middle devonian olistostromes in the rheno hercynian zone rheinisches schiefergebirge an indication of back arc rifting on the southern shelf of laurussia abstract the rheinisches schiefergebirge in the rheno hercynian zone of the mid european variscides was geographically situated on laurussia s southern shelf during the devonian in its eastern part lahn dill area givetian coarse clastic sediments formerly misinterpreted as transgressional conglomerates are recognised as olistostrome deposits intercalated in basinal sediments that document a phase of rifting on the southern shelf of laurussia the clastic components redeposited as olistolithes and rounded boulders are locally derived rocks of emsian and givetian age the olistostromes show facies patterns comparable with other syn rift deposits the proximal facies is characterised by olistolithes and slideflow debrites overlain by debrites and intercalated with turbidites the distal facies is represented by intercalation of debrites and turbidites both grade upwards into basinal sedimentation based on conodont stratigraphy the redeposition lasted from the middle varcus zone to the disparilis zone during the middle varcus zone footwall uplift raised the sedimentary column of the source area by about 500 m above which brought it above sea level rifting of the laurussian shelf as a result of hinterland transtension and extension caused by the oblique collision of peri gondwana with laurussia provides the most plausible explanation for the development of the olistostromes olistolithes and slideflow debrites in the eastern rheinisches schiefergebirge

implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary non collisional and collisional suturing orogens

late archean prograde amphibolite to granulite facies relations in the fiskenaesset region southern west greenland the tasiusarsuaq terrane in the fiskenaesset nuuk region of southern west greenland is interpreted as a late archean tilted crustal section modified by deformation the northern part of the terrane was affected by granulite facies metamorphism at about 2810 ma but large areas were subsequently retrogressed to amphibolite facies in the prograde amphibolite to granulite facies transition in inner bjornesund a relatively sharp but highly irregular front that crosses structures and lithological boundaries on outcrop scale separates orthopyroxene free from orthopyroxene bearing rocks outcrop relations resemble in some ways examples of arrested charnockite formation described from high grade regions in southern india and sri lanka relations in the transition zone in bjornesund appear to be the result of late local influx of a fluid presumably rich in co 2 that permitted orthopyroxene to grow at the expense of hornblende regional considerations suggest a genetic connection between granulite faci

amphibole plagioclase and clinopyroxene geochemistry of the archean fiskenaesset complex at majorqap qâva southwestern greenland implications for archean petrogenetic and geodynamic processes abstract despite multiple phases of deformation and metamorphism the archean ca 2 97 ga fiskenaesset complex southwestern greenland contains well preserved igneous olivine pyroxene amphibole predominantly hornblende and plagioclase this study presents new major and trace element data for hornblende plagioclase and clinopyroxene from the majorqap q va stratigraphic section of the complex petrographic observations and coherent trace element patterns of hornblende clinopyroxene and plagioclase grains in samples within and between stratigraphic units indicate that trace elements including lile ree and hfse in these minerals were generally immobile during amphibolite to granulite facies metamorphism their mineral chemistry therefore should reflect the trace element composition of the melt from which they grew two distinct petrogenetic suites of rocks are recognized using the trace element composition of hornblende the earlier suite a is characterized by hree depleted hornblende whereas the later suite b contains hree enriched hornblende within suite a hornblendes can be grouped into type a1 type a2 and type a3 hornblendes type a1 hornblendes characterized by low ree nb and ta contents and the absence of eu anomalies most likely have derived from a primitive liquid type a2 hornblendes which have high concentrations of incompatible elements including nb zr and ree and negative eu anomalies appear to have precipitated from a residual liquid following the extensive crystallization of plagioclase type a3 hornblendes are slightly enriched in mree compare to lree and hree displaying negative nb zr and ba anomalies suite a typifies a hydrous crystallization sequence of olivine clinopyroxene orthopyroxene hornblende plagioclase hornblende intercumulus this crystallization sequence played an important role in the formation of anorthosite layers in the fiskenaesset complex within suite b hornblendes are grouped into four types type b1 type b2 type b3 and type b4 hornblendes irrespective of eu anomalies type b1 type b2 and type b3 hornblendes display parallel patterns with coherent hree enrichment over lree in contrast type b4 hornblendes are enriched in lree suite b illustrates a crystallization of sequence of olivine plagioclase hornblende hornblende the presence and chemistry of the igneous hornblendes suggest the fiskenaesset complex formed in an archean convergent plate margin setting

rapid burial and exhumation during orogeny thickening and synconvergent exhumation of thermally weakened and thinned crust variscan orogen in western europe we present a model demonstrating that the presence of a thinned and thermally weakened crustal domain is a prerequisite for the formation of an orogenic root in mountain belts furthermore this mechanism permits burial to 50 to 70 km in time periods of less than 20 myr and exhumation from these depths in less than 10 myr our example from the variscan belt is based on field geology petrology of metamorphic rocks and detailed geochronology it describes an evolution from i sedimentation and magmatism during back arc extension ii compression leading to burial thickening of the lithosphere and granulite facies metamorphism followed by iii exhumation formation of dehydration melts migmatites and late intrusive granites the existing u pb ar ar data constrain this evolution to no more than 20 to 25 myr thermorheological modelling starts from a hot thinned crust forming a narrow 50 km wide orogenic root within 15 myr at a maximum of 50 km depth and subsequent exhumation by extrusion within 7 myr the model explains exhumation rates of 3 to 4 mm year by compressional extrusion and shows that extension cannot do the same job within the same time extrusion transported the internal thermal structure into the upper crust within a short time followed by rapid cooling whereas exhumation through extension would cause mantle upwelling and the formation of a long lived thermal anomaly the latter would contradict the geochronological and petrological database the model explains orogeny as a thermorheological process without needing to involve an external heat source from the mantle and presents a new explanation of the high temperature and low pressure conditions very typical of the variscan orogenic crust

the rotations of the variscides during the carboniferous collision paleomagnetic constraints from the vosges and the massif central france abstract this study aims at completing and refining results from previous paleomagnetic investigations carried out in various outcrops of the european variscan belt sampling was performed in visean effusive and intrusive rocks of the southern vosges basin and the northern massif central all emplaced in the time range 345 340 ma stepwise thermal demagnetization of 254 specimens from 13 sites of the doller valley and nine sites of the massif central has yielded a very similar paleomagnetic record for both areas with the exception of post carboniferous overprints isolated in the southern vosges two groups of directions with both normal and reversed polarity have been evidenced the first group of directions labelled c occurs in both areas and is characterized by scarce pre tectonic magnetizations two sites and by syn tectonic overprints five sites acquired during the late visean convergence i e in the time range 340 328 ma the mean pole co is 28 s 45 e k 139 95 5 the main group labelled b shows a negative fold test and a positive reversal test indicating that the magnetizations are post tectonic overprints acquired after the main tightening phase i e after 328 ma at a few sites in both areas were evidenced syn tectonic b magnetizations that are in favour of an overprinting in the context of post visean extension tectonics the mean post tectonic b direction of the doller valley southern vosges 62 25 k 52 95 6 n 11 vgp 8 n 127 e is consistent with the mean post tectonic b direction 72 23 k 126 95 5 n 6 vgp 3 n 114 e obtained from the tuffs the grey microgranite and the dyke intrusive into the microgranite of the massif central the occurrence of similar paleomagnetic records in the eastern and central variscides implies that from the massif central to the sudetes the belt has probably moved as one block since early carboniferous the change in inclination of the c components suggests a southward drift of the moldanubian and saxothuringien variscides in the late visean 340 328 ma according to geological features in the same time the rhenohercynian crust was underplating the saxothuringian arc and fore arc zone subsequent to 328 ma the drift turned into a rapid clockwise rotation by about 70 around a pole located on southeastern corner of baltica leading to the closure of the rhenohercynian oceanic basin in the namurian large scale overprinting b components likely related to post rotation extension affected the internal belt the convergence continuing the variscides underwent a second clockwise rotation by about 45 in the late namurian westphalian 320 305 ma stacking of the rhenohercynian sedimentary cover and thrusting over the london brabant basement may be related with this motion

tectonic evolution of the european variscan belt constrained by palaeomagnetic structural and anisotropy of magnetic susceptibility data from the northern vosges magmatic arc eastern france geochronology structural and anisotropy of magnetic susceptibility data from the northern vosges batholith which belongs to the northern vosges mid german crystalline rise arc show contrasting emplacement modes of southern granodiorites and northern granites the ene wsw trending fabrics of granodiorites 346 334 ma are parallel to the metamorphic cleavage affecting the host rocks developed during regional compression the nnw sse trending fabrics of younger granitoids c 330 ma reveal an extensional emplacement mode associated with a normal shear zone separating the two magmatic suites palaeomagnetism shows that the switch from a compressive to an extensional regime coincides with a regional counterclockwise rotation the 330 325 ma extension is further supported by palaeomagnetic and seismic data indicating southeastward tilt of the whole batholith finally the system is rotated clockwise without any structural overprint data from the early carboniferous northern vosges magmatic arc and remote bohemian massif suggest that this evolution is valid for the whole eastern branch of the european variscan belt for which the following model is proposed 1 late devonian early carboniferous east west shortening of the variscides above the rhenohercynian subduction zone 2 axial nnw sse shortening of the assembled variscan orogen associated with 3 internal sinistral rotation of inherited rhenohercynian transform faults and shortening of intervening blocks 4 east west extension and dextral external rotation of blocks between dextrally reactivated transforms supplementary materials analytical procedures for geochronology magnetism anisotropy of magnetic susceptibility and palaeomagnetism are available at www geolsoc org uk sup18635

hafnium neodymium isotope systematics of the 2 7 ga gadwal greenstone terrane eastern dharwar craton india implications for the evolution of the archean depleted mantle abstract the neoarchean gadwal greenstone belt in the eastern dharwar craton india hosts a well preserved metavolcanic sequence that is dominated by tholeiitic and calc alkaline basalt andesite dacite rhyolite series which includes boninitic geochemical varieties bulk rock lu hf and sm nd isotope systematics of these apparently arc related volcanic rocks yield indistinguishable ages of 2 701 0 024 ga and 2 702 0 026 ga respectively on the basis of the close spatial association and identical ages of the different rock types we suggest 2 70 0 03 ga as the age of crystallization of the different rock types within the gadwal metavolcanic sequence in contrast bulk rock pb pb isotope systematics of the same samples yield a significantly younger and less precise age of 2 466 ga 0 068 0 110 ga we tentatively interpret this younger age to represent a metallogenic and crustal reworking event in the dharwar craton which disturbed the u pb system but not the lu hf or sm nd systems the gadwal metavolcanic rocks have positive initial ehf 2 70ga 1 6 to 8 7 and slightly negative to positive end 2 70ga 0 1 to 3 0 values consistent with an origin from a long term depleted source relative to a chondritic reservoir at 2 7 ga lack of correlation between initial isotopic compositions and major or trace element indices of fractionation and alteration suggest that the observed isotope variability probably reflects compositional variation in the gadwal source similar to that observed in modern day island arcs two boninitic samples of the gadwal sequence have ehf 8 3 and 8 7 and are more radiogenic than average depleted mantle for the time period 3 2 to 2 5 ga ehf 4 to 6 early perhaps hadean differentiation events that led to a depleted and heterogeneous mantle are apparent in the nd and hf isotope systematics of 3 7 3 8 ga isua supracrustal rocks the radiogenic hf isotopes of the gadwal boninites and the hf nd isotope systematics of rocks from other locations in the 3 4 to 2 5 ga time period are consistent with the survival of fragments of an early depleted mantle later in the archean from 2 0 ga to present the time integrated 176lu 177hf and 147sm 144nd of the depleted mantle appears nearly constant and similar to the present day average morb source these data indicate that progressive elimination of early 4 5 ga formed heterogeneities in the depleted mantle dominated the history of the archean mantle and that portions of early depleted reservoirs survived through the mesoarchean these results have implications for the mixing scales for the early terrestrial mantle and the timing of the initiation of present day plate tectonics

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

complex calc alkaline volcanism recorded in mesoarchaean supracrustal belts north of frederikshab isblink southern west greenland implications for subduction zone processes in the early earth we present new geochemical data for three mesoarchaean supracrustal belts ravns storo bjornesund and perserajoorsuaq situated north of frederikshab isblink in southern west greenland for which we propose the collective name the ikkattup nunaa supracrustal association they comprise mainly amphibolites of tholeiitic basalt composition and leucoamphibolites of calc alkaline andesite composition both lithological units are cut by aplite sheets of tonalite trondhjemite granodiorite ttg composition with u pb zircon ages of c 2900 ma lu hf and sm nd isochrons based on whole rock amphibolite and leucoamphibolite samples yield ages of 2990 41 ma and 3020 78 ma respectively which are within error of the age of the fiskenaesset complex situated 5 25 km to the north leucoamphibolites from the three supracrustal belts show apparent geochemical mixing trends between tholeiitic amphibolites and ttg gneisses as the end members by assimilation fractional crystallisation afc modelling we can show that one group of leucoamphibolites can indeed be explained by contamination of the parental melts by a ttg like end member and another group of high p2o5 la and nb leucoamphibolites can be explained by contamination involving a hypothetical low silica adakite end member however the leucoamphibolites are juvenile with epsilon nd 2970 ma from 2 1 to 3 5 and epsilon hf 2970 ma of 3 5 to 4 3 thus the mafic source of the felsic contaminant melts must have been derived from a depleted mantle source more or less at the same time 60 ma as the volcanism took place our preferred interpretation of the geochemical and isotope data is that the protoliths of the supracrustal rocks formed in an island arc setting where early tholeiitic volcanism gave way to calc alkaline volcanism in a maturing arc the apparent afc trends are thus explained by in situ partial melting of basaltic arc crust to form juvenile ttg and adakite melts that mixed with mafic magmas or contaminated their mantle source to produce the calc alkaline leucoamphibolite protolith this model has important implications for the general interpretation of other archaean supracrustal belts because afc and geochemical mixing trends towards a ttg end member are not uniquely diagnostic of crustal contamination but may rather reflect processes operating at lower crustal or source levels in arcs such as melting assimilation storage homogenisation mash or slab melt low silica adakite metasomatism of the mantle source respectively c 2012 elsevier b v all rights reserved less

new age ca 2970 ma mantle source composition and geodynamic constraints on the archean fiskenaesset anorthosite complex sw greenland the archean fiskenaesset complex sw greenland consists of an association of ca 550 meter thick layered anorthosite leucogabbro gabbro and ultramafic rocks peridotite pyroxenite dunite hornblendite the complex was intruded by tonalite trondhjemite and granodiorite ttg sheets now orthogneisses during thrusting that was followed by several phases of isoclinal folding the trace element systematics of the fiskenaesset complex and associated volcanic rocks are consistent with a supra subduction zone geodynamic setting the fiskenaesset anorthosites leucogabbros gabbros and ultramafic rocks collectively yield an sm nd errorchron age of 2973 28 ma mswd 33 with an average initial epsilon nd 3 3 0 7 consistent with a long term depleted mantle source regression of pb isotope data define an age of 2945 36 ma mswd 44 and the regression line intersects the average growth curve at 3036 ma slightly lower pb pb errorchron age is interpreted as reflecting partial disturbance of the u pb system in gabbros leucogabbros and ultramafic rocks during intrusion of ttgs complex internal structures in zircons from orthogneisses reveal several episodes of zircon growth and recrystallization taking place between ca 3200 and 2650 ma zircon ages peak at about 3200 3100 3000 2950 2820 and 2750 ma the 3200 3000 ma zircon cores are interpreted as inherited xenocrysts from older reworked crustal rocks 2950 ma is considered as an approximate intrusion age of sampled ttgs the 29402650 ma ages are attributed to metamorphic overgrowth and recrystallization in response to multiple tectonothermal events that affected the fiskenaesset region on the basis of recently published trace element data and new nd and pb isotope and u pb zircon age data a three stage geodynamic model is proposed to explain the evolution of the fiskenaesset complex stage 1 represents the formation of depleted shallow mantle source 3000 ma epsilon nd 3 3 0 7 for the complex stage 2 corresponds to the development of an intra oceanic island arc between 3000 2950 ma stage 3 is characterized by the collision of the island arc with either a passive continental margin or with an older arc between 2950 2940 ma c 2010 elsevier b v all rights reserved less

an experimental study of trace element partitioning between zircon and melt as a function of oxygen fugacity abstract the partitioning of p sc ti sr y nb the rare earth elements ree hf ta th and u between zircon and a synthetic andesitic melt was determined as a function of oxygen fugacity f o 2 over a range of fourteen log units from qfm 8 to qfm 6 where qfm is the quartz fayalite magnetite oxygen buffer at 1300 c and 1 atm using sims and la icp ms the partition coefficients of ce and eu were found to vary systematically with f o 2 relative to those of the other ree producing ce and eu anomalies similar to those that are characteristic of natural igneous zircon these anomalies coexist at terrestrial f o 2 s and were used to quantify eu 3 eu 2 eu 3 and ce 4 ce 3 ce 4 in the melt the partition coefficients of the redox invariant trivalent cations are in excellent agreement with lattice strain theory the values of d ree are related to those determined for natural samples by the expected dependence on temperature the incorporation of ree 3 in zircon was independent of the presence of p d u d th was found to vary systematically with f o 2 indicating the presence of u 4 u 5 and u 6 in the melt at terrestrial conditions d ti was independent of f o 2 despite an expectation of significant ti 3 in the most reduced experiments

origin of mesoarchaean arc related rocks with boninite komatiite affinities from southern west greenland we report whole rock elemental and sm nd isotope geochemical data from mafic ultramafic supracrustal rocks from the nunatak 1390 area in southern west greenland additionally we report the metamorphic temperature history for these rocks as derived from tourmaline thermometry on a tourmalinite inlier as well as in situ u pb hf and o isotopic data from zircons extracted from tonalite trondhjemite granodiorite ttg gneisses that intruded the mafic ultramafic sequence the supracrustal rocks from the nunatak 1390 area have a minimum age of c 2900 ma defined by u pb zircon ages of cross cutting aplite sheets of ttg composition the supracrustal sequence comprises mafic rocks with pillow structures and ultramafic rocks with no evidence of their protolith they all have amphibolite facies mineral assemblages and a peak metamorphic temperature of approximately 550 degrees c the mafic sequence has relatively flat trace element patterns la n sm n of 0 70 2 4 and mostly negative nb anomalies nb nb of 0 30 1 0 and resembles modern island arc tholeiites the mafic sequence can be divided into a high and low ti group where the former group has lower mgo and significantly higher contents of incompatible elements such as tio2 p2o5 zr nb and th the ultramafic rocks have major and trace element compositions similar to ti enriched karasjok type komatiites described in the literature however there are no textural indications that the ultramafic rocks from nunatak 1390 are komatiites sensu stricto the low ti group of the mafic sequence appears to have been derived from a n morb source whereas the high ti group and the ultramafic rocks appear to have been derived from a mantle source that is more enriched than the n morb source however there is no difference in the initial epsilon nd of the mafic and ultramafic rocks additionally assimilation fractional crystallisation afc modelling is consistent with this enrichment being caused by introduction of juvenile low silica adalcite slab melt into the mantle source region accordingly we propose that the mafic and ultramafic rocks were derived from a similar type of mantle source but that the ultramafic rocks were derived from a previously depleted mantle source that was refertilised by slab melts in a subduction zone setting the high mgo contents of the ultramafic rocks could thus reflect a second stage of partial melting of a refractory mantle in a process similar to that which is suggested for the formation of modern boninites we propose that the mafic ultramafic sequence represents an island arc that evolved initially as a juvenile complex c 3000 ma however inherited zircon grains in aplites and hf isotope data recorded by the second intrusive ttg phase c 2850 2870 ma show that mixing with older pre existing crust occurred during this event because the regional crust is dominated by ttgs of this younger age our data suggests that it likely formed by accretion and melting of arcs of different ages and or contamination of juvenile arcs by pre existing continental crust rather than entirely by juvenile arc differentiation or melting our data thus supports melting of thickened mafic crust in an accretionary setting rather than direct slab melting as a mechanism for archaean crust formation c 2012 elsevier b v all rights reserved

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

resetting of smnd systematics during metamorphism of 3 7 ga rocks implications for isotopic models of early earth differentiation abstract new sm nd isotopic results for sixteen samples from the garbenschiefer amphibolite unit of the 3700 ma isua supracrustal belt of west greenland are reported the data illustrate the difficulty in identifying primary nd t values in poly metamorphic and poly deformed rock sequences the investigated samples are interpreted as derived from a mg and al rich igneous protolith which can be argued to have been essentially isotopically homogeneous at least on the outcrop scale at the time of its emplacement however the data do not give uniform nd t values even from single outcrops the wide variation of nd t which we obtain 1 0 to 5 0 demonstrates that the sm nd systematics of the samples were disturbed by one or more metamorphic event s which took place some considerable time after the original crystallization of the isua belt a sm nd isochron age of 2849 116 ma has been obtained on plagioclase and amphibole separates which represent the oldest metamorphic assemblage now preserved at isua this result indicates that the earliest amphbolite facies metamorphism of the isua belt for which pressure and temperature conditions can be obtained p 5 kbar t 500 600 c is late archaean and not early archaean as had been assumed earlier by some authors 2850 ma is also thought to represent the main time of disturbance of the sm nd system in the garbenschiefer amphibolite unit a consideration of available isotopic data suggests that many of the rocks from isua underwent some nd isotopic re equilibration and or fractionation of their 147 sm 144 nd ratio during the 2850 ma metamorphic episode in these rocks the computed nd t are apparent values only and need not represent either the primary isotopic composition of their protoliths or early archaean mantle values only nd rich rocks 20 ppm such as some felsic schists and the grey gneisses adjacent to the belt are arguably sufficiently robust to retain nd t values which are close to their original composition these average 2 0 which may represent a close approximation of the nd value of the earth s mantle at 3800 ma the results presented in this paper show that great care should be exercised in the use of sm nd data from very early archaean rocks as constraints for isotopic models of early earth s evolution particularly in terranes which have been affected by several metamorphic episodes

material transfer and subduction channel segmentation at erosive continental margins insights from scaled analogue experiments abstract whereas tectonically erosive convergent margins make up nearly 60 of all present day convergent margins processes and amounts of material transfer and recycling remain enigmatic removal of material from the frontal forearc leaves no features which could be imaged seismically or probed through drilling only a few scaled laboratory analogue experiments have analyzed material transfer at convergent margins characterized by long term subduction erosion and none has focused on processes and amounts of material transfer and recycling therefore in this study we attempt to identify potentially relevant parameters controlling material transfer in brittle tectonically erosive forearcs this is addressed by a series of sandbox experiments performed with granular materials we here mainly focus on the amount of sediment that can be transported downdip through the subduction channel sc we analyzed our experiments with regard to the evolution of internal and basal material transfer material transfer mode patterns wedge geometry and sc evolution to achieve a truly erosive scenario we built a large initial wedge representing the brittle forearc featuring a moderately high friction basal detachment and composed of strong granular material there was no incoming material the slope of the wedge initially had a critical taper through opening a subduction gap sg the wedge was subject to rear material loss our observations point to a close interaction between sc segmentation and wedge deformation basal erosion mainly took place beneath the middle upper slope our experiments confirmed the strong influence of the sc on the wedge evolution a larger amount of material loss led to the development of a sc with an inhomogeneous distribution of velocities along the base of the wedge segmenting the sc and slope geometry the latter was comparable to observations in natural forearcs which are segmented into lower middle and upper slopes

mesoarchaean aluminous rocks at storo southern west greenland new age data and evidence of premetamorphic seafloor weathering of basalts abstract metamorphosed meso to neoarchaean supracrustal rocks in the central part of the island of storo nuuk region southern west greenland show field and geochemical evidence of premetamorphic chemical alteration this alteration changed basaltic precursors into aluminous lithologies and following amphibolite grade metamorphism and penetrative ductile deformation these garnet biotite schists now resemble adjacent metapelitic schists of sedimentary origin mass balance calculations isocon method suggests that most major elements si fe mg ca na and p were leached during alteration the calculated overall net mass changes are between 18 and 45 consistent with breakdown of olivine pyroxenes plagioclase and apatite in the basaltic precursor rocks major and trace elements such as k cs rb ba pb zn la and ce were added during this alteration process whereas high field strength elements ti al zr hf and nb remained essentially immobile and were thus residually enriched interestingly th which is generally assumed to be immobile in fluids was also added during this process these chemical changes reflect interaction between a basaltic protolith and hydrous fluids that established a new equilibrium and thus a different mineral assemblage it is proposed that the premetamorphic alteration at storo was due to low temperature interaction between seawater and oceanic crust and thus essentially represents in situ submarine seafloor weathering this interpretation is consistent with the mass balances reported from well documented examples in younger settings new u pb zircon geochronology from the arc related mafic sequences at storo shows that they comprise at least two distinct age groups an older anorthosite complex dated at 3051 3 2 6 ma and a younger supracrustal sequence with age brackets between 2840 and 2710 ma the allochthonous nature of these two mafic igneous to sedimentary stacks is consistent with accretionary processes in island arc complexes and a compressional archaean tectonic setting

tectonic erosion and the removal of forearc lithosphere during arc continent collision evidence from recent earthquake sequences and tomography results in eastern taiwan a very common but important observation of collided and accreted volcanic arcs is that the forearc regions of these arcs are largely or even entirely missing the processes and mechanisms responsible for the removal and transport of the forearc materials from the collisional belts are thus important issues in understanding tectonics and crustal growth the young and ongoing collision between the luzon volcanic arc and the eurasian continental margin that forms the island of taiwan provides a rare opportunity to examine these processes and mechanisms as they occur from observations of a new detailed 3 d tomography combined with relocated hypocenters of two earthquake sequences occurred in 2003 and 2006 we found that the luzon forearc lithosphere initially underthrusts westward after the collision began as the collision proceeds the forearc basement then subducts eastward beneath the colliding and accreting luzon arc along a major fault system in eastern taiwan thus the luzon forearc lithosphere appears to be removed by tectonic erosion and is being transported eastward into the mantle our results from the active taiwan orogen will provide important insights for interpreting rock records from many old arc continent collisional belts in the world

the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland the mesoarchean ca 3075 ma ivisaartoq greenstone belt contains well preserved primary magmatic structures such as pillow lavas volcanic breccias and clinopyroxene cumulate layers picrites despite the isoclinal folding and amphibolite facies metamorphism the belt also includes variably deformed gabbroic to dioritic dykes and sills actinolite schists and serpentinites the ivisaartoq rocks underwent at least two stages of post magmatic metamorphic alteration including seafloor hydrothermal alteration and syn to post tectonic calc silicate metasomatism between 3075 and 2961 ma these alteration processes resulted in the mobilization of many major and trace elements the trace element characteristics of the least altered rocks are consistent with a supra subduction zone geodynamic setting and shallow mantle sources on the basis of geological similarities between the ivisaartoq greenstone belt and phanerozoic forearc ophiolites and intra oceanic island arcs we suggest that the ivisaartoq greenstone belt represents a relic of dismembered mesoarchean supra subduction zone oceanic crust this crust might originally have been composed of a lower layer of leucogabbros and anorthosites and an upper layer of pillow lavas picritic flows gabbroic to dioritic dykes and sills and dunitic to wehrlitic sills the sm nd and u pb isotope systems have been disturbed in strongly altered actinolite schists in addition the u pb isotope system in pillow basalts appears to have been partially open during seafloor hydrothermal alteration gabbros and diorites have the least disturbed pb isotopic compositions in contrast the sm nd isotope system appears to have remained relatively undisturbed in picrites pillow lavas gabbros and diorites as a group picrites have more depleted initial nd isotopic signatures end 4 23 to 4 97 than pillow lavas gabbros and diorites end 0 30 to 3 04 consistent with a variably depleted heterogeneous mantle source in some areas gabbros include up to 15 cm long white inclusions xenoliths these inclusions are composed primarily 90 of ca rich plagioclase and are interpreted as anorthositic cumulates brought to the surface by upwelling gabbroic magmas the anorthositic cumulates have significantly higher initial end 4 8 to 6 0 values than the surrounding gabbroic matrix 2 3 to 2 8 consistent with different mantle sources for the two rock types

trace element systematics of the neoarchean fiskenaesset anorthosite complex and associated meta volcanic rocks sw greenland evidence for a magmatic arc origin abstract new major and high precision icp ms trace element data on the neoarchean ca 2970 ma layered fiskenaesset complex and associated volcanic rocks southern west greenland provide new constraints on the petrogenesis and geodynamic setting of the complex the complex appears to have been emplaced as multiple sills of magma and crystal mush into oceanic crust tholeiitic basalts and gabbros forming an association of ca 550 m thick anorthosite leucogabbro gabbro and peridotite layers the fiskenaesset complex and the associated volcanic rocks were intruded by neoarchean tonalite trondhjemite and granodiorite ttg sheets during thrusting that was followed by several phases of isoclinal folding despite the intense deformation and amphibolite to granulite facies metamorphism primary cumulate textures and igneous layering are locally well preserved throughout the complex the presence of calcic plagioclase an75 95 and igneous amphibole in anorthosites gabbros and leucogabbros and hornblendite veins in peridotites suggests a hydrous magma source s the major and trace element compositions of tholeiitic basalts amphibolites suggest that they are petrogenetically related to the fiskenaesset complex by fractional crystallization the trace element systematics of the least altered anorthosites gabbros leucogabbros peridotites tholeiitic basalts and calc alkaline high magnesian andesites hma is collectively consistent with a supra subduction zone geodynamic setting on the log transformed tectonic discrimination diagram including la th sm th yb th and nb th ratios tholeiitic basalts display a trend projecting from mid ocean ridge basalt morb field to island arc basalt iab field this trend is interpreted as reflecting a transition from the neoarchean depleted upper mantle to a subarc mantle wedge following the initiation of intra oceanic subduction and arc migration collectively on the basis of field relationships petrographic features and geochemical characteristics the fiskenaesset complex is interpreted as a fragment of a neoarchean oceanic island arc on the basis of ree patterns anorthosites are divisible into four major groups 1 group 1 displays moderately depleted to slightly enriched lree and hree patterns 2 group 2 possesses strongly enriched lree and moderately depleted hree patterns 3 group 3 has strongly enriched lree and depleted hree patterns and 4 group 4 garnet anorthosite exhibits concave upward ree patterns that appear to have resulted from contamination of anorthositic magma by basaltic rocks or magma the different ree patterns in group 1 2 and 3 anorthosites are interpreted to reflect various depths of partial melting and variably enriched in incompatible elements source compositions the majority of anorthosites and leucogabbros were derived from depths above the garnet stability field

the theoretical effect of metasomatism on sm nd isotopic systems abstract if sm and nd are mobilized during fluid rock interaction fractionation of the ratio between sm and nd in geological units may take place the fractionation can be characterized by a fractionation factor f which can attain any value between d sm d nd and 1 d sm d nd depending on the fluid rock ratio during ree transfer if such fractionations are not recognized this will induce errors in sm nd isotopic interpretations of age and provenance of the affected geological units the apparent ages and nd values are functions of the true values f and the time lapse between petrogenesis and secondary disturbance the functional relationships can be closely approximated by direct proportionalities

the neoarchaean storo supracrustal belt nuuk region southern west greenland an arc related basin with continent derived sedimentation we present new major and trace element data as well as sm nd isotope compositions for archaean supracrustal rocks and a gabbro anorthosite complex from the island of storo in godthabsfjord southern west greenland we also provide new u pb isotope data for zircon extracted from these rocks the storo rocks have experienced amphibolite facies metamorphism and the entire sequence forms an east dipping frontal thrust ramp in tectonic contact with mesoarchaean nuk gneisses to the west and eoarchaean itsaq gneisses to the east these orthogneisses record a complex regional accretion history as established by previous work the present study aims at explaining the geochemical features of the storo supracrustal belt ssb which comprises metavolcanic and metasedimentary rocks that are situated within this collisional zone the ssb has a maximum age of ca 2800 ma constrained by the youngest detrital zircon population found in a metasedimentary unit the minimum age of the ssb is 2707 8 ma defined by previously published re os isotope data the metavolcanic rocks have a tholeiitic basaltic composition with relatively flat primitive mantle normalised trace element patterns and generally negative nb anomalies nb nb 0 30 0 90 they plot above the mantle array in th yb nb yb space consistent with a subduction zone affinity as also proposed by previous studies a thin fault contact separates the ssb from a gabbro anorthosite complex formally defined here as the storo anorthosite complex which has an age of ca 3050 ma the sm nd isotope data of the ssb suggest that these metavolcanic rocks experienced contamination by a crustal source that was isotopically similar to the storo anorthosite complex sac this in turn suggests that the sac could have formed the basement for the younger volcanic sequence of the ssb or alternatively that the mantle source of the ssb was contaminated by melts derived from sac aged crust or sediments the metasedimentary rocks of ssb show a mixed mafic felsic component with variable degrees of maturity highly mature metasediments of the ssb contain several age populations of regionally well known magmatic events and thus support a significant local crustal provenance furthermore the youngest documented detrital zircon is found in a thin metasedimentary unit within the metavolcanic rocks and thereby shows that the ssb formed in close proximity to subaerially exposed continental crust i e a back arc environment and not a distal island arc setting as previously proposed finally this suggests that relatively cool lower continental crust which was capable of supporting subaerial mountains existed at least locally during the meso neoarchaean

a morb arc basalt adakite association in the 2 5 ga wutai greenstone belt late archean magmatism and crustal growth in the north china craton abstract the wutai fuping and hengshan complexes form part of a major 2 5 ga late archean arc system in the northern part of the north china craton the wutai complex is a well preserved greenstone belt dominated by mafic intermediate and felsic volcanic rocks with a mid ocean ridge basalt morb arc back arc basalt adakite association multi element plots for morb type mafic volcanic rocks are characterised by depletion of th nb ta and la nearly flat or lree depleted ree patterns and nb u and nb th ratios greater than primitive mantle values back arc basin basalt babb type rocks have nearly flat trace element patterns with nb ti and y troughs and lree depleted or near flat ree patterns island arc basalt iab type rocks are characterized by negative anomalies of nb ta zr hf ti and y and by lree enrichment the felsic volcanic rocks conform to most compositional criteria for cenozoic adakites including low y and yb high la smn la yb and zr sm ratios and similar trace element and ree patterns trends defined by the association are consistent with magma mixing between morb like melts and subduction slab related melts intra oceanic subduction coupled with contemporary morb type mantle upwelling related to back arc basin extension can account for the interaction between the two components subduction accretion or back arc basin collapse led to accretion of the wutai intra oceanic arc back arc complexes with the magmatic arcs of the fuping and hengshan terranes that are dominated by high grade tonalite trondhjemite granodiorite ttg gneisses this geodynamic interpretation is different from many tectonic models for archean greenstone belts in which magmatism and crustal growth were dominated by mantle plumes or interaction between subduction zones and mantle plumes the final disposition of the arc complexes was controlled by the collision of the eastern and western blocks of the north china craton during the paleoproterozoic resulting in extensive tectonic interleaving within a major orogenic belt that extended throughout the central portion of the north china craton

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

sedimentary record of arc continent collision along mesozoic sw north america siuna belt nicaragua the western margin of the caribbean plate is a typical example where oceanic and continental terranes have amalgamated by subduction collision and strike slip processes the boundaries between these blocks as well as their tectonostratigraphic records are generally covered by younger deposits and dense tropical vegetation which may hamper reconstructing the accretionary evolution of the convergent margin in that context the study of overlap sedimentary assemblages represents an important tool to constrain the accretion timing of terranes in northern central america the geology of the suture zone between the chortis block and the exotic siuna intraoceanic arc indicates that the two terranes were assembled together during a hauterivian arc continent collision approximately 134 131 ma the exotic origin of the intraoceanic arc is based on the nature of metamorphic blocks within and the kinematics of the siuna serpentinite melange the short duration of the collision event is suggested by coeval exhumation of the siuna serpentinite melange and chortis derived coarse sedimentation el amparo formation along the suture zone rapidly followed by onset of pelagic sedimentation rio matis formation although the collision appears to have been short lived and preserved only in the suture zone postcollisional extension affected intra arc to back arc settings of the chortis block and led to the formation of kilometer thick extensional basins we envisage that the convergent margin inboard of sw mexico represented by the fringing guerrero intraoceanic arc and the mixteca continental terrane underwent similar postcollisional extension whereas the western margin of the proto caribbean oceanic realm experienced onset of wsw dipping subduction beneath the accreted siuna intraoceanic arc

pb u and th diffusion in natural zircon zircon zrsio4 is one of the most widely used minerals for determining the age origin and thermal history of rocks by u th pb geochronology but the parameters describing the solid state volume diffusion rates of these elements in natural zircon which are themselves important for establishing the limits on the applicability of zircon for geochronological studies have remained poorly quantified1 this is because of the measurement difficulties associated with the low p p m concentrations and low diffusion rates of these elements in natural zircon and the chemical and physical heterogeneity present in most crystals here we present direct measurements of the uranium thorium and lead loss from a thermally treated gem quality natural zircon and show that lead diffuses much faster than uranium or thorium we find that the u th pb isotopic system in natural zircon will typically have a closure temperature greater than 900 c which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high grade metamorphism and partial melting of the host rock

crustal structure and transform margin development south of svalbard based on ocean bottom seismometer data abstract the barents sea is located in the northwestern corner of the eurasian continent where the crustal terrain was assembled in the caledonian orogeny during late ordovician and silurian times the western barents sea margin developed primarily as a transform margin during the early tertiary in the northwestern part south of svalbard multichannel reflection seismic lines have poor resolution below the permian sequence and the early post orogenic development is not well known here in 1998 an ocean bottom seismometer obs survey was collected southwest to southeast of the svalbard archipelago one profile was shot across the continental transform margin south of svalbard which is presented here p wave modeling of the obs profile indicates a caledonian suture in the continental basement south of svalbard also proposed previously based on a deep seismic reflection line coincident with the obs profile the suture zone is associated with a small crustal root and westward dipping mantle reflectivity and it marks a boundary between two different crystalline basement terrains the western terrain has low 6 2 6 45 km s 1 p wave velocities while the eastern has higher 6 3 6 9 km s 1 velocities gravity modeling agrees with this as an increased density is needed in the eastern block the s wave data predict a quartz rich lithology compatible with felsic gneiss to granite within and west of the suture zone and an intermediate lithological composition to the east a geological model assuming westward dipping caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust as well as the observed structuring due to the transform margin setting the tectonic thinning of the continental block during opening of the norwegian greenland sea is restricted to the outer 35 km of the continental block and the continent ocean boundary cob can be located to within 5 km in our data distinct from the outer high commonly observed on transform margins the upper part of the continental crust at the margin is dominated by two large rotated down faulted blocks with throws of 2 3 km on each fault apparently formed during the transform margin development analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile and that the downfaulting dominates the whole margin segment from spitsbergen to bjornoya south of bjornoya the faulting leaves the continental margin to terminate as a graben 75 km south of the island adjacent to the continental margin there is no clear oceanic layer 2 seismic signature however the top basement velocity of 6 55 km s 1 is significantly lower than the high 7 km s 1 velocity reported earlier from expanding spread profiles esps and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7 8 km thick sedimentary overburden

recycling of heterogeneous material in the subduction factory evidence from the sedimentary melange of the internal ligurian units italy in the northern apennine italy the internal ligurian units consist of middle late jurassic ophiolites covered by thick sedimentary deposits whose top is represented by the early paleocene bocco shale this formation is characterized by mass transport deposits interlayered with thin bedded siliciclastic turbidites the sedimentological and structural features of these mass transport deposits reveal a long lived history of recycling of heterogeneous material in a subduction setting this history started with the frontal accretion of a fragment of oceanic crust into an accretionary prism whose lower slope was subsequently affected by tectonic erosion and consequent instability leading to the production of mass transport deposits and the transfer of material to the lower plate these mass transport deposits were subsequently underthrust and then again transferred to the base of the accretionary prism by coherent underplating before their exhumation to the surface the bocco shale is thus representative of a subduction setting where both accretionary and erosive events occurred depending on changing boundary conditions the reconstructed history for the bocco shale indicates that the sedimentary and gravitational processes both at the prism front and on the prism slope possibly induced by alternating accretion and erosion events are the most efficient mechanisms of lithological mixing and recycling in subduction margins

dacitic ocelli in mafic lavas 3 8 3 7 ga isua greenstone belt west greenland geochemical evidence for partial melting of oceanic crust and magma mixing abstract mafic volcanic rocks in the 3 8 3 7 ga isua greenstone belt southern west greenland contain randomly distributed 1 to 10 centimeters long white spheroidal structures in this study these structures are called ocelli in the western part of the belt ocelli bearing lavas are enclosed in basaltic to picritic flows mgo 9 21 wt with a subduction zone geochemical signature the ocelli are composed predominantly of polycrystalline na plagioclase and quartz with minor hornblende and biotite whereas the surrounding amphibolite matrix basaltic host is composed mainly of hornblende ca plagioclase and quartz the ocelli are devoid of radial or concentric internal structure and display all stages of coalescence contacts between the ocelli and surrounding amphibolite matrix are sharp to gradational compositionally the ocelli are calc alkaline dacites sio2 62 9 72 0 wt mgo 0 60 3 50 wt ni 58 143 ppm cr 250 510 ppm whereas the surrounding matrix is tholeiitic basalt sio2 46 6 50 6 wt mgo 8 70 12 30 wt ni 119 175 ppm cr 330 600 ppm in terms of major element composition the isua ocelli closely resemble plagiogranites in phanerozoic supra subduction zone ophiolites field and petrographic observations and geochemical data sio2 54 2 60 7 wt mgo 3 95 7 72 wt ni 127 158 ppm cr 500 570 ppm on the transitional areas between the ocelli and the matrix suggest magma mixing between dacitic and basaltic melts on a chondrite normalized diagram the basaltic host is characterized by variably depleted lree patterns la smcn 0 30 0 94 gd ybcn 1 03 1 45 whereas the dacitic ocelli display lree enriched patterns la smcn 1 30 2 60 gd ybcn 1 32 2 58 the strongly depleted ree patterns in the basaltic host are attributed to lree loss during carbonate alteration partial melting of a forearc mantle wedge is favoured for the origin of the protolith of the basaltic host the geochemical characteristic of the ocelli cannot be explained by post magmatic alteration slab melting fractional crystallization of tholeiitic melts or liquid immiscibility we suggest that the dacitic ocelli might have been derived from hydrous melting of the fragments of oceanic crust high mg volcanic rocks that fell into the magma chamber suggesting magma crust interaction in the early earth formation of dacitic volcanic rocks by partial melting of altered oceanic crust may have played an important role in the generation of felsic crust in the early archean

abandonment of the south penninic austroalpine palaeosubduction zone central alps and shift from subduction erosion to accretion constraints from rb sr geochronology abstract we present new age data for the evolution of the suture zone between lower plate south penninic and upper plate austroalpine units in the central european alps rb sr deformation ages for mylonitized rocks of the south penninic palaeosubduction melange and for deformed austroalpine basement eastern switzerland shed light on the pre alpine and alpine deformation history along the suture as well as on synsubduction interplate mass transfer rb sr age data define two age groups the first group reflects pre alpine events within the upper plate basement with varying degree of resetting by subsequent alpine overprints the second group marks the waning of subduction related deformation along the south penninic austroalpine suture zone at around 50 ma and termination at c 47 ma identical rb sr ages for pervasively deformed austroalpine and south penninic lithologies point to tectonic erosion of the upper plate during subduction we propose that underplating of the middle penninic micro continent at c 50 ma led to the cessation of deformation within the south penninic melange and shifted the zone of active deformation into the footwall this also caused a contemporaneous upper plate surface uplift and shutoff of sedimentation in alpine gosau type forearc basins

hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago earth s lithosphere probably experienced an evolution towards the modern plate tectonic regime owing to secular changes in mantle temperature radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time with some estimates suggesting that over 70 of the present continental crustal reservoir was extracted by the end of the archaean eon patterns of crustal growth and reworking in rocks younger than three billion years gyr are thought to reflect the assembly and break up of supercontinents by wilson cycle processes and mark an important change in lithosphere dynamics in southern west greenland numerous studies have however argued for subduction settings and crust growth by arc accretion back to 3 8 gyr ago suggesting that modern day tectonic regimes operated during the formation of the earliest crustal rock record here we report in situ uranium lead hafnium and oxygen isotope data from zircons of basement rocks in southern west greenland across the critical time period during which modern like tectonic regimes could have initiated our data show pronounced differences in the hafnium isotope time patterns across this interval requiring changes in the characteristics of the magmatic protolith the observations suggest that 3 9 3 5 gyr old rocks differentiated from a 3 9 gyr old source reservoir with a chondritic to slightly depleted hafnium isotope composition in contrast rocks formed after 3 2 gyr ago register the first additions of juvenile depleted material that is new mantle derived crust since 3 9 gyr ago and are characterized by striking shifts in hafnium isotope ratios similar to those shown by phanerozoic subduction related orogens these data suggest a transitional period 3 5 3 2 gyr ago from an ancient 3 9 3 5 gyr old crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3 2 gyr ago that involved juvenile crust generation by plate tectonic processes

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

meso and neoarchaean geological history of the bjornesund and ravns storo supracrustal belts southern west greenland settings for gold enrichment and corundum formation to understand the mesa and neoarchaean geological history of the bjornesund region southern west greenland ca 62 63 degrees n we have undertaken an integrated study combining field observations with u pb zircon geochronology by la icp ms metamorphic petrology and microstructural analyses the bjornesund region links the southern part of the fiskenaesset anorthosite complex to the well preserved metavolcanic rocks of the ravn story supracrustal belt and the bjornesund supracrustal belt so the development of this region is of key interest to unravelling the archaean geology of the entire region in this study we couple the observed phases of gold enrichment and corundum formation to the geological history the leucogabbroic rocks of the bjornesund region which are part of the fiskenaesset complex intruded at ca 2 95 ga into the tholeiitic and calc alkaline metavolcanic rocks of the two studied belts following isoclinal folding the rocks were intruded by diorite 2 92 ga and granodiorite 2 91 and 2 88 ga which are now texturally orthogneisses the area was affected by three major phases of folding f1 undated f2 at 2 85 2 83 ga and f3 at 2 71 2 70 ga the former deformation event is associated with the gold enrichment in alteration zones in shear folds in metavolcanic amphibolite near the contact with the orthogneiss intrusion of granitic sheets during the f3 deformation triggered the formation of corundum which is locally of gem quality at the contact between leucogabbroic and ultramafic rocks peak metamorphic conditions were calculated as 580 630 degrees c and 6 4 kbar at 2 85 2 83 ga later retrogression in the southern part of the area at ca 500 450 degrees c and 4 5 3 kbar and extensional fault reactivation overprinted some of the older structures c 2014 elsevier b v all rights reserved less

melt composition control of zr hf fractionation in magmatic processes zircon zrsio4 and hafnon hfsio4 solubilities in water saturated granitic melts have been determined as a function of melt composition at 800 and 1035 c at 200 mpa the solubilities of zircon and hafnon in metaluminous or peraluminous melts are orders of magnitude lower than in strongly peralkaline melt moreover the molar ratio of zircon and hafnon solubility is a function of melt composition although the solubilities are nearly identical in peralkaline melts zircon on a molar basis is up to five times more soluble than hafnon in peraluminous melts accordingly calculated partition coefficients of zr and hf between zircon and melt are nearly equal for the peralkaline melts whereas for metaluminous and peraluminous melts dhf dzr for zircon is 0 5 to 0 2 consequently zircon fractionation will strongly decrease zr hf in some granites whereas it has little effect on the zr hf ratio in alkaline melts or similar depolymerized melt compositions the ratio of the molar solubilities of zircon and hafnon for a given melt composition temperature and pressure is proportional to the hf zr activity coefficient ratio in the melt the data imply that this ratio is nearly constant and probably close to unity for a wide range of peralkaline and similar depolymerized melts however it changes by a factor of two to five over a relatively small interval of melt compositions when a nearly fully polymerized melt structure is approached for most ferromagnesian minerals in equilibrium with a depolymerized melt dhf dzr typical values of dhf dzr range from 1 5 to 2 5 for clinopyroxene amphibole and titanite because of the change in the hf zr activity ratio in the melt the relative fractionation of zr and hf by these minerals will disappear or even be reversed when the melt composition approaches that of a metaluminous or peraluminous granite it is thus not surprising that fractional crystallization of such granitic magmas leads to a decrease in zr hf whereas fractional crystallization of depolymerized melts tends to increase zr hf there is no need to invoke fluid metasomatism to explain these effects results demonstrate that for ions with identical charge and nearly identical radius crystal chemistry does not alone determine relative compatibilities rather the effect of changing activity coefficients in the melt may be comparable to or even larger than elastic strain effects in the crystal lattice

devonian permian magmatic pulses in the northern vosges mountains ne france result of continuous subduction of the rhenohercynian ocean and avalonian passive margin

trace element composition and u pb age of zircons from estherville constraints on the timing of the metal silicate mixing event on the mesosiderite parent body abstract mesosiderites are a group of stony iron meteorites which are thought to be the result of mixing of silicates with fe ni metal in this study we combined textural observations with geochemical and chronological studies of two zircon grains found in the estherville mesosiderite one of the zircons zrc1 occurs with pyroxene plagioclase troilite and silica and the other zrc2 is located at a boundary between fe ni metal and a silicate part mainly composed of pyroxene and plagioclase the textural observations demonstrate that zrc1 is relatively homogenous whereas zrc2 is composed of at least two chemically distinct domains trace element analyses of zrc2 resolve large concentration gradients within this single grain with variations that are an order of magnitude for rare earth elements ree and two orders of magnitude for u and th the lowest trace element concentration in zrc2 is more than an order of magnitude lower than those of lunar and eucritic zircons however it is similar to those of zrc1 and a zircon from the vaca muerta mesosiderite the calculated ree composition of the melt in equilibrium with zrc2 shows that zrc2 and perhaps also zrc1 did not crystallize from a melt that was produced by fractional crystallization of the primary magmatic mineral assemblages the zircons with low ree u and th concentrations can be interpreted to have formed in a residual melt after incorporation of large amounts of ree u and th into secondary phosphate minerals which formed during the metal silicate mixing event the large concentration gradients observed in zrc2 suggest significant heterogeneities in the melt from which the zircon crystallized alternatively either mixing or diffusion between a relict zircon and a newly formed zircon could explain the observed concentration gradients however the ree patterns of zrc2 cannot be explained by mixing or diffusion between the two distinct generations of zircons these considerations suggest that zrc1 and zrc2 formed during a high temperature reheating event which is probably related to the metal silicate mixing event the weighted average 207 pb 206 pb age obtained by sims from both zircons is 4521 26 ma 2 this age is younger than that of a primary magmatic zircon from vaca muerta 4563 15 ma and probably corresponds to the timing of the metal silicate mixing event or a later impact event

81kr kr age and multiple cosmic ray exposure history of the vaca muerta mesosiderite abstract noble gas isotopic compositions were measured for a eucritic pebble and bulk material of a silicate metal mixture from the vaca muerta mesosiderite as well as pyroxene and plagioclase separated from the eucritic pebble by total melting and stepwise heating methods trapped noble gases were degassed completely by a high temperature thermal event probably at the formation of the vaca muerta parent body vmpb the presence of fissiogenic xe isotopes from extinct 244pu in the bulk samples might be a result of rapid cooling from an early high temperature metamorphism high concentrations of cosmogenic noble gases enabled us to determine precise isotopic ratios of cosmogenic kr and xe spallogenic ne from na and unique ar isotopic compositions were observed the 81kr kr exposure age of 168 8 myr for the silicate pebble is distinctly longer than the age of 139 8 myr for the bulk samples the precursor of the pebble had been irradiated on the surface of the vmpb for more than 60 myr first stage irradiation with subsequent incorporation into bulk materials approximately 4 gyr ago the vaca muerta meteorite was excavated from the vmpb 140 myr ago second stage irradiation relative diffusion rates among the cosmogenic ar kr and xe based on data obtained by stepwise heating indicate that kr and xe can be partially retained in pyroxene and plagioclase under the condition that resets the k ar system this result supports the presence of fission xe and of excess concentration of cosmogenic kr which could have survived the thermal event approximately 3 8 gyr ago

arc generated blocks with crustal sections in the north atlantic craton of west greenland crustal growth in the archean with modern analogues abstract the ca 700 km long archean craton of west greenland consists of six meso neoarchean ca 3000 2720 ma shear zone bounded crustal blocks that display similar cross sections from south to north ivittuut kvanefjord bjornesund sermilik fiskefjord maniitsoq each block has a southerly upper and a northerly lower zone thus each faces upwards to the south upper zones have prograde amphibolite facies mineralogy and have never been in the granulite facies whereas lower zones reached granulite facies and were partly retrogressed to amphibolite facies upper and lower zones consist predominantly of tonalite trondhjemite granodiorite ttg orthogneisses geochemistry suggests generation by slab melting in subduction settings of island arcs and active continental margins the gneisses contain km thick metavolcanic amphibolite layers typically bordered by km thick layers containing anorthosite and leucogabbro most upper zones contain upper greenschist to amphibolite facies metavolcanic belts including volcaniclastic andesitic rocks the two most prominent metavolcanic belts in the fiskefjord block at qussuk andesitic volcaniclastic rocks garde a a 2007 a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland journal of the geological society london 164 565 579 and ivisaartoq mafic ultramafic rocks and anorthosite leucogabbro from upper and lower parts of a supra subduction zone system polat a frei r appel p w u dilek y fryer b ordonez calderon j c yang z 2008 the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland lithos 100 293 321 have island arc geochemical signatures the 2 km thick fiskenaesset complex bjornesund block comprises chromite layered anorthosites leucogabbros and gabbros and local pillow bearing roof pendants from overlying metavolcanic amphibolite the style of deformation changes downwards within crustal blocks upper zones are characterised by linear metavolcanic belts deformed by mostly one major phase of isoclinal folding and lower zones by kilometre scale double triple fold interference patterns everywhere ttg protoliths have intruded anorthositic and volcanic rocks typically along ductile shear zones often so extensively that only anorthositic or amphibolitic lenses are preserved the meso neoarchean crust was thickened by a combination of thrusting isoclinal folding and continued ttg injection dissimilarities in the proportions of anorthositic and metavolcanic rocks in the six blocks suggest that they evolved in several different microcontinents but by similar processes these crustal blocks provide an exceptional example of how continents evolved in the meso neoarchean comparable archean examples in kapuskasing and pikwitonei canada and modern analogues in fiordland new zealand kohistan himalayas southern california batholith peruvian andes and hidaka japan demonstrate that processes of continental growth from island arc to continental arc magmatism and by implication to continental collision were broadly similar throughout most of earth history

differential preservation in the geologic record of intraoceanic arc sedimentary and tectonic processes abstract records of ancient intraoceanic arc activity now preserved in continental suture zones are commonly used to reconstruct paleogeography and plate motion and to understand how continental crust is formed recycled and maintained through time however interpreting tectonic and sedimentary records from ancient terranes after arc continent collision is complicated by preferential preservation of evidence for some arc processes and loss of evidence for others in this synthesis we examine what is lost and what is preserved in the translation from modern processes to the ancient record of intraoceanic arcs composition of accreted arc terranes differs as a function of arc continent collision geometry forward facing collision can accrete an oceanic arc on to either a passive or an active continental margin with the arc facing the continent and colliding trench and forearc side first in a backward facing collision involving two subduction zones with similar polarity the arc collides backarc first with an active continental margin the preservation of evidence for contemporary sedimentary and tectonic arc processes in the geologic record depends greatly on how well the various parts of the arc survive collision and orogeny in each case preservation of arc terranes likely is biased towards those that were in a state of tectonic accretion for tens of millions of years before collision rather than tectonic erosion the prevalence of tectonic erosion in modern intraoceanic arcs implies that valuable records of arc processes are commonly destroyed even before the arc collides with a continent arc systems are most likely to undergo tectonic accretion shortly before forward facing collision with a continent and thus most forearc and accretionary prism material in ancient arc terranes likely is temporally biased toward the final stages of arc activity when sediment flux to the trench was greatest and tectonic accretion prevailed collision geometry and tectonic erosion vs accretion are important controls on the ultimate survival of material from the trench forearc arc massif intra arc basins and backarc basins and thus on how well an ancient arc terrane preserves evidence for tectonic processes such as subduction of aseismic ridges and seamounts oblique plate convergence and arc rifting forward facing collision involves substantial recycling melting and fractionation of continent derived material during and after collision and so produces melts rich in silica and incompatible trace elements as a result forward facing collision can drive the composition of accreted arc crust toward that of average continental crust

u pb zircon study of tectonically bounded blocks of 2940 2840 ma crust with different metamorphic histories paamiut region south west greenland implications for the tectonic assembly of the north atlantic craton abstract new fieldwork map interpretation petrography and single zircon u pb geochronology has allowed the identification of different crustal blocks in the paamiut region in the southern portion of the west greenland archaean craton changes of metamorphic grade from only amphibolite facies to granulite facies some subsequently retrogressed corresponds with zones of archaean high strain ductile deformation mylonites u pb zircon dates are presented for the ttg tonalite trondhjemite granodiorite protoliths from each block in the paamiut region and the southern portion of the previously identified tasiusarsuaq terrane lying to the north the southern part of the tasiusarsuaq terrane contains 2880 2860 ma ttg rocks and underwent amphibolite facies metamorphism structurally underneath the tasiusarsuaq terrane to the south is the sioraq block containing 2870 2830 ma ttg rocks partly retrogressed from granulite facies structurally underneath and to the south is the paamiut block dominated by 2850 2770 ma granodioritic rocks that have only undergone amphibolite facies metamorphism also structurally overlying the paamiut block but cropping out separately from the sioraq block is the neria block this appears to be dominated by 2940 2920 ma gneisses that have been totally retrogressed from granulite facies and strongly deformed in the southernmost part of the region the neria block overlies the greenschist to lowermost amphibolite facies sermiligaarsuk block that contains the 2945 ma tartoq group rocks from all the blocks record ancient loss of pb from zircons and some new zircon growth at 2820 ma interpreted to indicate a high grade metamorphic event at that time including granulite facies metamorphism in the sioraq and neria blocks the blocks of different metamorphic grade are interpreted to have moved to their current positions after the 2820 ma metamorphism explaining the change in metamorphic history across some mylonites and ductile shear zones which deform and retrogress granulite facies textures the juxtaposed blocks and their contacts were subsequently folded under amphibolite facies conditions the contacts are cut by undeformed palaeoproterozoic dolerite dykes which post date amphibolite facies metamorphism these results together with previously published data from the godthabsfjord region north of paamiut shows that the north atlantic craton in west greenland from ivittuut in the south to maniitsoq in the north 550 km consists of a mosaic of ductile fault bounded packages that attained their present relative positions in the late archaean

orogenic wedge growth during collision constraints on mechanics of a fossil wedge from its kinematic record rhenohercynian ftb central europe abstract the kinematic evolution of the rhenohercynian fold and thrust belt of the central european variscides can be reconstructed from incremental strain and palaeostress data synkinematic metamorphism geochronology synorogenic sediments and the geometry of shortening this data set allows to infer the mechanical development with respect to the mohr coloumb theory of critically tapered orogenic wedges growth of the rhenohercynian orogenic wedge was kinematically partitioned into a broad orogen normal moving and a narrow obliquely moving domain the ratio of the final widths of these domains varies from 1 6 to 2 3 this low ratio indicates strong basal coupling growth averages are 14 km ma of shortening and a strain rate of 6 10 16 s 1 the mode of mass transfer from lower to upper plate which controlled the accretion geometry and the particle paths evolved in the long term from dominantly basal to frontal accretion this evolution was accompanied by a decrease in taper from precollisional 18 28 to final 5 8 4 6 accretionary subsystems assemble the wedge each is characterised by a sequence of basal to frontal accretion both rooting in a common detachment the development of the subsystems is supported by the observation of stepwise migration of a foreland bulge ponding flysch subbasins these features imply shorter termed ca 2 ma cyclic mass transfer above a high friction detachment with episodes of basal accretion followed by episodes of a quickly forward propagating thrust front transfer of the basal detachment from ductile mid crustal to brittle conditions via a crustal ramp and particle flow across the ramp provoked large scale adjustment of wedge geometry internal wedge thickening is spatially and chronologically related to the formation of the ramp and was supported by surficial erosion of ca 3 kbar for keeping a critical taper three growth stages of the wedge can be reconstructed the high taper of the precollisional stage plots in the tectonically erosive field in a taper stability diagram then taper decreases to 10 after emplacement of a flysch nappe stack during the final stage of contraction an eastern and a western branch develop with geometries fluctuating around a critically stable state the two branches link with differences in large scale wedge architecture bulk contraction and detachment geometry the latter being controlled by the former basin geometry

volcanism and reef development in the devonian a case study from the lahn syncline rheinisches schiefergebirge germany abstract new data from the rheinisches schiefergebirge in germany offer insights into a complex marine basinal facies setting on the southern shelf area of the old red continent this interdisciplinary approach has the aim of providing a reconstruction of depositional and palaeoecological conditions of volcanic island induced reef growth during the middle devonian time devonian volcanic activity culminated in a givetian frasnian phase producing mainly alkali basaltic to basanitic melts some volcanic buildups reached sea level and gave rise to the development of reefs during times of reduced volcanic activity reef communities in the lahn syncline were dominated by corals and stromatoporoids in terms of conodont stratigraphy they began to flourish during the middle varcus subzone in the givetian and lasted until the late falsiovalis subzone in the frasnian a facies model is presented for the entire area that clarifies the association of volcanism and reef development

two metamorphic gold mineralization events confirmed by lu hf isotope dating of garnet in the late archean storo au deposit nuuk region of sw greenland abstract we report lu hf isotope ages for garnet from the late archean storo supracrustal belt ssb ranging from 2697 8 ma to 2646 13 ma these ages essentially overlap with two previously reported re os isotope ages for arsenopyrite associated with au mineralization within the ssb which implies that both au forming events took place while garnet was stable and thus occurred during amphibolite facies metamorphic conditions furthermore we report bulk rock lu hf isotope data for amphibolite and aluminous gneiss from the ssb the latter rock type has previously been suggested to represent the pre metamorphic hydrothermal alteration product associated with low temperature epithermal au formation however the initial hf isotope compositions of these two rock types are significantly different and therefore demonstrate that they do not share a common origin because the lu hf isotope system is essentially fluid immobile instead the aluminous gneiss represents a pelitic rock with mixed mafic and felsic sources which is in agreement with the formation of both au mineralization events during orogenic amphibolite facies metamorphic processes which were therefore structurally controlled a too young bulk rock errorchron age of 2553 72 ma for the amphibolite unit 2720 ma is interpreted to reflect resetting of the system during the intrusion and melt infiltration of the ca 2560 ma qorqut granite complex

trace element zoning and incipient metamictization in a lunar zircon application of three microprobe techniques we have determined major si zr hf minor al y fe p and trace element ca sc ti ba ree th u concentrations and raman spectra of a zoned 200 microns zircon grain in lunar sample 14161 7069 a quartz monzodiorite breccia collected at the apollo 14 site analyses were obtained on a thin section in situ with an ion microprobe an electron microprobe and a laser raman microprobe the zircon grain is optically zoned in birefringence a reflection of variable incomplete metamictization resulting from zo nation in u and th concentrations variations in the concentrations of u and th correlate strongly with those of other high field strength trace elements and with changes in raman spectral parameters concentrations of u and th range from 21 to 55 ppm and 6 to 31 ppm respectively and correlate with lower raman peak intensities wider raman peaks and shifted si o peak positions concentrations of heavy rare earth elements range over a factor of three to four and correlate with intensities of fluorescence peaks correlated variations in trace element concentrations reflect the original magmatic differentiation of the parental melt approx 4 b y ago degradation of the zircon structure as reflected by the observed raman spectral parameters has occurred in this sample over a range of alpha decay event dose from approx 5 2 x 10 exp 14 to 1 4 x 10 exp 15 decay events per milligram of zircon as calculated from the u and th concentrations this dose is well below the approx 10 exp 16 events per milligram cumulative dose that causes complete metamictization and indicates that laser raman microprobe spectroscopy is an analytical technique that is very sensitive to the radiation induced damage in zircon

petrogenesis of neoarchaean volcanic rocks of the macquoid supracrustal belt a back arc setting for the northwestern hearne subdomain western churchill province canada abstract rocks exposed in the macquoid gibson lakes region northwest hearne subdomain western churchill province canada comprise three major lithotectonic assemblages the principal volcanic belt the metasedimentary macquoid homocline and the cross bay plutonic complex neoarchaean supracrustal rocks of the belt range in age from 2700 ma crust likely did not occur ndt 2680 ma values for three specimens one from each of the arc like suites and one babb like basalt are slightly lower than the remainder suggesting very minor incorporation of slightly older crust these features imply that the processes that generated the macquoid supracrustal belt required simultaneous tapping of geochemically distinct mantle reservoirs with concomitant anatexis of sialic crust garnet stability field and fractionation of felsic magmas in upper crustal magma chambers shallow water deposition of abundant volcaniclastic rocks and semipelite along with minor conglomerate and quartzite was broadly contemporaneous with this magmatism we envisage a geodynamic setting characterized by tectonomagmatic processes similar to those of modern supra subduction zone back arc marginal basins such as the sea of japan therein an extensional back arc setting likely proximal to continental crust provides an explanation for a broad swath of diverse mantle derived rocks intercalated with less common felsic rocks as well as an abundance of immature clastic metasedimentary rocks

exposed plate interface in the european alps reveals fabric styles and gradients related to an ancient seismogenic coupling zone 1 we present observations from a continuous exposure of an ancient plate interface in the depth range of its former seismogenic zone in the central alps of europe related to late cretaceous early tertiary subduction and accretion of the south penninic lower plate underneath the adriatic upper plate the material forming the exposed plate interface zone has experienced flow and fracturing over an extended period of time followed by syncollisional exhumation thus reflecting a multistage evolution fabric formation and metamorphism however chiefly record the deformation conditions of the precollisional setting along the plate interface we identify an unstable slip domain from pseudotachylytes occurring in the temperature range between 200 and 300 c this zone coincides with a domain of intense veining in the subduction melange with mineral growth into open cavities indicating fast possibly seismic rupture evidence for transient near lithostatic fluid pressure as well as brittle fractures competing with mylonitic shear zones continues into the region below the occurrence of pseudotachylytes possibly reflecting a zone of conditionally stable slip the zone above the unstable slip area is devoid of veins but displays ample evidence of fluid assisted processes similar to the deeper zone solution precipitation creep and dehydration reactions in the melange matrix hydration and sealing of the base of the upper plate seismic rupture here is possibly expressed by ubiquitous localized deformation zones we hypothesize that trenchward sealing of parts of the plate interface as well as reaction enhanced destruction of upper plate permeability is an important component localizing the unstable slip zone this relation may result from the competition of the pervasive presumably interseismic pressure solution creep destroying permeability and building elevated fluid pressure until the strength threshold is reached with seismic failure

onset of plate tectonics the further back we look into the geological past the more obscured the view masked by an increasingly fragmentary geological record this has resulted in a controversy on the rates and mechanisms of early continental crust formation and whether plate tectonics the dominant crust forming process over at least the past 2 5 billion years operated the same way or even at all during early earth history 1 2 on page 434 of this issue shirey and richardson 3 shed light on this issue by looking at it from a new angle from the bottom up they investigated the composition of a suite of minerals occurring as inclusions in diamonds dredged up by young kimberlite volcanoes these diamonds derive from great depths 125 to 175 km within the ancient lithospheric mantle keels that underpin the stable continental crustal regions known as cratons because the minerals can be precisely dated they can provide a snapshot of the subcontinental lithospheric mantle sclm composition at the time when the crust was being formed

archaean andesite petrogenesis insights from the graedefjord supracrustal belt southern west greenland we present new whole rock major trace and platinum group element data as well as sm nd and lu hfisotope data for meta volcanic rocks from the mesoarchaean graedefjord supracrustal belt gsb locatedwithin the tasiusarsuaq terrane southern west greenland we also present new in situ zircon u pbisotope data by la icp ms for associated felsic rocks this region has experienced amphibolite to lowergranulite facies metamorphism causing re equilibration of most mineral phases including zircon an intrusive tonalite sheet with a zircon u pb age of 2888 6 8 ma yields a minimum age for the gsb the sm nd and lu hf isotope data do not provide meaningful isochron ages but the isotope compositionsof the mafic rocks are consistent with the ca 2970 ma regional volcanic event which is documented in pre vious studies of the tasiusarsuaq terrane the major and trace element data suggest a significant crustalcontribution in the petrogenesis of andesitic volcanic rocks in the gsb the trace element variation of theseandesitic leucoamphibolites cannot be explained by bulk assimilation fractional crystallisation afc processes involving local basement rather the observed patterns require binary mixing between basalticand felsic end member magmas with between 50 and 80 contributions from the latter depending onthe assumed felsic composition hf isotope constraints point to contamination with pre existing conti nental crust with an age of ca 3250 ma basement gneisses of this age were previously described at twolocalities in the tasiusarsuaq terrane which supports the mixing hypothesis thus the felsic end memberlikely represents melts derived from the local basement ultramafic rocks 18 35 22 80 wt mgo in gsb have platinum group element pge patterns that aresimilar to magmas derived from high degree melting of mantle but they have relatively enriched traceelement patterns we propose that the ultramafic rocks represent arc related picrites or alternativelywere derived by melting of metasomatised sub continental lithospheric mantle overall these new geochemical data from the mesoarchaean graedefjord supracrustal belt and thepetrogenetic mixing model in particular are similar to observations from modern continental subductionzone environments which also require large degrees of mixing with felsic basement melts therefore we propose that the metavolcanic rocks formed in a modern style subduction zone geodynamic setting which due to the hotter archaean mantle conditions allowed for substantial amounts of partial meltingand magma mixing rather than assimilating pre existing continental crust less

crustal forensics in arc magmas abstract the geochemical characteristics of continental crust are present in nearly all arc magmas these characteristics may reflect a specific source process such as fluid fluxing common to both arc magmas and the continental crust and or may reflect the incorporation of continental crust into arc magmas either at source via subducted sediment or via contamination during differentiation resolving the relative mass contributions of juvenile mantle derived material versus that derived from pre existing crust of the upper plate and providing these estimates on an element by element basis is important because 1 we want to constrain crustal growth rates 2 we want to quantitatively track element cycling at convergent margins and 3 we want to determine the origin of economically important elements and compounds traditional geochemical approaches for determining the contributions of various components to arc magmas are particularly successful when applied on a comparative basis studies of suites from multiple magmatic systems along arcs for which differentiation effects can be individually constrained can be used to extrapolate to potential source compositions in the lesser antilles arc for example differentiation trends from individual volcanoes are consistent with open system evolution however such trends do not project back to a common primitive magma composition suggesting that differentiation modifies magmas that were derived from distinct mantle sources we propose that such approaches should now be complemented by petrographically constrained mineral scale isotope and trace element analysis to unravel the contributing components to arc magmas this innovative approach can 1 better constrain true end member compositions by returning wider ranges in geochemical compositions among constituent minerals than is found in whole rocks 2 better determine magmatic evolution processes from core rim isotopic or trace element profiles from the phases contained in magmas and 3 constrain rates of differentiation by applying diffusion controlled timescales to element profiles an example from nguaruhoe volcano new zealand underscores the importance of such a microsampling approach showing that mineral isotopic compositions encompass wide ranges that whole rock isotopic compositions are consequently simply element weighted averages of the heterogeneous crystal cargo and that open system evolution is proved by core rim variations in sr isotope ratios nguaruhoe is just one of many systems examined through microanalytical approaches the overwhelming conclusion of these studies is that crystal cargoes are not truly phenocrystic but are inherited from various sources the implication of this realization is that the interpretation of whole rock isotopic data including the currently popular u series needs careful evaluation in the context of petrographic observations

eoarchean within plate basalts from southwest greenland comment jenner et al 2013 1 reported the occurrence of what they interpret as earth s oldest ocean island basalts oibs on the island of innersuartuut southwest greenland however this interpretation hinges critically on the incompatible trace element contents of the presented rocks compared

new evidence for an old idea geochronological constraints for a paired metamorphic belt in the central european variscides abstract new geochronological data reveal a prolonged tectonothermal evolution of the variscan odenwald spessart basement being part of the mid german crystalline zone in central europe we report the results from i secondary ion mass spectrometry sims u pb dating of zircon rutile and monazite ii sims zircon oxygen isotope analyses iii laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms zircon lu hf isotope analyses and iv la icpms zircon and rutile trace element data for a suite of metamorphic rocks five amphibolite and eclogite facies mafic meta igneous rocks and one granulite facies paragneiss the protoliths of the mafic rocks formed from juvenile as well as depleted mantle sources in distinct tectonic environments at different times magmatism took place at a divergent oceanic margin possibly in a back arc setting at 460 ma in an intraoceanic basin at ca 445 ma and at a continental margin at 329 ma regardless of lithology zircon in eclogite amphibolite and high temperature paragneiss provide almost identical carboniferous ages of 333 7 4 1 ma eclogite 329 1 1 8 to 328 4 8 9 ma amphibolite and 334 0 2 0 ma paragneiss respectively rutile yielded ages of 328 6 4 7 and 321 4 7 0 ma in eclogite and amphibolite and monazite in high temperature paragneiss grew at 330 1 2 4 ma all ages are quoted at the 2 level the data constrain coeval high pressure eclogite and high temperature granulite facies metamorphism of the odenwald spessart basement at ca 330 ma amphibolite facies conditions were attained shortly afterwards the lower plate eclogite formed in a fossil subduction zone and the upper plate high temperature low pressure rocks are the remains of an eroded carboniferous magmatic arc the close proximity of tectonically juxtaposed units of such radically different metamorphic conditions and thermal gradients is characteristic for a paired metamorphic belt sensu miyashiro 1961 thus the odenwald spessart basement represents the first recognised paired metamorphic belt in the european variscides

thermal history of northwest africa 5073 a coarse grained stannern trend eucrite containing cm sized pyroxenes and large zircon grains we report on the bulk chemical composition petrology oxygen isotopic composition trace element composition of silicates and degree of self irradiation damage on zircon grains of the eucrite northwest africa nwa 5073 to constrain its formation and postcrystallization thermal history and to discuss their implications for the geologic history of its parent body this unequilibrated and unbrecciated meteorite is a new member of the rare stannern trend eucrites it is mainly composed of elongated zoned pyroxene phenocrysts up to 1 2 cm plagioclase laths up to 0 3 cm in length and is rich in mesostasis the latter contains zircon grains up to 30 m in diameter metal sulfide tridymite and ca phosphates textural observations and silicate compositions coupled with the occurrence of extraordinary fe rich olivine veins that are restricted to large pyroxene laths indicate that nwa 5073 underwent a complex thermal history this is also supported by the annealed state of zircon grains inferred from raman spectroscopic measurements along with u and th data obtained by electron probe microanalyses

subsidence and extension at a convergent plate margin evidence for subduction erosion off costa rica subduction erosion rather than subduction accretion is proposed as the dominant process currently occurring at the nicoya segment of the convergent plate margin off costa rica based on new results from odp drilling cores and our interpretations of published seismic data we present a tectonic model of subsidence and extension due to tectonic erosion of the forearc wedge and landward migration of the nicoya coastline high seismic velocities in the outermost part of the forearc wedge off the nicoya peninsula below the boss bottom of slope sediment reflector indicate the seaward continuation of the nicoya ophiolite complex into the basement of the forearc wedge odp site 1042 revealed neritic sediment that points to strong subsidence of the forearc basement progradation of the sedimentary succession and landward migration of the coastline tilted block structures are explained by substantial fore arc extension in our interpretation the removal of material from the base of the forearc wedge by the process of basal subduction erosion leads to progressive subsidence and thus to landward migration of the coastline landward shift of the active volcano chain in the same order of magnitude as the coastline migration is consistent with this interpretation

3430 to 3417 ma calc alkaline volcanism in the mcphee dome and kelly belt and growth of the eastern pilbara craton abstract the mcphee dome and kelly belt in the eastern pilbara craton contain well preserved proximal shallow marine dominantly intermediate volcanic successions these have shrimp u pb zircon ages of 3430 3 and 3417 9 ma respectively which are younger than geochemically similar 3 52 and 3 47 to 3 45 ga suites in other eastern pilbara greenstone belts basalts andesites and dacites from the mcphee dome are similar in composition to modern arc rocks whereas dacites and rhyodacites from the kelly belt have geochemical characteristics of archaean tonalite trondhjemite granodiorite ttg magmas the geochemistry of the mcphee dome basalts basaltic andesites and dacites most likely reflects infracrustal fractional crystallization of a mantle derived magma combined with assimilation of older crust or a silicic melt in contrast the kelly belt dacites and rhyodacites were most likely derived by melting of subducted mafic crust the extensional submarine environment of pre 3 4 ga dominantly mafic and intermediate volcanism in the eastern pilbara differs from that of approximately coeval granitoid magmatism the pre 3 4 ga granitoids have the major and trace element features of ttg suites and appear to have been emplaced into already thick crust culminating in core complex formation available data are most consistent with growth of the eastern pilbara craton in convergent tectonic settings involving pulses of extensional shallow marine arc like volcanism between 3 52 and 3 42 ga overlapping with and or punctuated by periods of crustal thickening and the intrusion of complex ttg granitoid batholiths

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

a change in the geodynamics of continental growth 3 billion years ago models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout earth s history the oxygen isotopic composition of zircons for which uranium lead and hafnium isotopic data provide age constraints is a key archive of crustal reworking we identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time growth of continental crust appears to have been a continuous process albeit at variable rates a marked decrease in the rate of crustal growth at 3 billion years ago may be linked to the onset of subduction driven plate tectonics

isua supracrustal belt greenland a vestige of a 3 8 ga suprasubduction zone ophiolite and the implications for archean geology abstract the mafic ultramafic units of the 3 8 ga isua supracrustal belt isb in greenland occur in a two armed arcuate zone eastern and western arms and are grouped into two major tectonostratigraphic units based on their lithological and geochemical characteristics 1 undifferentiated amphibolites ua and 2 garbenschiefer amphibolites ga the ua contains all major lithological units of a typical penrose type complete ophiolite sequence the ga is composed dominantly of volcaniclastic and volcanic rocks commonly found in immature island arcs the available geochemical data from ua and ga show distinct differences between the two units compared with the geochemical evolution of some of the well known phanerozoic ophiolites the pillow lavas and associated dikes of the ua show a compositional range that is similar to typical morb type ligurian ophiolites in the western alps apennines and those displayed by lip type caribbean ophiolites the ga is characterized by island arc tholeiite iat to boninite like rocks and defines a magmatic evolution that is comparable to that of suprasubduction zone ssz ophiolites in the mediterranean region our proposed geodynamic model for the isb suggests that the ua was built by primary to differentiated mantle generated melts during seafloor spreading little to moderately affected by subduction processes and that the iat to boninitic like rocks of the ga formed at a later stage by melting from a strongly subduction affected depleted and hydrated mantle our interpretation of the isb is that the ua and ga represent early and late stages respectively in the formation of a ssz ophiolite this implies that phanerozoic type plate tectonic processes such as seafloor spreading and subduction were operating by 3 8 ga in the palaeoarchean

formation of the neoarchean bad vermilion lake anorthosite complex and spatially associated granitic rocks at a convergent plate margin superior province western ontario canada abstract the bad vermilion lake anorthosite complex henceforth the bvla complex in western ontario is one of the well exposed anorthosite bearing archean layered intrusions in the superior province canada this study presents new whole rock major and trace element data for the various units of the complex oxygen isotope data for the anorthosite and major and trace element data for the spatially associated granitic rocks intruding the bvla complex to constrain their petrogenetic and geodynamic origin zircons from granitic rocks have yielded a 207pb 206pb age of 2716 18 ma constraining the minimum intrusion age of the complex despite deformation and greenschist facies metamorphism primary igneous textures are locally well preserved in the bvla complex its whole rock major and trace elemental compositions and the oxygen isotopic systematics appear not to have been substantially modified by deformation and metamorphism mantle like oxygen isotope signatures and major and trace element compositions are inconsistent with significant crustal contamination of the bvla complex during its emplacement the existence of primary calcic igneous plagioclase coherent negative nb anomalies nb nb 0 08 0 88 and geochemical similarities between gabbros from the bvla complex and gabbros from cenozoic arcs collectively suggest an intra oceanic subduction zone geodynamic setting for the complex near flat ree patterns in the various units of the bvla complex suggest that they were derived from melting of a shallow source beneath a subarc mantle wedge trends in immobile major e g mgo and trace e g zr element data indicate that the mineralogical composition of the complex can be explained by fractional crystallization and accumulation of olivine orthopyroxene clinopyroxene plagioclase and possibly amphibole compositionally the bordering granitic rocks are a2 type and strongly enriched in th and ree 100 times chondrite and depleted of ba sr eu and ti we suggest that they formed in a post collisional extensional tectonic regime following emplacement of the bvla complex in an oceanic arc

statistical empirical index of chemical weathering in igneous rocks a new tool for evaluating the degree of weathering abstract chemical weathering indices are useful tools in characterizing weathering profiles and determining the extent of weathering however the predictive performance of the conventional indices is critically dependent on the composition of the unweathered parent rock to overcome this limitation the present paper introduces an alternative statistical empirical index of chemical weathering that is extracted by the principal component analysis pca of a large dataset derived from unweathered igneous rocks and their weathering profiles the pca analysis yields two principal components pc1 and pc2 which capture 39 23 and 35 17 of total variability respectively the extent of weathering is reflected by variation along pc1 primarily due to the loss of na 2 o and cao during weathering in contrast pc2 is the direction along which the projections of unweathered felsic intermediate and mafic igneous rocks appear to be best discriminated therefore pc1 and pc2 represent independent latent variables that correspond to the extent of weathering and the chemistry of the unweathered parent rock subsequently pc1 and pc2 were then mapped onto a ternary diagram mfw diagram the m and f vertices characterize mafic and felsic rock source respectively while the w vertex identifies the degree of weathering of these sources independent of the chemistry of the unweathered parent rock the w index has a number of significant properties that are not found in conventional weathering indices first the w index is sensitive to chemical changes that occur during weathering because it is based on eight major oxides whereas most conventional indices are defined by between two and four oxides second the w index provides robust results even for highly weathered sesquioxide rich samples third the w index is applicable to a wide range of felsic intermediate and mafic igneous rock types finally the mfw diagram is expected to facilitate provenance analysis of sedimentary rocks by identifying their weathering trends and thereby enabling a backward estimate of the composition of the unweathered source rock

formation origin and geographic typing of corundum ruby and pink sapphire from the fiskenaesset complex greenland abstract metamorphic petrology observations on rubies found in situ in their host rock are combined with geochemical measurements and optical microscopy observations on the same rubies with the aim of connecting the ruby forming metamorphic reaction to a unique fingerprint for these minerals the fiskenaesset complex in greenland is used as an area of this case study isochemical pressure temperature sections were calculated based on electron microprobe and whole rock geochemistry analyses and compared to field observations rubies formed from reaction between olivine serpentine and anorthite triggered by the intrusion of a 2 71 ga pegmatite al is sourced from the anorthite reacting to calcic amphibole silica from the pegmatite reacts with olivine serpentine to anthophyllite cr3 is mobile in the pegmatitic fluid giving colour to the rubies the ruby forming reaction occurs at about 640 c and 7 kbar in order to establish the unique fingerprint for this ruby bearing ultramafic complex laser ablation inductively coupled plasma mass spectrometry trace element measurements oxygen isotope compositions optical microscopy and scanning electron microscopy were applied due to the setting in an ultramafic rock anorthosite leucogabbro complex the fingerprint of the rubies from the fiskenaesset complex is rather unique compared to rubies from other localities fiskenaesset complex rubies contain high cr intermediate fe and low v ga and ti concentrations low oxygen isotope values 1 6 4 2 and a rarely observed combination of optical growth features and mineral inclusions like anthophyllite biotite results for other greenland localities are presented and discussed as well even though these are derived from ultramafic rock settings too they record different trace element ratios and oxygen isotope values resulting from variations in the archaean ruby forming reaction

evidence for precollisional subduction erosion in ancient collisional belts the case of the mid european variscides analysis of the fore arc remnants and suture zone rocks in the european variscides using criteria established for the circum pacific margins geometry sedimentary evolution and style of deformation shows that precollisonal orogenic evolution can also be reconstructed for ancient collisional belts the collision zone of the lower plate thrust belt rhenohercynian zone and the upper plate arc and fore arc remnants saxothuringian zone and saar basin indicate subduction erosion of the upper plate by some 50 km length of upper plate lithosphere prior to collision removal started during early convergence in the famennian ca 365 ma and lasted until the late visean onset of collision ca 330 ma part of this material may have been underplated below the upper plate and has since resurfaced as high pressure rocks contributing to the metamorphic core of the orogen

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

mineral chemistry and the petrogenesis of eucrites ii cumulate eucrites abstract we carried out an ion microprobe study of rees and other selected trace and minor element concentrations in orthopyroxene augite lamellae and plagioclase of two cumulate eucrites moama and moore county in order to reevaluate the relationships between cumulate and noncumulate eucrites and among cumulate eucrites pyroxene and plagioclase in these two meteorites are more homogeneous in major minor and trace elements than in noncumulate eucrites and trace and minor element concentrations in moore county are higher than in moama although subsolidus equilibration erased and original igneous zoning it did not affect the ree patterns of these minerals both pigeonite and plagioclase of moama and moore county have higher lree hree ratios than those phases in noncumulate eucrites this indicates that cumulate eucrites derived from melts with fractionated ree patterns and thus are not related to noncumulate eucrites in addition the inconsistency between degrees of fractional crystallization deduced from major and trace elements and the differences of lree hree ratios recorded in pigeonite and plagioclase also suggest that moama and moore county did not form in the same fractional crystallization sequence this study confirms earlier observations and conclusions of ma and schmitt 1979 and poses problems for prevalent models

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

a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland a relict oceanic island arc complex in the eastern akia terrane godthabsfjord southern west greenland constitutes a magmatic and geotectonic link between c 3 05 3 0 ga tonalitic orthogneiss and enclaves of older supracrustal amphibolite the relict arc forms isoclinally folded panels of volcaniclastic meta andesite with major and trace element island arc signatures intercalated with volcano sedimentary schist tholeiitic amphibolite and opx rich cumulate rocks a zircon u pb age of 3071 1 ma obtained from a volcano sedimentary schist is marginally older than the main orthogneisses and is the first depositional age reported from a supracrustal rock within akia terrane granite sheets were emplaced at c 3005 2980 ma synkinematically with the isoclinal folding and were followed by peak metamorphism at 2990 2970 ma with substantial recrystallization of volcanic zircon and mobility of large ion lithophile elements the identification of the arc complex provides new insight into mid archaean continental crustal accretion in west greenland and substantiates previous ideas that the orthogneisses are products of slab melting in convergent plate tectonic settings the presence of the arc complex also implies that archaean high grade orthogneiss amphibolite associations may not represent plate tectonic environments distinct from granite greenstone associations but expose deeper sections of the same convergent systems

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

geochemistry and provenance of devono carboniferous volcano sedimentary sequences from the southern vosges basin and the geodynamic implications for the western moldanubian zone

back arc basin basalt systematics the mariana east scotia lau and manus back arc basins babs have spreading rates that vary from slow 100 mm yr and extension axes located from 10 to 400 km behind their island arcs axial lava compositions from these babs indicate melting of mid ocean ridge basalt morb like sources in proportion to the amount added of previously depleted water rich arc like components the arc like end members are characterized by low na ti and fe and by high h2o and ba la the morb like end members have the opposite traits comparisons between basins show that the least hydrous compositions follow global morb systematics and an inverse correlation between na8 and fe8 this is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures lau manus hotter than mariana scotia this interpretation accords with numerical model predictions that faster subduction induced advection will maintain a hotter mantle wedge the primary compositional trends within each bab a positive correlation between fe8 na8 and ti8 and their inverse correlation with h2o 8 and ba la are controlled by variations in water content melt extraction and enrichments imposed by slab and mantle wedge processes systematic axial depth as a proxy for crustal production variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate hydrous fluxing enhances decompression melting allowing depleted mantle sources just behind the island arc to melt extensively producing shallow spreading axes flow of enriched mantle components around the ends of slabs may augment this process in transform bounded back arcs such as the east scotia basin the re circulation by mantle wedge corner flow to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the east lau spreading center manus southern rifts and mariana trough and the very depleted lavas of east scotia segments e8 e9 the crust becomes mid ocean ridge mor like where the spreading axes further away from the island arc and subducted slab entrain dominantly fertile mantle

subduction zone recycling processes and the rock record of crustal suture zonesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent offshore observations at modern ocean margin subduction zones omszs reveal that bodies of accreted material are commonly volumetrically small or missing that crustal thinning and subsidence 3 5 km has occurred and that most trench axes lie close 5 30 km to the seaward tapering edge of coastal basement rock onshore mapping commonly documents missing or only narrow terranes of former forearc rock and the inboard migration of the arc magmatic front these observations are evidence that subduction is accompanied by the removal of sediment and crustal material from the submerged forearc by the kindred tectonic processes respectively of sediment subduction and subduction erosion subduction erosion truncates the margin migrates the trench inboard at 2 5 km ma onshore observations at ancient crust suturing subduction zones csszs imply that collisional suturing is accompanied by sediment subduction and truncation of both upper and lower plates during a protracted period of suturing 20 50 millio

geochemistry of anorthositic differentiated sills in the archean 2970 ma fiskenaesset complex sw greenland implications for parental magma compositions geodynamic setting and secular heat flow in arcs abstract the fiskenaesset complex sw greenland is one of the best preserved layered archean intrusions in the world consisting of an association of ca 550 meter thick anorthosite leucogabbro gabbro and ultramafic rocks dunite peridotite pyroxenite and hornblendite despite poly phase deformation and amphibolite to granulite facies metamorphism primary cumulate textures and igneous layering are well preserved in the complex this study reports new major and trace element data for three variably thick 1 to 5 m differentiated dunite through peridotite pyroxenite gabbro leucogabbro to anorthosite sequences sequences 1 2 and 3 in the sinarssuk area of the fiskenaesset region on several variation diagrams samples from these sequences plot along a well defined liquid line of descent consistent with in situ fractional crystallization the average chemical compositions of these sequences are used to constrain their approximate parental magma compositions petrographic observations and geochemical data suggest that sequences 2 and 3 solidified from evolved magmas that underwent olivine fractionation prior to their intrusion in contrast sequence 1 appears to have been derived from a near primary parental magma sio2 43 wt mgo 20 wt al2o3 16 wt cao 9 3 wt ni 840 ppm mg number 80 the trace element patterns of this parental magma are comparable to those of phanerozoic boninites consistent with a supra subduction zone geodynamic setting if the relative thickness of ultramafic layers the sum of dunite peridotite and pyroxenite layers in differentiated sequences is taken as an analog for the original complex emplaced into archean oceanic crust the fiskenaesset complex might have had a minimum thickness of 1000 m with a 500 m thick ultramafic unit at the bottom the thickness of the ultramafic unit in the preserved complex is less than 50 m suggesting that more than 90 of the original ultramafic unit was either delaminated and recycled back into the mantle as a residual cumulate or was destroyed during thrusting and ttg intrusion both the fiskenaesset complex and associated tholeiitic basalts display similar th nb lree patterns and plot along the same differentiation trend on zr versus incompatible trace element diagrams suggesting a possible petrogenetic link between the two suites of rocks however basalts do not display the same differentiation trend as the complex on several major and trace element diagrams in addition basalts and parental magma to the complex do not plot along the same fractionation line on al2o3 tio2 versus incompatible trace element diagrams implying that the fiskenaesset layered intrusive rocks were not derived from tholeiitic basalts through fractional crystallization as previously thought accordingly we infer that the fiskenaesset complex and spatially associated basalts were derived from different mantle sources parental magmas to the fiskenaesset complex originated from a mantle source that was metasomatized by highly aluminous slab derived melts layered anorthosite complexes are mostly restricted to the archean and typically associated with tonalite trondhjemite granodiorite ttg gneisses the petrogenesis of both suites appears to have been controlled mainly by slab melting endorsing independent evidence for a secular change from slab dominated melting to wedge dominated melting in arc magmatism at the end of the archean reflecting higher geothermal gradients in archean arcs than post archean counterparts

geochemistry and geochronology of the bimodal volcanic rocks of the suguti area in the southern part of the musoma mara greenstone belt northern tanzania abstract the suguti volcanic rocks of the southern musoma mara greenstone belt in northern tanzania comprise mainly of a bimodal suite of tholeiitic basalts basaltic andesites and calc alkaline rhyolites with a subordinate amount of intermediate rocks zircon u pb and whole rock sm nd geochronology suggests that the two suites are cogenetic and were emplaced at 2755 1 ma with a common initial nd value of 2 1 the tholeiitic basalts are characterised by relatively flat chondrite normalised ree patterns with la ybcn ratios of 0 8 1 6 mean 1 0 the basalts also exhibit negative ti and nb anomalies in primitive mantle normalised multi element diagrams the flat ree patterns the presence of prominent negative nb anomalies and the positive initial nd value of 2 1 suggest that the basalts were formed by low pressure melting of a mantle wedge in an active continental margin setting compared to the tholeiitic basalts the calc alkaline rhyolites are characterised by low abundances of the transition elements cr

isotopic evidence for rapid continental growth in an extensional accretionary orogen the tasmanides eastern australia the trace element signature of earth s continental crust resembles that of arc lavas but the continents may have formed during ancient igneous pulses that are hard to reconcile with supra subduction zone magmatism we explore the role of coupled arc back arc accretionary processes in crust generation by considering the tectonic context of whole rock nd isotope and zircon hf o isotope data from igneous rocks of the australian tasmanides 515 230 ma which is thought to have evolved by the repeated opening and closure of sediment filled back arc basins behind a long lived subduction zone the significance of this process for continental crust formation has yet to be evaluated from an isotopic perspective granitic rocks in this area define striking secular end ehf 18o trends that correlate with the pattern of deformational events and register changes in magma source during tectonic activity these trends reveal that juvenile magmatic input was enhanced during extensional back arc rifting episodes that immediately followed crustal thickening suggesting a relationship between slab rollback and continental growth interaction between juvenile magma and sedimentary units deposited during a preceding back arc rifting cycle was integral to the formation of stable continental material this highlights the importance of back arc environments for both the generation and differentiation of continental crust the juvenile component within the tasmanide igneous rocks increased from the cambrian to the triassic consistent with a diminished input from cratonderived metasedimentary material as the subduction zone migrated outboard of the gondwana margin subduction zone retreat formed large tracts of new crust in eastern australia at comparable rates to crust generation at modern island arcs providing a mechanism for rapid continental growth at convergent margins using isotopic tracers to link tectonic evolution and crust generation in modern and ancient orogens can lead to a better understanding of the geodynamic controls on planetary differentiation

the petrogenesis of gorgona komatiites picrites and basalts new field petrographic and geochemical constraints gorgona island colombia is remarkable not only because it contains the only phanerozoic komatiites but also because it has mafic to ultramafic lavas with a wide range of compositions from moderately enriched to extremely depleted relative to bulk earth the komatiite flows are in many respects similar to archaean komatiites they formed from mgo rich 18 liquids and have upper spinifex zones and lower cumulate zones the cumulate zones of archaean komatiites contain many solid grains in contrast more than 90 of the olivine in the gorgona cumulates is highly skeletal this combined with the fact that the gorgona cumulate zones are thinner than those in archaean komatiites suggests that the komatiite magma became strongly superheated en route to the surface the komatiites have trace element contents intermediate between those of the basalts and the ultramafic tuffs some basalts have isotope compositions indicative of long term enrichment in incompatible elements whereas other basalts and ultramafic volcanics have isotopic signatures that imply corresponding depletion it is apparent that the plume source region of the gorgona magmas was markedly heterogeneous with at least two source components contributing to the observed variation in composition this heterogeneity may have resulted from the incorporation of different components into the plume source or it may be the result of complex melting and melt extraction processes during the ascent of a heterogeneous plume despite earlier suggestions that there may have been a significant age gap between depleted komatiite and basalt flows and the enriched basal new 4oar 39ar dating of basalts and gabbros are more consistent with all being generated at 87 ma during formation of the caribbean colombian plateau possibly at the galapagos hotspot