Basaltic Members Research Articles

Cenozoic volcanism, tectonics and mineralisation of Woodlark Island (Muyuw), eastern Papua

Cenozoic geology of Woodlark Island was influenced by events prior to the onset of continental rifting at ca 8 Ma and the slightly later commencement of seafloor spreading in the eastern Woodlark Basin between ca 6 and 3.6 Ma. The embryonic Woodlark Island consisted of latest Paleocene(?)–Eocene shallow marine, deep-water shelf and turbidite deposits (Loluai Formation) and small submarine and emergent mafic volcanic centres (Utavai Basalt Member). Platform carbonates extended across the Woodlark Basin in the late Oligocene–middle Miocene (Nasai Limestone). Onset of middle Miocene eruptions in the north-northeast-trending Okiduse Volcanic Zone (OVZ) was localised in the similarly trending Kulumadau Horst and Graben Zone (KHGZ). Volcanic centres flanked major structures in the 12.5 km wide horst and graben zone, suggesting a link between volcanism and extensional tectonics. It was not an island arc settling, and there are neither andesites nor an obvious subduction zone. Offshore the KHGZ is connected with the similarly trending Nubara Fault, the plate boundary between the Solomon Sea Plate and Woodlark Plate. Contrasting volcanic deposits were erupted from volcanoes of the OVZ, viz. Mt Kabat Eruptive Centre, Uvarakoi Caldera, Watou Mountain Eruptive Centre and Suloga Harbour Tuff Cone. The epithermal Busai and Kulumadau gold deposits are located in the KHGZ. They are hosted by porous and permeable crystal lithic lapilli tuff beds in the Monasiy Tuff (Uvarakoi Caldera). Emplacement of thick dolerite sills (Auyed Dolerite) occurred post-middle Miocene. Sill emplacement coincided with the ca 6–3.6 Ma commencement of seafloor spreading in the eastern Woodlark Basin. A major phase of extensional tectonics along the KHGZ commenced post-middle Miocene and has persisted to the Holocene. Uplift was accompanied by rapid erosion and the widespread deposition of poorly to chaotically sorted gravels (Na’ku’iel Conglomerate). KEY POINTS Middle Miocene volcanism occurred from four volcanic centres in the north-northeast-trending Okiduse Volcanic Zone. Volcanism was confined to the similarly north-northeast-trending Kulumadau Horst and Graben Zone. Volcanism was not located in an island arc setting, as there are neither andesites nor an obvious subduction zone. A major phase of extensional tectonics along the horst and graben zone commenced post-middle Miocene and has persisted to the Holocene. Both the epithermal Busai and Kulumadau gold deposits exhibit lithological controls (porosity and permeability) on ore deposition.

Syn-sedimentary Mafic Volcanics in the Eocene Coal-bearing Tanjung Formation, Senakin Peninsula, South Kalimantan (Borneo), Indonesia

DOI:10.17014/ijog.7.1.65-85Syn-sedimentary mafic volcanism has been identified within a rift setting (Eocene Tanjung Formation) in the Senakin Peninsula, southeast Kalimantan. Fine-grained, dark-grey basalt lava occurs and has prominent vertically oriented columnar jointing. Petrographically, the basalt is composed of small euhedral pyroxene, olivine, and lath-shaped plagioclase phenocrysts within a very fine-grained dark coloured groundmass. A volcaniclastic unit also occurs and in outcrop has sharp contacts with underlying and overlying sedimentary mudstone. The unit is composed of cm-scale clasts of fine-grained to glassy textured basalt with vesicles of varying size and abundance. Euhedral pyroxene phenocrysts are observed within the clasts, although some with overprinting alteration. Palagonite alteration on the margins of some clasts is noted and is indicative of mafic composition volcanic material that has come into contact with sea water. Presence of bivalve and coral fragments in sandstone and mudstone underlying the volcaniclastic unit indicates emplacement into a marine environment. Core description from 33 locations over an 18 km transect length show that both the basalt and volcaniclastic sediments are extensive throughout the east Senakin area. Lithological relationships and compositional similarities between the basalt and volcaniclastic sediment suggest they are related and were contemporaneous with sedimentation within the Tanjung Formation. It is proposed that the basalt unit is designated the Tanah Rata Basalt Member of the Tanjung Formation. If a wider distribution occurs for the volcaniclastic unit it is proposed that it is termed the Gumbil Volcaniclastic Member of the Tanjung Formation.

Custom-Made Reinforcement Structures Made of Inorganic Fibers Challenges, Chances and Technical Approaches

Ceramic fibers are just as glass and basalt member of the group of inorganic fibers. Like most types of inorganic fibers ceramic fibers have a high tear resistance but a limited flexibility. [1] Ceramic fibers are characterized by their extraordinary high temperature and chemical resistance. These properties make them interesting for different high technical applications, as they occur in aerospace, chemical-and energy technology. In this field, they are applied especially as a reinforcement component in composite materials. Not only the partially high material price, but although the typical brittleness of ceramic fibers bring huge problems during the textile production chain, which limits the availability of complex textile preforms in the market. Often, a radical revision of the machine and processing concept is necessary to enable an economical production process. The Application Center for Textile Fiber Ceramics TFK at Fraunhofer-Center for High Temperature Materials and Design HTL develops and modifies textile production processes to make them suitable for the special requirements of ceramic fibers. One and multilayer woven fabrics, braids and tape structures for the winding process have already been successfully implemented. A further development complex is the intensive investigation of three-dimensional textile reinforcement structures. Regarding the high material costs, these research activities are very important. If the textile reinforcement is placed only where needed, the amount of used fiber material can be reduced significantly.

Using 40Ar/39Ar ages of intercalated silicic tuffs to date flood basalts: Precise ages for Steens Basalt Member of the Columbia River Basalt Group

To establish causality between flood basalt eruptions and extinction events and global environmental effects recorded by isotopic excursions in marine sediments, highly accurate and precise ages for the flood basalts are required. But flood basalts are intrinsically difficult to date. We illustrate how 40Ar/39Ar feldspar ages for silicic tuffs intercalated with and overlying sections of Steens Basalt, the earliest lavas of the Middle Miocene Columbia River Basalt Group in the northwestern United States, provide high-precision ages that, for the first time, make it possible to resolve age differences with stratigraphic position within a section of these flood lavas. The stratigraphically lowest rhyolitic tuff, a fall deposit, yielded an age of 16.592±±0.028 Ma (FCs = 28.02 Ma), and the uppermost, the alkali rhyolite ignimbrite Tuff of Oregon Canyon, is 16.468±±0.014 Ma. The argon and stratigraphic data indicate that Steens Basalt eruptions occurred from ∼16.64 to 16.43 Ma in the southern end of its distribution. We estimate that the Steens Mountain geomagnetic reversal occurred at 16.496±±0.028 Ma (±0.18 Ma total error). Our estimates of the timing for initiation of volcanism and volumetric eruptive rates do not seem to support volcanic forcing by the initial stages of Columbia River Basalt Group eruptions as an explanation for the abrupt warming and carbonate dissolution at the beginning of the Miocene Climatic Optimum.

UPPER CRETACEOUS-TERTIARY GEOLOGY/STRATIGRAPHY OF PERTEK AND ITS VICINITY (TUNCELİ, TURKEY)

The basement units of the study area are Paleozoic-Mesozoic Keban metamorphics and Late Cretaceous Elazig magmatics. As a result of tectonic events caused by final collision in Middle Maastrichtian, all these basement units were juxtaposed, uplifted, eroded and acted as a source area for the Maastrichtian-Paleogene basins. Upper Cretaceous Paleogene deposits were systematically sampled in order to determine their ages and lithologies. Due to new age findings and stratigraphical positions the marine units were identified as; Harami, Seske and Kirkgecit formations. Late Oligocene-Early Miocene Alibonca formation and Pliocene Karabakir formation are the other units of the study area. Maastrichtian-Thanetian Harami formation is formed by grainstone intercalated with sandstone at the bottom and white limestone at the top. Early Lutetian Seske formation consists of conglomerate at the bottom and much nummulitic, carbonate cemented sandstone with ophiolitic fragments at the top. Kirkgecit formation consists of sandstone-siltstone-limestone intercalation, and it is early Bartonian early Chattian. Late Chattian-middle Burdigalian Alibonca formation consists of conglomerate at the bottom, reefal limestone in the middle and clayey limestone intercalated with algal limestone at the top. Pliocene Karabakir formation is formed by conglomerate-mudstone and locally by intercalations of pyroclastics and lavas. Pertek Andesite and Basalt Members are both vertically and laterally transitional with sedimentary lithologies of the Karabakir formation. There is no data to date the lithology; however, it was assumed that it had been Pliocene in age according to the lithostratigraphical succession. In the study area, macrotectonic events were experienced in Late Cretaceous, Early Eocene, middle-late Lutetian, Middle?-Late Miocene and in the Latest Pliocene. During the last tectonic phase, the Anatolian plate bounded by the East Anatolian Fault (EAF) in E-SE began to move westward, and the right lateral strike slip Pertek Fault activated in the study area.

Pillow lava and spasmodic submarine fire fountaining in the middle Miocene marginal basin, Sado Island, Japan

AbstractThe Sawasaki basalt unit of the Ogi Basalt Member is a middle Miocene submarine volcano formed in a marginal basin, cropping out on the coast of the Ogi Peninsula, Sado Island, Japan. It is approx. 100 m thick, more than 4 km wide, and is composed of pillow lavas (Sawasaki pillow lava) and overlying fire fountain deposits (Sawasaki pyroclastic rocks). The Sawasaki pillow lava is a pile of nearly‐horizontal lava flows. Massive sheet lavas and lava pods grade into pillow lavas. The flat surfaces of the sheet lavas suggest that they spread over a summit plain. The margin of the Sawasaki pillow lava is dominated by thin elongated pillow lobes (20–30 cm across) dipping at 45–60°, contrast with the large pillows (40–100 cm across) in the core. The proximal lithofacies of the Sawasaki pyroclastic rocks is scoria agglomerate which is composed of tens of stratified beds. Individual beds have a range of clast sizes and abundance of subaqueous bombs (sometimes up to >70%), which suggests that they were formed by submarine spasmodic fire fountain eruptions. Progressive aggradation from a turbulent flow allowed settling of well‐preserved fluidal‐shaped subaqueous bombs in a fines‐depleted matrix. The basal layer shows lamina, scour and imbrication of scoria clasts. The thick scoria agglomerate beds with large subaqueous bombs, derived from energetic fire fountains, were emplaced during transition to higher degrees of hydrovolcanic fragmentation. Alternating scoria lapilli tuff and tuff beds in the distal area may represent the coupled topset and foreset beds formed by deposition from dilute, turbulent flows, and cascading of volcanic debris from the crest of the volcanic hill.

PETROGRAPHY, GEOCHEMISTRY AND GEOCHRONOLOGY OF THE METAVOLCANIC ROCKS OF THE MESOPROTEROZOIC LEERKRANS FORMATION, WILGENHOUTSDRIF GROUP, SOUTH AFRICA - BACK-ARC BASIN TO THE AREACHAP VOLCANIC ARC

The Leerkrans Formation of the Wilgenhoutsdrif Group is a succession of highly sheared metasedimentary and metavolcanic rocks separating the western margin of the Archean Kaapvaal Craton from the polydeformed and highly metamorphosed Proterozoic Namaqua Sector of the Namaqua-Natal Province. This study presents the results of a comprehensive petrographic, geochemical and isotopic study of the metavolcanic rocks of the Leerkrans Formation. Rocks of a basaltic member, a quartz porphyry and a second stratigraphically higher basaltic unit were sampled, with those of the lower basaltic member further subdivided based on their petrographic features and geochemical characteristics. Extensive alteration (chloritisation and epidotisation) of the volcanic rocks make identification of the precursor lithology difficult. Highly chloritised and epidotised metabasalts from the Lower Basalt are typically flow-banded, massive, vesicular or amygdaloidal, and have primitive tholeiitic, MORB-like geochemical characteristics and positive epsilon Nd(t) (e Nd(t) ) values (+4.2 to +6.1), with enriched large ion lithophile element (LILE) contents relative to primitive mantle suggesting derivation from a depleted mantle source. Some of the basalts of this unit have alkaline within-plate tholeiite characteristics. The Upper Quartz Porphyry is comprised of less altered felsic volcanic rocks which have pyroclastic or tuffaceous textures. The geochemistry of these rocks, which display a tholeiitic affinity with fractioned REE patterns and LILE enrichment relative to the high field strength elements (HFSE) and primitive mantle, along with negative e Nd(t) values (−4.8) indicate that the rocks are derived from crustal melting. U-Pb single zircon SHRIMP dating of zircons from a felsic pyroclastic rock from this unit yields an age of 1289 ± 9 Ma. The Upper Basalt and Mixed Zone of the Leerkrans Formation are comprised of basaltic lavas showing similar geochemical features to the Lower Basalt. The metavolcanic rocks of the Leerkrans Formation overlap in age with the oldest units of the ~1.3 to ~1.23 Ga Areachap Group, the polydeformed and highly metamorphosed remnants of a volcanic arc that separates the western margin of the Kaapvaal Craton from the Namaqua Sector. The Leerkrans Formation likely represents the remnants of a related back-arc basin to the volcanic arc which was accreted onto the western margin of the craton. The Leerkrans Formation escaped high-grade metamorphism and polyphase deformation that affected the Areachap arc during the Namaquan Orogeny due to being thrust back onto the Kaapvaal Craton during early arc accretion.

Magmatic characteristics and geochronology of Ordovician igneous rocks from the Cadia – Neville region, New South Wales: implications for tectonic evolution*

The Ordovician volcanic and intrusive rocks of the Cadia – Neville region, in the southern Molong Volcanic Belt section of the Macquarie Arc in central-western New South Wales, display a temporal progression from shoshonitic basaltic volcanism (e.g. Mt Pleasant Basalt Member) in the late Darriwilian to Gisbornian (ca 460 – 453 Ma) to small-volume dacitic medium-K calc-alkaline magmatism (e.g. Copper Hill-type dacite) in the late Eastonian to early Bolindian (ca 450 – 445 Ma) to large-volume, high-K calc-alkaline to shoshonitic, mafic to highly evolved magmatism (e.g. Nullawonga Latite Member and Cadia Intrusive Complex) at about 445 – 440 Ma. The two episodes of shoshonitic magmatism are separated by at least 15 (but up to ∼20) million years, during which time limestones, associated with regional uplift, were deposited broadly coincident with emplacement of the medium-K calc-alkaline dacitic intrusions. Despite this, the lavas and intrusions associated with both episodes of shoshonitic magmatism have strikingly similar geochemistry, indicating that a similar mantle source and partial melting processes were involved over this time interval. The arc-type geochemistry of the volcanic and intrusive rocks in the Cadia – Neville region is very similar to the lavas and intrusions forming the Pliocene Tavua Caldera in Viti Levu, Fiji. The Pliocene shoshonitic rocks at Tavua Caldera were emplaced following cessation of arc magmatism when the arc moved into an extensional phase. By analogy with the Pliocene tectonic setting in Fiji, the arc-like signature of the Ordovician magmas of the Macquarie Arc may be due to pre-processing of the mantle above a subduction zone prior to extension and fragmentation of arc lithosphere that led to generation of the subduction-modified shoshonite-dominated magmas.

Physical evolution of Grande Ronde Basalt magmas, Columbia River Basalt Group, north-western USA

In this paper we present what is, to the best of our knowledge, the first comprehensive study of clinopyroxenes and plagioclases contained in the flows of the Grande Ronde Basalt member of the Columbia River Basalt Group (northwestern USA). The rocks have MgO(wt%)<6%, and trace amounts of Cr and Ni. About 56% of extracted solid containing normative clinopyroxene and plagioclase explains the liquid line of descent from the more mafic sample (MgO wt%=5.89) to the most evolved. The most ubiquitous phases in the basalts are plagioclase and augite. Ilmenite and magnetite are accessories in all rocks. Olivine is present in small amount only in one sample (RT 89-7). Based on principles of Ca–Na plagioclase–liquid exchange, estimates of pre-eruptive magmatic water are < 2.4 wt%. From clinopyroxene–liquid equilibria, calculated pressures and temperatures of ascending magmas are between 1 atm and 0.617 GPa, and 1068 °C and 1166 °C, respectively. Compositions of magnetite–ilmenite pairs and olivine–clinopyroxene–oxide assemblages yield post-eruptive oxygen fugacities of ΔNNO=−1.923, and one pre-eruptive value of ΔNNO=− 2.455. A simple model of asthenospheric melting and magma ponding in the lower crust fits the physical parameters.

The Lesser Antilles volcanic chain: a study in arc magmatism

The Lesser Antilles volcanic arc is related to subduction of the American plate under the Caribbean plate. The rate of subduction is low, 2–4 cm a −1, and this has been reflected, at least over the past 0.1 Ma, in relatively low magma production rates (3–5 km 3 Ma −1 km −1 of arc). The arc is segmented; a northern segment trends 330° and the Benioff zone dips at 50–60°, whilst the southern segment trends 020° and the dip varies from 45° to 50° in the north to vertical in the south. Pleistocene–Recent volcanism (<2 Ma) occurs in narrow zones less than 10 km wide and seems to define three segments, the break between the central and southern segments being in the same location as the kink in the Benioff zone. Magma production over the past 0.1 Ma has been higher in islands of the central segment (8–40 km 3) than in the northern and southern segments (0–5 km 3); the variations may be related to the degree of obliquity of subduction along the arc. Cenozoic volcanic rocks of the arc are divided into low-K and medium-K series, each of which contains basaltic (MgO>6%) members ranging from hypersthene- to nepheline-normative. It is likely that all the Lesser Antilles eruptives had picritic (or, more rarely, ankaramitic), possibly silica-undersaturated, primary magmas. The medium-K rocks show wide variations in trace-element and isotopic characteristics. A generalised sequence of phenocryst assemblages, applicable to both groups, is: olivine+spinel±clinopyroxene→olivine+spinel+clinopyroxene+plagioclase→plagioclase+clinopyroxene+titanomagnetite+orthopyroxene±amphibole±quartz. Phenocryst crystallisation temperatures were: basalts 1180–1130°C; basaltic andesites 1060–1050°C; and andesites–dacites 960–740°C. Magmas inferred to be primary to the eruptive suites equilibrated within the spinel peridotite facies in the mantle wedge at pressures between 1.5 and 3 GPa. fO 2 conditions of magma crystallisation were rather oxidising (NNO +0.5 to NNO +3). Estimates of magmatic water contents, using direct and indirect methods, give conflicting results. Generalisations, based on melt inclusion data, are that water contents increased from 1–2 wt.% in primary/parental magmas to 5–6 wt.% in dacitic and rhyolitic melts. Primary magmas were generated in normal mid-ocean ridge basalts (N-MORB)-type mantle, modified by the addition of a fluid component derived mainly from subducted basaltic crust and a component derived from partial melting of subducted sediment. The proportions of each component in the different magma types are still debated, as is the importance of crustal contamination in producing their trace-element and isotopic characteristics. Compositional variations in the magmatic suites are a result mainly of polybaric fractional crystallisation, accompanied, particularly in some central islands, by crustal contamination, and by minor magma mixing. The amount of contamination may be related to volumetric volcanic production. The influence of the sediment component relative to that of hydrous fluids generally increases towards the south. However, neighbouring islands, and different centres within islands, may show different fractionation histories, indicating that the factors which controlled magma compositions, such as water concentrations in the source rocks and magma ascent rates, vary on the scale of tens of kilometers. The presence in individual centres on Grenada of two series with differing major- and trace-element and isotopic characteristics implies mantle sources which are heterogeneous on the scale of single plumbing systems.

Miocene stratigraphy and tectonic process of the Echigo-Yuzawa Basin, Northern Fossa Magna region, central Japan.

This study clarified the stratigraphy of the Miocene series that occur in contact with pre-Neogene basement rocks in the Echigo-Yuzawa region. Based on new lithologic assemblages, the Miocene stratigraphy and the tectonic process of the Echigo-Yuzawa Basin were reconstructed. The Miocene formations are distributed in the Shiozawa and Tsuchitaru areas in the Echigo-Yuzawa region. The Shiozawa area comprises the newly proposed Shiozawa Formation (Maiko Breccia Member, Yoshiyama Shale Member and Komatsuzawa Basalt Member) and the Tsuchitaru area comprises the redefined Matsukawa Formation (newly proposed Tsuchitaru Sandstone and Basalt Member, Nakazato Tuff Member and Kandatsu Shale Member). A new record of planktonic foraminifer indicative of Blow's N.9 (Blow, 1969) was discovered from the Kandatsu Shale Member. The tectonic development of the Miocene Echigo-Yuzawa Basin is as follows : [Stage I] Basin generation due to collapse, forming a lacustrine lake (18-17 Ma).[Stage II] Subsidence accompanied with marine transgression and bimodal volcanism (17-15Ma).[Stage III] Continuous subsidence depositing mud (15 Ma).

東北日本弧，稲庭岳地域の火山層序とＫ‐Ａｒ年代 著しくＫ２Ｏに乏しいマグマの活動時期

The Inaniwadake volcano, situated on the northern part of the volcanic front of Northeast (NE) Japan arc, comprises two groups of volcanic rocks which overlie the Kiritoshi Formation and the lower part of the Itsukaichi Formation. The lower group is the Joboji Andesite Member belonging to the Itsukaichi Formation, distributed in the northern and eastern flanks of the Inaniwadake volcano, and composed of lavas with pyroclastics. The Inaniwadake Formation, the upper group, is distributed in the most part of the Inaniwadake volcano, and can be divided into the Takamagezawa (lower), Hanakosodemine (middle) and Takamagehara (upper) Basalt Members, all of which are composed of lavas with minor pyroclasts. Three samples were chosen for K-Ar dating. The sample from the Takamagehara Basalt Member, the youngest lava of the study area, shows 2.67±0.23 Ma, while the other two samples from the Joboji Andesite Member show 6.52±0.26 Ma and 6.95±0.26 Ma. All the Inaniwadake volcanic rocks including above samples are characterized by extremely low K2O suites like as the other late Miocene to Pliocene volcanic rocks from Mitaki and Kurohanayama, the southern part of NE Japan arc. These Tertiary volcanic rocks are mainly basalts and basaltic andesites in composition, and distinctively lower in K2O content than those of Quaternary volcanoes on the volcanic front. These extremely low-K volcanism coincide contemporaneously with the formation of a number of late Miocene to Pliocene calderas of NE Japan arc.

Adakitic magmas: modern analogues of Archaean granitoids

Both geochemical and experimental petrological research indicate that Archaean continental crust was generated by partial melting of an Archaean tholeiite transformed into a garnet-bearing amphibolite or eclogite. The geodynamic context of tholeiite melting is the subject of controversy. It is assumed to be either (1) subduction (melting of a hot subducting slab), or (2) hot spot (melting of underplated basalts). These hypotheses are considered in the light of modern adakite genesis. Adakites are intermediate to felsic volcanic rocks, andesitic to rhyolitic in composition (basaltic members are lacking). They have trondhjemitic affinities (high-Na 2O contents and K 2O/Na 2O∼0.5) and their Mg no. (0.5), Ni (20–40 ppm) and Cr (30–50 ppm) contents are higher than in typical calc-alkaline magmas. Sr contents are high (>300 ppm, until 2000 ppm) and REE show strongly fractionated patterns with very low heavy REE (HREE) contents (Yb≤1.8 ppm, Y≤18 ppm). Consequently, high Sr/Y and La/Yb ratios are typical and discriminating features of adakitic magmas, indicative of melting of a mafic source where garnet and/or hornblende are residual phases. Adakitic magmas are only found in subduction zone environments, exclusively where the subduction and/or the subducted slab are young (<20 Ma). This situation is well-exemplified in Southern Chile where the Chile ridge is subducted and where the adakitic character of the lavas correlates well with the young age of the subducting oceanic lithosphere. In typical subduction zones, the subducted lithosphere is older than 20 Ma, it is cool and the geothermal gradient along the Benioff plane is low such that the oceanic crust dehydrates before it reaches the solidus temperature of hydrated tholeiite. Consequently, the basaltic slab cannot melt. The released large ion lithophile element (LILE)-rich fluids rise up into the mantle wedge, inducing both its metasomatism and partial melting. Afterwards, the residue is made up of olivine+clinopyroxene+orthopyroxene, such that the partial melts are HREE-rich (low La/Yb and Sr/Y). Contrarily, when a young (<20 Ma) and hot oceanic lithosphere is subducted, the geothermal gradient along the Benioff plane is high, so the temperature of hydrated tholeiite solidus is reached before dehydration occurs. Under these conditions, garnet and/or hornblende are the main residual phases giving rise to HREE-depleted magmas (high La/Yb). The lack of residual plagioclase accounts for the Sr enrichment (high Sr/Y) of the magma. Experimental petrologic data show that the liquids produced by melting of tholeiite in subduction-like P– T conditions are adakitic in composition. However, natural adakites systematically have higher Mg no., Ni and Cr contents, which are interpreted as reflecting interactions between the ascending adakitic magma generated in the subducted slab and the overlying mantle wedge. This interpretation has been recently corroborated by studies on ultramafic enclaves in Batan lavas where olivine crystals contain glass inclusions with adakitic compositions [Schiano, P., Clochiatti, R., Shimizu, N., Maury, R., Jochum, K.P., Hofmann, A.W., 1995. Hydrous, silica-rich melts in the sub-arc mantle and their relationships with erupted arc lavas. Nature 377 595–600.]. This is interpreted as demonstrating that adakitic magmas passed through the mantle wedge and interacted with it. Sajona [Sajona, F.G., 1995. Fusion de la croûte océanique en contexte de subduction collision: géochimie, géochronologie et pétrologie du magmatisme plioquaternaire de Mindanao (Philippines). Unpublished thesis, Brest University, France, 223 pp.] also considers that the high-Nb basalts, which are associated with adakites, reflect mantle–adakite interactions. Recent structural studies have demonstrated that plate tectonics operated during the first half of Earth history. The very strong similarities that exist between modern adakites and Archaean tonalite, trondhjemite and granodiorite (TTG) attest that both have the same source and petrogenesis. Consequently, when Archaean-like P– T conditions are exceptionally realised in modern subduction zones, Archaean-like magmas are generated. Contrarily, hot spots never produce TTG-like magmas, thus, strongly supporting the hypothesis of the generation of the Archaean continental crust within a subduction environment. However, Archaean TTG are poorer in Mg, Ni and Cr than adakites, indicating that mantle–magma interactions were less efficient, probably due to the shallower depth of slab melting. In this case, the slab-derived magmas rise through a thinner mantle wedge, thus, reducing the efficiency of the interactions. This is corroborated by the absence of a positive Sr anomaly in TTG, which indicates that plagioclase could have been a residual phase during their genesis.

Geochemistry of the Tertiary volcanism of Northern Ireland

The Antrim Plateau (Northern Ireland) forms the southwestern part of the British Tertiary Igneous Province and is dominated by basaltic lavas. Previous work divided the province into three stratigraphic formations, viz. the Lower, Middle and Upper Formations, with the Middle Formation having a basaltic member known as the Causeway Member. New chemical and SrNd isotopic data covering the spectrum of lava types are presented. Three distinct geochemical types are recognised: (1) basalts of the Lower Formation and most of those from the Upper Formation are LREE-enriched but have characteristic convex-up REE patterns with a maximum around Nd; (2) some of the Upper Formation basalts and the majority of the Causeway Member basalts are LREE-enriched with no convex shape in their REE pattern; and (3) some of the Causeway Member basalts are LREE-depleted and similar to N-MORB. Isotopic compositions of Sr and Nd show considerable variation throughout the three formations with ε Sr( t) = − 22 to + 107 and ε Nd( t) = − 11 to + 8.5. The exceptionally wide ranges of isotopic compositions are believed to have been produced by assimilation of a crustal (mainly Dalradian derived) component. The REE patterns indicate that most of the basalts were derived from a LREE-depleted mantle and with the exception of the Causeway Member, most have convex-up REE patterns. These are interpreted as being due to residual garnet in the mantle source, which indicates melting at depths in excess of 80 km. This suggests that at the start of Tertiary volcanism, the melt regime was controlled by a thick lithosphere which thinned with time such that the Causeway Tholeiites were produced at shallower levels. The return to the convex-up patterns of the Upper Formation means that simple models of lithospheric stretching and rifting are not able to explain the Antrim situation.

Bimodal cyclical Archean basalts and rhyolites from the Michipicoten (Wawa) greenstone belt, Ontario: geochemical evidence for magma contributions from the asthenospheric mantle and ancient continental lithosphere near the southern margin of the Superior Province

The petrographic and geochemical variations in the middle to late Archean volcanic rocks of the Michipicoten (Wawa) greenstone belt (MGB), Ontario, record bimodal basalt-rhyolite magmatic activity at approximately 2.9, 2.75 and 2.7 Ga along what has previously been interpreted to be a southward-dipping convergent plate margin which is underlain by continental crust. Cycle 1 komatiitic basalts have elevated Mg-number (0.51–0.83) compared to cycle 2 and 3 tholeiites (Mg-number 0.44–0.56), and dominantly flat REE patterns with negative TaNb anomalies ( La Ta 4–35). Cycles 2 and 3 are more evolved than cycle 1, but all three cycles have broadly similar ratios of the incompatible trace elements. Compared to modern oceanic basalts, the basaltic members of cycles 1–3 have a moderate TaNb anomaly ( La Ta 4–46) and low [ La Yb ] N (1.18–1.56). Low La Ta is a feature of oceanic lavas, whereas high La Ta is a feature of magmas which contain contributions from a crustal reservoir. The mafic volcanic rocks are believed to be derived from asthenospheric mantle with assimilation of older continental crust. The felsic volcanic rocks have elevated Th Nb (0.7–1.6), and [ Ce Yb ] N = 10.2−16.4 , and are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE) relative to rhyolites from other Superior Province greenstone belts. We suggest that the rhyolites record the geochemical traits of their sources, and these could be interpreted to be moderate to high degree melts of tonalitic upper crust rather than fractionated magmas or partial melts derived from the underlying mafic volcanics. This contrasts with models for many other rhyolites in Superior Province greenstone belts, such as the Southern Volcanic Zone of the Abitibi greenstone belt, where variations require a more mafic crustal source to generate the fractionated low La Sm , Gd Yb , and high Zr Th rhyolites believed to be locally associated with volcanogenic massive sulphide deposits. The absence of significant volumes of compositionally intermediate volcanic rocks and the evidence for crustal contributions to MGB cyclical magmatism raises the possibility that these are Archean analogues of the Phanerozoic flood basalts which straddle the margins of continental shield areas, such as the Java-Ontong sequence, and are not relics of either Archean oceanic crust or subduction-related volcanics.

Petrology and Geochemistry of Cretaceous Ultramafic Volcanics from Eastern Kamchatka

The origin, evolution and primary melt compositions of late Cretaceous high-K ultramafic volcanics and associated basalts of Eastern Kamchatka are discussed an the basis of a study of the mineralogy and geochemistry of the rocks and magmatic inclusions in phenocrysts. The exceptionally primitive composition of the phenocryst assemblage [olivine-Fo(88-95), Cr-spinel-Cr/(Cr + Al) up to 85] provides direct evidence of the mantle origin of primary melts, which were highly magnesian compositions (MgO 19-24 wt%). The rocks and melts are characterized by strong high field strength element (HFSE) depletion in comparison with rare earth elements, and high and variable levels of enrichment in large ion lithophile elements (LILE), P, K and H2O (0.6-1.2 wt% in picritic to basaltic melts). epsilon(Nd) values lie in a narrow range (+10.7 to +9.1), typical of N-MORB (mid-ocean ridge basalt), but Sr-87/Sr-86 (0.70316-0.70358) is slightly displaced from the mantle array. High-K ultramafic melts from Kamchatka are considered as a new magma type within the island-are magmatic spectrum; basaltic members of the suite resemble are shoshonites. The primary melts were produced under high-pressure (30-50 kbar) and high-temperature (1500-1700 oC) conditions by partial melting of a refractory peridotitic mantle.

Perspective. Certified Ontario reference materials programme at the Geoscience Laboratories, Sudbury, Ontario, Canada

The Ontario Geoscience Laboratories (hereafter referred to as the Geoscience Laboratories) have produced 30 in-house materials since 1970. This experience, the limited availability of international rock standards in volume, and the recent acquisition of specialized equipment (capable of homogenizing up to 1 t of sample), has led to the development of the Certified Ontario Reference Materials programme for the production of certified reference materials (CRMs). Given the geological diversity of Ontario, the decision was made to create two streams of reference materials. One is a suite of unaltered and altered volcanic rock reference materials that reflected the wide compositional range of Archean rocks of Ontario. Sample sites are clustered in the Abitibi Greenstone Belt which includes the Timmins and Kirkland Lake mining camps. This region also hosts some of the world's largest and most prolific mines of Au and Cu–Zn, smaller Ni deposits and one of the two international type-localities of komatiite. Advantages of such a choice are extensive. The rocks in this area have been studied by the Ontario Geological Survey (OGS) for 50 years and this expertise is readily available both in terms of appropriate chemistry and in sampling sites. Geologically, Archean volcanic rocks were typically extruded into a subaqueous environment and cooled quickly. This facilitates a high degree of homogeneity. There are a wide variety of compositions available, clear association with mineral deposits and thus the reference materials may also provide excellent baseline material for environmental studies. In the 1970s, the OGS devised internationally used major element classification criteria which divided Archean volcanic rocks into three major clans that have several subtypes (komatiitic clan has ultramafic and basaltic members; tholeiitic clan has Fe-, Mg-, basaltic, dacitic, andesitic, and rhyolitic members; and calc-alkalic clan has basaltic, andesitic, dacitic and rhyolitic members). These rock types typically show different styles of alteration near ore deposits (i.e., chloritization, sericitization, carbonatization). The Archean volcanic rock suite will be comprised of these unaltered and altered rock types. The other group will focus on materials that fill a specific geoanalytical need. Based on the information above and the capacity of the Geoscience Laboratories geostandards programme (5 CRMs per year), it is anticipated that several rock types will be prepared yearly. A calc-alkaline rhyolite and an ultramafic komatiite have now been collected and other sites have been identified. Collection has commenced. After the sample is bottled, the homogeneity will be tested in our laboratories using validated methods. This will provide a tentative composition and will be followed by round-robin determinations. The reference materials will be released in 100 and 500 g plastic bottles to both the geoanalytical community and the exploration industry to monitor precision and accuracy in commercial surveys.

Geochemical evolution of an Ordovician island arc, South Mayo, Ireland

Early Ordovician volcanic rocks exposed in the South Mayo region of western Ireland document the history of a volcanic arc complex, produced following the initiation of south-dipping subduction within the Iapetus Ocean in the Late Cambrian or Early Ordovician. Lavas of the Lough Nafooey Group (Tremadoc-Arenig) show an eruption history marked by an initial period of tholeiitic basaltic volcanism, followed by a shift to increasing silica compositions of calc-alkaline affinity. Subsequent volcanism of the Tourmakeady Group (Arenig) shows a mixture of rhyolitic and andesitic compositions, which are in turn succeeded by tuffs of the Rosroe and Mweelrea Formations of the Murrisk Group (Llanvirn). These are also rhyolitic and andesitic but are generally more siliceous. The overall increase in silica contents may reflect increasing fractional crystallization due to crustal thickening. Trace element studies of the basaltic members of the Lough Nafooey Group show a strong subduction zone influence. While absolute levels of immobile trace and rare earth elements decrease upsection, relative depletion of the most incompatible elements decreases over the same time period. These trends suggest decreasing degrees of partial melting of a progressively more depleted source mantle upsection. The basaltic Bohaun Volcanic Formation (Tremadoc-Arenig ?) shows greater depletion in its incompatible trace elements than the Lough Nafooey Group, so indicating a more depleted mantle source. Comparison with modern arc systems suggests that the Bohaun Volcanic Formation was probably generated shortly after the initiation of subduction and was erupted in a more trenchward position than the Lough Nafooey Group (i.e. in the forearc). This confirms a south-dipping polarity to the subduction zone, and suggests an age of initiation during the Late Cambrian or earliest Ordovician.

The development of Early Permian sequences and hydrothermal alteration in the Myall Syncline, central eastern New South Wales

The Myall Syncline, within the southern Tamworth Belt of the New England Fold Belt, contains a sequence of concentric meridionally folded Carboniferous to Early Permian shallow marine to fluvial‐lacustrine sedimentary and bimodal terrestrial volcanic rocks. The sequence spans a cycle of marine regression and transgression. The Late Carboniferous Koolanock Sandstone is overlain by the Muirs Creek Conglomerate, a possibly diachronous unit, which is succeeded apparently conformably by the Early Permian Markwell Coal Measures. Deposition of the coal measures took place during the intervals between extrusion of the members of the Alum Mountain Volcanics: the basal rhyolitic to dacitic predominantly ash flow Sams Road Rhyolite Member (new name); the Burdekins Gap Basalt Member; and the flow dominated Lakes Road Rhyolite Member. The Markwell Coal Measures had previously been interpreted as overlying the Alum Mountain Volcanics. The Late Permian marine Bulahdelah Formation disconformably overlies the Alum Mountai...

Le Complexe d'Ascot des Appalaches du sud du Québec: pétrologie et géochimie

This paper reviews new geochemical data obtained from Ordovician volcanics in the Ascot Complex of the Quebec Appalachians. The Ascot Complex is divided into three distinctive lithotectonic assemblages: the Sherbrooke, Eustis, and Stokes domains. The Sherbrooke Domain includes basalts, pyroclastic breccias, and felsic tuffs. The Eustis Domain is composed of volcanoclastic and pyroclastic rocks. The Stokes Domain is an assemblage of rhyolitic tuffs and basalts. The geochemical composition of the volcanics varies between basaltic and rhyolitic end members. Basaltic rocks are arc tholeiites (La/Yb = 0.5–2). Some basalts found in the Sherbrooke Domain show a composition very close to that of frontal-arc boninites (La/Yb = 1–2) and are depleted of rare-earth elements. Felsic rocks from the Sherbrooke Domain are poorer in light rare-earth elements (La/Yb = 0.5–0.7) than those from the Stokes Domain (La/Yb &gt; 2). All of the felsic rocks are characterized by a negative Eu anomaly (Eu*/Eu = 1.3–3). The Stokes felsites are derived from an enriched magma contaminated by a continental crust component. The Sherbrooke Domain felsites are derived from a source impoverished in incompatible elements and enriched in high-partition-coefficient elements. Volcanic rocks from Sherbrooke and Eustis are thought to represent an incipient intra-oceanic arc. The rocks of the Stokes assemblage represent a more mature section of the same arc, explaining the contamination of generated magmas by continental crust components. The modern architecture of the Ascot Complex is essentially tectonic and may not correspond to the initial internal structure of this Ordovician arc. [Journal Translation]