bluuebunny/crossref_metadata_2025 huggingface dataset:
The role of shale deformation and growth faulting in the Late Cretaceous evolution of the Bight Basin, offshore southern Australia
Authors: Totterdell J.M. , Krassay A.A.
Published: April 2008

The Jurassic-Cretaceous Bight Basin of offshore southern Australia is a large, under-explored basin with little stratigraphic control. Sequence stratigraphic and structural interpretation of regional-scale 2-D seismic data has revealed the presence of two large progradational delta systems of mid-Late Cretaceous age, which are vastly different in terms of geometry and stratal architecture. The Cenomanian White Pointer Delta is characterized by shale deformation and growth faulting. Growth fault-controlled depocentres occupy a wide band across the Ceduna Subbasin of the Bight Basin that records the progradation of the delta across the palaeo-shelf during the Cenomanian. The growth faults are generally listric and basinward dipping (regional). Counter-regional faults are uncommon. The updip extensional features are accompanied downdip by a zone of diapirs that coincides with an outer basin high and, in deeper water at the toe of slope, a region of contractional deformation and toe-thrusts. In comparison, the younger (Late Santonian-Maastrichtian) Hammerhead Delta exhibits strongly progradational stratal geometries with little evidence of shale tectonics except in the SW part of the Ceduna Sub-basin. There, sustained progradation under a high sediment supply regime caused gravitational instability and the formation of listric growth faults at the palaeo-shelf margin with associated down-dip contractional deformation.
The role of shear stress in mobilizing deep-seated mud volcanoes: geological and geomechanical evidence from Trinidad and Taiwan
Authors: Yassir Najwa
Published: April 2008

Deep-seated mud volcanoes are observed in a variety of geological settings, which has led to considerable debate on their origin. This paper summarizes the geological features common to mud volcanoes around the world and possible mechanisms of their extrusion. Field and laboratory data from Trinidad and Taiwan are discussed to assess the possible sources and causes of the volcanoes. A close association between mud volcanoes and compressional tectonics leads to the conclusion that tectonic activity plays an important part in mud volcano development. Experimental data are presented to explain the role of tectonic activity and the association between shear stresses and mud volcanoes. It is demonstrated that shear stresses applied to low permeability sediments can produce a dramatic increase in pore pressure and can cause sediment flow. This is proposed as one possible contributing cause of mud volcanoes.
Boron and boron isotopes as tracers for diagenetic reactions and depth of mobilization, using muds and authigenic carbonates from eastern Mediterranean mud volcanoes
Authors: Deyhle A. , Kopf Achim J. , Aloisi G.
Published: April 2008

Authigenic carbonates and muds from six mud volcanoes in the eastern Mediterranean Sea were recovered during the French/Dutch MEDINAUT cruise utilizing the submersible
Nautile
in November 1998. The mud volcanoes are active seafloor vents in two areas at the plate boundary between the converging African and Eurasian Plates: the Mediterranean Ridge accretionary prism near Crete (Greece) and the Anaximander Mountains south of Turkey. B contents and δ
11
B signatures were measured with the aim of identifying the diagenetic processes and source depths of the material in the collision zone. B concentrations of the carbonate precipitates cover a range of 8–45 ppm and vary isotopically from +15.6 to +22.9‰ (corresponding to a parent solution of 34.9–42.2‰ at pH 7). Both the B-enrichment and a δ
11
B valve slightly lower than seawater suggest the mud domes originate from a moderately deep fluid source, with local admixture of seawater. B contents and δ
11
B of the mud show distinct differences between the areas: the Mediterranean Ridge mud domes have lower B contents and higher δ
11
B (average 3.9‰) compared to Anaximander Mountains mud volcanoes (δ
11
B average −0.6‰). These B results attest the release of structurally-bound B from clay mineral lattices, probably due to stronger deformation near Turkey. These mudstones, which had previously been affected by deep-seated thrusting beneath the Antalya Complex, may have been liquefied and remobilized in their present setting. By contrast, the mud on the Mediterranean Ridge represents offscraped clay-rich strata that was incorporated into the large accretionary wedge.
On the phylogeny and evolutionary history of pterosaurs
Authors: Unwin David M.
Published: April 2008

Previous cladistic studies of pterosaur relationships suffer from restricted numbers of taxa and characters, incomplete data sets and absence of information on characters, tree structure and the robustness of tress. Parsimony analysis of a new character data set (60 characters, 20 terminal taxa, 97.75% complete) yielded six trees. In the strict consensus tree
Preondactylus
is the most basal taxon followed, stepwise, by the Dimorphodontidae and the Anurognathidae. Beyond this basal group, more derived pterosaurs (Campylognathoididae (Rhamphorhynchidae + Pterodactyloidea)) share a suite of characters principally associated with elongation of the rostrum. The Pterodactyloidea consists of four major clades. The Ornithocheiroidea is the most basal taxon consisting, stepwise, of
Istiodactylus
, the Ornithocheiridae,
Nyctosaurus
and the Pteranodontidae. The remaining taxa, Ctenochasmatoidea, Dsungaripteroidea and Azhdarchoidea, are weakly united in a clade of non-ornithocheiroid pterodactyloids, but their inter-relationships are difficult to resolve.
Cycnorhamphus
is the basal-most ctenochasmatoid, while the remaining taxa (
Pterodactylus
, Lonchodectidae, Ctenochasmatidae) form an unresolved trichotomy. The Dsungaripteroidea (
Germanodactylus
+Dsungaripteridae) is strongly supported by features of the skull and dentition. The Azhdarchoidea (
Tapejara
[
Tupuxuara
+ Azhdarchidae]) is united by cranial characters such as elevation of the antorbital region, and relative shortening of the wing finger. The pattern of pterosaur evolution suggested by the results of this analysis is broadly similar to traditional ideas, but has greater resolution, more complexity and reveals several previously unrecognized ‘events’.
Morphological evolution of the pectoral girdle of pterosaurs: myology and function
Authors: Bennett S. Christopher
Published: April 2008

The musculature of the pectoral region of representative rhamphorhynchoid (
Campylognathoides
) and large pterodactyloid (
Anhanguera
) pterosaurs was reconstructed in order to examine the function of various muscles and the functional consequences of the evolution of the advanced pectoral girdle of large pterodactyloids. The reconstructions suggest that
m. supracoracoideus
was not an elevator of the wing, but instead depressed and flexed the humerus.
m. latissimus dorsi, m. teres major, m. deltoides scapularis
, and
m. scapulohumeralis anterior
were wing elevators. Comparison of the origin, insertion and function of muscles in the rhamphorhynchoid and the large pterodactyloid suggests that the evolution of the advanced pectoral girdle: (1) straightened the pull of
m. pectoralis, m. deltoides scapularis
and
m. teres major
, improving their function in wing elevation; (2) allowed ligaments rather than muscles to resist the tendency of those muscles to move the scapula; and (3) braced the pectoral girdle against the vertebral column so that the tendency of
m. latissimus dorsi
and of aerodynamic lift on the wing to move the scapulocoracoid medially and dorsally, thereby compressing the thorax, could be resisted. The osteological and myological complexity of the advanced pectoral girdle, its uniqueness among tetrapods and its association with other complex osteological features argue that the advanced pectoral girdle is a synapomorphy complex of a single clade of large pterodactyloids, rather than a mere correlate of large size evolved convergently in various lineages.
New specimens of Pterosauria (Reptilia) with soft parts with implications for pterosaurian anatomy and locomotion
Authors: Frey Eberhard , Tischlinger Helmut , Buchy Marie-Céline , Martill David M.
Published: April 2008

New specimens of pterosaurs with soft-part preservation from the Solnhofen Lithographic Limestone (S Germany) and the Crato Formation (northeastern Brazil) yield hitherto unknown and unexpected details of pterosaur anatomy: the presence and internal anatomy of softtissue crests, the internal anatomy of the brachiopatagium, including a blood vessel system and structural details of foot and hand. Some consequences for pterosaurian flight, thermoregulation and aspects of evolution are discussed.
Middle- and bottom-decker Cretaceous pterosaurs: unique designs in active flying vertebrates
Authors: Frey Eberhard , Buchy Marie-Céline , Martill David M.
Published: April 2008

Among Pterosauria there are three types of scapulocoracoid construction. In the ornithocheirid scapulocoracoid the scapula is oriented almost horizontally; it is shorter than the coracoid and the glenoid fossa is level with the ventral margin of the vertebral column. In the azhdarchid scapulocoracoid the scapula is curved ventrally and is as long as the coracoid. In this construction the glenoid fossa lies approximately in the mid-horizontal plane of the chest. In the tapejaroid construction, the scapula is about one-third longer than the coracoid, which is oriented subhorizontally, and the glenoid fossa is level with the dorsal rim of the sternal plate. Both latter conditions are hitherto unknown among flying vertebrates and result in an unstable, but manoeuvrable flight, probably powered with wing beats.
Pterosaur swim tracks and other ichnological evidence of behaviour and ecology
Authors: Lockley Martin G. , Wright Joanna L.
Published: April 2008

New finds of pterosaur tracks (cf.
Pteraichnus
) from the Summerville and Sundance formations (Late Jurassic) of western North America associated with elongate scrape marks and small circular paired depressions indicate that pterosaurs could swim, or at least float in water, and that they may have fed on small animals living at the sediment surface in shallow water. In this respect their behaviour resembled certain seabirds. The ichnological evidence for pterosaur ‘swim’ and ‘feeding’ traces consists of scrape marks interpreted as traces left when the paddling limb of a pterosaur registered on the substrate, and small circular paired depressions as traces left when the beak of a pterosaur was probing for food. Such traces resulting from aquatic activity are consistent with the nearshore environments in which pterosaur bones and trackways have been found.

There is growing evidence for an extensive and complex
Pteraichnus
ichnofacies in the Late Jurassic of western North America; over 80 specimens have been collected with many more remaining in the field. In particular we draw attention to the large quantitative data base that is available for morphometric, size-frequency studies and the potential for behavioural studies of individual locomotion and flocking. In addition the sites hold considerable promise for understanding what appears to be the world’s largest pterosaur ichnofacies in the context of ancient depositional environments and regional sequence stratigraphy.
Triassic mid-ocean ridge basalts from the Argolis Peninsula (Greece): new constraints for the early oceanization phases of the Neo-Tethyan Pindos basin
Authors: Saccani Emilio , Padoa Elisa , Photiades Adonis
Published: April 2008

The Middle Unit of the central-northern Argolis Peninsula, in NE Peloponnesus (Greece), is composed of several tectonic slices, locally including intact sequences of mafic volcanic rocks topped by radiolarian cherts. Although some of these sequences are Jurassic in age, many of them display a Triassic age based on biostratigraphical evidence. The petrological studies presented in this paper indicate that the Triassic volcanic rocks were generated in a mid-ocean ridge setting, and that they represent the oldest remnants of the Pindos oceanic crust so far recognized in the Subpelagonian zone. On the basis of immobile trace element analyses, two chemically distinct groups of Triassic lavas can be recognized in the various volcanic sequences. One group is represented by transitional-type mid-ocean ridge basalts (T-MORBs) displaying moderate light rare earth element (LREE) enrichment, and incompatible element abundances very similar to those observed in present-day T-MORBs. The other group exhibits a range of characteristics typical of many normal-type MORBs: that is, variable LREE depletion and flat N-MORB normalized patterns of incompatible element abundance. Moreover, many geochemical characteristics indicate that the various N-MORB type volcanic sequences originated from chemically distinct (heterogeneous) sub-oceanic mantle sources. Analogous to similar basalts from ophiolitic mélanges of the Dinaride-Hellenide belt, the T-MORBs from the Argolis Middle Unit are interpreted as having originated from a primitive mantle source variably enriched by an ocean-island basalt (OIB)-type component. In contrast, the contemporaneous occurrence of N-MORBs implies that, during the Mid-Late Triassic, oceanic spreading of the Pindos basin had already reached, at least in some sectors, a quasi-steady state involving only sub-oceanic mantle sources and their partial melt derivatives. Our model for the Triassic opening of the Pindos oceanic basin and its related tectonomagmatic evolution is largely supported by comparison with the Red Sea embryonic ocean, a modern analogous setting.
Yarlung Zangbo ophiolites (Southern Tibet) revisited: geodynamic implications from the mineral record
Authors: Hébert Réjean , Huot François , Wang Chengshan , Liu Zhifei
Published: April 2008

We present mineral chemistry data and petrological evidence from the Yarlung Zangbo suture zone ophiolites (Southern Tibet) suggesting that they represent a collage of heterogeneous massifs. Mantle sections in these ophiolites consist of harzburgite and lherzolite cut by several generations of gabbroic to diabasic intrusions, all affected by high-temperature deformation. Pyroxenitic bands are parallel to the mantle foliation. Crustal plutonic sections, consisting of dunite, wehrlite and gabbro, are thin or absent and have been observed only in the Dazhuqu massif. Plagioclase is an additional phase associated with crustal peridotites. The mineral chemistry of silicate minerals and spinel in the mantle and crustal rocks varies widely and is believed to reflect complex melt percolation and reaction. The massifs record polybaric exhumation steps from at least 50 km depth to the plagioclase stability field. Pyroxene has re-equilibrated compositions from 1200 °C down to medium-grade metamorphic conditions. The mantle peridotites are interpreted as the residues of 10–40% partial melting of a fertile lherzolitic source. High Cr number, low TiO
2
content and relatively high Fe
3+
number of spinels suggest that the ophiolitic massifs were generated in a suprasubduction zone (arc or back-arc) environment.
Tectonic implications of boninite, arc tholeiite, and MORB magma types in the Josephine Ophiolite, California-Oregon
Authors: Harper Gregory D.
Published: April 2008

The Josephine Ophiolite is a large complete ophiolite flanked by arc complexes, including rifted arc facies, and overlain by volcanopelagic and volcaniclastic sedimentary rocks. The extrusive sequence and sheeted dyke complex record a wide range in magma types and degree of fractionation. The upper part of the extrusive sequence, as well as late dykes in the ophiolite, have mid-ocean ridge basalt (MORB) affinities and include unusual highly fractionated Fe-Ti basalts. The sheeted dyke complex and lower pillow lavas are dominated by transitional island-arc tholeiite (IAT) to MORB, but about 10% consist of low-Ti, high-Mg basalts and andesites. Whole-rock chemistry and Cr-spinel compositions indicate that the low-Ti rocks range from boninite (BON) to primitive arc basalt. The low-Ti samples have trace element characteristics indicating a greater subduction component than the IAT-MORB or MORB samples, as well as derivation from a wide range of sources ranging from depleted to enriched relative to an average N-MORB mantle source. Mixing of low-Ti and MORB magmas may have produced the IAT-MORB magma type that is most characteristic of the ophiolite. Podiform chromites and late magmatic features in the mantle peridotite, described by previous workers, appear to have been formed from the low-Ti magmas. Regional geological relationships and the presence of boninitic magmas suggest that arc rifting and initial sea-floor spreading to form the Josephine Ophiolite occurred in the forearc of a west-facing arc built on edge of the North American plate. Arc magmatism appears to have jumped westward, at which time the Josephine basin became situated in a back-arc setting, analogous to the inferred evolution of the modern Lau back-arc basin. Alternatively, the Josephine Ophiolite may have formed in a setting analogous to the north end of the Tonga Trench or the south end of the North Fiji basin, both sites of modern boninites, where a back-arc spreading centre has propagated across an arc into the forearc. Rift propagation during formation of the Josephine Ophiolite is consistent with the presence of highly fractionated Fe-Ti basalts.
Forearc extension and sea-floor spreading in the Thetford Mines Ophiolite Complex
Authors: Schroetter Jean-Michel , Pagé Philippe , Bédard Jean H. , Tremblay Alain , Bécu Valérie
Published: April 2008

The Ordovician Thetford Mines Ophiolite Complex (TMOC) is an oceanic terrane accreted to the Laurentian margin during the Taconic Orogeny and is affected by syn-obduction (syn-emplacement) deformation and two post-obduction events (Silurian backthrusting and normal faulting, and Acadian folding and reverse faulting). The southern part of the TMOC was tilted to the vertical during post-obduction deformation and preserves a nearly complete cross-section through the crust. From base to top we distinguish cumulate Dunitic, Pyroxenitic and Gabbroic Zones, a hypabyssal unit (either sheeted dykes or a subvolcanic breccia facies), and an ophiolitic extrusive-sedimentary sequence, upon which were deposited sedimentary rocks constituting the base of a piggy-back basin. Our mapping has revealed the presence of subvertically dipping, north-south- to 20°-striking faults, spaced
c.
1 km apart on average. The faults are manifested as sheared or mylonitic dunites and synmagmatic breccias, and correspond to breaks in lithology. The fault breccias are cut by undeformed websteritic to peridotitic intrusions, demonstrating the pre- to synmagmatic nature of the faulting. Assuming that rhythmic cumulate bedding was originally palaeo-horizontal, kinematic analysis indicates that these are normal faults separating a series of tilted blocks. In the upper part of the crust, the north-south-striking fault blocks contain north-south-striking dykes that locally constitute a sheeted complex. The faults correspond to marked lateral changes in the thickness and facies assemblages seen in supracrustal rocks, are locally marked by prominent subvolcanic breccias, and have upward decreasing throws suggesting that they are growth faults. The base of the volcano-sedimentary sequence is a major erosional surface in places, which can penetrate down to the Dunitic Zone. The evidence for coeval extension and magmatism, and the discovery of a locally well-developed sheeted dyke complex, suggest that the TMOC formed by sea-floor spreading. The dominance of a boninitic signature in cumulate and volcanic rocks suggests that spreading occurred in a subduction zone environment, possibly in a forearc setting.
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

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

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

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

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

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

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

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

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

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

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

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

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.
Linking growth episodes of zircon and metamorphic textures to zircon chemistry: an example from the ultrahigh-temperature granulites of Rogaland (SW Norway)
Authors: Möller Andreas , O’Brien Patrick J. , Kennedy Allen , Kröner Alfred
Published: April 2008

In-situ
U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland anorthosite-norite intrusive complex; and (b) to constrain the age of M
2
ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M
3
event. All samples yield magmatic inherited zircon of
c.
1035 Ma, some an additional group at
c.
1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite ± osumilite-bearing) migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results for the M
1
metamorphic event at
c.
1000 Ma
BP
are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence for zircon growth during the M
2
metamorphic episode.

Zircon intergrown with or included within M
2
metamorphic minerals (magnetite, spinel, orthopyroxene) give an age of 927 ± 7 Ma (2
σ, n
= 20). The youngest observed results are found in zircon outside M
2
minerals, some overgrown by M
3
mineral assemblages (late garnet coronas, garnet + quartz and orthopyroxene + garnet symplectites) and yield a slightly younger pooled age of 908 ± 9 Ma (2
σ, n
= 6). These textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the UHT event. These youngest age groups are consistent with the emplacement age of the Rogaland intrusive complex and the last magmatic activity (Tellnes dyke intrusion), respectively. This is direct and conclusive evidence for UHT metamorphism in the regional aureole being caused by the intrusions, and corrects earlier notions that the events are not linked. Trace element behaviour of zircon (Tb/U and Y content) has been tracked through time in the samples and shows variations both within and between samples. This heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of ‘rules of thumb’ in the interpretation of zircon chemistry, but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism.
Improving precision of Sm-Nd garnet dating by H
2
SO
4
leaching: a simple solution to the phosphate inclusion problem
Authors: Anczkiewicz Robert , Thirlwall Matthew F.
Published: April 2008

Sm-Nd garnet geochronology is often hampered by the presence of submicroscopic inclusions of rare earth element-rich phosphates, which lower age precision, lead to inaccurate ages or make dating impossible. We propose a single-step sulphuric acid leaching technique as a very efficient tool in eliminating phosphate inclusions, which helps to achieve more precise and more accurate Sm-Nd garnet dates. Examples from silimanite grade metapelites demonstrate the much higher efficiency of this method in comparison with previously proposed techniques based on HF and HCl.
147
Sm/
144
Nd ratios obtained on garnets leached by sulphuric acid were twice as high as those obtained by HF and HCl leaching. This led to age precision better than 3% for Tertiary samples. Comparison of leached and unleached nearly inclusion-free garnets from high pressure granulites, demonstrates that there is no Sm/Nd fractionation induced by H
2
SO
4
leaching. Our new technique eliminates phosphates, but does not attack garnet. This considerably reduces the necessity for hand-picking and lowers the amount of sample required for analysis making Sm-Nd garnet dating a more easily applied geochronometer.