Tholeiitic Andesites Research Articles

Chemostratigraphic division of the Blake River Group, Rouyn–Noranda area, Abitibi, Quebec

The effusive rocks of the Blake River Group in the Abitibi volcanic belt, Rouyn–Noranda region, belong to a bimodal sequence in which andesites and rhyolites clearly dominate. The identification of calc-alkaline and tholeiitic affinities is made upon examination of the major, trace, and rare earth element (REE) content. Thus, the andesites (58% normalized SiO2 value without volatiles) of the calc-alkaline units have average K (4800 ppm), Ba (160 ppm), and Rb (13 ppm) values that are greater than the average K (1800 ppm), Ba (130 ppm), and Rb (3 ppm) values for andésites (57% SiO2) that belong to associated tholeiitic units. Furthermore, tholeiitic andesites have distinctive average values of Ti (9600 ppm) and Y (40 ppm) that are higher than the average values of Ti (6700 ppm) and Y (25 ppm) of calc-alkaline andesites. An effective discrimination between the calc-alkaline and tholeiitic affinities is obtained using the Zr/Y and Ti/Zr ratios, which are, respectively, less than 4 and greater than 70 in andesites of the tholeiitic units.REE profiles of tholeiitic andesites are flat when compared with those of calc-alkaline andesites, which show an enrichment in light rare earths.The tholeiitic units of the Blake River Group are found in the proximity of the Porcupine-Destor and Larder Lake – Cadillac faults, the major faults of the region, and at the periphery of an ensemble of calc-alkaline units. Four of the five tholeiitic units are differentiated, showing an enrichment of iron passing from basalt to andesite. These units possess felsic variole-bearing flows, the result of an unmixing, which was probably responsible for the formation of minor associated quantities of porphyries and rhyolitic volcaniclastites.A progressive increase in the concentration of hygromagmatophile elements (REE, Zr, Nb) is observed in the tholeiitic units from the Pelletier unit beginning at the base and passing through the Trémoy, Destor, and Dufresnoy units, at the top of the Blake River Group. The calc-alkaline units are characterized by an alternation of rhyolitic complexes and calc-alkaline andesites. This cyclic repetition occurs without significant modification of the calc-alkaline andesite composition.It is proposed that the volcanism responsible for the formation of the Blake River Group was restricted to a concentric zone centring on a continental environment. Mafic magmas nourished the central reservoir where melting of the sialic crust took place. The rhyolitic magma occupying the upper part of this reservoir mixed with the basaltic magma, producing calc-alkaline andesites. Successive mantle melt products were also emplaced into subsidiary reservoirs peripheral to the central chamber. Injections of tholeiitic magma in the peripheral reservoirs underwent differentiation and unmixing during emplacement of a part of the magma at the surface.

Origin of calc-alkalic andesites, Nasu zone, northeastern Japan: Kuno revisited.

KUNO (1950, 1959) first identified the existence of two rock series in the Japanese volcanic arc and proposed the origin of one from the other by assimilation of granitic material. In the Nasu zone in northeastern Japan, both the tholeiitic suite with pigeonite in the groundmass and the calc-alkalic suite with hypersthene as the Ca-poor pyroxene in the groundmass have been shown to have erupted from one volcano and to have differences in major element chemistries (KAWANO and AOKI, 1960; KAWANO et al., 1961). MASUDA and AOKI (1979), using mainly trace elements, showed that the calc-alkalic suite has higher Mg, K, Co, Cr, Ni, Th, and U than the tholeiitic suite, and also that each suite had a separate fractional crystallization. A model is presented here which is intended to explain both the mineralogical and chemical differences. KUNO's idea of assimilation is considered from a different point of view; that is, calc-alkalic basaltic andesite is formed by the partial assimilation of mafic rather than granitic material by a silicic tholeiitic andesite. Changes in the chemistry, mainly in the Mg/Fe ratio in the contaminated magma, prevents the crystallization of pigeonite; a drop in temperature as KUNO suggested is not as important although lower temperatures will help. It is demonstrated how such elements as K, Th, and U, which tend to be concentrated in granitic material, can be increased in the calc-alkalic rocks by addition of mafic xenoliths. This process is offered as an alternative to magma mixing, since it is believed that the mineralogical features used to support magma mixing could be explained equally well by assimilation.

Geochemical and isotopic evidence for the age, orogenic setting and petrogenesis of the Nychum Volcanic association, North Queensland

Abstract Rb‐Sr and K‐Ar age determinations agree with palaeobotanical evidence in assigning an age of 270 Ma (Lower Permian) to the Nychum Volcanics, North Queensland. Geological and geochemical evidence (moderate ‐REE, La/Yb, Th, Zr/Y, Hf/Yb, Sc/Ni) indicate that the volcanics were erupted on a thin, active continental margin. Five magma groups are present: high‐alumina basalts, andesite‐dacites, acid rocks, tholeiitic andesites, and pitchstone with a high La/Yb ratio. The high‐alumina basalts and calc‐alkaline andesites have a parent‐daughter (source rock‐partial melt) relationship. The acid rocks share high HREE, Zr, Hf, Zn, Sc, Fe/Mg and low Al with, and may be fractionated from, the tholeiitic subduction‐zone andesites. Low‐pressure fractional crystallisation is evident in all five magma groups.

Chemical stratigraphy, Iceland Research Drilling Project, Reydarfjordur, eastern Iceland

Major element data are reported for lavas and dikes from a 3.2‐km section through the eastern Iceland lava pile. The section includes a 1.2‐km subaerial exposure and a 1.9‐km sequence drilled by the Iceland Research Drilling Project (IRDP). Alteration increases down‐section, although primary magma chemistry may be interpreted from FeO* Al2O3, TiO2, P2O5 and (to a lesser extent) SiO2 in addition to immobile trace elements. Lava compositions range from MgO‐rich tholeiite to SiO2‐rich icelandite, while dikes range from tholeiite to tholeiitic andesite. Three stratigraphic divisions based on volcanologic and trace element criteria (e.g., Ce/Yb and Zr/Y) are confirmed by major elements. Upper and lower groups are predominantly basaltic while the middle group includes a high proportion of silicic lavas. Variation within groups conforms to simple models combining fractional crystallization and accumulation of phenocryst plagioclase. During build‐up of the pile, eruptive phases commenced with silica‐rich magma and frequently terminated with basaltic or intermediate plagioclase cumulates. Dike chemistry resembles that of upper group lavas and also the nearby Breiddalur volcano. Middle group lavas probably derive from a buried silicic complex, while the lower group may represent the flank of a more deeply buried complex. Chemical stratigrapy is consistent with iterative burial with spreading of lenticular basaltic‐silicic complexes. Magma supply, fractionation, and mixing are considered in the framework of such kinematic models, although it is premature to draw direct analogies to the active Neogene zone of Iceland.

Petrogenesis of the Tertiary Anorogenic Volcanic Series of Southern Queensland, Australia, in the Light of Trace Element Geochemistry and O, Sr and Pb Isotopes

The magmas of the Tertiary volcanic province of S. Queensland are chemically bimodal, and occur in numerous volcanic centres, at least three representing original shield volcanoes. The mafic lavas are dominantly hawaiites and tholeiitic andesites, whereas the silicic magmas comprise mainly trachytes, rhyolites, and comendites.The silicic rocks exhibit variable trace element abundance patterns. There is a progressive depletion of Sr, Ba, V, Mg, Ni, Cr, Mn, and P, through the trachytes to the rhyolites and comendites while the behaviour of Zr, Nb, LREE, Y and Zn is very variable. Rb, Th, and to a lesser extent Pb exhibit a more regular behaviour, becoming most generally concentrated in the comendites and rhyolites. These trace element patterns are modelled by application of the Rayleigh distillation model, using partition coefficients based on analysed phenocrysts from the S. Queensland silicic lavas. Trace mineral phases, namely zircon, chevkinite, and allanite, are shown to be important in the probable control of LREE, Zr, and Th abundances, while Nb and Zn are probably controlled during fractionation by magnetite. Trace element data for the hawaiites and tholeiitic andesites also indicate extensive although variable levels of fractional crystallization of these magmas.The Sr and O isotopic compositions of the mafic lavas, trachytes, comendites and rhyolites are as follows: initial 87Sr/86Sr ratios; 0.70357-0.70456, 0.70432-0.70589, 0.70495-0.70917, and 0.70708-0.70863 respectively. δ18O range between 5.6-7.0 (mafic lavas), 4.9-8.7 (trachytes), 5.0-7.6 (comendites) and 8.1-10.4 per mil (rhyolites). Pb isotopic compositions are variable, showing a variation of 6.7 per cent for 206Pb/204Pb ratios through the range of volcanic compositions. The rhyolites exhibit a much greater divergence in their O, Sr, and Pb isotopic compositions compared with those of associated mafic lavas, than is found in the trachytes and comendites. Within the silicic volcanics, positive correlations exist between δ18O and initial Sr ratios, and between Pb isotopic compositions and initial Sr ratios (with one group of trachytes providing a noteworthy exception). These correlations are not so clearly defined for the mafic lavas, although these do exhibit positive correlations between differentiation index, δ18O, and initial Sr isotope ratios.The development of the silicic magmas, excepting two groups, is interpreted in terms of a model in which assimilation and fractional crystallization occur concurrently, involving a basalt or hawaiite magma component and a crustal component (modelled on the analysed Carboniferous basement greywackes outeropping in the region); the data indicate, however, that differentiation continued in isotopically closed systems (i. e. isolated from the wallrocks). The highly depleted Sr and Ba abundances of the rhyolites and comendites suggest that contamination did not occur after differentiation had ceased. The rhyolites have the highest isotopic input of the crustal components and are interpreted as crustal anatectic melts, produced locally within the crust in response to basalt/hawaiite magma intrusion, whereas most of the trachytes and comendites are interpreted as primarily the differentiated products from original mafic parental magmas, with variable assimilation of crustal wallrock components. The isotopic data suggest that only the Minerva Hills trachytic lavas, and a Glass House comendite, have not been significantly modified by wallrock assimilation processes. The erpted mafic magmas were also evidently modified by isotopic crustal wallrock interactions, which independent petrological data suggest has occurred at intermediate to lower crustal depths.

Petrogenesis of Lewisian amphibolite dykes from Clashnessie Bay, Near Stoer, Sutherland

Synopsis Six thin amphibolite dykes cut the c. 2 7 b.y. old Scourian gneisses in Clashnessie Bay, near Stoer, Sutherland. They have a different orientation to the Scourie dykes, a prominent c. 2.4 b.y. old tholeiitic dyke swarm. These amphibolite dykes were intruded, metamorphosed and folded some time between the end of the Scourian granulite facies metamorphism and the intrusion of the Scourie dykes. They are possibly associated with the Inverian period of folding and retrogression. They are chemically distinct from Scourie dykes and have a very uniform composition being low-K tholeiitic andesites with a SiO 2 content of 54–57 wt%. They could have formed from a tholeiitic magma by hornblende fractionation.

The mineral chemistry of dolerites and gabbros from the central Complex of Slieve Gullion, NE Ireland

SynopsisThe dolerites and gabbros of the Tertiary central complex of Slieve Gullion, NE Ireland, represent a suite of tholeiitic magmas (varying from tholeiitic basalt to tholeiitic andesite, Gamble 1979a) which crystallized in the root-zone of a shield volcano. The ubiquity of biotite suggests that the magmas were hydrous and, in addition, both silicate (olivine) and oxide (Ti-magnetite) phases show evidence for oxidation at high temperatures.

Magmatism of the British Tertiary Volcanic Province

Synopsis This review concentrates on those aspects of research into the magmatism of the British Tertiary Volcanic Province (BTVP) which have seen rapid accumulation of data and changes of views during the last few years. There seems to be general agreement at present amongst those who have made detailed geochemical studies of the problem that the pervasive hydrothermal metamorphism which affects most BTVP igneous rocks has not caused significant changes in the compositions (other than hydrogen and oxygen isotopes) of samples collected away from the immediate vicinities of the major passageways for hydrothermal fluids. Wide ranges of silica saturation characterize the basic members of both the major traditional BTVP magma types—Plateau and Non-Porphyritic Central (NPC). It is only the evolved members of these suites, the ne-normative Plateau-type hawaiites and the Q-normative NPC tholeiitic andesites, which show clearly the long-emphasized contrast between alkalic and tholeiitic compositions. Prior to their final uprise and eruption or emplacement, the Plateau-type magmas appear to have equilibrated near the base of the continental crust, whereas the NPC magmas evolved at upper-crustal depths. The NPC basalts (as traditionally defined) seem to be polygenetic in terms of mantle-derived magma types. Most of them are related to the MORB(Mid Ocean Ridge Basalt)-like Preshal Mhor basalts of Skye and Mull but some appear to be batches of Plateau-type magma which equilibrated in the upper crust. The Rio Grande Rift, southwestern U.S.A., provides a present-day analogue to the Palaeocene magmatic plumbing envisaged beneath the BTVP. The mantle-derived basic magmas upwelling beneath the BTVP were affected by fractional crystallisation and sialic contamination to varying extents as they rose through the continental crust. The earlier magma batches mostly interacted only with granulite-facies Lewisian gneisses, from which they appear to have melted selectively the most-fusible minor acid members. In addition, there is little doubt that Pb equilibrated selectively, in a vapour or non-silicate liquid phase, between the magmas and Archean sial, whilst Sr- and Nd-isotope ratios in the BTVP igneous rocks may also be in part the products of selective magma-crust interactions. If those BTVP basic rocks which contain negligible components of sialic contaminant are considered, it is apparent that there are consistent differences in various incompatible-element ratios between the Plateau-type basalts of Skye and Mull, the two areas studied in most detail at present. All of the BTVP Plateau-type basalts are exceptionally depleted in the strongly-incompatible elements, relative to worldwide examples of basalts with similar major-element compositions. This feature is consistent with genesis from a volume of upper mantle which had previously lost a very small fraction of strongly-alkalic melt. The Permian lamprophyres of the region are tentatively identified as samples of this postulated pre-Tertiary alkalic magma. A case is also made for considering the Caledonian Iapetus Suture as the tectonic feature along which most of the BTVP igneous centres are distributed. Seen from this point of view, the origins of BTVP magmatism are to be found in upper-mantle convection during and after the subduction which closed Iapetus—subsequently re-activated by the regional tension and Palaeocene mantle convection associated with the opening of the North Atlantic. At the climax of BTVP magmatism, when the bulk of the dyke swarms and the plutons of the central complexes were emplaced, a final increment of the progressive partial fusion of the upper mantle beneath the region gave rise to the MORB-like Preshal Mhor-type magmas. Although it is apparent from their Sr-, Nd- and Pb-isotope ratios that most of the BTVP acid magmas contained appreciable fractions from sialic sources, there are few instances where mantle-derived magma does not appear to have been the dominant ingredient in the hybrid liquids which evolved—by fractional crystallisation—to acid residua. Nevertheless, when the sequence of granite emplacement within individual BTVP intrusive centres is considered, it is apparent that the evolution of each acid magma batch was a complex and variable process, involving combinations of such mechanisms as fractional crystallisation, fusion of both sialic crust and earlier BTVP acid rocks, and mixing of magmas at various states of evolution.

Petrology of tholeiitic lavas from Tanna Island (New Hebrides): Importance of cumulative processes in Island arc magmatism

Tanna, one of the southernmost islands of the New Hebrides volcanic arc, is made of Late Pliocene to Recent island arc tholeiitic basalts and andesites, with SiO2 contents ranging from 45 to 57%. These lavas are highly porphyritic (30–50% in volume): phenocrysts of plagioclase are the most abundant, together with olivine and clinopyroxene. The groundmass contain plagioclase, augite, olivine, magnetite and glass; pigeonite, tridymite, sanidine and, rarely, biotite may also occur. The olivines and clinopyroxenes show an iron enrichment from the cores of phenocrysts to their rims and the groundmass crystals, but their compositional variations are not correlated with the Mg/Fe ratio of bulk host rocks, the most Fe-rich compositions being found in Mg-rich lavas. Plagioclase compositions range from An95 to An60 in the basalts and An60 to An50 in the andesites, but, within each group, they are not correlated with SiO2 or Na2O contents of host lavas. Consequently, the bulk major element compositions of Tanna volcanic rocks cannot be considered as primarily controlled by crystal separation from successive liquids. The oxyde-SiO2 variations diagrams, and the modal compositions and mineral chemistry show that crystal accumulation is the predominant mechanism accounting for bulk rock compositions. However, this does not exclude fractional crystallization: the variation of the calculated groundmass mineralogy strongly suggest the occurrence of crystal removal mainly clinopyroxene and magnetite.

The mineralogy and chemistry of the anorogenic tertiary silicic volcanics of S.E. Queensland and N.E. New South Wales, Australia

The Late Oligocene‐Early Miocene volcanism of this region is chemically strongly bimodal; the mafic lavas (volumetrically dominant) comprise basalts, hawaiites, and tholeiitic andesites, while the silicic eruptives are mainly comendites, potassic trachytes, and potassic, high‐silica rhyolites. The comendites and rhyolites have distinctive trace element abundance patterns, notably the extreme depletions of Sr, Ba, Mg, Mn, P, Cr, V, and Eu, and the variable enrichment of such elements as Rb, Zr, Pb, Nb, Zn, U, and Th. The trachytes exhibit these characteristics to lesser degrees. The comendites are distinguished from the rhyolites by their overall relative enrichment of the more highly charged cations (e.g., LREE, Nb, Y, and especially Zr) and Zn. The phenocryst mineralogy of the trachytes and rhyolites comprises various combinations of the following phases: sodic plagioclase (albite‐andesine), calcic anorthoclase, sanidine, quartz, ferroaugite‐ferrohedenbergite, ferrohypersthene, fayalitic olivine, ilmenite, titanomagnetite, and rarely biotite (near annite) and Fe‐hastingsitic amphibole. Accessories include apatite, zircon, chevkinite (ferrohedenbergite‐bearing rhyolites only), and allanite (amphibole and botite rhyolites only). The comendites generally contain Ca‐poor anorthoclase‐sanidine, quartz, fluorarfvedsonite, aegirine and aegirine‐augite (Zr‐bearing), aenigmatite, and ± ilmenite. Coexisting Fe‐Ti oxides are absent in the comendites and relatively uncommon in the rhyolites and trachytes. Where present, they indicate equilibration temperatures of 885°–980°C and fo2 between QFM and WM buffers. The magmas are thus interpreted to have been strongly water undersaturated during phenocryst equilibration, which is also consistent with the general paucity of pyroclastics, the rarity of hydrous mineral phases, and the extreme Fe‐enriched ferromagnesian phenocryst compositions. The chemical and mineralogical data are interpreted to indicate the operation of extreme fractionation processes controlling the development of the silicic magmas, and the comendites, trachytes, and certain trachyte‐rhyolite series are considered to have evolved from a mafic parentage. The available oxygen, Sr, and Pb isotopic data, however, point to some modification of the magnas through crustal equilibration processes. The remaining high‐silica rhyolites are considered to most likely represent crustal partial melts but are again modified by extensive fractionation.

The pre-basaltic sediments and the Lower Basalts at Kangerdlugssuaq, East Greenland: their stratigraphy, lithology, palaeomagnetism and petrology

This paper presents a new 1:40 000 topographic and geological map of the area around Miki Fjord and l. C. Jacobsen Fjord, East Greenland. The post-Precambrian sedimentary succession, the Kangerdlugssuaq Group, begins with the Ryberg Formation (Campanian to Danian (?)) and continues into the Vandfaldsdalen Formation of Late Paleocene age. These sediments were laid down in a basin which subsequently became filled and covered by basaltic rocks of the Blosseville Group, including lavas, hyaloclastites, tuffs and breccias. Considerable facies variations are apparent in both the sediments and volcanics indicating that the basin deepened in an easterly direction. Palaeomagnetic measurements confirm previous results for the Blosseville Group on the Lower Basalts but do not yet entirely eliminate the possibility that some of the succession has normal polarity. Petrographically the volcanics include picrites (oceanites and ankaramites), olivine tholeiites, tholeiites and tholeiitic andesites. They have almost all suffered alteration up to greenschist facies and some show evidence of sedimentary contamination. They are all of tholeiitic affinity and are Fe and Ti enriched when compared to normal ocean ridge basalts. They are however much more variable in composition than the overlying Plateau Basalts and have not been produced in such large volumes. It is suggested that a primary picritic magma gave rise to the oceanites and ankaramites by olivine and clinopyroxene fractionation and accumulation. The olivine tholeiites, which appear to be separated from the picrites by a compositional gap, may be derived from a different parental magma. Petrological parallels are drawn with other provinces.

Rare earth patterns for tholeiitic andesite and its associated calc-alcali andesite.

Rare earth elements were determined by isotope dilution method for calc-alkali andesite from Hakone volcano and for tholeiitic basic andesite, tholeiitic andesite and calc-alkali andesite from Iwate volcano. Using previously reported REE data for tholeiitic basalt and andesite form Hakone volcano, petrogenetic relationships between tholeiitic andesite and closely associated calc-alkali andesite were discussed. The REE data suggest that tholeiitic andesite and associated calc-alkali andesite were derived from a common parental magma. However, associated calc-alkali andesite is considered to have been affected by slight contamination. The slight contamination could not widely change the original REE pattern of the parental magma.

Geochemistry of a continental tholeiite suite: late Palaeozoic quartz dolerite dykes of Scotland

AbstractThe late Carboniferous quartz dolerite suite of Scotland consists mainly of quartz tholeiites, with subordinate olivine tholeiites and tholeiitic andesites. The low pressure evolution of the magmas was controlled by fractionation of olivine–plagioclase–pyroxene–oxides assemblages from more magnesian compositions and plagioclase–pyroxene–oxides–apatite removal from intermediate compositions. A higher pressure stage dominated by olivine fractionation is suggested by the presence of olivine nodules in a magnesian basalt dyke from Fife.The suite is of high-Fe-Ti type, closely comparable to certain basalts erupted in areas of active lithospheric spreading or “hot spots”, such as Iceland and Hawaii. The ppO2 can be inferred to have been rather higher in the Scottish rocks than in comparative suites, promoting earlier separation of Fe-Ti oxides, with the consequent effects on trace element distribution.Apart from varying degrees of fractionation, chemical variations in the dykes are of three types: rather minor variations along individual dykes, variations across certain thicker dykes, and minor and trace element variations reflecting chemical heterogeneities in the mantle sources. The dykes and sills are inferred to have been fed from a plexus of small, partly independent, magma reservoirs.New trace element data on tholeiitic lavas from the Oslo Rift confirm recently revived suggestions that the Scottish and northern English quartz dolerites are part of a larger province extending into Scandinavia.

Geology of Sierra del Gallego Area: Comparison with Pena Blanca Uranium District, Mexico: ABSTRACT

A 1,000-m section of volcanic rocks overlying Lower Cretaceous limestone in the Sierra del Gallego area 200 km south of El Paso can be divided into four lithologic sequences. About 44 to 38 m.y. ago, at least five separate crystal-poor sanidine-bearing outflow units of rhyolitic tuff covered parts of the area. The tuffaceous activity was closely followed by lava flows of fritted feldspar-bearing tholeiitic andesite. These flows were followed about 37-35 m.y. ago by a sequence of rhyolite flows, flow domes, and intrusions. A 6-m.y. period of quiescence was followed about 29 m.y. ago by massive outpourings of basalt with minor associated rhyolite tuff. Parts of the oldest sequence are probably correlative with stratigraphic units in the Pena Blanca area in New Mexico. The P zos limestone conglomerate at the deposit has a counterpart in the Sierra del Gallego area. Petrographic similarities, thickness relations, geodes, and K-Ar dating directly link the H member in the lower part of the Liebres Formation in the Sierra del Gallego area with the Nopal Formation at Pena Blanca. The Pozos, Nopal, Escuadra, Pena Blanca, and Mesa Formations are, as a group, lithologically and chronologically similar to the oldest sequence (the Liebres Formation) of the Sierra del Gallego area. End_of_Article - Last_Page 755------------

Icelandite and aenigmatite-bearing pantellerite from the McDermitt caldera complex, Nevada-Oregon

Intermediate lavas from the middle Miocene McDermitt caldera complex, Nevada-Oregon, have very high FeO* (= total Fe as FeO; 9.1 to 10.2 wt %) and low MgO (0.4 to 2.1 wt %) contents at SiO 2 contents of 59.5 to 62 wt %. With the exception of significantly higher K 2 O contents (3.1 to 4.7 wt %), these lavas are compositionally very similar to icelandite (“tholeiitic andesite”) from oceanic areas. In addition to the voluminous comendite tuffs of the complex, a thin unit of densely welded crystal-rich pantellerite tuff of probable air-fall origin is exposed on the northern wall of the McDermitt caldera. The pantellerite tuff contains 1.5% aenigmatite phenocrysts. The high FeO*/MgO ratios of the icelandites and the peralkaline nature of the silicic rocks suggest differentiation under conditions of low f O 2 and f H 2 O. The existence at McDermitt of a continuous series of rocks ranging from potassic icelandite to comendite and pantellerite is consistent—as are other geologic, geochemical, and geophysical data—with the derivation of the icelandites and ultimately of the voluminous peralkaline silicic rocks of the complex from mafic magma generated from upwelling mantle material. Labile U released during the crystallization of late subvolcanic bodies of peralkaline silicic magma may have provided an appreciable proportion of the epigenetic U now present in the upper part of the caldera complex. Reducing conditions produced by the very Fe-rich glassy icelandites may have contributed to the fixation of U at the Aurora and Bretz prospects.

Trace element and strontium isotope characteristics of volcanic rocks from Isla Tortuga: a young seamount in the Gulf of California

Isla Tortuga is a small isolated central volcano which is located near an actively spreading trough in the Gulf of California. The basalt lavas from Tortuga which have the highest Mg/Fe and Ni contents have trace element abundances and ratios and 87Sr/ 86Sr which are similar to those of mid-ocean ridge tholeiite. The major element, rare earth element and Sr abundances of fractionated tholeiite (low Mg/Fe) and tholeiitic andesite of Tortuga are consistent with an origin by closed-system fractional crystallization. This hypothesis is not supported by K, Na, Rb and Ba abundances in the lavas nor by their variable 87Sr/ 86Sr (0.7024–0.7035). It is proposed that the apparent decoupling of light rare earth elements, other incompatible trace elements and 87Sr/ 86Sr is due to contamination of some Tortuga magmas while they are fractionated in a high-level crustal magma chamber. The mantle source of least-contaminated, high Mg/Fe basalt lavas of Tortuga is similar, although not identical to the source of normal mid-ocean ridge tholeiite; significant differences exist. The reasons for these differences are not yet known.

The geochemistry and petrogenesis of dolerites and gabbros from the Tertiary central volcanic complex of Slieve Gullion, North East Ireland

Basaltic magmas emplaced into the root zone of the Slieve Gullion volcano have crystallised to rocks varying in texture from dolerite to gabbro. A mineralogical variation from olivine bearing to quartz bearing varieties has been recorded and with the presence of coexisting Ca-rich and Ca-poor pyroxenes, a tholeiitic assemblage is indicated. Geochemical data confirm this prognosis and a continuous spread of compositions from basalt to tholeiitic andesite are interpreted in terms of fractional crystallisation. Modelling of the fractional crystallisation processes indicate an approach to cotectic conditions with fractionation at low pressures involving olivine, plagioclase, clinopyroxene and Ti-magnetite. Primitive magma compositions, indicated by low values of D.I. and 100 Mg/Mg + Fe2+ (atomic)>61, show low concentrations of the large ion lithophile elements such as Rb, Ba, Zr, Y, and K. The relatively high CaO content (>11 %) of these rocks invite comparison with high-calcium low-alkali tholeiitic liquids recognised in extrusive and intrusive magmas elsewhere in the North Atlantic Tertiary volcanic province and with magmas currently erupted at active spreading ridges. In north west Britain the field and stratigraphic distribution of these high-calcium low-alkali magmas suggests that they occupy a distinct chronological niche towards the top of the Palaeocene-Eocene volcanic succession succeeding eruption of mildly alkali and transitional basalts. As such, the refractory (high CaO, MgO, etc.) and large ion lithophile depleted geochemistry can be explained either by differential partial melting in the upper mantle source region or melting of a depleted and refractory mantle source which has already contributed to basalt genesis.

Volcanic rocks of the Witu Islands, Papua New Guinea: The origin of magmas above the deepest part of the New Britain Benioff zone

The Witu Islands are Quaternary volcanoes that overlie the deepest (about 300–580 km) part of the New Britain Benioff zone. The islands are about 100 km south of the transcurrent-divergent plate boundary that crosses the Bismarck Sea, and they surmount the southeastern end of the Willaumez-Manus Rise. The rocks are olivine- and quartz-normative tholeiitic basalts, low- and high-SiO2 andesites, dacites, and rhyolites. Alkaline rocks that overlie the deep (greater than 300 km) parts of other Benioff zones have not been found in the Witu Islands. Compared to the Witu Islands rocks, those with similar SiO2 contents from New Britain volcanoes that overlie progressively shallower parts of the Benioff zone to the south, are, for example, generally poorer in Na+K, Ti, and P, and higher in Ca and Al. There are similar progressive changes in trace-element abundances, but Zr and Nb contents are distinctly richer in Witu Islands rocks.87Sr/86Sr values range between 0.70311 and 0.7038, which are typical for rocks from New Britain as a whole and from other island arcs in the southwest Pacific. Two143Nd/144Nd values of 0.512211 and 0.512271, taken together with the Sr isotopic results, define a source region equivalent to those for oceanic-island basalts; there is no evidence for sea-water contamination of the sources.

Stratigraphy of the Lamington Volcanics in far Northeastern New South Wales

Abstract The stratigraphic nomenclature for the Lamington Volcanics in the southern part of the Tweed Shield Volcano is revised. An alkaline formation (the Kyogle Basalt) containing discontinuous rhyolitic‐agglomerate (the Homeleigh Agglomerate Member) and rhyolite flow (the Georgica Rhyolite Member) horizons is the basal unit of the volcanic succession. The Kyogle Basalt thins eastward where it is overlain by a subalkaline sequence comprising the Lismore Basalt, Nimbin Rhyolite, and Blue Knob Basalt respectively. The Lismore Basalt is composed predominantly of tholeiitic andesite with minor icelandite and sporadic flows of mildly alkaline rocks; tholeiitic andesites predominate in the Blue Knob Basalt. The Nimbin Rhyolite consists of rhyolite flows, and a relatively minor pyroclastic component (principally lapilli tuff).

Geology, petrology, and geochemistry of Isla Tortuga, a recently formed tholeiitic island in the Gulf of California

Isla Tortuga is a recently formed volcanic island located on a fracture zone of an active spreading trough in the Gulf of California. The volcano is composed of tholeiitic basalt lava flows and vitric tuffs and minor volumes of tholeiitic andesite. It was built in at least two stages by a northward-migrating point source of volcanism. The latest stage of activity culminated in caldera collapse, extrusion of the surficial flows, and the formation of a lava lake. The suite of basalts and Fe-Ti–rich basalts on Tortuga is chemically very similar to suites of ocean-ridge tholeiite. The chemical variation observed in the Tortuga basalts can be explained by removal of the low-pressure phase assemblage Pl + Cpx + Ol from a relatively “primitive” (high Mg/Fe) tholeiitic basalt parental magma. The chemical variation of coarse-grained basaltic pegmatoidal segregations, which occur in thick flows on Tortuga, is similar to that of the basaltic lavas. These pegmatoids have fractionated in situ and have produced SiO 2 -rich liquids (interstitial glass) by the separation of Pl + Cpx + Ol + ilmenomagnetite and/or ilmenite from basaltic liquids. The most primitive Tortuga basalts are similar to ocean-ridge tholeiite in 87 Sr/ 86 Sr ratio and large-ion lithophile element abundance, but have higher sodium contents. This may be due to halite assimilation by the Tortuga basalt liquids.