magma types and the tectonic evolution of the swedish proterozoic information on tectonic evolution can be obtained from the succession of magmatic types represented i type and s type granitoids and volcanics basic dykes and alkali complexes the earliest proterozoic magmatic activity resulted in the formation of greenstone belts within the karelian continent area probably in the age range 2500 2100 ma this was followed by an event similar to andinotype orogeny at around 1900 ma with i type granitoids and calc alkaline volcanics this progressed to an event of hercynotype which was concluded by about 1750 ma subsequent magmatic activity i type granitoids and volcanics suggests a new phase of andino type orogeny until about 1630 ma s and c sweden or 1500 ma n sweden tension related magmatism at around 1550 ma basic dykes alkaline complexes and gabbro may relate to rifting prior to the independant evolution of sw sweden which includes intrusion of i s type granitoids metamorphism followed by continental rotation and collision with n america at about 1050 ma basic dykes and s type granites are related to subsequent uplift 950 850 ma ago

magnetic fabrics as constraints on the kinematic history of a pre tectonic granitoid intrusion kristineberg northern sweden abstract outcrop scale correlations of deformation fabrics with low field anisotropy of magnetic susceptibility ams measurements revealed a two stage structural evolution of the pre tectonic palaeoproterozoic viterliden intrusion in the skellefte district sweden the first deformation event reflected n s compression during basin inversion and comprised reverse dip slip shearing along major e w faults whereas the low strain lenses in between experienced penetrative deformation with a component of ne sw elongation along the main foliation this event is largely responsible for the present structural geometry regionally and locally and also for the magnetic fabric of the rocks in particular the sub vertical maximum principal susceptibility axes k max within the high strain zones are related to early dip slip deformation and were virtually unaffected by subsequent dextral strike slip reactivation which is recorded by sub horizontal rock lineations the strike slip deformation reflects e w bulk shortening and may regionally be correlated with reverse faulting along a n s trending major shear zone east of the study area

late orogenic svecofennian deformation in sw finland constrained by pegmatite emplacement ages late stages of the svecofennian orogeny in sw finland were related to a tranpressional stress field and dextral movements along crustal scale shear zones under decreasing temperature and pressure conditions in the kemio area a minimum estimate for the time span of movements along one of these shear zones is obtained from the ages of 1840 to 1830 myr old microcline granite sheets related to early ductile deformation and by dating structurally late rare mineral granite pegmatites related to brittle deformation one pegmatite was emplaced when the rheological conditions in the gabbro changed from ductile to semi ductile and brittle it has u pb ferrotapiolite ages ranging between 1807 0 2 9 myr 2 s and 1803 1 2 9 2 0 myr 2 s another pegmatite emplaced under brittle conditions has an u pb ferrotapiolite age of 1802 9 1 3 myr 2 s these pegmatites were emplaced preferentially in gabbroic rocks that showed brittle to semi ductile deformation at a time when their more felsic host rocks still showed ductile deformation the age range bracketed by the microcline granite sheets and the structurally late pegmatites indicates that ductile deformation related to the transpressional late svecofennian tectonic regime in southwestern finland persisted for at least 30 40 myr

magnetic fabric development in the lower seve thrust from the cosc 1 drilling swedish caledonides abstract the cosc 1 project drilled the several hundred meters thick basal shear zone of the lower seve nappe with mylonites in mica schists amphibole schists and gneisses in zones of high magnetic susceptibility from 1910 to 2450 m we studied magnetic and petrographic fabrics and magnetic mineralogy borehole imaging allowed for geographic reorientation of the samples and offered the opportunity to study anisotropy of magnetic susceptibility ams in relation to tectonic evolution of the seve thrust we measured ams at room temperature added low temperature and field dependent ams for a subset of samples and compared magnetic with petrographic fabrics triaxial and prolate magnetic fabrics with degree of anisotropy p up to 3 2 together with abundant s c fabrics and strain partitioning around porphyroclasts indicate dominant simple shear until 2300 m magnetite and ilmenohematite mimic the rock fabric due to fabric parallel alignment and or magnetic interaction and either contribute to increase or decrease of p depending on the dominating rock fabric elements field dependency of pyrrhotite and magnetite in kmax direction further increases p homogeneous and oblate petrographic and magnetic fabrics in the greenschist grade overprinted rocks below 2300 m with subhorizontal kmax kint girdle distributions indicate dominant flattening ams depicts shear fabrics including magnetite and ilmenohematite and is additionally increased by retrograde magnetite rutile intergrowth in ilmenohematites we interpret that shape and degree of ams are controlled by a tectonic deformation and strain b alteration and magnetic grain interaction and c field dependency of deformed pyrrhotite and or magnetite we observed that all petrographic and magnetic subfabrics are coaxial and lineations are mainly e w to se nw directed confirming the transport direction of the caledonian allochthonous from our microstructural and ams results we suggest that thrusting of the lower seve unit commenced under simple shear conditions at higher metamorphic grades and subsequently switched to more pure shear under greenschist grade conditions

retention of u and pb in zircons from shocked granite in the siljan impact structure sweden the siljan ring structure central sweden was formed by a devonian impact the u pb zircon isotopic system of the shocked proterozoic granite basement appears to be undisturbed although the zircons were mechanically disrupted by the impact

discriminating assimilants and decoupling deep vs shallow level crystal records at mount adams using 238u 230th disequilibria and os isotopes article a suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the mount adams volcanic field has been analyzed for u th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis all of the lavas have 230 th 238 u n1 and span a large range in 230 th 232 th ratios

magnetic fabric development in a highly anisotropic magnetite bearing ductile shear zone seve nappe complex scandinavian caledonides magnetite bearing mylonitic garnet micaschists close to the major suture between the baltica and iapetus terranes seve nappe complex scandinavian caledonides show very high anisotropy of magnetic susceptibility ams with corrected degree of anisotropy p up to 4 8 three different magnetic fabric types can be distinguished they correspond to protomylonite type i p 3 respectively the orientation of the ellipsoid axes from all applied magnetic fabric methods in this study is similar with shallow dips of the metamorphic foliation toward wsw and subhorizontal mostly nw se trending mineral lineation differences between subfabrics were minimized under high shear strain as all markers tend to align parallel with the shear plane the very high anisotropies and mostly oblate ellipsoid shapes of type iii correlate with high magnetic susceptibility kmean up to 55 10 3 si units and are related to the concentration of magnetite aggregates with shape preferred orientation they show a distinct field dependence of magnetic susceptibility of up to 10 in the kmax direction we attribute this field dependence to a memory of high strains in the domain walls of the crystals acquired during synkinematic magnetite growth during shear zone fabric development at temperatures of 550 570 c

how well do precambrian paleomagnetic data agree with the phanerozoic apparent polar wander path a baltica case study abstract precambrian paleomagnetic data are crucial for various tectonic and geophysical applications the reconstructions of fusion and fission of supercontinents in particular due to many objective reasons the variety of stability tests for these data are abridged in comparison to phanerozoic results and is further hindered by the rather common refusal to consider the resemblance between precambrian and younger poles in this paper we selected precambrian paleomagnetic data with ages of

magma flow directions in dikes of the koolau complex oahu determined from magnetic fabric studies we compared the magnetic fabric with the macroscopic surface lineation direction previously assumed to be parallel with magma flow direction for 35 dikes of the koolau complex oahu anisotropy of magnetic susceptibility ams was determined for 530 samples from 59 dikes including a sill and a pluglike intrusive body and two filled lava tubes and statistically significant ams clusters were found in all but five dikes for the 35 lineated dikes the mean maximum ams direction 1 coincides with the macroscopic lineations to within 25 in 18 and to within 15 in eight the ams ellipsoid shape in about half of the samples is prolate cigar shaped and in about half is oblate pancake shaped we interpret the ams to be determined by shape anisotropy of ferromagnetic grains the mean total anisotropy is 2 2 and total anisotropy h ranges from 0 24 to as high as 14 12 the principal ams axes for samples from a dike are tightly grouped on equal area plots and for most of the dikes the minimum ams axes plot near to the pole of the dike our ams measurements enable us to infer absolute magma flow directions for half the dikes studied the maximum ams axes of samples from one side of the dike form a tight cluster on an equal area plot and those from the other side of the dike form another tight cluster the two clusters are close together separated by 10 30 for narrow dikes 100 cm wide and are farther apart 30 60 for wide dikes and are symmetrically disposed on either side of the plane of the dike we infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient and hence indicate the flow azimuth i e the absolute magma flow direction nearly half of the dikes measured 25 gave paired ams plots in 12 the magma flowed up at low oblique angles 5 30 from the horizontal in five it flowed down at 5 20 in four it flowed horizontally 5 in three it flowed up at a steep angle 50 75 and in one it flowed down at a steep angle the magma flow in general was rarely vertical or horizontal

granite emplacement in an extensional setting an ams study of the magmatic structures of monte capanne elba italy abstract monte capanne elba is an excellent example of a granite pluton 42 km 2 emplaced 6 m y ago in a well known extensional environment that of the tyrrhenian sea its internal fabric has been studied using the magnitude of magnetic susceptibility and anisotropy measurements at 74 regularly distributed stations susceptibility magnitudes reveal a normal and asymmetrical petrographical zoning in the pluton the magnetic lineations are remarkably well organized mostly of nw se azimuth with slight plunges and always normal to the walls of the late aplitic dykes these lineations are also parallel to the stretching lineations of the immediate country rocks of the pluton in the western and probably deepest part of the pluton a ne sw trending magnetic magmatic lineation is also imprinted the following scenario is proposed for the emplacement of monte capanne during the opening of the tyrrhenian sea an approximately e w trending sinistral transform fault is taken as being responsible for the creation of a pull apart void that permitted upwelling of the magma orientations of the magmatic structures at depth are parallel to the elongation of the feeding zone of this void i e ne sw the very top part of the pluton was sheared parallel to the granite country rock interface i e nw se parallel to the local stretching of the tyrrhenian sea at the time of emplacement

transpressive shear related to arc magmatism the paleoproterozoic storsjon edsbyn deformation zone central sweden 1 the polyphase 15 30 km wide and 200 km long storsjon edsbyn deformation zone sedz apparently separates two major paleoproterozoic provinces in central sweden four main phases of deformation have been recognized along the zone associated high strain rocks are characterized by their mineralogy microstructures kinematic patterns magnetic signatures and relative ages the mineral and stretching lineations are consistently steeply to moderately plunging analysis of mesoscopic shear zone populations of different generations suggests that most displacements are compatible with north south to northeast southwest bulk regional shortening the structural analysis suggests that the main ductile deformation along the sedz was due to dextral transpression with a steep coaxial component field evidence combined with published age data constrain the main deformation to the time interval circa 1 7 1 3 ga synkinematic titanite from a ductile protomylonite in the sedz yields an u pb age of 1674 6 ma the combined results imply transpressive deformation broadly synchronous with and spatially related to volcanism and plutonism in a continental margin magmatic arc setting

effects of magnetic anisotropy on total magnetic field anomalies modeling of magnetic anomaly data is a powerful technique to gain information on the shape of subsurface rock bodies most models are based on the assumption that the magnetization in the source body is parallel to the direction of the earth s magnetic field it has long been recognized that remanent magnetization affects the magnetization direction intensity and shape of the anomaly and therefore the interpreted structure the effects of anisotropy however have only received little attention so far this study uses synthetic models and a case study to investigate how anisotropy affects magnetization and anomalies over a thick dipping sheet and determines expected errors in interpreted magnetic properties and geometry of the source body for various anisotropy degrees and field inclinations anisotropy affects both the shape and amplitude of anomalies for an oblate uniaxial fabric with the minimum susceptibility normal to the sheet and p 1 5 errors in interpreted dip are up to 12 depending on the field inclination dip and profile orientation and errors in estimated mean susceptibility are up to 30 20 if anisotropy is not taken into account during modeling these effects are larger for higher degrees of anisotropy a case study over the mcu ive layer in the bjerkreim sokndal layered intrusion norway investigates the contributions of an isotropic induced and remanent magnetizations to the total field anomalies there the influence of anisotropy is mainly related to remanence deflection the results shown here will help to further improve interpretation of magnetic potential field data

shallow and deep crustal control on differentiation of calc alkaline and tholeiitic magma article u 230 th isotopes 87 sr 86 sr lower crust assimilation the role of changing crustal interaction and plumbing geometry in modulating calc alkaline vs tholeiitic magma affinity is well illustrated by the influence of 70 km thick crust beneath volcan parinacota changes in petrologic affinity correlate with periods of cone building sector collapse and rebuilding of the volcano over the last 52 ka and are well explained by changes in magma recharge regime with increasing recharge and magma output lavas transition from low fe strongly calc alkaline phenocryst rich silicic compositions to medium fe near tholeiitic mafic and aphanitic characteristics strontium isotope data show that the change in magma regime did not affect all parts of the system simultaneously these are characterized by distinctive 87 sr 86 sr ratios which suggest an initially compartmentalized system relatively high 230 th 232 th activity ratios of 0 72 in early erupted calc alkaline lavas are consistent with interaction with high u upper crust low 230 th 232 th activity ratios of 0 55 and up to 33 th excess in younger near tholeiitic lavas correlate with steep ree patterns indicating lower crustal interaction thorium excesses at the time of eruption approach the maximum that can be generated via small degree garnet residual melting in the lower crust or mantle and imply that transit time through the crustal column for the most tholeiitic magmas had to be short on the order of b2 10 4 yr in contrast lavas with greatest calc alkalinity are also at or near secular

osmium isotope constraints on lower crustal recycling and pluton preservation at lassen volcanic center ca osmium isotope compositions of intermediate to silicic composition calc alkaline volcanic rocks from the lassen volcanic region of the cascade arc are significantly more radiogenic qos 23 to 224 than typical mantle these evolved arc rocks in the lassen region have unradiogenic sr nd and pb isotope compositions which overlap with those of contemporaneous mafic lavas crystal fractionation of mafic to intermediate composition magmas produces re os ratios that are high enough to evolve to very radiogenic os isotope compositions in only a few million years providing a potential fingerprint for detecting the involvement of such young relatively mafic crust in magmatic systems however the sr nd and pb isotope compositions will remain constant over such short time intervals due to relatively low parent daughter enrichment during magmatic evolution the radiogenic os isotope compositions in typically evolved lassen rocks are interpreted to reflect significant interaction with lower crustal material that has radiogenic os isotope compositions beneath this section of the cascade arc large amounts of such high qos lower crust may have formed and been isolated from mash zone mixing and homogenization processes during the pliocene or late miocene the results from this study indicate that os isotopes may provide a unique glimpse into lower crustal processes such as recycling in primitive orogenic arcs 9 2002 elsevier science b v all rights reserved

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

origins and ages of proterozoic granitoids in the bothnian basin central sweden isotopic and geochemical constraints abstract the paleoproterozoic bothnian basin in central sweden and south central finland is dominated by a 10 km thick sequence of metasediments mainly metaturbidites these have been intruded by granitoids which have traditionally been classified as early late and postorogenic relative to the c 1 9 1 8 ga s vecofennian orogeny the early orogenic granitoids are part of a calcalkaline suite ranging in composition from ultramafic to granitic while the late orogenic granitoids mainly are purely granitic and associated with abundant pegmatites the postorogenic group forms large massifs of mainly granitic composition and associated pegmatites are rare an early orogenic tonalite yields a u pb zircon age of 1877 6 ma which is within the age range for s vecofennian granitoids of this tectonic position elsewhere in sweden and finland a late orogenic granite belonging to the harno granites from the southern part of the bothnian basin gives a u pb monazite age of 1822 5 ma this provides a minimum age for the main phases of svecofennian deformation in the region and a correlation with the 1 84 1 83 ga old granite migmatite zone of southern finland is suggested a postorogenic granite belonging to the revsund granitoids gives a u pb zircon age of 1798 8 ma together with previously published age data this indicates that the revsund granitoids were emplaced successively during an age interval of at least between 1 80 and 1 77 ga ree patterns and trace element model calculations indicate that late orogenic granites could have been produced by partial melting of metasediments similar to those exposed at the surface a contribution from the early orogenic granitoids is possible but not discerned by these calculations the postorogenic granites on the other hand have too high concentrations of sr ba y and hree to be derived from such a metasedimentary source the initial e nd values for the early orogenic granitoids range between 2 and 1 corresponding values for both late and postorogenic granites range between 1 and 3 which is on average slightly higher than those for surrounding metasediments the initial sr isotope ratios for the harno granites are intermediate between those for the metasediments and the early orogenic granitoids while the initial sr isotope ratios for the revsund granites are distinctly lower the geochemical and isotopic data support a model in which the harno granites were derived mainly from bothnian basin metasediments with a contribution from early orogenic granitoids the composition of the revsund granitoids is however inconsistent with such a derivation for these rocks a deeper as yet unknown crustal source with a significant pre 1 9 ga crustal component is indicated

magnetic fabrics and paleomagnetism of the storsjon edsbyn deformation zone central sweden abstract a regional deformation zone separating tectonomagmatic blocks in the central part of the fennoscandian shield has been studied using anisotropy of magnetic susceptibility and paleomagnetism in order to define the tectonic character and age of the zone and to put the deformation into a plate tectonic context the deformation zone the storsjon edsbyn deformation zone sedz separates the ratan granite of the trans scandinavian igneous belt tib and the southern svecofennian subprovince in the southwest from older svecofennian granitoids of the central svecofennian subprovince in the northeast the sedz is the most prominent deformation zone in a regional pattern of nw se trending shear zones on basis of ams data the width of the zone which cuts through 1 7 1 9 ga old rocks is estimated at 10 12 km in its central southern part and 20 25 km in its northern part the orientations of the susceptibility axes vary systematically across the zone in a way that suggests that the sedz is a pure shear dominated transpression zone caused by a compressive stress from the present ssw paleomagnetic data from a dolerite sill plat 10 plon 166 and from a dyke plat 25 plon 190 that cut the sedz indicate that no plastic deformation has occurred along the zone during the past 1 25 ga and probably not for the past 1 5 1 6 ga ams data do not point to any deformation of the 1 70 ga old ratan granite which further limits the time span of deformation the main phase of deformation most likely took place between 1 80 and 1 70 ga ago the geological evolution of this part of the fennoscandian shield at the time between 1 80 and 1 70 ga ago is very complex and in part still enigmatic this time period involves magmatism of the tib which is related to eastward subduction it also comprises the beginning of the gothian orogeny and the development of a regional system of transpressive deformation zones the compressive stress field from the present ssw indicated in this study is interpreted to be related to an initial stage of plate collision between the southwestern scandinavian subprovince and older parts of the fennoscandian shield this collision resulted in a system of regional nw se trending intra plate deformation zones one of which is the sedz

age and geochemistry of granites associated with momineralizations in western bergslagen sweden abstract the bergslagen area south central sweden where large amounts of fe cu zn pb au ag mo and w have been produced for several centuries is one of the oldest mining districts in europe two episodes of magmatism and ore formation can be distinguished 1 an older early svecofennian phase that generated 1890 1850 ma old calc alkaline continental margin volcanic and plutonic units related to the svecofennian orogeny and 2 a younger phase comprising potassiumrich granitoids which intruded over a period extending from 1825 to 1680 ma during the final stage of or subsequent to the regional metamorphism and deformation related to the svecofennian orogeny in recent years considerable confusion has dominated the discussion concerning the timing and setting of the mo w forming granites in western bergslagen in this study one of the most important intrusions of undeformed granite in western bergslagen has been investigated in regard to geochemistry and u pb zircon age the granite skalhojden has a peraluminous silica and potassium rich chemistry as well as a uniformly flat ree pattern with a marked negative eu anomaly the u pb data yielded an age of 1758 8 ma these data show convincingly that the granite is comparable to other 1800 1680 ma mo w bearing granitoids elsewhere in the fennoscandian shield but are in strong conflict with hypotheses on early svecofennian rift related crust and ore forming processes recently proposed for western bergslagen a revision of the models of crustal growth and ore formation in that area is suggested

u series evidence for crustal involvement and magma residence times in the petrogenesis of parinacota volcano chile in this study we present th u disequilibria as well as radiogenic and trace element data for recent volcanic rocks from the nevados de payachata volcano which erupted through 70 km of continental crust in the central volcanic zone of the andes 18 s 69 w both lavas and mineral separates were analyzed by mass spectrometry for 238u 230th disequilibria the lavas are characterized either by 230th enrichment or depletion relative to its parent nuclide 238u mineral separates are used to derive u th isochron ages and these ages compare favorably with inferred stratigraphic ages or k ar ages although in one case the u th age is significantly older than the stratigraphic age despite relatively constant sr nd and pb isotope ratios the lavas display inverse trends in 230th 238u versus ce yb or ba hf diagrams these trends cannot be interpreted by simple two component mixing rather there must be three and perhaps four components involved in the genesis of the parinacota lavas a mantle wedge a slab fluid and a lower crustal component can be identified a sediment component is more difficult to detect as it is difficult to decipher its signature because of the strong crustal influence the existence of binary arrays can be explained by variable amounts of crustal material the process of crust mantle interaction must have been short enough to preserve u th disequilibrium 300 ka

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

rapid magma ascent and generation of 230 th excesses in the lower crust at puyehue cordon caulle southern volcanic zone chile basaltic to rhyolitic lavas and tephras erupted over the last 70 kyr at the puyehue cordon caulle volcanic complex in the andean southern volcanic zone svz were analyzed for major and trace element sr isotope and u th isotope compositions to constrain the timescales of magmatic processes and identify the subducted and crustal components involved in magma genesis internal u th mineral isochrons from five lavas and three tephra fall deposits are indistinguishable from their eruption ages indicating a short period b1000 yr of crystal residence in the magma prior to eruption the 230 th 232 th ratios define a narrow range 0 80 0 83 compared to that of all svz lavas 0 72 0 97 suggesting that puyehue basalt was derived from a relatively uniform mantle source dacites and rhyolites have the largest u excesses and likely evolved via fractional crystallization of a plagioclase dominated mineral assemblage in contrast basalts have 1 to 6 230 th excesses a characteristic not previously observed in frontal arc stratovolcanoes of the andean svz the 230 th excesses are interpreted to reflect a relatively small degree of fluid flux melting coupled with assimilation and melting of the lower crust lower crustal processes therefore have dampened the 238 u excesses that were generated during fluid addition to the mantle wedge although prior 238 u 230 th 226 ra studies of lavas from other southern svz stratovolcanoes 36 to 41 s have inferred that slab additions and the extent of mantle melting were nearly constant along strike of the arc our results suggest that mash processes envisioned by hildreth and moorbath w hildreth s moorbath crustal contributions to arc magmatism in the andes of central chile contrib mineral petrol 98 1988 455 489 in the northern svz also occur in the southern svz where the crust is relatively thin

geology geochemistry age and geotectonic evolution of the dala granitoids central sweden abstract the dala granitoids and their associated volcanic products are part of a major palaeoproterozoic igneous complex the 1 85 1 67 ga transscandinavian igneous belt tib in the fennoscandian shield tib constitutes the south western border of the svecofennian domain which was formed by an orogenic episode at 1 93 1 83 ga earlier plate tectonic models proposed that tib is a postorogenic batholith complex which was formed in a compressional tectonic regime representing an andino type environment whereas an extensional tectonic regime has been proposed for the anorogenic 1 65 1 51 ga rapakivi granites in the svecofennian domain in this paper a key segment of tib the dala granitoids is discussed in order to focus attention on the post to anorogenic magmatic evolution in the svecofennian domain three types of granitoids jarna siljan and garberg can be distinguished within the dala granitoid complex the jarna granitoids are the most primitive and were emplaced at a significant depth in the crust at ca 1 79 ga the 1 70 1 68 ga siljan and garberg granites are more evolved and were emplaced at a shallow level in the crust closely associated in space time and origin with the volcanic dala porphyries field relationships as well as geochemical and geochronological data indicate magma genesis within a compressional tectonic regime for the 1 79 ga jarna granitoids however most of the dala granitoids appear to have formed in an extensional tectonic regime which was initiated during the last phase of tib magmatism this extensional igneous component is represented by the 1 70 1 68 ga high level siljan and garberg granites and their closely associated volcanic products the dala porphyries an improved model for the formation of the post to anorogenic magmatism in the svecofennian domain suggests that the dala granitoids represent a post to anorogenic igneous key complex which reflects the transition from a compressional to an extensional tectonic environment along a plate boundary

the mesozoic rimrock lake inlier southern washington cascades implications for the basement to the columbia embayment the mesozoic rimrock lake inlier lies between the north cascades blue and klamath mountains and thus has significant implications for the basement to the columbia embayment the inlier consists of the late jurassic indian creek complex and the voluminous late jurassic early cretaceous russell ranch complex the indian creek complex consists of trondhjemitic to gabbroic rocks some of which underwent dynamo thermal medium grade metamorphism that was probably broadly synchronous with plutonism lithologies and geochemical data indicate that the complex represents the roots of an arc the russell ranch complex is a tectonic melange consisting mainly of arkose and mudstone with subordinate chert conglomerate and pillowed greenstone and minor green tuff deposition may have occurred on an inner fan or the inner part of a mid fan sandstones were derived from a plutonic terrane presumably a dissected are the geochemistry of greenstones apparently is most compatible with eruption as within plate tholeiites or morb the complex displays a block in matrix mesoscopic fabric greenstone and the foliated twin sisters lakes silicic metavolcanic unit occur as kilometer scale blocks predominantly steep faults riddle the complex overlapping time relations between normal and thrust faults several generations of folds and cleavages record protracted deformation steep faults separate the units and are bracketed between early cretaceous and earliest eocene as is deformation of the russell ranch complex these are mostly brittle structures but one mylonite zone has a gentle stretching lineation suggestive of strike slip large displacements probably occurred between the indian creek and russell ranch complexes but an original basement and cover relationship cannot be precluded the complexes are broadly correlative with rocks to the north the late jurassic are represented by the indian creek complex may include rocks in other inliers in central washington the melange belts of north western washington and the san juan islands the russell ranch complex most closely resembles parts of the western melange belt and the constitution formation late jurassic arc type rocks also occur in the klamath and blue mountains and the russell ranch complex may correlate with the yolla bolly terrane of the franciscan complex these correlations suggest that lithological belts are more continuous between washington and california than implied by models which propose major tectonic dispersal the mesozoic arc type crust and continentally derived clastic rocks in the inlier demonstrate that the basement to the columbia embayment includes a variety of tectonic slices or terranes and that the embayment is not simply underlain by tertiary or mesozoic oceanic crust as is commonly proposed

re os isotopic systematics of primitive lavas from the lassen region of the cascade arc california abstract rhenium osmium isotopic systematics of primitive calc alkaline lavas from the lassen region appear to be controlled by mantle wedge processes lavas with a large proportion of slab component have relatively low re and os abundances and have radiogenic os and mid ocean ridge basalt like sr and pb isotopic compositions lavas with a small proportion of slab component have higher re and os elemental abundances and display mantle like os sr nd and pb isotopic compositions assimilation with fractional crystallization can only generate the re os systematics of the lassen lavas from a common parent if the distribution coefficient for re in sulfide is 40 1100 times higher than most published estimates and if most incompatible element abundances decrease during differentiation high re os ratios in mid ocean ridge basalts makes subducted oceanic crust a potential source of radiogenic os in volcanic arcs the slab beneath the southernmost cascades is estimated to have 187os 188os ratios as high as 1 4 mixing between a slab component and mantle wedge peridotite can generate the os isotopic systematics of the lassen lavas provided the slab component has a sr os ratio of 7 5 105 and os abundances that are 100 600 times higher than mid ocean ridge basalts for this model to be correct os must be readily mobilized and concentrated in the slab component perhaps as a result of high water and hcl fugacities in this subduction environment another possible mechanism to account for the correlation between the magnitude of the subduction geochemical signature and os isotopic composition involves increasing the stability of an os bearing phase in mantle wedge peridotites as a result of fluxing with the slab component melting of such a source could yield low os magmas that are more susceptible to crustal contamination and hence have more radiogenic os isotopic compositions than magmas derived from sources with a smaller contribution from the slab thus the addition of the slab component to the mantle wedge appears to result in either the direct or indirect addition of radiogenic os to arc magmas

40ar 39ar and k ar chronology of pleistocene glaciations in patagonia during the pleistocene east of lago buenos aires argentina at 46 5 s at least 19 terminal moraines were deposited as piedmont glaciers from the patagonian ice cap advanced onto the semi arid high plains adjacent to the southern andes exceptional preservation of these deposits offers a rare opportunity to document ice cap fluctuations during the last 1 2 m y 4 0 ar 3 9 ar incremental heating and unspiked k ar experiments on four basaltic lava flows interbedded with the moraines provide a chronologic framework for the entire glacial sequence the 4 0 ar 3 9 ar isochron ages of three lavas that overlie till 90 km east of the cordillera at lago buenos aires and another 120 km from the andes along rio gallegos at 51 8 s that underlies till strongly suggest that the ice cap reached its greatest eastward extent ca 1100 ka at least six moraines were deposited within the 256 k y period bracketed by basaltic eruptions at 1016 10 ka and 760 14 ka similarly six younger more proximal moraines were deposited during an 651 k y period bracketed by an underlying 760 14 ka basalt and the 109 3 ka cerro volcan basalt flow that buried all six moraines coupled with in situ cosmogenic surface exposure ages of moraine boulders the 109 ka age of cerro volcan implies that moraines deposited during the penultimate local glaciation correspond to marine oxygen isotope stage 6 further westward toward lago buenos aires six additional moraines younger than the cerro volcan basalt flow occur surface exposure dating of boulders on these moraines combined with the 1 4 c age of overlying varved lacustrine sediment indicates deposition during the last glacial maximum lgm 23 16 ka although antarctic dust records signal an important patagonian glaciation at 60 40 ka moraines corresponding to marine oxygen isotope stage 4 are not preserved at lago buenos aires apparently these were overrun and obliterated by the younger ice advance at 23 ka notwithstanding the overall pattern of glaciation in patagonia is one of diminishing eastward extent of ice during successive glacial advances over the past 1 m y we hypothesize that tectonically driven uplift of the patagonian andes which began in the pliocene yet continued into the quaternary in part due to subduction of the chile rise spreading center during the past 2 m y maximized the ice accumulation area and ice extent by 1 1 ma subsequent deep glacial erosion has reduced the accumulation area resulting in less extensive glaciers over time

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

potassium argon geochronology of a basalt andesite dacite arc system the mount adams volcanic field cascade range of southern washington high precision k ar dating and detailed mapping have established an eruptive chronology for a cascade stratovolcano and its surrounding array of coeval basaltic centers mount adams is a 200 km 3 andesitic cone that stands at the center of a coeval 1250 km 2 quaternary volcanic field that contains 60 discrete vents k ar ages were measured for 74 samples from 63 of the 136 volcanic units defined in the course of the mapping within analytical error there are no discrepancies between k ar ages and stratigraphic sequence major activity began in the area ca 940 ka and inception of the central stratovolcano took place at ca 520 ka a plot of cumulative volume erupted versus time shows that between 940 and 520 ka the eruptive rate was 3 k y and 80 of the products were basaltic andesites are volumetrically dominant and were emplaced in three main cone building episodes centered at 500 450 and 30 ka at eruptive rates of 1 6 5 km 3 k y at a lower rate of 0 05 0 1 km 3 k y the magmatic system remained almost continuously active between the main pulses although breaks in activity as long as 30 k y are permitted by the k ar data andesitic dacitic activity in the focal region and dominantly basaltic activity on the periphery have coexisted for 520 k y and their products are interstratified the last main episode of cone construction occurred ca 40 10 ka the oldest andesite identifiably derived from within the present day edifice having an age of 33 14 ka andesites forming the south summit rim and the true summit have ages of 13 8 ka and 15 8 ka respectively the time volume composition data bear upon several fundamental questions concerning the long term behavior of arc volcanoes stratovolcanoes commonly grow in spurts but can stay active between the widely spaced episodes of peak productivity large stratocone systems may remain active for half a million years subdivision of complex stratovolcanoes into eruptive or constructional stages without detailed mapping geochronology and compositional data should be treated skeptically discussion of volumetric eruptive rates can be misleading without an adequate time scale stratovolcanoes need never develop large upper crustal magma chambers basalt erupts peripherally but its ascent is suppressed beneath stratovolcanoes by deep crustal domains of magma storage and hybridization that form where concentrated injection of basalt amplifies crustal melting

melting of a subducting oceanic crust from u th disequilibria in austral andean lavas understanding crustal genesis at convergent plate boundaries is important for determining mass transfer between different geochemical reservoirs in the earth s mantle and for deciphering the long term growth of the continental crust most arc magmas are thought to be generated from fluid induced melting of the mantle wedge above slabs of subducting oceanic crust1 such magmas frequently display 238u enrichments or radioactive equilibrium2 3 between 238u and its radiogenic product 230th but where a young and hot oceanic crust is being subducted it may itself partially melt and produce calc alkaline andesites and dacites termed adakites4 here we report a uniform excess of 230th over 238u but variable th isotope ratios in young adakites from the andean austral volcanic zone south of the triple junction where the chile ridge subducts beneath south america we show that these results are compatible with the adakites having been formed by approximately 20 equilibrium melting due to amphibole decomposition in a heterogeneous5 oceanic crust moreover both the degree of melting of the oceanic crust and its thermal structure appear to be uniform under most of the andean austral volcanic zone such partial melting of subducted oceanic slabs may have occurred throughout the earth s history where young oceanic plates were subducted

observations of blowing snow at the south pole 1 observations of blowing snow from visual observers records as well as ground based infrared and lidar measurements at south pole station are analyzed to obtain the first climatology of blowing snow over the antarctic plateau occurrence frequencies of blowing snow wind direction and speed during blowing snow events typical snow layer heights as well as optical depths are determined blowing snow is recorded in roughly one third of the visual observations and occurs under a narrow range of wind directions blowing snow layers are usually less than 400 m in thickness but can exceed 1000 m during blowing snow conditions these near surface layers are apparent in lidar backscatter profiles these layers emit radiances similar to those from optically thin clouds frequently seen over the antarctic plateau because the near surface blowing snow layers are frequently present they are a factor in space borne laser altimetry and other satellite remote sensing

rapid remobilization of magmatic crystals kept in cold storage we lack thermal histories for magma reservoirs but here the magma under mount hood oregon usa is shown to have been too cold to mobilize for most of the time it has been stored which implies that magma mobilizes at which point it can be imaged geophysically very quickly prior to eruption this paper presents a novel approach to constraining the thermal history of the subsurface magma bodies that feed eruptions by combining temporal information from a variety of sources kari cooper and adam kent combine timescales derived from uranium series disequilibria textural information and trace element zoning in crystals they show that at mount hood oregon only a small fraction of the total storage duration has been spent at temperatures above the critical crystallinity the point at which magma is easily mobilized they conclude that largely liquid bodies that could be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption the processes involved in the formation and storage of magma within the earth s upper crust are of fundamental importance to volcanology many volcanic eruptions including some of the largest result from the eruption of components stored for tens to hundreds of thousands of years before eruption1 2 3 although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes they remain relatively poorly known4 5 eruptions of crystal rich magma are often suggested to require the mobilization of magma stored at near solidus conditions6 7 8 however accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures7 8 9 what has been lacking in this debate is clear observational evidence linking the thermal and therefore physical conditions within a magma reservoir to timescales of storage that is thermal histories here we present a method of constraining such thermal histories by combining timescales derived from uranium series disequilibria crystal sizes and trace element zoning in crystals at mount hood oregon usa only a small fraction of the total magma storage duration at most 12 per cent and probably much less than 1 per cent has been spent at temperatures above the critical crystallinity 40 50 per cent at which magma is easily mobilized partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near solidus temperatures is common magma storage at low temperatures indicates that although thermobarometry calculations based on mineral compositions may record the conditions of crystallization they are unlikely to reflect the conditions of most of the time that the magma is stored our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption

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

recent eruptions of mount adams washington cascades usa the postglacial eruption rate for the mount adams volcanic field is 0 1 km3 k y four to seven times smaller than the average rate for the past 520 k y ten vents have been active since the last main deglaciation 15 ka seven high flank vents at 2100 2600 m and the central summit vent of the 3742 m stratocone produced varied andesites and two peripheral vents at 2100 and 1200 m produced mildly alkalic basalt eruptive ages of most of these units are bracketed with respect to regional tephra layers from mount mazama and mount st helens the basaltic lavas and scoria cones north and south of mount adams and a 13 km long andesitic lava flow on its east flank are of early postglacial age the three most extensive andesitic lava flow complexes were emplaced in the mid holocene 7 4 ka ages of three smaller holocene andesite units are less well constrained a phreatomagmatic ejecta cone and associated andesite lavas that together cap the summit may be of latest pleistocene age but a thin layer of mid holocene tephra appears to have erupted there as well an alpine meadow section on the southeast flank contains 24 locally derived holocene andesitic ash layers intercalated with several silicic tephras from mazama and st helens microprobe analyses of phenocrysts from the ash layers and postglacial lavas suggest a few correlations and refine some age constraints approximately 6 ka a 0 07 km3 debris avalanche from the southwest face of mount adams generated a clay rich debris flow that devastated 30 km2 south of the volcano a gravitationally metastable 2 to 3 km3 reservoir of hydrothermally altered fragmental andesite remains on the ice capped summit and towering 3 km above the surrounding lowlands represents a greater hazard than an eruptive recurrence in the style of the last 15 k y

origin of continental crust of 1 9 1 7 ga age nd isotopes in the svecofennian orogenic terrains of sweden initial nd isotopic ratios are reported for 57 samples representing all crustal components in the central and southern parts of the 1 9 1 7 ga old svecofennian orogenic terrains of sweden and the 1 8 1 7 ga old transscandinavian granite porphyry belt transscandinavian igneous belt u pb zircon ages are determined where the magmatic ages were not sufficiently constrained the nd isotopic results show that there are no archaean crustal blocks within the studied 1 9 1 7 ga old crustal segment this is consistent with results from finland the main input of recycled archaean crust to the svecofennian was in the form of sediments which are concentrated in the central svecofennian subprovince of north central sweden this is consistent with the vicinity of this subprovince to the archaean craton of the baltic shield these turbidite sediments contain up to 40 archaean material but s type granitoids associated with them have a smaller archaean component around 20 most of the quantitatively predominant i type granitoids and volcanics as well as all the basaltic rocks studied have nd from 1 to 3 and consist of 90 material newly derived from the mantle 1 9 1 7 ga ago we argue that the archaean material was introduced in the form of sediments but we cannot distinguish between pelagic sediments delivered on subducting oceanic crust and turbidite accumulations like the ones seen in north central sweden in either case an overall 15 archaean contribution for the swedish svecofennian was reasonably well homogenized with mantle derived magmatic products for this to occur an active near surface tectonic environment such as a complex of subduction zones must have been necessary

u th pa ra study of the kamchatka arc new constraints on the genesis of arc lavas abstract the 238u 230th 226ra and 235u 231pa disequilibria have been measured by mass spectrometry in historic lavas from the kamchatka arc the samples come from three closely located volcanoes in the central kamchatka depression ckd the most active region of subducted related volcanism in the world the large excesses of 226ra over 230th found in the ckd lavas are believed to be linked to slab dehydration moreover the samples show the uncommon feature of 230th 238u activity ratios both lower and higher than 1 the u series disequilibria are characterized by binary trends between activity ratios with 231pa 235u ratios all 1 it is shown that these correlations cannot be explained by a simple process involving a combination of slab dehydration and melting we suggest that they are more likely to reflect mixing between two end members a high magnesia basalt hmb end member with a clear slab fluid signature and a high alumina andesite haa end member reflecting the contribution of a slab derived melt the u th ra characteristics of the hmb end member can be explained either by a two step fluid addition with a time lag of 150 ka between each event or by continuous dehydration the inferred composition for the dehydrating slab is a phengite bearing eclogite equilibrium transport or dynamic melting can both account for 231pa excess over 235u in hmb end member nevertheless dynamic melting is preferred as equilibrium transport melting requires unrealistically high upwelling velocities to preserve fluid derived 226ra 230th a continuous flux melting model is also tested in this model 231pa 235u is quickly dominated by fluid addition and for realistic extents of melting this process cannot account for 231pa 235u ratios as high as 1 6 as observed in the hmb end member the involvement of a melt derived from the subducted oceanic crust is more likely for explaining the haa end member compositions than crustal assimilation melting of the oceanic crust is believed to occur in presence of residual phengite and rutile resulting in no 226ra 230th disequilibrium and low 231pa excess over 235u in the high alumina andesites consequently it appears that high alumina andesites and high magnesia basalts have distinct origins the former being derived from melting of the subducted oceanic crust and the latter from hydrated mantle it seems that there is no genetic link between these two magma types in contrast with what was previously believed

the extent of u series disequilibria produced during partial melting of the lower crust with implications for the formation of the mount st helens dacites the extent to which u series disequilibria can be produced during partial melting of mafic lower crust is quantified using a simple batch melting model and both experimental and theoretical partition coefficients for u th and ra we show that partial melting of mafic lower crust can only produce small disequilibria between 238u 230th and 226ra crystallisation of basalt and mixing between young basalt and crustal derived melts will have a similar or smaller effect consequently u series disequilibrium in arc andesites and dacites can generally only be an inherited feature derived from a mantle parent unless the timescales of silicic magma production within the crust are short compared to the half life of 226ra our results have profound implications for several recent models of silicic magma production by thermal incubation and partial melting of the lower crust we show that the 226ra excess observed in most arc andesites and dacites requires extremely rapid differentiation and or the involvement of mantle derived basalts less than a few thousand years old application to mount st helens suggests that crystallisation of young mantle derived magma is likely to be the dominant process in the formation of these dacites

timescales of crustal magma reservoir processes insights from u series crystal ages the dynamic processes operating within crustal magma reservoirs control many aspects of the chemical composition of erupted magmas and crystals in volcanic rocks provide a temporally constrained archive of these changing environments in this review i compile 238 u 230 th ages of accessory phases and 238 u 230 th 226 ra ages of bulk mineral separates of major phases these data document that crystals in individual samples can have ages spanning most of the history of a volcanic centre age populations for accessory phases show protracted pre eruptive crystal residence times but few crystals predate magmatic activity at a given centre these data have been interpreted in the context of residence times of the host magmas or timescales of the storage of crystals within a largely crystalline portion of the reservoir system in contrast less than half of the bulk separate 238 u 230 th 226 ra ages for major phases are more than 10 kyr older than the eruption many of these apparently conflicting observations of ages of major and accessory phases can be reconciled within the context of a model where a crystal mush was remobilized during processes leading to eruption overall the compiled data show that crystals contain rich archives of magmatic processes in crustal reservoirs especially when combined with other crystal scale geochemical data

plagioclase residence times at two island arc volcanoes kameni islands santorini and soufriere st vincent determined by sr diffusion systematics the diffusive relaxation of trace element profiles in plagioclase phenocrysts may provide important constraints on magma residence times in crustal magma chambers initial trace element profiles in plagioclase phenocrysts are governed by variations in the concentration of a trace element in the melt and by the plagioclase melt partition coefficient trace element diffusion will subsequently act to modify this initial profile and given enough time produce a profile that is in equilibrium with the anorthite variations within the crystal we argue that the trace element partition coefficient da b between two parts a and b of a plagioclase crystal of variable anorthite content is equal to the ratio of their crystal liquid partition coefficients and that the equilibrium profile of the crystal can be calculated the time required to establish diffusive equilibrium is dependent on the wavelength and amplitude of the initial trace element concentration range and on the diffusivity of the trace element in plagioclase strontium plagioclase melt partition coefficients and diffusivities are calculated for a range of magmatic temperatures and plagioclase compositions a one dimensional diffusion model is developed that describes the diffusive destruction of oscillatory trace element zoning with time and allows the calculation of upper limits for plagioclase crystal residence times in a magma reservoir the model is tested using major and trace element concentrations measured along crystal traverses of plagioclase phenocrysts from the kameni island dacites santorini and from the 1979 soufriere andesite st vincent three out of eight plagioclase phenocrysts have sr concentration profiles that are not in diffusive equilibrium for these the diffusion model is employed to calculate maximum crystal residence times from incomplete diffusive equilibration of trace element zoning in plagioclase maximum crystal residence times range from 100 to 450 years this is in good agreement with estimates from crystal size distribution and from ra th disequilibrium studies for the kameni islands for soufriere however such short residence times are incompatible with u th mineral errorchron data that suggest residence times of 40 ka in a thermally buffered magma reservoir to reconcile these apparently different ages we invoke a more complicated magmatic history for soufriere where an initially buffered magma reservoir is disturbed by magma mixing and suffers limited additional crystal fractionation prior to eruption

the 40ar 39ar dating of core recovered by the hawaii scientific drilling project phase 2 hilo hawaii 1 the hawaii scientific drilling project phase 2 hsdp 2 recovered core from a 3 1 km thick section through the eastern flanks of mauna loa and mauna kea volcanoes we report results of 40ar 39ar incremental heating by broad beam infrared laser of 16 basaltic groundmass samples and 1 plagioclase separate mostly from k poor tholeiites the tholeiites generally have mean radiogenic 40ar enrichments of 1 3 and some contain excess 40ar however isochron ages of glass poor samples preserve stratigraphic order in all cases a 246 m thick sequence of mauna loa tholeiitic lavas yields an isochron age of 122 86 kyr all errors 2 at its base beneath the mauna loa overlap sequence lie mauna kea s postshield and shield sequences a postshield alkalic lava yields an age of 236 16 kyr in agreement with an age of 240 14 kyr for a geochemically correlative flow in the nearby hsdp 1 core hole where more complete dating of the postshield sequence shows it to have accumulated at 0 9 0 4 m kyr from about 330 to 200 ka mauna kea s shield consists of subaerial tholeiitic flows to a depth of 1079 m below sea level then shallow submarine flows hyaloclastites pillow lavas and minor intrusions to core bottom at 3098 m most subaerial tholeiitic flows fail to form isochrons however a sample at 984 m yields an age of 370 180 kyr consistent with ages from similar levels in hsdp 1 submarine tholeiites including shallow marine vitrophyres clasts from hyaloclastites and pillow lavas were analyzed however only pillow lava cores from 2243 2614 and 2789 m yield reliable ages of 482 67 560 150 and 683 82 kyr respectively a linear fit to ages for shield samples defines a mean accumulation rate of 8 6 3 1 m kyr and extrapolates to 635 kyr at core bottom alternatively a model relating mauna kea s growth to transport across the hawaiian hot spot that predicts downward accelerating accumulation rates that reach 20 m kyr at core bottom depaolo and stolper 1996 is also consistent with all reliable ages except the deepest

delineation and character of the archaean proterozoic boundary in northern sweden abstract before the deposition of a proterozoic cover and the repeated proterozoic reworking of the older rocks the presently exposed archaean areas in northern sweden formed part of a coherent craton in the present study we have used sm nd isotopic analyses of proterozoic granitoids and metavolcanics to delineate the archaean palaeoboundary in a regional context the transition from strongly negative nd t values in the northeast to positive values in the southwest is distinct and approximately defines the border of the old craton the archaean palaeoboundary extends in a wnw direction and is subparallel to the longitudinal axis of the skellefte sulphide ore district but it is situated 100 km farther to the north the 1 9 ga old granitoids on the two sides of the palaeoboundary were all formed in compressional environments but those situated to the north have higher contents of lile and lree at similar contents of si this indicates that they were generated in an area with thicker crust and supports the location of the archaean proterozoic palaeoboundary there is no simple correlation between the archaean palaeoboundary as defined by the isotopic results and any of the major fracture systems as interpreted from regional geophysical measurements reflection seismic work indicates that juvenile volcanic arc terrains to the south have been thrust onto the archaean craton possible thrust faults have been identified from aeromagnetic measurements rifting of the archaean craton created a passive margin 2 0 ga ago spreading shifted to convergence with subduction beneath the archaean continent 1 9 ga ago subsequently the resulting juvenile volcanic arc collided with the old continent and the archaean palaeoboundary as existing today was formed by a collision characterized by overthrusting the boundary then was disturbed by later deformation predominantly along nne trending fracture systems

contrasting timescales of crystallization and magma storage beneath the aleutian island arc abstract geologic chronologic and u th isotope data from pleistocene recent basaltic to rhyolitic lavas and their phenocrysts constrain the long term evolution of magmatic processes at seguam island aleutian island arc and suggest a model in which erupted magmas were derived from a single deep seated reservoir for 130 kyrs of its eruptive history the monotonic evolution in 230 th 232 th 0 ratios is consistent with radiogenic ingrowth of 230 th in a long lived magma reservoir between 142 and 9 ka internal u th mineral isochrons from seven lavas and one ignimbrite are indistinguishable from their eruption ages as constrained by 40 ar 39 ar dating which implies a short period i e 10 3 yrs or less of crystal residence in the magma prior to eruption these results can be reconciled if small batches of magma are repeatedly extracted from a deep thermally buffered basaltic reservoir followed by decompression driven crystallization and differentiation in small chambers or conduits in the upper crust immediately prior to eruption stratovolcano collapse at 9 ka produced a 4 km diameter caldera and a 70 m thick dacitic ignimbrite covering 12 km 2 the ignimbrite as well as subsequent rhyolitic and basaltic lava flows signal a decrease in 230 th 232 th 0 and 87 sr 86 sr ratios the abrupt change in magma composition at 9 ka most likely reflects disruption of the long lived reservoir by rapid ascent of basaltic magma with low 230 th 232 th and 87 sr 86 sr ratios and excess 226 ra while the physiochemical connection between cone collapse and rise of new magma into the shallow crust remains to be explored the shift in isotopic composition and evidence for a protracted 100 kyr period of magma storage would have been missed had the focus been on only the most recent historical eruptions our results suggest that not all arc basalt produced in the mantle wedge separates and ascends rapidly magma ascent rates and storage times may vary significantly over periods of 10 4 to 10 5 yrs at a single arc volcano and u th isotopes offer a means of better understanding relationships between explosive eruptions and changes in magma reservoir dynamics in island arcs provided a sufficiently long period of volcanic activity is examined

new u pb ages from the wiborg rapakivi area constraints on the temporal evolution of the rapakivi granite anorthosite diabase dyke association of southeastern finland abstract new u pb data on zircons monazites and baddeleyite suggest that the wiborg rapakivi batholith and associated mafic rocks in southeastern finland were emplaced mainly between 1650 and 1625 ma the earliest anorogenic magmatism related to the intrusion of the rapakivi granites was the emplacement of some diabase dykes about 1665 ma ago while the youngest porphyries intruded the rapakivi granites at 1615 ma the process involved three peaks of diabase activity at 1665 1645 and 1635 ma and two major granite events at 1640 5 and 1630 5 ma the former of which comprises also the intrusion of minor gabbroic anorthosite bodies on the whole the result was the emplacement of at least 105 km3 of rock material over a period of 50 ma within southern finland the rapakivi magmatism continued until 1540 ma by emplacement of the west finnish intrusions which combined are as extensive as the wiborg area globally the proterozoic rapakivi event probably represents the largest pulse of intracratonic magmatism which occurred during geological history and may be a consequence of rapid growth of continental masses in the early proterozoic

geophysical and geochemical evidence of proterozoic collision in the western marginal zone of the baltic shield new continental crust was formed in the svecofennian domain of the baltic shield c 1 9 ga ago approximately 0 1 0 15 ga later new crust accreted to the sw part of the shield in this paper an attempt is made on the basis of gravity measurements and lithogeochemistry to describe the tectonic processes responsible for the continental growth c 1 75 1 8 ga ago the transscandinavian granite porphyry belt tgpb separates the svecofennian domain from the polymetamorphic terrain of the sw swedish gneiss region red orthogneisses occurring immediately west of the tgpb are the deformed equivalents of the tgpb type granitoids while grey orthogneisses displaying a tonalitic granodioritic trend and situated further west were generated in a volcanic arc environment the tgpb granitoids and the red sw swedish gneisses represent a transition from this volcanic arc type rock to contemporaneous within plate type granites intruded in the svecofennian crust the volcanic arc was forced against the svecofennian crust in which large tensional fracture zones ensued with strike directions normal to the collision front in such tensional environments the withinplate type granites were generated in the collision zone the crust was down warped and huge amounts of granitic melts were generated at the base of the crust this tgpb magma rose upwards utilizing the fracture zone between the arc rocks generated slightly earlier and the svecofennian crust a relatively thin upper part of the tgpb that spread laterally westwards became strongly deformed during the collision i e the red sw swedish gneisses while the major deep reaching tgpb root zone that was not completely solidified yet acted as a buffer against the foliation front

os and u th isotope signatures of arc magmatism near mount mazama crater lake oregon abstract interaction of mantle melts with the continental crust can have significant effects on the composition of the resulting melts as well as on the crust itself and tracing this interaction is key to our understanding of arc magmatism lava flows and pyroclastic deposits erupted from 50 to 7 7 ka at mt mazama crater lake oregon were analyzed for their re os and u th isotopic compositions mafic lavas from monogenetic vents around mt mazama that erupted during the buildup to its climactic eruption have lower 187 os 188 os ratios 0 1394 to 0 1956 and high 230 th excess 230 th 238 u 0 of 1 180 to 1 302 whereas dacites and rhyodacites tend to have higher 187 os 188 os ratios 0 2292 to 0 2788 and significant 238 u excess 230 th 238 u 0 of 0 975 to 0 989 the less radiogenic os isotope compositions of the mafic lavas can be modeled by assimilation of young 2 5 to 7 ma mafic lower crust that was modified during regional extension whereas the more radiogenic os isotope compositions of the dacites and rhyodacites can be attributed to assimilation of older 10 to 16 ma mid to upper crust that acquired its composition during an earlier period of cascade magmatism production of th excesses in the lower crust requires very young garnet formation accompanying dehydration melting in the lower crust at less than a few 100 ka by heat from recent basaltic magma injection the results from this study suggest that the combination of os and th isotopes may be used to provide insights into the timescales of evolution of the continental crust in arc settings as well as the influence of the crust on erupted magmas and suggest a link between the age and composition of the lower and upper crust to regional tectonic extension and or earlier cascade magmatism

optical properties of blowing snow calc ulation procedures are developed and results shown for the exact calculation of extinction and meteorological visual range using the blowin g snow mass in the atmosp here and particle radius results of the calculations show i for monochromatic radiation geometrical optics approx imations of the extin ction efficiency are found to provide results of only moderate accuracy in calculating the extinction of radiation by a single particle 2 for broad band radiation the geomet rical optics appro ximation is sufficiently accurate for many single particle measurement instru ments and appli cations except in the infra red band where mie theory should be used 3 for typical blowing s now particle size dist ributions the shape parameter of the distribution of particle radii and the mean particle radius are very important in broa d band extinction and visua l range modelling estimates of blowing snow quantities from broad band extinction measurements or visual range from blowin g snow quantities should address the shape and mean value of the snow particle radius distribution

geological geochemical and geochronological evidence for a new palaeoproterozoic terrane in southeastern sweden the geological and geochronological relationships between a rhyolite belonging to the transscandinavian igneous belt tib and an intrusive suite the backaby intrusion southeast sweden have been investigated by means of bedrock mapping geochemistry and radiometric age determinations u pb on zircons field relations suggest that the rhyolite rests unconformably on the backaby intrusion which previously has been correlated with 1 9 ga granitoids within the svecofennian domain the rhyolite yielded an age of 1800 8 ma and a tonalite belonging to the backaby intrusion basement yielded an age of 1834 3 ma the age of the rhyolite is slightly older 10 15 ma than adjacent granitoids belonging to the transscandinavian igneous belt and is considered as the earliest magmatic expression of tib in the area the age and the geochemical trends for the backaby intrusion is significantly different from that of the surrounding tib granitoids and indicate a rock forming event at about 1830 1840 ma in southeastern sweden represented by the oskarshamn jonkoping belt this event is significantly younger than the older svecofennian rocks within the svecofennian domain proper indicating a hitherto unidentified geological terrane in southeastern sweden

a new view into the cascadia subduction zone and volcanic arc implications for earthquake hazards along the washington margin in light of suggestions that the cascadia subduction margin may pose a significant seismic hazard for the highly populated pacific northwest region of the united states the u s geological survey usgs the research center for marine geosciences geomar and university collaborators collected and interpreted a 530 km long wide angle onshore offshore seismic transect across the subduction zone and volcanic arc to study the major structures that contribute to seismogenic deformation we observed 1 an increase in the dip of the juan de fuca slab from 2 7 to 12 where it encounters a 20 km thick block of the siletz terrane or other accreted oceanic crust 2 a distinct transition from siletz crust into cascade arc crust that coincides with the mount st helens seismic zone supporting the idea that the mafic siletz block focuses seismic deformation at its edges and 3 a crustal root 35 45 km deep beneath the cascade range with thinner crust 30 35 km east of the volcanic arc beneath the columbia plateau flood basalt province from the measured crustal structure and subduction geometry we identify two zones that may concentrate future seismic activity 1 a broad because of the shallow dip possibly locked part of the interplate contact that extends from 25 km depth beneath the coastline to perhaps as far west as the deformation front 120 km offshore and 2 a crustal zone at the eastern boundary between the siletz terrane and the cascade range

effects of tectonic deformation on the remanent magnetization of rocks paleomagnetic investigations have usually been impossible in deformed rocks because of the effects of distortion on the remanent magnetization vector deformation produces a magnetic anisotropy in rocks which can deflect the remanence away from the minimum susceptibility axis however the assumption that the intersection points of great circles joining these two directions represent an improved estimate of the undisturbed remanence is invalid because the amount of the deflection of each remanence is not really known anisotropy of magnetic susceptibility ams provides a quantitative basis for strain determination the principal axes of the magnetic anisotropy ellipsoid have the same directions as those of the strain ellipsoid a linear relationship between the normalized principal susceptibility differences and the logarithmic strains has been found in several rock types if the correlation method used is valid ams observations can be used to establish the magnitudes and directions of the principal strains strain modifies bedding planes and realigns the grains that carry magnetic remanence compensation for deformation requires application of a modified bedding tilt correction and correction of the remanent vector for distortional strain the effects of strain on bedding can be corrected using passive marker theory attempts to compensate for distortional strain by treating the remanent vector as a passive marker have proved inconclusive during deformation linear and planar elements deflect away from the axis of maximum shortening the passive marker correction shifts the remanent vectors toward the local shortening axis for the site and reduces the directional scatter tests of the passive marker method in strongly deformed rocks resulted in a remanence distribution dominated by the shortening axis directions the passive marker model becomes inapplicable when the deformation results in recrystallization of the rock matrix

new osmium isotope evidence for intracrustal recycling of crustal domains with discrete ages new 187os 188os ratios of quaternary mount adams volcanic rocks from the cascade arc in southern washington vary by 300 187 os 188 os 5 0 165 0 564 and fall into high 0 319 and low 0 166 to 0 281 groups of 187 os 188 os ratios that are sub stantially more radiogenic than mantle values these os isotope compositions and groupings are interpreted to reflect recycling of discrete intracrustal domains with high 187os 188os ratios but dif fering ages thus recording the process of crustal hybridization and homogenization os isotope compositions provide new constraints on amounts of intracrustal recycling in young subduction zone en vironments that reflect the magmatic history of the arc sr nd hf and pb isotope variations in this young mafic arc complex are too small to allow such constraints

lead isotopic evidence for the origin of 1 8 1 4 ga ores and granitoids in the southeastern part of the fennoscandian shield abstract new data for the lead isotopic composition of galena from ore deposits related to 1 8 1 4 ga granitoids in the southeastern part of the fennoscandian shield are presented and discussed together with earlier published information on granitoids and associated ore deposits two types of sulphide deposits genetically related to the granites can be recognized based on general metallogenetic features and isotopic composition granite hosted deposits including sn bearing greisen veins in rapakivi granites and peri intrusive deposits including contact metasomatic skarn deposits like pitkaranta and gruvasen in addition epigenetic deposits with a spatial but not a genetic link to the granites occur at various sites an important metallogenetic belt related to 1 7 1 5 ga granitoids can be followed in an east west direction across the shield from lake siljan in sweden to salmi at lake ladoga within this belt a major difference in metal derivation can be recognized ore deposits southwest of the raahe ladoga line i e in areas without archaean basement indicate metal derivation from remelted 1 9 ga svecofennian crust whereas ore deposits in areas underlain by archaean rock units indicate metal derivation from highly metamorphosed archaean sources

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

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

40ar 39ar k ar and 230th 238u dating of the laschamp excursion a radioisotopic tie point for ice core and climate chronologies abstract a brief period of enhanced 10 be flux that straddles the interstadial warm period known as dansgaard oeschger event 10 in greenland and its counterpart in antarctica the antarctic isotope maximum 10 is but one consequence of the weakening of earth s magnetic field associated with the laschamp excursion this 10 be peak measured in the grip ice core is dated at 41 250 y b2k before year 2000 ad in the most recent gicc05 age model obtained from the northgrip core via multi parameter counting of annual layers uncertainty in the age of the 10 be peak is however no better than 1630 y at the 95 confidence level reflecting accumulated error in identifying annual layers the age of the laschamp excursion guillou h singer b s laj c kissel c scaillet s jicha b 2004 on the age of the laschamp geomagnetic excursion earth planet sci lett 227 331 343 is revised on the basis of new 40 ar 39 ar unspiked k ar and 238 u 230 th data from three lava flows in the massif central france together with the 40 ar 39 ar age of a transitionally magnetized lava flow at auckland new zealand combined these data yield an age of 40 700 950 y b2k where the uncertainty includes both analytical and systematic 40 k and 230 th decay constant errors taking the radioisotopic age as a calibration tie point suggests that the layer counting chronologies for the northgrip and gisp2 ice cores are more accurate and precise than previously thought at depths corresponding to the laschamp excursion

new age for the skalamaelifell excursion and identification of a global geomagnetic event in the late brunhes chron article ar 39 ar geochronology paleomagnetism sixteen lava flows sampled from isolated hills including skalamaelifell hill on the reykjanes peninsula in southwest iceland record excursional geomagnetic field directions that correspond to virtual geomagnetic poles vgps grouped at 12 s 250 e in the eastern pacific ocean the vgps are similar to those obtained in earlier studies of these lavas for which the name skalamaelifell excursion was previously assigned based on unspiked k ar dating of nine samples from six hills it has been accepted for more than two decades that the transitionally magnetized lavas reflect the 41 ka laschamp excursion however nine new 40 ar 39 ar incremental heating experiments on four of the transitionally magnetized lavas give an age of 91 13 ka moreover 238 u 230 th isochrons determined for two of these lava flows yield a weighted mean age of 96 10 ka 2 the weighted meanof the

diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase residence times of plagioclase crystals in magma reservoirs can be determined by modeling the compositional zoning of trace elements in these crystals we present a formulation to model diffusion of trace elements in plagioclase paying special attention to certain thermodynamic and kinetic aspects in particular we account for the compositional dependence on anorthite content of the chemical potential and diffusion coefficients of trace elements e g mg the choice of suitable boundary conditions and potential effects of diffusion in more than one dimension we show that contrary to intuition diffusive fluxes of trace elements may be coupled to major element concentration gradients and ignoring such coupling can lead to incorrect estimates of timescales we illustrate application of the model using plagioclase crystals from a suite of gabbroic xenoliths from a holocene dacitic lava flow of volcan san pedro chilean andes 36 s the inferred timescale for metasomatism of the xenoliths by evolved liquids is on the order of 100 30 to 148 yr and serves to illustrate how trace element zoning in plagioclase provides a window into timescales of magmatic processes inaccessible by isotopic or other methods

post svecokarelian andinotype evolution in central sweden granitoids and associated volcanics postorogenic in relation to the svecokarelian orogeny constitute a belt in central sweden evidence presented in this paper suggest that the belt originated in an andinotype orogenic environment the inferred development of the belt was characterized by fracture controlled high level emplacement of batholiths in an ensialic setting voluminous venting of ignimbrites and formation of grabens through cauldron subsidence accompanied the recurrent intrusive activity block movements and episodes of burial metamorphism appear to have been prominent features throughout the history of the post svecokarelian belt