petrogenesis of the laguna del maule volcanic complex chile 36 s from 33 42 s in central south chile there are numerous volcanoes which form part of the andean magmatic arc caused by subduction of the nazca plate beneath western south america the 0 3 m y old laguna del maule volcanic complex at 36 s is in a transition region between volcanoes at 33 34 s formed dominantly of hornblende bearing andesite and volcanoes south of 37 s dominantly composed of basalt and basaltic andesite the laguna del maule complex ranges in composition from basalt 0 3 m y to rhyolite post glacial although there is abundant evidence for magma mixing basalt and rhyolite have similar sr and nd isotopic ratios thereby requiring that the mixing members had the same isotopic ratios 87sr 86sr 0 70419 and 143nd 144nd 0 51274 in contrast dacitic dikes and a volcanic neck which also have evidence for magma mixing are isotopically distinct major and trace element abundances are consistent with a genetic relationship between the basalt and rhyolite either by low pressure plagioclase dominated fractional crystallization or by partial melting of a plagioclase rich assemblage there is no evidence that the rhyolites contain more of a crustal component than the associated basic volcanics

role of subduction erosion in the generation of andean magmas subduction erosion or scraping off of the continental crust by subducting oceanic lithosphere is an important tectonic process along parts of the andean convergent plate boundary estimated rates of subduction erosion increase northward north of the southem andes due to decreasing subduction angle and decreasing sediment supply to the trench west of the central andes subduction erosion may remove as much as 535 km 3 of crust per 1 km of trench per 1 m y or 7 vol of the subducting oceanic lithosphere the higher 87 sr 86 sr ratios of mafic magmas erupted north of the southern andes may be explained by the higher rates of subduction erosion and increased amounts of contamination of the sub arc mantle magma source

a facies model for a quaternary andesitic composite volcano ruapehu new zealand ruapehu composite volcano is a dynamic volcanic sedimentary system characterised by high accumulation rates and by rapid lateral and vertical change in facies four major cone building episodes have occurred over 250 ka from a variety of summit flank and satellite vents eruptive styles include subplinian strombolian phreatomagmatic vulcanian and dome related explosive eruptions and extrusion of lava flows and domes the volcano can be divided into two parts a composite cone of volume 110 km3 surrounded by an equally voluminous ring plain complementary portions of ruapehu s history are preserved in cone forming and ring plain environments cone forming sequences are dominated by sheet and autobrecciated lava flows which seldom reach the ring plain the ring plain is built predominantly from the products of explosive volcanism both the distal primary pyroclastic deposits and the reworked material eroded from the cone much of the material entering the ring plain is transported by lahars either generated directly by eruptions or triggered by the high intensity rain storms which characterise the region ring plain detritus is reworked rapidly by concentrated and hyperconcentrated streams in pulses of rapid aggradation immediately following eruptions and more gradually in the longer intervals between eruptions

proterozoic metamorphism and cooling in the southern lake superior region north america and its bearing on crustal evolution abstract metamorphism along the southern margin of the archean superior province has been historically attributed to the penokean orogeny a narrow corridor of amphibolite facies rocks north of the main suture does record 1 83 1 80 ga metamorphic monazite u th pb ages that mark the culmination of arc accretion however subsequent widespread amphibolite facies metamorphism and associated magmatism is recorded along the regions of greatest penokean crustal thickening the tectonically buried archean proterozoic continental margin in minnesota new monazite geochronology reveals a profound midcrustal metamorphic imprint caused by emplacement of the 1 775 ga east central minnesota batholith at moderate depths in northern wisconsin and upper peninsula michigan metamorphic monazite growth at 1 78 1 745 ga and far from geon 17 intrusions reflect a previously little recognized regional amphibolite facies metamorphic event associated with ca 1 76 ga yavapai interval accretion not solely penokean induced crustal collapse south of the penokean suture penokean terrane rocks were twice metamorphosed to upper greenschist facies first during yavapai accretion and again during geon 16 mazatzal accretion geon 16 overprinting also affected a small part of the continental margin in the northeast orogen the peavy metamorphic node south directed basement thrusts there likely accommodated substantial mazatzal foreland shortening suggesting thick skinned deformation mazatzal amphibolite facies metamorphism occurred throughout iowa and southernmost wisconsin south of the baraboo quartzite 40 ar 39 ar mineral cooling ages from eastern wisconsin reveal a limited metamorphic aureole associated with the intrusion of the 1 47 ga wolf river batholith in part reflecting its rapid emplacement at shallow crustal levels a local area of anomalously young 40 ar 39 ar mica ages away from known midcontinent rift exposures probably reflect shallow crustal level reheating during rifting not exhumation the pattern and degree of proterozoic metamorphism preserved across the northern interior of the north american midcontinent is highly variable and is a reflection of multiple accretionary and intrusive events between 1 87 and 1 47 ga collectively the geo and thermochronologic data reveal a progressive tectonic younging toward the south and overall waning of proterozoic metamorphism in this region these results demonstrate a changing tectonic environment from active margin to continental interior as laurentia grew southward and stabilized over a few hundred million years

compositional and dynamic controls on mafic silicic magma interactions at continental arc volcanoes evidence from cordon el guadal tatara san pedro complex chile

petrology and geochemistry of lavas from antuco volcano a basaltic volcano of the southern andes 37 25 s sierra velluda plio pleistocene cerro condor pleistocene and antuco holocene volcanoes form a volcanic group located in the southern andes at latitude 37 25 s lavas from these volcanoes become less silicic with decreasing age antuco volcano consists of two units pre and post caldera collapse antuco i and antuco ii and a parasitic cone antuco lavas are basalts and those from antuco i and ii are relatively uniform in mineral and chemical composition common phenocrysts are plagioclase an65 an81 18 33 vol olivine fo 78 3 18 vol and clinopyroxene 0 5 3 vol the same phenocrysts occur in the parasitic cone lavas but plagioclase is much more abundant key compositional features of antuco lavas are 52 sio2 17 al2o3 3 3 4 2 na2o 1 2 tio2 0 9 k2o and mg mg fe ratios 0 60 these major element features characterize many circumpacific basalts in island arc and continental margin environments the antuco i and ii lavas have light ree abundances equal to 26 33 chondrites and heavy ree abundances equal to 8 9 chondrites some samples have positive 10 eu anomalies but no negative eu anomalies occur in these lavas trace element abundances in the antuco lavas are similar to those in theoretical liquids derived by 5 10 partial melting of garnet free peridotite followed by 10 fractionation of a clinopyroxene 40 and olivine 60 cumulate thus a plausible model is that the lavas were derived from the mantle by partial melting followed by mafic mineral fractionation

crustal xenoliths from calbuco volcano andean southern volcanic zone implications for crustal composition and magma crust interaction crustal xenoliths in the 1961 andesite flow of calbuco volcano in the southern southern volcanic zone ssvz of the andes consist predominantly of pyroxene granulites and hornblende gabbronorites the granulites contain plagioclase pyroxene magnetite amphibole and have pronounced granoblastic textures small amounts of relict amphibole surrounded by pyroxene plagioclase magnetite glass symplectites are found in some specimens these and similar textures in the gabbronorites are interpreted as evidence of dehydration melting mineral and bulk rock geochemical data indicate that the granulites are derived from an incompatible trace element depleted basaltic protolith that underwent two stages of metamorphism a moderate pressure high temperature stage accompanied by melting and melt extraction from some samples followed by thermal metamorphism after entrainment in the calbuco andesite lavas high nd t values 4 0 to 8 6 nd isotope model ages of 1 7 2 0 ga and trace element characteristics like chondrite normalized la yb and la nb 1 indicate that the protoliths were oceanic basalts similar oceanic metabasalts of greenschist to amphibolite facies are found in the paleozoic metamorphic belt that underlies the chilean coastal ranges mineral and bulk rock compositions of the gabbronorite xenoliths indicate that they are cognate crystallizing from the basaltic andesite magma at calbuco crystallization pressures for the gabbros based on total al contents in amphibole are 6 8 kbar these pressures point to middle to lower crustal storage of the calbuco magma neither granulite nor gabbro xenoliths have the appropriate geochemical characteristics to be contaminants of calbuco andesites although an ancient sedimentary contaminant is indicated by the lava compositions the presence of oceanic metabasaltic xenoliths together with the sedimentary isotopic imprint suggests that the lower crust beneath the volcano is analogous to the coastal metamorphic belt which is an accretionary complex of intercalated basalts and sediments that formed along the paleozoic gondwanan margin if this is the case the geochemical composition of the lower and middle crust beneath the ssvz is significantly different from that of most recent ssvz volcanic rocks

crustal assimilation during turbulent magma ascent ata new isotopic evidence from the mull tertiary lava succession n w scotland assimilation of crustal rocks with concomitant fractional crystallisation afc is a well documented phenomenon in many igneous suites but geochemical evidence from the tertiary mull lava succession suggests that in these magmas crustal contamination occurred by a distinctly different mechanism lavas from the lower half of the mull plateau group mpg can be divided into two broad sub types high 8 mgo basalts with elevated ba and k and lower mgo 8 basaltic hawaiites with lower ba and k the lower crust and most of the upper crust beneath mull is probably of lewisian age the sr nd and pb isotope compositions of local lewisian crustal samples yield the following ranges 87sr 86sr 0 71002 0 72348 143nd 144nd 0 51045 0 51058 and 206pb 204pb 14 0 14 6 ten lavas have also been analysed and yield the following ranges 87sr 86sr 0 7028 0 7042 143nd 144nd 0 51214 0 51230 and 206pb 204pb 15 1 17 9 however within this range it is predominantly the more primitive mafic compositions with elevated mg ba and k that show the lowest nd and pb and the highest sr isotope values modelling of these isotopic results in conjunction with major and trace element data show that 1 contamination by lewisian lower crustal material does occur 2 that the process involved was not one of assimilation with concomitant fractional crystallisation afc the proposed contamination process is one whereby the hottest most mgo rich magmas have assimilated acidic partial melts of lewisian lower crust during turbulent ascent ata through thin poorly connected dyke and sill like magma chambers the chemical composition of the contaminated lavas can be modelled successfully through addition of 5 acidic lewisian crust to an uncontaminated lava in contrast the more evolved magmas which probably fractionated at sub crustal levels were either not hot enough to molt significant amounts of crust or did not ascend turbulently because of their higher viscosity and so are less contaminated with crust

paleoproterozoic basin development and sedimentation in the lake superior region north america abstract the peneplaned archean craton in the lake superior region was the platform upon which a continental margin assemblage was deposited extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform seas transgressed onto the continent several times and an ocean basin opened south of the present day lake superior island arcs that formed during subduction collided with the craton margin as the ocean basin closed oceanic crust is poorly preserved as a dismembered ophiolite sequence the arc volcanics are preserved as the wisconsin magmatic terranes the collision resulted in a fold and thrust belt known as the penokean orogen to the north of the fold and thrust belt a northward migrating foreland basin the animikie basin developed thick turbidite successions were deposited along the basin axis and terrigenous clastics and lake superior type iron formation were deposited on the shelf along the northern margin of the basin the primary paleoclimatic indicators are 1 glaciogenic rocks at the base of the paleoproterozoic succession in michigan indicating ice house conditions 2 remnants of a paleosol on the glaciogenic rocks indicative of deep weathering probably under subtropical conditions and therefore of greenhouse conditions and 3 carbonate minerals after gypsum halite and anhydrite in stromatolitic dolomite indicative of aridity three second order depositional sequences are bounded by major unconformities and can be correlated throughout the lake superior region

geochemistry of the active azufre planchon peteroa volcanic complex chile 35 15 s evidence for multiple sources and processes in a cordilleran arc magmatic system

buffering of melt oxygen fugacity by sulphur redox reactions in calc alkaline magmas calculated oxygen fugacities of magmas from lascar volcano northern chile and the 1991 eruption of mount pinatubo phillipines are seen to increase with decreasing temperature relative to the fayalite magnetite quartz buffer isopleths of constant h2s so2 at moderate pressures 2 4 kbar lie parallel to the data we conclude that magma mixing prior to eruption liberated large quantities of so2 gas by oxidation of dissolved sulphide in the mafic end member oxygen fugacity of the magmas was then buffered by reduction of this gas to h2s leading to increasingly oxidized magmas such a process might be used to explain the highly oxidized nature of other sulphur rich evolved calc alkaline magmas notably the el chichon trachyandesite

volcanism and erosion during the past 930 k y at the tatara san pedro complex chilean andes geologic mapping together with 73 new k ar and 40ar 39ar age determinations of 45 samples from 17 different volcanic units plus paleomagnetic orientations geochemical compositions and terrestrial photogrammetry are used to define the chronostratigraphy of the tatara san pedro complex an eruptive center at 36 s on the volcanic front of the andean southern volcanic zone the tatara san pedro complex preserves 55 km3 of lavas that erupted from at least three central vent regions remnant unconformity bound sequences of lavas are separated by lacunae that include significant periods of erosion quaternary volcanism commenced ca 930 ka with eruption of voluminous dacitic magma followed 100 k y later by the only major rhyolitic eruption from 780 ka onward more than 80 of the preserved volume is basaltic andesite 52 57 sio2 but petrographically and geochemically diverse dacitic magmas 63 69 sio2 erupted sporadically throughout this younger dominantly mafic phase of activity a few basaltic lavas 49 52 sio2 are present mainly in portions of the complex older than 230 ka the number of vents the petrologic and geochemical diversity and the temporal distribution of mafic and silicic lavas are consistent with emplacement of many separate batches of mafic magma into the shallow crust beneath the tatara san pedro complex over the past million years nearly two thirds of the preserved volume of the tatara san pedro complex comprises the two youngest volcanoes which were active between ca 188 83 ka and 90 19 ka repeated advances of mountain glaciers punctuated growth of the complex with major erosional episodes that removed much of the pre 200 ka volcanic record particularly on the south flank of the complex dating the inception of a glaciation on the basis of preserved material is difficult but the age of the oldest lava above a lacuna may be used to estimate the timing of deglaciation on this basis the argon ages of basal lavas of multiple sequences indicate minimum upper limits of lacunae at ca 830 790 610 400 330 230 110 and 17 ka these are broadly consistent with global ice volume peaks predicted by the oxygen isotope based astronomical time scale and with age brackets on north american glacial advances estimated growth rates for the two young volcanoes are 0 2 to 0 3 km3 k y these are three to five times greater than a growth rate estimated from all preserved lavas in the complex 0 06 km3 k y removal of up to 50 95 of the material erupted between 930 and 200 ka by repeated glacial advances largely explains this discrepancy and it raises the possibility that episodic erosion of mid latitude frontal arc complexes may be extensive and common

the loma seca tuff and the calabozos caldera a major ash flow and caldera complex in the southern andes of central chile a 26 14 km composite ring structure caldera of late pleistocene age has been discovered and mapped near the andean crest in central chile 35 30 s rhyodacitic to dacitic zoned ash flow sheets each representing 150 to 300 km 3 of magma were emplaced 0 8 0 3 and 0 15 m y ago the youngest of the associated collapses was closely followed by resurgent doming of the caldera floor and development of a longitudinal graben postcaldera eruptions of dacite and andesite have persisted into holocene time and active hot springs are abundant along caldera marginal and resurgent fault systems suggesting a significant geothermal energy resource the pleistocene eruption rate of this district and the abundance of older quaternary to miocene ash flow remnants in the 33 s to 36 s segment of the glaciated southern andes indicate that ash flow magmatism has been no less important here than in the arid central andes 16 s 28 s where ash flow sheets are far better preserved

recent volcanism in the puyehue cordon caulle region southern andes chile 40 5 s petrogenesis of evolved lavas

petrology of lavas from the puu oo eruption of kilauea volcano iii the kupaianaha episode 1986 1992 the puu oo eruption has been remarkable in the historical record of kilauea volcano for its duration over 13 years volume 1 km3 and compositional variation 5 7 10 wt mgo during the summer of 1986 the main vent for lava production moved 3 km down the east rift zone and the eruption style changed from episodic geyser like fountaining at puu oo to virtually continuous relatively quiescent effusion at the kupaianaha vent this paper examines this next chapter in the puu oo eruption episodes 48 and 49 and presents new icp ms trace element and pb sr and nd isotope data for the entire eruption 1983 1994 nearly aphyric to weakly olivine phyric lavas were erupted during episodes 48 and 49 the variation in mgo content of kupaianaha lavas erupted before 1990 correlates with changes in tilt at the summit of kilauea both of which probably were controlled by variations in kilauea s magma supply rate these lavas contain euhedral olivines which generally are in equilibrium with whole rock compositions although some of the more mafic lavas which erupted during 1990 a period of frequent pauses in the eruption accumulated 2 4 vol olivine the highest forsterite content of olivines 85 in kupaianaha lavas indicates that the parental magmas for these lavas had mgo contents of 10 wt which equals the highest observed value for lavas during this eruption the composition of the puu oo lavas has progressively changed during the eruption since early 1985 episode 30 when mixing between an evolved rift zone magma and a more mafic summit reservoir derived magma ended the normalized to 10 wt mgo abundances of highly incompatible elements and cao have systematically decreased with time whereas ratios of these trace elements and pb sr and nd isotopes and the abundances of y and yb have remained relatively unchanged these results indicate that the hawaiian plume source for puu oo magmas must be relatively homogeneous on a scale of 10 20 km3 assuming 5 10 partial melting and that localized melting within the plume has apparently progressively depleted its incompatible elements and clinopyroxene component as the eruption continued the rate of variation of highly incompatible elements in puu oo lavas is much greater than that observed for kilauea historical summit lavas e g ba y 0 09 a 1 vs 0 03 a 1 this rapid change indicates that puu oo magmas did not mix thoroughly with magma in the summit reservoir thus except for variable amounts of olivine fractionation the geochemical variation in these lavas is predominantly controlled by mantle processes

the origin and evolution of magmas from the san pedro pellado volcanic complex s chile multicomponent sources and open system evolution the san pedro pellado volcanic complex is located at 36 s in the chilean andes the eruptive rocks of the complex record the development and collapse of a caldera followed by voluminous largely basaltic andesite volcanism at each stage of evolution crystal fractionation was accompanied by variable degrees of contamination and mixing large variations in incompatible element ratios cannot be produced by closed system evolution correlations between indices of differentiation and incompatible element ratios together with high 18o values indicate that basaltic andesites have assimilated crust to generate the evolved volcanic rocks at san pedro pellado even in the most mafic rocks however incompatible element characteristics are variable as a result of source heterogeneity and deep level processes the restricted ranges in isotope ratios of sr nd and pb among san pedro pellado rocks are due to the small contrast in isotopic compositions between magma and wallrock three source components are recognized as contributing to parental magmas at san pedro pellado although the relative contributions of each cannot be quantified the volumetrically dominant source component is the sub arc asthenospheric mantle morb source the major source of lile is thought to be slab derived fluids which modified the sub arc mantle other incompatible elements may also have been enriched by interaction with the continental lithosphere mantle and or lower crust during ascent

implications of a large flattening strain for the origin of a bedding parallel foliation in the early proterozoic thomson formation minnesota abstract a bedding parallel foliation exists in an area of multiple deformation in the early proterozoic thomson formation of minnesota comparison of strain measurements in a region where the bedding parallel foliation occurs with those from an area where it does not indicates that a large flattening strain accompanied the development of the foliation this and the relationship of the foliation to other structures demonstrates that it is a tectonic foliation and not a result of inherited depositional orientation or mimetic recrystallization along bedding

paricutin re examined a classic example of crustal assimilation in calc alkaline magma following its birth on the 20th of february 1943 the mexican volcano paricutin discharged a total of 1 38 km3 of basaltic andesite and andesite before the eruption came to an end in 1952 until 1947 when 75 of the volume had been erupted the lavas varied little in chemical or isotopic composition all were basaltic andesites with 55 to 56 sio2 18o of 6 9 to 7 0 and 87sr 86sr ratios close to 0 7038 subsequent lavas were hypersthene andesites with silica contents reaching 60 18o values up to 7 6 and 87sr 86sr of 0 7040 to 0 7043 the later lavas were enriched in ba rb li and k2o and depleted in mgo cu zn cr ni sr and co the isotopic and other chemical changes which appeared abruptly over a few months in 1947 are interpreted as the result of tapping a sharply zoned and density stratified magma chamber xenoliths of partially fused felsic basement rocks in the lavas have silica contents greater than 70 18o of 5 6 to 9 9 and 87sr 86sr between 0 7043 and 0 7101 in many respects they resemble samples of basement rocks collected from nearby outcrops three analysed samples of the latter have silica contents of 65 to 67 18o of 7 7 to 8 6 and 87sr 86sr between 0 7047 and 0 7056 these new data provide strong support for the original interpretations of wilcox 1954 who explained the chemical variations by a combination of fractional crystallization and concurrent assimilation of up to 20 weight continental crust except for a few trace elements particularly ba sr and zr the chemical and isotopic compositions of the xenoliths and basement rocks that crop out nearby match the type of contaminant required to explain the late stage lavas some of the discrepancies may be explained by postulating a contaminant that was older and richer in ba sr and zr than those represented by the analysed xenoliths others can be attributed to chemical changes accompanying disequilibrium partial melting contact metamorphism and meteoric hydrothermal alteration of the country rock many of the xenoliths show evidence of having been affected by such processes the lavas were erupted from a zoned magma chamber that had differentiated by liquid fractionation prior to the eruption the order of appearance of the lavas can be explained in terms of withdrawal of stratified liquids of differing densities and viscosities

the influence of water on the petrogenesis of subductionrelated igneous rocks the presence of dissolved water can significantly change the sequence in which different minerals crystallize from a silicate magma1 3 this will alter the compositional path followed by residual liquids the liquid line of descent and modify the types and compositions of crystals that accumulate at the site of cooling the sharp decrease in the solubility of water in silicate melts with decreasing pressure makes direct measurements of pre eruptive concentrations in magmas difficult because much of the dissolved water boils off before eruption here we report experimental results on the crystallization of basalts that contain substantial dissolved water up to 6 wt and show how the results can be used to infer the amount of dissolved h2o involved in the petrogenesis of lavas that preserve a record of their liquid lines of descent or the accumulated minerals left behind after solidification the presence of abundant magmatic water is a signature of subduction zone volcanism4 and can be used to associate a suite of magmas or igneous cumulates with a convergent margin back arc or interarc tectonic setting

on the origin of early proterozoic gneiss domes and metamorphic nodes northern michigan biotite 40 r 39 r cooling ages from medium pressure 500 600 mpa rocks in the watersmeet district northern michigan suggest significant cooling uplift and concomitant deformation during gneiss dome formation at 1755 ma well after the close of the 1870 1830 ma penokean orogeny however an 1822 ma hornblende plateau date indicates that the isograds surrounding the dome are penokean in age we attribute gneiss dome formation and doming of penokean aged isograds to an episode of orogenic collapse superimposed on an earlier history of crustal shortening this contrasts with the compressional origin for gneiss domes preserved in the low pressure 200 300 mpa republic district the different origins may reflect the fact that collapse was localized along the overthickened region of the orogenic belt in contrast to the watersmeet area hornblende and biotite 40ar 39ar ages obtained from the republic area are 1720 1680 ma given the relatively shallow depth of this region it is unlikely that temperatures re

strain variations across the proterozoic penokean orogen usa and canada abstract strata in the huron 2 5 2 0 ga and animikie 2 2 1 85 ga basins were deposited on the southern margin of the archean superior province these rocks were deformed during the penokean orogeny 1850 ma followed by subsequent accretionary orogens to the south at 1750 ma yavapai and 1630 ma mazatzal strain patterns are unique to each orogenic belt with no far field effect archean wawa terrane rocks in the penokean foreland preserve deformation associated with archean accretion with no younger penokean yavapai or mazatzal strain overprint the penokean orogeny deformed huron animikie basin sediments into a north vergent fold and thrust belt with no yavapai or mazatzal strain overprint yavapai orogen strains sw ne margin parallel shortening are unique when compared to the younger mazatzal shortening n20 w shortening with no strain overprint penokean deformation is characterized by shortening from the south including uplifted archean gneisses and a northerly thin skinned fold and thrust belt with north vergent nappes and a gently dipping foreland our study of finite and calcite twinning strains n 60 along 1500 km and across 200 km the penokean belt indicate that this orogeny was collisional as layer parallel shortening axes are parallel across the belt or parallel to the tectonic transport direction n s penokean nappe burial near the margin resulted in vertical shortening strain overprints some of which are layer normal the sudbury impact layer 1850 ma is found across the animikie basin and provides a widespread deformation marker with many local unique strain observations we also report new geochronology u pb zircon and apatite for the gneiss mafic dike rocks at wissota chippewa falls wi and arbutus black river falls wi dams respectively which bears on penokean yavapai deformation in the archean marshfield terrane which was accreted during the penokean orogen pseudotachylite formation was common in the superior province archean basement rocks especially along terrane boundaries reactivated by contemporaneous penokean trans hudson cape smith and new quebec deformation in the hinterland south the younger yavapai orogen 1750 ma n 8 deformation is preserved as margin parallel horizontal shortening sw ne in yavapai crust and up to 200 km to the north in the penokean thrust belt as a strain and barrovian metamorphic overprint mazatzal deformation 1630 ma n 16 is preserved in quartzites on yavapai and penokean crust with layer parallel and layer normal shortening strains oriented n20 w

the penokean orogeny in the lake superior region abstract the penokean orogeny began at about 1880 ma when an oceanic arc now the pembine wausau terrane collided with the southern margin of the archean superior craton marking the end of a period of south directed subduction the docking of the buoyant craton to the arc resulted in a subduction jump to the south and development of back arc extension both in the initial arc and adjacent craton margin to the north a belt of volcanogenic massive sulfide deposits formed in the extending back arc rift within the arc synchronous extension and subsidence of the superior craton resulted in a broad shallow sea characterized by volcanic grabens menominee group in northern michigan the classic lake superior banded iron formations including those in the marquette gogebic mesabi and gunflint iron ranges formed in that sea the newly established subduction zone caused continued arc volcanism until about 1850 ma when a fragment of archean crust now the basement of the marshfield terrane arrived at the subduction zone the convergence of archean blocks of the superior and marshfield cratons resulted in the major contractional phase of the penokean orogeny rocks of the pembine wausau arc were thrust northward onto the superior craton causing subsidence of a foreland basin in which sedimentation began at about 1850 ma in the south baraga group rocks and 1835 ma in the north rove and virginia formations a thick succession of arc derived turbidites constitutes most of the foreland basin fill along with lesser volcanic rocks in the southern fold and thrust belt tectonic thickening resulted in high grade metamorphism of the sediments by 1830 ma at this same time a suite of post tectonic plutons intruded the deformed sedimentary sequence and accreted arc terranes marking the end of the penokean orogeny the penokean orogen was strongly overprinted by younger tectonic and thermal events some of which were previously ascribed to the penokean principal among these was a period of vertical faulting in the archean basement and overlying paleoproterozoic strata this deformation is now known to have post dated the terminal penokean plutons by at least several tens of millions of years evidence of the penokean orogen is now largely confined to the lake superior region comparisons with more recent orogens formed by similar plate tectonic processes implies that significant parts of a once more extensive penokean orogen have been removed or overprinted by younger tectonic events

a fifty year perspective of magmatic evolution on ruapehu volcano new zealand verification of open system behaviour in an arc volcano geochemical and petrological data for samples from well documented eruptions that occurred at ruapehu volcano over the period 1945 1996 can be used to illustrate the complexity of short term geochemical variation in an arc type volcano collectively data from ruapehu volcano show trends with time of increasing sio2 abundance and rising 87sr 86sr ratios consistent with broad control by assimilation and crystal fractionation processes afc however the magmas emplaced during the past fifty years show geochemical variability that spans most of the range shown by lavas erupted over the entire history of the volcano magma compositions fluctuate through wide ranges over relatively short time intervals reflecting the effects of processes associated with magma recharge events within the volcano these complex trends are also manifested when the geochemistry and petrography of sequences of prehistoric lavas are examined in detail and they arise from short term effects that are imposed during recharge on the overall afc trend we show that the temporal geochemical and petrographic variations among erupted magmas are modulated by processes of mixing and mingling between fresh magma from below and stagnant melt and entrained crystals from earlier events remaining in the volcanic edifice probably in dikes and sills these processes are probably replicated over longer time periods hundreds to thousands of years as melts arrested at different levels in the near surface conduit system are progressively displaced by new magma batches arc type volcanoes such as ruapehu are characterised by pulsatory growth in which bursts of high magma production are superimposed on a background of subdued but more or less continuous activity this style of activity is difficult to predict through the usual seismicity ground deformation lake water geochemistry volcano monitoring techniques and petrology and geochemistry may provide the basis for an alternative strategy

magma supply and storage at kilauea volcano hawaii 1956 1983 abstract shallow crustal magma reservoirs beneath the summit of kilauea volcano and within its rift zones are linked in such a way that the magma supply to each can be estimated from the rate of ground deformation at the volcano s summit our model builds on the well documented pattern of summit inflation as magma accumulates in a shallow summit reservoir followed by deflation as magma is discharged to the surface or into the rift zones magma supply to the summit reservoir is thus proportional to summit uplift and supply to the rift zones is proportional to summit subsidence the average proportionality constant is 0 33 10 6 m 3 rad this model yields minimum supply estimates because it does not account for magma which escapes detection by moving passively through the summit reservoir or directly into the rift zones calculations suggest that magma was supplied to kilauea during july 1956 april 1983 at a minimum average rate of 7 2 10 6 m3 month roughly 35 of the net supply was extruded the rest remains stored within the volcano s east rift zone 55 and southwest rift zone 10 periods of relatively rapid supply were associated with the large kapoho eruption in 1960 and the sustained mauna ulu eruptions in 1969 1971 and 1972 1974 bursts of harmonic tremor from the mantle beneath kilauea were also unusually energetic during 1968 1975 suggesting a close link between kilauea s deep magma supply region and shallow storage reservoirs it remains unclear whether pulses in magma supply from depth give rise to corresponding increases in shallow supply or if instead unloading of a delicately balanced magma transport system during large eruptions or intrusions triggers more rapid ascent from a relatively constant mantle source

petrology of the plio quaternary volcanism of the south central and meridional andes plio quaternary volcanoes lay upon a continental crust which is thick and of precambrian paleozoic age in the south central andes sca 18 28 s whereas it is relatively thin and of mesozoic cenozoic age in the meridional andes ma 32 42 s eleven calc alkaline volcanic massifs or groups of small volcanoes and two shoshonitic volcanoes have been studied in the sca eleven calc alkaline volcanic massifs have been studied in the ma one alkaline volcano has been approached in argentinian patagonia calc alkaline lavas are mostly andesites and dacites in the sca and high al basalts and basic andesites in the ma shoshonitic lavas are absarokites shoshonites and banakites no cognate xenoliths were found in the lavas the texture of the lavas is microlitic with abundant calc alkaline lavas or rare shoshonitic lavas phenocrysts distribution of phenocrysts opposes calc alkaline lavas of the sca with hornblende and biotite to those of the ma with olivine in calc alkaline lavas plagioclase is the most abundant mineral phase from high al basalts an90 to rhyolites an20 olivine is fo78 65 phenocrysts in basic lavas clinopyroxene ca mg rich augite and orthopyroxene bronzite show little compositional variation with lava differentiation amphibole is pargasitic or edenetic hornblende crystal clots which result from resorption of hornblende are found biotite is ti rich phenocrysts of quartz are found in rhyolites and rimmed by pyroxenes in basic lavas of the sca microlites of magnetite are ubiquitous and phenocrysts occur only in the sca in the sca geothermometers and geobarometers give the following results equilibration temperatures which decrease from 1000 c in andesites to 900 c in dacites h2o pressures vary between 1 and 4 kbar and oxygen fugacities are higher than the ni nio buffer basic lavas may have equilibrated at 50 km depth in shoshonitic lavas plagioclase and k feldspar are present olivine is fo85 phenocrysts and fo65 microlites clinopyroxene is cr rich orthopyroxene is bronzite in shoshonites microlites and banakites microlites and phenocrysts hornblende is pargasitic biotite is t rich in banakites conditions of differentiation of the lavas are high temperatures 850 1050 c water pressures inferior to 2 kbar oxygen fugacities above the ni nio buffer and total pressures which correspond to depths of 80 120 km major and trace element distributions emphasize differences between calc alkaline associations of the sca and ma and between calc alkaline and shoshonitic associations in shoshonitic lavas decreases in mgo and ni cao and cr al2o3 and sr correspond to respective olivine clinopyroxene and plagioclase fractionations incompatible elements circum pacific characteristics high al2o3 constant na2o3 increasing k2o with differentiation andean characteristics low cao and peculiar characteristics ma lavas have lower mgo olivine fractionation in the sca lavas have lower tio2 and feo fractionations of magnetite pyroxenes and hornblende the origin of the andean magmatism is not consistent with single stage models anatexis of continental crust crystal fractionation of basaltic magma under amphibolite or eclogite facies partial melting of peridotitic upper mantle material a three stage model is proposed the first stage consists of partial melting of upper mantle material loss in fluids of the subducted oceanic crust induces the melting bubbles of primary magma are generated the second stage consists of selective crustal contamination during the stay of the magma at the base of the continental crust the third stage consists of low pressure crystal fractionation which governs the differentiation of the lava series the present model considers transversal and longitudinal zonation of the andes high degrees of melting low rate of contamination and reduced crystal fractionation characterize the ma whereas low degrees of melting high rate of contamination and considerable crystal fractionation characterize the sca garnet is involved in the genesis of shoshonitic lavas

evolution of galapagos magmas mantle and crustal fractionation without assimilation summary of helium isotopic data from the gala pagos islands and dredged samples from the gala pagos platform data from graham et al 1993 and kurz et al 1993 data are normalized to the atmospheric value r ra most gala pagos samples are intermediate betweenmorb value of 8 1 and the fernandina value 22 material in those magmas the exception is floreana structure first the depth to the magma chambers mayisland which ranges from 14 5 to 7 7 ra it is possible be due to the rate at which magma is supplied from thethat the low value at floreana re ects assimilation of mantle as the volcano is carried away from the focus ofmaterial with excess radiogenic 4 he by a magma with the hotspot the volcanoes in a juvenile growing stage 3 he 4 he ra 14 most floreana lavas contain abundant of evolution those that have had numerous historicdisaggregated debris from ultrama c xenoliths bow eruptions are dominated by lavas with mgo con geist 1992 a potential source of helium contamination centrations between 5 and 6 e g reynolds geist 1995 a narrow deep caldera characterizes the juvenilestage e g cerro azul wolf and fernandina and iteventually broadens and shallows because of lling with

assimilation of felsic crust by basaltic magma thermal limits and extents of crustal contamination of mantle derived magmas basaltic magma is the most widely used probe of mantle chemistry and structure chemical characterization of the sources for mantle derived magma is possible only after quantitative evaluation of possible contamination by crustal wall rocks the most commonly invoked contamination mechanisms are the combined processes of crustal assimilation and magmatic fractional crystallization afc because of the energetic link between these processes predicting assimilation and crystallization progress requires modeling both heat and mass transfer in the magma country rock system using the melts software package we modeled isenthalpic heat balanced afc between two common types of basaltic magma and several types of felsic crust at 1 kbar our simulations show that during the early stages of isenthalpic afc the ratio of rates of assimilation to crystallization r may be substantially greater than one 2 0 2 7 allowing assimilation of a mass of country rock up to 5 18 of the initial magma mass with only 3 7 crystallization the second stage of afc beginning with plagioclase and or pyroxene saturation is characterized by lower r 0 5 1 0 the initial high r stage results from suppression of crystallization associated with the change in magma composition as assimilation progresses under certain conditions even small degrees of crystallization of olivine alone coupled with cooling of the magma can accommodate relatively large amounts of crustal assimilation and cause large shifts in isotopic and trace element geochemical indices with little differentiation

postcollisional cooling of the penokean orogen in east central minnesota new 40ar 39ar thermochronologic data from the internal zone of the penokean orogen in east central minnesota indicate rapid cooling from mid crustal temperatures early proterozoic metamorphosed cover rocks yield concordant hornblende and biotite plateau ages of 1755 ma underlying archean basement gneiss yields concordant muscovite and biotite plateau ages of 1705 ma these are some of the oldest cooling ages recorded in rocks buried during the 1830 1860 ma penokean collisional orogeny cooling was coeval with regional rhyolitic volcanism and granite emplacement suggesting a period of tectonic activity in this region during mid to late 1700 ma based on similarities with phanerozoic extensional regimes we suggest preliminarily that these data might be explained by a previously unrecognized major extensional episode in the lake superior region

inherited argon in a pleistocene andesite lava 40ar 39ar incremental heating and laser fusion analyses of plagioclase by using 40 ar 39 ar incremental heating and laser fusion techniques xenocrystic plagioclase was discovered in a late pleistocene andesitic lava that erupted through the andean cordillera inherited argon in the xenocrysts is as much as 450 times older than the host lava the age of which is independently known and is an obstacle to dating the lava by using a whole rock sample the xenocrysts are impossible to identify from petrography or chemical parameters such as their k ca ratios holocrystalline groundmass carefully separated from plagioclase and other phenocrysts gives an accurate 40 ar 39 ar age for the lava the xenocrysts could not have been degassed for more than several days in the magma and probably were assimilated from paleozoic rocks buried under kilometers of mesozoic and tertiary arc rocks composing the cordillera in central chile the required magma ascent velocity on the order of kilometers day is extraordinarily high compared to the 10 4 km day minimum implied by the 226 ra excess in continental arc lavas these data permit magma migration and storage in the mantle and lower crust for as much as thousands of years followed abruptly by rapid ascent to the surface

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

geochemical evidence for invasion of kilauea s plumbing system by mauna loa magma from the beginning of the study of hawaiian volcanism there has been controversy over possible relationships between the neighbouring active volcanoes mauna loa and kilauea1 5 seismic activity thought to reflect upward migration of magma reveals that the magmatic plumbing systems apparently converge at depth to form a broad funnel within the mantle6 although on rare occasions they have erupted concurrently the brief historical eruptive record appears to show that when kilauea is most active mauna loa is in repose and vice versa suggesting that they may be competing for the same magma supply5 7 petrological geochemical and isotope data however require a diametrically opposite conclusion distinct differences in major element trace element and isotope compositions of lavas are regarded as compelling evidence that the two volcanoes have separate magmatic plumbing systems supplied by parental magmas from physically and geochemically distinct mantle sources8 13 here we present preliminary geochemical data which show that in the past 2 000 years kilauea has erupted a spectrum of lava compositions resembling historical kilauea lavas at one end and mauna loa lavas at the other we discuss the cause of this diversity and speculate that magma from mauna loa may have invaded kilauea s high level magmatic plumbing system

estimation of co2 saturation during both co2 drainage and imbibition processes based on both seismic velocity and electrical resistivity measurements summary in order to monitor injected carbon dioxide co2 simultaneous measurements of seismic velocity and electrical resistivity are employed during the drainage co2 injection and imbibition water injection processes of a berea sandstone supercritical co2 10 mpa at 40 o c was injected into a water saturated berea sandstone in the drainage stage and monitored via simultaneous measurements after the injection of supercritical co2 fresh distilled water was injected into the co2 injected sandstone during the imbibition stage electrical resistivity and p wave velocity measurements acquired during the drainage and imbibition stages were employed to evaluate co2 saturations sco2 based on the resistivity index and the gassmann fluid substitution equations respectively comparing estimated values for sco2 saturation against those from volume derived sco2 based on analysis on injected and drained fluid volumes in the drainage process we conclude that gassmann brie and resistivity index are suitable for the evaluation based on p wave velocity and electrical resistivity respectively rt based estimation properly tracks the variation in sco2 even when sco2 is large 0 15 while vp based estimation is sensitive to the variation in sco2 when sco2 is small 0 1 while properly estimating sco2 when sco2 is small 0 1 monitoring the co2 imbibition process confirms residual co2 saturation within the sample

compositional control of magnetic anisotropy in the thomson formation east central minnesota abstract anisotropy of magnetic low field susceptibility ams and anisotropy of anhysteretic susceptibility aas of the middle precambrian thomson formation east central minnesota generally record structural features but orientations and axial ratios of the susceptibility ellipsoids depend on local magnetic mineralogy the northern structural zone of the thomson formation is characterized by an upright east west slaty cleavage the susceptibility ellipsoids plot in the flattening field of a flinn diagram with minimum axes normal to the cleavage aas ellipsoids are coaxial with ams ellipsoids but are generally more prolate anisotropy increases with mean susceptibility at one outcrop interpreted as mixing of weakly anisotropic paramagnetic chlorite with more strongly anisotropic magnetite a second outcrop shows the opposite pattern anisotropy decreases with increasing mean susceptibility suggesting that here the magnetite may be relatively isotropic or possibly post deformational the southern structural zone of the thomson formation is characterized by a subhorizontal foliation s 1 and an upright crenulation cleavage s 2 here the magnetic susceptibility minima are coincident with poles to s 1 and maxima are parallel to the s 1 s 2 intersection correlation between directional susceptibilities and mean susceptibility technique of henry and daly 1983 suggests that paramagnetic minima are poles to the early foliation while possibly interdeformational magnetite minima record the second deformation this interpretation however is not supported by the aas fabric by thin section observations or by high field measurements of paramagnetic susceptibility alternatively both directional and mean susceptibilities may be controlled by syndeformational chlorite

non explosive constructional evolution of the ice filled caldera at volcan sollipulli chile a radar and gravity survey of the ice filled caldera at volcan sollipulli chile indicates that the intra caldera ice has a thickness of up to 650 m in its central part and that the caldera harbours a minimum of 6 km3 of ice reconnaissance geological observations show that the volcano has erupted compositions ranging from olivine basalt to dacite and have identified five distinct volcanic units in the caldera walls pre or syn caldera collapse deposits the sharkfin pyroclastic unit comprise a sequence which evolved from subglacial to subaerial facies post caldera collapse products which crop out along 17 of the 20 km length of the caldera wall were erupted almost exclusively along the caldera margins in the presence of a large body of intra caldera ice the alpehue crater formed by an explosive eruption between 2960 and 2780 a bp in the southwest part of the caldera is shown to post date formation of the caldera sollipulli lacks voluminous silicic pyroclastic rocks associated with caldera formation and the collapse structure does not appear to be a consequence of a large magnitude explosive eruption instead lateral magma movement at depth resulting in emptying of the magma chamber may have generated the caldera the radar and gravity data show that the central part of the caldera floor is flat but within a few hundred metres of the caldera walls the floor has a stepped topography with relatively low density rock bodies beneath the ice in this region this coupled with the fact that most of the post caldera eruptions have taken place along the caldera walls implies that the caldera has been substantially modified by subglacial marginal eruptions sollipulli caldera has evolved from a collapse to a constructional feature with intra caldera ice playing a major role the post caldera eruptions have resulted in an increase in height of the walls and concomitant deepening of the caldera with time

comparative mass balance of volcanic edifices at the southern volcanic zone of the andes between 33 s and 46 s volcano edifice volume calculations are presented for 65 volcanoes of the 1400 km long chilean southern volcanic zone svz as a basic step in subduction zone mass budgets volume calculations are performed in a geographical information system that integrates digital elevation models based on of shuttle radar topography mission as well as aster gdem topographic data landsat satellite images and geological maps the method of volume calculation is straightforward for isolated morphologically well defined stratovolcanoes uncertainties increase for volcanic edifices that formed on pre existing rugged terrain for multi phase eruptive centers as well as for eroded edifices a revised segmentation of the arc is used to describe the spatial volume distribution of extruded magma along the svz and to discuss controlling tectonic factors peak volumes between arc and back arc are offset by 400 km the total volcanic extrusion is in the range of 10 13 km3 km ma major differences between the svz and the central american subduction system are notable with regard to volcano density and maximum volumes

bundled slaty cleavage in laminated argillite north central minnesota abstract exceptional bundled slaty cleavage defined herein has been found in drill cores of laminated folded weakly metamorphosed argillite at several localities in the early proterozoic animikie basin of north central minnesota the cleavage domains are more closely spaced within the cleavage bundles than outside them the mean tectosilicate grain size of siltstone layers measured normal to cleavage is less in the cleavage bundles than outside them and the cleavage bundles are enriched in opaque phases and phyllosilicates relative to extra bundle segments these facts suggest that pressure solution was a major factor in bundle development if it is assumed that opaque phases have been conserved during pressure solution the modal differences in composition between intra bundle and extra bundle segments of beds provide a means for estimating bulk material shortening normal to cleavage argillite samples from the central part of the animikie basin have been shortened a minimum of about 22 as estimated by this method these estimates are similar to the shortening values derived from other strain markers in other rock types interbedded with the argillite and are also consistent with the regional pattern of deformation

age and duration of the matuyama brunhes geomagnetic polarity reversal from 40ar39ar incremental heating analyses of lavas abstract constraints on the timing of geomagnetic polarity reversals have come mainly from k ar or more recently 40 ar 39 ar age determinations of lavas or their k rich phenocrysts that erupted prior or subsequent to particular geomagnetic events we have obtained 40 ar 39 ar isochron ages using incremental heating techniques on groundmass separates phenocryst poor whole rock samples or plagioclase from eight basaltic to andesitic lavas that erupted during the matuyama brunhes m b polarity transition at four geographically dispersed sites these eight lavas range from 784 6 7 1 ka to 770 8 5 2 ka 1 errors the weighted mean 778 7 1 9 ka gives a high precision age that is remarkably consistent with revised astronomical age estimates for the m b polarity transition 6 12 13 despite uncertainties in absolute calibration of 40 ar 39 ar ages relative to mineral standards used as neutron fluence monitors our age determinations are consistent with five other 40 ar 39 ar studies focused on the m b transition these results confirm that the earlier k ar based geomagnetic polarity time scale underestimated the age of the m b reversal by about 6 none of the eight isochron ages are distinguishable from one another at the 95 confidence level however we are tantalizingly close to testing for the duration of this reversal one lava at the base of a sequence of transitionally magnetized flows in chile and the uppermost lava in a similar sequence on maui are only just indistinguishable in age at the 95 confidence level and preserve different magnetic orientations we suggest that the 12 kyr difference in age represents an upper limit for the duration of the reversal and is similar to the period of low magnetic field intensity associated with records of the m b reversal from deep sea sediment cores together with the short duration 2 kyr of the directional reversal observed in several different marine sediment sections our data suggest that reversal of the field s direction could have occurred at slightly different times depending on the position of the recording site

titanium substitution in biotite an empirical model with applications to thermometry o2 and h2o barometries and consequences for biotite stability abstract an empirical model for solution of ti in biotite is developed and calibrated on the basis of various equilibria among biotite and ti saturating assemblages sources of data used for the calibration include both experimental products and published analyses of natural assemblages ti solution in biotite can be satisfactorily modelled by means of the exchange component tife 2 whose non ideal behavior can be approximated empirically by means of a linear function of the relative concentrations of ti and octahedral al in biotite within assemblages which fix the chemical potential of the exchange component tife 2 the concentration of this component in biotite is very sensitive to temperature and oxygen fugacity making it possible to use biotite oxide assemblages to obtain reliable t and f o 2 estimates in igneous and metamorphic rocks in assemblages which also contain alkali feldspar it is possible to use breakdown equilibria of ti additive components in biotite to estimate h 2 o fugacity such uses are likely to find important applications in rocks in which other assemblages sensitive to temperature f o 2 and or f h 2 o are absent in general rocks containing a single oxide phase plus biotite as the only mafic silicate phase alkali feldspar the empirical model for solution of ti in biotite also allows to calculate the loci of various terminal reactions for the assemblage titanian biotite quartz which by comparison with equivalent equilibria in ti free systems provide insight into the effect of ti on the stability of biotite solubility of ti in biotite increases with temperature in a strongly non linear fashion as a consequence calculations predict that the effect of ti on the stability of biotite is very small up to conditions typical of the beginning of the granulite facies of metamorphism but beyond those conditions ti concentration increases markedly and biotite is rendered very refractory at mid to deep crustal pressures 10 kbar the vapor absent solidus of biotite quartz in rocks containing ti saturating phases is likely to exceed 900 c even in bulk compositions with subequal concentrations of fe and mg

numerical studies on co2 injection brine extraction process in a low medium temperature reservoir system co2 sequestration in deep saline formations has been proved to be an effective method for reducing greenhouse emissions into the atmosphere however pure sequestration of co2 will add to the costs incurred by both industries and governments a win win method of co2 injection and hot brine water extraction can become attractive to the investors as it will not only increase the storage capacity of the injected co2 but also offset the costs by selling and using the produced hot water for industrial agricultural or household purposes for instance water from very hot geothermal reservoirs t 150 c can be used for electricity generation in power plants and water from low medium temperature reservoirs the most predominant in natural systems are more popular for direct use e g in heating systems household hot water baths aquaculture etc in this paper low medium geothermal reservoirs widely distributed in china especially those in the ordos basin were selected for the numerical case studies using tough2mp with the eco2n module for the simulations generally simulation parameters were taken from the ordos basin where the first full integration co2 sequestration project had been operated since 2010 the simulations in the base case study lasted 35 years based on the lifespan of a normal geothermal project shallow re injection systems were also considered to investigate the influence of thermal breakthrough pressure perturbation etc results show that injection of cold co2 causes sharp decrease in temperature in the reservoir region near the injection well which is enlarged with continuous injection the region near the production well is dominated by different fluid phases during the co2 driven process including a single water phase a two phase fluid water and co2 and a phase of almost pure co2 results also show that the co2 breakthrough lags far behind the pressure response in the geothermal production system before breakthrough the injected co2 pressurizes the reservoir improving the overall performance of the geothermal reservoir furthermore the heat extraction efficiency of co2 based system is obviously higher than h2o based system

reply to comment on crustal contributions to arc magmatism in the andes of central chile by w hildreth and s moorbath

sr and nd isotopic and trace element compositions of quaternary volcanic centers of the southern andes isotopic compositions of samples from six quaternary volcanoes located in the northern and southern extremities of the southern volcanic zone svz 33 46 s of the andes and from four centers in the austral volcanic zone avz 49 54 s range for 87sr 86sr from 0 70280 to 0 70591 and for 143nd 144nd from 0 51314 to 0 51255 the ranges are significantly greater than previously reported from the southern andes but are different from the isotopic compositions of volcanoes in the central and northern andes basalts and basaltic andesites from three centers just north of the chile rise trench triple junction have 87sr 86sr 143nd 144nd la yb ba la and hf lu that lie within the relatively restricted ranges of the basic magmas erupted from the volcanic centers as far north as 35 s in the svz of the andes the trace element and sr and nd isotopic characteristics of these magmas may be explained by source region contamination of subarc asthenosphere with contaminants derived from subducted pelagic sediments and seawater altered basalts by dehydration of subducted oceanic lithosphere in the northern extremity of the svz between 33 and 34 s basaltic andesites and andesites have higher 87sr 86sr rb cs and hf lu and lower 143nd 144nd than basalts and basaltic andesites erupted farther south in the svz which suggests involvement of components derived from the continental crust in the avz the most primitive sample high mg andesite from the southernmost volcanic center in the andes 54 s has sr and nd isotopic compositions and kr b and ba la similar to morb the high la yb of this sample suggests formation by small degrees of partial melting of subducted morb with garnet as a residue samples from centers farther north in the avz show a regionally regular northward increase in sio2 k2o rb ba ba la and 87sr 86sr and decrease in mgo sr kr b rb cs and 143nd 144nd suggesting increasingly greater degrees of fractional crystallization and associated intra crustal contamination

crustal sources involved in continental arc magmatism a case study of volcan mocho choshuenco southern chile volcan mocho choshuenco a pleistocene holocene basaltic andesite to dacite shield volcano located at lat 40 s in the southern volcanic zone svz of the andes is built on the thinnest svz continental crust 30 km which consists largely of paleozoic sedimentary and metamorphic rocks and mesozoic plutons sr pb nd and o isotopic ratios of mocho choshuenco lavas 87 sr 86 sr 0 70399 to 0 70422 206 pb 204 pb 18 57 to 18 61 207 pb 204 pb 15 60 to 15 63 208 pb 204 pb 38 45 to 38 57 143 nd 144 nd 0 512787 to 0 512881 18 o 5 8 to 6 8 standard mean ocean water are similar to those of other svz centers and exhibit no correlation with indices of differentiation however incompatible trace element systematics indicate that evolved lavas have been contaminated by crustal plutonic rocks by 5 to 15 rb ba k ba rb la and k la ratios increase with increasing sio 2 content as a result of assimilation of orogenic calc alkalic plutonic rocks enriched in rb and k relative to ba and la xenoliths of plutonic rocks recovered from mocho choshuenco lavas are slightly more radiogenic in 87 sr 86 sr less radiogenic in 143 nd 144 nd and comparable in pb isotopic ratios and they have variable 18 o contents 87 sr 86 sr 0 70482 to 0 70557 206 pb 204 pb 18 60 to 18 64 207 pb 204 pb 15 60 to 15 62 208 pb 204 pb 38 53 to 38 56 143 nd 144 nd 0 512686 18 o 3 3 to 5 5 smow contamination of mocho choshuenco parental magmas by crustal melts with small isotopic contrast imposed little variation on isotopic heterogeneities existing in the parental compositions these heterogeneities probably were produced during magma crust interaction at the crust mantle boundary or in the lower crust

crust magma interactions and the evolution of arc magmas the san pedro pellado volcanic complex southern chilean andes field petrographic and trace element data indicate that volcanic rocks of the quaternary san pedro pellado complex lat 36 s chile have incorporated a silicic component that may be derived from the crust isotope ratios of sr nd and pb vary little among the volcanic rocks this is due primarily to the limited isotopic contrast between the mesozoic tertiary crust and the young magmas isotope and element compositions of san pedro pellado rocks are similar to those reported from other localities in the southern volcanic zone of the chilean andes we suggest that crustal assimilation is a significant process in the genesis of differentiated volcanic rocks in the southern volcanic zone the san pedro pellado example illustrates the importance of considering all types of evidence not just isotopic data in the evaluation of crustal contributions to arc magmas

matuyama brunhes transition recorded in lava flows of the chilean andes evidence for dipolar fields during reversals the transitional behavior exhibited by earth9s magnetic field during polarity reversals holds the key to understanding the complexities of the field interpretation of the severely limited reversal data set however remains controversial discussion centers around the nature of pole paths during transitions and their possible relation to other geophysical phenomena paleomagnetic data from volcan tatara san pedro in the central chilean andes lat 36 s long 71 w provide the first record of the matuyama brunhes reversal from south america and only the third transitional record from the southern hemisphere a stratigraphic section of ten lava flows yielded intermediate pole positions that center in australia the mean pole is 16 8 s 133 0 e k ar analyses of two of these flows provided dates of 768 8 ka and 763 14 ka ages coincident with the established matuyama brunhes boundary the clustering of these transitional poles strengthens the hypothesis that earth9s magnetic field retains a strong dipole component during reversals

tertiary tectonic history of the southern andes the subvolcanic sequence to the tatara san pedro volcanic complex lat 36 s the tatara san pedro volcanic complex a quaternary to holocene composite volcano in central chile is underlain by late miocene 6 ma plutons and tertiary metavolcanic rocks the plutons were intruded into greenschist facies metavolcanic rocks at a depth of about 4 5 km together these rocks reveal important information about the development of continental arcs in some cases apparent eruptive ages of metavolcanic rocks are younger than ages of intruding plutons indicating that there was 1 pervasive reheating of metavolcanic rocks or 2 that metavolcanic rocks were locally derived and intruded by their own magma chambers plutons we have estimated uplift or denudation rates of nearly 1 mm yr since late miocene time normal faulting currently dominates the brittle behavior of the upper crust therefore uplift is probably related to magmatic addition to the crust rather than to compression and shortening although plutonic and metavolcanic rocks display expected arc geochemical affinities the isotopic similarity of metavolcanic and plutonic rocks to quaternary volcanic rocks is strong this makes it difficult to use subvolcanic rocks as upper crustal contaminants to evaluate the igneous evolution of the quaternary deposits most plutonic metavolcanic and quaternary volcanic rocks exhibit the isotopic characteristics of juvenile crust some samples however exhibit sparse but unmistakable evidence 87 sr 86 sr and 40 ar 39 ar isotopic data for the presence of older probably precambrian through triassic radiogenic crust beneath the volcano crustal structure in this arc can be described by a three layer model in which a central section of precambrian through triassic rock is underplated by juvenile material is intruded by juvenile material or has juvenile material transported through it to be emplaced at or near the surface continued erosion of juvenile material at the surface may be balanced to some degree by continued volcanism and epizonal plutonism

the petrology and geochemistry of the handkerchief mesa mixed magma complex san juan mountains colorado the handkerchief mesa mixed magma complex is one of several late cenozoic volcanic complexes in the southeastern san juan mountains characterized by mingling and limited mixing of basalt and rhyodacite stratigraphy in the dissected vent complex at handkerchief mesa records three phases of volcanism the first and third displaying evidence for coeruption of mafic and silicic magmas phases 1 and 2 erupted silicic pyroclastics and basaltic lava flows respectively phase 3 eruptions were dominated by rhyodacite lava flows rhyodacite dikes and abundant mingled and mixed hybrid lavas pre and syneruptive basalt rhyodacite mixing of phase 3 eruptions is shown by 1 inclusions of quenched basalt in rhyodacite 2 partially disaggregated basalt inclusions in mixed hybrids and rhyodacites 3 interfingering lenses of mixed hybrid lavas and rhyodacite whole rock major and trace element analyses support a two component mixing model whereby intermediate hybrids are produced by mixing of basalt and rhyodacite up to 30 basalt 70 rhyodacite disequilibrium phenocryst textures and mineral compositions are consistent with multistage mixing culminating in an eruptive mixing event protracted mixing along a boundary zone at the base of a rhyodacite magma chamber may be responsible for stabilizing fe rich olivine phenocrysts in some hybrids basalt rhyodacite mixing is inhibited by rapid crystallization in the basalt shortly after inclusion within the lower temperature melt the degree to which mechanical dispersion and blending ensues is a critical function of the initial temperature contrast ti between the two magmas thermal models simulating the conductive cooling histories for basalt spheres in rhyodacite reservoirs suggest that at large ti s 200 rapid cooling of the inclusion leads to disequilibrium crystallization with concomitant depression of equilibrium solidi grain boundary wetting by residual liquids and limited disaggregation of the inclusion imposed by movement of the host for small ti s 100 temperatures within the inclusion can be maintained above the solidus for prolonged time periods enhancing the possibility of producing homogeneous mixed hybrids through mechanical blending and diffusion both mechanisms operated at handkerchief mesa and contributed to the range of observed textures and compositions

illegitimate magmas of the galapagos insights into mantle mixing and magma transport roughly 1 2 of the flows erupted from flank vents of the western galapagos shield volcanoes have anomalous compositions we call these illegitimate magmas because of their uncertain parentage because some illegitimate magmas are compositionally indistinguishable from lavas of an adjacent volcano and erupt from the flank facing the adjacent volcano such magmas apparently result from lateral intrusion of magma from the adjacent volcano other illegitimate magmas come from parts of the galapagos plume that have incompletely mixed or result from unusually advanced melting of part of the mantle

evidence for nappe development during the early proterozoic penokean orogeny minnesota two major phases of early proterozoic penokean folding are now recognized in the thomson formation of east central minnesota effects of the first deformation are found only in the southern two thirds of the exposed thomson formation the second deformation affected the entire region of outcrop several lines of evidence indicate that northward directed nappes developed during the early phase of folding this evidence includes lithologic differences between the areas of one and two deformations the pervasive nature of an s 1 foliation in the area of its occurrence facing directions of f 2 folds and the refraction pattern of the s 1 foliation in graded metagraywacke slate beds taken together the data show that an area of several hundred square kilometres is on the upper limb or limbs of a large northward directed recumbent fold or folds nappes of the early phase of penokean deformation

geochemistry of the 1984 mauna loa eruption implications for magma storage and supply the 1984 eruption of mauna loa volcano hawaii produced about 220 106 m3 of lava of remarkably homogeneous composition no significant compositional changes occurred during the 22 day eruption and along the 20 km vent system even though crystallinity increased substantially accompanying a decline in the eruption rate such homogeneity in lava composition is typical of many historic mauna loa eruptions the 1984 lava along with most other homogeneous mauna loa lavas is multiply saturated crystallizing olivine clinopyroxene and plagioclase bulk compositions are perched at the intersection of an olivine control trend with a trend for more differentiated lavas there is no evidence to indicate that the 1984 lava is a differentiate of the 1975 magma or that it results from mixing a new batch of magma with differentiated melt remaining in the magmatic plumbing system after the 1975 eruption it is proposed that these lavas are erupted directly from a small continuously replenished magma reservoir located about 3 4 km beneath the summit caldera successive eruptions therefore provide a sampling of magma chamber processes and small but discernible changes in lava composition reflect the competing processes of magma supply differentiation and eruption

time scales of magmatic processes new insights from dynamic models for magmatic assimilation we present a computational model that predicts the consequences of time dependent isenthalpic assimilation fractional crystallization afc our model combines an existing paradigm for fractional crystallization based on equilibrium thermodynamics with a kinetic model for predicting rates of mineral dissolution the isenthalpic constraint directly links the sensible heat of the magma and the latent heat of crystallization to the energy necessary for assimilation thus we predict liquid lines of descent rates of crystallization rates of cooling and ratios of assimilation to crystallization r as a function of time model simulations predict the time scale of afc processes weeks to years and show the extent to which temperature and composition of the assimilant control these time scales

geochemical variations in andean basaltic and silicic lavas from the villarrica lanin volcanic chain 39 5 s an evaluation of source heterogeneity fractional crystallization and crustal assimilation at 39 5 s in the southern volcanic zone of the andes three pleistocene recent stratovolcanoes villarrica quetrupillan and lanin form a trend perpendicular to the strike of the andes 275 to 325 km from the peru chile trench basalts from villarrica and lanin are geochemically distinct the latter have higher incompatible element abundances and la sm but lower ba la and alkali metal la ratios these differences are consistent with our previously proposed models involving a a west to east decrease in an alkali metal rich high ba la slab derived component which causes an across strike decrease in degree of melting or b a west to east increase in the contamination of subduction related magma by enriched subcontinental lithospheric mantle silicic and mafic lavas from the stratovolcanoes have overlapping sr nd and o isotopic ratios silicic lavas also have geochemical differences that parallel those of their associated basalts e g rhyolite from villarrica has lower la sm and incompatible element contents than high sio2 andesite from lanin at each volcano the most silicic lavas can be modelled by closed system fractional crystallization while andesites are best explained by magma mixing apparently crustal contamination was not an important process in deriving the evolved lavas basaltic flows from small scoria cones 20 35 km from villarrica volcano have high incompatible element contents and low ba la like lanin basalts but trend to higher k rb 356 855 and lower 87sr 86sr 0 70361 0 70400 than basalts from either stratovolcano however all basalts have similar nd pb and o isotope ratios the best explanation for the unique features of the cones is that the sources of svz magmas e g slab derived fluids or melts of the subcontinental lithospheric mantle have varying alkali metal and radiogenic sr contents these heterogeneities are not manifested in stratovolcano basalts because of extensive subcrustal pooling and mixing this model is preferable to one involving crustal contamination because it can account for variable sr isotope ratios and uniform nd and pb isotope ratios among the basalts and the divergence of the cones from across strike geochemical trends defined by the stratovolcanoes

a complex magma mixing origin for rocks erupted in 1915 lassen peak california the eruption of lassen peak in may 1915 produced four volcanic introduction rock types within 3 days and in the following order 1 hybrid magma mixing is an important process among interblack dacite lava containing 2 undercooled andesitic inclusions mediate magmas in volcanic arcs e g turner 3 compositionally banded pumice with dark andesite and light campbell 1986 philpotts 1990 and references therein dacite bands and 4 unbanded light dacite all types represent laboratory studies of synthetic analogs of silicate magma stages of a complex mixing process between basaltic andesite systems and mathematical modeling of mixing viscous and dacite that was interrupted by the eruption they contain liquids have suggested a range of possible mixing mechdisequilibrium phenocryst assemblages characterized by the coanisms e g cashman bergantz 1991 however use existence of magnesian olivine and quartz and by reacted and of these results to interpret magmatic systems is limited unreacted phenocrysts derived from the dacite the petrography and by the applicability of synthetic analogs to magmatic crystal chemistry of the phenocrysts and the variation in rock conditions and by the imprecise knowledge of the effective compositions indicate that basaltic andesite intruded dacite magma viscosity of silicate liquid crystal mixtures at magmatic and partially hybridized with it phenocrysts from the dacite magma temperatures and pressures if magmas mix incompletely were reacted cooling crystallization and vesiculation of the hybrid i e mingle features such as compositional banding or andesite magma converted it to a layer of mafic foam the decreased undercooled inclusions are usually apparent but if they density of the andesite magma destabilized and disrupted the foam mix completely mineralogical disequilibrium may be the blobs of foam rose into and were further cooled by the overlying dacite magma forming the andesitic inclusions disaggregation of only direct evidence for a mixing origin andesitic inclusions in the host dacite produced the black dacite and if the thermal and compositional contrasts between light dacite magmas formation of foam was a dynamic process two magmas are great and the ratio of silicic to mafic removal of foam propagated the foam layer downward into the magma is large there is little interaction between them hybrid andesite magma eventually the thermal and compositional and the mafic magma is undercooled e g bacon 1986 contrasts between the hybrid andesite and black dacite magmas were many undercooled inclusions contain reacted phereduced then they mixed directly forming the dark andesite magma nocrysts inherited from their host silicic magma heiken about 40 50 andesitic inclusions were disaggregated into the eichelberger 1980 and bacon 1986 pointed out host dacite to produce the hybrid black dacite thus disaggregation that undercooled inclusions are typically formed from of inclusions into small fragments and individual crystals can be hybrid magmas in general the formation of undercooled an efficient magma mixing process disaggregation of undercooled inclusions retards further mixing sakuyama 1984 inclusions carrying reacted host magma phenocrysts produces cothompson dungan 1985 sparks marshall 1986 existing reacted and unreacted phenocryst populations koyaguchi blake 1991 however fragmentation and or disaggregation of undercooled inclusions plays an important role in hybridization in some magma systems thompson dungan 1985 coulon et al 1986 clynne christiansen 1987 clynne 1989 linneman myers

improved understanding of velocity saturation relationships using 4d computer tomography acoustic measurements a recently developed laboratory method allows for simultaneous imaging of fluid distribution and measurements of acoustic wave velocities during flooding experiments using a specially developed acoustic sample holder that combines high pressure capacity with good transparency for x rays it becomes possible to investigate relationships between velocity and fluid saturation at reservoir stress levels high resolution 3d images can be constructed from thin slices of cross sectional computer tomography scans ct scans covering the entire rock core volume and from imaging the distribution of fluid at different saturation levels the x ray imaging clearly adds a new dimension to rock physics measurements it can be used in the explanation of variations in measured velocities from core scale heterogeneities computer tomography gives a detailed visualization of density regimes in reservoir rocks within a core this allows an examination of the interior of core samples revealing inhomogeneities porosity and fluid distribution this mapping will not only lead to an explanation of acoustic velocity measurements it may also contribute to an increased understanding of the fluid flow process and gas liquid mixing mechanisms in rock immiscible and miscible flow in core plugs can be mapped simultaneously with acoustic measurements the effects of core heterogeneity and experimentally introduced effects can be separated to clarify the validity of measured velocity relationships

homogenization and lowering of 18o 16o in mid crustal rocks during extension related magmatism in eastern nevada the oxygen isotope systematics of tertiary volcanic rocks of east central nevada and of plutonic and metamorphic rocks of the ruby mountains east humboldt range core complex provide complementary evidence for major18o depletion and 18o 16o homogenization of mid crustal rocks during metamorphism and magmatism the 18o value of crustal source material for silicic volcanic rocks decreased from between 9 and 11 to between 7 and 8 over 5 ma mid crustal metasedimentary and granitic rocks in the east humboldt range have 18o values very similar to the volcanic rocks and values are lower and more homogeneous at deeper structural levels exchange with deep seated mantle derived igneous rocks or fluids derived therefrom is the most plausible18o depletion mechanism intrusion of these mafic magmas promoted crustal melting and fluid migration homogenization of 18o 16o resulted from migration of high temperature fluids and melts at mid crustal levels and was less effective at higher structural levels where the crust was dominated by less permeable carbonate rocks

evaluation of magmatic processes for the products of the nevado del ruiz volcano colombia from geochemical and petrological data abstract trace element concentrations of the eruptive products from the nevado del ruiz volcano indicate that at least for the last 1 ma the major controlling factor in the evolution of the magma has been a simple fractional crystallization the zr hf ratios suggest that the source material is mantle like in origin and whilst subduction related contamination of the source material has taken place there is no evidence that assimilation of continental crust has affected the series no convincing geochemical evidence for magma mixing was found and data on banded pumices of pleistocene historic and the 1985 eruptions strongly support a crystal fractionation model to explain the compositional variation of their glasses therefore if mixing has taken place it can only be a mixing of layers within a stratified or partially stratified magma chamber modeling of fractional crystallization using a stepwise program has been undertaken and least squares approximations with small residuals are commensurate with analyzed samples crystal fractionation models with plagioclase dominating two pyroxenes and iron oxides give good fits for all calculations we conclude that simple crystal fractionation is the process most consistent with major and trace element variations in the ruiz series this closed system model requires at least 77 crystal fractionation of a basaltic parent to generate the observed compositions of dacites

eruptive stratigraphy of the tatara san pedro complex 36 s southern volcanic zone chilean andes reconstruction method and implications for magma evolution at long lived arc volcanic centers

contribution of ground surface altitude difference to thermal anomaly detection using satellite images application to volcanic geothermal complexes in the andes of central chile abstract detection of thermal anomalies in volcanic geothermal areas using remote sensing methodologies requires the subtraction of temperatures not provided by geothermal manifestations e g hot springs fumaroles active craters from satellite image kinetic temperature which is assumed to correspond to the ground surface temperature temperatures that have been subtracted in current models include those derived from the atmospheric transmittance reflectance of the earth s surface albedo topography effect thermal inertia and geographic position effect we propose a model that includes a new parameter k that accounts for the variation of temperature with ground surface altitude difference in areas where steep relief exists the proposed model was developed and applied using aster satellite images in two andean volcanic geothermal complexes descabezado grande cerro azul volcanic complex and planchon peteroa azufre volcanic complex where field data of atmosphere and ground surface temperature as well as radiation for albedo calibration were obtained in 10 selected sites the study area was divided into three zones northern central and southern zones where the thermal anomalies were obtained independently k value calculated for night images of the three zones are better constrained and resulted to be very similar to the environmental lapse rate elr determined for a stable atmosphere elr 7 c km using the proposed model numerous thermal anomalies in areas of 90 m 90 m were identified that were successfully cross checked in the field night images provide more reliable information for thermal anomaly detection than day images because they record higher temperature contrast between geothermal areas and its surroundings and correspond to more stable atmospheric condition at the time of image acquisition

time lapse crosswell seismic and vsp monitoring of injected co2 in a brine aquifer seismic surveys successfully imaged a small scale co2 injection 1 600 ton conducted in a brine aquifer of the frio formation near houston texas these time lapse borehole seismic surveys crosswell and vertical seismic profile vsp were acquired to monitor the co2 distribution using two boreholes the new injection well and a pre existing well used for monitoring which are 30 m apart at a depth of 1 500 m the crosswell survey provided a high resolution image of the co2 distribution between the wells via tomographic imaging of the p wave velocity decrease up to 500 m s the simultaneously acquired s wave tomography showed little change in s wave velocity as expected for fluid substitution a rock physics model was used to estimate co2 saturations of 10 20 from the p wave velocity change the vsp survey resolved a large 70 change in reflection amplitude for the frio horizon this co2 induced reflection amplitude change allowed estimation of the co2 extent beyond the monitor well and on three azimuths the vsp result is compared with numerical modeling of co2 saturations and is seismically modeled using the velocity change estimated in the crosswell survey

magmatic interactions as recorded in plagioclase phenocrysts of chaos crags lassen volcanic center california the silicic lava domes of chaos crags in lassen volcanic national and mingling of this material into the silicic host these processes park contain a suite of variably quenched hybrid basaltic andesite are commonplace in some orogenic magma systems and may be magmatic inclusions the inclusions represent thorough mixing elucidated by isotopic microsampling and analysis of the plagioclases between rhyodacite and basalt recharge liquids accompanied by some crystallizing from them mechanical disaggregation of the inclusions resulting in crystals mixing into the rhyodacite host preserved by quenching on dome emplacement sr sr ratios 0 7037 0 7038 of the inclusions are distinctly lower than those of the host rhyodacite

moving window poisson analysis of gravity and magnetic data from the penokean orogen east central minnesota analytical correlation of gravity and magnetic data through moving window application of poisson s theorem is useful in studying the complex precambrian geology of central minnesota linear regression between the two data sets at each window position yields correlation intercept and slope parameters that quantitatively describe the relationship between the gravity and magnetic data and in the case of the slope parameter are often accurate estimates of magnetizatons to density ratios mdr of anomalous sources in this study gridded gravity and magnetic data from a 217 6 217 6 km area in central minnesota were analyzed using a 8 5 8 5 km window the study area includes part of the early proterozoic penokean orogen and an archean greenstone granite terrane of the superior province the parameters derived by the moving window analysis show striking relationships to many geologic features and many of the mdr estimates agree with rock property data inversely related gravity and magnetic anomalies ar

pb and nd isotope and trace element constraints on the origin of basic rocks in an early proterozoic igneous complex minnesota abstract a suite of early proterozoic basic to granitic rocks exposed near st cloud minnesota are cut by northeast trending basaltic dikes pb isotope data for all of these rocks overlap and plot about an 1800 ma pb pb correlation line the basic rocks which are light ree enriched have nd values between 0 4 and 4 8 petrogenetic considerations suggest that the basic rocks were derived from a light ree enriched source which had an fe mg ratio greater than that for pyrolite the enrichment in fe mg is probably a result of the addition of basic melts to the source the light ree enrichment may have a mantle origin by the addition of mantle derived light ree enriched basic melts or fluids or a sedimentary origin by the addition of light ree enriched fluids or melts derived from subducted sediments with an early proterozoic provenance in either case the nd isotopes suggest that the light ree enriched component existed since c 2300 ma the igneous complex may have formed at a convergent margin an anorthositic gabbro near mora has an nd of 5 indicating that it was derived from a mantle source with a history of light ree depletion this gabbro may have been part of an early proterozoic ocean crust

comment on crustal contributions to arc magmatism in the andes of central chile by w hildreth and s moorbath in their paper crustal contributions to arc magmatism in the andes of central chile hildreth and moorbath conclude that the stratovolcanos forming the volcanic front at the northern end between 33 37 5 s of the southern volcanic zone svz 33 46 s of the andes show along arc changes in isotopic and elemental ratios that demonstrate a northward increase in crustal contributions to magma genesis stern et al 1984a b futa and stern 1988 and stern 1988 previously reached this same conclusion by comparing the chemistry of three of the northernmost volcanos in the svz between 33 34 s with those from south of 36 s however the processes by which crustal components are incorporated in andean magmas and specifically what is causing crustal components to be incorporated in increasing amounts in the magmas erupted at the northern end of the svz remain controversial with regard to andean volcanism in general hildreth and moorbath suggest that although the principal magma source is presumed to lie within the mantle wedge fluxed by a slab derived contribution the base level isotopic and chemical values for each volcano are established by blending of subcrustal and deep crustal magmas in zones of melting assimilation storage and homogenization mash at the mantle crust transition they further suggest that the along arc changes in isotopic and elemental ratios observed at the northern end of the svz are due to changing composition age and most significantly increased proportional contribution of the lowermost crust in conjunction with increased depth to the mash zone as the crust thickens northward below the volcanic front in contrast stern et al 1984a futa and stern 1988 and stern 1988 1991 argue that the distinctive petrochemical characteristics of the volcanoes at the northernmost end of the svz reflect a northward change in the composition and amount of continental components subducted into the mantle source region hildreth and moorbath s preference for intra crustal upper plate influence on the along arc variations in magma chemistry at the northern end of the svz appears to be based on both geological and geochemical arguments that i find in many cases debatable i discuss these separately below followed by a brief comment on the relative effectiveness of lower crustal mash versus mantle source region contamination processes in explaining along arc chemical variations observed in andean magmas

transpressive tectonism in the eastern penokean orogen canada consequences for proterozoic crustal kinematics and continental fragmentation abstract results of a structural analysis of archean basement huronian cover and various igneous rocks in the sudbury area ontario and regional tectonometamorphic correlations suggest that structures in the eastern penokean orogen formed dominantly by dextral transpression bulk horizontal shortening started with the ca 2 4 to 2 2 ga blezardian orogeny and was accompanied by peak metamorphism and local granitoid magmatism during the 1 89 to 1 83 ga penokean orogeny horizontal dextral shear parallel to the curved axis of the eastern penokean orogen gained in importance and is most likely related to docking of proterozoic magmatic arc terranes to the superior province margin this tectonic activity led to distributed heterogeneous deformation of archean basement rocks and exhumation of deep crustal rocks in the sudbury area and the kapuskasing structural zone but may have failed to accrete paleoproterozoic terranes in the lake huron area the lack of evidence for horizontal extension following blezardian and penokean contraction suggests that crustal fragmentation of the archean supercontinent started at 2 45 ga finally the great lakes tectonic zone and the murray fault master dislocations in the penokean orogen may have acted as crustal weakness zones which influenced the orientation and development of 1 1 ga midcontinent rift segments

fernandina volcano s evolved well mixed basalts mineralogical and petrological constraints on the nature of the galapagos plume fernandina volcano the most active of galapagos volcanoes and the one most directly overlying the galapagos hotspot is one of a small number of active plume related oceanic island volcanoes with a well documented recent eruptive record whole rock and glass analyses of lava and tephra from all 12 known eruptions from 1958 to the present show them to be evolved mg mg mg fe2 of 0 47 0 56 plagioclase phyric clinopyroxene and olivine bearing tholeiites thermodynamic modeling indicates that this phase assemblage erupted at 1100 1130 c and is stable only at low pressures 0 1 0 2 gpa thereby representing crystallization within a shallow magma chamber underlying the well defined 850 m deep 5 by 6 5 km caldera these tholeiites are remarkably homogenous as is shown by narrow ranges in zr y 5 7 6 3 la sm n 1 4 1 7 and la yb n 3 8 4 3 prehistoric fernandina lavas selected for petrographic and geographic variation over the entire subaerial volcano are of remarkably similar composition e g zr y is 5 2 6 1 la sm n is 1 5 1 6 and la yb n is 3 4 4 1 with a single example of a more primitive lava having an mg of 0 63 with 8 5 feo at 8 mgo cr spinel and glass melt inclusions in plagioclase together with zoned olivine give evidence for a range of parental melts in the recent shallow magmatic system with mg as high as 0 62 extensive shallow magma mixing is required to explain these variations in mineral and inclusion chemistry indicating more thermal variation within the shallow magmatic system than is implied by the relatively monotonous whole rock compositions apparently fernandina s shallow magma chamber has physically blocked passage of more primitive magmas to the surface and homogenized the erupted lavas by extensive mixing small variations in lava isotopic and compatible element composition combined with the need to resupply melt to maintain thermal balance indicate that replenishment of the magma chamber is semicontinuous with resupply occurring of the order of a decade or less plagioclase dominated evolved fernandina lavas are in sharp contrast to more primitive olivine dominated hawaiian lavas the total lack of primitive melts in the shallow fernandina magmatic system implies that substantial fractionation and heat loss occurred during melt transportation from the mantle source to the edifice magma chamber despite the fact that these melts were transported through relatively warm young and thin lithosphere adjacent to the cocos nazca ridge the diffuse nature of galapagos volcanism with 20th century eruptions from eight distinct volcanoes over a 20 000 km2 area further supports this interpretation and suggests that the galapagos plume is relatively diffuse weak and significantly less thermally intense than the vigorous hawaiian plume it may in fact be waning

interpretation of phase reversals in seismic reflections from attenuating targets phase reversals are commonly observed during exploration seismic amplitude versus offset analysis and usually modelled with the zoeppritz equations in practice when multiple fluids are present we often deal with reflections from interfaces having a contrast in both elastic and anelastic properties in this paper we present a new phenomenological model for phase reversals in cases in which velocity dispersion and attenuation are present we demonstrate strong qualitative differences in behaviour between the elastic and anelastic cases influencing both the amplitude and phase of the reflection coefficient analysis of seismic data showing a phase reversal from a gas reservoir in the vienna basin shows a striking agreement with the modelling we conclude that the influence of fluid induced dispersion on reflection data is a significant and measurable phenomenon in cases of practical interest that recognition of the phenomena may be used as a novel fluid indicator and that frequency dependent rock physics analysis may be an important tool for both industrial geophysics and seismic monitoring of co2 storage

bounds on low frequency seismic velocities in partially saturated rocks the most common technique for estimating seismic velocities in rocks with mixed pore fluid saturations is to use gassmann s relations with an effective fluid whose density and compressibility are averages of the individual pore fluid properties this approach is applicable only if the gas oil and brine phases are mixed uniformly at a very small scale so the different wave induced increments of pore pressure in each phase have time to diffuse and equilibrate during a seismic period in contrast saturations that are heterogeneous over scales larger than the characteristic diffusion length i e patchy saturation will always lead to higher seismic velocities than if the same fluids are mixed uniformly at a fine scale critical saturation scales separating uniform from patchy behavior are typically of the order 0 1 1 cm for laboratory measurements and tens of centimeters for field seismic frequencies for low seismic frequencies velocities corresponding to patchy and homogeneous saturations represent a

calbuco volcano and minor eruptive centers distributed along the liquine ofqui fault zone chile 41 42 s contrasting origin of andesitic and basaltic magma in the southern volcanic zone of the andes calbuco volcano is a late pleistocene holocene composite stratovolcano located at 41 20 s in the southern region of the southern volcanic zone of the andes ssvz 37 46 s in contrast to basalt and basaltic andesite which are the dominant lava types on the volcanic front from 37 to 42 s calbuco lavas are porphyritic andesites which contain a wide variety of crustal xenoliths they have sio2 contents in the 55 60 range and have comparatively low k2o rb ba th and lref abundances relative to other ssvz centers incompatible element abundance ratios are similar to those of most ssvz volcanics but 87sr 86sr and 143nd 144nd are respectively higher and lower than those of adjacent volcanic centers basalts from nearby osorno stratovolcano 25 km to the northeast are similar to other basaltic ssvz volcanoes however basalts from several minor eruptive centers mec located east of calbuco and osorno volcano along the liquine ofqui fault zone lofz are enriched in ba nb th and lree and have higher la yb and lower ba la k la and rb la 87sr 86sr and 143nd 144nd in mec basalts are respectively lower and higher than those of osorno and calbuco lavas we suggest that mec basalts were produced by lower extents of mantle melting than basalts from osorno and other ssvz stratovolcanoes probably as a result of lower water content in the source of mec basalts calbuco andesites formed from basaltic parents similar to osorno basalts by moderate pressure crystallization of a hornblende bearing assemblage accompanied by crustal assimilation hornblende stability in the calbuco andesites was promoted by the assimilation of hydrous metasedimentary crustal rocks which are also an appropriate endmember for isotopic trends together with magma storage at mid crustal depths the unique characteristics of calbuco volcano i e the stability of hornblende at andesitic sio2 contents low 143nd 144nd and high 87sr 86sr and abundant crustal xenoliths provide evidence for crustal assimilation that is not apparent at more northerly volcanoes in the ssvz