Ocean Floor Basalts Research Articles

Additional evidence of existence of ancient rocks in the mid-Atlantic ridge and the age of the opening of the Atlantic

A metabasalt dredged at a junction of the median valley with the Atlantis fracture zone (30°01/tN, 42°04/tW) in the Mid-Atlantic Ridge shows complete recrystallization under a metasomatic condition, though the original igneous texture of a coarse-grained basalt is still recognizable. There is strong circumstantial evidence suggesting that this rock is not an ice-rafted erratic, but an authentic Mid-Atlantic Ridge rock. The 40Ar- 39Ar age of this sample is 169 m.y. (Jurassic) which should represent the time of recrystallization. The initial value ( 87Sr/ 86Sr ) O is 0.720, far above the values previously observed in oceanfloor basalts, including both tholeiitic and alkalic rocks (0.701–0.704). Sr with such a high isotopic ratio is considered to have been introduced by metasomatism during metamorphism by a solution coming from a continental mass or masses which were then located very close to the Mid-Atlantic Ridge. The 40Ar− 39Ar age of sample AM50 may approximate the time of the commencement of the opening of the Atlantic. All these data support the possible existence of ancient rock masses in the Mid-Atlantic Ridge, as was formerly claimed by Bonatti, Melson and others.

Statistical Analysis of Major Element Patterns in Basalts

Discriminant analysis enables basalts from different tectonic settings to be identified on the basis of their complete major element patterns. By using discriminant functions it is possible to represent visually the separation of six tectonically-defined magma types in three dimensions instead of the original eight. Analysis of variance confirms that this separation is highly significant. Numerical classification is possible by applying Bayes' decision rule, using either normal distribution functions or empirical discriminant functions. Tests on known samples showed that ocean-floor basalts, low-potassium tholeiites (from island arcs), calc-alkali basalts and shoshonites (from volcanic arcs) and within-plate basalts could be correctly classified about 90 per cent of the time by all these methods. A subdivision of within plate basalts into ocean island and continental basalts could not however be achieved chemically with any great success. Classification of weathered and metamorphosed ocean-floor basalts showed that alteration can greatly reduce the success rate of the classification. Application of the numerical and visual methods to lavas of 'unknown' tectonic settings (Archaean greenstones and high-potash basalts) mostly gave geologically consistent results.

Basic intrusions in the Ordovician of North Wales—geochemical data and tectonic setting

Selected geochemical data (principally immobile elements, including rare earth elements), are presented for a restricted basic instrusive suite to the west of the Conway Valley, North Wales. Immobile element data are used to characterise the rocks in terms of magma type, differentiation and tectonic environment by comparison with modern fresh volcanics. The intrusives have suffered varying degrees of low-grade hydrous metamorphism and are now meta-dolerites exhibiting an admixture of primary and secondary phases. The meta-dolerites compare closely to sub-alkaline, high alumina basalts and in general are little differentiated. They were probably generated near the margin of a stable continental plate; that is. continental calc-alkali volcanism, ‘Andean-type’. A calc-alkali island arc environment seems less likely. One unexpected geochemical feature is the discovery of lateral chemical variation and symmetry along the strike of the intrusive outcrop with the development of ‘primitive’, low immobile element, basaltic rocks (akin to ocean floor basalts) in the central zone.

Sr-isotopic evidence for an old mantle source region for French Polynesian volcanism

Research Article| December 01, 1976 Sr-isotopic evidence for an old mantle source region for French Polynesian volcanism R. A. Duncan; R. A. Duncan 1Research School of Earth Sciences, Australian National University, Canberra, A.C.T., Australia Search for other works by this author on: GSW Google Scholar W. Compston W. Compston 1Research School of Earth Sciences, Australian National University, Canberra, A.C.T., Australia Search for other works by this author on: GSW Google Scholar Author and Article Information R. A. Duncan 1Research School of Earth Sciences, Australian National University, Canberra, A.C.T., Australia W. Compston 1Research School of Earth Sciences, Australian National University, Canberra, A.C.T., Australia Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1976) 4 (12): 728–732. https://doi.org/10.1130/0091-7613(1976)4<728:SEFAOM>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. A. Duncan, W. Compston; Sr-isotopic evidence for an old mantle source region for French Polynesian volcanism. Geology 1976;; 4 (12): 728–732. doi: https://doi.org/10.1130/0091-7613(1976)4<728:SEFAOM>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract It has been proposed that the volcanic islands of French Polynesia (south-central Pacific Ocean) owe their origin either to the activity of several hot spots (plumes) or to propagating tensional fractures. The isotopic composition of strontium has been determined for Miocene to Holocene volcanic rocks from island members of three linear island chains in French Polynesia. A high correlation between Sr87/Sr86 and Rb87/Sr86 is evident and, interpreted as an isochron, indicates that chemical heterogeneities in the mantle source region for these rocks have been maintained for one billion years or more. This ancient age implies that the source region is divorced from the source regions for ocean-floor basalts. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The basalt clan

The major element compositions of the various rocks that belong to the basalt clan are examined, and the basalt clan is defined as consisting of those volcanic rocks that contain between 44.0% and 53.5% silica. After examining both the diverse tectonic environments in which the basaltic rocks are found, and also the various petrographic suites to which they belong, or appear to belong, new chemical criteria were devised to divide them into the following groups: (1) low-potash basalt, (2) high-alumina basalt, (3) continental-flood basalt, (4) komatiitic basalt, (5) sodic basalt, (6) hawaiite, (7) phonolitic basalt, (8) potassic basalt, (9) trachybasalt, (10) leucitite, (11) sodic transitional basalt, and (12) potassic transitional basalt. The lunar mare-basalts are regarded as being transitional in their chemical character between the low-potash, ocean-floor basalts and the komatiitic basalts. While it was relatively easy to discover petrogenetic models that were able to account for the origin and evolution of the common sub-alkalic basalts, and even the sodic basalts, a variety of possible petrogenetic models had to be explored in order to account for the origin and evolution of the different basaltic rocks of the potash- and high-potash series.

Significance of Cs/Rb ratios in volcanic rocks as exemplified by the Nohi rhyolite complex, central Japan.

Cs/Rb ratio was examined on the Nohi rhyolite complex in Central Japan, which is composed mainly of rhyolitic ignimbrite. The variation in the Cs/Rb ratio of the magma through crystallization differentiation was estimated using a Rayleigh fractionation model and available distribution coefficients between phenocryst minerals and co-existing groundmass. It was found that the K/Rb ratio of this complex decreased from 273 to 125, but the Cs/Rb ratio remained constant through the differentiation process. The initial Cs/Rb ratio of this magma is 0.030 ± 0.005. The value is very close to the average Cs/Rb ratio of the ignimbrite in the Taupo region, Northern New Zealand, but is distinctly heigher than that of ocean floor basalts. To estimate the effect of contamination the Rb-Sr whole rock isochron method was used.

Chemical and isotopic constraints on the origin of low-silica latite and andesite from the Andes of central Peru

Low-silica latite highly enriched in large-ion-lithophile elements and moderately potassic low-Si andesite were erupted in central Peru during late Cenozoic time. FeO*/MgO ratios of 0.93 to 1.25 at 53 wt percent SiO/sub 2/ indicate a definitely calcalkalic character. The combination of low FeO*/MgO ratios, low SiO/sub 2/, and high Cr, Co, and Ni with large-ion-lithophile and light rare-earth elements makes it very unlikely that the parent magmas were produced by high-pressure partial melting of subducted ocean-floor basalt (eclogite). The data are more compatible with small degrees of partial melting of ultramafic material or mixtures of basalt and ultramafic mantle. The initial melts probably contained 52 to 53 percent SiO/sub 2/ and had a content of large-ion-lithophile elements nearly as high as that of the rocks. /sup 87/Sr//sup 86/Sr ratios of from 0.7042 to 0.7051 and low to very low Rb/Sr ratios indicate an isotopically variable source region that, at least in part, had earlier been depleted in Rb relative to Sr.

Oceanic mafic rocks in the Eastern Alps

The Eastern Alps in Austria have been interpreted as a pile of thrust sheets resulting from the collision of two continental masses. The only remains of the ocean-floor which may once have separated these continents could be the highly deformed greenschists, metasediments and serpentinites found in the lower thrust sheets. To test this hypothesis, a total of sixty mafic rocks from the Grosglockner, Mooserboden, Fusch, Hochtor, Matrei Zone and Strobl localities have been analysed for the stable trace elements, Ti, Zr, Y, Nb and Cr, and the less stable elements K, Rb, and Sr. Visual and statistical comparison of the stable elements with known magma types reveals that five of the sample groups classify clearly as tholeiitic ocean-floor basalts, while one group, the Fusch locality, classifies as within-plate (probably ocean island) basalts. It is suggested that the tectonic units containing such rocks comprise a melange of disrupted oceanic crust, upper mantle and seamounts, pelagic sediments and continental margin sediments. The rocks may have formed in a large ocean basin, rather than a marginal basin behind an island arc.

Yttrium geochemistry applied to petrogenesis utilizing calcium-yttrium relationships in minerals and rocks

A survey of Y data from all sources shows that Y behaves systematically in igneous, metamorphic and sedimentary rock series, due to its incorporation in a predictable and uniform manner in Ca minerals. Compared with average calc-alkali basalt melts, plagioclase, kaersutite, augitic clinopyroxene and calcite have low Y for their CaO contents; whereas hornblende, garnet, orthopyroxene, apatite, sphene, zircon and most K, Na rich minerals other than plagioclase have high Y for their Ca contents. In sedimentary processes, Ca/Y becomes lower in shales and sandstones, but higher in limestones than their source. In metamorphic processes Y appears to be inert. In all igneous series for which Y data has been assembled Ca/Y falls as Ca falls. These series can be classified into three categories: (i) a standard calc-alkali trend, used as a reference (ii) J-type trends which become progressively impoverished in Y as Ca falls when compared with the standard trend, and (iii) L-trends which become progressively enriched in Y as Ca falls when compared with the standard trend. Despite little knowledge of partition coefficients, the J-type trends appear to have a significant component of hornblende control in their fractionation processes, whereas plagioclase and augitic clinopyroxene dominate the L-type trend. Alternative names for these series might therefore be the standard series, the hornblendic and the pyroxenic series respectively. Modern ocean floor basalts appear to be significantly richer in Y than modern calc-alkali basalts, and these in turn may be richer in Y than their Archaean counterparts.

Role of underthrust oceanic crust in the genesis of a Fijian calc-alkaline suite

A test of the proposition that calc-alkaline magmas are direct partial melts of underthrust oceanic crust is presented. It involves numerical evaluations of whether or not the major and trace element and isotopic composition of a Fijian calc-alkaline rock suite is consistent with these rocks representing unfractionated partial melts of oceanic crust at high pressures. Experimental data for one of the samples constrain the calculations. When compositions of liquidus minerals at 27 kb are combined with compositions of the volcanic rocks, close approximations can be made to the composition of oceanic crust only if the degree of partial melting is between ∼20% (dacite) and ∼45% (basaltic-andesite), and if accessory minerals are refractory phases. Concentrations of elements such as K, Rb, Sr, Pb, Th, and U, and 87Sr/86Sr ratios in the Fijian suite can be satisfactorily explained only if the parental material consisted of altered rather than fresh ocean floor basalt. Sediments are not likely to have been involved. Concentrations of Na, Ni, Co, Cr, Sc, V, the REE, Y, Zr, Hf, and Nb cannot be explained unless, or in some cases even if, several accessory phases are partially refractory. Therefore, partial melting of underthrust lithosphere does not seem likely to produce magmas with the composition of at least one quite typical calc-alkaline suite.

Lavas from Niuafo'ou Island, Tonga, Resemble Ocean–Floor Basalts

Major element, trace element, and mineral analyses of rocks from the island of Niuafo9ou in the western Pacific Ocean show that Niuafo9ou rocks have a marked similarity to ocean-floor basalt, especially with basalt of relatively high FeO*/MgO ratios.

Trace elements and tectonic relationships of basaltic rocks in the Ballantrae igneous complex, Ayrshire

SummaryIn order to obtain further evidence for the tectonic environment of generation of the basaltic rocks of the Ballantrae igneous complex, specimens of pillow lavas, dolerites, glaucophane schists and amphibolites were analysed for Ti, Zr, Y and Nb. Using Ti–Zr–Y, Ti–Zr and Y/Nb diagrams, three different genetic groups have been distinguished – (i) hot-spot basalts round Downan Point, round Bennane Head as far N as Balcreuchan Port, and also within serpentinite around Lendalfoot, (ii) island arc low-potassium tholeiites between Balcreuchan Port and Gamesloup, and (iii) possible ocean floor basalts at Knockormal and Knocklaugh. The interrelationships of these groups are in doubt. The hot-spot basalts may have been erupted through oceanic crust, but they could have formed prior to, or subsequent to, the island arc activity. More likely, the hot-spot basalts formed (possibly on oceanic crust) at a considerable distance from the island arc, and the two were later brought together tectonically, probably at the subduction zone associated with the arc.

Aspects of magmatism and plate tectonics in the Precambrian of England and Wales

AbstractThe geological features of igneous, metamorphic and sedimentary rocks comprising the Precambrian of England and Wales suggest formation in one or more Precambrian orogenic cycles. They are now interpreted in terms of plate tectonics. Evidence for Late Proterozoic plate subduction in the Mona Complex of Anglesey is suggested by the association of pillow lavas, cherts, high P/T metamorphic rocks and by the occurrence of gabbros and serpentinites with similar features to rocks believed to comprise the oceanic crust. Precambrian rocks in England and South Wales include calc‐alkaline plutonic complexes (Malvern and Johnston Complexes), calc‐alkaline lavas (Uriconian and Charnian) and basic and intermediate intrusions of tholeiitic affinity (dykes in the plutonic complexes and granophyric diorites in Charnwood Forest). The features of these rocks indicate formation in a continental margin setting and this is consistent with features of the Rushton Schist and Primrose Hill “gneiss” which suggest that they predate the Late Proterozoic orogenic activity. This evidence is consistent with plate tectonic models involving oceanic plate subduction below the Mona Complex from an ocean to the northwest, or from a small ocean basin southeast of the complex. The Warren House lavas show some affinities to ocean floor basalts and are problematic with regard to the Precambrian history of the area.

Zr contents of glaucophane-bearing meta-basalts of Western Crete, Greece

Glaucophane-bearing meta-basalts of Western Crete are compared with basalts of different crustal environments with respect to their Zr contents and Ti/Zr correlations. Two groups of meta-basalts are distinguished. The first group (90–140 ppm Zr) corresponds to ocean-floor basalts while the second one (210–430 ppm Zr) nearly fits in oceanic alkali basalts. The first group would represent the more primitive, the second one the more differentiated members of a basalt suite which are transformed into glaucophane-lawsonite schists during the subduction of an oceanic plate.

TiO2 and a Possible Guide to Past Oceanic Spreading Rates

THE work reported here shows that there is some sort of systematic relationship between the titanium content of present-day ocean-floor basalts and spreading rate. This simple geochemical indicator can be applied to estimate ancient spreading rates using basalts and spilites from ophiolite complexes.

Proceedings

23 May 1973 Volcanic Studies Group Meeting The geochemistry of volcanic rocks relative to their tectonic setting Trace elenlent geochemistry related to the tectonic environment of basalt eruption J. R. Cann &amp; J. A. Pearce, University of East Anglia. Recent investigations have shown that there is a good correspondence between the trace element geochemistry of basic and intermediate volcanic rocks and the tectonic environment of their eruption. Major elements, with the possible exception of Mg, show less discrimination between tectonic settings (Ti and K are counted here as trace elements). The following tectonic-cure-chemical classes of basalts and basaltic andesites can be distinguished from one another: ocean-floor basalts (characteristic of relatively fast-spreading plate boundaries); low-potassium tholeiites, calc-alkaline volcanics and shoshonites of island arcs; and within-plate basalts, of which alkalic and tholeiitic varieties may be distinguished. There is some overlap for almost all elements yet investigated between some ocean-floor basalts and some low-potassium tholeiites of island arcs, but the other classes are reasonably distinct except where transitional varieties of volcanic rock are found. The tectonic classification of environment of eruption cannot be considered final, especially as within-plate basalts also appear too to be associated with slowly-spreading plate boundaries, and because of the possible tectonic importance of plumes rising from the lower mantle. The petrogenetic processes leading to these geochemical differences are far from clear but it seems that the variation observed can be explained by a model involving migration of a hydrous, incompatible-rich fluid in the mantle, with consequent changes in phases on the basalt liquidus.

Tectonic setting of basic volcanic rocks determined using trace element analyses

Analyses for Ti, Zr, Y, Nb and Sr in over 200 basaltic rocks from different tectonic settings have been used to construct diagrams in which these settings can usually be identified. Basalts erupted within plates (ocean island and continental basalts) can be identified using a Ti-Zr-Y diagram, ocean-floor basalts, and low-potassium tholeiites and calc-alkali basalts from island arcs can be identified using a Ti-Zr diagram (for altered samples) and a Ti-Zr-Sr diagram (for fresh samples). Y/Nb is suggested as a parameter for indicating whether a basalt is of tholeiitic or alkalic nature. Analyses of dykes and pillow lavas from the Troodos Massif of Cyprus are plotted on these diagrams and appear to the tholeiitic ocean-floor rocks.

Paleozoic basaltic komatiite and ocean-floor type basalts from northeastern Newfoundland

Basic pillow lavas from northeastern Newfoundland are shown to be chemically similar to basaltic komatiites and ocean-floor basalts. New trace element data presented for basaltic komatiites indicate that they were derived from a magma more primitive than that producing ocean-floor basalts. A model for the derivation of basaltic komatiite by extensive partial melting of pyrolitic upper mantle at sites of plate separation is proposed.

Ocean Floor Basalt Affinity of Precambrian Glaucophane Schist from Anglesey

THE Mona Complex of Anglesey and Lleyn comprises a thick geosynclinal sequence of arenaceous and argillaceous sedimentary rocks1, which were folded, variably metamorphosed and subjected to granite intrusion during the late Precambrian2. The complex includes separated outcrops of spilitic lavas, basic glaucophane schists and gabbros and serpentinized ultramafic rocks3. Where these rocks coexist in an orogenic environment, they have been termed an ophiolitic association. Such associations may represent thrust slices of ocean crust and upper mantle emplaced at destructive plate margins4. An origin for the Mona Complex at such a plate margin during the late Precambrian has been suggested by Dewey5. But the relationship between the different rock units in Anglesey is obscure and their affinity with oceanic crust is unproven. Here I discuss some chemical features of the glaucophane schists which suggest that these rocks have originated by metamorphism of basaltic rocks with an affinity to ocean-floor tholeiite.

Ophiolite origin investigated by discriminant analysis using Ti, Zr and Y

Samples of rocks from four known or suspected ophiolite complexes were compared with five groups of Cenozoic volcanic rocks using their contents of Ti, Zr and Y. Triangular plots of Ti-Zr-Y and plots of Ti-Zr were constructed, and the problem was also approached using discriminant analysis. The elements Ti, Zr and Y were chosen because of their apparent invariance with respect to such secondary processes as weathering and metamorphism which have often affected ophiolitic rocks. The five groups of volcanic rocks nominated as standards were a group of ocean-floor basalts, one of Hawaiian tholeiites, one of alkali olivine basalts from Flores, Azores, one of island-arc andesites, and one of island-arc tholeiites from Japan. These could all be well discriminated using the three elements chosen. The four ophiolites selected included two, Troodos and Oman, which have been postulated on structural grounds to be fragments of ocean crust, and two others, from the Grossglockner region, Austria, and from the Othrys region, Greece, which were less well-defined. Most of the samples from all four ophiolites showed the greatest affinity with the ocean-floor basalt group, except for seven andesites from Greece. It is thus very probable that most of the samples measured were generated as ocean crust at spreading plate boundaries, and have been thrust into their present positions during continental collision.