the structure of archean ketilidian crust along the continental shelf of southwestern greenland from a seismic refraction profile the velocity structure of the continental crust on the outer shelf of southwestern greenland is determined from dense wide angle reflection refraction data obtained with large air gun sources and ocean bottom seismometers along a 230 km seismic line this line crosses the geological boundary between the archean block and the ketilidian mobile belt although the data have high noise levels p and s wave arrivals from within the upper intermediate and lower crust and at the moho boundary can be consistently identified and correlated with one dimensional wkbj synthetic seismograms in the archean p and s wave velocities in the upper crust are 6 0 and 3 4 km s while in the intermediate crust they are 6 4 and 3 6 km s these velocities match for the upper crust a quartz feldspar gneiss composition and for the intermediate crust an amphibolitized pyroxene granulite in the ketilidian mobile belt p and s wave velocities are 5 6 and 3 3 km s for the upper crust and 6 3 and 3 6 km s for the intermediate

reevaluation of the oxygen isotopic composition of planktonic foraminifera experimental results and revised paleotemperature equations cultured planktonic foraminifera orbulina universa symbiotic and globigerina bulloides nonsymbiotic are used to reexamine temperature 18o relationships at 15 25 c relationships for both species can be described by linear equations equations for o universa grown under low light ll and high light hl share a slope of 4 80 0 21 c 1 with a hl ll offset of 0 33 due to symbiont photosynthetic activity the effect of co32 on o universa is 0 002 mol 1 kg 1 and is insensitive to temperature for g bulloides ontogenetic effects produce size related trends in temperature 18o whereby larger shells are enriched in 18o relative to smaller specimens the o universa temperature 18o relationships are more accurate than previously published equations for describing plankton tow data our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic cibicidoides data below 10 c temperature 18o relationships for g bulloides provide good agreement with field data for this species from the northeast pacific

geochemistry and origin of archaean quartz cordierite gneisses from the godthabsfjord region west greenland we report new data on the major minor and trace element compositions of metasedimentary quartzcordierite gneisses qcg an important member of the archaean malene supracrustal suite found throughout the godthabsfjord region of west greenland the analyzed qcg contain assemblages of quartz cordierite biotite garnet anthophyllite gedrite staurolite sillimanite plagioclase with abundant accessory zircon and minor rutile monazite and allanite and broadly resemble cordierite orthoamphibole rocks found in a great number of other metamorphic terrains chemically the qcg are characterized by 1 high but variable sio2 59 87 wt relative enrichments in mgo feo and al2o3 mg 0 35 0 85 and depletions in na2o k2o and especially cao 2 low concentrations of sc cr co ni and sr 3 high concentrations of y nb zr hf ta th u and ree rare earth elements with prominent negative eu anomalies in each case 5 high concentrations of ga 18 55 ppm with variable ga al ratios that are significantly higher than average crustal material low cr and ni together with enriched and fractionated ree displaying negative eu anomalies distinguish the malene qcg from published accounts of most other archaean sedimentary rocks furthermore all of the above mentioned trace element characteristics distinguish the qcg from ordinary malene clastic metasediments quartzites psammites and pelites suggesting a separate origin for the qcg these data point towards chemically evolved felsic igneous rocks being the source of the qcg consequently we propose that the malene qcg represent metamorphosed felsic volcaniclastic sediments that underwent hydrothermal alteration by heated seawater prior to metamorphism which resulted in gain of mg and fe loss of alkalis and lime and possibly eu and sr the overall trace element characteristics of the qcg elevated ga zr nb ree etc are features shared by a type granites and their volcanic equivalents such igneous rocks may represent the ultimate source material for the qcg protolith

dating the oldest terrestrial rocks fact and fiction abstract this paper is a review of well documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks mainly using rb 1bsr sm 1bnd and u 1bpb decay schemes the oldest known stable sialic crust of true continental character predominantly composed of calc alkaline orthogneisses was produced during major mantle differentiation some 3 7 3 5 ga ago and is represented on several continents enclaves of older supracrustal rocks as well as a major remnant of a volcano sedimentary sequence of greenstone belt affinity dated at 3 8 ga are preserved within the surrounding gneisses in southern west greenland recent ionmicroprobe evidence for 4 1 ga old detrital zircon grains in a younger early archaean metasedimentary sequence in western australia is also discussed several cases are reviewed where claims for early archaean 3 4 ga dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data some of the geological and geochemical mechanisms which have given rise to claims for spuriously old sm 1bnd u 1bpb and pb pb dates are critically discussed to supplement published data we present several new sets of whole rock sm 1bnd and pb pb isotopic data from west greenland labrador zimbabwe and eastern india relevant to the overall topic of this review

late archaean terrane accretion in the godthab region southern west greenland tectono stratigraphic terranes fault bounded blocks of the earth s crust characterized by a geological history distinct from that of adjacent terranes1 are now widely recognized in phanerozoic and proterozoic orogenic belts and their present configurations are thought to result from plate tectonic processes similar to those in operation today2 4 there has been much debate about whether such processes operated in archaean times5 6 here we present new data from the archaean high grade gneiss complex of the godthab now nuuk region of southern west greenland which show that discrete tectonic units recognized in the south of the region7 can be more widely identified at least four terranes are recognized each of which evolved separately until their juxtaposition in the late archaean this suggests that at least some archaean high grade gneiss complexes may resemble more recent orogenic belts formed by plate tectonic processes

a shear zone related greenstone belt hosted gold mineralization in the archean of west greenland a petrographic and combined pb pb and rb sr geochronological study abstract widespread gold showings have been found in an archean greenstone belt on storo in godthabsfjord west greenland the storo greenstone belt sgb comprises metasedimentary and metavolcanic rocks with minor magnetite rich bands gold mineralization is hosted by amphibolites mica schists and garnetite rocks gold occurs in various mineral associations but its presence in lollingite arsenopyrite pyrrhotite parageneses is prevalent to a lesser extent native gold occurs within seriticized plagioclase in sheeted quartz veins and as inclusions in garnet based on petrographic observations and comparison with similar ore mineral textures from other deposits we propose the following model for the deposition of the gold bearing lollingite arsenopyrite pyrrhotite assemblage on storo in the first stage arsenopyrite likely formed with gold in solid solution in the second stage gold bearing arsenopyrite was replaced by lollingite and pyrrhotite this took place during regional prograde metamorphism gold accumulated within lollingite as discrete grains during subsequent retrograde metamorphic conditions lollingite and pyrrhotite reacted and formed arsenopyrite and gold was subsequently exsolved and preferably deposited as discrete grains or films along arsenopyrite lollingite grain boundaries during a later event a garnet overgrowth partly affected the gold bearing lollingite arsenopyrite pyrrhotite assemblage the isotopic composition of pb released from these garnets by pb stepwise leaching experiments pbsl and the bulk pb isotopic compositions of arsenopyrite and pyrrhotite indicate a common source of pb in the fluid from which first the sulfides and subsequently garnet were formed bulk pb analyses and pbsl experiments of arsenopyrite and pbsl experiments of garnet yield ages of 2863 24 ma 2 mswd 0 2 for arsenopyrite and 2748 62 ma 2 mswd 220 for garnet the 2863 24 ma age is thus a minimum age for the mineralization rb sr isotopic analyses of cogenetic biotite from within strongly mineralized zones reveal middle proterozoic ca 1 65 ga mineral ages well in accordance with a regional reheating event known to have affected the area and are thus interpreted as cooling ages thereof this regional reheating event has apparently not affected the pb pb isotopic system of arsenopyrite implying that the blocking temperature of the u pb system in arsenopyrite is likely to be above 300 c the late archean ages for arsenopyrite and garnet are consistent with the regional amphibolite facies metamorphism related to major terrane assemblies in the study area occurring at that time the ore textures preserved today are interpreted as those formed under this regional metamorphic event

evidence for a fold nappe in the high grade basement of central sri lanka terrane assembly in the pan african lower crust abstract structural analysis in the kadugannawa complex kc in central sri lanka reveals the presence of an early steep gneissic layering s 1 which has been overprinted by a younger event d 2 in the structurally deepest levels d 2 folds become progressively tighter towards the contact with the underlying highland complex hc the distribution of lithologies in the kc the distribution of steep s 1 and subhorizontal s 2 and the strain gradient towards the kc hc contact suggest that the kc is a d 2 fold nappe overlying the hc vergence of asymmetric sheath folds in sections parallel to a strong nnw sse lineation is towards nnw indicating a top to the nnw displacement granulite facies assemblages were formed during and after the final stages of d 2 in the kc d 2 was the main fabric forming event in the hc where meta igneous and metasedimentary gneisses show l s fabrics defined by the peak granulite facies assemblages the sense of shear deduced from asymmetric foliation boudinage is consistent with the top to the nnw displacement in the kc both units have been refolded around large scale upright d 3 folds the nappe model and nd model ages suggest that the kc and the eastern part of the hc may represent distinct lower crustal terranes juxtaposed during pan african high grade high strain events d 2 the contact between them is interpreted as a terrane boundary in a lower crustal environment the top to the nnw movement sense suggests that the kc may have been originally located south of the eastern highland complex this and the similarity in isotope signatures indicate a pre assembly link of the kc the western highland complex and the vijayan complex of eastern sri lanka

comparative sedimentology of shelf carbonates of the middle ordovician st paul group central appalachians abstract observations of present day environments are commonly used to aid in the interpretation of ancient sedimentary sequences this approach is used to determine the paleoclimate and depositional processes operating during formation of the limestones and dolostones of the lower middle ordovician st paul group row park and new market formations of maryland and southern pennsylvania a 200 m thick carbonate platform deposit within the group six facies defined by suites of sedimentary structures faunal assemblages and early diagenetic features are arranged in a large scale regressive transgressive sequence these consist of facies i lower burrowed peloidal facies facies ii burrowed skeletal facies facies iii lower laminated peloidal facies facies iv stromatolitic peloidal facies facies v upper laminated peloidal facies and facies vi upper burrowed peloidal facies a comparison of the st paul group deposits with those of the modern peritidal carbonate environments of shark bay western australia the abu dhabi and trucial coast areas of the persian gulf and andros island and the great bahama bank reveals 1 an overall similarity in features between the st paul group and the sediments of andros island and the great bahama bank and 2 a dissimilarity between those two deposits and the sediments of shark bay western australia and shoreline areas of the persian gulf for example the supratidal zones of the latter two typically contain evaporite minerals e g gypsum and anhydrite whereas the supratidal deposits of andros island and the st paul group are freshwater algal marsh and lake sediments the subtidal deposits of shark bay and the persian gulf generally are mechanically layered skeletal sand the subtidal deposits of the great bahama bank and the st paul group are bioturbated peloidal micrites the st paul group and the great bahama bank deposits are similar on a facies scale down to fine details observed microscopically bahamian modern analogs of the facies of the st paul group are for facies i shallow restricted bank and subtidal pond environments for facies ii a normal marine subtidal environment similar to the shoals which rim parts of the great bahama bank for facies iii and v tidal channel levee pond and marsh subenvironments of the channeled belt environment hardie and garrett 1977 for facies iv levee marsh and freshwater lake subenvironments of the interior of andros island and for the facies vi a semirestricted subtidal bank environment based on the features of the st paul group and by analogy with modern deposits the st paul group was deposited in a low energy rainy environment where deposition was controlled by storm processes many facies display penecontemporaneous marine cements and dissolution features

preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend gneiss complexes are the world s most horrendously complexrocks to understand see cover of this issue yet they form a largeamount of the continents albeit they are normally hidden fromview by sedimentary sequences the last half century has seenenormous advances in understanding these rocks which form notonly a large amount of the continental crust but also hold much of earth s oldest rock record the archaean disciplines medicine and health sciences social and behavioral sciences publication details nutman a p 2014 preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend precambrian research 255 part 3 771 773 this journal article is available at research online http ro uow edu au smhpapers 2672 preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend

reconstructing mid to high latitude marine climate and ocean variability using bivalves coralline algae and marine sediment cores from the northern hemisphere quantifying the role and contribution of the world s oceans in past present and future global change is an essential goal in climate paleoclimate and environmental studies although the global oceans interact and influence climate greatly the marine environment is substantially under represented in key climate assessment reports especially during the last millennium ipcc 2007 see palaeoclimate chapter 6 6 the last 2000 years the under representation of marine records in key climate documents likely results from the often imprecise chronologies associated with many marine based archives which greatly hinders singular climate comparisons lag lead phasing relationships with well dated and or annually resolved archives however several marine archive records have excellent chronological constraint in particular many marine bivalve taxa and coralline algae have annual increments that form within their carbonate framework that can be used to establish an absolutely dated chronology via cross dating techniques from the marine environment additionally in some cases where sedimentation rates are high and alternative chronological dating methods exist e g tephrochronology other than radiocarbon measurements often greater than 40 years uncertainty sediment archives can provide continuous sub decadal records of environmental change for centuries to millennia this brief introductory article and accompanying special issue will focus on the utilization of bivalves coralline algae and high resolution marine sediment cores in paleoclimate and environmental studies within the most recent millennium with a focus on the northern hemisphere

proposal for a terrane based nomenclature for the lewisian gneiss complex of nw scotland the current nomenclature for the lewisian gneiss complex has evolved from lithological and structural correlations made prior to any dating initial flawed geochronological studies gave some names an apparent chronological standing but as work advanced fitting events into a coherent regional framework became increasingly difficult modern dating studies have shown that the lewisian gneiss complex was progressively assembled from disparate blocks of archaean continental crust and juvenile proterozoic arcs which satisfy the definition of terranes each terrane had its own separate accretionary and metamorphic history followed by a common history once juxtaposed against other terranes based on a new compilation of modern geochronology allied to the many detailed structural and metamorphic studies this paper proposes a new systematic terminology for the lewisian gneiss complex that is more applicable to this new tectonic framework

gold occurrences of the archean north atlantic craton southwestern greenland a comprehensive genetic model abstract the north atlantic craton of southwestern greenland hosts several orogenic gold occurrences although to date none is in production four gold provinces are distinguished and include godthabsfjord tasiusarsuaq paamiut and tartoq in the godthabsfjord gold province the hypozonal gold occurrences are aligned along the major ca 2660 2600 ma ivinnguit fault orogenic gold mineralization correlates temporally with and is related to ductile deformation along this first order structure the northern part of the tasiusarsuaq gold province is characterized by small hypozonal gold occurrences that are controlled by 2670 2610 ma folds and shear zones auriferous fluids were focused into the structures in both gold provinces during west directed accretion of the kapisilik terrane 2650 2580 ma to the already amalgamated terranes of the north atlantic craton in the southern part of the tasiusarsuaq gold province hypozonal gold mineralization is hosted in back thrusts sermilik prospect and thrusts bjornesund prospect that formed at 2740 ma and 2860 2830 ma respectively the deformation is related to the ca 2850 ma accretion of the sioraq block and the tasiusarsuaq terrane and the 2800 2700 ma accretion of the tasiusarsuaq terrane and the faeringehavn and tre brodre terranes mesozonal orogenic gold mineralization is hosted in an accretionary complex in the paamiut and tartoq gold provinces gold occurrences cluster over a strike extent of approx 40 km in thrusts and complex strike slip settings in lateral ramps the timing of the e vergent terrane accretion in both areas is unknown and could either be at ca 2850 ma or 2740 ma in the eastern part of the paamiut gold province quartz veins and associated alteration zones were overprinted by granulite facies metamorphism and show evidence for partial melting these outermost parts of the accretionary complex were involved in burial exhumation tectonics during crustal accretion mainly three different orogenic stages related to gold mineralization are distinguished in the north atlantic craton between ca 2850 ma and 2610 ma these are generally accretionary tectonic episodes and gold mineralization is hosted either in reactivated fault systems between terranes or accretionary complex structures along the deformed cratonic margin the larger orogenic gold occurrences formed at ca 2740 2600 ma that appears to be a period of orogenic gold mineralization globally although significant gold resources in the north atlantic craton have yet to be identified

the first 800 million years of earth s history the earth grew initially from hot planetesimals of reduced material and began to differentiate early into a metal rich core a silicate rich mantle and a volcanic surface accretion ended with volatile rich oxidized material which did not interact with the core because of a mantle barrier quenching of magmas at the surface and foundering of cool material was effective in cooling the earth by 1600 j per gram of magma magmas transported radioactive elements and volatiles to the outer 50 km the lower mantle became stranded below the critical adiabat for melting the core may have been fully molten radiogenic and non radiogenic heat sources produced a surface heat flow that decreased from ca 1 3 x 10 14 w at 4 45 ga b p to the present value of 4 x 10 13 w in spite of uncertainties in the production and transport of heat it seems safe to conclude that intense volcanism occurred throughout the pre archaean era before survival of crust from phase equilibrium data for peridotites and calc alkaline rocks it is deduced that a range of ultrabasic to basic magmas was produced from a peridotitic upper mantle and that calc alkaline magmas were produced during foundering of crust an anorthositic crust was not formed from observations of lunar basins it is concluded that a minimum of ca 500 1000 impact basins were formed on earth in ca 100 200 ma before 3 95 ga b p plausible calculations suggest that ca 20 times as many basins were formed in the preceding ca 300 ma impact debris would be cooled by water in wet crustal rocks and oceans it is assumed that after 4 0 ga b p impacts would have only temporary local effects on the crust and emphasis is placed on upper mantle convection as the main driving force for geochemical transport sedimentation and metamorphism were important factors during foundering of crust oceanic9 and continental9 regions developed above up welling and down welling segments of convection cells and these regions resembled greenstone belts and high grade regions of the archaean era as heat flow declined polygonal tectonics was replaced by symmetrical linear tectonics during the archaean era and then by asymmetrical linear tectonics during the proterozoic era

field and geochemical characteristics of mesoarchean to neoarchean volcanic rocks in the storo greenstone belt sw greenland evidence for accretion of intra oceanic volcanic arcs abstract the storo greenstone belt southern west greenland consists of thrust imbricated slices of mesoarchean 3060 ma and neoarchean ca 2800 ma mafic to ultramafic volcanic rocks volcaniclastic sediments and gabbro anorthosite associations the belt underwent polyphase metamorphism at upper amphibolite facies conditions between 2650 and 2600 ma the contacts between the mesoarchean and neoarchean volcanic rocks and surrounding eoarchean to neoarchean tonalite trondhjemite granodiorite ttg gneisses are tectonic and typically bounded by high grade mylonites regardless of age the volcanic rocks are dominated by mafic amphibolites with a tholeiitic basalt composition near flat to slightly enriched light rare earth element lree patterns la smcn 0 91 1 48 relatively flat to slightly depleted heavy ree hree gd ybcn 1 0 1 28 and pronounced negative nb ta anomalies nb nb 0 34 0 73 on chondrite and primitive mantle normalized diagrams these geochemical characteristics are consistent with subduction zone geochemical signatures and partial melting of a shallow

radiocarbon as a thermocline proxy for the eastern equatorial pacific an ocean model is used to test the idea that sea surface delta 14 c behaves as a thermocline proxy in the eastern equatorial pacific the orca2 model which includes delta 14 c as a passive tracer has been forced with reanalysis fluxes over 1948 1999 and the output is compared with a previously reported galapagos delta 14 c record the model reproduces the abrupt increase in the seasonally minimum delta 14 c in 1976 77 found in the data this increase is associated with neither a shift of thermocline depth over the nino3 region nor a change in the relative proportion of northern southern source waters rather it is due to a decrease in the sub antarctic mode water samw component of the upwelling water thereby representing a decrease in entrainment of water from below the base of the directly ventilated thermocline

subseasonal variations in marine reservoir age from pre bomb donax obesulus and protothaca asperrima shell carbonate abstract two donax obesulus and two protothaca asperrima shells collected prior to the nuclear testing of the 1950 s were micromilled at sub seasonal resolution to yield new reservoir effect r estimates for the coast of peru shells from northern 4 40 s to 8 14 s and central 13 52 s peru produced r values of 123 50 and 110 49 years respectively we found such values statistically indistinguishable from each other while reporting intra annual r variability along shells ontogeny in agreement with previously published regional r values this similarity suggests radiocarbon dating of short lived

a new fragment of the early earth crust the aasivik terrane of west greenland abstract the aasivik terrane is a 1500 km 2 complex of gneisses dominated by 3600 ma components which has been discovered in the archaean craton of west greenland 20 50 km south of the paleoproterozoic nagssugtoqidian orogen the aasivik terrain comprises granulite facies tonalitic to granitic gneisses with bands of mafic granulite which include disrupted mafic dykes four gneiss samples of the aasivik terrain have given imprecise shrimp u pb zircon ages of 3550 3780 ma with strong loss of radiogenic lead and new growth of zircon probably associated with a granulite facies metamorphic event s at 2800 2700 ma to the southeast the aasivik terrane is in tectonic contact with a late archaean complex of granitic and metapelitic gneisses with apparently randomly distributed mafic and ultramafic units here named the ukaleq gneiss complex two granitic samples from the ukaleq gneiss complex have u pb zircon ages of 2817 10 and 2820 12 ma and t zircon e nd values of 2 3 5 4 given their composition and positive e nd values they probably represent melts of only slightly older juvenile crust a reconnaissance shrimp u pb study of a sample of metasedimentary rock from the ukaleq gneiss complex found 2750 2900 ma zircons of probable detrital origin and that two or more generations of 2700 2500 ma metamorphic zircons are present this gneiss complex is provisionally interpreted as a late archaean accretionary wedge a sample of banded granulite facies gneiss from a complex of banded gneisses south of the aasivik terrain here named the tasersiaq gneiss complex has yielded two zircon populations of 3212 11 and 3127 12 ma contacts between the three gneiss complexes are mylonites which are locally cut by late post kinematic granite veins with shrimp u pb zircon ages of 2700 ma the isotopic character and the relationships between the lithologies from the different gneiss complexes suggest the assembly of unrelated rocks along shear zones between 2800 and 2700 ma the collage of archaean gneiss complexes were intruded by a type granites here named the umiatsiaasat granites at 2700 ma later than the tectonic intercalation of the gneiss complexes

late archaean tectonics in the faeringehavn tre brodre area south of buksefjorden southern west greenland the faeringehavn tre brodre area consists of three distinct terranes tectonically jux taposed by a previously unrecognized event contacts between the terranes are mylonitic shear zones truncating lithological units in adjacent terranes the terrenes are 1 faeringehavn terrane largely composed of early archaean amitsoq gneiss cut by younger archaean granitic gneiss defined here as the satut gneiss 2 tre brodre terrane comprising mid archaean malene supracrustal rocks anorthosite complex and polyphase nuk gneisses 3 tasiusarsuaq terrane largely comprising mid archaean nuk gneisses affected by c 2800 ma granulite facies metamorphism the tasiusarsuaq terrane is structurally above both the faeringehavn and tre brodre terranes which at c 2800 ma experienced a lower grade of metamorphism juxtaposition of the terranes took place between 2800 and 2500 ma and involved thrusting and crustal thickening subsequent re equilibration involved folding steeply inclined shear belts intrusion of synkinematic granitoids under amphibolite facies conditions and retrogression of granulite facies assemblages this thrusting post dates the 2800 ma granulite facies metamorphism it is younger and distinct from the thrusting postulated to explain the intercalation of the early archaean amitsoq gneisses and the mid archaean malene supracrustal rocks associated with intrusion of the nuk gneiss precursors described from godthabsfjord the tectonic breaks provide for the first time marker horizons which can be used to assess the amount and type of late archaean deformation in the southern godthabsfjord region

the termination of the equatorial undercurrent in the eastern pacific abstract the termination of the equatorial undercurrent euc in the eastern tropical pacific has been studied through analysis of the historical hydrographic station data in the region bounded by 5 n 10 s 80 w and 100 w three distinctive hydrographic features associated with the euc are used along with dynamic topography to trace the mean path of the euc and to investigate aspects of its seasonal variation these features are 1 the 13 c thermostad 2 the high salinity core and 3 the high dissolved oxygen concentration tongue the thermostad is thickest just south of the equator to the west of the galapagos islands with a decreasing maximum extending southeast to the coast of south america near 7 s the thermostad is poorly developed in the mean near the equator east of the galapagos islands this pattern appears to be produced by convergence of eastward flow in the euc and in the southern subsurface countercurrent as the euc flows southeastward from the galapagos this interpretation is supported by the geostrophic flow calculated from the mean dynamic topography the isolated high salinity core is found to the south of the equator and is traced all the way to the coast of south america the high oxygen tongue associated with the euc splits at the galapagos with the southward projecting branch disappearing rapidly as an identifiable feature the other branch of the tongue continues around the islands to the north and along the equator to the coast of ecuador some evidence of recirculation of euc water in the south equatorial current is seen to the northwest of the galapagos a strong seasonal pulsation of the euc affects these features in a consistent way with the pulse occurring first west of the galapagos and later along the coast the thermostad first reaches maximum development and then a pronounced downwelling of the thermocline occurs rapidly leading to minimum thermostad extent associated wtih this downwelling is an eastward protruding high salinity tongue near the equator which is later seen as an isolated core along the coast at 4 s and 6 s pressure gradients along the equator and the coast inferred from dynamic topography suggest that this pulse is associated with an increase in the flow of the euc and ultimately the peru chile undercurrent the pressure gradient reversal along the equator occurs prior to the annual wind reversal suggesting that remote forcing plays a role in the seasonal pulse of the euc in the eastern pacific

clamr a statistical evaluation of isotopic and temperature records in sclerochronologic studies abstract isotope sclerochronology has been widely used to reconstruct paleoclimate from the hard part remains of a broad range of taxa from many different environmental settings and geological and cultural contexts oxygen isotope 18o time series derived from these ultra high resolution daily seasonal annual records have been used to estimate annual averages seasonal ranges and seasonal extremes these time series however inherently include uncertainties such as unconstrained environmental noise changes in seasonal growth rates resulting in weighting annual temperature estimates toward seasons of maximum growth growth cessation and reliance on the most extreme 18o values in the time series to estimate seasonal temperature range etc to better quantify uncertainties in paleoclimate reconstructions using sclerochronologic data we developed an open source r package clamr the clamr evaluation tool improves upon the previous best fit sinusoid approach by introducing additional statistical computation and analysis including discrete fourier transform to initialize parameters for the sinusoidal function nelder mead method to optimize parameters jackknife error estimation and window size calculation for sinusoidal smoothing it can automatically calculate and plot statistical errors for oxygen isotope or temperature data series and provide statistical estimation of temperature and isotopic variables such as annual averages seasonal ranges and seasonal extremes moreover it allows evaluation of the influence of environmental and or physiological noise added to the climate signal therefore clamr enables more comprehensive and justified comparison between paleoclimate reconstructions and modern instrument records and ultimately advances our interpretation of seasonal isotope records from biogenic hard parts

food for thought mathematical approaches for the conversion of high resolution sclerochronological oxygen isotope records into sub annually resolved time series abstract oxygen isotope 18 o records of incrementally banded marine calcifiers provide high resolution information about modern and past environmental conditions however given their biological origin these records are often distorted by ontogenetic and seasonal growth pattern to evaluate their reliability many studies correlate skeletal 18 o records with instrumentally measured time series of environmental conditions using a variety of methods to recalculate their temporal framework lacking consistency and a limited traceability of many temporal alignment methods underline the need for standardized and flexible method for the temporal alignment of sclerochronological records here two methods for the temporal alignment of sclerochronological data and their application on measured oxygen isotope signatures of a fish otolith and a marine bivalve shell are presented and compared the first method is a flexible approach using a linear interpolation between carefully selected temporal reference points for recalculating ontogenetic growth effects with an optional extension accounting for seasonal growth oscillations the second approach uses the seasonally oscillating von bertalanffy growth function parameterized based on the oxygen isotope record to reconstruct the underlying growth model and reconstruct thereby its temporal framework using these methods we re align 18 o based water temperature records of a carlarius heudelotii otolith and a venus crebrisulca shell with measured sea surface temperature time series and compare the resulting correlation as well as the different underlying growth models our results show that both methods represent useful tools for the temporal alignment of sclerochronological records however underlying growth models strongly differ depending on the temporal alignment method and the parameters used for the reconstruction of the temporal framework this underlines the importance of a careful selection of a particular alignment method depending on the complexity of distortion due to seasonal and ontogenetic growth trajectories moreover we show that both organisms represent reliable paleoenvironmental archives providing sub seasonally resolved water temperature records

stratabound scheelite in altered archaean komatiites west greenland extensive scheelite bearing calc silicate zones with elevated contents of chromium nickel copper zinc yttrium and barium are found in archaean supracrustal rocks of the nuuk area west greenland the calc silicate zones are largely stratabound and occur in mafic and ultramafic metavolcanic rocks the calc silicates are interpreted as amphibolite facies metamorphosed alteration zones in komatiitic intrusive and extrusive rocks as well as in tholeiitic pillow lavas the alteration which antedates the amphibolite facies metamorphism and the main deformation of the supracrustal rocks presumably took place at the sea floor or immediately below the sea floor the brines which caused the alteration were enriched in elements such as tungsten boron sulphur iron copper and zinc the boron was precipitated as thin tourmalinites and the iron in massive to semi massive sulphide bodies the scheelite in the calc silicates is mainly found as disseminated grains as stringers and as veinlets rare veins up to 15 cm wide with massive scheelite are encountered channel sampling revealed grades up to 0 35 w over a width of 2 5 m highly anomalous bromine contents up to 418 ppm and equally anomalous chlorine bromine ratios down to 3 have been found in scheelite bearing calc silicates somewhat similar anomalous bromine contents and cl br ratios have been reported from serpentinites in the outokumpu ore body finland at mount isa northern australia and in the antimony line in the murchinson greenstone belt southern africa anomalous cl br ratios have been observed the possible role of bromine in the ore forming processes is as yet poorly understood

geochemical characteristics of granulite facies rocks in the archean varpaisjarvi area central fennoscandian shield abstract archean lower crustal granulites are exposed as fault bounded blocks in the varpaisjarvi area central finland the prevailing rock types in these blocks are igneous enderbites tonalitic trondhjemitic granodioritic migmatites and mafic granulites although metamorphosed at moderate pressures 8 10 kbar the varpaisjarvi granulites are not in a geochemical sense good representatives of depleted restitic lower crust having invariably negative or negligible eu anomalies only the enderbites have k rb ratios showing depleted granulite trends the enderbites are orthopyroxene bearing igneous rocks ranging in composition from diorite to tonalite they have distinctive flat lree patterns with la sm from 2 03 2 86 which may be due to the removal of a lree rich phase during fractionation if not the result of infiltration by a co 2 rich fluid mafic granulites occur as intercalations within the tonalitic trondhjemitic granodioritic gneisses and while all are tholeiitic in composition they fall into two groups on the basis of major and trace element abundances the group 1 tholeiites are mostly basaltic with steeply increasing alkali contents over a short range in sio 2 ti zr ratios are close to the chondritic value of 110 the group 2 tholeiites vary from picrobasaltic to andesitic in composition with the latter being dominant ti zr ratios lie below the chondritic value the group 1 rocks have higher ni and cr and lower zr y ratios than their group 2 counterparts the varpaisjarvi area represents at least two accreted blocks or terranes that exhibit marked lithological and geochemical differences with the jonsa block being the most distinctive the main differences are the two contrasting groups of tholeiitic mafic granulites the jonsa block having only the group 2 metatholeiites and the presence of quartz cordierite and cordierite orthoamphibole orthopyroxene rocks only in the jonsa block where they are found as interlayers amongst the group 2 tholeiites the ti zr and zr y ratios of these latter lithologies are closely comparable with those of the group 2 tholeiites the jonsa block cordierite orthoamphibole orthopyroxene rocks have high mgo mgo feo tot ratios compared to most cordierite orthoamphibole rocks produced by seawater basalt interaction their major and trace element geochemistry nevertheless indicates that they were derived from a mafic igneous precursor

marine radiocarbon reservoir age variation in donax obesulus shells from northern peru late holocene evidence for extended el nino for at least 6 m y el nino events have posed the greatest environmental risk on the peruvian coast a better understanding of el nino is essential for predicting future risk and growth in this tropical desert to achieve this we analyzed archaeological and modern pre bomb shells from the surf clam donax for the radiocarbon reservoir effect r to characterize late holocene coastal upwelling conditions in northern peru 8 14 s mean r values from these shells suggest that modern upwelling conditions in this region were likely established between a d 539 and a d 1578 our radiocarbon data suggest that upwelling conditions ca a d 539 were less intense than those in modern times the observed coastal water enrichment in 14 c may be consequence of frequent strong el nino events or extended el nino like conditions these r inferred marine conditions are in agreement with proposed extended el nino activity in proxy and archaeological records of ca a d 475 530 extended el nino conditions have been linked to political destabilization societal transformation and collapse of the moche civilization in northern peru a return to such conditions would have significant impacts on the dense population of this region today and in the near future

the complex age of orthogneiss protoliths exemplified by the eoarchaean itsaq gneiss complex greenland shrimp and old rocks abstract field studies integrated with cathodoluminescence petrography and shrimp u pb dating of zircons from 150 orthogneisses and metatonalites from the eoarchaean itsaq gneiss complex southern west greenland shows that only a minority contain 3840 ma zircons whereas the majority carry only younger ones rocks containing 3840 ma zircons vary from very rare single phase metatonalites to more common complexly banded tonalitic migmatites the former metatonalites have simple oscillatory zoned 3840 ma zircon with limited recrystallisation and overgrowth whereas the more common migmatites have much more complicated zircon populations with both 3840 ma and 3650 3600 ma oscillatory zoned zircon more extensive recrystallisation and widespread complex core rim multiple growth relationships with only 100 160 ppm zr in the tonalites and likely melt generation temperatures of 1000 c the experimentally determined zircon solubility melt composition relationships established by other workers shows that the precursor melts to the itsaq complex tonalites were strongly undersaturated in zircon thus any entrained xenocrystic zircon would have been rapidly dissolved therefore the 3840 ma oscillatory zoned zircons crystallised out of tonalitic melt and gives magmatic age of the rock in which they occur with an established igneous age of 3840 ma established from such relationships we interpret the correlated variation between the field nature of these rocks and their zircon petrography age structure as due to superimposition onto 3840 ma tonalite protoliths of variable amounts of heterogeneous strain heterogeneous distribution of melt patches formed during in situ anatexis at up to 800 c plus granitic veining this explains why geologically simple metatonalites have simple zircon populations whereas complex orthogneisses have complex zircons the large amount of integrated field geochemical and zircon data rule out an alternative interpretation that the 3840 ma zircons represent an igneous xenocrystic component present in younger rocks to varying degrees if this were true then the structurally simple less reworked rocks should still display complex zircon populations gneisses with 3840 ma zircon are commonest on akilia and neighbouring islands in itilleq fjord 65 km east akilia and on the north of ivisaartoq 150 km northeast of akilia these include from itilleq a 3891 6 ma gneissic tonalite with minor neosome which is currently the oldest rock recognised in the itsaq gneiss complex overall the 3840 ma tonalites are a widespread and unevenly distributed in the itsaq gneiss complex and they are a volumetrically minor component compared with 3800 3750 and 3700 ma tonalite generations using the subset of our data covering itilleq and the neighbouring fjords migmatite samples with 3800 ma igneous zircon are mutually exclusive from migmatite samples with 3700 ma igneous zircon this suggests that prior to an amalgamation event followed by 3660 3600 ma high grade metamorphism 3840 ma tonalites might have resided in a terrane discrete from 3700 ma tonalites this is in accord with interpretation of the non migmatised part of the complex in the isua area where a terrane of 3800 ma tonalites with a minor associated 3840 ma component and a terrane with 3700 ma tonalites were tectonically juxtaposed at 3660 ma

smnd age of the fisken sset anorthosite complex west greenland abstract the fisken sset anorthosite complex of southern west greenland is a sheet like layered igneous body which has been fragmented by granitoid intrusions multiply deformed and folded metamorphosed as high as granulite facies and in some places retrogressed to lower amphibolite or greenschist facies in some localities however such as at majorqap qa va some 20 km inland from the coast these effects are minimal here metamorphic grade reached only amphibolite facies with no apparent retrogression igneous textures and structures are spectacularly preserved detailed stratigraphic sequences have been established and in some samples primary mineralogy remains we report a sm nd isotopic study on samples from this locality attempting to constrain the age of crystallization of the fisken sset complex this complex represents a useful marker unit with which the age relationships of surrounding units may be compared a five point isochron including data for whole rock samples of anorthosite metagabbro metaperidotite and separates of calcic plagioclase and mafic matrix from a coarse megacrystic leucogabbro corresponds to an age of 2 86 0 05 ga mswd 2 5 with initial end of 2 9 0 4 the sm nd age is significantly older than previous pb pb whole rock ages of the complex but is sufficiently close to the 2 8 ga high grade metamorphic event which affected the area to cause partial resetting of the sm nd system to be considered however based on the integrity of the whole rock sm nd isotopic system in similar anorthositic complexes of higher metamorphic grade and the apparent preservation of magmatic ree patterns in the analysed plagioclase megacryst and coexisting mafic matrix we conclude that 2 86 0 05 ga represents the time of crystallization of the fisken sset complex this would imply a relatively short time interval on the order of 70 ma during which anorthosite formation tonalite emplacement and high grade metamorphism took place and this must be accounted for in any tectonic model for late archean events in southern west greenland in any case the positive end value of the fisken sset complex requires that it was derived from a source with long term depletion in light ree these results add to the increasing body of data indicating the widespread presence of depleted mantle sources during the archean

reconstructing past upwelling intensity and the seasonal dynamics of primary productivity along the peruvian coastline from mollusk shell stable isotopes 1 we present here a potential new method to evaluate past variations of the mean intensity of peruvian coastal upwelling and of the seasonal timing of phytoplankton blooms this method uses a combination of the monthly carbon and oxygen isotopic signals preserved in fossil mollusk shells and a series of corrections to extract the variations of the dissolved inorganic carbon dic 13c based on the analysis of five shell samples 85 shells in total from the southern peruvian coast we suggest that the mean coastal upwelling intensity can be determined from a linear relationship between average values of corrected shell 13c and 18o this new potential proxy would bring additional independent information valuable to interpret paleoproductivity changes reconstructed from marine sediment of the nearby continental shelf results obtained on fossil samples from the middle holocene show an increase in upwelling intensity during this period associated to a spatial reorganization of upwelling centers along the south peruvian coast at the seasonal scale corrected shell 13c enrichment indicates a phytoplankton bloom seasonal timing of phytoplankton blooms can be estimated by the lag with the annual temperature cycle reproduced by shell 18o monthly variations the results obtained with two modern shell samples indicate phytoplankton blooms occurring during summer and fall consistently with in situ productivity observations our method relies on revisited assumptions about the influence of temperature and metabolism in mollusk shell 13c we further discussed the validity of these assumptions and the potential implications for the interpretation of similar data sets

mesoarchaean collision of kapisilik terrane 3070ma juvenile arc rocks and 3600ma isukasia terrane continental crust greenland abstract the mesoarchaean kapisilik and eoarchaean isukasia terranes in the nuuk region of southern west greenland were tectonically juxtaposed in the archaean the north of the isukasia terrane is distal from the kapisilik terrane and has only rare growth of 2690 ma metamorphic zircon and no 2980 2950 ma metamorphic zircon the southern part of the isukasia terrane lies between two 2690 ma shear zones and has locally preserved high pressure granulite facies assemblages and widespread growth of 2980 2950 ma metamorphic zircon and also sporadic growth of 2690 ma metamorphic zircon within this southern part of the isukasia terrane there is a folded klippe of mylonitised mesoarchaean detrital meta sedimentary rocks carrying 3600 and 3070 ma detrital zircons mafic and ultramafic rocks with 2970 ma metamorphic zircon overgrowths south of the isukasia terrane is the kapisilik terrane containing 3070 ma arc related volcanic rocks gabbro anorthosites and meta tonalites intruded by 2970 2960 ma granites zircons of an ivis rtoq supracrustal belt 3075 ma intermediate volcanic rock have initial hf values of 2 to 5 thus are juvenile crustal additions 3070 ma tonalites along the northern edge of the kapisilik terrane have whole rock positive initial nd values and thus are also juvenile crustal additions in contrast igneous zircons in 2960 ma granites intruded into juvenile 3075 ma supracrustal rocks of the kapisilik terrane have initial hf values of 5 to 10 and must have involved the partial melting of 3600 ma isukasia terrane rocks the integrated structural and zircon u th pb hf isotopic data show that at 2980 2950 ma the kapisilik terrane juvenile arc components collided with and over rid the isukasia terrane the southern edge of the isukasia terrane came to lie in the deep crust under the ivis rtoq supracrustal belt and melted at 2970 2960 ma to produce granites these granites derived from ancient crust rose into the upper crust where they intruded the overlying allochthonous juvenile 3075 ma ivis rtoq supracrustal belt arc assemblages the southern edge of the isukasia terrane is interpreted as an interior nappe of eoarchaean basement rocks interfolded with a klippe of mesoarchaean metasedimentary and mafic ultramafic rocks both of which are affected by 2980 2950 ma metamorphism the mixed eoarchaean mesoarchaean detrital provenance suggests that the klippe could be dismembered components of an accretionary prism or forearc crust the northern part of the isukasia terrane is interpreted as foreland free of 2980 2950 ma high grade metamorphic overprint this shows that the isukasia terrane is not a coherent block but contains ancient rocks that are parautochthonous or allochthonous to each other with contrasting later metamorphic history at 2690 ma the crustal architecture arisen from mesoarchaean collision between an older continental block and an island arc was reworked along intra crustal shear zones coeval with amphibolite facies metamorphism this reworking followed on from major terrane assembly at 2710 2700 ma in the southern part of the nuuk region when the eoarchaean faeringehavn terrane was juxtaposed with 2840 2825 ma arc rocks thus the 2980 2950 ma assembly of the isukasia and kapisilik terranes is distinct from the later 2710 2700 ma terrane assembly further south in the nuuk region

reconstructing el nino southern oscillation activity and ocean temperature seasonality from short lived marine mollusk shells from peru abstract a critical need exists for quantitative reconstructions of long term el nino southern oscillation enso variability in the eastern tropical pacific presented here is a method to quantitatively estimate past changes 1 in the seasonal amplitude of sea surface temperature sst in the peruvian coastal upwelling system and 2 in the amplitude of enso related interannual variability in the eastern tropical pacific the seasonal amplitude of sst t along the length of the peruvian coast is strongly correlated with the nino1 2 index we show that the frequency distribution of t values provided by a modern sample of 13 mesodesma donacium shells faithfully reflects modern enso variability at the regional scale including the range of anomalies from la nina to moderate el nino events but excludes extreme warm anomalies because of high shell mortality we propose to use the frequency distribution of enso anomalies in paleoclimate studies for comparisons between shell records coral records and gcm simulations reconstruction uncertainties can be quantified using monte carlo simulations the method presented here opens new perspectives for quantitative paleo enso reconstructions in the eastern pacific since it may be applied with any mollusk species from peru provided at least one annual cycle of sst is faithfully recorded by shell 18 o

3820 ma zircons from a tonalitic armisoq gneiss in the godthab district of southern west greenland abstract zircons from a sample of the tonalitic amitsoq gneisses near godthhb west greenland exhibit several stages of crystal growth measurements of the u th pb isotopic composition of 30 m areas within individual grains using the ion microprobe shrimp reveal that components of magmatic zircon from the original igneous protolith are at least 3822 5 ma l old slightly but significantly older than the age of zircons from a fclsic volcanic unit in the isua supracrustal belt as measured at high precision by the same technique and older than all ages previously determined by mineral and whole rock isotopic techniques for amitsoq gneisses at godthab and at isukasia overgrowths of younger zircon formed at ca 3630 ma coinciding with an episode of major pb loss from the older grains these results indicate that previous whole rock isochron ages refer to later isotopic redistribution and or that other phases of the amitsoq gneisses intruded during this later period and dominate the isotopic systematics comparison of the ion probe results with previous conventional u th pb data for the same zircon sample demonstrates the strong influence of discordant high uranium parts of zircon on the mean isotopic composition of multi grain samples

miogeoclines and suspect terranes of the caledonian appalachian orogen tectonic patterns in the north atlantic region the caledonian appalachian orogen was formed by the closing of a paleozoic iapetus ocean the continental margins of iapetus are identified in the deformed early paleozoic miogeoclines of the caledonian appalachian orogen ophiolitic vestiges of iapetus its oceanic plateaus microcontinents and volcanic arcs are caledonian appalachian suspect terranes these were assembled in interior parts of the orogen and locally they were emplaced structurally upon the adjacent miogeoclines the modern north atlantic ocean opened along an axis that traversed the paleozoic orogen longitudinally its opening dispersed the elements of the paleozoic orogen and led to the present arrangement of disjunct paleozoic miogeoclines and suspect terranes throughout the north atlantic borderlands the western or north american margin of iapetus is represented by the miogeoclines along the west flank of the north american appalachians and caledonides of east greenland a small north american miogeoclinal segment occurs in the britis

marine or estuarine radiocarbon reservoir corrections for mollusks a case study from a medieval site in the south of england abstract mollusk shells are frequently radiocarbon dated and provide reliable calibrated age ranges when the regional marine reservoir correction is well established for mollusks from an estuarine environment the reservoir correction may be significantly different than the regional marine reservoir correction due to the input of bedrock or soil derived carbonates some mollusk species such as oysters are tolerant of a significant range of salinities which makes it difficult to determine which reservoir correction is appropriate a case study is presented of an anomalous radiocarbon age for an oyster shell paint dish found in the fabric of the ruined nave walls of st mary s church shoreham by sea west sussex england stable isotopes 18 o and 13 c were used to establish the type of environment in which the oyster had lived paired marine and terrestrial samples from a nearby medieval site were radiocarbon dated to provide an appropriate reservoir correction

upwelling signals in radiocarbon from early 20th century peruvian bay scallop argopecten purpuratus shells abstract we quantified 14c 18o and 13c cycles along ontogeny within four bay scallop argopecten purpuratus shells collected from callao bay salaverry and sechura bay peru following the 1907 1908 non el nino years and the 1925 1926 el nino 14c and 13c generally covary 14c and 18o vary inversely simultaneous decreases in 14c and increases in 18o in non el nino shells are followed by constant 14c and gradually decreasing 18o which we interpret as evidence for discrete marine upwelling events followed by warming of the initially cold upwelled water upwelling changes from el nino events are detectable with difficulty in mollusk shell 14c

testing the model of late archean terrane accretion in southern west greenland a comparison of the timing of geological events across the qarliit nunaat fault buksefjorden region abstract the prevailing tectonic model for the archean gneiss complex of southern west greenland holds that terranes with different metamorphic histories and protolith ages were juxtaposed between 2 80 and 2 72 ga this model was tested by using detailed mapping and u pb geochronology to compare the timing of geological events across the qarliit nunaat fault the boundary between terranes in the buksefjorden region the fault was interpreted as a terrane boundary mostly because it is a narrow zone of mylonitic rocks that separates granulite or retrograde amphibolite facies gneisses of the tasiusarsuaq terrane tt from prograde amphibolite facies gneisses of the akulleq terrane at and it separates tonalitic gneisses of different age 3 87 3 60 and 2 80 ga in the at versus 2 92 2 86 ga in the tt dating of similar homogeneous gneisses in each terrane yielded ages of 2843 3 ma in the tt tonalite and 2824 2 ma in the at granodiorite ikkattoq gneiss granulite facies metamorphism in the tt is thought to have coincided with intrusion of a 2805 2 ma orthopyroxene pegmatite and zircon and titanite from nearby amphibolitic gneiss yielded ages of 2779 2 and 2660 10 ma respectively metamorphic minerals in the at are younger zircon and titanite from amphibolitic gneiss are 2717 3 and 2510 10 ma respectively and monazite from pelitic schist is 2732 5 and 2708 4 ma a concordant pegmatite layer that underwent a substantial amount of shearing along the qarliit nunaat fault was dated at 2725 2 other pegmatites that intruded relatively late in the structural history crystallized at 2710 2 in the tt and 2541 2 ma in the at these results support the prevailing tectonic model by showing that i similar homogeneous gneisses in each terrane are different age ii 2805 ma granulite facies metamorphism in the tt predated 2740 2700 ma amphibolite facies metamorphism in the at and iii shearing along the terrane boundary occurred after 2725 ma

radiocarbon dating reservoir effects and calibration abstract we summarize how radiocarbon measurements are made using accelerator mass spectrometry we also discuss some complications inherent in the radiocarbon dating method when calibrating radiocarbon dates to calendar dates for example measurements of radiocarbon 14c in some types of materials are complicated by a reservoir effect caused by an apparent age of the source reservoir that differs from the contemporary atmospheric surface 14c value in other cases mobile carbon sources in nature can produce mixed sources of carbon we explore these effects and discuss their possible implications for 14c measurements and how we can deal with them

archaean fluid assisted crustal cannibalism recorded by low δ 18 o and negative e hf t isotopic signatures of west greenland granite zircon the role of fluids during archaean intra crustal magmatism has been investigated via integrated shrimp u pb 18o and la mc icpms 176hf isotopic zircon analysis six rock samples studied are all from the nuuk region southern west greenland including two 3 69 ga granitic and trondhjemitic gneisses a 3 64 ga granitic augen gneiss a 2 82 ga granodioritic ikkattoq gneiss a migmatite with late neoarchaean neosome and a homogeneous granite of the 2 56 ga qorqut granite complex qgc all zircon grains were thoroughly imaged to facilitate analysis of magmatic growth domains within the zircon analysed there is no evidence for metamictization initial ehf zircon values n 63 are largely sub chondritic indicating the granitic host magmas were generated by the remelting of older un radiogenic crustal components zircon from some granite samples displays more than one 207pb 206pb age and correlated with 176hf 177hf compositions can trace multiple phases of remelting or recrystallization during the archaean model ages calculated using lu hf arrays for each sample indicate that the crustal parental rocks to the granites granodiorites and trondhjemites segregated from a chondrite like reservoir at an earlier time during the archaean corresponding to known formation periods of more primitive tonalite trondhjemite granodiorite ttg gneisses zircon from the 3 69 ga granite the migmatite and qgc granite contains eoarchaean cores with chondritic 176hf 177hf and mantle like 18o compositions the age and geochemical signatures from these inherited components are identical to those of surrounding tonalitic gneisses further suggesting genesis of these granites by remelting of broadly tonalitic protoliths zircon oxygen isotopic compositions n 62 over nine age populations six igneous and three inherited have weighted mean or mean 18o values ranging from 5 8 0 6 to 3 7 0 5 the 3 64 ga granitic augen gneiss sample displays the highest 18o with a mildly supra mantle composition of 5 8 0 6 inherited eoarchaean ttg derived zircon shows mantle like values igneous zircon from all other samples spanning more than a billion years of archaean time record low 18o sub mantle compositions these are the first low 18o signatures reported from archaean zircon and represent low 18o magmas formed by the remelting and metamorphism of older crustal rocks following high temperature hydrothermal alteration by meteoric water meteoric fluid ingress coupled with crustal extension associated high heat flow and intra crustal melting are a viable mechanism for the production of the low 18o granites granodiorites and trondhjemites reported here both high and low 18o magmas may have been generated in extensional environments and are distinct in composition from phanerozoic i type granitic plutonic systems which are typified by increasing 18o during intra crustal reworking this suggests that archaean magmatic processes studied here were subtly different from those operating on the modern earth and involved extensional tectonic regimes and the predominance of remelting of hydrothermally altered crystalline basement

adjacent terranes with ca 2715 and 2650 ma high pressure metamorphic assemblages in the nuuk region of the north atlantic craton southern west greenland complexities of neoarchaean collisional orogeny abstract in the gneiss complex of the nuuk region of the north atlantic craton in southern west greenland terrane juxtaposition was followed by neoarchaean folding under amphibolite facies conditions with widespread low pressure recrystallisation 5 kbar and 550 700 c the complex metamorphic overprinting requires that the p t history related to actual terrane assembly has to be extracted from very small relicts of older metamorphic assemblages combined with the u pb dating petrology and geochemistry of metamorphic zircons in the south of the region the faeringehavn terrane eoarchaean orthogneisses is tectonically overlain by a supracrustal package of amphibolites and paragneisses ca 2840 ma felsic volcano sedimentary protoliths this package is juxtaposed against a higher tectonic level represented by the tre brodre terrane 2825 ma orthogneisses and the tasiusarsuaq terrane 2920 2810 ma orthogneisses the terranes were assembled by 2710 2720 ma as shown by dating of granitic sheets intruded along the terrane boundary mylonites in the faeringehavn terrane and in the overlying 2840 ma supracrustal package relict early high pressure assemblages 12 8 kbar 700 750 c are clinopyroxene garnet plagioclase quartz hornblende in mafic rocks and garnet kyanite rutile bearing assemblages in paragneisses these are commonly replaced by lower pressure assemblages 7 5 kbar such as cordierite sillimanite garnet in paragneisses and hornblende plagioclase quartz garnet in mafic rocks in situ partial melting took place during both low and high pressure regimes metamorphic zircon in the high and low pressure assemblages yields dates of ca 2715 ma mostly with errors of the kapisilik terrane 3050 2960 ma orthogneisses and another supracrustal assemblage of amphibolites and ca 2800 ma quartzo feldspathic metasedimentary rocks are exposed north of the faeringehavn terrane and within fold cores along its western margin are bounded by folded neoarchaean mylonites the kapisilik terrane and this supracrustal assemblage include high pressure metamorphic remnants in amphibolites and metasediments metamorphic segregations with garnet clinopyroxene and kyanite respectively that formed at ca 2650 ma these remnants are overprinted by high temperature but lower pressure metamorphic events at ca 2630 2610 and 2580 ma thus remnants of early metamorphism reveal mutually exclusive ca 2715 and 2650 ma high pressure events in adjacent tectonically juxtaposed terranes repeated high pressure metamorphism in adjacent terranes is observed in younger complex collisional orogens

petrological and whole rock isotopic characteristics of tectonically juxtaposed archaean gneisses in the okak area of the nain province labrador relevance for terrane models in a recent model schiotte et al 1990 the archaean nain province of northern labrador is described as a tectonic collage of terranes with different origins and metamorphic histories that was assembled in the late archaean 3200 3260 ma granulite facies gneisses that are intercalated with the regional 3500 ma uivak gneisses in the northern saglek block of the nain province are interpreted as tectonic slivers of a terrane of mid archaean rocks represented by the maggo gneisses of the hopedale block 200 km farther south this terrane is thought to correlate with the akia terrane in southern west greenland in the nain province as opposed to west greenland the tectonic juxtaposition is thought to have taken place along subvertical planes there is overlap in the range of chemical compositions exhibited by the early and mid archaean orthogneisses in the okak area of the saglek block but on average the mid archaean gneisses are more mafic and less silica rich their major element chemistry exhibits much simpler evolutionary trends the ree chemistry is not related to the major element chemistry in any straightforward way in either early or mid archaean gneisses ree patterns show some correlation with the local concentration of accessory ree bearing minerals and are subject to metamorphic redistribution as late as 2560 ma in spite of this redistribution late archaean nd values can be used as a rough guide to the duration and complexity of the pre juxtaposition history nd 2560 ma of individual samples ranges from 4 4 to 7 0 in the mid archaean gneisses as opposed to values 7 7 in the early archaean gneisses the difference in metamorphic history also shows up in the whole rock pb isotopic signature the mid archaean granulite facies gneisses were juvenile intrusions the source of which can be estimated from model 1 values of 7 8 8 0 they crystallized in granulite facies sometimes between their emplacement at 3235 ma and an event of granitoid veining at 2990 ma they do not exhibit the complexities or distinctly non juvenile 207pb204pb ratios recorded from the early archaean gneisses the major volume of exposed early archaean rocks in the saglek block recrystallized under granulite facies metamorphism between 2780 and 2740 ma a suite of early archaean granitic granodioritic gneisses in granulite facies from the okak area differs from previously investigated uivak gneisses by showing evidence for larger pre metamorphic variation in upb 207pb204pb ratios that range from 14 02 to 16 30 and lower thu secondary u loss during late archaean metamorphism gave rise to a pb pb palaeoisochron a late archaean sm nd isochron with a strongly negative initial nd has been obtained on this gneiss suite the suite also exhibits other chemical peculiarities such as high cr contents a complex petrogenesis within a strongly differentiated early archaean crust is suggested there is no straightforward correlation between possible subterranes within terranes consisting of early archaean rocks in the nain province and west greenland

abrupt shift in subsurface temperatures in the tropical pacific associated with changes in el nino radiocarbon 14c content of surface waters inferred from a coral record from the galapagos islands increased abruptly during the upwelling season july through september after the el nino event of 1976 sea surface temperatures ssts associated with the upwelling season also shifted after 1976 the synchroneity of the shift in both 14c and sst implies that the vertical thermal structure of the eastern tropical pacific changed in 1976 this change may be responsible for the increase in frequency and intensity of el nino events since 1976

radiocarbon in annual coral rings from the eastern tropical pacific ocean sixty radiocarbon measurements were performed on aragonite from annually banded corals collected from three sites in the galapagos islands preanthropogenic 14c values of coral that grew around a d 1930 averaged 70 this is substantially lower than average values previously reported 51 for corals from florida and belize in the western north atlantic ocean a decrease of 6 was noticed in coral that grew from 1930 to 1954 this decrease could be interpreted as a suess effect in surface ocean water the 100 increase in 14c for coral that grew from 1954 to 1973 is the result of bomb produced 14c that was introduced to the surface ocean waters the 14c levels in corals that grew during el nino years were considerably higher than those for normal years these higher values are attributed to the absence of up welling at the equator during el nino events

lead isotopic fingerprinting of tectono stratigraphic terranes east central alaska common lead isotopic compositions have been determined on feldspars from meta igneous rocks from nine tectono stratigraphic terranes or subterranes in east central alaska most of the terranes have distinct and well defined signatures in terms of isotopic composition thus most can be distinguished on conventional lead isotopic diagrams lead isotopic ratios provide evidence for 1 possible sources for the igneous rocks 2 time of metamorphism 3 correlation of terrane fragments and 4 delineation of juxtaposed terranes determination of lead isotopic ratios from igneous rocks can be useful in characterizing tectono stratigraphic terranes as to mantle or crustal origin and in correlation particularly where terrane relationships are enigmatic

felsic igneous rocks within the 3 3 to 3 5 ga barberton greenstone belt high crustal level equivalents of the surrounding tonalite trondhjemite terrain emplaced during thrusting felsic igneous rocks within the circa 3 5 ga ophiolitic onverwacht group of the barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession these felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic ultramafic rocks of the onverwacht group rather the data indicate that these felsic units are high level equivalents of the widespread gneissic to isotropic trondhjemite tonalite plutons which surround and intrude the greenstone belt the felsic igneous rocks are predominantly shallow level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the onverwacht group by 40 50 ma the felsic intrusions were preferentially emplaced along thrust zones cutting the ophiolite basement and its overlying sedimentary cover structural and chemical data imply that the hydrated ophiolitic rocks of the barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting such structural imbrication of simatic crust can be best incorporated in a model advocating archean intraoceanic obduction associated with shallow angle subduction similar to that documented for the neotectonics of the western aleutians

titanite petrochronology linked to phase equilibrium modelling constrains tectono thermal events in the akia terrane west greenland abstract the mesoarchean akia terrane in west greenland contains a detailed magmatic and metamorphic mineral growth record from 3 2 ga to at least c 2 5 ga this time span makes this region an important case study in the quest to track secular changes in geodynamic style which may ultimately inform on the development of plate tectonics as a globally linked system of lateral rigid plate motions the common accessory mineral titanite has recently become recognised as a powerful high temperature geochronometer whose chemistry may chart the thermal conditions of its growth furthermore titanite offers the potential to record the time temperature history of mafic lithologies which may lack zircon although titanite suffers from higher levels of common pb than many other u pb chronometers we show how measurement of 207pb 206pb in texturally coeval biotite may assist in the characterization of the appropriate common pb composition in titanite titanite extracted from two samples of mafic gneisses from the akia terrane both yield u pb ages of c 2 54 ga although coeval their chemistry implies growth under two distinctly different processes in one case the titanite has elevated total ree high th u and grew from an in situ partial melt consistent with an identical date to granite dyke zircon in contrast the second titanite sample contains greater common pb lower total ree lower th u and grew from dominantly hydrothermal fluids zr in titanite thermometry for partial melt derived titanite with activities constrained by phase equilibrium modelling indicates maxima of c 690 c elsewhere in the akia terrane coeval metamorphism linked to growth of hydrothermal titanite is estimated at temperatures of c 670 c these new results when coupled with existing findings indicate punctuated repeated metamorphic events in the akia terrane in which high temperature conditions re occurred at least three times between 3 0 and 2 5 ga but crucially changed in style across a c 3 0 ga change point we interpret this change in metamorphism as reflecting a fundamental shift in geodynamic style in west greenland at 3 0 ga consistent with other estimates for the onset of widespread plate tectonic type processes

archaean crustal growth processes in southern west greenland and the southern superior province geodynamic and magmatic constraints abstract eo to mesoarchaean greenstone belts e g 3800 3700 ma isua c 3075 ma ivisaartoq 3071 ma qussuk occur within orthogneisses of the southern west greenland craton greenstone belts are composed mainly of metavolcanic rocks with minor ultramafic and sedimentary schists compositionally volcanic rocks are dominantly tholeiitic basalts boninites and picrites with minor intermediate to felsic volcanic rocks these greenstone belts appear to have formed in convergent margin geodynamic settings detailed field observations contrasting ages and metamorphic and structural histories suggest that this craton was assembled in several accretionary tectonothermal events involving accretion of arcs back arcs forearcs and continental fragments by horizontal tectonics the superior province of canada was also built by the amalgamation of oceanic and continental fragments ranging in age from 3700 to 2650 ma during five discrete tectonothermal events over 40 ma between 2720 and 2680 ma the neoarchaean 2750 2670 ma wawa greenstone belts are composed of tectonically juxtaposed fragments of oceanic plateaux oceanic island arcs back arcs and siliciclastic trench turbidites following juxtaposition these diverse lithologies were collectively intruded by syn to post kinematic granitoids with subduction zone geochemical signatures oceanic island arc lavas are easily distinguished from oceanic plateau counterparts because they possess positively fractionated rare earth element la sm cn 1 and gd yb cn 1 and high field strength element depleted nb th pm pm c 1000 km scale subduction accretion complex that formed along an intra oceanic convergent plate margin during trenchward migration of the magmatic arc axis several first order geological observations on archaean greenstone belts of sw greenland and the superior province suggest that phanerozoic style plate tectonic models can provide an elegant explanation for their structural lithological metamorphic and geochemical characteristics

p t t development of archaean granulites in varpaisjarvi central finland ii dating of high grade metamorphism with the u pb and sm nd methods exposed blocks of lower crustal archaean granulites separated by palaeoproterozoic faults occur near the western boundary of the karelian craton in central finland the southwesternmost granulite block in the study area the jonsa block differs from other granulites by having younger zircon u pb and sm nd tdm model ages the u pb ages on zircons and monazites from leucosomes and mesosomes of mafic and pelitic granulites in the jonsa block are ca 2 63 ga which is interpreted as the age of granulite metamorphism igneous enderbites which make up a considerable part of the bedrock in granulite blocks are ca 50 ma older outside the jonsa block both conventional and ion probe zircon ages of mesosomes of migmatitic mafic granulites are 3 2 3 1 ga which are the protolith ages sm nd tdm model ages of the mafic granulites are 2 7 2 9 ga in the jonsa block but ca 3 2 ga in the other granulites the age differences are interpreted to represent terrane accretion that juxtaposed 3 2 ga rocks next to younger rocks all granulites were metamorphosed in similar pt conditions at 2 63 ga granulite metamorphism affected the whole area but was not able to reset zircons in older rocks in all blocks the sm nd garnet whole rock ages are younger ranging from 2 48 2 59 ga this reflects relatively low closure temperature of the sm nd system in garnet in these rocks the u pb age of a palaeoproterozoic dolerite which cuts the granulites is 2 3 ga close to the k ar age of amphibole in a retrogressed fracture

terrane accretion in the southwestern yilgarn craton evidence from a deep seismic crustal profile abstract interpretation of a seismic gravity and magnetic traverse across the western margin of the yilgarn craton undertaken approximately 110 km north of perth together with new rb sr and u pb geochronological data from the southwestern portion of the craton have led to a re evaluation of the geological relationships and to the development of a terrane accretion model for the region the geophysical data established the three layered nature of the crust with e dipping reflectors present throughout its thickness of 32 40 km reflectors in the upper crust correlate with shear zones mapped at the surface and are interpreted as major thrusts that have transported crustal slices from the east these sole out at a depth of about 10 km along a well defined decollement and indicate thin skinned tectonism the shear zones define the boundaries of three terranes which from west to east are herein defined as the balingup boddington and lake grace terranes the balingup terrane is composed of trough facies metasediments believed to have been deposited at 2 8 ga and subsequently metamorphosed under medium pressure amphibolite facies conditions this terrane contains some of the youngest granitoids in the yilgarn craton 2612 and 2527 ma and these have been extensively sheared by late archaean movement along the darling fault zone the boundary with the boddington terrane to the east is not precisely defined partly due to lack of exposure it is marked in the south by a narrow 100 km long strip of tectonised quartz monzonite and further north by a change in the rb sr biotite ages of granitoids from 400 500 ma in the west to 2300 2600 ma in the east the boddington terrane contains a sequence of archaean shelf facies metasediments part of the jimperding metamorphic belt believed to have been deposited at 3 1 ga and metamorphosed up to amphibolite granulite transition facies under low pressure conditions these sequences occur mainly in flat lying nappes which may have been tectonically transported from the east during assembly with the lake grace terrane the boddington terrane also contains two low grade greenstone belts formed between 2650 and 2670 ma and intruded by 2640 ma granitoids the eastern boundary of the boddington terrane is marked by a major change in gravity which also corresponds with a series of shear zones at the surface a change in structural style and a major change in the nature of the granitoids east of this boundary in the lake grace terrane granitoids of two ages have been recognised charnockites formed at 2640 ma and post tectonic granodiorites formed at 2580 ma metasedimentary rocks metamorphosed to granulite facies occur as rafts and lenses with steep upright folds and are enclosed in leucocratic garnetiferous granitic gneiss greenstones also metamorphosed to granulite facies and recording ages of around 2790 ma are also enclosed within the charnockites and are thus substantially older than the low grade greenstones in the boddington terrane to the west assembly of the balingup and boddington terranes appears to have been completed prior to the emplacement of the logue brook granite at 2612 ma the boddington and lake grace terranes came together after the 2640 ma granulite facies metamorphic event but the precise timing of assembly is not currently constrained it was certainly completed by 2411 ma the time of emplacement of a major set of e w trending mafic dykes the widgemooltha dyke suite which extend for over 500 km across the yilgarn craton although the widgemooltha dyke suite appears to be absent from the balingup terrane this is possibly related to extensive nw shearing during the proterozoic which also led to extensional re activation of the decollement that marks the base of the upper crust in the region

deformation of a high grade gondwana fragment sri lanka abstract the sri lankan gondwana fragment is a high grade gneiss terrain consisting of at least three crustal units namely the wanni complex wc the highland complex hc and the vijayan complex vc which were juxtaposed during the pan african collision at least six phases of ductile deformation were recognized in the smallest gondwana fragment sri lanka there is microscopic and mesoscopic evidence for a first deformation d 1 which occurred during prograde metamorphism but before the main granulite facies metamorphism this first deformation is represented by straight to curved and crenulated inclusion trails in garnet porphyroblasts microfolds in some metapelites and quartzo feldspathic gneisses and by a s 1 foliation preserved in crenulation folds and some rootless intrafolial folds d 2 has two main stages of which the second phase is the most intense and dominant deformation which produced most part of the major composite and transposed compositional layering as well as the prominent stretching lineation l 2 the first stage of d 2 may have been responsible for the formation of a crenulation cleavage of s 1 as seen in garnet porphyroblasts in some metapelites the compositional layering s 2 in metapelites and metasemipelites appears to have formed through the development of a crenulation cleavage f 2 folds are mesoscopic isoclinal rootless and occur as intrafolial folds and their limbs are highly attenuated major part of the deformation during the latter stage of d 2 was non coaxial many minor and large scale recumbent isoclinal folds f 3 were produced during d 3 both d 2 and d 3 were coeval with the peak conditions of granulite facies metamorphism d 4 produced very large gentle nearly e w upright folds probably during the beginning of uplift of the entire high grade assemblage from deeper crustal levels still under granulite facies conditions d 5 is the second strongest deformation and was responsible for the large scale upright folds f 5 which control the present map pattern of sri lanka the superimposition of d 5 folds on d 4 folds gave rise to a large scale type i fold interference pattern of ramsay 1967 d 6 deformation caused local refolding of the f 5 folds and was accompanied by amphibolite facies conditions during uplift the deformation represented by the majority of vein type structures such as patchy charnockite veins cordierite bearing veins and graphite veins clearly post date d 5 it is argued that the wc the hc and the vc in sri lanka may be three distinctly different proterozoic crustal units brought into contact by tectonism in late proterozoic at two different stages

deep crustal structure in the southwestern yilgarn craton western australia abstract the archaean rocks of the yilgarn craton comprise the largest geological entity in western australia and one of the largest areas of archaean crust anywhere in the world however poor outcrop has prevented a thorough understanding of the structure of the crust in the region a subject of particular interest being the relationship between high grade metamorphic rocks in the west and lower grade granitoid greenstone terrains further east interpretation of previously unpublished seismic refraction data combined with the re interpretation of published data allows crustal velocity structure in the southwest of the craton to be resolved overall the structure is similar to shield areas elsewhere in the world the data defining a two layered crust with an average thickness of about 35 km however significant lateral variations in velocity structure are also observed and these coincide with a terrane boundary previously postulated on the basis of geological observations and seismic reflection data the terrane boundary dips to the east extends through the entire crust and is notable for an area of anomalously high seismic velocity in the lower crust two possible interpretations of the high velocity zone are proposed firstly it is due to the presence of mafic to ultramafic intrusions secondly and the preferred interpretation the high velocity zone is a fault bounded mega sliver which may be a suspect terrane in its own right perhaps with oceanic affinities the results of the seismic experiment strongly support tectonic models for yilgarn craton based on terrane tectonics the possibility that the terrane boundary is being reactivated by contemporary stresses also provides a plausible explanation for the local intra plate seismicity

stable isotopes and sclerochronology of the bivalve mesodesma donacium potential application to peruvian paleoceanographic reconstructions reconstructing the past enso el nino southern oscillation interannual variability requires very high resolution sea surface temperature sst proxies in order to compensate for the lack of coral archive in the eastern pacific cold tongue we studied the potentiality of the bivalve species mesodesma donacium as a sst tracer for the peruvian coast biomineralisation cycles and growth line formation in the shells are shown to be controlled by tides the sclerochronology based on fortnightly cycles allowed us to compare local sst data with high resolution shell records as a result of the high correlation between sst and aragonite delta 18 o we calculated a linear sst fractionation relationship for the 14 20 degrees c range t degrees c 17 41 1 15 3 66 0 16 delta 18 o arag pdb delta 18 o wat smow the difference from the commonly used grossman and ku 1986 grossman e l ku t l 1986 oxygen and carbon fractionation in biogenic aragonite temperature effect chemical geology 59 59 74 equation suggests the potential existence of a species dependence of vital effect within bivalves this preliminary equation allows monthly scale sst reconstructions with a 1 5 degrees c precision using m donacium archaeological shells from southern peru giving potential insights into the problem of past enso behaviour c 2005 elsevier b v all rights reserved

strong el nino events during the early holocene stable isotope evidence from peruvian sea shells intrashell oxygen and carbon stable isotope profiles 18o 13c were established for mesodesma donacium shells from the preceramic archaeological site quebrada de los burros in coastal southern peru to reconstruct the sea surface temperature sst variations and to estimate changes of the el nino southem oscillation enso interannual variability from the early holocene to the present very strong oxygen isotopic excursions attributed to el nino events are evident in three shells dated between 9 and 7 9 ka this indicates that short and strong warm events were breaking the mean cooler surface conditions 2 3 c cooler compared with today ssts in relation with an intense upwelling system at 4 8 ka slightly lower mean sst cooler than today by 0 7 c and a reduced seasonality confirms the previous hypothesis of a weak enso variability at this period

lithological structural and geochemical characteristics of the mesoarchean târtoq greenstone belt southern west greenland and the chugach prince william accretionary complex southern alaska evidence for uniformitarian plate tectonic processes the mesoarchean t rtoq greenstone belt southern west greenland consists of tectonically imbricated slices of metamorphosed basalt gabbro peridotite and sedimentary rocks and is intruded by fel

shell midden sclerochronology abstract sclerochronological analysis of shell midden faunal remains offers a unique opportunity to study climate and environmental change in relation to human activity middens exist near most aquatic habitats almost worldwide for much of the late pleistocene and holocene they afford good preservation of accretionary calcium carbonate remains such as mollusk shells and fish otoliths in comparatively well dated contexts as such midden samples are a source of sequential proxy records of past climate and environmental conditions and may provide detailed pictures of human environment interaction however midden samples present several challenges to the development and application of sclerochronological proxies including unique taphonomic concerns here these challenges are outlined within descriptions of the major applications of midden sclerochronology archeological concerns including season of capture reconstructing the history of anthropogenic environmental impacts and paleoclimate environment proxy reconstructions

a review of structural patterns and melting processes in the archean craton of west greenland evidence for crustal growth at convergent plate margins as opposed to non uniformitarian models abstract the archean craton of west greenland consists of many fault bounded eoarchean to neoarchean tectonic terranes crustal blocks these tectonic terranes are composed mainly of tonalite trondhjemite granodiorite ttg gneisses granitic gneisses metavolcanic dominated supracrustal belts layered anorthositic complexes and late to post tectonic granites rock assemblages and geochemical signatures in these terranes suggest that they represent fragments of dismembered oceanic island arcs consisting mainly of ttg plutons tholeiitic to calc alkaline basalts boninites picrites and cumulate layers of ultramafic rocks gabbros leucogabbros and anorthosites with minor sedimentary rocks the structural characteristics of the terrane boundaries are consistent with the assembly of these island arcs through modern style of horizontal tectonics suggesting that the archean craton of west greenland grew at convergent plate margins several supracrustal belts that occur at or near the terrane boundaries are interpreted as relict accretionary prisms the terranes display fold and thrust structures and contain numerous 10 cm to 20 m wide bifurcating ductile shear zones that are characterized by a variety of structures including transposed and redistributed isoclinal folds geometrically these structures are similar to those occurring on regional scales suggesting that the archean craton of west greenland can be interpreted as a continental scale accretionary complex such as the paleozoic altaids melting of metavolcanic rocks during tectonic thickening in the arcs played an important role in the generation of ttgs non uniformitarian models proposed for the origin of archean terranes have no analogs in the geologic record and are inconsistent with structural lithological petrological and geochemical data collected from archean terranes over the last four decades the style of deformation and generation of felsic rocks on outcrop scales in the archean craton of west greenland and the mesozoic sulu orogenic belt of eastern china are similar consistent with the formation of archean continental crust by subduction zone processes

an overview of the geochemistry of eoarchean to mesoarchean ultramafic to mafic volcanic rocks sw greenland implications for mantle depletion and petrogenetic processes at subduction zones in the early earth abstract this study reviews the geochemical characteristics of eoarchean to mesoarchean ultramafic to mafic volcanic rocks now amphibolites in sw greenland and compares them with those of cenozoic oceanic island arc basalts to evaluate archean subduction zone petrogenetic processes emphasis is placed on the th ree hfse zr ti and nb systematics of the ca 3800 and ca 3700 ma arc suites in the isua greenstone belt the ca 3075 ma ivisaartoq ujarassuit greenstone belt and amphibolites associated with the ca 2970 ma fiskenaesset layered anorthosite complex on n morb normalized diagrams the isua ivisaartoq ujarassuit and fiskenaesset volcanic rocks are all characterized by depletion of nb relative to th and lree consistent with a supra subduction forearc arc backarc geodynamic setting similarly on the th yb nb yb projection these suites plot within the field of cenozoic oceanic island arc basalts on log transformed immobile trace element ratio nb th la th sm th and yb th diagrams they display a trend projecting from morb mid ocean ridge basalt to iab island arc basalt on the iab crb continental rift basalt oib ocean island basalt morb diagram as for cenozoic oceanic island arc basalts accordingly these trace element compositions are interpreted as reflecting the enrichment of archean depleted upper mantle morb source by subduction derived melts and fluids following the initiation of intra oceanic subduction and arc migration concentrations of mgo and ni in sw greenland archean basalts overlap with but extend to 2 to 4 times higher than those in cenozoic oceanic island arc counterparts in contrast the majority of archean basalts have ree and hfse concentrations 2 to 4 times lower than cenozoic oceanic island arc basalts consistent with more depleted sub arc mantle wedge peridotites in the archean than in cenozoic counterparts such depletion reflects the extraction of large volumes of mafic to ultramafic melts from hotter archean mantle we infer that less refractory compositions of the present day depleted upper mantle the source of morb and conservative hfse in arc basalts have resulted from re mixing with the less depleted to enriched deeper mantle material transferred to shallower depths by mantle convection and plumes in post archean times archean basalts in sw greenland share the negative nb and ti anomalies of cenozoic oceanic island arc basalts on average archean basalts however have lower th nb la nb sm tio2 and gd tio2 ratios than cenozoic oceanic island arc counterparts given that rutile controls the nb and ti budgets in arc magmas the lower th nb la nb sm tio2 and gd tio2 ratios in sw greenland basalts are attributed to the mobility of nb and ti in slab derived melts involving rutile fusion in archean subduction zones these elements are less mobile in fluids originating from cenozoic subducting oceanic crust where rutile appears to be generally stable accordingly it is suggested that higher geothermal gradients in the archean may have provided optimized conditions for slab melting and metasomatism of the sub arc mantle wedge by slab derived melts

early precambrian crustal development changing styles of mafic magmatism the work of sutton watson 1951 was one of the cornerstones in the interpretation of the evolution of early precambrian archaean and early proterozoic gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted archaean cratons since then the concept of geological terranes and terrane accretion has been extended back to the archaean and different events can be distinguished by increasingly precise isotopic dating techniques there is growing evidence that with certain clear provisos the principle of uniformitarianism can be applied as equally to the early precambrian as it can through the rest of the geological record in terms of mafic magmatism there are similarities between some ancient and modern suites both intrusive and extrusive but there are also some major differences such as the presence of komatiites in the archaean and the formation of large scale layered intrusions and extensive dyke swarms at the end of the late archaean and in the early proterozoic the mantle has evolved continuously with the progressive extraction of mafic magmas but allowing for this continental tholeiites appear to have changed little with time ancient mid ocean ridge basalts are elusive because of their poor preservation record the geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present day arc systems the relative abundance of early precambrian intrusive mafic suites parallels the crustal growth curve reflecting the global crustal accretion differentiation superevent towards the close of the archaean bimodal tholeiitic noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle mafic crust continental crust system this in turn gives a possible link between the respective evolutions of archaean to recent komatiitic noritic and boninitic suites

the whole rock sm nd age for the 2825 ma ikkattoq gneisses greenland is 800 ma too young insights into archaean ttg petrogenesis abstract the ikkattoq gneisses of the archaean gneiss complex in the nuuk region southern west greenland are the orthogneiss component within the amphibolite facies tre brodre terrane they have mostly granodioritic compositions with a small amount of quartz diorite sm nd isotopic data for a quartz diorite and five granodiorite ikkattoq gneiss samples from within 5 km of the ikkattoq fjord type locality yielded a regression with a slope equivalent to 2005 52 ma mswd 0 72 regardless of the low mswd this cannot be the true age of the ikkattoq gneisses because all ikkattoq gneisses yield u pb zircon dates of c 2825 ma and they are cut by the undeformed 2560 ma qorqut granite complex this anomalously young regression age resulted instead from mixing of different nd components indicating that the ikkattoq gneisses are derived from mixed source materials taking the true age of the ikkattoq gneisses as 2825 ma from u pb zircon dating the range of initial e nd in the ikkattoq gneisses is 7 1 to 1 8 the negative initial e nd values mean that older light rare earth enriched sialic crust contributed to the igneous precursors of the ikkattoq gneisses this nd evidence for contribution of older sialic crust is supported by positive e sr values for the ikkattoq gneisses with e nd values as low as 7 1 this older crustal component has to be eoarchaean the presence of scarce quartz diorites low sio 2 high mgo suggests that ultramafic rocks upper mantle metasomatised by the passage of fluids or silicic melts were another contributing source the ikkattoq gneisses are proposed as a complex suite incorporating material derived from melting of much older sialic crust and probably upper mantle the intercalation of tectonostratigraphic terranes during collisional orogeny at c 2720 ma destroyed the architecture of this 2825 ma magmatic system and the ikkattoq gneisses now form a slice tectonically isolated from their source region in terms of trace element parameters the ikkattoq gneisses resemble phanerozoic volcanic arc granites thus an andean style arc setting for the generation of the ikkattoq gneiss precursors is possible other archaean ttg suites of the nuuk region are generally thought to represent predominantly juvenile additions to the crust in the broadest sense they do because isotopic work over the past 30 years has demonstrated that they do not represent wholesale recycling of considerably older crust however in detail within these broadly juvenile suites a contribution from older crust can be detected thus c 3000 ma type nuk gneisses from around nuuk town show a spread in e nd values down to 1 7 in this case the likely older crustal component was 3230 ma quartz diorite that occurs as enclaves in the c 3000 ma suite thus to a lesser or greater degree some meso to neoarchaean ttg suites in the nuuk region display the same internal complexities and evidence for mixed sources as modern arc suites developed near the margins of older crust

3800 to 2500 ma crustal evolution in the anshan area of liaoning province northeastern china shrimp u pb zircon geochronology has revealed several generations of granitoids and gneisses from the anshan area of northeastern china two samples of the baijiafen granite sensu lato 3804 5 and 3812 4 ma chentaigou granite 3306 13 ma lishan granite 3142 7 ma east and west anshan granites 3001 4 and 2994 8 ma tiejiashan granite 2962 4 and 2964 6 ma and qidashan granite 2475 ma the 3800 ma baijiafen granite consists of banded migmatitic trondhjemitic gneisses and forms only a small part of the area nonetheless they are the oldest known rocks in asia and are one of very few locations in the world where rocks of this antiquity have been found the younger suites are predominantly granite sensu stricto in composition and although foliated they have not been transformed into banded gneisses nd isotopic data suggest that the 3800 ma granites were produced at least in part by remelting of very ancient lree enriched sources perhaps like the exposed remnants of the 3800 ma rocks the chentaigou supracrustals contain layered mafic and ultramafic rocks banded iron formation bif and siliceous layered rocks of probable volcanogenic origin a sample of the latter has yielded a concordant u pb zircon age of 3362 5 ma interpreted as the age of deposition a trondhjemitic sheet cutting these rocks has yielded a u pb zircon age of 3342 6 ma the chentaigou supracrustals are the oldest recognised in the area younger supracrustal sequences collectively known as the anshan group contains the main economic bif units in the area bif is intruded by the 2475 ma qidashan granite indicating that the anshan group is archaean or earliest proterozoic in age

investigating δ13c and δ14c within mytilus californianus shells as proxies of upwelling intensity 1 along the west coast of north america climate and marine productivity is affected by seasonal to interannual changes in coastal upwelling our understanding of upwelling variability in the past is limited by the short duration of instrumental records changes in upwelling intensity are expected to affect the 13c 12c 13c and radiocarbon 14c content of marine dissolved inorganic carbon dic due to variable mixing of old upwelled seawater into surface waters if seasonal variations in the 13c of dic are recorded in marine bivalve shells they may provide valuable information about the extent of upwelling in the past comparison of modern mytilus californianus shells from south california with a 5 year time series of coastal seawater 14c and 13c allows an assessment of the suitability of mytilus shell 14c and 13c as proxies of upwelling intensity we show that both absolute values and the seasonal range in seawater 14c are preserved in shell 14c allowing its use as an indicator of upwelling intensity interpretation of shell 13c is more problematic with the 13c of shell carbonate lower than seawater dic by variable amounts ranging from 0 5 to 1 5 due to the incorporation of metabolic carbon the spatial and temporal variability observed in specimens that grew during the severe el nino event of 1997 1998 demonstrates how a transect of shells spanning the western north american coastline can be used to reconstruct large scale patterns of seawater 14c variability for specific intervals of interest such as those associated with el nino southern oscillation type phenomena

u pb ages of single zircons within upernavik metasedimentary rocks and regional implications for the tectonic evolution of the archaean nain province labrador detrital zircons and their postdepositional overgrowths from three units of the upernavik supracrustal association in the northern saglek block of the archaean nain province have been dated with the ion microprobe shrimp in one unit from the granulite facies area in inner saglek fiord the zircon population is dominated by early archaean grains thought to be derived from the uivak gneisses recrystallization and growth of new zircon within this metasediment took place during granulite facies metamorphism at 2761 12 ma 2 which is also a younger limit on the age of deposition in a second unit from the amphibolite facies area in outer saglek fiord detrital zircons have predominantly mid and late archaean ages the mid archaean zircons are comparable in age to the 3235 ma lister gneisses the ages of the late archaean detrital zircons 2800 2960 ma do not correspond to any known rock complex in the saglek block but plutonic rocks associated or correlative with the ca 2840 ma kanairiktok plut

the u th pb systematics of zircons from the type nuk gneisses godthabsfjord west greenland abstract u pb analyses were made on zircons separated from different lithologies within the nuk gneisses taken from godthab nuuk townsite and from the south west shore of bjorneoen size fractions of the zircons from each rock yield linear discordia which give apparent crystallisation ages for the parents of the nuk gneisses of 2890 3065 myr this spread of ages is not a result of multiple intrusion but more likely results from different degrees of diffusion and metamorphic episodic lead loss by the zircons since u pb and th pb discordance are similar loss of radiogenic lead and not u or th loss gain by radiation damage diffusion and through metamorphism is the main factor causing discordance of the u pb dates the results show that the discordia line given by the analyses of fractions of a single zircon population may not give a true crystallisation age when the rock has been strongly metamorphosed this interpretation is supported by rb sr whole rock analyses of the same rocks in which the nuk townsite rocks scatter about a reference isochron of 2840 myr while the bjorneoen samples give an isochron mswd 0 55 of 3076 27 myr and the initial ratio at 0 70205 0 00009 the zircon results do not rule out the possibility that more than one metamorphic episode affected the original nuk gneiss parents prior to 2520 myr qorqut granite intrusion a date of 2660 myr was obtained from an aplite intruded during a period of plutonic activity that included the formation of a linear belt of intense ductile deformation that extends through south western bjorneoen and godthab town

shell of the great scallop pecten maximus as a high frequency archive of paleoenvironmental changes 1 we investigate the environmental and biological controls on oxygen isotope ratios in the shells of modern pecten maximus bivalves collected alive in the bay of brest france seasonal 18o profiles obtained by drilling the daily calcite ridges deposited at the surface of the left valve were compared with in situ measurements of temperature and salinity we show that p maximus 18o values accurately track seasonal variations in bottom water temperature shell growth rate has no significant effect on shell 18o values our study demonstrates that daily variations in water temperature can be reconstructed within about 0 5 c temperatures estimated with the paleotemperature equation established in this study were compared with temperatures derived from previously published equations the comparison indicates that the most commonly used paleotemperature equation for biogenic calcite epstein et al 1953 provides inaccurate temperature estimates but the kim and o neil 1997 equation established from abiogenic calcite precipitation provides results very similar to ours and should therefore be used for scallop individuals coming from populations where proper empirical calibration cannot be done pecten maximus bivalves precipitate calcite in isotopic equilibrium with seawater produce large daily growth striae are stenohaline and are well preserved in archeological and geological deposits making them an excellent high frequency archive of paleoenvironmental change