chronology and mechanism of depletion in lewisian granulites the pattern of th u and rb depletion has been investigated in a suite of contrasting lithologies from the classic lewisian granulite terrain at scourie n w scotland the study is based on lithologies ranging from ultramafic to tonalitic gneisses together with calcsilicate and pelitic metasediments sampled on the metre scale because of their close proximity all of these lithologies have experienced identical p t t histories since depletion however their expected dehydration and melting characteristics are markedly different pb isotope analyses of the samples define the latest time of u pb fractionation as 2 665 0 026 ga and indicate substantial u and th depletion relative to pb in tonalitic and mafic lithologies and in some metasediments in contrast ultramafic samples show no u and th depletion sm nd isotope analyses of the mafic and ultramafic samples define an age of 2 707 0 052 ga which is interpreted as the time of igneous differentiation of the mafic ultramafic bodies this time is indistinguishable from the latest time of th pb and u pb fractionation overall there is a straightforward relationship between the magnitude of depletion lithology and the stability of mineral phases tonalitic and mafic gneisses are depleted in th and u relative to pb whereas ultramafic gneisses which retain amphibole show no evidence for depletion the observations are consistent with loss of u th and rb in fluids produced by metamorphic dehydration and melting a role for externally derived co2 in the depletion process is not excluded by the data but there is no evidence for it having existed limits are placed on the length scale of transport for several elements u th and rb are depleted regionally from mafic and tonalitic lithologies transport distances must have been on the kilometre scale in contrast movements of sm and nd have been more restricted

geochemical evidence for the geotectonic setting of early proterozoic metavolcanic sequences in lapland systematic geochemical study of early proterozoic metavolcanic rocks from the northern part of the baltic shield supports the concept that a complex mosaic of continental masses volcanic arcs and marginal basins were welded into one cratonic massif during the svecokarelian orogeny 1900 1750 ma in the north in finnmark and northern norrbotten tholeiitic metabasalts together with thick shallow water clastic metasediments form the main part of the karelian supracrustal sequences which in turn overlie archaean basement the trace element signatures of these metabasalts are comparable to those of post palaeozoic continental tholeiites which erupted in zones of lithosphere attenuation and which suffered contamination by sialic crust it is not however possible to determine by geochemistry alone whether the continental rift basin in question lay in an intraplate or back arc setting metabasalts with a volcanic arc geochemical signature become an increasingly important component of supracrustal sequences as the diffuse southern margin of the karelian continent is approached in the skellefte field an ensialic volcanic arc developed and fed considerable quantities of immature clastic sediment into the deep water norrland geosyncline which may have been fully oceanic in character subduction magmatism continued during the svecokarelian orogeny giving rise to synorogenic metavolcanic suites such as the kiruna porphyries the evidence for marked spatial variation of magmatic activity in the early proterozoic is in contrast to the rather monotonous pattern of greenstone belt development reported from the archaean we believe that this spatial variability is a consequence of geotectonic processes similar to those of modern day plate tectonics and present a plate tectonic model for the early proterozoic development of the baltic shield

sm nd evidence for diachronous crustal accretion in the lewisian complex of northwest scotland sm nd whole rock analyses of several cogenetic rock suites from the late archaean lewisian complex of northwest scotland yield chronological data for a crustal accretion differentiation superevent cads in this terrane three layered mafic ultramafic bodies predate badcallian high grade metamorphism 2660 ma but their temporal relationship to the igneous precursors of the host quartzofeldspathic gneiss complex remains unclear regression of data from these samples yield the following achiltibuie 2850 95 ma with nd t 1 93 0 52 drumbeg 2910 55 ma with nd t 1 70 0 32 scouriemore 2670 110 ma with nd t 0 98 0 73 these data suggest that contemporaneous sub lewisian mantle had a long term light rare earth element depleted history relative to a chondritic reservoir composition three suites of quartzofeldspathic gneisses the pyroxene granulites of scourie the hornblende granulites of gruinard bay and the amphibolites of the northern region north of loch laxford have small ranges of sm nd ratios and do not yield precise regression results if these gneisses were derived from a relatively homogeneous source region perhaps represented by the mafic ultramafic bodies then sm nd model ages suggest a diachronous sequence of crustal generation decreasing in age from scourie terrane average tdm 2930 ma through gruinard bay 2860 ma to the northern region 2780 ma a diachronous sequence such as this suggests that for a cads in which separate accretion and differentiation episodes may be resolved final high grade metamorphism may not be related to magmatic crustal generation processes

lewisian crustal history from in situ sims mineral chronometry and related metamorphic textures abstract a combination of detailed petrography and in situ sims 207 pb 206 pb micro chronometry of monazite has been carried out on samples from the lewisian high grade metamorphic terrain northwest scotland two episodes of granulite facies metamorphism have been recognized the thermal maxima of which are estimated at or before 2760 12 ma 2 and at 2526 8 ma respectively pegmatites at scourie crystallised at 2480 ma the amphibolite facies metamorphism of the main laxfordian event is estimated at 1752 12 ma and the formation of kyanite and staurolite is associated with this event the oldest ages of 2760 ma have been obtained from micro inclusions of monazite in garnet it appears that garnet which has a high closure temperature for the u pb system plays an important role in the behaviour of the monazite u pb system it serves to shield monazite from reaction with the other phases and to armour monazine from diffusive pb loss or from resetting

tectonics of the kola collision suture and adjacent archaean and early proterozoic terrains in the northeastern region of the baltic shield as preparation for the deep seismic and other geophysical experiments along the polar profile which transects the granulite belt and the kola collision suture structural field work has been performed in northernmost finland and norway and published geological information including data from the neighbouring soviet territory of the kola peninsula have been compiled and reinterpreted based on these studies and a classification according to crustal and structural ages the northeastern region of the baltic shield is divided into six major tectonic units these units are separated and outlined by important low angle ductile shear or thrust zones of late archaean to early proterozoic age the lateral extension of these units into soviet territory and their involvement in large scale crustal deformation structures are described using the view down the plunge method a generalised tectonic cross section that predicts the crustal structures along the polar profile is compiled and the structures around the kola deep drill hole are reinterpreted the kola suture belt through parts of which the kola deep bore hole has been drilled is considered to represent a ca 1900 ma old arc continent and continent continent collision suture it divides the northeastern shield region into two major crustal compartments a northern compartment comprising the murmansk and sorvaranger units and a southern compartment including the inari unit the granulite belt and the tanaelv belt as well as the more southernly situated south lapland karelia craton of the karelian province of the svecokarelian fold belt the kola suture belt is outlined by a 2 40 km wide and ca 500 km long crustal belt composed of 1 1 early proterozoic ca 2400 2000 ma old metavolcanic and metasedimentary sequences which originally formed part of the attenuated margin of the northern archaean compartment and 2 2 the remains of a ca 2000 1900 ma old predominantly andesitic island arc terrain this island arc terrain was built up above a sw plunging subduction zone initiated ca 2000 ma ago in the southern part of a newly formed oceanic domain the kola ocean due to continued subduction and complete consumption of this ocean the northern passive margin deposits and the island arc terrain were brought into tectonic juxtaposition and during the final arc continent and continent continent collision they were overthrusted onto the northern archaean continent along its southern boundary the kola suture belt is tectonically overlain by the archaean rocks of the inari unit this unit was derived from a microcontinent split from the southern compartment the depositional basin of the protoliths of the granulite belt being formed to the south of the microcontinent the inari microcontinent appears to have wedged out towards the southeast as the continuation of the granulite belt north of the white sea is in direct tectonic contact with the kola suture belt the granulite belt is composed of high grade paragneisses and minor amounts of meta igneous rocks the paragneisses formed from thick turbidite and mass flow deposits lain down in a back arc basin south of the inari microcontinent a thermal anomaly beneath the partly oceanic basement of the back arc basin is believed to have contributed to the ca 2000 1900 ma old granulite facies metamorphism of the granulite assemblages granulite facies conditions still prevailed when the inari microcontinent overrode the granulites and when the granulite belt as such was formed and was overthrusted for at least 100 km towards the southwest in conjunction with the latter event the rocks of the basement of the basin also became involved in thrust movements these now form the tanaelv belt which shows gradational tectonic contacts towards underlying cover and basement rocks of the south lapland karelia craton although not all parts of this craton were affected by the svecokarelian deformation it is considered to belong to the karelian province of the svecokarelian fold belt a ca 1900 1800 ma old episode of wrench faulting and the intrusion of 1790 1770 ma old post kinematic granites concluded the svecokarelian evolution of the northeastern shield region

low frequency and broad band source models for the 2009 l aquila italy earthquake summary the 2009 l aquila italy earthquake mw 6 3 produced an unprecedented number of seismological records for a normal faulting event and hence represents an important case study of damaging moderate magnitude earthquakes occurring in densely inhabited areas we have investigated the source process of the 2009 l aquila earthquake in both the low and broad band frequency ranges by analysing the teleseismic and strong motion data a low frequency 0 5 hz source model has been determined by waveform inversion of teleseismic and near field strong motion data yielding slip distributions with a major asperity located about 8 km southeast of the hypocentre the broad band 0 2 10 hz source characteristics were estimated using the empirical green s function method providing the strong motion generation area of about 42 km2 located southeast of the hypocentre below the village of onna this coincides with the location of the asperity from the inversion analysis indicating that the low frequency and broad band ground motions produced during the earthquake were generated at similar locations on the fault plane the position of the strong motion generation area also correlates well with the distribution of observed macroseismic mcs intensities reported by ingv the maximum macroseismic intensities of ix x were reported southeast of the epicentre with an intensity of x measured in onna the source properties of the 2009 l aquila mainshock reveal the estimated size of the strong motion generation area to be in agreement with predictions made using empirical source scaling relation for inland crustal earthquakes implying a stress drop of approximately 10 mpa this further indicates that the 2009 l aquila earthquake involved a stress drop similar to that of previously recorded earthquakes in the central and southern apennines

a sm nd isotopic study of the south harris igneous complex the outer hebrides sm nd geochronology may be used to bracket the age of metamorphism in rocks which are difficult to date by other methods by coupling whole rock sm nd analyses of the principal members of the south harris igneous complex with sm nd mineral isochrons on two anothositic gabbros the age of granulite facios metamorphism has been defined whole rock analyses of three pairs of closely spaced samples of the anorthosite give consistent ages averaging 2 18 0 06 gyr but in general the data from the anorthosite do not define an isochron as a result of variable contamination of the evolving magma chamber whole rock data on the tonalite indicate that it is younger than 2 06 gyr its mean tchur age is 1 86 0 05 gyr garnet pyroxene amphibole plagioclase mineral isochrons on two anorthosite samples give identical 1 87 0 04 gyr ages which date cooling after the high pressure granulite facies metamorphism together with the tonalite whole rock data this defines the age of that metamorphism and confirms dearnley s original assignment of an early laxfordian age

an overview of the upb geochronology of the paleoproterozoic torngat orogen northeastern canada abstract the paleoproterozoic torngat orogen northern labrador and northeastern quebec canada is the result of oblique collision between the nain north atlantic and rae cratons a comprehensive set of u pb ages allows the chronology of events to be closely bracketed subduction occurred below the nain craton from ca 1910 1870 ma and collision between the cratons may have occurred between 1869 and 1844 ma sinistral deformation continued along the intervening abloviak shear zone until ca 1824 ma and convergence was ultimately impeded by the partial subduction of the southern rae craton post collisional isostatic uplift of the southern rae craton initiated deformation in the komaktorvik zone ca 1798 ma a prominent bend in the abloviak shear zone may be the result of differential uplift of the western side of the orogen in the south sinistral strike slip and east as well as west side up deformation continued episodically in the komaktorvik zone until at least 1710 ma the tectonic development of the orogen can be explained in terms of modern plate tectonic processes

geochronological studies of proterozoic crustal evolution in the northern outer hebrides abstract new data on the proterozoic thermal and tectonic crustal history of the lewisian complex have been determined using sm nd and 40 ar 39 ar isotopic analyses igneous assemblages from two mafic ultramafic masses indicate crystallization at 2 54 and 2 14 ga from magmas with initial nd isotopic composition lower than predicted depleted mantle at those times sm nd isotope systematics t dm 2 45 ga in two samples of metasediment from the leverburgh belt imply that they contain a significant post archaean component and cannot therefore be part of an archaean stratigraphic sequence sm nd mineral isotope data on three samples confirm the proterozoic age of high pressure granulite facies metamorphism in the south harris igneous complex the most precise estimate is 1 827 0 016 ga on a metagabbro which was intruded after deformation of the host anorthosite under granulite facies conditions localized granulite facies assemblages in younger basic bodies further north are significantly younger at 1 63 ga whole rock sm nd data on two samples from the 1 7 ga granite injection complex of south harris indicate nd between 11 5 and 13 7 at the time of intrusion providing a strong indication that they were produced by remelting of pre existing crust similar to the exposed grey gneisses which had values close to 12 at that time 40 ar 39 ar data from 13 hornblendes generally show complex argon release patterns with indications of excess argon in most samples flat sections of the spectra range in age between 1 63 and 1 78 ga previous rb sr data on biotite showed a distinct break in age pattern across the langavat belt there is no indication of such a break in the hornblende data and it is inferred that despite variations in the timing of peak metamorphism the whole of the northern outer hebrides cooled through hornblende closure temperatures in the interval 1 7 1 6 ga

tectono metamorphic events in the north atlantic region in the palaeoproterozoic from the view point of high grade metamorphic rocks in the lewisian complex south harris nw scotland abstract the tectono thermal history of the lewisian complex in south harris south harris complex was inferred from its geologic and metamorphic characteristics the lithological assemblages and geochemical features of the complex suggest that its precursory rocks were composed of the subduction related accretionary complex formed in the palaeo convergent margin the complex has suffered the ultra high temperature uht metamorphism that was contemporaneous with the igneous activity to make the south harris igneous complex shic and the subsequent continent continent collisional activity a similar complex recording the geological processes of the subduction the uht metamorphism and the collision has been recognized in the lapland kola belt and new quebec in the palaeoproterozoic this suggests an assembly of micro continents to form the palaeoproterozoic supercontinent in the north atlantic region

a new understanding of the provinces of the amazon craton based on integration of field mapping and u pb and sm nd geochronology abstract new conventional and sensitive high resolution ion microprobe zircon u pb dating has led to a new understanding of the subdivision and evolution of the amazon craton during precambrian time with major improvements and changes made to the previous rb sr based model the interpretation of u pb and sm nd isotopic data identifies eight main precambrian tectonic provinces in the craton with ages ranging from 3 1 to 0 99 ga some of the provinces were generated by accretional arc related processes carajas transamazonic tapajos parima and rondonia juruena and others by recycling of continental crust central amazon rio negro and sunsas the exposed archean crust is restricted to the east carajas and south amapa in brazil and north imataca in venezuela of the craton indicating that the amazon craton is largely a proterozoic crust the carajas imataca 3 10 2 53 ga and transamazonian 2 25 2 00 ga provinces are composed predominantly of granite greenstone terranes the tapajos parima 2 10 1 87 ga and rondonia juruena 1 75 1 47 ga provinces represent new crust added as orogenic belts while the rio negro 1 86 1 52 ga and sunsas 1 33 0 99 ga provinces originated mainly by magmatic tectonic recycling of the above two orogenic belts the only zone with a prominent northeast trend is the poorly known k mudku shear belt characterized by a 1 20 ga shear zone which deforms the rocks of at least three different provinces rio negro tapajos parima and transamazonic the central amazon province comprises mostly orosirian volcano plutonic rocks uatuma magmatism and is a terrane in which the exposed crustal structure and deformation are pluton related the sm nd tdm model ages and nd suggest that the central amazon province was generated by the partial melting of archean continental crust carajas province perhaps related to underplating that began at the end of the tapajos parima orogeny 1 88 1 86 ga

a model of ocean crust accretion for the superior province canada abstract one of the keys to understanding the origin of archaean greenstone belts lies in the geological relationships between mafic and ultramafic greenstones felsic to intermediate volcanic rocks and terrigenous sediments traditional models for greenstone belt evolution have been based on in situ stratigraphic relationships most of these models for example an oceanic island arc developed on oceanic basement back arc basins and the recently popular plume model predict concordant stratigraphic relationships among the various greenstone belt lithologies however rather than being depositional in nature several authors have indicated that many of the relationships between the different lithologies in greenstone belts are in fact tectonic suggesting an allochthonous origin for most greenstone sequences all of these latter models make analogies to phanerozoic tectonic processes involving accretion of oceanic materials with volcanism related to both plate subduction and rifting in this paper we have evaluated the geological relationships between volcanic rocks and sediments in three regions in the superior province where the accretion of oceanic material can be documented and direct comparisons are made to geological processes in phanerozoic accretionary complexes in the malartic area in the southeastern abitibi subprovince 3 to 4 km thick slices of komatiite and tholeiite with intercalated terrigenous sediment are tectonically imbricated and are overlain by calc alkaline volcanics which postdate tectonic stacking in both the larder lake region of the southwestern abitibi belt and in the beardmore geraldton belt at the south eastern limit of the wabigoon belt slices of iron rich tholeiite and chemical sediments of an oceanic origin are tectonically imbricated with terrigenous sediment the malartic val d or area is considered to be an example of accretion of an archaean oceanic plateau while the larder lake and the beardmore geraldton regions are potentially typical of accretion of normal oceanic crust in an arc environment phanerozoic accretion of oceanic crust is accompanied by a step back in subduction and in this paper we suggest that oceanic crust accretion may have been the principal mechanism by which the locus of subduction migrated towards the south of the superior province asthenospheric upwelling associated with the isolated sinking plate may have been responsible for widespread late magmatism this scenario requires that magmas be erupted through previously accreted volcanic plutonic and sedimentary material furthermore later ridge subduction will result in transpressional tectonics and eruption of mafic sequences over mature and immature volcano plutonic sequences the combined result of the plate tectonic scenario envisaged would result in the well described cyclic stratigraphy of many granite greenstone sequences

an example of ultrahigh temperature metamorphism orthopyroxene sillimanite garnet sapphirine quartz and spinel quartz parageneses in al mg granulites from in hihaou in ouzzal hoggar quartz al mg granulites exposed at in hihaou in ouzzal nw hoggar preserve an unusual high grade mineral association stable at temperatures up to 1050 c involving the parageneses orthopyroxene sillimanite garnet quartz sapphirine quartz and spinel quartz the phase relationships within the fmas system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event the following mineral reactions involving sillimanite are deduced 1 grt qtzopx sil 2 opx silgrt spr qtz 3 grt sil qtzcrd 4 grt silcrd spr 5 grt sil sprcrd spl 6 grt silcrd spl 7 grt crd silspl qtz and 8 grt silspl qtz minerals in quartz al mg granulites display compositional variations consistent with the observed reactions the mg mg fe2 range of the main minerals is as follows cordierite 0 81 0 97 sapphirine 0 77 0 88 orthopyroxene 0 65 0 81 garnet 0 33 0 64 and spinel 0 23 0 56 the reaction textures and the evolution of the mineral assemblages in the quartz al mg granulites indicate a clockwise p t trajectory characterized by peak conditions of at least 10 kbar and 1050 c followed by decompression from 10 to 6 kbar at a temperature of at least 900 c

stress directions and shear wave anisotropy observations from local earthquakes in southeastern sicily italy the spatial distribution of 414 earthquakes 1 0 m l 4 6 recorded from 1994 to 2002 in southeastern sicily italy has been analyzed the seismicity generally coincides with mapped plio quaternary faults including the north north west south southeast striking offshore fault system which is the most important tectonic structure of the area for the best located events we computed 70 focal mechanisms by combining p wave polarities with s wave polarizations a predom inance of strike slip and normal faults was observed focal mechanisms were then inverted for stress tensor parameters by using the algorithm of gephart and forsyth the results highlighted a region governed mainly by a north northwest south south east to northwest southeast compressional stress regime moreover anisotropy anal ysis of shear waves showed a polarization of fast s waves coherently aligned with this stress direction a finer scale analysis of the stress tensor evidenced three regions characterized by slightly differing orientation of the greatest principal stress axis r1 the eastern sector displays a nearly horizontal r1 trending northwest southeast the central sector is affected by a low dip north northwest south southeast r1 whereas in the western sector a north northwest south southeast oriented r1 with a higher dip angle was detected finally the comparison of the spatial distribution of seismicity occurring during 1994 2002 with locations of previous instrumental earthquakes and larger m 5 0 historical events showed that the seismicity patterns are persistent

lithostratigraphy and composition of 2 1 ga greenstone belts of the west african craton and their bearing on crustal evolution and the archean proterozoic boundary we have determined the lithostratigraphy of three early proterozoic 2 1 ga birimian greenstone belts in the west african craton and measured the concentrations of major and trace elements in samples of volcanic rocks from those belts the nangodi belt largely consists of tholeiitic basalt flows with mg numbers of 37 to 69 and flat to slightly lree depleted chondrite normalized ree patterns overlain by sodic na 2 o k 2 o 3 7 29 calc alkaline dacitic tuffs and lavas that are hree depleted la yb 4 5 8 9 times chondrites yb 3 9 8 7 times chondrites the volcanic sequence is capped by calc alkaline andesitic crystal tuffs with high mg numbers 66 75 and a high la yb ratio 10 5 times chondrites similar to archean and modern sanukitoids the dixcove belt largely consists of tholeiitic hyaloclastites and pillow lavas mg numbers 52 61 that grade upward into high mg mg numbers 59 calc alkaline andesitic tuffs and flows the winneba belt largely consists of massive tholeiitic h

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

the northern margin of the capricorn orogen western australia an example of an early proterozoic collision zone abstract the early proterozoic 2200 1600 ma capricorn orogen is a major zone of deformation metamorphism and granite emplacement between the archaean yilgarn and pilbara cratons of northwest australia capricorn orogen structures are recognized along the southern margin of the pilbara craton deforming the archaean granite greenstone terrain of the sylvania inlier together with rocks of the hamersley ashburton and blair basins in the sylvania inlier deformation is characterized by large scale shear zones interpreted as nne directed thrusts in the hamersley basin orogenic deformation forms the ophthalmia fold belt and can be divided into two groups of structures in the southwest large scale dome and basin folds pre ashburton basin are present in the southeast folds and faults are part of a younger syn ashburton basin n directed foreland fold and thrust belt directly linked to deformation in the sylvania inlier the deformed ashburton basin and adjacent parts of the hamersley basin form the ashburton fold belt two periods of deformation are recognized early recumbent folding d1a post ashburton basin pre blair basin and later dextral wrench faulting and associated folding d2a syn or post blair basin capricorn orogen structures are interpreted as the result of an oblique continent continent collision between the pilbara and yilgarn cratons along the southern pilbara margin collision first occurred in the southeast and migrated northwest with the ashburton basin evolving as a foreland basin late stage dextral wrench faulting was the result of westward extrusion of material caught between the opposing craton margins

purtuniq ophiolite cape smith belt northern quebec canada a reconstructed section of early proterozoic oceanic crust the crustal part of a dismembered ophiolite is preserved in the structurally highest thrust sheets of the early proterozoic cape smith belt in northern quebec canada the rocks of the ophiolite represent two magmatic suites each with a mantle source that is nd isotopically distinct the older suite comprises pillowed and massive volcanic rocks sheeted dikes dominantly gabbroic layered cumulate rocks and rare plagiogranite sills and dikes in the volcanic sequence this 5 km thick tholeiitic suite is ca 1998 ma and is compositionally and petrographically similar to rocks formed at modern oceanic spreading ridges the younger suite 4 km comprises sheeted mafic dikes and layered ultramafic to mafic cumulate rocks that are similar to tholeiitic rocks found in modern plume generated oceanic islands such as hawaii the composite section is in excess of 9 km although the total thickness of the purtuniq ophiolite may not be typical of that for oceanic crust formed at early proterozoic spreading ridges elsewhere it does suggest that ancient ridge formed crust was at least as thick as that in modern oceans

u pb isotopic evidence for the accretion of different crustal blocks to form the lewisian complex of northwest scotland single zircon and titanite u pb shrimp data presented for tonalite trondhjemite granodiorite ttg suite gneisses and an ultramafic rock from the northern and central regions of the lewisian complex of northwest scotland show that protolith ages of tonalitic gneisses in the northern region 2800 2840 ma are significantly younger than those in the central region 2960 3030 ma further evidence of a major 2490 2480 ma metamorphic event in the central region is documented by a metamorphic zircon associated with a granulite facies ultramafic body a dioritic gneiss from the northern region has also been dated at c 2680 ma the northern region therefore does not comprise reworked central region rocks and consequently the old models for the evolution of the lewisian which were based upon this concept need replacing it is instead proposed that two distinct crustal blocks now the northern and central regions were tectonically juxtaposed along a boundary corresponding to the laxford front juxtaposition would appear to have occurred in proterozoic times as it must have postdated the 2490 2480 ma inverian metamorphism recorded only in the central region and the emplacement of granite sheets restricted to the northern side of the boundary the first recorded event common to both regions is resetting of titanite ages associated with c 1750 ma laxfordian amphibolite facies metamorphism zircon inheritance in rocks of both regions is scarce within one zircon from the northern region a c 3550 ma core was found this represents the oldest known material from the region

two stages of sapphirine formation during prograde and retrograde metamorphism in the palaeoproterozoic lewisian complex in south harris nw scotland

metamorphic evolution of aluminous granulites from labwor hills uganda sapphirine cordierite quartz and spinel cordierite quartz form relic assemblages of probable archaean age in fe rich aluminous metapelites from labwor hills uganda and reflect an unusually high temperature metamorphism 1 000 c at pressures in the vicinity of 7 9 kbars and a o2 near the magnetite hematite buffer subsequent reaction textures include the replacement of spinel and cordierite by sillimanite and hypersthene and formation of sapphirine hypersthene k feldspar quartz symplectites which are interpreted as pseudomorphs after osumilite a petrogenetic grid appropriate to these assemblages suggests these reaction textures may be due to cooling at constant or increasing pressure and constant a o2 or decreasing a o2 at constant temperature and pressure the former interpretation is supported by the coexistence of ilmenohematite and magnetite during the development of the reaction textures and by the comparatively low al2o3 contents of secondary hypersthene this pressure temperature path implies that 1 metamorphism occurred at deep levels within normal thickness crust probably less than 40 45 km thick due to an extreme thermal perturbation induced either by emplacement of mantle derived magmas or by thinning of the subcontinental lithosphere in an extensional tectonic regime 2 the excavation and surface exposure of the granulites is due to a subsequent postgranulite facies metamorphism crustal thickening most probably involving their incorporation into an allochthonous upper crustal thrust sheet during the formation of the mozambique foldbelt