geochemistry and nd isotope systematics of archaean granites of the eastern goldfields yilgarn craton australia implications for crustal growth processes abstract the archaean granites and granitic gneisses of the eastern goldfields in the yilgarn craton western australia can be divided into two major groups high ca and low ca and three minor groups high hfse high field strength elements mafic and syenitic the high ca group 68 77 sio 2 with high al 2 o 3 na 2 o and sr and low y shares many features with typical archaean tonalite trondhjemite suites but has higher k 2 o rb and th contents the low ca group 70 76 sio 2 differs from the high ca group in having lower al 2 o 3 cao and na 2 o but higher k 2 o rb th zr y la and ce contents granites of the high hfse mafic and syenitic groups form a minor component 10 20 of the eastern goldfields granites the siliceous 74 77 sio 2 high hfse granites are restricted to a narrow nnw trending zone and are characterised by high tio 2 total feo mgo y zr and ce but only moderate rb th and pb contents the a type syenites 50 68 sio 2 are distinguished by their high total alkalies and mainly occur along tectonic lineaments the mafic group 55 70 sio 2 is lithologically diverse and exhibits a wide range of k 2 o rb th la and ce contents granite emplacement occurred between 2 69 and 2 60 ga contemporaneous with or post dating greenstone formation nd values for the high ca mafic and syenitic groups are similar mostly 0 0 to 2 5 whereas those for the low ca group show a pronounced polarity from 4 5 in the west to 2 0 in the east nd depleted mantle model ages for the low ca granites range from 3 2 ga in the west to 2 75 ga in the east model ages for the high ca group 2 75 2 9 ga overlap with ages for inherited zircons from these granites the high ca granites appear to have been derived at high pressures by partial melting of a mafic to intermediate source subducted oceanic crust crustal underplate or thickened crust or remelting of an older tonalitic source derived by such a process if the high ca granites were largely crust derived then their extensive source rocks must have formed less than 250 ma prior to granite formation the low ca granites appear to be the products of crustal reworking probably from a tonalitic to granodioritic protolith that was progressively younger towards the east perhaps in the order of several hundred million years younger or involved an additional younger component the localisation of the crustally derived high hfse granites suggests either geochemical zonation of the crust or specific tectonic processes such as rifting the mafic granites and possibly the syenites imply a significant mantle contribution and new crustal growth although the extent is equivocal the crust of the eastern goldfields appears to have been generated by both lateral and vertical accretion over a significant period from or before 3 0 ga until the time of granite emplacement

role of crustal contamination in heterogeneous archean volcanics from the leonora region western australia abstract archean supracrustal volcanic successions in the leonora laverton region of the eastern goldfields province western australia are subdivided into two stratigraphic associations 1 an early succession ten km thick containing homogeneous high ti tholeiite basalts displaying nearly flat chondrite normalized incompatible lithophile element patterns with la sm n ratios ranging from 0 88 to 1 12 and 2 an unconformably overlying heterogeneous succession four km thick consisting of a early siliceous low ti tholeiites and associated intermediate calc alkaline volcanics grading to b high ti tholeiite similar to those of association 1 and c late siliceous high mg spinifex textured basalts fractionation models and trace element distribution in high ti basalts suggest that all the flows were derived from similar but not identical parental magmas and that subvolcanic gabbroic fractionation was characteristic basaltic end members of the siliceous low ti suite had identical fractionation patterns but hornblende joined the assemblage in contaminated intermediate calc alkaline members producing low hree fractionates the siliceous basalts exhibit variable enrichment in large ion lithophile lile and light rate earth lree elements with a continuum of la sm n ratios up to 2 75 they also have marked negative anomalies for nb p and ti several samples also exhibit a positive anomaly for zr on normalized distribution diagrams low sr concentrations are inconsistent with mantle enrichment processes and compositions of siliceous volcanics are explained instead by up to 30 contamination of high ti tholeiite liquids by archean crust composed of hornblende tonalite gneiss geochemistry of the volcancis is consistent with eruption in a continental back arc basin tectonic setting

the santa catarina granulite complex of southern brazil a review abstract the santa catarina granulite complex scgc is mainly composed of intermediate to felsic orthogneisses of trondhjemitic tonalitic and granodioritic compositions ttg intruded by subsidiary basic ultrabasic rocks and a small metasedimentary component the ortho derived rocks belong to different igneous suites nd model ages tdm of 2 7 2 8 ga date the first major mantle crust differentiation in the area while 2 6 ga rb sr whole rock isochron ages in both depleted and undepleted granulites and nd model ages of 2 4 2 3 ga record subsequent magmatic events regional granulite facies which reached 800 c at ca 5 8 kb finished at 2 3 ga according to u pb zircon and sm nd mineral isochron ages and was also followed by a regional amphibolite facies metamorphism at 2 0 ga recorded by pb pb rb sr whole rock isochrons in rocks with little orthopyroxene and predominant amphibole tectonic stabilization was complete at the end of the transamazonian cycle at 1 9 ga as indicated by k ar mineral ages and apart from local reactivation along faults at the margins of the complex and granite intrusion the scgc was not affected by the neoproterozoic brasiliano cycle during which it acted as a microplate and was incorporated onto the eastern border of the parana craton isotopic and geochemical characteristics of brasiliano crustal granites which intrude the complex suggest that it is not an adequate source for the melts and thus the scgc may be underlain by different paleoproterozoic 2 0 ga crustal material

boron and lithium in high grade rocks and minerals from the wawa kapuskasing region ontario there is no preferential partitioning of boron among the principal rock forming minerals in high grade rocks of the kapuskasing structural zone ksz and the wawa domal gneiss region wdg lithium is strongly concentrated in biotite and other ferromagnesian minerals but does not show consistent partitioning between these and the sialic minerals the distribution of b and li within a rock may be studied using an alpha track image which shows that the inconsistencies in partitioning may be largely attributed to disturbance of mineral equilibria by postmetamorphic low grade alteration that deposited b and li boron has similar concentrations in all the rock types studied although it is an incompatible element that elsewhere accumulates in pegmatites lithium concentrations are low in the anorthositic rocks but are otherwise very variable in some but not all rocks higher than usual b and li can be attributed to introduction during alteration boron occurs at low concentrations 2 3 ppm throughout both the k

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

major and trace element geochemistry of the archaean ntem plutonic complex south cameroon petrogenesis and crustal evolution the ntem complex crops out in south cameroon and belongs to the congo craton it corresponds to a 2 8 ga basement which in the northern part is made up of plutonic rocks 1 charnockites and 2 soo tonalites both have similar compositions and define a typical tonalite trondhjemite and granodiorite ttg suite the study of isotopes isr major elements and ree allows us to propose a petrogenetic model that may summarized as follows 1 1 partial melting of the mantle gave rise to tholeiites 2 2 tholeiites transformed into garnet bearing amphibolites underwent partial fusion thus generating the parental magma of the ttg suite 3 3 fractional crystallization mainly of hornblende and plagioclase produced the differentiated ttg suite this scheme is typical of archaean ttg suites and is supposed to have occurred in a subduction like geodynamic environment where because of the high archaean geothermal gradients the subducted oceanic crust underwent fusion before dehydration operated it is also possible to show that the ttg suite was not emplaced during the granulite facies metamorphism but that this occurred later affecting only some parts of the basement consequently the ntem complex appears to be an archaean high grade terrane where the granulitic assemblages may be early proterozoic in age characteristic that seems to be more widespread among archaean high grade terranes

archaean cratonization and deformation in the northern superior province canada an evaluation of plate tectonic versus vertical tectonic models abstract the archaean minto block northeastern superior province is dominated by tonalite trondhjemite enderbite pyroxene tonalite granodiorite and granite with subordinate mafic rocks and supracrustal belts the plutons have been interpreted as the batholithic roots of andean type plate margins and intra oceanic arcs existing horizontal tectonic models propose that penetrative recrystallization and transposition of older fabrics during terrane assembly at 2 77 and 2 69 ga produced a n nw tectonic grain in the douglas harbour domain northeastern minto block tonalite and trondhjemite dominate the faribault thury complex 2 87 2 73 ga and enderbite constitutes 50 100 km scale ovoid massifs troie and qimussinguat complexes 2 74 2 73 ga magmatic muscovite and epidote in tonalite trondhjemite have corroded edges against quartz plagioclase suggesting resorption during ascent of crystal charged magma foliation maps and air photo interpretation show the common development of 2 10 km scale ovoid structures throughout the douglas harbour domain outcrop and thin section scale structures imply that many plutons experienced a phase of syn magmatic deformation typically followed by high temperature sub magmatic overprints thermobarometric data for plutons indicate near solidus recrystallization at 4 6 kbar pressures the common preservation of syn magmatic fabrics in plutons of different ages seems incompatible with the origin of these fabrics through superimposed regional orogenesis the broad uniformity of intrusion ages and lithologies throughout the minto block and the rarity of shallowly dipping planar fabrics also seem inconsistent with accretion of disparate older terranes each of which should preserve distinct histories a possible alternative explanation for these features is provided by vertical tectonic models whereby buoyant felsic magmas ascended as crystal slurries while dense supracrustal rocks and solidified felsic intrusions emplaced into them subsided as cold fingers 10 20 km scale instabilities shear between upwelling and downwelling limbs would have concentrated in the weak intrusions generating steeply plunging syn magmatic fabrics and producing ductile overprints in solidified rocks

ree geochemistry of early precambrian charnockites and tonalitic granodioritic gneisses of the qianan region eastern hebei north china abstract the charnockites and gneisses from the qianan area eastern hebei province china were investigated for their isotope and trace element abundances the majority of the charnockites are acid in composition only a few samples have a composition corresponding to intermediate to basic rocks a rb sr whole rock isochron of the charnockites gives an age of 2 65 0 05 ga the highly fractionated ree patterns and the variable ce yb n ratios in these rocks are interpreted as being a result of igneous processes it is believed that the charnockitic rocks are related to each other by fractional crystallization under high temperatures and low p h 2 o conditions the major precipitating phases are thought to be orthopyroxene and plagioclase by contrast the qianan gneisses are mainly of tonalitic and granodioritic composition however a few samples have composition corresponding to granite s s probably produced by metasomatic processes a rb sr whole rock isochron of the qianan gneisses yields an age of 2 13 0 06 ga and this age probably represents a remobilization event the qianan gneisses do not contain hypersthene they have strongly fractionated ree patterns similar to those found in many archaean tonalitic rocks of the world in addition the total ree abundances and the magnitudes of the eu anomaly show regular variation with the sio 2 content this may suggest that the qianan gneisses might have evolved under hydrous conditions with hornblende exerting a major control on their trace element chemistry

geochemistry petrogenesis and tectonic significance of the early proterozoic loch maree group amphibolites of the lewisian complex nw scotland

late archaean felsic alkaline igneous rocks in the eastern goldfields yilgarn craton western australia a result of lower crustal delamination numerous felsic alkaline intrusions were emplaced in the granite greenstone terrain of the eastern goldfields yilgarn craton between c 2650 and 2630 ma intrusion coincided with the waning stages of voluminous regional granitic magmatism the alkaline rocks are localized along and adjacent to major crustal structures that separate lithostratigraphic greenstone terranes and are divided into four geographically and compositionally distinct suites viz the mount monger emu claypan and ninnis suites the rocks have features in common with a type granites including high total alkalis an anhydrous primary mineralogy an apparent anorogenic setting and enrichment in some hfse although they also have low concentrations of nb and y they show a range of features that together point to a crustal source e g the alkaline rocks are not associated with voluminous mafic magmatism they have high lile concentrations they carry zircon xenocrysts of similar age to whole rock sm nd model ages they show evidence for regional source heterogeneity suites have constant and distinct la y and nb y ratios and the rocks show many compositional similarities to the slightly older voluminous crust derived granites extreme enrichments in sr and ba in the mount monger suite however suggests that their source may have been metasomatized before or during melting intrusion of the alkaline rocks heralds a poorly recognized third tectonothermal event in the evolution of the eastern part of the yilgarn craton and was broadly synchronous with the intrusion of voluminous felsic fractionated granites crustal extension and widespread au mineralization the late archaean geology of the eastern goldfields shows many features in common with phanerozoic collisional belts thought to be profoundly influenced by processes of lithospheric delamination or convective thinning this newly recognized third tectonothermal event in the eastern goldfields may relate to the delamination or convective thinning of a dense garnet rich lower crust that resulted from extraction of earlier voluminous granite magma and it is possible that such processes have had a greater influence on late archaean crustal evolution than previously recognized

investigating the maximum resolution of µxrf core scanners a 1800 year storminess reconstruction from the outer hebrides scotland uk micro x ray fluorescence xrf core scanning is capable of measuring the elemental composition of lake sediment at sub millimetre resolution but bioturbation and physical mixing may degrade envir

assimilation and partial melting of continental crust evidence from the mineralogy and geochemistry of autoliths and xenoliths this paper presents a model for the partial melting of quartz diorite and greywacke in the upper crust based on the mineralogy and geochemistry of enclaves within the loch doon granitic intrusion of southern scotland the melting of quartz diorite was modelled using autoliths which represent fragments of cogenetic igneous rocks that became incorporated in the fractionating magma compared to their quartz diorite parents the autoliths are enriched to varying degrees in some elements notably rb nb ta sm y yb and depleted in others sr and ba eu and p are also depleted in the more assimitated autoliths the compositions of melts that could be derived from assimilation of the autoliths have also been calculated their ree patterns reveal a light ree enrichment low concentrations of heavy rees 1 3 x chondrite and a positive eu anomaly the calculated degrees of melting vary from 35 in the least assimilated to 84 in the most assimilated autolith assuming a bulk distribution coefficient of 10 for the most compatible element results from modelling of xenolith compositions derived from metasediments are also reported but because of uncertainties in the composition of the parental sediment these data are subject to larger errors they do however indicate that resultant partial melts are distinctly different from those derived by partial melting of autoliths in particular the ree pattern of a greywacke derived melt shows a slight enrichment in light rees greater concentrations of heavy rees 10 x chondrite and a small negative eu anomaly the calculated degrees of melting of the xenoliths fall in the range of 66 88 assuming a bulk distribution coefficient of 10 for the most compatible element the results have direct implications for assimilation and melting of the upper crust by taking into account how the nature of residual phases is likely to change with depth it can be demonstrated that some archaean tonalite gneisses could represent liquids derived by partial melting of igneous material

tectonic evolution in the archaean and proterozoic kre high heat flow probably favoured global hotspot activity with crustal growth of iceland type through vertical magmatic accretion the tectonic environment for the generation of about 3 9 2 5 ga granite gneiss greenstone terrains with volurnlnous production of tonalite trondhjemite granodlorite magmas is still uncertain and both piate ma and intrapiate scenarios arc possible deformanon atylea in greenstones and adjacent high grade orthognersses with intercalated shallow water supracrustal assemblages suggest extensive horizontal shortening and crustal interstacking this probably resulted from collisional and or rotational motion and produced significant intracrustal melting as early as 3 9 ga by the end of the archaean and locally much earlier crustal thicknesses reached 35 50 km or more and the cxtl aordinary hhlding to erogenic deformation may have been due to delamination and crustal subduction towards the end of the proterozoic evidence for modern style plate tectonics becomes abundant through the recognition of laland a16 accretion ophiolite obducdon foreland thrust and fold belts and exotic terranes one of the best documented example for this tectonic environment is the arabian nubian shield 4lthough it is doubtful whether modern plate tectonic settings can serve as analogues for all precambrian tectonic styles it i clear that global crustal evolution since the early archaean was governed by the motion of plates which repeatedly came togethel tv fotm supercontinents and then dlspersetl again we shall not understand the detailed mechanism of precambrian crustal itlton cs before we learn horn lo irltejpret recent large scale crustal deformation and its causes

tectonics of the pliocene homs basalts syria and implications for the dead sea fault zone activity timing of activity along the yammuneh segment lebanon syria of the dead sea fault zone and its northward continuation is still a subject of controversy our field structural analysis and observations on radar landsat and digital elevation model imagery of the homs region syria are of concern for plio quaternary tectonics of the whole northern part of the dead sea fault zone we show in this paper that the northern dead sea fault zone has remained active since the onset of the homs basalts at c 6 ma continuing movement in recent times is indicated by the occurrence of quaternary pull apart structures and offset of active ravines along the fault the homs basalts are related to the distinct oval shaped shin volcanic edifice of which the long axis trends nw the volcano was fed through nw striking tension fractures which now form dykes and volcanic ridges these patterns are consistent with a ne sw extension that occurred c 6 ma ago the northwestern end of the shin volcano is left laterally displaced c 20 km yielding a c 3 3 mm a 1 mean rate of relative movement between the arabian and african plates in the northern part of the dead sea fault zone the overall trace of the main active dead sea fault zone is not a single transform but forms an irregular plate boundary composed of transform fault and collision belt segments fine grained mylonite developed in the fault corridor may have favoured aseismic deformation in the shin volcano area

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

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

some aspects of the geochemistry provenance and palaeoclimatology of the torridonian of nw scotland geochemical data mostly from mudstones are used in an attempt to investigate some aspects of the palaeoclimatology and provenance of the torridonian succession in nw scotland the basal part of the stoer group which has been interpreted as glaciogenic is thought to have formed in a warm arid setting the presence of oscillation ripples and desiccation cracks in mudstones containing isolated clasts is thought to preclude their interpretation as ice rafted debris some aspects of the major element geochemistry of the stoer group mudstones suggest deposition in an environment with little weathering but chemical weathering is inhibited in both frigid and arid settings and sedimentological evidence favours the latter the stoer group mudstones are enriched in mg ca and na relative to the torridon group samples which are much closer in composition to an average shale estimate paas the cr and ni contents of the stoer mudstones are significantly higher than those of the torridon group samples there is however no evidence that enrichment in these elements was due to contamination with volcanic material like that of the stac fada member the scourian basement appears to be an adequate source of the ni and cr in the stoer group sediments th sc ratios are higher in the torridon than in the stoer mudstones this difference is interpreted as indicating that the stoer mudstones were largely derived from local archaean sources but that the torridon group mudstones incorporated materials from a much wider provenance including younger and recycled materials evidence from rare earth elements also provides evidence for different source terrains for the stoer and torridon groups the torridon group mudstones are more weathered have a higher ree content and display a more prominent eu anomaly lower value for eu eu than do the stoer samples these results support evidence from th sc ratios that the torridon group was derived from a more varied recycled and granite rich terrain than the stoer much of which appears to have relatively local provenance the geochemistry of mudstones can shed considerable light on problems of provenance and palaeoclimatology because many useful trace elements tend to be concentrated in such fine grained rocks and some aspects of their major element chemistry may be related to climatic factors

regional metamorphism of the vassalboro formation south central maine usa a case study of the role of fluid in metamorphic petrogenesis progressively metamorphosed carbonate rocks of the vassalboro formation serve a natural laboratory for the investigation of fluid rock interactions during metamorphism an integrated study of their whole rock chemistry mineralogy mineral chemistry and mineral abundances has led to the following results a prograde mineral reactions involved hydrolysis as well as dehydration and decarbonation the hydrolysis reactions have resulted in an almost quantitative extraction of k and na from high grade metacarbonates b prograde mineral reactions buffered the x co 2 of coexisting metamorphic fluid during almost all of the rocks metamorphic history c the carbonate rocks were infiltrated by large volumes of h 2 o rich fluid while the buffering reactions progressed during the metamorphic event some high grade metacarbonates were infiltrated by at least 3 rock volumes of fluid combined buffering and infiltration appears to be a general metamorphic phenomenon in carbonate bearing rocks to depths of 30 km in the crust d there is an excellent positive correlation between the heat budget of individual metacarbonate rock samples and calculated fluid rock ratios convective heat transfer by metamorphic fluids is therefore likely to be an important item in the heat budget of metamorphic terrains consideration of these results leads to a model of regional metamorphism which involves infiltration of rock by large volumes of acid h 2 o rich fluid large scale acid infiltration metasomatism in this model the infiltrating fluid exerts enormous control over the mineralogical evolution of the carbonate rocks during the metamorphic event it drives mineral reactions it changes whole rock chemistry and it supplies heat

petrological evolution of the crust and mantle in its simplest form crust mantle evolution can be thought of as a study of the processes by which material is lost from the mantle and forms new crust the overall goal is achievement of mass balance between the average composition of material known to have been added to the crust and the estimated average composition of the entire crust attainment of this goal for the earth has been elusive primarily because the materials available for study have generally been involved in several stages of evolution including fractionation within both the crust and mantle and recycling of material from crust to mantle thereby complicating the interpretation of available data although the overall composition of the continental crust generally has been modelled as andesitic taylor and mclennan 1981a the present upper continental crust is granodioritic and requires a more mafic lower crust it has been proposed taylor and mclennan 1981a that originally the archean crust had an andesitic composition throughout its volume and that partial melting in the lower crust developed more felsic melts that rose to enrich the upper crust leaving a more mafic residuum of lower crust this concept has been challenged by weaver and tarney 1981a who argue that the granulites of presumed lower crustal origin do not have the correct compositions for this model if one considers more recent additions to the crust from the mantle over the past several hundred million years it appears that these latest additions are basaltic with very little if any granitic additions in fact barker 1981 in his introduction to a special issue of jgr on granites and rhyolites states to date no one has demonstrated that such magmas may form in the mantle indeed much experimental and geochemical work indicates that such siliceous liquids cannot originate there therefore such magmas must be generated in the crust in island arc environments where the average composition of added material normally is thought to be andesitic a anderson 1982 has shown that the parent melt is basaltic not andesitic in addition kay 1980 has shown that the k content of the newly added material is less than the average k content of the previously existing crust in the oceanic crust the additions from the mantle are basaltic and according to o hara 1982 probably are derived from very mafic picritic melts as parents continental flood basalts which have also added material from mantle to crust also are clearly derived from basaltic parental melts basaltic volcanism study project 1981 thus the evidence for the nature of material subtracted from the mantle and added to the crust for the past several hundred million or perhaps a billion years points to very mafic melts this contrasts significantly with the overall andesitic composition of continental crust it implies furthermore that if the overall archean crustal composition was andesitic as estimated by taylor and mclennan 1981a then additions to the crust have become more mafic with time a concept in direct conflict with most models in which secular trends are from mafic to felsic with time veizer and jansen 1979 and mclennan et al 198o a question which is beginning to appear either explicitly or implicitly is whether crustal volcanic rocks or the crust as a whole really represents the parent melt that formed in the mantle a anderson 1982 o hara 1982 and nisbet and walker 1982

geochemical and sr and nd isotopic constraints on the origin of late proterozoic volcanics and associated tin bearing granites from the franklin mountains west texas late proterozoic volcanic and granitic rocks from the franklin mountains in west texas define a continuous tholeiitic trend from rhyolite granite to trachyte with a small compositional gap betwee

elemental evidence for an enriched small fraction melt input into tertiary mull basalts western scotland continental basalts enriched in the incompatible trace elements have recently been interpreted as mixtures of depleted asthenospheric melt and an enriched small fraction melt lithospheric component hebridean tertiary basalts are relatively depleted in incompatible trace elements and it has been suggested that this is because the enriched lithospheric component had already been extracted beneath that region during the permo carboniferous it was therefore not widely available to contaminate tertiary magmas a flow by flow geochemical study of the mull lava succession has nevertheless revealed the presence of lava flows relatively enriched in the incompatible trace elements at the base of the succession in some parts of the island fractional crystallization contamination by archean and moinian crust variation in the degree of mantle melting and an asthenospheric ocean island basalt like source have all been ruled out as possible mechanisms of enrichment in favour of 5 10 contamination by an enriched fusible small fraction melt from the lithospheric mantle the overall lack of basalts enriched in trace elements in the british tertiary igneous province argues strongly against the widespread presence of an enriched tertiary lithospheric mantle below the hebrides the origin of the a sodic rather than potassic contaminant within the hebridean lithosphere is also discussed

an isotopic and geochemical study of the northern kaapvaal craton and the southern marginal zone of the limpopo belt are they juxtaposed terranes abstract to test existing lateral accretion models for the archean kaapvaal craton in south africa metapelites and leucocratic granitoids from two different provinces the granulite facies southern marginal zone of the limpopo belt and the northern part of the adjacent greenschist to amphibolite facies kaapvaal craton have been studied both provinces resemble each other in terms of isotope and trace element characteristics they show a general depletion of incompatible elements independent from the grade of metamorphism this observation conflicts with theories involving mobilisation during granulite facies metamorphism a further common fingerprint is the unusually high cr and ni concentrations in the metapelites in addition they define a very narrow range of nd model ages 2 9 3 05 ga constraining the average crustal age of their cratonic source region the granitoid gneisses show somewhat larger variations of nd model ages indicating some age heterogenity within the crust however the data from both provinces completely overlap the southern marginal zone and the kaapvaal craton also show uniform low 207 pb 204 pb relative to their 206 pb 204 pb ratios that differ clearly from other southern african provinces these results support the view that the southern marginal zone represents a part of the kaapvaal craton exposed at a deeper crustal level it rules out a hypothesis that both provinces are separate amalgamated terranes furthermore there is very little difference between granitoids and metapelites which indicates that the latter largely derived from the former the exposure of the granulite facies metapelites of the southern marginal zone at the same level as the lower grade metamorphic rocks of the kaapvaal craton indicates uplift along a sharp tectonic boundary

geochemical and nd isotopic compositions of the palaeoproterozoic metasedimentary rocks in the kongling complex nucleus of yangtze craton south china block implications for provenance and tectonic evolution

relation between archaean high grade gneiss and granite greenstone terrain in western australia abstract two major regions of archaean crust occur in western australia called the pilbara and yilgarn blocks all the pilbara and most of the yilgarn block consist of granite greenstone terrain with metamorphic grades in greenschist or low amphibolite facies but these terrains developed at different times in the two regions 3500 3000 ma ago in the pilbara and 3000 2600 ma ago in the yilgarn high grade gneisses are only known to occur in a belt along the western margin of the yilgarn block in a region called the western gneiss terrain direct observations of the relation between this high grade gneiss terrain and typical granite greenstone terrain can therefore only be made in this region two kinds of relation are seen in the north the high grade gneiss terrain developed between 3800 3300 ma ago it is distinctly older than and contains a different assemblage supracrustal rocks from the granite greenstone terrain the gneiss terrain could have formed a sialic basement to the greenstones but direct evidence of this relation is not preserved the gneiss terrain is intruded by granites similar to those of the granite greenstone terrain and the two terrains are deformed and metamorphosed together prograde metamorphism of the granite greenstone terrain increases from mainly greenschist facies in the east to high amphibolite facies in the west where it is associated with retrograde metamorphism of the gneiss terrain from granulite to high amphibolite facies in the south a portion of the western gneiss terrain also consists of gneisses which pre date the granite greenstone terrain however most of the gneiss terrain consists of younger granitoid intrusions similar to those that form a major component of the granite greenstone terrain but metamorphosed in granulite facies as in the north a regional eastward decrease in metamorphic grade suggests that the exposed crustal section is tilted eastwards the tectonic significance of the high grade gneiss and granite greenstone terrains remains uncertain regional relationships on the scale of the yilgarn block suggest three possible models 1 cratonic basinal model whereby large greenstone basins formed on gneissic basement 2 continental marginal model whereby greenstone terrains developed adjacent to pre existing protocontinent or 3 collision involving two unrelated segments of crust

crustal geochemistry in the wawa foleyet region ontario fifty three rock samples from the kapuskasing structural zone ksz and 56 from the wawa domal gneiss terrane wgt ontario have been analysed for major elements 32 trace elements and 18o d was measured in a few samples average chemical compositions for the ksz and wgt regions have been calculated from map unit averages weighted by regional abundance compared with estimates of the composition of the upper continental crust ucc the ksz is enriched in al fe mg ca p transition elements sc and sr depleted in si b most rare earth elements zr hf nb ta li na k u th ba and rb but similar in composition to estimates of the lower continental crust the wgt is closer in composition to the ucc these data support the interpretation on geophysical and petrological grounds that the crust here is layered and has been uplifted the wgt and ksz regions representing progressively deeper levels igneous rock and orthogneiss 18o values in the ksz and wgt show good correlation with the weigh

the structural evolution of the lower crust of orogenic belts present and past summary the lower crust of orogenic belts may be exposed by post growth uplift in magmatic arcs or by tectonic uplift on post collisional crustal scale thrusts each tectonic zone of a belt can be expected to have a different type of lower crust and there is a similar tectonic zonation in collisional belts ranging from the early proterozoic to the tertiary tonalitic gneisses make up the bulk of the lower crust of andean type arcs deep sections of sutures may expose nepheline gneisses blueschists and granulites exposed basic dyke swarms represent the feeders to eroded lavas in aulacogens in continental forelands post collisional thrusts uplift earlier lower crust of amphibolite facies gneisses or granulites of either igneous or sedimentary parentage the result of the many factors controlling variable uplift in the different tectonic zones of an orogenic belt is that a deep seated cross section can only be constructed by compilation of type sections from different belts crustal generation in the archaean was dominantly magmatic and broadly similar to that seen in the deeper parts of andean type arcs but on a much more extensive scale magmatic underplating associated with strong horizontal thrusting produced a strongly foliated lower crust with interlayering of marie and silicic components archaean crust probably had a primary vertical zonation with respect to the proportion of these components and of heat producing elements

origin and early growth rate of continents abstract data suggest that early continents formed over mantle sinks and grew rapidly between 2 7 and 3 0 ga ago nd isotopic results indicate that archean tonalites which are largely responsible for early continental growth are derived chiefly from ancient depleted mantle sources trace element and oxygen isotope data however indicate enriched sources for such tonalites these constraints can be explained if the mantle is fractionated into depleted dmr and enriched emr reservoirs during early degassing of the earth 4 4 ga blocks of emr are recycled back into the mantle and the emr dmr mixtures serve as sources for archean basalts these basalts are enriched in incompatible elements by deep sea alteration metasomatism in sinks or by other mantle processes and then undergo wet partial melting to produce tonalite magmas enrichment must occur not more than 150 ma prior to tonalite production basalt plateaus accumulate at sinks and as cooling continues geotherms at sinks pass into the eclogite stability field dense eclogite roots form on the basalt plateaus which pulls them into the mantle between 2 7 and 3 0 ga ago resulting in rapid tonalite production and thus in rapid continental growth

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

geochemistry of precambrian gneisses relevance for the evolution of the east antarctic shield archaean granulite facies orthogneisses of the napier complex in enderby land metamorphosed 3070 maago comprise two chemically distinct suites the more abundant mainly of tonalitic to granodioritic composition shows strong y depletion explicable by hydrous partial melting of a garnet bearing source garnet amphibolite or possibly eclogite it apparently represents new continental crust other gneisses predominantly of trondhjemitic to granitic s s composition do not show y depletion and have higher tio2 zr nb la ce and ga al and lower cao sr and mg mg total fe they probably originated by relatively dry melting of predominantly felsic crystal rocks both suites show evidence for loss of rb relative to k th and u during metamorphism late archaean 2800 ma amphibolite facies gneisses of macrobertson land are of undepleted type and may be representative of a higher crustal level than those of enderby land late proterozoic 1000 ma granulite facies gneisses of enderby land rayner complex are to a large extent remetamorphosed napier complex rocks of igneous derivation in contrast gneisses of similar age in macrobertson land include a much higher proportion derived either directly or by partial melting from sedimentary protoliths

zircon u pb geochronology and geochemistry of low grade metamorphosed volcanic rocks from the dantazi complex implications for the evolution of the north china craton abstract the late neoarchean witnessed the cratonization of the north china craton ncc through amalgamation of several micro blocks to form a coherent basement the archean orthogneisses and supracrustal rocks in this craton have experienced various grades of metamorphism ranging up to upper amphibolite and granulite facies at 2500 ma recently a suite of low grade metamorphosed greenschist to lower amphibolite facies volcanic rocks was discovered in the late neoarchean dantazi complex in northern hebei province these meta volcanic rocks consist of bimodal basalt andesite and trachyte dacite with a sio2 gap between 54 4 wt and 60 7 wt here we report shrimp zircon u pb ages of 2490 19 ma mswd 2 0 and 2502 8 ma mswd 0 83 from the meta mafic and meta felsic volcanics respectively representing the timing of igneous activity all the meta mafic volcanic rocks display coherent trace element and ree patterns which are characterized by enriched lile and lree but depleted hfse and hree la yb n 6 29 15 10 combining these trace element features with the positive zircon ehf t values 1 3 to 6 6 we propose that the mafic rocks were likely derived from partial melting of a previously metasomatized lithospheric mantle in the primitive mantle normalized diagram the felsic rocks display uniform patterns enriched in lile but depleted in nb and ta similar to those of lower crust furthermore their strongly fractionated ree la yb n 15 24 61 20 lower hree concentrations yb 0 47 1 65 ppm and positive zircon ehf t values 1 6 to 5 3 suggest that they were derived from partial melting of the lower crust with garnet in the residue this coeval occurrence of metasomatized mantle derived mafic magmas and potassic felsic magmas from different source regions reflects an intracontinental extensional setting during the late neoarchean to earliest paleoproterozoic following the cratonization of the ncc our new data combined with previous published data allow us to speculate that the intracontinental extensional regime might have prevailed in the ncc from late neoarchean onwards

crustal growth in south eastern australia evidence from lower crustal eclogitic and granulitic xenoliths mafic and ultramafic xenoliths in a basaltic cone at the anakies in south eastern australia are geochemically equivalent to continental basaltic magmas and cumulates the xenolith microstructures range from recognizably meta igneous for intrusive rocks to granoblastic for garnet pyroxenites contact relationships between different rock types within some xenoliths suggest a complex petrogenesis of multiple intrusive metamorphic and metasomatic events at the crust mantle boundary during the evolution of south eastern australia unaltered spinel lher zolite typical of the uppermost eastern australian mantle is interleaved with or veined by the metamorphosed intrusive rocks of basaltic composition geothermobarometry calculations by a variety of methods show a concordance of equilibration temperatures ranging from 880 c to 980 c and pressures of 12 to 18 kbar 1200 1800 mpa these physical conditions span the gabbro to granulite to eclogite transition boundaries the water vapour pressure during equilibration is estimated to be about 0 5 of the load pressure using amphibole breakdown data large fluid inclusions of pure co2 are abundant in the mineral phases in the xenoliths and it is suggested that flux of co2 from the mantle has been an important heat source and fluid medium during metamorphism of the mafic and ultramafic protoliths at the lower crust upper mantle boundary the calculated pressures and temperatures suggest that the south eastern australian crust has sustained a high geothermal gradient in addition the nature of the mineral assemblages and the contact relationships of granulitic rock with spinel lherzolite characteristic of mantle material suggest that the moho is not a discrete feature in this region but is represented by a transition zone approximately 20 km thick these inferences are in agreement with geophysical data including seismic heat flow and electrical resistivity data determined for south eastern australia underplating at the crust mantle boundary by continental basaltic magmas may be an important alternative or additional mechanism to the conventional andesite model for crustal accretion

a neoarchean subduction recorded by the eastern hebei precambrian basement north china craton geochemical fingerprints from metavolcanic rocks of the saheqiao shangying qinglong supracrustal belt abstract the saheqiao shangying qinglong supracrustal belt ssqb in the northern eastern hebei precambrian basement ehpb is located in the northern margin of the eastern block eb of the north china craton ncc the shangying terrane constitutes the middle segment of the ssqb and contains primarily metamorphic volcanics and plutonic tonalitic gneisses the metamorphic volcanics mainly consist of pyroxene plagioclase amphibolites garnet plagioclase amphibolites biotite plagioclase amphibole gneisses and amphibole plagioclase gneisses zircon u pb lu hf isotopic analyses reveal that the metavolcanic rocks from the shangying terrane crystallized at 2506 2613 ma with t dm hf values of 2541 2944 ma these metamorphic volcanic rocks are subdivided into four groups based on their lithological and chemical features group i consists chiefly of tholeiites that are characterized by slightly light rare earth element lree depleted patterns and flat multi element spider diagrams which are similar to back arc basin basalt babb like rocks and were derived from the partial melting of the depleted mantle the tholeiites in group ii have slightly fractionated rare earth element ree patterns without nb anomalies exhibit an affinity to nb enriched basalt neb like rocks and were produced by the partial melting of hfse enriched mantle peridotites group iii is composed of slightly lree enriched tholeiites with negative nb ta anomalies that resemble island arc tholeiites group iv comprises calc alkaline basalts and andesites with highly enriched lrees and evident nb ta and ti depletions that are geochemically similar to the products of island arcs the island arc tholeiites and calc alkaline basalt andesites originated from the partial melting of sub arc mantle peridotites that were previously metasomatized by slab derived fluids melts with the fractional crystallization of ferromagnesian minerals collectively the babb like rocks the nebs arc tholeiites and calc alkaline basalt andesites recorded magmatism from a subduction regime similarly neb arc tholeiite andesite dacites and back arc related rocks were recognized in the saheqiao and qinglong areas in the western and eastern parts of the ssqb r guo et al 2013 2015 which further supports the existence of a neoarchean convergent margin in the eastern hebei precambrian basement ehpb compared to the northwestern ehpb the southeastern ehpb underwent lower metamorphism contains arc dacites rhyolites and greater contributions of the ancient continental crust implying that the southeastern ehpb might have been or near a back arc setting thus we conclude that the ssqb experienced a southeastward dipping subduction which is consistent with studies of the adjacent regions e g western liaoning province when combining the 2 5 2 6 ga supracrustal rocks and plutonic granitoid gneisses with depleted mantle origins along the northern margin of the eb of the ncc we concluded that 2 5 2 6 ga was a significant period for juvenile crustal growth and that lateral accretion played a dominant role in crustal growth

the western border of the archaean province of the baltic shield evidence from northern sweden restricted areas of acid to intermediate gneisses in northernmost sweden are known to be archaean and several other gneissic granodioritic to tonalitic massifs have been suggested to be of this age to delimit the western border of the archaean province of the baltic shield and to characterize the swedish archaean we have studied the 2 83 ga old soppero gneiss and 8 of the proposed archaean massifs in northern sweden by sm nd isotopic analyses u pb zircon dating in two cases and geochemistry

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

smnd isotopes and ree geochemistry of madras granulites india an introductory statement abstract major and trace elements including ree as well as sm nd isotope analyses of eight granulite samples from the madras area are presented the data provide evidence for the origin of some of the granulites but in other cases demonstrate metasomatic changes in major and trace element contents including lree the data furthermore demonstrate that the analysed granulites are not cogenetic however when plotted in a sm nd isochron diagram the samples yield a linear array which corresponds to an age t 2555 140 ma and nd t 1 1 4 this age of 2555 140 ma could represent the time of granulite facies metamorphism immediately preceded by the formation of the protolith the nd t 1 value may be indicative of a near chondritic mantle source or alternatively of mixing between a depleted mantle and a crustal component

an open system fractional crystallization model for very early continental crust formation abstract a model is discussed which seeks to explain the origin of archaean tonalitic trondhjemitic granodioritic ttg crust by an open system fractional crystallization mechanism following the assumption that in the pre and early archaean the melt which formed above sites of large scale mantle upwelling was too voluminous and too hot to solidify an almost global near surface partially molten zone could form the magma in this layer would converge above mantle sinks and undergo fractional crystallization recharged fractionating reservoirs would form at sites of convergence the major element composition of the magma in such reservoirs would evolve towards one resembling that of ttg rocks provided that amphibole was a significant fractionating phase from such reservoirs their incompatible as well as compatible trace element abundances would also become similar to those of ttg terrains it is proposed that such terrains resulted from the consolidation of these recharged fractionating reservoirs it is shown that the reservoirs would remain indistinguishable from the primitive mantle in terms of radiogenic isotopes so that model ages cannot be greater than the age of their consolidation consideration of the main aspects of greenstone belts and archaean high grade terrains indicates no contradictions with the model the intrusion of late archaean adamellites and granites is interpreted as an indirect effect of pressure induced breakdown of amphibole in the cumulate underlying the ttg crust which in turn resulted from crustal shortening cratonization is seen as the result of consolidation not only of the crust but also of the underlying cumulate as subcontinental lithosphere

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