metallogenic evolution of convergent margins selected ore deposit models abstract metallogenic patterns in convergent margin arcs are related to thickness of the arc crust and its petrologic evolution which are themselves linked by a positive correlation between average silica content of arc rocks and average thickness of arc crust in general tin mercury antimony and fluorine are most abundant in areas of thickest crust although the composition of ore deposits is strongly controlled by deep crustal and mantle features the actual form and type of many ore deposits is controlled by upper crustal features particularly the relative abundance of carbonate and silicate country rock for instance epithermal precious metal deposits are commonly associated with subaerial silicic volcanism which generally forms in the later stages of arc evolution deposits of this type will be absent if arc volcanism is terminated prematurely or removed if subduction of thickened or hot oceanic crust causes extreme uplift and consequent erosion of the upper parts of the arc similarly chimney manto limestone replacement lead zinc silver deposits are common only in terranes with extensive thick limestone units which are commonly formed in shallow shelf environments several important types of convergent margin deposits remain poorly understood acid sulfate deposits in primitive oceanic volcanic arcs are associated with bimodal volcanic suites that appear to have formed at an early stage in the evolution of the arc alkaline porphyry copper deposits are closely associated with calc alkaline porphyry copper deposits commonly in the same volcanic arc although they appear to have formed farther back from the trench or during periods of oblique subduction or arc reversal massive iron oxide deposits and related base and precious metal mineralization are closely associated with silicic magmas that form during later stages of arc evolution in addition to tectonic and petrologic factors metallogenic patterns probably reflect global forcing mechanisms the most important of which are global seawater anoxia and plate reorganizations global anoxia produces deep water environments that favor sedex mineralization and can promote subsurface ore deposition where anoxic bottom waters recharge underlying aquifers plate reorganizations can lead to ore deposition by providing extensional environments in the upper crust and possibly by facilitating partial melting of oceanic crust that has been modified by earlier subducting slabs

thermal evolution of convergent plate margins thermal modeling and reevaluation of isotopic ar ages for blueschists in the franciscan complex of california modeling of heat flow through a wedge shaped accretionary complex emplaced beneath the overriding plate at a convergent plate margin provides insight into the tectonic significance of ar isotopic ages and metamorphic progressions across blueschist belts if convergence rates are maintained at rates of a few centimeters per year or more material recrystallized at depth where the accretionary wedge is narrow can be rapidly heated to high temperatures and then rapidly cooled to the low temperatures where ar loss by diffusion and recrystallization effectively ceases maximum temperatures are lower and the periods of heating and cooling are longer at shallower levels where the wedge is wider because of this difference in thermal history k ar and 40ar 39ar ages from blueschists recrystallized in the upper part of the accretionary wedge can be younger by several tens of million years than ages from material initially recrystallized at the same time but at greater depth application of this thermal modeling to the franciscan subduction complex of northern california indicates that the coherent blueschists in the eastern belt typically with 115 to 125 m y ar isotopic ages and the high grade blueschist blocks in the mud matrix melanges of the western portion of the central belt typically with 140 to 150 m y ar isotopic ages could both have formed during a single prolonged period of continuous convergence

precise ages and isotopic constraints for the lewis hills bay of islands ophiolite preservation of an arc spreading ridge intersection the lewis hills the southernmost massif of the bay of islands ophiolite exposes a prominent shear zone the origin of this shear zone is pivotal with respect to the interpretation of the tectonic setting of the lewis hills and the genesis of the bay of islands ophiolite different models suggest that the shear zone represents a fracture zone that separates either oceanic lithospheres of similar origin or the lithospheres of an island arc and a marginal basin a contrasting model disputes the fracture zone setting new u pb zircon ages of the lewis hills combined with sm nd systematics are used to resolve this controversy in the western part of the lewis hills western lewis hills magmatic zircons from two trondhjemite bodies yield u pb ages of 500 6 2 0 ma and 503 7 3 2 ma initial nd 502 ma values for trondhjemites and gabbros range from 1 5 to 2 0 both the mineral ages and nd signature are similar to those of the island arc related little port complex which is located to the north of the western lewis hills in contrast apatite from a gabbro of the eastern part of the lewis hills eastern lewis hills yields a concordant u pb age of 485 0 1 0 ma this gabbro has an nd 485 ma of 7 4 the age and isotope signature link the eastern lewis hills with the northern marginal basin related massifs of the bay of islands ophiolite the significant differences of ages and isotope characteristics of the western and eastern lewis hills support the interpretation of the shear zone as the major tectonic boundary between an island arc and a marginal basin this complex tectonic setting may have resulted in a wide spectrum of magma compositions that involved both island arc and marginal basin sources

plagiogranite from the andaman ophiolite belt bay of bengal india plagiogranites occur on the eastern margin of the southern part of south andaman in the outer sedimentary arc of the andaman nicobar islands a part of the sunda burmese double chain arc system in the bay of bengal they intrude gabbros that are associated with pillow basalt east coast volcanic rocks basalt basaltic andesite to acid differentiates radiolarian cherts conglomerate and grit the plagiogranite characterized by the occurrence of interstitial vermicular and micrographic intergrowths of quartz and plagioclase shows some geochemical similarities with ophiolitic plagiogranites but rb yb nb ta and y abundances in the andaman plagiogranite are characteristic of volcanic arc granites rather than the ocean ridge granites it is suggested that the andaman plagiogranite has been generated by low pressure crystal fractionation of a low k tholeiitic magma derived from a mantle source in a supra subduction zone tectonic setting further it is suggested that this plagiogranite intruded the gabbro of the andaman ophiolite probably during mid eocene times as the andaman ophiolitic rocks late cretaceous to early eocene are believed to have been obducted onto the leading edge of the eurasian continent during the mid eocene to late oligocene subduction event prior to the currently active andaman java subduction system that was initiated during the late miocene

traces of frost activity and ice segregation in pleistocene loess deposits and till of northern italy deep seasonal freezing or permafrost abstract this paper describes traces of ice lensing preserved in loessic deposits in till and in shelters and cave infillings most of the observed traces correspond to seasonal frost activity rather well preserved in poorly drained sites or in the shadow of shelters no distinct evidence of permafrost dating to the last glaciation has been found the preservation of ice lensing traces in soil is poor due to intense biological activity and dry climate conditions this shows that during the glacial time the po plain belonged to the southeastern european domain though presently included in the western sub mediterranean climate zone

mid ocean ridge or marginal basin origin of the east taiwan ophiolite chemical and isotopic evidence the east taiwan ophiolite eto occurs as allochthonous fragments embedded in the plio pleistocene lichi melange in the coastal range of eastern taiwan it is probably the youngest among the known ophiolitic complexes of the world the overall geochemical and isotopic characteristics of the representative rock types are very oceanic and strongly argue for a mid ocean or marginal basin origin of the eto the best criteria include 1 major element compositions 2 ree distribution patterns 3 nd isotopic ratios some have nd 12 4 pb isotopic ratios and 5 the presence and nature of the red shale this conclusion is supported by previous studies of metamorphic evolution of the eto

comment on paleoclimatic record of the past 22 000 years in venice northern italy biostratigraphic evidence and chronology by serandrei barbero et al quaternary international 140 141 37 52 interstadials or phases of accumulation of reworked pollen

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

interactions between magma and hydrothermal system in oman ophiolite and in iodp hole 1256d fossilization of a dynamic melt lens at fast spreading ridges the transition between the small melt lens observed on top of fast spreading ridge magma chambers and the overlying sheeted dike complex marks the interface between magma and the hydrothermal convective system it is therefore critical to our understanding of fast spreading ridge accretion processes we present maps of two areas of the oman ophiolite where this transition zone is observed as continuous outcrops our observations which include the base of the sheeted dike being crosscut by gabbros are consistent with episodic dike injections in a steady state model but also suggest that the root of these dikes is commonly erased by vertical movements of the top of the melt lens dike assimilation is a possible mechanism for incorporating hydrated phases which result from hydrothermal alteration to the melt lens during upward migrations of its upper boundary upward migrations are also responsible for a granoblastic overprint of the root of the dikes that is also observed in the stoped diabase xenoliths this granoblastic overprint attests to reheating of previously hydrothermally altered lithologies which can even trigger hydrous partial melting due to the lowering of the solidus of mafic lithologies by the presence of a water activity clinopyroxenes present in these granoblastic lithologies are typically low in ti and al content thus strongly contrasting with corresponding magmatic clinopyroxene this may attest to the recrystallization of clinopyroxenes after amphiboles under the peculiar conditions present at the root zone of the sheeted dike complex downward migrations of the top of the melt lens result in the crystallization of the isotropic gabbros at its roof which represent the partly fossilized melt lens melt lens fossilization eventually occurs when magma supply is stopped or at the melt lens margins where the thermal conditions become cooler melt lens migration recrystallization of hydrothermally altered sheeted dikes during reheating stages and assimilation processes observed in the oman ophiolite are consistent with the observations made in iodp hole 1256d we propose a general dynamic model in which the melt lens at fast spreading ridges undergoes upward and downward movements as a result of either eruption replenishment stages or variations in the hydrothermal magmatic fluxes

origin and tectonic significance of the aldrich mountains serpentinite matrix melange northeastern oregon the permian triassic aldrich mountains serpentinite melange consists of a matrix of massive and foliated serpentinite lizardite chrysotile that encases a variety of metamorphic igneous and sedimentary block types serpentinite matrix foliation trends ne sw dips steeply 75 and is parallel to long dimensions of blocks at block matrix contacts the foliation traces block margins the volumetrically most abundant block types are greenstone chert and metagabbro subordinate block types are amphibolite tectonites epiclastic sediments breccia conglomerate sandstone and argillite silicic volcanic rocks silicic plutonic rocks and ultramafic rocks three types of metamorphism are evident from textures and mineral assemblages of blocks and matrix 1 serpentinization and related hydrous alteration confined to matrix ultramafic blocks and margins of mafic blocks 2 lower to middle greenschist and prehnite actinolite facies metamorphism characterized by static non penetrative fabrics of some blocks and 3 dynamothermal epidote amphibolite to upper amphibolite lower granulite metamorphism exhibited by strongly schistose amphibolite tectonites mineral assemblages and mineral chemistry indicate amphibolite tectonites formed at moderate to high temperatures 550 780 c and low to moderate pressures 3 5 kbar most blocks were affected by variable degrees of post metamorphic brittle deformation prior to immersion in serpentinite matrix the melange is interpreted to have formed within a permian to late triassic sediment starved tectonically active forearc fragments of abyssal oceanic crust and early permian ensimatic arc crust were mixed by tectonic and sedimentary processes within the forearc highly mobile serpentinite generated beneath the forearc was protrusively emplaced into the tectonized forearc and mixed with the blocks forming the melange melange generation probably began in the late early permian and ceased by the late triassic

structures in the canyon mountain ophiolite indicate an island arc intrusion abstract lithosphere created in an interarc basin is expected to be characterized by features distinguishing it from normal oceanic lithosphere apart from island arc geochemical affinities and from the occurrence of hydrous high t parageneses in the mantle and deep crustal sequences it is expected that due to a low rate of spreading vertical transport prevails over lateral drifting the canyon mountain complex located in an island arc environment of permo triassic age offers a remarkable illustration of these expected geological characteristics in particular mantle diapirism is deduced from the structural study smaller diapirs are formed in crustal formations the intrusions took place at variable temperatures 1300 800 c and were accompanied by multistage melting in hydrous conditions

environmental and climatic conditions at a potential glacial refugial site of tree species near the southern alpine glaciers new insights from multiproxy sedimentary studies at lago della costa euganean hills northeastern italy abstract it has been hypothesized that refugia of thermophilous tree species were located in northern italy very close to the alps though this hypothesis has yet to be tested thoroughly in contrast to central and southern italy with its relative wealth of data only a few fragmentary records are currently available from northern italy for the last glacial wurm weichselian our new study site lago della costa lies adjacent to the catchment of the megafans of the alpine forelands and the braided rivers of the northeastern po plain that have so far inhibited the recovery of continuous glacial and late glacial records we analyze pollen plant macrofossils charcoal and ostracods to reconstruct the vegetation fire and lake history for the period 33 000 16 000 cal bp we compare our data with glacial records from southern europe to discuss similarities and dissimilarities between these potential refugial areas a comparison with independent paleoclimatic proxies allows to assess potential linkages between environmental and climatic variability new macrofossil and pollen data at lago della costa unambiguously document the local persistence of boreal tree taxa such as larix decidua and betula tree species around the study site during the last glacial the regular occurrence of pollen of temperate trees in the organic lake sediments fine detritus calcareous gyttja suggests that temperate taxa such as corylus avellana quercus deciduous tilia ulmus fraxinus excelsior carpinus abies alba and fagus sylvatica most likely survived the last glacial maximum lgm at favorable sites in the euganean hills the percentage values of temperate trees are comparable with those from southern europe e g monticchio in southern italy we conclude that the euganean hills were one of the northernmost refugial areas of temperate taxa in europe however the relative and absolute abundances of pollen of temperate trees are highly variable pollen inferred declines of temperate tree communities e g quercetum mixtum and low ostracod inferred water levels at lago della costa correspond to the cold heinrich events h 2 lgm 23 000 19 000 cal bp and h 3 around 28 000 cal bp as recorded in the marine sediments of the north atlantic similar patterns of significant temperate tree population collapses during cold heinrich events are recorded at southern mediterranean sites e g monticchio and the alboran sea these findings suggest close linkages between northern atlantic and south central european climates during the past glacial

the roof of an axial magma chamber a hornfelsic heat exchanger the heat flux from an axial magma chamber into the convecting hydrothermal system at mid ocean ridges is governed by the conductive boundary layer separating them the nature of this critical interface has been modeled and seismically imaged but its characteristics have hitherto rarely been documented in ocean crust here hornfelsic rocks from the sheeted dike gabbro transitions at two tectonic exposures of fast spreading east pacific rise crust at the pito and hess deeps and in the oman and troodos ophiolites are described these rocks record thermal metamorphism to hornblende and pyroxene hornfels at 700 1000 c these temperatures in conjunction with their geological relationships imply that the hornfels are preserved fragments of conductive boundary layers their distribution at the base of sheeted dike complexes in narrow contact aureoles and dikes intruding the uppermost gabbros tracks the vertical migration of axial magma chambers over minimum depth ranges of 200 400 m the heat flux across the hornfelsic aureoles 11 44 mw km is comparable to hydrothermal fluxes released along fast spreading ridges confirming that the heat driving hydrothermal convection is transferred across conductive boundary layers

reconstruction of full glacial environments and summer temperatures from lago della costa a refugial site in northern italy abstract vegetation and climate during the last ice age and the last glacial maximum lgm 23 000 19 000 cal bp were considerably different than during the current interglacial holocene cold climatic conditions and growing ice sheets during the last glaciation radically reduced forest extent in europe to a restricted number of so called refugia mostly located in the southern part of the continent on the basis of paleobotanical analyses the euganian hills colli euganei in northeastern italy have previously been proposed as one of the northernmost refugia of temperate trees e g deciduous quercus tilia ulmus fraxinus excelsior acer abies alba fagus sylvatica carpinus and castanea in europe in this study we provide the first quantitative vegetation independent summer air temperature reconstruction for northern italy spanning the time 31 000 17 000 cal yr bp which covers the coldest periods of the last glacial including the lgm and heinrich stadials 1 to 3 chironomids preserved in a lake sediment core from lago della costa 7m a s l a small lake at the south eastern edge of the euganean hills allowed quantitative reconstruction of full and late glacial summer air temperatures using a combined swiss norwegian temperature inference model based on chironomid assemblages from 274 lakes chironomid and pollen evidence from lago della costa derives from finely stratified autochthonous organic gyttja sediments which excludes major sediment mixing or reworking after reconstructing paleo temperatures we address the question whether climate conditions were warm enough to permit the local survival of temperate tree species during the lgm and whether local expansions and pollen inferred contractions of temperate tree taxa coincided with chironomid inferred climatic changes our results suggest that chironomids at lago della costa have responded to major climatic fluctuations such as temperature decreases during the lgm and heinrich stadials the vegetation of the euganean hills shows responses to these climatic oscillations although the effects of temperature changes were probably also strongly influenced by changes in humidity reconstructed july air temperatures at lago della costa never fell below 10 13 c error range of reconstruction 1 5 1 6 c which is considerably above the limit considered necessary for forest growth 8 10 c instead rather mild climatic conditions prevailed 31 000 17 000 cal yr bp with average summer temperatures between 12 and 16 c which most likely allowed survival of temperate tree taxa in the warmest and moistest microhabitats of the euganean hills during the lgm only assuming local survival is it possible to explain the repeated expansions and collapses of temperate trees at lago della costa which faithfully accompanied the climatic oscillations

oxygen isotope composition of a section of lower oceanic crust odp hole 735b 1 the oxygen and hydrogen isotope compositions of vein minerals and 18o values of 64 whole rocks from the deeper kilometer of the 1 5 km lower crustal section of odp hole 735b in the sw indian ocean were analyzed in order to investigate seawater penetration into and reaction with the lower oceanic crust vein amphiboles have d values of 35 to 65 indicating formation from seawater fluids and hydrothermal fluids containing a component of low d magmatic fluid crystal plastic shear zones reacted with 18o enriched fluids at high temperatures and have 18o values of 6 0 7 0 indistinguishable from surrounding rocks reaction with seawater dominated fluids over a range of temperatures 350 700 c led to local 18o depleted rocks but much of the section exhibits 18o enrichment 18o up to 7 6 as the result of static alteration of plagioclase over a range of temperatures 250 to 600 c and fluid compositions chloritic veins and associated alteration by 18o enriched fluids occurred at temperatures of 150 250 c and late carbonates and fe oxyhydroxides formed from cold 10 seawater associated with open fault related fractures bulk rock 18o values of the lower 1 km range from 3 7 to 10 2 but are mostly 5 6 7 6 and the section has a mean bulk 18o of 6 5 1 0 in contrast rocks from the shallower 500 m exhibit net 18o depletion 5 1 1 0 reflecting high temperature deformation and seawater hydrothermal alteration stakes 1991 the weighted average 18o for the entire gabbro section of hole 735b is 6 0 exhibiting little or no change compared to unaltered crust although tectonism at slow spreading rates may allow exposure of lower crust to greater amounts of low temperature alteration and 18o enrichment there is no evidence for a consistent difference in the bulk 18o of the plutonic sections of oceanic crust formed at fast versus slow spreading rates

stability and composition relations of calcic amphiboles in ultramafic rocks calcic amphiboles are observed in ultramafic rocks that have equilibrated under a broad span of geological conditions and might prove to be good indicators of metamorphic grade if their stabilities could be determined as a function of their compositions experiments were performed on the stability of tremolite plus forsterite in the system h2o cao mgo sio2 from 5 to 20 kbar a univariant curve was fitted to the experimental brackets using volume water fugacity and heat capacity data the results indicate that the maximum stability of tremolite in the presence of forsterite is about 825 c at 5 kbar addition of al2o3 to this system increases the stability of tremolitic amphibole by only 20 40 c and induces solubility of 5 7 wt al2o3 in the amphibole as determined from quantitative sem analyses of individual amphibole crystals thus substitution of the tschermakite component ca2 mg3al2 si6al2 o22 oh 2 alone cannot lead to the greatly enhanced al2o3 contents or thermal stability of natural calcic amphiboles

an island arc origin for the canyon mountain ophiolite complex eastern oregon u s a abstract the canyon mountain ophiolite oregon is exceptional in lacking sheeted dikes basaltic pillow lavas and sediments that are characteristic of many other ophiolites instead the uppermost portion of the complex consists of a significant volume of plagiogranites which in addition to minor basalts intrude a large section of keratophyres believed to be of volcanic origin the trend of intrusive rocks and of bedding in the keratophyres is mostly parallel to layering in the underlying gabbroic cumulates and to contacts between units in the remainder of the ophiolite it is suggested that the plagiogranites basalts and keratophyres comprise a sill complex both the plagiogranites and the keratophyres are similar respectively to low k2o plutonic and extrusive rocks of island arcs the mineralogy and penetrative deformation structures of the ultramafic and some of the gabbroic rocks of the ophiolite indicate greater depth of formation related to magmatism and diapirism above a benioff zone radiometric age dates of plagiogranites confine the minimum age of the complex to the early permian the canyon mountain ophiolite may thus be correlative with other fragments of a lower permian arc terrane throughout northeastern oregon which were chaotically mixed during renewed subduction in middle to late triassic time

an archean oceanic felsic dyke swarm in a nascent arc the hunter mine group abitibi greenstone belt canada abstract the 2730 ma old hunter mine group hmg a dominantly felsic subaqueous volcanic sequence was formed during early arc construction in the abitibi greenstone belt quebec canada the western part of the hmg contains a felsic dyke swarm up to 1 5 km wide and traceable up section for 2 5 km five distinct generations were identified 1 aphanitic to feldspar phyric dykes 2 quartz feldspar phyric dykes with nd values suggest that an older sialic substrate was not involved in their genesis we infer that the felsic rocks were generated by melting of mafic oceanic crust the swarm was emplaced during nascent oceanic island arc development and was related to rifting of the arc the conformably overlying morb like basalts and basaltic komatiites of the stoughton roquemaure group used the same conduits and further indicate splitting of the arc hmg and associated parts of the abitibi greenstone belts bear a strong resemblance to modern rifted intraoceanic arcs of the western pacific

moist morb axial magmatism in the oman ophiolite the evidence against a mid ocean ridge origin the oman ophiolite has an axial volcanic suite and associated sheeted dike complex similar in composition to modern mid oceanic ridge basalt morb its internal structure is regarded by many as being directly comparable to ocean lithosphere from the east pacific rise however there has long been controversy over the geodynamic setting in which the ophiolite formed and the extent to which the analogy can be drawn because the morb like axial volcanics are overlain by lavas that include depleted arc tholeiites and boninites to some this implies that the entire ophiolite formed above a subduction zone others maintain that it formed at a true open ocean mor and that the water required to generate the arc like magmas derived from descending near axis hydrothermal fluids or from ancient subduction a popular compromise posits that the axial suite formed at a true mor and the later magmatism documents the initiation of obduction we test these models by reexamining the morb like character of the early axial lavas and dikes we show that fractionation trends require the presence of water at concentrations significantly higher than any open ocean morb instead trends are identical to those of backarc basin and intraoceanic forearc volcanics by showing that the entire ophiolite formed in a hydrous system we rule out all models in which the oman ophiolite was generated at an open ocean mor instead it formed at a submarine spreading center above a probably newly initiated subduction zone

analysis of the wallowa baker terrane boundary implications for tectonic accretion in the blue mountains province northeastern oregon the baker terrane exposed in the blue mountains province of northeastern oregon is a long lived ancient late paleozoic early mesozoic accretionary complex with an associated forearc this composite terrane lies between the partially coeval wallowa and olds ferry island arc terranes the northern margin of the baker terrane is a broad zone 25 km wide of fault bounded imbricated slabs and slices of meta igneous and metasedimentary rocks faulted into chert argillite melange of the elkhorn ridge argillite metaplutonic rocks within tectonic units in this zone crystallized between 231 and 226 ma and have low initial 87 sr 86 sr ratios 0 7033 0 7034 and positive initial e nd values 7 7 to 8 5 in contrast siliceous argillites from the chert argillite melange have initial 87 sr 86 sr values ranging from 0 7073 to 0 7094 and initial e nd values between 4 7 and 7 8 we interpret this broad imbricate fault zone as a fundamental tectonic boundary that separates the distal wallowa island arc terrane from the baker accretionary complex terrane we propose that this terrane boundary is an example of a broad zone of imbrication made up of slabs and slices of arc crust tectonically mixed within an accretionary complex providing an on land ancient analogue to the actualistic arc arc collisional zone developed along the margins of the molucca sea of the central equatorial indo pacific region

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

reorganization of geological sciences and particularly of metamorphic geology by the advent of plate tectonics a personal view abstract plate tectonics having being formulated in geophysics in the 1960 s was introduced into and caused reorganization of geological sciences in about 1968 1975 an anticipated system of reorganized geology was formulated around 1970 by a few pioneering geologists with a strong background of global scale regional geology pre existing dutch school tectonics with an explicit uniformitarian tendency played an important role in the beginning development and acceptance of plate tectonic viewpoints global scale views in metamorphic geology that had begun prior to the advent of plate tectonics were developed into tectonic models of regional metamorphism as well as discussions of the p t t paths and of secular changes in the type of regional metamorphism

petrological evolution of the crust and mantle in its simplest form crust mantle evolution can be thought of as a study of the processes by which material is lost from the mantle and forms new crust the overall goal is achievement of mass balance between the average composition of material known to have been added to the crust and the estimated average composition of the entire crust attainment of this goal for the earth has been elusive primarily because the materials available for study have generally been involved in several stages of evolution including fractionation within both the crust and mantle and recycling of material from crust to mantle thereby complicating the interpretation of available data although the overall composition of the continental crust generally has been modelled as andesitic taylor and mclennan 1981a the present upper continental crust is granodioritic and requires a more mafic lower crust it has been proposed taylor and mclennan 1981a that originally the archean crust had an andesitic composition throughout its volume and that partial melting in the lower crust developed more felsic melts that rose to enrich the upper crust leaving a more mafic residuum of lower crust this concept has been challenged by weaver and tarney 1981a who argue that the granulites of presumed lower crustal origin do not have the correct compositions for this model if one considers more recent additions to the crust from the mantle over the past several hundred million years it appears that these latest additions are basaltic with very little if any granitic additions in fact barker 1981 in his introduction to a special issue of jgr on granites and rhyolites states to date no one has demonstrated that such magmas may form in the mantle indeed much experimental and geochemical work indicates that such siliceous liquids cannot originate there therefore such magmas must be generated in the crust in island arc environments where the average composition of added material normally is thought to be andesitic a anderson 1982 has shown that the parent melt is basaltic not andesitic in addition kay 1980 has shown that the k content of the newly added material is less than the average k content of the previously existing crust in the oceanic crust the additions from the mantle are basaltic and according to o hara 1982 probably are derived from very mafic picritic melts as parents continental flood basalts which have also added material from mantle to crust also are clearly derived from basaltic parental melts basaltic volcanism study project 1981 thus the evidence for the nature of material subtracted from the mantle and added to the crust for the past several hundred million or perhaps a billion years points to very mafic melts this contrasts significantly with the overall andesitic composition of continental crust it implies furthermore that if the overall archean crustal composition was andesitic as estimated by taylor and mclennan 1981a then additions to the crust have become more mafic with time a concept in direct conflict with most models in which secular trends are from mafic to felsic with time veizer and jansen 1979 and mclennan et al 198o a question which is beginning to appear either explicitly or implicitly is whether crustal volcanic rocks or the crust as a whole really represents the parent melt that formed in the mantle a anderson 1982 o hara 1982 and nisbet and walker 1982

temporal variation of mineralogy and petrology in cognate gabbroic enclaves at arenal volcano costa rica gabbroic enclaves ejected during the current eruption phase a 1 and during the latest prehistoric eruption phase a 2 of arenal volcano show systematic variations in texture mineralogy and composition as a function of host rock chemistry and timing of eruption the most differentiated enclaves occur in the more differentiated a 2 lavas enclaves in the a 1 volcanics are consistently less evolved within the current a 1 eruption the most mafic enclaves are amphibole bearing rocks that were erupted during the first 2 3 years of activity 1968 1970 these enclaves occur in the most differentiated a 1 volcanics and are not in equilibrium with their host rocks they crystallized from a hydrous melt that was slightly more mafic than anything erupted during the current cycle we interpret the enclaves as sidewall crystallization products of a melt possibly a high alumina basalt that was immediately parental to the a 1 lavas enclaves that occur in a 1 rocks erupted after 1970 and all of the a 2 enclaves are amphibole free and less mafic than the early a 1 enclaves their chemistry suggests that they formed during the early to intermediate crystallization of their host lavas none of the enclaves contain minerals that might have equilibrated with a primary mantle derived melt geothermometry is consistent with geochemistry with amphibole bearing a 1 enclaves yielding the highest pyroxene temperatures ave 1090 c and a 2 enclaves the lowest ave 1030 c geobarometry suggests mid to upper crustal depths for the crystallization of all enclaves the enclaves are cognate and reflect pre eruptive crystallization of arenal magmas they record evolution from a hydrous basaltic magma to the drier basaltic andesites that characterize the current eruption volatiles appear to have been lost due to depressurization during the slow ascent of the magmas through the upper levels of the crust following the initial explosive eruption volatile loss and depressurization resulted in the destabilization and the progressive resorption of amphibole the a 2 lavas may represent the long term fractionation products of basaltic andesite magmas similar in composition to the a 1 lavas anorthitic plagioclase commonly thought of as a phase stabilized by high ca na and high water pressure continued to crystallize in a system with relatively low ca na and which had dehydrated and or depressurized to the point at which amphibole was no longer stable this suggests that compositional characteristics other than high ca na or high water content may have stabilized the anorthite in the basaltic and basaltic andesite melts at arenal we speculate that the high alumina content of the arenal magmas may be the stabilizing factor

late paleozoic gabbroic rocks of the bridge river accretionary complex southwestern british columbia geology and geochemistry gabbroic bodies in the bralorne gold bridge area of southwestern british columbia are associated with the oceanic bridge river complex of the western canadian cordillera one of the suspect terranes accreted to north america in the jurassic the gabbros are locally cut by tonalites and are structurally interleaved with ultramafic rocks phyllites graphitic cherts and carbonate lenses that comprise the lower part of the bridge river complex their late carboniferous crystallization age overlaps the depositional age of affiliated supracrustal rocks mississippian jurassic some of which have been metamorphosed to blueschist facies compositionally the gabbros resemble mafic plutonic rocks of ophiolitic complexes and gabbroic rocks of the nearby shulaps range they display some affinity to oceanic island arc tholeiitic suites the bralorne and shulaps gabbros include cumulates and appear to have been derived from a single light ree depleted peridotitic source by melting and subsequent fractional crystallization accumulation of various combinations of plagioclase pyroxenes and olivine the tonalites are compositionally distinct from typical ophiolitic plagiogranites but might be related to the associated gabbros the gabbroic bodies occur within tectonic slivers derived from the oceanic crust that floored a deep ocean basin that existed during the late paleozoic and early mesozoic the bridge river complex comprises fragments of oceanic crust that were tectonically incorporated into an east verging accretionary prism during a middle late triassic to jurassic collisional event

root zone of the sheeted dike complex in the oman ophiolite the root zone of the sheeted dike complex representing a thin zone hundred meters thick of extreme thermal gradient 5 c m is regarded as a thermal boundary between the convective magma chamber system below and the main convective hydrothermal circuit which closes above at the base of this root zone the root zone of the sheeted dike complex is located at the top of the high level foliated gabbro unit where the foliation steepens and where the first diabase dikes appears it is a complex zone characterized by mutual intrusions of microgabbros dikes that we call protodikes with brownish microgranular contacts against the gabbro matrix upward viscous flow in the protodikes and in the reheated enclosing gabbros generate a diffuse transition to the sheeted complex protodike margins stretched in the enclosing flowing doleritic gabbros form a complicated network which can be depicted thanks to microstructural analysis later diabase dikes cross cut the section these relationships are obscured by the hydrothermal circulation which has generated in particular isotropic amphibole gabbro veins these veins tend to propagate horizontally they may be interpreted as the downward closure of the main hydrothermal convective circuit

field based evidence for devolatilization in subduction zones implications for arc magmatism metamorphic rocks on santa catalina island california afford examination of fluid related processes at depths of 15 to 45 kilometers in an early cretaceous subduction zone a combination of field stable isotope and volatile content data for the catalina schist indicates kilometer scale transport of large amounts of water rich fluid with uniform oxygen and hydrogen isotope compositions the fluids were liberated in devolatilizing relatively low temperature 400 degrees to 600 degrees c parts of the subduction zone primarily by chlorite breakdown reactions an evaluation of pertinent phase equilibria indicates that chlorite in mafic and sedimentary rocks and melange may stabilize a large volatile component to great depths perhaps 100 kilometers depending on the thermal structure of the subduction zone this evidence for deep volatile subduction and large scale flow of slab derived water rich fluids lends credence to models that invoke fluid addition to sites of arc magma genesis

large scale deformation related to the collision of the aleutian arc with kamchatka the far western aleutian island arc is actively colliding with kamchatka westward motion of the aleutian arc is brought about by the tangential relative motion of the pacific plate transferred to major right lateral shear zones north and south of the arc early geologic mapping of cape kamchatka a promontory of kamchatka along strike with the aleutian arc revealed many similarities to the geology of the aleutian islands later studies support the notion that cape kamchatka is the farthest west aleutian island and that il has been accreted to kamchatka by the process of arc continent collision deformation associated with the collision onshore kamchatka includes gravimetrically determined crustal thickening and formation of a narrow thrust belt of intensely deformed rucks directly west of cape kamchatka the trend of the thrust faults is concave toward the collision zone indicating a radial distribution of maximum horizontal compressive stress offshore major crustal faults trend either oblique to the kamchatka margin or parallel to major aleutian shear zones these offshore faults are complex accommodating both strike slip and thrust displacements as documented by focal mechtmisms and seismic reflection data earthquake activity is much higher in the offshore region within a zone bounded to the north by the northernmost aleutian shear zone and to the west by an apparent aseismic front analysis of focal mechanisms in the region indicate that the present day arc continent contact zone is located directly east of cape kamchatka in modeling the dynamics of the coiiision zone using thin viscous sheet theory the rhealogical parameters are only partially constrained to values of n the effective power law exponent 2 3 and ar the argand number s 30 these values are consistent with a fnrcarc thermal profile of kamchatka previously determined from heat flow modeling the thin viscous sheet modeling also indicates that onshore thrust faulting is a consequence not only of compressive stresses resulting from the west directed collision but also of sediment induced coupling of the subducting pacific plate

a study of rare earth element ree sio 2 variations in felsic liquids generated by basalt fractionation and amphibolite melting a potential test for discriminating between the two different processes the origin of felsic magmas 63 sio2 in intra oceanic arc settings is still a matter of debate two very different processes are currently invoked to explain their origin these include fractional crystallization of basaltic magma and partial melting of lower crustal amphibolite because both fractionation and melting can lead to similar major element trace element and isotopic characteristics in felsic magmas such lines of evidence have been generally unsuccessful in discriminating between the two processes a commonly under appreciated aspect of rare earth element ree solid liquid partitioning behavior is that d ree for most common igneous minerals especially hornblende increase significantly with increasing liquid sio2 contents for some minerals e g hornblende and augite ree partitioning can change from incomptatible d 1 at high liquid sio2 when this behavior is incorporated into carefully constrained mass balance models for mafic basaltic amphibolite melting intermediate andesitic amphibolite melting lower or mid to upper crustal hornblende present basalt fractionation and mid to upper crustal hornblende absent basalt fractionation the following general predictions emerge for felsic magmas e g 63 to 76 sio2 partial melting of either mafic or intermediate amphibolite should regardless of the type of melting equilibrium fractional accumulated fractional yield ree abundances that remain essentially constant and then decrease or steadily decrease with increasing liquid sio2 content at high liquid sio2 contents lree abundances should be slightly enriched to slightly depleted i e c l c o 2 to 0 2 while hree abundances should be slightly depleted c l c o 1 to 0 2 lower crustal hornblende bearing basalt fractionation should yield roughly constant ree abundances with increasing liquid sio2 and exhibit only slight enrichment c l c o 1 2 mid to upper crustal hornblende bearing basalt fractionation should yield steadily increasing lree abundances but constant and then decreasing hree abundances at high liquid sio2 contents lree abundances may range from non enriched to highly enriched c l c o 1 to 5 while hree abundances are generally non enriched to only slightly enriched c l c o 1 to 2 hornblende absent basalt fractionation should yield steadily increasing ree abundances with increasing liquid sio2 contents at high sio2 contents both lree and hree are highly enriched c l c o 3 to 4 it is proposed that these model predictions constitute a viable test for determining a fractionation or amphibolite melting origin for felsic magmas in intra oceanic arc environments where continental crust is absent

the cooling history and the depth of detachment faulting at the atlantis massif oceanic core complex 1 oceanic core complexes occs are domal exposures of oceanic crust and mantle interpreted to be denuded to the seafloor by large slip oceanic detachment faults we combine previously reported u pb zircon crystallization ages with u th he zircon thermochronometry and multicomponent magnetic remanence data to determine the cooling history of the footwall to the atlantis massif occ 30 n mar and help establish cooling rates as well as depths of detachment faulting and gabbro emplacement we present nine new u th he zircon ages for samples from iodp hole u1309d ranging from 40 to 1415 m below seafloor these data paired with u pb zircon ages and magnetic remanence data constrain cooling rates of gabbroic rocks from the upper 800 m of the central dome at atlantis massif as 2895 1276 1162 c myr 1 from 780 c to 250 c the lower 600 m of the borehole cooled more slowly at mean rates of 500 125 102 c myr 1 from 780 c to present day temperatures rocks from the uppermost part of the hole also reveal a brief period of slow cooling at rates of 300 c myr 1 possibly due to hydrothermal circulation to 4 km depth through the detachment fault zone assuming a fault slip rate of 20 mm yr from u pb zircon ages of surface samples and a rolling hinge model for the sub surface fault geometry we predict that the 780 c isotherm lies at 7 km below the axial valley floor likely corresponding both to the depth at which the semi brittle detachment fault roots and the probable upper limit of significant gabbro emplacement

gabbros from iodp site 1256 equatorial pacific insight into axial magma chamber processes at fast spreading ocean ridges the odp iodp multileg campaign at odp site 1256 cocos plate eastern equatorial pacific provides the first continuous in situ sampling of fast spreading ocean crust from the extrusive lavas through the sheeted dikes and down into the uppermost gabbros this paper focuses on a detailed petrographic and microanalytical investigation of the gabbro section drilled during iodp expedition 312 the marked patchy and spotty features that can be observed in many hole 1256d gabbros is mostly due to a close association of two different lithological domains in variable amounts 1 subophitic domains and 2 a granular matrix major and trace element mineral compositions geothermometry and petrological modeling suggest that subophitic and granular domains follow one single magma evolution trend formed by in situ fractionation the subophitic domains correspond to the relative primitive high temperature end member compositionally similar to the basalts and dikes from the extrusive unit upsection while the granular domains fit with a magma evolution by crystal fractionation to lower temperatures up to a degree of crystallization of 80 our results support the following scenario for the fossilization of the axial melt lens at odp site 1256 relatively primitive morb melts under near liquidus conditions fill the melt lens and feed the upper extrusive crust near the melt lens sheeted dike boundary at lower temperatures crystallization starts with first plagioclase before clinopyroxene in a mushy zone forming the subophitic domains at decreasing temperatures the subophitic domains continue to crystallize finally forming a well connected framework evolved residual melt is finally trapped within the subophitic network crystallizing at near solidus conditions to the granular matrix another important textural feature in hole 1256d gabbros is the presence of microgranular domains which are interpreted as relics of stoped assimilated sheeted dikes transformed to granoblastic dikes by contact metamorphism all these different domains can be observed in close association often at the thin section scale demonstrating the extremely complex petrological record of combined crystallization assimilation processes ongoing in the axial melt lens very similar gabbros with a marked spotty patchy appearance and bearing the same close association of lithological domains as observed at site 1256 are known in the so called varitextured gabbro unit from the oman ophiolite located at the same structural level between cumulate gabbros and granoblastic dikes the close petrological similarity of the gabbro dike transition between both iodp hole 1256d and the oman ophiolite suggests that in situ fractionation and dike assimilation contamination are major magmatic processes controlling the dynamics and fossilization of the axial melt lens at fast spreading oceanic ridges

la sio2 and yb sio2 systematics in mid ocean ridge magmas implications for the origin of oceanic plagiogranite analytical expressions for the variation in d la and d yb with increasing liquid sio2 for olivine plagioclase augite hornblende orthopyroxene magnetite and ilmenite brophy in contrib mineral petrol 2008 online first have been combined with numerical models of hydrous partial melting of mid ocean ridge mor cumulate gabbro melting and fractional crystallization of slightly hydrous mid ocean ridge basalt morb magma to assess a melting versus fractionation origin for oceanic plagiogranite for felsic magmas 63 wt sio2 the modeling predicts the following mor cumulate gabbro melting should yield constant or decreasing la and constant yb abundances with increasing liquid sio2 the overall abundances should be similar to those in associated mafic magmas morb fractional crystallization should yield steadily increasing la and yb abundances with increasing sio2 with overall abundances significantly higher than those in associated mafic magmas application to natural occurrences of oceanic plagiogranite indicate that both mor cumulate gabbro melting and morb fractionation are responsible application of the model results to icelandic rhyolites strongly support a fractional crystallization rather than a crustal melting origin

dissolution reprecipitation of igneous zircon in mid ocean ridge gabbro atlantis bank southwest indian ridge abstract zircons recovered from oceanic gabbro exposed on atlantis bank southwest indian ridge typically display oscillatory and sector zoning consistent with igneous crystallization from mafic magmas in one rock of twenty investigated weak oscillatory zonation patterns are overprinted by secondary textural features characterized by mottled convoluted and wavy internal zonation patterns that are frequently associated with secondary micron to submicron scale micro porosity these zircons are hosted in a felsic vein that intruded an oxide gabbro both of which are cross cut by monomineralic amphibole and quartz rich veinlets zircons with weak oscillatory zonation patterns record a weighted average 206pb 238u age of 12 76 0 20 ma mswd 1 5 and have high trace element concentrations e g rees 0 4 2 2 wt y 0 6 2 8 wt p 0 4 0 9 wt and th u 0 1 0 5 these zircons are anomalously old 1 myr relative to the magnetic age for this portion of oceanic crust 11 75 ma in contrast zircons with non igneous secondary textures have a younger weighted average 206pb 238u age of 12 00 0 16 ma mswd 1 7 and have lower trace element concentrations e g rees 0 2 0 8 wt y 0 3 1 0 wt p 0 1 0 3 wt and slightly lower th u 0 1 0 3 the weighted average age of these zircons is similar to the magnetic anomaly age and other 206pb 238u ages of nearby rocks we do not observe a correlation between crystallographic misorientation internal texture or trace element chemistry we suggest that the decrease in trace element concentrations associated with the development of non igneous alteration textures is attributed to the purging of non essential structural constituent cations from the zircon crystal lattice at amphibolite facies conditions the mechanism of alteration re equilibration was likely an interface coupled dissolution reprecipitation processes that affected pre existing anomalously old zircons during shallow level magmatic construction of atlantis bank at 12 0 ma

late jurassic magmatism metamorphism and deformation in the blue mountains province northeast oregon an early to mid mesozoic record of sedimentation magmatism and metamorphism is well developed in the blue mountains province of northeast oregon detailed studies both north and south of the blue mountains province e g terranes of the intermontane belt klamath mountains and western sierra nevada have documented a complex middle to late jurassic orogenic evolution however the timing of magmatic metamorphic and deformational events in the blue mountains and the significance of these events in relationship to other terranes in the western north american cordillera remain poorly understood in this study we investigate the structural magmatic and metamorphic histories of brittle to semibrittle deformation zones that indicate widespread late jurassic orogenesis in the blue mountains province folding and faulting associated with contractional deformation are primarily localized along terrane boundaries e g baker wallowa and baker izee olds ferry boundaries and within the composite baker oceanic melange terrane e g bourne greenhorn subterrane boundary these brittle to semibrittle deformation zones are broadly characterized by the development of e w oriented slaty to spaced cleavage in fine grained metasedimentary rocks of the baker terrane e g elkhorn ridge argillite approximately n s bivergent folding and n and s dipping reverse and thrust faulting on opposite flanks of the baker terrane similarly oriented contractional features are also present in late middle triassic to early late jurassic i e oxfordian stage ca 159 ma sedimentary rocks of the john day and huntington areas of northeast oregon radiometric age constraints from youngest detrital zircons in deformed sedimentary rocks and crystallization ages of postkinematic plutons which intrude the deformation zones limit deformation to between ca 159 and ca 154 ma we suggest that the widespread approximately n s directed contractional features in the blue mountains province record a short lived intense early late jurassic deformational event and preserve an example of upper crustal strain localization associated with terminal arc arc collision between the olds ferry and wallowa island arc terranes the age interval of deformation in the blue mountains province is younger than middle jurassic deformation in the canadian cordillera and klamath mountains siskiyou orogeny and predates classic nevadan orogenesis

palynological evidence for climatic change anthropogenic activity and outflow of black sea water during the late pleistocene and holocene centennial to decadal scale records from the black and marmara seas abstract previous marine palynology studies of pollen and dinoflagellate cysts dinocysts in the black sea mediterranean corridor were made on short cores allowing only millennial century scale reconstruction of past climates we now describe pollen accumulation rates for 5 cores with eleven 14c ages over 30 000 years and new decadal scale data for a holocene core with fourteen 14c ages pollen influxes show pinus abies quercus forest steppe and deteriorating climate from 29 8 24 kyr bp then colder drier pre and post lgm conditions with artemisia cheno ams and ephedra that lasted through the bolling interstade deciduous oak returns by the younger dryas and rapidly expands followed by pistacia by 9 3 kyr bp foraminiferal 18o shows 4 c cooler than present late glacial summers and rapid holocene warming by 9 bp influxes from quercus cerris tilia fagus castanea ulmus shade ferns aquatics swamp plants and 18o indicate warm winters 5 c or more and year round precipitation 600 1000 mm not the dry conditions that would be required for black sea drawdown there is mid holocene expansion of carpinus evergreen quercus and pistacia walnut olive and cereal pollen suggest early cultivation attempts but no sustained farming before 4 5 kyr bp dinocyst assemblages show major changes traceable throughout the corridor variability in process length marks low diversity n 13 pleniglacial early holocene spiniferites cruciformis assemblages suggesting fluctuating salinity like the modern caspian sea with brackish to saline conditions 5 16 distributions of euryhaline cysts spiniferites mirabilis and spiniferites bentorii show the aegean and marmara seas were connected by 11 kyr bp and linked to black sea by 9 3 kyr bp there is no palynological evidence of either freshwater salinity

plagiogranites from the ocean crust and ophiolites rock types similar to those found in ophiolites have been recovered from the floors of ocean basins1 3 this is critical evidence for the theory that ophiolites represent the ocean crust4 5 aumento6 has described diorites quartz diorites and trondhjemites dredged from the mid atlantic ridge mar near 45 n and noted that these were the first rocks analogous to the low k acid differentiates in ophiolites collectively referred to as oceanic plagiogranites 7 8 to be recognized amongst those recovered from the modern ocean floor only a few plagiogranites from the ocean floor ocean crust plagiogranites have since been described these were dredged from widely spaced localities on the mar9 11 the amani plateau and the kyushu palau ridge north west pacific 12 however the kyushu palau ridge could be the product of destructive plate margin magmatism13 as are the potassic granites from the aves ridge caribbean 14 plagiogranite xenoliths also occur in extrusives on surtsey and askja iceland 15 that only some of the petrological compositions of ophiolite plagiogranites are represented in the ocean crust is not surprising in view of the small sample of ocean crust plagiogranites it is less obvious however if ophiolites represent the ocean crust why certain acidic rock types from the oceans have not been found in ophiolites i show here that the difference between the two groups of plagiogranites is evidence that ophiolites do not represent all kinds of ocean crust and that a greater range of magma compositions and or conditions of magmatic differentiation is represented by the ocean crust

island arc elements and arc related ophiolites abstract all major structural elements in island arc systems fore arc magmatic arc back arc basins and remnant arcs are potential ophiolite sources and those features that allegedly characterise ophiolites of ocean ridge origin sheeted dyke complexes mantling pelagic rocks hydrothermal metamorphism and associated mineralization can also arise within arc settings age relations are critical in the interpretation of arc related ophiolites remnants of oceanic lithosphere identified by a pre arc initiation age are restricted to fore arc magmatic arc and remnant arc elements as are ophiolite masses generated at the inception of underthrusting the latter apparently common in ancient fore arc terrains form in nascent arc systems in which the rate of role back of the subduction hinge exceeds the rate of convergence spreading occurs above a foundering slab resulting in some arc like compositional features in simple arc systems later ophiolitic rocks have formed either in the active back arc basin or the magmatic arc only those ophiolites that have resided within or very close behind magmatic arcs should show calcalkaline or arc tholeiite magmatic affinities or be intruded or overlain by these rocks volcanic derived sediment or pelagic material may mantle ophiolites from all arc settings but pelagic rocks will generally dominate in stratigraphic sequences above remnant arcs and on back arc basin floors except adjacent to the magmatic arc ophiolites generated at major ocean ridges are unlikely to be immediately overlain by sediment with a significant volcanic component whereas such detritus may lie directly on arc inception arc and back arc ophiolites some arc derived ophiolites are preserved in their original tectonic position others can be identified from their internal features their relationship to other tectonic elements and the nature of associated rocks

burst of high temperature seawater injection throughout accreting oceanic crust a case study in oman ophiolite following the discovery of a high temperature ht 800 c and a very high temperature 1000 c hydrothermal alteration in the crust of the oman ophiolite a systematic structural and petrological study was conducted throughout the entire ophiolite with supporting isotopic geochemistry the published results showed that the crustal gabbros are extensively altered down to moho by a large seawater flux which was channelled through an identified recharge and discharge circuit microcracks constituting the recharge system propagated through the hot gabbros accreting at the ridge and in spite of their submillimetre width provided the conduit for the large volume of seawater necessary for the observed alteration building on these results we show here that these microcracks opened and were active over a time of a few tens of thousands years while the newly accreted gabbros were drifting away from the ridge microcrack activity was highly episodic with bursts of seawater ingression lasting a few days to a few weeks followed by quiescence periods of a few tens of years this model of ht oceanic hydrothermal alteration has several implications concerning fast spreading oceanic ridges

sulfur volcanism on io abstract in february 2003 march 2003 and january 2004 pele plume transmission spectra were obtained during jupiter transit with hubble s space telescope imaging spectrograph stis using the 0 1 wide slit and the g230lb grating the stis spectra covered the 2100 3100 a wavelength regions and extended spatially along io s limb encompassing the region directly above and northward of the vent of the pele volcano the s2 and so2 absorption signatures evident in these data indicate that the gas signature at pele was temporally variable and that an s2 absorption signature was present 12 from the pele vent near 6 5 s and 264 15 w suggesting the presence of another s2 bearing plume on io contemporaneous with the spectral data uv and visible wavelength images of the plume were obtained in reflected sunlight with the advanced camera for surveys acs prior to jupiter transit the dust scattering recorded in these data provide an additional qualitative measure of plume activity on io indicating that the degree of dust scattering over pele varied as a function of the date of observation and that there were several other dust bearing plumes active during the observations we present constraints on the composition and variability of the gas abundances of the pele plume as well as the plumes detected by acs and recorded within the stis data as a function of time

protracted construction of gabbroic crust at a slow spreading ridge constraints from 206pb 238u zircon ages from atlantis massif and iodp hole u1309d 30 n mar sensitive high resolution ion microprobe shrimp u pb zircon ages of 24 samples from oceanic crust recovered in integrated ocean drilling program iodp hole u1309d and from the surface of atlantis massif mid atlantic ridge mar 30 n document a protracted history of accretion in the footwall to an oceanic detachment fault ages for 18 samples of evolved fe ti oxide gabbro and felsic dikes collected 40 1415 m below seafloor in u1309d yield a weighted mean of 1 20 0 03 ma mean square of weighted deviates 7 1 however the ages range from 1 08 0 07 ma and 1 28 0 05 ma indicating crustal construction occurred over a minimum of 100 200 ka the zircon ages along with petrologic observations indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka the oldest ages are observed below 600 mbsf an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault the data are most consistent with a multiple sill model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1 4 km zircon ages from broadly spaced samples collected along the southern ridge of atlantis massif yield a detachment fault slip rate of 28 7 6 7 mm a and imply significant asymmetric plate spreading up to 100 on the north american plate for at least 200 ka during core complex formation

cooling rates and the depth of detachment faulting at oceanic core complexes evidence from zircon pb u and u th he ages oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid ocean ridges but many questions persist about the thermal evolution and depth of faulting we present new pb u and u th he zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the mid atlantic ridge ocean drilling program odp holes 1270d and 1275d near the 15 20 n transform and atlantis massif at 30 n closure temperatures for the pb u 800 c 850 c and u th he 210 c isotopic systems in zircon bracket acquisition of magnetic remanence collectively providing a temperature time history during faulting results indicate cooling to 200 c in 0 3 0 5 myr after zircon crystallization recording time averaged cooling rates of 1000 c 2000 c myr assuming the footwalls were denuded along single continuous faults differences in pb u and u th he zircon ages together with independently determined slip rates allow the distance between the 850 c and 200 c isotherms along the fault plane to be estimated calculated distances are 8 4 4 2 km and 5 0 2 1 km from holes 1275d and 1270d and 8 4 1 4 km at atlantis massif estimating an initial subsurface fault dip of 50 and a depth of 1 5 km to the 200 c isotherm leads to the prediction that the 850 c isotherm lies 5 7 km below seafloor at the time of faulting these depth estimates for active fault systems are consistent with depths of microseismicity observed beneath the hypothesized detachment fault at the tag hydrothermal field and high temperature fault rocks recovered from many oceanic detachment faults

rare earth element sio 2 systematics of mid ocean ridge plagiogranites and host gabbros from the fournier oceanic fragment new brunswick canada a field evaluation of some model predictions the two most commonly invoked processes for generating plagiogranites in mid ocean ridge environments are extended fractional crystallization of mid ocean ridge basalt morb magma and hydration melting of hot dry mor gabbro initiated by the influx of seawater derived hydrothermal fluids within localized zones of shear brophy contrib mineral petrol 158 99 111 2009 has proposed on theoretical grounds that for liquids greater than 62 wt sio2 hydration melting should yield among other features a negative correlation between rare earth element ree abundances and increasing sio2 while fractional crystallization should yield a positive correlation if correct the ree sio2 systematics of natural systems might be used to distinguish between the two processes the ordovician fournier oceanic fragment new brunswick canada contains mor gabbro hosted plagiogranite veins and dikes that are believed to have formed from hydration melting thus forming an appropriate location for field verification of the proposed ree sio2 systematics for such a process in addition to a negative correlation between liquid sio2 and ree abundance for liquids in excess of 62 sio2 other important model features include the following 1 relative to a gabbro source rock the degree of enrichment at liquids of 62 and 75 sio2 decreases from the lree to the hree 2 the degree of enrichment at 75 sio2 approaches 1 for the hree 3 the rate of change of the degree of enrichment with increasing liquid sio2 i e the slope diminishes from the lree to the hree all of these predicted features are observed in the fournier plagiogranites assuming an initial source rock equivalent to the host gabbro an additional strongly lree enriched component must be added prior to melting in order to make the absolute ree abundances agree with the model values the most likely candidates are the very seawater derived hydrothermal fluids that triggered hydration melting in the first place

perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon abstract the formation of oceanic plagiogranite has been attributed primarily to either 1 extreme fractional crystallization of a mantle melt or 2 partial melting of hydrated mafic crust with support for the latter from field evidence and recent melting experiments remelting of hydrothermally altered ocean crust could yield rocks and minerals with diverse primary magmatic 18 o values forming proximal to the magmatic center where crustal growth is occurring to constrain the magmatic 18 o of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis we characterized the 18 o of zircons in 22 plagiogranite samples tonalite and trondhjemite from 8 different ophiolites and one dacite sampled along the east pacific rise using secondary ion mass spectrometry sims the 18 o values of 202 magmatic zircons from ophiolites range from 3 9 to 5 6 n 244 spots average 4 9 0 6 2sd extending 1 below typical zircon in equilibrium with mantle and from gabbroic massifs along slow spreading mid ocean ridges 4 7 5 9 east pacific rise dacite zircons range from 4 6 to 5 0 n 12 spots plagiogranite from the dike gabbro transition zone of the northern oman ophiolite yield the lowest 18 o zrn with rock average values of 4 3 5 0 the low 18 o values are best explained by remelting of crust altered by hydrothermal fluids with seawater like isotopic compositions at high temperatures possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high temperature hydrothermal system in the overlying crust if the partial melt was assimilated into a fractionating melt lens with morb like 18 o as envisioned for km scale plagiogranite bodies in oman up to 20 contamination by a protolith with 18 o 2 would be required previous oxygen isotope constraints from quartz in oman plagiogranite suggested melting of both high and low 18 o crust had occurred comparison of quartz zircon pairs indicates that quartz has been modified in most samples and we find no evidence for the involvement of high 18 o rocks during plagiogranite formation

petrology and geochemistry of plagiogranite in the canyon mountain ophiolite oregon plagiogranites in the canyon mountain ophiolite oregon include a wide range of rock types ranging from diorite to trondhjemite the plagiogranites are mostly concentrated as an intrusive sill swarm at the top of a section of gabbroic cumulates the plagiogranites are typically low in k2o and high in na2o and are enriched 10 20 times chondrites in ree and overlap with abundances in basic rocks from canyon mountain all samples of plagiogranite are relatively depleted in lree with more silicic samples characterized by a slightly lesser degree of lree depletion total ree content is not consistently correlated with contents of major and other trace elements

hydrothermal circulation and the dike gabbro transition in the detachment mode of slow seafloor spreading one of the most ubiquitous boundaries within our planet is between sheeted dikes and gabbros in fast spreading ocean crust this boundary marks the brittle ductile transition at the ridge crest and is localized by a decametric conductive boundary layer between hydrothermal circulation in the sheeted dike layer and a shallow quasi steady state melt lens in contrast at slow spreading ridges the crustal structure appears chaotic with no consistent sheeted dike layer and widespread occurrences of gabbro and serpentinized peridotite on the seafloor recent work suggests that as much as 50 of the atlantic ocean crust formed by a detachment mode of seafloor spreading including the formation of oceanic core complexes capped by long lived convex upward detachment faults these detachment faults are often associated with large hydrothermal systems in which the location of any magmatic heat source is uncertain here we show that detachment faults can act as thermal boundaries between gabbroic melt in the fault footwall and hydrothermal circulation in the fault zone and hanging wall thus explaining the link between faulting and black smoker systems we suggest that interaction between magmatism and hydrothermal circulation means that detachment faults can act as the dike gabbro transition in the detachment mode of spreading inevitably leading to exposure of gabbros on the seafloor through continued faulting this concept provides a means of unifying apparently contrasting processes and crustal structures at different spreading rates

kinematics of plate convergence deduced from mesozoic structures in the western cordillera mesozoic structures in the western cordillera of the united states yield important constraints for kinematic reconstruction of plate motions relative to north america structures in the mesozoic arc s of the northern sierra nevada and of northeastern oregon are remarkably uniform in timing style and with late cretaceous and or cenozoic rotations removed orientation rocks of both regions experienced three major episodes of contraction occurring in the triassic early jurassic the late jurassic and the early cretaceous cross folds were developed after the first two deformations but were not associated with significant shortening the three major deformations are nearly homoaxial and have shortening axes oriented subperpendicular ne sw to the regional trend of the arc system in the back arc region of the northwestern great basin deposition was continuous during the triassic and early jurassic and the region was shielded from tectonism in the arc to the west development of a regional fold thrust belt associated with major nw se shortening was initiated in the back arc region during the middle or late jurassic and continued through the early cretaceous corresponding deformation did not occur in the arc and the arc and back arc were decoupled by a major left lateral fault system that subparallelled the arc axis in the mid cretaceous approximately 100 m y b p regional compression in the back arc abruptly changed to ne sw and both the arc and back arc experienced major shortening development of homoaxial structures in the sierran arc is probably analogous to shortening in present day arc systems undergoing oblique subduction in which the shortening axis is parallel to the normal component of convergence thus the dominant structures in the sierran arc simply may be a reflection of the orientation of the trench and may not constrain the sense of obliquity of convergence strike slip faults and related compressional structures such as the back arc left lateral fault and associated fold thrust belt are probably the response to the strike slip component of convergence and indicate a period of left oblique subduction relative plate motions constrained by paleomagnetic data are in agreement with regional structures and indicate strong right lateral components of motion for the last 100 m y before 100 m y b p relative plate motions are inadequately constrained by paleomagnetic data and regional structures indicate a period of left oblique convergence from the middle or late jurassic to approximately 100 m y b p during the triassic and early jurassic the relative sense of convergence is equivocal and the possibility exists that continuous left oblique convergence occurred throughout the late paleozoic and early mesozoic until 100 m y b p based on the available data we cannot discount the possibility that a complex convergence history existed however with changes from right to left convergence in the middle or late jurassic and from left to right convergence in the mid cretaceous

geochemistry of the neoproterozoic tilemsi belt of iforas mali sahara a crustal section of an oceanic island arc abstract the pre pan african neoproterozoic tilemsi belt of the western iforas mali sahara comprises a predominantly bimodal mafic and felsic volcanic assemblage associated with numerous intrusions mainly of mafic composition the volcanics include island arc tholeiites and the calc alkali rocks rhyolites dacites and andesites the volcanic domain shows similarities to recent oceanic island arc systems as well as to archaean greenstone belts the tilemsi magmatic sequence represents an example of a section through the middle and upper levels of a neoproterozoic intra oceanic island arc crust that is composed nearly exclusively of igneous material the middle crustal section is mainly of mafic composition while the upper crustal level contains more silica rich plutonic volcanic and volcaniclastic assemblages the mafic intrusions include a large proportion of cumulates indicating that fractional crystallization played an important role during the evolution of the oceanic arc suites compositionally the arc changed through time from an immature to a relatively developed system the rocks became more silica and incompatible element rich with higher sr isotopic ratios this trend reflects the increasing growth and maturity of the arc

reply to the comment by c ravazzi on paleoclimatic record of the past 22 000yr in venice northern italy biostratigraphic evidence and chronology quaternary international 140 141 37 52

cold moho boundary beneath island arc systems an example from the north kamchatka arc a suite of amphibole bearing ultramafic and mafic xenoliths representing the crust mantle boundary was sampled in the northern segment of the kamchatka arc these nodules exhibit low equilibration temperatures at elevated pressures suggesting relatively cold conditions at moho depths which are uncommon in modern island arcs a model including successive stages of subduction initiation subduction cessation resulting in cooling of the lithospheric wedge followed by intra arc rifting due to upwelling of astenospheric diapirs is proposed to explain the observed textural petrologic and chemical characteristics of sampled rock assemblage

radiocarbon calibration curve spanning 0 to 50 000 years bp based on paired 230 th 234 u 238 u and 14 c dates on pristine corals abstract radiocarbon dating is the most widely used dating technique in the world recent advances in accelerator mass spectrometry ams and sample preparation techniques have reduced the sample size requirements by a factor of 1000 and decreased the measurement time from weeks to minutes today it is estimated that more than 90 percent of all measurements made on accelerator mass spectrometers are for radiocarbon age dates the production of 14 c in the atmosphere varies through time due to changes in the earth s geomagnetic field intensity and in its concentration which is regulated by the carbon cycle as a result of these two variables a radiocarbon age is not equivalent to a calendar age four decades of joint research by the dendrochronology and radiocarbon communities have produced a radiocarbon calibration data set of remarkable precision and accuracy extending from the present to approximately 12 000 calendar years before present this paper presents high precision paired 230 th 234 u 238 u and 14 c age determinations on pristine coral samples that enable us to extend the radiocarbon calibration curve from 12 000 to 50 000 years before present we developed a statistical model to properly estimate sample age conversion from radiocarbon years to calendar years taking full account of combined errors in input ages and calibration uncertainties our radiocarbon calibration program is publicly accessible at http www radiocarbon ldeo columbia edu along with full documentation of the samples data and our statistical calibration model

geochemistry of pillow basalts from bompoka andaman nicobar islands bay of bengal india abstract pillow lavas in bompoka island of the andaman nicobar islands forming a part of sunda burmese forearc are composed of plagioclase and clinopyroxene microphenocrysts in a fine grained ferruginous groundmass along with glass they are also characterized by several quench plagioclase and clinopyroxene morphologies zr tio2 versus nb y relationship of these pillow lavas show that these are tholeiitic basalts in composition these basalts have low mgo 5 19 6 12 wt ni 84 118 ppm and cr 144 175 ppm abundance and high feo t mgo 1 71 1 92 ratios reflecting their fractionated nature in th yb versus nb yb and ti yb versus nb yb binary diagrams they show n morb affinity however la nb y and ce nb th nb relationships along with a slight lree depleted lan ybn 0 75 0 82 pattern and high ba zr 0 28 0 40 ratios and lile k rb ba sr and th enrichment relative to n morb suggest their back arc basin basalt affinity it is inferred that these pillow basalts have been derived from a metasomatised n morb like mantle source in a trench distal wider back arc basin probably near the leading edge of the eurasian continent during early to late cretaceous times prior to the currently active andaman java subduction system