possible crustal contamination of midcontinent rift igneous rocks examples from the mineral lake intrusions wisconsin interlayered mafic telsic intrusions from the mineral lake intrusive complex in northwest wisconsin reflect the typical bimodal basalt rhyolite compositional pattern of the midcontinent rift flood

petrology petrochemistry and stromatolites of the middle to late proterozoic beck spring dolomite eastern mojave desert california the beck spring dolomite is the medial unit of the middle to late proterozoic pahrump group the oldest sequence of sedimentary rocks in eastern california stratigraphic sections of the beck sprin

a continuum model of melt pond evolution on arctic sea ice 1 during the northern hemisphere summer absorbed solar radiation melts snow and the upper surface of arctic sea ice to generate meltwater that accumulates in ponds the melt ponds reduce the albedo of the sea ice cover during the melting season with a significant impact on the heat and mass budget of the sea ice and the upper ocean we have developed a model designed to be suitable for inclusion into a global circulation model gcm which simulates the formation and evolution of the melt pond cover in order to be compatible with existing gcm sea ice models our melt pond model builds upon the existing theory of the evolution of the sea ice thickness distribution since this theory does not describe the topography of the ice cover which is crucial to determining the location extent and depth of individual ponds we have needed to introduce some assumptions we describe our model present calculations and a sensitivity analysis and discuss our results

a transitional alkalic dolerite dike suite of mesozoic age in southeastern new england dike rocks from the new england platform of rhode island and adjacent massachusetts consist of premetamorphic and post metamorphic suites the older group includes metamorphosed dolerite minette and schistose dioritic rocks post metamorphic dikes consist of dolerite and sparse monchiquite the post metamorphic dolerites are of comparable age to the eastern north american dolerite suite associated with the mesozoic basins along the eastern seaboard of north america however the southeastern new england dolerites exhibit mineralogy and chemistry more typical of a transitional alkalic suite compared to the more subalkalic tholeiitic dolerites of the eastern north american suite both suites are compatible with a rift tectonic setting but the more alkalic dolerites may represent a deeper source of small volume melts compared to the eastern north american dolerites these more alkaline melts may have concentrated at local centers or they may be typical of flank dolerites as opposed to the less alkalic varieties that occur within the central axial rift

magma dynamics at the base of an evolving mafic magma chamber incompatible element evidence from the partridge river intrusion duluth complex minnesota usa abstract a characteristic feature of the partridge river intrusion of the keweenawan duluth complex is the approximately fivefold to ninefold increase in the concentrations of incompatible elements in the lower zone compared with cumulates stratigraphically higher the concentrations of incompatible elements decrease from the lower zone upward to steady state values which is ascribed to variations in the proportions of trapped liquid rather than variable degrees of fractional crystallization of a single parental magma the calculated average composition of trapped liquid using our algorithm is similar to typical keweenawan low alumina high ti p basalts associated with the duluth complex but is different from the leading edge ferrodioritic liquid quenched in the chilled margin of the intrusion this difference suggests that the chilled margin does not represent the original parental magma composition from which the whole intrusion solidified and that the enrichment of incompatible elements may be related to the local flotation of magmatic suspensions to test the latter hypothesis numerically we have used heat mass transfer models assuming a sheet like magma chamber to calculate the parameters of the model that best reproduce the observed distribution of incompatible elements in a mush zone at the base of the partridge river intrusion the results indicate that a mush zone enriched in the incompatible elements is produced if the velocity of movement of the lower solidification front into the magma body was less than the floating velocity of the bulk crystal mush the dynamic parameters that best reproduce the observed distribution of incompatible elements include a magma emplacement pressure of 2 kbar critical crystallinities of 50 68 in the mush zone from which the liquid is being expelled and an emplacement temperature of 1160 c for the initial magma

late proterozoic and devonian plutonic terrane within the avalon zone of rhode island the u th pb radiometric age of zircons demonstrates that much of rhode island consists of a late proterozoic plutonic complex that subsequently was intruded by a large devonian alkalic to subalkalic igneous complex two groups of late proterozoic rocks can be recognized 1 the esmond granite and related plutonic rocks in northern rhode island ranging in composition from gabbro to granite and yielding an upper concordia intercept of 621 8 m y and 2 quartz rich recrystallized gneissic rocks ten rod granite gneiss and hope valley alaskite gneiss in southern rhode island giving an upper concordia intercept age of 601 5 m y the lower intercept of the zircon discordia for the gneissic rocks indicates the isotopic systems were disturbed during the late paleozoic alleghanian orogeny whereas zircons from the esmond granite and related plutonic rocks have been subjected mainly to loss of lead attributable to recent dilatancy rocks determined to be of devonian age include the scituate granite and plutonic rocks of the east greenwich plutonic suite the quincy granite of quinn 1971 in northeastern rhode island may be of a similar devonian age or it may be slightly younger zircon data from these rocks fall upon a chord that gives an upper concordia intercept of 370 7 m y and a lower intercept close to the origin collectively these rocks form a composite pluton with an area 700 km 2 the devonian rocks exhibit petrologic characteristics distinct from slightly older acadian orogenic rocks that occur in lithostratigraphic zones to the west and are more comparable to anorogenic ordovician to devonian alkalic granitoids that trend north northeast across massachusetts and the gulf of maine the new age determinations confirm that much of rhode island contains plutonic rocks intruded during the avalonian orogeny but that a hertofore unrecognized major devonian plutonic episode also occurred the distinctly different ages probably had gone unrecognized because of generally similar lithologic and textural features exhibited by the late proterozoic and devonian rocks we present new petrologic and geochemical criteria that should assist in distinguishing the rocks of different ages as additional mapping and geologic study proceeds

sea ice thickness distribution of the bellingshausen sea from surface measurements and icesat altimetry abstract although sea ice extent in the bellingshausen amundsen ba seas sector of the antarctic has shown significant decline over several decades there is not enough data to draw any conclusion on sea ice thickness and its change for the ba sector or for the entire southern ocean this paper presents our results of snow and ice thickness distributions from the simba 2007 experiment in the bellingshausen sea using four different methods aspect ship observations downward looking camera imaging ship based electromagnetic induction em sounding and in situ measurements using ice drills a snow freeboard and ice thickness model generated from in situ measurements was then applied to contemporaneous icesat satellite laser altimetry measured freeboard to derive ice thickness at the icesat footprint scale errors from in situ measurements and from icesat freeboard estimations were incorporated into the model so a thorough evaluation of the model and uncertainty of the ice thickness estimation from icesat are possible our results indicate that icesat derived snow freeboard and ice thickness distributions asymmetrical unimodal tailing to right for first year ice 0 29 0 14 m for mean snow freeboard and 1 06 0 40 m for mean ice thickness multi year ice 0 48 0 26 and 1 59 0 75 m respectively and all ice together 0 42 0 24 and 1 38 0 70 m respectively for the study area seem reasonable compared with those values from the in situ measurements aspect observations and em measurements the em measurements can act as an appropriate supplement for aspect observations taken hourly from the ship s bridge and provide reasonable ice and snow distributions under homogeneous ice conditions our proposed approaches 1 of using empirical equations relating snow freeboard to ice thickness based on in situ measurements and 2 of using isostatic equations that replace snow depth with snow freeboard or empirical equations that convert freeboard to snow depth are efficient and important ways to derive ice thickness from icesat altimetry at the footprint scale for antarctic sea ice spatial and temporal snow and ice thickness from satellite altimetry for the ba sector and for the entire southern ocean is therefore possible

geochemistry and petrogenesis of proterozoic mafic dykes in north kerala southwestern indian shield preliminary results abstract mafic dyke intrusions occur in three distinct orientations nnw sse nw se and ne sw in north kerala region the southwestern part of the indian shield dating of two nnw sse trending dykes by k ar method has yielded middle proterozoic ages ca 1660 ma and ca 1420 ma respectively our initial geochemical results on these dyke rocks 0 65 0 15 wt k 2 o 0 37 0 38 wt p 2 o 5 3 30 1 00 wt tio 2 11 1 p p m rb 250 90 p p m sr 230 40 p p m ba 160 40 p p m zr and 30 10 chondrite rare earth elemental abundances indicate a transitional character between abyssal and plateau tholeiites petrogenetic modelling suggests that the dyke compositions have been derived by different degrees of partial melting of a heterogenous source mantle the mantle sources with accessory amphibole and or garnet variably enriched in lree and lil elements is compatible with the observed geochemical data

nature and origin of proterozoic a type granitic magmatism in the southwestern united states of america the mountain ranges of arizona and adjacent california and nevada contain large areas underlain by proterozoic anorogenic granites comprising the southwesternmost portion of a transcontinental belt of 1 4 1 5 ga old anorogenic complexes that extends across north america northeast into labrador of these a two mica monazite bearing granitic suite resides in central and southeastern arizona as part of a peraluminous subprovince that is bordered on the south southern arizona to sonora and west western arizona and adjacent portions of california and nevada by marginally metaluminous granites bearing biotite sphene hornblende and fluorite all of these 1 4 ga granites are distinctly more potassic iron enriched relative to mg and depleted in ca mg and sr in contrast to typical orogenic granitoids in general the large ion lithophile element enriched composition is a consequence of limited melting of a water deficient crustal source at depths greater than 25 37 km for the peraluminous granites this contrast is less extreme perhaps resulting from a larger degree of melting as a consequence of a greater metasedimentary component and water in its crustal source the anorogenic granitic magmas intruded into the upper crust at depths of 8 17 km or shallower at temperatures up to 790 c the most dramatic variation in the crystallization intensive parameters resides in the oxygen fugacity which spans three orders of magnitude relative to other anorogenic suites all of the magmas crystallized at elevated levels of o2 as reflected in their assignment to the anorogenic magnetite series yet a regionally significant rise in primary o2 levels unmatched elsewhere in the transcontinental belt occurs for plutons in western arizona including the holy moses and hualapai granites the most extreme case is the hualapai granite whose biotite fe fe mg ratios drop due to high o2 to a low of 0 27 down from more typical levels of 0 54 to 0 75 such extreme variations in primary levels of oxygen fugacity must be an indirect imprint of regional changes of the level of oxidation of the lower crust the high fo2 holy moses and hualapai plutons have intruded near the regional boundary between the metaluminous and peraluminous granites and appear to be imaging a major change in the level of oxidation of the lower crust this boundary is also approximately equivalent to significant changes in the nd and pb isotopic compositions of these granites and the metamorphic and magmatic character of the older orogenic terrane on a global scale the crust forming orogenies ended by 1 6 ga ago and the continents entered a long lived era dominated by localized extension and transcontinental intrusion of anorogenic potassic rapakivi granite mafic dike swarms charnockite and anorthosite the absence of orogenic deformation implies that plate consumption became intraoceanic during this time the profuse and widespread nature of the igneous activity has no phanerozoic analogue and is considered to be unique to the proterozoic a crustal overturn model ties the magmatism to heating within a largely undepleted subcontinental mantle the eventual rise of mantle plumes and the transfer of heat into the youthful undifferentiated proterozoic crust subsequent melting and rise of potassic granitic magmas from the lower crust leads to considerable crustal reorganization a process that would continue until both the mantle and crust reached a stable configuration

on the origin of an amphibole rich vein in a peridotite inclusion from the lunar crater volcanic field nevada u s a abstract late cenozoic alkali basaltic lavas of the lunar crater volcanic field lcvf located in the center of the great basin of the western u s a contain a diverse suite of nodule samples of the lower crust and upper mantle this paper documents a composite nodule from the marcath flow in which an amphibole bearing wehrlite 59 olivine 30 clinopyroxene 6 amphibole is cut by a 6 9 mm wide vein of andesine amphibolite 80 kaersutite 15 andesine 3 ilmenite aside from nodule basalt reaction at the nodule exterior there is little chemical variation either within or between individual grains of hydrous and anhydrous phases in the vein and host wehrlite furthermore there is no systematic compositional zoning in the wehrlite relative to vein proximity the whole rock major and trace element composition of the vein is similar to a primitive mg mg fe 0 692 basaltic liquid and has al fe mg ca mn na k zr y and sr contents similar to basalts observed in the lcvf in contrast to the sr isotopic equilibrium displayed by vein feldspar and vein amphibole sr isotopic disequilibrium is exhibited between the vein 0 70318 4 wehrlite 0 70322 4 and host basalt 0 70357 5 n 3 however the sr isotopic ratios of older lcvf basalts 0 7030 0 7038 n 14 overlap those of the vein and wehrlite and the magmatic activity leading to vein and wehrlite formation could be related to this older phase of lcvf volcanism petrographic and geochemical evidence is not consistent with a metasomatic origin for the vein and instead supports the view that the vein originated by the intrusion into a wehrlite mass and subsequent crystallization of a relatively primitive alkali basaltic magma in the lower crust or upper mantle the wehrlite contains olivine of fo71 and probably originated by crystal separation and accumulation from a relatively differentiated basaltic magma in the lower crust or upper mantle

zoned minerals in garnet peridotite nodules from the colorado plateau implications for mantle metasomatism and kinetics some garnet peridotite nodules from the thumb a minette neck on the colorado plateau in the southwestern united states contain zoned minerals zoning does not exceed 1 5 wt for any oxide but some relative changes are large in one garnet tio2 and cr2o3 ranges are 0 05 0 65 and 3 5 5 0 wt respectively in two porphyroclastic nodules garnet rims are depleted in mg and enriched in fe ti and na compared to cores and one garnet is irregularly zoned in ti and cr olivine crystals in these rocks are unzoned and pyroxene zoning is slight yet matrix olivine and pyroxene contain more fe and ti and less mg and cr than inclusions of these phases in garnet in three coarse nodules garnet rims are ti rich compared to cores and ca fe mg and cr zoning patterns are complex several nodules appear to have partially equilibrated near 1200 c and 35 kb and under these conditions cation mobility in pyroxene was greater than in garnet

co 2 co fluid inclusions in a composite peridotite xenolith implications for upper mantle oxygen fugacity fluid inclusions occur in a composite xenolith from the lunar crater volcanic field nevada u s a the xenolith is an amphibole bearing wehrlite that is cut by an andesine amphibole vein the compositions of individual fluid inclusions in both portions of the xenolith have been determined using microthermometry and micro laser raman spectroscopy fluids in the host wehrlite are nearly pure co2 99 mol whereas those in the vein contain from 8 5 to 12 0 mol co in co2 chemical modelling shows that the composition of the vein fluids at t room is representative of the composition at the high p t conditions of trapping graphite has not been observed by optical microscopy in any of the fluid inclusions graphite is probably absent although stable at t 800 c most probably because of the kinetically unfavorable co decomposition reaction and rapid quenching by combining the measured fluid compositions with fluid p v t data and the chemical equilibrium co2 co 1 2 o2 we have calculated the oxygen fugacity of the fluid inclusions at 1200 c log f o 2 8 6 vein and 6 host if the f o 2 of the fluid in the vein represents that in equilibrium with the magma that crystallized to produce the vein then the f o 2 of the basalt magma is near qfm at 1200 c and 10 3 kbar this is similar to values reported for extrusive basaltic lavas if the much lower intrinsic oxygen fugacity values for divines and spinels from alkali basalt nodules are representative of upper mantle conditions then oxidation of basaltic magmas must occur in the upper mantle prior to ascent to the surface implications for the origin of co2 rich fluids and carbon isotope geochemistry are also discussed

a one dimensional percolation model of flooding and snow ice formation on antarctic sea ice a one dimensional thermodynamic sea ice growth model is developed with emphasis on seawater flooding and snow ice formation on antarctic sea ice to examine the possibility that flooding is the result of brine percolation through a porous ice matrix and the consequences of this percolation on the salinity structure and mass balance of the ice flooding is modeled in a semiheuristic fashion by examining two possible regimes for seawater and brine infiltration through the porous ice into the snowpack a standard model assumes that the upward flow of brine is horizontally homogeneous and brine flow is modeled using a darcian scheme a simple model assumes that the infiltrating brine does not interact with the internal ice brine network but instead percolates upward through cracks or is restricted in spatial extent results of simulations are shown for the climatology in the ross sea and compared to field data from one autumn and one winter cruise aboard the r v nathaniel b palmer in 1995 sensitivity studies are performed to show the effects of variations in climate forcing the dependence of brine percolation and snow ice formation on key thermophysical parameters is investigated results indicate that the mass balance depends very strongly on the brine flow regime in the standard model ice salinities and brine volumes are greatly affected by brine percolation creating a strong feedback between flooding and future snow ice production snow ice formation for the simple model was much more predictable and depended primarily on snow load comparison of results with field data indicate that if brine percolation is the major mechanism for flooding and snow ice formation then there must be either substantial convective redistribution of salt to maintain ice porosity or the flow must be spatially inhomogeneous

petrology and geochemistry of late stage intrusions of the a type mid proterozoic pikes peak batholith central colorado usa implications for petrogenetic models abstract the 1 08 ga anorogenic a type pikes peak batholith front range central colorado is dominated by coarse grained biotite amphibole syenogranites and minor monzogranites collectively referred to as pikes peak granite ppg the batholith is also host to numerous small late stage plutons that have been subdivided into two groups e g wobus 1976 studies in colorado field geology colorado school of mines professional contributions colorado 1 a sodic series sio 2 44 78 wt k na 0 32 1 36 composed of gabbro diabase syenite quartz syenite and fayalite and sodic amphibole granite and 2 a potassic series sio 2 70 77 wt k na 0 95 2 05 composed of biotite granite and minor quartz monzonite differences in major and trace element and nd isotopic characteristics for the two series indicate different petrogenetic histories potassic granites of the late stage intrusions appear to represent crustal anatectic melts derived from tonalite sources based on comparison of their major element compositions with experimental melt products in addition nd isotopic characteristics of the potassic granites e nd 1 08 ga 0 2 to 2 7 overlap with those for tonalites granodiorites ca 1 7 ga boulder creek intrusions e nd 1 08 ga 2 4 to 3 6 exposed in the region some of the partial melts evolved by fractionation dominated by feldspar the late stage potassic granites share geochemical characteristics with most of the ppg which is also interpreted to have an anatectic origin involving tonalitic crust the origin of monzogranites associated with the ppg remains unclear but mixing between granitic and mafic or intermediate magmas is a possibility syenites and granites of the sodic series cannot be explained as crustal melts but are interpreted as fractionation products of mantle derived mafic magmas with minor crustal input high temperature and low oxygen fugacity estimates e g frost et al 1988 american mineralogist 73 727 740 support a basalt fractionation origin as do e nd values for sodic granitoids e nd 1 08 ga 2 2 to 0 7 which are higher than e nd values for colorado crust at 1 08 ga ca 1 0 to 4 0 enrichments in incompatible elements e g rare earth elements rb y and depletions in compatible elements e g cr sr ba in the sodic granitoids compared to coeval mafic rocks are also consistent with fractionation accessory mineral fractionation release of fluorine rich volatiles and or removal of pegmatitic fluids could have modified abundances of ce nb zr and y in some sodic granitoid magmas gabbros and mafic dikes associated with the sodic granitoids have e nd 1 08 ga of 3 0 to 3 5 which are lower than depleted mantle at 1 08 ga and their trace element characteristics suggest derivation from mantle sources that were previously affected by subduction related processes however it is difficult to characterize the mantle component in these magmas because assimilation of crust during magma ascent could also result in their observed geochemical features the pikes peak batholith is composed of at least two petrogenetically different granite types both of which exhibit geochemical characteristics typical of a type granites models proposed for the petrogenesis of the granitoids imply the existence of mafic rocks at depth and addition of juvenile material to the crust in central colorado at 1 1 ga

proterozoic multistage ca 1 1 and 0 8 ga extension recorded in the grand canyon supergroup and establishment of northwest and north trending tectonic grains in the southwestern united states the grand canyon supergroup records at least two distinct periods of intracratonic extension and sedimentation in the late mesoproterozoic and neoproterozoic new 40 ar 39 ar age determinations indicate that the mesoproterozoic unkar group was deposited between ca 1 2 and 1 1 ga basins in which the unkar group was deposited and the related northwest striking faults were created by northeast southwest extension which was contemporaneous with regional northwest southeast grenville contraction new u pb data indicate that the neoproterozoic chuar group was deposited between 800 and 742 ma sedimentary and tectonic studies show that chuar deposition took place during east west extension and resulting normal slip across the butte fault this event is interpreted to be an intracratonic response to the breakup of rodinia and initiation of the cordilleran rift margin laramide monoclines of the grand canyon region have north and northwest trends reactivate faults that originated at the time of unkar and chuar deposition and can be traced for great distances hundreds of kilometers from the grand canyon we use the distribution of monoclines in the southwest to infer the extent of proterozoic extensional fault systems the 1 1 ga northwest trending structures and ca 800 700 ma north trending extensional structures created regional fault networks that were tectonically inverted during formation of the ancestral rocky mountains and laramide contraction and reactivated during tertiary extension

comments on tectonic implications of the age composition and orientation of lamprophyre dikes navajo volcanic field arizona by a w laughlin m j aldrich jr m shafiqullah and j husler commentaire sur l article de a w laughlin m j aldrich jr m shifiqullah et j husler paru dans earth planet sci le hers nld 1979 43 279 284

the chengwatana volcanics wisconsin and minnesota petrogenesis of the southernmost volcanic rocks exposed in the midcontinent rift the southernmost exposed rocks of the north american midcontinent rift system 1100 ma consist of 3000 m of mafic volcanic flows and minor interflow sediment exposed along the st croix river in minnesota and wisconsin the flows are mostly high fe tholeiitic basalt with plagioclase phenocrysts and ophitic to subophitic clinopyroxene abundant secondary chlorite epidote and actinolite indicate the group was metamorphosed to greenschist facies 350 c low sodium m4 site and tetrahedral aluminum aliv contents of actinolite indicate low pressure metamorphism 0 25 gpa and imply a geothermal gradient of 45 50 c km low magnesium mg 0 37 0 58 and ni contents 36 185 ppm indicate the basalts have undergone significant fractionation and are not primary mantle melts incompatible element abundances are inversely correlated with mg and most samples plot within either high or low trace element groups e g ti p zr the basalts are enriched in the light rare earth elements and th and are v

40ar 39ar ages from the narragansett basin and southern rhode island basement terrane their bearing on the extent and timing of alleghenian tectonothermal events in new england pennsylvanian sedimentary rocks record the effects of progressive barrovian metamorphism and polydeformation in the narragansett basin of rhode island whole rock phyllite samples from low grade portions of this terrane display internally concordant 40 ar 39 ar age spectra with plateau ages of 248 to 258 m y four samples biotite from higher grade rocks records plateau ages of 240 to 248 m y eight samples similar 40 ar 39 ar biotite ages are displayed westward across the southern rhode island basement terrane 239 to 250 m y nine samples although coexisting hornblende records slightly older plateau ages of 255 m y two samples the 40 ar 39 ar plateau ages are interpreted to date times of post metamorphic cooling through temperatures required for argon retention in the various mineral phases examined the ages suggest that a distinct late paleozoic alleghenian thermal event affected much of southern rhode island together with the depositional ages of the variably metamorphosed pennsylvanian rocks westphalian b stephanian a or b the 40 ar 39 ar data permit calibration of the sequence of alleghenian tectonothermal events recorded in the narragansett basin the controls suggest a permian event lasting about 40 to 50 m y between initial barrovian regional metamorphism and synchronous d 1 d 2 ductile deformation between 295 and 275 m y and post d 3 cooling below argon retention temperatures between 250 and 240 m y

lamprophyric lavas in the colima graben sw mexico the colima graben located in sw mexico is one of three grabens which intersect about 50 km ssw of guadalajara forming a triple junction the 90 km long 20 60 km wide colima graben represents a n s rift of the e w trending mexican volcanic belt since the early pliocene the colima graben has served as a locus for the eruption of alkaline lavas the most recent of which are basanites and minettes erupted from late pleistocene cinder cones luhr and carmichael 1981 in this paper we report on older alkaline lavas which crop out in the graben s walls these rocks include phlogopite and hornblende bearing lamprophyres a phlogopite kalsilite ankaratrite and high k andesites these lavas crop out throughout the colima graben area and are intimately associated with calc alkaline lavas in the field compared to these the alkaline rocks are strikingly enriched in the incompatible elements particularly ba sr p and the lree unlike the younger late pleistocene alkaline cinder cone lavas most of the graben wall lamprophyres and the high k andesites represent magmas that appear to have undergone significant evolution since their generation including fractionation crustal contamination and possible magma mixing least squares modeling indicates that the cinder cone minettes represent reasonable parental magmas for the graben lamprophyres the occurrence of these alkaline lavas in an active calc alkaline volcanic arc is unusual and we suggest that they are a manifestation of the rifting processes which produced the colima graben

origin of coexisting minette and ultramafic breccia navajo volcanic field trace element evidence indicates that at the buell park diatreme navajo volcanic field the felsic minette can be best explained by crystal fractionation from a potassic magma similar in composition to the mafic minettes compatible trace element cr ni sc abundances decrease while concentrations of most incompatible elements ce yb rb ba sr remain constant or increase from mafic to felsic minette in particular the nearly constant ce yb ratio of the minettes combined with the decrease in cr ni and sc abundances from mafic to felsic minette is inconsistent with a model of varying amounts of partial melting as the process to explain minette compositions the uniformity of rare earth element ree abundances in all the minettes requires that an accessory mineral apatite dominated the geochemistry of the ree during fractionation a decrease in p2o5 from mafic to felsic minette and the presence of apatite in cognate inclusions are also consistent with apatite fractionation higher initial87sr 86sr ratios in the felsic minettes relative to the proposed parental mafic minettes however is inconsistent with a simple fractionation model also a separated phlogopite has a higher initial87sr 86sr ratio than host minette these anomalous isotopic features probably reflect interaction of minette magma with crust the associated ultramafic breccia at buell park is one of the navajo kimberlites but ree concentrations of the matrix do not support the kimberlite classification although the matrix of the breccia is enriched in the light ree relative to chondrites and has high la rb ba and sr concentrations relative to peridotites the concentrations of these elements are significantly lower than in south african kimberlites a high initial87sr 86sr ratio combined with petrographic evidence of ubiquitous crustal xenoliths in the navajo kimberlites suggests that the relatively high incompatible element concentrations are due to a crustal component apparently navajo kimberlites are most likely a mixture of comminuted mantle wall rock and crustal material there is no evidence for an incompatible element rich magma which is characteristic of south african kimberlites if the mafic minettes are primary magmas derived from a garnet peridotite source with chondritic ree abundances then ree geochemistry requires very small less than 1 degrees of melting to explain the minettes alternatively the minettes could have formed by a larger degree of melting of a metasomatized relatively light ree enriched garnet peridotite the important role of phlogopite and apatite in the differentiation of the minettes supports this latter hypothesis

active megadetachment beneath the western united states geodetic data interpreted in light of seismic imaging seismicity xenolith studies and the late quaternary geologic history of the northern great basin suggest that a subcontinental scale extensional detachment is localized near the moho to first order seismic yielding in the upper crust at any given latitude in this region occurs via an m7 earthquake every 100 years here we develop the hypothesis that since 1996 the region has undergone a cycle of strain accumulation and release similar to slow slip events observed on subduction megathrusts but yielding occurred on a subhorizontal surface 5 10 times larger in the slip direction and at temperatures 800 c net slip was variable ranging from 5 to 10 mm over most of the region strain energy with moment magnitude equivalent to an m7 earthquake was released along this megadetachment primarily between 2000 0 and 2005 5 slip initiated in late 1998 to mid 1999 in northeastern nevada and is best expressed in late 2003 during a magma injection event at moho depth beneath the sierra nevada accompanied by more rapid eastward relative displacement across the entire region the event ended in the east at 2004 0 and in the remainder of the network at about 2005 5 strain energy thus appears to have been transmitted from the cordilleran interior toward the plate boundary from high gravitational potential to low via yielding on the megadetachment the size and kinematic function of the proposed structure in light of various proxies for lithospheric thickness imply that the subcrustal lithosphere beneath nevada is a strong thin plate even though it resides in a high heat flow tectonic regime a strong lowermost crust and upper mantle is consistent with patterns of postseismic relaxation in the southern great basin deformation microstructures and low water content in dunite xenoliths in young lavas in central nevada and high temperature microstructures in analog surface exposures of deformed lower crust large scale decoupling between crust and upper mantle is consistent with the broad distribution of strain in the upper crust versus the more localized distribution in the subcrustal lithosphere as inferred by such proxies as low p wave velocity and mafic magmatism

evolution of archaean southern indian lithospheric mantle a geochemical study of proterozoic agali coimbatore dykes agali coimbatore dolerite dykes constitute an important proterozoic magmatic event that affected the south indian shield rb sr whole rock isotope data yield an errorchron of 2369 400 ma 2 error which is within error of the reported 2030 65 ma k ar age the dyke magmas were evolved fe rich tholeiitic melts produced by fractionation of clinopyroxene orthopyroxene and olivine in the initial stages plagioclase became a fractionation phase during the latter stages of crystallization the dykes characteristically have high 87sr 86sri 0 703 0 706 and are enriched in large ion lithophile and light rare earth elements relative to primordial mantle values and show negative nb anomalies these compositional characteristics are interpreted as source mantle characteristics whereas some crustal effects are visible in some samples with high initial 87sr 86sr peridotite with minor hydrous metasomatic phases like amphibole and phlogopite within the shallow lithospheric mantle could be a potential source material for the dykes however at this stage we cannot convincingly differentiate whether the source of the parent magmas is solely lithospheric or a product of asthenosphere lithosphere mixing the 18o values of the dykes range from 5 2 to 7 2 per mil vs standard mean oceanic water initial nd isotope values at the time of dyke intrusion nd at t 2 0 ga range from 2 3 to 4 8 whole rocks define a correlation on an sm nd isochron plot with a slope equivalent to an age of 3 15 0 53 ga 2 error sm nd crustal residence ages average at 2 87 ga the isochron age does not appear to be the result of systematic mixing with an older crustal component these results together with trace element geochemistry suggest that the south indian mantle lithosphere developed by addition of enriched melts fluids at about 3 0 ga synchronously with major crustal gene ration in the south indian shield

geochronology of the proterozoic basement of southwesternmost north america and the origin and evolution of the mojave crustal province the proterozoic baldwin gneiss in the central transverse ranges of southern california a part of the mojave crustal province is composed of quartzofeldspathic gneiss and schist augen and granitic gneiss trondhjemite gneiss and minor quartzite amphibolite metagabbro and metapyroxenite sensitive high resolution ion microprobe shrimp data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 12 and 1675 19 ma adjacent to or through thinned archean crust high u th rims on zircons in most samples suggest an early metamorphic event at 1741 ma but peak amphibolite facies metamorphism and penetrative west vergent deformation occurred after 1675 ma the baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a proterozoic shelf slope sequence joshua tree terrane we hypothesize that emplacement of this regional allochthon occurred during a late early or middle proterozoic arc continent collision along the western margin of laurentia

petrological significance of mineral chemistry in the agathla peak and the thumb minettes navajo volcanic field the agathla peak and the thumb minettes from the navajo province southwestern usa are characterized by macrocrysts of phlogopite and diopside plus rare olivine sim fo 84 set in a crystalline to glassy groundmass containing diopside phlogopite sanidine apatite ti magnetite and analcime the diopside has low al ti and na like late stage diopside in kimberlite groundmass ti rich phlogopite is zoned with increasing fe ti and ba from a cr ni bearing core high sr in the minettes is expressed in groundmass sanidine up to 1 2 wt sro apatite 0 5 0 9 and diopside 0 03 0 11 xenocrystal apatite is chemically distinct from phenocrystal apatite in the thumb minette in having low sro 0 04 wt and a distinct ree distribution which matches apatites in garaet granulite xenoliths all chemical properties of the minerals are consistent with derivation of the minette magmas from a region of the upper mantle containing olivine clinopyroxene phlogopite and ilmenite and enriched in k ti b

basaltic tephra from monogenetic marcath volcano central nevada abstract monogenetic volcanic eruptions are generally assumed to last to tens of years and produce frequent intermittent explosive activity that creates eruption columns up to 10 km in elevation although this view fits the historically observed eruptions few data have been collected about the range of durations and eruption sizes possible from monogenetic events examining other eruption deposits can help clarify the range of eruption styles and sizes possible from future explosive monogenetic eruptions the 38 ka marcath event at lunar crater volcanic field nevada produced a scoria cone two tephra fall deposits and a lava field we reconstruct the activity that produced the largest tephra fall deposit explosive activity produced an eruption column up to 7 km and a volume of about 0 018 km 3 the marcath tephra forming activity was small compared to other characterized scoria cones post eruptive remobilization of the deposit has occurred especially around the margins through both fluvial and eolian processes and has likely removed at least 0 001 km 3 of fine material from the distal portions of the deposit

geologic and geochemical evidence for the nature and development of the middle proterozoic keweenawan midcontinent rift of north america abstract general models of active continental rifting must take into account the large plateau basalt provinces many of which have been precursors of or associated with continental breakup the keweenawan middle proterozoic midcontinent rift mcr of north america 2300 km long is one such major continental rift but it aborted before significant crustal separation was achieved these plateau basalt rifts including the mcr differ from the east african type in being dominated by broad subsidence rather than rift valleys tholeiitic flood basalts rather than alkalic central volcanoes and large positive gravity anomalies geologic and geophysical evidence is reviewed which contradicts some recent models which have been proposed for the development of the mcr the basic causes of the pre rift subsidence the u shaped nature of the structure and the abrupt termination of its activity are obscure a triple junction in the usual sense does not appear to have been formed

grain boundary enrichment of incompatible elements in some mantle peridotites electron microprobe analyses sensitive to 15 40 ppm at 3 confidence have been made for minerals in a spinel dunite from san carlos arizona u s a and a spinel lherzolite from ichinomegata akita ken japan whereas both rocks contain an appreciable amount of k2o p2o5 and tio2 the constituent minerals contain only trace amounts of these elements microprobe analyses across the grain boundaries on polished sections and on mineral surfaces before and after acid leaching show that most of the incompatible elements in the rocks are contained within a surface film al k emission spectroscopy suggests that the surface film is neither an aggregate of fine grained intergranular minerals nor a quenched glass it could be an adsorbate on the irregular sites of crystal surfaces and was probably formed by any of diffusion from interiors of minerals i e segregation mantle metasomatism and partial melting the contribution of incompatible elements from the grain boundaries into partial melts has been documented by heating experiments on peridotite chips under controlled fo2 the spinel dunite yields at 1300 c a 4 5 vol partial melt with about 1 k2o 1 6 na2o 0 14 p2o5 0 8 tio2 210 ppm sr 140 ppm ba and 40 ppm ce along the grain boundaries grain boundaries are an important storage of incompatible elements in peridotites and they in turn can be the dominant source of these elements in magmas during partial melting of the upper mantle

a grenville age layered intrusion in the subsurface of west texas petrology petrography and possible tectonic setting the nellie intrusion is a thick more than 4420 m mafic to ultramafic layered intrusion with a radiometric age of 1163 ma rock types change abruptly with stratigraphic height and include norite pyroxenite gabbronorite hornblende gabbro gabbro anorthosite harzburgite and lherzolite norite is most abundant but gabbro and hornblende gabbro are locally abundant rare olivine rich layers are also present the general order of crystallization was olivine orthopyroxene plagioclase clinopyroxene and hornblende mg s expressed as 100 mg mg fe range from 76 3 to 85 8 for olivine 56 7 to 84 9 for orthopyroxene 62 5 to 90 3 for clinopyroxene and 52 4 to 82 8 for amphibole mg s vary with height and display abrupt reversals which indicate open system addition of new mafic magma eleven cyclic units were identified on the basis of evidence for injection of basaltic magma these can be grouped into three megacyclic units the abundance of orthopyroxene and mineral compositional evidence for

petrogenesis of mesoproterozoic granitic plutons eastern llano uplift central texas usa abstract the llano uplift of central texas is a gentle structural dome exposing 1370 to 1230 ma metaigneous and metasedimentary rocks of grenville affinity along the southern margin of laurentia the metamorphic rocks were subsequently intruded by 1119 to 1070 ma late syn to post tectonic granites collectively known as the town mountain granite tmg the eastern most of the tmg the marble falls mf kingsland kl and lone grove lg plutons are metaluminous to marginally peraluminous high k calc alkaline ferroan biotite calcic amphibole granites fe fe mg 0 71 0 92 and 0 78 0 91 for biotite and calcic amphibole respectively displaying distinct variation trends with increasing silica content they are chemically and texturally zoned and have mineralogical and chemical characteristics similar to a type granites i e 1 fe rich biotites calcic amphiboles accessory fluorite and sporadic rapakivi texture 2 high k2o 4 wt 3 low al2o3 on granite discrimination diagrams k2o na2o cao vs zr nb ce y ppm and zr ppm vs ga al 10 000 the kl and mf plutons plot within the a type field whereas the lg pluton compositions are divided between a type and fractionated granite fields i s and m types on tectonic discrimination diagrams y vs nb the mf and kl granites plot in the within plate granite field but the lg pluton plots across several fields including within plate and volcanic arc plus syn collisional fields consequently the tectonic classification on a geochemical basis for the lg pluton is unclear based on thermal metamorphic mineral assemblages normative q ab or plots and q ab or h2o experimental data johannes and holtz 1996 crystallization temperatures and pressures are estimated to range from 750 to 850 c and 200 to 500 mpa respectively the assemblage of titanite magnetite quartz suggests crystallization at low fo2 confirmed by fe fe mg vs 4 al microprobe analyses of calcic amphibole and a water content of less than 1 5 wt wones 1989 like other town mountain type plutons the mf kl and lg granites display comparable iron contents at similar alkali and silica enrichments melting models ba vs sr suggest the mf kl and lg plutons may have evolved from the partial melting anatexis of juvenile tonalitic lower crustal rocks followed by plagioclase and pyroxene dominated fractionation nd isotopic data for the mf pluton end 3 4 at 1 06 ga patchett and ruiz 1989 and whole rock 18o values for the mf kl and lg plutons 7 0 10 1 rangel et al 2008 suggest that the magmas in the eastern llano uplift may contain a significant mantle component whereas relatively high 18o values 9 3 to 9 7 bebout and carlson 1986 for other coeval tmg rocks suggest that a significant crustal component is involved whole rock and trace element chemistry indicate that the mf and kl plutons along with the coarser grained textures of the lg pluton are a type granites however with no coeval mafic dikes syenitic compositions or volcanic rocks it is clear that the tmg plutons do not represent anorogenic granites the available evidence is most compatible with emplacement of the tmg plutons in a post orogenic grenville relaxation and extensional i e slab breakoff setting

partition coefficients for ree between garnets and liquids implications of non henry s law behaviour for models of basalt origin and evolution partition coefficients for the rare earth elements ree ce sm and tm between coexisting garnets and hydrous liquids have been determined at high pressure and temperatures 30 kbar and 1300 and 1500 c two synthetic systems were studied mg3al2si3o12 h2o and ca3al2si3o12 h2o in addition to a natural pyrope bearing system deviations from henry s law behaviour occur at geologically relevant ree concentrations at concentrations 3 ppm ce 12 ppm sm 80 ppm tm in pyrope and 100 ppm ce 250 ppm sm 1000 ppm tm in grossular at 30 kbar and 1300 c dgarnet liquidree increases as the ree concentration in the garnet decreases at higher concentrations dree is constant dgrossular liquidree also constant when the garnet contains less than about 2 ppm sm or tm the ree concentration at which dree becomes constant increases with increasing temperature decreasing ree ionic radius and increasing ca content of the garnet partitioning behaviour of ce sm and tm between a natural pyrope rich garnet and hydrous liquid is analogous to that in the synthetic systems and substantiates the substitution model proposed by harrison and wood 1980 values of dreegarnet liquid for which henry s law is obeyed are systematically higher for grossular than for pyrope dpyrope liquid 0 067 ce 0 108 sm 0 155 tm and dgrossular liquid 0 65 ce 0 75 sm 4 55 tm the implications of non henry s law partitioning of ree for models of basalt petrogenesis involving garnet are far ranging deviations from henry s law permit refinements to be made to calculated ree abundances once basic model parameters have been defined

petrological evolution of the crust and mantle in its simplest form crust mantle evolution can be thought of as a study of the processes by which material is lost from the mantle and forms new crust the overall goal is achievement of mass balance between the average composition of material known to have been added to the crust and the estimated average composition of the entire crust attainment of this goal for the earth has been elusive primarily because the materials available for study have generally been involved in several stages of evolution including fractionation within both the crust and mantle and recycling of material from crust to mantle thereby complicating the interpretation of available data although the overall composition of the continental crust generally has been modelled as andesitic taylor and mclennan 1981a the present upper continental crust is granodioritic and requires a more mafic lower crust it has been proposed taylor and mclennan 1981a that originally the archean crust had an andesitic composition throughout its volume and that partial melting in the lower crust developed more felsic melts that rose to enrich the upper crust leaving a more mafic residuum of lower crust this concept has been challenged by weaver and tarney 1981a who argue that the granulites of presumed lower crustal origin do not have the correct compositions for this model if one considers more recent additions to the crust from the mantle over the past several hundred million years it appears that these latest additions are basaltic with very little if any granitic additions in fact barker 1981 in his introduction to a special issue of jgr on granites and rhyolites states to date no one has demonstrated that such magmas may form in the mantle indeed much experimental and geochemical work indicates that such siliceous liquids cannot originate there therefore such magmas must be generated in the crust in island arc environments where the average composition of added material normally is thought to be andesitic a anderson 1982 has shown that the parent melt is basaltic not andesitic in addition kay 1980 has shown that the k content of the newly added material is less than the average k content of the previously existing crust in the oceanic crust the additions from the mantle are basaltic and according to o hara 1982 probably are derived from very mafic picritic melts as parents continental flood basalts which have also added material from mantle to crust also are clearly derived from basaltic parental melts basaltic volcanism study project 1981 thus the evidence for the nature of material subtracted from the mantle and added to the crust for the past several hundred million or perhaps a billion years points to very mafic melts this contrasts significantly with the overall andesitic composition of continental crust it implies furthermore that if the overall archean crustal composition was andesitic as estimated by taylor and mclennan 1981a then additions to the crust have become more mafic with time a concept in direct conflict with most models in which secular trends are from mafic to felsic with time veizer and jansen 1979 and mclennan et al 198o a question which is beginning to appear either explicitly or implicitly is whether crustal volcanic rocks or the crust as a whole really represents the parent melt that formed in the mantle a anderson 1982 o hara 1982 and nisbet and walker 1982

structural implications of some radiometric ages of igneous rocks in southeastern new england u th pb zircon rb sr whole rock and k ar hornblende isotopic ages reveal two major episodes of igneous activity in southeastern new england one in late proterozoic z time 600 650 m y ago and the other in devonian to ordovician time 375 450 m y ago the dated rocks are discussed in the context of several individual structural provinces that evolved more or less independently of each other prior to middle to late paleozoic assembly these provinces generally are bounded by major faults and or abrupt changes in metamorphic grade and from east to west include i the milford dedham zone ii the putnam nashoba zone iii the merrimack synclinorium and iv the bronson hill zone from the present and other recently published studies reasonably accurate ages representative of the two major episodes can now be assigned to many of the igneous rocks other units remain less well constrained but probably may be placed into one or the other of the general age groupings case by case examination of the geochronologic data makes us cautious in accepting some apparent fine structure age resolution within each grouping until factors such as inherited zircon and metamorphic overprinting can be better evaluated with the above mentioned qualifications in mind igneous rock ages summarized according to structural province are as follows i milford dedham zone a late proterozoic z dedham granite and related rocks 630 15 m y 595 15 m y 612 646 m y milford granite 630 15 m y 591 50 m y hope valley alaskite gneiss and scituate granite gneiss ages uncertain unnamed granite near assonet 600 650 m y b devonian to ordovician alkalic rocks including quincy cape ann and peabody granites wenham monzonite and nahant gabbro ii putnam nashoba zone a late proterozoic z no plutonic rocks identified but may include some metavolcanic rocks b devonian to ordovician sharpners pond diorite 430 5 m y andover granite 408 22 m y 450 22 m y muscovite granite near west berlin age uncertain preston gabbro 424 5 m y ill merrimack synclinorium a late proterozoic z granitic orthogneiss unit of the massabesic gneiss complex 600 620 m y metavolcanic unit of the massabesic gneiss complex 646 m y or older b devonian to ordovician fitchburg complex at maiden hill 402 11 m y fitchburg complex at rollstone quarry 390 15 m y newburyport complex 450 15 m y ayer granite 433 5 m y chelmsford granite age uncertain muscovite biotite granite at millstone hill 372 7 m y canterbury gneisses 395 10 m y 392 9 m y iv bronson hill zone a late proterozoic z dry hill gneiss of the pelham dome 600 m y b devonian to ordovician monson gneiss 440 10 m y these distinctive age patterns together with contrasts in petrography and texture of the igneous rocks in stratigraphy of the country lock and in metamorphic grade accentuate the separate geologic development of each structural province no correlation involving preassembly geology should be attempted across province boundaries without due consideration of the exotic character of each province

colorado plateau magmatism and uplift by warming of heterogeneous lithosphere the forces that drove rock uplift of the low relief high elevation tectonically stable colorado plateau are the subject of long standing debate while the adjacent basin and range province and rio grande rift province underwent cenozoic shortening followed by extension the plateau experienced approximately 2 km of rock uplift without significant internal deformation here we propose that warming of the thicker more iron depleted colorado plateau lithosphere over 35 40 myr following mid cenozoic removal of the farallon plate from beneath north america is the primary mechanism driving rock uplift in our model conductive re equilibration not only explains the rock uplift of the plateau but also provides a robust geodynamic interpretation of observed contrasts between the colorado plateau margins and the plateau interior in particular the model matches the encroachment of cenozoic magmatism from the margins towards the plateau interior at rates of 3 6 km myr 1 and is consistent with lower seismic velocities and more negative bouguer gravity at the margins than in the plateau interior we suggest that warming of heterogeneous lithosphere is a powerful mechanism for driving epeirogenic rock uplift of the colorado plateau and may be of general importance in plate interior settings

a network model for fluid transport through sea ice the flow of liquid through porous sea ice is a fundamental process affecting problems in polar biology oceanography and geophysics the geometry and connectedness of the pore microstructure of sea ice determine its fluid permeability which depends strongly on temperature here we analyze a simple pipe network as a basis for modeling fluid flow through the complex porous microstructure and for numerical approximations of the fluid permeability of sea ice as a function of temperature for slow flow the fluid system is equivalent to an electrical resistor network and the network is solved using a fast multi grid method the radii of the pipes in the network are chosen randomly from distributions describing measured cross sectional areas of brine inclusions in sea ice at this stage the model reflects only the most general features of brine microstructure and its evolution with temperature preliminary results for a basic implementation in two dimensions are presented they are consistent with theoretical bounds on the vertical fluid permeability of sea ice found recently moreover the results agree roughly with laboratory data for higher porosities for lower porosities and colder temperatures the fully connected network of pipes in the model albeit with smaller radii overestimates observed values this finding provides evidence that the brine network becomes more disconnected with lower temperatures which is consistent with transitional behavior near a percolation threshold

rare earth element geochemistry of garnet lherzolite and megacrystalline nodules from minette of the colorado plateau province abstract mineral and whole rock ree abundances in garnet lherzolite and megacrystalline nodules from the thumb display broad correlations with major element compositions lherzolites with 12 modal clinopyroxene plus garnet high ca al lherzolites have relatively flat chondrite normalized whole rock ree patterns lherzolites poor in clinopyroxene and garnet low ca al lherzolites have lower hree in clinopyroxenes and garnets and higher whole rock lree hree it is concluded that the low ca al lherzolites may have undergone lree metasomatism after depletion of the major element compositions by partial melting and that much of the garnet now present was originally dissolved in aluminous orthopyroxene the high ca al lherzolites may be interpreted either as primordial mantle samples or as products of equilibration with very lree enriched liquids the megacrystalline nodules are medium to ultracoarse grained intergrowths and megacrysts with mineral compositions similar to discrete nodule suites in kimberlites the ree abundances of the megacrystalline minerals are consistent with an origin as cumulates from magma with extremely fractionated ree similar to minette or kimberlite the patterns of correlation of ree and major elements in this inclusion suite are similar to the patterns observed in the garnet lherzolite and discrete nodule suites of southern african kimberlites both of the subcontinental mantle provinces represented by these suites contain three distinct petrogenetic components refractory garnet lherzolite enriched in lree and depleted in hree fertile garnet lherzolite with generally chondritic ree abundances and a suite of ultracoarse minerals precipitated from magma with extremely fractionated ree generally similar to the host magmas

intrusion of horizontal dikes tectonic significance of middle proterozoic diabase sheets widespread in the upper crust of the southwestern united states initially horizontal sheet intrusions of middle proterozoic diabase are abundant in a region 650 by 300 km across in arizona and california the diabase forms discordant sheets in basement granite and gneiss and sills in overlying shelf sedimentary sequences massive granite is the most common basement host for the sheets probably because it fractured more easily than foliated hosts during sheet emplacement steep feeder dikes are rare compared to the sheets the diabase in many places is exposed in fault blocks that were tilted during tertiary tectonic extension structure sections restored from the map patterns of upended blocks show that the sheets were intruded at levels throughout the upper crust to depths of at least 13 km sheet intrusion implies a vertical orientation of the least compressive stress so i conclude that the crust was under tectonic compression or in an isotropic state of stress at the time of diabase intrusion about 1 1 ga magma overpressures water encountered by rising magma vertical changes in the crustal stress regime and flotation of low density granite all may be important factors for sheet intrusion the stress conditions suggested by the presence of the sheets argue against an extensional tectonic regime earlier proposed for the diabase event the diabase province contrasts structurally with similar age provinces of basaltic magmatism elsewhere in north america that show evidence of tectonic extension such as the midcontinent rift the consistent orientations of the sheets commonly allow them to be used as structural markers for postdiabase deformation of basement blocks sheet intrusions in the geologic record may be seriously underreported their recognition is important for the interpretation of seismic reflection profiles of continental crust sheets in arizona may be responsible for the bagdad reflection sequence which extends to depths of at least 15 km

delineation of subsurface proterozoic unkar and chuar group sedimentary basins in northern arizona using gravity and magnetics implications for hydrocarbon source potential subsurface occurrences of mesoproterozoic unkar group 1 25 1 1 ga and neoproterozoic chuar group 0 9 0 7 ga rocks have been delineated in regions east and southeast of the grand canyon of northern arizona gravity and aeromagnetic data combined with gravity modeling an analysis of seismic velocities geologic cross sections and geologic structures have identified two major proterozoic extensional regimes in the region 1 a mesoproterozoic northwest to southeast trend indicating northeast southwest directed extension throughout the study area and 2 a neoproterozoic mostly north south directed trend indicating predominant east to west extension evident mostly in northern regions of the study area a more ancient northeast southwest aligned system of basement shear zones has also been identified these shear zones correlate with many of the exposed major fault systems of the region such as the bright angel and mesa butte fault systems unkar group graben and half graben formation was constrained by the preexisting northeast directed shear zones chuar group basin formation was influenced by the reactivation of northwest to southeast unkar group basin structures the more ancient northeast trending shear zones and by neoproterozoic age north to south oriented extension this superimposed network of deep seated normal faults led to the development of a complex array of surface structures during the laramide by delineating proterozoic basin structures in the region this study helps to delineate the probable subsurface occurrences of hydrocarbon source rocks of the walcott member of the neoproterozoic chuar group john m seeley is a research geologist and assistant director for the pan american center for earth and environmental studies at the university of texas at el paso he received his b s 1985 and m s 1990 degrees from texas a i university and his ph d 1999 from the university of texas at el paso his research interests include depositional systems sequence stratigraphy gravity and magnetics and tectonics g randy keller holds the l a nelson professorship he is also chief scientist of university of texas at el paso s nasa earth science research center his research interests stress the geological applications of geophysics and he is particularly interested in the integration of diverse data sets and the use of information technology he is a longtime member of the aapg society of exploration geophysicists geological society of america american geophysical union and the european association of exploration geophysicists

an unmetasomatized source for the malaitan alnoite solomon islands petrogenesis involving zone refining megacryst fractionation and assimilation of oceanic lithosphere the ma tan alnijite contains a rich and varied megacryst suite of unprecedented compositional range we have undertaken trace element and isotope modeling in order to formulate a petrogenetic scheme which links the host alniiite to its entrained megacrysts this requires that a proto alntiite magma is the product of zone refining initiated by diapiric upwelling where the initial melt passes through 200 times its volume of mantle isotopic evidence indicates the source of the proto alniiite contains a time integrated lree depleted signature 143nd 44nd 0 51274 impingement upon the rigid lithosphere halts or dramatically slows the upward progress of the mantle diapir at this point the magma cools and megacryst fractionation begins with augites crystallizingjirst followed by subcalcic diopsides and finally phlogopites garnet probably crystallizes over the entire range of clinopyroxene fractionation estimated proportions of fractionating phases are 30 augite 24 5 subcalcic diopside 27 garnet 12 9 phlogopite 5 bronzite 0 5 ilmenite and 0 1 zircon as this proto alnoite magma crystallizes it assimilates a subducted component of seawater altered basalt which underplates the ontong java plateau this is witnessed in the isotopic composition of the megacrysts and alnoite

stable isotope clues to the formation and evolution of refrozen melt ponds on arctic sea ice author posting american geophysical union 2018 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in journal of geophysical research oceans 123 12 2018 8887 8901 doi 10 1029 2018jc013797

simulation of primary phase relations and mineral compositions in the partridge river intrusion duluth complex minnesota implications for the parent magma composition in order to describe the composition and crystallinity of the initial parental magma of the partridge river intrusion of the keweenawan duluth complex and thereby understand the mode of emplacement and solidification of the intrusion we have applied a numerical simulation technique called geochemical thermometry frenkel et al 1988 the parental magma was a low alumina high ti p olivine tholeiite similar to typical keweenawan low alumina high ti p basalts associated with the duluth complex and from the nearby portage lake area of the lake superior region the parental magma was emplaced as a crystal liquid suspension followed by chilling of an evolved leading edge ferrodioritic liquid in the basal zone of the intrusion the conditions of emplacement at the present crustal location were 1 150 c 2 kbar and fo2 slightly above the wustite magnetite wm buffer the main differentiation process after emplacement was the sorting and redistribution of plagioclase and olivine crystals on a local scale accompanied by less efficient convection and minor settling of olivine calculated crystallization sequence for the parental magma is olivine plagioclase 1 240 c olivine plagioclase magnetite 1 146 c wm 0 5 olivine plagioclase magnetite augite 1 140 c wm 0 5 the calculated compositions of the cumulus olivine and plagioclase in equilibrium with the parent magma at 1 150 c are fo66 7 1 1 and an64 5 2 5 respectively and are similar to the estimated average composition of primary olivine fo69 1 2 8 and the average composition of plagioclase core an66 3 2 8 measured in drill core samples through the intrusion chalokwu and grant 1987

dependencies of high latitude plasma convection consideration of interplanetary magnetic field seasonal and universal time factors in statistical patterns 1 the database of the nine radars of the super dual auroral radar network superdarn in the northern hemisphere has been analyzed for information on factors that influence the convection of plasma in the high latitude ionosphere the velocity measurements were collected over the period 1998 2002 the data were first used to derive a new statistical model of convection that improves upon the earlier one radar model of ruohoniemi and greenwald 1996 in its specification of the dependence of the convection pattern on the magnitude and direction of the imf in the gsm y z plane we then derived average patterns for secondary sortings by season year and radar such dependencies as emerged were most clearly seen by contrasting the results for by and by the seasonal effect in the convection pattern is found to have similarities to that of the sign of by in particular the combination of by summer by winter reinforces the tendency of the by sign factor to sculpt the dusk and dawn cells into more round crescent crescent round shapes and to shift the crescent cell across the midnight mlt meridian however these combinations are associated with lower estimates of the total cross polar cap potential drop pc while the nonreinforcing combinations produce elevated pc especially by summer there is an overall tendency for pc to increase from winter to summer although the pure seasonal effect on the potential drop is weaker than that of the by sign season factor we did not find pronounced differences among the patterns derived for the 5 individual years which spanned the most recent interval of solar cycle maximum sorting by radar we found few differences among the patterns for by but for by variations emerged that are consistent with a possible dependence on universal time ut the impacts of season and ut on convection in the high latitude ionosphere thus depends on the imf especially the sign of by we speculate that variability in the ionospheric conductivity has a greater effect on magnetosphere ionosphere coupling under by conditions

the eagle and east eagle sulfide ore bearing mafic ultramafic intrusions in the midcontinent rift system upper michigan geochronology and petrologic evolution 1 the eagle and east eagle intrusions are small subvertical dike like mafic ultramafic bodies that cut proterozoic sedimentary strata in the baraga basin in northern michigan the eagle intrusion hosts a newly discovered magmatic ni cu pge deposit the nearby east eagle intrusion also contains sulfide mineralization but the extent of this mineralization has yet to be determined by further drilling both intrusions contain olivine bearing rocks such as feldspathic peridotite melatroctolite and olivine melagabbro sulfide accumulations range from disseminated at both eagle and east eagle to semimassive and massive at eagle u pb baddeleyite dating gives a crystallization age of 1107 2 5 7 ma for the eagle intrusion coeval with eruption of picritic basalts at the base of the volcanic succession in the midcontinent rift system mrs the fo contents of olivine cores in the eagle and east eagle intrusions vary between 75 and 85 mol higher than those of olivine in larger layered intrusions in the mrs such as the duluth complex the feo mgo ratios and al2o3 contents of the parental magmas for the eagle and east eagle intrusions inferred from olivine and spinel compositions are similar to those of picritic basalts in the base of the mrs volcanic succession these petrochemical data suggest that the eagle and east eagle intrusions are the intrusive equivalents of high mgo basalts that erupted in the early stages of continental magmatism associated with the development of the rift variations in mineral compositions and incompatible trace element ratios suggest that at least three major pulses of magmas were involved in the formation of low sulfide rocks in the eagle intrusion lower fo contents of olivine associated with semimassive sulfides as compared to that of olivine in low sulfide rocks suggest that the magma associated with the semimassive sulfide was more fractionated than the parental magmas of the low sulfide rocks in the eagle intrusion accumulation of suspended olivine crystals and sulfide droplets from ascending magmas as they passed through wide parts of the conduits at eagle and east eagle played a critical role in the genesis of olivine rich rocks and sulfide ores in the intrusions the eagle ni cu pge deposit typifies the conduit style of magmatic sulfide deposition that is associated with continental basaltic magmatism

geochemistry and petrogenesis of proterozoic diabase in the southern death valley region of california diabase sills and dikes of proterozoic age intrude crystalline basement and the overlying crystal spring formation in the southern death valley region of california despite pervasive deuteric alteration analyses of relict plagioclase an66 45 titaniferous augite and ilmenite permit the calculation of initial crystallization temperatures of 1 165 25 c for plagioclase and 1 110 25 c for augite with an oxygen fugacity of 10 11 atm the early crystallization of plagioclase is consistent with the generally subophitic texture of the diabase

metasomatised lower crustal and upper mantle xenoliths from north queensland chemical and isotopic evidence bearing on the composition and source of the fluid phase a diverse range of ultramafic and mafic granulite xenoliths from cone 32 north queensland display mineralogical and chemical evidence for enhanced fluid activities in the lowermost crust and uppermost mantle metasomatism of spinel lherzolite garnet and spinel websterite and garnet granulite produced variable amounts of amphibole mica locally resulting in development of hornblendite as the end product of reaction between spinel websterite and c o h fluids in contrast associated felsic and 2 pyroxene granulite xenoliths have remained relatively unaffected the garnet granulites and spinel websterites are interpreted as products of underplating of the lower crust by cenozoic magmatic activity whereas nd sr isotope systematics for the 2 pyroxene and felsic granulite xenoliths suggest that they may be of proterozoic age the development of amphibole and mica in the mafic and ultramafic xenoliths was accompanied by increases in tio2 na2o k2o a12o2 sr ba zr nb lree and fe2o3 feo and decreased mgo sio2 cr and ni virtually anhydrous spinel websterite xenoliths have 87sr86sr and 143nd144nd almost identical to amphibole rich variants and hornblendite and very similar values to the host nepheline mugearite and an amphibole megacryst the chemical and isotopic data suggest that the metasomatism is a relatively young feature caused by oxidized fluids exsolved from alkaline magmas similar to the host as they ascended through the lithosphere this style of metasomatism is considered a relatively localized phenomenon which is unlikely to provide an important contribution to the source regions of alkali basaltic magmas

shrimp rg u pb zircon geochronology of mesoproterozoic metamorphism and plutonism in the southwesternmost united states abstract mesoproterozoic intrusive and granulite grade metamorphic rocks in southern california have been inferred to be exotic to north america on the basis of perceived chronologic incompatibility with autochthonous cratonal rocks ion microprobe geochronology indicates that zircons in granulite grade gneisses dated at 1 4 ga using conventional methods are composed of 1 68 1 80 ga cores and 1 19 ga rims these early proterozoic gneisses were metamorphosed at extremely high temperatures and moderate pressures during emplacement of the 1 19 ga san gabriel anorthosite complex the lack of a 1 4 ga metamorphic event suggests that proterozoic rocks in this region rather than being exotic to north america may in fact be a midcrustal window into mesoproterozoic crustal evolutionary processes in southwestern north america

microbiotas from the late proterozoic chuar group northern arizona and uinta mountain group utah and their chronostratigraphic implications abstract samples of shales from the dominantly clastic upper proterozoic chuar and uinta mountain groups in northern arizona and utah yielded relatively abundant assemblages of acritarchs these are taxonomically varied and are interpreted as consisting of openwater cosmopolitan eukaryotic plankton the assemblages are compared with late proterozoic late riphean and early vendian c 800 700 ma sequences in the southern urals of the ussr the russian platform northern and southern scandinavia svalbard and east and north west greenland in all instances both the assemblages and discrete form taxa predate the varangerian vendian glacial event of the north atlantic region and this is taken to indicate a comparable age for glaciogenic rock units in western north america one new acritarch species vandalosphaeridium walcottii is erected and the partly megascopic species chuaria circularis is emended

tertiary minette and melanephelinite dikes wasatch plateau utah records of mantle heterogeneities and changing tectonics a swarm of minette and melanephelinite dikes is exposed over 2500 km2 in and near the wasatch plateau central utah along the western margin of the colorado plateaus in the transition zone with the basin and range province to date 110 vertical dikes in 25 dike sets have been recognized strikes shift from about n80 w for 24 ma dikes to about n60 w for 18 ma to due north for 8 7 m y these orientations are consistent with a shift from east west oligocene compression associated with subduction to east west late miocene crustal extension minettes are the most common rock type mica rich minette and mica bearing melanephelinite occurs in 24 ma dikes whereas more ordinary minette is found in 8 7 ma dikes one melanephelinite dike is 18 ma these mafic alkaline rocks are transitional to one another in modal and major element composition but have distinctive trace element patterns and isotopic compositions they appear to have crystallized from primitive magmas major trace element and nd sr isotopic data indicate that melanephelinite which has similarities to ocean island basalt was derived from small degree melts of mantle with a chondritic sm nd ratio probably located in the asthenosphere but it is difficult to rule out a lithospheric source in contrast mica bearing rocks mica melanephelinite and both types of minette are more potassic and have trace element patterns with strong nb ta depletions and sr nd isotopic compositions caused by involvement with a component from heterogeneously enriched lithospheric mantle with long term enrichment of rb or light rare earth elements ree epsilon nd as low as 15 in minette light ree enrichment must have occurred anciently in the mid proterozoic when the lithosphere was formed and is not a result of cenozoic subduction processes after about 25 ma foundering of the subducting farallon plate may have triggered upwelling of warm asthenospheric mantle to the base of the lithosphere melanephelinite magma may have separated from the asthenosphere and while rising through the lithosphere provided heat for lithospheric magma generation varying degrees of interaction between melanephelinite and small potassic melt fractions derived from the lithospheric mantle can explain the gradational character of the melanephelinite to minette suite

common pb isotopic compositions of the lima arequipa and toquepala segments in the coastal batholith peru implications for magmagenesis abstract common pb isotopic compositions for 65 feldspar samples from the 188 37 ma old gabbroic to monzogranitic plutons of the peruvian coastal batholith show changes along strike that can be related to variable contamination of mantle derived magmas by the local precambrian basement results have the following isotopic ranges 206 pb 204 pb 17 580 20 803 207 pb 204 pb 15 555 15 709 and 208 pb 204 pb 38 104 41 177 with averages of 206 pb 204 pb 18 630 207 pb 204 pb 15 610 and 208 pb 204 pb 38 500 the gabbroic dioritic and tonalitic plutons north and east of lima the lima segment have ratios similar to the isotopically homogeneous reservoir identified by tilton 1979 tilton and barreiro 1980 and barreiro and stern 1982 for rocks in central and southern chile the homogeneous reservoir has been suggested to be enriched subcontinental mantle more siliceous rocks granodiorites and monzogranites in the lima segment have considerably higher ratios 206 pb 204 pb 18 680 20 803 207 pb 204 pb 15 610 15 709 and 208 pb 204 pb 38 500 41 177 nine of ten monzogranites from the ring dike complexes in the lima segment define a linear array with a slope of 0 1143 correlation coefficient 0 9398 and a pb isochron age of 1 9 ga this isochron age has no simple chronological meaning although it may represent a component of sedimentary debris derived from 1 9 ga crustal materials feldspars from plutons in the arequipa and toquepala segments south of lima tend to have low pb isotopic ratios 206 pb 204 pb 17 580 18 603 207 pb 204 pb 15 555 15 633 and 208 pb 204 pb 38 104 38 749 the ratios plot between the isotopic signatures of local precambrian granulites and gneisses and the chile type enriched subcontinental magmatic reservoir indicating that these end members were the principal magma sources the oldest plutonic rocks in the segments early to mid jurassic are closest to the pb isotopic ratios of the precambrian basement contamination of plutons in the arequipa and toquepala segments by unradiogenic pb from the precambrian basement and lack of it in the lima segment supports crustal models by couch et al 1981 and jones 1981 which show a thick precambrian basement layer throughout southern peru and an extremely thin layer beneath the lima segment

the presence of a stable megablock in the southwestern north american proterozoic craton in northern mexico abstract the present study based on combined geophysical geochronological and isotopic data identifies a cratonic block which we call the western chihuahua mesa central megablock wcmb in northern mexico new bouguer and isostatic gravity anomaly maps revealed a conspicuously distinct spatially extensive gravity minima over a region extending from north western chihuahua to as far south as the mesa central mc this region can be differentiated from the surrounding mobile belts with significantly higher gravity anomaly values gravity modeling combined with sparse seismically determined crustal thickness estimates indicate that the proposed wcmb region has crustal thicknesses ranging from 38 to 42 km the regions surrounding wcmb are the spatially constrained mobile belts that are characterized by basinal and trough like structures and are associated with a type granites and bimodal volcanism spatial distribution of enriched initial sr isotopic data 0 706 confirms the sialic character of the crust associated with the gravity minimum at least in western chihuahua the mobile belts surrounding the wcmb contain geologic records of the events since 1 8 0 9 ga these ages from within and just adjacent to the block correlate well with the rodinia neoproterozoic supercontinent that was assembled 1 3 0 9 ga we propose that the northwest southeast directed wcmb is a cratonic region underlain by proterozoic crust the wcmb is most likely separated from the deformed and active southwestern part of the north american proterozoic craton by repeated thermotectonic activities within the surrounding mobile belts these events have helped to dislocate and transport the block to its present location through processes of repeated compression extension and transform faulting

timing of arrival of the avalon zone in the northeastern appalachians a new look at the straddling granite the straddling granite was previously thought to straddle the hermitage bay dover fault system which marks the boundary between the avalon and gander zones of eastern newfoundland and hence to fix the earliest juxtaposition of these two zones before 504 12 ma more detailed investigation shows that these rocks are divisible into two distinct geochemical suites the indian point granite to the west and the hardy9s cove intrusive suite to the east of the samples that provided the 504 ma isochron five were obtained from the hardy9s cove suite and one from within the fault zone hence the isochron must be taken as representing only a typical date for plutonic rocks within the avalon zone the indian point granite intrudes the gaultois granite dated at 350 18 ma and is thus considerably younger than the hardy9s cove suite these observations now allow for juxtaposition of the avalon and gander zones to have been as late as early carboniferous

interplanetary magnetic field control of fast azimuthal flows in the nightside high latitude ionosphere superdarn radar data from both hemispheres for the interval january 1999 to november 2006 have been inspected for evidence of high speed azimuthal flows in the nightside high latitude ionosphere in the northern hemisphere 88 of the westward directed fast flows occurred during imf by negative intervals and 83 of the eastward flows occurred during imf by positive intervals in the southern hemisphere 82 of the westward flows occurred during imf by positive intervals and 74 of the eastward flows occurred during imf by negative intervals 74 of these fast flows occurred when the imf was within the 30 100 clock angle range and the ae index was below 150 nt these results support the hypothesis that highly dynamic magnetospheric activitycan be associated with predominantly northward but by dominated imf driving conditions and subsequent non substorm reconnection in a twisted magnetotail

a method to automatically determine sea level for referencing snow freeboards and computing sea ice thicknesses from nasa icebridge airborne lidar abstract the nasa icebridge flights have obtained critical observations for earth s polar ice since icesat stopped collecting data in 2009 this study develops an automatic method in processing icebridge airborne topographic mapper atm altimeter l1b data one elevation per 3 4 m horizontally to derive a local sea level height for referencing snow freeboards and then computing sea ice thicknesses four 30 km l1b profiles a b c and d flown on october 21 2009 over the bellingshausen sea in antarctica are selected the local sea level reference is first obtained by visual examination of atm l1b heights over leads or thin ice identified on images simultaneously acquired from the digital mapping system camera called manual selection this sea level reference is then used as ground truth to validate sea level heights derived by automatic calculations using five thresholds of 2 1 0 5 0 2 and 0 1 of the lowest l1b data the l1b 0 2 method gives a similar sea level height as from the l1b manual selection by mean absolute difference of 0 01 0 06 m the sea level heights demonstrate a near linear gradient of 0 01 m km to 0 03 m km within each 30 km l1b profile along the flight track from section a to d the resulting mean snow freeboards are 0 59 m 0 67 m 0 53 m and 0 60 m on sections a b c and d respectively three empirical equations and the buoyancy equation with zero ice freeboard assumption all give similar statistics in ice thickness estimation with mean ice thicknesses of 1 91 m for section a 2 16 m for b 1 76 m for c and 1 94 m for d the sea level over leads cannot be accurately resolved from the atm l2 data 60 m x 80 m horizontal averaging however by using a sea level reference obtained from the l1b data the atm l2 data can achieve reasonable ice thickness estimates with mean absolute difference of only 0 10 m compared to thickness derived from the l1b data

garnet peridotite from colorado plateau ultramafic diatremes hydrates carbonates and comparative geothermometry crystal fragments of pyrope from diatremes of ultramafic microbreccia in the navajo province of the colorado plateau contain inclusions of olivine pyroxene spinel chlorite amphibole chlorapatite and dolomite the included suite supports earlier hypotheses that hydrous phases and carbonates were primary parts of some garnet peridotite assemblages in the plateau lithosphere garnets with spinel and orthopyroxene inclusions likely all were sampled at pressures less than 36 kb and perhaps as low as 15 20 kb no evidence was found for inclusions from greater depths temperature estimates are 800 900 c for garnet clinopyroxene equilibration but only 500 700 c for garnetolivine equilibration inherent differences between geothermometry methods account for only part of the discrepancy pronounced fe mg zoning in garnet at olivine contacts and the lack of such zoning at clinopyroxene contacts are evidence that the difference in part relates to relative reequilibration rates with cooling the garnet olivine temperature estimates may be the best approximations to mantle temperatures before eruption our data are compatible both with the hypothesis that the garnet peridotite was emplaced in the mantle by large scale horizontal transport in the lithosphere and with the hypothesis that rocks were sampled from precambrian lithosphere cooled to temperatures like those along a low heat flow geotherm discordances between the geothermometers here and in other lherzolite localities may be keys to evaluating tectonic histories of lherzolite masses

paleontology of proterozoic shales and mudstones examples from the belt supergroup chuar group and pahrump group western usa abstract proterozoic black shales and other fine grained siliciclastic rocks contain a variety of fossils these fossils are illustrated by examples from three proterozoic units in the western usa the belt supergroup middle proterozoic the chuar group late proterozoic and the upper pahrump group late proterozoic a variety of microfossils and megafossils occur in the belt supergroup western montana acritarchs and filamentous microfossils are moderately well preserved in sub greenschist facies black shale in the little belt mountains the biologic affinity of the acritarchs is uncertain they could represent the vegetative or encystment stage of eukaryotic algae or the outer envelope of colonial coccoid cyanobacteria most of the filaments in these shales are of a size and morphology that is consistent with prokaryotic affinities however rare 20 and 40 m wide filaments could be either prokaryotic or eukaryotic higher grade sub greenschist facies dark gray to black mudstones in glacier national park contain poorly preserved acritarchs and spheroidal and filamentous pseudomicrofossils they are of interest because they provide information useful for distinguishing authentic microfossils from pseudofossils in mudstones and shales that have undergone substantial burial metamorphism several authentic megafossils occur in lower belt shales in the little belt and big belt mountains montana one of these is the carbonaceous compression beltina danai which could represent either fragments of a microbial mat or fragments of a sheet like megascopic alga the megafossil grypania spiralis has the form of a coiled ribbon and tentatively is interpreted as a megascopic alga green to purple mudstones and muddy sandstones in the lower belt supergroup in glacier national park contain markings that resemble a string of beads they are interpreted as megafossils that may have eukaryotic algal affinities black shales in the late proterozoic kwagunt formation chuar group grand canyon arizona contain a variety of acritarchs and filamentous microfossils of special interest is the vase shaped microfossil melanocyrillium which is interpreted as the sporangium of a eukaryotic and probably megascopic alga melanocyrillium also occurs in black mudstone in the late proterozoic pahrump group southeastern california

precambrian olistoliths masquerading as sills from death valley california olistolith production and magmatism are processes commonly associated with extensional tectonic settings such as rift basins we present a cautionary exemplar from one such precambrian basin in which we reinterpret metabasite bodies previously documented as sills to be olistoliths we nevertheless demonstrate that on the basis of field observation alone the previous but erroneous sill interpretation is parsimonious indeed it is only by using isotopic age and compositional analysis that the true identities of these metabasite olistoliths are revealed we present new data from metabasites and metasedimentary strata of the kingston peak formation cryogenian and crystal spring formation mesoproterozoic of death valley usa these include field observations u pb apatite ages u pb zircon ages detrital and igneous and whole rock geochemistry these data also provide a new maximum age for the base of the pahrump group and suggest that the crystal spring diabase was more tholeiitic than previously thought similar sill olistolith misinterpretations may have occurred elsewhere potentially producing erroneous age and tectonic setting interpretations of surrounding strata this is particularly relevant in precambrian rocks where fossil age constraints are rare this is illustrated herein using a potential example from the neoproterozoic literature of the lufilian belt africa we caution others against precambrian olistoliths masquerading as sills supplementary material details of a meta igneous boulder from p12 of the silurian hills la icp ms and whole rock geochemistry methods and standards and u pb apatite and zircon isotopic data including standards and selected cathodoluminescence images are available at https doi org 10 6084 m9 figshare c 3990639

origin of gneisses in the aureole of the san gabriel anorthosite complex and implications for the proterozoic crustal evolution of southern california granulite facies gneisses along the southwestern margin of the san gabriel anorthosite complex include compositionally banded locally migmatitic felsic gneisses and mafic granulites with rare interlayered augen gneiss and garnet hypersthene spinel aluminous gneiss previous workers accepted a 1 4 ga age for these granulites and a 1 2 ga age for the adjacent anorthosite complex and hence that this proterozoic basement is allochthonous with respect to the southwestern united states part of the north american craton solvus geothermometry in felsic and mafic granulites yields minimum metamorphic temperatures of 900 950 c and new u pb zircon data yield a minimum metamorphic age of 1186 ma coincident with that of the adjacent anorthosite we reinterpret these rocks as a 1190 ma anorthosite complex and its granulite grade thermal aureole geochemical data suggest that the anorthosite complex intruded early proterozoic rocks correlative with the mojave crustal province representing the north american craton exposed to the northeast we conclude that these proterozoic basement rocks represent the southwestern margin of the north american craton and not an exotic terrane accreted in mesozoic or tertiary time

successive substorm expansions during a period of prolonged northward interplanetary magnetic field 1 we have studied successive substorm expansions that occurred in spite of a period of prolonged northward interplanetary magnetic field imf after the northward imf persisted for almost 19 h a series of 11 very weak to moderate substorm expansions occurred during the following 6 h interval of northward imf on 19 january 1998 separated from each other by a few tens of minutes most of these substorm expansions did not accompany significant changes of the solar wind and the imf as for typical substorms that occur under southward imf conditions spacecraft and ground based instruments observed auroral breakups and enhancements of the westward auroral electrojet and large scale convection in the ionosphere together with plasmoids and dipolarizations in the magnetotail in association with the substorm expansion onsets these signatures suggest that the mechanism of substorm expansion onsets is the same as for typical substorms even under prolonged northward imf conditions we suggest that the large imf by affects large scale convection and energy accumulation in the magnetotail and that the enhanced convection due to the large imf by effect and substorm expansions can be an important factor for the promotion of succeeding substorm expansion onsets

models of basalt petrogenesis lower keweenawan diabase dikes and middle keweenawan portage lake lavas upper michigan lower keweenawan diabase dikes from marquette baraga counties and middle keweenawan portage lake lavas from upper michigan can each be subdivided into two chemically distinct groups a low tio2 p2o5 group characterized by higher a12o3 content higher mg ratio and lower total fe and ree abundances than a high tio2 p2o5 group both groups which are indistinguishable in the field are enriched in the lree relative to the hree and have similar normalized ree abundance patterns

petrogenesis of ultramafic xenoliths from the 1800 kaupulehu flow hualalai volcano hawaii the 1800 kaupulehu flow on hualalai volcano hawaii contains abundant xenoliths of dunite wehrlite and olivine clinopyroxenite with minor gabbro troctolite anorthosite and websterite the petrography and mineral compositions of 41 dunite wehrlite and olivine clinopytoxenite xenoliths have been studied and clinopyroxene separates from eight of these have been analyzed for ba k rb sr rare earth elements 87sr 6sr and 143nd 14 nd temperatures of equilibration obtained by olivine spinel and pyroxene geothermometry range from 1000 to 1200 c mineralogical data combined with published fluid inclusion data indicate depths of origin in the range of 8 30 km the rarity of orthopyroxene the presence of fe rich olivine fog 89 and clinopyroxene fs3 2x and the occurrence of high tio2 in spinel 0 9 2 8 wt and clinopyroxene 0 35 1 33 wt all indicate that the xenoliths are cumulates not residues from partial fusion the separated clinopyroxenes have 87sr 6sr 0 70348 0 70367 and nd nd o 51293 o 51299 values that are different from sr and nd isotope ratios of pacific abyssal basalts 0 5130 respectively also clinopyroxenes and spinels in the xenoliths have generally higher tio2 contents o 35 and 0 91 wt respectively than their counterparts in abyssal cumulates 0 40 and 0 70wt respectively these differences indicate that the xenoliths are not a normal component of oceanic crust because the xenoliths and alkalic to transitional hualalai lavas have similar values for cr cr al

large scale imaging of high latitude convection with super dual auroral radar network hf radar observations the hf radars of the super dual auroral radar network superdarn provide measurements of the e b drift of ionospheric plasma over extended regions of the high latitude ionosphere with the recent augmentation of the northern hemisphere component to six radars a sizable fraction of the entire convection zone approximately one third can be imaged nearly instantaneously 2 min to date the two dimensional convection velocity has been mapped by combining line of sight velocity measurements obtained from pairs of radars within common volume areas we describe a new method of deriving large scale convection maps based on all the available velocity data the measurements are used to determine a solution for the distribution of electrostatic potential expressed as a series expansion in spherical harmonics the addition of data from a statistical model constrains the solution in regions of no data coverage for low order expansions the results provide a gross characterization of the global convection we discuss the processing of the radar velocity data the factors that condition the fitting and the reliability of the results we present examples of imaging that demonstrate the response of the global convection to variations in the interplanetary magnetic field imf in the case of a sudden polarity change from northward to southward imf the convection is seen to reconfigure globally on very short 6 min timescales

paleomagnetism of middle proterozoic diabase sheets from central arizona paleomagnetic results from 1090 ma diabase sheets of the southwestern u s a diabase province exposed in central arizona yield two distinct remanent magnetizations herein termed adn and adr in accordance with the findings of previous investigations magnetization adn is well defined and has an in situ mean direction of d 283 3 i 45 1 k 17 7 95 8 7 n 17 independent observations a mean pole after correction of paleomagnetic site means for a net 5 clockwise rotation of the colorado plateau and transition zone is located at 22 7 n 179 3 e k 21 9 a95 7 8 the second magnetization adr gives an in situ mean direction of d 161 1 i 87 5 k 22 2 95 19 9 n 4 independent observations with a poorly defined pole at 37 6 n 247 6 e k 6 5 a95 38 9 rock magnetic and alternating field and thermal demagnetization characteristics indicate the adn and adr magnetizations are both carried by low ti titanomagnetite both magnetizations are interpreted to be primary

petrology and geochemistry of the early mesozoic caraquet dyke new brunswick canada the caraquet dyke of new brunswick is one of three early mesozoic dykes recognized in atlantic canada it has a quartz normative tholeiitic composition intermediate in character between low ti and high ti appalachian diabase major element e g mgo sio2 and trace element concentrations e g ba zr suggest that the dyke is less evolved than the shelburne dyke but slightly more evolved than the avalon dyke although related to continental rifting and opening of the atlantic ocean the dykes of the atlantic provinces show little in common with continental rift volcanic rocks they resemble morb except that their k rb and ba concentrations are much higher semi quantitative modelling suggests that the elevated large ion lithophile element concentrations reflect assimilation of continental crust differences in composition among the three dykes may be due to variations in the quantity and composition of assimilated material as well as the degree of differentiation as with other early mesozoic dyk

juxtaposed mesozoic diabase dike sets from the carolinas a preliminary assessment aeromagnetic anomaly maps of the north carolina piedmont provide confirming evidence of juxtaposed mesozoic diabase dike sets dikes in north carolina may be divided into four domains on the basis of dike orientations most dikes are olivine normative but there are apparent systematic differences in chemical compositions that are reflected in different orientations and ages two orientations dominate northwest and north south the northwest trending set dominates in the south carolina piedmont and the north south set extends through much of the central virginia piedmont and under the coastal plain in south carolina the north south dikes are part of a set that converges near charleston and georgetown south carolina and diverges northward into virginia preliminary information suggests that the north south dikes are younger than the more common northwest trending dikes current models proposed for stress regimes associated with the breakup of pangea can accommodate the northwest set but not the north south convergent set this convergent set may be evidence for crustal deformation above a mantle plume centered near charleston and georgetown in middle jurassic time it is possible that presumed mafic plutons under the coastal plain near charleston regional seismicity patterns and orientations of the diabase dikes are all related to an intersection of zones of crustal weakness inherited from mid mesozoic time the western margin of northwest trending dikes may be related to tectonic features inherited from pre mesozoic time

petrology and geochemistry of hawaiite lavas from crater flat nevada hawaiite type lavas were erupted in three cycles 3 7 1 2 and 0 3 m y at crater flat nevada the compositions of all three cycles considered together form a straddling alkalic series as defined by miyashiro in which the less evolved basalts plot near the normative olivine diopside divide and the more evolved basalts project into hypersthene or nepheline fields fractionation modeling based on the oldest cycle allows the removal of olivine clinopyroxene and amphibole to arrive at the more evolved hawaiite compositions in general fractionation of phlogo pite or feldspar is limited by the fractionation modeling and by eu ree relations in detail all hawaiites within one cycle 3 7 m y need not be derived from a single parent magma varied parentage is more evident between cycles although all cycles are consistently of hawaiite composition basalts of the youngest two cycles are generally enriched in trace elements superimposed on this enrichment is a lack of rb variation leading to rb sr ratios far lower than required to generate the high 87sr 86sr ratio 0 707 typical of basalts in this region the very low rb sr ratios limit processes that may lead to trace element enrichment during magma evolution cyclic recharge of a fractionating magma chamber decreased fractions of mantle melting leaving phlogopite in the residuum or an earlier event of metasomatic transport from phlogopite bearing mantle rocks into a phlogopite absent mantle assemblage might explain the observed trace element enrichment with low rb sr

peridotite xenoliths in silica rich potassic latite from the transition zone of the colorado plateau in north central arizona potassic latite in the transition zone of the colorado plateau near chino valley arizona contains abundant eclogite and amphibolite xenoliths and minor websterite and pyroxenite xenoliths one unit contains peridotite xenoliths analyzed samples have mg ratios of 68 and 71 58 and 63 wt sio2 and are enriched in potassium and other large ion lithophile lil elements rare earth element ree patterns are light ree enriched with la greater than 100 times chondritic abundance the peridotite xenoliths are partly to totally altered but contain remnant olivine orthopyroxene and clinopyroxene one harzburgite nodule also contains spinel mineral compositions from the xenoliths are relatively refractory and similar to those in other spinel peridotite xenoliths from the colorado plateau geothermometry on olivine spinel and two pyroxene pairs indicates equilibration temperatures of less than 800 c for the peridotite nodules the relatively low temperatures calculated from mineral equilibria are consistent with temperature estimates for other mantle nodules from under the colorado plateau

superposed epoch analysis of the ionospheric convection evolution during substorms imf by dependence we present superposed epoch analyses of the average ionospheric convection response in the northern and southern hemispheres to magnetospheric substorms occurring under different orientations of the interplanetary magnetic field imf observations of the ionospheric convection were provided by the super dual auroral radar network superdarn and substorms were identified using the far ultraviolet fuv instrument on board the imager for magnetopause to aurora global exploration image spacecraft we find that during the substorm growth phase the expected imf by dependent dawn dusk asymmetry is observed over the entire convection pattern but that during the expansion phase this asymmetry is retained only in the polar cap and dayside auroral zone in the nightside auroral zone the convection is reordered according to the local substorm electrodynamics with any remaining dusk dawn asymmetry being more closely related to the magnetic local time of substorm onset itself only weakly governed by imf by owing to the preponderance of substorms occurring just prior to magnetic midnight the substorm asymmetry tends to be an azimuthal extension of the dusk convection cell across the midnight sector a manifestation of the so called harang discontinuity this results in the northern southern hemisphere nightside auroral convection during substorms generally resembling the expected pattern for negative positive imf by when the preexisting convection pattern in the northern southern hemisphere is driven by positive negative imf by the nightside auroral convection changes markedly over the course of the substorm to establish this same harang configuration

petrogenesis of mid proterozoic granitic magmas examples from central and west texas abstract circa 1 1 ga granitic magmatism in texas was manifested as two compositional groups 1 the 1 12 ga red bluff granitic suite in west texas and 2 1 12 1 07 ga granites of the llano uplift of central texas both suites share some characteristics typical of anorogenic granites e g potassium and iron rich bulk compositions fe rich hydrous silicates emplacement conditions involving low oxygen fugacities and water contents and exhibit similar isotopic characteristics however rock associations mineral chemistries and trace element compositions of the two suites are distinct and no single petrogenetic model for the two suites is possible the red bluff granitic suite includes cogenetic syenites quartz syenites and granites transitional ferrobasaltic dikes are also present in contrast syenitic and mafic rocks are not associated with the llano granites the llano granites contain biotite and calcic amphibole with lower fe fe mg ratios compared to those occurring in the red bluff rocks alkali amphiboles e g arfvedsonite occur in the red bluff granites but not in the llano granites the red bluff granitoids are characterized by high feo t mgo ratios high na2o k2o high concentrations of hfse and rare earth elements ree and other features typical of a type within plate granites e g the pikes peak batholith ppb the llano granites are geochemically distinct with generally higher p2o5 and sr lower na2o feo t mgo zr y and ree and much lower ta and nb nd isotopic data overlap between the two granite suites and have juvenile signatures however trace element data suggest different petrogeneses for the two suites the red bluff suite is interpreted as having a direct derivation from mantle sources via extended fractional crystallization of basaltic parental magmas with minor crustal assimilation the llano granites appear to represent anatectic melts derived from slightly older juvenile crustal sources some melts underwent fractional crystallization controlled by feldspar and accessory minerals the petrology and geochemistry of 1 1 ga granites in texas indicate that they should not be considered as part of a single anorogenic magmatic event the red bluff granitic suite was emplaced into a shelf sequence north of the grenville front within a broad zone characterized by mild extension in contrast llano granites are late stage intrusions emplaced into multiply deformed and metamorphosed crust south of the grenville front during or after waning stages of grenville orogenesis

thermodynamics of slush and snow ice formation in the antarctic sea ice zone snow over antarctic sea ice is often flooded by brine or seawater particularly in spring forming slush and snow ice here we evaluate the representation of the thermodynamics of slush and snow ice formation in large scale sea ice models using laboratory experiments nacl solutions poured into grated ice in an isolated container scaling analysis highlights latent heat as the main term of the energy budget the temperature of the new sea ice immediately after flooding is found very close to the saltwater freezing point whereas its bulk salinity is typically 20 g kg large scale sea ice models faithfully represent such physics yet the uncertainty on the origin of flooding saltwater impacts the calculated new ice temperature because of the different salinities of seawater and brine the laboratory experiments also suggest a potential limitation to the existing physical representations of flooding for brine fractions 60 ice crystals start floating upon saltwater natural sea ice observations suggest that the isolated system assumption holds for a few hours at most after which rapid heat and salt exchanges mostly destroy the initial flooding signature on temperature and salinity a small footprint on ice salinity remains however natural snow ice is found 3 5 g kg more saline than other forms of sea ice

geochemistry of north shore hypabyssal dikes and sills in the midcontinent rift of minnesota an example the 47th avenue sill this study presents geochemical data for several of the numerous small to large dikes and sills including the 47th avenue sill exposed along the shore of lake superior in and north of duluth minnesota these intrusions are late magmatic features of the proterozoic midcontinent rift system and together form the north shore hypabyssal group the dikes are geochemically distinct from the sills and when the two are exposed together the younger dike intrudes the older sill dikes are primitive with mg up to 68 have positive end values and are oriented approximately north south with steep westerly or near vertical dips the older sills are more evolved usually have end values near or below 0 and have the same gentle easterly dip as the thick sequence of north shore volcanic group flows they intrude dike compositions correlate best with a mixture of widespread basalt compositions types 4 and 5 with primitive geochemistry and positive end values whereas sill compositions are similar to widespread ba