u pb ages of neoarchean granitoids from the black hills south dakota usa implications for crustal evolution in the archean wyoming province abstract zircons in basement rocks from the eastern wyoming province black hills south dakota usa have been analyzed by ion microprobe shrimp in order to determine precise ages of archean tectonomagmatic events in the northern black hills nbh near nemo phanerozoic and proterozoic meta sedimentary rocks are nonconformably underlain by archean biotite feldspar gneiss bfg and little elk gneissic granite leg both of which intrude older schists the archean granitoid gneisses exhibit a pervasive nw se trending fabric whereas an earlier ne sw trending fabric occurs sporadically only in the bfg which is intruded by the somewhat younger leg zircon crystals obtained from the leg and bfg exhibit double terminations oscillatory zoning and th u ratios of 0 6 0 3 thereby confirming a magmatic origin for both lithologies in situ analysis of the most u pb concordant domains yields equivalent 207 pb 206 pb ages upper intercept u pb concordia of 2559 6 and 2563 6 ma both 2 for the leg and bfg respectively which constrains a late neoarchean age for sequential pulses of magmatism in the nbh unzoned in bse patches of 2560 ma zircon commonly truncate coeval zonation in the same crystals with no change in th u ratio suggesting that deuteric fluid assisted recrystallization accompanied post magmatic cooling a xenocrystic core of magmatic zircon observed in one leg zircon yields a concordant age of 2894 6 ma 2 this xenocryst represents the oldest crustal material reported thus far in the black hills whether this older zircon originated as unmelted residue of 2900 ma crust that potentially underlies the black hills or as detritus derived from 2900 ma crustal sources in the wyoming province cannot be discerned in the southern black hills sbh the peraluminous granite at bear mountain bmg of previously unknown age intrudes biotite plagioclase schist zircon crystals from the bmg are highly metamict and altered but locally preserve small domains suitable for in situ analysis a u pb concordia upper intercept age of 2596 11 ma 2 obtained for zircon confirms both the late neoarchean magmatic age of the bmg and a minimum age for the schist it intrudes taken together these data indicate that the neoarchean basement granitoids were emplaced at 2590 2600 ma sbh and 2560 ma nbh most likely in response to subduction associated with plate convergence final assembly of supercontinent kenorland in contrast thin rims present on some leg bfg zircons exhibit strong u pb discordance high common pb and low th u ratios suggesting growth or modification under hydrothermal conditions as previously suggested for similar zircons from se wyoming the leg bfg zircon rims yield a nominal upper intercept date of 1940 2180 ma which may represent a composite of multiple rifting events known to have affected the nemo area between 2480 and 1960 ma together these observations confirm the existence of a paleoproterozoic rift margin along the easternmost wyoming craton moreover the 2480 1960 ma time frame inferred for rifting in the black hills nemo area corresponds closely to a 2450 2100 ma time frame previously inferred for the fragmentation of supercontinent kenorland

thermo rheological shear heating model for leucogranite generation metamorphism and deformation during the proterozoic trans hudson orogeny black hills south dakota abstract this paper evaluates thermotectonic models for metamorphism and leucogranite generation during the proterozoic trans hudson orogeny as recorded in rocks exposed in the black hills sd intrusion of the harney peak granite and associated pegmatites at 1715 ma occurred at the waning stages of regional deformation and staurolite grade regional metamorphism published consortium for continental reflection profiling cocorp results indicate that proterozoic sedimentary rocks were thrust over the archean wyoming province during the trans hudson collision isotopic compositions of the harney peak granite suggest that the exposed proterozoic and archean metasedimentary rocks in the black hills represent source rocks of the granites numerical simulations of the regional metamorphism and harney peak granite generation assuming crustal thickening by thrusting coupled with erosion show the following 1 doubling of the crust with normal distribution of radioactive elements does not yield sufficiently high temperatures to cause anatexis anywhere in the crust or growth of garnet in the now exposed part of the crust 2 a 35 km drop off length for internal heat production can yield sufficient temperature for garnet growth at the current erosion level it is however insufficient to produce staurolite and melting can occur only in the deepest parts of the crust 3 temperatures in crust with stable 70 km thickness for 40 ma and 35 km drop off length for heat production could become sufficient to produce staurolite at the current erosion level and subsequent rapid denudation of the crust could potentially trigger decompression melting of lower crustal rocks although this model could potentially explain the observed temporal relationship between regional metamorphism and leucogranite generation it is inconsistent with melting of upper crustal proterozoic source rocks that is indicated by isotopic compositions of the granites with lack of evidence for rapid denudation of the trans hudson orogen and with confinement of the leucogranites to the deformed proterozoic metapelitic rocks production of the harney peak granite and its relationship to regional metamorphism of the country rocks are best explained by shear heating at the interface between the wyoming province and overthrusted sedimentary rocks we suggest that with reasonable rheologic properties of metapelites and rates of plate convergence shear heating sufficiently perturbs locally the geotherms to cause anatexis in a deep shear zone system and growth of staurolite in the overlying crust modeling rheology of the lithologically stratified thickened crust with granitic basement and metapelitic upper plate shows that the currently exposed part of the crust and the granite source region were ductile through much of the orogeny which explains regional folding of the schists and predicts ductile shear zones in the granite source region because of the lithologic stratification the granitic basement is likely to become significantly weaker during crustal thickening than the upper crust dominated by schists a weak basement under a folded upper crust is likely to contribute to the observed relatively flat topography of high plateaus over thickened orogens

new constraints on the timing of early proterozoic tectonism in the black hills south dakota with implications for docking of the wyoming province with laurentia a question regarding the 1900 1600 ma assembly of laurentia is whether the wyoming province west central united states was part of the hearne province west central canada prior to the early proterozoic trans hudson orogeny or a separate entity welded later to a hearne superior continent central canada new 40 ar 39 ar mica dates help to address this question by extending a middle proterozoic geochronologic front long established along the southern wyoming province into the black hills of south dakota this suggests that previously unexplained north directed fold nappes f 1 in the black hills resulted from island arc accretion to the south ca 1780 ma north northwest trending upright f 2 folds which formed during east west collision of the wyoming and superior provinces must therefore be younger new 40 ar 39 ar hornblende dates recently published age data and crustal heat flow considerations further suggest that this collision began at or before ca 1770 ma and culminated with posttectonic magmatism beginning ca 1715 ma the harney peak granite this tectonic magmatic interval is 50 60 m y younger than that reported for the hearne superior collision trans hudson orogeny in canada comparably young metamorphic dates 1810 1710 ma also typify the eastern and northern wyoming province periphery western dakotas and southwestern montana collectively these data suggest that the hearne and wyoming provinces were once separate continents that were ultimately welded to the superior province and to each other during distinct early proterozoic orogenies regional relationships further suggest that final docking of the eastern wyoming province with laurentia began during the ca 1780 1740 ma interval of island arc accretion along the southern margin of the growing craton

does reactive surface area depend on grain size results from ph 3 25 c far from equilibrium flow through dissolution experiments on anorthite and biotite abstract laboratory determined mineral weathering rates need to be normalised to allow their extrapolation to natural systems the principle normalisation terms used in the literature are mass and geometric and bet specific surface area ssa the purpose of this study was to determine how dissolution rates normalised to these terms vary with grain size different size fractions of anorthite and biotite ranging from 180 150 to 20 10 m were dissolved in ph 3 hcl at 25 c in flow through reactors under far from equilibrium conditions steady state dissolution rates after 5376 h anorthite and 4992 h biotite were calculated from si concentrations and were normalised to initial and final mass and geometric geometric edge biotite and bet ssa for anorthite rates normalised to initial and final bet ssa ranged from 0 33 to 2 77 10 10 mol feldspar m 2 s 1 rates normalised to initial and final geometric ssa ranged from 5 74 to 8 88 10 10 mol feldspar m 2 s 1 and rates normalised to initial and final mass ranged from 0 11 to 1 65 mol feldspar g 1 s 1 for biotite rates normalised to initial and final bet ssa ranged from 1 02 to 2 03 10 12 mol biotite m 2 s 1 rates normalised to initial and final geometric ssa ranged from 3 26 to 16 21 10 12 mol biotite m 2 s 1 rates normalised to initial and final geometric edge ssa ranged from 59 46 to 111 32 10 12 mol biotite m 2 s 1 and rates normalised to initial and final mass ranged from 0 81 to 6 93 10 12 mol biotite g 1 s 1 for all normalising terms rates varied significantly p 0 05 with grain size the normalising terms which gave least variation in dissolution rate between grain sizes for anorthite were initial bet ssa and initial and final geometric ssa this is consistent with 1 dissolution being dominated by the slower dissolving but area dominant non etched surfaces of the grains and 2 the walls of etch pits and other dissolution features being relatively unreactive these steady state normalised dissolution rates are likely to be constant with time normalisation to final bet ssa did not give constant ratios across grain size due to a non uniform distribution of dissolution features after dissolution coarser grains had a greater density of dissolution features with bet measurable but unreactive wall surface area than the finer grains the normalising term which gave the least variation in dissolution rates between grain sizes for biotite was initial bet ssa initial and final geometric edge ssa and final bet ssa gave the next least varied rates the basal surfaces dissolved sufficiently rapidly to influence bulk dissolution rate and prevent geometric edge ssa normalised dissolution rates showing the least variation simple modelling indicated that biotite grain edges dissolved 71 132 times faster than basal surfaces in this experiment initial bet ssa best integrated the different areas and reactivities of the edge and basal surfaces of biotite steady state dissolution rates are likely to vary with time as dissolution alters the ratio of edge to basal surface area therefore they would be more properly termed pseudo steady state rates only appearing constant because the time period over which they were measured 1512 h was less than the time period over which they would change significantly

4 d constrained depth conversion for reservoir compaction estimation application to ekofisk field ekofisk field in the norweigian sector of the north sea is a remarkable example of a strongly compacted chalk reservoir since the start of production in 1971 the water bottom has subsided by over 8 m as a result of reservoir compaction hermansen et al the field wide waterflood begun in 1989 was expected to halt pressure depletion and reservoir compaction after a few years of waterflooding ekofisk reservoir pressure began increasing but compaction and subsidence continued laboratory studies on chalk cores indicated that high porosity oil filled chalk compacts strongly when flushed with seawater these studies along with reservoir flow simulation and geomechanic modeling led to a model of a water weakening compaction sylte et al ongoing efforts deployed to monitor the reservoir compaction and seabed subsidence include twice a year compaction logging in a monitoring well daily gps measurements at the platforms and periodic bathymetry surveys nine surveys from 1970 to 1999

four dimensional seismology and true direct detection of hydrocarbons the petrophysical basis figure 1 published by m king hubbert in 1962 and again in 1969 shows the exploitation of fossil fuels in a historical perspective according to this graph about half of the fossil energy on earth is being used up in a period of about 200 years half of which is by now over

nd o and sr isotopic constraints on the origin of precambrian rocks southern black hills south dakota abstract the nd o and sr isotopic characteristics of precambrian metasedimentary metavolcanic and granitic rocks from the black hills of south dakota are examined two late archean granites 2 5 2 6 ga have t dm ages of 3 05 and 3 30 ga suggesting that at least one of the granites was derived through the melting of significantly older crust early proterozoic metasedimentary rocks have t dm ages that range from 2 32 to 2 45 ga these model ages in conjunction with probable stratigraphic ages ranging from 1 9 to 2 2 ga indicate that mantle derived material was added to the continental crust of this region during the early proterozoic previous studies of the harney peak granite complex have reported u pb and rb sr ages of about 1 71 ga and most granite samples examined in this study have sr isotopic compositions consistent with that age two granite samples taken from the same sill however give two point rb sr and sm nd ages of 2 08 0 08 and 2 20 0 20ga 2200 nd 15 5 respectively in addition whole rock and apatite samples of the spatially associated tin mountain pegmatite give a sm nd isochron age of 2000 100 ma 2200 nd 5 8 1 8 the sm nd o and rb sr isotopic systematics of these granitic rocks have been complicated to some degree by both crystallization and post crystallization processes and the age of the pegmatite and parts of the harney peak granite complex remain uncertain processes that probably complicated the isotopic systematics of these rocks include derivation from heterogeneous source material assimilation mixing of ree between granite and country rock during crystallization via a fluid phase and post crystallization mobility of sr the nd isotopic compositions of the pegmatite and the harney peak granite indicate that they were not derived primarily from the exposed metasedimentary rocks

petrogenesis of early proterozoic pelitic schists of the southern black hills south dakota constraints on regional low pressure metamorphism the late archean to early proterozoic terrain exposed in the black hills south dakota records a history of deformation metamorphism and igneous activity extending from 2 5 to 1 7 ga metamorphism began during crustal thickening however maximum temperatures were attained after crustal thickening during a regional low pressure high temperature event coinciding with the emplacement of the harney peak granite at 1 7 ga metamorphic grade increased over a distance of 10 to 15 km from garnet grade in the northern part of the terrain to sillimanite k feldspar grade in the southern part isograds mapped in the terrain include the first appearance of staurolite biotite andalusite biotite sillimanite sillimanite biotite garnet and sillimanite k feldspar in the extreme western exposure of the terrain kyanite biotite occurs instead of andalusite biotite geobarometry indicates that metamorphism occurred between 2 0 and 4 4 kbar geothermometry indicates that temperature increased from 469 500 c in the garnet zone to 528 555 c in the sillimanite zone on the northern flank of the harney peak granite higher temperatures are recorded along the southern flank of the granite in the sillimanite biotite garnet zone 585 c and in the sillimanite k feldspar zone 662 c the distributions of isograds and isotherms indicate that the harney peak granite was at the locus of a regional thermal high and that doming during granite emplacement was partly responsible for exposure of the low pressure high temperature metamorphic terrain

petrogenesis and tectonic implications of paleoproterozoic mafic rocks in the black hills south dakota abstract four suites of mafic to intermediate intrusions and flows that occur in the precambrian core of the black hills were analyzed for major and trace elements to establish tectonic settings of magmatism during the late archean and early proterozoic history of the region most of the mafic rocks were later metamorphosed to amphibolites during the black hills orogeny the collision of the wyoming and superior cratons which began at 1760 ma compositions of the mafic suites define at least two distinct tectonic settings of magmatism between 2480 and 1880 ma the 2 48 ga blue draw intrusion located on the eastern edge of the precambrian core is differentiated with compositions ranging from gabbro to granodiorite it is a calcalkaline intrusion with relatively high sio2 from 51 8 to 60 5 wt and k2o mostly 1 wt ree patterns are slightly concave upward and show lree enrichment the intrusion is also enriched in ba rb th and zr these and other geochemical discriminants are typical of modern calcalkaline basalts the intrusion s calcalkaline character is similar to the neighboring slightly older 2 56 ga little elk granite gneiss these two intrusions support the presence of late archean early proterozoic arc magmatism on the eastern margin of the wyoming craton the three other mafic suites comprising metamorphosed flows and sills occur within a sequence of deep marine sediments including black shales limestones iron formations and cherts u pb zircon ages suggest their emplacement between 2 01 and 1 88 ga mafic rocks in the eastern half of the precambrian core in the mt rushmore and pactola dam quadrangles and near bear mountain on the western margin of the precambrian core have slightly depleted to enriched tholeiitic compositions with sio2 50 wt and k2o generally 2 5 wt and the lowest sio2 concentrations typically

geochronology and geochemistry of precambrian gneisses metabasites and pegmatite from the tobacco root mountains northwestern wyoming craton montanathis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh t e krogh deceased april 2008 the middle mountain metamorphic domain of the montana metasedimentary terrane northwestern wyoming craton within the northwestern tobacco root mountains mainly comprises migmatized tonalitic gneiss interlayered with amphibolitic hornblende gneiss both of which are cut by metamorphosed mafic rocks together these gneisses are defined as middle mountain gneiss archean tonalitic gneiss from west of and amphibolitic gneiss from east of the bismark fault give from chemically and air abraded zircon grains u pb id tims ages of 3325 5 1 7 and 3340 ma respectively these results reflect primary magmatic ages and show that the middle mountain gneiss extends into the northern area of the central fault block between the bismark and mammoth faults older crustal processes in the tonalitic gneiss are evidenced by inherited grains the oldest of which is 3460 ma a metabasite hosted in tonalitic gneiss in the bismark fault selvage zone yields a zircon age of 2468 ma which is interpreted as the time of

mantle and slab contributions in arc magmas destructive plate margins are major sites of terrestrial magmatism that have long had a key role in models for the generation of continental crust and the development of chemical heterogeneities in the upper mantle in these models it is important to evaluate the nature and size of contributions from preexisting crust in the generation of magmas above subduction zones and to constrain the size of the fluxes of different elements from the subducted crust and the overlying mantle wedge in arc magmas kay 1 980 hawkesworth ellam 1 989 hawkesworth et al 1 99 1 a mcculloch gamble 1 99 1 the more mobile components of the subducted oceanic crust are the products of element fractionation processes in the upper continental crust the ocean basins and hydrothermal systems on the ocean floor they therefore have distinctive isotope and even trace element ratios that in principle should be readily distinguished from those in peridotite in the mantle wedge prior to subduction in practice however element ratios are almost certainly fractionated during shallow level dehydration of sub ducted material and at greater depths where material from the slab appears to contribute in the generation of arc magmas moreover the isotope signal of slab derived material may be heavily diluted by exchange with

rocks under strain strain induced time lapse time shifts are observed for depleting reservoirs pressure depletion as a result of oil and gas production creates changes in the stress and strain fields of the rock material both inside and outside the reservoir seismic waves propagating through these rocks have different traveltimes before and after depletion because the layer thicknesses change and the seismic velocity is altered by the changes in the stress and strain fields the cumulative change in two way traveltimes with depth is referred to as time lapse time shift

rock physics analysis and time lapse rock imaging of geochemical effects due to the injection of co2 into reservoir rocks the fundamental concept of time lapse seismic monitoring is that changes in physical parameters such as saturation pore fluid pressure temperature and stress affect rock and fluid properties which in turn alter the seismic velocity and density increasingly however time lapse seismic monitoring is called upon to quantify subsurface changes due in part to chemical reactions between injected fluids and the host rocks this study springs from a series of laboratory experiments and high resolution images assessing the changes in microstructure transport and seismic properties of fluid saturated sandstones and carbonates injected with co2 results show that injecting co2 into a brine rock system induces chemo mechanical mechanisms that permanently change the rock frame injecting co2 into brine saturated sandstones induces salt precipitation primarily at grain contacts and within small pore throats in rocks with porosity lower than 10 salt precipitation reduces permeability and increases p and s wa

electron probe ultrachron microchronometry of metamorphic monazite unraveling the timing of polyphase thermotectonism in the easternmost wyoming craton black hills south dakota a metapelite from the easternmost wyoming craton black hills south dakota has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800 1700 ma assembly of southern laurentia three deformational fabrics are recognized in oriented thin sections an ene trending s 1 fabric preserved as oblique inclusion trails in garnet porphyroblasts a nnw trending s 2 fabric preserved as microlithons in the rock matrix and a flattening fabric s 3 which transposed s 1 s 2 and dominates the matrix a complex monazite porphyroblast has been analyzed in situ with the electron microprobe ultrachron to constrain the timing of s 1 s 3 fabric formation associated with monazite growth the core of this grain uniquely preserves the s 1 s 2 fabrics as sigmoidal inclusion trails the mean total pb age of this domain is 1750 10 ma all dates reported at 95 confidence n 39 spots which is equivalent to the published 2 0 7 pb 2 0 6 pb age for the same domain these results validate the total pb dating method in general and the ultrachron in particular for reliable age determination in low th monazite and are interpreted as 1750 ma minimum ages for the s 1 s 2 fabrics and sequential d 1 d 2 collisional events that imposed them n directed arc accretion and e w continental collision respectively a higher th y rim of this same rosetta grain truncates the s 1 s 2 sigmoid and is associated with resorption textures in garnet porphyroblasts coupled release of y and an s 3 fabric that pervasively overprinted s 1 s 2 in the rock matrix the mean ultrachron date of this domain is 1692 5 ma n 17 spots which is slightly younger that the published isotopic age for all monazite rims combined these results support a 1715 1690 ma timeframe for localized doming d 3 related to granite magmatism the onset of which has been dated independently at 1715 3 ma the timing of post d 3 cooling through 350 and 300 c is constrained by 4 0 ar 3 9 ar dates of 1610 and 1480 ma obtained for separates of d 3 matrix muscovite and biotite respectively which are interpreted as closure ages this study shows that fabrics in poly deformed rocks can be dated by linking monazite spot ages to key microtextures further the results of this micrometer scale study enhance previous knowledge of local thermotectonism black hills and regional terrane assembly laurentia

hydraulic and acoustic properties as a function of porosity in fontainebleau sandstone laboratory measurements have been made of the permeability k free porosity l compressional velocities vp or ve and compressional attenuations qp or qe in fontainebleau sandstone over a continuous range of porosities from 3 to 28 this large variation was achieved without any composition change fontainebleau sandstone is made of fine quartz grains with regular grain size 250 m permeability was measured with a falling head permeameter velocities and attenuations were obtained either through an ultrasonic experiment for frequencies around 500 khz or through a resonant bar technique experiment for frequencies around 5 khz and in both cases with varying water saturation the results show an excellent correlation between permeability k and total porosity for all our samples for low porosities 3 to 9 permeability in millidarcies is 2 75 10 5 7 33 while for high porosities 9 to 28 permeability k in millidarcies is given by 0 303 3 05 the correlation is also excellent between free porosity and total porosity on the other hand the correlation between acoustic properties and total porosity is not as clear as for hydraulic properties whatever the frequency 500 khz or 5 khz or the water saturation on the average velocity decreases and attenuation roughly increases with increasing total porosity velocity and attenuation values are related to the variation of grain contact structure and two samples with the same porosity and permeability may exhibit different velocities and attenuations the clear correlation between hydraulic properties and porosity is related to constant grain size while the lack of correlation for acoustic properties emphasizes the importance of the microstructure

the generation and crystallization conditions of the proterozoic harney peak leucogranite black hills south dakota usa petrologic and geochemical constraints the mineralogy petrology and geochemistry of the proterozoic harney peak granite black hills south dakota were examined in view of experimentally determined phase equilibria applicable to granitic systems in order to place constraints on the progenesis of peraluminous leucogranites and commonly associated rare element pegmatites the granite was emplaced at 3 4 kbar as multiple sills and dikes into quartz mica schists at the culmination of a regional high temperature low pressure metamorphic event principally along the periphery of the main pluton and in satellite intrusions the sills segregated into granite pegmatite couplets the major minerals include quartz k feldspar sodic plagioclase and muscovite biotite mg no molar mgo mgo feo 0 32 0 38 is the predominant ferromagnesian mineral in the granite s core whereas at the periphery of the main pluton and in the satellite intrusions tourmaline mg no 0 18 0 48 is the dominant ferromagnesian phase almandine spessartine garnet is also found in the outer intrusions there is virtually a complete overlap in the wide concentration ranges of sio2 cao mgo feo sr zr w of the biotite and tourmaline bearing granite suites with no discernable differentiation trends on harker diagrams precluding the derivation of one suite from the other by differentiation following emplacement this is consistent with the oxygen isotope compositions which are 11 5 0 6 for the biotite granites and 13 2 0 8 for the tourmaline granites suggesting derivation from different sources the concentrations of tio2 and possibly ba are higher and of mno and b are lower in the biotite granites the normative orthoclase albite ratio is extremely variable ranging from 0 26 to 1 65 in the biotite granites to 0 01 1 75 in the tourmaline granites very few sample compositions fall near the high pressure watersaturated haplogranite minima eutectic trend indicating that the granites for the most part are not minimum melts generated under conditions with a text h text 2 text o 1 instead most biotite granites are more potassic than the water saturated minima and eutectics and in analogy with experimentally produced granitic melts they are best explained by melting at 6 kbar a text h text 2 text o 1 and temperatures 800 c such high temperatures are also indicated by oxygen isotope equilibration among the constituent minerals nabelek et al 1992 several of the tourmaline granite samples contain virtually no k feldspar and have oxygen isotope equilibration temperatures 716 775 c therefore they must represent high temperature accumulations of liquidus minerals crystallized under equilibrium conditions from melts more sodic than the water saturated haplogranite minima or during fractionation of intruded melts into granite pegmatite couplets accompanied by volatile aided differentiation of the alkali elements the indicated high temperatures a text h text 2 text o 1 the relatively high tio2 and ba concentrations and the relatively low partial 18 text o values of the biotite granites suggest that they were generated by high extent biotite dehydration melting of an immature archean metasedimentary source the ascent of the hot melts may have triggered low extent muscovite dehydration melting of schists higher in the crust producing the high b low ti melts comprising the periphery of the main pluton and the satellite intrusions alternatively the different granite types may be the result of melting of a vertical section of the crust in response to the ascent of a thermal pulse with the low partial 18 text o biotite granites generated at a deeper hotter region and the high partial 18 text o tourmaline granites at a higher cooler region of the crust the low ti and high b concentrations in the high partial 18 text o melts resulted in the crystallization of tourmaline rather than biotite which promoted the observed differentiation of the melts into the granitic and pegmatitic layers found along the periphery of the main pluton and the satellite intrusions

archean rocks of the black hills south dakota reworked basement from the southern extension of the trans hudson orogen the little elk and the bear mountain terranes on the margin of the precambrian core of the black hills provide the only known exposures of archean rocks between the archean wyoming and superior provinces the little elk terrane consists of a supracrustal sequence dominated by biotite feldspar gneiss meta arkose or metatuff that is intruded by the slightly metaluminous to peraluminous calc alkaline little elk granite which has a u pb intercept age of 2 549 11 m y a biotite plagioclase schist metagraywacke dominates the bear mountain terrane and is intruded subparallel to foliation by the peraluminous granite at bear mountain an isochron calculated from six severely discordant u pb zircon analyses yields an age of 2 393 230 m y the archean terranes participated in the trans hudson orogenic event suggested by a published rb sr age for the little elk granite 1 850 m y 1 87 1 97 gaigneous activity in the black hills and the similarity in orientation of the proterozoic deformational fabric with geophysical anomalies interpreted to be associated with the trans hudson orogen the metamorphic conditions estimated for the archean rocks may be the result of a thermal event that produced the proterozoic harney peak granite and thermal high northeast of lead south dakota the unexposed precambrian basement is probably dominated by granites similar to the little elk granite and supracrustal rocks consisting of continental margin deposits based on the ree contents of the archean schists and on the heavy mineral assemblages in the uraniferous nature of and the metasedimentary clasts in the lower proterozoic box elder creek formation the ages and the relatively long lived crustal sources for these archean granites and the similarity of the inferred basement characteristics to those of rocks in the wyoming province suggest that the black hills archean rocks may be the easternmost exposures of this province the probable trans hudsonian reworking of the black hills proterozoic rocks and their similarity to those in northern saskatchewan imply that the black hills occupies a tectonic position comparable to that of rocks described from the cree lake zone of saskatchewan

the cheyenne belt analysis of a proterozoic suture in southern wyoming abstract the cheyenne belt of southeastern wyoming is a major shear zone which separates archean rocks of the wyoming province to the north from 1800 1600 ma old eugeoclinal gneisses to the south miogeoclinal rocks 2500 2000 ma old unconformably overlie archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift formed continental margin intrusive tholeiitic sills and dikes are interpreted as rift related intrusions and a date of 2000 ma on a felsic differentiate of these intrusions gives the approximate age of rifting there are no known post 2000 ma felsic intrusions north of the cheyenne belt volcanogenic gneisses and abundant syntectonic calc alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks the volcanics are a bimodal basalt rhyolite assemblage plutons include large gabbroic complexes and quartz diorite 1780 ma syntectonic granitoids 1730 1630 ma and post tectonic anorthosite and granite 1400 ma there is no evidence for archean crust south of the cheyenne belt structural data thrusts in the miogeoclinal rocks vertical stretching lineations and the same fold geometries north and south of the shear zone suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane island arc over the northern terrane craton and miogeocline probably as a continuation of the south dipping subduction which generated calc alkaline plutons of the southern terrane a metamorphic discontinuity across the shear zone with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south also suggests thrusting of the southern terrane deeper crustal levels over the northern terrane shallower levels the cheyenne belt may be a deeply eroded master decollement perhaps analogous to a ramp in the master decollement in the southern appalachians this interpretation of the cheyenne belt as a proterozoic suture zone provides an explanation for the geologic geochronologic geophysical metallogenic and metamorphic discontinuities across the shear zone

geochemistry and origin of archean granites from the black hills south dakota the little elk granite 2549 ma and granite at bear mountain bmg 2 5 ga of the black hills formed as a result of a collisional event along the eastern margin of the wyoming province during th

formation dry out from co2 injection into saline aquifers 1 effects of solids precipitation and their mitigation f ormation d ry o ut from co 2 i njection into s aline a quifers p art 1 e ffects of s olids p recipitation and their m itigation karsten pruess 1 and nadja muller 2 earth sciences division lawrence berkeley national laboratory university of california berkeley ca 94720 shell international exploration and production b v exploratory research rijswijk the netherlands abstract injection of co 2 into saline aquifers may cause formation dry out and precipitation of salt near the injection well which may reduce formation porosity permeability and injectivity this paper uses numerical simulation to explore the role of different processes and parameters in the salt precipitation process and to examine injection strategies that could mitigate the effects the main physical mechanisms affecting the dry out and salt precipitation process include 1 displacement of brine away from the injection well by injected co 2 2 dissolution evaporation of brine into the flowing co 2 stream 3 upflow of co 2 due to gravity effects buoyancy 4 backflow of brine towards the injection point due to capillary pressure gradients that oppose the pressure gradient in the co 2 rich gas phase and 5 molecular diffusion of dissolved salt the different mechanisms operate on a range of spatial scales co 2 injection at constant rate into a homogeneous reservoir with uniform initial conditions is simulated in 1 d radial geometry to resolve multi scale processes by taking advantage of the similarity property i e the evolution of system conditions as a function of radial distance r and time t depends only on the similarity variable r 2 t simulations in 2 d vertical cross sections are used to examine the role of gravity effects we find that counterflow of co 2 and brine can greatly increase aqueous phase salinity and can promote substantial salt precipitation even in formations with low dissolved solids salt precipitation can accentuate effects of gravity override we find that injecting a slug of fresh water prior to commencement of co 2 injection can reduce salt precipitation and permeability loss near the injection well

the reappearance of the huronian in wyoming rifting and drifting of ancient continents striking stratigraphic and sedimentological similarities between the early proterozoic huronian supergroup of the canadian shield and the snowy pass supergroup of wyoming suggest that they were deposited in a single broad epicratonic basin developed atop a large archean continent that included the superior and wyoming geological provinces breakup of the continent after the 2 2 ga intrusion of widespread gabbro sheets and dykes resulted in the separation of the archean superior and wyoming cratons and their early proterozoic covers these crustal fragments were subsequently reassembled during early proterozoic 1 85 ga orogenesis the end result being the present 2000 km separation of the huronian and snowy pass supergroups and their archean basements

the geochemistry of mafic gneisses from the renzy terrane western grenville province quebec implications for the geodynamic setting of the early mesoproterozoic laurentian margin abstract this study presents new geochemical data on mafic gneisses from the renzy terrane rt located in the grenvillian parautochthonous belt of western quebec this region is of particular interest since it contains one of the rare ni cu co massive sulfide deposits ever mined in the grenville province rt mafic gneisses have been divided into 3 groups based mainly on major element compositions trace element profiles normalized to the primitive mantle and incompatible trace element ratios the behaviour of high field strength elements hfse ta zr hf ti th and large ion lithophile elements lile ba rb are particularly useful for discriminating rt mafic gneisses major and trace element compositions of group 1 mafic ultramafic gneisses are very similar to those of the rt ultramafic sheets the mafic gneisses are therefore interpreted as melagabbroic sills cogenetic with these sheets the trace element composition of group 1 mafic gneisses is typical of magmas formed from a previously metasomatized depleted mantle source these rocks have high th ta ratios and show strong to moderate negative anomalies in ti zr and hf on multi element plots normalized to the primitive mantle mafic gneisses of group 2 show geochemical features transitional between extensional and compressional settings these rocks generally have low concentrations of hfse and high concentrations of lile typical features of mafic rocks formed in a compressional environment however some of them are fe ti rich with minor or no negative anomalies in hfse and low to moderate concentrations of lile on multi element diagrams normalized to primitive mantle features more typical of an extensional environment mafic gneisses of group 3 have a geochemical signature typical of extensional environments or of ocean island basalt oib these gneisses have no negative anomalies in hfse and have low concentrations of lile rt mafic gneisses are similar in composition to mafic granulites in the bondy gneiss complex in the central metasedimentary belt and to shawanaga domain amphibolites from the central gneiss belt two paleoarc back arc environments in the grenville province based on the contrasting geochemical signatures found in the rt mafic gneisses and the similarities with known arc back arc settings in the grenville province we propose that the rt was formed by arc back arc magmatic activity most likely associated with a period of arc magmatism along the proterozoic laurentian margin oib like mafic gneisses found in the rt are similar to those from the kipawa region formed by intraplate continental magmatism in the parautochthonous belt of the grenville province

chemostratigraphy of paleoproterozoic carbonate successions of the wyoming craton tectonic forcing of biogeochemical change the archean wyoming craton is flanked on the south and east by belts of paleoproterozoic supracrustal successions whose correlation is complicated by lack of geochronologic constraints and continuous outcrop however carbonate units in these successions may be correlated by integrating carbon isotope stratigraphy with lithostratigraphy the 10 km thick paleoproterozoic snowy pass supergroup in the medicine bow mountains was deposited on the present day southern flank of the wyoming craton it contains three discrete levels of glacial diamictite correlative with those in the huronian supergroup on the southern margin of the superior craton the nash fork formation of the upper snowy pass supergroup is significantly younger than the uppermost diamictite and was deposited after the end of the paleoproterozoic glacial epoch carbonates at the base of the nash fork formation record remarkable 13 cenrichment up to 28 v pdb whereas those from overlying members of the lower nash fork formation have d 13 c values between 6 and 8 carbonates from the upper nash fork formation above the carbonaceous shale have carbon isotope values ranging between 0 and 2 5 the transition from high carbon isotope values to those near 0 in the nash fork formation is similar to that at the end of the ca 2 2 2 1 ga carbon isotope excursion in fennoscandia this chemostratigraphic trend and deposition of bifs mn rich lithologies carbonaceous shales and phosphorites at the end of the global ca 2 2 2 1 ga carbon isotope excursion are likely related to ocean overturn associated with the final breakup of the kenorland supercontinent correlative carbonates from the slaughterhouse formation in the sierra madre wy and from the whalen group in the rawhide creek area in the hartville uplift wy have highly positive carbon isotope values in contrast carbonates from other exposures of the whalen group in the hartville uplift and all carbonate units in the black hills sd have carbon isotope values close to 0 combined with existing geochronologic and stratigraphic constraints these data suggest that the slaughterhouse formation and the succession exposed in the rawhide creek area of the hartville uplift are correlative with the lower and middle nash fork formation and were deposited during the ca 2 2 2 1 ga carbon isotope excursion the estes and roberts draw formations in the black hills and carbonates from other exposures in the hartville uplift postdate the ca 2 2 2 1 ga

the east bull lake intrusive suite remnants of a 2 48 ga large igneous and metallogenic province in the sudbury area of the canadian shield the 2 49 to 2 475 ga intrusions of the east bull lake intrusive suite occur in an east northeast trending discontinuous belt along the presently exposed boundary between the archean superior and the proterozoic southern provinces of the canadian shield near sudbury ontario the east bull lake intrusive suite is part of a regional paleoproterozoic magmatic event extending from 2 49 to 2 44 ga which also includes bimodal volcanic rocks felsic plutons and the regionally extensive hearst and matachewan dike swarms this regional magmatic event is thought to be the result of a mantle plume driven intracontinental rifting event that led to the development of a major basin to the south that was subsequently filled by sedimentary rocks of the huronian supergroup the east bull lake intrusive suite appears to have been emplaced along a major axial rift fault related to this rifting event compelling field and geochemical evidence summarized here indicates that the three largest east bull lake suite intrusions the east bull lake agnew lake and river valley intrusions crystallized from similar low ti high al tholeiitic parent magmas that originated in deeper more basic chambers it is proposed that the primary magmas for the east bull lake suite intrusions were second stage melts derived from a sublithospheric depleted mantle source that had been modified by neoarchean subduction and accretion events most of the petrological characteristics of the east bull lake suite intrusions reflect plagioclase dominated fractional crystallization that generated a pronounced fe enrichment trend in the residual magmas orthopyroxene is the high temperature pyroxene and occurs as a cumulus phase through most of the stratigraphy of each intrusion of the suite local olivine rich cumulates present in all of the major east bull lake suite plutons in some instances appear to be comagmatic with associated plagioclase rich cumulates a distinctive marginal facies comprising brecciated and locally thermally recrystallized and or partially melted footwall rocks is a common feature of the east bull lake suite plutons and indicates that a high energy flow regime existed during the initial stages of emplacement this marginal unit typically grades into a heterolithic inclusion rich gabbronorite that contains erratically distributed mafic autoliths and footwall derived xenoliths chemical contamination of the mineralized matrix of these inclusion bearing rocks can be extreme at a local scale the inclusion rich zone is overlain by a thick interval of undifferentiated plagioclase rich cumulates that locally display spectacular glomerocrystic textures rhythmically and irregularly modally layered leucogabbronorite and gabbronorite make up much of the middle parts of the stratigraphy massive to crudely layered ferrogabbro and ferrosyenite only occur in the uppermost part of the agnew lake intrusion varitextured gabbro and pegmatitic gabbro occur as irregular layers and local veins and pods throughout the stratigraphy the east bull lake suite is currently being explored for contact type platinum group element pge cu ni mineralization that has a clear spatial association with orthopyroxene rich cumulates which are otherwise poorly represented in these intrusions disseminated pge and cu rich sulfide mineralization is concentrated within the inclusion bearing zones but extends upward into overlying plagioclase cumulates and appears to have collected along the lower margins of these intrusions through the combined action of vigorous convection of the resident magma and downward percolation of dense mafic residual liquid the resultant narrow zones of sulfide mineralization were thus derived from a much larger volume of parent magma feeder dikes to the mineralized parts of the intrusions appear to have been saturated in sulfide upon emplacement and had relatively high background pge contents compared to those for other known magmatic pge deposits however the economic potential of the contact type mineralization hinges on the efficacy of the physical concentration process which upgraded the metal content and size of the original sulfide particles inside the magma chambers based on the proposed basal accumulation model the best prospective mineralization is most likely to occur in embayments along the sidewall and floor of the intrusions

proterozoic collisional tectonism in the trans hudson orogen saskatchewan isotopic and structural data from the juvenile reindeer zone of the trans hudson orogen northern saskatchewan indicate a pre 1 85 ga thermotectonic event possibly reflecting arc continent collision followed by a more extensive nappe forming ca 1 83 1 80 ga thermotectonism during terminal continent continent collision preliminary data from the adjacent ensialic cree lake zone suggest high grade reworking of archean crust by the pre 1 85 ga event in the rae province to the west high grade metamorphism and reworking of archean crust occurred about 2 0 ga and may be related to the formation of the coeval taltson magmatic zone

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

2480 ma mafic magmatism in the northern black hills south dakota a new link connecting the wyoming and superior cratons the laramide black hills uplift of southwest south dakota exposes a precambrian crystalline core of 2560 2600 ma basement granitoids nonconformably overlain by two paleoproterozoic intracratonic rift successions in the northern black hills a 1 km thick layered sill the blue draw metagabbro that intrudes the older rift succession provides a key constraint on the timing of mafic magmatism and of older rift basin sedimentation ion microprobe spot analyses of megacrysts of magmatic titanite from a horizon of dioritic pegmatite in the uppermost sill portion yield a 207pb 206pb upper intercept age of 2480 6 ma all age errors 2 comparable to two point 207pb 206pb errorchron ages obtained by pb stepwise leaching of the same titanites nearly concordant domains in coexisting magmatic zircon yield apparent spot ages ranging from 2458 16 to 2284 20 ma i e differentially reset along u pb concordia and hornblende from an associated metadiorite yields a partially reset date with oldest apparent a

40ar 39ar evidence for middle proterozoic 1300 1500 ma slow cooling of the southern black hills south dakota midcontinent north america implications for early proterozoic p t evolution and posttectonic magmatism 40ar 39ar total gas and plateau dates from moscovite and biotite in the southern black hills south dakota provide evidence for a period of middle proterozoic slow cooling early proterozoic 1600 1650 ma mica dates were obtained from metasedimentary rocks located in a synformal structure between the harney peak and bear mountain domes and also south of bear mountain metamorphic rocks from the dome areas and undeformed samples of the 1710 ma harney peak granite hpg yield middle proterozoic mica dates 1270 1500 ma two samples collected between the synform and bear mountain dome yield intermediate total gas mica dates of 1550 ma we suggest two end member interpretations to explain the map pattern of cooling ages 1 subhorizontal slow cooling of an area which exhibits variation in mica ar retention intervals or 2 mild folding of a middle proterozoic 1500 ma 300 c isotherm according to the second interpretation the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure and downwarping of relatively cold rocks during a period of approximately east west contraction and slow uplift during the middle proterozoic the mica data together with hornblende data from the black hills published elsewhere indicate that the ambient country rock temperature at the 3 4 kbar depth of emplacement of the hpg was between 350 c and 500 c suggesting that the average upper crustal geothermal gradient was 25 40 c km prior to intrusion the thermochronologic data suggest hpg emplacement was followed by a 200 m y period of stability and tectonic quiescence with little uplift we propose that crust thickened during the early proterozoic was uplifted and erosionally thinned prior to 1710 ma and that the hpg magma was emplaced into isostatically stable crust of relatively normal thickness we speculate that uplift and crustal thinning prior to hpg intrusion was the result of differential thinning of the subcrustal lithosphere beneath the black hills if so this process would have also caused an increase in mantle heat flux across the moho and triggered vapor absent melting of biotite to produce the hpg magma this scenario for posttectonic granite generation is supported in part by the fact that in the whole of the black hills the hpg is spatially associated with the deepest exposed early proterozoic country rock

the effect of chemical and physical processes on the acoustic properties of carbonate rocks carbonate rocks have major economic significance 60 of the world s oil reserves lie in carbonate reservoirs and the potential for additional gas reserves is huge however the relationship in carbonates between measured geophysical data and rock properties is complex due to the large variety of textures that arise during postdepositional diagenesis and to the chemical processes i e dissolution and replacement by newly formed phases that characterize carbonate forming minerals most experimental and theoretical rock physics research has focused on siliciclastic and shaly rocks however applying insights gained from clastics to carbonates is rarely straightforward and thus is questioned in the literature

lead isotopic evidence for mixed sources of proterozoic granites and pegmatites black hills south dakota usa abstract the lead isotopic compositions of k feldspars separated from the ca 1700 ma harney peak granite complex and spatially associated granitic pegmatites indicate that these rocks were derived from at least two sources it has been reported previously that the core of the harney peak granite complex is dominated by relatively lower gd18o avg 11 5 granites whereas higher gd18o avg 13 2 granites occur around the periphery of the complex the higher 18 o granites and one simple pegmatite have low values of 207 pb 204 pb for their 206 pb 204 pb thus they likely were derived from a source with a short crustal residence time this source may have been the pelitic schists into which the harney peak granite complex and pegmatites were intruded feldspars from granites with lower gd18o values have significantly higher 207 pb 204 pb for their 206 pb 204 pb the data define a linear array with a slope equivalent to an age of ca 2 6 ga with t2 defined to be 1 7 ga such a slope could represent a mixing array or a secondary isochron for the source these low 18o granites could have been derived from a source with a high u pb and with a crustal residence beginning before the proterozoic the source s of these granites may have been a sediment derived from late archean continental crust the highly evolved tin mountain pegmatite has lead isotopic systematics intermediate between those of the two granite groups suggesting either a mixed source or contamination two late archean granites the little elk granite and the bear mountain granite had precursors with high u pb and low th u histories the th u history of the bear mountain granite is too low for this rock to have been an important component of the source of the proterozoic granites however crustal rocks with lead isotopic compositions similar to those of the little elk granite were an important source of lead for some of the proterozoic granitic rocks