Oceanic Volcanic Rocks Research Articles

Paleozoic ocean plate stratigraphy unraveled by calcite U-Pb dating of basalt and biostratigraphy

Oceanic mafic volcanic rocks preserve unique information regarding the nature and evolution of tectonic plates. However, constraining their age is commonly challenging because of their lack of datable minerals and high degrees of alteration. We present in situ laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of calcite phases in altered basalts in a Paleozoic subduction complex (eastern Australia). Calcite enclosed in amygdules and filled in fractures yielded two distinctive ages with contrasting geochemical signatures. These results, combined with new biostratigraphic and whole-rock geochemical data, suggest that oceanic islands formed in the Panthalassa Ocean at about 365 million years ago, accreted to eastern Gondwana at about 330 million years ago, and underwent brittle deformation at about 305 million years ago. Calcite U-Pb geochronology is valuable to help constrain minimum formation ages of volcanic rocks and their deformation history, ultimately improving ability to unravel the geological record of accretionary complexes, and more generally ancient underwater volcanic systems.

A Systematic Assessment of Stable Sr Isotopic Compositions of Vent Fluids in Arc/Back-Arc Hydrothermal Systems: Effects of Host Rock Type, Phase Separation, and Overlying Sediment

Variations in the stable isotopic composition of seawater Sr (δ88Sr) is a new tool for estimating the rates of global carbonate sedimentation over geologic time, yet the isotope compositions of the major sources and sinks of Sr to the world oceans are still in need of further constraint. We report δ88Sr values of vent fluids from arc/back-arc seafloor hydrothermal systems in the western Pacific. In the sediment-starved hydrothermal fields of the Manus Basin, Izu-Bonin Arc, and Mariana Trough, the δ88Sr values of end-member fluids for each site showed little variation (0.29–0.30‰) and were close to the average value of oceanic volcanic rocks, reflecting dissolved Sr sourced from host rocks. Chlorine-depleted fluids from phase-separated hydrothermal systems in the North Fiji Basin had the end-member δ88Sr values of 0.26, 0.28, and 0.29‰. Thus, both sediment-starved and phase-separated vent fluids had the end-member δ88Sr values indistinguishable from or very close to the range of oceanic volcanic rocks. Therefore, the δ88Sr compositions in these hydrothermal sites are controlled predominantly by Sr sourced from host rock with a small influence from secondary mineral precipitation/re-dissolution. Fluids from the sediment-hosted hydrothermal fields of the Okinawa Trough, however, were characterized by low δ88Sr values of approximately 0.22‰ and high 87Sr/86Sr ratios, indicating interactions with sedimentary carbonates. As for the modern oceanic δ88Sr budget, the sediment-hosted sites lower the global hydrothermal δ88Sr. Since both sediment-starved and -hosted hydrothermal systems provide a long-term control on the global Sr cycle, the end-member δ88Sr value is an important constraint on the evolution of Sr cycling in past oceans.

Volcanic Rocks in the Udyl’ Segment of the Kiselevka–Manoma Accretionary Terrane (Sikhote-Alin): Petrogeochemistry, Formation Conditions, and Tectonic Setting

New petrogeochemical data were used to determine the conditions of the formation and tectonic setting of volcanic rocks of the Lake Udyl’ area. These rocks were compared with similar rocks in the region. The volcanic rocks of the Lake Udyl’ area are subdivided into two genetic types: (1) within-plate oceanic volcanic rocks ascribed to the accretionary volcanogenic–siliceous complex (Tithonian–Valanginian–Hauterivian–Barremian) similar to accretionary complexes in other segments of the Kiselevka–Manoma Terrane, and (2) suprasubduction-zone volcanic rocks from the island-arc volcanogenic-terrigenous complex (Valanginian–Hauterivian–Aptian–Albian–Cenomanian). A synthesis of new geological and petrogeochemical data, as well as previous geochronological and paleomagnetic data indicates the co-occurrence of genetically diverse complexes in the Udyl’ segment.

Discriminant and factor analyses as tools for comparison of terrestrial and Venusian volcanic rocks

Statistical methods of multi-dimensional analysis (discriminant functions and factor analysis) were applied to compare the chemical analyses obtained by Venera-13, -14, and Vega-2 landers (contents of major oxides except for sodium) with petrochemical data compiled into the data base on terrestrial ocean. It is shown that the distribution of major petrogenic elements in the terrestrial rocks ascribed to different geodynamic settings (spreading zones, hot spots, and subduction zones) is determined by crystallization differentiation. This process is best manifested in hot spot volcanics (volcanic islands). In spite of the difficulties related to the poor precision of chemical determinations of Venusian rocks, obtained data indicate that the rocks from the Venera-13 and Vega-2 landing sites have no petrochemical analogues among terrestrial oceanic volcanic rocks. Rocks analyzed in the Venera-14 landing site may resemble the mid-ocean ridge volcanic rocks, although geological setting in the Venera-14 landing site ellipse strongly differs from terrestrial spreading zones.

Structural evolution and deformation kinematics of a subduction-related serpentinite-matrix mélange, Santa Elena peninsula, northwest Costa Rica

Detailed structural analysis of the Isla Pelada serpentinite-matrix mélange on Santa Elena Peninsula, Pacific coast of northwest Costa Rica, elucidates for the spatial distribution and temporal progression of a décollement-related deformation. This décollement zone constitutes the fault rock of a plate boundary in a subduction to collision setting. The décollement zone is defined by a thick tectonic mélange that includes blocks and belts of polymictic breccia, turbiditic greywacke, argillite, radiolarite, basalt, gabbro and serpentinized peridotite in a variably foliated serpentinite matrix. The chemical-mineral data of the peridotite blocks in the mélange indicate that they are fragments of abyssal mantle, different of the very depleted forearc mantle peridotites of the overlying Santa Elena ultramafic nappe.The structural evolution of the mélange can be divided into four main events. Early D1 deformation took place during underthrusting in the footwall of the décollement and is characterized by subhorizontal layer-parallel extension and subsequent by heterogeneous shearing of trench-fill deposits, oceanic volcanic rocks and variably serpentinized peridotites, resulting asymmetric boudinage, mesoscopic porphyroclasts and pervasive (S1) S–C fabrics. D1 is followed by layer-parallel contraction with early D2 folding and local development of an S2 pressure-solution cleavage, and late-D2 thrusting and large-scale duplexing. Therefore, D2 deformation evolves from pervasive coaxial subhorizontal contraction to a more localized non-coaxial strain (along relatively narrow thrust surfaces, in the mélange). D2 deformation mainly occurred in the hanging-wall of the décollement and was related to the downward migration of the subduction thrust. The result is the incorporation of slices of the subducted sequence by underplating into the overriding plate, typical of accretionary prisms.Kinematic indicators in the mélange are consistent with a general eastward-directed shearing during D1 and D2 deformations. This kinematics can be correlated with the direction of emplacement of the overlying Santa Elena ultramafic nappe during the early Upper Cretaceous. This also suggests the western-directed subduction of the oceanic lower plate. After the mélange was incorporated into the accretionary prism, the whole tectonic edifice was deformed in pre-late Campanian time by WNW–ESE trending D3 cylindrical upright folding and S-directed D4 narrow brittle thrusting. The recorded evolution of deformation in rocks already lithified, combined with the exotic origin of the oceanic lithologies and the serpentinite nature of the matrix points to disruption and mélange formation mainly during tectonic burial and underthrusting. In this context, the serpentinite matrix of the mélange is the result of the low-T hydration, metamorphism and deformation of mantle peridotites in a relatively shallow subduction setting.

Study on the geochemical characteristics of ocean-ridge and oceanic-island volcanic rocks in the Nan-Uttaradit zone, northern Thailand

Field investigations and laboratory integrated research as indicated that ophiolite melange in the Nan-Uttaradit zone, northern Thailand, consists of fragments of tectonites such as metamorphic peridotite (extremely silicified serpentinite), cumulates (pyroxenolite, gabbro, and gabbro-diorite), ocean-ridge basalt, oceanic-island basalt and radiolarian silicalite, and it was formed during D3-P. The rock series, rock types and petrogeochemical characteristics of metamorphic tholeiites in the Nan area of the Nan-Uttaradit zone are similar to those of ocean-ridge basalts (C1) in China’s Ailaoshan zone. As for the Hawaiites in the Nan area of the Nan-Uttaradit zone, their major elements, REEs and trace elements are similar to those of oceanic-island basalts in China’s Jinshanjiang zone (P1 1). In the Uttaradit area of this zone the metamorphic alkaline basalts show transitional petrogeochemical characteristics between ocean-ridge basalts and oceanic-island basalts, which were still formed in oceanic-island environments. The above-described basalts are all oceanic volcanic rocks and they are the most important part of the Paleo-Tethys oceanic crust in the Nan-Uttaradit zone.

Geochemical characteristics of the oceanic island-type volcanic rocks in the Chiang Mai zone, northern Thailand

The oceanic island volcanic rocks in the Chiang Mai zone, northern Thailand, are usually covered by Lower Carboniferous and Upper Permian shallow-water carbonate rocks, with the Hawaii rocks and potash trachybasalt being the main rock types. The alkaline series is dominant with sub-alkaline series occurring in few cases. The geochemical characteristics are described as follows: the major chemical compositions are characterized by high TiO2, high P2O5 and medium K2O; the rare-earth elements are characterized by right-inclined strong LREE-enrichment patterns; the trace element patterns are of the upward-bulging K-Ti enrichment type; multi-component plots falling within the fields of oceanic island basalts and alkali basalts, belonging to the oceanic island-type volcanic rocks, which are similar to the equivalents in Deqin and Gengma (the Changning-Menglian zone) of Yunnan Province, China.

New data on Mesozoic Radiolaria of Serbia and Bosnia, and implications for the age and evolution of oceanic volcanic rocks in the Central and Northern Balkans

The Dinaridic Ophiolite Belt (DOB) includes Middle Triassic cherty limestones, Upper Triassic radiolarian cherts associated with basaltic lavas and volcaniclastic sediments, and also Jurassic continental slope sequences. Sedimentary sequences grade upwards into Cretaceous red limestones with planktonic foraminifers of the Genus Hedbergella. Many of the Late Triassic to Late Jurassic cherts probably accumulated in an oceanic setting, and now crop out as blocks in olistostromal deposits. Jurassic radiolarite sequences of the margin of the Dinaride Ophiolite Belt are represented by Aalenian, Upper Bajocian–Lower Bathonian and Upper Bathonian–Lower Callovian, Middle Callovian–Oxfordian, Upper Callovian–Upper Oxfordian and Kimmeridgian–Tithonian-aged strata. These are overlain by Upper Tithonian–Valanginian sedimentary sequences with spongolite–radiolarites, volcaniclastic sediments and other continental slope deposits. The oldest units outcropping in the DOB are Mid-Triassic (Ladinian) red cherts and limestones. These are associated with alkaline and tholeiitic basalts in the Bistrica area, and with tholeiitic basalts in the Visoka area. In contrast, in the Vardar Zone Western Belt the oldest basalt–radiolarite formation is represented by cherts associated with tholeiitic basalt in the Ovčar Banja area. Upper Triassic red radiolarites of the Čačak area lie in direct stratigraphical contact with MORB-type pillow lavas and contain a Carnian–Norian radiolarian assemblage. The youngest radiolarian-bearing units are in the Upper Cretaceous part of the Vardar Zone Western Belt, in the Struganik area, where Coniacian–Santonian radiolarians occur in chert–clay–tuff sediments. In addition, cherty limestones from the northern slope of Kozara Mt. contain a Campanian radiolarian assemblage, together with Campanian–Maastrichtian planktonic foraminifers. Volcanic units associated with the siliceous sediments of Dinaride Ophiolite Belt and the Vardar Zone Western Belt are of MOR- and SSZ types, respectively. The ocean-related basalts of the Dinaride Ophiolite Belt and the Vardar Zone Western Belt show some geochemical differences, irrespective of age. Compared to those of the Dinaride Ophiolite Belt, the basalts of the Vardar Zone Western Belt are more variable chemically and exhibit much higher LREE/HREE and Th/REE ratios. Finally, possible paleo-reconstructions of the radiolarian sediments and the volcanic rocks are proposed.

Protolith of eclogites in the north Qaidam and Altun UHP terrane, NW China: Earlier oceanic crust?

An early Paleozoic ultrahigh pressure metamorphic belt occurs in the north Qaidam–Altun mountains and was offset about 400 km southwestward by the Altyn Tagh strike-slip fault. Eclogites in the belt consist of major basaltic and minor picritic rock types and can be subdivided into three groups: high TiO 2 (2–5 wt%), medium TiO 2 (1–2 wt%) and low TiO 2 (<1 wt%). Geochemical evidence shows that the protoliths of the eclogite were basaltic rocks from different tectonic environments, including mid-ocean ridge basalt, island arc basalt and ocean island basalt. The Nd isotopes of these rocks are similar to those of MORB, characterized by mainly positive and minor negative εNd(0) values, providing further evidence that the eclogite protoliths were ocean floor basalts to which minor crustal components were added during subduction. Geochronological data indicate that the UHPM occurred about 500–440 Ma ago, and that there were two eclogite protoliths with ages of 800–750 and ∼1000 Ma, respectively. Geochemically and isotopically, the eclogites are similar to basaltic rocks in Luliangshan, north Qaidam mountains, which have Sr and Nd ages of 768±39 and 780±22 Ma. The age and composition of these volcanic rocks suggest that they were one type of the protoliths for the eclogites. A later series of calc-alkaline, island arc volcanic rocks was formed at about 500 Ma ago, essentially contemporaneous with the UHP metamorphism. We propose the following tectonic model for the evolution of the north Qaidam UHP belt. At about 1000 Ma ago, a number of continents were amalgamated to form the Rodinian continent in this general area, which contained some oceanic volcanic rocks, possibly as ophiolitic fragments. This part of Rodinia was then rifted at about 800–750 Ma ago to form an oceanic basin with a variety of MORB and ocean island basalts. Closure of this ocean basin produced the Neoproterozoic ophiolites and granitic gneisses in the north Qaidam mountains. In the early Paleozoic another Qilian ocean basin formed and subduction of this oceanic lithosphere formed the island arc volcanic rocks at about 500 Ma ago and the subduction-related granites (470–450 Ma). Once the oceanic lithosphere was consumed, the subsequent continent–continent collision led to deep subduction of continental crust, which was tectonically mixed with the eclogites, to cause the ultrahigh pressure metamorphism.

Experimental constraints on crystallization differentiation in a deep magma ocean

Major and trace element mineral/melt partition coefficients are presented for phases on the liquidus of fertile peridotite at 23–23.5 GPa and 2300 °C. Partitioning models, based on lattice-strain theory, are developed for cations in the ‘8-fold’ sites of majorite and Mg-perovskite. Composition-dependant partitioning models are made for cations in the 12-fold site of Ca-perovskite based on previously published data. D min/melt is extremely variable for many elements in Ca-perovskite and highly correlated with certain melt compositional parameters (e.g. CaO and Al 2O 3 contents). The 8-fold sites in Mg-perovskite and majorite generally have ideal site radii between 0.8 and 0.9 Å for trivalent cations, such that among rare-earth-elements (REE) D min/melt is maximum for Lu. Lighter REE become increasingly incompatible with increasing ionic radii. The 12-fold site in Ca-perovskite is larger and has an ideal trivalent site radius of ∼1.05 Å, such that the middle REE has the maximum D min/melt. Trivalent cations are generally compatible to highly compatible in Ca-perovskite giving it considerable leverage in crystallization models. Geochemical models based on these phase relations and partitioning results are used to test for evidence in mantle peridotite of preserved signals of crystal differentiation in a deep, Hadean magma ocean. Model compositions for bulk silicate Earth and convecting mantle are constructed and evaluated. The model compositions for primitive convecting mantle yield superchondritic Mg/Si and Ca/Al ratios, although many refractory lithophile element ratios are near chondritic. Major element mass balance calculations effectively preclude a CI-chondritic bulk silicate Earth composition, and the super-chondritic Mg/Si ratio of the mantle is apparently a primary feature. Mass balance calculations indicate that 10–15% crystal fractionation of an assemblage dominated by Mg-perovskite, but with minor amounts of Ca-perovskite and ferropericlase, from a magma ocean with model peridotite-based bulk silicate Earth composition produces a residual magma that resembles closely the convecting mantle. Partition coefficient based crystal fractionation models are developed that track changes in refractory lithophile major and trace element ratios in the residual magma (e.g. convecting mantle). Monomineralic crystallization of majorite or Mg-perovskite is limited to less than 5% before certain ratios fractionate beyond convecting mantle values. Only trace amounts of Ca-perovskite can be tolerated in isolation due to its remarkable ability to fractionate lithophile elements. Indeed, Ca-perovskite is limited to only a few percent in a deep mantle crystal assemblage. Removal from a magma ocean of approximately 13% of a deep mantle assemblage comprised of Mg-perovskite, Ca-perovskite and ferropericlase in the proportions 93:3:4 produces a residual magma with a superchondritic Ca/Al ratio matching that of the model convecting mantle. This amount of crystal separation generates fractionations in other refractory lithophile elements ratios that generally mimic those observed in the convecting mantle. Further, the residual magma is expected to have subchondritic Sm/Nd and Lu/Hf ratios. Modeling shows that up to 15% crystal separation of the deep mantle assemblage from an early magma ocean could have yielded a convecting mantle reservoir with 143Nd/ 144Nd and 176Hf/ 177Hf isotopic compositions that remain internal to the array observed for modern oceanic volcanic rocks. If kept in isolation, the residual magma and deep crystal piles would grow model isotopic compositions that are akin to enriched mantle 1 (EM1) and HIMU reservoirs, respectively, in Nd-Hf isotopic space.

Geochemical and geochronological constraints on the origin and emplacement of the Yarlung Zangbo ophiolites, Southern Tibet

Abstract The Indus-Yarlung Zangbo suture zone in southern Tibet marks the Eocene collision of the Indian continent and the Lhasa Block of Eurasia. It is characterized, particularly in its central portion, by an east-west belt of ophiolitic and related oceanic volcanic and sedimentary rocks that form a number of structurally juxtaposed geological terranes. Although tectonically disrupted in many places, almost complete ophiolite sequences exist at Luobusa and Zedong in the east and near Xigaze in the west. In Luobusa, the ophiolite sequence is thrust over the Tertiary molasse deposits of the Luobusa Formation or onto plutonic rocks of the Gangdese batholith. A mantle sequence dominates the ophiolite massif and consists chiefly of harzburgite and clinopyroxene-bearing harzburgite with abundant podiform chromitites enveloped by dunite. The Luobusa ophiolite formed the basement to an intra-oceanic volcanic arc, the Zedong terrane, which developed between the Mid-Jurassic and Mid-Cretaceous. Farther to the west, complete ophiolite sequences exist at Dazhuqu and near Xigaze. These ophiolites have suprasubduction zone geochemical signatures but there is no apparent development of a volcanic arc. Sensitive high-resolution ion microprobe U-Pb zircon analyses yield an age of 126 Ma for the crystallization of a quartz diorite from the Dazhuqu massif. Amphibolites that occur as large blocks in mélanges at the base of the ophiolites are considered to be remnants of dynamothermal metamorphic soles produced early in the ophiolite obduction process. Ar/Ar geochronology on amphibole and biotite separates from these rocks yields ages of 80 and 90 Ma, respectively, for this event, which is considered to have occurred as the Indian continental margin entered the intra-oceanic subduction zone. Continued northward subduction of the remaining portion of the Neo-Tethyan ocean floor beneath the southern margin of Eurasia produced the Gangdese continental arc on the southern margin of the Lhasa Block and led to the final closure of the ocean with the collision of India and Eurasia in the Eocene.

Sources and Depositional Environments of Some Permian and Triassic Cherts: Significance of Rb‐Sr and Sm‐Nd Isotopic and REE Abundance Data

Rb‐Sr and Sm‐Nd isotopic data, rare earth element (REE ) abundances, and major‐element compositions are reported for the Triassic cherts of the Mino Terrane in the Inner Zone of southwest Japan and for the Permian and Triassic cherts of the Sambosan Terrane in the Outer Zone of southwest Japan. Rb‐Sr isotopic data of the Mino and Sambosan cherts define separate isochron lines, and the Rb‐Sr ages of ca. 210 and 240 Ma reflect the chert deposition and the end of the subsequent chemical diagenesis during which amorphous silica was transformed into quartz. It is concluded that the Rb‐Sr isotopic system of the cherts becomes essentially homogenized among biogenic silica, detrital components and hydrogeneous components during deposition, and the subsequent chemical diagenesis before lithification due to high mobility of Rb and Sr. The Sr initial ratio (0.71363) of the Triassic Mino cherts is clearly higher than those (0.7079 and 0.7068) of the Triassic and Permian Sambosan cherts, which are close to the estimated oceanic Sr ratios of 0.7066–0.7081 in the Permian and Triassic. On the other hand, Sm‐Nd isotopic data do not define isochron lines. This might be due to incomplete homogenization of the system among biogenic silica, detrital components and hydrogeneous components during deposition, and chemical diagenesis because of low mobility of REE. Initial Sr and Nd isotopic ratios of the Mino cherts can be interpreted by mixing typical continental crustal rocks, represented by aeolian loess compositions, with a small amount of Precambrian rocks (<10%), while those of the Sambosan cherts can be explained by mixing continental crustal rocks with oceanic volcanic rocks such as MORB (mid‐ocean ridge basalt). In the mixing model between Chinese loess and MORB, a proportion of loess is ca. 30%–60% for the Middle Permian Sambosan cherts and ca. 60%–90% for the Triassic Sambosan cherts. Al2O3/(Al2O3 + Fe 2O3) ratios and Ce anomalies in the REE patterns also show a higher contribution of continental components in the Triassic Sambosan and Mino cherts than the Permian Sambosan cherts, suggesting deposition in a marginal sea or on the continental shelf and slope for the Triassic Sambosan and Mino cherts. Our study shows that the geochemical and isotopic features of the Mino and Sambosan cherts essentially retain the nature of their formation and subsequent chemical diagenesis before lithification. Alteration or metamorphic effects during subduction and accretion processes cannot be found in the chemical and isotopic features of these cherts. Our result requires the reexamination of the previous paleogeographical reconstructions of the Japanese Islands as well as the relationship between the Mino and Sambosan Terranes in Permian and Triassic time.

Geochemistry of Oceanic Igneous Rocks—Ridges, Islands, and Arcs—with Emphasis on Manganese, Scandium, and Vanadium

A database on a number of elements in oceanic volcanic rocks is presented, including the principal major-element oxides-SiO2, TiO2, Al2O3, Fe2O3(T), MnO, MgO, CaO, Na2O, K2O, and P2O5 (where T refers to total iron)–and the trace elements–Ba, Ce, Cr, Cu, Ni, Sc, Sr, V, Pb (mainly by isotope dilution), Yb, Zn, and Zr. Interpretations are given for transition metals, with emphasis on Mn, Sc, and V, in order to determine the concentration of the elements in primitive melts and assess their trends in magmatic differentiation. Transition metals are not enriched in plagioclase, so all are incompatible with pure plagioclase removal–that is, they become enriched in the melt. Both Cr and Ni are known to be highly compatible with olivine separation-i.e., they are depleted in the melt early in differentiation. Also, Sc is compatible with clinopyroxene (Cpx) removal from the melt and is depleted by separation of Cpx. Copper does not fit well in any of the principal silicates, but Cu, like Ni, is greatly enriched in sulfides that may remain in the source or separate from the magma. Decreasing Ni abundances and increasing Cu contents during differentiation are a sign of olivine separation. In the analysis presented herein, V–in the absence of Cpx separation–is found to behave remarkably like the moderately incompatible element Zn, and these two elements add to the list of element pairs of similar incompatibility whose ratios are insensitive to differentiation and to submarine weathering as well. Both are enhanced in titanomagnetite, so both would be compatible during titanomagnetite separation. When Cpx separates, however, V becomes compatible like Sc, but Zn remains incompatible. Thus, decreasing V (and Sc) contents and increasing Zn contents during differentiation are a sign of Cpx separation. Manganese often behaves much like Zn and therefore is moderately incompatible, but Mn is less compatible than Zn and V in titanomagnetite. Thus, decreasing Zn and V with increasing Mn is an indication of titanomagnetite removal. Dual compatible and incompatible trends with differentiation are found chiefly for Cu, Sc, and Sr. Distinguishing mid-ocean ridge basalts (MORB), oceanic-island volcanic rocks (OIV), and island-arc volcanic rocks (IAV) may be accomplished by plots of Ce/Yb versus Ba/Ce, where OIV plot to higher values of Ce/Yb than do MORB, and IAV data plot to higher values of Ba/Ce than do those of MORB. These ratios do not seem to be significantly affected by submarine weathering.

Iapetus Ocean floor stuffed into a suture zone: xenolith Nd isotopic evidence for Dunnage-equivalent basement in central Newfoundland

Granulite-facies xenoliths from Late Jurassic alkaline lamprophyres may represent basement to the Dunnage Zone in north-central Newfoundland (Notre Dame Bay area). At 143 Ma the xenoliths had positive εNd values between 0.9 and 4.7. They give Nd depleted mantle model ages around 700 Ma and have trace element and major element compositions reminiscent of oceanic arc-related intermediate volcanic and sedimentary rocks. Their positive εNd values and associated "young" Nd model ages argue against their representing Grenvillian crust. Similarly, Gander Zone basement to the east produced granitic rocks with strongly negative εNd values unlike those of the xenoliths. Positive εNd values for Avalonian granites indicate that the xenoliths could represent Avalon-type basement; however, there are 100–200 km of Gander and Dunnage zone rocks between the xenoiith locality and the Avalon Zone. Early orogenic volcanic rocks and some late orogenic to postorogenic granitic rocks in the central to northern portion of the Gander Zone have positive εNd values, consistent with extraction from a depleted mantle at the same time as material forming the xenoliths. Similarities between the xenolith chemistry and that of early orogenic (Cambrian) arc-related intermediate volcanic rocks of the Dunnage Zone indicate that the xenoliths and basement in the Notre Dame Bay area are composed of Iapetus Ocean floor relics (volcanic or volcanic-rich sedimentary rocks) stuffed into a collisional suture zone during ocean closure.

The Hf isotopic composition of ferromanganese nodules and crusts and hydrothermal manganese deposits: Implications for seawater Hf

We present Hf and Pb isotopic data, and chemical compositions of the outermost layers of marine ferromanganese deposits of different types (hydrogenous and hydrothermal) with a worldwide distribution. The Hf isotopic compositions display a broad range and refine previously reported regional differences as follows: Atlantic Ocean ɛ Hf = −4to+2, Indian Ocean ɛ Hf = +2to+4, Pacific Ocean ɛ Hf = +3to+10. The most radiogenic Hf isotopic compositions in the Pacific samples are for hydrothermal manganese deposits that also have low 207Pb 204Pb, demonstrating that this signature reflects a contribution from hydrothermal venting of Hf leached from oceanic volcanic rocks rather than from riverine inputs, volcanic ash, or eolian dust. Hafnium concentrations in the deposits increase from 20 ppb to 20 ppm with decreasing ɛ Hf, The Hf and Pb isotopic compositions for ferromanganese crusts define an apparent mixing trend between literature values of average continental crust and MORB. The range in ɛ Hf for ferromanganese crusts is narrower than it is for 206Pb 204Pb compared to the differences in isotopic composition of the sources of Hf and Pb. This is consistent with Hf having a longer residence time than Pb. The concentration of Hf in ferromanganese crusts has been found to co-vary with growth rate, and inversely correlates with Hf isotopic compositions. Hf isotope ratios may be used to determine not only the source of Hf, but possibly the source of Fe and Mn. Measurements of ɛ Hf and Hf concentrations in nodule tops, bottoms and associated sediments show that the ɛ Hf of nodules is sensitive to sedimentary oxic and sub-oxic diagenetic processes and thus most nodules may not reliably reflect the isotopic composition of Hf in seawater.

Magma sources and plumbing systems during break-up of the SE Greenland margin: preliminary results from ODP Leg 152

Abstract ODP Leg 152 drilled a transect across the SE Greenland seaward-dipping reflector sequence. The earliest, and most landward, lava flows (Hole 917A, Lower and Middle Series) represent a pre-break-up continental sequence showing an overall variation from olivine basalt to dacite. Evolution in deep crustal magma reservoirs, with a dwindling supply of primitive magma and an increasing role of crustal contamination, can account for the variation. Eruption of picrite and olivine basalt magmas (917A, Upper Series) marked a dramatic change in the style of magmatism to one of unrestrained passage of primitive magma from the mantle to the surface during the final phase of break-up. The younger parts of the seaward-dipping reflector sequence (915A and 918D) comprise compositionally uniform basalt (7.6 ± 0.8% MgO), suggesting that an effective magmatic filtering system was established, soon after break-up, in magma chambers associated with a spreading axis. The continental to oceanic magmatic transition is marked by clear changes in composition. The continental magmas had a garnet lherzolite source (indicated by low Y/Zr) and were contaminated with continental lithospheric mantle and crust (high Ba/Zr). The majority of the break-up and oceanic volcanic rocks show no evidence for lithospheric contamination (low Ba/Zr) and appear to have had a source in progressively shallower, depleted (?MORB-source) asthenosphere (increasing Y/Zr, low Nb/Zr).

Western Newfoundland, Canada, a Lower Paleozoic Basin with Hydrocarbon Potential

Western Newfoundland hosts an early Paleozoic passive margin that was destroyed by later convergence and is now preserved in a foreland fold and thrust belt. Autochthonous Cambrian-Ordovician carbonate and siliciclastic rift, shelf, and foreland basin cover sequences are structurally overlain by allochthonous comprising of coeval deep-water periplatform and oceanic sedimentary, volcanic, and ophiolitic rocks. Geologic relationships indicate that allochthonous deep-water sediments were partly emplaced above the autochthon by Late Ordovician Taconian orogenesis. Recent mapping of a post-Silurian, pre-Mississippian triangle zone in the offshore, and of an imbricated thrust shelf carbonates in the onshore indicates that the present disposition of lower Paleozoic rocks is the product of Acadian foreshortening of at least 100 km. The stratigraphy, geologic setting, and history of the lower Paleozoic rocks and the presence of oil shows in wells, seeps, and bitumin throughout the area mark western Newfoundland as a potential hydrocarbon basin similar to other lower Paleozoic examples dolomite reservoirs are common in the autochthon and occur locally in the allochthon. Source rocks occur in both the deep-water and foreland basin sequences. Allochthonous deep-water shales have total organic carbon values of up to 8.37%, whereas foreland basin black graptolitic shales range up to 2%. Pyrolysis datamore » and conodont color alteration indices document immature to mature thermal maturation in the southern part of the area and mature to overmature maturation in the northern part of the area.« less

La-Ce and Sm-Nd systematics of siliceous sedimentary rocks: A clue to marine environment in their deposition

La-Ce isotopic data, together with Sm-Nd isotopic data, were determined on siliceous sedimentary rocks (cherts) in order to elucidate the rare earth element (REE) character of their sources and the nature of their depositional environments. The cherts studied are a late Archean chert from the Gorge Creek Group in the Pilbara block of Western Australia, Triassic cherts from central Japan, and Cretaceous and Paleogene deep-sea cherts from the central Pacific and the Caribbean Sea. The Archean chert from the Gorge Creek Group shows chondritic Ce and Nd isotope ratios at its sedimentation age which indicate that its sources had a time-integrated chondritic REE pattern. Triassic cherts from Japan have initial Ce and Nd isotope ratios that show a direct derivation from their continental source. On the other hand, for Cretaceous and Paleogene deep-sea cherts having negative Ce anomalies in their REE patterns, two different sources for Ce and Nd are revealed from their initial Ce and Nd isotope data: Ce from long-term light-REE-depleted oceanic volcanic rocks and Nd from light-REE-enriched continental rocks. The reverse nature observed for deep-sea cherts is considered to be a reflection of their depositional environment far from a continent. These results confirm that the La-Ce isotopemore » system is highly useful in determining the nature and cause of Ce anomalies observed in marine sedimentary rocks such as chert.« less

Bedrock geology and tectonic evolution of the Wrangellia, Peninsular, and Chugach Terranes along the Trans‐Alaska Crustal Transect in the Chugach Mountains and Southern Copper River Basin, Alaska

The Trans‐Alaskan Crustal Transect in the southern Copper River Basin and Chugach Mountains traverses the margins of the Peninsular and Wrangellia terranes, and the adjacent accretionary oceanic units of the Chugach terrane to the south. The southern Wrangellia terrane margin consists of a polymetamorphosed magmatic arc complex at least in part of Pennsylvanian age (Strelna Metamorphics and metagranodiorite) and tonalitic metaplutonic rocks of the Late Jurassic Chitina magmatic arc. The southern Peninsular terrane margin is underlain by rocks of the Late Triassic (?) and Early Jurassic Talkeetna magmatic arc (Talkeetna Formation and Border Ranges ultra‐mafic‐mafic assemblage) on Permian or older basement rocks. The Peninsular and Wrangellia terranes are parts of a dominantly oceanic superterrane (composite Terrane II) that was amalgamated by Late Triassic time and was accreted to terranes of continental affinity north of the Denali fault system in the mid‐ to Late Cretaceous. The Chugach terrane in the transect area consists of three successively accreted units: (1) minor greenschist and intercalated blueschist, the schist of Liberty Creek, of unknown protolith age that was metamorphosed and probably accreted during the Early Jurassic, (2) the McHugh Complex (Late Triassic to mid‐Cretaceous protolith age), a melange of mixed oceanic, volcaniclastic, and olistostromal rocks that is metamorphosed to prehnite‐pumpellyite and lower greenschist facies that was accreted by middle Cretaceous time, and (3) the Upper Cretaceous Valdez Group, mainly magmatic arc‐derived flysch and lesser oceanic volcanic rocks of greenschist facies that was accreted by early Paleocene time. A regional thermal event that culminated in early middle Eocene time (48–52 Ma) resulted in widespread greenschist facies metamorphism and plutonism.

Roberts victor eclogites: Ancient oceanic crust

The two groups of Roberts Victor eclogites may be further distinguished by their 18O/16O ratios; one above (I) and one below (II) the mantle value (5.7). In the group II suite systematic variation of the major elements with oxygen isotopes matches that calculated for oceanic volcanic rocks altered by circulating seawater in ridge crest hydrothermal systems. A negative linear correlation between 18O/16O and 87Sr/86Sr(WR) and an associated positive correlation with K2O (and Rb) of the anhydrous eclogite assemblage with 87Sr/86Sr(WR) are also products of hydrothermal alteration. The chemical and isotopic similarity of the eclogites to hydrothermally altered oceanic crust supports the conclusion that they are subducted fragments that have been metamorphosed from metabasalts to eclogites. During metamorphism the elements are fractionated into two components. One, enriched in the radiogenic isotopes of U, Th, K, and Rb, and light rare earth elements, is concentrated in a hydrous, interstitial, mobile component, and the other, enriched in Nd relative to Sm, is concentrated in the anhydrous, immobile, garnet and clinopyroxene host. The mobile component is comparable in chemistry to rocks of the MARID suite and may well play an important role in mantle metasomatism. Nd/Sm and Pb/Pb ages indicate an Archean age (approximately 2.47 b.y.) for the metamorphism. Depth estimates indicate a skewed distribution with most eclogites coming from the depth range 165–190 km. The concentration of eclogites correlates with the inflection in the Lesotho geotherm and appears to mark a disrupted layer of subducted Archean oceanic crust separating a shallower region of undeformed, refractory, Mg‐rich peridotites from deformed, Fe‐rich, more “fertile” pyrolite. This boundary may be interpreted as the “petrologic” base to the continental lithosphere.