late holocene and younger dryas glaciers in the northern cairngorm mountains scotland we present 17 cosmogenic 10be ages of glacial deposits in coire an lochain cairngorm mountains which demonstrate that glacial and nival deposits cover a longer timescale than previously recognis

post eruption erosion and deposition in the 1980 crater of mount st helens washington determined from digital maps the crater of mount st helens shows one of the world s highest known rates of mass wasting on many summer days rockfall is almost continuous and many large rock and dirty snow avalanches have travelled several kilometres from their sources on the crater walls since formation of the crater on 18 may 1980 talus cones exceeding 100 m in thickness have formed at the base of the unstable 600 m high crater walls to estimate rates of erosion and deposition a series of digitized topographic maps made from aerial photographs taken of the crater in 1980 1981 1983 1986 and 1988 were analysed using a geographic information system between 1980 and 1988 30 106 m3 of rock were eroded from the crater wall representing a mean retreat rate of 2 1 m yr 1 to account for the volume increase that occurs when bedrock is transformed into scree this volume is multiplied by 4 3 this provides an estimate of the rock debris volume supplied to the crater floor of 40 106 m3 the actual volume of deposits that accumulated during this 8 year period however is 68 106 m3 the difference of 28 106 m3 is presumably the volume of snow intercalated between insulating layers of rock debris similar calculations for each of four time intervals between 1980 and 1988 suggest that wall erosion and thus talus accumulation rates are declining but that rates will probably remain high for decades to come

u pb zircon ages from the eastern savant lake crow lake metavolcanic metasedimentary belt northwest ontario five rocks from three areas in the eastern part of the savant lake crow lake metavolcanic metasedimentary belt have been dated by u pb analysis of zircons the sioux lookout area is composed of a sequence of generally fault bounded metavolcanic and metasedirnentary belts the age of a felsic tuff in the central volcanic belt is 2732 72 ma this is possibly the youngest sequence in the area the savant lake volcanic belt comprises the jutten volcanic unit at its base separated by an angular unconforrnity from the handy lake volcanic sequence the age of a dacitic tuff near the base of the handy lake volcanic sequence is 2745 22 i ma the south sturgeon lake volcanic belt is composed of four volcanic cycles the age of a felsic tuff from the top of the uppermost cycle is 2717 92 ma the felsic beidelman bay pluton and the mafic pike lake pluton which both intruded the lower art of the sequence give ages of 2733 82 and 2732 721 a respectively this establishes a minimum time span of 15 921 ma for deposition of the sequence this interval along with the 27 ma span between the ages of the handy lake sample and the volcanic sample from the south sturgeon lake area suggests that the sequences were laid down by a series of discrete volcanic episodes for samples subjected to multistage lead loss the reliability of ages has been greatly enhanced by air abrasion of zircons several cases of suspected biasing of analyses by the air abrasion process have been encountered it appears that this biasing problem can be overcome by selection and abrasion of low uranium grains that are free of imperfections

modeling the dynamic response of a crater glacier to lava dome emplacement mount st helens washington usa the debris rich glacier that grew in the crater of mount st helens after the volcano s cataclysmic 1980 eruption was split in two by a new lava dome in 2004 for nearly six months the eastern part of the glacier was squeezed against the crater wall as the lava dome expanded glacier thickness nearly doubled locally and surface speed increased substantially as squeezing slowed and then stopped surface speed fell and ice was redistributed downglacier this sequence of events which amounts to a field scale experiment on the deformation of debris rich ice at high strain rates was interpreted using a two dimensional flowband model the best match between modeled and observed glacier surface motion both vertical and horizontal requires ice that is about 5 times stiffer and 1 2 times denser than normal temperate ice results also indicate that lateral squeezing and by inference lava dome growth adjacent to the glacier likely slowed over a period of about 30 days rather than stopping abruptly this finding is supported by geodetic data documenting dome growth

sm nd age of kambalda and kanowna greenstones and heterogeneity in the archaean mantle a precise sm nd age of 2 790 30 myr and an initial ratio corresponding to nd 3 4 0 6 has been determined for the kambalda sequence from the eastern yilgarn block of western australia the high initial 143nd 144nd implies a long lived light rare earth element depleted high sm nd source for the kambalda volcanics and therefore provides definitive evidence for ancient heterogeneities in the archaean mantle

glacier reconstruction and mass balance modelling as a geomorphic and palaeoclimatic tool the reconstruction of former mountain glaciers has long been used to examine the implications of rapid climate shifts for example at the last glacial interglacial transition and for evaluating asynchronous behaviour of mountain glaciers compared with mid latitude ice sheets during the late quaternary glacier reconstruction has also been used as a source of palaeoclimatic information based on the recognition of empirical relationships between glaciers and climate this paper reviews the application and implications of a recently revised method of glacier reconstruction carr and coleman 2007 based around glaciological principles of mass balance this study examines how this approach can be used to test geomorphological interpretations of former mountain glaciation and also to infer precipitation fields at sites of former glaciation sites of younger dryas niche and icefield glaciation in the british isles demonstrate how this method can verify interpretations of marginal glaciation and begin to understand the different behaviour of outlet glaciers within the same environmental regime examination of a site of former niche glaciation in southern africa demonstrates how glacier reconstruction may be used to infer annual and seasonal precipitation values and strongly supports the idea that winter precipitation in lesotho and se south africa was substantially greater than present day values during the last glacial cycle copyright 2010 john wiley sons ltd

basement terrane correlations and crustal recycling in the western superior province nd isotopic character of granitoid and felsic volcanic rocks in the wabigoon subprovince n ontario canada abstract the western superior province represents a collage of mesoarchean continental 3 2 2 8 ga and neoarchean continental arc and oceanic 2 8 2 66 ga crustal fragments nd isotope data presented here for 89 felsic volcanic and plutonic rocks have been combined with compiled data to generate a data set of over 240 analyses from the wabigoon and winnipeg river subprovinces these samples from both oceanic and continental environments have been used as crustal probes their nd isotopic character used to investigate the extent of basement terranes the ancestry of crustal fragments their interaction with each other and the degree of sialic crustal reworking the wabigoon and winnipeg river subprovinces include three isotopically distinct terranes 1 the winnipeg river terrane comprises the winnipeg river subprovince the north central wabigoon and the north eastern wabigoon subprovince this region is dominated by neoarchean granitoid rocks with mesoarchean vestiges of previous workers have suggested that the boundary between the western wabigoon and winnipeg river terranes represents a cryptic suture resulting from a collision at circa 2 71 ga the boundary between the western wabigoon and marmion terranes although obscured by late intrusions may be a continuation of this suture zone the boundary between the winnipeg river and marmion terranes in the central wabigoon is also cryptic isotopic data suggest that the marmion terrane is allochthonous with respect to the winnipeg river terrane common magmatism in these two terranes at 2 93 2 87 ga suggests that they may have collided during this time in the eastern wabigoon subprovince this collision zone is speculated to occur at the humboldt bay high strain zone a complex history of crustal recycling throughout the mesoarchean and neoarchean is revealed which demonstrates the importance of autochthonous development of granite greenstone terranes in a continental environment the data also support models of tectonic accretion during the archean indicated by the presence of granite greenstone terranes of allochthonous origin

stratigraphy and petrology of the mulcahy lake layered gabbro an archean intrusion in the wabigoon subprovince ontario

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

sedimentary and structural evidence for 2 7 ga continental arc oceanic arc collision in the savant sturgeon greenstone belt western superior province canada the western superior province sustained rapid crustal growth in the interval 2 72 2 68 ga through amalgamation of microcontinental crustal blocks and juvenile oceanic terranes recent field isotopic and geophysical surveys provide insight on the nature timing and scale of this accretionary growth however few places offer the rich tectono stratigraphic and structural detail with which to establish accretion of oceanic and continental blocks as does the savant sturgeon area here 3 4 2 8 ga continental crust of the winnipeg river terrane is juxtaposed with 2 775 2 718 ga juvenile oceanic rocks of the western wabigoon terrane across a 2 85 2 75 ga southwest facing continental margin sequence the continental margin was reactivated at 2 715 ga with the establishment of an arc recorded by 2 715 2 70 ga tonalite and associated intermediate volcanic rocks this magmatic activity is interpreted to reflect north and east dipping subduction that led to consumption of a small tract of oceanic crust betwe

surface exposure dating of moraines in the kromer valley silvretta mountains austria evidence for glacial response to the 8 2 ka event in the eastern alps 10 be dating of a pronounced glacier advance kromer stadial in the western part of the silvretta mountains western austria yielded ages ranging from 8010 360 to 8690 410 years with a mean age for moraine stabilization of 8410 690 years hence the kromer stadial which was previously considered as early preboreal may represent the glacier response to the early phase of the 8 2 ka event as it is recorded in the greenland ice cores the glacier advanced to a position beyond its little ice age lia limit the end moraines are situated 750 1000 m downvalley from the glacier ends in 1850 the corresponding drop of the equilibrium line altitude relative to the lia datum was 75 m the glacier advance was followed by a period of glacier recession and rock glacier development in total the climatic fluctuation may have lasted for about 500 200 years the age and duration of the climatic fluctuation is in good accordance with the misox cold phase and ce 3 climatic fluctuation as recorded in switzerland climate during the kromer stadial was characterized by more humid conditions than today along the northern fringe of the alps and slightly negative to moderately positive changes in precipitation in the central part of the austrian alps summer temperatures changes were most likely in the order of i to 0k mean annual temperature was 1 5k to 2k lower than today at least during the second phase of the climatic fluctuation

oxygen isotope perspective on precambrian crustal growth and maturation in this study the authors contrast insights on precambrian crustal growth and maturation from radiogenic and oxygen isotope systematics in the superior 3 0 2 7 ga and grenville 1 3 1 0 ga provinces of the canadian shield oxygen isotope ratios in zircon provide the best evidence of supracrustal input into ancient orogens archean superior province zircons have relatively low delta sup 18 o values and a limited range 5 7 per thousand 0 6 per thousand while proterozoic grenville province zircons have elevated delta sup 18 o values and a wider range 8 2 per thousand 1 7 per thousand these data reflect fundamental differences in crustal evolution and maturation between the superior and the grenville provinces in the grenville province radiogenically juvenile supracrustal material with high delta sup 18 o values was buried or subducted to the base of the crust within 150 m y of initial crust production causing high magmatic delta sup 18 o values delta sup 18 o zircon ge 8 per thousand in anorthosite suite and subsequent plutons information about large volumes and rapid recycling of grenville crust is not accessible from radiogenic isotope data alone the grenville data contrast with the restricted delta sup 18 o values of superior province magmatism where subtle approximately 1 per thousand elevation in delta sup 18 o occurs only in volumetrically minor late to postorogenic sanukitoid plutons differences in sediment delta sup 18 omore values between the superior and grenville provinces are predominantly a function of the delta sup 18 o of source materials rather than differences in chemical maturity or erosion styles this study shows that zircon is a robust reference mineral to compare igneous processes in rocks that have undergone radically different histories less

sm nd and rb sr isotope systematics of an archean anorthosite and related rocks from the superior province of the canadian shield sm nd and rb sr isotopic data for the bad vermilion lake anorthosite complex bvl in the rainy lake area of the superior province of northwestern ontario show that direct ages of archean anorthosites can be obtained with these isotopic systems despite the effects of low grade metamorphism there is sufficient spread in sm nd between plagioclase megacrysts and coexisting mafic groundmass to allow the determination of reasonably precise internal sm nd isochrons anorthosite samples from bvl show an unusually large range in lil concentrations such that there is sufficient spread in rb sr for a whole rock isochron 2 69 0 10 ga isr 0 70079 8 this variability may have been caused by rb introduction during hydrothermal alteration and or low grade metamorphism the sm nd isochron for bvl 2747 58 ma end 2 0 1 4 includes data for anorthosite gabbro and metabasalt and is consistent with the consanguineity of these units in the rainy lake area the age is interpreted as the time of crystallization of the anorthosite complex and related mafic plutonics and volcanics visibly altered samples show evidence for disturbance of the sm nd and rb sr isotopic systems in one altered porphyritic dike plagioclase appears to have exchanged light ree with the relatively ree rich basaltic matrix this sample yields an internal sm nd age of 2 16 0 05 ga which may correspond to the time of local heating or represent a partial resetting from a still younger event initial isotopic ratios of nd and sr determined here add to the growing body of data indicating that the superior province is underlain by depleted mantle

dynamics of seismogenic volcanic extrusion at mount st helens in 2004 05 the 2004 05 eruption of mount st helens exhibited sustained near equilibrium behaviour characterized by relatively steady extrusion of a solid dacite plug and nearly periodic shallow earthquakes here we present a diverse data set to support our hypothesis that these earthquakes resulted from stick slip motion along the margins of the plug as it was forced incrementally upwards by ascending solidifying gas poor magma we formalize this hypothesis with a dynamical model that reveals a strong analogy between behaviour of the magma plug system and that of a variably damped oscillator modelled stick slip oscillations have properties that help constrain the balance of forces governing the earthquakes and eruption and they imply that magma pressure never deviated much from the steady equilibrium pressure we infer that the volcano was probably poised in a near eruptive equilibrium state long before the onset of the 2004 05 eruption

roots of an archean volcanic arc complex the lac des iles area in ontario canada abstract the lac des iles area is located in the wabigoon subprovince an archean granite greenstone terrain in the superior province in ontario canada the area consists of several unmetamorphosed plutons of biotite and hornblende tonalite and mafic and ultramafic intrusions which are emplaced into an older tonalitic gneiss the mafic ultramafic bodies are concentric in shape and consist of periodotite pyroxenite gabbro and hornblende gabbro cumulates and diorite the largest intrusion the lac des iles complex consists of several mafic and ultramafic centers an internal sm nd isochron from the southern ultramafic complex gives an intrusion age of 2736 28 ma mswd 1 5 nd t 1 2 this age is also assumed to be representative of the time of emplacement of the other mafic and ultramafic bodies as well as the unmetamorphosed tonalite plutons the tonalitic gneiss represents older continental crust and had an nd value of 1 at this time the nd t values of the felsic and mafic ultramafic suites are positive ranging from 0 to 2 this range can be explained if primary magmas derived from a depleted mantle reservoir assimilated continental crust such as the tonalite gneiss during their ascent this implies that the tonalite magmas in the lac des iles area represent mixtures of the tonalite country rock and mantle magmas which formed by partial melting of depleted mantle or of mafic rocks underplated beneath the continental lithosphere the morphology and the rock assemblages of the mafic and ultramafic intrusions in the lac des iles area resemble alaskan type intrusions which are common components of phanerozoic magmatic arcs associated with subduction zones in addition the crystallization sequence in the mafic ultramafic intrusions suggests that they are derived from a parental magma with high al basalt affinities the mafic ultramafic rocks are depleted in high field strength elements but enriched in large ion lithophile elements and the tonalites belong to calc alkaline rock series displaying fractionation trends towards trondhjemitic compositions thus the archean lac des iles area and the root zones of phanerozoic active continental margins share many geological geochemical and petrological features which would suggest that the lac des iles area formed in an andean type continental margin

geochronology of wabigoon belt granitoids northwestern ontario rb sr isochrons for seven late tectonic plutons whole rock rb sr isochrons are presented for seven late tectonic granitoid plutons intruded into supracrustal rocks of the wabigoon volcanic plutonic belt age estimates range from 2461 78 ma ottertail lake stock to 2645 158 ma bears passage stock a mean age estimate by weighted pooling of the isochrons gives 2555 41 ma while pooling 39 samples into one regression gives 2568 35 ma these ages concur with estimates for the duration of the kenoran orogeny error limits on our isochrons incorporate the duration of polydiapirism aplitic dyking and deuteric metasomatism available ages indicate that syntectonic and late tectonic granitoid plutonism spanned a geologically short time interval centered at 2565 62 ma that cannot be differentiated by the rb sr technique consistently low initial sr ratios of 0 7008 0 0006 indicate a mantle or low rb sr source

emplacement of a silicic lava dome through a crater glacier mount st helens 2004 06 the process of lava dome emplacement through a glacier was observed for the first time after mount st helens reawakened in september 2004 the glacier that had grown in the crater since the cataclysmic 1980 eruption was split in two by the new lava dome the two parts of the glacier were successively squeezed against the crater wall photography photogrammetry and geodetic measurements document glacier deformation of an extreme variety with strain rates of extraordinary magnitude as compared to normal alpine glaciers unlike normal temperate glaciers the crater glacier shows no evidence of either speed up at the beginning of the ablation season or diurnal speed fluctuations during the ablation season thus there is evidently no slip of the glacier over its bed the most reasonable explanation for this anomaly is that meltwater penetrating the glacier is captured by a thick layer of coarse rubble at the bed and then enters the volcano s groundwater system rather than flowing through a drainage network along the bed

correction for deviations due to magnetic anisotropy of palaeomagnetic directions from the mulcahy gabbro north west ontario summary the mulcahy gabbro is an archean layered intrusion in the wabigoon subprovince of north western ontario alternating field and thermal demagnetization of specimens from it have provided consistent within site characteristic remanence chrm directions in eight of 19 sites antipodal polarities of remanence are recovered and the overall in situ formation mean is d 240 i 51 k 21 this direction is unlike any reported archean direction from the superior province of north america the degree of anisotropy of low field magnetic susceptibility is high with k k3 values up to 1 3 cooling selected specimens in weak fields indicate that trm is deflected towards the magnetic fabric plane accordingly the in situ chrm was corrected for the effect of magnetic anisotropy two methods of correction give similar results however the resultant formation mean remains essentially unchanged as a consequence of sufficient variability in the attitudes of k k planes although uncertainty in the initial dip renders the tilt correction problematical partial tilt correction 36 per cent results in a formational mean with better precision than the in situ formation mean furthermore partial untilting of remanence directions results in a positive reversal test with a better classification than that obtained using in situ directions this partially tilt corrected result d 202 i 49 k 35 yields a pole latitude 62 n longitude 42 e which is archean in aspect

magma mixing in late archean tonalitic and mafic rocks of the lac des iles area western superior province abstract plutonic rocks in the lac des iles area of the western superior province provide a record of contemporaneous late archean mafic and tonalitic magmatism this suite of rocks consists of mafic to ultramafic intrusions mafic to intermediate dikes hornblendite and hornblende tonalite the tonalite contains net veined and disaggregated mafic to intermediate dikes mafic schlieren and zones of hornblendite to mela tonalite with skeletal hornblende these textures are interpreted to be the result of magma mixing the hornblende tonalite is geochemically characterized by high al 2 o 3 high na 2 o k 2 o low rb moderate sr and fractionated ree the hornblendite mafic to intermediate dikes and tonalite show whole rock trends of decreasing total ree increasing depletion of hree and increasing eu eu with increasing sio 2 amphiboles range in composition from edenitic and pargasitic hornblende to hornblende in dike rocks mela tonalite and hornblendite and from hornblende to actinolite in tonalite the development of skeletal amphiboles with strong normal zoning and resorbed hornblende with weak reverse zoning are attributed to conditions of supercooling and superheating respectively in zones of magma mixing the least fractionated dike rocks geochemically resemble lamprophyres derived from alkali basaltic magma variation of ree among the mafic to intermediate dikes suggests that they fractionated hornblende hornblendites and mela tonalites can be modeled as hornblende cumulates from the dike rocks the amphiboles probably crystallized rapidly during mixing leaving a feldspathic residue variation in hornblende tonalites is consistent with mixing of sio 2 rich tonalite melt with mafic to intermediate dike phases and associated hornblende cumulates and feldspathic residue open system magmatic processes at lac des iles result in a continuum of disequilibrium textures reflecting emplacement of mafic magma into a crystallizing felsic host the plutonic rocks probably formed from a zoned magma chamber with a mafic base and tonalitic top breccia zones in the tonalite suggest that explosive degassing of the chamber occurred structures similar to those reported here are common in other granitoids in the superior province and suggest that mafic magmatism and magma mixing played an important role in the development of late archean granitoid plutons

crystallization of apatite in natural magmas under high pressure hydrous conditions with particular reference to orogenic rock series two separate series of hydrous experiments involving 1 imposing apatite saturation on a series of igneous rock compositions from basanite to rhyolite and 2 crystallizing similar natural rock compositions progressively until apatite appears demonstrate a close dependence between apatite saturation and silica content of the magma and determine p2o5 levels at a given silica value and temperature at which that composition may be expected to crystallize apatite the effect of pressure on apatite solubility is not great and is most significant for silicic compositions

the lac des iles complex ontario petrology and platinum group elements mineralization in an archean mafic intrusion the lac des iles complex is a late archean mafic to ultramafic complex that is host to 20 4 106 t pt pd and au mineralization with an average grade of 6 34 g t platinum group elements pge the 30 km2 complex located in the wabigoon subprovince consists of several coalescing mafic to ultramafic intrusive centers the complex is emplaced into early gneissic tonalite and is contemporaneous with late granitoids the magmatic evolution of the complex reflects the emplacement of multiple pulses of previously fractionated magma some of which underwent subsequent in situ fractionation distinct magma sequences are recognized on the basis of intrusive relationships differences in the cumulus minerals crystallization order and trace element chemistry these sequences are 1 hornblende gabbro 2 gabbro and gabbronorite and 3 ultramafic where there is evidence of intrusive relations the most primitive sequences are emplaced late and occur toward the north end of the complex microprobe analyses in

petrogenetic and geodynamic origin of the neoarchean dore lake complex abitibi subprovince superior province canada the neoarchean ca 2728 ma anorthosite bearing dore lake complex in the northeastern abitibi subprovince quebec was emplaced into an association of intra oceanic tholeiitic basalts and gabbros known as the obatogamau formation the obatogamau formation constitutes the lower part of the roy group which is composed of two cycles of tholeiitic to calc alkaline volcanic and volcaniclastic rocks siliciclastic and chemical sedimentary rocks and layered mafic to ultramafic sills in this study we report major and trace element results and nd sr pb and o isotope data for anorthosites leucogabbros gabbros and mafic dykes from the dore lake complex and spatially associated basalts and gabbros of the obatogamau formation to assess their petrogenetic origin and geodynamic setting field and petrographic observations indicate that the dore lake complex and associated volcanic rocks underwent extensive metamorphic alteration under greenschist facies conditions resulting in widespread epidotization 20 40 and chloritization 10 40 of many rock types plagioclase recrystallized mainly to anorthite and albite endmembers erasing intermediate compositions metamorphic alteration also led to the mobilization of many elements e g lile and transition metals and to significant disturbance of the rb sr and u pb isotope systems resulting in 1935 150 and 3326 270 ma errorchron ages respectively the sm nd isotope system was less disturbed yielding an errorchron age of 2624 160 ma on many binary major and trace element diagrams the least altered anorthosites and leucogabbros and the gabbros and mafic dykes of the dore lake complex plot in separate fields signifying the presence of two distinct magma types in the complex the gabbros and mafic dykes in the dore lake complex share the geochemical characteristics of tholeiitic basalts and gabbros in the obatogamau formation suggesting a possible genetic link between the two rock associations initial end 2 6 to 5 0 and 18o 6 1 to 7 9 values for the dore lake complex and gabbros of the obatogamau formation end 2 8 to 4 0 18o 7 3 to 8 0 are consistent with depleted mantle sources all rock types in the dore lake complex and the roy group share the trace element characteristics of modern arc magmas suggesting a suprasubduction zone setting for these two lithological associations on the basis of regional geology and geochemical data we suggest that the dore lake complex and the obatogamau formation represent a dismembered fragment of a suture zone like many phanerozoic ophiolites resulting from closure of a back arc basin between 2703 and 2690 ma

zircon u pb ages from the wabigoon manitou lakes region wabigoon subprovince northwest ontario three distinct stratigraphic units have been recognized in the crow lake savant lake belt these are a basal unit consisting of high magnesium tholeiitic flows a middle unit consisting mostly of tholeiitic to calc alkaline pyroclastic rocks and flows and an upper unit of high iron tholeiitic flows a zircon u pb study has been carried out on seven rocks from the wabigoon manitou lakes region of this belt in the manitou lakes area a minimum age of 2755 ma on a porphyry pluton intruded into the wapageisi volcanics gives a minimum age for the high magnesium tholeiitic sequence a trondhjemite phase of the atikwa batholith near wabigoon lake that is dated at 2732 2 2 9 ma has the same age within experimental error as a rhyolite flow from the lower wabigoon volcanics in the same area that is dated at however a trondhjemite phase of the atikwa batholith at eagle lake that is dated at is distinctly younger than a dacite flow collected nearby from the lower wabigoon volcanics that is dated at both thes

structure and origin of holocene cold events abstract the present interglacial the holocene spans the period of the last 11 700 years it has sustained the growth and development of modern society the millennial scale decreasing solar insolation in the northern hemisphere summer lead to northern hemisphere cooling a southern shift of the intertropical convergence zone itcz and a weakening of the northern hemisphere summer monsoon systems on the multidecadal to multicentury scale periods of more stable and warmer climate were interrupted by several cold relapses at least in the northern hemisphere extra tropical area based on carefully selected 10 000 year long time series of temperature and humidity precipitation as well as reconstructions of glacier advances the spatiotemporal pattern of six cold relapses during the last 10 000 years was analysed and presented in form of a holocene climate atlas hoclat see http www oeschger unibe ch research projects holocene atlas a clear cyclicity was not found and the spatiotemporal variability of temperature and humidity precipitation during the six specific cold events 8200 6300 4700 2700 1550 and 550 years bp was very high different dynamical processes such as meltwater flux into the north atlantic low solar activity explosive volcanic eruptions and fluctuations of the thermohaline circulation likely played a major role in addition internal dynamics in the north atlantic and pacific area including their complex interaction were likely involved

stratigraphy and petrology of the lower part of the mulcahy gabbro northwestern ontario origin of reverse and normal fractionation trends and implications for tectonic setting of late archean mafic magmatism the mulcahy gabbro 2 73 ga is an exceptionally well preserved tholeiitic layered intrusion located in the wabigoon subprovince of the superior province the intrusion has a 900 m thick marginal z

evidence for holocene climate variability from the sediments of a scottish remote mountain lake high resolution sampling of sediment cores covering approximately the last 4000 yr from lochan uaine a small corrie loch in the scottish cairngorm mountains show quasi periodic cycles in organic matter measured as percentage loss on ignition analysis of these cycles show correspondences between loss on ignition d 13 c values and chironomid head capsule abundance we interpret the changes as reflecting changes in lake productivity and hypothesise that they are driven by climate variability however it is not yet clear whether the periods of relatively high organic matter production and preservation are associated with colder or with warmer conditions nevertheless the results indicate the value of using sediments from remote undisturbed mountain lakes as recorders of holocene climate variability copyright 2001 john wiley sons ltd

rapid low cost photogrammetry to monitor volcanic eruptions an example from mount st helens washington usa we describe a low cost application of digital photogrammetry using commercially available photogrammetric software and oblique photographs taken with an off the shelf digital camera to create sequential digital elevation models dems of a lava dome that grew during the 2004 2008 eruption of mount st helens msh volcano renewed activity at msh provided an opportunity to devise and test this method because it could be validated against other observations of this well monitored volcano the datasets consist of oblique aerial photographs snapshots taken from a helicopter using a digital single lens reflex camera twelve sets of overlapping digital images of the dome taken during 2004 2007 were used to produce dems and to calculate lava dome volumes and extrusion rates analyses of the digital images were carried out using photogrammetric software to produce three dimensional coordinates of points identified in multiple photos the evolving morphology of the dome was modeled by comparing successive dems results were validated by comparison to volume measurements derived from traditional vertical photogrammetric surveys by the us geological survey cascades volcano observatory our technique was significantly less expensive and required less time than traditional vertical photogrammetric techniques yet it consistently yielded volume estimates within 5 of the traditional method this technique provides an inexpensive rapid assessment tool for tracking lava dome growth or other topographic changes at restless volcanoes

scaling factors for the rates of production of cosmogenic nuclides for geometric shielding and attenuation at depth on sloped surfaces abstract the decrease in rates of production of cosmogenic nuclides occurs because of shielding of cosmic rays by mountains sloped surfaces and local rock formations that block them when a large part of the sky is blocked this correction is large and requires detailed model calculations this paper considers three geometries a rectangular obstruction a triangular obstruction and a sloped surface other geometries can be considered as a combination of these the results are presented in terms of formulas and graphs so that the reader can easily apply them to common field situations any use of cosmogenic nuclides in the study of geomorphic processes or forms must consider factors that introduce variations in the production of nuclides

mantle derivation of archean amphibole bearing granitoid and associated mafic rocks evidence from the southern superior province canada amphibole bearing late archean 2 73 2 68 ga granitoids of the southern superior province are examined to constrain processes of crustal development the investigated plutons which range from tonalite and diorite to monzodiorite monzonite and syenite share textural mineralogical and geochemical attributes suggesting a common origin as juvenile magmas despite variation in modal mineralogy the plutons are geochemically characterized by normative quartz high al2o3 15 wt na rich fractionation trends mol na2o k2o 2 low to moderate rb generally 150 ppm are comagmatic with the evolved granitoids and indicate that the suites are mantle derived isotopic studies of archean monzodioritic rocks have shown lree enrichment and initial 143nd 144nd ratios indicating derivation from mantle sources enriched in large ion lithophile elements lile shortly before melting mineral assemblages record lower ph2o with increased alkali contents of the suites this evidence in conjunction with experimental studies suggests that increased alkali contents may reflect decreased ph2o during mantle melting these features indicate that 2 73 ga tonalitic rocks are derived from more hydrous mantle sources than 2 68 ga syenitic rocks and that the spectrum of late archean juvenile granitoid rocks is broader than previously recognized comparison with phanerozoic and recent plutonic suites suggests that these archean suites are subduction related

zircon u pb ages from the kakagi lake area wabigoon subprovince northwest ontario five rocks have been dated from the kakagi lake area of the wabigoon subprovince by means of u pb analysis of zircons using the techniques of air abrasion and high gradient magnetic separation zi

zircon dating of polycyclic volcanism at sturgeon lake and implications for base metal mineralization the south sturgeon lake sequence consists of four volcanic cycles the lower two cycles host cu zn massive sulfide mineralization economic mineralization has not been found in the third and fourth cycles trace element and rare earth analyses performed previously and in the present study indicate that the lower three cycles have a similar petrogenetic history and are cogenetic with the beidelman bay pluton a subvolcanic trondhjemite sill which intrudes the base of the lower cycle the upper cycle is distinctly different u pb zircon analyses have been performed on seven rocks from the lower three cycles in an effort to resolve the ages all age analyses agree within error giving an average of 2 735 5 m y with a range of 1 5 m y a previously determined age of 2 733 8 super 1 4 sub 1 3 m y on the subvolcanic intrusion is marginally younger the uppermost cycle was previously dated at 2717 9 super 2 7 sub 1 5 m y 18 m y younger than the lower cycles moreover rare earth element data indicate that this upper cycle is not comagmatic with the lower cycles the data indicate rapid evolution of a volcanic pile followed by a long period of quiescence and later reactivation from a different magmatic source the younger age of the beidelman bay pluton indicates that it probably is not directly related to volcanism and ore forming hydrothermal activity however it may occupy the magma chamber which previously fed rhyolitic volcanism and acted as a heat source driving hydrothermal convection which transported and deposited base metals this would require the subvolcanic magma chamber to be coupled to a much larger magma system which fed eruptive and high level intrusive activity remaining molten for about 1 7 m y

assessment of the u th pb system in two archean metabasalts deciphering the complex histories of sulphides and silicates using acid leaching methods abstract a u th pb study of archean metabasalts from two greenstone belts in the eastern wawa subprovince of the canadian shield indicated variable disturbances had occurred in both whole rock systems changes in the pb content appear to predominate over loss of parent elements and meaningful pb isochron ages could not be obtained from either of the metabasalts detailed leaching experiments on the rocks and analysis of associated disseminated sulphides indicate that the pb isotopic compositions of the whole rocks are dominated by updated but for the most part cogenetic sulphides model ages for pb released by sulphides and metabasalt acid leaches suggest that the sulphide pb has been updated or remobilized during discrete thermal episodes the validity of the inferred events is supported by ages indicated by other isotopic systems for nearby rocks the silicate residues of the acid leached volcanics give well defined pb isochron ages the gamitagama belt metabasalt has a pb isochron age of 2694 54 ma zircons from this rock unit have been dated by u pb at 2691 ma demonstrating that the acid leaching technique on whole rocks can isolate residues which give meaningful ages sulphides in this metabasalt appear to be affected by an event at 2 55 ga a similar rock from the michipicoten belt has undergone a multistage history and yields a pb isochron age of 2761 36 ma consistent with a zircon u pb date for overlying felsic metavolcanic rocks later events affecting this basalt occurred at approximately 2 4 2 2 and 1 6 ga the results show that through acid leaching the primary ages of metabasalts and their later overprinting histories can be approximated initial pb ratios for these metabasalts have been estimated it appears that both depleted and enriched mantle reservoirs contributed to volcanism in this area acid leaching combined with measurement of u and th contents appears to be a useful tool for better understanding the pb isotope systematics of archean metabasalts

high resolution numerical simulation of younger dryas glaciation in scotland golledge n r hubbard a sugden d e 2008 high resolution numerical simulation of younger dryas glaciation in scotland quaternary science reviews 27 9 10 888 904 sponsorship nerc

evidence for a caledonian amphibolite to eclogite facies pressure gradient in the middle allochthon lindas nappe sw norway the proterozoic anorthosite mangerite charnockite complex dominating the lindas nappe in the scandinavian caledonides was locally eclogitized in the southwestern part of the nappe during the caledonian orogeny whereas only amphibolite facies assemblages are recorded in the rest of the nappe sveconorwegian granulites of anorthositic to jotunitic composition in the northernmost eclogite free exposures of the nappe exhibit large garnet phenoblasts ca 900 c that are fractured and partly replaced by a caledonian symplectitic amphibolite facies assemblage ca 515 c metamorphic zircon attributed to this garnet breakdown is dated by id tims u pb at 430 3 ma suggesting that the amphibolite facies overprint was coeval with the formation of eclogite 30 km further south probably implying that the section across the nappe represents a caledonian pressure gradient the rocks also preserve a complex sveconorwegian history including an age of 969 6 ma which we interpret as dating magmatic emplacement of jotunitic anorthositic portions of the complex 936 12 ma reflecting the granulite facies metamorphism and 908 16 ma representing a late generation of zircon best explained as having formed by metasomatic processes caledonian shearing severely deformed zircon grains in an amphibolite facies shear zone resetting their u pb systems and forming new ones hereby also demonstrating a case of resetting and recrystallization of low u zircon our data gained from diverse lithologies illustrate several processes involved in making and resetting zircon as well as indicate the contemporaneous evolution and similar origin of the lindas nappe and the jotun nappe complex

climate at the equilibrium line of glaciers the relationships between temperature precipitation and radiation on glacier equilibrium lines are investigated using 70 glaciers for which the mass balance and meteorological observations have been carried out for sufficiently long periods it is found that the characteristic climate at glacier equilibrium lines can be described using the summer 3 months temperature in a free atmosphere annual total precipitation and the sum of global and long wave net radiation all of these are measured at or very near the equilibrium line altitudes then it is shown how the shift of the equilibriumline will occur as a result of a climatic change finally the effect of the shift of the equilibrium line on the annualmean specific mass balance is analytically derived and compared with observations the present results make it possible to identify the altitudes in climate models where glacierization should begin and to evaluate the mass balance changes as a result of possible future changes in the climate

trimlines blockfields mountain top erratics and the vertical dimensions of the last british irish ice sheet in nw scotland periglacial trimlines separating glacially eroded lower slopes from blockfield covered plateaus on british and irish mountains have been interpreted either 1 in terms of representing the maximum altitude of the last ice sheet during the last glacial maximum lgm or 2 as a thermal boundary separating wet based ice at pressure melting point from cold based ice on summit plateaus we test these competing hypotheses through 10be exposure dating of high level erratic boulders above trimlines on five mountains in nw scotland nine out of 14 erratics yielded post lgm exposure ages ranging from 14 0 0 7 ka to 16 5 0 9 ka or from 14 9 0 9 ka to 17 6 1 1 ka depending on the 10be production rate employed in exposure age calculation these ages refute hypothesis 1 as they imply that the last ice sheet overtopped the mountains preservation of apparently intact blockfields on the summits implies cold based ice cover supporting hypothesis 2 as altitudinally consistent high level trimlines extend from our sampled sites across much of nw scotland and the hebrides our conclusions apply to all trimlines in this broader area and probably to all high level trimlines elsewhere in the british isles preservation of blockfields under cold based ice is consistent with blockfield evolution on plateaus throughout much or all of the quaternary averaged exposure ages of 15 16 ka for plateau top erratics implies nunatak emergence from the downwasting ice sheet prior to a regional readvance of the ice margin the wester ross readvance and before rapid warming at 14 7 ka at the onset of the lateglacial interstade but after the timing of ice sheet thinning as retrodicted by recent proxy climate driven thermo mechanical coupled models our findings provide an additional constraint on the future development of such models by implying that high level trimlines represent the altitude of a former transition zone between ice at pressure melting point and ice below pressure melting point

geomorphological effects of the 1840 ahora gorge catastrophe on mount ararat eastern turkey abstract ahora gorge is a 400 m deep canyon located along the north eastern flank of mt ararat turkey a compound volcanic complex covered by an ice cap in the past several diarists and scientific authors reported a calamitous event on july 2 1840 when a landslide triggered by a volcanic eruption and or an earthquake obliterated several villages located at the foot of the volcano the reasons and effects of this ahora gorge catastrophe agc event have been obscure and ambiguous to reappraise the 1840 catastrophe and the geomorphic evolution of the ahora gorge we used high resolution satellite images remote sensing thermal data supplemented by observations collected during two field surveys according to our interpretation an earthquake in 1840 triggered an abrupt volcanic eruption which caused the consequent rapid melting of glacial ice and mountain snow this event produced significant mud and ice flow along the ahora gorge that inundated the villages downslope the flooding which we interpret as a lahar deposited a wide alluvial fan at the foot of the volcano today an elongated landform present within the ahora gorge has alternately been interpreted as a landslide deposit our multi data integrated analysis and ground based observations demonstrate that this feature is an unmapped glacier the shape of the landform coupled with aster thermal satellite data and ground based observations reveal that a debris covered glacier dcg formed soon after the catastrophic lahar our discovery of the ahora gorge glacier in context of a reappraisal of the ahora gorge catastrophe demonstrates the contribution of glaciers in shaping the landscape near ice capped volcanoes moreover our data underscore the importance of being more aware of the potential future geo hazards posed by mount ararat particularly where glaciers are still present along several valleys and seismically induced lahar events might occur

posteruption glacier development within the crater of mount st helens washington usa the cataclysmic eruption of mount st helens on may 18 1980 resulted in a large north facing amphitheater with a steep headwall rising 700 m above the crater floor in this deeply shaded niche a glacier here named the amphitheater glacier has formed tongues of ice containing crevasses extend from the main ice mass around both the east and the west sides of the lava dome that occupies the center of the crater floor aerial photographs taken in september 1996 reveal a small glacier in the southwest portion of the amphitheater containing several crevasses and a bergschrund like feature at its head the extent of the glacier at this time is probably about 0 1 km2 by september 2001 the debris laden glacier had grown to about 1 km2 in area with a maximum thickness of about 200 m and contained an estimated 120 000 000 m3 of ice and rock debris approximately one third of the volume of the glacier is thought to be rock debris derived mainly from rock avalanches from the surrounding amphitheater walls the newly formed amphitheater glacier is not only the largest glacier on mount st helens but its aerial extent exceeds that of all other remaining glaciers combined

isotopic evidence from the eastern canadian shield for geochemical discontinuity in the proterozoic mantle most workers agree that proterozoic anorthosite massifs represent the crystallization products of mantle derived magmas1 2 although the composition of the parental melts is a major unsolved petrological problem3 as mantle derived rocks the massifs can be used as geochemical probes of their late precambrian upper mantle sources we report here nd and sr isotopic compositions of anorthosites and related rocks from the grenville and nain provinces of the eastern canadian shield here 75 of the earth s known anorthosite is found in a 1 600 km belt from the adirondack mountains of northern new york state to the eastern coast of labrador4 fig 1 the results indicate that the massifs were derived from at least two distinct mantle source regions which were established before 1 650 myr ago and were episodically involved in magmatism over 500 myr one reservoir below the grenville province and probably below much of the eastern superior province was isotopically similar to the depleted modern day mid ocean ridge basalt morb source the other reservoir was chondritic to moderately enriched and is most easily identified in the nain province but may have occurred scattered throughout the superior province

a multi proxy study of lake development in response to catchment changes during the holocene at lochnagar north east scotland abstract this paper describes a multi core lake sediment study using pollen diatoms and chironomids together with magnetics and sediment biogeochemistry as biotic and abiotic proxies to infer lake development in response to environmental change during the holocene at lochnagar in the eastern highlands of scotland diatoms are used to infer ph chironomids to infer temperature with pollen and plant megafossils acting as an independent proxy to validate these records and to provide insights into changes in catchment vegetation and soils lipid biomarkers are explored for their potential to provide additional information on lake productivity the results indicate highly distinctive fluctuations in the loss on ignition loi record which are in phase with changes in some biotic chironomid head capsule concentration and abiotic coarse silt particle size fraction and lipid and chlorine fractions variables catchment driven changes due to the development and degredation of soils and the natural succession and human intervention on terrestrial catchment vegetation have the strongest influence on the diatom and chironomid assemblages these catchment processes resulted in the natural acidification of the lake water post industrial acidification of the lake was also influential on the lake biota climate driven temperature change appears to have had only a weak influence on the biota with declines in cold stenothermic chironomid taxa in response to early holocene warming and declines in thermophilic chironomids in response to cooling at about 2600 cal yr bp

timing of magmatism and deformation in the kamiskotia kidd creek area western abitibi subprovince canada abstract the kamiskotia kidd creek area comprises well preserved granitoid greenstone belt lithologies of the western most abitibi subprovince similar in composition and structural style to other late archean granitoid greenstone terranes e g kalgoorlie norseman western australia belingwe zimbabwe slave province canada it contains the kidd creek volcanogenic massive sulfide deposit and lesser deposits of the kamiskotia district and lode gold deposits associated with the destor porcupine fault zone near the prolific porcupine gold district the magmatic and tectonic history for the area is provided through detailed structural mapping and high precision u pb geochronology field observations support the following sequence of magmatic and structural events in the kamiskotia area 1 formation of older mafic and intermediate volcanic rocks followed by the emplacement of the kamiskotia gabbroic complex kgc and cogenetic massive sulfide bearing bimodal volcanic rocks kv also accompanied by coeval intrusion of granitoid a into the crystallizing kgc 2 regional bulk rotation about a sub horizontal axis of this stratigraphy to form a ne facing monocline 3 emplacement of three large granitoid intrusions imparting their respective contact strain aureoles on the stratigraphy 4 regional sub horizontal bulk north south shortening that formed a non penetrative east west flattening fabric with an element of 5 late dextral transpression that formed high strain zones related to western destor porcupine fault zone dpfz within less competent volcanic units and 6 emplacement of alkalic dikes associated with the late stages of transpressive deformation u pb ages help substantiate this chronology and aid in the correlation of kamiskotia lithologies across the southern abitibi subprovince zircon ages for the kgc and a kamiskotia rhyolite are 2707 2 ma and 2705 2 ma respectively these ages are coeval with the reid rhyolite with a zircon age of 2706 2 ma but are significantly younger than the kidd creek rhyolite and the prosser rhyolite with zircon ages of 2717 2 ma data refined by the addition of an abraded concordant analysis and 2716 6 3 ma respectively two of the granitoid intrusions have zircon ages of 2696 1 5 and 2694 4 ma the older granitoid intrusion has a titanite population with a 207 pb 206 pb minimum age of 2692 5 ma which may represent a protracted cooling history or a metamorphic event after the setting of the zircon u pb system two zircon fractions from the younger granitoid intrusion exhibit inheritance with 207 pb 206 pb minimum ages up to 2926 ma indicating the presence of significantly older crust in this region timing of north south compression and late transpression may be constrained by a u pb garnet titanite age on a late to post tectonic alkalic dike at 2687 3 ma

age of the mulcahy lake intrusion northwest ontario and implications for the evolution of greenstone granite terrains abstract zircon data from a 63 km 2 layered mafic intrusion in the wabigoon subprovince the mulcahy lake gabbro show that the gabbro crystallized at 2733 2 0 9 1 0 ma the largely unaltered and unmetamorphosed gabbro intrudes tholeiites of the crow lake savant lake greenstone belt which were generated over at least a 10 ma time span rb sr and sm nd data were obtained for pristine whole rock and mineral separates from the intrusion the rb sr data show disturbances in the system particularly in the case of pyroxenes samples with low rb sr ratios indicate an initial sr ratio of 0 7007 for an age of 2733 ma the sm nd age is 2744 55ma and an epsilon nd value of 2 6 1 2 indicates a light rare earth element depleted source the sm nd data show less disturbance than the rb sr data granitic and mafic intrusives with similar initial ratios occur 80 km to the south in the quetico subprovince primary hornblende was analyzed for 40 ar 39 ar the age obtained is 2703 20ma clearly younger than the zircon age indicating some argon loss the mulcahy lake gabbro is one of four layered mafic plutons of similar petrographic character and stratigraphic age which are arrayed in an arc in the crow lake savant lake greenstone belt the plutons appear to represent subvolcanic chambers which developed simultaneously and which crystallized at the end of the volcanic cycle all intrude basalts and are intruded in turn by the atikwa batholith a massive granitic intrusion of over 1000 km 2 in area previously published zircon ages for several sample sites within the batholith indicate crystallization at 2731 8 1 3 1 6 ma to 2732 2 2 9ma these data show that the batholith overlaps the age of the gabbros or is at most 5 ma younger this virtual contemporaneity is hidden in the rb sr and sm nd data the cessation of mafic magma production and the immediate if not parallel voluminous production of calc alkaline liquid may represent a process important in the development of greenstone granite belts a process not apparent when ages are not so precisely determined

oxygen isotope ratios of archean plutonic zircons from granite greenstone belts of the superior province indicator of magmatic source zircons commonly preserve primary magmatic oxygen isotope ratios even through high grade metamorphic events unlike quartz which more readily loses its primary 18o value by subsolidus exchange intrusive bodies from the superior province canada have an average 18o zircon of 5 8 0 6 n 59 rocks those from granite greenstone belts have an average 18o zrc 5 8 0 6 n 55 in contrast to those from metasedimentary belts that average 18o zrc 6 7 0 2 n 4 the 18o zircon values from granite greenstone belts indicate a maximum allowable input of 14 supracrustal high 18o material into archean magmas of the superior province relative to mantle 18o values igneous bodies of the superior province have been separated into two groups based on geochemistry unenriched plutons and sanukitoids magmatic oxygen isotope ratios inferred from these zircons indicate up to a subtle but significant division of 1 between the two geochemical sets of rocks this distinction is apparent only in the 18o zrc and not in more commonly analyzed less refractory minerals such as quartz and feldspar oxygen isotope data correlate with geochemical modeling of sanukitoids the average 18o zrc for sanukitoids is 6 4 0 2 in comparison to 5 5 0 4 for unenriched plutons there is almost no overlap in values the elevation of sanukitoid 18o zrc indicates input from a high 18o source such as fluids from the subducting oceanic crust melts of subducted sediments or continental crust all of the plutons analyzed in this study have been dated by u pb zircon techniques and the combination of u pb ages and 18o zrc indicates little variability of 18o versus age during archean plutonism in the superior province therefore recycling of sediments and other high 18o sources was not a dominant process during crustal growth from 3003 to 2670 ma in the superior province

3 5 ga old amphibolites from eastern hebei province china field occurrence petrography sm nd isochron age and ree geochemistry amphibolite blocks frequently occur in association with grey gneisses within a small area near the villages of caozhuang and huangbaiyu in eastern hebei they are interpreted as remnants of an earlier bimodal suite now engulfed in late archaean k rich granitic magma and are further cut by pegmatite dykes they are considered together as giant enclaves within late archaean granite plutons u pb zircon ages 2 5 2 6 ga the amphibolites have been dated by the sm nd isochron method the results yielded t 3470 107 ma ind 0 50827 11 and nd t 2 7 0 6 this age represents the time of protolithic basalt eruption and possibly suggests a period of formation of primitive mafic crust this is the oldest reliable age so far obtained for the archaean rocks of china the sm nd results of grey gneiss samples yielded model ages tdm of about 3 24 ga it is not clear whether this younger age in comparison with that of the amphibolites indicates the intrusive nature of these rocks or is a result of contamination by later granite about 2 5 ga ago the rb sr and common pb isotopic systems have been highly disturbed and no isochron ages could be established the common pb data show retarded growth of radiogenic pb and hence imply reduction of u pb ratios in the past this is consistent with the petrographic study which suggests that the amphibolites have undergone an earlier period of granulite facies metamorphism and were subsequently retrogressed to the present amphibolite facies mineral assemblages the positive nd t value of 2 7 for the amphibolites implies that the protolithic basalt magmas were generated from mantle sources which had undergone long term depletion in light rare earth elements lree the depletion event must have taken place prior to 4 ga ago if their source had a chemical nature similar to the modern oceanic mantle it could have occurred as early as 4 5 ga ago if their source was less impoverished in lree if the mantle depletion was related to extraction of continental or sialic material the sm nd data of the caozhuang amphibolites lend support to the existence of sialic crust if not of a continental size before 4 ga ago

did large ice caps persist on low ground in north west scotland during the lateglacial interstade recent research based primarily on exposure ages of boulders on moraines has suggested that extensive ice masses persisted in fjords and across low ground in north west scotland throughout the lateglacial interstade greenland interstade 1 ca 14 7 12 9 ka and that glacier ice was much more extensive in this area during the older dryas chronozone ca 14 0 ka than during the younger dryas stade ca 12 9 11 7 ka we have recalibrated the same exposure age data using locally derived 10be production rates this increases the original mean ages by 6 5 12 implying moraine deposition between ca 14 3 and ca 15 1 ka and we infer a most probable age of ca 14 7 ka based on palaeoclimatic considerations the internal consistency of the ages implies that the dated moraines represent a single readvance of the ice margin the wester ross readvance pollen stratigraphic evidence from a lateglacial site at loch droma on the present drainage divide demonstrates deglaciation before ca 14 0 ka and therefore implies extensive deglaciation of all low ground and fjords in this area during the first half of the interstade ca 14 7 14 0 ka this inference appears consistent with lateglacial radiocarbon dates for shells recovered from glacimarine sediments and a dated tephra layer our revised chronology conflicts with earlier proposals that substantial dynamic ice caps persisted in scotland between 14 and 13 ka that large active glaciers probably survived throughout the lateglacial interstade and that ice extent was greater during the older dryas period than during the younger dryas stade copyright 2011 john wiley sons ltd