short paper the age and origin of boulders in the highland border complex constraints on terrane movements mid proterozoic rb sr muscovite ages on metasedimentary boulders within the highland border complex together with archean t dm ages on the same clasts provide constraints on terrane movements in the scottish caledonides an origin for these clasts from cratonic north america is ruled out combined radiometric age t dm and fauna1 province studies on basement materials provide a potentially productive method of determining the provenance of terranes

graptolite derived organic matter in the wufeng longmaxi formations upper ordovician lower silurian of southeastern chongqing china implications for gas shale evaluation abstract the organic rich shales of the wufeng longmaxi formations ordovician silurian are one of the most important hydrocarbon source rocks and shale gas exploration targets in china graptolites are abundant in these shales but their contribution to the dispersed organic matter om has been overlooked this lack of attention could seriously affect the evaluation of om type om maturity hydrocarbon generation potential reservoir characteristics and gas content of these shales optical microscope and focused ion beam scanning electron microscope fib sem techniques were employed to describe the optical characteristics and micro nanopores of the graptolites to determine the contribution of the graptolites and to evaluate the thermal maturity of the wufeng longmaxi formations non granular graptolites which show a stronger anisotropy and higher reflectance under polarized light than the granular graptolites were primarily observed in these sediments graptolites account for 20 93 of the dispersed om in the wufeng longmaxi formations and the level of their contribution has been attributed to the radiation of graptolites at the late ordovician kaptian and the recovery of graptolites at the early silurian rhuddanian the bireflectance values of the non granular graptolites fall between 2 22 and 4 32 indicating a behavior similar to biaxial anthracitic vitrinite the equivalent vitrinite reflectance eqvro values of the sediments in the wufeng longmaxi formations calculated from the mean maximum reflectance of non granular graptolite fall in the range of 3 08 4 29 and these values have a precision that is greater than in previous studies on the wufeng longmaxi formations abundant organic pores were observed in the non granular graptolites and they are aligned parallel to the fusellar layers of the graptolites this indicates that the fusellar layers of non granular graptolites controlled the development and distribution of organic pores such abundant organic pores played a significant role in the accumulation of shale gas in the wufeng longmaxi formations of the upper yangtze platform

chemistry of proterozoic orogenic processes at a continental margin in northern sweden abstract sm nd isotopic major trace and rare earth element data for two separate age groups of plutonic rocks have been used to characterize early proterozoic crust formation mechanisms in the vicinity of the exposed archaean of northern sweden the granitoid rocks of the older 1 89 1 87 ga group show calc alkaline trends diorite quartz monzonite adamellite and have negative nd values ranging from about 8 to 5 they represent variable contamination of isotopically juvenile mantle melts with archaean crustal material the younger 1 80 ga group of plutonic rocks are of true granitic composition they show a restricted range of nd values which is encompassed by those of the older group trace and major element data are consistent with the derivation of the younger group of rocks by remobilization of the earlier formed proterozoic continental crust this is in contrast to the temporally similar but isotopically more juvenile granitoids of the skellefte district to the south

garnet provenance studies juxtaposition of laurentian marginal terranes and timing of the grampian orogeny in scotland a study of the composition and zoning patterns of detrital garnets from ordovician greywackes from the northern belt of the southern uplands terrane of scotland reveals characteristics of the metamorphic sources very similar to the dalradian supergroup of the scottish grampian terrane the radiometric cooling and uplift ages of the dalradian metamorphic zones and the depositional ages of the southern uplands greywackes support the hypothesis for local provenance detrital metamorphic garnet identical to dalradian garnet has also been identified in the upper ordovican sandstones of the highland border complex these observations do not support proposals that the grampian midland valley and southern uplands terranes were exotic to each other in the late ordovician time these new results together with a review of published age dates clarify the late ordovician palaeogeography for this part of the laurentian margin the distances between the grampian midland valley and southern uplands terranes may have been similar to the present day it is concluded that large rivers flowed out of the uplifting mountainous grampian terrane and across the midland valley into a southern uplands trench during the late ordovician time the main orogeny i e mountain building in the grampian terrane was therefore post cambrian producing the first high mountains and resultant flysch in the caradoc

testing for the presence of a terrane boundary within neoproterozoic dalradian to cambrian siliceous turbidites at callander perthshire scotland the southern highland group dalradian and keltie water grit formation which includes the lower cambrian leny limestone form an inverted 1 4 km thick largely arenaceous sequence at callander the grits have the same detrital mineralogy throughout mainly quartz plagioclase an 1 3 muscovite and biotite chlorite formed from detrital biotite during low grade regional metamorphism t 40 ar 39 ar laser fusion dating of detrital muscovites yields an age spectrum with a peak at 1600 1800 ma in the dalradian rocks similar old ages occur in the keltie water grits but are diluted by ages of 507 886 ma we interpret these new data as showing that the rocks were most likely deposited as a single sequence possibly with a disconformity in neoproterozoic to early cambrian times before the onset of grampian orogenesis in the early palaeozoic no major structural or stratigraphical breaks have been identified and there is no direct evidence for the presence of two separate terranes

a cryptic caledonian flysch terrane in scotland a substantial part of the lower old red sandstone crawton group in the north east midland valley of scotland has a first cycle provenance from the south and east i e from a source block within the midland valley from the clast assemblages in proximal conglomerates it is clear that this provenance comprised a flysch terrane cut by granodioritic and tonalitic plutons angular clasts of lithic arenite and metagreywacke derived from this cryptic flysch sequence show it to be distinct from a metasedimentary sequence which flanked the lower old red sandstone basin to the north and north east the basin must overlie or lie adjacent to a boundary between two quite different sedimentary assemblages petrographic examination and chemical analysis of the clasts show that the southerly cryptic succession includes an important mafic ultramanc provenance implying that ophiolitic source rocks may have originally contributed to it sr and nd isotopic compositions for the clasts establish that the midland valley flysch is isotopically unrelated to either southern uplands shales and greywackes or dalradian metasedimentary rocks the clasts have lower sr contents than either suite and sr isotopic compositions 87 sr 86 sr in the range 0 7099 0 7176 at 450 ma which are more radiogenic than the southern uplands but less radiogenic than the dalradian crustal residence ages t dm have a narrow range of 1 76 1 94 ga the mean of 1 87 0 06 ga compares with a mean residence age of 1 49 0 21 ga for southern uplands greywackes and shales mixing calculations demonstrate that the incorporation of young ophiolite derived basaltic detritus into the caledonian sedimentary mass can explain properties of the nd data previously attributed to a bimodal proterozoic provenance the distinct compositions of the southern uplands and midland valley flysch sediments suggest they originally tapped different parts of the laurentian margin and have subsequently been brought together structurally as displaced fore arc slivers the diverse isotopic compositions of outboard flysch terranes has implications for the nature of the sedimentary component incorporated in inboard magma suites

pedologic and climatic controls on rn 222 concentrations in soil gas denver colorado soil gas radon concentrations are controlled seasonally by factors of climate and pedology in a swelling soil of the semiarid western united states soil gas radon concentrations at 100 cm depth increase in winter and spring due to increased emanation with higher soil moisture and the capping effect of surface water or ice increased soil moisture results from a combination of higher winter and spring precipitation and decreased insolation in fall and winter lowering soil temperatures so that water infiltrates deeper and evaporates more slowly radon concentrations in soil drop markedly through the summer and fall the increased insolation of spring and summer warms and dries the soil limiting the amount of water that reaches 100 cm as the soil dries radon emanation decreases and deep soil cracks develop these cracks aid convective transport of soil gas increase radon s flux into the atmosphere and lower its concentration in soil gas probable controls on the distribution of uranium within the soil column include its downward leaching its precipitation or adsorption onto b horizon clays concretions or cement and the uranium content and mineralogy of the soil s granitic and gneissic precursors

petrogenesis of mount rainier andesite magma flux and geologic controls on the contrasting differentiation styles at stratovolcanoes of the southern washington cascades quaternary mount rainier washington usa of the cascades magmatic arc consists of porphyritic calc alkaline andesites and subordinate dacites with common evidence for mingling and mixing with less evolved magmas encompassing andesites basaltic andesites and rarely basalts basaltic andesites and amphibole andesites spessartites that erupted from vents at the north foot of the volcano represent some of mount rainier s immediate parents and overlap in composition with regional basalts and basaltic andesites geochemical major and trace elements and isotopic sr nd pb o compositions of mount rainier andesites and dacites are consistent with modest assimilation typically 20 wt of evolved sediment or sediment partial melt sandstones and shales of the eocene puget group derived from the continental interior are exposed in regional anticlines flanking the volcano and probably underlie it in the middle to lower crust accounting for their assimilation mesozoic and cenozoic igneous basement rocks are unsuitable as assimilants due to their high 143 nd 144 nd diverse 206 pb 204 pb and generally high 18 o the dominant cause of magmatic evolution at mount rainier however is inferred to be a version of in situ crystallization differentiation and mixing langmuir 1989 wherein small magma batches stall as crustal intrusions and solidify extensively yielding silicic residual liquids with trace element concentrations influenced by accessory mineral saturation subsequent magmas ascending through the intrusive plexus entrain and mix with the residual liquids and low degree re melts of those antecedent intrusions producing hybrid andesites and dacites mount st helens volcanic rocks have geochemical similarities to those at mount rainier and may also result from in situ differentiation and mixing due to low and intermittent long term magma supply accompanied by modest crustal assimilation andesites and dacites of mount adams isotopically overlap the least contaminated mount rainier magmas and derive from similar parental magma types but have trace element variations more consistent with progressive crystallization differentiation probably due to higher magma fluxes leading to slower crystallization of large magma batches allowing time for progressive separation of minerals from melt mount adams also sits atop the southern projection of a regional anticlinorium so eocene sediments are absent or are at shallow crustal levels and so are cold and difficult to assimilate differences between southwest washington stratovolcanoes highlight some ways that crustal geology and magma flux are primary factors in andesite generation

sedimentary facies analysis mineralogy and diagenesis of the mesozoic aquifers of the central perth basin western australia abstract assessment and successful exploration for and exploitation of aquifers in the perth basin require knowledge of their petrophysical and sedimentological characteristics and geometries this study provides a sedimentological and lithofacies analysis of core from the mesozoic yarragadee formation cadda formation cattamarra coal measures eneabba formation and lesueur sandstone of western australia recovered from pinjarra 1 cockburn 1 gingin 1 and gingin 2 wells provide insights into changes in depositional environment over time and space in the central perth basin in studied sections nine different fluvial dominated lithofacies have been identified in the yarragadee formation cadda formation and cattamarra coal measures that occur in similar proportions whereas the lesueur sandstone is dominated by coarse grained deposits of high energy fluvial systems vertical variation in lithofacies on the order of 10 cm to approximately 2 m vertical scale is observed in all formations and wells changes in the lithofacies linked to local depositional environments are probably associated with fluvial alluvial to fluvio deltaic systems that have complex 3d architecture including braided and to probably meandering systems and are affected by channel avulsion point counting and automated mineral analysis of thin sections of core showed detrital mineralogy dominated by monocrystalline quartz with rare polycrystalline quartz a moderate amount of alkali feldspar and very minor amounts of garnet organic fragments muscovite and biotite these data show a lithofacies dependence of the proportion of minerals with little variation between equivalent lithofacies from different depths formations or wells grain size and sorting that have a first order control on porosity are strongly lithofacies dependent individual lithofacies types show a trend of decreasing porosity with depth due to the increasing effects of compaction quartz overgrowth cementation and authigenic clay mineral development lithofacies and depth are the main controls on permeability and so lithofacies distribution exerts a key control on hydraulic behaviour

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

the origin and evolution of mineralizing fluids in a sediment hosted orogenic gold deposit ballarat east southeastern australia the hydrothermal fluids responsible for gold mineralization at the ballarat east gold deposit the second largest orogenic gold deposit in the western lachlan orogen are thought to have links to a variety of processes including metamorphism sedimentation and or magmatism in the current study noble gases and halogens have been used as fluid tracers to reevaluate the origin and evolution of the gold related fluids at the ballarat east deposit gold bearing quartz and carbonate veins from the ballarat east contain low salinity 4 wt nacl equiv aqueous h2o and mixed h2o co2 fluid inclusions these fluid inclusions have variable molar br cl values of between 1 2 10 3 and 2 9 10 3 and i cl values between 150 10 6 and 500 10 6 and br is strongly correlated with i defining a mixing line with a br i ratio of 5 6 the fluid inclusions have 40ar 36ar ratios ranging from 322 close to the atmospheric 40ar 36ar ratio of 296 up to a maximum of 4503 40ar is strongly correlated with cl and defines a mixing line with a 40are cl ratio of 4 6 10 4 40are denotes excess 40ar the fluid inclusions contain 5 1 to 32 ppm 40are by mass and exhibit minimum 36ar concentrations ranging from 3 1 to 11 ppb which exceed air saturated water asw levels by several parts per billion asw 1 3 2 7 ppb fluid inclusion 84kr 36ar and 130xe 36ar values are uniformly enriched in kr and xe relative to air but exhibit limited variation these data provide strong evidence for the involvement of two noble gas and halogen reservoirs this data is compatible with a deeply sourced fluid possibly originating by devolatilization of altered volcanic rocks e g basalts that acquired additional noble gases and organic br plus i by interaction with sedimentary rocks including organic rich shales that are found beneath and surrounding the deposit the data are also consistent with mixing deeply sourced metamorphic fluids with sedimentary formation waters however both interpretations favor the involvement of metamorphic fluids and sedimentary components and highlight the significance of fluid rock interaction as controls on fluid compositions in victorian deposits the data are compatible with genetic models for orogenic gold in which gold mineralization was initiated by metamorphic devolatilization in the lower crust and was linked to lachlan orogenesis at ca 440 ma

colonsay and islay a suspect terrane within the scottish caledonides colonsay and the western part of islay southern hebridean islands of scotland consist of unfossiliferous deformed low grade metasedimentary rocks which rest with sheared unconformity upon gneissic basement the area has a long history of investigation but its stratigraphic affinities have been a persistent problem the speculative assignment of the basement rocks to the lewisian has never been substantiated the age of the cover sequence remains unclear its deformation differs from that of nearby caledonian successions small igneous bodies have yielded a tentative intrusion age of ca 600 ma which is abnormal for the region because the area is bounded by major strike slip faults it is suggested that the colonsay western islay terrane is allochthonous probably emplaced between 620 and ca 540 ma the terrane concept explains the long standing difficulties of correlation recognition of suspect terranes even of this small size within orogens such as the orthotectonic caledonides can help resolve some long standing problems

mantle flow through the northern cordilleran slab window revealed by volcanic geochemistry the northern cordilleran slab window formed beneath western canada concurrently with the opening of the californian slab window beneath the southwestern united states beginning in late oligocene miocene time a database of 3530 analyses from miocene holocene volcanoes along a 3500 km long transect from the northern cascade arc to the aleutian arc was used to investigate mantle conditions in the northern cordilleran slab window using geochemical ratios sensitive to tectonic affi nity such as nb zr we show that typical volcanic arc compositions in the cascade and aleutian systems derived from subduction hydrated mantle are separated by an extensive volcanic fi eld with intraplate compositions derived from relatively anhydrous mantle this chemically defi ned region of intraplate volcanism is spatially coincident with a geophysical model of the northern cordilleran slab window we suggest that opening of the slab window triggered upwelling of anhydrous mantle and displacement of the hydrous mantle wedge which had developed during extensive early cenozoic arc and backarc volcanism in western canada high heat fl ow throughout the western canadian cordillera is broadly coincident with the fi eld of intraplate volcanism and is linked to slab window induced mantle upwelling

localized uplift in the scottish dalradian rb sr mica ages are reported here from the dalradian meta sediments of the angus region in the scottish highlands fig 1 in a study of the uplift history of this classic metamorphic terrain the samples come from across the barrovian metamorphic zones1 and provide evidence of diachroneity in both metamorphism and later cooling thus the oldest mineral ages and hence earliest cooling are found in the highest grade metamorphic rocks to the north the results show the importance of tectonically controlled localized uplift processes given that erosion is an important influence on the thermal development of metamorphic rocks2 the constant or steadily decaying isostatic uplift rates often assumed in models of orogenic belts2 3 may be a marked oversimplification of the real events

episodic granite plutonism in the scottish caledonides boulders from a lower devonian conglomerate complete the record of a phase of ordovician plutonism associated with island arcs and blue schist metamorphism indicating high angle subduction following a phase of sedimentary accretion late silurian plutonism was the result of low angle subduction the two granite suites have distinct isotopic trends

isotopic insights into the degassing and secondary hydration of volcanic glass from the 1980 eruptions of mount st helens abstract the magmatic degassing history of newly erupted volcanic glass is recorded in its remaining volatile content however this history is subsequently overprinted by post depositional secondary hydration the rates and origins of which are not yet adequately constrained here we present the results of a natural experiment using products of the 1980 eruptions of mount st helens we measured water concentration dglass and 18obsg 18o of the bulk silicate glass of samples collected during the dry summer months of 1980 and compared them with material resampled in 2015 from the same deposits samples collected from the subsurface near gas escape pipes show elevated water concentrations near 2 0 wt and these are associated with lower dglass 110 to 130 and 18obsg 6 0 to 6 6 values than the 1980 glass 70 to 100 and 6 8 to 6 9 respectively samples collected in 2015 from the surface to 10 cm subsurface of the 1980 summer deposits have a small increase in average water contents of 0 1 0 2 wt but similar 18obsg 6 8 6 9 values compared to the 1980 glass values these samples however show 15 higher dglass values exchange with meteoric water is expected to yield lower dglass values we attribute higher dglass values in the upper portion of the 1980 deposits collected in 2015 to rehydration by higher d waters that were degassed for several months to a year from the hot underlying deposits which hydrated the overlying deposits with relatively high d gases our data also contribute to magmatic degassing of crystal rich volcanoes using the 1980 samples our reconstructed d h2o trends for the dacitic mount st helens deposits with rhyolitic groundmass yield a trend that overlaps with the degassing trend for crystal poor rhyolitic eruptions studied previously elsewhere suggesting similar behavior of volatiles upon exsolution from magma furthermore our data support previous studies proposing that exsolved volatiles were trapped within a rapidly rising magma and started degassing only at shallow depths during the 1980 eruptions

the origin of mount st helens andesites abstract mount st helens volcano has intermittently produced mainly dacitic products but occasionally erupted a more diverse suite of lavas including basalts and andesites petrogenetic relations between these magmas provide insight into the dynamics of the subjacent magma system mineralogical and geochemical features of representative lavas erupted during the past 2200 years can distinguish three basaltic and three andesitic variants the mafic lavas include a transitional olivine plagioclase basalts with low k 2 o and incompatible trace element abundances b calc alkaline olivine plagioclase clinopyroxene basalts enriched in k 2 o tio 2 and incompatible trace elements and c calc alkaline olivine plagioclase basaltic andesites with incompatible trace element contents transitional between the two basalt types intermediate lavas include a tholeiitic two pyroxene andesites b calc alkaline plagioclase two pyroxenes olivine amphibole mafic andesites 56 59 wt sio 2 and c calc alkaline plagioclase two pyroxenes amphibole high silica andesites 61 62 wt sio 2 eruption of these magmatic variants within the same eruptive phase implies the existence of different petrogenetic lineages and that the plumbing system is sufficiently complex to simultaneously isolate and preserve numerous magma batches it is unlikely that any of the andesites or dacites are derived by fractional crystallization of the recognized basaltic variants formation of the andesites simply by contamination or assimilation fractional crystallization of basaltic magma is also improbable more plausibly the andesites represent mixing between basaltic and dacitic end member magmas each of which may be somewhat heterogeneous or vary in composition with time in this model efficient mixing must occur in some parts of the magma plumbing system while some conduits or storage reservoirs must be effectively isolated

comparative study on micro pore structure of marine terrestrial and transitional shales in key areas china abstract the purpose of this article is to make a comparative study on the pore structure of different sedimentary facies shales developed in typical areas china results show that the surface area sa and pore volume pv of the over mature marine shales are in the range of 17 83 29 49 m 2 g and 3 65 18 26 ml kg respectively which are much higher than that of the highly mature transitional and less mature terrestrial shales in the range of 0 9 1 9 m 2 g and 2 5 10 9 ml kg respectively in the view of the pore size distribution psd marine shales concentrate at the fine mesopores

is the colonsay west islay block of sw scotland an allochthonous terrane evidence from dalradian tillite clasts in the scottish caledonides context the colonsay west islay terrane has seemed anomalous and therefore likely to be allochthonous new studies suggest however that its basement is not necessarily exotic but represents the missing early proterozoic link between greenland and scandinavia recent interpretations of the scottish mainland deformation history imply that the age of the grampian events there resembles that of the late proterozoic deformation previously identified in the colonsay group cover succession reconnaisance studies of dalradian tillite provenance suggest that the colonsay west islay basement was a source of dalradian sediment and perhaps floored that basin there is therefore less reason now to regard the colonsay west islay terrane as allochthonous and recent hypotheses of its tectonic emplacement being the cause of the grampian orogeny should be viewed with caution

geological and geochemical characteristics of marine continental transitional shale from the lower permian taiyuan formation taikang uplift southern north china basin abstract in recent years extensive studies have been conducted on conventional shale gas resources but little work has been performed on the resource potential of marine continental shales this paper takes the lower permian taiyuan shale in the taikang uplift of the southern north china basin as an example to characterize a transitional shale reservoir the results indicate that the taiyuan shale has high toc values averaging 2 67 and contains type iii gas prone organic matter the shale has high ro values averaging 3 44 and tmax ranges from 424 c to 601 c with respect to fracability or brittleness the content of brittle minerals in the taiyuan shale ranges from 21 to 71 averaging 45 55 and the average content of clay minerals is 44 1 based on sem and sem with ar ion milling observation interparticle pores intraparticle pores and microfractures are developed in the taiyuan shale which can provide reservoir space for the accumulation of shale gas organic matter pores rarely appear due to the organic type and high maturity the results of this study can guide the shale gas exploration of taiyuan formation in the taikang uplift of the southern north china basin

short paper the depositional age of the dalradian supergroup u pb and sm nd isotopic studies of the tayvallich volcanics scotland zircons from a keratophyre associated with the tayvallich volcanics in the dalradian rocks of the sw scottish highlands have been dated by u pb methods yielding an age of 595 4 ma this age indicates that most or all of the dalradian is precambrian in age and that dalradian sedimentation may have lasted for about 200 million years the age also constrains the time interval between cessation of dalradian sedimentation and subsequent crustal thickening and regional metamorphism during the grampian orogeny sm nd isotopic data for the tayvallich volcanins and related metadolerite sills yield initial ond values of 2 to 4 which are thought to reffect the effects of melting of lithospheric mantle

the caledonian strike swing and associated lineaments in nw ireland and adjacent areas sedimentation deformation and igneous intrusion patterns part of the major swing in strike associated with the greenland labrador promontory of the north atlantic caledonides is exposed in considerable detail in the metasedimentary and meta igneous rocks of the neo proterozoic dalradian supergroup in northwest ireland we show that the strike swing controlled dalradian stratigraphy and sedimentation patterns and is therefore at least c 600 ma in age our interpretation of the stratigraphy allows a reconstruction of this promontory in the laurentian continental margin and shows that it separated a ne trending gently inclined and broad continental shelf in the north from an e trending more steeply inclined and narrower shelf to the south once established in the geometry of the dalradian sequence the strike swing and basement promontory had a major influence on caledonian deformation patterns it controlled the nucleation of major folds the geometry of the regionally developed primary cleavages the facing direction of the early folds and it also acted as a major buttress in perturbing and deflecting on a regional scale the orogenic transport vector accurate restoration of the sinistral displacement on a late caledonian fault system shows that the apices of the strike swing at different stratigraphic levels and their associated sedimentary facies changes define a line trending approximately n12oe this line is coincident with again after fault restoration six out of the eight devonian donegal granites the vast majority of the mantle related ultrabasic appinite suite with most of these bodies concentrated at the intersection of this line and the ne trending main donegal granite shear zone and the highest density of the neo proterozoic basic tholeiite suite thes e and other data are used to suggest that the donegal lineament is the expression of a major steeply inclined fault in the basement beneath the dalradian which is intimately related to the morphology of the promontory the fault which might be as old as 1800 ma probably reached down into the lithospheric mantle controlling igneous activity including the ascent emplacement site and perhaps sources of the magmas another complementary strike swing and parallel lineament is described from the dalradian rocks of the adjacent part of scotland it is suggested that these and other major lineaments i e old pre caledonian faults which have either nne ne or ese trends reflect not so much the location of the northern british isles at a 120o triple junction during iapetus rifting but rather the exploitation of these pre existing trends in the pre caledonian atlantic borderlands during iapetus opening

combined sm nd and rb sr isotope systematics in the donegal granitoids and their petrogenetic implications the metaluminous to peraluminous granitoids of the donegal batholith northwest ireland were emplaced at c 400 ma into greenschist amphibolite facies metasediments of the dalradian supergroup sm nd and rb sr isotopic data are provided for eleven granitoid samples from six of the plutons and one specimen from the northeast granodiorite pluton of the newry complex co down the donegal results reveal essentially similar initial sr isotope ratios 0 7051 0 7068 but highly variable initial e nd values 1 2 to 8 3 and 0 5 for newry certain granitoids have distinctive nd isotopic compositions characteristic of the involvement of old lree enriched continental crust in some cases or young crust and or mantle derived magmas in others the nd and sr isotopic variations can be explained by a variety of mixing hypotheses

petrologic diversity in mount st helens dacites during the last 4 000 years implications for magma mixing mount st helens has explosively erupted dacitic magma discontinuously over the last 40 000 years and detailed stratigraphic data are available for the past 4 000 years during this last time period the major element composition of the dacites has ranged from mafic 62 64 wt sio2 to felsic 65 67 wt sio2 temperature has varied by about 150 c 770 920 c and crystallinity has ranged between 20 and 55 water content of these dacites has also fluctuated greatly although the source for the dacitic magmas is probably partial melting of lower crustal rocks there is strong physical evidence such as banded pumices thermal heterogeneities in single pumices phenocryst disequilibrium contrasts between compositions of glass inclusions and host matrix glass and amphibole reaction rims that suggests that magma mixing has been prominent in the dacitic reservoir indeed we suggest that the variations in major and trace element abundances in mount st helens dacites indicate that magma mixing between felsic dacite and mafic magma has controlled the petrologic diversity of the dacitic magmas magma mixing has also controlled the composition of andesites erupted at mount st helens and thus it appears that the continuum of magmatic composition erupted at the volcano is controlled by mixing between felsic dacite or possibly rhyodacite and basalt the flux of the felsic endmember to the reservior appears to have been relatively constant whereas the flux of basalt may have increased in the past 4 000 years as suggested by the apparently increased abundance of mafic dacite and andesite erupted in this period

the isotopic and chemical evolution of mount st helens isotopic and major and trace element analysis of nine samples of eruptive products spanning the history of the mt st helens volcano suggest three different episodes 1 40 000 2500 years ago eruptions of dacite with end 5 esr 10 variable 18o 206pb 204pb 18 76 ca sr 60 rb ba 0 1 la yb 18 2 2500 1000 years ago eruptions of basalt andesite and dacite with end 4 to 8 esr 7 to 22 variable 18o thought to represent melting of differing mantle crust reservoirs 206pb 204pb 18 81 18 87 variable ca sr rb ba la yb and high zr 3 1000 years ago to present day eruptions of andesite and dacite with end 6 esr 13 18o 6 variable206pb 204pb ca sr 77 rb ba 0 1 la yb 11 none of the products exhibit eu anomalies and all are lree enriched there is a strong correlation between87sr 86sr and differentiation indices these data are interpreted in terms of a mantle heat source melting young crust bearing zircon and garnet but not feldspar followed by intrusion of this crustal reservoir by mantle derived magma which caused further crustal melting and contaminated the crustal magma system with mafic components since 1000 years ago all the eruptions have been from the same reservoir which has displayed a much more gradual re equilibration of pb isotopic compositions than other components suggesting that pb is being transported via a fluid phase the nd and sr isotopic compositions lie along the mantle array and suggest that the mantle underneath mt st helens is not as depleted as morb sources there is no indication of seawater involvement in the source region

re examination of crystal ages in recent mount st helens lavas implications for magma reservoir processes u series data for recent mount st helens lavas suggest that crystallization preceded eruption by more than 0 5 ka but are complicated by possible evidence of crystal recycling and or addition of radium to the liquid after crystallization we report new ion and electron microprobe trace and major element data for plagioclase and pyroxene in these recent mount st helens lavas and use these data to reassess 226ra 230th crystal ages by taking into account differences in the partitioning behavior of radium and barium and the effects of impurities in mineral separates revised 226ra 230th model crystallization ages are 2 4 ka for plagioclase with the exception of the 1982 dacite and 0 15 5 7 ka for pyroxene in contrast to previous interpretations no late stage addition of ra to the liquid after precipitation of the minerals is required the variability of ba concentrations measured in plagioclase is too large to be consistent with progressive crystallization from the same liquid or with diffusive re equilibration of xenocrysts with a new host liquid ba heterogeneity limits the residence time of the crystals in a magma at high temperatures and also suggests that in most cases ra th ages have not been significantly modified by ra diffusion into or out of the crystals high 226ra ba in plagioclase in the 1982 dacite relative to the host liquid likely reflects crystallization processes that precluded bulk crystal liquid chemical equilibrium one possibility is that of growth entrapment of surface enrichments during rapid crystallization which could lead to less discrimination between ra and ba than predicted by calculated bulk partition coefficients 226ra 230th crystal ages for the castle creek andesite and basalt that are younger than 230th 238u ages of the same crystals could be explained by mixing of crystals into melts with different 230th 232th ratios by combinations of older and younger crystal growth within the same magma or for the basalt by diffusion of ra into crystals at high magmatic temperatures average plagioclase ages in most of the samples of 2 ka imply that some significant mass fraction of the crystals in each flow was present simultaneously beneath the volcano this observation could be consistent either with simultaneous storage of physically distinct magma batches or with incorporation of a population of similarly aged crystals into each successive magma batch

geological and geochemical characteristics of marine continental transitional shale from the upper permian longtan formation northwestern guizhou china abstract organic rich shale deposited in a marine continental transitional facies was well developed in the upper yangtze area during the late permian it is considered to contain a large amount of shale hydrocarbon resources this paper takes the upper permian longtan shale in the central guizhou uplift as an example to characterize the transitional shale reservoir based on organic geochemistry data x ray diffraction xrd analysis and scanning electron microscopy sem observation the geochemical characteristics of this transitional shale were systematically studied and the shale gas potential was investigated the results indicate that the longtan shale has fair to good hydrocarbon potential with high total organic carbon toc and contains type iii gas prone organic matter its high thermal maturity favors the generation of gas ro 1 10 2 74 xrd analyses show that the mineral components of the transitional shale are dominated by high clay content 58 7 wt the clay minerals are dominated by illite 44 and mixed illite smectite 43 sem observation shows that intra particle pores inter particle pores and microfracture were well developed in the longtan shale which can provide reservoir space for the accumulation of shale gas organic matter pores appear rare due to its organic type in summary the longtan shale has high organic content high maturation and high gas content all of which demonstrate its great potential as a shale gas resource but the high clay content of the longtan shale may be a challenge to shale gas production

lead isotopic data from sulfide minerals from the cascade range oregon and washington abstract lead isotopic studies of mineral deposits associated with tertiary plutons found in the cascade range of oregon and washington demonstrate a rather uniform isotopic composition in various sulfide minerals 206 pb 204 pb 18 84 to 19 05 207 pb 204 pb 15 57 to 15 62 208 pb 204 pb 38 49 to 38 74 show less variation than data from the volcanic rocks of the cascade range and fall within the mixing array defined by the morb regression line and continental sediments an evaluation of the role of crustal assimilation by hydrothermal convection during emplacement was made on five sulfide deposits associated with a single composite batholith the cloudy pass pluton the pb isotopic data and mass balance calculations suggest that only minor amounts of the lead were derived from the overlying precambrian swakane biotite gneiss during emplacement the bulk of the metal that occurs in sulfide deposits in the cascade mineral belt appears to have been derived from subducted continental detritus the variation of the pb isotopic signature of sulfides from specific districts or deposits suggests that there is a correlation with age and structure of the crust 206 pb 204 pb is greater than 18 92 in northern washington and southern oregon where deposits have intruded mesozoic or older crust however the ore deposits between the northern oregon border and central oregon south of eugene have intruded younger crust composed largely of mafic and andesitic volcanic rocks and 206 pb 204 pb lies between 18 84 and 18 92 this region previously called the columbia embayment appears to be underlain by tertiary volcanic rocks lead isotopic data may be used to define the boundaries between discontinuous blocks of mesozoic crust and tertiary volcanic cover

granites and early proterozoic crustal evolution in sweden evidence from smnd upb and o isotope systematics abstract u pb zircon and whole rock rb sr sm nd and o isotope results are reported for granitoids from three proterozoic provinces in sweden that represent different crustal environments the radiometric ages span the period 1 89 1 65 ga and record events ranging from original igneous crystallisation to late metamorphic resetting these events occurred at different times in the crustal segments the o isotope ratios identify different crustal source materials for the granitoids the sm nd results indicate that the major crustal source components common to most of the granitoids formed shortly before 1 9 ga close in time to the formation of the earliest granitoids contributions from recycled archaean basement are minimal the geographically widespread occurrence of such crustal sources indicates that the period around 2 ga was of great importance for the growth of new continental crust in sweden

investigation of pore structure and fractal characteristics of organic rich shale reservoirs a case study of lower cambrian qiongzhusi formation in malong block of eastern yunnan province south china abstract in order to study pore structure and fractal characteristics of the organic rich marine shale fourteen shale samples from lower cambrian qiongzhusi formation in malong block of eastern yunnan province were investigated by organic geochemical analysis total organic carbon content analysis and thermal maturity analysis x ray diffraction xrd analysis porosity and permeability tests field emission scanning electron microscopy fe sem low pressure nitrogen adsorption and methane adsorption experiments fractal dimensions d 1 and d 2 at relative pressure of 0 0 5 and 0 5 1 respectively were obtained from the nitrogen adsorption data using the fractal frenkel halsey hill fhh method not only have the relationships among pore structure parameters of shale the relationships between toc content mineral compositions pore structure parameters and fractal dimensions been discussed but also the significance of two fractal dimensions d 1 and d 2 and the impact of fractal dimensions on adsorption capacity have been investigated the results showed that fourteen shale samples have toc content ranging from 1 25 to 7 72 two fractal dimensions both increase with the increasing toc content and gradually come to a standstill the curves present the shape of parabola the major mineralogical compositions of shales are quartz and clay minerals the quartz contents are between 25 5 and 42 7 the clay contents are between 26 6 and 44 2 fractal dimension d 1 has a negative correlation with quartz contents and a positive correlation with clay minerals contents but fractal dimension d 2 has no apparent relationship with quartz and clay minerals contents the specific surface area is in the range of 4 98 m 2 g 19 66 m 2 g the total pore volume is between 0 00479 cm 3 g and 0 01765 cm 3 g and the average pore diameter is between 3 37 nm and 6 02 nm two fractal dimensions increase with the increasing surface area and pore volume and also increase with the decreasing average pore diameter because of the complicated pore surface and structure of small pores further investigation indicates that d 1 represents fractal characteristics from the irregular pore surface while d 2 represents fractal characteristics related to the complicated pore structure and shale samples with larger fractal dimensions have higher methane adsorption capacity therefore fractal analysis is helpful to have a better understanding of pore structure and adsorption capacity of marine shale

the genesis of arc dacites the case of mount st helens wa throughout the last 35 ka mount st helens has been the most active volcano in the cascade arc but the origin of its voluminous dacites remains controversial these dacites were traditionally interpreted as a result of melting metabasaltic lower crust yet recent studies have challenged this view and suggested an origin dominated by differentiation of mafic magmas through assimilation fractional crystallization afc processes to address this discrepancy on the origin of dacites at mount st helens we conduct an interdisciplinary study using a combination of thermal and geochemical modeling our results show that 45 crystallization of a basaltic andesite parent reproduces the compositions of the dacites with a maximum of 20 30 assimilation of lower crustal lithologies amphibole textures and compositions support such a differentiation trend in a polybaric mush system combined with recent geophysical imaging and experimental data we suggest that mount st helens dacites are generated by 1 mantle derived arc magma evolving by afc to intermediate compositions in a lower crustal magma reservoir and 2 ascent of these magmas to a mid to upper crustal reservoir where they reach high crystallinity without significant further chemical differentiation and are subject to frequent recharge that leave a clear mixing mingling overprint

depositional environment and thermal maturity of the coal bearing longtan shale in southwest guizhou china implications for shale gas resource potential abstract significant knowledge has been assembled on the marine longmaxi shale in the sichuan basin over the past ten years however the distribution of shale gas resources on the periphery of the sichuan basin and in other source rock formations are still not sufficiently understood to improve our understanding on the depositional environment and thermal maturity and their relevance to the source rock quality and shale gas potential of the thick coal bearing longtan shale 36 organic rich shale samples were selected from a single borehole located in the southwestern guizhou province for geochemical and petrographic analysis total organic carbon toc contents are high with 7 7 wt on average reaching partly up to 28 21 wt while carbonate contents are low vitrinite reflectance vrr values and rock eval tmax temperatures range from 1 48 to 1 84 and 482 to 583 c respectively indicating a high thermal maturity which is in accordance with aromatic molecular parameters the remaining present day hydrocarbon generation potential rock eval s2 is rather low due to the high maturity microscopic investigations further show that the organic matter in the rocks is mainly composed of higher land plant particles present as large and poorly rounded vitrinite and inertinite particles indicating a rather short transport distance toc vs total sulfur ts relationships as well as specific hydrocarbon ratios e g pristane phytane and framboidal pyrite size distributions indicate a variable either marine or freshwater environment for the shales in the coal bearing sequence and suggest that original kerogen was either type iii or type ii iii bottom water during deposition was probably oxygen depleted but not anoxic high sedimentation rates can be assumed for the now 700 m thick compacted sequence favorable for the preservation of organic matter overall the present results are in accordance with previous studies of the longtan shale which initially had a high hydrocarbon mainly gas generation potential furthermore the thickness of the formation is much larger in the here studied southwestern part of the guizhou province than in the previously studied northern parts

cummingtonite and the evolution of the mount st helens washington magma system an experimental study hydrothermal experiments have been conducted on the 3 5 ka cummingtonite bearing yn tephra from mount st helens washington to determine the stability field of cummingtonite in volcanic rocks and to gather additional data on the evolutionary history of the mount st helens magmatic system cummingtonite is not stable in experiments conducted at or above 810 c at pressures 2 o analyses of melt inclusions that yield preeruption melt h 2 o contents of 6 4 wt the yn tephra belongs to a series of cummingtonite bearing dacites that erupted early in the history of mount st helens these early dacites have relatively homogeneous compositions and are characterized by low temperature 800 c high water pressure and high oxygen fugacity nno 1 phenocryst crystallization silicic magmas that erupted after eruption of basaltic andesite at mount st helens 2 3 ka are more heterogeneous in composition and last equilibrated under higher temperatures 900 c lower water pressures and lower oxygen fugacities than the early dacites these changes in the magma chamber conditions appear to have been related to the intrusion of mafic magma into an upper crustal dacitic magma chamber

the highland border complex scotland a paradox resolved a previously inexplicable difference between the highland border complex scotland and its correlative in ireland the clew bay complex is that rocks of caradoc ashgill age occur only in the former we reject evidence from supposed chitinozoa for this dichotomy no sedimentary rocks of proven age younger than arenig occur in the highland border complex consequentially the stratigraphy must be totally recast and the exotic terrane model replaced by one in which a largely autochthonous highland border complex in stratigraphical continuity with the dalradian grampian terrane was overridden by the highland border ophiolite midland valley terrane tectonic models for se laurentia are thereby considerably simplified

isotopic dating in metamorphic belts isotopic dating in metamorphic rocks is concerned with the timing of processes whose operation is controlled by continually changing physical conditions of these temperature has a pervasive influence and the use of physical models to elucidate the ways in which temperature is likely to vary is briefly reviewed the central concept of closure temperature is critically discussed and quantitative estimates for biotite and hornblende are evaluated in order to illustrate the factors which control closure temperatures and limit their application in practice the possibility of dating crystallization using minerals which crystallize below their respective closure temperatures is examined taking examples involving k ar rb sr sm nd and u pb analyses the indirect dating of metamorphism via structurally dated plutons is briefly considered and the problems of interpreting both discordant u pb zircon ages and rb sr whole rock isochrons in metamorphosed granites are discussed the value of near concordant zircon ages is emphasized the extent of isotopic equilibration during metamorphism specifically of sr isotopes in metasediments is shown not to be a simple function of metamorphic grade the flow of fluids produced by metamorphic reactions appears to be crucial and there is a potential for dating metamorphism using whole rock samples where such flow can he demonstrated independently the concluding discussion briefly examines existing data for the dalradian and indicates the potential for further progress using both well established and novel approaches

textural and chemical variation in plagioclase phenocrysts from the 1980 eruptions of mount st helens usa this study presents major and trace element chemistry of plagioclase phenocrysts from the 1980 eruptions of mount st helens volcano despite the considerable variation in textures and composition of plagioclase phenocrysts distinct segments have been cross correlated between crystals the variation of sr and ba concentration in the melt as calculated from the concentration in the phenocrysts using partition coefficients suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector in both of these melts sr and ba are correlated despite the abundance of plagioclase in the 1980 dacites we propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower sr and higher ba as documented in the rims of the phenocrysts although the cores of the phenocrysts crystallised at relatively shallow depths they preserve the sr and ba of the deep seated melts as they ascended from a deeper region further magma ascent resulted in microlite nucleation which is responsible for a similar shift to even lower sr concentration as observed in the groundmass of post 18 may 1980 samples

high silica rhyolites of niijima volcano in the northern izu bonin arc japan petrological and geochemical constraints on magma generation and supply abstract niijima volcano in the northern part of the quaternary izu bonin volcanic arc japan consists predominantly of rhyolitic volcanic units associated with minor basaltic and andesitic units most of the rhyolites are characterized by a phenocryst assemblage of orthopyroxene opx cummingtonite cum biotite bt and hornblende which are classified into four types opx cum cum cum bt and bt in eruption order the major element compositions of the rhyolites are typified by high sio2 73 78 wt and k2o 1 5 3 3 wt contents and the trace elements are indicative of a highly differentiated nature which differs significantly from rhyolites at other sites in the izu bonin arc the presence of cummingtonite phenocrysts and mineralogical and whole rock chemical compositions of the rhyolite lavas clearly indicate that these magmas were produced under low temperatures 13 units in a shallow magma system at the same time and at deeper levels basaltic and rhyolitic parental magmas mixed to produce andesitic magmas between the basaltic and rhyolitic parental magmas in some cases basaltic and andesitic magmas in deeper level ascended and mingled with shallow rhyolitic magmas before eruption the eruption of rhyolitic magmas from shallow levels might have been triggered by the injection of basaltic and andesitic magmas from deeper levels the complex magmatic system beneath niijima volcano formed within a relatively thick crustal sequence on the rear arc side of the izu bonin volcanic arc

the glen kyllachy granite and its bearing on the nature of the caledonian orogeny in scotland the tomatin findhorn granite in the nw grampian highlands has been separated into two distinct complexes a late tectonic glen kyllachy granite tectonically foliated and a post tectonic findhorn granite flow foliated for the glen kyllachy complex rb sr analyses of muscovites from the granite and from an associated suite of cross foliated pegmatites yield an emplacement age of 443 5 15 ma whole rock rb sr data support field and textural evidence that the pegmatite and granite emplacement was late tectonic to the last f3 phase of caledonian folding initial granite 87 sr 86 sr of 0 7176 supports field and geochemical evidence of derivation from upper crustal metasediments first metamorphosed during the grenville event for the findhorn granite concordant rb sr and k ar mineral data establish an age of 413 5 ma and an initial 87 sr 86 sr of c 0 706 indicates a lower crustal and or mantle source this age and isotopic contrast between these granites is characteristic of the whole grampian region in which a plutonic hiatus between c 440 and 415 ma coincides with the peak of sedimentary accretion in the southern uplands and may be explained in terms of the lack of hydrous materials passing down the associated subduction zone structural metamorphic and radiometric evidence suggests a that the late f3 glen kyllachy pegmatites are comparable with the 442 7 ma old syn f3 pegmatites in the n highlands both pegmatite suites are situated in the axial zone of the metamorphic caledonides displaced by the great glen fault and b that the caledonian c 454 ma old peak tectonic metamorphic event d2 previously documented in the n highlands coincides with a caradocian basin closure along the highland boundary fault for silurian events in the n highlands we propose a separate subduction history in this region the peak of calc alkaline plutonism was accompanied by continuous folding and westward thrusting at the same time as a plutonic hiatus in the grampian highlands current evidence suggests that the c 160 km or greater sinistral movements on the great glen fault were not initiated until late silurian early devonian time

the genesis of silicic arc magmas in shallow crustal cold zones abstract a number of currently popular models for the genesis of evolved arc magmas from basaltic andesite to dacite invoke repeated intrusion partial melting and differentiation at the base of the crust however several observations suggest that this may be the exception rather than the norm 1 geobarometry often indicates shallow pressure 0 1 0 3 gpa evolution 2 incongruent melting of amphibolite at elevated pressures should yield magmas in equilibrium with high pressure phases like garnet but rare earth element patterns almost ubiquiously preclude this 3 compositionally zoned caldera forming eruptions suggest differentiation at near surface depths 4 u series data most commonly indicate differentiation over millennia time scales this requires rapid cooling that in turn is most easily explained by relatively small magma volumes undergoing crystal fractionation within the shallow i e cool crust to further test these ideas we combined published experimental data for liquidus equilibria with appropriate silicic arc magma compositions on projections of the ternary liquidus system nepheline silica olivine recent data for tongan silicic lavas plot either on or close to low pressure 1 atm cotectics for the rocks phenocryst phases suggesting low pressure differentiation using our own and published data from arc volcanoes around the world we find that the majority are consistent with differentiation at shallow depths regardless of total crustal thickness combined with the typical timescales of differentiation we estimate that the volumes of magma stored during differentiation in shallow crustal zones are usually on the order of only a few km 3 there is also a clear role for mixing and recharge that involves magmas that are more deeply sourced and primitive in character typically evolved basalts and basaltic andesites whether the latter differentiated in the lower crust or at the crust mantle boundary has important implications for the constitution and average composition of arc crust at present we can only conclude that evidence for more silicic arc magma generation at these depths is generally lacking

paleogeography and shale development characteristics of the late permian longtan formation in southeastern sichuan basin china abstract the late permian longtan formation is extensively developed within the southeastern sichuan basin which lies on the western margin of the yangtze plate in southwestern china the stratigraphy and sequence boundaries of the longtan formation were interpreted from a combined study of field data well log data and various data analysis in order to determine the extent and thickness of shale sequences this interpretation defined the broad sequence stratigraphic framework and separated the longtan formation into three 3rd order sequences in addition the formation was divided into five sedimentary facies using sedimentary paleontological geophysical and other data the facies identified were fluvial low energy shore tidal flat melanged accumulated shelf and platform basin three paleogeographic maps were created based on the 3rd order sequences sq1 sq2 and sq3 the study shows that sq1 sq3 sedimentary environment of the study area which extends from the southwest to northeast was developed followed by fluvial shore tide shallow water melanged accumulation shelf deep water melanged accumulation shelf and platform basin the mud shale rock types of the longtan formation that were identified are carbonaceous mud shale silty mudstone and lime mudstone according to the divided standard of mud shale effective thickness there are three mud shale assemblages present in the sequences sq1 sq2 sq3 namely mud shale with sandstone mud shale with limestone and mud shale sq1 has the thickest mud shale identified on well logs mud shale with sandstone was well developed in the southwest of the study area and was interpreted to have been deposited in a shore or tidal flat sedimentary environment this rock assemblage has a large effective thickness and extensive lateral distribution in contrast the assemblage of mud shale with limestone was mainly developed in the northeast of the study area and was interpreted to have formed in a melanged accumulation shelf sedimentary environment the effective thickness of the mud shale with limestone is small and the distribution is limited there is developed mud shale in above two type s sedimentary environment but the distribution is also limited the longtan formation s mud shale reached stage b diagenesis and is principally composed of quartz and clay minerals the average quartz content is 43 40 and the average clay mineral content which is mainly composed of illite mixed illite smectite chlorite and kaolinite is 39 58 the sedimentary environment has controlled the distribution of minerals within the sequences the clay mineral content gradually decreases as the sedimentary environment changes from shore tidal flat in the southwest to deep water melanged accumulation shelf in the northeast therefore this comprehensive analysis shows that the southwest of the study area is the key area for shale gas exploration in the longtan formation

a high precision u pb age for the ben vuirich granite implications for the evolution of the scottish dalradian supergroup high precision high accuracy u pb analyses of zircons from the ben vuirich granite in the scottish dalradian succession were performed on carefully picked high integrity abraded fractions of specific morphologies zircon needles which grew during crystallization give an age of 590 2 ma stubby prisms show inheritance with an upper concordia intercept of 1448 7 ma the age of intrusion at 590 ma contrasts strongly with a previous u pb zircon age of 514 6 7 ma determined on bulk fraction unabraded grains which it is argued must have suffered significant pb loss as the ben vuirich granite was emplaced between the regional d2 and d3 deformation events in the dalradian block the new data imply that all dalradian sedimentation was precambrian supposed lower cambrian fauna1 links between the dalradian succession and laurentia are thus discounted and the possibility that the dalradian supergroup has non laurentian affinities must be considered the early d1 d2 nappe forming grampian structures are also precambrian and may be associated with terrane docking along a tectonically active ocean margin later d3 structures and associated peak barrovian metamorphism suggest a subsequent thickening event possibly c 520 ma ago

time scales of magma formation ascent and storage beneath subduction zone volcanoes there is now sucient information to attempt an integrated model for melt gener ation transfer and storage beneath subduction zone volcanoes fluid release from the subducting oceanic crust into the mantle wedge may occur over a period ranging from a few hundred kyr to as little as less than 1 kyr before eruption this supports models in which uid addition is closely linked to partial melting though there may also be evidence for a component of decompression melting the timing of the onset of uid addition may be linked to the rate of subduction i e water supply rate and the angle of subduction and consequently the thermal structure of the mantle wedge in contrast contributions from subducted sediments to subduction zone lava sources appear to occur some 350 kyr 4 myr before eruption evidence for partial melting of the sediment component combined with the short uid transfer times phenocryst equilibration temperatures and other observations all point to quite high mantle wedge temperatures close to the interface with the subducting plate new 226 ra data permit only a short period of time between uid addition and eruption this requires rapid melt segregation magma ascent by channelled ow and minimal residence time within the lithosphere typically the evolution from basalt to andesite occurs rapidly during ascent or in magma reservoirs inferred from some geophysical data to lie within the lithospheric mantle mineral isochron data suggest that some andesitic magmas subsequently stall in more shallow crustal level magma chambers where they can evolve to dacitic compositions via fractionation typically combined with assimilation on time scales of a few thousand years or less

metamorphism and cooling of the ne dalradian the buchan area of dalradian rocks represents a distinct tectonothermal domain within the scottish highlands and the portsoy duchray hill lineament on the western margin of this block is one of the best examples of a long lived domain boundary rb sr mica cooling ages from metamorphic rocks from across the trace of this lineament document a prolonged ordovician cooling history delayed but then relatively rapid cooling to the west of the portsoy duchray hill lineament is recognized whereas to the east the age data point to a high heat flow regime lasting more than 50 ma the isotopic ages suggest that c 460 ma was a time of active tectonism resulting in final cooling of the low grade rocks and differential uplift and rapid cooling of deeper structural levels to the west the radiometric ages do not support previous tectonic models for the region that have invoked thrust thickening along the line of the portsoy duchray hill lineament and a new model for the metamorphism is proposed the overprinting of andalusite bearing assemblages by kyanite to the west of the portsoy duchray hill lineament is linked to pressure increases associated with the intrusion of the newer gabbros and represents a magmatic loading related to the same tectonothermal event that generated the low pressure metamorphism rather than a separate tectonic episode

aleutian lead isotopic data additional evidence for the evolution of lithospheric plumbing systems abstract lead isotopic ratios and concentrations have been measured in lavas from the aleutian volcanic centers of adak 12 and atka 12 whereas lavas from atka have very small pb isotopic ranges 206 pb 204 pb 18 78 18 86 207 pb 204 pb 15 55 15 62 208 pb 204 pb 38 31 38 52 those from adak have ranges nearly twice as large 206 pb 204 pb 18 34 18 86 207 pb 204 pb 15 47 15 63 208 pb 204 pb 37 81 38 56 on lead lead isotopic diagrams atka data form tight clusters whereas those from adaak define a well developed linear trend extending from the atka field toward less radiogenic values at a given 206 pb 204 pb the lavas from both islands plot in the morb trend for 208 pb 204 pb but atka lavas plot above the morb 207 pb 204 pb 206pb 204pb trend and those from adak fall both within and above the trend lead contents in lavas from atka increase four fold 5 21 ppm over the compositional range of the volcanic suite 49 68 sio2 in contrast actak lavas 49 65 have concentration levels of 0 6 13 ppm and display no simple correlation with sio2 the lead isotopic data alone can be explained by three different processes model 1 assigns lead isotopic differences to original magma source heterogeneity according to model 2 the isotopic ratios of a primary non radiogenic component from the mantle are elevated by the addition of an isotopically enriched slab derived component in contrast model 3 assumes a primary radiogenic magma produced by melting of the slab is contaminated by a non radiogenic lithospheric component during conduit formation because these models all adequately explain the lead isotopic data supplementary geologic petrographic geochemical and isotopic data must be used to select the most likely model careful consideration of the evidence suggests model 3 best explains our extensive lead isotopic data as well as other characteristics of aleutian lavas our study suggests detailed isotopic studies of individual volcanic centers arc or otherwise can be extremely useful in understanding the complex processes of magma generation extraction ascent and evolution

provenance of dust in the pacific ocean eolian dust preserved in deep sea sediment cores provides a valuable indicator of past atmospheric circulation and continental paleoclimate in order to identify the provenance of eolian dust nd and sr isotopic compositions and rb sr and rare earth element ree concentrations have been determined for the silicate fractions of deep sea sediments from the north and central pacific ocean different regions of the pacific ocean are characterized by distinct air borne inputs producing a large range in end 10 to 1 stsr 86sr 0 705 0 721 la yb 5 15 eun eu 0 6 1 0 and sr nd 4 33 the average nd isotopic composition of pacific deep sea sediments eno 6 is more radiogenic than the average from the atlantic ena 8 in contrast the average 1475m 14and ratio for pacific sediments 0 114 is identical to that of atlantic sediments and to that of global average riverine suspended material the values of ena and 147sm 144nd are positively correlated for the pacific samples but negatively correlated for atlantic samples reflecting a fundamental difference between the dominant components in the end members with radiogenic nd island arc components in the pacific and lree enriched intraplate ocean island components in the atlantic samples from the north central pacific have distinctive unradiogenic end values of 10 87sr 86sr 0 715 high la yb 12 and low eun eu 0 6 and sr nd 3 6 these data are virtually identical to the values for loess from asia and endorse the use of these sediments as indicators of asian paleoclimate and paleowind directions island arc contributions appear to dominate in the northwest pacific resulting in higher end 1 to 1 and lower 87sr 86sr 0 705 and la yb 5 sediments from the eastern pacific tend to have intermediate sr and nd isotopic compositions but regionally variable sr nd and ree patterns they appear to be derived from the west margin of the north and south american continents rather than from asia our results confirm that dust provenance can be constrained by isotopic and geochemical analyses which will facilitate reconstructions of past atmospheric circulation and continental paleoclimate

genetic link between emi and emii an adakite connection abstract geochemical modeling of the origin of enriched mantle i emi and enriched mantle ii emii is conducted from the perspective of adakite production for the model the average composition of adakites is re estimated from published data for eighteen trace elements although the concentrations determined for highly incompatible elements are very high about 100 times of primitive mantle these high concentrations can be explained by melting of oceanic crust without sediment contribution the compiled data further suggest that the mantle slab melt reaction would play a major role in the production of basic adakites in addition crystal fractionation in the magma chamber should produce additional chemical variations in adakites in particular for acidic adakites to examine the effect of chemical variations on the isotopic composition of recycled adakites broad correlations between trace elements and sio2 concentrations and the melts program are employed the results suggest that recycling of a basic adakite sio2 55 can account for emi isotopic signatures with storage times of about 2 0 gyr the isotopic compositions of less basic adakites and their evolved magmas shift towards emii values with increasing sio2 concentrations in particular evolved acidic adakite can yield emii isotopic signatures these lines of evidence suggest that the recycling of adakites at various stages of evolution can conceivably produce the entire isotopic range between emi and emii reservoirs consequently adakite recycling via sediment subduction or subduction erosion can account for the origins of emi and emii reservoirs in this context residual garnet under high pressure and plagioclase fractionation at low pressure might play an essential role in producing the chemical variations among adakites that ultimately govern the isotopic compositions of these geochemical reservoirs

the age and tectonic significance of the bute amphibolite highland border complex scotland sm nd and k ar isotopic determinations on an amphibolite from the highland border complex yield cambrian ages collisional events which might have generated the amphibolite appear incompatible with the passive margin character of laurentia at that time a possible explanation is that the part of the highland border complex containing the amphibolite and associated rock units was generated outside laurentia

isotopic and elemental evidence for scabland flood sediments offshore vancouver island abstract geological records contain evidence for catastrophic ice sheet processes such as megafloods and massive ice discharges such large scale phenomena associated with ice sheet collapse rapid sea level rise and disruptions to ocean circulation have never been directly observed but are forecast as a consequence of global warming here we use potassium argon k ar ages and neodymium nd isotopes as provenance tools to show that cyclical megafloods from the cordilleran ice sheet of western north america traveled through the channeled scabland of washington and transported sediments to the continental slope of vancouver island during the last glacial laying down a sequence of rhythmically bedded sediments in deep sea core md02 2496 this work addresses longstanding questions about the absolute timing of scabland floods their cyclicity and the fate of their sediments and freshwater in the marine realm our data suggest that between 19 3 and 14 9 ka scabland floods repeatedly generated far traveled sediment water plumes in the ne pacific ocean requiring a hydrologically active ice sheet system capable of producing catastrophic megafloods for about 4500 years

sedimentary and diagenetic controls on the multiscale fracturing pattern of a carbonate reservoir the madison formation sheep mountain wyoming usa a multidisciplinary approach has been applied to characterize the madison formation in the sheep mountain anticline at different scales in order to illustrate the relationship between sedimentology diagenesis and fracturing pattern sedimentary facies are organized into three types of elementary facies sequences showing different vertical evolutions of their petrophysical properties this is controlled by the double influence of the initial sedimentary facies and the diagenetic evolution three main sets of fractures related to the sevier and laramide compressive phases and to the folding of sheep mountain have been described at three orders of scale at small scale the sevier related set i and folding related set iii are the most abundant set ii dated from the early stage of the laramide compression exhibits the highest fracture intensity there is also a clear faciologic control on the fracture intensity small scale mechanical units correspond to entire facies sequences when no or little petrophysical property contrasts between beds are observed conversely small scale mechanical units correspond to each bed of a facies sequence in the case of high contrasts of petrophysical properties between beds at intermediate scale set ii is not present and sets i and iii show the highest fracture intensities the latter are partly controlled by the mechanical unit thickness intermediate and large scale mechanical unit distribution is mainly controlled by the overall faciologic vertical stacking pattern and by the position across the sheep mountain anticline forelimb or backlimb sheep mountain outcrops provide a good spatial representation of the carbonate reservoir heterogeneities from the micro to the field scale illustrating the complexity of fractured carbonate reservoirs

dynamic palaeogeographic reconstructions of the wuchiapingian stage lopingian late permian for the south china block abstract as an important tool in natural resource exploration palaeogeography provides a portrayal of deep time earth the south china block located in the eastern palaeotethys includes multiple natural resources e g coal shale gas bauxite etc in the permian which are well known for their important economic significance in this paper data gathered from literature amount to 431 stratigraphic sections of wuchiapingian age from south china these data were quantitatively analyzed and dynamically reconstructed in the global context under the revised plate motion model by gplates software for the first time a schematic 3 d model was constructed based on the high resolution reconstructions in this paper showing two relatively large highlands developed in the south china block during the wuchiapingian including the kangdian highland in the west and the cathaysia highland in the east and a small yunkai highland in the south a broad platform dominated depositional system with small uplifts and depressions was developed between these two highlands these reconstructions show great potential to explore enriched coal seams and shale gas in the peripheral areas of the two ancient highlands in the west and east of the south china block based on the dynamic evolution of this depositional system south china experienced an overall sea level rise with frequent secondary transgressions and regressions during the wuchiapingian this is in contrast to the widespread continental and evaporite deposits on much of pangea and its surrounding margins after the end guadalupian global regression compilation of conodont biostratigraphic data suggest that the temporal and spatial constraints on the unconformity across the guadalupian lopingian boundary glb resulted from dongwu uplift movement the regression began during the jinogondolella shannoni zone and ended in the clarkina leveni zone but the response time varies in different localities of south china these results provide detailed information about sea level eustacy tectonic uplift sedimentary hiatus resource accumulation and palaeoclimate change from the south china block during the wuchiapingian

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

marine hydrocarbon source rocks of the upper permian longtan formation and their contribution to gas accumulation in the northeastern sichuan basin southwest china abstract identification of the main hydrocarbon source rocks of the large puguang gas field northeastern sichuan basin southwest china has been the subject of much discussion in recent years a key aspect has been the lack of a comprehensive understanding of the development of hydrocarbon source rocks of the upper permian longtan formation which had been thought to contain mainly coal seams and thick carbonate layers in this paper based on geological data from more than ten wells and outcrops and their related mineralogy and geochemistry we investigated the depositional environment and main factors controlling organic matter enrichment in the longtan formation we propose a model which combines information on the geological environment and biological changes over time in the model organic matter from prolific phytoplankton blooms was deposited in quiescent platform interior sags with rising sea levels during the longtan period the area from bazhong to dazhou was a platform interior sag with relatively deep water and a closed environment which was controlled by multiple factors including syngenetic fault settling isolation of submarine uplifts and rising sea levels leading to water column stratification although the bottom water was anoxic the phytoplankton were able to bloom in the well lit upper euphotic zone thus giving rise to a set of sapropelic black shales and marlstones containing mostly algal organic matter with minor terrestrial contributions as a consequence these rocks have a high hydrocarbon generation potentials and can be classified as high quality source rocks the area from bazhong to dazhou is a center of hydrocarbon generation being the main source of reservoired paleo oils and presently discovered as pyrobitumen in the puguang gas field the identification of these source rocks is very important to guide future petroleum exploration in the northeastern sichuan basin

coupled sm nd and u pb systematics in late caledonian granites and the basement under northern britain the british late caledonian granites display a close correspondence between inherited u pb systematics in zircons and initial end indicating that both were controlled by the age of a major component of continental crust the data confirm and refine previous models of a basement comprising young crust in the south which includes arc and oceanic crust scrapings and progressively older continental crust northwestwards across scotland

soil gas co 2 ch 4 and h 2 distribution in and around las canadas caldera tenerife canary islands spain abstract diffuse degassing of co 2 ch 4 and h 2 was investigated at the surface environment of canadas caldera canary islands during the gas survey carried out in the summer of 1995 soil co 2 concentration varied significantly from atmospheric levels to 30 while soil ch 4 and h 2 contents ranged from 5 to 851 ppm and from 0 5 to 620 ppm respectively soil co 2 ch 4 and h 2 distribution suggests that high diffuse degassing at canadas caldera is volcanic structurally controlled anomalous soil h 2 concentrations were identified at the summit of teide and outside caldera boundaries where the most recent eruption of tenerife island occurred 13 c co 2 data showed a magmatic mixed magmatic biogenic and biogenic origin while a biogenic origin is suggested for soil ch 4 at canadas caldera and its surroundings by coupling the co 2 3 he ratio with the 3 he 4 he ratio of fumarolic gas samples from the summit of teide we propose three possible sources for carbon morb type organic carbon and carbonate

a neoarchean subduction recorded by the eastern hebei precambrian basement north china craton geochemical fingerprints from metavolcanic rocks of the saheqiao shangying qinglong supracrustal belt abstract the saheqiao shangying qinglong supracrustal belt ssqb in the northern eastern hebei precambrian basement ehpb is located in the northern margin of the eastern block eb of the north china craton ncc the shangying terrane constitutes the middle segment of the ssqb and contains primarily metamorphic volcanics and plutonic tonalitic gneisses the metamorphic volcanics mainly consist of pyroxene plagioclase amphibolites garnet plagioclase amphibolites biotite plagioclase amphibole gneisses and amphibole plagioclase gneisses zircon u pb lu hf isotopic analyses reveal that the metavolcanic rocks from the shangying terrane crystallized at 2506 2613 ma with t dm hf values of 2541 2944 ma these metamorphic volcanic rocks are subdivided into four groups based on their lithological and chemical features group i consists chiefly of tholeiites that are characterized by slightly light rare earth element lree depleted patterns and flat multi element spider diagrams which are similar to back arc basin basalt babb like rocks and were derived from the partial melting of the depleted mantle the tholeiites in group ii have slightly fractionated rare earth element ree patterns without nb anomalies exhibit an affinity to nb enriched basalt neb like rocks and were produced by the partial melting of hfse enriched mantle peridotites group iii is composed of slightly lree enriched tholeiites with negative nb ta anomalies that resemble island arc tholeiites group iv comprises calc alkaline basalts and andesites with highly enriched lrees and evident nb ta and ti depletions that are geochemically similar to the products of island arcs the island arc tholeiites and calc alkaline basalt andesites originated from the partial melting of sub arc mantle peridotites that were previously metasomatized by slab derived fluids melts with the fractional crystallization of ferromagnesian minerals collectively the babb like rocks the nebs arc tholeiites and calc alkaline basalt andesites recorded magmatism from a subduction regime similarly neb arc tholeiite andesite dacites and back arc related rocks were recognized in the saheqiao and qinglong areas in the western and eastern parts of the ssqb r guo et al 2013 2015 which further supports the existence of a neoarchean convergent margin in the eastern hebei precambrian basement ehpb compared to the northwestern ehpb the southeastern ehpb underwent lower metamorphism contains arc dacites rhyolites and greater contributions of the ancient continental crust implying that the southeastern ehpb might have been or near a back arc setting thus we conclude that the ssqb experienced a southeastward dipping subduction which is consistent with studies of the adjacent regions e g western liaoning province when combining the 2 5 2 6 ga supracrustal rocks and plutonic granitoid gneisses with depleted mantle origins along the northern margin of the eb of the ncc we concluded that 2 5 2 6 ga was a significant period for juvenile crustal growth and that lateral accretion played a dominant role in crustal growth

relationship of diagenetic chlorite rims to depositional facies in lower cretaceous reservoir sandstones of the scotian basin the relationship between diagenetic chlorite rims and depositional facies in deltaic strata of the lower cretaceous missisauga formation was investigated using a combination of electron microprobe bulk geochemistry and x ray diffraction data the succession studied comprises several stacked parasequences the delta progradational facies association includes i fluvial or distributary channel sandstones some with tidal influence ii thick bedded delta front graded beds of sandstone interpreted as resulting from fluvial hyperpycnal flow during floods and storms and iii more distal muddier delta front and prodeltaic facies the transgressive facies association includes lag conglomerate siderite cemented muddy sandstone and mudstone and bioclastic sandy limestone chlorite rims are absent in the fluvial facies and best developed in thick sandstones lacking mudstone baffles good quality chlorite rims are well correlated with ti in bulk geochemistry ti is a proxy for fe availability principally from the breakdown of abundant detrital ilmenite fetio3 under conditions of sea floor diagenesis the abrupt decrease in sedimentation rate at transgressive surfaces caused progressive shallowing of the sulphate depletion level and of the overlying eh controlled diagenetic zones resulting in conditions suitable for diagenetic formation of berthierine to migrate upwards through the packet of reservoir sandstones this early diagenetic berthierine suppressed silica cementation and later recrystallized to chlorite thick euhedral outer chlorite rims were precipitated from formation water in sandstone lacking muddy baffles on this chlorite substrate and inhibited late carbonate cementation this study thus shows that the preservation of porosity by chlorite rims is a two stage process rapidly deposited delta front turbidite facies create early diagenetic conditions that eventually lead to the formation of chlorite rims but the best quality chlorite rims are restricted to sandstones with high permeability during burial diagenesis

the western border of the archaean province of the baltic shield evidence from northern sweden restricted areas of acid to intermediate gneisses in northernmost sweden are known to be archaean and several other gneissic granodioritic to tonalitic massifs have been suggested to be of this age to delimit the western border of the archaean province of the baltic shield and to characterize the swedish archaean we have studied the 2 83 ga old soppero gneiss and 8 of the proposed archaean massifs in northern sweden by sm nd isotopic analyses u pb zircon dating in two cases and geochemistry

textural evolution of the rapakivi granites south greenland sr o and h isotopic investigations the development of rapakivi texture in feldspars from the ketilidian granitoids of south greenland has been investigated using sr o and h isotopes a low temperature signature is found in the sr and o data which seemingly contradicts some textural features that point to a magmatic origin of the plagioclase mantles around the k feldspar ovoids an origin for these mantles involving exsolution from an original alkali feldspar solid solution is proposed which involves growth of mantles over a range of conditions determined by the mobility of the exsolving sodic feldspar this mobility may be enhanced at high temperatures in the presence of melts or increased fluid pressures and at lower temperatures by the processes responsible for the transformation of k feldspar to microcline rapakivi granites with both white and dark green feldspar occur in south greenland but show no major isotopic differences although the dark alkali feldspars contain significantly more fluid equivalent fluids in the white alkali feldspars may have escaped during plagioclase exsolution

uplift patterns and orogenic evolution in the scottish dalradian rb sr and k ar mineral ages from the dalradian rocks of angus and perthshire indicate considerable diachronism of cooling following the caledonian regional metamorphism the oldest mineral ages 515 ma are similar to the independent estimates of peak metamorphic age and hence indicate very rapid cooling they occur in the highest grade samples of the angus region and are associated with localized uplift regional rapid uplift events at 460 440 ma and 410 390 ma may also be recognized and correlated with structural and igneous activity and metamorphic age dates within the scottish highlands thermal models show the importance of localized and discontinuous uplift in determining both the timing of metamorphism and the shape of the p t paths they suggest that the major diachronous metamorphism and granite production in the scottish highlands is depth controlled rather than representing major lateral diachronism of thickening or heat sources localized uplift may exercise considerable control on mineral facies variation within orogenic belts the local uplift in angus introduces hot high pressure tarfside nappe rocks into higher level tay nappe rocks thus the uniform block uplift models frequently used in the interpretation of metamorphic and structural phenomena in orogenic belts are an oversimplification of the real events

jorn an early proterozoic intrusive complex in a volcanic arc environment north sweden abstract the jorn complex is an early svecokarelian granitoid suite intruded into a volcanic arc environment on the southern margin of a major proterozoic continental area both the complex and its country rocks have been affected by hydrothermal alteration and low grade regional metamorphism four phases can be distinguished on geological chemical and geophysical criteria in particular a heterogeneous calcic outer zone and a more evolved central diapir the rock types vary from granodiorite to granite and have major and trace element abundances similar to those phanerozoic subduction related granitoids from relatively immature volcanic arcs zircon populations from the different phases are rather heterogeneous but do not show significant differences in u distribution or u pb age between the phases while pooled u pb data 13 points give an upper intercept of 1890 7 8 ma a two stage model assuming an age difference between cores and rims gives a more probable crystallisation age of 1876 2 ma with significant lead loss at 1720 ma and 410 ma epsilon nd values range from 1 8 to 3 2 indicating that the source material for the jorn complex was derived from an lree depleted mantle with a small contribution of older crustal material and had a relatively short crustal residence period metaluminous compositions and oxygen isotope ratios suggest non pelitic sources the jorn complex probably originated as a series of subduction related melts of basic to intermediate composition which underwent varying degrees of subsequent differentiation and secondary alteration

holocene geomagnetic secular variation recorded by volcanic deposits at mount st helens washington a compilation of paleomagnetic data from volcanic deposits of mount st helens is presented in this report the database is used to determine signature paleomagnetic directions of products from its holocene eruptive events to assign sampled units to their proper eruptive period and to begin the assembly of a much larger database of paleomagnetic directions from holocene volcanic rocks in western north america the paleomagnetic results from mount st helens are mostly of high quality and generally agree with the division of its volcanic deposits into eruptive episodes based on previous geologic mapping and radiocarbon dates the muddy river andesite s paleomagnetic direction however indicates that it is more likely part of the pine creek eruptive period rather than the castle creek period in addition the two fingers andesite flow is more likely part of the middle kalama eruptive period and not part of the goat rocks period the paleomagnetic data from mount st helens and mount hood document variation in the geomagnetic field s pole position over the last 2 500 years a distinct feature of the new paleosecular variation psv record similar to the fish lake record oregon indicates a sudden change from rapid clockwise movement of the pole about the earth s spin axis to relatively slow counterclockwise movement at 800 to 900 years b p

ediacaran cambrian ordovician silurian and permian shales of the upper yangtze platform south china deposition thermal maturity and shale gas potential abstract for many years the yangtze platform has been successfully explored and exploited for petroleum the lower silurian longmaxi shale proved to contain commercial quantities of gas but several other formations might also have a high potential for shale hosted gas exploration this paper comprises of geochemical petrographical petrophysical and mineralogical information on the major proterozoic and paleozoic gas shales in the upper yangtze area which provides insights on the depositional environment the thermal maturity of organic matter shale gas storage capacity and the fracability of these rocks the total organic carbon toc content varies from 0 1 to 22 5 with an average value of 2 7 and most of these shales were deposited in an oxygen depleted marine environment equivalent vitrinite reflectance vrr values range from 2 20 to 4 25 indicating that all samples are in the gas generation window and lost a large amount of primary organic carbon porosity varies between 1 5 and 13 6 with an average value of 5 8 while the excess sorption capacity ranges from 0 10 to 0 22 mmol g rock minerology data show that quartz is the predominant mineral except for the upper permian longtan formation in which clay minerals account for 29 to 84 new data were combined with those from other publications and compared to information on other well studied shale hosted gas systems

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

quantitative stratigraphy of the wufeng and lungmachi black shales and graptolite evolution during and after the late ordovician mass extinction article i nfo graptolite bearing black shales are widely distributed on the yangtze platform in south china based on 19 sec tions systematically studied recently we conducted a quantitative stratigraphic analysis of the graptolite biodi versity change during and after the late ordovician mass extinction the combination of graphic correlation and rarefaction methods make it possible to evaluate the effect of sampling intensity on biodiversity estimates graptolite diversity reached its peak in the dicellograptus complexus chron and then a gradual decline to the p pacificus chron it was followed by a two step mass extinction the first and major phase occurred in the metabolograptus extraordinarius chron and the second and minor phase in the latest m persculptus chron the second phase of the extinction was mostly represented by the elimination of the diplograptina survivors from the major extinction event but did not show significant decline in standardized diversity curve thereafter a moderate increase in richness appeared in the early and mid parakidograptus acuminatus chron which was followd by a considerable decrease of graptolite richness in the late p acuminatus chron and early c vesiculosus chron the present study also indicates slow and uneven sedimentation rates in the studied sections from mid katian to early rhuddanian including a relatively slower sedimentation rate during the hirnantian epoch the latter andthehiatusatthe honghuayuanandguanwusections probablyreflecttheeffectsofthe hirnantian glaciation and regional tectonic movements

inclusions in mount st helens dacite erupted from 1980 through 1983 abstract inclusions of plutonic metavolcanic and volcanic rocks are abundant in dacite pumice and lava from the 1980 1986 eruption sequence at mount st helens point counts of inclusions exposed in talus blocks from the dome from 1980 through 1983 show that inclusions form approximately 3 5 vol of the lava eighty five percent of the inclusions are medium grained gabbros with an average diameter of 6 cm additional rock types include quartz diorite hornfelsic basalt dacite andesite and vein quartz disaggregated inclusions are common and define shear planes within the dome these fragmented inclusions may significantly contaminate analyses of the dacite the gabbroic inclusions are of four distinct types all with mineral assemblages consistent with crystallization pressures of less than 9 kb textures and major element compositions indicate that most of the gabbroic inclusions are cumulates the most abundant inclusion type is laminated gabbronorite which contains up to 9 interstitial glass derived from partial melting the presence of quartz veins and hornblende bearing veins within sheared zones in the laminated gabbronorite indicates that the source of these inclusions was holocrystalline rock that had been penetrated by water rich fluids the gabbronorite contained sufficient water to be susceptible to partial melting when the magma that fed the 1980 1986 eruption sequence was emplaced nearby various types of gabbroic inclusions including the laminated gabbronorite are common in mount st helens lavas of approximately the last 3000 years this coincides with the interval in which mount st helens first erupted basalt and basaltic andesite lavas these observations together with the fact that the gabbroic inclusions are compositionally unlike any of the tertiary intrusive rocks in the mount st helens area strongly suggest that the inclusions are related to the introduction of basalt to the mount st helens magmatic system the source of the gabbros could be a layered mafic pluton formed through crystal accumulation from multiple batches of basaltic magma emplaced at mid to upper crustal depths beneath the volcano the prevalence of explosive eruptions at mount st helens may play a part in bringing the inclusions to the surface the eruptive products of the cataclysmic eruption of may 18 1980 contain notably fewer inclusions than the pyroclastic flows and dome lavas erupted subsequently this suggests that the may 18 eruption shattered conduit wall rock that was subsequently stoped into the magma and carried to the surface later in the eruption series

a multicomponent dual polarity palaeomagnetic regional overprint from the moine of northwest scotland abstract palaeomagnetic remanence may be reset by orogeny in a manner analogous to isotopic systems the heavy mineral seams and associated psammite in the deformed and metamorphosed lower morar division of the moine assemblage of northwest scotland have a palaeomagnetic signature that is characteristic over a wide area the various components of the magnetization are resolved by detailed thermal demagnetization these components are post tectonic as they are not affected by any structural fabric they may be classified according to blocking temperature and when statistically distinguishable show a systematic change of direction from high to lower temperature possibly associated with apparent polar wander comparison to the european palaeomagnetic record suggests a late silurian to early devonian age of magnetization for the localities south of the strath conon fault the distribution of remanance directions and blocking temperatures is not consistent with the pattern of potassium argon ages in this part of the british caledonides if the magnetization is purely of thermal origin the palaeomagnetic record indicates higher temperatures at later time than does the potassium argon record this remagnetization event is related to the closing stages of the caledonian orogeny

fractured shale gas systems the first commercial united states natural gas production 1821 came from an organic rich devonian shale in the appalachian basin understanding the geological and geochemical nature of organic shale formations and improving their gas producibility have subsequently been the challenge of millions of dollars worth of research since the 1970s shale gas systems essentially are continuous type biogenic predominant thermogenic or combined biogenic thermogenic gas accumulations characterized by widespread gas saturation subtle trapping mechanisms seals of variable lithology and relatively short hydrocarbon migration distances shale gas may be stored as free gas in natural fractures and intergranular porosity as gas sorbed onto kerogen and clay particle surfaces or as gas dissolved in kerogen and bitumen five united states shale formations that presently produce gas commercially exhibit an unexpectedly wide variation in the values of five key parameters thermal maturity expressed as vitrinite reflectance sorbed gas fraction reservoir thickness total organic carbon content and volume of gas in place the degree of natural fracture development in an otherwise low matrix permeability shale reservoir is a controlling factor in gas producibility to date unstimulated commercial production has been achievable in only a small proportion of shale wells those that intercept natural fracture networks in most other cases a successful shale gas well requires hydraulic stimulation together the devonian antrim shale of the michigan basin and devonian ohio shale of the appalachian basin accounted for about 84 of the total 380 bcf of shale gas produced in 1999 however annual gas production is steadily increasing from three other major organic shale formations that subsequently have been explored and developed the devonian new albany shale in the illinois basin the mississippian barnett shale in the fort worth basin and the cretaceous lewis shale in the san juan basin in the basins for which estimates have been made shale gas resources are substantial with in place volumes of 497 783 tcf the estimated technically recoverable resource exclusive of the lewis shale ranges from 31 to 76 tcf in both cases the ohio shale accounts for the largest share

characterizing long term radon concentration changes in a geothermal area for correlation with volcanic earthquakes and reservoir temperatures a case study from mt aso southwestern japan abstract the purpose of this study is to characterize in detail the temporal changes in rn radon 222 concentration in soil gases near fumaroles and clarify its correlation with volcanic earthquakes and temperatures in two geothermal reservoirs mt aso crater in southwest japan which has two reservoirs on its western side estimated by magnetotelluric survey to be at about 2 km in depth was selected for this study for the long term survey the scintillation counter method was used weekly for 12 5 years at the three hot springs within a 2 km range rn concentrations were calculated using the cras method a calculation method that considers radioactive equilibrium or nonequilibrium state of the soil gas rn concentrations generally showed similar fluctuation patterns among the sites cras was used as a new indicator for evaluating the age of the soil gas this age corresponds to the elapsed time determined from the generation of rn based on the measurement of the numbers of atoms of rn and its daughter 218po at the start of measurement in comparing the rn data with the history of earthquakes in the aso caldera volcanic seismicity was identified as a major controlling factor in the sudden increase and decrease in rn concentration as a function of age for more precise detections of change rn concentrations were measured continuously at one site by pumping soil gas from a borehole and using an ionization chamber over 2 5 years five chemical components he h2 n2 ch4 and co2 were then measured by gas chromatography at 1 week intervals because rn concentrations are affected strongly by atmospheric temperatures the residual components were obtained by subtracting the trend of the components from the original data chemical component data were used to estimate the temperature and pressure in the reservoir at the site temperatures ranged from 229 to 280 c average 265 c average pressure 80 mpa residual rn concentrations showed a clear correlation with temporal changes in estimated temperature and to a degree with pressure considering the migration mechanism of rn by molecular diffusion and advection under rapid gas upflow conditions change in the diffusion coefficient is regarded as a possible factor to have induced temporal changes in rn concentrations in conjunction with the temperature changes in the reservoirs in addition the increase of the amount of degassed rn from the advective convective hydrothermal fluids which is associated with wall rock fracturing is another important factor earthquakes likely enhance the permeability of a reservoir by generating fractures which in turn can cause an increase in the reservoir temperature upflow fluid velocity and gas flux this provides one possible interpretation for the significant correlation of rn concentrations with earthquakes

sequence stratigraphy sedimentary facies and reservoir quality of es4s southern slope of dongying depression bohai bay basin east china abstract thin bedded beach bar sandstone reservoirs are common in the eocene shahejie formation es4s of niuzhuang sag along the southern gentle slope of the dongying depression here we report on the link between sequence stratigraphy sedimentary facies and diagenetic effects on reservoir quality seismic data wireline logs core observations and analyses are used to interpret depositional settings and sequence stratigraphic framework petrographic study based on microscopic observation of optical cathodoluminescence cl confocal laser scanning lscm and scanning electron microscope sem and x ray diffraction xrd analyses were used to describe the fabric texture allogenic and authigenic mineralogy of these highly heterogeneous sandstone reservoirs the es4s interval is interpreted as third order sequence composed of a lowstand systems tract lst a transgressive systems tract tst and a highstand systems tract hst on the fourth order twenty nine parasequences and seven parasequence sets have been identified sand bodies were deposited mainly in the shoreface shallow lake beach bar clastic beach bar semi deep lake carbonate beach bar and the fluvial channels the depositional and diagenetic heterogeneities were mainly due to the following factors 1 fine grain size poor sorting and continuous thin inter bedded mud layers with siltstone fine sandstone having argillaceous layers in regular intervals 2 immature sediment composition and 3 even with the dissolution of grains and several fractures destruction of porosity by cementation and compaction secondary pores from feldspar dissolution are better developed in sandstones with increased cementation grain coating smectite clays preserved the primary porosity at places while dominating pore filling authigenic illite and illite smectite clays reduced permeability with little impact on porosity due to the high degree of heterogeneity in the es4s beach bar interval it is recognized as middle to low permeable reservoir the aforementioned study reflects significant insight into the understanding of the properties of the beach bar sands and valuable for the comprehensive reservoir characterization and overall reservoir bed quality

the caledonian metamorphic core an alpine model the neogene evolution of the central swiss alps is outlined and used as an analogue for the grampian orogeny of scotland a direct comparison is made between the three tectonothermal stages which characterize the alps and similar events in the scottish highlands geometric similarities are emphasised using geological cross sections kinematic and mechanistic similarities are inferred a net convergence model which links the metamorphic and uplift denudation history of the alps to its tectonic development in a crustal scale duplex is argued to describe appropriately the history of the dalradian and northern scottish highlands

a precise u pb zircon age for the inishtrahull syenitic gneiss county donegal ireland smd zircon fractions from the deformed alkaline igneous complex on inishtrahd county donegal ireland yield near concordant u pb ages of 1779 3 ma interpreted as dating the igneous crystallization of the gneiss9s precursor sm nd depleted mantle model ages range from 1912 ma to 1978 ms the similarity of the model ages and the crystallization age indicates that the protoliths were derived from a depleted mantle source essentially uncontaminated by older crust the isotopic data confirm previous tentative correlations of the inishtrahull rocks with those of islay in southwestern scotland archaean crust is apparently absent from this region consistent with recent models placing a southern limit on the archaean along the line of the great glen fault zone pre caledonian orthogneisses in scotland and ireland south of the great glen fault zone have generally been correlated with the late archaean lewisian complex of northwestern scotland e g wilkinson 1907 mccallien 1930 bowes hopgood 1975 dixon et al 1990 although aftalion max 1987 concluded that the lewisian was absent from the annagh division gneisses of north mayo ireland there are now several lines of evidence to suggest that archaean crust may be absent southeast of the great glen fault zone e g fitches et al 1990 daly 1990 halliday et al 1985 a large area of pre caledonian basement here termed the rhinns complex is thought to be present within the fault bounded colonsay west islay block fig 1 bentley et al 1988 fitches et al 1990 marcantonio et al 1988 which extends from colonsay in

the chemistry of lava seawater interactions ii the elemental signature abstract the flow of lava into the ocean at the shoreline of kilauea volcano during the ongoing pu u o o eruption has allowed a detailed study of the geochemical interaction between lava and seawater this paper focuses on the chemistry of the major and minor elements in the fluids that resulted from this interaction the elemental enrichments in these fluids are dominated by three processes 1 evaporation of water from seawater which creates solutions enriched in the major elements found in seawater 2 congruent dissolution of the basalt glass matrix which is limited by the solubility of some of the elements in seawater and 3 removal of volatile phases from the lava on contact with seawater using a simple model of volatile emanation using published emanation coefficients and congruent dissolution we are able to explain the concentrations observed for the majority of elements in precipitation from the steam plume at the shoreline lava entry and in water allowed to interact with molten lava in controlled experiments fe al ti and some of the rare earth elements rees in precipitation samples from the steam plume at the lava entry were 10 000 times enriched over their ambient seawater concentrations suggesting that these elements may be useful for identifying submarine eruptions the flux of elements from the kilauea ocean lava entry is greater than that from a typical midocean ridge hydrothermal vent field for al cd co and the rees whereas the opposite is true for the remainder of the elements studied

milankovitch and sub milankovitch cycles of the early triassic daye formation south china and their geochronological and paleoclimatic implications article the mass extinction at the end of permian was followed by a prolonged recovery process with multiple phases of devastation restoration of marine ecosystems in early triassic the time framework for the early triassic geological biological and geochemical events is traditionally established by conodont biostratigra phy but the absolute duration of conodont biozones are not well constrained in this study a rock magnetic cyclostratigraphy based on high resolution analysis 2440 samples of magnetic susceptibility ms and anhysteretic remanent magnetization arm intensity variations was developed for the 55 1 m thick early triassic lower daye formation at the daxiakou section hubei province in south china the lower daye formation shows exceptionally well preserved lithological cycles with alternating thinly bedded mud stone marls and limestone which are closely tracked by the ms and arm variations power spectral wavelet and amplitude modulation am analyses of the arm and ms series reveal strong evidence for the presence of milankovitch to sub milankovitch frequencies dominated by precession index signal and 4 5 ka cycles cy cles expressed by variations in ms and arm were likely controlled by changes in the input of fine grained detrital magnetite which in turn may have been driven by astronomically induced changes in monsoon in tensity in the equatorial eastern paleotethys during the early triassic greenhouse period on the basis of the 100 ka tuning results the astronomically constrained duration of the induan stage is 1 16 ma with the griesbachian and dienerian substages of 490 ka and 670 ka respectively the new astro nomical time scale also provides time constraints for the conodont and bivalve biozones and the carbonate carbon isotope 13 c records of the lower triassic daye formation time constraints for the conodont bio zones include 34 ka for hindeodus parvus 2 4 ka forisarcicella staschei i isarcica 366 ka for neogondolella planata ng carinata 66 ka for neogondolella discreta 255 ka for neospathodus kummeli and 415 ka for neospathodus dieneri the duration for the negative 13 c shift from high 13