Stratigraphically above the McQueens Valley Formation, but unconformably overlain by the main volcanic dome sequence, is a unit of rhyolite (Gebbies Pass Rhyolites) which is not directly related to the earlier or later basaltic volcanism. The rhyolite was probably formed during intracrustal melting which was related to the rise of basaltic magma into the crust.Between 12 and 9.7 m.y. a large volcanic dome, composed mainly of hawaiite, was built at Lyttelton. Dykes, which intrude the Lyttelton volcanic sequence, range in composition from basalt to trachyte. Late, mildly alkalic, basaltic flank flows (7.5–5.8 m.y.) occur in several areas and they, and the differentiated rocks of the dyke swarm can be related by a crystal fractionation model which has been quantitatively tested.Following construction of the Lyttelton dome a second larger dome was built at Akaroa between 9 and 7.5 m.y. The rocks of the Akaroa Volcano are principally hawaiites but rocks ranging in composition through to trachyte also occur. The differentiated rocks of the Akaroa volcano have derived from the basaltic rocks by a crystal fractionation controlled process, operating during ascent through the crust.None of the Banks Peninsula basalts appear to have derived from primitive (pyrolitic) mantle material, but progressive changes in the chemistry of the basalts with time implies that the mantle source regions were evolving geochemically as partial melting proceeded. Later lavas tend to be more alkalic and to have lower MgO/FeO ratios than earlier lavas. The volcanic rocks of the Banks Peninsula volcanoes were derived by fractional removal of olivine, plagioclase, clinopyroxene, magnetite and apatite from ascending basaltic magma batches. Variations between the suites reflect differences between the parental magma batches.     

Trace-element data relevant to the origin of trachytic and pantelleritic lavas in the East African Rift system

Determination of the trace-elements Ba, Ce, La, Nb, Rb, Sr and Zr have been made on lavas from six trachytic and pantelleritic volcanoes in the Kenyan and Ethiopian Rifts. Consideration of these data shows that Ce, La, Nb and Zr have behaved as truly residual elements. In the peralkaline suites examined, plotting other chemical parameters against a residual-element such as Zr appears to be informative. The individual residual-element ratios are constant at any one volcano suggesting that the salic and basic lavas are geochemieally related and that the trachytes and pantellerites do not have an origin independent of the associated basalts.Different values of individual residual-element ratios characterize different groups of volcanic centres. In the northern part of the Kenyan Rift, progressive change in these ratios with time can be traced.In the salic lavas, Ba and Sr show patterns of extreme depletion relative to the alkali basalts and these data suggest that the salic rocks are the result of protracted crystal-fractionation.The relative volumes of the extruded lava-types are discussed and it is suggested that the trachytic and pantelleritic centres developed above cupolas of salic magma situated on top of a large basaltic reservoir. It is likely that the preponderance of salic rocks and the scarcity of lavas of intermediate composition are not criteria by which one can rule out an origin of differentiation by crystal-fractionation.