bluuebunny/crossref_metadata_2025_split huggingface dataset:
Cr-spinel compositions, metadunite petrology, and the petrotectonic history of Blue Ridge ophiolites, Southern Appalachian Orogen, USA
Authors: Raymond Loren A. , Swanson Samuel E. , Love Anthony B. , Allan James F. 
Published: April 2008

Abstract
Resolution of the petrotectonic history of Blue Ridge ophiolites of the Southern Appalachian Orogen has remained enigmatic because of metamorphism and tectonic fragmentation of ultramafic bodies. Understanding of this history is confounded by the presence of five partial metamorphic overprints and by similar Ti enrichments in spinels from Blue Ridge and modern mid-ocean ridge basalt ultramafic rocks that result from different processes. Chrome spinels from oceanic ultramafic lithosphere show increases in Ti caused by metasomatism induced by passing mafic melts, which create both dunite melt channels within harzburgite wall rocks and associated troctolite impregnation zones. In the Blue Ridge Belt, the oldest metadunite mineral association generally lacks high-Ti spinel, whereas the higher Ti spinels are relatively low in Al and Mg and occur in three amphibolite- to greenschist-facies retrograde metamorphic associations that occur in deformed, metasomatized ultramafic bodies with high aspect ratios. Some spinel compositions in the oldest mineral association are similar to those from arc-suprasubduction zone ultramafic lithosphere. Together, available data are consistent with the hypothesis that: (1) the Blue Ridge ophiolites are fragmented, metamorphosed, very slow-spreading ridge, Xigaze-type ophiolites, consisting of mafic rocks, minor plutonic rocks, and a sublithospheric ultramafic tectonite base; (2) the metadunites represent sublithospheric melt channels and zones of high melt flux, perhaps formed in a suprasubduction zone setting; (3) pre-Taconic subduction may have been west-directed rather than east-directed. The Taconic orogenesis deformed, fragmented, and metamorphosed the ophiolites; and later Taconic, Acadian, and Alleghenian metamorphism hydrated the bodies, while associated deformation exaggerated their elongation.
Multi-stage evolution of the Tertiary Mineoka ophiolite, Japan: new geochemical and age constraints
Authors: Hirano N. , Ogawa Y. , Saito K. , Yoshida T. , Sato H. , Taniguchi H. 
Published: April 2008

Abstract

The Mineoka ophiolite in the southern Boso Peninsula is situated in a unique tectonic setting in the collisional zone between the Izu and Honshu arcs in Japan. The ophiolitic rocks are composed mainly of tholeiitic pillow basalts and dolerites, alkali-basaltic sheet flows, and calc-alkaline dioritic to gabbroic rocks. The tholeiitic basalts show variable trace element compositions ranging from mid-ocean ridge basalt to island-arc basalt, whereas the alkali-basalts have a within-plate affinity. High-Fe and -Ti tholeiitic basalt and within-plate alkali-basalt have Ar/Ar ages of 49 ± 13 Ma and 19.62 ± 0.90 Ma, respectively. Three plutonic rocks have K-Ar ages of
c.
25, 35 and 40 Ma. These ages are inconsistent with the known ages from the Pacific or Philippine Sea Plate. We infer that the Mineoka ophiolitic assemblage was part of another Tertiary oceanic plate, the ‘Mineoka Plate’, which underwent island-arc volcanism in the Miocene as a result of subduction initiation at a fracture zone or a transform fault system due to a change in the position of the Euler rotation pole of the Pacific Plate at
c.
43 Ma. Eruption of within-plate type alkali basalts on the Mineoka Plate took place near the palaeo-Japan continental arc just before the emplacement of the Mineoka ophiolite into the Japanese continental margin.
Subduction zone polarity in the Oman Mountains: implications for ophiolite emplacement
Authors: Searle M. P. , Warren C. J. , Waters D. J. , Parrish R. R. 
Published: April 2008

Abstract

Two end-member models have been proposed to account for the structure and metamorphism of rocks beneath the Semail ophiolite in the Oman mountains. Model A involves a single, continuous NE-directed subduction away from the continental margin during the late Cretaceous. The ophiolite and underlying thrust sheets of distal to proximal oceanic sediments were emplaced a minimum of 250 km SW onto the continental margin. Subduction of Triassic-Jurassic oceanic basalts to
c.
10 kbar (
c.
39 km depth) led to the accretion of amphibolite-facies rocks to the base of the ophiolite. Thrusting propagated towards the continental margin and ended with subduction of the thinned continental crust to
c.
20 kbar (
c.
78 km depth), choking the subduction zone. Buoyancy forces caused the rapid exhumation of eclogites, blueschists and carpholite-grade HP rocks along the NE margin of the continental plate. During the later phase of foreland-propagating thin-skinned thrusting in the SW, NE-facing backfolding and backthrusting occurred in the hinterland, with the final exhumation of the HP rocks. Model B follows recent suggestions that a nascent SW-dipping subduction zone, dipping beneath the continental margin, existed between 130 and 95 Ma, prior to formation and emplacement of the ophiolite. A major NE-facing fold-nappe structure in the pre-Permian basement rocks of Saih Hatat is interpreted as reflecting subduction beneath the margin. Two high-pressure metamorphic events have been suggested, the first predating ophiolite emplacement, the second caused by ophiolite loading. This model is untenable, being based on a misinterpretation of the NE-facing structures in northern Saih Hatat, and on some dubious older
40
Ar/
39
Ar cooling ages from the eclogite-facies rocks of As Sifah. We conclude that all structures in northern Oman and all the reliable geochronology point to a single emplacement-obduction event lasting from Cenomanian-Turonian time (
c.
95 Ma) when amphibolites were accreted along the metamorphic sole of the ophiolite, to Campanian time, when the continental margin was subducted to the NE producing blueschists and eclogites, to the final thin-skinned emplacement of all thrust sheets, which ended before the Late Maastrichtian, at
c.
68 Ma.
Ophiolites in China: their distribution, ages and tectonic settings
Authors: Zhang Q. , Wang Y. , Zhou G. Q. , Qian Q. , Robinson Paul T. 
Published: April 2008

Abstract

Ophiolites are widespread and abundant in China, where they lie along suture zones delineating major tectonic blocks. They range in age from Proterozoic to Tertiary and generally belong to the Palaeo-Asian, Qinling-Qilian-Kunlun, Tethyan and Circum-Pacific systems. A few possible Proterozoic ophiolites may reflect rifting of Rodinia at
c.
800–1000 Ma. The Palaeo-Asian ophiolites, which range in age from Cambrian to Carboniferous, crop out abundantly in the northern parts of the country whereas the Qinling-Qilian-Kunlun ophiolites occur in north-central China, along the boundaries of the Tarim, North China and Yangtze blocks. Tethyan ophiolites are confined to southwestern China and Tibet whereas those of the Circum-Pacific belt are found in Taiwan and northeastern China. Chinese ophiolites are typically tectonically disrupted mélanges composed of isolated blocks of peridotite, gabbro and basalt. Sheeted dykes are rare or absent. Many of the ophiolites are compositionally complex, containing mixtures of island-arc tholeiite and boninite with lesser amounts of mid-ocean ridge basalt and ocean-island basalt. Most show evidence of having been formed or assembled in suprasubduction zone environments. However, Palaeo-Tethyan ophiolites generally lack suprasubduction zone signatures and may have formed in small, intracontinental basins. A rapidly expanding database of high-precision age dates and detailed geochemical analyses on Chinese ophiolites is providing new insight into the nature and timing of the tectonic events that shaped this part of Asia.
Petrological diversity and origin of ophiolites in Japan and Far East Russia with emphasis on depleted harzburgite
Authors: Ishiwatari Akira , Sokolov Sergei D. , Vysotskiy Sergei V. 
Published: April 2008

Abstract
Ophiolites are divided into lherzolite-type (L-type) and harzburgite-type (H-type) by the lithology of their mantle peridotites. Rare depleted harzburgite-type (DH-type) is distinguished from the normal H-type by the more refractory nature of its mantle peridotite and the occurrence of orthopyroxene-type cumulate rocks including iron-rich harzburgite and orthopyroxenite. The Shelting (Sakhalin) and Krasnaya (Koryak Mountains) ophiolites in Far East Russia, which have both depleted harzburgite and orthopyroxene-type cumulate rocks, belong to this newly defined DH-type. The ophiolites in SW Japan-Primorye, NE Japan-Sakhalin, and the Koryak Mountains in the northwestern Pacific margin have diverse ophiolite types ranging from L- to DH-types. The wide petrological diversity, the common occurrence of DH-type, and the presence of thick crustal sections in these ophiolites suggest regionally inhomogeneous, commonly very high degrees of mantle melting over subduction zones, as in the modern Mariana forearc environment. The ophiolites of Japan and Far East Russia range in age from Early Palaeozoic to Cenozoic and are tectonically underlain by younger blueschists and accretionary complexes. The spatial association of these ophiolites with blueschists is analogous to the ophiolite-blueschist assemblages recovered from the Mariana forearc. This association might have formed in a period of non-accretion at an oceanic subduction zone that was followed by voluminous accretion of sediments, facilitating subsequent uplift of the ophiolites and blueschists.
Ophiolites in accretionary complexes along the Early Cretaceous margin of NE Asia: age, composition, and geodynamic diversity
Authors: Sokolov S. D. , Luchitskaya M. V. , Silantyev S. A. , Morozov O. L. , Ganelin A. V. , Bazylev B. A. , Osipenko A. B. , Palandzhyan S. A. , Kravchenko-Berezhnoy I. R. 
Published: April 2008

Abstract
The existing published data, combined with our own new field, petrographic, and geochemical observations and data show that ophiolites of the West Koryak fold system originated in a variety of tectonic environments. This fold system stretches along the boundary shared by two of NE Asia’s largest tectonic units, the Verkhoyansk-Chukotka and Koryak-Kamchatka foldbelts. The fold system abounds in Palaeozoic and Mesozoic ophiolites and sedimentary and volcanic island-arc assemblages. The ophiolites are Palaeozoic and Mesozoic in age. The variety of geological and geochemical signatures implies ophiolite origin in diverse tectonic settings. The Early Palaeozoic ophiolites of the Ganychalan accreted terrane and Devonian(?) ophiolites of the Ust-Belaya accreted terrane are fragments of the Panthalassan oceanic lithosphere. Serpentinite mélange in the Ust-Belaya terrane contains some blocks of island-arc provenance. They are probably Late Palaeozoic-Early Mesozoic in age as determined by K-Ar measurements, which require validation by other techniques. Mesozoic, chiefly Late Jurassic-Early Cretaceous ophiolites of the Beregovoi and Kuyul accreted terranes, originated in a suprasubduction-zone (SSZ) setting (ensimatic island arc and back-arc basin). Among the Mesozoic ophiolites, one finds blocks of oceanic assemblages in serpentinite mélanges as well. Basalt and chert blocks of clearly oceanic derivation are viewed as detached fragments of the upper part of the oceanic lithosphere. The ophiolites have experienced a variety of accretionary scenarios. Palaeozoic ophiolites docked onto the Koni-Taigonos island arc (of Late Palaeozoic-Early Mesozoic age), probably in the Late Palaeozoic or Early Mesozoic, whereas Mesozoic ophiolites accreted onto the Uda-Murgal island arc (of Late Jurassic-Early Cretaceous age) in the terminal Early Cretaceous. Sedimentary deposits, whose base is late Albian in age, make a post-accretionary sequence. These island arcs portray the overall history of the convergent boundary between the North Asian continent and NW Pacific. Ophiolites of the Ganychalan and Ust-Belaya terranes consist of thrust sheets and, jointly with Yelistratov Peninsula ophiolites, make up the basement to the forearc of the Uda-Murgal island arc, ophiolites of Cape Povorotny and Kuyul terrane being incorporated in accretionary prisms of the same arc. Ophiolites and associated metamorphic, volcanic, and sedimentary rocks of Palaeozoic-Early Cretaceous age underwent three deformation phases, each reflecting a different stage in the evolution of the NE Asian continental margin and readily correlative with principal tectonic events in the northern Circum-Pacific region.
Rocas Verdes ophiolites, southernmost South America: remnants of progressive stages of development of oceanic-type crust in a continental margin back-arc basin
Authors: Stern Charles R. , De Wit Maarten J. 
Published: April 2008

Abstract
The Mesozoic Rocas Verdes are a group of mafic igneous complexes in the southernmost Andes. They consist of pillow lavas, dykes and gabbros, interpreted as the upper parts of ophiolites formed along mid-ocean-ridge-type spreading centres that rifted the southwestern margin of the Gondwana continental crust, during the onset of spreading in the South Atlantic, to form the mafic igneous part of the floor of a back-arc basin behind a contemporaneous convergent plate boundary magmatic arc. Mafic dykes and gabbros intrude older continental lithologies along both flanks of the Rocas Verdes, and these same leucocratic country rocks are engulfed in the Rocas Verdes mafic complexes. These relations indicate that the Rocas Verdes ophiolites formed in place and are autochthonous. Zircon U/Pb ages, as well as both chemical and lithostructural characteristics of these ophiolite complexes, suggest that the Rocas Verdes basin formed by ‘unzipping’ from the south to the north, with the southern part beginning to form earlier, and developing more extensively, than the northern part of the basin. The Rocas Verdes ophiolites contain a wealth of information about progressive stages of continental rifting during back-arc basin formation, magmatic and metamorphic processes along mid-ocean-ridge-type spreading centres, and as analogues to Archaean greenstone belts.
Proterozoic ophiolites of the Arabian Shield and their significance in Precambrian tectonics
Authors: Dilek Yildirim , Ahmed Zulfiqar 
Published: April 2008

Abstract

Neoproterozoic ophiolites, ranging in age from
c.
870 Ma to
c.
627 Ma, occur in several discrete suture and/or fault zones within the Arabian Shield and display a record of riftdrift, sea-floor spreading and collision tectonics during the evolution of the East African Orogen. The ophiolites within the Yanbu and Bir Umq suture zones in the west are among the oldest (870–740 Ma) in the Shield, locally show a Penrose-tyep complete pseudostratigraphy, and have chemical compositions typical of modern forearc oceanic crust. They are spatially associated with coeval and younger volcanic arc assemblages and were incorporated into the Arabian Shield during a series of collisional events that amalgamated these ensimatic arc terranes. The ophiolites of the Hulayfah-Ruwah suture zone in the central Arabian Shield are coeval with and/or slightly younger (
c.
843–821 Ma) than the ophiolites in the west and probably developed in a rifted ensimatic arc system that evolved as a volcanic archipelago near the Afif continental plate. Younger ophiolites (
c.
694 Ma) of the Halaban and Al Amar suture zones in the eastern Arabian Shield were incorporated into a subduction-accretion complex that evolved at the Andean-type active margin along the eastern edge (in present coordinate system) of the Afif continental plate. The Halaban suture zone ophiolites represent forearc oceanic crust, whereas the Al Amar suture zone ophiolites are scraped-off fragments of Mozambique ocean floor, seamounts and/or ocean island(s); the Abt Schist between them corresponds to a Franciscan-type accretionary prism of the ‘Halaban’ subduction zone. The incorporation of these ophiolites and the continental plates (Afif and Ar Rayn) into the Arabian Shield during 640–620 Ma marks a major shift in the direction of convergence (from northerly to westerly) during the assembly of the Shield and distinct episodes of continental collisions during closure of the Mozambique Ocean. The ophiolites of the Nabitah-Hamdah fault zone within the Asir terrane are the youngest (
c.
627 Ma) in the Shield, post-collisional in origin, display mid-ocean ridge basalt chemical affinity, and represent Ligurian-type oceanic crust developed in an intracontinental pararift zone. The ophiolite tectonics of the Arabian Shield indicates an eastward progression of continental growth through time as the East African Orogen was built during the late Neoproterozoic, following the breakup of Rodinia.
Intra-oceanic subduction systems: introduction
Authors: Leat P. T. , Larter R. D. 
Published: April 2008

Abstract
Intra-oceanic arcs are the simplest type of subduction systems in that they occur where overridding plates of subduction zones consist of oceanic rocks, contrasting with arcs built on continental margins. They comprise some 40% of the subduction margins of the Earth. The better-known examples include the Izu-Bonin-Mariana arc, the Tonga-Kermadec arc, the Vanuatu arc, the Solomon arc, the New Britain arc, the western part of the Aleutian arc, the South Sandwich arc and the Lesser Antilles arc. They are thought to represent the first stage in the generation of continental crust from oceanic materials. They are generally more inaccessible than continental arcs, but, for a variety of reasons, provide insights into processes in subduction zones that are impossible or difficult to glean from the better-studied continental arcs. Intra-oceanic arcs typically have a simpler crustal structure than arcs built on continental crust, although there are significant differences between examples. Geochemically, magmas erupted in intra-oceanic arcs are not contaminated by ancient sialic crust, and their compositions more accurately record partial melting processes in the mantle wedge. They are also the sites of generation of intermediate-silicic middle crust and volcanic rocks, probably representing the earliest stage of generation of andesitic continental crust by partial melting of basaltic lower crust. They are the best locations in which to study mantle flow in the vicinity of subducting slabs using both geophysical and geochemical methods. They are the sites of significant hydrothermal activity and metallogenesis. The fact that their hydrothermal discharges typically occur shallower in the ocean than those from mid-ocean ridge vents means that they have the potential for greater environmental impact.
Some geochemical constraints on hot fingers in the mantle wedge: evidence from NE Japan
Authors: Tamura Y. 
Published: April 2008

Abstract

Mantle melting and the production of magmas along the NE Japan arc may be controlled by hot regions in the mantle wedge (hot fingers) that move toward the volcanic front along upward-sloping trajectories. At depths equivalent to 1–2 GPa, where magmas are expected to segregate from mantle diapirs, the hot-finger structures result in a decreasing thermal gradient away from volcanic front. Low-alkali tholeiite is therefore formed by the greater degree of diapiric melting near the volcanic front; high-alumina basalt and alkali olivine basalt are produced by lesser degrees of diapiric melting to the west. The grouping of volcanoes at the volcanic front is interpreted as being controlled by thermal structure in the mantle wedge, and groups are concentrated above the tips of the hot fingers. Map-view variations of minimum
87
Sr/
86
Sr of NE Japan volcanoes are interpreted as resulting from transport of fertile and high-
87
Sr/
86
Sr mantle material into the magma source region in the hot fingers. Given that mantle diapirs are formed in the lower part of the mantle wedge, a greater proportion of fertile material will be contained in the diapirs at the tips of the hot fingers, resulting in higher
87
Sr/
86
Sr magmas along the volcanic front. Conveyor-like return flow carries the sheet-like remnants of the fingers to depth along the top of the subducting slab.
Magmatism in the South Sandwich arc
Authors: Leat P. T. , Smellie J. L. , Millar I. L. , Larter R. D. 
Published: April 2008

Abstract

The South Sandwich Islands are one of the world’s classic examples of an intraoceanic arc. Formed on recently generated back-arc crust, they represent the earliest stages of formation of arc crust, and are an excellent laboratory for investigating variations in magma chemistry resulting from mantle processes, and generation of silicic magmas in a dominantly basaltic environment. Two volcanoes are examined. Southern Thule in the south of the arc is a complex volcanic edifice with three calderas and compositions that range from mafic to silicic and tholeiitic to calc-alkaline. It is compared to the Candlemas-Vindication edifice in the north of the arc, which is low-K tholeiitic and strongly bimodal from mafic to silicic. Critically, Southern Thule lies along a cross-arc, wide-angle seismic section that reveals the velocity structure of the underlying arc crust. Trace element variations are used to argue that the variations in both mantle depletion and input of a subducted sediment component produced the diverse low-K tholeiite, tholeiite and calc-alkaline series. Primitive, mantle-derived melts fractionally crystallized by
c.
36% to produce the most Mg-rich erupted basalts and a high-velocity cumulitic crustal keel. Plagioclase cumulation produced abundant high-Al basalts (especially in the tholeiitic series), and strongly influenced Sr abundances in the magmas. However, examination of volumetric and geochemical arguments indicates that the silicic rocks do not result from fractional crystallization, and are melts of amphibolitic arc crust instead.
Resolving mantle components in oceanic lavas from segment E2 of the East Scotia back-arc ridge, South Sandwich Islands
Authors: Harrison D. , Leat P. T. , Burnard P. G. , Turner G. , Fretzdorff S. , Millar I. L. 
Published: April 2008

Abstract

The East Scotia Ridge, situated in the South Atlantic, is the back-arc spreading centre to the intra-oceanic South Sandwich arc. Samples from the ridge show a wide diversity in erupted magma compositions. Segment E2, in the northern part of the ridge, has an axial topographic high, which contrasts with the rift-like topography common to most of the ridge. Lava compositions in the segment have been modelled by mixing of magmas derived from normal mid-ocean ridge basalt (N-MORB)-like mantle, a mantle plume component similar in composition to that sampled by Bouvet Island and mantle modified by addition of components from the subducting slab. The ‘Bouvet’-like plume signature has higher
87
Sr/
86
Sr,
206
Pb/
204
Pb, Nb/Yb, and lower
143
Nd/
144
Nd and
4
He/
3
He, than the local upper mantle. It can be traced geochemically from the Bouvet Island hot spot to segment E2, via the South American-Antarctic Ridge, which connects the Bouvet triple junction to the South Sandwich subduction system.

Four samples dredged from segment E2 have
4
He/
3
He ratios of 85 000–90 200 (8.5–8.0
R/R
A
, where)
R/R
A
is the
4
He/
3
He ratio normalized to air) and three wax core samples taken from the segment axis have values of 104 300, 101 560 and 176 620 (6.9, 7.1 and 4.1
R/R
A
). These latter data are similar to values from the South American-Antarctic Ridge which have no discernable plume input. Whilst the dredge samples have a measurably lower
4
He/
3
He ratio than the South American-Antarctic Ridge and samples from the segment axis, these He isotope data contrast with a dominant plume signature recorded by other petrogenetic tracers. This is interpreted to be due to re-melting of an entrained plume component, with an inherent low He concentration, incorporated into the E2 mantle. Helium depletion from the plume component can be seen to be a consequence of mantle processing and does not imply shallow-level degassing prior to entrainment within the upper-mantle-melting zone. As a consequence, He is characterized in the back-arc by values more similar to the upper mantle, whereas lithophile tracers are more influenced by the plume component.