lagrangian simulation of multidimensional anomalous transport at the made site 1 contaminant transport through regional scale natural geological formations typically exhibits several anomalous features including direction dependent spreading rates channeling along preferential flow paths trapping of solute in relatively immobile domains and or the local variation of transport speed simulating these plume characteristics can be computationally intensive using a traditional advection dispersion equation ade because anomalous features of transport generally depend on local scale subsurface properties here we develop an alternative simulation approach that solves the full nonlocal multidimensional spatiotemporal fractional order ade with variable coefficients in a lagrangian framework using a novel non markovian random walk method this model allows us to simulate anomalous plumes without the need to explicitly define local scale heterogeneity the simple model accurately simulates the tritium plume measured at the extensively characterized made test site

constraints on mantle evolution from 187os 188os isotopic compositions of archean ultramafic rocks from southern west greenland 3 8 ga and western australia 3 46 ga initial 187os 188os isotopic compositions for geochronologically and geologically well constrained 3 8 ga spinel peridotites from the itsaq gneiss complex of southern west greenland and chromite separates from 3 46 ga komatiites from the pilbara region of western australia have been determined to investigate the osmium isotopic evolution of the early terrestrial mantle the measured compositions of 187os 188os 0 0 10262 2 from an olivine separate and 0 10329 3 for a spinel separate from 3 8 ga peridotite g93 42 are the lowest yet reported from any terrestrial sample the corrections for in situ decay over 3 8 ga for these low re os phases are minimal and change the isotopic compositions by only 0 5 and 2 2 for the spinel and the olivine respectively resulting in 187os 188os 3 8 ga 0 1021 0 0002 and 0 1009 0 0002 respectively these data extend direct measurement of os isotopic compositions to much earlier periods of earth history than previously documented and provide the best constraints on the os isotopic composition of the early archean terrestrial mantle analyses of pilbara chromites yield 3 46 ga mantle compositions of 0 1042 0 0002 and 0 1051 0 0002 these new data combined with published initial os isotopic compositions from late archean and early proterozoic samples are compatible with the mantle or at least portions of it evolving from a solar system initially defined by meteorites to a modern composition of 187os 188os 0 0 1296 0 0008 as previously suggested from peridotite xenolith data meisel et al 2001 the associated 187re 188os 0 0 435 0 005 thus chondritic 187os 188os compositions were a feature of the upper mantle for at least 3 8 billion years requiring chondritic re os ratios to have been a characteristic of the very early terrestrial mantle in contrast nonchondritic initial compositions of some archean komatiites demonstrate that os isotopic heterogeneity is an ancient feature of plume materials reflecting the development of variable re os mantle sources early in earth history the lower average 187os 188os 0 1247 for abyssal peridotites snow and reisberg 1995 indicate that not all regions of the modern mantle have evolved with the same re os ratio the relative sizes of the various reservoirs are unknown although mass balance considerations can provide some general constraints for example if the unradiogenic 187os 188os modern abyssal peridotite compositions reflect the prevalent upper mantle composition then the complementary high re os basaltic reservoir must represent 20 to 40 by mass of the upper mantle taken here as 50 of the entire mantle depending on the mean storage age the difficulties associated with efficient long term storage of such large volumes of subducted basalt suggest that the majority of the upper mantle is not significantly re depleted rather abyssal peridotites sample anomalous mantle regions the existence of 3 8 ga mantle peridotites with chondritic 187os 188os compositions and with os concentrations similar to the mean abundances measured in modern peridotites places an upper limit on the timing of a late accretionary veneer these observations require that any highly siderophile element rich component must have been added to the earth and transported into and grossly homogenized within the mantle by 3 8 ga either large scale mixing of impact materials occurred on very short 0 100 myr timescales or the interpretation preferred here the late veneer of highly siderophile elements is unrelated to the lunar terminal cataclysm estimated to have occurred at 3 8 to 3 9 ga

multiple 3 8 3 1 ga tectono magmatic events in a newly discovered area of ancient rocks the shengousi complex anshan north china craton abstract near anshan city in the north china craton the oldest rocks in asia 3300 ma have been thoroughly documented at two localities baijiafen and dongshan in this paper we report the full geological context for more ancient rocks from a third anshan locality the polyphase migmatite shengousi complex shrimp u pb zircon dating indicates a protracted tectono magmatic history for the shengousi complex the oldest recognised component is banded trondhjemitic gneiss 3773 6 ma which is veined by strongly deformed granitic pegmatite these occur with a second generation of trondhjemitic rocks 3454 8 and 3448 9 ma the next generation of plutonic rocks is a composite suite of iron enriched mafic dykes 3332 6 and 3331 8 ma with broadly coeval felsic veins 3311 4 ma finally there was intrusion of monzogranite 3129 6 ma strong deformation has generally brought these 3773 3129 ma plutonic phases into close concordance to form banded rocks however locally cross cutting relationships are preserved in small lower strain domains that give a relative chronology agreeing with the absolute zircon u pb chronology all of this complex history is recorded in a single some of the the 3450 ma igneous phases discovered in the shengousi complex are new ages for igneous rocks in the anshan area the precambrian geology of the anshan area is thus marked by polyphase eoarchaean orthogneisses the newly recognised 3450 ma phases widespread 3360 3300 magmatic activity found in association with mesoarchaean plutonic rocks and the neoarchaean anshan group and palaeoproterozoic liaohe group metasedimentary rocks this shows that the geology of the anshan area has a strong similarity with the narryer gneiss complex of western australia which contains the polyphase eoarchaean meeberrie gneisses the 3490 3440 ma eurada gneiss association 3300 ma granites and migmatisation and is intercalated with younger precambrian metasedimentary rocks of different ages containing hadean detrital zircons no other ancient gneiss complexes in the world show such a close match in their history with the anshan area rocks this means that the anshan area is prospective for locating new occurrences of hadean crustal components intercalated with younger sedimentary rocks

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

alteration and geochemical patterns in the 3 7 3 8 ga isua greenstone belt west greenland abstract volcanic rocks of the 3 7 3 8 ga isua greenstone belt have undergone heterogeneous poly phase post magmatic alteration resulting in mobilisation of many elements including lree and th in contrast nb ta ti zr y and hree were relatively immobile during alteration processes the degree of element mobility appears to have been controlled by chemical composition primary volcanic structures carbonate and silica enrichment and proximity to ultramafic units and shear zones on the basis of the geochemical characteristics of the least altered samples two distinct associations have been recognised in structurally separated lithotectonic sequences these magmatic associations are a low hfse association tio2 0 20 0 40 wt zr 12 30 ppm nb 0 13 0 80 ppm y 6 14 ppm in the central arc sequence resembling tertiary boninites and a high hfse association tio2 0 50 1 14 wt zr 34 77 ppm nb 1 2 2 7 ppm y 11 28 ppm in the outer and inner arc sequences comparable to phanerozoic island arc basaltic to picritic volcanic rocks these two geochemically distinct sequences were likely to have been juxtaposed as a consequence of phanerozoic style plate tectonic processes operating in the early earth the isua volcanic rocks are characterised by sub chondritic nb ta 10 16 ratios post magmatic alteration can be ruled out as the cause of these low ratios rather these ratios reflect a compositional characteristic of the paleoarchean mantle source s the sub chondritic nb ta ratios may have resulted from either preferential sink of nb into the core as a weakly siderophile element or preferential retention of nb in the subducted oceanic crust during subduction zone petrogenetic processes alternatively the building blocks of the earth were not composed of ci like chondritic material

the origin of earth s first continents and the onset of plate tectonics the growth and recycling of continental crust has resulted in the chemical and thermal modification of earth s mantle hydrosphere atmosphere and biosphere for 4 0 b y however knowledge of the protolith that gave rise to the first continents and whether the environment of formation was a subduction zone still remains unknown here tonalite melts are formed in high p t experiments in which primitive oceanic plateau starting material is used as an analogue for eoarchean 3 6 4 0 ga oceanic crust generated at early spreading centers the tonalites are produced at 1 6 2 2 gpa and 900 950 c and are mixed with slab derived aqueous fluids to generate melts that have compositions identical to that of eoarchean continental crust our data support the idea that the first continents formed at ca 4 ga and subsequently through the subduction and partial melting of 30 45 km thick eoarchean oceanic crust modified earth s mantle and eoarchean environments and ecosystems

from granulites to eclogites in the sesia zone italian western alps a record of the opening and closure of the piedmont ocean the sesia zone italian western alps offers one of the best preserved examples of pre alpine basement reactivated under eclogite facies conditions during the alpine orogenesis a detailed mineralogical study of eclogitized acid and basic granulites and related amphibolites is presented in these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished a a medium to low p granulite stage giving rise to the development of orthopyroxene garnet plagioclase brown amphibole ilmenite biotite in basic granulites and garnet k feldspar plagioclase cordierite sillimanite biotite ilmenite in acid granulites b a post granulite re equilibration associated with the development of shear zones producing discrete amphibolitization of the basic granulites and widespread development of biotite sillimanite cordierite spinel in the acid rocks c an eo alpine eclogite stage giving rise to the crystallization of high p and low t assemblages in an effort to quantify this evolution independent well calibrated thermobarometers were applied to basic and acid rocks for the granulite event p t estimates are 7 9 kbar and 700 800 c and for subsequent retrograde evolution p t was 4 5 kbar and 600 c for the eo alpine eclogite metamorphism pressure and temperature conditions were 14 16 kbar and 550 c the inferred p t path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks in the light of the available geochronological constraints we propose to relate the pre alpine granulite and post granulite retrograde evolution to the permo jurassic extensional regime the complex granulite eclogite transition is thus regarded as a record of the opening and of the closure of the piedmont ocean

high pressure metamorphism of the lanzo peridotite and its oceanic cover and some consequences for the sesia lanzo zone northwestern italian alps abstract this paper presents new data on the alpine high pressure evolution of the lanzo peridotite massif and its metasedimentary cover western alps field relations in the northwestern lanzo massif show that the serpentinized part is covered by a metasedimentary sequence including local occurrences of metaophicarbonate metabasalt calcschist metaquartzite and gneiss remnants of granulite facies mafic gneiss associated with metaophicarbonate are found within the serpentinite major element garnet zonation profiles in metaquartzite and mafic gneiss were used to explore the contrasting origin of metasediments bell shaped mn zoning in garnet from metaquartzite indicates a one stage alpine metamorphic evolution while inverse zoning in garnet of the mafic gneiss shows that the protolith equilibrated under high grade metamorphic conditions in pre alpine times similar to kinzigites within the neighbouring sesia zone phase equilibria in metaquartzite and mafic gneiss point to 500 50 c at pressures of 0 9 to 1 3 gpa for the alpine metamorphic equilibration that contrasts with the 550 620 c and pressures in excess of 2 0 gpa obtained for kyanite talc chloritoid and talc chloritoid garnet domains in a metagabbro dike from the internal part of the lanzo massif these different p t trajectories can be explained by a much stronger retrograde overprint at the basement cover contact than in the internal part of the lanzo massif

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

eoarchaean crustal growth in west greenland itsaq gneiss complex and in northeastern china anshan area review and synthesis eoarchaean crust in west greenland the itsaq gneiss complex 3870 3600 ma is 80 by volume orthogneisses derived from plutonic tonalite trondhjemite granodiorite ttg suites 3850 c 3810 and c 3710 mahave some compositional similarities to modern island arc basalts iab suggesting their origin by hydrous fluxing of a suprasubduction zone upper mantle wedge most of the eoarchaean tonalites match in composition highsilica low magnesian adakites whose petrogenesis is dominated by partial melting of garnetiferous mafic rocks at high pressure however associated with the tonalites are volumetrically minor more magnesian quartz diorites whose genesis probably involved melting of depleted mantle to which some slab derived component had been added this assemblage is evocative of suites of magmas produced at phanerozoic convergent plate boundaries in the case where subducted crust is young and hot thus eoarchaean subduction first gave rise to short lived episodes of mantle wedge melting by hydrous fluxing yielding iablike basalts boninites in the hotter eoarchaean earth flux dominated destructive plate boundary magma generation quickly switched to slab melting of subducted oceanic crust this latter process produced the voluminous tonalites that were intruded into the slightly older sequences consisting of tectonically imbricated assemblages of iab like pillow lavas sedimentary rocks gabbros and upper mantle peridotite slivers zircon datingshows that eoarchaean ttg production in the itsaq gneiss complex was episodic 3870 3850 3840 3820 3810 3795 3760 3740 3710 3695 and 3660 ma in each case emplacement of small volumes of magma was probably followed by 10 40 maquiescence which allowed the associated thermal pulse to dissipate this explains why greenland eoarchaean crustal growth did not have granulite facies metamorphismdirectly associated with it instead 3660 3600 ma granulite facies metamorphism s in the itsaq gneiss complex were consequential to collisional orogeny and underplating upon termination of crustal growth similar eoarchaean crustal history is recorded in the anshan area of china where a few wellpreserved rocks as old as 3800 ma have been found including high mgo quartz diorites for 3800 ma rocks this is a rare if not unique situation outside of the itsaq gneiss complex the presence of volumetrically minor 3800 mamantle derived high mgo quartz diorites in both the itsaq gneiss complex and the anshan area indicates either that eoarchaean subduction zones were overlain by a narrow mantlewedge or that the shallow subduction trapped slivers of upper mantle between the conserved and consumed plates the geological society of london 2009

petrogenesis of plagiogranites in the muslim bagh ophiolite pakistan implications for the generation of archaean continental crust high sio2 rocks referred to as oceanic plagiogranites are common within the crustal sequences of ophiolites however their mode of petrogenesis is controversial with both late stage fractional crystallization and partial melting models being proposed here we present new whole rock data from plagiogranitic dyke like bodies and lenses from the lower and middle sections of the sheeted dyke complex of the cretaceous muslim bagh ophiolite northwestern pakistan the plagiogranites have similar geochemical signatures that are inconsistent with them being the fractionation products of the mafic units of the muslim bagh ophiolite however the plagiogranites all display very low tio2 contents 0 4 wt implying that they formed by partial melting of mafic rocks melt modelling of a crustal gabbro from the muslim bagh ophiolite shows that the trace element signature of the plagiogranites can be replicated by 5 10 melting of a crustal hornblende gabbro with amphibole as a residual phase resulting in a concave up middle rare earth element pattern compositional similarities between the muslim bagh ophiolite plagiogranites and archaean ttg trondhjemite tonalite granodiorite has implications for the generation of juvenile archaean continental crust as the muslim bagh ophiolite was derived in a supra subduction zone it is suggested that some archaean ttg may have been derived from melting of mafic upper crust in early subduction like settings however due to the small volume of muslim bagh ophiolite plagiogranites it is inferred that they can be instructive on the petrogenesis of some but not all archaean ttg

stratigraphic and geochemical evidence for the depositional environment of the early archaean isua supracrustal belt southern west greenland the highly deformed c 3800 ma isua supracrustal belt is a fragment of a more extensive early archaean sedimentary and volcanic succession intruded by and tectonically intercalated with tonalitic and granitic amftsoq gneisses in the period 3800 3600 ma the supracrustal rocks recrystallised under amphibolite facies conditions between 3800 and 3600 ma in the late archaean and locally at c 1800 ma layered sequences of rock of sedimentary and probable volcanic origin form over 50 of the belt bodies of high mg al basic rocks and ultramafic rocks were intruded into the layered sequences prior to isoclinal folding and intrusion of amitsoq gneisses the layered rocks which are 1 km thick are divided into two sequences that are in faulted contact with each other the way up of these sequences has been determined from facing directions of locally preserved graded layering in felsic metasediments at several localities the overall upwards change in sedimentary succession is interpreted as showing change from dominantly basic to dominantly felsic volcanism which provided the major clastic component of the sediments clastic sedimentation took place against a background of chemical sedimentation shown by interlayers of banded iron formation metachert and calc silicate rocks throughout the sequences the felsic rocks locally preserve graded bedding and possible conglomerate structures indicating deposition from turbidite flows and possibly as debris flows nodules in the felsic rocks contain structures interpreted as fiamme there is an irregular enrichment in k2o na2o in many of the felsic rocks at constant sio2 and al2o3 content interpreted as owing to alteration of original andesitic to dacitic volcanic rocks banded iron formations locally contain conglomeratic structures suggesting sedimentary reworking possibly under shallow water conditions lithological and geochemical characters of the clastic components of the supracrustal sequences are consistent with derivation from felsic and basic volcanic rocks and do not require a continental source

a modern structural regime in the paleoarchean 3 64 ga isua greenstone belt southern west greenland abstract the footwall gneisses beneath the western part of the paleoarchean 3 8 ga isua greenstone belt southern west greenland are interpreted here in terms of a 3 64 ga stack of mylonitic crystalline thrust nappes the oldest example known on earth in present coordinates the kinematic history of the thrust nappe stack is couched in terms of initial longitudinal strike parallel thrusting towards the southwest followed by transverse thrusting to the northwest and subsequent extensional collapse of the thickened crust toward the southeast diorite and tonalite that form the western margin of granitoids structurally overlying the western part of the isua greenstone belt and its footwall contain 3 5 ga mafic dykes some of which are deformed and or truncated at fault contacts within the granitoids accordingly a component of the deformation structurally above the isua greenstone belt occurred after 3 5 ga significantly later than the formation of the underlying mylonitic nappes probably during the neoarchean the structural regime of mylonitic thrust nappe stacking is very similar to that in modern mountain belts it would appear that the deformational behaviour rheological constitution and overall strength of paleoarchean and modern continental crust were similar

modeling tracer transport at the made site the importance of heterogeneity 1 we examine three stochastic transport models of the macrodispersion experiment made site using high resolution conductivity fields derived from a new geostatistical interpretation of the flowmeter data evaluation of the spatial continuity of the hydraulic conductivity data revealed a hole effect structure indicating the occurrence of periodic structures i e clustered lenses or facies alternatively we examine geostatistical models based on indicator variables and found a similarity to bivariate gaussian properties tritium transport was simulated in kriged fields and in random fields generated using different geostatistical models at a grid spacing equal to the vertical support scale of the flowmeter measurements to explicitly represent small scale heterogeneity only those simulations obtained using the hole effect model resulted in a subset of realizations which reproduced the strong anomalous tracer spreading neglecting the hole effect structure of the spatial model in the gaussian simulations resulted in a reduced tailing of the tracer illustrating the importance of preferential flow on anomalous solute transport at the columbus aquifer furthermore we found that a multivariate gaussian random function is adequate to model the spatial distribution of hydraulic conductivity at the made site based on the results of the indicator transport simulations and the univariate and bivariate normality detected in the analysis of the flowmeter data we conclude that when small scale variability of hydraulic conductivity is correctly modeled at the flowmeter measurement support scale the ade is capable of reproducing the tracer spreading results suggest that the main contributor to the operational memory function used in previous successful mass transfer models is mostly a reflection of the suppressed spatial variation of hydraulic conductivity at the model scale

complex 3670 3500 ma orogenic episodes superimposed on juvenile crust accreted between 3850 and 3690 ma itsaq gneiss complex southern west greenland abstract the itsaq gneiss complex of the nuuk region southern west greenland is dominated by 3850 3690 ma tonalites intruded into and intercalated with lesser amounts of different 3850 3700 ma supracrustal units published whole rock sr and nd isotopic studies demonstrate that the tonalites are juvenile crustal additions from a depleted mantle source from our field studies and shrimp u pb zircon dating we argue that this juvenile crustal accretion was spread over ca 170 m yr 3850 3690 ma following 3850 3690 ma juvenile crustal accretion events the evolution of the itsaq gneiss complex continued with numerous crustal reworking events between 3670 and 3500 ma examples of these reworking events are as follows 1 there is intrusion of several generations of geochemically diverse granites sensu stricto with subordinate gabbros and diorites 2 there are superimposed multiple episodes of amphibolite to granulite facies metamorphism illustrated with akilia island samples by detailed dating of

field study of dispersion in a heterogeneous aquifer 4 investigation of adsorption and sampling bias the declining mass balance trend observed for bromide and three fluorobenzoate tracers during a natural gradient experiment at a site near columbus mississippi prompted investigations related to the reactivity of the tracers and the representativeness of the tracer samples obtained from multilevel samplers a laboratory soil column study indicated that adsorption of bromide during the field experiment was of the order of 20 and that up to 10 of the three fluorobenzoates was adsorbed the presence of iron oxides and kaolinite in the alluvial aquifer combined with the low groundwater ph of 4 8 produced geochemical conditions conducive to adsorption of the anionic tracers multilevel samplers mls used in the field experiment were evaluated by comparing bromide concentrations from the mls with water samples extracted from adjacent soil cores vertically averaged bromide concentrations for the mls were 21 lower than those for the cores a matrix diffusion process in conjunction with a natural tendency for preferential sampling from permeable regions in the heterogeneous alluvial sediments is proposed as an explanation for the apparent bias in the mls sample concentrations this process is shown to be qualitatively consistent with the tracer mass balance observed during the natural gradient experiment 14 refs 11more figs 8 tabs less

the iron rich suite from the amitsoq gneisses of southern west greenland early archaean plutonic rocks of mixed crustal and mantle origin a distinctive group of augen gneisses and ferrodiorites termed the iron rich suite is a component of the early archaean amitsoq gneisses of southern west greenland the iron rich suite outcrops south of the mouth of ameralik fjord in an area that underwent granulite facies metamorphism in the early archaean the iron rich suite forms approximately 30 of the amitsoq gneiss of this area and occurs as sheets and lenses up to 500 m thick the rest of the amitsoq gneisses are predominantly tonalitic granodioritic banded grey gneisses despite intense deformation and polymetamorphism there is local field evidence that the iron rich suite was intruded into the grey gneisses after they had been affected by tectonism and metamorphism the banded grey gneisses are interpreted as 3 700 to 3 800 ma old u pb zircon ages from the iron rich suite give concordia intercepts at circa 3 600 ma

no water no granites no oceans no continents water is essential for the formation of granite and granite in turn is essential for the formation of continents earth the only inner planet with abundant water is the only planet with granite and continents the moon and the other inner planets have little or no water and no granites or continents

establishment of a 3 83 ga magmatic age for the akilia tonalite southern west greenland a recurring challenge to ion microprobe u pb zircon geochronology has been to discriminate between preservation of original igneous zircon populations and inherited grains this has proved particularly problematic in studying the polyphase metamorphic rocks that dominate early archean gneissic terranes for example differing interpretations exist for the origin of complex zircon populations investigated by u pb ion microprobe zircon geochronology from a granitoid orthogneiss body previously used to establish a minimum age for a supracrustal enclave on akilia island in southern west greenland we describe a method whereby the geochemistry of the akilia orthogneiss coupled with a u th pb vs age depth profile in a zircon from the same rock permits direct assessment of zircon inheritance results reveal evidence for three phases of concentric zircon growth at 3 83 ga v3 6 ga and 2 7 2 5 ga zircon growth at both v3 6 ga and 2 7 2 5 ga is consistent with precipitation from a metamorphic fluid depth profile u th pb data demonstrate that only the s 3 8 ga zircon core could have crystallized from the host rock we conclude that the magmatic age of this rock is 3 83 7 0 01 ga not v3 65 ga as has been previously proposed the u th pb zircon depth profile technique has wider applications to resolving other geochronological debates in highgrade metamorphic terranes where zircons populations are diverse and individual grains record complex overgrowth and dissolution re precipitation features 2002 elsevier science b v all rights reserved

geochemistry of basic dikes in the lanzo massif western alps petrogenetic and geodynamic implications abstract the alpine peridotite massif of lanzo italy contains three generations of basic dikes gabbros and basalts the older gabbros are plagioclase rich mantle segregates while the younger gabbro dikes are cumulates very similar in chemical composition to recent oceanic gabbros and gabbros from ophiolitic complexes they both were derived from the n type mid ocean ridge basalt morb magmas which were progressively more depleted in incompatible elements and were probably generated during a dynamic melting of a rising mantle diapir the basaltic dikes are the n type morb and closely resemble the alpine apennine ophiolitic basalts they were derived from a different upper mantle source than the parental magmas of the gabbros the source of the basalts was less depleted in light ree the presence of basic magmas with n type morb affinities in the lanzo massif is consistent with the close genetic relationship between the alpine peridotite body and the ophiolites of the liguro piemontese basin

3850 ma bif and mafic inclusions in the early archaean itsaq gneiss complex around akilia southern west greenland the difficulties of precise dating of zircon free protoliths in migmatites abstract the southern part of the early archaean itsaq gneiss complex southern west greenland on akilia and adjacent islands consists of polyphase dioritic tonalitic granitic injection components with inclusions of metavolcanic amphibolites chemical sediments such as banded iron formation bif gabbros and ultramafic rocks incipient in situ partial melting and strong deformation during several archaean tectonothermal events strongly modified these injection components so that they are now mostly banded schlieric migmatites with neosome produced during several events the margins of many inclusions have been loci for either segregation of neosome and or higher strain obliterating the relationship between the inclusions and the older components of the migmatites an added complication is that none of the inclusion lithologies in the southern part of the complex contain protolith zircon which would provide precise direct dates instead minimum ages of inclusions are obtained by dating invasive components of the granitic sensu lato migmatites previous age determinations of the inclusions were centred on a 200 m long body of amphibolites bif and ultramafic rocks on the southwestern corner of akilia geochronology of this locality has been controversial with our proposed age of 3850 ma making the inclusion the world s oldest dated sedimentary and mafic rocks we continue the debate on the age of the inclusion on southwest of akilia and we broaden it by presenting mapping and zircon dating studies of inclusions on islands with 20 km of akilia this 1 addresses the contentious ages of these rocks 2 examines the broader issue of how the complex field relationships in migmatites can lead to geochronological controversies and 3 addresses the problem of precise dating of rocks in gneiss complexes which do not carry zircon from their protoliths on qilanngaarsuit island a gneiss sheet cutting a metaperidotite inclusion has an age of 3850 ma on an islet 2 km west of akilia tonalite and quartz diorite which intrude and envelop two mafic amphibolite and hornblendite inclusions have dates of 3850 ma also with our proposed ages of 3850 ma these inclusions are older than similar rocks in the 3700 3800 ma isua supracrustal belt at other investigated localities 3850 ma zircon dates have not been obtained from the orthogneisses enveloping or intruding inclusions at two localities invasive leucogranite neosome and a discordant diorite sheet cutting inclusions have ages of 3600 3660 ma a period of known tectonothermal events including crustal melting and intrusion of gabbros and diorites at three localities banded schlieric orthogneisses adjacent to supracrustal gabbroic and ultramafic rocks contain 3800 3700 ma igneous components in these cases the inclusions could still be 3850 ma and be equivalent in age to similar rocks on akilia but this has been masked by the local tectonothermal history and greater amount of neosome development an alternative explanation is that some younger

effects of melt density on magma mixing in calc alkaline series lavas we discuss here the melt density variations expected to occur during differentiation in calc alkaline magma systems and show how these density changes can influence the generation and chemical composition of mixed magmas we assume that fractional crystallization of basaltic melt is the dominant process in generating the association high alumina basalt hab basaltic andesite ba andesite dacite and rhyolite1 during differentiation of a basaltic parent there is an overall decrease in the density of derivative calc alkaline liquids2 4 but associated with the decline in density are density increases and related minima at medicine lake highland petrological studies5 7 have identified mixed ba mixed andesite and mixed dacite lavas we show how liquid density variations may explain the origin of the mixed basaltic andesites and andesites at medicine lake we emphasize that our mechanism may be limited to aphyric calc alkaline suites similar to those found at medicine lake volcano and we do not propose magma mixing as a general model for the origin of all andesites densities of residual liquids produced by differentiation of primary basaltic magmas at elevated pressures pass through a minimum and many of the basalts and basaltic andesites in island arcs have compositions that coincide with this minimum fractionation at crustal pressures may create a density filter that prohibits the eruption of denser primary magmas

the early archaean itsaq gneiss complex of southern west greenland the importance of field observations in interpreting age and isotopic constraints for early terrestrial evolution geochemical and isotopic studies of small volumes of variably preserved 3600 ma rocks in gneiss complexes are crucial for documenting early earth history in the itsaq gneiss complex of the nuuk region west greenland there is dispute whether the granitic sensu lato orthogneisses dominating it are mainly products of a single ca 3650 ma crust formation super event or whether they formed in several unrelated events between ca 3850 and 3560 ma which of these interpretations of the dates is correct has major implications regarding what the whole rock radiogenic isotopic record pb pb sm nd rb sr reveals about continental crust formation and early terrestrial differentiation there is also debate whether some west greenland metasedimentary rocks with 12c 13c data interpreted as evidence for life are 3850 ma or only 3650 ma old establishing the correct age for these rocks is important for debates concerning early surficial environments and origin of life controversies have arisen because of different approaches taken by different workers specifically with respect to how much emphasis is placed on field geology in interpreting dates and isotopic data in this paper field observations and sampling from low strain zones where the origin and geological context of the rocks are best preserved and understood are closely integrated with u pb zircon dates and cathodoluminescence cl imagery of the zircons this approach shows that most single phase well preserved meta granitoid samples have simple zircon populations dominated by oscillatory zoned prismatic grains formed when their host magmas crystallized on the other hand migmatites and some strongly deformed banded gneisses have much more complex zircon populations the combined field evidence and zircon geochronology on the itsaq gneiss complex demonstrate that 1 some areas contain exposed orthogneisses formed during multiple magmatic thermal events between ca 3850 and 3560 ma and are not as suggested by kamber and moorbath 1998 dominated by ca 3650 ma granitoids containing abundant 3650 ma zircons inherited from cryptic unexposed older rocks 2 abundant 3750 ma granitoids are present which are locally well preserved 3 some water lain sediments reported as showing c isotope evidence for life were deposited as early as 3850 ma 4 the whole rock sm nd isochron approach fails to distinguish with any confidence 3650 ma from 3800 ma rocks 5 however it reinforces previous indications for markedly depleted 2 5 nd domains in the pre 3750 ma mantle

deformation and metamorphism in the bard area of the sesia lanzo zone western alps during subduction and uplift the structure microstructure and petrology of a small area close to the village of bard in val d aosta italy has been studied in detail the area lies across the contact between the gneiss minuti gm and the eclogitic micaschist ems complexes of the lower element of the sesia portion of the sesia lanzo zone western alps both complexes have undergone high pressure metamorphism but the metamorphic assemblages indicate a sudden increase in pressure in going across the contact from the gm to the ems therefore we interpret the contact as a thrust dividing the lower element of the sesia into two sub elements this interpretation is supported by structural evidence the early alpine 90 70 ma metamorphic history is best preserved in the ems and is one of increasing pressure associated with thrusting the maximum p t recorded in the ems is 1500 mpa 15kbar and 550 c and in the gm is 1500 1300 mpa 15 13 kbar and 500 550 c we suggest that the rocks were probably in an active benioff zone during this time from then on the histories of the gm and ems are the same deformation continued and the thrust and thrust slices were folded during decreasing pressure we interpret the first postthrusting deformation in terms of uplift associated with continued shortening of the crust and underplating after the benioff zone had become inactive and a new benioff zone had developed further to the north west a still later deformation and the lepontine metamorphism 38 ma are related to continued uplift much of this deformation is characterized by structures indicative of vertical shortening and lateral spreading as the mountains rose above the general level of the surface

the zircon geochronology of the akilia association and isua supracrustal belt west greenland the akilia association and the isua supracrustal belt are interpreted as both derived from a single volcano sedimentary sequence because they occupy the identical position in local stratigraphic sequences show a similar association of lithologies and are both intruded by geologically and geochronologically correlated orthogneisses zircon from the akilia association yields a concordia intersection age of3587 38ma while that from the isua supracrustal belt gives 3813 14 21 ma the isua supracrustal date is close to the original age of crystallisation of the zircon while the akilia intercept age results from the resetting of the zircon date by granulite facies metamorphism close to 3600 ma ago

new pieces to the archaean terrane jigsaw puzzle in the nuuk region southern west greenland steps in transforming a simple insight into a complex regional tectonothermal model in the south of the nuuk region of west greenland our 1980s mapping recognized four archaean gneiss terranes f ringehavn tre brodre tasiusarsuaq and akia terranes with different protolith ages and separate early tectonothermal histories later in the archaean these were juxtaposed and then experienced the same 2700 2500 ma tectonothermal events here we abandon extrapolation of only these four terranes across the whole region and distinguish two new terranes in the ne the northernmost isukasia terrane previously regarded as the northernmost exposure of the f ringehavn terrane consists of palaeoarchaean rocks 3600 ma tectonically bounded on its south by the 3075 2960 ma kapisilik terrane these were juxtaposed and metamorphosed together by 2950 ma the previously recognized f ringehavn terrane to the sw is another separate entity of palaeoarchaean rocks that was juxtaposed with adjacent terranes only after c 2800 ma hence in an increasingly complex regional model there were several mid to neoarchaean terrane assembly events with superimposed orogenies from c 2950 ma until after 2700 ma although the f ringehavn and isukasia terranes were incorporated into the later archaean terrane collage at different times they might be fragments from a larger palaeoarchaean complex rifted apart from c 3500 ma onwards

timing and nature of multiple 3700 3600 ma tectonic events in intrusive rocks north of the isua greenstone belt southern west greenland one of the oldest well preserved sequences of tectonic events on earth is recorded in intrusive rocks that lie north of the isua greenstone belt southern west greenland detailed mapping and u pb dating of zircon and titanite build upon previous work to better define the timing and nature of the 3700 3600 ma tectonic history voluminous 3698 2 ma tonalite was converted into gneiss during a deformation event d 1 that predated intrusion of 3659 2 ma diorite dikes the dikes and the earliest gneissosity s 1 are most abundant in a zone of low regional deformation d 2 located 8 km north of the isua greenstone belt f 2 folding produced the dominant gneissosity s 2 during injection of meter wide granite and pegmatite sheets along s 2 at ca 3650 ma d 3 shearing transposed s 2 into a narrow mylonite zone at the contact between the intrusive rocks and the isua greenstone belt motion on this contact must have occurred in the early archean mylonites are cut by dolerite dikes that are correlated with nearby middle archean dikes titanite from the low d 2 strain zone yielded concordant u pb dates of 3606 3 ma indicating that these rocks were little affected by late archean metamorphism in contrast significant late archean heating of rocks adjacent to the isua greenstone belt is shown by titanite that lost considerable pb and or was largely recrystallized at 2915 32 ma late archean strain was also restricted to rocks adjacent to the belt this sequence of overprinting deformation and metamorphism is similar to that in younger orogenic belts

empirical garnet muscovite geothermometry in metapelites abstract two empirical garnet muscovite geothermometers assuming no ferric iron model a and 50 ferric iron model b in muscovite respectively were calibrated under the physical conditions of p 3 0 14 0 kbar and t 530 700 c the input temperatures and pressures were determined by simultaneously applying the garnet biotite thermometer am mineral 85 2000 881 and the gasp geobarometer am mineral 86 2001 1117 to natural metapelites to confirm internal thermodynamic consistency holdaway s am mineral 85 2000 881 garnet mixing properties were adopted muscovite was treated as a symmetric fe mg al vi ternary solid solution and its margules parameters were derived in this work the resulting two formulae reproduced the input garnet biotite temperatures well within 50 c and gave identical results for a great body of natural samples moreover they successfully distinguished the systematic changes of temperatures of different grade rocks from a prograde sequence inverted metamorphic zone and thermal contact aureole pressure estimation has almost no effect on the two formalisms of the garnet muscovite geothermometer assuming analytical error of 5 for the relevant components of both garnet and muscovite the total random uncertainty of the two formulations will generally be within 5 c the two thermometers derived in this work may be used as practical tools to metamorphic pelites under the conditions of 480 to 700 c low to high pressure in the composition ranges xalm 0 51 0 82 xpyr 0 04 0 22 and xgros 0 03 0 24 in garnet and fe tot 0 03 0 17 and mg 0 04 0 14 atoms p f u in muscovite

earliest part of earth s stratigraphic record a reappraisal of the 3 7 ga isua greenland supracrustal sequence investigations of the isua supracrustal rocks in west greenland allow us to identify the protoliths and alteration history for most isua rocks the protoliths consisted of alternating basalt and banded iron formation this sequence was invaded by dunitic sills metamorphosed and later intruded by felsic gneisses pervasive carbonation and k metasomatism produced a sequence of lithologies mimicking those found in modern platform deposits however the protoliths could have originated in a purely oceanic environment with no sialic detrital components the isua sequence probably consists of several tectonic panels some of which are cut by 3810 ma felsic intrusive rocks

analysis of nonlinear effects on tracer migration in heterogeneous aquifers using lagrangian travel time statistics advective transport in a heterogeneous two dimensional aquifer is simulated and quantified by the statistical moments of tracer travel time and transverse displacement these moments depend on the

secular changes in tonalite trondhjemite granodiorite composition as markers of the progressive cooling of earth archean tonalite trondhjemite granodiorite associations ttg are classically thought to generate through partial melting of hydrous metabasalts however the chemical composition of the least differentiated ttg parental magmas evolved from 4 0 to 2 5 ga during this interval the mg as well as the ni and cr contents increased which is interpreted as reflecting increased interactions between felsic melts generated by metabasalt melting and mantle peridotite similarly cao na2o and sr also increased over time thus reflecting an increase in the abundance of plagioclase in the melt residue the presence or absence of residual plagioclase is interpreted in terms of melting depth the demonstrated interaction between ttg parental magmas and the mantle rules out their genesis by fusion of previously underplated metabasalt and favors the melting of subducted slab material at 4 0 ga earth s geothermal gradient was sufficiently high to allow slab melting at shallow depths where plagioclase was stable consequently due to the small thickness of the overlying mantle wedge felsic magmas interacted little with the mantle at 2 5 ga however owing to lower geothermal gradients the melting depth was greater and plagioclase became no longer stable in the thick mantle wedge overlying the subducted slab as a result felsic magmas reacted strongly with the mantle peridotite the changes of ttg composition during archean time can be thus interpreted as reflecting the progressive cooling of earth

geochronology and isotopic variation of the early archaean amitsoq gneisses of the isukasia area southern west greenland abstract relatively unaltered samples of the amitsoq gneiss complex in the isukasia area were identified and analysed despite the effects of metamorphism and local proterozoic hydrothermal alteration and contamination the parents of the three lithologies that make up the amitsoq gneisses of the area were emplaced over about 320 ma taking a weighted average of dates from u pb on zircons rb sr on wr sm nd on wr and pb pb on wr the most probable ages of intrusion are grey gneisses at 3690 50 ma white gneisses at 3590 40 ma and pegmatitic gneisses at 3370 60 ma the rb sr isochron initial 87 sr 86 sr ratios for the grey white and pegmatitic gneisses are 0 7006 1 0 7016 1 and 0 8248 3 respectively the corresponding average nd t values are 1 0 0 7 and 0 2 since the ree chemistry of the rocks that were the direct parents of the grey gneisses is not known the 1 0 average nd t of the grey gneisses need not imply generation from a depleted mantle the sr initial ratio and nd t values for the white and pegmatitic gneisses are compatible with consecutive derivation at two discrete intervals by partial melting of crust mainly comprised of rocks similar to the grey gneisses the progressive evolution of nd with t for the amitsoq gneisses in the early archaean shows that values of nd t near zero in granitoids do not necessarily indicate derivation from the undepleted mantle chur

petrogenetic grids for metacarbonate rocks pressure temperature phase diagram projection for mixed volatile systems isothermal or isobaric phase diagram sections as a function of fluid composition x f are widely used for interpreting the genetic history of metacarbonate rocks this approach has the disadvantages that 1 the influence of a key metamorphic variable either pressure p or temperature t is obscured 2 the diagrams are inappropriate for systems that are not fluid saturated these problems are avoided by constructing phase diagram projections in which the volatile composition of the system is projected onto a p t coordinate frame i e a petrogenetic grid the univariant curves of such p t projections trace the conditions of the invariant points of isothermal or isobaric phase diagram sections thereby defining the absolute stability of high variance mineral assemblages with and without a coexistent fluid phase petrogenetic grids for metacarbonate rocks are most useful for the study of regional metamorphism and for systems in which fluid composition has not been externally controlled a calculated example of a p t projection for the system cao mgo sio2 h2o co2 suggests that many assemblages e g calcite tale enstatite fluid magnesite tremolite antigorite diopside dolomite and calcite forsterite tremolite in mixed volatile systems have stability fields that make them useful as p t indicators consideration of the principles governing projection topology demonstrates that the univariant curves around a fluid present invariant point cannot be oriented independently with respect to the direction of compositional variation in the fluid phase this has the interesting predictive implication that if the direction of compositional variation along one univariant curve around an invariant point is known then the direction of compositional variation along the remaining curves can be determined solely from topologic constraints the same constraints can be applied to systems containing simple mineral solutions or melts in order to predict compositional variations

geology age and origin of supracrustal rocks at akilia west greenland new mapping geochronology and geochemistry of the supracrustal enclave and associated orthogneisses at akilia west greenland support a 3825 6 ma age for the sequence and a chemical sedimentary origin for the controversial fe rich quartz pyroxene gneisses lithologies of the enclave comprise laterally continuous mappable units of mafic amphibolite ultramafic rocks and two fe rich quartz pyroxene units a minor anthophyllite garnet rock with chemical characteristics suggesting a sedimentary protolith was identified the earliest foliation parallels lithologic contacts this foliation was isoclinally folded about a steep hinge surface and then tightly refolded about a steep ns hinge plane the latest folding produced limbs exhibiting contrasting strains the full deformation history is shared by all lithologies of the enclave two previously unidentified orthogneiss sheets preserve deformed magmatic crosscutting relations with amphibolite and ultramafic units in the low strain limb the supracrustal anthophyllite garnet rock experienced zircon growth at 3600 and 2700 ma in common with amphibolites these ages are interpreted to reflect metamorphism at granulite and amphibolite facies respectively consistent with independent regional evidence the crosscutting orthogneiss sheets preserve a complex history of zircon growth but only earliest cores indicate th u exchange equilibrium with the bulk rock at magmatic conditions weighted mean 207 pb 206 pb ages of cores of 3730 7 ma and 3825 6 ma date the crystallization ages of the sheets indicating that the akilia supracrustal rocks were deposited 3819 ma quartz pyroxene rocks on akilia have been interpreted as metamorphosed chemical sediments that may preserve carbon isotope evidence for life at the time of their formation an alternative proposal is an origin by metasomatic alteration our results show that these rocks experienced a history of deformation and zircon growth identical to other lithologies of the supracrustal body metamorphic zircons give a minimum 207 pb 206 pb age of 3589 13 ma regardless of origin the two models of formation of quartz pyroxene rocks were evaluated using field petrologic and geochemical tests the distribution of the units the absence of field or petrologic indicators of metasomatism and geochemistry oxygen and nontraditional stable isotopes immobile elements and ree together support a sedimentary but not a metasomatic origin because their protolith was sedimentary the quartz pyroxene rocks are a primary part of the supracrustal sequence and have the same minimum age despite strong deformation and polymetamorphism the akilia supracrustal enclave contains information about terrestrial surficial processes at 3819 ma

the age and pb loss behaviour of zircons from the isua supracrustal belt as determined by ion microprobe ion microprobe uthpb analyses of zircons from a conglomeratic metavolcanic unit in the isua supracrustal belt of west greenland show that their magmatic age is3807 1ma and failed to detect any older zircon cores or xenocrysts low u areas within zircons at a 30 m scale lost only small amounts of radiogenic pb and only in the recent past whereas high u areas within grains lost much greater proportions of their pb both recently and as early as the archaean some areas in the zircons have concordant upb and thpb ages the ion probe results agree with earlier determinations for the isua supracrustals and clarify minor discrepancies between previous multigrain and single crystal analyses

boninite like volcanic rocks in the 3 7 3 8 ga isua greenstone belt west greenland geochemical evidence for intra oceanic subduction zone processes in the early earth abstract the 3 7 3 8 ga isua greenstone belt of southwest greenland is characterized by variably metamorphosed metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks the voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units the sedimentary rocks consist mainly of banded iron formation cherts conglomerates and siliciclastic turbidites the least altered metavolcanic amphibolites the garbenschiefer unit from the central tectonic domain of the isua greenstone belt are characterized by high mg number 0 60 0 80 mgo 7 18 wt al2o3 14 20 wt ni 60 645 ppm and cr 60 1920 ppm contents but low tio2 0 20 0 40 wt zr 12 30 ppm y 6 14 ppm and rare earth element ree concentrations these compositional features collectively represent a coherent mafic to ultramafic suite chondrite normalized ree patterns are concave upward on the primitive mantle normalized extended trace element diagrams they are characterized by relative depletion of nb but with an enrichment of zr relative to neighboring rees alteration deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition the average initial end value of these metavolcanic rocks is 2 collectively these geochemical characteristics are comparable to those of phanerozoic boninites given the observation that in the tertiary boninites are exclusively associated with intra oceanic subduction environments e g izu bonin mariana subduction system this suggests that intra oceanic subduction zone like geodynamic processes were operating as early as 3 7 3 8 ga

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

chloritoid bearing assemblages in eclogitised metagabbros of the lanzo peridotite body western italian alps abstract dykes of metagabbro from the lanzo peridotite body in the western italian alps contain mg chloritoidtalc garnet na pyroxenes chlorite or fe chloritoid glaucophane garnet na pyroxenes eclogitic assemblages of cretaceous age in these metagabbros the metamorphic equilibration is incomplete and controlled by local equilibrium conditions here the chemical gradients which document the magmatic microstructural domains and the diffusion patterns during metamorphism survive the eclogitic crystallisation concentration of elements like al mg and fe near the boundaries between the magmatic domains favours the crystallisation in reaction coronas of mg rich chloritoid in mg metagabbro and of fe rich chloritoid in fe ti metagabbro phase compatibilities in the al2o3 feo mgo sio2 h2o system suggest temperatures around 400 500 c at approximate pressures of 15 18 kbar for the eclogitic metamorphism of the lanzo rocks in spite of the difficulties and of the inaccuracies of these thermobarometric estimates the inferred p t values are in good agreement with determinations involving na pyroxenes and garnets

femg partitioning between coexisting garnet and phengite at high pressure and comments on a garnet phengite geothermometer abstract the fe mg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite mg 67 in the presence of quartz and water at pressures of 20 35 kb and temperatures of 800 1000 c compositions of coexisting garnet and mica indicate a linear relation between both the inkd fe mg garnet fe mg phengite and temperature and inkd and pressure in the above p t range this fe mg exchange reaction between garnet and phengite is shown to be dependent on the ca content of the garnet and on the mg number of the bulk composition these two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46 and by using a synthetic basaltic composition the overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems respectively however because of non ideality in the exchange reaction this geothermometer should not be used in any practical application outside the composition ranges studied also careful consideration of the presence of fe3 in phengite must be made if the fe3 content of the natural phengite is unknown then the temperatures obtained will be maximum temperatures only

early archaean amitsoq tonalites and granites of the isukasia area southern west greenland development of the oldest known sial amphibolite facies early archaean amitsoq gneisses envelop and intrude the c 3 800 ma isua supracrustal belt isukasia area southern west greenland most of these gneisses are strongly deformed but in a c 75 km2 augen of lower deformation the amitsoq gneisses are seen to comprise predominantly 3 750 3 700 ma tonalitic grey gneisses that were intruded first by thin bodies of mafic to dioritic composition known collectively as the inaluk dykes and then by c 3 600 ma white gneisses and finally by sporadic c 3 400 ma pegmatitic gneiss sheets the grey gneisses could have formed by partial melting of crust consisting predominantly of basic rocks the inaluk dykes are interpreted as strongly fractionated basic melts of mantle origin contaminated by crustal material the white gneisses consist mostly of medium grained granite and occur as lenses and anastomosing sheets throughout their host of grey gneisses with subordinate inclusions of supracrustal rocks the white gneisses have chemistry compatible with formation by partial melting at depth of a source dominated by grey gneisses the igneous chemistry including ree abundances of the grey gneisses and white gneisses has been modified to varying degrees by metasomatism and assimilation reactions during the crystallisation of the white gneisses and also during subsequent tectonometamorphic events the white gneisses are evidence for considerable reworking by anatexis of sialic crust in the early archaean 150 to 100 ma after its formation the white gneisses and the pegmatitic gneisses show that granitic rocks s s were important in the earliest archaean and are further evidence of the diversity of the oldest known sial

solute transport in aquifers of arbitrary variability a time domain random walk formulation solute transport in three dimensional aquifers with spatially varying hydraulic conductivity of arbitrary point distribution is investigated the basis of our study is a multiindicator model mim representation of the heterogeneity combined with a self consistent approximation for groundwater flow and particle transport a time domain random walk tdrw approach is presented for computing the expected mass arrival along the longitudinal transport direction that is simple and honors the hydrodynamics of flow for any variability using hydraulic conductivity measurements at the made site and the mim it is shown that the travel time distribution for large variability cannot be well reproduced by the common distributions used for modeling hydrological transport such as the log normal distribution or the inverse gaussian distribution the proposed tdrw approach directly relates to the lagrangian trajectory formulation and is appropriate for applications where occurrence of negative flow velocities is small these results open new possibilities for modeling solute transport in aquifers of arbitrary variability by the time domain random walk that can readily account for a wide range of mass transfer reactions

chromite in the early archean akilia association ca 3 800 m y ivisartoq region inner godthabsfjord southern west greenland

hadean continental crust in the southern north china craton evidence from the xinyang felsic granulite xenoliths abstract an increasing number of hadean detrital and xenocrystic zircon crystals have been discovered in china although its oldest crustal component remains enigmatic four xenoliths of garnet free felsic granulites entrained by the mesozoic xinyang volcanic rocks in the southern north china craton ncc were selected for u pb dating and lu hf isotopes of zircon as well as whole rock major and trace element analysis their petrography and geochemical composition reveal that the xenoliths are of igneous origin the magmatic zircons yield ages including 3 5 3 6 ga and ca 2 1 ga the 3 5 3 6 ga zircons have widely variable e hf t of 7 8 to 4 0 and model ages t crust of 3 51 4 12 ga the ca 2 1 ga zircon grains have very negative e hf t 29 0 to 26 2 and hadean model ages t crust 3 96 4 10 ga the metamorphic zircons from the studied xenoliths record several thermal episodes at ca 2 73 ga 2 13 2 35 ga and 1 77 1 86 ga these results suggest that hadean crustal components may exist in the deep crust of the southern ncc and probably suffered complex modification during paleoarchean paleoproterozoic time combined with published data the hf crustal model ages of zircons from the ncc peak at ca 3 94 ga are older than those from the south china craton peak at ca 3 77 ga this means that there is much more likelihood of finding more hadean continental crust in the ncc than in the south china craton by comparison with the ancient crustal components in the anshan and caozhuang areas of the northern ncc the xinyang area in the southern ncc may be more prospective for further occurrences of hadean zircons and or rocks

recognizable primary volcanic and sedimentary features in a low strain domain of the highly deformed oldest known 3 7 3 8 gyr greenstone belt isua west greenland a low strain domain has been identified in the metamorphosed mostly highly deformed volcanic and sedimentary rocks of the early archaean isua supracrustal belt this domain contains well preserved volcanic and sedimentary features including basaltic pillow lavas pillow breccia heterogeneous volcanic breccia amygdules in metabasalt and polymict conglomerate dominated by recrystallized chert and volcanic clasts the low strain domain is bounded by highly deformed rocks mostly derived from basalt chert and banded iron formation these discoveries demonstrate that some primary features have escaped the pervasive metasomatism dominant in other parts of the belt and furthermore strengthen the characterization of the isua supracrustals as a typical greenstone belt

thermodynamic properties of white micas on the basis of high pressure experiments in the systems k2omgoal2o3sio2h2o and k2ofeoal3o3sio2h3o abstract experiments in the piston cylinder apparatus were undertaken to synthesize the assemblage of phengite garnet kyanite and quartz or coesite in the system k2o mgo al2o3 sio2 h2o kmash compositional data of phengite were obtained for 13 experimental runs in the range of 30 to 45 kbar and 850 c to 1100 c the same was achieved for 23 runs in the system k2o feo al2o3 sio2 h2o kfash between 600 c and 900 c and 15 to 55 kbar on the basis of the latter runs and 6 additional synthesis experiments the dependence of the lattice dimensions on the composition of 2m1 phengites in kfash was determined to derive thermodynamic data we evaluated the above experiments and those reported by massonne and schreyer 1987 1989 on kmash phengites coexisting with other assemblages considering simple activity model like ams ms we obtained for the mg al celadonite and fe2 al celadonite theoretical endmembers hf0 5832415 3040 and 5492287 6267 j mol s0 288 527 4 008 and 303 148 6 827 j k mol v0 13 870 0 027 and 13 962 0 067 j bar mol the nonideal behavior of the muscovite ms mg al celadomite series was modelled using an asymmetric margules formalism with the following parameters w12h w21h 0 w12s 58 598 8 804 w21s 15 920 3 393 w12v 0 735 0 118 w21v 0 187 0 026 with units as above for the muscovite fe2 al celadonite series we used a symmetric model with wh 0 ws 24 083 3 088 j k mol and wv 0 3394 0 1120 j bar mol the above data were applied to construct a petrogenetic grid the example chosen is related to the kfash system with potassic white mica quartz and h2o in excess it covers the t range from 150 c to 550 c at a p up to 28 kbar the new thermodynamic data are also discussed in regard to their use in geothermobarometry

rate limited mass transfer or macrodispersion which dominates plume evolution at the macrodispersion experiment made site we present a model of solute transport that explains the large scale behavior of the solute tracer test plumes at the macrodispersion experiment made site as the result of advection and rate limited mass transfer between mobile and small scale immobile domains this model does not consider the process of dispersion and yet provides an alternative explanation of the evolution of the observed concentration profiles compared to the macrodispersion model the mass transfer model better represents the change in mobile dissolved mass with time the peak of the concentration profile and the profile asymmetry specifically unlike the macrodispersion model the mass transfer model explains the facts that the observed mass of the plume was greater than the injected mass in early snap shots of the plume and less than the injected mass at late times we suggest that the injected mass advects through the mobile domain and diffuses into and out of the immobile domain the immobile domain consists of a combination of low permeability zones on the scale of centimeters to decimeters the darcy scale immobile domain and intragranular porosity dead end pores and surface sorption the pore scale immobile domain we suggest that the mobile domain was sampled preferentially when water was extracted therefore at early times relatively clean water in the immobile domain was not sampled and incorrectly assumed to contain high solute concentrations similarly the mass at late times was underestimated because solute trapped in the pore scale immobile domain was not extracted during sampling and therefore ignored the combination of advection and slow mass transfer is consistent with the fact that the peak of the plume migrated only 5 m by the termination of the experiment as well as the different behavior of bromide and tritium tracers

comment on investigating the macrodispersion experiment made site in columbus mississippi using a three dimensional inverse flow and transport model by heidi christiansen barlebo mary c hill and dan rosbjerg

archaean subduction inferred from seismic images of a mantle suture in the superior province plate tectonics provides the basis for the interpretation of most current terrestrial tectonic activity and is widely accepted as having been active over much of the earth s history1 yet the timing of initiation of this process is subject to debate2 9 so far the earliest seismic evidence for plate tectonics has come from a fossil mantle suture in the svecofennian orogen 1 89 gyr ago 10 and from inferred plate convergence subduction and accretion in the trans hudson orogen 1 91 1 79 gyr ago 11 as yet seismic data from archaean areas have been able to demonstrate only the importance of compression in the construction of the continental crust12 15 here we present seismic data from a collision zone in the superior province of canada involving the abitibi granite greenstone subprovince and the plutonic arc related opatica belt we interpret dipping seismic reflections that extend 30km into the mantle as representing a relict 2 69 gyr old suture associated with subduction although crustal structure lithospheric thicknesses and convergence rates may have differed from those seen today these seismic data provide direct evidence that plate tectonics was active in late archaean times 800 myr earlier than indicated by previous seismic reflection surveys

setting of the 2560 ma qorqut granite complex in the archean crustal evolution of southern west greenland the archean gneiss complex of west greenland contains packages of unrelated rocks created during relatively short periods of time in arc like magmatic environments and having similarities to rocks formed at phanerozoic convergent plate boundaries the terranes of new archean crust were amalgamated by collisional orogeny and then partitioned by post assembly tectonic processes having summarized the origin of west greenland archean crust in arc like environments this paper then focuses on new data concerning the latest neoarchean post terrane assembly intra continental tectonic and magmatic evolution of the region following the youngest documented high pressure metamorphism in a clockwise p t t loop at 2650 ma attributed to tectonic thickening of the crust there is in west greenland a 150 million year record of intermittent production of crustally derived granite shearing and folding under amphibolite facies conditions this is exemplified by the ssw nne orientated neoarchean qorqut granite complex qgc which forms a myriad of closely spaced coeval sheets ne of nuuk town shrimp u pb zircon dating of a homogeneous gray granite sheet gives a magmatic age of 2561 11 ma with 3800 to 3600 and 3070 to 2970 ma zircon xenocrysts the 40 km long fjringehavn straight belt a lower amphibolite facies vertical shear zone runs from the qgc s se margin and contains strongly deformed granite sheets with a u pb zircon age of 2565 8 ma but is cut by undeformed granite sheets dated at 2555 12 ma the 60 km long ivisaartoq fault consisting of lowermost amphibolite facies mylonite runs from the northwestern end of the main mass of granite it formed post 2630 ma because granites of that age are truncated by it near the qgc s northeastern extent 2559 3 ma granitic lithons in a folded mylonite are cut by 2521 72 ma granite sheets at a deep structural level at the northern end of the qgc deformed granitic neosomes give ages of 2567 9 and 2563 5 ma therefore at 2560 ma the ages within error for strongly deformed to non deformed granite bodies shows that the qgc is not a largely post kinematic intrusion as previously thought but was coeval with lowermost amphibolite facies metamorphism and shear zones with an important strike slip component late in the development of regional non cylindrical upright folds the main body of the qgc appears to be essentially post kinematic only because it was emplaced in a node of dilation during the heterogeneous predominantly strike slip deformation melting at this node may have been triggered by meteoric water percolating down dilational fractures causing metasomatism melting of these altered rocks gave rise to the low 18o signature of qgc igneous zircons due to the hydrous nature of the melting event the qgc was emplaced immediately above its migmatitic generation zone these late neoarchean shear zones of the nuuk region partition and disrupt the earlier formed mosaic of amalgamated terranes of unrelated rocks such tectonic patterns are seen more recently for example in holocene asian intra continental tectonics along the north side of the himalayas

vestiges of life in the oldest greenland rocks a review of early archean geology in the godthabsfjord region and reappraisal of field evidence for 3850 ma life on akilia abstract the godthabsfjord region of west greenland contains the most extensive best exposed and most intensely studied early archean rocks on earth a geological record has been described of numerous magmatic events between 3 9 and 3 6 ga and evidence of life at 3 85 ga and 3 8 3 7 ga has been proposed from two widely separated localities some of these claims have recently been questioned and the nature of the best preserved remnants of the oldest known terrestrial volcanic and sedimentary rocks in the isua greenstone belt are being reinvestigated and substantially reinterpreted the first part of this article reviews the evolution of geological research and interpretations outlining the techniques by which the geological history has been determined and the ensuing controversies the second part re examines crucial field evidence upon which the antiquity of the oldest terrestrial life is claimed from the island of akilia

initial pb of the amitsoq gneiss revisited implications for the timing of early archaean crustal evolution in west greenland reply abstract the rapid change in 207 pb 206 pb ratio during early earth history provides a powerful constraint for temporal resolution of archaean crustal evolution new and published data for amitsoq gneiss feldspars and whole rocks define the earth s least radiogenic silicate pb known until now the most important feature of the data is that independent of the exact slope of the regression in a common pb diagram a very well defined intercept with plausible terrestrial pb evolution lines is obtained between 3 63 and 3 66 ga identical to a combined pb pb regression age of 3654 73 ma mswd 17 6 we conclude that the magmatic precursors of the amitsoq orthogneisses separated from a depleted mantle like source at ca 3 65 ga this contrasts sharply with claims based on ion probe u pb zircon dating that the amitsoq gneisses had a prolonged episodic magmatic evolution extending from ca 3 87 to 3 60 ga pb isotopes offer no evidence that any amitsoq gneiss analysed until now began its evolution in the crust as long ago as 3 85 ga we propose that zircons significantly older than 3 65 ga were inherited from chemically evolved rocks as old as ca 3 85 ga geochemical arguments are presented to support this proposal new feldspar and whole rock pb isotope data are also reported for sheets of quartz dioritic gneiss on akilia island that yield the oldest zircon u pb age clusters in the range 3 84 3 87 ga these sheets are discordant to a metasedimentary enclave containing accessory apatite with graphite inclusions whose c isotope ratios are regarded as biogenic in origin as above pb isotopes in the sheets are consistent with a magmatic age of 3 65 ga which we regard as the true minimum age for these waterlain sediments and their possibly biogenic markers additional pb isotope data as well as limited published sm nd and u pb zircon data on the akilia enclaves themselves suggest an age between ca 3 65 and 3 70 ga for their original deposition some 150 200 ma younger than proposed previously thus the question of overlap of earliest life with a bolide impact scenario terminating 3 80 ga as on the moon becomes irrelevant

field study of dispersion in a heterogeneous aquifer 1 overview and site description results are presented for a large scale natural gradient tracer experiment conducted in a heterogeneous alluvial aquifer at a site near columbus mississippi the study was initiated with a 48 hour pulse injection of 10 m3 of groundwater containing bromide and three organic tracers pentaflourobenzoic acid o trifluoromethylbenzoic acid and 2 6 diflourobenzoic acid over a 20 month period seven comprehensive samplings of the tracer plume were performed at approximately 1 to 4 month intervals using an extensive three dimensional sampling well network the dominant feature of the tracer plume that evolved during the study was the highly asymmetric concentration distribution in the longitudinal direction this asymmetry was produced by accelerating groundwater flow along the plume travel path that in turn resulted from an approximate 2 order of magnitude increase in the mean hydraulic conductivity between the near field and far field regions of the site the columbus study is distinct from previous natural gradient experiments because of the extreme heterogeneity of the aquifer the large scale spatial variations in groundwater velocity and the extensive set of hydraulic conductivity measurements for the aquifer

simultaneous boninitic and arc tholeiitic volcanisms in the izu forearc region during early arc volcanism based on odp leg 125 site 786 early arc volcanism during eocene to oligocene in the izu forearc region was investigated during odp legs 125 and 126 in 1989 and effusive and intrusive volcanics were recovered from leg 125 site 786 these rocks were all classified into boninites and associated rocks by leg 125 shipboard scientific party and they concluded that boninitic volcanism had occurred before 40 ma and arc tholeiitic volcanism began after 40 ma in this study lava flows and breccias that classified into boninite series are divided into two groups tholeiite and boninite based on petrographical and petrological properties both series are also distinguished by bulk rock composition it is considered that the sources of both rock types have similar depleted compositions because of their similar very low bulk hfse concentrations we suggest that boninitic and tholeiitic volcanism occurred closely in time and space and reflected different temperature and water condition

syn convergent exhumation implied by progressive deformation and metamorphism in the valle dell orco transect nw italian alps structural metamorphic and geochronological data from the valle del9orco are best explained by an orogenic wedge model in which crustal slices from continent and ocean acted as passive blocks at different times in the alpine orogeny deformation generally migrated westwards in this section and metamorphic recrystallization at any given point was largely controlled by the pervasiveness of strain the data suggest a dynamic crustal stacking system lasting from 70 to 35 ma rather than continental collision following long lived b type subduction and offer a relevant and simple model for eclogite exhumation that requires exhumation of ductile shear bounded slices within the orogenic wedge the easternmost internal unit of the sesia lanzo zone the eclogitic micaschist complex was subducted and pervasively recrystallized under eclogite facies conditions during the early stages of convergence in the late cretaceous during the development of the major preserved structures the eastern eclogitic micaschist complex was undergoing general non coaxial shear with an overall top down to the east vorticity suggesting a potential mechanism of exhuming an eclogitic 9pip9 slices of pre alpine basement the vasario sparone subunit were incorporated into the lower western parts of this unit during the latest part of this phase but do not show penetrative effects of the early alpine deformation they were passively folded during the early decompressional part of the p t history prior to the unit being juxtaposed against the underlying gneiss minuti complex under blueschist greenschist facies conditions the gneiss minuti complex itself was then recrystallized at high strain in the greenschist facies in the latest eocene fabrics developed at this stage indicate top down to east extension which may have accommodated the exhumation of eclogitized piemont oceanic material and the european continental material of the gran paradiso massif evidence of northwest directed thrusting within the gran paradiso unit at the same time suggests again that the crustal block was transported on a deeper reverse sense shear zone that presumably links to the more external thrusts now exposed to the west high pressure mineral lineations within the piemont zone cannot be unequivocally assigned to either the burial or exhumation paths but are consistent with the generally east west transport directions observed in the structurally higher units

protoliths of the 3 8 3 7 ga isua greenstone belt west greenland abstract the isua greenstone belt fig 1 contains the oldest known relatively well preserved metavolcanic and metasedimentary rocks on earth the rocks are all deformed and many were substantially altered by metasomatism but both the deformation and metasomatism were heterogeneous transitional stages can be seen from relatively well preserved primary volcanic and sedimentary structures to schists in which all primary features have been obliterated likewise different kinds and different episodes of metasomatic alteration can be seen that produced a diversity of different compositions and metamorphic mineral assemblages from similar protoliths new geological mapping has traced out gradations between the best preserved protoliths and their diverse deformed and metasomatised equivalents by this means the primary nature of the schists that make up most of the isua greenstone belt was reinterpreted and a new map that better portrays the primary nature of the rocks has been produced the previously mapped stratigraphy was found to be of little value in understanding the geology stratigraphic units were defined by different and diverse criteria such as current composition structure metamorphic texture and inferred protoliths much of this stratigraphy represents a misinterpretation of the primary nature of the rocks the new work indicates that most of the isua greenstone belt consists of fault bounded rock packages mainly derived from basaltic and high mg basaltic pillow lava and pillow lava breccia chert bif and a minor component of clastic sedimentary rocks derived from chert and basaltic volcanic rocks a previously mapped extensive unit of felsic volcanic rocks was found to be derived from metasomatised basaltic pillow lava and pillow breccia intruded by numerous sheets of tonalite

transport upscaling in highly heterogeneous aquifers and the prediction of tracer dispersion at the made site we thank daniel fernandez garcia for sharing the data for the two concentration snapshots of the made 2 experiment and alraune zech and aldo fiori for sharing the made 1 data with us the authors acknowledge the financial support of the european research council through the project mhetscale grant agreement no 617511 and the spanish ministry of science and innovation through a severo ochoa project no cex2018 000794 s and the project hydropore pid2019 106887gb c31 juan j hidalgo acknowledges the support of the spanish ministry of science and innovation through a ramon y cajal fellowship no ryc 2017 22300 alessandro comolli acknowledges support through the prodex grant chypi

very early archean crustal accretion complexes preserved in the north atlantic craton the north atlantic craton contains very early archean supracrustal rocks orthogneisses and massive ultramafic rocks most units of supracrustal rocks are dominated by mafic volcanic rocks layered gabbros and banded iron formations bust some also contain abundant felsic volcanic sedimentary rocks quartzites and marbles some quartzites contain detrital zircons derived from rocks identical in age to felsic volcanic sedimentary rocks in these sequences ca 3800 ma and also from older ca 3850 ma sources the presence of the ca 3850 ma detrital zircons suggests that the supracrustal units containing them were deposited on or close to ca 3850 ma sialic crust the massive ultramafic rocks have chemical affinities to upper mantle rocks the voluminous suites of tonalitic gneisses are dominated by 3700 3730 ma bodies that intrude the supracrustal sequences but they also locally contain components with ages between 3820 and 3920 ma the diverse supracrustal units upper mantle rocks and ge 3820 ma components in the gneisses were tectonically interleaved in very early archean convergent plate boundaries giving rise to accretion complexes in the period 3700 3730 ma voluminous tonalitic magmas produced by partial melting of predominantly mafic rocks in the base of the accretion complexes were emplaced at higher levels forming juvenilemore continental crust and leaving behind a refractory lower crustal to upper mantle substrate less

u pb geochronology of 3810 3630 ma granitoid rocks south of the isua greenstone belt southern west greenland abstract the oldest well preserved granitoid rocks on earth envelope the isua greenstone belt southern west greenland the timing and nature of geological events that affected granitoid rocks south of the isua belt southern gneisses are refined with new mapping and u pb geochronology these results and previous work allow for a detailed comparison between the southern gneisses and similar granitoid rocks north of the arcuate isua belt northern gneisses zircons from tonalitic gneiss and meta tonalite that yielded ages of 3810 3795 ma are interpreted as having grown during igneous crystallisation of their protoliths based on the consistency of ages within and between samples and the oscillatory internal zoning that is typical of magmatic zircons two dated tonalitic rocks are particularly important because they occur as concordant layers within supracrustal rocks amphibolite with deformed pillow lava structures in the southwestern part of the isua belt and quartz rich gneiss likely derived from a chemical sediment that forms an enclave in the southern gneisses although these layers lack cross cutting intrusive relationships they are interpreted as having intruded as sheets into 3800 ma supracrustal rocks most tonalite was converted into gneiss before intrusion of diorite dated at 3658 3 1 2 ma and deformed again during or shortly after intrusion of 3650 3630 ma granitic rocks whereas a few small lenticular zones of meta tonalite escaped nearly all strain the timing of amphibolite facies metamorphism may be recorded by zircon and titanite growth in amphibolitic gneiss at 3632 3620 ma other thermal events were recorded by zircon growth at 3550 and 2660 ma and titanite growth or pb loss at 2650 and 2600 ma these new data and interpretations suggest that the southern gneisses underwent many geological events along with the northern gneisses including 3658 ma deformation 3658 ma diorite intrusion and 3655 3630 ma crystallisation of granitic rocks during significant metamorphism and deformation the major differences are that most tonalite in the southern gneisses is 100 my older than the northern tonalite and the intensity of the neoarchaean tectonothermal activity increased southward any large scale juxtaposition of the southern gneisses against the northern gneisses across the isua belt likely occurred before intrusion of the 3658 ma diorite

flat to steep transition in subduction style we use a thermal history of the mantle to calculate the evolution of oceanic crustal thickness over earth history and use residual depth anomalies from the present day pacific to find the crustal thickness range 9 2 11 6 km where the subduction style changes from mainly flat to mainly steep we find that steep subduction was well developed by 2 5 ga which coincides with a major change in sedimentary rare earth element patterns over 50 of all oceanic crust subducted steeply by 2 0 1 6 ga the same interval over which the average thickness of continental plates declined rapidly because producing thick plates requires many episodes of flat subduction our model can explain why there are no known thick plates

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

anatomy of an early archean gneiss complex 3900 to 3600 ma crustal evolution in southern west greenland the early archean complex of southern west greenland consists of polyphase tonalitic trondhjemitic granodioritic ttg and granitic amitsoq gneisses with inclusions of volcanic and sedimentary rocks gabbros and ultramafic rocks in this complex rocks of similar appearance and composition were found to be of different ages by u pb zircon dating the amitsoq gneisses comprise 3870 3820 3810 3760 3730 3700 and 3625 ma ttg and 3660 3650 and 3625 ma granites and their inclusions belong to several supracrustal sequences with a similar spread of ages these results show that the complex grew by episodic addition of new ttg and welding together of rocks of different ages a possible plate tectonics scenario is as follows melting of subducted mafic oceanic crust formed 3700 ma microcontinents consisting of ttg suites with predominantly mafic inclusions at 3650 ma collision between microcontinents caused crustal thickening high grade metamorphism and emplacement of leucogranites at 3625 ma subduction at the edge of the 3625 ma continental mass created a new crustal addition comprising both ttg and granite while granites were emplaced into the 3625 ma continental mass