Tholeiitic Lavas Research Articles

Volcanism in the Sumisu Rift, II. Subduction and non-subduction related components

A bimodal suite of volcanic rocks collected from the Sumisu Rift by alvin provide present day examples of the first magmatic products of arc rifting during the initiation of back-arc spreading. The trace element and isotopic composition of these rocks, which are contemporaneous with island arc tholeiite lavas of the Izu-Ogasawara arc 20 km to the east, differ from those of arc rocks and N-MORB in their relative incorporation of both subduction-related and non-subduction-related components. Subduction-related components, i.e., those that distinguish volcanic arc basalts from N-MORB, are less pronounced in rift lavas than in arc lavas. Alkali and alkaline earth to high field strength element and REE ratios as well as 87Sr/ 86Sr are intermediate between those of N-MORB and Izu arc lavas and indicate that Sumisu Rift basalts are similar to BABB erupted in other, more mature back-arc basins. These results show that back-arc basins may begin their magmatic evolution with BABB rather than more arc-like lavas. Evidence of non-subduction related components remains after the effects of subduction related components are removed or accounted for. Compared to the arc, higher HFSE and REE concentrations, contrasting REE patterns, and ⩽ε Nd in the rift reflect derivation of rift lavas from more enriched components. Although SR basalt resembles E-MORB in many trace element ratios, it is referred to as BABB because low concentrations of Nb are similar to those in volcanic arcs andH 2O/REE andH 2O/K 2O exceed those of E-MORB. Differences in HREE pattern andε Nd require that the E-MORB characteristics result from source heterogeneities and not lower degrees of melting. Enriched mantle beneath the rift may reflect enriched blobs entrained in a more depleted matrix, or injection of new, more enriched mantle. High 208Pb/ 204Pb and moderate 207Pb/ 204Pb ratios with respect to Pacific MORB also reflect ancient mantle enrichment. Trends on Pb isotopic diagrams and ⩽ε Nd in the rift than in the arc indicate that recent sediment recycling is not an important process in the generation of these back-arc lavas.

Volcanism in the Sumisu Rift, I. Major element, volatile, and stable isotope geochemistry

A bimodal volcanic suite with K Ar ages of 0.05–1.40 Ma was collected from the Sumisu Rift using alvin. These rocks are contemporaneous with island arc tholeiite lavas of the Izu-Ogasawara arc 20 km to the east, and provide a present day example of volcanism associated with arc rifting and back-arc basin initiation. Major element geochemistry of the basalts is most similar to that of basalts found in other, more mature back-arc basins, which indicates that back-arc basins need not begin their magmatic evolution with lavas bearing strong arc signatures. Volatile concentrations distinguish Sumisu Rift basalts from island arc basalts and MORB. H 2O contents, which are at least four times greater than in MORB, suppress plagioclase crystallization. This suppression results in a more mafic fractionating assemblage, which prevents Al 2O 3 depletion and delays the initiation of Fe 2O 3 (tot) and TiO 2 enrichment. However, unlike arc basalts,Fe 3+/ΣFe ratios are only slightly higher than in MORB and are insufficient to cause magnetite saturation early enough to suppress Fe 2O 3 (tot) and TiO 2 enrichment. Thus, major element trends are more similar to those of MORB than arcs. H 2O, CO 2 and S are undersaturated relative to pure phase solubility curves, indicating exsolution of an H 2O-rich mixed gas phase. HighH 2O/S, highδD, and low (MORB-like)δ 34S ratios are considered primary and distinctive of the back-arc basin setting.

A Rb-Sr whole-rock isochron of 1.5 Ga in early Proterozoic pillow lavas in SW Finland

Abstract Svecofennian pillowed metalavas from SW Finland give Rb-Sr whole-rock ages close to 1.5 Ga and an initial strontium isotope ratio around 0.705. The low age is synchronous with the ages of two large rapakivi massives north and west of the pillow lavas, and it is suggested that it dates the thermal halo around the massives. The primitive and uniform initial strontium isotope ratios for the chemically slightly different tholeiitic pillow lavas show that they are partial melts of similar source rocks and that their different trace element composition is not due to contamination with older crustal material.

Calc-alkaline, Shoshonitic, and Primitive Tholeiitic Lavas from Monogenetic Volcanoes near Crater Lake, Oregon

Calc-alkaline, Shoshonitic, and Primitive Tholeiitic Lavas from Monogenetic Volcanoes near Crater Lake, Oregon

The early Proterozoic Birimian Supergroup of Ghana and some aspects of its associated gold mineralization

Rocks of the Birimian Supergroup of early Proterozoic age (Sm-Nd isochron age = 2166 ± 66 Ma) form part of the West African craton. Ghana contains the eastern portion of the Birimian terrane. Five parallel, evenly spaced, several-hundred-kilometre-long volcanic belts, consisting mainly of low-grade metamorphic, tholeiitic lavas (MORB chemistry), characterize the Birimian Supergroup of Ghana. The belts are separated by basins containing isoclinally folded dacitic volcaniclastics, wackes and argillitic sediments as well as granitoids. Lavas and sedimentary rocks were deposited contemporaneously as lateral facies equivalents. Within the sedimentary basins, various lithofacies are distinguished. In the transition zone between belt volcanics and basin sediments a chemical facies occurs. This is defined by cherts, manganiferous and carbon-rich sediments, Fe-Ca-Mg carbonates, and sulphide mineral disseminations. After their emplacement, Birimian lavas and sediments were folded in the course of the Eburnean tectono-thermal event, intruded by various types of granitoids, uplifted and eroded. The erosion products were deposited as sediments of the Tarkwaian Group in long, narrow intramontane grabens which formed due to rifting preferentially in the central portions of all five Birimian volcanic belts. Tarkwaian sediments were deformed by gravity tectonics. Two major types of Eburnean granitoids in Ghana are characterized as follows: basin granitoids (= Cape Coast type) occur as synorogenic foliated batholiths chiefly in the central portions of Birimian sedimentary basins; they are peraluminous and generally granodioritic in composition. Belt granitoids (= Dixcove type) mostly form late-orogenic unfoliated intrusions in volcanic belts. They possess a metaluminous character and commonly are of tonalitic composition. Bongo granitoids are of post-Tarkwaian age and K-rich. In contrast to these granitoid types which have strong mantle affinities, the Winneba type had an Archean sialic precursor. A model involving small-scale, equidimensional, parallel, and contemporaneously operating convection cells in the upper mantle which caused rifting of a highly attenuated protocrust as well as linear eruptions of tholeiitic magmas is proposed. Recent Sm-Nd, Pb-Pb and Rb-Sr data of volcanic and plutonic rocks are incorporated. Most Ghanaian Birimian gold occurrences and mines are concentrated in narrow ‘corridors’ of 10–15 km width in the transition zone between volcanic belts and sedimentary basins, as are the chemical facies and regionally extensive shear zones at the volcanics-sediment interface. Birimian gold in Ghana is present as two major types: (1) the disseminated sulphide type which is generally lithofacies controlled, i.e. controlled by chemical sediments, and to a lesser extent by selvages of gold-quartz veins; and (2) the quartz vein type which is exclusively structure controlled. Data on geochemical underground traverses across both types of gold mineralization are briefly summarized.

Platinum-group elements and gold in komatiitic rocks from the Agrilia Formation, Othrys ophiolite complex, Greece

Highly magnesian lavas (32 wt.% MgO) from the Agrilia Formation, Othrys ophiolite complex, central Greece, were analyzed for major and trace elements. Average platinum-group elements (PGE) concentrations (all in ppb) are: Os 3.6, Ir 0.68, Ru 4, Pt 1.2, Pd 8.5, Rh < 0.5 and Au 19. Although the studied lavas from the Agrilia Formation have unusually high Mg, Cr and Ni contents, they differ from boninites in respect to the dominance of olivine phenocrysts, the lack of orthopyroxene and primary hornblende and the preferential association of chromite with groundmass. The higher Pd than Pt concentrations and the lack of detectable Rh appear to be characteristic of the Agrilia ultramafic lavas. The PGE concentrations, the relatively low values of Pd/Ir ratios (∼ 12), the chondrite-normalized PGE patterns — which are almost flat, with exception of a Pt anomaly — and the mineral chemistry of the studied lavas are comparable to typical komatiites, suggesting that these lavas may represent primitive magmas. The derived parental magma for the studied lavas (in equilibrium with Fo 90.5) contains ∼ 17 wt.% MgO and is similar to those which gave rise to modern analogues of the Archean komatiites from Gorgona island, Colombia. However, the variation in Ti/V and Ti/Sc ratios in Agrilia lavas and the enrichment in some incompatible elements (Sr, Rb, Ba, La) may indicate a modification of the magma composition in the mantle source region by a subduction component. The occurrence of the Triassic komatiitic (and boninitic) lavas in combination with tholeiitic lavas with mid-ocean ridge basalt (MORB) affinities of the Othrys complex, may suggest changes in the geotectonic setting during the development of the Othrys complex from a fore-arc (formation of the highly magnesian lavas) to back-arc tectonic setting (formation of the main complex).

Non-depleted sub-continental mantle beneath the Superior Province of the Canadian Shield: Nd-Sr isotopic and trace element evidence from Midcontinent Rift basalts

Midcontinent Rift flood basalts represent a sample of the relatively shallow, sub-continental upper mantle beneath the Canadian Shield at 1.1 Ga. A thick sequence of olivine tholeiite lavas, including minor intermediate to rhyolitic lavas, from the Portage Lake Volcanics (PLV) in northern Michigan have initial Nd and Sr isotopic compositions which cluster near Bulk Earth values. The effects of assimilation of old LREE-enriched continental crust into mantle-derived fractionating liquids are isotopically discernible in evolved lavas as well as in olivine tholeiites from the lowest portion of the volcanic pile. However, the effects of crustal contamination decrease with stratigraphie height and are absent in more primitive lavas in the upper half of the section. Therefore, the youngest olivine tholeiites preserve the isotopic characteristics of their sub-continental mantle source area which, at 1095 Ma, had ϵ Nd (T) and ϵ Sr (T) values of about +0.8 and +2, respectively. Incompatible trace element compositions from the PLV olivine tholeiite suite support the interpretation of a mantle source slightly enriched in LIL elements relative to chondritic compositions as opposed to a suite of hybrid magmas resulting from mixtures between depleted mantle and enriched crustal end-members. The source for PLV tholeiites is substantially less depleted than previously reported mantle values from the Superior Province. An origin for the PLV source is compatible with either of several mantle evolution models. The PLV source may have been associated with upwelling of a LIL element-enriched, asthenospheric plume which emplaced non-depleted material from deeper sources into the shallow subcontinental mantle beneath the Midcontinent Rift during continental break-up. Alternatively, the PLV source may have originated by enrichment of refractory sub-continental lithospheric mantle which was previously depleted in incompatible trace elements during Archean-aged melt extraction and continental crust formation. Concurrent generation of carbonatite magmas in other areas beneath the Superior Province indicates the widespread presence of sub-continental mantle with substantially higher ϵ Nd (T) and lower ϵ Sr (T) than the PLV source. Combined tholeiite and carbonatite data indicates the presence of large, chemically distinct regions in the upper mantle beneath the Superior Province at 1.1 Ga.

The origin of andesite from Rajmahal traps, eastern India: A quantitative evaluation of a fractional crystallization model

Major-element mass balance calculation supplemented by trace-element modelling indicates that the andesite and tholeiitic andesites in the northwestern portion of Rajmahal traps, Bihar, eastern India might have originated by about 50% fractional crystallization of a quartz-tholeiitic magma (represented by the Rajmahal traps), leaving a cumulate phase of plagioclase (An 67–74), clinopyroxene (augite), orthopyroxene (hypersthene), and magnetite in the proportion 23.19:20.38:2.67:3.76. Scanty occurrences of coarse-grained gabbroic rocks within the tholeiitic traps of adjacent areas have been noted; modal, major- and trace-element compositions of these rocks show marked similarities with those of the computed cumulate phase and furnish support to the fractionation model.The temperatures and oxygen fugacities estimated from coexisting iron-titanium oxide equilibration range from 920 to 955°C, and from 10−10.85 to 10−10.35 bar, respectively, for the andesites. The temperature estimated from the two-pyroxene geothermometer for the trap-tholeiites is c. 1120°C, while the pressure estimated from the olivine-orthopyroxene-quartz assemblage is of the order of 9 kbar. However, the oxygen fugacities estimated from the two-oxide geothermometer range from 10−11.1 to 10−10.45 bar and the temperatures from 978 to 1015°C, which is possibly the quenching temperature of the tholeiitic lava.

Basaltic volcanism in Ethiopia: Constraints on continental rifting and mantle interactions

Middle to late Cenozoic mafic lavas from the Ethiopian volcanic province exhibit considerable chemical and isotopic diversity that is linked to eruption age and eruption location. These variations provide a geochemical framework in which continental rifting can be examined. Trace element and Sr, Nd, and Pb isotopic data are interpreted to indicate involvement of up to two depleted and two enriched mantle reservoirs throughout Cenozoic rift development in Ethiopia. Superimposed on the characteristics imparted by varying degrees of melting of these distinct reservoirs are the effects of crystal fractionation and, in some instances, crustal contamination. Initial stages of Oligocene rifting and volcanism, as manifested by the rift‐bounding plateau flood basalts, are attributed to asthenospheric upwelling and melting of a heterogeneous, enriched subcontinental lithospheric mantle. Mildly alkaline lavas were produced from an enriched source with characteristics similar to those of the inferred source of other mantle‐derived lavas and xenoliths from east Africa (LoNd array, EMI to HIMU). Contemporaneous tholeiitic lavas were derived from a source similar to that producing oceanic basalts from Samoa and the Society Islands (EMII). As lithospheric thinning and rifting continued into the Miocene, upwelling depleted asthenosphere (depleted OIB reservoir, PREMA) interacted with the lithospheric sources producing lavas with hybrid elemental and isotopic characteristics (11–6 Ma plateau and rift margin basalts). Crustal contamination is most evident in the Oligocene to Miocene plateau basalts and is suggested to have taken place primarily at middle to lower crustal levels during initial stages of continental rifting. By 4–5 Ma b.p. continental breakup had begun in Afar, with basalts during this period being derived almost entirely from a depleted PREMA‐type reservoir. In the Main Ethiopian Rift, where continental breakup is less advanced, young rift basalts retain a geochemical signature consistent with enriched (LoNd)‐depleted (PREMA) mantle hybridization. During the Holocene, proto‐oceanic crust and oceanic crust characterize the Afar and Red Sea/Gulf of Aden, respectively, and input from a depleted MORB source first becomes apparent. Chronologic and tectonic control on mantle melting, mantle reservoir interactions, and crust‐mantle interactions is a theme common to many extensional regions. Another common feature is the apparent role of a depleted PREMA‐type reservoir in these regions, supporting the idea that this reservoir is located at depth within the convecting asthenosphere. Involvement of enriched mantle during continental extension‐related magmatism is also prevalent, but the geochemical signature of this component varies from region to region, suggesting a strong link to local crust formation history and local enrichment events such as subduction‐driven lithospheric recycling.

Patterns of volcanism along the southern margin of the Colorado Plateau: The Springerville Field

The mostly 2.1‐ to 0.3‐m.y.‐old Springerville volcanic field is the southernmost of the Colorado Plateau‐margin fields and encompasses −3000 km2, with a volume of −300 km3. Mapping documents 409 flow units, most emanating from one of the −400 predominantly pyroclastic cones. Alkali olivine basalt constitutes about one‐half the outcrop area, with hawaiite and tholeiite each composing about a quarter of the area, and mugearite and benmoreite each covering less than 1% of the area. The composition of the lavas shows a progression away from tholeiitic and toward alkalic affinities with decreasing age, in marked contrast to that found in both the southeast and western margins of the Colorado Plateau. Estimates of volume effusion rates show an early eruptive episode dominated by large volumes of tholeiitic lavas, a middle period of alkali olivine basalt eruption, and a late pulse of evolved alkalic rocks. Changes in chemistry [Na2O + K2O, MgO/(MgO + FeO*), K/P] with respect to location of lava sources that might reflect thermal and/or chemical differences appear to be minimal. Eastward migration of alkali olivine basalt volcanism within the Springerville volcanic field is essentially identical to that seen in the San Francisco volcanic field for the same time period, supporting the suggestion of Tanaka et al. (1986) that it may reflect the westward motion of the North American continent above relatively fixed mantle sources. The late Cenozoic migration of volcanic fields toward the Colorado Plateau suggested by Best and Hamblin (1978) and by Luedke and Smith (1978) does not seem to be reflected by vent migrations from 2.1 Ma to the present within the Springerville and San Francisco fields.

Petrology of recrystallized ultramafic xenoliths from Merelava volcano, Vanuatu

Xenoliths in primitive olivine tholeiite lavas from Merelava Volcano, Vanuatu, include recrystallized wehrlites and harzburgites characterized by extremely fine grain size (0.02–2 mm) and equigranular textures. The harzburgites display mineral segregations, have highly variable ratios of ol: opx, minor clinopyroxene and accessory Cr-spinel, and are interpreted as the residues of high degrees of melting of upper mantle peridotite. Annealed Cr-spinel aggregates in harzburgite sample # 31564B enclose numerous small inclusions of sodic sanidine and minor plagioclase, attributed to infiltration of the harzburgite by a residual melt derived from an earlier period of island arc magmatism. The recrystallized wehrlites have high ol/cpx ratios and depleted REE patterns compatible with a cumulus origin. The refractory nature of the phases in both groups of recrystallized xenoliths compares closely with phases in ‘Alpine-type’ peridotites and primitive arc lavas, and is incompatible with compositions of abyssal peridotites. The recrystallized wehrlites give equilibration temperatures of 1070–1130° C and are interpreted as cumulates derived from an earlier period of Vanuatu Arc magmatism. The range of composition displayed by phases in the harzburgites is greater than phase variation in the wehrlites, and reflects a more complex thermal history. Textural, mineralogical, and geothermometric considerations indicate the harzburgites underwent cooling to 800°/900° C before being re-heated to 1000–1100° C by the current magmatic regime. A shallow crustal origin for these xenoliths is indicated by gravity data and tectonic considerations which strongly imply the presence of an ophiolite body beneath Merelava, representing the northward extension of the Pentecost Ophiolite. These interpretations are compatible with a published model for generation of the host basalts by partial melting at the crust/mantle boundary (ca. 17 km). Sr isotopic data show that the harzburgites are incompatible with residues of ocean-floor magmatism, or with residues of Merelava and Central Chain magmatism, but suggest an affinity with Vitiaz Arc magmatism of Eocene-lower Miocene age. Both groups of xenoliths were apparently entrained from wall rocks during ascent of the host magmas.

Origin of Sr, Nd and Pb isotopic systematics in high-Sr basalts from central Arizona

Alkalic and tholeiitic basalts were erupted in the central Arizona Transition Zone during Miocene-Pliocene time before and after regional faulting. The alkalic lava types differ from the subalkaline lavas in Sr, Nd and Pb isotopic ratios and trace element ratios and, despite close temporal and spatial relationships, the two types appear to be from discrete mantle sources. Pre-faulting lava types include: potassic trachybasalts (87Sr/86Sr = 0.7052 to 0.7055, ɛNd= −9.2 to −10.7); alkali olivine basalts (87Sr/ 86Sr = 0.7049 to 0.7054, ɛNd= −2 to 0.2); basanite and hawaiites (87Sr/86Sr = 0.7049 to 0.7053, ɛNd= −3.5 to −7.8); and quartz tholeiites (87Sr/86Sr = 0.7047, ɛNd= −1.4 to −2.6). Post-faulting lavas have lower 87Sr/86Sr (<0.7045) and ɛNd from −3.2 to 2.3. Pb isotopic data for both preand post-faulting lavas form coherent clusters by magma type with values higher than those associated with MORB but within the range of values found for crustal rocks and sulfide ores in Arizona and New Mexico. Pb isotopic systematics appear to be dominated by crustal contamination. Effects of assimilation and fractional crystallization are inadequate to produce the Sr isotopic variations unless very large amounts of assimilation occurred relative to fractionation. It is impossible to produce the Nd isotopic variations unless ancient very unradiogenic material exists beneath the region. Moreover the assumption that the alkalic lavas are cogenetic requires high degrees of fractionation inconsistent with major- and trace-element data. Metasomatism of the subcontinental lithosphere above a subduction zone by a slab-derived fluid enriched in Sr, Ba, P and K could have produced the isotopic and elemental patterns. The degree of metasomatism apparently decreased upward, with the alkalic lavas sampling more modified regions of the mantle than the tholeiitic lavas. Such metasomatism may have been a regional event associated with crustal formation at about 1.6 Ga. Disruption and weakening of the subcontinental lithosphere in the Transition Zone of the Colorado Plateau by volcanism probably made deformation possible.

伊豆・小笠原孤の横断掘削 ＯＤＰ１２６節成果報告

The Izu-Bonin intra-oceanic island arc-trench system is a product of the subduction of Pacific lithosphere since the Eocene. The purposes of Ocean Drilling Program (ODP) Leg 126 were to study the following three important and poorly understood aspects of this system, namely (1) the origin and evolution of the forearc basin, investigated by drilling a series of holes through the sediment piles and into the basement of the forearc basin (Sites 787 and 792-793); (2) the process and products of arc rifting, investigated by drilling holes into the center (Sites 790/791) and eastern footwall (Sites 788/789) of the Sumisu backarc depression ; and (3) recycling of subducted lithosphere and evolution of the mantle, investigated indirectly from the composition of the volcanic rocks as well as volcaniclastics recovered from all sites.The principal drilling results of Leg 126 show that the forearc basin formed about 35-30 Ma by separation of the formerly contiguous frontal and outer arc highs. Igneous basement beneath the center of the forearc basin includes high-Mg andesites, andesites with boninitic affinities, and low-Mg tholeiitic lavas. The forearc basin was rapidly buried immediately after its formation with volcaniclastic turbidites and debris flow deposits in the late Oligocene age. Following a minimum of volcanic output between 24 and 13 Ma, there has been a steady increase in explosive volcanic materials in the forearc with a dramatic increase in the late Quaternary. Paleomagnetic evidence shows that the forearc has been northwardly translated about 15° since 30 Ma. Benthic foraminiferal assemblage data suggest 1-2km of basement uplift has occurred since the mid-Oligocene. Pore waters in the Oligocene volcanogenic sediments of the forearc basin are the most extensively altered pore water of seawater origin ever sampled by DSDP/ODP. Low-temperature alteration of the volcanogenic sediments has produced fluids extremely enriched in Ca and depleted in Mg, Si and sulfate.The present stage of rifting together with deep sea explosive volcanic eruption of the basaltic mousse in the backarc Sumisu Rift at 31°N began between 3.56 and 1.1 Ma with both present-day and pre-rift volcanism along the volcanic front dominated by rhyolitic pumice eruptions. The footwall of the Sumisu Rift has been uplifted 200-1700m, and rift basement depth prior to 1.1 Ma exceeded 2 km. The basement of the rift is formed by early rift basaltic lavas and intrusives, as well as by arc pyroclastics metamorphosed to zeolite to greenschist facies. Bimodal volcanic activity with intra-rift basaltic eruptions (basaltic mousse) and rhyolitic eruptions was common but explosive arc volcanic activity dramatically increased 200 ka. at the Sumisu and South Sumisu Calderas along the volcanic front. Unlike the forearc region, fluids other than sea water are not circulating locally through the sediments in the Sumisu Rift.

The death of an accretion zone as evidenced by the magmatic history of the Sumail ophiolite (Oman)

The ophiolitic crustal sequence of the Salahi block in the northern part of Oman shows a complex magmatic history. The oceanic accretion episode led to the formation of a main plutonic sequence consisting of layered cumulates, isotropic gabbros, a sheeted dike complex and a first tholeiitic lava series (V1) depleted in compatible elements. The second magmatic episode immediately following the accretion stage is illustrated by predominantly ultrabasic intrusions crosscutting the main plutonic sequence and by a second volcanic suite (V2) directly linked, as can be seen from field evidence, to more differentiated high-level intrusive plutons. This volcanic unit includes very primitive members and is in general characterized by depletion in incompatible elements. A third volcanic event (V3) exhibiting a transitional character enriched in LREE's. caps the ophiolite. These V3 lavas are separated from the V2 unit by a 15 m thick pelagic sedimentary section. A geotectonic evolution of the Sumail ophiolite is proposed on the basis of field, mineralogical and geochemical data. After the oceanic accretion phase that gave birth to the main ophiolitic sequence, a second-stage melting of the upper mantle is suggested to explain the genesis of the second magmatic episode. Such a model accounts well for the depleted character of the V2 lavas. The compression linked to the beginning of the intra oceanic detachment allows the extraction from the mantle of these second-stage melts which are then emplaced in the crust and partly extruded. The V3 volcanism postdates the intraoceanic detachment of the ophiolite, but precedes its obduction onto the Arabian continental margin. It is interpreted as an intraplate seamount-type magmatism.

A magmatic model of Medicine Lake Volcano, California

Medicine Lake volcano is a Pleistocene and Holocene shield volcano of the southern Cascade Range. It is located behind the main Cascade arc in an extensional tectonic setting where high‐alumina basalt is the most commonly erupted lava. This basalt is parental to the higher‐silica calc‐alkaline and tholeiitic lavas that make up the bulk of the shield. The presence of late Holocene, chemically identical rhyolites on opposite sides of the volcano led to hypotheses of a large shallow silicic magma chamber and of a small, deep chamber that fed rhyolites to the surface via cone sheets. Subsequent geophysical work has been unable to identify a large silicic magma body, and instead a small one has apparently been recognized. Some geologic data support the geophysical results. Tectonic control of vent alignments and the dominance of mafic eruptions both in number of events and volume throughout the history of the volcano indicate that no large silicic magma reservoir exists. Instead, a model is proposed that includes numerous dikes, sills, and small magma bodies, most of which are too small to be recognized by present geophysical methods.

Origin of differentiated lavas at Kilauea Volcano, Hawaii: Implications from the 1955 eruption

Kilauea's 1955 eruption was the first major eruption (longer than 2 days) on its east rift zone in 115 years. It lasted 88 days during which 108 × 106 m3 of lava was erupted along a discontinuous, 15-km-long system of fissures. A wide compositional range of lavas was erupted including the most differentiated lavas (5.0 wt% MgO) from a historic Kilauea eruption. Lavas from the first half of the eruption are strongly differentiated (5.0–5.7 wt% MgO); later lavas are weakly to moderately differentiated (6.2–6.7 wt% MgO). Previous studies using only major-element compositions invoked either crystal fractionation (Macdonald and Eaton 1964) or magma mixing (Wright and Fiske 1971) as models to explain the wide compositional variation in the lavas. To further evaluate these models detailed petrographic, mineralogical, and whole-rock, major, and trace element XRF analyses were made of the 1955 lavas. Plagioclase and clinopyroxene in the early and late lavas show no petrographic evidence for magma mixing. Olivines from both the early and late lavas show minor resorption, which is typical of tholeiitic lavas with low MgO contents. Core-to-rim microprobe analyses across olivine, augite, and plagioclase mineral grains give no evidence of disequilibrium features related to mixing. Instead, plots of An/Ab vs distance from the core (D) and %Fo vs (D)4.5 generated essentially linear trends indicative of simple crystal fractionation. Least-squares, mass-balance calculations for major- and trace-element data using observed mineral compositions yield excellent results for crystal fractionation (sum of residuals squared <0.01 for major elements, and <5% for trace elements); magma mixing produced less satisfactory results especially for Cr. Furthermore, trace-element plots of Zr vs Sr, Cr, and A12O3 generate curved trends indicative of crystal fractionation processes. There is no evidence that mixing occurred in the 1955 lavas. Instead, the data are best explained by crystal fractionation involving a reservoir that extends at least 15 km along Kilauea's east rift zone. A dike was intruded into the rift zone from the summit reservoir eight days after the eruption started. Instead of causing magma mixing, the dike probably acted as a hydraulic plunger forcing more of the stored magma to be erupted.

Tertiary volcanism in the Kangerdlugssuaq region, E Greenland: trace-element geochemistry of the Lower Basalts and tholeiitic dyke swarms

Summary Tholeiitic dyke swarms in the Kangerdlugssuaq area can be divided, like the Lower Basalts, into two broad categories: (1) an early tholeiitic ‘Picrite-ankaramite Series’ (PAS) with relatively high levels of incompatible elements; and (2) a less incompatible-element enriched ‘Tholeiitic Series’ (TS) to which the majority of the dykes belong. The latest of the tholeiitic dykes tend to have the lowest incompatible element contents. The PAS dyke samples have steep light-enriched rare earth patterns and strongly fractionated spidergrams ([Nb/Yb] N &gt;20), with no conclusive elemental evidence of significant crustal contamination. TS dykes have less steep rare earth element (REE) patterns and spidergrams, the latter having negative Sr anomalies. Corresponding PAS and TS lavas have similar patterns, although the Nb/La, Ta/La and P/Nd ratios are significantly lower. The PAS dyke samples are more enriched in highly incompatible elements than typical ocean island basalt (OIB) tholeiites. Derivation of these magmas from a sub-lithospheric OIB source, such as that presently contributing to Icelandic volcanism, is tenable only if the picritic magmas underwent subsequent enrichment by incorporation of incompatible elements from sub-continental lithosphere or from continental crust. It is difficult to derive the observed enrichment entirely from the continental crust. The alternative is to envisage an OIB-source jet (the precursor to that currently situated under Iceland) that penetrates upward into the sub-continental lithosphere, and supplies parental picritic magmas which, in passing to the surface, scavenge incompatible elements from enriched lithospheric mantle. Later tholeiitic lavas and dykes tend to be characterized by less steep incompatible-element profiles, resulting in two late dykes with spidergrams identical to sub-aerial Icelandic tholeiites. This trend may be interpreted in terms of a decreasing contribution of highly incompatible elements from the progressively thinner lithosphere.

Geochemical evolution of the Juan Fernandez Islands, SE Pacific

The two main islands of the Juan Fernandez group, some 180 km apart are intra-plate volcanoes related to a single hot spot. K-Ar determinations suggest an age difference of c. 3 Ma which is in accord with a spreading rate of c. 6 cm a −1 in this part of the Nazca plate. The younger island of Alexander Selkirk is a relatively recent volcano (&lt;1 Ma) composed of olivine tholeiites, many of them picrites, together with occasional quartz tholeiites and rare trachytes. Robinson Crusoe is an older, deeply dissected island with several separate volcanic centres. The older lavas are predominantly olivine tholeiites, again often picritic, with an occasional quartz tholeiite flow. Alkali basalt flows, never abundant, become more common in the higher part of the succession. Basanite flows are restricted to younger parasitic centres, but basanite dykes and sills cut the older formations. The tholeiites of Alexander Selkirk have a low pressure phenocryst assemblage and their low Ba contents, low Nb/Zr ratios and REE patterns suggest higher degrees of mantle partial melting than their closest counterparts on Robinson Crusoe. The basanites of the latter island have a higher pressure phenocryst assemblage and have higher La/Sm ratios than the tholeiitic lavas. The basanites are not always distinct from the Robinson Crusoe tholeiites on the basis of incompatible trace elements. 86 Sr/ 87 Sr ratios are fairly uniform ( c. 0.7035) in all the rock types but 143 Nd/ 144 Nd ratios are more variable, attaining their lowest values in some of the basanites. Trace element and isotopic data suggest remelting of source regions modified by migration of small degree partial melts over a long period.

Open System Magmatic Evolution of the Taos Plateau Volcanic Field, Northern New Mexico: II. The Genesis of Cryptic Hybrids

Tholeiitic lavas of the Servilleta Basalt exhibit only subtle textural and mineralogical evidence for a hybrid origin, but elemental and isotopic analyses of these basalts are best modeled in terms of mixing Servilleta parent magma with a range of contemporaneous andesite and dacite magmas. Cryptic compositional heterogeneities in some flows interpreted as hybrids apparently reflect incomplete homogenization following pre-emptive magma mingling. The general scarcity of mixing-related textural disequilibrium is ascribed in part to mixing of mineralogically similar end-members. Eradication of some phenocrysts during post-mixing residence and evolution in a convecting magma body may be an even more important factor. Eruptions of hybrid magmas may frequently be triggered by magma mixing events (i.e. injection or replenishment), and mingling of compositionally diverse magmas may ensue as a consequence of tapping a compositionally graded or layered magma chamber. These hybrids are instantly recognizable by the preservation of disequilibrium textures and mineral assemblages, and by discontinuous compositional heterogeneities. Cryptic hybrids, which have not preserved this record, will be recognizable as mixed magmas primarily by geochemical evidence for open system evolution.

Manganese‐rich rock coatings from Iceland

AbstractManganese‐rich rock coatings are widespread in Iceland, especially in fractures in basalt lavas. Three common types of coating are described. A thick subsurface coating is found on whaleback forms on high plateaux. The most common type is thinner and occurs in hydrothermal clay‐filled fracture systems which are especially well developed in tholeiite lavas. A third type occurs in younger rocks and is associated with tephra‐rich, loessic sediment which fills open fractures and vesicles. There are differences in the chemistry, morphology, and mineralogy of these coatings which are interpreted as being due to the different microenvironments in which they form. The fracture microenvironment is regarded as being of much more importance to coating thickness and chemistry than simple age and in this respect the hydrothermal clays seem to favour rapid manganese concentration. On the other hand, there are variations in coating type, both laterally and vertically through the lava succession, which are related to regional distribution of hydrothermal alteration and rock type. The regional patterns are the result of Iceland's lateral drift and as a result there is an incidental connection between coating type and rock age. There is some evidence to suggest that the manganese coatings may be both forming and being degraded rapidly in the present environment.