bluuebunny/crossref_metadata_2025_split huggingface dataset:
What constitutes ‘emplacement’ of an ophiolite?: Mechanisms and relationship to subduction initiation and formation of metamorphic soles
Authors: Wakabayashi John , Dilek Yildirim 
Published: April 2008

Abstract
Ophiolites have long been recognized as on-land fragments of fossil oceanic lithosphere, which becomes an ophiolite when incorporated into continental margins through a complex process known as ‘emplacement’. A fundamental problem of ophiolite emplacement is how dense oceanic crust becomes emplaced over less dense material(s) of continental margins or subduction-accretion systems. Subduction of less dense material beneath a future ophiolite is necessary to overcome the adverse density contrast. The relationship of subduction to ophiolite emplacement is a critical link between ophiolites and their role in the development of orogenic belts. Although ophiolite emplacement mechanisms are clearly varied, most existing models and definitions of emplacement concern a specific type of ophiolite (i.e. Oman or Troodos) and do not apply to many of the world’s ophiolites. We have defined four prototype ophiolites based on different emplacement mechanisms: (1) ‘Tethyan’ ophiolites, emplaced over passive continental margins or microcontinents as a result of collisional events; (2) ‘Cordilleran’ ophiolites progressively emplaced over subduction complexes through accretionary processes; (3) ‘ridge-trench intersection’ (RTI) ophiolites emplaced through complex processes resulting from the interaction between a spreading ridge and a subduction zone; (4) the unique Macquarie Island ophiolite, which has been subaerially exposed as a result of a change in plate boundary configuration along a mid-ocean ridge system. Protracted evolutionary history of some ocean basins, and variation along the strike of subduction zones may result in more complicated scenarios in ophiolite emplacement mechanisms. No single definition of emplacement is free of drawbacks; however, we can consider the inception of subduction, thrusting over a continental margin or subduction complex, and subaerial exposure as critical individual stages in ophiolite emplacement.
Forearc ophiolites: a view from the western Pacific
Authors: Milsom John 
Published: April 2008

Abstract

Many of the world’s largest ophiolite masses are now being interpreted as remnants of oceanic forearcs stranded on continental margins in the course of arc-continent collision. This interpretation implies a former association between the ophiolite and a volcanic arc at a distance of 100–200 km. The New Caledonia region of the SW Pacific is one of the few areas where there is good evidence for the presence of such an arc. On New Caledonia itself, the Grand Massif du Sud ophiolite has been thrust over the Norfolk Ridge continental fragment, while the coral islands of the Loyalty Group,
c.
100 km to the NE, cap large sub-sea edifices with volcanic morphology.

New Caledonia can be correlated geologically with New Guinea, despite the considerable width of open ocean that now separates the two. In central New Guinea, where ophiolites emplaced along the northern flank of the main mountain spine lie some 100 km south of exposures of arc-volcanic basement in the north coast ranges, the similarities include the relationship between New Caledonia and the Loyalty Islands. There is, however, no obvious comparable relationship in the case of the largest of the New Guinea ophiolites, the Papuan Ultramafic Belt of the eastern peninsula, which is backed to the north and east by the oceanic Solomon Sea. An associated volcanic arc can be recognized in this area only by assuming a complicated history of collision, post-collision arc-forearc separation and sea-floor spreading, followed by renewed contraction and a very recent and continuing second collision. The case for such a sequence of events can be made on a number of grounds. If the processes responsible are general in their nature, they could explain the apparent absence of arc-volcanic belts in association with many other supposedly forearc ophiolites.
Mineral chemistry of ultramafic massifs in the Southern Uralides orogenic belt (Russia) and the petrogenesis of the Lower Palaeozoic ophiolites of the Uralian Ocean
Authors: Spadea P. , Zanetti A. , Vannucci R. 
Published: April 2008

Abstract

Ophiolites of the southern Uralides arc-continent collisional orogen include fertile mantle lherzolites and minor harzburgites in the Nurali and Mindyak massifs located along the Main Uralian Fault suture of the East European craton margin and the Magnitogorsk island arc. We present the first
in situ
analyses of pyroxene from Nurali spinel ± plagioclase-bearing lherzolites and harzburgites and Mindyak spinel lherzolites and harzburgites. Based on the trace element signatures of pyroxene, the Nurali peridotites are divided into: Group 1, consisting of plagioclase-bearing spinel lherzolites with moderately to extremely light rare earth element (LREE)-depleted clinopyroxenes, consistent with ⩽8% fractional melting followed by impregnation by incremental to mid-ocean ridge basalt (MORB)-like melts; Group 2, formed by a spinel peridotite with strongly LREE- to middle REE (MREE)-depleted to enriched clinopyroxenes that testify to re-equilibration with large volumes of melt of tholeiitic affinity; Group 3, consisting of amphibole-bearing spinel harzburgites that underwent pervasive percolation of alkali-enriched melts or fluids. Clinopyroxenes from the Mindyak peridotites are strongly depleted and re-equilibrated by reactive porous flow of infiltrating tholeiitic melts. Two alternative petrogenetic models are proposed. In Model 1 the peridotites derive from oceanic lithosphere generated by mid-ocean ridge processes and affected by refertilization via melt percolation. In Model 2 the peridotites were subcontinental lithospheric mantle that experienced deep-seated magmatism and sub-solidus re-equilibration prior to the opening of the Uralian Ocean, and interacted with melts generated in the asthenospheric mantle by extension-related decompression partial melting during the opening of the Uralian Ocean. In both models the final pre-orogenic events are related to the subduction of the Uralian oceanic lithosphere and to mantle wedge processes, notably intrusion of gabbro-diorite at
c.
400 Ma into the Moho sections.
A general model of arc-continent collision and subduction polarity reversal from Taiwan and the Irish Caledonides
Authors: Clift Peter D. , Schouten Hans , Draut Amy E. 
Published: April 2008

Abstract
The collision of the Luzon Arc with southern China represents the best example of arc-continent collision in the modern oceans, and compares closely with the Early Ordovician accretion of the Lough Nafooey arc of Connemara, Ireland, to the passive margin of Laurentia. We propose a general model for steady-state arc-continent collision in which arc crust is progressively added to a passive margin during a process of compression, metamorphism and magmatism lasting 3–10 Ma at any one location on the margin. Depending on the obliquity of the angle of collision, the timing of active collision may be diachronous and long-lived along the margin. Magmatism accompanying accretion can be more enriched in incompatible trace elements than average continental crust, contrasting with more depleted magmatism prior to collision. Accretion of a mixture of depleted and enriched arc lithologies to the continental margin allows the continental crust to grow through time by arc-passive margin collision events. During the collision the upper and middle arc crust are detached from the depleted ultramafic lower crust, which is subducted along with the mantle lithosphere on which the arc was founded. Rapid (2–3 Ma) exhumation and gravitational collapse of the collisional orogen forms the Okinawa and South Mayo Troughs in Taiwan and western Ireland, respectively. These basins are filled by detritus eroded from the adjacent collision zone. During subsequent subduction polarity reversal, continuous tearing and retreat of the oceanic lithosphere along the former continent-ocean transition provides space for the new subducting oceanic plate to descend without need for breaking of the original slab.
Felsic volcanism in the Kermadec arc, SW Pacific: crustal recycling in an oceanic setting
Authors: Smith Ian E.M. , Worthington Timothy J. , Stewart Robert B. , Price Richard C. , Gamble John A. 
Published: April 2008

Abstract
Large-scale felsic volcanic systems are a common, but not ubiquitous, feature of volcanic arc systems in continental settings. However, in oceanic volcanic arcs the erupted materials are dominated by basalts and basaltic andesites, whereas intermediate compositions are rare and dacites and rhyolites relatively uncommon. The Kermadec arc is an intraoceanic convergent system in the SW Pacific. Volcanoes occur as a continuous arc that is mainly submarine. Despite its simple tectonic setting, felsic magmatism is widespread. In the Kermadec Islands, Macauley Volcano is a basaltic volcano that produced a large felsic eruption about 6000 years ago. A comparable pattern of magmatic evolution is seen on adjacent Raoul Volcano, where basaltic activity built the main edifice of the volcano and where activity during the last 3000 years has been characterized by felsic eruptions of varying size. Elsewhere in the Kermadec arc and in its northward extension, the Tonga arc, felsic eruptions are recorded from 11 of the 30 volcanoes for which petrographic information is available, and in many cases these are the most recent eruptions. Felsic europtions are a widespread recent feature of the arc, and the scale and extent of this magmatism appears to be unusual for a tectonically simple oceanic subduction system. One explanation of the origin of the felsic magmatism is prolonged fractional crystallization from a parental basalt composition, but modelling of the chemical compositions of the felsic rocks does not support this. A second explanation, albeit apparently at odds with the oceanic setting, is crustal anatexis. An important feature of the felsic eruptives from the Kermadec arc is that each tephra sequence or occurrence has a unique chemical composition, although all show the same generalized characteristics. We suggest that this feature supports a model of crustal anatexis rather than fractionation of a range of parental magmas. We also suggest that in the thermal evolution of an oceanic arc system the processes of underplating, together with the continuous magmatic (and thermal) flux, can generate a crustal thickness in which dehydration melting of underplated arc material generates felsic magmas. Further, this condition can represent a unique ‘adolescent’ stage in a developing oceanic arc, as once the felsic melts are extracted the lower crust becomes an infertile residue.
Chemically rich and diverse submarine hydrothermal plumes of the southern Kermadec volcanic arc (New Zealand)
Authors: Massoth Gary J. , De Ronde Cornel E.J. , Lupton John E. , Feely Richard A. , Baker Edward T. , Lebon Geoffrey T. , Maenner Stacy M. 
Published: April 2008

Abstract

The New Zealand American PLUme Mapping Expedition (NZAPLUME) provided the first systematic survey of chemical emissions along a submarine volcanic frontal arc. Chemical plumes emanated from seven of 13 volcanoes that line a 260 km-long section of the southern Kermadec arc northeast of New Zealand. Hydrothermal plumes ranged in depth from <200 to 1500 m and are generally more shallow than plumes over mid-ocean ridges (MORs). The chemical signatures of plumes along the southern Kermadec arc are unusually diverse and have concentration anomalies for CO
2
, H
2
S and Fe that can exceed those for MOR settings by 5–10 times, or more. Projected end-member fluid concentrations of carbon and sulphur gases at some volcanoes require a magmatic vapour source, while unusually high Fe concentrations and Fe/Mn values are consistent with venting an iron-rich magmatic brine. Thus, vent-fluid emissions on the Kermadec arc volcanoes often appear as hybrid mixtures of hydrothermally
evolved
sea water influenced by water-rock reaction with compositionally diverse arc lavas, and
exsolved
magmatic fluid present as gaseous (CO
2
and SO
2
+H
2
S) and liquid (Fe-rich brines) components. While rock-buffered fluids in arc settings are expected to vary compositionally from one another and from MOR fluids, it is the magmatic components that clearly differentiate arc emissions as being super-enriched in sulphur gases and ionic metals. These first systematic observations of spatially frequent and chemically robust fluid emissions from southern Kermadec arc forecast arcs as being a potentially important source of chemicals to the oceans.
Submarine hydrothermal venting on the southern Kermadec volcanic arc front (offshore New Zealand): location and extent of particle plume signatures
Authors: Baker E. T. , Feely R. A. , De Ronde C. E. J. , Massoth G. J. , Wright I. C. 
Published: April 2008

Abstract

Hydrothermal activity on submarine volcanic arcs in the western Pacific Ocean is known but mostly unexplored. In March 1999, the New Zealand American PLUme Mapping Expedition (NZAPLUME) cruise conducted the first systematic exploration of hydrothermal venting along a sizeable section of an intra-oceanic arc, visiting 13 volcanoes along 260 km of the southern Kermadec arc, just northeast of New Zealand. Conclusive evidence of hydrothermal plumes exists for seven of the 13 volcanoes; at two other volcanoes plume indications were weak and uncertain. The hydrothermal origin of the particle plumes was confirmed by positive anomalies in the ratios of sulphur, iron and copper to titanium relative to non-plume particles, in mass concentrations similar to particles collected from hydrothermal plumes over mid-ocean ridges. The spatial density of active sites along the southern Kermadec arc is at least 2.7 per 100 km (2.7/100 km), probably not significantly different from the weakly constrained value of
c.
1/100 km on slow- and intermediate-rate mid-ocean ridges. An analysis of the number of hydrothermal fields produced for the magma delivery rate in each of these environments suggests that the southern Kermadec arc presently has relatively abundant hydrothermal activity. While this result cannot yet be generalized to other Pacific arcs, submarine volcanoes may contribute significantly to the global hydrothermal budget.
Geodynamic setting of Izu-Bonin-Mariana boninites
Authors: Deschamps Anne , Lallemand Serge 
Published: April 2008

Abstract
The Izu-Bonin-Mariana (IBM) forearc is characterized by the occurrence of boninite-like lavas. The study of the Cenozoic setting of the genesis of these boninitic lavas in light of modern geodynamic contexts in the Tonga and Fiji regions lead us to define three tectonic settings that favour the formation of boninites in back-arc basins in addition to previous settings that involve the presence of a mantle plume: (1) propagation at low angle between a spreading centre and the associated volcanic arc; (2) intersection at a high angle of an active spreading centre and a transform fault at the termination of an active volcanic arc; and (3) intersection at a right angle between an active spreading centre and a newly created subduction zone. A geodynamic model of the Philippine Sea Plate shows that boninites in the Bonin Islands are related to the second mechanism mentioned above, whereas Mariana forearc boninites are relevant to the third mechanism. In the early Eocene, the transform plate boundary bounding the eastern margin of the Philippine Sea Plate at the location of the present-day Mariana arc evolved into a subduction zone that trends perpendicular to the active spreading centre of the West Philippine Basin, somewhere around 43–47 Ma. The presence of a mantle plume in the vicinity of the subduction zone bounding the northern IBM arc explains boninites that erupted in its northern part, but only in early Eocene time.
Mantle genesis and crustal evolution of primitive calc-alkaline basaltic magmas from the Lesser Antilles arc
Authors: Pichavant M. , Macdonald R. 
Published: April 2008

Abstract

Most eruptive rocks in the Lesser Antilles arc are compositionally evolved. However, lavas with primitive characteristics do occur including, in the central part of the arc, a suite of rocks from Soufriere, St Vincent, and the Ilet à Ramiers basalt from Martinique. High-pressure experiments performed on a Soufriere basalt point to a spinel lherzolite source. Glass inclusion data and phase equilibria analysis suggest extraction of the Soufriere melt under relatively dry conditions (
c.
2 wt% H
2
O in melt). Using estimates of the H
2
O content of mantle sources fluxed by an hydrous slab-derived component, H
2
O concentrations as high as 5 wt% are considered possible for primary mantle melts in the Lesser Antilles arc. Experiments at low pressures (4 kbar) simulate the evolution of primitive melts within the arc crust. For elevated melt H
2
O concentrations (6–8.5 wt%), derivative liquids ranging from low-MgO basalt to basaltic andesite are generated at 1050–1100°C. Their crystallization at 950–1000°C yield andesitic liquids similar to those erupting at active volcanic centres such as Mt Pelée, Martinique, and Soufriere Hills, Montserrat. Therefore, experimental data support the derivation of Lesser Antilles arc eruptives by different degrees of fractionation from primary mantle melts.
Structure and tectonic evolution of the South Sandwich arc
Authors: Larter Robert D. , Vanneste Lieve E. , Morris Peter , Smythe David K. 
Published: April 2008

AbstractDetailed analysis of marine magnetic profiles from the western part of the East Scotia Sea confirms continuous, organized back-arc spreading since at least 15 Ma ago. In the eastern part of the East Scotia Sea, the South Sandwich arc lies on crust that formed at the back-arc spreading centre since 10 Ma ago, so older back-arc crust forms the basement of the present inner forearc. Interpretations of two multichannel seismic reflection profiles reveal the main structural components of the arc at shallow depth, including evidence of trench-normal extension in the mid-forearc, and other features consistent with ongoing subduction erosion. The seismic profile interpretations have been used to constrain simple two-dimensional gravity models. The models were designed to provide constraints on the maximum possible thickness of the arc crust, and it is concluded that this is 20 and 19.2 km on the northern and southern lines, respectively. On the northern line the models indicate that the forearc crust cannot be much thicker than normal oceanic crust. Even with such thin crust, however, the magmatic growth rate implied by the cross-section of the arc crust is within the range recently estimated for two other arcs that have been built over a much longer interval.
Back-arc spreading and mantle flow in the East Scotia Sea
Authors: Livermore Roy 
Published: April 2008

Abstract
The East Scotia Ridge exhibits systematic variations in axial morphology and basalt geochemistry. Central segments have morphology typical of intermediate-rate spreading centres and erupt mainly normal mid-ocean ridge basalt (N-MORB). Segments near the ridge ends exhibit anomalous, inflated, axial morphology and erupt more evolved basalts, influenced by the Bouvet plume in the north. As the end segments lie closer to the volcanic arc, these variations could be caused by coupled flow within the mantle wedge, as inferred from similar studies in the Lau Basin. Three of the four zones of crustal accretion defined from the Lau Basin may be identified in the East Scotia Sea, although there is no counterpart to a zone of diminished magma supply observed at the East Lau Spreading Centre. Superimposed on the pattern of plate-driven flow is a ridge-parallel flow related to inflow of Atlantic mantle into the East Scotia Sea back-arc region at both ends of the South Sandwich slab. The inflow causes enhanced magmatism and propagation of the end segments towards the middle of the back-arc region, and may be related to trench-parallel flow beneath the rapidly retreating slab. Alternatively, it may be driven by buoyancy flux from Atlantic hot spots. There is no evidence that retreat was ever driven by escape flow of Pacific mantle.
Rare earth elements in zircon: a review of applications and case studies from the Outer Hebridean Lewisian Complex, NW Scotland
Authors: Whitehouse Martin J. 
Published: April 2008

Abstract
Rare earth element (REE) analysis of zircon coupled with high spatial resolution U-Pb geochronology and imaging is emerging as a useful petrogenetic tool capable of providing a link between dated zircon growth and contemporaneous magmatic conditions or metamorphic reactions. Notable amongst recent studies has been the link established between zircon growth in equilibrium with garnet and characteristic heavy REE depletion in zircon which has enabled the dating of specific pressure-temperature (P-T) evolution. Links to other REE-bearing minerals have yet to be established but have similar potential for dating P-T pathways. A brief review of recent applications in both igneous and metamorphic zircons is presented here to illustrate the range of investigations in this rapidly developing area. The method is then applied to two case studies from the late-Archaean Lewisian Composite Terrane of the Outer Hebrides, NW Scotland. In both cases, these new zircon REE analyses reveal previously unknown events in the early history of these samples, as well as clarifying the relationship of different zircon growth phases in relation to the geochronology.
Oxygen isotope equilibrium between ultrahigh-pressure metamorphic minerals and its constraints on Sm-Nd and Rb-Sr chronometers
Authors: Zheng Young-Fei , Zhao Zi-Fu , Li Shu-Guang , Gong Bing 
Published: April 2008

Abstract
In the Sm-Nd and Rb-Sr isotopic geochronology of metamorphic rocks, an important question is whether radiometric systems of mineral isochrons have achieved isotopic equilibrium during a given metamorphic event and preserved the equilibrium afterwards. An analogue to mineral chronometry is O isotope geothermometry. Because the rates of Sm-Nd, Sr and O diffusion in metamorphic minerals are comparable in many cases, the state of O isotope equilibrium between metamorphic minerals can provide a test for the validity of mineral Sm-Nd and Rb-Sr chronometers. In order to illustrate this applicability, O isotope geothermometry was carried out for Sm-Nd and Rb-Sr isochron minerals from ultrahigh-pressure (UHP) eclogites and gneisses at Shuanghe in the Dabie terrane of east-central China. Although the Sm-Nd isochrons give consistent Triassic ages of 213 to 238 Ma for UHP metamorphism, the Rb-Sr isochrons give Jurassic ages of 171 to 174 Ma for the same samples. O isotope geothermometry of the gneiss, eclogite and amphibolite minerals yields two sets of temperatures of 600 to 720 °C and 420 to 550 °C, respectively, corresponding to cessation of isotopic exchange by diffusion at about 225 ± 5 Ma during high pressure eclogite-facies recrystallization and at about 175 ± 5 Ma during amphibolite-facies retrogression. The preservation of Triassic Sm-Nd isochron ages, but the occurrence of Jurassic Rb-Sr isochron ages and the regular O isotope temperatures for the same samples, suggest that rates of Sr and O diffusion in such hydroxyl-bearing minerals as biotite and hornblende are faster than rates of Nd diffusion in garnet and Sr diffusion in phengite on the scale of a hand-specimen during the amphibolite-facies retrogression. While the mineral with slow diffusivity has exerted the primary control on the homogenization rate of initial isotope ratios among isochron minerals during retrograde metamorphism, the mineral with high parent/daughter ratio has exerted the principal control on the initiation of the mineral isochron clock in response to retrogression. Valid mineral isochrons can be expected to date the timing of metamorphic resetting only if the mineral with high parent/daughter ratio has a fast rate of radiogenic isotope diffusion during the metamorphic resetting.