rare earth thhfta and large ion lithophile element variations in metabasites from the proterozoic amphibolite granulite transition zone at arendal south norway abstract the proterozoic arendal metabasites are minor intrusions which possess wholly metamorphic textures and mineral assemblages data are presented for eight selected samples chondrite normalised ree patterns are relatively flat between 10 and 20 chondrite abundances they are characterised by 1 ce n yb n n nd n 1 normalised to primordial mantle values the incompatible elements reveal variable enrichment in the large ion lithophile elements except th and low ta they are similar to many destructive plate margin basalts on a th hf ta diagram the rocks plot in a sub linear trend reflecting variation in th hf samples with high th hf ratios come from the zone of highest metamorphic grade where the host gneisses are deficient in th consequently the patterns in the metabasites could not have been produced by any secondary metamorphic fractionation or by bulk contamination by the country rocks there is no evidence for any post magmatic mobility of any of the elements except rb which is depleted in the highest grade rocks relative to their lower grade counterparts the trace element distributions are largely reflecting those of the parental magmas but consideration of bulk distribution coefficients shows how they may have been modified in some aspects e g ce n nd n by the imposition of the metamorphic mineralogies the data are consistent with a model involving a derivation from a mantle source region which had been affected by subduction related hydrous metasomatism or partial melting and b crystallisation of the magmas directly under the high grade conditions

pb isotopic evidence for u th pb behaviour in a prograde amphibolite to granulite fades transition from the lewisian complex of north west scotland implications for pb pb dating abstract two end member types of u pb fractionation behaviour are recognised type i in which pb isotopic homogenisation occurs and type ii in which pb isotopic homogenisation is incomplete or absent type ii can be further subdivided into three categories depending upon the relationship of the post fractionation 3 238 u 204 pb value to prefractionation 2 in type iia 3 is unrelated to 2 in type iib 3 k 2 where k is a constant in type iic 3 values are uniform each of these types of behaviour results in a predictable effect upon observed uranogenic pb isotope systematics which may lead to an incorrect geochronological interpretation an example of the relationship between these specific types of u pb behaviour and metamorphic grade is provided from the late archaean lewisian complex of n w scotland this complex was metamorphosed during the 2660 ma badcallian event three suites of tonalitic granodioritic grey gneisses have been analysed pyroxene granulite facies from scourie hornblende granulite facies from gruinard bay and amphibolite facies from the northern region the present day pb isotopic composition of these suites can be interpreted as representing a transition from type lib behaviour in the lowest grade rocks resulting in a rotated isochron through type ha in the hornblende granulites which yield a scatter of data points to type i in the pyroxene granulites which yield an isochron corresponding to the badcallian event this transition presumably reflects increasing u depletion with metamorphic grade and ultimately at the highest grade pb mobility and or depletion in addition a consideration of the thorogenic pb isotopic data reveals a systematic variation during prograde metamorphism an attempt to test the model of 2660 ma initial ratio inhomogeneity by analysis of k feldspars from the amphibolite facies gneiss suite reveals 1700 ma laxfordian resetting of their pb isotope systematics this study has implications for pb pb geochronological data particularly in lower grade metamorphic terrains it suggests that reliable isochrons may only be generated by type i behaviour or by the imposition of high 3 values during type iia behaviour at higher grades of metamorphism

granulite amphibolite facies metasediments from the serre calabria southern italy their protoliths and the processes controlling their chemistry abstract in the northwestern serre a near continuous section through the palaeozoic lower continental crust crops out this paper deals with granulite facies metasedimentary rocks of this section fifty five chemical analyses major elements sc v cr co ni rb sr y nb and ba are given chemical data indicate that the original sedimentary sequence was made up of arenites in the lower part and mainly fe and mg rich pelites in the upper part the metapelites experienced anatexis and removal of some melt whereas the meta arenites suffered only incipient anatexis and became dehydration granulites the various rock types display an overall relatively low k rb ratio due mainly to the stability of k and rb retaining phases a weighted mean composition of the fragment of the lower continental crust outcropping in the serre has been established by integrating the chemical data for metasediments 55 analyses metabasites 92 analyses and dioritic and tonalitic gneisses 10 analyses with the field data this composition compared to the compositions of the bulk continental crust and of other sections considered as belonging to the lower crust confirms the hypothesis that 1 the lower crust is heterogeneous and 2 it is more basic than the continental crust

uranium and boron distributions related to metamorphic microstructure evidence for metamorphic fluid activity the distribution of uranium and boron in polymetamorphosed granulite facies schists and gneisses has been studied using particle track methods the concentration and distribution of these elements when examined in relation to mineralogy and microstructure provide an insight into 1 the behaviour of u and b in metamorphism 2 the activity of a fluid phase in the metamorphic processes and 3 the nature of chemical processes during schistosity development a low concentration of primary u occurs in micro inclusions of apatite and zircon many are metamict in the granulite facies m1 assemblage this assemblage which lacks b except for zoned sillimanite has undergone a localized retrograde metamorphism rm1 characterised by hydrous alteration products containing abundant u and b the rm1 metamorphism is attributed to fluids generated during granulite facies dehydration reactions a schistosity s2 defined by m2 fibrolite aggregates overprints the m1 events it is associated with 1 intragranular u concentrated in m2 apatite and titanium bearing minerals and 2 abundant intergranular u within the fibrolite aggregates high b contents also occur with the fibrolite the s2 schistosity appears to develop in a metamorphic environment containing a fluid enriched in u and b

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

the geochemistry of the trotternish sills isle of skye crustal contamination in the british tertiary volcanic province geochemical and isotopic data are presented for a suite of palaeocene picrites picrodolerites and crinanites from the trotternish sills northern skye the whole rock geochemistry of the trotternish sills is comparable with the overlying skye main lava series although the sills are relatively mg rich up to 30 wt mgo due to abundant cumulus olivine rare earth element patterns exhibit overall lree enrichment relative to chondrite and suggest an alkali basalt parental magma incompatible element patterns show ba and k enrichment relative to nb th and u sr nd and pb isotope ratios indicate that crinanites which are the most fractionated magmas are the most contaminated isotope ratios suggest up to 22 amphibolite facies lewisian gneiss may have been assimilated during fractionation in the upper crust prior to multi phase intrusion as sills this is in contrast to the skye main lava series which show evidence of crustal contamination by granulite facies material in the lower crust but is comparable with the skye preshal mhor basalts and the rhum intrusion the trotternish sills and the overlying plateau lavas therefore have a more complicated genetic relationship than was previously supposed

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

mineralogical changes occurring during the retrogression of archaean gneisses from the lewisian complex of nw scotland abstract gneisses metamorphosed at granulite facies ca 2 7 ga were subsequently retrogressed to amphibolite facies during a prolonged period of retrogression perhaps lasting as long as 200 m y the scourie dykes were emplaced towards the end of this event localised laxfordian shear zones further modified the mineral assemblages the retrogression caused the production of a uniform plagioclase hornblende quartz biotite assemblage a study of hornblende composition shows that it depends on metamorphic grade host rock composition and paragenesis the sequence of mineral assemblages suggests that retrogression took place on a falling temperature path beginning at about 650 50 c post tectonic muscovite indicates that temperatures were still in excess of 500 c after the formation of laxfordian shear zones this indicates that the lewisian complex was uplifted and cooled extremely slowly

the transformation of amphibolite facies gneiss to charnockite in southern karnataka and northern tamil nadu india amphibolite facies metamorphic grade gives way southward to the granulite grade in southern karnataka as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes petrologic investigations in the transitional zone south of mysore have established the following points 1 the transition is prograde amphibole bearing gneisses intimately associated with charnockite at kabbal and several similar localities are not retrogressive after charnockite as proved by patchy obliteration of their foliation by transgressive very coarse grained charnockite high fluorine content of biotite and amphibole in gneisses and high large ion lithophile element contents in gneisses and charnockites these features are in contrast to very low fluorine in retrogressive amphiboles and biotites very low large ion lithophile element contents and zonal bleaching of charnockite in clearly retrogressive areas as at bhavani sagar tamil nadu 2 metamorphic temperatures in the transitional areas were 700 800 c pressures were 5 7 kbar and h2o pressures were 0 1 0 3 times total pressures based on thermodynamic calculations using mineral analyses dense co2 rich fluid inclusions in the kabbal rocks confirm the low h2o pressures at the first appearance of orthopyroxene farther to the south in the nilgiri hills and adjacent granulite massif areas peak metamorphic temperatures were 800 900 c pressures were 7 9 kbar and water pressures were very low so that primary biotites and amphiboles those with high f contents are rare 3 the incipient granulite grade metamorphism of the transitional areas was introduced by a wave of anatexis and k metasomatism this process was arrested by drying out under heavy co2 influx charnockites so formed are hybrids of anatectic granite and metabasite of metabasite and immediately adjacent gneiss or are virtually isochemical with pre existing gneiss despite gross recrystallization to granulite mineralogy these features show that partial melting and metasomatism are attendant rather than causative in charnockite development copious co2 from a deep crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis granulite metamorphism of both neosome and paleosome followed the process is very similar to that deduced for the madras granulites by weaver 1980 the massif charnockites for the most part extremely depleted in lithophile minor elements show many evidences of having gone through the same process

the scourie dyke suite petrogenesis and geochemical nature of the proterozoic sub continental mantle the petrogenesis of bronzite picrite olivine gabbro norite and quartz tholeiite dykes which make up the 2 39 b y scourie dyke swarm cutting the archaean lewisian gneisses of n w scotland is interpreted on the basis of their major and trace element geochemistry most of the dykes bear primary amphibole and or phlogopite and with one exception are all hypersthene or quartz normative apart from one tholeiite dyke which shows relative light rare earth element depletion all the dykes show enrichment in light rare earths and large ion lithophile elements they do not however show an equivalent enrichment in other incompatible high field strength ions such as nb and ta and in this respect resemble island arc and calc alkaline basalts the different dyke types have distinctive rare earth patterns and other trace element ratios which are maintained over a range of major element compositions

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

reply to comment by y f zheng on pb isotopic evidence for u th pb behaviour in a prograde amphibolite to granulite facies transition from the lewisian complex of north west scotland implications for pb pb dating abstract in a recent paper whitehouse 1989a pb isotopic arrays obtained from a prograde amphibolite to granulite facies transition in the lewisian complex were interpreted in terms of different degrees of pb isotopic homogenisation and different styles of u pb fractionation during the 2660 ma badcallian metamorphism zheng 1990 comments that post 2660 ma u pb disturbance might have contributed significantly to the observed pb isotopic arrays thus requiring a re evaluation of the proposed model for u th pb behaviour at 2660 ma this comment raises an important point with regard to the general interpretation of pb and other isotopic data from ancient poly phase metamorphic terranes namely the extent to which particular events may be evaluated as significant in multi stage isotopic evolution or dismissed as insignificant thus reducing the number of stages and permitting more definitive modelling of earlier events in this reply it is argued that for the samples studied by whitehouse 1989a post badcallian events are not represented or are unable to generate the observed pb isotopic arrays the modelling of zheng 1989 for these samples is considered to be inappropriate and misleading the pb isotopic data of moorbath et al 1969 for which measured u pb ratios are available appear to indicate an u pb disturbance at 900 ma when reduced using a three dimensional isochron plane model zheng 1990 similar results are obtained using the approach of gale and mussett 1973 for reducing three stage data to two stage however ages obtained from both these methods are highly dependent upon the assumption of initial ratio homogenisation during badcallian metamorphism geochemical considerations discussed by whitehouse 1989a and reiterated here question the validity of this assumption and any modelling based upon it

regional granulite facies metamorphism in the ivrea zone is the mafic complex the smoking gun or a red herring one widely accepted paradigm for the development of continental lower crust is that regional granulite facies metamorphism is caused by intrusion of mafic magma beneath or into the crust magmatic accretion the amphibolite to granulite facies supracrustal section exposed in the ivrea zone southern alps northern italy is commonly cited as a classic example establishing this postulated genetic relationship our interpretation of the pattern of metamorphic isograds compositional trends in high grade metasedimentary rocks and textural evidence in metapelite however indicates that final emplacement of the mafic plutonic rocks the mafic complex occurred subsequent to the regional thermal maximum field and petrographic relations suggest that a spatially restricted contact melting event in crustal rocks accompanied the emplacement of the mafic complex this inference is consistent with leucosome compositions in migmatites and a low pressure high temperature metamorphic overprint recorded by mineral assemblages in wall rocks proximal to the intrusion therefore evidence of anatexis and metamorphism of crustal rocks associated unequivocally with emplacement of the mafic complex is found only within an 2 km wide contact aureole overlying the intrusion the narrow aureole associated with emplacement of the mafic complex demonstrates that in some cases emplacement of large volumes of mafic magma within the crust does not inexorably lead to regional scale granulite facies metamorphism and large ion lithophile element depletion by melt loss

the heat flow from the continents the terrestrial heat flow is defined as the quantity of heat escaping per unit time from the earth s interior across each unit area of the earth s solid surface the quantity varies from place to place over the surface of the earth as well as with time throughout earth history the total heat being lost from the earth at a given time is the integral of the heat flow taken over the entire surface of earth the heat flow from the continents is therefore but a part of the larger picture of global heat loss indeed the lesser part when compared to the heat loss through the floor of the ocean basins pollack 1980 sclater et al 1980 davies 1980 this is so not only because of the lesser continental area but also because of the generally smaller heat flow found on the continents than in the ocean basins thus a full discussion of the thermal regime of the earth and the mechanisms by which it exhausts heat from the interior must center on the heat loss through the oceans a topic which lies beyond the scope of this review nonetheless because of the geological antiquity of at least some of their constituent terranes the continents provide the only direct evidence about the thermal tectonic and petrologic processes of the earth at times before the birth of any of the modem ocean basins some two hundred million years ago because inferences about earlier times are drawn wholly from materials now preserved in continents one must be cautious not to overgeneralize about global tectonothermal processes from observations confined to continents

multi element geochemical modelling of crust mantle interactions during late archaean crustal growth the closepet granite south india the closepet granite in the dharwar craton of south india is a large late archaean magmatic body its composition can be explained as a result of interactions between mantle derived melts and pre existing continental crust ttg gneisses using geochemical modelling based upon major and trace element compositions the following petrogenetic model is proposed for the formation of the batholith i an enriched mantle garnet and amphibole bearing lherzolite melts to produce a basaltic liquid ii the basaltic liquid undergoes limited fractional crystallization of biotite and amphibole f 0 9 iii the differentiated liquid rises into the old continental crust and induces water saturated anatexis of the ttg gneisses k feldspar qtz plagioclase melt iv both mantle derived and anatectic liquid mix and give rise to magma compositions ranging from quartz monzonite to granite the origin of the mantle enrichment is discussed in the context of the regional geological setting the most likely possibility seems to be metasomatism by slab melts the metasomatism of a mantle wedge by slab melts this suggests a two stage evolution for the dharwar craton during the late archaean i a subduction related event with the formation of ttgs sanukitoids and with associated mantle metasomatism and ii re melting of the metasomatized mantle this evolutionary history implies that at least some of the k rich late archaean granites are juvenile rather than products of intracrustal reworking as frequently assumed

a 3500 ma plutonic and volcanic calc alkaline province in the archaean east pilbara block variably foliated predominantly granodioritic plutonic rocks from the northern part of the shaw batholith in the east pilbara archaean craton are dated at 3 499 22 ma 2 errors by a whole rock pb pb isochron these rocks intrude the surrounding greenstone sequence and their age is indistinguishable from that sequence high strain grey gneisses which occupy much of the western and southern shaw batholith are chemically and isotopically similar to the north shaw suite and are inferred to have been derived from this suite by tectonic processes felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc alkaline volcanic and plutonic province at 3 500 ma this volcanic and plutonic suite is similar to modern calc alkaline suites on the basis of major element rare earh element and most other trace element contents the archaean suite contrasts with modern equivalents only in having lower concentrations of hree and higher concentrations of ni and cr

the timescale and mechanism of granulite formation at kurunegala sri lanka incipient charnockite formation at kurunegala in sri lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss mineral equilibria in the charnockite assemblage record pressure temperature p t conditions of 738 60 c and 6 9 1 2 kbar at 17 0 1 2 log fo2 and ah2o 0 18 0 16 wholerock trace element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of sm rb pb u sr nd with a fractionation of rb sr sm nd and th u ratios and crystallization of the charnockite assemblage at 535 5 ma major element fe mg ca and sm nd equilibration between minerals occurred at 524 9 ma whereas pb and rb sr underwent continued exchange to 501 5 ma and 486 1 ma respectively trace element data for both amphibolite and charnockite minerals show that depletion on a whole rock scale can be accounted for either by changes in mineral modes or trace element abundances within the immediate area of dehydration the fractionation of sm nd on a whole rock scale is controlled by the breakdown of amphibole without the growth of a major new host phase for sm in the charnockite rubidium and sr are dependent on the relative behaviour of biotite plagioclase and alkali feldspar modelling of dehydration melting involving the breakdown of amphibole biotite and alkali feldspar reproduces the observed sm nd and rb sr fractionation and indicates the loss of small melt fractions on a cm scale from the charnockite these observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation

timing of magmatism and metamorphism in the gruinard bay area of the lewisian gneiss complex comparison with the assynt terrane and implications for terrane accretion reply u pb zircon data from selected gneiss samples provide estimates of the timing of igneous protolith emplacement and granulite facies metamorphism in the gruinard bay badcall bay and lochinver areas of the mainland lewisian gneiss complex of north west scotland zircons from the gneisses contain complex zoning patterns making them amenable to shrimp ion microprobe analysis with inherited zircon distinguishable from magmatic growth and metamorphic recrystallisation from gruinard bay a tonalitic granulite yielded a protolith age of 2 825 8 ma for oscillatory zoned zircon cores with recrystallised low u rims preserving an age of 2 733 12 ma an inherited 2 905 15 ma age component was recognised in a sample of trondhjemitic gneiss the protolith of which is estimated to have been emplaced at 2 858 11 ma low u rims yielding an age of 2 729 20 ma are also present which together with the rim age from the granulite are considered to date granulite facies metamorphism in the gruinard region for comparison with the gruinard bay gneisses a trondhjemitic partial melt and trondhjemite sheet were analysed from the granulite facies rocks at badcall bay in the assynt terrane age data do not show definitive relationships with zoning patterns in the zircons and show a spread along concordia typical of previous ion microprobe age data from the scourie gneisses both samples do however show a significant isotopic disturbance at 2 480 ma the most recent estimate for the badcallian metamorphism protolith ages for components of the assynt terrane range between ca 2 900 and 3 030 ma and appear to be older than the 2 860 2 825 ma gneiss protoliths from the gruinard bay area the timing of granulite facies metamorphism also appears to be different between the two areas suggesting that the two granulite regions of the mainland may not be related as previously thought instead they are interpreted in terms of a terrane accretion model whereby the two areas evolved as separate blocks prior to their juxtaposition during the palaeoproterozoic the boundary between the two terranes is thought to lie to the south of lochinver corresponding with the strathan line because of the observed 2 480 ma age signature to the north and 2 730 ma signature to the south by defining a new mainland terrane the gruinard terrane the number of mainland blocks which includes the assynt central rhiconich northern and southern terranes has been expanded

granulite fades nd isotopic homogenization in the lewisian complex of northwest scotland a published sm nd whole rock isochron of 2 920 50 myr obtained from a wide range of lithologies in the lewisian complex of north west scotland was interpreted1 as the time of protolith formation this date is 260 myr older than estimates for the timing of high grade metamorphism in the complex at 2 660 myr2 3 and this period is considered to represent the duration of the lewisian crustal accretion differentiation superevent cads 4 here we give new sm nd data obtained specifically from granulite facies tonalitic gneisses that yield a date of 2 600 155 myr although depleted mantle model ages tdm suggest 200 myr of premetamorphic crustal residence the regression date and its associated initial nd isotopic parameters demonstrate nd isotopic homogenization during the high grade event as well as the probability of general rare earth element ree mobility models for selective element depletion in the complex have previously assumed ree immobility since 2 920 myr but the data presented here suggest that a reappraisal of the depletion mechanism is required

rare earth element distributions among coexisting granulite facies minerals scourian complex nw scotland the major rock forming mineral phases pyroxenes plagioclase garnet hornblende of a suite of granulite facies gneisses from the scourian complex nw scotland have been analyzed for their rare earth element ree content although host rock compositions range from felsic to ultramafic ree abundances and patterns for each mineral group show only limited variation the rees exhibit regular and consistent distribution patterns for each mineral which suggest together with major element and textural considerations that the observed distribution coefficients approach equilibrium total ree content follows the sequence hornblende clinopyroxene garnet plagioclase orthopyroxene and mass balance calculations show that even in the felsic gneisses 60 of the rees reside in the major rock forming minerals comparisons of both relative ree abundances and distribution coefficients with those in other rock types reveal a striking resemblance with patterns observed in mineral liquid pairs of dacitic rocks these similarities may have arisen during a partial melting episode in which granite granodiorite melts were generated and removed from the scourian complex leaving a residuum which is severely depleted in the incompatible elements including the rees

formation of the earliest continental crust inferences from the scourian complex of northwest scotland and geophysical models of the lower continental crust abstract in this paper the structure composition and pre metamorphic lithology of the scourian complex at its type locality are compared with models of the lower continental crust proposed by several authors on the basis of seismic reflection profiling and exposures in the ivrea zone and the vredefort dome it is shown that the scourian complex resembles these models in major characteristics consisting of gently dipping granulite gneisses of intermediate bulk composition formed from partly or largely supracrustal precursors in addition seismic investigations of northwest scotland indicate that granulites of the scourian type make up most of the 26 km of continental crust in the area it is therefore proposed that the scourian complex provides a representative exposure of the lower continental crust if this is so it implies that the lower crust is dominantly of supracrustal origin with normal superposition age relationships except for localized thrusting and intrusion such age relationships in turn imply that isotopic investigations sm nd and rb sr date only later additions to the early crust either intrusions or supracrustal additions not formation of entire crustal segments it is therefore proposed that older continental crust may underlie even the oldest exposed rocks that primordial crust may survive near the moho and that the earliest continental crust was formed largely by andesitic volcanism that produced a widespread possibly global layer now represented by granulites of the scourian type

tectonic cycles in southern africa abstract the recurrence of major geological events within the 3600 ma stratigraphic record of southern africa suggests a periodicity of320 25ma for at least the last 3200 ma this periodicity is reflected in major episodes of continental volcanism granitoid plutonism orogeny reciprocity of terrestrial and marine dominated sequences platform carbonate sedimentation glaciation and first order changes in sea level these serve to define 11 completed megacycles each introduced by a major rifting event accompanied by listric faulting alluvial sedimentation in restricted fault bound troughs volcanism and a type granitoid plutonism prior to 3200 ma continental rift magmatism occurred subaqueously due to very high early archean sea levels resulting in pillow lavas and a lack of interbedded terrigenous siliciclastics the long term reciprocity of terrestrial and marine dominated sequences records eustatic first order changes in sea level with highstands characterized by generally humid warm continental climates low relief and tropical afforestation on the other hand lowstand climates were hot and arid with high relief and widespread desertification thick marine sequences accumulate in areally extensive epeirogenic basins without significant volcanism or faulting and commonly are divided into an early phase of open shelf sedimentation followed by deposition in a brackish marine foreland basin within openshelf environments an early phase of biochemical precipitation and or siliciclastic reworking along linear shorelines is separated from a later phase involving rapid deposition of immature siliciclastics along lobate shorelines evolution of the foreland basin is related to orogenic activity and the isolation of an epeiric sea which sometimes developed the characters of a barred basin marine dominated sequences display a deterministic stacking of lithofacies important glacial epochs commonly occur midway through first order regression sometimes with lesser events closely following peak transgression potentially they display 320 ma periodicity but with an unexplained mid proterozoic gap glaciation is related to the combined effect of climatic cooling associated with first order regression and depletion of atmospheric co2 by biological uptake associated with platform carbonate development terrestrial volcanism is believed to be the main contributor of atmospheric co2 climatic change is accompanied by cyclical patterns of cratonic weathering and pedogenesis

k rb sr ba u and th geochemistry of the lapland granulites fennoscandia lile fractionation controlling factors the lile geochemical patterns of the three main lithological units graywacke shale metasedimentary sequence tholeiitic metaigneous rocks and migmatitic rocks of the lapland granulite belt are described k ba sr and th concentrations in metasediments are nearly similar to average continental crust whereas rb and u are unevenly impoverished in particular graphitic metashales and calcsilicate rocks are not significantly depleted in uranium tholeiitic metaigneous rocks comprises metavolcanics which present k rb ratios similar to metasediments and metaplutonics with lile abundances close to those of the low k tholeiites migmatites show wide range in lile content metatexites and diatexites have higher k rb th and u concentrations and similar k rb ratios with respect to equivalent unmobilized rocks potassic pegmatoids are strongly enriched k rb ba and th but moderately in sr and u plagioclasic pegmatoids and ferromagnesian restites are rich in sr and poor in other lil elements

multi chronometric ages and origin of archaean tonalitic gneisses in finnish lapland a case for long crustal residence time the tojottamanselka gneisses of the koitelainen region northern finland have been dated by the sm nd and the common pb methods the sm nd data of seven samples from a small area 100 m 100 m define an isochron oft 3 06 0 12 2 ga with correspondingind 0 50848 9 2 orend t 3 7 1 8 this age is in good agreement with the zircon u pb discordia age 3 1 ga reported by kroner et al 1981 and is interpreted as the time of magmatic emplacement the distinctly negativeend t value is found for the first time for archean tonalitic gneisses and implies derivation of these magmas by remelting of continental material with a long 200 500 ma crustal residence time a few samples on the other hand possessend t values close to zero hence they are thought to be derived by partial melting of basaltic sources with nearchondritic ree distribution patterns

the role of fluids in the development of a granulite facies transition zone in s india the bodinayakkanur area of tamil nadu lies in the transition zone p 5 kbar t 700 c from amphibolite to granulite facies metamorphism in the archaean high grade terrain of s india within the zone 3 closely associated rock units occur 1 precursor grey gneiss 2 pink migmatitic and metasomatic gneiss 3 green charnockitic granulites field observation shows that the complex inter relations between these 3 rock types can be related to influx of fabric and layer guided co 2 and h 2 o k 2 o rich fluid phases derived from a deep seated source

predicting the topographic limits to a gully network using a digital terrain model and process thresholds a digital terrain model is used with process thresholds to predict the extent of a stable gully network in a 5 km2 catchment of the southeastern highlands of australia the model developed by dietnch et al 1992 1993 predicts the topographic controls on channel networks and interprets these in terms of a critical shear stress for channel incision c applied by saturation overland flow we adapt the model slightly to compare the shear stress applied by hortonian overland flow to that applied by saturation overland flow the limits to gully erosion in the catchment are controlled strongly by a topographic threshold that has an inverse relationship between upslope catchment area and local gradient the topographic threshold for channel incision is reproduced using a simple model of hortonian overland flow and a c appropriate for incision into a degraded grass surface c 245 dyn cm2 this is consistent with historical evidence for the timing of gully erosion the study confirms a strong topographic control on the extent of the channel network in a catchment significantly different from the western north america catchments where the topographic threshold was first demonstrated despite its simplicity the model for incision by overland flow appears capable of distinguishing the hydrological processes responsible for channel incision when these are reflected in the relationship between channel network and landscape morphology the model requires relatively simple inputs suggesting it may be useful for mapping gully erosion hazard in actively eroding catchments

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

radioactive element distribution in the archean granulite terrane of jequie bahia brazil the radioactive element distribution in the archean granulite facies rocks of jequie brazil has been determined the k rb ratios show a large spread with majority of the values falling within the normal field k rb 300 the th u ratios fall into two groups one group with values in the range of 1 7 to 2 6 whereas the other group show th u values greater than 7 7 to 25 these distributions can be explained on the basis of the presence of accessory minerals apatite and zircon the relatively high concentrations of the radioactive and other elements in jequie granulites probably represents the relict primary geochemical character little modified during metamorphism the radiogenic heat production data for the area are calculated and using the measured regional heat flow value from nearby area an estimate of the relative thickness of the continental lithosphere is made using the model developed by oxburgh 1981

nd isotopic systematics and chemistry of central australian sapphirine granulites an example of rare earth element mobility abstract lower proterozoic sapphirine bearing and associated granulites from central australia exhibit the greatest range of present day 143 nd 144 nd ratios nd o 26 5 to 112 3 yet reported for rocks believed to be cogenetic the nd isotopic data and ree abundances of these rocks demonstrate extreme fractionation of the rare earths during the formation of stratiform cu pb zn sulfide deposits with which they are closely associated field relationships petrography and chemistry of the sapphirine granulites suggest that their protoliths comprised chlorite rich rocks which were generated by hydrothermal alteration of a range of rock types prior to metamorphism calculations employing ree abundances of the sapphirine granulites and associated rocks combined with bulk solid fluid distribution coefficient data yield high fluid rock ratios consistent with a pre metamorphic hydrothermal origin for the unusual ree patterns the sm nd data for these rocks define an age of 1760 75ma which is significantly younger than the crust formation age of the terrain 2070 125ma but indistinguishable from the rb sr whole rock age for granulite facies metamorphism 1790 35ma these data are interpreted in terms of major hydrothermal fractionation of the rare earths shortly perhaps tens of millions of years before granulite facies metamorphism followed by redistribution of nd isotopes or small fractionations of the sm nd ratio during the granulite facies event and possibly also during intense retrogression which reset rb sr whole rock and u pb zircon and monazite systematics at about 1700 ma

water activity changes across an amphibolite granulite facies transition broken hill australia phase compositions in pelitic and mafic gneisses place tight constraints on pressure ranging from 3 up to 6 kb and to a lesser extent on temperature 500 up to 800 c during prograde regional metamorphism of the willyama complex broken hill se australia these limits allow an evaluation of water activity across the terrain using various equilibria in pelitic and mafic gneisses the stability of cummingtonite and biotite over much of the terrain places upper limits on temperature and the presence of syn metamorphic partial melts in the metasediments places lower limits on a h2o garnet biotitesillimanite k feldspar quartz relations combined with the partial melting data suggest a decrease in water activity from near 1 0 in the lower grade zones to 0 5 0 2 in the broken hill little broken hill part of the two pyroxene zone this result is compatible with less precise hornblende orthopyroxene clinopyroxene quartz relations

conflicting mineral and whole rock isochron ages from the late archaean lewisian complex of northwestern scotland implications for geochronology in polymetamorphic high grade terrains we report major and trace element including rare earth element ree geochemistry and whole rock smnd geochronology on two suites of mafic ultramafic inclusions in an agmatite complex within a trondhjemitic host at gruinard bay in the late archaean lewisian complex of northwest scotland a suite of medium grained equigranular amphibolites main amphibolite suite mas has broadly chondritic high field strength element hfse ratios ti zr ca 115 ti y ca 318 y cr ca 0 39 flat heavy ree patterns at five to fifteen times chondrites usually with mild light ree enrichment lan 50 and small eu anomalies this is consistent with an origin as basaltic liquids which may represent suitable mafic precursors for tonalite trondhjemite granodiorite ttg suite magmas as argued previously by rollinson and fowler 1987 a smnd regression line mswd 3 4 obtained from the mas yields an age of 2 943 0 091 ga with an initial end value of 2 69 0 62 a suite of coarser grained hornblendites and metagabbros hornblendite metagabbro suite hms displays significant geochemical differences from the mas exemplified by significantly light ree enriched patterns in which the hornblendites have the highest total ree with lan ndn 1 and large negative eu anomalies and the most felsic metagabbros have low total ree and large positive eu anomalies samples from the hms yield a smnd isochron mswd 0 52 age of 2 846 0 073 ga with an initial end value of 1 10 0 72 the hms geochemistry is consistent with an origin as amphibole and plagioclase cumulates to the local ttg magmas combining the new data with previously published data from the host trondhjemites at gruinard bay whitehouse 1989a yields a smnd isochron mswd 1 09 age of 2 795 0 028 ga with an initial end value of 0 66 0 38 the relatively low initial end values for the hms and combined hms trondhjemite isochrons compared to contemporaneous depleted mantle end 2 8 ca 2 1 to 3 3 is consistent with derivation of a parental magma from light ree enriched components of a mafic basaltic precursor which was itself derived from a depleted mantle source approximately 100 200 ma previously the mas suite at gruinard bay may be representative of such a mafic precursor our new geochronological data differ significantly from the ca 3 3 ga mineral smnd and pbpb regression ages presented recently for two members of the hms by burton et al 1994 which if correct require radical revision of the early geological history of the lewisian complex comparison of the relatively old mineral ages with our new whole rock age contradicts the conventional assumption that mineral isotope systematics are less robust to later disturbance than whole rock systematics however we argue that extensive retrogression evident in all lithologies at gruinard bay makes them less than ideal samples for a mineral isotopic study and it is likely that the ca 3 3 ga ages may be spurious the product of disturbance of isotope systematics during the laxfordian on the basis of the geochronology presented here evolution of the lewisian closely follows generally accepted models for late archaean crust formation

u pb zircon systematics at gruinard bay northwest scotland implications for the early orogenic evolution of the lewisian complex the gruinard bay area of the mainland lewisian complex comprises a metamorphosed suite of archaean trondhjemites and minor granites enclosing remnants of older tonalitic gneiss and mafic to ultramafic enclaves the u pb zircon dating yields ages of 2731 14 ma and 2728 2 ma for two trondhjemite and 2732 4 ma for one granite sample also revealing the presence of large amounts of inherited xenocrystic zircons although the region has been pervasively overprinted by retrogressive events in amphibolite to greenschist facies the textural relations between biotite hornblende quartz and titaniferous minerals indicate that these minerals are pseudomorphs of pyroxene and high ti amphibole formed in hornblende granulite facies structural relations link this metamorphism to a steep northeast trending fabric coeval with the intrusion of the trondhjemites dated at 2730 ma dating of zircon in amphibolite and tonalite enclaves yields complex internal isotopic relations with apparent ages ranging from 2825 to 2740 ma this age range reflects new growth during the 2730 ma metamorphic metasomatic events superimposed on older zircon phases which include combinations of xenocrystic cores and magmatic and or metamorphic growth phases whose mode of formation cannot clearly be resolved by imaging techniques e g cathodoluminescence alone a pegmatitic vein that escaped the d3 strain and related isotopic disturbances yields a precise age of 2792 2 ma which constrains to some degree the earliest orogenic events in the area age relationships displayed in the central block at scourie badcall and in the gruinard bay area indicate that petrogenetic events in both areas were comparable about 2800 ma and that both areas underwent trondhjemitic magmatism about 2730 ma in contrast at gruinard bay there is no isotopic evidence for a period of high grade metamorphism and magmatism at 2490 2480 ma that drastically affected the scourie block indicating that at this stage the two regions occupied different levels of the crust

the mafic ultramafic complex near finero ivrea verbano zone i chemistry of morb like magmas geochemical data are presented for the meta igneous mafic ultramafic complex near finero this complex is in contact with a phlogopite bearing mantle peridotite and is subdivided into the internal gabbro unit the amphibole peridotite unit and the external gabbro unit the internal gabbro and the amphibole peridotite units consist of coarse grained chemically heterogeneous cumulates whereas the external gabbro unit is generally massive chemically more uniform and approximately representative of the residual melt with mgo contents between 6 6 and 9 1 and mg numbers between 38 and 58 both whole rock and mineral contents of ni and cr are significantly higher at similar mg numbers in the amphibole peridotite unit than in the internal gabbro unit the most straightforward interpretation of this is that the amphibole peridotite unit accumulated after the influx of fresh mafic or ultramafic magma into the magma chamber major element chemical trends are continuous from the amphibole peridotite unit to the external gabbro unit and are consistent with closed system fractionation with no further addition of magma or contamination by wall or roof rock assimilation in the external gabbro unit total feo and tio2 contents are strongly correlated with each other and with p2o5 and zr and reach values as high as 19 and 4 respectively indicating an advanced degree of crystal fractionation along a tholeftic trend the external gabbro samples have generally smooth normalized trace element patterns which are consistent with being representative of a liquid composition the residual nature of the external gabbro magma is also indicated by negative eu and sr anomalies clear evidence for prior feldspar fractionation ree patterns are otherwise indistinguishable from n type morb but th and u are significantly more depleted than in morb this th and u depletion is similar to that found in olivine basalts and picrites on iceland and hawaii its origin is not well understood no evidence is seen for any assimilation of crystal material in sharp contrast with the situation of the igneous complex in val sesia near balmuccia where the magma composition is dominated by assimilation of crust we suggest that the heat provided by at most two injections of magma near finero was insufficient to induce crystal anatexis in contrast with the excess heat supplied by multiple magma injections at balmuccia

the geochemistry of prograde and retrograde charnockite gneiss reactions in southern india abstract exposures in many quarries in southern india exhibit field evidence for incipient charnockitization of tonalitic and granitic gneiss prograde relationship or retrogression of charnockite to produce tonalitic gneiss retrograde relationship few systematic geochemical relationships exist between adjacent gneisscharnockite sample pairs during either prograde or retrograde reactions most elements and element ratios exhibit inconsistent variations however prograde chamockites appear enriched in ta pb volatiles chiefly co 2 and in transition metals relative to mg and depleted in ree and y compared to adjacent gneiss protoliths retrograde gneisses have higher rb pb th hf zn relative to co nb relative to ta hf relative to zr and volatiles chiefly h 2 o compared to parental charnockites of those elements u th rb cs pb significantly depleted in high pressure charnockites exposed south of the prograde transition zone only pb is significantly replenished during retrogression evidence suggests that prograde fluids are relatively rich in co 2 and retrograde fluids in h 2 o and that the typical non systematic geochemical variations during prograde and retrograde reactions reflect local effects at the wave front

metamorphism of reduced granulites in low co 2 vapour free environment pervasive flooding of co2 has been proposed as the cause of granulite facies metamorphism that is capable of producing many distinctive characteristics of the deep continental crust reduced water activity orthopyroxene bearing assemblages depletion of large ion lithophile lil elements and dehydration1 6 the calculations presented here of c o h fluid composition for conditions of granulite facies metamorphism show that oxygen fugacity f o2 estimates from many terranes are sufficiently low that the addition of co2 rich fluid causes graphite to precipitate for values of pressure p and temperature t common to granulites and with f o2 slightly below the quartz fayalite magnetite buffer qfm the addition of co2 sufficient to grow 10 vol orthopyroxene requires the precipitation of 1 5 vol graphite as 0 1 vol graphite is readily recognizable but is not reported in most low f o2 granulites these rocks have not been flooded by co2 and low f h2o is probably due to extraction of a magma or recrystallization of an already dry rock7 8 granulite terranes may thus result from a combination of these three processes and the dominance of any one cannot now be demonstrated on a regional basis

light rare earth element zoning in sphene and allanite during granitoid fractionation sphene and allanite rare earth element ree and whole rock light ree concentrations have been found to increase during granitoid fractionation the ree concentrations in both sphene and allanite crystals are zoned with relative depletion occurring at the crystal rims the ree concentration variability and zoning in these minerals can be explained by changes in the melt structure that occur during differentiation and final solidification of the granitoid our data suggest that crystal melt separation controls light ree enrichment or depletion in granitoids and that the end result depends on liquidus accessory and or restite phases present during the crystal melt separation

layered ultramafic gabbro bodies in the lewisian of northwest scotland geochemistry and petrogenesis abstract layered ultramafic gabbro bodies occur widely in the archaean of northwest scotland they were metamorphosed at granulite or high amphibolite facies and were tectonically thinned and broken up during deformation they comprise repeated ultramafic gabbro layers locally with ni poor sulphide rich tops each rhythmic unit showing decreasing mgo ni and normative anorthite with stratigraphic height major trace and rare earth element data are presented for the range of rock types in ultramafic rocks mgo varies from 22 to 37 wt ni from 1000 to 2500 ppm and tio 2 from 0 08 to 0 40 wt while the mgo content of the gabbros ranges from 14 to 6 wt the ree patterns are flat to lree enriched with no significant eu anomalies in ultramafic rocks ree are from 4 to 10 times chondrite and in the gabbros lree range from 8 to 30 times chondrite and hree from 6 to 15 times chondrite study of incompatible elements ti zr y which are relatively immobile during metamorphism shows that neither garnet nor hornblende were involved in fractionation trace element modelling shows it is improbable that the ultramafic rocks represent primary mgo rich liquids even though their incompatible element contents are quite high the chemical trends are interpreted in terms of olivine and pyroxene settling from a tholeiitic high mg magma with 15 20 wt mgo derived by 30 40 partial melting of an undepleted mantle the ultramafic rocks are the cumulates and the gabbros the derived liquids

geochemistry and origin of granitic rocks scourian complex nw scotland concordant granite sheets from the granulite facies scourian complex n w scotland exhibit the following features 1 a common planar fabric with their host pyroxene granulites 2 the presence of an exsolved ternary feldspar phase 3 a low pressure water saturated minimum composition 4 k rb ratios 450 1 350 distinctly higher than most upper crustal granites but similar to the surrounding granulites 5 low absolute concentrations of the rare earth elements rees light ree enrichment and large positive eu anomalies

large ion lithophile element characteristics of an amphibolite facies to granulite facies transition at gruinard bay north west scotland abstract lewisian grey gneisses from gruinard bay north west scotland retain mineralogical and geochemical evidence for scourian horn blende granulite facies metamorphism and they may be used to assess current models of elemental depletion at granulite grade their immobile major and trace element geochemistry is indistinguishable from that of lewisian amphibolite and pyroxene granulite facies counterparts the k rb th and u contents of the gruinard bay gneisses are depleted relative to amphibolite facies gneisses but generally the abundances of these elements are above those of comparable pyroxene granulites u and th have reached an advanced stage of depletion but allanite appears to be crucial in maintaining significantly higher u and th abundances at gruinard bay than in pyroxene granulites k and rb loss is less extreme and depends on the stability of the rock forming minerals k feldspar biotite and amphibole early removal of k and rb has resulted in a small rise in k rb but further preferential rb loss would have been required to generate the characteristically high k rb ratios of lewisian pyroxene granulites the residence of u and th in the accessory minerals of granulite facies gneisses which are often correlated with the residua of intracrustal partial melting renders unlikely their extreme incompatibility required by such models even if such phases are ignored high mineral melt partition coefficients for silicic melts argue against partial fusion as an efficient depletion mechanism on the other hand the advanced stage of u and th depletion reached in gruinard bay gneisses which were still partly hydrous severely restricts the role played by co2 dominated fluids and a hydrous medium is preferred

lewisian gneiss geochemistry and archaean crustal development models abstract the geochemistry of lewisian amphibolite facies gneisses from northwest scotland is described with particular reference to the rare earth elements ree and compared with the geochemistry of lewisian granulite facies gneisses the results show that there are no significant differences between laxfordian amphibolite facies and scourian granulite facies gneisses in terms of ree and other immobile trace elements at equivalent silica levels although the mobile radioactive heat producing elements k rb th u are significantly lower in the granulites in both types the basic gneisses have moderately fractionated ree patterns while the intermediate and acid gneisses have strongly fractionated ree patterns with low heavy ree abundances and decreasing levels of total ree with increasing sio 2 the most silicic gneisses develop large positive europium anomalies these gross chemical similarities between gneisses from intermediate amphibolite facies and lower granulite facies crustal levels constrain models for the evolution of the archaean crust the depletion of k rb cs th and u in granulites but not other incompatible trace elements cannot be explained by magmatic processes the positive eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation fractionation of radioactive heat producing elements from other trace elements is a result of granulite facies metamorphism with these elements being removed by an active fluid phase the apparent lack of partial melting in lower crustal granulites suggests a model for archaean crustal growth largely through underplating by primary tonalitic magmas

geochemistry of the archean low to high grade transition zone southern india the transition zone between archean low and high grade rocks in southern india represents eroded crustal levels representative of 15 20 km it is comprised chiefly of tonalitic gneisses with some varieties showing incipient charnockitization and of minor amounts of granitic gneiss and charnockite both of which appear to have developed from the tonalitic gneisses