k feldspar megacrysts in granites phenocrysts not porphyroblasts abstract k feldspar megacrysts in granitoid plutons have been interpreted as either phenocrysts or porphyroblasts most of the microstructural mineralogical and chemical evidence e g shape alignment concentration ba content zoning inclusions and twinning favours a phenocryst origin the main features that have been used to support a porphyroblast origin are occurrence of megacrysts 1 across aplite vein boundaries 2 in country rocks and 3 in or across boundaries of microgranitoid enclaves mafic inclusions however these features can be explained by the phenocryst hypothesis in particular megacrysts in microgranitoid enclaves can be explained by growth or mixing in magma before a globule of that magma or a fragment of the resulting igneous rock was incorporated as an enclave all available evidence favours or is consistent with a phenocryst origin for k feldspar megacrysts in granitoid rocks and their enclaves the large size of the megacrysts is evidently due to nucleation difficulties for k feldspar in granitic melts though k feldspar is commonly the last mineral to begin crystallizing in granitic magmas abundant melt is still present at that stage allowing sufficient space for the megacrysts to grow the reason for the common lack of megacrysts in volcanic rocks may be that the phenocrysts do not grow large enough to be called megacrysts until the magma contains such a high proportion of crystals that it cannot erupt

pacific ocean and cenozoic evolution of climate 1 the pacific ocean has played a major role in climate evolution throughout the cenozoic 65 0 ma it is a fundamental component of global heat transport and circulation the dominant locus of primary productivity and consequently the largest reservoir for carbon exchange between the oceans and the atmosphere a satisfactory understanding of the cenozoic evolutionary history of the pacific and its impact on global climate is currently data limited nevertheless the large dynamic range of cenozoic conditions sets the stage to greatly expand our understanding of global climate and biogeochemical cycles past earth experiments are particularly useful to understand interactions between climate and geosystems under different greenhouse gas loads we highlight in this review four important problems in which the pacific played a major role the effect of changing geographic boundary conditions on ocean circulation interactions between the carbon cycle and climate the pacific ocean s influence on north american climate and its water cycle and the gradual evolution of climate systems

tectonostratigraphy of the north alpine foreland basin correlation of tertiary depositional cycles and orogenic phases abstract tectonostratigraphic analysis of the depositional history of the north alpine foreland basin indicates that temporal relationships exist between short term events in the foredeep and the adjacent alpine orogen the foredeep can be divided stratigraphically into an alpine foredeep thanetian bartonian pre molasse basin latest bartonian earliest rupelian and molasse basin rupelian tortonian their general and tectonostratigraphic development includes time significant cycle sets and so called cenozoic rift and foredeep crf sequences which are apparently correlative to both foreland and orogen deformation events as well as to changes in global sea level individual basin fill sequences and consequently general stratigraphic and palaeogeographical aspects of the evolving basin system are thus found to be frequently defined by contemporaneous widely distributed tectonics and eustasy successive dominant depositional states underfilled steady state and overfilled episodes and facies flysch molasse are correlated with different concurrently progressing phases of the convergence and subduction history of the european and apulian plates

stable isotope and fluid inclusion evidence for the origin of the brandberg west area sn w vein deposits nw namibia the brandberg west region of nw namibia is dominated by poly deformed turbidites and carbonate rocks of the neoproterozoic damara supergoup which have been regionally metamorphosed to greenschist facies and thermally metamorphosed up to mid amphibolite facies by neoproterozoic granite plutons the meta sedimentary rocks host damaran age hydrothermal quartz vein hosted sn w mineralization at brandberg west and numerous nearby smaller deposits fluid inclusion microthermometric studies of the vein quartz suggests that the ore forming fluids at the brandberg west mine were co2 bearing aqueous fluids represented by the nacl cacl2 h2o co2 system with moderate salinity mean 8 6 wt naclequivalent temperatures determined using oxygen isotope thermometry are 415 521 c quartz muscovite 392 447 c quartz cassiterite and 444 490 c quartz hematite at brandberg west the oxygen isotope ratios of quartz veins and siliciclastic host rocks in the mineralized area are lower than those in the rocks and veins of the surrounding areas suggesting that pervasive fluid rock interaction occurred during mineralization the o and h isotope data of quartz muscovite veins and fluid inclusions indicate that the ore fluids were dominantly of magmatic origin implying that mineralization occurred above a shallow granite pluton simple mass balance calculations suggest water rock ratios of 1 88 closed system and 1 01 open system the co2 component of the fluid inclusions had similar 13c to the carbonate rocks intercalated with the turbidites it is most likely that mineralization at brandberg west was caused by a combination of an impermeable marble barrier and interaction of the fluids with the marble the minor deposits in the area have quartz veins with higher 18o values which is consistent with these deposits being similar geological environments exposed at higher erosion levels

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

the trace element compositions of s type granites evidence for disequilibrium melting and accessory phase entrainment in the source within individual plutons the trace element concentrations in s type granites generally increase with maficity total iron and magnesium content and expressed as atomic fe mg in this study the degree of variability in trace element concentration also expands markedly with the same parameter the strongly peraluminous high level s type granites of the peninsular pluton cape granite suite south africa are the product of biotite incongruent melting of a metasedimentary source near the base of the crust leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties represent mixtures of melt and peritectic garnet and ilmenite trace elements such as rb ba sr and eu that are concentrated in reactant minerals in the melting process show considerable scatter within the granites this is interpreted to reflect compositional variation in the source in contrast elements such as lree zr and hf which are concentrated within refractory accessory phases zircon and monazite show well defined negative correlations with increasing sio2 and increase linearly with increasing maficity this is interpreted to reflect coupled co entrainment of accessory minerals and peritectic phases to the melt leucocratic rocks cannot have evolved from the more mafic compositions in the suite by a process of fractional crystallisation because in this case they would have inherited the zircon saturated character of this hypothetical earlier magma trace element behaviour of granites from the peninsular pluton has been modelled via both equilibrium and disequilibrium trace element melting in the disequilibrium case melts are modelled as leaving the source with variable proportions of entrained peritectic phases and accessory minerals but before the melt has dis solved any accessory minerals thus the trace element signature of the melt is largely inherited from the reactants in the melting reaction with no contribution from zircon and monazite dissolution in the equilibrium case melt leaves the source with entrained crystals after reaching zircon and monazite saturation a significant proportion of the rocks of the peninsular pluton have trace element concentrations below those predicted by zircon and monazite saturation in the case of the most leucocratic rocks all compositions are zircon undersaturated whilst the majority of the most mafic compositions are zircon oversaturated however in both cases zircon is commonly xenocrystic thus the leucocratic rocks represent close to pure melts which escaped their sources rapidly enough that some very closely match the trace element disequilibrium melting model applied in this study zircon dissolution rates allow the residency time for the melt in the source to be conservatively estimated at less than 500 years

the origin of low δ18o granites and related rocks from the seychelles the 750 ma seychelles granites have whole rock 18o values that range from 1 2 to 7 5 differences in 18o values between quartz feldspar and whole rock for samples that have a range of 18o values suggest that these minerals are in magmatic equilibrium and that the whole rock 18o values were little affected by post crystallization interaction with fluids two suites of granites the mahe type and the praslin type have previously been recognized on the basis of chemical and radiogenic isotope composition the former have a mean whole rock 18o value of 5 25 0 65 1 whereas the latter are much more variable with a mean whole rock 18o value of 3 00 3 27 biotite and amphibole separates from both granite types range in d from 69 to 116 and show a positive correlation with whole rock 18o values dolerite dykes that intruded the granites during or shortly after their crystallization also have whole rock 18o values mean 2 24 1 93 lower than that expected for mantle derived basaltic rocks but these values appear to be the result of fluid rock interaction we suggest that the mahe type granites are derived mainly from juvenile mafic to intermediate crust with normal d and 18o values whereas the praslin type granites are mixtures of this source and older crust that acquired its low d and 18o values by extensive interaction with meteoric water at high temperature it is unlikely that meteoric water sufficiently depleted in d and 18o was available at 750 ma because of the relatively low latitude of the region at that time we suggest that alteration of the source took place significantly before production of the granite magmas depletion in 18o of the seychelles granites does not necessarily require a regional extensional tectonic setting at 750 ma as has been proposed

john day formation of oregon a distal record of early cascade volcanism the john day formation in oregon consists largely of silicic to intermediate pyroclastic material ranging in age from about 19 to 37 m y stratigraphic and lithologic variations within the formation indicate that the bulk of this material was derived from vents west of the 121st meridian in or near the present day cascade range voluminous dacitic to andesite air fall material was probably derived from volcanoes within the western cascade range whereas rhyolitie ash flow tuffs and lava flows were erupted from vents farther east sparse alkali basalt and trachyandesite flows compositionally distinct from cascade range lavas were erupted from local vents within the john day outcrop area initiation of john day volcanism about 37 m y ago signified a shift in the locus of calc alkaline volcanic activity from the blue mountains to the cascade range and marked the emergence of the cascade range as a major volcanic feature

hydrogen and oxygen isotope geochemistry of ascension island lavas and granites variation with crystal fractionation and interaction with sea water lavas and pyroclastics on ascension island contain plutonic blocks that include fluid inclusion bearing peralkaline granite 18o 16o ratios f and cl have been analysed on whole rocks and or minerals for lavas and granites and d h ratios and h2o for comenditic obsidians and granites whole rock 18o 16o ratios of fresh alkali basalt hawaiite trachyandesite trachyte and comendite range from 6 0 to 6 9 with 18o tending to increase with increase in sio2 the 18o values of the granites are from 0 0 to 0 3 depleted in 18o relative to the comendites comenditic obsidians have d 80 4 and h2o 0 3 wt while amphiboles from the granites have d 56 2 the o isotope trend of the lavas is consistent with a crystal fractionation model fresh igneous rocks with 18o values greater than 7 involve processes in addition to crystal fractionation of a basaltic magma the d h ratios and cl contents 3 000 ppm of the h2o poor comenditic obsidians represent undegassed primary magmatic values the h isotope compositions and low h2o and cl 167 ppm contents of the granites are consistent with the major degassing loss of 90 of initial h2o of an h2osaturated magma derived from the interaction of sea or possibly meteoric water with the h2o undersaturated comenditic melt it is proposed that associated with caldera subsidence and stoping water was sucked in around the residual magma before the system had time to be sealed up the h2o undersaturated magma consumed this h2o with possibly some minor partial dehydration and dewatering of the hydrated volcanic roof blocks at a pressure of about 1 5 kb the granites are the plutonic equivalents of rhyolitic pyroclastics and not directly of the comendites granites from oceanic islands may in general be a result of generating an h2o saturated acid melt by such direct or indirect crustal water magma interaction processes

multi proxy isotopic tracing of magmatic sources and crustal recycling in the palaeozoic to early jurassic active margin of north western gondwana abstract we trace source variations of active margin granitoids which crystallised intermittently over 300 ma in varying kinematic regimes by combining zircon lu hf isotopic data from early palaeozoic to early jurassic igneous and metaigneous rocks in the merida andes venezuela and the santander massif colombia with new whole rock rb sr and sm nd isotopic data and quartz o isotopic data these new data are unique in south america because they were obtained from discrete magmatic and metamorphic zircon populations providing a high temporal resolution dataset and compare several isotopic systems on the same samples collectively these data provide valuable insight into the evolution of the isotopic structure of the continental crust in long lived active margins phanerozoic active margin related granitoids in the merida andes and the santander massif yield zircon lu hf model ages ranging between 0 77 ga and 1 57 ga which clearly define temporal trends that can be correlated with changes in tectonic regimes the oldest lu hf model ages of 1 3 ga are restricted to granitoids which formed during barrovian metamorphism and crustal thickening between 499 ma and 473 ma these granitoids yield high initial 87sr 86sr ratios suggesting that evolved rb rich middle to upper crust was the major source of melt granitoids and rhyolites which crystallised during subsequent extension between 472 ma and 452 ma yield younger lu hf model ages of 0 80 ga 1 3 ga and low initial 87sr 86sr ratios suggesting that they were derived from much more juvenile rb poor sources such as mafic lower crust and mantle derived melts the rapid change in magmatic sources at 472 ma can be attributed either to reduced crustal assimilation during extension or a short pulse of crustal growth by addition of juvenile material to the continental crust between 472 ma and 196 ma lu hf model ages remain mostly constant between 1 0 and 1 2 ga the large scatter and the absence of definite trends in initial 87sr 86sr ratios suggest that both mafic rb poor and evolved rb rich sources were important precursors of active margin magmas in colombia and venezuela throughout the palaeozoic to the early jurassic previous studies have shown that the genesis of arc magmas may be stimulated by heat advection to the crust during the underplating of mantle derived melt but the absence of permanent younging trends in lu hf model ages from 472 ma to 196 ma suggests that very little new crust was generated during this period in the studied region an overwhelming majority of the analysed igneous rocks yield zircon lu hf model ages of 1 ga which may be accounted for by documented local crustal end members of 1 ga 1 6 ga and do not require contributions from the depleted mantle therefore recycling of 1 ga and older crust was a dominant process in the north western corner of gondwana between 472 ma and 196 ma this study shows that whole rock sm nd and zircon lu hf data can be interpreted similarly regarding the age of the source regions whereas rb sr and o isotope data from the same rocks yield valuable information regarding the geochemical nature of the source

granitic magmas with i type affinities from mainly metasedimentary sources the harcourt batholith of southeastern australia the high k calcalkaline granitic rocks of the 370 ma post orogenic harcourt batholith in southeastern australia have i type affinities but are mildly peraluminous and have remarkably radiogenic isotope characteristics with 87sr 86srt in the range 0 70807 to 0 714121 and endt in the range 5 6 to 4 3 this batholith appears to be a good example of magmas that were derived through partial melting of distinctly heterogeneous source rocks that vary from intermediate meta igneous to mildly aluminous metasedimentary rocks with the balance between the two rock types on the metasedimentary side such transitional s i type magmas formed from mainly metasedimentary source rocks may be more common than is generally realised the harcourt batholith also contains mainly granodioritic igneous microgranular enclaves imes like their host rocks the imes are peraluminous and have rather radiogenic isotope signatures 87sr 86srt of 0 71257 0 71435 and endt of 7 3 to 4 3 though some are hornblende bearing origins of these imes by mixing a putative mantle end member with the host granitic magma can be excluded because of the variability in whole rock isotope ratios and for the same reason the ime magmas cannot represent quench cumulates autoliths from the host magmas less abundant monzonitic to monzosyenitic imes cannot represent accumulations of magmatic biotite and or alkali feldspar because k feldspar is absent and there is no co enrichment of k2o and feo mgo nor can they be mixtures of anything plausible with the host rock magma the granodioritic imes probably originated through high degrees of assimilation of a range of crustal materials partial melts by basaltic magmas in the deep crust and the monzonitic imes as melts of enriched subcontinental mantle such enclave suites provide little or no information on the chemical evolution of their host granitic rocks

hydrogen and oxygen isotope geochemistry of a type granites in the continental margins of eastern china hydrogen and oxygen isotope studies of both whole rock and mineral separates were carried out for five representative atype granites in eastern china from north to south the whole rock d d and d 18 o values vary from 2145 to 282 and from 21 2 to 5 0 for nianzishan pluton from 2135 to 2110 and from 1 6 to 6 7 for shanhaiguan pluton and from 2145 to 2111 and from 1 5 to 9 1 for laoshan massif respectively the highest whole rock dd and d 18 o values are observed in suzhou pluton and range from 281 to 259 and from 3 5 to 9 2 respectively homogeneous d d and d 18 o values are found in kuiqi massif ranging from 2113 to 299 and from 5 6 to 8 5 respectively the results show that the large variabilities in the hydrogen and oxygen isotope ratios are mainly related to the diverse processes of magma evolution such as magma degassing fractional crystallization as well as post magmatic alteration by local meteoric water under subsolidus conditions the initial d d and d 18 o values of the a type granites in eastern china are constrained to be about 250 5 and 7 5 1 0 respectively this is consistent with mantle like protoliths for granitoid rocks and thus precludes the possibility that the a type granites is derived from the anatexis of the 18 o rich upper crust however recycling of crustal materials by earlier subduction cannot be excluded in the processes of generating the a type granitic magmas the variation in the ddepletion of the granites with latitude implies that the paleogeographic localities of the a type granites have not changed significantly since the mesozoic q 2000 elsevier science b v all rights reserved

mafic magmatism and associated tectonism of the central high cascade range oregon volcanism in the central high cascade range has been dominated since late miocene time by the construction of a mafic platform of coalescent shield volcanoes within an intra arc graben that developed in response to extensional plate tectonics graben development along the cascade crest was possibly activated by failure of crust that had become thermally weakened during previous episodes of calc alkaline magmatism lithospheric extension along the central cascade arc is attributed to a decrease in the convergence rate of the farallon north american plate system since early tertiary time geochemical variations compiled from new and existing data are used to delineate four major catagories of basalts sio2 47 53 wt and basaltic andesites sio2 53 60 wt 1 early high cascade high alumina olivine tholeiitic haot basalts 2 normal high cascade haot basalts 3 mount washington mw type basaltic andesites and 4 north sister ns type basaltic andesites compositional variations among the four types possibly reflect source differences although much of the variation especially in basaltic andesites might be related to open system evolution of primary magmas early high cascade basalts erupted during late miocene to early pliocene time display depleted light rare earth element ree and other large ion lithophile element llle abundances similar to those observed in mid ocean ridge baslts morb later basalts erupted since the pliocene have variably enriched lile and a wide range of ree fractionation e g la yb ratios and chemically resemble ocean island basalts oib pliocene to holocene basaltic andesites exhibit ranges in total ree zr hf and sc abundances that are similar to those in normal basalts yet the abundances of these elements in ns types are overall lower than the average basalt contents fractionation among the relative abundances of ree zr hf and sc between ns and mw types are not appreciable compared to normal basalts both types of basaltic andesite exhibit uniformally elevated abundances of mobile lile cs rb k sr ba and th and marginally depleted abundances of high field strength elements hfse ta and nb typical of magmas erupted in magmatic arcs chemical signatures in central high cascade basalts imply that the mantle beneath the central cascades has been subjected to variable influences of morb and oib like components the implication of oceanic mantle beneath the central cascades is consistent with the existence of the columbia embayment into which a segment of oceanic lithosphere was compressed and thickened relatively high mobile lile and somewhat depleted hfse in the basaltic andesites considered to be secondary effects to oib morb mantle evolution are believed to believed to result from interaction of aqueous subduction derived fluids with evolving magmas or their sources in lower crustal or upper mantle regions the preponderance of haot basalts that exhibit within plate chemical signatures argues for less input relative to typical calc alkaline magmatic arcs of aqueous fluids due to subduction processes the petrologic signicance of a subduction zone beneath the cascade arc may be restricted to the tectonic overprinting of an extensional system caused by a reduction in convergence rate on a waning of calc alkaline magmatism physical models of the high cascade subduction system are presented to explain the apparent chemical decoupling between calc alkaline and tholeiitic magmatism

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

erosion by rivers and transport pathways in the ocean a provenance tool using 40ar 39ar incremental heating on fine grained sediment measured from 39 ar and 37 ar respectively are consistent with typical values for k and ca bearing minerals calculations show that the bulk mineralogy is reflected in the outgassing k ca spectra and identify plagioclase as the dominant mineral contributing to the plateau defining portion of the age spectra a second model predicts bulk sediment ages from integrated bedrock cooling age area estimates in order to examine whether bulk sediment plateau ages are representative of the average cooling age of rocks from a given river basin calculated and observed ages are notably similar in three river basins when topographic and lithologic effects are accounted for overall this technique shows considerable promise not only in tracking individual terrigenous sources in the marine realm but also for understanding processes such as erosion and sediment transport in terrestrial systems

andean tectonics as a consequence of sea floor spreading abstract the evolution of sea floor spreading directions and rates in the southeast pacific shows a close relationship with the andean structural directions and the plutonic episodicity as well as the succession of short compressive orogenic phases alternating with relatively longer periods of extension the orogenic periods coincided with the beginning of periods of relatively higher rates of plate convergence immediately after periods of relative quiescence with the following periodicity 110 85 76 70 63 60 49 45 35 33 16 13 7 m y the presence of oceanic structures as aseismic ridges or fracture zones produces weak benioff zones shallow and poorly defined without a corresponding active volcanism on the other hand ancient continental structures such as the precambrian paleozoic orogenic belts have active volcanism and neat deep and well developed benioff zones

stable isotope study of the archaean rocks of the vredefort impact structure central kaapvaal craton south africa the vredefort dome in the kaapvaal craton was formed as a result of the impact of a large meteorite at 2 02 ga the central core of archaean granitic basement rocks is surrounded by a collar of uplifted and overturned strata of the witwatersrand supergroup exposing a substantial depth section of the archaean crust orthogneisses of the core show little variation in whole rock 18o value with the majority being between 8 and 10 with a mean of 9 2 n 35 quartz and feldspar have per mil differences that are consistent with o isotope equilibrium at high temperatures suggesting minimal interaction with fluids during subsequent cooling these data refute previous suggestions that the outer granite gneiss ogg and inlandsee leucogranofels ilg of the core represent middle and lower crust respectively granulite facies greenstone remnants from the ilg have 18o values that are on average 1 5 higher than the ilg host rocks and are unlikely therefore to represent the residuum from the partial melting event that formed the host rock witwatersrand supergroup sedimentary rocks of the collar which were metamorphosed at greenschist to amphibolite facies conditions generally have lower 18o values than the core rocks with a mean value for metapelites of 7 7 n 45 overall through an 20 km thick section of crust there is a general increase in whole rock 18o value with increasing depth this is the reverse of what is normal in the crust largely because the collar rocks have 18o values that are unusually low in comparison with metamorphosed sedimentary rocks worldwide the collar rocks have d values ranging from 35 to 115 average 62 n 29 which are consistent with interaction with water of meteoric origin having a d of about 25 to 45 we suggest that fluid movement through the collar rocks was enhanced by impact induced secondary permeability in the dome structure

archaean plate tectonics revisited 1 heat flow spreading rate and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents a simple model which relates the rate of seafloor creation and the age of the oceanic lithosphere at subduction to the rate of continental accretion can successfully explain the apparent differences between archaean and phanerozoic terrains in terms of plate tectonics the model has been derived using the following parameters 1 the spreading rate at mid ocean ridges 2 the age of the oceanic lithosphere at the time of subduction 3 the area age distribution of the seafloor 4 the continental surface area as a fraction of the total surface area of the earth and 5 the erosion rate of continents as a function of continental surface area and the total number of continental masses observations in phanerozoic terranes suggest that there are profound differences in the nature and volume of subduction zone igneous activity depending upon the age of the oceanic lithosphere being subducted and the nature of the overriding plate that is either continental or oceanic the subduction of young oceanic lithosphere less than 50 m y old which is thermally buoyant appears to result in a reduced volume of igneous activity most of the igneous activity caused by subduction of young oceanic lithosphere is either siliceous plutonism or bimodal tholeiitic rhyolitic volcanism when very young lithosphere is being subducted 50 m y old appears to result in greater volumes of igneous activity including the eruption of andesitic magmas thus andesites could only begin to be abundant in the rock record when older oceanic lithosphere began to be subducted our model predicts that as the earth aged and as heat flow from the interior of the earth diminished the proportion of old oceanic lithosphere being subducted increased fundamentally changing the nature of subduction zone igneous activity and the rate of continental accretion if the subduction of old oceanic lithosphere results in an 8 10 times greater volume of subduction zone magmatism our model predicts or explains all of the following observed features of earth history 1 archaean terranes appear to record two periods of rapid continental accretion between 3 8 and 3 5 b y ago and between 3 1 and 2 6 b y ago 2 there are very few differences and many marked similarities between rocks from archaean terranes and equivalent rocks from phanerozoic terranes 3 the total continental area appears to have remained essentially constant for the past 2 b y 4 archaean andesites are comparatively rare and the relative abundances of mafic and siliceous rocks appear to change during the archaean and the proterozoic with siliceous volcanics becoming proportionately more abundant in the geologic record with time 5 plutonic tonalites and trondhjemites appear to have been relatively much more abundant during the archaean plate tectonics is thus shown to have evolved over time due to a gradually decreasing rate of creation of oceanic lithosphere meaning that archaean tectonics and phanerozoic tectonics are but two points on an evolutionary continuum

the 2011 2012 submarine eruption off el hierro canary islands new lessons in oceanic island growth and volcanic crisis management forty years after the eruption of the teneguia volcano on la palma 1971 the last volcanic event in the canary islands a submarine eruption took place in 2011 off shore el hierro the smallest an

the four plutonic belts of the transhimalaya himalaya a chemical mineralogical isotopic and chronological synthesis along a tibet nepal section

oxygen isotope trajectories of crystallizing melts insights from modeling and the plutonic record elevated oxygen isotope values in igneous rocks are often used to fingerprint supracrustal alteration or assimilation of material that once resided near the surface of the earth the 18 o value of a melt however can also increase through closed system fractional crystallization in order to quantify the change in melt 18 o due to crystallization we develop a detailed closed system fractional crystallization mass balance model and apply it to six experimentally and naturally determined liquid lines of descent llds which cover nearly complete crystallization intervals melt fractions of 1 to 0 1 the studied llds vary from anhydrous tholeiitic basalts to hydrous high k and calc alkaline basalts and are characterized by distinct melt temperature sio 2 trajectories as well as crystallizing phase relationships our model results demonstrate that melt fraction temperature sio 2 relationships of crystallizing melts which are strongly a function of magmatic water content will control the specific 18 o path of a crystallizing melt hydrous melts typical of subduction zones undergo larger increases in 18 o during early stages of crystallization due to their lower magmatic temperatures greater initial increases in sio 2 content and high temperature stability of low 18 o phases such as oxides amphibole and anorthitic plagioclase versus albite conversely relatively dry tholeiitic melts only experience significant increases in 18 o at degrees of crystallization greater than 80 total calculated increases in melt 18 o of 1 0 to 1 5 can be attributed to crystallization from 50 to 70 wt sio 2 for modeled closed system crystallizing melt compositions as an example application we compare our closed system model results to oxygen isotope mineral data from two natural plutonic sequences a relatively dry tholeiitic sequence from the upper and upper main zones uumz of the bushveld complex south africa and a high k hydrous sequence from the arc related dariv igneous complex mongolia these two sequences were chosen as their major and trace element compositions appear to have been predominantly controlled by closed system fractional crystallization and their llds have been modeled in detail we calculated equilibrium melt 18 o values using the measured mineral 18 o values and calculated mineral melt fractionation factors increases of 2 3 and 1 1 5 in the equilibrium melts are observed for the dariv igneous complex and the uumz of the bushveld complex respectively closed system fractional crystallization model results reproduce the 1 increase observed in the equilibrium melt 18 o for the bushveld uumz whereas for the dariv igneous complex assimilation of high 18 o material is necessary to account for the increase in melt 18 o values assimilation of evolved supracrustal material is also confirmed with sr and nd isotope analyses of clinopyroxene from the sequence beginning with a range of mantle derived basalt 18 o values of 5 7 pristine mantle to 7 0 heavily subduction influenced mantle our model results demonstrated that high silica melts i e granites with 18 o of up to 8 5 can be produced through fractional crystallization alone lastly we model the zircon melt 18 o fractionations of different llds emphasizing their dependence on the specific sio 2 t relationships of a given crystallizing melt wet relatively cool granitic melts will have larger zircon melt fractionations potentially by 1 5 compared to hot dry granites therefore it is critical to constrain zircon melt fractionations specific to a system of interest when using zircon 18 o values to calculate melt 18 o

volcanic glass as a proxy for cenozoic elevation and climate in the cascade mountains oregon usa abstract after deposition volcanic glass hydrates with ambient water recording the average hydrogen isotope ratio d or 2h of local meteoric water during the hydration period previous researchers have used ancient glass d values to reconstruct paleotopography and paleoclimate while others have questioned the long term reliability of the proxy as a recorder of ancient meteoric water in this study we sampled volcanic glasses ranging in age 33 ma to 1 ma and local meteoric water converted to glass d values throughout the study area we propose a paleoenvironmental interpretation of volcanic glass results that resolves previously published isotopic data and agrees well with the petrologic structural and stratigraphic record namely the oregon cascades have been a significant topographic barrier since at least the mid miocene and likely as far back as the oligocene since reaching a topographic maximum during the eruption of columbia river flood basalts in the mid miocene surface elevations in oregon have decreased while the northern cascades in washington continue to rise

origin of rare element mineralized damara belt pegmatites a geochemical and light stable isotope study abstract neoproterozoic li cs ta lct pegmatites and pegmatitic sheeted leucogranites slgs of the damara belt namibia although coeval are variably mineralized and have intruded multiple tectono stratigraphic units major and trace element data together with oxygen and hydrogen isotopes were used to identify possible sources for these pegmatite melts two groups of pegmatites can be identified on the basis of o isotopes group a pegmatites which include u enriched pegmatitic slgs with a nb y f nyf pegmatite affinity kinnaird and nex 2007 and li be rich lct pegmatites with 18o 11 2 12 9 are metaluminous to peraluminous suggesting that they evolved from a partial melt of a mixed metasedimentary and igneous source group b pegmatites which contain sn li sn and tourmaline mineralization have higher 18o values 14 16 consistent with their derivation from a purely s type source with possible incorporation of or exchange with carbonates crystallization temperatures calculated using ti in quartz and o isotope thermometry are in agreement with each other and range from 420 to 575 c for lct pegmatites and 650 c for pegmatitic slgs the juxtaposition of coeval post tectonic lct pegmatites and pegmatitic slgs with an nyf affinity in the damara belt precludes tectonic setting as the only source of compositional variability in lct and nyf pegmatite melts

geochronology and sources of late neoproterozoic to cambrian granites of the saldania belt the saldania belt sb located in the southernmost part of south africa contains s i and a type granites whole rock sm nd data for the saldania granites indicate the presence of a juvenile as well as inherited crustal signature the earlier s type granites have end t values from 4 2 to 3 28 for t 550 ma in contrast the intermediate i type and youngest a type and highly fractionated i type granites display end values ranging from 1 44 to 3 68 for t 540 ma and from 3 66 to 5 1 for t 530 ma respectively the u pb single zircon data of a type granites exposed in the western branch of the sb yielded dates from 524 8 to 510 4 ma whereas an s type granite situated in the southern branch of the sb and represented by the syn to late tectonic rooiklip granite yielded an age of 527 8 ma the volcano sedimentary rocks intruded by these granites display nd model ages from ga to 1 67 ga and end t values from 6 58 to 3 34 for t 560 ma with isotope signature similar to those of the granites the s and i type granitic magmatism is mostly a product of melting of an earlier crust mesoproterozoic to paleoproterozoic with different degree of juvenile contribution the obtained isotope data and field relationship support the hypothesis that the lithological units of the sb were affected by the late neoproterozoic to early cambrian tectonism related to compressive deformational processes at the southern margin of the kalahari plate and probably correlated with the sierra la ventana belt basement

volcanic signature of basin and range extension on the shrinking cascade arc klamath falls keno area oregon 1 detailed geologic mapping of the klamath falls keno area revealed the complex relationship between subduction crustal extension and magmatic composition of the southern oregon cascade volcanic arc volcanism in the study area at 7 4 ma consisted of calc alkaline basaltic andesite and andesite lava flowing over a relatively flat landscape local angular unconformities are evidence that basin and range extension began at by at least 4 ma and continues today with fault blocks tilting at a long term rate of 2 ma to 3 ma minimum nw se extension is 1 5 km over 28 km 5 high alumina olivine tholeiite haot or low k low ti transitional high alumina olivine tholeiite lklt erupted within and adjacent to the back edge of the calc alkaline arc as the edge receded westward at a rate of 10 km ma at 2 7 0 45 ma the volcanic front migrated east much slower than the back arc migrated west 0 km ma for 6 0 4 ma calc alkaline rocks 0 7 km ma if 6 ma haot lklt is included and 1 km ma if highly differentiated 17 30 ma volcanic rocks of the early western cascades are included declining convergence probably decreased asthenospheric corner flow decreasing width of calc alkaline and haot lklt volcanism and the associated heat flow anomaly the margins of which focused on basin and range extension and leakage of haot lklt magma to the surface this declining corner flow combined with steepening slab dip shifted the back arc west compensation of extension by volcanic intrusion and extrusion allowed growth of imposing range front fault scarps only behind the trailing edge of the shrinking arc

petrogenesis of ca 1 95 ga meta leucogranites from the jining complex in the khondalite belt north china craton water fluxed melting of metasedimentary rocks abstract anatexis occurs in response to changing p t fluid conditions during orogenesis is thus crucial for understanding the tectonic evolution of orogenic belts here we report an integrated petrological geochemical whole rock oxygen isotopic and zircon u pb hf o isotopic study of meta leucogranites from the jining complex north china craton to unravel its petrogenesis and constrain the tectonic evolution of the paleoproterozoic khondalite belt the meta leucogranites are weakly peraluminous with homogeneous minimum melt compositions and are characterized by low rb contents and rb sr ratios high ba and sr contents and sr y ratios and moderately fractionated ree patterns without obvious eu anomalies these trace element characteristics differ from that of leucogranites derived from fluid absent melting considering their low whole rock zr saturation temperatures 746 780 c and comparable compositions with experimentally generated melts produced by water fluxed melting we infer that the meta leucogranites are low temperature granites and were generated by water fluxed melting of metasedimentary rocks at 6 8 kbar sims zircon u pb analyses suggest that the meta leucogranites were emplaced at 1 95 ga or slightly earlier and were subsequently metamorphosed and cut by a pegmatite dike at 1 92 ga the abundant inherited zircons yield nearly concordant 207 pb 206 pb ages ranging from 2 24 to 1 98 ga which are consistent with the detrital zircon age spectra of the metasedimentary rocks from the khondalite belt the hf isotopic compositions of inherited and magmatic zircons e hf t 0 2 to 7 7 closely overlap with the time evolved e hf range of detrital zircons from metasediments in the khondalite belt different types of zircons inherited magmatic and metamorphic zircons have similar 18 o values of 7 2 9 2 which is lower than that of metamorphic zircons 10 0 13 6 from surrounding metasediments thus we propose that the meta leucogranites were formed by water fluxed melting of metasedimentary rocks from the khondalite belt at 1 95 ga during the prograde upper amphibolite facies metamorphism

late eocene early oligocene tectonism volcanism and floristic change near gray butte central oregon mid tertiary strata near gray butte central oregon record volcanism and sedimentation on the margin of a west tilted basin that was subsequently disrupted by a northeaststriking fault system compositional characteristics of the volcanic rocks support correlation of the section to the upper eocene oligocene part of the john day formation the 1 2 kmthick section contains five fossil floras documenting climatic change in late eocene early oligocene time and a progression between better known eocene and oligocene floras in the region the presence of the transitional floras is a consequence of the subsidence of the gray butte basin to accommodate a section that is nearly four times thicker than better studied correlative strata 50 km to the northeast that consist almost entirely of volcanic rocks the lowest flora is within flood plain facies whereas the other four are hosted in lacustrine strata although alteration of volcanic rocks most closely associated with the floras precludes development of a precise isotopic age chronology regional correlations and several accurate isotopic age determinations indicate that the principal interval of climatic cooling may have been in late eocene time ca 38 39 ma rather than at the eocene oligocene boundary the paleoclimate interpretation is tempered however by the low diversity of the floras 12 24 species and possible taphonomic biases in comparing flood plain and lacustrine environments mapping established the presence of the cyrus springs fault zone a large 1 2 km vertical displacement possibly 7 km dextral offset shear zone that is possibly a surface expression of the klamath blue mountain gravity anomaly lineament the orientations of this fault zone subsidiary sinistral structures dikes and fold axes suggest that the presumed mesozoic structure marked by the lineament was reactivated as a dextralnormal fault by east northeast west southwest oriented compressive stress this stress is consistent with early oligocene north america farallon convergence but is inconsistent with northwest southeast to northsouth compression suggested by structures farther east stress either varied temporally or was partitioned into complex local strain domains the cyrus springs fault zone may have become active at about 28 30 ma resulting in uplift and southward tilting of part of the gray butte basin fill and ceased activity before deposition of the horizontal upper miocene pliocene deschutes formation the gray butte area was also an eruptive center for rhyolitic and alkaline mafic lava and tuff both before and after initiation of movement on the cyrus springs fault mid tertiary volcanism and sedimentation near the western end of the blue mountains heretofore not clearly related to active structures may have taken place within a regional transtensional regime associated with stress orientations different from those of neogene time local basins with higher subsidence rates accumulated relatively thick sequences of lacustrine tuffaceous strata and their fossil floras show progressive climate change through the eocene oligocene boundary interval

diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks recently obtained data on oxygen diffusion in feldspars quartz and hornblende permit the prediction of the apparent18o16o temperatures that would be measured in a rock that consisted only of those three minerals and cooled slowly from high temperature the computed temperatures would be based on the differences in the18o16o ratios between coexisting pairs of minerals the present calculation takes into account the diffusion rates for oxygen as a function of temperature the cooling rate of the rock the mineral grain sizes and the mode of the rock for mineral grains 1 mm in radius and a cooling rate of 10 c m y the minimum difference in apparent temperature between quartz feldspar and feldspar hornblende pairs will be 115 c despite the assumption of a normal uneventful slow cooling history to room temperature further the apparent quartz hornblende temperature will range over 30 c 590 620 c depending on the mode of the rock for a cooling rate of 1000 c m y the apparent difference in temperature can be as much as 400 c consequently consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high grade metamorphic rocks or igneous rocks which are cooled slowly departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature or a complex history for the rock for some minerals including hornblende the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens the choice of the specimens used for such calibrations needs to be re evaluated in light of these findings this may result in a change in the equilibrium equation constants an example from the literature the san jose tonalite baja california mexico was modelled and yields 18o concentrations in the minerals that correspond closely with the measured values this suggests that the model used is appropriate that the rock has had a simple thermal history and that it cooled at 100 200 c m y over the temperature range 800 500 c the set of paleotemperatures obtained for a rock will in general yield neither the mineral closure temperatures nor the formation or crystallization temperatures on the other hand the cooling rate of the rock may be derived from the data this in turn may have important tectonic implications with regard to denudation and uplift rates

oxygen and sulfur isotope geochemistry revealing a significant crustal signature in the genesis of the post collisional granitoids in central anatolia turkey abstract late cretaceous granitoid rocks from central anatolia comprise s i a type plutons derived from the collisional stages of the neo tethyan convergence system in central turkey these granitoids intrude the tectonic imbrication zone consisting of blocks of supra subduction zone type ssz type central anatolian ophiolite and crustal metasediments which are present in the izmir ankara erzincan suture zone the plutons are overlain by late palaeocene early eocene or younger detrital sediments granitoid formation is thought to be related to magma generation processes occurring in a post collisional lithospheric detachment related geodynamic setting that resulted from slab break off or lithospheric delamination whole rock s and quartz feldspar o isotope data from these plutons yields a broad range of values and both parameters indicate a nearly exclusively supracrustal origin for the s type granites as well as a significant crustal contribution in the genesis of the hybrid i type and a type granitoids the more mafic i type and a type granitoid rocks of any given suite have lower s and o values indicative of their larger degree of mantle component the combined stable isotope geochemical compositions when coupled with major trace and ree geochemistry and regional geology provide evidence that the significant crustal contribution originated from a metasomatized mantle layer which was affected by earlier ssz derived fluids and then accreted into the subcontinental lithosphere as collision occurred the partial melting of such a metasomatized mantle layer in a post collisional extensional geodynamic setting supplied either by the slab break off or lithospheric delamination mechanisms provided the significant crustal signature in the hybrid magmas of the late cretaceous i type and a type granitoids in central anatolia turkey

migmatites formed by water fluxed partial melting of a leucogranodiorite protolith microstructures in the residual rocks and source of the fluid abstract the opatica subprovince in the canadian shield is a late archaean 2761 2702 ma plutonic arc formed above a north dipping subduction zone anatexis 2690 2677 ma of leucogranodiorite and leucotonalite orthogneisses in the opatica generated migmatites in an area of north vergent back thrusts visible at the surface and in l ithoprobe seismic profile 48 schollen diatexite migmatites occur in the thrusts and metatexite migmatites between them the modal mineralogy microstructure and whole rock major trace and oxygen isotope compositions of the protolith and migmatites were investigated to 1 determine the melting reaction 2 find microstructural criteria for identifying residual rocks in leucocratic systems where there is no melanosome and 3 to determine the source of the fluid involved in anatexis partial melting of the protolith did not change the mineral assemblage but the abundance of quartz and microcline both declined and plagioclase and biotite increased in the residual rocks quartz plagioclase and microcline show evidence for dissolution and biotite does not thus water fluxed melting of quartz plagioclase microcline occurred a mass balance indicates 25 30 partial melting the melting reaction consumed the microcline and created essentially monomineralic domains of plagioclase extraction of 80 90 of the melt left a thin film of melt on the grain boundaries and crystallization of these in the plagioclase domains created diagnostic microstructures microcline fills the last remaining pore space and forms high aspect ratio crystals between plagioclases or triangular crystals at grain junctions quartz shows a range of morphologies from high aspect ratio films through the string of beads to isolated rounded grains as the microstructure progressively equilibrated after crystallisation most accessory phases including zircon remained in the residuum however almost all the schollen migmatites have high contents of th u nb ta and ree relative to the protolith due to contamination by accessory phases derived from mafic rocks disaggregation of the mafic rocks may have been facilitated by the high strain in the back thrusts where the schollen diatexites formed average whole rock 18 o for the protolith and migmatites are similar ca 8 2 and the small difference between melt rich 8 6 and residuum rich rocks 8 0 is consistent with fractionation thus the fluid that caused melting was probably of metamorphic origin with 18 o similar to the protolith the seismic profile shows several reflectors extending to a present depth of 20 km ca 40 km in the late archaean under the migmatites these are the paths along which the metamorphic fluid migrated and generated the migmatites now at the surface a new type of neosome reported in this study may have formed along fractures that the fluids migrated along however these are peripheral pathways in the metatexites adjacent to the back thrusts and schollen diatexites

metamorphic record of high pressure dehydration of antigorite serpentinite to chlorite harzburgite in a subduction setting cerro del almirez nevado filabride complex southern spain

on the causes of 18 o depletion and 18 o 16 o homogenization during regional metamorphism the east humboldt range core complex nevada previous stable isotope studies at lizzies basin revealed that metasedimentary rocks are 18o depleted relative to protolith values particularly in the lower parts of the section lower zone where the rocks are also isotopically homogeneous on a scale of hundreds of meters quartz 18o 9 0 to 9 6 per mil in contrast metasedimentary rocks at higher levels at lizzies basin upper zone are less 18o depleted and more heterogeneous in 18o in order to understand more fully the isotopic evolution of this terrane a series of detailed meter scale traverses across various metamorphic and igneous lithologies were completed at lizzies basin and at the structurally higher angel lake locality traverses in the lizzies basin lower zone and in the lower parts of angel lake angel lake lower sequence across various silicate lithologies including abundant granitoids reveal similar degrees of homogeneity although the average 18o values are higher at angel lake in contrast traverses which include substantial thicknesses of marble and calc silicate gneiss and very little granitoid have more heterogeneous quartz 18o values 11 9 to 13 4 per mil and also have a higher average 18o 12 9 per mil than observed elsewhere the scale of 18o 16o homogeneity in quartz observed at lizzies basin and angel lake meters to hundreds of meters requires fluid mediated isotope exchange which accompanied tertiary metamorphism there is a correlation between the degree of 18o depletion in metasedimentary rocks 18o 16o homogenization between lithologies and the proportion of granitoids leucogranites in particular within any part of the section and a corresponding anticor relation with the proportion of marble this points to a causal relationship whereby the leucogranites as well as the tertiary hornblende diorite and biotite monzogranite acted as both a relatively low 18o reservoir and a source of fluids to enhance exchange while the marbles hindered isotope deplction and homogenization by acting as relatively high 18o reservoirs and impermeable layers material balance calculations help delineate the plausible mechanisms of exchange between granitoids and metasediments single pass infiltration of magmatic fluids from the granitoids is not capable of reproducing all of the observations fluidmediated exchange by convective recirculation of magmatic fluids on a scale of meters is the mechanism which explains all of the observations the generalized model for the isotopic evolution of the east humboldt range core complex provides and excellent opportunity to establish the main causes and controlling factors of 18o depletion and 18o 16o homogenization during regional metamorphism

hotspots in the southern oceans an absolute frame of reference for motion of the gondwana continents abstract the geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time that is motion of an order of magnitude less than the relative motion between plate pairs in most cases it is concluded that inter hotspot movement cannot be discerned for the period 100 m y to present and that widely distributed hotspots in the atlantic and indian oceans provide a frame of reference for plate motions following the disintegration of gondwanaland which is independent of paleomagnetism this frame of reference is absolute in that it gives the motion of the lithosphere with respect to the mantle hotspots the absolute motion model indicates that africa and antarctica are now moving only very slowly that there has been significant relative movement between east and west antarctica since the cretaceous and prescribes the relative motion between the somali and african plates

new evidence for cretaceous strike slip faulting in the united states cordillera and implications for terrane displacement deformation patterns and plutonism early to mid cretaceous dextral strike slip faulting involving up to 400 km displacement has been documented in the southern sierra nevada of california in western idaho the salmon river suture juxtaposes unrelated oceanic and continen tal rocks along a major near vertical shear zone that was active in the early to mid cretaceous these two structural zones have been treated as separate entities however new data indicate that they likely connect via a structural discontinuity in northwest nevada and southeast oregon herein called the western nevada shear zone wns current structures within the wns record dip slip contractional deformation however regional relations structural and stratigraphic indicate a mismatch in the early mesozoic geology of rocks across the wns that is best explained by dextral strike slip displacement of several 100 km these relations argue that an early strike slip structure along the wns was reactivated and obscured during later regional shortening deformation most likely related to the cretaceous sevier orogeny the wns is along strike and broadly coeval with the salmon river suture zone and strike slip boundaries in the sierra nevada we hypothesize that all these features are related elements of a major structural boundary in the western united states cordil lera this boundary is interpreted to have accommodated significant dextral strike slip displacement of outboard arc terranes of the cordillera in the early cretaceous followed by reactivation as a localized contractional zone in the mid cretaceous during the sevier orogeny cretaceous batholiths of the united states cordillera closely parallel the boundary which we suggest formed a major crustal flaw that localized and facilitated emplacement of arc magmas

stable isotope geochemistry of caledonian granitoids from the british isles and east greenland abstract the british caledonian granites bcg comprise a compositionally diverse group of intrusive rocks ranging from diorite to leucogranite which were emplaced across the british isles in and adjacent to the caledonian metamorphic belt during the early paleozoic although the largest volume of caledonian magmatic activity in britain postdates the polyphase deformation and metamorphism of the grampian orogeny a small number of plutons were emplaced prior to the main orogenic event and another group are essentially contemporaneous with it o isotope ratios for the bcg are extremely variable and essentially overlap the full range of o isotope variations documented for orogenic granitoids the margins of two plutonic complexes have been affected by subsolidus hydrothermal exchange with meteoric fluids for the older 440 ma bcg plutons 18 o 6 6 12 1 and d 83 to 45 whereas for the younger c 400 ma plutons 18 o 7 1 12 4 and d 97 to 44 there is a general positive correlation between o and sr isotope variations in the post tectonic granitoids which is absent in their pre tectonic counterparts caledonian plutonic rocks occur in east greenland in the southern half of the fold belt between 70 75 n the earliest intrusions are sheets of monzonite and granodiorite emplaced slightly before the climax of caledonian orogenic activity subsequently a suite of post tectonic granites was emplaced c 450 370 ma two of the east greenland plutons have been affected by post emplacement meteoric hydrothermal alteration the 18 o range for the unaltered east greenland plutons is 8 5 13 9

oxygen isotope exchange kinetics of mineral pairs in closed and open systems applications to problems of hydrothermal alteration of igneous rocks and precambrian iron formations the systematics of stable isotope exchange between minerals and fluids are examined in the context of modal mineralogical variations and mass balance considerations both in closed and in open systems on mineral pair 18 o plots samples from terranes that have exchanged with large amounts of fluid typically map out steep positively sloped non equilibrium arrays analytical models are derived to explain these effects these models allow for different exchange rates between the various minerals and the external fluids as well as different fluid fluxes the steep arrays are adequately modelled by calculated isochron lines that involve the whole family of possible exchange trajectories these isochrons have initially steep near vertical positive slopes that rotate toward a 45 equilibrium slope as the exchange process proceeds to completion the actual data point array is thus analogous to the hand of an isotopic clock that measures the duration of the hydrothermal episode the dimensionless ratio of the volumetric fluid flux to the kinetic rate parameter u k determines the shape of each individual exchange trajectory in a fluid buffered system u k 1 the solutions to the equations 1 are independent of the mole fractions of the solid phases 2 correspond to taylor s open system water rock equation and 3 yield straight line isochrons that have slopes that approach 1 f where f is the fraction reacted of the more sluggishly exchanging mineral the isochrons for this simple exchange model are closely congruent with the isochrons calculated for all of the more complex models thereby simplifying the application of theory to actual hydrothermal systems in nature in all of the models an order of magnitude of time in units of kt separates steep non equilibrium arrays e g slope 10 from arrays approaching an equilibrium slope of unity on a diagram because we know the approximate lifetimes of many hydrothermal systems from geologic and heat balance constraints we can utilize the 18 o 16 o data on natural mineral assemblages to calculate the kinetic rate constants k s and the effective diffusion constants d s for mineral h 2o exchange these calculated values k qtz 10 14 k feld 10 13 10 12 agree with experimental determinations of such constants in nature once the driving force or energy source for the external infiltrating fluid phase is removed the disequilibrium mineral pair arrays will either 1 remain frozen in their existing state if the temperatures are low enough or 2 re equilibrate along specific closed system exchange vectors determined solely by the temperature path and the mineral modal proportions thus modal mineralogical information is a particularly important parameter in both the open and closed system scenarios and should in general always be reported in stable isotopic studies of mineral assemblages these concepts are applied to an analysis of 18 o 16 o systematics of gabbros plagioclase clinopyroxene and plagioclase amphibole pairs granitic plutons quartz feldspar pairs and precambrian siliceous iron formations quartz magnetite pairs in all these examples striking regularities are observed on and plots but we point out that plots have many advantages over their equivalent diagrams as the latter are more susceptible to misinterpretation using the equations developed in this study these regularities can be interpreted to give semiquantitative information on the exchange histories of these rocks subsequent to their formation in particular we present a new interpretation indicating that precambrian cherty iron formations have in general undergone a complex fluid exchange history in which the iron oxide magnetite precursor has exchanged much faster with low temperature 400 c fluids than has the relatively inert quartz

hydrogen and oxygen isotope variations in the high himalaya peraluminous manaslu leucogranite evidence for heterogeneous sedimentary source abstract the manaslu granite belongs to the high himalaya leucogranitic belt which was produced by melting of the crust during postcollisional thrusting d and 18o values have been determined for whole rock and coexisting minerals from the 8 km thick manaslu massif and its 50 km long dyke sheet its country rocks and the formation 1 f1 paragneisses which are the source of the granite for the granite dmusc range from 70 to 85 and 18o w r from 10 9 to 12 8 h and o isotope fractionations among minerals are consistent with high temperature equilibrium and for oxygen closed system evolution a few samples coming mainly from the dyke swarm have very low d values down to 188 and biotite muscovite h isotope fractionations indicative of disequilibrium the d h ratios of associated magmatic tourmaline are essentially unmodified from the distribution of d values in the granite and its country rocks circulation of very low deuterium meteoric hydrothermal waters was extremely localized because these waters are depleted in deuterium by up to 50 relative to modern meteoric waters the manaslu area was either at an altitude substantially higher than that of today 2500 6000 m for the analyzed samples or a mountain chain once existed to the south the f1 gneisses have 18oquartz between 12 and 14 3 which confirms that the granite was generated from f1 but d values are 20 higher than in the granite such a difference can be a result of degassing of the magma and or introduction of fluid in the melting zone infiltration of low d fluid 90 into the hot but dry f1 probably triggered partial melting these fluids could have come from the dehydration of the midlands sediments which are separated from the overlying f1 by the main central thrust the correlations among 18o 87 sr 86 sr 20 ma and nd values in both f1 and the granite indicate that the variations of these isotopic ratios in the manaslu are inherited from those in f1 at the time of melting in turn these ratios in f1 are related to the proportion of quartz and phyllosilicates for the isotopic ratios of nd and o and to the quantity of radiogenic sr generated within the sediment which is a function of age and rb content amount of phyllosilicate some other himalayan leucogranites require either other crustal source rocks or the 18o and 87 sr 86 sr ratios of f1 vary along the himalaya

snow peak oregon latest miocene low k tholeiite volcanism in the cascadia forearc abstract snow peak oregon is a moderate size basaltic shield volcano 50 52 wt sio2 7 4 km3 located within the forearc of the cascadia subduction zone 50 km west of the current arc front herein we present new whole rock geochemistry mineral chemistry and 11 new 40ar 39ar ages together with petrologic modeling that allow us to constrain the timing and origin of volcanism in contrast to previous k ar ages that suggested volcanism occurred at 3 ma our new 40ar 39ar ages show that snow peak formed between 5 3 and 6 million years ago the volcano lies unconformably on 30 ma volcanic rocks of the western cascades volcanism occurred over a total duration of 14 wt mgo snow peak lavas have trace element characteristics transitional between the calc alkaline basalt cab and low k tholeiite lkt primary magma types recognized throughout the cascade range but are closer to lkt and are classified as such estimates based on phase equilibria models and plagioclase hygrometers suggest that the primary magmas contained moderate amounts of water 1 5 2 wt consistent with lile hfse ratios that are greater than morb values snow peak is part of a widespread suite of lkt magmas that erupted between 5 8 ma throughout the central oregon cascade range in response to intra arc rifting and snow peak shows that lkt magmatism at this time extended well into the forearc of the central oregon cascade range overall lkt magmas of this age occur over a distance of 120 km normal to the arc front we speculate that the presence of snow peak may relate to the western extension of the brothers fault zone into the cascade forearc

back arc convection model for columbia river basalt genesis abstract genesis of the columbia river basalt group crbg may be accounted for by the progressive thermal erosion of an attenuated heterogenous subcontinental lithosphere scl by back arc convection behind the cascade arc assembly of the source components of the crbg began during the early mesozoic when slices of former oceanic lithosphere were isolated below terranes accreted to the continental margin subsequent lithospheric shearing accompanying northward translation of the terranes allowed the injection of small degree partial melts from the asthenosphere which solidified in the scl as pyroxenite veins the scl source of the crbg underwent a further episode of shearing accompanied by attenuation during the eocene and oligocene when it was caught between contrasting right lateral terrane translation and e w extension regimes in the northern cordillera and basin and range province respectively the attenuation resulted in the raising of the geotherm close to that of the pyroxenite and iherzolite solidi melting was triggered by back arc convection behind the cascade arc the pyroxenite was the first component mobilised constituting the principal source of the imnaha formation continued back arc convection resulted in the failure of the scl and the generation of the voluminous grande ronde formation from pyroxenite and the surrounding scl the change to wanapum volcanism marks the stage of exhaustion of in situ sources and their replacement with a combination of proterozoic scl from the east and asthenospheric material by back arc convection the saddle mountains formation was derived entirely from proterozoic scl small magma volumes resulted in a susceptibility to local contamination such that the distinct isotopic arrays of the saddle mountains formation comprise three mixing lines and do not have any age significance

origins of igneous microgranular enclaves in granites the example of central victoria australia to investigate their genesis and relations with their host rocks we study igneous microgranular enclaves imes in the c 370 ma post orogenic high level felsic plutons and volcanic rocks of central victoria australia the imes are thermally quenched magma globules but are not autoliths and they do not form mixing series with their host magmas these imes generally represent hybrids between mantle derived magmas and very high t crust derived melts modified by fractionation ingestion of host derived crystals and to a lesser extent by chemical interactions with their hosts isotopic and elemental evidence suggests that their likely mafic progenitors formed by partial melting of subcontinental mantle but that the ime suites from different felsic host bodies did not share a common initial composition we infer that melts of heterogeneous mantle underwent high t hybridisation with melts from a variety of crustal rocks which led to a high degree of primary variability in the ime magmas our model for the formation of the central victorian imes is likely to be applicable to other occurrences especially in suites of postorogenic granitic magmas emplaced in the shallow crust however there are many different origins for the mingled magma globules that we call imes and different phenomena seem to occur in differing tectonic settings the complexity of ime formation means that it is difficult to unravel the petrogenesis of these products of chaotic magma processes nevertheless the survival of fine grained non equilibrium mineralogy and texture in the imes suggests that their tenure in the host magmas must have been geologically brief

neoproterozoic to early cambrian crustal evolution of the pan african saldania belt south africa abstract the saldania belt forms part of a system of neoproterozoic mobile belts that surround and weld older cratons on the african continent it is a poorly exposed low grade orogenic belt composed of a number of inliers unroofed in mega anticlinal hinges of the permo triassic cape fold belt along the southern tip of africa deformed meta volcanosedimentary units are contained within tectonostratigraphic terranes separated by deep rooted sinistral dextral strike slip fault zones which display evidence of prolonged reactivation sedimentary rocks and volcanics of the boland terrane malmesbury group lower kango group and gamtoos group are considered to be distal facies of rift successions overlying the mid proterozoic crystalline basement on the southwestern edge of the kalahari craton these lithologies have been deposited in semi pull apart basins that formed in response to the breakup of rodinia and progressive opening of the proto atlantic adamaster ocean from 780 750 ma fragments of juvenile pan african crust with wpb morb characteristics occur in the bridgetown formation swartland terrane malmesbury inlier and attest to ocean floor spreading in this belt the thick turbidite successions of the swartland and tygerberg terranes malmesbury group reflect deeper water conditions and were deposited partly on oceanic crust in an evolving ocean continental margin basin marked by increasing sea levels and reduction of continental freeboard reversal of spreading and closing of the adamastor ocean possibly initiated by the opening of iapetus occurred in the period 600 570 ma although this movement vector caused sinistral transpressional reactivation of the gariep and saldanian margins there is no proper collision orogen developed in the saldania belt the bridgetown formation represents the suture between the southern rifted margin succession of the kalahari craton and a poorly developed southern orogenic belt represented in part by the swartland and tygerberg terranes this geosuture is similar to that of the south atlantic which formed in the analogous gariep belt between 575 and 543 ma multiphase s i and a type granites of the cape granite suite intruded the saldania belt in a pervasive transpressive regime between 550 and 510 ma comparing well with the timing of peak metamorphism collision in the gariep belt 545 ma the absence of a proper collisional orogen and the strongly developed northwesterly structural grain in the saldania belt seem to suggest that the cape granite suite was generated in a strike slip regime a discrepancy of approximately 50 ma between the age of the collision granites of the dom feliciano belt and cape granite suite suggests that they are genetically unrelated similarities in structural styles magmatism and dated events between the saldania ross delamarian chain of orogens however indicate a common history the evolution of this chain and that of the gariep damara dom feliciano belts can be accommodated in a single plate tectonic model of simultaneous sinistral transpressive subduction driven by the rifting of laurentia from south america and opening of the iapetus ocean loading and depression of the western and southern kalahari craton margin by thrust stacks of the late neoproterozoic orogens led to the formation of syn to post orogenic peripheral foreland basins such as those represented by the nama and vanrhynsdorp groups lowering of sea level and increased continental freeboard deposition of the unconformity based upper kango group and possibly the molassic franschhoek formation occurred in intra orogen pull apart basins which possibly developed between 530 and 510 ma in the saldania belt

tertiary meteoric hydrothermal systems and their relation to ore deposition northwestern united states and southern british columbia tertiary meteoric hydrothermal systems have altered the rocks exposed over more than 5 of the land surface of the northwestern united states and southern british columbia including at least 25 000 km 2 in idaho the systems typically involved convective circulation of fluid derived from ordinary meteoric groundwaters around crystallizing calc alkaline epizonal plutons emplaced into coeval volcanic cover rocks these individual systems had widely ranging lifetimes of 10 3 to 10 6 years and operated locally throughout the cenozoic although the most profound development of such activity occurred during eocene time individual systems varied in size from a few tens of square kilometers yankee fork idaho or less to several thousand square kilometers sawtooth and castro ring zones idaho typically regional propylitization aacompanied the fluid circulation although the higher temperature alteration assemblages were developed locally as were intense alteration effects e g silicification sericitization etc near some veins and in mining districts a significant amount probably 25 50 of the mineral production and potential in the region is closely related to tertiary meteoric hydrothermal systems oxygen and hydrogen isotopic data clearly demonstrate the close geologic association of meteoric hydrothermal systems and mineralization in 1 the paleocene cu zn pb mn main stage mineralization at butte montana 2 numerous eocene epithermal deposits principally valued for au and ag but also including significant deposits of cu pb zn f sb etc as at republic washington and in several mining districts in the idaho batholith and the challis volcanic field 3 several eocene skarn deposits valued for w ima idaho and cu mackay idaho 4 important lead silver vein and replacement deposits of tertiary bluebell british columbia and of probable cretaceous and early tertiary age wood river idaho 5 several potentially economic mo be and u bearing eocene porphyry plutons and 6 miocene epithermal deposits most prominently the au and ag bearing veins at silver city and delamar idaho the hg deposits at the mcdermitt caldera nevada and oregon and at weiser idaho and au deposits in the western cascade range and lake county oregon a close spatial association has been demonstrated between ore deposits and rocks having anomalous 18 o values and low d values the most important deposits are associated with relatively small generally 5 300 km 2 zones of low 18 o values and they are particularly closely linked with zones of very steep 18 o 16 o gradients in the altered rocks these associations hold much promise for the use of 18 o and d contour maps in future exploration efforts

uncertainties and implications of the late cretaceous and tertiary position of north america relative to the farallon kula and pacific plates we present updated global plate reconstructions and calculated uncertainties of the pacific kula and farallon vancouver plates relative to north america for selected times since 68 ma improved magnetic data from the indian ocean decrease the uncertainties in the global plate circuit approach these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots major differences between these results and those of others are due to our use of more detailed africa north america reconstructions separate vancouver and farallon plate reconstructions and the assumption of a rigid antarctica plate during cenozoic time the uncertainties in the relative positions of the pacific and north america plates since the time of anomaly 7 26 ma range up to 100 km in position or from 1 to 3 m y in time if the mendocino triple junction initiated at about 28 5 ma its position would have been at 31 3 n 130 km relative to fixed north america unacceptable overlap of oceanic crust of the pacific plate with continental crust of western north america in the anomaly 10 30 ma reconstruction is a minimum of 340 200 km along an azimuth of n60 e and may be accounted for by basin and range extension pacific north america displacement in the past 20 ma is found to be considerably less than that calculated by fixed hotspot reconstructions farallon vancouver north america convergence velocity decreased greatly between the times of anomalies 24 and 21 56 to 50 ma prior to the 43 ma age of the hawaiian emperor bend and the often quoted 40 ma end of the laramide orogeny a change in direction of farallon north america convergence occurred sometime between 50 and 42 ma and also may not correlate with the time of the hawaiian emperor bend the lack of data from subducted parts of the farallon and kula plates permits many possibilities regarding the position of the kula farallon ridge the age of subducted crust or the position of oceanic plateaus during the laramide orogeny leaving open the question of the relationship between plate tectonic scenarios and tectonic style during laramide time displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed north america during the interval between anomalies 30 31 and 18 68 to 42 ma are found to be pacific plate 1700 200 km northward farallon plate 3200 400 km northeastward vancouver plate 3000 400 km northeastward kula plate if attached to the pacific plate after a24 time 2500 400 km northward

the geological history of northwestern south america from pangaea to the early collision of the caribbean large igneous province 290 75ma abstract northwestern south america preserves a record of the assembly of western pangaea its disassembly and initiation of the far western tethys wilson cycle subsequent pacific margin magmatism and ocean plateau continent interaction since the late cretaceous numerous models have been presented for various time slices although they are based on either spatially restricted datasets or dates that are inaccurate estimates of the time of crystallisation here we review a very large quantity of geochronological geochemical thermochronological sedimentological and palaeomagnetic data that collectively provide tight constraints for geological models these data have been collected over a trench pacific parallel distance of 1500 km colombia and ecuador and reveal important temporal trends in rifting and subduction the temporal framework for our model constraints are obtained from robust concordant zircon u pb ages of magmatic rocks during 290 75 ma the late cretaceous thermal history of the margin

timing of tectonic and magmatic events in the central andes of peru the study of fault displacements in a region of well dated sequential granitic intrusions suggests a close time relationship between two peaks of deformation and intrusive activity mantle derived basic magmas were associated with both episodes which are dated as mid cretaceous and early palaeocene it is concluded that in each case a period of tensile tectonics with mafic dyke injection preceded the intense compressive deformation which accompanied the generation and rise of granitoid magmas these events may be related to alternations of slow and rapid subduction

temporal variations in plate convergence and eruption rates in the western cascades oregon new k ar age determinations on basalts basaltic andesites and ash flow tuffs from the central western cascades in oregon range in age from 32 to 3 ma the ages decrease from west to east and with increasing elevation volcanism has been continuous throughout the evolution of the western cascades with some periods of greater activity 13 16 ma 22 26 ma and 29 31 ma relative volume estimates indicate that eruption rates decreased by a factor of 6 from 35 ma to the present the eruption rates during formation of the western cascades volcanic arc were influenced by changes in the direction and rate of convergence between the farallon and north american plates we have developed a model of the tertiary convergence of these two plates based on the mantle fixed hotspot reference frame which indicates a factor of 5 decrease in convergence rate 16 0 to 3 2 cm a clockwise rotation of the western margin of the north american plate led to a decrease in the convergence angle since about 35 ma apparently slower more oblique subduction resulted in a decrease in the volume of erupted magmas

regional paleoprecipitation records from the late eocene and oligocene of north america abstract the marine 18o record indicates precipitous cooling or significant expansion of antarctic ice volume across the eocene oligocene epoch boundary at 33 7 ma this climatic step is also found in estimates of decreased paleoprecipitation from paleosols in oregon montana and nebraska but is only a part of a long term decline across the eocene oligocene transition rather than a sudden shift this suggests that climate was driven by a gradual process rather than a single perturbation paleoprecipitation levels stabilized during the oligocene coincident with a stabilization in 18o values possibly indicating a new climatic steady state

preserved magnetic fabrics vs annealed microstructures in the syntectonic recrystallised george granite south africa the saldanian basement of the cape fold belt of south africa outcrops in the kaaimans inlier with granite plutons intruded in low grade pelitic and quartzitic metasediments around 535 ma new field data support a ubiquitous saldanian top to the north thrust kinematics coeval with granite emplacement with no substantial cape tectonic overprint the granites and their contact aureoles display both synkinematic and post kinematic fabrics this and the high strain zone commonly observed all along the contact between the kaaimans inlier and the cape fold belt suggest a structural decoupling between the basement and its cover microstructures in the kaaimans inlier and in the george pluton establish a post kinematic pervasive and thermal overprint of saldanian age granites and country rocks record a medium temperature high strain deformation phase followed by a strong low temperature static recrystallisation two sets of andalusite porphyroblasts occur systematically in the contact aureoles of the studied plutons and cannot be explained by successive magmatic pulses the granites studied by the anisotropy of magnetic susceptibility ams technique are paramagnetic 20 km 300 si biotite is mostly at the origin of the bulk rock susceptibility although minor contributions of tourmaline or ferromagnetic phases may occur the contribution of biotite alone to the bulk magnetic susceptibility is supported by two quantitative models based respectively on whole rock compositions curie weiss law and on intrinsic mineral susceptibilities the magnetic foliations and lineations are homogeneous throughout the george pluton and are consistent with field structures the ams results mainly from the magneto crystalline anisotropy of biotite and from its lattice preferred orientation lpo in the rock the magnetic fabric reveals the biotite subfabrics that had been acquired before static recrystallisation and which was not modified by the subsequent thermal metamorphic event the magnetic fabric therefore preserves the emplacement related deformation fabric

oxygen isotope fractionation in quartz albite anorthite and calcite laboratory measurements of equilibrium oxygen isotope fractionation in quartz albite anorthite and calcite have been carried out by anhydrous exchange between silicates and calcite at temperatures of 600 c and above exchange in these systems is as rapid as exchange between silicates and water fractionation factors can be summarized in the following equations qc 0 38 106t 2 abc 0 57 106t 2 anc 1 59 106t 2 from which the silicate pair fractionations are readily obtained these results are compared with published theoretical estimates as well as data derived from hydrothermal experiments some significant differences are found in particular it is difficult to reconcile all of the hydrothermal data either with theoretical calculations or with the present experimental data the new experiments provide an internally consistent set of fractionation factors suitable for isotopic thermometry and for test of disequilibrium in natural systems

temporal and spatial evolution of northern cascade arc magmatism revealed by la icp ms u pb zircon dating we present thirty new laser ablation inductively coupled plasma mass spectrometry u pb zircon dates for intermediate to silicic plutons of the northern cascade arc with emphasis on the chilliwack batholith mount baker magmatic focus located in southwestern british columbia and northern washington chilliwack magmatism commenced at 35 ma in southwestern british columbia and the most voluminous plutons define a cluster at 32 29 ma documenting an early flare up during the same interval the index squire creek and cascade pass intrusions were emplaced south of the chilliwack batholith north of the chilliwack maximum pluton ages become progressively younger northward tracking the northerly migration of the edge of the farallon juan de fuca explorer plate system relative to north america chilliwack magmatism continued from 29 ma to 22 ma at a slightly reduced flux followed by a lull from 22 to 11 ma during which magmatism shifted north to the mount barr batholith 18 ma chilliwack magmatism res

age progressive volcanism in the new england seamounts and the opening of the central atlantic ocean radiometric ages k ar and oar 39ar methods have been determined on dredged volcanic rocks from seven of the new england seamounts a prominent northwest southeast trend ing volcanic lineament in the northwestern atlantic ocean the oar 39ar total fusion and increnental heating ages show an increase in seamount construction age from southeast to northwest that is consistent with northwestward motion of the north american plate over a new england hot spot between 103 and 82 ma a linear volcano migration rate of 4 7 cm yr fits the seamount age distribution these ages fall within a longer age progression from the corner seamounts 70 to 75 ma at the eastern end of the new england seamounts to the youngest phase of volcanism in the white mountain igneous province new england 100 to 124 ma the new england hot spot estimated from the new radio metric ages and motion of north america in the hot spot reference frame to be near 28on 33ow probably generated a short line of mid to late tertiary age seamounts on the african plate but appears to be presently inactive the hot spot reference frame is used to calculate the motion of the north american plate away from the african plate from early cretaceous time to the present rominent magnetic anomalies recorded in the central atlantic seafloor give the positions of the spreading ridge for a bange of known ages hot spots that now underlie the african plate in the eastern central atlantic new england canary madeira cape verde azores could have produced cretaceous seamount and island chains on the north american plate during the early opening of the central atlantic each of these hot spots has been overridden by spreading ridges at pre dictable times some of these hot spot crossings are expressed as geochemical anomalies in the oceanic crust now far removed from spreading ridges

oxygen and hydrogen isotope geochemistry of s and i type granitoids the cape granite suite south africa abstract the late precambrian cape granite suite of the southwestern cape south africa can be subdivided into six major batholiths saldanha vredenburg darling malmesbury paarl wellington stellenbosch kuils river and peninsula individual plutons have been classified as a i and s type granites on the basis of petrography and chemical composition quartz and feldspar from the cape granites have 18o values which range from 8 6 to 13 4 and 5 0 to 13 6 respectively the 18o value of quartz is a very effective discriminator between s and i type granites and provides the best means to estimate the 18o value of the original magma the s type granites crystallized from magmas having 18o values between 9 5 and 11 4 mean 10 6 the a and i type granites crystallized from magmas having 18o values between 6 6 and 9 9 mean 7 8 there is almost no overlap of 18o values between the s and the i type granites with the boundary at 9 5 of the 31 quartz feldspar pairs analysed the vast majority have values of qtz fsp between 0 and 4 however only eight samples have values of qtz fsp between 0 7 and 1 55 which would be consistent with equilibrium under closed system conditions the remaining samples were affected by minor fluid rock interaction biotite d values range from 113 to 47 and show no correlation with estimated magma 18o value biotite water content or whole rock loss on ignition the variation in biotite d is probably caused by a combination of varying degree of magma degassing and minor low temperature alteration

permo triassic anatexis continental rifting and the disassembly of western pangaea crustal anatectites are frequently observed along ocean continent active margins although their origins are disputed with interpretations varying between rift related and collisional we report geochemical isotopic and geochronological data that define an 1500 km long belt of s type meta granites along the andes of colombia and ecuador which formed during 275 223 ma these are accompanied by amphibolitized tholeiitic basaltic dykes that yield concordant zircon u pb dates ranging between 240 and 223 ma a model is presented which places these rocks within a compressive permian arc setting that existed during the amalgamation of westernmost pangaea anatexis and mafic intrusion during 240 223 ma are interpreted to have occurred during continental rifting which culminated in the formation of oceanic crust and initiated the break up of western pangaea compression during 275 240 ma generated small volumes of crustal melting rifting during 240 225 ma was characterized by basaltic underplating the intrusion of tholeiitic basalts and a peak in crustal melting tholeiitic intrusions during 225 216 ma isotopically resemble depleted mantle and yield no evidence for contamination by continental crust and we assign this period to the onset of continental drift dissected ophiolitic sequences in northern colombia yield zircon u pb dates of 216 ma the permo triassic margin of ecuador and colombia exhibits close temporal faunal and geochemical similarities with various crustal blocks that form the basement to parts of mexico and thus these may represent the relict conjugate margin to nw gondwana the magmatic record of the early disassembly of pangaea spans 20 ma 240 216 ma and the duration of rifting and rift drift transition is similar to that documented in cretaceous tertiary rift settings such as the west iberia newfoundland conjugate margins and the taupo lau havre system where rifting and continental disassembly also occurred over periods lasting 20 ma

sedimentary record of subduction to forearc transition in the rotated eocene basin of western oregon systematic changes in compositions of eocene sandstones in the southern part of the oregon coast range constrain proposed models of tectonic accretion and rotation of this crustal fragment three basic petrofacies of lithic arkosic and volcanolithic composition can be defined sequentially from the base to the top of the section the lithic petrofacies was deposited prior to rotation and was derived from the nearby klamath mountains during collision of the subjacent seamount terrane the transition to arkosic composition coincided with the end of collision and the initiation of forearc basin subsidence the arkosic sands contain muscovite and potassium feldspar derived from a continental plutonic source area their presence in the forearc sequence suggests that the coast range block lay farther to the east and closer to interior parts of the mesozoic batholith belt prior to tectonic rotation volcanolithic sandstones were deposited after rotation of the coast range had begun and they mark the beginning of volcanic activity in the adjacent cascade arc

stable isotopic evidence for large scale seawater infiltration in a regional metamorphic terrane the trois seigneurs massif pyrenees france oxygen isotopic analyses of 95 metamorphic and igneous rocks and minerals from a hercynian metamorphic sequence in the trois seigneurs massif pyrenees france indicate that all lithologies at higher metamorphic grades than the andalusite in isograd have relatively homogeneous 18o values the extent of homogenization is shown by the similarity of 18o values in metacarbonates metapelites and granitic rocks 11 to 13 and by the narrow range of oxygen isotopic composition shown by quartz from these lithologies these values contrast with the 18o values of metapelites of lower metamorphic grade 18o about 15 homogenization was caused by a pervasive influx of hydrous fluid mass balance calculations imply that the fluid influx was so large that its source was probably high level groundwaters or connate formation water hydrogen isotopic analyses of muscovite from various lithologies are uniform and exceptionally heavy at d 25 to 30 suggesting a seawater origin many lines of petrological evidence from the area independently suggest that metamorphism and anatexis of pelitic metasediment occurred at depths of 6 12 km in the presence of this water rich fluid the composition of which was externally buffered deep penetration of surface waters in such environments has been hitherto unrecognized and may be a key factor in promoting major anatexis of the continental crust at shallow depth three types of granitoid are exposed in the area the leucogranites and the biotite granite quartz diorite are both mainly derived from fusion of local paleozoic pelitic metasediment because all these rocks have similar whole rock 18o values 11 to 13 the post metamorphic biotite granodiorite has a distinctly different 18o 9 5 to 10 0 and was probably derived from a deeper level in the crust rare mafic xenoliths within the deeper parts of the biotite granite quartz diorite also have different 18o 8 0 to 8 5 and possibly represent input of mantle derived magma which may have provided a heat source for the metamorphism