The alkali basalt andesite association of Grenada Lesser Antilles

Grenada is the southernmost volcanic island of the Lesser Antilles. A series of volcanic centres ranging from Pliocene to Recent in age are present overlying a folded Lower to Middle Tertiary volcano-sedimentary formation. Eruptions of silica-undersaturated alkali basalt and picrite magmas have occurred repeatedly during the evolution of these centres. Calc-alkaline andesites and dacites show a close field association with the basalts and picrites. Recent activity on the island has been explosive in nature. A model of variable volumes of Upper Mantle partial melting is proposed to account for the diversity of major, trace and Rare Earth element compositions and strontium isotope ratios of the basalts and picrites. Geochemical, petrographic and mineralogical criteria suggest that the andesites and dacites are related to these basic melts by fractional crystallisation processes. In addition the chemical compositions and strontium isotope ratios of the andesites and dacites reflect the diverse compositions of the parental basalt magmas. The petrography and mineralogy of the andesites and dacites is similar to calc-alkaline suites elsewhere in the arc. Some of the basalts and picrites contain abundant olivine and sector and oscillatory zoned clinopyroxene phenocrysts. In some basalts, phenocryst amphibole is present. An origin by partial melting of an Upper Mantle pericotite source is proposed. Alternative sources are examiner: but partial melting of a subducted lithospheric plate does not appear to be a significant petrogenetic process in Grenada, Fractional crystallisation of olivine, clinopyroxene and spinel is mainly responsible for the development of a normal calc-alkaline trend towards increasing silica-saturation in the magmas. Subsequent crystallisation of plagioclase feldspar and then amphibole is also important in the development of a trend towards silica- rather than alkali-enrichment in the residual melts. The significant feature of the Grenada vulcanicity is the occurrence within a restricted geographic range of magmas of contrasted geochemical characteristics. The local volcanic and tectonic history of the southern part of the Lesser Antilles island arc are probably the most important factors in the development of these unusual characteristics

Geochemistry and cosmochemistry, proceedings of the 27th international geological congress, vol. 11 : Moscow 4-14 August 1984, N. A. Bogdanov (Gen. Sec. Organizing Cte.). VNU Science Press. 1984, 605p., DM 164.00

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26133/1/0000209.pd

The significance of source versus process in the tectonic controls of magma genesis

Despite the association of certain characteristic trace-element signatures with particular tectonic environments of eruption, there are accumulating data which would result in significant tectonic misassignments. Ambiguity of signals appears in active arc/back-arc systems of the southwestern Pacific and particularly in some intracontinental plate suites. Given the selective preservation of continental as opposed to oceanic lithosphere, inappropriate paleotectonic inferences are probable using trace-element criteria alone.Strong relative fractionation of the alkalis and alkaline earth elements (AEE) with respect to the rare earth elements (REE) in the majority of arc-related magmas and a number of intraplate continental basalts is strongly suggestive of the involvement of hydrous fluids at some stages in the respective petrogenetic processes occurring in these two tectonic regimes. In contrast, fractionation of high-field-strength elements (HFSE) such as Nb and Ta with respect to the REE in the same suites is most readily explained by the involvement, at some stage in the magma formation process, of high-SiO2 melts. A number of widely applied tectonic discriminants makes use of AEE/HFSE fractionation, but the processes and sources involved in subduction-zone petrogenesis may be duplicated during interaction of mantle-derived basalt with the heterogeneous components of continental lithosphere, both mantle and crust. A significant role for both volatile-dominated fluids and silicate melts is implicated in collision and some intracontinental plate magmatism.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26680/1/0000224.pd

Landforms predict phylogenetic structure on one of the world's most ancient surfaces

<p>Abstract</p> <p>Background</p> <p>The iconic Pilbara in northwestern Australia is an ancient geological and biophysical region that is an important zone of biodiversity, endemism and refugia. It also is overlain by some of the oldest erosion surfaces on Earth, but very little is known about the patterns of biotic diversity within the Pilbara or how they relate to the landscape. We combined phylogenetic and spatial-autocorrelation genetic analyses of mitochondrial DNA data on populations of the gekkotan lizard <it>Lucasium stenodactylum </it>within the Pilbara with geological, distributional and habitat data to test the hypothesis that ancient surface geology predicts current clade-habitat associations in saxicoline animals.</p> <p>Results</p> <p>This is the first detailed phylogenetic examination of a vertebrate organism across the Pilbara region. Our phylogeny provides strong support for a deep and ancient phylogenetic split within <it>L. stenodactylum </it>that distinguishes populations within the Pilbara region from those outside the Pilbara. Within the Pilbara region itself, our phylogeny has identified five major clades whose distribution closely matches different surface geologies of this ancient landscape. Each clade shows strong affinities with particular terrain types and topographic regions, which are directly related to different geological bedrock.</p> <p>Conclusion</p> <p>Together our phylogenetic, distributional, geological and habitat data provide a clear example of ecological diversification across an ancient and heterogeneous landscape. Our favoured hypothesis is that ancestors of the Pilbara lineages radiated into the region at the onset of aridity in Australia approximately 5 mya and locally adapted to the various ancient and highly stable terrain types and the micro-habitats derived from them. In terms of specimen recovery and analysis, we are only beginning to reconstruct the biotic history of this ancient landscape. Our results show the geological history and the habitats derived from them will form an important part of this emerging story.</p

Iron isotopic evidence for convective resurfacing of recycled arc-front mantle beneath back-arc basins

Geophysical observations suggest sub-arc convective flow transports melt-exhausted and metasomatized wedge mantle into deeper mantle regions. Reciprocally, asthenospheric, fertile mantle may supply back-arc ridges distal to the trench by shallow, latera

The importance of talc and chlorite "hybrid” rocks for volatile recycling through subduction zones; evidence from the high-pressure subduction mélange of New Caledonia

The transfer of fluid and trace elements from the slab to the mantle wedge cannot be adequately explained by simple models of slab devolatilization. The eclogite-facies mélange belt of northern New Caledonia represents previously subducted oceanic crust and contains a significant proportion of talc and chlorite schists associated with serpentinite. These rocks host large quantities of H2O and CO2 and may transport volatiles to deep levels in subduction zones. The bulk-rock and stable isotope compositions of talc and chlorite schist and serpentinite indicate that the serpentinite was formed by seawater alteration of oceanic lithosphere prior to subduction, whereas the talc and chlorite schists were formed by fluid-induced metasomatism of a mélange of mafic, ultramafic and metasedimentary rocks during subduction. In subduction zones, dehydration of talc and chlorite schists should occur at sub-arc depths and at significantly higher temperatures (∼ 800°C) than other lithologies (400-650°C). Fluids released under these conditions could carry high trace-element contents and may trigger partial melting of adjacent pelitic and mafic rocks, and hence may be vital for transferring volatile and trace elements to the source regions of arc magmas. In contrast, these hybrid rocks are unlikely to undergo significant decarbonation during subduction and so may be important for recycling carbon into the deep mantl

Boninite and Harzburgite from Leg 125 (Bonin-Mariana Forearc): A Case Study of Magma Genesis during the Initial Stages of Subduction

Holes drilled into the volcanic and ultrabasic basement of the Izu-Ogasawara and Mariana forearc terranes during Leg 125 provide data on some of the earliest lithosphere created after the start of Eocene subduction in the Western Pacific. The volcanic basement contains three boninite series and one tholeiite series. (1) Eocene low-Ca boninite and low-Ca bronzite andesite pillow lavas and dikes dominate the lowermost part of the deep crustal section through the outer-arc high at Site 786. (2) Eocene intermediate-Ca boninite and its fractionation products (bronzite andesite, andesite, dacite, and rhyolite) make up the main part of the boninitic edifice at Site 786. (3) Early Oligocene intermediate-Ca to high-Ca boninite sills or dikes intrude the edifice and perhaps feed an uppermost breccia unit at Site 786. (4) Eocene or Early Oligocene tholeiitic andesite, dacite, and rhyolite form the uppermost part of the outer-arc high at Site 782. All four groups can be explained by remelting above a subduction zone of oceanic mantle lithosphere that has been depleted by its previous episode of partial melting at an ocean ridge. We estimate that the average boninite source had lost 10-15 wt% of melt at the ridge before undergoing further melting (5-10%) shortly after subduction started. The composition of the harzburgite (<2% clinopyroxene, Fo content of about 92%) indicates that it underwent a total of about 25% melting with respect to a fertile MORB mantle. The low concentration of Nb in the boninite indicates that the oceanic lithosphere prior to subduction was not enriched by any asthenospheric (OIB) component. The subduction component is characterized by (1) high Zr and Hf contents relative to Sm, Ti, Y, and middle-heavy REE, (2) light REE-enrichment, (3) low contents of Nb and Ta relative to Th, Rb, or La, (4) high contents of Na and Al, and (5) Pb isotopes on the Northern Hemisphere Reference Line. This component is unlike any subduction component from active arc volcanoes in the Izu-Mariana region or elsewhere. Modeling suggests that these characteristics fit a trondhjemitic melt from slab fusion in amphibolite facies. The resulting metasomatized mantle may have contained about 0.15 wt% water. The overall melting regime is constrained by experimental data to shallow depths and high temperatures (1250°C and 1.5 kb for an average boninite) of boninite segregation. We thus envisage that boninites were generated by decompression melting of a diapir of metasomatized residual MORB mantle leaving the harzburgites as the uppermost, most depleted residue from this second stage of melting. Thermal constraints require that both subducted lithosphere and overlying oceanic lithosphere of the mantle wedge be very young at the time of boninite genesis. This conclusion is consistent with models in which an active transform fault offsetting two ridge axes is placed under compression or transpression following the Eocene plate reorganization in the Pacific. Comparison between Leg 125 boninites and boninites and related rocks elsewhere in the Western Pacific highlights large regional differences in petrogenesis in terms of mantle mineralogy, degree of partial melting, composition of subduction components, and the nature of pre-subduction lithosphere. It is likely that, on a regional scale, the initiation of subduction involved subducted crust and lithospheric mantle wedge of a range of ages and compositions, as might be expected in this type of tectonic setting

The 1994–2001 eruptive period at Rabaul, Papua New Guinea: Petrological and geochemical evidence for basalt injections into a shallow dacite magma reservoir, and significant SO 2 flux

The eruptions that began at Rabaul Caldera on 19 September 1994 had two focal points, the vents Tavurvur and Vulcan, located 6 km apart on opposing sides of the caldera. Vulcan eruptives define a tight cluster of dacite compositions, whereas Tavurvur eruptives span an array from equivalent dacite compositions to mafic andesites. The eruption of geochemically and mineralogically identical dacites from both vents indicates sourcing from the same magma reservoir. This, together with previously reported H₂O-CO₂ volatile contents of dacite melt inclusions, a caldera-wide seismic low-velocity zone, and a seismically active caldera ring fault structure are consistent with the presence at 3–6 km depth of an extensive, tabular dacitic magma body having volume of about 15–150 km³. The Tavurvur andesites form a linear compositional array and have strongly bimodal phenocryst assemblages that reflect dacite hybridisation with a mafic basalt. The moderately large volume SO₂ flux documented in the Tavurvur volcanic plume (and negligible SO₂ flux in the Vulcan plume) combined with high dissolved S contents of basaltic melt inclusions trapped in olivine of Tavurvur eruptives, indicate that the amount of degassed basaltic magma was ~ 0.1 km³ and suggest that the injection of this magma was confined to the Tavurvur-side (eastern to northeastern sector) of the caldera. Circumstantial evidence suggests that the eruption was triggered and evolved in response to a series of basaltic magma injections that may have commenced in 1971 and continued up until at least the start of the 1994 eruptions. The presence of zoned plagioclase phenocrysts reflecting older basalt-dacite interaction events (i.e. anorthite cores overgrown with thick andesine rims), evaluation of limited available data for the products of previous eruptions in 1878 and 1937–1943, and the episodic occurrence of major intra-caldera seismo-deformational events indicates that the shallow magma system at Rabaul Caldera is subjected to repeated mafic magma injections at intervals of several years to several decades.We thank Shane Nancarrow, formerly of Geoscience Australia, for making many aspects of this project possible and AusAID for providing financial support to HP to undertake research at the Australian National University (ANU) into the 1994 and historical eruptions at Rabaul

Geodynamic implications for zonal and meridional isotopic patterns across the northern Lau and North Fiji Basins

We present new Sr-Nd-Pb-Hf-He isotopic data for sixty-five volcanic samples from the northern Lau and North Fiji Basin. This includes forty-seven lavas obtained from forty dredge sites spanning an east-west transect across the Lau and North Fiji basins, ten ocean island basalt (OIB)-type lavas collected from seven Fijian islands, and eight OIB lavas sampled on Rotuma. For the first time we are able to map clear north-south and east-west geochemical gradients in 87Sr/86Sr across the northern Lau and North Fiji Basins: lavas with the most geochemically enriched radiogenic isotopic signatures are located in the northeast Lau Basin, while signatures of geochemical enrichment are diminished to the south and west away from the Samoan hotspot. Based on these geochemical patterns and plate reconstructions of the region, these observations are best explained by the addition of Samoa, Rurutu, and Rarotonga hotspot material over the past 4 Ma. We suggest that underplated Samoan material has been advected into the Lau Basin over the past ∼4 Ma. As the slab migrated west (and toward the Samoan plume) via rollback over time, younger and hotter (and therefore less viscous) underplated Samoan plume material was entrained. Thus, entrainment efficiency of underplated plume material was enhanced, and Samoan plume signatures in the Lau Basin became stronger as the trench approached the Samoan hotspot. The addition of subducted volcanoes to the Cook-Austral Volcanic Lineament material, first from the Rarotonga hotspot, then followed by the Rurutu hotspot, contributes to the extreme geochemical signatures observed in the northeast Lau Basin

Variation in sub-arc mantle oxygen fugacity during partial melting recorded in refractory peridotite xenoliths from the West Bismarck Arc

This work was funded bygrants (DP120104240 and DE120100513) to Richard J Arculus andOliver Nebel from the Australian Research Council