magma mixing in the aleutian arc evidence from cognate inclusions and composite xenoliths abstract the complexity of igneous processes in the aleutian calc alkaline magma series can be inferred from study of xenolithic fragments composite xenoliths and cognate inclusions provide direct evidence for magma magma and wall rock magma mixing processes using distributions of cr in clinopyroxene compositional endmembers involved in mixing are identified within the xenoliths the basaltic mixing endmember is more mafic than calc alkaline lavas in the arc magma mixing and wall rock assimilation within calc alkaline basaltic to andesitic magmas is identified in phenocrystic assemblages as well as in xenoliths and appears to be a widespread phenomenon in aleutian calc alkaline magmas

a noncognate relationship between megacrysts and host basalts from the tuoyun basin chinese tian shan abstract abundant kaersutite and alkali feldspar megacrysts occur together with mantle derived and lower crustal xenoliths including kaersutite hornblendite and syenite in the early cretaceous alkaline basalts of the tuoyun basin in the westernmost chinese tian shan particularly several kaersutite hornblendite cumulate xenoliths have the assemblage of kaersutite alkali feldspar titanian mica titanomagnetite clinopyroxene in which kaersutite occurs as an early stage cumulus phase crystallized from a parental magma and alkali feldspar occurs as a late stage intercumulus phase from the evolved magma between accumulating kaersutite grains the megacrystic and xenolithic kaersutite has much higher mg values 0 70 0 96 than the host basalt 0 47 0 60 the kaersutite and alkali feldspar megacrysts have similar initial ratios 0 7032 0 7033 and 0 7035 0 7038 respectively significantly different from their host basalts 0 7047 0 7054 xenolithic petrography and megacrystic sr isotope geochemis

the metagabbros orthogneisses and paragneisses of the connemara complex western ireland although now largely disappeared within the 400 ma old galway granite the syntectonic connemara metagabbro gneiss complex which crystallized at 490 ma arenig is the largest intrusion into the dalradian rocks it is part of widespread cambro ordovician volcanism and plutonic injection into continental crust in the appalachian caledonide orogenic belt being synchronous in crystallization with the widely obducted 490 ma ophiolites within the orogeny the peridotites and hornblende gabbros were injected broken up and shouldered aside by quartz diorite granodiorite granite and trondhjemite gneisses and it is shown that these orthogneisses are not the in situ differentiation products of the gabbro magma the hornblende in the metagabbros is mainly igneous but is partly metamorphic and replaces olivine fo 75 92 orthopyroxene en 65 92 clinopyroxene wo 48 5 en 50 39 fs 5 14 and plagioclase typically an 80 92 range an 50 96 an aureole at least 10 km wide in the sillimanite zone was imposed and cordierite garnet sillimanite dalradian hornfelses were partially melted at temperatures up to 1000 o c near to the gabbros at 4 5 1 kbar corresponding to c 18 km depth this suggests that the complex forms part of the root of a now eroded batholith of calc alkaline affinities the gabbros corresponding to the transition zone to a basic bottom which is often supposed to underlie many batholiths within the orthogneisses are sheets of paragneiss formed from dalradian metasediments which have been folded up from the floor and down into sides of the magma chamber which process could be illustrative of the means by which basic magma can intimately interact with crustal material the 18 o 16 o d h nd and sr isotope evidence and the existence of inherited zircons in the gneisses indicate that the gabbro and gneiss magmas were significantly contaminated with crustal possibly dalradian material both before and after intrusion with the lowest initial 87 sr 86 sr at 490 ma being 0 708 even in samples far removed from metasediment the metagabbros are tholeiitic to high alumina basalt in character but the gneisses have distinct calc alkaline geochemistry the chemistry igneous hornblende and highly calcic plagioclase match many arc plutons in the western usa the syntectonic nature and the ubiquitous gneissic fabric in the quartz diorites trondhjemites and granites is a consequence of early movements which were probably in part connected with the separation of the connemara massif and which culminated in slicing off the largely inverted complex and carrying it southwards on the mannin thrust which underlies the whole of connemara the complex is thus a fragment of a now disrupted cambro ordovician continental magmatic arc or continent island arc environment which can be traced along the se edge of the laurentian continent

composition and structure of the central aleutian island arc from arc parallel wide angle seismic data new results from wide angle seismic data collected parallel to the central aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry the 3 d ray tracing 2 5 d inversion of this sparse wide angle data set which incorporates independent phase interpretations and new constraints on shallow velocity structure produces a faster and smoother result than a previously published velocity model middle crustal velocities of 6 5 7 3 km s over depths of 10 20 km indicate an andesitic to basaltic composition high lower crustal velocities of 7 3 7 7 km s over depths of 20 35 km are interpreted as ultramafic mafic cumulates and or garnet granulites the total crustal thickness is 35 37 km this result indicates that the aleutian island arc has higher velocities and thus more mafic compositions than average continental crust implying that significant modifications would be required for this arc to be a suitable building block for continental crust lateral variations in average crustal velocity below 10 km roughly correspond to trends in major element geochemistry of primitive mg 0 6 lavas the highest lower crustal velocities and presumably most mafic material are detected in the center of an arc segment between unmak and unalaska islands implying that arc segmentation exerts control over crustal composition

geochemistry of the 1989 1990 eruption of redoubt volcano part ii evidence from mineral and glass chemistry abstract early stages december 1989 of the 1989 1990 eruption of redoubt volcano produced two distinct lavas both lavas are high silica andesites with a narrow range of bulk composition 58 64 wt and similar mineralogies phenocrysts of plagioclase hornblende augite hypersthene and fe ti oxides in a groundmass of the same phases plus glass the two lavas are distinguished by groundmass glass compositions one is dacitic and the other rhyolitic sharp boundaries between the two glasses in compositionally banded pumices lack of extensive coronas on hornblende phenocrysts and seismic data suggest that a magma mixing event immediately preceeded the eruption in december 1989 textural disequilibrium in the phenocrysts suggests both magmas dacitic and rhyolitic glasses had a mixing history prior to their interaction and eruption in 1989 sievey plagioclase and overgrowths of magnetite on ilmenite are textures that are at least consistent with magma mixing the presence of two hornblende compositions one a high al pargasitic hornblende and one a low al magnesiohornblende in both the dacitic and rhyolitic groundmasses indicates a mixing event to yield these two amphibole populations prior to the magma mixing in december 1989 the pargasitic hornblende and the presence of ca rich overgrowths in the sievey zones of the plagioclase together indicate at least one component of this earlier mixing event was a mafic magma either a basalt or a basaltic andesite eruptions in 1990 produced only andesite with a rhyolitic groundmass glass glass compositions in the 1990 andesite are identical to the rhyolitic glass in the 1989 andesite cognate xenoliths from the magma chamber or conduit are also found in the 1990 lavas magma mixing probably triggered the eruption in 1989 the eruption ended when this rather viscous rhyolitic groundmass glass magma capable of entraining sidewall xenoliths magma stabalized within the conduit

the northeast brazilian rift system the northeast brazilian rift basins provide important data critical to the understanding of continental rifting processes associated with the opening of the south atlantic these basins represent the locus of intersection of the southern and equatorial branches and some basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases similar data are not found in its counterpart in africa especially for the neocomian these early cretaceous rift basins of northeast brazil illustrate key three dimensional geometries of intracontinental rift systems mainly controlled by the basement structural framework during the main rift phase syn rift ii neocomian early barremian extensional deformation was distributed over three main rift axes 1 the gabon sergipe alagoas gsa trend 2 the reconcavo tucano jatoba rtj trend and 3 the cariri potiguar cp trend during this phase extensional deformation jumped west from the easternmost basins gsa trend to a series of ne trending intracratonic basins rtj and cp trends characterized by a set of asymmetric half grabens separated by basement highs transfer faults and or accommodation zones these basins are typically a few tens of kilometers wide and trend ne sw roughly perpendicular to the main extension direction during the neocomian preexisting upper crustal weakness zones like the dominantly ne sw trending shear zones of the brazilian pan african orogeny controlled the development of intracrustal listric normal faults internal transverse structures such as transfer faults and accommodation zones were also controlled by the local basement structural framework the megashear zones of pernambuco brazil and ngaundere africa seem to have behaved like a huge accommodation zone accommodating extensional deformation along the rtj gsa trends with simultaneous extension along the cp trend during the late barremian syn rift phase iii a significant change in rifting kinematics occurred when the cp trend was aborted and major rifting initiated at the equatorial branch during the aptian while the equatorial branch and benue trough africa experienced the main rift phase the rtj trend was aborted and the gsa trend developed a transitional phase between the rift and drift stage the gsa trend and the offshore potiguar basin represent the site of continued evolution into passive margin basins following the main rift episode

the major ion composition of silurian seawater one hundred fluid inclusions in silurian marine halite were analyzed in order to determine the major ion composition of silurian seawater the samples analyzed were from three formations in the late silurian michigan basin the a 1 a 2 and b evaporites of the salina group and one formation in the early silurian canning basin australia the mallowa salt of the carribuddy group the results indicate that the major ion composition of silurian seawater was not the same as present day seawater the silurian ocean had lower concentrations of mg2 na and so42 and much higher concentrations of ca2 relative to the ocean s present day composition furthermore silurian seawater had ca2 in excess of so42 evaporation of silurian seawater of the composition determined in this study produces kcl type potash minerals that lack the mgso4 type late stage salts formed during the evaporation of present day seawater the relatively low na concentrations in silurian seawater support the hypothesis that oscillations in the major ion composition of the oceans are primarily controlled by changes in the flux of mid ocean ridge brine and riverine inputs and not global or basin scale seawater driven dolomitization the mg2 ca2 ratio of silurian seawater was 1 4 and the k ca2 ratio was 0 3 both of which differ from the present day counterparts of 5 and 1 respectively seawaters with mg2 ca2 2 e g modern seawater facilitate the precipitation of aragonite and high magnesian calcite therefore the early paleozoic calcite seas were likely due to the low mg2 ca2 ratio of seawater not the pco2 of the silurian atmosphere

the cold bay volcanic center aleutian volcanic arc the cold bay volcanic center has experienced two major stages of eruptive activity early m series acitivity produced bimodal hi alumina basalt and calc alkaline andesite lavas while later fpk series activity produced only calc alkaline andesite the spectrum of basalt compositions is believed to be due to high pressure 8 kb fractionation at or near the base of the crust abundant mineralogical and geochemical evidence support a lower pressure mixing origin for all andesites inspection of the mineralogical data has shown that the earliest m series andesites were produced by mixing of basalt 53 wt sio2 and silicic andesite 60 5 to 62 5 wt while later fpk series andesites resulted from the mixing of basaltic andesite 53 to 56 wt and less silicic andesite 58 5 to 60 0 wt the major element and trace element geochemical data are consistent with a low pressure fractionation origin for the silicic endmember magmas and support the temporal variations in both mafic and silicic endmember compositions the complete lack of crustal inclusions in all lavas is taken as evidence for a minimal crustal melting and or assimilation role in the origin of the silicic endmembers many of the features of all andesites including the important long term convergence of endmember magma compositions are consistent with the process of liquid fractionation accompanied by large scale magma mixing a deduced upper limit of 62 5 wt sio2 for the silicic endmember magmas suggests that liquid fractionation in the absence of major crustal melting cannot produce more silicic magmas a possible explanation is the presence of a rheological barrier based on the concept of critical crystallinity marsh 1981 which prohibits more silicic liquids from being extracted from a crystal liquid suspension

a petrologic re investigation of the adak volcanic center central aleutian arc alaska the adak volcanic center is located in the central part of the aleutian arc and consists of three main volcanic vents andrew bay volcano the oldest center has been mostly removed by erosion the next youngest vent mount adagdak was built in three major volcanic stages whereas mount moffett the largest volcanic edifice consists of a main cone and a parasitic cone each with several magmatic phases adak is unique compared to other modern aleutian volcanic centers in that it contains two xenolith suites conrad and kay 1984 debari et al 1987 one suite consisting predominantly of mafic xenoliths occurs on mount moffett whereas an assemblage of ultramafic and mafic xenoliths is found on mount adagdak lavas erupted at adak span the compositional range from 48 4 to 65 0 wt sio2 and are characterized by significant variations in al2o3 mgo sr ni and cr on harker diagrams this variability produces compositional trends with significant scatter the adak suite has total ree contents that vary from 32 to 154 ppm but do not correlate systematically with silica layb n ratios range from 2 41 to 21 72 with the majority of lavas between 2 41 and 6 06 on process identification diagrams the adak suite plots as steeply sloping trends that contrast with the horizontal patterns of most other aleutian centers measured isotopic ranges are large and nearly equal to those for the entire arc although they span similar silica ranges subtle geochemical and isotopic differences distinguish the different volcanic vents of adak on mount moffett a geochemically and isotopically distinct group of andesites 55 5 57 9 sio2 the mafic andesites occur on its ne flank these lavas have elevated mgo ni and cr but are depleted in al2o3 relative to other mount moffett andesites with similar silica they also have more heterogeneous ree abundances and isotopic ratios than most of the other andesites significant compositional differences exist between adak and the other volcanic centers of the central aleutian arc although these differences are characteristic of all geochemical systems they are greatest for major and rare earth elements and isotopic ratios the lack of coherent relationships on major and trace element harker diagrams the isotopic variability as well as the steeply sloping trends on ree process identification diagrams suggest that the adak volcanic suite was not formed predominantly by closed system crystal fractionation but must be the product of a complex open system process es the significant isotopic variability displayed by the suite suggests that contamination by an isotopically distinct contaminant must also have been an important petrologic component in the evolution of the suite ree data are also suggestive of a role for magma mixing such a complex petrologic evolution is consistent with an immature lithospheric plumbing system based on ree systematics the xenolith suites of adak cannot as previously proposed be related to the host lavas or the rest of the adak suite through crystal fractionation schemes rather they are probably accidental fragments derived from various depths along lithospheric conduits in light of their relation to xenolith bearing units the mafic andesites of adak presumably represent hybrid magmas formed during the interaction of ascending magmas with lithospheric wall rock they are therefore characteristic of immature volcanic centers and unlikely to be related directly to the magmatic processes responsible for the generation of primary arc magmas because of the close proximity of the vents and the subtle compositional differences between their lavas the adak volcanic center was probably supplied by a single deep lithospheric plumbing system that fed separate crustal magma chambers the absence of historic volcanic activity on adak suggests this plumbing system was abandoned before complete conduit development this decline in magmatism may reflect a re adjustment of volcano spacing within this part of the aleutian arc

evidence for transform margin evolution from the ivory coast ghana continental margin the relative motions between lithospheric plates can be reduced to three major types convergence divergence and transform motion convergence leads to an active margin and eventual collision divergence results in the construction of a rifted passive margin transform motion generates a transform or sheared margin although passive margins have been extensively studied1 2 and many models have been proposed for their origin and subsequent evolution3 5 little is known about transform margins with the exception of a few studies of their morphologies6 7 shallow structures or crustal sections8 9 here we present the main conclusions derived from a recent study of the northern gulf of guinea margins particularly off the eastern ivory coast and ghana where the continental margin is one of the best preserved examples of an extinct transform margin10 12 our observations support a four stage model for transform margin evolution technically active transform contacts first between normal continental crusts and then between thinned margins induce characteristic structures such as pull apart grabens and shear folds the next stage in which thermal exchange between oceanic and continental lithos pheres controls a complex subsidence is followed by the transition to a true intra oceanic fracture zone

composite transform convergent plate boundaries description and discussion abstract the leading edge of the overriding plate at an obliquely convergent boundary is commonly sliced by a system of strike slip faults this fault system is often structurally complex and may show correspondingly uneven strain effects with great vertical and translational shifts of the component blocks of the fault system the stress pattern and strain effects vary along the length of the system and change through time these margins are considered to be composite transform convergent ctc plate boundaries examples are given of structures formed along three ctc boundaries the aleutian ridge the solomon islands and the philippines the dynamism of the fault system along a ctc boundary can enhance vertical tectonism and basin formation this concept provides a framework for the evaluation of petroleum resources related to basin formation and mineral exploration related to igneous activity associated with transtensional processes

constraints on the composition of the aleutian arc lower crust from vp vs 1 determining the bulk composition of island arc lower crust is essential for distinguishing between competing models for arc magmatism and assessing the stability of arc lower crust we present new constraints on the composition of high p wave velocity vp 7 3 7 6 km s lower crust of the aleutian arc from best fitting average lower crustal vp vs ratio using sparse converted s waves from an along arc refraction profile we find a low vp vs of 1 7 1 75 using petrologic modeling we show that no single composition is likely to explain the combination of high vp and low vp vs our preferred explanation is a combination of clinopyroxenite 50 70 and alpha quartz bearing gabbros 30 50 this is consistent with aleutian xenoliths and lower crustal rocks in obducted arcs and implies that 30 40 of the full aleutian crust comprises ultramafic cumulates these results also suggest that small amounts of quartz can exert a strong influence on vp vs in arc crust

early cretaceous rifts of western and central africa an overview abstract the structure and evolution of early cretaceous rift basins in western and central africa are described two stages of rift development and fracturing have been identified 1 from neocomian to early aptian roughly e w and nw trending troughs upper benue n cameroon s chad sudan etc opened in response to a submeridian extensional regime in central africa while in western africa the n s trending transsaharian fault zone acted as a sinistral wrench 2 from middle aptian to late albian large northwest trending troughs e niger sudan sirte etc opened in response to a northeast extensional regime while the central african fault zone from benue to sudan exhibited strike slip movements generating pull apart basins these rift and fracture systems delimit three large blocks within the african plate a western block an arabian nubian block and an austral block the arabian nubian block tends to separate from the two other blocks migrating towards the north during the first stage of basin development and then towards the ne during the second stage the opening of the atlantic ocean was the dominant driving force for the western and austral blocks while the arabian nubian block probably moved in response to the opening of the indian ocean and to the evolution of the tethyan margin

the origin and significance of groundwater seepage gypsum from bristol dry lake california usa gypsum and anhydrite fabrics observed in trenches and deep 500 m cores from bristol dry lake california usa exhibit a vertical alignment of crystals similar to the fabric seen in bottom nucleated brine pond gypsum however geochemical and sedimentological evidence indicate that the gypsum formed in bristol dry lake precipitated displacively within the sediment where groundwater saturated with respect to gypsum recharges around the playa margin groundwater seepage gypsum evidence for displacive growth of gypsum is i the geometry of the deposit ii stable isotopic data and the water chemistry of the brine and iii inclusions of matrix which follow twin planes and completely surround crystals as they grow the bulk of the gypsum precipitated in the playa occurs around the edges of the playa in the playamargin facies and completely rings the lake sulphate concentrations in the groundwater increase toward the gypsum zone in the playa margin basinward of this zone sulphate concentrations decrease sharply to trace element levels in the basin centre brine authigenic gypsum is rare in the centre of the playa stable 18o values measured for gypsum waters of crystallization gwc are similar to the values calculated for groundwater in the playa margin and alluvial fan sediments 6 0 whereas measured brine 18o values range from 0 5 to 3 7 0 deuterium values measured for groundwater are 70 0 gwc are 60 to 65 0 and brine values are 57 0 the geometry of the deposit and the chemical data suggest that the water precipitating the gypsum is more closely associated with the groundwater than the brine however some mixing between groundwater and brine is likely within 100 m of the surface the gypsum dehydrates to anhydrite although the same vertically aligned fabric is retained through the diagenetic process the similarity of displacive vertically aligned gypsum and anhydrite fabrics seen in bristol dry lake to subaqueously deposited gypsum in modern brine ponds indicates that the criteria used to define subaqueous fabrics must be better constrained

extracting mg isotope signatures of ancient seawater from marine halite a reconnaissance abstract the mg ca ratio of seawater fluctuated remarkably over the phanerozoic the processes that controlled the temporal variation in seawater mg ca ratio may also have caused the secular changes in mg isotope ratios of seawater therefore the mg isotope composition of seawater may be useful to understand the long term variation in seawater chemistry different sedimentary carbonates have been proposed as archives of ancient seawater mg isotope signatures but each type of carbonate has its limitations halite is a common mineral in evaporite deposits and their fluid inclusions may be used to infer the mg isotope composition of contemporary seawater in this study we developed a method to measure the isotope composition of mg in halite with an accuracy of better than 0 1 in 26mg the accuracy of the method was verified systematically using synthetic nacl solutions as well as holocene lacustrine halite samples from the dead sea the 26mg values of marine halite samples show a large variation ranging from 0 1 to 1 6 relative to the dsm3 international mg isotope standard the 26mg values of some ancient salt samples are lower than a published model curve for 26mg of seawater and such inconsistency could be explained by 1 isotopic evolution of brines due to mineral precipitation 2 entrapment of evaporite minerals into halite during halite precipitation or post depositional deformation 3 recrystallization and dissolution of pre existing mg minerals and 4 limitations of the model curve of seawater 26mg values collectively we suggest that mg isotopes in halite are a new geochemical proxy to study the secular variations in seawater chemistry syndepositional and post depositional processes of evaporite deposits our data highlight the importance of understanding the sedimentary background and textural details of evaporites for appropriate interpretations of mg isotope data from halite

tholeiitic aleutian arc plutonism the finger bay pluton adak alaska the earliest representatives in the sequence of tertiary to recent magmatic rocks on adak island in the central aleutians are the finger bay volcanics and associated small shallow level gabbroic intrusives the tholeiitic finger bay pluton gabbro to quartz monzodiorite is among the least altered representatives of this earliest period the field relations mineralogy and geochemistry of the finger bay pluton contrast with the more recent calcalkaline plutons in the arc e g on adak hidden bay pluton 33 m y and on kagalaska kagalaska pluton 15 m y compared with the hidden bay pluton the finger bay pluton is smaller 8 km2 versus 100 km2 has a greater proportion of gabbro 84 versus 5 has a somewhat different mineralogy and has higher whole rock and mafic mineral feo mgo ratios higher k2o and higher concentrations of incompatable elements in rocks of equivalent sio2 particularly for the more siliceous units magmatic amphibole occurs only in the most siliceous units in the finger bay pluton whereas it is common in the calc alkaline plutons except for the size these differences parallel those between tholeiitic and calc alkaline volcanic rocks from the active aleutian volcanic centers textural miner alogical and trace element analyses indicate that mixing of magmas and gabbros generated some of the rocks of intermediate monzodiorite composition in the finger bay pluton most of the differences in magmatic trends between the calc alkaline and tholeiitic plutons are attributed to the physical conditions of magmatic evolution in the crust rather than to differences in initial magmatic types this conclusion rests on the similarity of fractionation independent isotope ratios and trace element ratios in aleutian magmas of all ages in particular compared to magmas of the north pacific ocean basin morb oceanic islands aleutian magmas show excess enrichment of ba k rb and u relative to ree and high 207pb 204pb ratios for a given 206pb 204pb ratio recognition of tholeiitic series plutons in the arc provides direct evidence for magmatic conditions accompanying fractionation and serves to emphasize the diversity of magmatic trends that are found over time and space at convergent plate margins

amphibole bearing inclusions from boisa island papua new guinea evaluation of the role of fractional crystallization in an andesitic volcano boisa island is the tip of a quaternary island arc volcano it consists of an older cone made up of porphyritic amphibole free mafic rocks and two younger cumulodomes of hornblende bearing high silica andesite one of the cumulodomes contains coarse grained igneous inclusions consisting of amphibole plagioclase clinopyroxene orthopyroxene olivine and spinel some of these minerals in the andesites themselves have both cognate and xenocrystal components both the anhydrous phenocryst assemblage in the basalts and the amphibple bearing assemblage of the inclusions provide low residuals in least squares linear mixing calculations suggesting that fractionation of either assemblage in the basalts could have given rise to the andesites however this is not supported by the results of rayleigh fractionation calculations for several incompatible elements four possible interpretations are 1 the rayleigh fractionation results are inappropriate and misleading 2 the andesites were formed by crystal f

aleutian tholeiitic and calc alkaline magma series i the mafic phenocrysts diagnostic mafic silicate assemblages in a continuous spectrum of aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the aleutian arc crust tectonic segmentation of the arc exerts a primary control on the variable mixing fractional crystallization and possible assimilation undergone by the magmas end members of the continuum are termed calc alkaline ca and tholeiitic th ca volcanic rocks e g buldir and moffett volcanoes have low feo mgo ratios and contain compositionally diverse phenocryst populations indicating magma mixing their ni and cr rich magnesian olivine and clinopyroxene come from mantle derived mafic olivine basalts that have mixed with more fractionated magmas at mid to lower crustal levels immediately preceding eruption high al amphibole is associated with the mafic end member in contrast th lavas e g okmok and westdahl volcanoes have high feo mgo ratios and contain little evidence for mixing evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma these lavas contain groundmass olivine fo 40 50 and lack ca poor pyroxene aleutian volcanic rocks with intermediate feo mgo ratios are termed transitional tholeiitic tth and calc alkaline tca tca magmas are common e g moffett adagdak great sitkin and kasatochi volcanoes and have resulted from mixing of high al basalt with more evolved magmas they contain amphibole high and low al or orthopyroxene or both and are similar to the japanese hypersthene series tth magmas e g okmok and westdahl contain orthopyroxene or pigeonite or both and show some indication of upper crustal mixing they are mineralogically similar to the japanese pigeonite series high al basalt lacks mg rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite the low pressure mineral assemblage of high al basalt results from crystallization at higher crustal levels

a long geoclimatic record from the permian a 260 000 year continuous annual geoclimatic time series has been compiled from the varved permian castile and bell canyon formations of the delaware basin southeastern new mexico and southwestern texas the last 200 000 years of the record was obtained from calcite laminated anhydrite and anhydrite laminated halite which formed as a result of seasonal and annual deposition in the evaporite basin the time series is based on measured thickness of individual varves analyses of calcium sulfate calcium carbonate and organic matter were made on 50 year intervals in the evaporite part of the sequence time series of the major components were examined by using smoothed graphic plots variance spectra and moving correlation coefficients the longest oscillation recorded in the series that may be of climatic origin has a period of about 100 000 years the calcium sulfate record contains 9 11 distinct oscillations with an average period of about 20 000 years the strongest oscillation has a preferred spectral period of 2700 years and is recorded throughout the series as thickness changes in calcium sulfate as events controlling halite deposition and as episodes of basin freshening a broad spectral response near a period of about 200 years reflects changes in sulfate carbonate and halite thickness shorter periods do not have consistent spectra the time series contains no preferred periods that have not already been identified in previously described records the climatic changes appear to be the result of a deterministic response to orbital effects at the longer periods and stochastic processes at the shorter periods an unusually strong response near 2700 years is associated with episodic freshening of the basin this period has been well established in the holocene paleoclimatic record and its presence in the permian suggests a deterministic origin

andesites from northeastern kanaga island aleutians kanaga island is located in the central aleutian island arc northeastern kanaga is a currently active late tertiary to recent calc alkaline volcanic complex basaltic andesite to andesite lavas record three episodes series of volcanic activity series i and series ii lavas are all andesite while series iii lavas are basaltic andesite to andesite four series ii andesites contain abundant quenched magmatic inclusions ranging in composition from high mgo low alumina basalt to low mgo highalumina basalt the spectrum of lava compositions is due primarily to fractional crystallization of a parental low mgo high alumina basalt but with variable degrees of crustal contamination and magma mixing the earliest series i lavas represent mixing between high alumina basalt and silicic andesite with maximum sio2 contents of 65 67 wt later series i and all series ii lavas are due to mixing of andesite magmas of similar composition the maximum sio2 content of the pre mixed andesites magmas is estimated at 60 63 wt the youngest lavas series iii are all non mixed and have maximum estimated sio2 contents of 59 wt the earliest series i lavas contain a significant crustal component while all later lavas do not it is concluded that the maximum sio2 contents of silicic magmas the contribution of crustal material to silicic magma generation and the role of magma mixing all decrease with time furthermore silicic magmas generated by fractional crystallization at this volcanic center have a maximum sio2 content of 63 wt all of these features have also been documented at the central aleutian cold bay volcanic center brophy 1987 based on data from these two centers a model of aleutian calc alkaline magma chamber development is proposed the main features are 1 a single low pressure magma chamber is continuously supplied by primitive low alumina basalt 2 non primary high alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase 3 sidewall crystallization accompanied by crustal melting produces buoyant silicic 63 wt sio2 liquids that pond at the top of the chamber and 4 continued sidewall crystallization now isolated from the chamber wall produces silicic liquids with 63 wt sio2 that increase the thickness and lowers the overall sio2 content of the upper silicic zone it is suggested that the maximum sio2 content of 63 imposed on fractionation generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal liquid mush along the chamber wall

petrologic constraints on the spatial distribution of crustal magma chambers atka volcanic center central aleutian arc the atka volcanic center the largest modern central aleutian magmatic complex covers 360 km2 with an estimated eruptive volume of 200 km3 the oldest exposed rocks the old harbor series are a suite of basaltic andesite to dacite flows pyroclastic rocks and lahars of presumed tertiary age the youngest phase of activity on atka probably began 1 2 ma ago near the center of the present volcanic field and consists of basaltic and basaltic andesite flows which produced a large shield volcano atka volcano subsequent to this activity satellite vents formed along the margin of this structure collapse of the ancestral atka volcano produced a caldera 5 km in diameter and was accompanied by the eruption of a large dacitic flow big pink after caldera formation four major volcanic cones korovin mount kliuchef sarichef and konia grew around the margins of atka caldera this study focuses on the atka volcano korovin and mount kliuchef volcanic centers atka lavas range from basalt through dacite with all major volcanic vents erupting lavas spanning similar compositional ranges they are porphyritic with phenocrysts of plagioclase 25 36 olivine 0 4 3 6 or orthopyroxene 2 8 clinopyroxene 1 12 and a single fe ti oxide 0 2 hydrous phases are lacking even in the most evolved lavas two generations of plagioclase occur in the korovin suite but not in lavas from mount kliuchef korovin plagioclase also displays resorption surfaces and inclusion zones which are either absent in the phenocrysts of mount kliuchef lavas or very rare similarly clinopyroxene in korovin has reaction rims and abundant glass inclusions which are absent in the mount kliuchef lavas lavas erupted in the atka volcanic field vary from 47 to 66 sio2 although the complex is dominated volumetrically by basaltic lavas on major element harker diagrams mafic and intermediate atka volcano lavas form linear trends but the most siliceous lavas do not fall along these trends by contrast mount kliuchef lavas are compositionally bimodal whereas korovin samples range from 51 to 62 wt silica with intermediate lavas clustering around 55 sio2 incompatible trace element abundances of korovin lavas do not increase with silica content but those of atka volcano and mount kliuchef do on incompatible incompatible element diagrams the three suites behave very differently differences also exist between the ree characteristics of the three atka suites the petrographic and geochemical differences between the three suites suggest formation by markedly different crustal magmatic processes the characteristics of the atka volcano suite suggest formation of the basalt to dacite suite by crystal fractionation of an anhydrous phenocryst assemblage incompatible incompatible element trends suggest the mafic mount kliuchef lavas were also produced by crystal fractionation a conclusion supported by quantitative mass balance calculations the difference in slope between the atka volcano and mount kliuchef suites indicates however that there were significant differences in these fractionation schemes modeling as well as geochemical characteristics suggest that the mount kliuchef dacites are not the product of crystal fractionation they may represent 1 crustal melts 2 complete melts of siliceous lava produced during earlier magmatic stages or 3 residual liquids from the atka volcano magmatic system in contrast to these suites the korovin lavas could not have been formed by crystal fractionation rather the andesites are probably the result of mixing basaltic and dacitic magmas the atka data suggest 1 large volcanic centers may be supplied by a series of non communicating crustal magma chambers 2 local factors may be important in determining liquid lines of descent and 3 very different magmatic processes may occur in adjacent magma chambers if the complexity of the atka suite is a common feature of arc magmatic plumbing systems reconstructing differentiation processes requires analytical sample sets which are well constrained spatially and temporally

criteria for the recognition of salt pan evaporites layered evaporites can accumulate in 1 ephemeral saline pans 2 shallow perennial lagoons or lakes and 3 deep perennial basins criteria for recognizing evaporites deposited in these settings have yet to be explicitly formulated the characteristics of the ephemeral saline pan setting have been determined by examining eight holocene halite dominated pans salt pans and their deposits marine and non marine from the u s mexico egypt and bolivia these salt pans are typified by alternating periods of flooding resulting in a temporary brackish lake evaporative concentration when the lake becomes saline and desiccation which produces a dry pan fed only by groundwater the resulting deposits consist of alternating layers millimetres to decimetres of halite and mud the layers of halite are characterized by 1 vertical and horizontal cavities rounded crystal edges and horizontal truncation surfaces due to dissolution during flooding 2 vertical chevrons and cornets grown syntaxially on the bottom during the saline lake stage 3 halite cements overgrowths and euhedral cavity linings and disruption of layering into metre scale polygons produced during the desiccation stage the muddy interbeds are characterized by displacive growth of halite during the desiccation stage immediately below the surface of the pan the halite layers are matured by repeated episodes of dissolution and diagenetic crystal growth this results in porous crusts with patches of chevron and cornet crystals truncated by dissolution clear diagenetic halite cement and internal sediment these layers of mature halite closely resemble the patchy cloudy and clear textures of ancient halite deposits holocene salt pans are known to cover thousands of square kilometres and cap halite deposits hundreds of metres thick so they are realistic models for ancient evaporites in scale e g permian salado formation of new mexico texas which preserves many primary salt pan features

displacive halite hoppers from the dead sea some implications for ancient evaporite deposits abstract large hopper shaped halite cubes up to 10 cm on a side grow displacively within holocene sediments near the southern end of the dead sea under conditions of high temperatures ionic concentrations and rapid rates of crystallization two theories of formation are presented downward diffusion of ions versus upward diffusion and evaporative pumping both alternatives are considered possible and the correct choice probably depends upon the location of the sediments at the time of precipitation whether subaqueous or subaerial a choice indeterminate at present because of human disturbance to the sediments prior to collection displacively formed halite crystals similar in morphology to the dead sea cubes are common in evaporite bearing carbonate rocks of many regions such as the salina fm u silurian michigan basin the kirkham mudstone u triassic cumbria england and the u miocene gessoso solfifera fm of sicily these are believed to have formed under conditions analogous to those in the present dead sea these observations taken together with those of other authors may now be used for the facies interpretation of ancient halite deposits

relation of lava compositions to volcano size and structure in el salvador abstract most of the lavas at the nine volcanic centers along the volcanic front of el salvador are basalts basaltic andesites and andesites the compositional variation within and among these centers can be explained by fractionation processes within the crust cognate gabbroic inclusions found in the lavas have appropriate mineralogy plagioclase olivine magnetite and augite to be cumulates formed by fractional crystallization two main variation trends occur depending on the proportion of plagioclase removal the more common or normal trend has a high 55 proportion of plagioclase being removed a less common al rich trend has a low 40 proportion of plagioclase being removed the al rich trend is found only at volcanoes that lack large negative bouguer gravity anomalies these volcanoes are unlikely to have large shallow magma chambers and fractionation probably occurs deeper in the crust where plagioclase removal is inhibited the incompatible element na 2 o k 2 o rb ba contents of lavas vary systematically with the volume of the volcanic centers at the same level of sio 2 large volcanic centers have higher incompatible element contents than small volcanic centers this suggests that open system fractionation in a periodically refilled chamber is the controlling factor the large difference in ba contents of lavas between eastern low and western high el salvador suggests a difference in the mantle source region

magmatism of the westernmost komandorsky segment of the aleutian island arc abstract four magmatic associations are distinguished in the westernmost komandorsky segment of the aleutian island arc basalt plagiorhyolite 45 28 ma trachybasalt teschenite 27 16 ma diorite granitoid 12 5 8 5 ma and dacite rhyolite 0 7 0 ma the latter association is represented by the lavas of a recently discovered active submarine volcano piip volcano about 40 km north of the komandorsky islands and 700 km west of the quaternary center on buldir is detailed petrogeochemical data including ree and 143 nd 144 nd and 87 sr 86 sr isotopic ratios show that the parental melts of the komandorsky is were of mantle origin and their sources remained isotopically homogeneous during the whole geological history of the aleutian arc 10be concentrations in lavas of the piip volcano 2 7 106 at g indicate subduction related incorporation of young pelagic sediments in their magmatic sources or contamination of ascending melts by non solidified sedimentary material directly beneath the volcano the komandorsky segment of the aleutian arc evolved from a depleted arc tholeiitic series through a markedly alkaline series to later calc alkaline andesites and dacites and finally to late stage calc alkaline dacites and rhyodacites erupted inboard of the older volcanic centers

hornblende and phlogopite bearing gabbroic xenoliths from volcan san pedro 36 s chilean andes evidence for melt and fluid migration and reactions in subduction related plutons

composition of brines in halite hosted fluid inclusions in the upper ordovician canning basin western australia new data on seawater chemistry analyses of primary and early diagenetic fluid inclusions in the halite from the late ordovician mallowa salt canning basin western australia indicate a ca rich composition and high concentration of parent brines in the basin which were close to sylvite and carnallite precipitation the salt bearing series in the sampled interval was overheated up to 62 c the recorded differences in gas compositions result from the input of several gas sources including dispersed organic matter in the salt series and hydrocarbon deposits in the underlying rocks the high concentration of the brines in fluid inclusions does not allow quantitative reconstruction of the chemical composition of late ordovician parent seawater using the information from early cambrian and late silurian basins as a proxy however the new data indicate that late ordovician seawater was undoubtedly ca rich and in comparison with modern seawater had a similar k content considerably lower mg content c 30 approximately three times the ca content and one third the so4 content

halite saltern in the canning basin western australia a sedimentological analysis of drill core from the ordovician silurian mallowa salt the late ordovician early silurian mallowa salt of the carribuddy group canning basin north west australia is the largest halite deposit known in australia attaining thicknesses of 800 m or more within an area of approximately 200 000 km2 study of 675 m of drill core from bhp utah minerals brooke no 1 well in the willara sub basin indicates that the mallowa salt accumulated within a saltern dominantly subaqueous evaporite water body that was subject to recurrent freshening desiccation and exposure textures and bromine signatures imply a shallow water to ephemeral hypersaline environment typified by increasing salinity and shallowing into evaporitic mudflat conditions toward the top of halite mudstone cycles type 2 and the less common dolomite anhydrite halite mudstone cycles type 1 the borate mineral priceite occurs in the capping mudstones of some cycles reinforcing the idea of an increasing continental influence toward the top of mudstone capped halite cycles the rock salt in both type 1 and type 2 cycles typically comprises a mosaic of large randomly orientated interlocking halite crystals that formed during early diagenesis it only partially preserves a primary sedimentary fabric of vertically elongate crystals some with remnant aligned chevrons intraformational hiati halite karst tubes and solution pits attest to episodic dissolution stacked type 2 cycles dominate occasional major recharges of less saline perhaps marine waters in the same area produced type i cycles the envisaged saltern conditions were comparable in many ways to those prevailing during the deposition of halite cycles of the permian salado formation in new mexico and the permian san andres formation of the palo duro basin area in texas however in the canning basin the cycles are characterized by a much lower proportion of anhydrite implying perhaps a greater degree of continental restriction to the basin the moderately high level of bromine in the mallowa salt 156 5 43 5 ppm br for primary halite 146 1 54 7 ppm br for secondary halite accords with evolved continental brines although highly evaporative minerals such as polyhalite and magnesite are absent the bromine levels suggest little or no dissolution reprecipitation of primary halite and yet paradoxically there is little preservation of the primary depositional fabric the preservation of early halite cements and replacement textures supports the idea of an early shutdown of brine flow paths probably at burial depths of no more than a few metres and the resultant preservation of primary bromine values in the secondary halite

depositional environments of marine dominated bedded halite permian san andres formation texas the interpretation of the depositional environments in which bedded halite is formed is a frontier of evaporite sedimentology facies studies supplemented by geochemical data define an evaporite shelf depositional environment for the halite of the san andres formation of the palo duro basin texas panhandle that is significantly different from modern and previously described ancient halite precipitating environments such as playas salinas sabkhas and barred basins a coastal palaeoenvironmental setting for the san andres halite is defined by its facies position between normal marine shelf carbonates and aeolian deposits the san andres brine pool extended over more than 10 000 km2 but was very shallow perennial brine pool conditions thick sequences of halite alternated with ephemeral brine pool conditions halite haloturbated mudstone halite mudstone interbeds and microkarst the depositional sequence in the perennial brine pool includes 1 flooding and minor dissolution of previously deposited halite 2 precipitation of a thin lamina of gypsum and 3 evaporation and precipitation of halite bottom growth forms of halite including chevrons and vertically elongated crystals are well preserved cumulates of foundered floating crystals and rafts were probably abundant but have been almost completely recrystallized mudstone interbeds in halite and microkarst features cut into bedded halite record the episodic interruption of brine pool deposition and the establishment of subaerial conditions vertically elongated karst pits originate at the palaeosurfaces cross cut primary brine pool fabrics and are filled with geopetal sediment and coarse halite cement mudstone interbeds and mixtures of mudstone and halite have been haloturbated by the alternating growth and dissolution of halite reflecting conditions of fluctuating salinity the geochemistry of halite from both ephemeral and perennial brine pool environments reflects the frequent influx of marine brine indicating that communication with the marine environments to the south was good considering that restriction was sufficient to precipitate halite

the border ranges ultramafic and mafic complex south central alaska cumulate fractionates of island arc volcanics a discontinuous elongate zone of mafic and ultramafic plutonic rock crops out in south central alaska for a distance of more than 1000 km intermediate and detailed scale geologic mapping petrographic study and compositional data suggest that the plutonic rocks are compositionally petrologically and mineralogically distinct from rocks in mid ocean ridge and back arc basin ophiolites the mafic and ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc the mafic ultramafic zone referred to as the border ranges ultramafic and mafic complex brumc is composed of ultramafic cumulates gabbronorite cumulates and massive gabbronorites a very minor amount of tectonized ultramafic rock of mantle origin is present in the southern part of the brumc a thick sequence of andesitic volcanic rocks the talkeetna formation of early jurassic age lies to the north of and structurally above the mafic ultramafic zone voluminous calcalkaline plutons composed of quartz diorite

millennial to centennial scale record of african climate variability and organic carbon accumulation in the coniacian santonian eastern tropical atlantic ocean drilling program site 959 off ivory coast and ghana millennial to centennial scale geochemical records of coniacian to santonian deposits from the deep ivorian basin are used to develop a model for the accumulation of black shales in equatorial regions during the final of the cretaceous oceanic anoxic events oae proxy records from ocean drilling program site 959 document a strong precessional signal in the occurrence of black shales the abundance of quartz and clay mineralogy during oae 3 we hypothesize that this signal reflects changes in atmospheric circulation patterns which in turn caused latitudinal shifts of continental climate belts across western africa furthermore we propose that the periodic deposition of black shales occurred in response to adjustments of oceanic circulation in the deep ivorian basin resulting from climate controlled fluctuations in continental runoff a new high resolution cyclostratigraphic framework allowed us to estimate a rapid change within 1000 yr from dysoxic background to anoxic or euxinic black shale environmental conditions followed by black shale deposition for 10 k y and a gradual return to the initial dysoxic conditions our findings imply a highly dynamic late cretaceous atmosphere ocean system

tectonic controls on tholeiitic and calc alkaline magmatism in the aleutian arc the tectonic position of aleutian arc volcanic centers and their magmatic differentiation trends calc alkaline or tholeiitic appear to correlate from 160 w to 175 e the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates the tholeiitic volcanoes are large primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow closed system differentiation the calc alkaline volcanoes are smaller more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers the chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma probably derived from mantle peridotite conditions for the derivation of the two trends through crystal fractionation support the proposed model the tholeiitic magmas show characteristics i e no hydrous phenocrysts fe enrichment trend parallel ree patterns vitrophyric and esites and dacites consistent with low pressure high temperature crystallization in large shallow magma chambers the calc alkaline magmas show characteristics i e some hydrous phenocrysts no fe enrichment trend nonparallel rare earth patterns porphyritic lavas consistent with higher pressure and lower temperature of crystallization than the tholeiitic series tertiary plutons also show both calc alkaline and tholeiitic trends and appear to be chemically similar to the quaternary volcanoes the larger plutons are calc alkaline and probably represent the extensive 1002 km magma chambers of the small calc alkaline volcanoes the small shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series

development of the cote d ivoire basin reading provenance sediment dispersal and geodynamic implications from heavy minerals the combined approach of conventional and varietal heavy mineral analysis was applied to constrain changes in source rock area dispersal routes of detritus and geodynamic setting of the upper cretaceous to neogene cote d ivoire basin the heavy mineral assemblage of albian to turonian sediments reflect a narrow source rock area and short transport distances a prominent change in the heavy mineral association of late cenomanian to neogene sediments indicates enlargement of source area and longer fluvial transport distances or longer storage of sediment within the littoral energy fence the sudden change within the heavy mineral associations marks the transition from the late syn rift stage to the early passive margin stage of the southern atlantic during albian to late cenomanian time the basin geometry was mainly controlled by steep slopes and sediments were transported directly to the depocenter the situation changed in late cenomanian time a small shelf area developed where sediments from different sources were mixed and reworked before deposition we believe that the w e direction of the long shore drift which transports today huge quantities of near shore sand was established at the end of the paleogene this resulted from the drifting of southern west africa across the paleoequator and subsequent change in surface wind directions

nodular and enterolithic gypsum the sabkha tization of salt flat playa west texas abstract the sediment in the continental sabkha of salt flat west texas is typically millimeter laminated and composed of evaporitic and diagenetic minerals including gypsum dolomite halite calcite and aragonite these laminated evaporites were originally deposited on the floor of a late pleistocene perennial brine lake and now constitute the matrix of the modern sabkha sediments in the playa as climate changed from the more humid phase of the pleistocene to the more arid conditions of the holocene the lake dried up and these laminated evaporites were deflated into gypsum dunes or overprinted by a sabkha related hydrology near surface gypsum rich layers were transformed by capillary evaporation into coalescing nodular horizons at the same time in shallow vadose sediments less than 1 2 m subsurface the gypsum was dissolved and replaced either by halite or dolomite diagenetic gypsum was co precipitated as gypsum overgrowths and a patchwork of cement from a highly saline interstitial brine avg 156 000 mg l tds these processes of early diagenetic enlargement and coalescence of gypsum crystals created holocene gypsum nodules within the laminated host sediment diffuse growth of nodules in the lower vadose zone created chicken wire textures gypsum growth in the upper vadose zone was often concentrated along particular layers and formed enterolithic bands pleistocene lacustrine evaporites were in effect sabkhatized by a fall in the lake water level

partial assimilative recycling of the mafic plutonic roots of arc volcanoes an example from the chilean andes at long lived arc volcanic centers open system interactions between new magma and the plutonic roots of the arc will increase with time relative to interactions with older crust assimilative recycling of mafic plutonic arc roots will have different isotopic consequences than incorporation of aged continental crust in the tatara san pedro complex 30 comagmatic basaltic lavas are characterized by xenocrysts and microxenoliths of olivine augite and plagioclase derived from solidified cumulates wide but poorly correlated ranges of incompatible and compatible major and trace elements and limited isotopic variability this mineralogical chemical signature reflects assimilation wherein grain boundary melting and disaggregation of xenoliths led to blending of incompatible element enriched melts derived mainly from hornblende phlogopite and plagioclase in combination with variable retention of xenocrysts hence variable but high compatible element concentrations

the hydrological setting occurrence and significance of gypsum in late quaternary salt lakes in south australia well exposed often laminated gypsum sequences occur in many quarternary salt lakes in southern south australia the gypsum in the salt lakes is classified by increasing grain size into gypsite gypsarenite and selenite the salt lakes are classified by age and hydrological setting into coastal salinas which are holocene sea water fed groundwater lakes and continental playas which are late pleistocene endorheic basins a study of the relationships between coastal salina hydrology and the associated gypsum deposition has shown the different types of gypsum form under distinct hydrological regimes as the hydrology above a coastal salina depositional surface changes through time so does the type of gypsum deposited application of a gypsum depositional model derived from a study of the coastal salina gypsum to those portions of a continental playa gypsum unit where deposition is no longer occurring confirms the applicability of the model to non salina gypsum deposits

sedimentology and evaporite genesis in a holocene continental sabkha playa basin bristol dry lake california bristol dry lake a 155 km2 continental sabkha playa basin in the mojave desert of south eastern california is filled with at least 300 m of interbedded terrigenous clastics gypsum anhydrite and halite evaporite facies conform approximately to a bull s eye pattern with gypsum and anhydrite surrounding a basin centre accumulation of halite transects through bristol dry lake from the alluvial fan to the centre of the playa reveal 1 crudely bedded alluvial fan clastics interfingering with 2 playa margin sand flat and wadi sand and silt followed by 3 gypsum anhydrite chaotic mud halite and clay of the saline mud flat and 4 salt pan halite beds terrigenous clastics were deposited in bristol dry lake by sheetflow and by suspension settling from ponded floodwater some sediment has been reworked by aeolian processes to form barchan dunes around the playa margin thin nodular like beds of anhydrite and several types of gypsum occur across most of the playa giant hopper shaped halite cubes are suspended in saline mud flat facies suggesting that they grew displacively in brine soaked sediment just below the surface thick beds 4 m of halite in the playa centre may have formed through a complex alternating history of subaqueous and intrasedimentary precipitation under the influence of periodic floods intense evaporation and brine level lowering and capillary discharge of brines the stratigraphy in the playa centre is cyclic an ideal cycle consists of 1 chaotic mud halite at the base overlain by 2 green to red clay with abundant giant hoppers and at the top 3 red clay gypsum and anhydrite with flaser to wavy bedded sand and silt this type of cycle probably records a gradual progradation of mud flat facies over salt pans bristol dry lake sediments are nearly identical to some of the permian evaporites of the permian basin region u s a and they can serve as modern analogues for ancient sabkha facies analysis

some thermal problems associated with magma migration abstract the mechanisms by which magma migrates from the point in the earth s interior where melting occurs to the earth s surface are poorly understood in this paper several aspects of this problem are examined magma can migrate upward due to its differential buoyancy on the scale of crystalline grains or as large dispairs magma transport is an effective means of heat transport magma transport at a rate of 0 15 cm yr is equivalent to a heat flow of 10 6 cal cm 2 s if magma encounters country rock witha lower melting point the original magma is likely to solidify while melting the country rock this would be an effective mechanism of purging silicic rocks and incomparable elements from the lower crust under some circumstances magma must penetrate up to 100 km or more of cold lithospheric rock in order for this magma to reach the surface without solidification a heated path must be provided the heating of this path requires the solidification of some magma it is estimated that magma penetrates the lithosphere in about 5000 years and that the crack is lined by several hundred meters of frozen basalt

the shallow structures of the guinea and ivory coast ghana transform margins their bearing on the equatorial atlantic mesozoic evolution abstract recent geological and geophysical data from a marine survey conducted off guinea and the ivory coast ghana region equatorial atlantic are discussed we interpret the southern guinean margin firstly as an area linked to the central jurassic atlantic and secondly as an area affected by the progressive transform opening of the equatorial atlantic in the early cretaceous off the ivory coast and ghana we have demonstrated distinct structural domains interpreted as consequences of the progressive transform contact between the west african and brazilian cratons also during the early cretaceous on both margins we observe a clear tectonic unconformity of probable late albian early cenomanian age we propose that this unconformity results from the final disruption between the two continental west african and brazilian crusts the geological structures generated along transform margins can be considered as good geological tools to constrain early phases of plate motions particularly around the equatorial atlantic

sedimentologic and geochemical evidence for middle ordovician near trench volcanism in the central appalachian orogen the jonestown volcanic rocks are a minor but important component of the greenwich slice of the hamburg klippe of eastern pennsylvania sedimentary rocks of the greenwich slice are arranged in faultbounded coarsening upward sequences of pelagic deposits early to middle ordovician overlain by hemipelagic and turbidite deposits middle ordovician petrographic and geochemical characteristics of the sedimentary rocks indicate derivation from continental rather than volcanic sources the coarsening upward sequences of the greenwich slice are inferred to record advance of a site on oceanic lithosphere toward and ultimately into a trench by middle ordovician time rocks of the jonestown volcanic suite were extruded onto and intruded into the inferred middle ordovician near trench and trench axis mudstone and sandstone immobile element concentrations of the jonestown suite are typical of mid oceanic ridge basalts and along with the lack of associated shallow water and or volcaniclastic sedimentary rocks a

the petroleum geology of the west africa margin an introduction the continental margin of west africa formed as result of the south to north rifting of gondwana and the progressive separation of the south american and african continents this margin has enjoyed a rich and varied exploration history and in the 70 years or so since the first significant exploration began in the onshore area the margin has emerged as a significant hydrocarbon producing region the amalgamation of hydrocarbon exploration approaches and imaginative ideas leveraged with modern technologies is yielding significant scientific and eco

rift shear architecture and tectonic development of the ghana margin deduced from multichannel seismic reflection and potential field data abstract the ghana margin displays one of the best known transform margins studies of the margin have provided the framework for a number of conceptual models aimed at understanding transform margin development worldwide however the deep structure of the margin is poorly known as knowledge is based only on wide angle refraction measurements obtained from two separate localities on the margin consequently complexities in the rift shear margin architecture have been overlooked by current interpretations of margin development based on combined analysis of a detailed grid of 2710 km multichannel mcs lines and potential field data we provide new insights into the structural architecture and tectonic development of the ghana margin in particular we outline the deep structure of the entire margin using a series of 2d gravity modelled transects constrained by mcs and published wide angle data our study reveals more complex rift shear margin architecture than previously envisaged we demonstrate that the main transform boundary representing the continental extension of the romanche fracture zone is actually composed of two distinct margin segments i e the ene wsw trending sheared margin segment of the cote d ivoire ghana ridge and the ne sw trending rift influenced sheared margin segment of the ghana platform these segments evolved under varying stress regimes and during different time intervals west of the transform margin divergent rifting during the early cretaceous initiated the development of the deep ivorian basin essentially as a single major pull apart structure however east of the shear zone oblique rifting resulted in the development of the eastern ghana slope basin as a composite of at least two coalescing pull apart basins displaced along strike slip faults our structural interpretation of the transform boundary geometry shows that the ridge and platform margin segments were each subjected to separate thermal influences from two different migrating spreading centres tectonic uplift of the ridge began through transpression during mid albian time following a change in relative direction of plate motion from ne sw to ene wsw however the ridge uplift was amplified by thermal heating from a previously undocumented spreading centre whose progressive westward migration along the ridge followed closely after the albian transpressional phase the structural architecture of the ghana margin resulted from a combination of factors notably pre existing basement structure plate boundary geometry the relative direction of plate motion and thermal heating

mid pleistocene basalt from the seguam volcanic center central aleutian arc alaska local lithospheric structures and source variability in the aleutian arc recent geophysical and tectonic studies make it possible to examine the relation between local structural and tectonic evolution of the modern aleutian arc and the sources and processes that produce the broad spectrum of aleutian basaltic lavas mid pleistocene basalt from the seguam island volcanic center in the central aleutian arc erupted through strongly extended arc crust contains 4 8 mgo is low in k2o 18 and is distinguished from aleutian basalt erupted through less extended crust by a low abundance of incompatible and rare earth elements ree and low sr and pb content the ba la 47 72 and pb la 0 7 1 5 ratios are high incompatible element ratios are variable and ree patterns cross one another the 87sr 86sr ratio 0 70365 0 00003 is the highest value obtained from the aleutian islands also 206pb 204pb 18 91 0 02 207pb 204pb 15 60 0 02 and 208pb 204pb 38 51 0 04 ratios are among the most radiogenic values known whereas the 143nd 144nd ratio 0 51297 0 00003 is among the lowest measured low abundances of incompatible elements high ba la and pb la ratios uniform sr nd and pb isotope ratios and high eruptive temperatures 1160 c suggest that basalt erupted at seguam ascends through extended subarc lithosphere without extensive chemical exchange with the upper mantle or lower crust partial melting of geochemically distinct portions of an isotopically uniform source region best explains observed geochemical characteristics trace element abundances rule out an eclogite source and suggest instead a peridotite dominated source with 5 10 of a subducted sediment component this peridotite may reside in the mantle wedge or in the amlia fracture zone afz of the pacific plate that was subducted beneath seguam 1 ma in addition the afz possibly enhanced the subduction of terrigenous sediment into the source region beneath seguam and this extra terrigenous component may explain the ba la pb la and sr nd and pb isotope ratios the distinctive composition of seguam basalt reflects a complex set of physical conditions including fracture zone subduction and intraarc extension that affected both the source region and thermo chemical history of magma ascent local structural and tectonic factors impact profoundly the nature of island arc magma sources and processes models that propose a common parental arc magma type or common processes on a global or arcwide scale are too generalized to explain the diversity actually observed in the aleutian arc

quantifying arc migration and the role of forearc subduction erosion in the central aleutians abstract the active aleutian arc is currently classified as an accreting margin as most of the arc has a well developed frontal accretionary prism that formed in the plio pleistocene we focus on the central aleutian island arc where the arc massif was built to near its current size by the late eocene and has since seen limited growth and endured episodic erosion prior to the development of the accretionary prism as a result the volcanic front has migrated northwards tens of kilometers since inception along with previous mapping dating and geochemical data sixteen new 40ar 39ar dates and new geochemical data for volcanic rocks on several central aleutian islands supplement recent geochronologic data from six other islands in this region and facilitate an attempt to understand how and where aleutian arc volcanism has migrated with time long term average arc migration rates range from 0 6 to 5 km ma in three distinct locations the arc migration rate increases to the west and is greatest over the last five million years as the arc migrated from a 7 5 ma volcanic front to its current location arc migration and erosion during this most recent period may have been expedited by either the subduction of the kula ridge which ceased spreading in the early eocene or clockwise rotation of the pacific plate in the late miocene some central aleutian lavas and plutonic rocks have elevated la yb sm yb and sr yb most of which were emplaced during times of arc migration or in the waning stages of a peak in magmatic activity the strongly fractionated trace element patterns are similar to the basaltic andesites to dacites from the western aleutian seafloor which are interpreted to be the result of residual garnet during partial melting of eclogite in the subducting oceanic crust however the existing subduction parameters coupled with the timing of the central aleutian adakite like lavas favors a model in which significant subduction erosion of the mafic forearc including erosion and subsequent melting of the trapped kula plate i e morb like oceanic crust as well as early aleutian arc basalts in the mid crust may be partly responsible for the eclogitic component observed in central aleutian magmas

paleobrine temperatures chemistries and paleoenvironments of silurian salina formation f 1 salt michigan basin u s a from petrography and fluid inclusions in halite abstract a 3 7 m stratigraphic section of the silurian salina group f 1 salt detroit salt company mine michigan basin was studied to document the paleodepositional conditions including brine chemistry and temperature of evaporite precipitation sedimentary features including rippled surfaces halite chevrons cumulate halite and dissolution surfaces confirm a shallow water depositional setting penecontemporaneous dissolution surfaces indicate that waters undersaturated with respect to halite were introduced into the michigan basin and that the basin brines were nonstratified during flooding allowing inflow waters to come into contact with halite crystals at the bottom fluid inclusions in primary chevron halite have homogenization temperatures between 5 and 25 c which represent brine temperatures during halite precipitation these temperatures are lower than expected for a marginal marine basin at a subtropical paleolatitude and they suggest a climatic cold spell for a period of thousands of years the major ions in fluid inclusion brines analyzed using the esem x ray eds method have relatively low concentrations of na mg2 and so42 and elevated concentrations of ca2 compared to modern seawater brine inclusion chemistries from the f 1 salt compare closely with the major ion composition of fluid inclusions from other silurian marine halites a 1 a 2 and b evaporites of the salina group and carribuddy group canning basin australia these results indicate a seawater parentage of the michigan basin f 1 salts and that the major ion chemistry of seawater remained more or less constant during the last 15 million years of the silurian detailed documentation of the temperatures brine chemistries and environments of halite deposition is a prerequisite for geobiological study of the entrapment of ancient microbes in salt the results reported here can help pinpoint the timing of entrapment of microorganisms in fluid inclusions in halite and can be used to argue that brine inclusions in the f 1 halites have been isolated and preserved in an unmodified state for more than 400 million years

new insights into prospectivity of liberia sierra leone basin because of improvements in seismic acquisition and processing this paper examines how the understanding of the prospectivity of the liberia sierra leone basin has developed over the last 40 years largely due to improvements in seismic data over this period a series of basins developed between major transform fault zones associated with the opening of the south atlantic ocean from the early cretaceous onward the liberia sierra leone basin forms part of the west african transform margin that extends from sierra leone to benin early hydrocarbon exploration 1972 1985 was targeted at shelfal structural synrift traps of lower cretaceous age none of the wells drilled in this period was regarded as a discovery in 2001 a new regional 2d seismic survey allowed a new play deepwater upper cretaceous channel fans to be identified more recent 3d seismic using the latest data acquisition and processing technologies including avo has enabled drillable prospects to be identified wells drilled in 2009 2010 offshore sierra leone have established that a working hydrocarbo

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

petrology of matthew and hunter volcanoes south new hebrides island arc southwest pacific abstract matthew and hunter the two southernmost active volcanoes of the new hebrides island arc southwest pacific differ markedly from the other mainly tholeiitic quaternary volcanoes of this arc geodynamically related to the new hebrides subduction zone they also lie close to the southern limb of the active expanding ridge of the north fiji basin both volcanoes are made up of acid medium k calcalkaline orogenic andesites containing a variety of inclusions pyroxene and gabbroic cumulates as well as doleritic cognate inclusions this paper presents the first systematic petrographic and chemical study of these volcanics and their inclusions trace element geochemistry and rare earth element modelling suggest that the two volcanoes developed from successive batches of similar parental magmas originating from limited partial fusion of garnet peridotite in the mantle wedge various degrees of fractional crystallization of these batches led to the formation of three volcanic suites hunter little fractionated west matthew moderately fractionated and east matthew highly fractionated moreover on matthew island no correlation exists between the degree of fractionation and the eruptive chronology the youngest edifice west matthew being less evolved than the older one east matthew

origin of depositional cycles in a permian saline giant the salado mcnutt zone evaporites of new mexico and texas two types of metre scale depositional cycles have been recognized within the mcnutt potash zone of the permian salado evaporites the cycles record progressive drawdown and concentration of brine in a shallow marginal marine basin type i cycles made of basal carbonate siliciclastic mudstone then anhydrite polyhalite pseudomorphous after primary gypsum overlain by halite and capped by muddy halite are interpreted as marine sea water dominated they formed by massive spillover of sea water through a connection with the permian ocean at high relative sea level stands lagoonal mudstone and anhydrite polyhalite after subaqueous gypsum followed by relative lowering of sea level and gradual basin restriction shallow saline lake and salt pan halite and ending with a desiccated basin isolated from the permian ocean salt pan halite and saline mudflat muddy halite cycle cap type ii cycles occur between the type i cycles and contain halite overlain transitionally by muddy halite they record a temporal evolution of environments from a shallow saline lake to an ephemeral salt pan saline mudflat complex and are interpreted as continental dominated sequences sourced by meteoric inflow from surrounding land areas that mixed with variable amounts of sea water either residual or introduced into the salado basin by seepage the vertical stacking of type i cycles is best explained by periodic invasions of sea water into the salado basin coincident with eustatic sea level rises the continental dominated type ii cycles formed during intervening periods of eustatic sea level fall and low stands the maximum time interval between major marine incursions is an average of 10 5 yr

epitaxial salt reefs and mushrooms in the southern dead sea salt usually accumulates in shallow ephemeral brines as simple flat beds however in permanent brines deepening in solar evaporation ponds in the southern dead sea salt accumulation is complicated by the growth of isolated vegetable like salt structures

deposition of a saline giant in the mississippian windsor group nova scotia and the nascent late paleozoic ice age abstract saline giants are vast marine evaporite deposits that currently have no modern analogues and remain one of the most enigmatic of chemical sedimentary rocks the mississippian windsor group ca 345 ma maritimes basin atlantic canada is a saline giant that consists of two evaporite rich sedimentary sequences that are subdivided into five subzones sequence 1 is composed almost entirely of thick halite belonging to subzone a osagean sequence 2 is in unconformable contact and comprised of stacked carbonate evaporite peritidal cycles of subzones b through e meramecian subzone b the focus of research herein documents the transition from wholly evaporitic to open marine conditions and thus preserves an exceptional window into the processes forming saline giants lithofacies stacking patterns in subzone b reveal that higher order fluctuations in relative sea level produced nine stacked parasequences interpreted to reflect high frequency glacioeustatic oscillations during the onset of the late paleozoic ice age each parasequence reflects progradation of intertidal and sabkha sediments over subtidal carbonate and evaporite deposits dissimilarities in cycle composition between sub basins imply the development of contrasting brine chemistries from differing recharge rates with the open ocean what the windsor group shows is that evaporite type is ostensibly linked to the amplitude and frequency of sea level rise and fall during deposition true saline giants like the basinwide evaporites of sequence 1 apparently require low amplitude long frequency changes in sea level to promote the development of stable brine pools that are only periodically recharged with seawater by contrast the high amplitude short frequency glacioeustatic variability in sea level that controlled the accumulation of peritidal evaporites in subzone b produce smaller subeconomic deposits with more complex facies relationships

isotope and trace element geochemistry of augustine volcano alaska implications for magmatic evolution k e johnson r s harmon j m richardson s moorbath and d f strong department of geology 1112 turlington hall university of florida gainesville fl 32611 usa us army research office p o box 12211 research triangle park nc 27709 usa geoscience branch ontario geological survey willet green miller centre sudbury ont pse 6b5 canada department of earth sciences university of oxford parks road oxford oxi 3pr uk centre for earth and ocean research university of victoria victoria b c v8w 3p6 canada

synsedimentary dissolution pits in halite of the permian salado formation southeastern new mexico abstract mining operations in the permian salado formation for the waste isolation pilot plant wipp reveal filled pits in halite beds which are interpreted as synsedimentary dissolution features the pits range from a few centimeters to as much as 1 m wide and 1 m deep and they clearly truncate thin underlying beds halite hopper crystals and possible chevron halite structures occurring in the pits and continuous undeformed bedding in overlying units demonstrate that the pits formed and filled before deposition of the overlying unit pseudomorphs of halite after gypsum swallow tail crystals are part of the evidence that the dissolution pits formed in shallow water

intracrustal controls on the coexistence of tholeiitic and calc alkaline magma series at aso volcano sw japan aso caldera sw japan was formed incrementally over a period of 300 kyr by the eruption of four basaltic to dacitic pyroclastic flow and fall deposits aso 1 4 these units are collectively referred to as the aso pyroclastic flow deposit apfd in which tholeiitic and calc alkaline magmas have been erupted in close temporal and spatial proximity detailed petrological and geochemical studies indicate that a major shift in magmatic composition occurred between aso 1 and 4 as the caldera forming stage evolved changing from a predominantly tholeiitic to a calc alkaline signature the mineralogical and geochemical characteristics of aso 1 4 suggest that magmatic evolution within each unit is controlled by fractional crystallization associated with magma mixing crustal assimilation by altering the fractional crystallization mineral assemblage as well as the relative amounts of fractional crystallization magma mixing and crustal assimilation the systematic shifts in differentiation trends and magmatic series observed between the four eruptive units can be reproduced this suggests that the coexistence of tholeiitic and calc alkaline magma series can be attributed to shallow level intracrustal processes

late cretaceous history of eolian deposition in the mid pacific mountains central north pacific ocean rea d k and janecek t r 1981 late cretaceous history of eolian deposition in the mid pacific mountains central north pacific ocean palaeogeogr palaeoclimatol palaeoecol 36 55 67 eolian dust preserved in deep sea sediment can provide a direct historical record of global atmospheric circulation data from a reasonably complete upper cretaceous section of pelagic sediments recovered at dsdp site 463 in the central north pacific provides a good record of eolian activity during the time period between about 112 hnd 66 m y ago we have isolated the eolian component from these sediments determined its mass accumulation rate and combined these data with the mineralogy of the inorganic fraction determined by others to construct a record of eolian deposition volcanic input is significant during aptian albian and maastrichtian times otherwise continentally derived minerals dominate mass accumulation rates of the continental eolian component range from over 500 mg cm 103yr during the late albian to a low of 5 mg cm2 10 yr during coniacian time for comparison the upper miocene to pleistocene rate averages about 20 mg cm2 103yr the temporal pattern of late cretaceous eolian accumulation of site 463 generally matches known changes in sea level suggesting that source availability is the dominant control of eolian sedimentation during that time

evaporites marine or non marine division of evaporites into those types that retain information about primary mineralogy textures and structures primary evaporites and those types so extensively altered that few or no primary features remain secondary evaporites only primary evaporites can give unequivocal data bout their marine or non marine origin to help evaluate such primary deposits they have been further divided using the following attributes 1 whether the depositional setting was marine or non marine 2 whether the source water was a seawater b meteoric c hydrothermal d diagenetic e volcanogenic or f mixed specific criteria for making distinctions are 1 kinds of fossils 2 sedimentology of the associated non saline facies 3 kinds of primary saline minerals 4 association of such saline minerals both in qualitative and quantitative terms 5 trace element isotope and fluid inclusion geochemistry of such saline minerals modified journal abstract

mid pleistocene lavas from the seguam volcanic center central aleutian arc closed system fractional crystallization of a basalt to rhyodacite eruptive suite in contrast to adjacent volcanic centers of the modern central aleutian arc seguam island developed on strongly extended arc crust k ar dates indicate that mid pleistocene late pleistocene and holocene eruptive phases constitute seguam this study focuses on the petrology of the mid pleistocene 1 07 07 ma turf point formation tpf which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst poor andesite to rhyodacite lavas increasing whole rock feo mgo from basalt to dacite the anhydrous plag ol cpx opx mt phenocryst assemblage groundmass pigeonite and the reaction ol liq opx preserved in the mafic lavas indicate a tholeiitic affinity thermometry and comparison to published phase equilibria suggests that most tpf basalts crystallized plag ol cpx mt at 1160 c between about 3 5 kb 1 2 h2o andesites crystallized plag cpx opx mt at 1000 c between 3 4 kb with 3 5 h2o and dacites crystallized plag cpx opx mt at 1000 c between 1 2 kb with 2 3 h2o all lavas crystallized at f o 2 close to the nno buffer mineral compositions and textures indicate equilibrium crystallization of the evolved lavas petrographic evidence of open system mixing or assimilation is rare mgo cao al2o3 cr ni and sr abundances decrease and k2o na2o rb ba zr and pb increase with increasing sio2 50 71 lree enrichment ce yb n 1 7 0 2 characterizes most tpf lavas total ree contents increase and eu anomalies become more negative with increasing sio2 relative to other aleutian volcanic centers tpf basalts and basaltic andesites have lower k2o na2o tio2 rb ba sr zr y and lree abundances 87sr 86sr ratios 0 70361 0 70375 and ratios of 206pb 204pb 18 88 18 97 207pb 204pb 15 58 15 62 208pb 204pb 38 46 38 55 are the highest measured for any suite of lavas in the oceanic portion of the aleutian arc conversely end values 5 8 to 6 7 are among the lowest from the aleutians sr nd and pb ratios are virtually constant from basalt through rhyodacite whereas detectable isotopic heterogenity is observed at most other aleutian volcanic centers major and trace element ree and sr nd and pb isotopic compositions are consistent with the basaltic andesitic andesitic dacitic and rhyodacitic liquids evolving from tpf basaltic magma via closed system fractional crystallization alone fractionation models suggest that removal of 80 wt cumulate 61 plag 17 cpx 12 opx 7 ol and 3 mt can produce 20 wt rhyodacitic residual liquid per unit mass of parental basaltic liquid petrologic and physical constraints favor segregation of small batches of basalt from a larger mid crustal reservoir trapped below a low density upper crustal lid in these small magma batches the degree of cooling crystallization and fractionation are functions of the initial mass of basaltic magma segregated the thermal state of the upper crust and the magnitude of extension tholeiitic magmas erupted at seguam evolved by substantially different mechanisms than did calc alkaline lavas erupted at the adjacent volcanic centers of kanaga and adak on unextended arc crust these variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew at seguam intra arc extension promoted eruption of voluminous basalt and its differentiates unmodified by interaction with lower crustal or upper mantle wallrocks

structure and composition of the aleutian island arc and implications for continental crustal growth we present results of a seismic reflection and refraction investigation of the aleutian island arc designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust the aleutian arc has the requisite thickness 30 km to build continental crust but it differs strongly from continental crust in its composition and reflectivity structure seismic velocities and the compositions of erupted lavas suggest that the aleutian crust has a mafic bulk composition in contrast to the andesitic bulk composition of continents the silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the aleutian intraoceanic arc therefore if island arcs form a significant source of continental crust the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin the pervasive deformation intracrustal melting and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc continent collision or through subsequent establishment of a continental arc the volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two

geochemistry of the 1989 1990 eruption of redoubt volcano part i whole rock major and trace element chemistry the 1989 1990 eruption of redoubt volcano produced medium k calc alkaline andesite and dacite of limited compositional range 58 2 63 4 sio2 and entrained quenched andesitic inclusions 55 sio2 which bear chemical similarities to the rest of the ejecta the earliest december 15 magmas are pumiceous often compositionally banded and the majority is relatively mafic 59 sio2 the most silicic magmas of the eruption are the late december to early january domes up to 63 4 sio2 subsequent magmas formed domes and rare pumices which converge on 60 sio2 chemical variations among ejecta comprise tight linear two component arrays for all elements for which the analytical uncertainty is much less than the compositional range the two component arrays are interpreted as mixing arrays between unrelated magmas because several of the arrays are at steep angles to the normal liquid line of descent additionally the felsic endmember cannot be easily related to the mafic endmember by normal high temperature igneous processes e g the silicic endmember has higher zr yet lower hf than the mafic endmember also relative enrichments of highly incompatible elements are dramatically different across the arrays the mixing event must have preceded eruption by a significant yet unspecified amount of time because groundmass glass compositions are homogeneous for all post december samples swanson et al 1994 this volume in spite of the whole rock chemical diversity this implies time for additional crystallization after the mixing event swanson et al 1994 this volume discuss evidence for a potentially different mixing event recorded only in december 15 magmas cognate cumulate xenoliths composed of pl cpx opx hb mt melt were recovered from january and april deposits these blocks differ from local batholithic country rock in their low concentrations of incompatible elements e g rb 60 wt they have mg cr ni sc and v contents higher than the andesites but lower than redoubt basalts and basaltic andesites thus they may be crystallization products of andesites but do not represent the cumulate residue of basalt fractionation the xenoliths were probably derived from a shallow or intermediate crustal chamber

strontium isotopic and selected trace element variations between two aleutian volcanic centers adak and atka implications for the development of arc volcanic plumbing systems major and trace element concentrations and initial 87sr 86sr ratios of lavas from the aleutian volcanic centers of adak and atka have been used to study the evolution of their respective lithospheric plumbing systems the centers are within 150 km of one another and show similar overall silica ranges 47 67 but adak 40 km3 is smaller than atka 200 km3 adak s lavas are chemically and isotopically heterogeneous 87sr 86sr 0 70285 0 70330 and two units contain lithospheric xenoliths the lavas of the much larger atka on the other hand have much less variability in major and trace elements as well as 87sr 86sr 0 70320 0 70345 we suggest that these characteristics are a measure of the relative maturity and cleanliness of the lithospheric plumbing systems that supply magma to these centers because aleutian volcanic centers often remain fixed for relatively long periods of time 5 m y once established magmatic passageways are repeatedly used young plumbing systems are relatively cool and contain large amounts of wallrock contaminant and ascending magmas undergo contamination as well as concurrent crystallization and fractionation with time however heat and mass transfer between ascending magmas and wallrock produce thermal and chemical boundary layers that insulate subsequent magmas in effect the plumbing system matures the chemical heterogeneity displayed by young dirty systems like adak reflects not only the magma source but also the wallrock encountered during ascent and possibly the effects of extensive crystal fractionation thus it is the petrologic data of mature clean systems like atka that yield the most direct and unambiguous information on the ultimate origin of the lavas and their near surface evolution

milankovitch band cyclicity in bedded halite deposits contemporaneous with late ordovician early silurian glaciation canning basin western australia abstract the mallowa salt of the early palaeozoic carribuddy group canning basin western australia is a halite mudstone anhydrite dolomite evaporite sequence of late ordovician to early silurian age deposited in barred marginal marine to ephemeral saltpan and saline mudflat environments bhp utah minerals potash exploration well gingerah hill no 1 obtained drill core of a 477 m stratigraphic interval of the mallowa salt which permitted the study of geochemical variations in a thick halite sequence contemporaneous with late ordovician early silurian glaciation on other continents a strip of uniform width was continuously ground from the core and the powdered rock bagged at 1 m intervals for geochemical assay the resulting geochemical stratigraphic series display conspicuous cyclicity several orders of cycles ranging from 3 m to100 m stratigraphic thickness are discernible in the plots of minor constituents br mgo and k2o in extracted soluble salts and also in the plots of na2o and total salts content fast fourier transform fft of the br mgo and k2o series gives principal spectral peaks in the 0 00 0 10 cycles m frequency band at 250 113 35 4 22 1 and 19 7 m as well as peaks at 65 m and between 2 8 and 13 5 m fft of the na2o and total salts series shows dominant spectral peaks at 220 259 and 109 114 m if the strongest spectral peak at 113 m is taken to represent the relatively stable eccentricity period of 100 ka and a constant net rate of accretion assumed for long sections the other main periods in the mallowa salt as exemplified by the br spectrum would be31 3 3 0 19 6 1 1 and17 4 1 1 ka these figures are consistent with predicted late ordovician early silurian 440 ma periods for obliquity 30 5 ka and precession 19 3 and 16 4 ka based on evolutionary change in the earth moon system additional spectral peaks identified with implied periods of approximately 206 233 57 5 8 12 and 2 5 3 5 ka also may be of palaeoclimatic relevance the relative amplitudes and structure of spectral peaks in the 0 00 0 10 cycles m frequency band support the identification of climatic oscillations forced by orbital cycles the data indicate a precession eccentricity dominated pattern which accords with the know low palaeolatitude

textural features of holocene perennial saline lake deposits of the taoudenni agorgott basin northern mali abstract the holocene salt lake deposits of the taoudenni agorgott basin northern mali mainly consist of sediments with a high glauberite na2ca so4 2 content the remainder of the deposits largely consists of salt beds with a bloedite na2mg so4 2 4h2o thenardite na2so4 or halite nacl composition a petrographical study of the deposits demonstrates that they formed in a perennial lake that experienced a gradual decrease in water depth textural features of the glauberite dominated deposits are found to be related to water depth through the control that this factor exerts on the sensitivity of the lake to changes in water supply and to short term variations in evaporation rates in this way layering due to variations in glauberite content and crystal size is inferred to be typical of deposits that formed in shallow water whereas unstratified deposits are the product of high lake level stages halite textures are found to be indicative of the place within the water column where crystal growth occurred along the lake bottom or higher which is mainly determined by water depth and partly by evaporation rates the oldest halite beds are largely unaltered cumulate deposits whereas the youngest layers developed exclusively through bottom growth the basal part of one thick halite bed at a level between these two groups of halite layers developed by an alternation of both types of growth in response to variations in evaporation rates variations in mineralogical composition between and within the salt beds that formed during the earliest periods with a higher salinity up to the first stage with halite formation record a change in lake water chemistry with time but they are in one instance also determined by an early diagenetic mineral transformation

petrology of basaltic xenoliths in andesitic to dacitic host lavas from martinique lesser antilles evidence for magma mixing some recent calc alkaline andesites and dacites from southern and central martinique contain basic xenoliths belonging to two main petrographic types the most frequent one has a hyalodoleritic texture h type with hornblende plagioclase fe ti oxides set in an abundant glassy and vacuolar groundmass the other one exhibits a typical porphyritic basaltic texture b type and mineralogy olivine plagioclase orthopyroxene clinopyroxene fe ti oxides and scarce or absent hornblende