a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

proterozoic accretionary tectonics at the southern margin of the archean wyoming craton the proterozoic cheyenne belt at the southern margin of the archean wyoming craton consists of strongly deformed lithotectonic blocks bound by major mylonite zones from north to south and structurally lowest to highest these blocks include 1 archean crystalline basement and associated early proterozoic miogeoclinal rocks 2 an amphibolite grade orthogneiss terrane of unknown age containing probable rift related mafic intrusive rocks 3 a 1750 1790 ma marginal basin succession consisting of upper amphibolite grade pelitic and volcano genie schist associated peraluminous granite and minor ultramafic rocks and 4 a 1750 1790 ma intermediate to mafic plutonic metamorphic complex interpreted as the deep roots of an island arc system the earliest deformation d1 produced a synmetamorphic penetrative horizontal transposition foliation and associated recumbent folds tectonic blocks were juxtaposed along major mylonite zones during d2 macroscopic anil microscopic structures associated with d2 indicate northward thrusting along low angle mylonite zones of successively deeper crustal blocks over supracrustal rocks of the wyoming craton at about 1750 ma thrusting occurred at minimum temperatures of 475 c and produced an inverted metamorphic gradient in pelitic rocks north of the belt mylonite zones were subsequently steepened and reactivated under greenschist facies conditions during a period of dextral strike slip d3 between 1750 and 1400 ma cataclasis d4 and brecciation associated with the laramide orogeny d5 locally overprint earlier structures oblique convergence between an island arc and the archean continental nucleus may explain accretion crustal shortening and subsequent strike slip the phanerozoic accretionary events of eastern and western north america provide an analog for this model presence of similar lithologies and shear zones south of the cheyenne belt suggest that the southern margin of the wyoming craton may have been a long lived zone of crustal accretion

metamorphic petrology of the northeast front range colorado the pingree park area the pingree park area in the northeast front range colorado is underlain by precambrian pelitic and semipelitic gneiss and schist that were metamorphosed about 1 75 b y ago forming a distinct zonal pattern of minerals indicating an increase in grade of metamorphism to the west and north three isograds have been identified the andalusite sillimanite isograd is defined by the first occurrence of sillimanite in the prograde direction and is interpreted to mark the reaction of andalusite to sillimanite a band as much as 1 km across contains both andalusite and sillimanite and represents a divariant band or reaction overstepping of 20 c or less the k feldspar sillimanite isograd separates microcline sillimanite from prograde muscovite quartz assemblages and is interpreted to represent the reaction k na muscovite quartz k na microcline sillimanite h 2 o despite the presence of significant amounts of sodium in both muscovite and microcline the reaction shows no perceptible divariant band there has been extensive reaction of microcline sillimanite to retrograde muscovite quartz the migmatite in isograd is defined by the first occurrence of migmatitic rocks in the prograde direction and is interpreted to represent the inception of partial melting the isograds form an eastward plunging set of synformal surface the south limb of which has a gentle dip and the north limb of which apparently has a much steeper dip based on the chemical composition of the minerals and comparison with experimentally calibrated equilibria the conditions of metamorphism at the k feldspar sillimanite isograd are estimated to have been p h 2 o p total 3 to 4 kbar t 650 30 c f o 2 10 12 to 10 15 bars and f hf 1 4 to 1 bar the geothermal gradient at the peak of metamorphism was 60 c km

evolution of the early proterozoic colorado province constraints from u pb geochronology the colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the archean wyoming craton during the early proterozoic between about 1790 and 1660 ma correspondence in ages between metamorphism deformation and plutonism association of volcanic rocks with comagmatic calc alkalic plutons and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton

revised interpretation of the age of allochthonous rocks of the uncompahgre formation needle mountains colorado the middle proterozoic uncompahgre formation consists of a deformed and mildly metamorphosed sequence of interlayered quartzites and metapelites previous workers advanced the notion that the uncompahgre formation represents an isolated remnant of a sequence of supracrustal sediments deposited on deeply eroded crystalline basement sometime after 1680 ma and deformed and metamorphosed before 1430 ma these conclusions followed from previous interpretations that the contact between uncompahgre lithologies and adjacent metamorphic units is an unconformity truncating dikes ranging in age from 1640 to 1680 ma a considerable body of evidence suggests however that the uncompahgre formation is in fact allochthonous and that age interpretations must be revised significant displacements between uncompahgre units and adjacent metamorphic sequences are suggested by an increase in accumulated strain toward the contact zone coupled with a 3 to 25 m thick zone of mylonites protomylonites and phyllonites in the contact zone itself the geometry of the contact and the presence of faults of similar attitude within the uncompahgre itself indicate an allochthonous sheet of uncompahgre units emplaced along a sole thrust of variable dip with imbricate internal north dipping thrusts and a northern thrust contact of similar attitude folding within the uncompahgre formation was intimately involved with southward emplacement and shortening of the allochthon because the uncompahgre formation is allochthonous time of deposition of the protolith cannot be bracketed by the ages of surrounding crystalline basement deformation in the uncompahgre formation must however postdate 1680 ma and predate intrusion of the eolus batholith at 1430 ma the uncompahgre formation may well be older than previously presumed and likely has equivalents in northern new mexico

sea surface temperature records in the northern south china sea from mid holocene coral sr ca ratios 1 three mid holocene sea surface temperature sst records spanning more than 30 years were reconstructed for the northern south china sea using sr ca ratios in porites corals the results indicate warmer than present climates between circa 6100 yr b p and circa 6500 yr b p with the mid holocene average minimum monthly winter ssts the average maximum monthly summer ssts and the average annual ssts being about 0 5 degrees 1 4 degrees c 0 degrees 2 0 degrees c and 0 2 degrees 1 5 degrees c higher respectively than they were during 1970 1994 summer ssts decrease from circa 6500 yr b p to circa 6100 yr b p with a minimum centered at circa 6300 yr b p the higher average summer ssts are consistent with a stronger summer monsoon during the mid holocene and the decreasing trend indicates a secular decrease of summer monsoon strength which reflects the change in summer insolation in the northern hemisphere el nino southern oscillation enso cycles were apparent in both the mid holocene coral and modern instrumental records however the enso variability in the mid holocene ssts was weaker than that in the modern record and the sst record with the highest summer temperatures from circa 6460 yr b p to 6496 yr b p shows no robust enso cycle this agrees with other studies that indicate that stronger summer monsoon circulation may have been associated with suppressed enso variability during the mid holocene

a reconstructed dynamic indian monsoon index extended back to 1880 the authors present a reconstruction of summer june july august mean dynamic indian monsoon index dimi back to 1880 based on a large number of historical surface observation data as well as information from the upper air data the reconstruction shows a satisfying skill in terms of both the value of reduction of error and an evaluation against other independent monsoon indices the skill of reconstruction increases over time with more predictor data in particular upper level data becoming available a comparison with the observed all indian summer monsoon rainfall index airi shows a high consistence in both inter decadal and inter annual variability the reconstruction shows stronger than normal monsoon during the 1880s 1915 1925 around 1920 and 1930 1945 around 1940 as the airi the el nino southern oscillation enso monsoon relationship is reasonably captured in the reconstruction powers concentrating within quasi biennial band stand out in the reconstruction as well as in the airi a comparison of the reconstruction against an atmospheric general circulation model simulation with specified sst and external forcing agents spanning 1901 1999 indicates a slightly higher reproducibility of monsoon circulation than monsoon rainfall in terms of interannual variability the relationship between the asian continent warming and the enso monsoon connection is also discussed by using the new dynamic index

lead isotopes as a provenance tool for quartz examples from plutons and quartzite northeastern minnesota usa abstract the u pb isotope system in plutonic quartz samples approximates the crystallization age of the host granitoid and thus provides a useful provenance tool for detrital quartz grains we present lead isotopic data and u and pb concentrations of quartz from three late archean granitoids and from the early proterozoic pokegama quartzite all in northeastern minnesota lead in granitic quartz is considerably more radiogenic than in its associated whole rock host but it is similar or only slightly more thorogenic than its host pb pb ages obtained from quartz whole rock leached feldspar from the late archean granitic plutons are consistent with existing geochronology suggesting that the u pb system in quartz remained closed since crystallization u pb isotopic data for detrital clear quartz populations in the pokegama quartzite record late archean ages 2550 2650 consistent with a source from the superior province lead isotopic compositions of detrital k feldspar populations and lead and neodymium isotopic compositions of whole rocks samples from the pokegama quartzite also support a superior province source this study highlights the potential of the lead isotope system in detrital quartz k feldspar and whole rocks in provenance studies of sandstones

the evolution of el nino past and future abstract we review forecasts of the future of el nino and the southern oscillation enso a coupled instability of the ocean atmosphere system in the tropical pacific with global impacts enso in the modern world is briefly described and the physics of the enso cycle is discussed particular attention is given to the bjerknes feedback the instability mechanism which figures prominently in enso past and future our knowledge of enso in the paleoclimate record has expanded rapidly within the last 5 yr the enso cycle is present in all relevant records going back 130 kyr it was systematically weaker during the early and middle holocene and model studies indicate that this results from reduced amplification in the late summer and early fall a consequence of the altered mean climate in response to boreal summer perihelion data from corals shows substantial decadal and longer variations in the strength of the enso cycle within the past 1000 yr it is suggested that this may be due to solar and volcanic variations in solar insolation amplified by the bjerknes feedback there is some evidence that this feedback has operated in the 20th century and some model results indicate that it will hold sway in the greenhouse future but it is very far from conclusive the comprehensive general circulation models used for future climate projections leave us with an indeterminate picture of enso s future some predict more enso activity some less with the highly uncertain consensus forecast indicating little change

coral δ18o based reconstruction of el nino southern oscillation from the northern south china sea since 1851 ad abstract el nino southern oscillation enso is the largest source of interannual climate variability over the globe knowledge of enso variability back to the pre instrumental period and earlier can help to enhance our understanding of its mechanisms and impacts here we use the coral record from the northern south china sea where the interannual climate variability is associated with enso activity to reconstruct enso variability over the past 150 years our record together with other enso chronologies indicated not only the weakened enso activity between 1930s and 1960s but also the larger variations of el nino activity relative to that of la nina based on the sliding window method the comparison between the scs and pacific enso band coral records revealed the relatively stronger climatic coupling between the scs and the equatorial central eastern pacific compared with that between scs and western pacific indicating the reliability of teleconnection relationship between the scs climate and enso our result indicated that the coral 18o record from the scs could contribute to the pan pacific enso reconstructions

seasonal and interannual variability of the mid holocene east asian monsoon in coral δ18o records from the south china sea understanding the full range of past monsoon variability with reference to specific monsoon seasons is essential to test coupled climate models and improve their predictive capabilities we present a 54 year long high resolution skeletal oxygen isotope 6180 record extracted from a well preserved massive porites sp coral at hainan island south china sea to investigate east asian monsoon variability during summer and winter similar to 4400 calendar yr ago analysis of modem coral 6180 confirms that porites from hainan island are well positioned to record winter monsoon forcing of sea surface temperature sst as well as the influence of summer monsoon rainfall on sea surface salinity sss the coral record for similar to 4400 yr ago shows similar to 9 amplification of the annual cycle of 6 delta o 18 in good agreement with coupled ocean atmosphere models showing higher summer rainfall lower coral 618 0 and cooler winter ssts higher coral delta o 18 in response to greater northern hemisphere insolation seasonality during the middle holocene mean ssts in the south china sea during the mid holocene were within 0 5 degrees c of modem values yet the mean 6 180 for the fossil coral is similar to 0 6 parts per thousand higher than that for the modem coral suggesting that the 6 180 of surface seawater was higher by at least similar to 0 5 parts per thousand relative to modem values the o 18 enrichment is likely to be driven by greater advection of moisture towards the asian landmass enhanced monsoon wind induced evaporation and vertical mixing and or invigorated advection of saltier o 18 enriched pacific water into the relatively fresh south china sea the o 18 enrichment of the northern south china sea similar to 4400 yr ago contributes to mounting evidence for recent freshening of the tropical western pacific today winter sst and summer sss variability in the south china sea reflect the interannual influence of enso and the biennial variability inherent to monsoon precipitation spectral analysis of winter ssts similar to 4400 yr ago reveals a strong enso cycle at 6 7 y which is significantly longer than the average 3 6 y cycle observed since 1970 the results suggest that the influence of enso on winter ssts in the south china sea was well established by similar to 4400 yr ago however spectral analysis of summer sss similar to 4400 yr ago shows no significant enso cycle suggesting that teleconnections between enso and summer monsoon rainfall were restricted taken together the results indicate marked differences in enso monsoon interactions during the winter and summer monsoon seasons in the past the fossil coral delta o 18 record also shows that the amplitude of interannual sst and sss variability was stronger similar to 4400 yr ago despite enso variability being significantly weaker in the pacific region thus it appears that the strengthened mid holocene monsoon was sensitive to forces other than enso that acted as alternative drivers of interannual monsoon variability if this is the case greater interannual climate variability could accompany the strengthening of the asian monsoon predicted to occur during the 21st century as transient greenhouse warming preferentially warms eurasia even if enso perturbations remain relatively stable c 2005 elsevier b v all rights reserved

proterozoic history and crustal evolution in southwestern colorado insight from u pb and sm nd data abstract new u pb age constraints and sm nd isotope data further establish the timing of magmatic and tectonic events from 1 8 to 1 4 ga in southwestern colorado and give insight into the interaction of crust and mantle systems during the assembly and evolution of orogenic belts along the southern margin of laurentia proterozoic orogenic events in the needle mountains led to crustal assembly thickening and tectonic reworking that was punctuated by periods of relative crustal stability pre 1 75 ga arc successions in southwestern colorado were produced and assembled into juvenile crust in an orogenic period that culminated in regional deformation and metamorphism that accompanied intrusion of synorogenic plutonic rocks from 1 73 to 1 71 ga emplacement of post orogenic plutonic rocks at 1 70 ga was followed by uplift and erosion and the deposition and deformation of siliciclastic sedimentary sequences paleoproterozoic crust underwent local melting and thermal metamorphism during invasion of felsic to mafic magmas at 1 43 ga that coincided with a pulse of intracratonic deformation invasion and melting of first cycle 1 8 1 7 ga crust at 1 71 and 1 43 ga involved heat and volatiles from mantle melts that were a catalyst for crustal melting and magmatic recycling sm nd isotopic signatures of 1 8 1 4 ga volcanic and plutonic rocks in southwestern colorado lend evidence to the emplacement of melts from depleted mantle and juvenile crust in the production continental crust with minor if any contamination by crust older than 1 8 ga the proterozoic magmatic record in southwestern colorado is consistent with continental growth by the addition of juvenile crust extracted from the mantle followed by periodic intracrustal growth during orogenic pulses in which mantle magmas underplate and melt the crust with consequent thickening

variations in the pacific decadal oscillation since 1853 in a coral record from the northern south china sea 1 the pacific decadal oscillation pdo has been shown to have significant climatic and environmental impacts across the pan pacific basin however there are no records of pdo activity from the south china sea scs the largest marginal sea in the northwest pacific ocean this study suggests that a series of geochemical profiles obtained from a modern coral in the northern scs records annual pdo activity dating back to 1853 these geochemical data are significantly correlated with the pdo index and their patterns of variation closely match those of the pdo index over the last century the relationship between the pdo and coral geochemistry may be related to the influence of the pdo on rainfall on hainan island rainfall patterns influence the volume of terrestrial runoff which in turn is a primary determinant of 18o and 18o values in coral however coral 13c values are also influenced by the 13c suess effect the results indicate that sr ca ratios in coral are affected by a combination of sea surface temperature and terrestrial runoff

monsoon tropical ocean interaction in a network of coral records spanning the 20th century abstract the 20th century evolution of basin wide gradients in surface ocean properties provides one essential test for recent models of the interaction between the asian monsoon and the tropical ocean because various feedback mechanisms should result in characteristic regional patterns of variability although the instrumental record of climate variability in the tropics is essentially limited to the last few decades the stable isotopic composition of living corals provides an effective means for extending the instrumental observations here we present two coral isotopic records from the indonesian maritime continent and we use these records with other previously published records to describe i the relationship between western pacific and central pacific climate variability over the past century with special emphasis on the biennial band and ii the strength of the west east indian ocean dipole we find that the amplitude of the biennial cycle in the pacific did not vary inversely with the strength of enso el nino southern oscillation as might be expected from some models of monsoonal feedback on the central pacific instead the biennial variability was modulated on decadal timescales throughout much of the pacific we also show that the zonal oxygen isotopic gradient in the indian ocean coral records was significantly correlated with central pacific sea surface temperature on a variety of timescales thus it is likely that this coral dipole was a product of strong enso like teleconnections over the indian ocean as opposed to being the result of unique indian ocean or monsoonal dynamics

proterozoic farwell mountain lester mountain suture zone northern colorado subduction flip and progressive assembly of arcs this paper considers the amalgamation of arc and oceanic terranes to be the main mechanism of ca 1 8 1 6 ga continental crustal growth in southwestern laurentia on the basis of geologic and seismic reflection data and teleseismic images we propose the farwell mountain lester mountain suture zone as the northern most paleoproterozoic arc arc suture north dipping farwell mountain seismic reflections project from 18 km depths to the surface and are interpreted to represent conjugate thrusting as the 1 79 1 77 ga green mountain arc was partially underthrust beneath the archean craton we speculate that a north dipping high velocity mantle slab in the teleseismic image is a continuation of this thrust zone south dipping lester mountain reflections project from 22 km depths to the surface and are interpreted to be a thrust zone between the green mountain arc and the 1 76 1 72 ga rawah block surface features of the farwell mountain lester mountain suture zone are 1 marble chert rock with sillimanite pods ultramafic rocks sulfide deposits and pillow basalts which we interpret to be a dismembered accretionary complex 2 an axial planar fabric to north verging isoclinal folds f 2 which we interpret to be part of a north vergent fold and thrust system and 3 a metamorphic break between 500 c rocks to the north and 610 c rocks to the south which we attribute to reactivation of the zone movement on the farwell mountain backthrust is inferred to relate to cheyenne belt suturing at 1 78 1 75 ga we correlate suturing in the farwell mountain lester mountain suture zone to s 1 d 1 1 746 1 74 ga in the soda creek fish creek shear zone we attribute the complexity of this broad suture zone to initial conjugate thrusting plus overprinting and steepening of accretionary structures by subsequent tectonism

the mount evans batholith in the colorado front range revision of its age and reinterpretation of its structure the mount evans batholith in the central front range of colorado is composed of a main phase of massive to conspicuously foliated monzogranite and granodiorite and undeformed aplite and pegmatite the mount evans batholith was previously considered to be part of the 1 7 ga routt plutonic suite new u pb zircon ages on four samples granodiorite monzogranite and granite however indicate that the batholith was emplaced at 1 442 2 ma and belongs to the berthoud plutonic suite most of the batholith has igneous textures and structures except in the vicinity of the idaho springs ralston shear zone where those features are tectonically recrystallized and foliated foliation elsewhere in the batholith is a flow structure zircons in two granodiorite samples collected near the shear zone just south of the colorado mineral belt are reversely discordant by about 0 8 2 1 with a considerable spread in 207 pb 206 pb ages many of the zircons from these samples contain apatite k feldspar and quartz inclusions that appear to replace zircon along cracks and imperfections from rim to core we suggest that these inclusions formed during a laramide ore forming event and incorporated early and middle proterozoic radiogenic lead scavenged from the country rock the excess radiogenic lead caused the scatter and reverse discordance in the data the mount evans batholith is anomalous in composition and structure compared to most other 1 4 ga plutons of the southwestern united states the differences probably reflect different sources of partial melting the specific tectonic setting where rocks of such disparate origin are temporally and spatially juxtaposed is not understood

the schwartzwalder uranium deposit ii age of uranium mineralization and lead isotope constraints on genesis u pb isotope analyses of ores from the schwartzwalder uranium mine colorado show that these ores have high amounts of initial common pb and that the initial pb was both variable and relatively radiogenic in its pb isotope ratios 206 pb 204 pb 26 30 as a result the only useful approach to dating these ores is with u pb isochrons and even so some means of dealing with the variable initial pb isotope ratios is required because the common pb in these ores was apparently derived from sources of similar age and th u the observed 208 pb 204 pb of these th free ores can be used either to identify sample suites with similar initial pb isotope ratios or to normalize for the variable initial pb isotope ratios the resulting u pb isochrons indicate an age of mineralization for both illinois vein and titan vein ores of 69 3 or 1 1 m y suggesting that the deposit was formed during the earliest stages of laramide uplift and under at least 3 km of phanerozoic cover the initial pb isotope systematics of the ores show that the metals in the schwartzwalder ores were derived from source s of 1 730 or 130 m y age with a th u of 2 2 or 0 2 and 238 u 204 pb of 30 to 60 these restrictions on the source s of the metals rule out the possibility that the deposit could have been formed by remobilization of a proterozoic uranium deposit and both the 69 3 m y age of the ores and their initial pb isotope ratios preclude any contribution of metals to the deposit by younger volcanics such as those in the denver formation

early proterozoic metasediments from north central colorado metamorphism provenance and tectonic setting early proterozoic metasediments from the big thompson canyon area in north central colorado increase in metamorphic grade from the biotite zone to the sillimanite zone representing a low pressure facies series that probably developed in response to a steep geotherm over a thermal high high grade rocks are enriched in k 2 o al 2 o 3 fe 2 o 3 mgo tio 2 cr sc cs rb ba and light ree and are depleted in sio 2 cao and zr relative to low grade rocks these differences chiefly reflect differences in the original sandstone shale ratio detrital grains in medium and coarse metasediments are quartz quartzite and feldspar and they suggest a recycled orogen provenance in which granitic rocks and quartzites dominate in the source the abundance of shale in the original succession also attests to the importance of granite in the source major and trace element contents of the rocks constrain the input of mafic to dacitic detritus to less than a few percent ree patterns in all rocks are similar to phanerozoic shales and cratonic sandstones the big thompson canyon metasediments may have been derived from an uplifted collisional orogen in southern wyoming in which unroofed proterozoic granitic rocks and quartzites of the snowy pass supergroup served as the principal sources

stratigraphy and depositional setting of the proterozoic snowy pass supergroup southeastern wyoming record of an early proterozoic atlantic type cratonic margin metasedimentary rocks in the medicine bow mountains and sierra madre are divided into four groups the 3 km thick phantom lake metamorphic suite contains strongly deformed metavolcanic and metasedimentary rocks that are crosscut by late archean granites the 2 5 km thick deep lake group unconformably overlies the phantom lake suite and late archean granites and contains fluvial sediments including radioactive quartz pebble conglomerates and glaciomarine deposits both successions are intruded by large sills of tholeiitic gabbro the 4 5 km thick lower libby creek group is inferred to be in thrust fault contact with older units and contains sediments recording transgressions and regressions across a macrotidal delta this succession is intruded by the 2 000 m y old gaps intrusion and comagmatic tholeiitic to weakly alkalic dikes the 3 km thick upper libby creek group is bounded by a thrust fault below and by the cheyenne belt above and contains carbonates and marine slates the early proterozoic deep lake lower libby creek and upper libby creek groups collectively are named the snowy pass supergroup lithologies and stratification sequence in the well preserved medicine bow mountain section suggest transgressive miogeoclinal sedimentation during the early proterozoic paleocurrent data indicate that fluvial then deltaic sedimentation of the deep lake and lower libby creek groups took place on a southwest dipping paleoslope parallel to the inferred south cratonic boundary of the wyoming province this and a few west directed paleocurrents suggest a continental or microcontinental block to the south bounding sedimentation in a northeast elongated basin a rift setting for deposition of these units explains the transgressional character of the sediments the deltaic sedimentation with paleocurrents parallel to the cratonic boundary and the 120 bend in the cheyenne belt between the medicine bow mountains and the sierra madre the upper libby creek group is interpreted to represent open marine conditions following separation of the two continental blocks tholeiitic sills in the deep lake group and tholeiitic to weakly alkalic dikes in the libby creek group are thought to be related to basaltic igneous activity associated with compound early proterozoic rifting between 2 300 and 2 000 m y ago

lead isotopic evidence for the origin of paleo and mesoproterozoic rocks of the colorado province u s a abstract lead isotopic ratios of k feldspars and whole rocks from 1 7 and 1 4 ga plutonic rocks of the colorado province are relatively non radiogenic for 207 pb 204 pb plotting below the average crust model curve of stacey and kramers 1975 indicating that the terrane was derived primarily from juvenile mantle material slightly more radiogenic ratios in the northern part of the terrane near the archean wyoming province suggest minor inclusion of an older component the data from 1 7 ga plutons plot in a broad field suggesting two episodes of re equilibration with whole rock pb probably related to heating events in the mesoproterozoic 1 4 ga and cretaceous 70 ma possible differences in calculated whole rock th u coupled with slight pb isotopic variations along the north south transect suggest either a terrane boundary through central colorado near salida and gunnison or fundamental differences in source rocks metasedimentary vs metavolcanic u pb analyses of multigrain splits of detrital zircons from quartzites throughout the colorado province have failed to identify archean detritus the oldest 207 pb 206 pb ages found in two samples of quartzite from northern colorado are about 2 0 ga perhaps derived from rocks of the trans hudson orogen in contrast to 2 75 ga detrital zircon in a paleoproterozoic quartzite from the southern part of the wyoming province while we are not yet able to discern if these ages are true provenance ages or mixtures of archean and paleoproterozoic components the absence of easily recognizable archean zircons supports other isotopic data and a conclusion that most of the paleoproterozoic crust of the colorado province was ultimately derived from a juvenile at 1 8 ga mantle reservoir

paired coral sr ca and δ18o records from the chagos archipelago late twentieth century warming affects rainfall variability in the tropical indian ocean understanding the relationship between sea surface temperature sst and precipitation is a significant challenge for climate models particularly for the tropics here we present a new monthly coral sr ca record from the tropical indian ocean chagos archipelago that extends from 1950 to 1995 the coral sr ca ratio shows a stationary relationship with local sst and documents a warming of 0 3 c since 1950 previous work has shown that the 18 o values measured in the same coral core provide a proxy record of precipitation in the tropical indian ocean the coral 18 o record shows a nonstationary relationship with local sst and a correlation between 18 o and sst only emerges in the 1970s it was proposed that this nonstationary behavior is due to an increase in mean ssts in the tropical indian ocean during the 1970s ssts reached a critical threshold 28 5 c beyond which small sst anomalies can have a significant impact on atmospheric convection as a result the covariance between sst and precipitation in the tropical indian ocean increased our new sr ca data confirm that the warming of the indian ocean during the late twentieth century affects atmospheric convection and rainfall variability moreover our proxy data show that the relationship between sst and precipitation is nonlinear and characterized by threshold behavior

proterozoic cuspate basement cover structure needle mountains colorado deformation zones along basement cover contacts which are commonly loci of significant displacement may also be zones of limited movement when cover and basement are folded together proterozoic basement gneisses and siliciclastic cover rocks of the uncompahgre group in the needle mountains colorado are in contact along steeply dipping deformation zones analysis of structures in the basement and cover and along their contacts provides evidence for 1 deformation of the basement prior to deposition of the uncompahgre group 2 north directed thin skinned thrusting and 3 cuspate infolding of cover into basement accompanied by upward and outward movement of the cover opposed senses of movement along basement cover contacts on opposite limbs of the synclinorium generated during cuspate infolding imply that the uncompahgre group is parautochthonous on basement conjugate strike slip shear zones in both units accommodated north northwest shortening deformation of basement and cover into cuspate structures may explain the presence of some cover sequences as isolated remnants within gneissic basement

producing characteristics and depositional environments of lower pennsylvanian reservoirs parkway empire south area eddy county new mexico the parkway empire area on the northwest shelf of the permian basin about 15 mi 24 km northeast of carlsbad new mexico produces gas from stratigraphic traps within four lower pennsylvanian zones lower and middle morrow sandstones atoka sandstones and strawn limestones the lower morrow sandstones which occur at about 11 400 ft 3 470 m are interpreted to be a prograding fluvial deltaic sequence of channels and point bars with a northwest source they trend toward the southeast generally normal to the morrowan paleoslope the lower morrow sands are separated from the middle morrow sandstones by a widespread dark gray organic lagoonal shale in southeastern new mexico middle morrow sandstones are a transgressive series of marine beaches and bars deposited along the northeast trending ancient shoreline stratigraphic traps are created in the lower and middle morrow sandstones by variations in cementation and depositional patterns productive atoka sandstones occur at approximately 10 700 ft 3 260 m and appear to be a series of prograding barri r bars deposited along the northeast trending shoreline strawn limestones produce from a series of small low relief algal banks developed along northeast depositional strike the strawn limestones occur between 10 250 and 10 500 ft 3 120 and 3 200 m the area is economically attractive payout of a typical well occurs in 1 2 years with a rate of return of 78 1 pennsylvanian completions in this area have typical ultimate recoveries of approximately 1 5 bcf 42 106 m3 of gas and 22 000 bbl 3 5 106 l of condensate the stacked nature of the reservoirs causes a success rate of 85 2 for this area the high success rate good production and attractive economics make this area of the morrow trend an important exploration target

early proterozoic oceanic crust in the northern colorado front range implications for crustal growth and initiation of basement faults 1 early proterozoic oceanic crust exposed in a shear zone in the northern colorado front range provides insight into the tectonic evolution of the colorado province tectonically intermingled mylonitic amphibolite locally pillowed metagabbro clinopyroxenite metachert and melange are preserved within the buckhorn creek shear zone west of fort collins whole rock geochemical data support formation of the igneous protoliths in an oceanic environment major and trace element data overlap samples from modern oceanic ridges and are distinct from modern arc trends metagabbros and clinopyroxenites preserve a subhorizontal lineation defined by igneous pyroxenes core and mantle microstructures indicate initial deformation at temperatures 900 c secondary amphiboles define both subhorizontal and subvertical lineations kinematic analysis indicates that the rocks experienced e w strike slip deformation prior to and during amphibolite facies metamorphism followed by n s shortening 40ar 39ar data from secondary hornblende yield age steps as old as 1 9 ga based on the chemical data and kinematic evidence for strike slip deformation we propose that the buckhorn creek rocks originated at an oceanic ridge transform intersection the high t pyroxene fabrics formed during ocean floor deformation prior to 1 7 ga regional deformation of the colorado province the buckhorn creek fragment likely represents original oceanic basement for 1 8 1 7 ga arc magmatism and sedimentation that ultimately formed the colorado province continental crust reactivation of original oceanic fracture zones may be responsible for the network of regularly spaced northeast trending shear zones in colorado once formed subvertical transform faults are likely to survive the transition from oceanic to continental crust and persist as long lived crustal discontinuities

u th pb systematics of zircon inclusions in rock forming minerals a study of armoring against isotopic loss using the sherman granite of colorado wyoming usa zircon inclusions were separated from the five major rock forming minerals of the sherman granite of southern wyoming in order to evaluate the degree of discordance as a possible function of host minerals u th pb isotopic ratios were determined for two size fractions of zircon inclusions from each mineral plus five size fractions from the bulk rock isotopic data from the inclusions have more than double the spread of data on a discordia obtained from the bulk sample thereby yielding better resolved concordia intercepts however isotopic ratios and morphologic characteristics indicate that the pb u systematics are complicated by inherited radiogenic lead although the data array cannot unequivocally be explained by the armoring process the proposed methodology has succeeded in identifying groups of zircon with different isotopic characteristics as such this technique can be used to decipher complex geologic isotopic histories and may be a useful addition to routine zircon geochronology

chronologic and isotopic framework for early proterozoic crustal evolution in the eastern mojave desert region se california the early proterozoic geologic evolution of the eastern mojave desert region as defined by characteristics of its supracrustal rocks granitoids metamorphism structural history and pb and nd isotopic signature contrasts sharply with other proterozoic provinces of the southwestern united states the oldest supracrustal rocks of the mojave desert region contain zircons over 2 0 ga corroborating nd isotopic evidence for a much older crust here than elsewhere in the southwestern united states granitoids widely emplaced within these supracrustal rocks range from 1 76 to 1 64 ga the earlier plutons and surrounding supracrustal rocks were metamorphosed to granulite and high amphibolite facies throughout the province at about 1705 ma in a migmatite producing event that we term informally the ivanpah orogeny subsequent granitoids emplaced from 1 69 to 1 67 ga were voluminous along a north trending belt in the middle of the mojave province younger plutons were emplaced at about 1 66 ga in several places and at about 1 64 ga along the extreme southern part of the province commonalities between the proterozoic evolutions of the mojave and arizona crustal provinces do not conclusively establish the time that the provinces were juxtaposed the data only suggest that the juxtaposition occurred between about 1 76 and 1 64 ga

assessing spatial variability in el nino southern oscillation event detection skill using coral geochemistry 1 the el nino southern oscillation enso drives interannual climate variability but evidence of its preinstrumental expression is limited to proxy data and model inference we develop a skill assessment technique using instrumental records of sea surface temperature sst and sea surface salinity sss and apply it to a suite of 23 coral 18o records a common enso proxy we compare coral proxy skill with pseudoproxy records constructed from instrumental data to establish the expected proxy response to enso events and constrain proxy sensitivity the central pacific has a balanced response toward enso events the western pacific warm pool favors el nino expression and the south pacific convergence zone preferentially records la nina events however all three regions display high sensitivity to enso variability presenting promising targets for future coral paleoclimate reconstructions eastern pacific coral records display surprisingly low enso sensitivity despite the presence of large enso related sst anomalies suggesting nonclimatic influences comparisons between the pseudoproxy and coral skill ratings provide a test to highlight where these nonclimatic impacts that may affect coral record skill networks of proxy data skill ratings may better delineate the spatial structure of the sst and sss response to enso variability a feature of the climate system that is currently poorly reproduced in climate models

proterozoic tectonic assembly of new mexico proterozoic rocks in northern and central new mexico originated in at least six tectonostratigraphic terranes which can be distinguished by differences in lithology age and or metamorphic grade boundaries between some terranes have the geometry of bedding parallel north verging ductile thrusts these were active during metamorphism but prior to the thermal peak one terrane boundary is a south verging ductile shear zone that has placed medium grade on top of high grade metamorphic rocks it was active at and after the peak of metamorphism and may be extensional in origin we interpret that terrane assembly occurred by ductile deformation in the middle levels of the continental crust we do not know the original geographic relations among terranes but the possibility exists of significant horizontal translation

u pb geochronology of detrital zircons from a continental margin assemblage in the northern coast mountains southeastern alaska metamorphic rocks within and west of the northern coast mountains in southeastern alaska consist of an upper proterozoic to upper paleozoic continental margin assemblage that we interpret to belong to the yukon tanana terrane u pb geochronologic analyses of single detrital zircon grains from four samples of quartzite suggest that the zircons were shed from source regions containing rocks of 495 ma 750 ma 1 05 1 40 ga 1 75 2 00 ga 2 3 ga 2 5 2 7 ga and 3 0 ga multigrain fractions from two samples yield upper intercepts between 2 0 and 2 3 ga but the scarcity of single grains of similar age suggests that these fractions comprise a mixture of 2 3 ga grains zircons in these rocks generally overlap in age with i detrital zircons in metasedimentary rocks of the yukon tanana terrane in eastern alaska and yukon ii detrital zircons in strata of the cordilleran miogeocline and iii plutonic and gneissic rocks that intrude or are overlain by miogeoclinal strata in addition the p

1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle 1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle

el nino southern oscillation enso and decadal scale climate variability at 10 n in the eastern pacific from 1893 to 1994 a coral based reconstruction from clipperton atoll we have developed a 101 year 1893 1994 subseasonal oxygen 18o and carbon 13c isotopic time series from clipperton atoll in the eastern pacific using the coral porites lobata in agreement with linsley et al 1999 we find that seasonal and interannual coral 18o variability at clipperton results from variability in both water temperature and salinity three new coral time series demonstrate the reproducibility of a secular coral 18o trend of 0 35 since 1910 but show varying 13c trends strong decadal scale variability in the 18o record appears related to the pacific decadal oscillation pdo through postulated changes in the strengths of the equatorial counter current and north equatorial current interannual variability in this coral 18o record is directly related to the el nino southern oscillation enso and isolation of this frequency band indicates reduced enso variability in the eastern equatorial pacific in the period 1925 to 1940 in agreement with instrumental and other pacific coral records

a sedimentary overlap assemblage links australia to northwestern laurentia at 1 6 ga abstract the columbia nuna supercontinent existed from approximately 1 9 ga to 1 3 ga laurentia was part of columbia and the western edge of laurentia current coordinates was likely proximal to a large landmass during parts of this interval here we present detrital zircon ages of a paleoproterozoic sedimentary succession in northern yukon canada that bear on the evolution of columbia the sedimentary succession is preserved as clasts within 1 60 ga hydrothermal megabreccias analyses of detrital zircon reveal abundant 1 78 1 68 ga zircon with evolved hf isotope values 16 1

coral δ18o records as an indicator of winter monsoon intensity in the south china sea we have used correlative analysis between mean december january february winter wind velocities measured at the xisha meteorological observatory 16degrees50 n 112degrees20 e in the middle of the south china sea and mean 518 0 data for the corresponding month from porites lutea coral collected in longwan waters 19degrees20 n 110degrees39 e to obtain a linear equation relating the two datasets this winter wind velocity for the south china sea wmiiscs can then be correlated to the coral delta 18 o by the equation wmiiscs 1 213 1 351 delta 18 o parts per thousand pdb r 0 60 n 40 p 0 01 from this the calculated wmiiscs delta18o series from 1944 to 1997 tends to decrease during the 1940s to the 1960s it increases slightly during the 1970s and then decreases again in the 1980s and 1990s the calculated decadal mean wmiiscs delta18o series had a obvious decrease from 5 92 to 4 63 m s during the period of 1944 1997 the calculated yearly mean wmiiscs delta18o value is 5 58 m s from 1944 to 1976 and this decreases to 4 85 m s from 1977 to 1998 that is the opposite trend to the observed yearly mean sst variation the yearly mean sst anomaly is 0 27degrees from 1943 to 1976 and this increases to 0 16degrees from 1977 to 1998 spectral analysis used on a 54 year long calculated wmiiscs delta18o series produces spectral peaks at 2 4 7 yr which can be closely correlated with the quasibiennial oscillation band qbo band 2 2 4 yr and the el nino southern oscillation band enso band 3 8 yr hence most of the variability of the winter monsoon intensity in the middle of the south china sea is mainly constrained by changes in the thermal difference between the land and the adjoining sea area perhaps due to global warming c 2003 elsevier science usa all rights reserved

early proterozoic central plains orogen a major buried structure in the north central united states a major early proterozoic orogen central plains more than 1000 km long and at least 500 km wide has been delineated in the subsurface of nebraska kansas and missouri as a result of a new compilation of precambrian basement drill data in the northern midcontinent the orogen is composed of metamorphic and granitoid rocks in the ringe 1 63 1 80 ga and it is interpreted on the basis of isotopic ages rock types and structure as reflected by magnetic and gravity anomalies as being the eastward extension of the early proterozoic foldbelt exposed in basement uplifts in colorado and southeastern wyoming it truncates the southern margin of the superior archean craton and its marginal early proterozoic orogens the penokean 1 83 1 90 ga and the trans hudson 1 85 1 95 ga the pattern of intersecting proterozoic mobile belts marginal to archean cratons is comparable to that in other continents such as western australia and southern africa

proterozoic polydeformation in basement rocks of the needle mountains colorado proterozoic rocks in the needle mountains include 1 755 m y old amphibolite grade gneisses and 1 690 m y old granites that comprise basement to the siliciclastic uncompahgre group the mafic and felsic gneisses which contain primary structures indicative of volcanic and plutonic protoliths under went two phases of isoclinal folding and foliation development prior to emplacement of the 1 690 m y old plutons synkinematic garnet porphyroblast textures suggest that these fabrics developed during a progressive deformation event d b subsequent d bc deformation caused folding of d b fabrics in the gneisses development of a subvertical east striking foliation in the granites and generation of a macroscopic sigmoidal foliation pattern throughout the area d bc structures in the basement are correlated with structures in the uncompahgre group and formed prior to pluton emplacement at ca 1440 ma gently east plunging mineral lineations in the granites and quartz c axis fabrics of both the gneisses and granites indicate subhorizontal extension on steeply dipping foliation surfaces during d bc asymmetric c axis fabrics and rare mesoscopic shear sense indicators in the gneisses record a component of dextral shear in domains of east striking foliation and sinistral shear in areas of northeast striking foliation in contrast symmetric c axis fabrics in the granites suggest nearly coaxial extension a model for d bc involving the development of conjugate strike slip shear zones in response to north northwest shortening is most consistent with the kinematic and fabric orientation data partitioning of deformation into subparallel zones of noncoaxial and coaxial deformation was controlled by the presence of a strong pre existing anisotropy in the gneiss complex the shortening direction during d bc is consistent with that related to north northwest directed thrusting in rocks of similar age in central arizona and northern new mexico therefore d bc structures are tentatively interpreted to have accommodated transverse shortening and orogen parallel extension within a foreland during regional crustal shortening between ca 1690 and 1400 ma

sr ca profile of long lived tridacna gigas bivalves from south china sea a new high resolution sst proxy abstract high resolution 18o and sr ca ratio profiles of a modern live caught and a fossil dead collected giant bivalve shell tridacna gigas from northern south china sea were analyzed the high resolution 18o profiles as documented in most of previous studies showed pronounced seasonal cycles and recorded the combined influence of seawater 18o and sea surface temperature the sr ca ratio profiles derived from icp oes had well defined annual cycles and significant and negative correlation with sea surface temperature while the sr ca profiles derived from la icp ms did not reveal clear annual cycles this difference was likely caused by the organics contents in t gigas shells which had a chemical composition different than caco3 crystals and a heterogeneous distribution across the shell the sampling methods and the experimental errors two calibration equations between sr ca ratios determined by icp oes of modern t gigas shell and instrumental sst were calculated based on all monthly and extremum sst data and the equation from extremum data produced better sst predictions these results suggested that the high resolution icp oes derived sr ca ratios of t gigas in the south china sea had the potential of providing a good proxy of historical sst

origin of thick first cycle quartz arenite successions evidence from the 1 7 ga ortega group northern new mexico abstract the 1700 ma old ortega group in northern new mexico represents a 1000 m thick succession of metamorphosed quartz arenites and subordinate pelites published ages of detrital zircon grains from the ortega group indicate that the sediments were derived mainly from 1760 1700 ma old volcano plutonic basement rocks and not from the archean wyoming province to the north evidence for recycling within the ortega group also is lacking consequently the ortega group quartz arenites are considered to be first cycle sediments the compositional and textural maturity of these sandstones is attributed to intense chemical and mechanical weathering abnormally high al contents of pelites within the ortega group imply intense chemical weathering in the provenance similar to present day low relief tropical regions quartz arenites in the ortega group were deposited exclusively within a shallow marine shelf environment influenced by tide storm and wave processes prolonged sediment transport especially on the tide dominated inner shelf in conjunction with inferred gradual subsidence optimized mechanical destruction of labile grains and rounding of quartz grains although the maturity of sediments in the ortega group can be accounted for the mode of origin of 850 m of quartz arenites remains enigmatic thick quartz arenite sequences of global distribution in the phanerozoic coincide with first order eustatic sea level rises world wide occurrences of 1700 ma old thick quartz arenite shelf successions suggest that the ortega group may have accumulated in response to a similar major eustatic sea level rise

paleoproterozoic rocks of central colorado accreted arcs or extended older crust paleoproterozoic 1770 1735 ma bimodal volcanic rocks in central colorado have been considered to represent southward growth of laurentia by arc accretion although the bimodality of these rocks suggests an extensional continental setting rather than continental or oceanic arcs there has been little evidence for pre 1800 ma crust south of the wyoming craton other than the 1840 1 ma elves chasm pluton in the upper granite gorge of the grand canyon arizona we report shrimp u pb ages of inherited zircons from metarhyolites and plutons in central colorado that are latest archean earliest proterozoic 2520 2000 ma and trans hudson penokean 1878 1814 ma associated quartzites contain detrital zircons with mean ages of 1735 ma indicating only local derivation a meta arkose however contains detrital zircons of trans hudson penokean and archean ages we believe it likely that the 1900 1800 ma trans hudson penokean orogens including archean enclaves extended farther south and west than is currently thought and were the source of the bimodal volcanic rocks and associated plutons during the period 1770 1700 ma

detrital zircon evidence for non laurentian provenance mesoproterozoic ca 1490 1450 ma deposition and orogenesis in a reconstructed orogenic belt northern new mexico usa defining the picuris orogeny detrital zircon and igneous zircon u pb ages are reported from proterozoic metamorphic rocks in northern new mexico these data give new insight into the provenance and depositional age of a 3 km thick metasedimentary succession and help resolve the timing of orogenesis within an area of overlapping accretionary orogens and thermal events related to the proterozoic tectonic evolution of southwest laurentia three samples from the paleoproterozoic vadito group yield narrow unimodal detrital zircon age spectra with peak ages near 1710 ma igneous rocks that intrude the vadito group include the cerro alto metadacite the picuris pueblo granite and the penasco quartz monzonite and yield crystallization ages of 1710 10 ma 1699 3 ma and 1450 10 ma respectively within the overlying hondo group a metamorphosed tuff layer from the pilar formation yields an age of 1488 6 ma and represents the first direct depositional age constraint on any part of the proterozoic metasedimentary succession in northern new mexico detrital zircon from the overlying piedra lumbre formation yield a minimum age peak of 1475 ma and 60 grains 25 yield ages between 1500 ma and 1600 ma possibly representing non laurentian detritus originating from australia and or antarctica detrital zircons from the basal metaconglomerate and the middle quartzite member of the marquenas formation yield minimum age peaks of 1472 ma and 1471 ma consistent with earlier results we interpret the onset of ca 1490 1450 ma deposition followed by tectonic burial regional al 2 sio 5 triple point metamorphism and ductile deformation at depths of 12 18 km to reflect a mesoproterozoic contractional orogenic event possibly related to the final suturing of the mazatzal crustal province to the southern margin of laurentia we propose to call this event the picuris orogeny

an evaluation of quantitative reconstruction of past precipitation records using coral skeletal sr ca and δ18o data abstract coupled records of 18o and sr ca in porites coral have been used to derive hydrological conditions by removing the sr ca inferred temperature component from the 18o signal nanwan a semi enclosed bay in southern taiwan provides an opportunity to demonstrate the feasibility of quantitatively reconstructing past precipitation history recurrence of seawater 18o offsets between wet and dry seasons in the early 1990s is well correlated with the precipitation record even though the hydrological signal only accounts for 20 of the total annual coral 18o variation of ca 1 offsets can be found in the residual 18o of modern corals after removing the thermal effect which contributes to the other 80 the observed timing and amplitude of the seasonal seawater 18o offsets in nanwan and their correlation with precipitation are reproduced by hydrological models in the mid holocene the seasonal anomaly of residual 18o was twice that of the modern value based on the 9 yr sr ca 18o data recorded in a 6 73 ka porites coral hydrological models suggest an annual rainfall of 1800 3000 mm yr at the window during mid holocene 20 higher than that of the average of 30 yr modern instrumental records of 1500 2500 mm consistent with the qualitative pollen record from lake sediments the seasonal decrease of residual 18o in 5 of 9 yr was earlier than the increase of the coral sr ca inferred temperature which implies that these rainy seasons probably occurred from the early mid spring to mid summer earlier than that from late spring to late summer today the driving force may be related to the changes of solar insolation and the east asian monsoon it is cautioned that the variability of hydrographic conditions imposes restrictions on a precise calculation of the amount of paleo precipitation the dynamic nature of local tectonics monsoons and water circulation should be further addressed to precisely quantify precipitation over the past 10 000 yr from coral geochemical records

lead isotopic compositions and paleohydrology of caldera related epithermal veins lake city colorado the uncompahgre caldera and the lake city caldera nested within it each have fossil hydrothermal systems and associated mineral deposits that formed during multiple episodes of mineralization during oligocene and miocene time new lead isotopic analyses for 51 ore samples mainly galena combined with previously obtained data for ore minerals and rocks suggest likely lead source rocks and fluid migration paths most values of 206pb 204pb 207pb 204pb and 208pb 204pb in galena from veins match the respective lead isotopic ratios for their host rocks for example all have 206pb 204pb typically in the range 18 4 19 0 the source for most vein lead is therefore probably the host unit for the vein some mixing of lead from other sources also occurred ratios of 206pb 204pb 19 0 probably indicate a component of more radiogenic lead from a middle proterozoic source lower lead isotopic ratios 206pb 204pb 18 5 suggest mixing with less radiogenic lead from either miocene rhyolitic volcanic and intrusive rocks or early oligocene intermediate composition volcanic rocks hydrothermal flow in the uncompahgre caldera was predominantly west to east down the topographic slope discharge was mainly in closed topographic lows marked by lacustrine deposits hydrothermal circulation in the lake city caldera was controlled by local topography and post caldera intrusions and was isolated from flow in the uncompahgre caldera and eureka graben richer ore deposits may be associated with ring fault related conduits that extend through the volcanic cover to more radiogenic middle proterozoic basement at depth as in the rest of the san juan mountains lead originally came from a predominantly 1450 ma source enough variation in 207pb 204pb was produced by orogenic events at ca 1450 ma ca 1760 ma and earlier to explain most of the 207pb 204pb variation present today in tertiary volcanic rocks and hydrothermal veins

recent 121 year variability of western boundary upwelling in the northern south china sea 1 coastal upwelling is typically related to the eastern boundary upwelling system whereas the powerful southwest asian summer monsoon can also generate significant cold nutrient rich deep water in western coastal zones here we present a sea surface temperature record a d 1876 1996 derived from coral porites sr ca for an upwelling zone in the northern south china sea the upwelling induced sea surface temperature anomaly record reveals prominent multidecadal variability driven by asian summer monsoon dynamics with an abrupt transition from warmer to colder conditions in 1930 and a return to warmer conditions after 1960 previous studies suggest the expected increase in atmospheric co2 for the coming decades may result in intensification in the eastern boundary upwelling system which could enhance upwelling of co2 rich deep water thus exacerbating the impact of acidification in these productive zones in contrast the weakening trend since 1961 in the upwelling time series from the northern south china sea suggests moderate regional ocean acidification from upwelling thus a stress relief for marine life in this region

a coral based reconstruction of sea surface salinity at sabine bank vanuatu from 1842 to 2007 ce climate variability associated with the el nino southern oscillation enso results in large sea surface temperature sst and sea surface salinity sss anomalies in many regions of the tropical pacific ocean we investigate interannual changes in sss driven by enso in the southwestern pacific at sabine bank vanuatu sbv 166 04 e 15 94 s using monthly variations in coral 18o from 1842 to 2007 ce we develop and apply a coral 18o sss transfer function which is assessed using a calibration verification exercise 1970 2007 ce the 165 year reconstructed sss record contains a prominent trend toward freshening from 1842 to 2007 ce mean sss for 1842 1872 ce is 35 46 0 28 psu which contrasts with a mean value of 34 85 0 31 psu for 1977 2007 ce with a freshening trend during the latter part of the 20th century that is not unprecedented with respect to the overall record variance in the record is concentrated in the interannual 42 and interdecadal 29 bands the sbv sss record matches well with a similarly reconstructed sss time series at malo channel vanuatu which is located 120 km to the east of sbv this regional signal is likely driven by enso related changes in the spcz and interdecadal changes in surface water advection the pattern of interdecadal variability at sbv agrees reasonably well with coral records of interdecadal variability from fiji and tonga especially in the pre 1940 portions of the records further evidence for the regional extent of the salinity signal at sabine bank vanuatu

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

isotopic zonations in silicic magma chambers many ash flow tuffs are zoned in radiogenic isotope ratios indicating that roofward assimilation of crust occurs in ash flow magma chambers prior to eruption cases where relatively well constrained calculations may be made regarding the percentage of assimilation in the roof zone indicate that the percentage of assimilation often exceeds the percentage of phenocrysts in the tuffs this relation in addition to the fact that assimilation gradients are opposite to that of the percentage of phenocrysts suggests that assimilation and crystallization in the silicic roof zones of crustal magma chambers are separated in time and space and that these processes are best modeled as two component mixing true assimilation fractional crystallization is probably restricted to the lower mafic parts most phenocrysts in the silicic upper parts of magma chambers crystallized after assimilation providing minimum estimates of time between assimilation and eruption 1 100 yr preservation of monotonic isotopic gradients suggests that convection is minor in the upper parts of silicic magma chambers during the late stages of evolution

crustal evolution of southern laurentia during the paleoproterozoic insights from zircon hf isotopic studies of ca 1 75 ga rocks in central colorado lu hf depleted mantle model tdm ages obtained by analysis of zircons previously dated by u pb methods demonstrate that ca 1 75 ga bimodal paleoproterozoic rocks in the gunnison salida region of central colorado and by extension in much of the southwestern u s were formed by partial melting of preexisting crustal rocks the ages of which mostly ages greater than 2 0 ga indicate that range from 1 83 to 1 87 ga some calculated lu hf tdm even older crust was probably involved consistent with the limited presence of ca 2 5 ga xenocrystic zircons in some rocks these results suggest that rocks related to the trans hudson and penokean orogens are cryptically present much farther to the south than previously believed coupled with the bimodality of the volcanic suite in central colorado and indeed in much of the southwestern u s these results indicate that in contrast to current juvenile arc accretion models melting of older crust related to extensional tectonics played an important role in the genesis of many magmatic rocks between 1 6 and 1 8 ga

plate tectonics model for proterozoic continental accretion in the southwestern united states proterozoic crustal provinces in the southwestern united states decrease in age south from the archean wyoming province supracrustal rocks define three major age provinces from north to south 1 72 to 1 80 1 65 to 1 72 and 1 1 to 1 2 b y successions within each province are similar and define two assemblages a bimodal volcanic assemblage commonly overlain by a quartzite shale assemblage precambrian granitic plutons intrude all three provinces geochemical and sr isotope data indicate a variably depleted upper mantle source for basalts and a short lived these observations can be explained by a plate tectonics model involving successive marginal basin closures and andean type orogenies associated with southward migrating arcs the net result is to accrete 1 300 km of continental crust to the southwestern margin of north america between 1 8 and 1 1 b y ago

polyphase suprastructure deformation in metasedimentary rocks of the uncompahgre group remnant of an early proterozoic fold belt in southwest colorado isolated remnants of supracrustal rocks of the uncompahgre group in the needle mountains and uncompahgre gorge area have been subjected to polyphase deformation and greenschist facies metamorphism phase 1 structures recognized only in the west needle mountains include bedding parallel ductile shear zones f 1 mesoscopic folds and cleavage s 1 continued crustal shortening during phase 2 resulted in upright macroscopic folding f 2 and cleavage s 2 development in addition basement cover contacts were folded further shortening during phase 3 deformation resulted in conjugate shearing of cover rocks with the development of both a macroscopic f 3 fold and an s 3 crenulation cleavage in addition basement cover contact zones were activated as phyllonitic oblique slip reverse faults conjugate shearing resulted in lateral offset of stratigraphic units with loss of section conjugate reactivation of phase 1 ductile deformation zones was manifested by oblique dextral and sinistral slip the structural evolution of the uncompahgre group records the transition from a ductile north directed thin skinned fold thrust belt to the formation of a basement involved megamullion structure which was subjected to conjugate strike slip faulting to accommodate further shortening the results of this study indicate that the basal unit of the uncompahgre group was parautochthonous on basement a departure from some previous interpretations this implies that the metasedimentary cover rocks are younger than surrounding ca 1750 ma basement gneisses and ca 1690 ma granitoids the over all style of deformation may be analogous to the deep foreland suprastructure of an orogenic belt that developed between ca 1690 ma and 1600 ma in the southwestern united states

age provenance and tectonic setting of paleoproterozoic quartzite successions in the southwestern united states new field studies combined with u pb zircon geochronology constrain the ages of deposition and sedimentary provenance of paleoproterozoic quartzite successions exposed in the southwestern united states orthoquartzites were deposited in short lived basins at two times ca 1 70 and 1 65 ga during crustal assembly of southern laurentia the more voluminous ca 1 70 ga successions occur in southern colorado northern new mexico and central arizona and are interpreted here to be time correlative though not necessarily deposited in the same basins detrital zircon from quartzites and metaconglomerates exposed in southern colorado and northern new mexico is characterized by a single population with a relatively narrow range of ages 1 80 1 70 ga and minimal archean input 5 of grains analyzed peak detrital zircon ages 1 76 1 70 ga vary slightly from location to location and mimic the age of underlying basement unimodal detrital populations suggest local sources and a first cycle origin of the orthoquartzites within a short time interval 1 70 1 68 ga during unroofing of local underlying basement the maximum age of quartzite exposed at blue ridge colorado is constrained by the 1705 1698 ma coarse grained granitoid basement on which quartzite was deposited unconformably the minimum age of ortega formation quartzite new mexico is constrained by ca 1680 1670 ma metamorphic monazite overgrowths these dates agree with direct ages on the lower mazatzal group arizona and suggest that orthoquartzite deposition occurred over a wide region during and soon after the ca 1 70 ga yavapai orogeny regional structural arguments and the thrust style of quartzite deformation suggest that the metasedimentary successions were deformed during the ca 1 66 1 60 ga mazatzal orogeny thus making them important time markers separating the yavapai and mazatzal orogenic events our model for syntectonic deposition involves extensional basin development followed by thrust closure possibly due to opening and closing of slab rollback basins related to outboard subduction the first cycle origin of orthoquartzites near the end of the arc collisions of the yavapai orogeny seems to contrast sharply with their extreme compositional maturity this can be explained in terms of protracted extreme diagenesis and or special environmental influences that enhanced chemical weathering but were unique to the transitional atmosphere and ocean chemistry of the proterozoic similarities among quartzites exposed throughout the southwestern united states and along the laurentian margin suggest that they represent a widespread regional and perhaps global episode of sedimentation involving a distinctive syntectonic setting and unique climatic conditions a combination that might make these units a signature lithology for paleoproterozoic time

pb isotope constraints on the provenance and diagenesis of detrital feldspars from the sudbury basin canada abstract residues after hf leaching of detrital feldspar grains from the early proterozoic chelmsford formation of the sudbury basin ontario record isotopic compositions very close to the initial pb isotopic composition of their late archean superior province source these grains have been recrystallized to albite and k feldspar residues of chelmsford feldspar grains lie within the range of reported 206 pb 204 pb compositions of feldspars from plutons of the adjacent abitibi subprovince of the superior province with a steep 207 pb 206 pb slope of 0 35 0 15 3 7 0 7 ga leaches are more radiogenic than residues but leach residue pairs do not record in situ u and th decay since either the late archean time of feldspar crystallization in the superior province or the early proterozoic time of recrystallization in the chelmsford formation instead leach residue pairs lie along an average 207 pb 206 pb slope of 0 250 0 036 representing mixing trends most whole rock samples from the chelmsford formation lie close to a 2 7 ga slope collinear with detrital feldspar compositions we interpret the pb data from the leaches as recording the passing of altering fluids these fluids likely carried pb derived from rocks or minerals with relatively high u pb th pb and th u within the sedimentary basin itself the residues appear to record very close to initial values because they lie along the very steep trend defined by feldspars of the superior province

lead isotopic heterogeneities within alkali feldspars implications for the determination of initial lead isotopic compositions abstract the mobility of uranium during metamorphism and alteration places constraints on the use of lead isotopic compositions as petrogenetic tracers in older rocks the problem of uranium mobility may be partially circumvented by analysis of alkali feldspar which because of its low u pb ratio should have a lead isotopic composition similar to the initial lead isotopic composition of the rock previous leaching and volatilization experiments however have demonstrated that alkali feldspars contain radio genic lead which can be removed preferentially to nonradiogenic lead this paper reports on the results of progressive acid leaching of a single alkali feldspar phenocryst and on albitic perthites separated from the phenocryst this experiment suggests that a number of distinct lead isotopic reservoirs are present within the alkali feldspar including a nonradiogenic reservoir approximating the initial lead isotopic composition of the system a radiogenic reservoir corresponding to albitic exsolution lamellae within the alkali feldspar loosely bound labile lead presumably associated with uranium and thorium occupying fractures and finally a radiogenic component corresponding to unidentified uranium thorium rich phases the commonly observed decrease in radiogenicity with progressive leaching of alkali feldspars is interpreted to be largely the result of preferential leaching of the albitic perthites relative to the host alkali feldspar containing the nonradiogenic reservoir the results of this experiment confirm that initial lead isotopic compositions may be estimated from alkali feldspars through severe acid leaching however this experiment also illustrates the complexity of the lead isotopic systematics of alkali feldspars the determination of the initial lead isotopic composition of an alkali feldspar requires experimental procedures which recognize and compensate for the presence of multiple lead reservoirs within typical alkali feldspars

a high resolution sr ca and δ18o coral record from the great barrier reef australia and the 1982 1983 el nino abstract a high resolution near weekly sr ca and oxygen isotopic record is presented for a coral from the pandora reef in the great barrier reef gbr of australia during the period 1978 to 1984 the records are well correlated except for periods of high rainfall when river runoff has significantly modified the 18o value of seawater using the sr ca temperature calibration of de villiers et al 1994 the sr ca records exhibit seasonally controlled cyclical sst sea surface temperature variations of from 21 to 28 c during the very strong el nino of 1982 1983 the sr ca systematics indicate a sharp drop in the winter sst to 18 5 c this represents a temperature anomaly of 3 c which is approximately twice that given by the 18o variations suggesting an 2 amplification of the anomaly by the sr ca system possibly due to the increasing dominance of inorganically controlled aragoniteseawater fractionation the oxygen isotopic systematics show the combined effects of both temperature and changing seawater 18o values the latter reflecting the influx of 18o depleted runoff during periods of high rainfall due to the extremely low 10 3 sr and ca contents of river run off relative to seawater it is possible to use the sr ca thermometer to calculate temperatures independent of major floods and hence deconvolve the combined effects in the oxygen isotopic record of variable temperature and the 18o value of seawater using this approach it is possible to quantitatively reproduce the volume of runoff from the burdekin river during the periods of major flooding that occurred in early 1979 and 1981 the results of this study demonstrate that the combined use of high resolution sr ca and 18o systematics in scleractinian corals is a powerful tool for providing quantitative constraints on past climate

interannual and decadal variability of the western pacific sea surface condition for the years 1787 2000 reconstruction based on stable isotope record from a guam coral 1 we present a monthly resolved 213 year stable isotope time series from a coral from guam 13 n 145 e which is located on the northern edge of the western pacific warm pool oxygen isotopic composition of the coral skeleton 18ocoral shows seasonal interannual and decadal variability which documents significant oceanographic changes related to thermal and hydrologic variations in this region the 18ocoral anomaly reflects sea surface temperature sst anomaly and sea surface salinity sss anomaly with significant r values of 0 69 and 0 49 respectively which are strongly linked to oceanographic changes that occur during el nino southern oscillation enso warm and cool phases we identified 46 enso warm el nino and 53 cool phases la nina in the coral record which are consistent with those phases reconstructed by nino 3 4 sst anomaly spectral analyses of the 18ocoral anomaly record for the years 1790 1999 identified significant peaks around 3 to 7 years these results indicate that the guam coral has recorded enso periodicity the 18ocoral anomaly shows decadal variability of 15 to 45 year periodicity with significant shifts 0 2 from warmer to cooler condition and vice versa an accumulative decrease in 18ocoral time series may imply 0 75 c warming of sst and 0 23 freshening of seawater 18o corresponding to a decrease of sss by 0 85 in the northwestern tropical pacific over the last 2 centuries

summer upwelling in the south china sea and its role in regional climate variations seasonal and interannual variations of summer upwelling off the south vietnam coast and the offshore spread of cold water are investigated using a suite of new satellite measurements in summer as the southwesterly winds impinge on annam cordillera a north south running mountain range on the east coast of indochina a strong wind jet occurs at its southern tip offshore east of saigon resulting in strong wind curls that are important for ocean upwelling off the coast in july and august an anticyclonic ocean eddy develops to the southeast advecting the cold coastal water offshore into the open south china sea scs the center of this cold filament is located consistently north of the wind speed maximum indicating that open ocean upwelling helps to cool the ocean surface corroborating evidence for the cold filament is found in ocean color observations that reveal a collocated tongue of high chlorophyll concentration the development of this cold filament disrupts the summer warming of the scs and causes a pronounced semiannual cycle in sst moreover the cold filament is an important player in interannual variability in the summer scs in 1998 the cold filament and mid summer cooling never took place giving rise to a strong basin wide surface warming interannual sst variance has a local maximum over the climatological cold filament and is much greater than the variance over the adjacent indian and western pacific oceans a cold filament index is constructed which displays significant lagged correlation with sst in the eastern equatorial pacific and indian oceans indicative of a teleconnection from el nino

geochemical and detrital mode evidence for two sources of early proterozoic sedimentary rocks from the tonto basin supergroup central arizona the tonto basin supergroup includes up to 6 5 km of early proterozoic sedimentary and volcanic rocks that were deposited in a relatively short period of time at about 1 7 ga in central arizona moderate correlations of rare earth elements ree and ti with al2o3 and ree distributions in detrital sediments of this supergroup suggest that these elements are contained chiefly in clay mica and or zircon fractions ree distributions including negative eu anomalies in most tonto basin sediments are similar to those in phanerozoic shales weak to moderate correlations of fe sc ni and co to al2o3 also suggest a clay mica control of these elements detrital modes and geochemical characteristics of sediments indicate two dominant sources for sedimentary rocks of the tonto basin supergroup a granitoid source and a volcanic source the granitoid source was important during deposition of the upper part of the succession the mazatzal group as shown by increases in k2o al2o3 and th in pelites with stratigraphic height and increases in zr and hf and decreases in eu eu cr and ni in in pelites of the maverick shale sediment provenance characteristics and paleocurrent indicators are consistent with deposition of the supergroup in a continental margin back arc basin the granitoid sediment source appears to have been the north american craton on the north and the volcanic source a more local source from an arc on the south

classifying el nino modoki i and ii by different impacts on rainfall in southern china and typhoon tracks based on their opposite influences on rainfall in southern china during boreal fall this paper classifies el nino modoki into two groups el nino modoki i and ii which show different origins and patterns of sst anomalies the warm sst anomalies originate in the equatorial central pacific and subtropical northeastern pacific for el nino modoki i and ii respectively thus el nino modoki i shows a symmetric sst anomaly distribution about the equator with the maximum warming in the equatorial central pacific whereas el nino modokiii displays an asymmetric distribution with the warmsstanomaliesextending fromthe northeastern pacifictotheequatorialcentralpacific additionally thewarmsstanomaliesintheequatorialcentralpacific extend farther westward for el nino modoki ii than for el nino modoki i similar to the canonical el nino el nino modoki i is associated with an anomalous anticyclone in the philippine sea that induces south westerly wind anomalies along the south coast of china and carries the moisture for increasing rainfall in southern china for el nino modoki ii an anomalous cyclone resides east of the philippines associated with northerly wind anomalies and a decrease in rainfall in southern china the canonical el nino and el nino modoki i are associated with a westward extension of the western north pacific subtropical high wnpsh whereas el nino modoki ii shifts the wnpsh eastward differing from canonical el nino and el nino modoki i el nino modoki ii corresponds to northwesterly anomalies of the typhoon steeringflow which are unfavorable for typhoons to make landfall in china

late quaternary loess in northeastern colorado part ii pb isotopic evidence for the variability of loess sources a new application of the pb isotopic tracer technique has been used to determine the relative importance of different silt sources for late wisconsin loess in the central great plains of eastern colorado samples of the peoria loess collected throughout the study area contain k feldspar derived from two isotopically and genetically distinct sources 1 glaciogenic material from early and middle proterozoic crystalline rocks of the colorado province and 2 volcaniclastic material from the tertiary white river group exposed on the northern great plains pb isotopic compositions of k feldspar in loess from two dated vertical sections at beecher island and last chance colorado vary systematically implying climatic control of source availability we propose a model whereby relatively cold conditions promoted the advance of front range valley glaciers discharging relatively little glaciogenic silt but strong winds caused eolian erosion of white river group silt due to a decrease in vegetation cover during warmer periods valley glaciers receded and discharged abundant glaciogenic silt while surfaces underlain by the white river group were stabilized by vegetation isotopic data from eastern colorado loess sections record two warm cold warm cycles during late wisconsin time between about 21 000 and 11 000 radiocarbon yr b p similar to results from other studies in the united states and greenland

identification of 2 0 to 2 4 ga nd model age crustal material in the cheyenne belt southeastern wyoming implications for proterozoic accretionary tectonics at the southern margin of the wyoming craton a detailed survey was conducted of the nd isotope compositions of precambrian metamorphic rocks both within and adjacent to the cheyenne belt a metamorphosed crustal segment in southeastern wyoming that defines the proterozoic suture zone between the archean wyoming craton and accreted proterozoic island arc s the results of the study indicate that proterozoic miogeoclinal sedimentary rocks exposed north of the cheyenne belt and deposited at the southern margin of the wyoming craton contain only archean crust derived material and no detritus from proterozoic island arcs approaching from the south similarly proterozoic crust exposed south of the cheyenne belt contains no crustal material derived from archean sources however within the cheyenne belt paragneiss of the barber lake block has 2 0 to 2 4 ga nd model ages and is interpreted as metamorphosed intermixed sedimentary detritus from both archean and proterozoic sources the paragneiss is thought to represent remnants of oceanic trench or foredeep sediments deposited in close proximity to both the archean craton and the proterozoic island arcs similar to sedimentary rocks with 2 0 to 2 4 ga nd model ages exposed in the penokean orogen in wisconsin the overall lack of 2 0 to 2 4 ga modal age crustal material in the vicinity of the cheyenne belt may reflect the fact that much of this crustal material was overthrust onto the wyoming craton during accretion and then eroded during postorogenic rebound of the continental margin

u pb geochronology in the trans hudson orogen northern saskatchewan canada we have obtained u pb ages on zircons from volcanic and plutonic units in several lithotectonic domains of the southern trans hudson orogen in northern saskatchewan these data constrain the timing of early proterozoic orogenic events in the region and enhance our understanding of both the relationships among local domains and the relationship of the trans hudson orogen to other early proterozoic orogens in north america with one exception all units studied so far yield zircon ages of 1890 1835 ma most of which are systematically earlier than previously reported rb sr isochron ages on the same or similar units suggesting open system behavior in the rb sr systems five metarhyolites from volcanic sequences in the la ronge domain glennie domain and hanson lake block give ages ranging from 1888 to 1876 ma most of the plutons we dated ranging from gneissic syntectonic tonalites and granodiorites to less deformed late intrusions such as the wathaman batholith and other smaller bodies yield ages of 18

u pb systematics of monazite and xenotime case studies from the paleoproterozoic of the grand canyon arizona monazite is accepted widely as an important u pb geochronometer in metamorphic terranes because it potentially preserves prograde crystallization ages however recent studies have shown that the u pb isotopic system in monazite can be influenced by a variety of processes that partially obscure the early growth history in this paper we attempt to interpret complex monazite and xenotime u pb data from three paleoproterozoic granite dikes exposed in the grand canyon single crystal monazite analyses from an unfoliated granite dike spread out along concordia from the crystallization age of the dike defined by u pb zircon data to be 1685 1 ma to 1659 2 ma a span of 26 million years back scattered electron bse imaging reveals that magmatic domains within most crystals from this sample are truncated by secondary domains associated with prominent embayments at the grain margin fragments of a single crystal yield contrasting concordant dates and fragments from the edges and tips of crystals yield the youngest dates based on these observations we suggest that the secondary domains formed at least 26 million years after the crystal formed monazite and xenotime dates from the second sample a sheared dike that cross cuts the previous dike spread out along concordia over 16 million years and range up to 2 4 normally discordant again bse imaging reveals secondary domains that truncate both magmatic zoning and xenocrystic cores fragments sliced from specific domains of a previously imaged monazite crystal demonstrate that the secondary domain is 13 million years younger than the core domain textures revealed in bse images suggest that the secondary domains formed by fluid mineral interaction normal discordance appears to result from both radiation damage accumulated at temperatures below 300 c and water mineral interaction monazite data from the third sample exhibit dispersion in both the 207pb 206pb dates 1677 1690 ma and discordance 1 6 to 3 1 reverse discordance in these monazites cannot be explained by incomplete dissolution or excess thorogenic 206pb sliced fragments from several crystals reveal dramatic intragrain u pb disequilibrium that does not correlate with either th or u concentration or position within the crystal we suggest that reverse discordance resulted from mechanisms that involve exchange or fractionation of elemental u or elemental pb and that neither the u pb dates nor the 207pb 206pb dates are reliable indicators of the rock s crystallization age given the large number of processes proposed in the recent literature to explain monazite u pb systematics from rocks of all ages our results can be viewed as another cautionary note for single crystal and multi crystal monazite geochronometry however we suggest that because individual crystals can preserve a temporal record of primary and secondary monazite growth micro sampling of individual monazite crystals may provide precise absolute ages on a variety of processes that operate during the prograde peak and or retrograde history of metamorphic terranes

petrogenesis of the 1 43 ga sherman batholith se wyoming usa a reduced rapakivi type anorogenic granite the 1 43 ga sherman batholith southeastern wyoming usa origin that are found throughout the proterozoic anorogenic granite provinces of the southwestern usa adirshows extreme a type petrochemical characteristics compared with other mid proterozoic granite batholiths of north america it ondacks eastern canada southern greenland and the fennoscandian shield anderson 1983 phanerozoic consists of 1 the sherman granite a coarse grained biotite hornblende granite that locally contains fayalite and pyroxenes 2 examples include paleozoic granites of southeastern australia collins et al 1982 and of corsica poitrasson et the lincoln granite a medium grained biotite granite 3 a porphyritic biotite hornblende granite that probably formed by interaction al 1995 and the mesozoic nigerian younger granites jacobsen et al 1958 and the white mountain magma of granitic and mafic magmas and 4 iron enriched mafic dikes and pods the ilmenite series metaluminous sherman granite series foland allen 1991 anorogenic granite refers to plutons that are not associated with compressional exhibits extreme values of feo feo mgo and is rich in structures and that were emplaced long after any known k ree nb and y it crystallized at temperatures exceeding orogenic event anorogenic or a type granites loiselle 900 c and a pressure of 2 5 kbar with water activity of 0 7 wones 1979 are characterized by low h2o and o2 and dlog fo2 of 0 1 to 0 5 the lincoln granite which is fugacities along with high feo feo mgo k2o peraluminous and has less extreme a type geochemical characteristics na2o and k2o contents a type granites typically are rich crystallized at temperatures as low as 750 c and dlog fo2 of in incompatible elements including rare earth elements around 0 5 units above fmq fayalite magnetite quartz the ree zr nb and ta but poor in co sc cr ni ba rocks of the sherman batholith are chemically equivalent to lavas sr and eu minimum magma liquidus temperatures are from the yellowstone hotspot like the yellowstone magmas the 900 1000 c clemens et al 1986 creaser white sherman batholith probably originated by partial melting of under1991 some workers suggest these rocks are derived plated mantle derived mafic rocks from melting of tonalitic or more felsic crust anderson cullers 1978 collins et al 1982 clemens et al 1986 creaser et al 1991 whereas others relate the granites

atmospheric circulation cells associated with the el nino southern oscillation abstract atmospheric circulation cells associated with the el nino southern oscillation enso are described and examined using the ncep ncar reanalysis field and the ncep sea surface temperature sst from january 1950 to december 1999 the divergent wind and pressure vertical velocity are employed for the identification of atmospheric circulation cells the warm phase of enso shows positive sst anomalies in the equatorial eastern pacific and along the east coast of asia and the west coast of north america and negative sst anomalies in the off equatorial western pacific and in the central north pacific associated with this sst anomaly distribution are variations of atmospheric zonal and meridional circulation cells over the pacific the equatorial zonal walker circulation cell is weakened consistent with previous schematic diagrams the anomalous meridional hadley circulation cell in the eastern pacific shows the air rising in the tropics flowing poleward in the upper troposphere sinking in the subt