geochronological constraints on paleoarchean thrust nappe and neoarchean accretionary tectonics in southern west greenland abstract major regional deformation and metamorphic events in the godthabsfjord region southern west greenland occurred at 3650 and 2820 2720 ma e g precambrian res 78 1996 1 new geochronological constraints u pb zircon sensitive high resolution ion microprobe shrimp and thermal ionisation mass spectrometry tims have been obtained from a stack of mylonitic crystalline thrust nappes in the footwall of the western part of the paleoarchean 3 8 3 7 ga isua greenstone belt isukasia a mylonitic tonalite sheet interpreted to have intruded synkinematically with respect to mylonitisation yields a magmatic crystallisation age of 3640 3 ma a cross cutting pegmatite and a post kinematic tonalite pluton yield magmatic crystallisation ages of 2948 8 and 2991 2 ma respectively accordingly we interpret the thrust nappe stack to have formed during the paleoarchean 3640 ma making it the oldest example known on earth the similarity of this structural regime to that of modern mountain belts suggests that paleoarchean and modern continental crust were comparable in terms of mechanical strength and constitution southern west greenland has been interpreted in terms of neoarchean accretion comparable with modern plate tectonics e g earth planet sci lett 142 1996 353 isukasia lies just east of a purported neoarchean accretionary boundary between the akia terrane to the northwest and the akulleq terrane to the southeast the akia terrane was previously considered to overthrust the akulleq terrane at 2820 2720 ma our geochronological and geological data indicate i that the two terranes as presently defined were stitched at 2991 2 ma and ii that thrusting across the boundary was directed toward the akia terrane therefore we suggest that the akia akulleq interface was not a fundamental tectonic structure during the neoarchean and we question its identification as an accretionary boundary

geochemistry and geodynamic origin of the mesoarchean ujarassuit and ivisaartoq greenstone belts sw greenland abstract the ivisaartoq ca 3075 ma and ujarassuit ca 3070 ma greenstone belts are the largest mesoarchean supracrustal lithotectonic assemblages in the nuuk region sw greenland both greenstone belts underwent polyphase deformation and amphibolite facies metamorphism and were in due course variously dismembered pillow lavas pillow breccia magmatic layering and relic sedimentary structures are well preserved in the ivisaartoq belt volcanic rocks include basalts and minor andesites and picrites boninite like rocks occur in the ujarassuit belt metasedimentary rocks in the ujarassuit belt occur as thin layers 0 5 1 0 m of biotite schists and in the ivisaartoq belt as 500 m thick biotite schists intercalated with minor quartzitic gneisses there is no field evidence indicating that the ivisaartoq and ujarassuit supracrustal rocks were deposited on older continental basement and their volcanic rocks do not exhibit any geochemical trends indicating contamination by archean upper continental crust four groups of amphibolites metavolcanic rocks were recognized in the ujarassuit greenstone belt on the basis of their ree and hfse characteristics 1 group 1 is characterized by near flat ree patterns la smcn 0 77 1 14 la ybcn 0 84 1 24 and moderate negative nb anomalies nb nb 0 60 0 79 2 group 2 displays lree depleted patterns la smcn 0 53 1 02 la ybcn 0 32 0 61 and pronounced negative nb anomalies nb nb 0 32 0 67 3 group 3 consists of lree depleted patterns la smcn 0 69 0 84 la ybcn 0 55 0 91 and absence of significant nb anomalies nb nb 0 92 1 15 and 4 group 4 has concave upward ree patterns la smcn 1 64 2 42 gd ybcn 0 57 1 01 and large negative nb anomalies nb nb 0 28 0 42 the depletion of lree in group 2 amphibolites appears to have resulted from mobility of these elements during metamorphism group 1 3 and 4 amphibolites retain their near primary geochemical signatures and their trace element patterns are comparable to those of phanerozoic oceanic island arc tholeiites iat normal mid ocean ridge basalts n morb and boninites respectively the trace element patterns of the least altered meta ultramafic rocks la ybcn 2 48 1 35 nb nb 0 31 0 60 are comparable to those of modern subduction related picrites amphibolites with an andesitic composition show large negative nb anomalies nb nb 0 21 0 55 enriched lree patterns la ybcn 6 29 15 64 and fractionated hree gd ybcn 2 61 3 12 implying deep melting in equilibrium with residual garnet in the source biotite schists and quartzitic gneisses have low chemical indexes of alteration values cia 46 to 62 and trace element characteristics indicating volcaniclastic sedimentary protoliths derived from poorly weathered felsic to mafic source rocks collectively the geochemical features of metavolcanic and metasedimentary rocks suggest that the ivisaartoq and ujarassuit greenstone belts represent dismembered fragments of mesoarchean supra subduction zone oceanic crust formed in an arc forearc backarc tectonic setting

a shear zone related greenstone belt hosted gold mineralization in the archean of west greenland a petrographic and combined pb pb and rb sr geochronological study abstract widespread gold showings have been found in an archean greenstone belt on storo in godthabsfjord west greenland the storo greenstone belt sgb comprises metasedimentary and metavolcanic rocks with minor magnetite rich bands gold mineralization is hosted by amphibolites mica schists and garnetite rocks gold occurs in various mineral associations but its presence in lollingite arsenopyrite pyrrhotite parageneses is prevalent to a lesser extent native gold occurs within seriticized plagioclase in sheeted quartz veins and as inclusions in garnet based on petrographic observations and comparison with similar ore mineral textures from other deposits we propose the following model for the deposition of the gold bearing lollingite arsenopyrite pyrrhotite assemblage on storo in the first stage arsenopyrite likely formed with gold in solid solution in the second stage gold bearing arsenopyrite was replaced by lollingite and pyrrhotite this took place during regional prograde metamorphism gold accumulated within lollingite as discrete grains during subsequent retrograde metamorphic conditions lollingite and pyrrhotite reacted and formed arsenopyrite and gold was subsequently exsolved and preferably deposited as discrete grains or films along arsenopyrite lollingite grain boundaries during a later event a garnet overgrowth partly affected the gold bearing lollingite arsenopyrite pyrrhotite assemblage the isotopic composition of pb released from these garnets by pb stepwise leaching experiments pbsl and the bulk pb isotopic compositions of arsenopyrite and pyrrhotite indicate a common source of pb in the fluid from which first the sulfides and subsequently garnet were formed bulk pb analyses and pbsl experiments of arsenopyrite and pbsl experiments of garnet yield ages of 2863 24 ma 2 mswd 0 2 for arsenopyrite and 2748 62 ma 2 mswd 220 for garnet the 2863 24 ma age is thus a minimum age for the mineralization rb sr isotopic analyses of cogenetic biotite from within strongly mineralized zones reveal middle proterozoic ca 1 65 ga mineral ages well in accordance with a regional reheating event known to have affected the area and are thus interpreted as cooling ages thereof this regional reheating event has apparently not affected the pb pb isotopic system of arsenopyrite implying that the blocking temperature of the u pb system in arsenopyrite is likely to be above 300 c the late archean ages for arsenopyrite and garnet are consistent with the regional amphibolite facies metamorphism related to major terrane assemblies in the study area occurring at that time the ore textures preserved today are interpreted as those formed under this regional metamorphic event

geochemistry and origin of archaean quartz cordierite gneisses from the godthabsfjord region west greenland we report new data on the major minor and trace element compositions of metasedimentary quartzcordierite gneisses qcg an important member of the archaean malene supracrustal suite found throughout the godthabsfjord region of west greenland the analyzed qcg contain assemblages of quartz cordierite biotite garnet anthophyllite gedrite staurolite sillimanite plagioclase with abundant accessory zircon and minor rutile monazite and allanite and broadly resemble cordierite orthoamphibole rocks found in a great number of other metamorphic terrains chemically the qcg are characterized by 1 high but variable sio2 59 87 wt relative enrichments in mgo feo and al2o3 mg 0 35 0 85 and depletions in na2o k2o and especially cao 2 low concentrations of sc cr co ni and sr 3 high concentrations of y nb zr hf ta th u and ree rare earth elements with prominent negative eu anomalies in each case 5 high concentrations of ga 18 55 ppm with variable ga al ratios that are significantly higher than average crustal material low cr and ni together with enriched and fractionated ree displaying negative eu anomalies distinguish the malene qcg from published accounts of most other archaean sedimentary rocks furthermore all of the above mentioned trace element characteristics distinguish the qcg from ordinary malene clastic metasediments quartzites psammites and pelites suggesting a separate origin for the qcg these data point towards chemically evolved felsic igneous rocks being the source of the qcg consequently we propose that the malene qcg represent metamorphosed felsic volcaniclastic sediments that underwent hydrothermal alteration by heated seawater prior to metamorphism which resulted in gain of mg and fe loss of alkalis and lime and possibly eu and sr the overall trace element characteristics of the qcg elevated ga zr nb ree etc are features shared by a type granites and their volcanic equivalents such igneous rocks may represent the ultimate source material for the qcg protolith

late archaean terrane accretion in the godthab region southern west greenland tectono stratigraphic terranes fault bounded blocks of the earth s crust characterized by a geological history distinct from that of adjacent terranes1 are now widely recognized in phanerozoic and proterozoic orogenic belts and their present configurations are thought to result from plate tectonic processes similar to those in operation today2 4 there has been much debate about whether such processes operated in archaean times5 6 here we present new data from the archaean high grade gneiss complex of the godthab now nuuk region of southern west greenland which show that discrete tectonic units recognized in the south of the region7 can be more widely identified at least four terranes are recognized each of which evolved separately until their juxtaposition in the late archaean this suggests that at least some archaean high grade gneiss complexes may resemble more recent orogenic belts formed by plate tectonic processes

refolded nappes formed during late archaean terrane assembly godthabsfjord southern west greenland recent work has provided a new tectono metamorphic framework for the archaean evolution of the godthabsfjord region which can be divided into discrete tectonic units each characterized by different magmatic structural and metamorphic histories these tectonic units were assembled in the late archaean post 2750 ma the present structural patterns seen in the region date from the period 2750 2600 ma and do not reflect earlier structural events as was previously considered late archaean structural evolution began with a period of interthrusting of early archaean sialic and late archaean oceanic crustal segments coeval with or followed by isoclinal nappe formation under amphibolite facies conditions linear fabrics in early archaean rocks were rotated into parallelism with those newly formed parallel to f1 hinge lines in the late archaean rocks a second major thrusting event emplaced granulite facies rocks structurally on top of the amphibolite facies rocks subsequent to this event a series of f2 upright or steeply inclined basin and dome structures formed some of which have sheath morphology this second folding episode acting upon the isoclinal nappes produced the regional scale interference patterns observed the early linear fabrics were re oriented around these second generation structures data are produced from selected areas to illustrate the fabric patterns and structural continuity that exists throughout the region

age significance of u th pb zircon data from early archaean rocks of west greenland a reassessment based on combined ion microprobe and imaging studies comment

preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend gneiss complexes are the world s most horrendously complexrocks to understand see cover of this issue yet they form a largeamount of the continents albeit they are normally hidden fromview by sedimentary sequences the last half century has seenenormous advances in understanding these rocks which form notonly a large amount of the continental crust but also hold much of earth s oldest rock record the archaean disciplines medicine and health sciences social and behavioral sciences publication details nutman a p 2014 preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend precambrian research 255 part 3 771 773 this journal article is available at research online http ro uow edu au smhpapers 2672 preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend

origin of granitic magma by crustal remobilisation rb sr and pb pb geochronology and isotope geochemistry of the late archaean qorqut granite complex of southern west greenland abstract rb sr and pb pb whole rock isochrons on the qorqut granite complex yield ages of 2530 30 myr initial 87 sr 86 sr 0 7081 0 0008 and 2580 80 myr respectively a model relating initial sr and pb isotopic compositions of the qorqut granites to the sr and pb isotopic compositions of the amitsoq gneisses ca 3700 myr and nuk gneisses ca 2900 myr at 2550 myr ago as well as sr and pb contents of the gneiss units suggests that between 40 and 50 of the qorqut granite magma was generated by partial melting of amitsoq gneisses and the remainder by partial melting of nuk gneisses

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

gold occurrences of the archean north atlantic craton southwestern greenland a comprehensive genetic model abstract the north atlantic craton of southwestern greenland hosts several orogenic gold occurrences although to date none is in production four gold provinces are distinguished and include godthabsfjord tasiusarsuaq paamiut and tartoq in the godthabsfjord gold province the hypozonal gold occurrences are aligned along the major ca 2660 2600 ma ivinnguit fault orogenic gold mineralization correlates temporally with and is related to ductile deformation along this first order structure the northern part of the tasiusarsuaq gold province is characterized by small hypozonal gold occurrences that are controlled by 2670 2610 ma folds and shear zones auriferous fluids were focused into the structures in both gold provinces during west directed accretion of the kapisilik terrane 2650 2580 ma to the already amalgamated terranes of the north atlantic craton in the southern part of the tasiusarsuaq gold province hypozonal gold mineralization is hosted in back thrusts sermilik prospect and thrusts bjornesund prospect that formed at 2740 ma and 2860 2830 ma respectively the deformation is related to the ca 2850 ma accretion of the sioraq block and the tasiusarsuaq terrane and the 2800 2700 ma accretion of the tasiusarsuaq terrane and the faeringehavn and tre brodre terranes mesozonal orogenic gold mineralization is hosted in an accretionary complex in the paamiut and tartoq gold provinces gold occurrences cluster over a strike extent of approx 40 km in thrusts and complex strike slip settings in lateral ramps the timing of the e vergent terrane accretion in both areas is unknown and could either be at ca 2850 ma or 2740 ma in the eastern part of the paamiut gold province quartz veins and associated alteration zones were overprinted by granulite facies metamorphism and show evidence for partial melting these outermost parts of the accretionary complex were involved in burial exhumation tectonics during crustal accretion mainly three different orogenic stages related to gold mineralization are distinguished in the north atlantic craton between ca 2850 ma and 2610 ma these are generally accretionary tectonic episodes and gold mineralization is hosted either in reactivated fault systems between terranes or accretionary complex structures along the deformed cratonic margin the larger orogenic gold occurrences formed at ca 2740 2600 ma that appears to be a period of orogenic gold mineralization globally although significant gold resources in the north atlantic craton have yet to be identified

field and geochemical characteristics of mesoarchean to neoarchean volcanic rocks in the storo greenstone belt sw greenland evidence for accretion of intra oceanic volcanic arcs abstract the storo greenstone belt southern west greenland consists of thrust imbricated slices of mesoarchean 3060 ma and neoarchean ca 2800 ma mafic to ultramafic volcanic rocks volcaniclastic sediments and gabbro anorthosite associations the belt underwent polyphase metamorphism at upper amphibolite facies conditions between 2650 and 2600 ma the contacts between the mesoarchean and neoarchean volcanic rocks and surrounding eoarchean to neoarchean tonalite trondhjemite granodiorite ttg gneisses are tectonic and typically bounded by high grade mylonites regardless of age the volcanic rocks are dominated by mafic amphibolites with a tholeiitic basalt composition near flat to slightly enriched light rare earth element lree patterns la smcn 0 91 1 48 relatively flat to slightly depleted heavy ree hree gd ybcn 1 0 1 28 and pronounced negative nb ta anomalies nb nb 0 34 0 73 on chondrite and primitive mantle normalized diagrams these geochemical characteristics are consistent with subduction zone geochemical signatures and partial melting of a shallow

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

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

thermal granulite facies metamorphism with diffuse retrogression in archaean orthogneisses fiskefjord southern west greenland around fiskefjord southern west greenland archaean amphibolite facies granulite facies and retrograde orthogneisses occur in lithological and structural continuity with each other the granulite facies rocks here and elsewhere in west greenland are surrounded by extensive areas of retrograde gneisses both the prograde and retrograde metamorphism took place in a major event of continental crust formation c 3000 ma ago which gave rise to granulite facies conditions in part of the rock complex exposed today in the fiskefjord area distributions of major and trace elements as well as strontium and lead isotopes show that the fades transformations were accompanied by pronounced metasomatism and mineral chemistry indicates that the hydrous retrograde metamorphism took place under amphibolite facies conditions and was gradual and incomplete the metamorphic and metasomatic processes in the fiskefjord area are believed to have been controlled by heat from continuous intracrustal injection of large masses of tonalitic magma which caused gradual dehydration and partial melting followed by liberation of aqueous fluids during crystallization of anatectic melts these fluids partially retrograded previously dehydrated gneisses in contrast south indian high grade gneisses have mainly prograde amphibolite granulite facies transitions which are distinct and well preserved later than penetrative deformation and are likely to have been controlled by co2 streaming these amphibolite granulite facies transitions are reported to be near isochemical it is suggested that there are at least two different kinds of granulite facies metamorphism a near isochemical prograde type in stabilized tectonic environments perhaps controlled by influx of co2 e g in south india and significantly post dating original crust formation and a fluid deficient type with widespread anatexis hydrous retrogression and metasomatism which takes place during accretion of continental crust and in which heat is the governing factor e g in southern west greenland

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

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

late archaean tectonics in the faeringehavn tre brodre area south of buksefjorden southern west greenland the faeringehavn tre brodre area consists of three distinct terranes tectonically jux taposed by a previously unrecognized event contacts between the terranes are mylonitic shear zones truncating lithological units in adjacent terranes the terrenes are 1 faeringehavn terrane largely composed of early archaean amitsoq gneiss cut by younger archaean granitic gneiss defined here as the satut gneiss 2 tre brodre terrane comprising mid archaean malene supracrustal rocks anorthosite complex and polyphase nuk gneisses 3 tasiusarsuaq terrane largely comprising mid archaean nuk gneisses affected by c 2800 ma granulite facies metamorphism the tasiusarsuaq terrane is structurally above both the faeringehavn and tre brodre terranes which at c 2800 ma experienced a lower grade of metamorphism juxtaposition of the terranes took place between 2800 and 2500 ma and involved thrusting and crustal thickening subsequent re equilibration involved folding steeply inclined shear belts intrusion of synkinematic granitoids under amphibolite facies conditions and retrogression of granulite facies assemblages this thrusting post dates the 2800 ma granulite facies metamorphism it is younger and distinct from the thrusting postulated to explain the intercalation of the early archaean amitsoq gneisses and the mid archaean malene supracrustal rocks associated with intrusion of the nuk gneiss precursors described from godthabsfjord the tectonic breaks provide for the first time marker horizons which can be used to assess the amount and type of late archaean deformation in the southern godthabsfjord region

archaean crustal evolution in southern west greenland a review based on observations in the buksefjorden region abstract thermal metamorphic and tectonic processes associated with the two major crust forming events expressed as voluminous generation of amitsoq and nuk tonalitic granitic gneisses ca 3750 ma and ca 2800 ma respectively in southern west greenland are reviewed briefly in the light of recent studies in the buksefjorden region

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

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

2635 ma amphibolite facies gold mineralisation near a terrane boundary suture on storo nuuk region southern west greenland abstract gold on storo in the nuuk region of southern west greenland occurs in a slice of strongly deformed amphibolite facies neoarchaean quartzo feldspathic metasedimentary rocks and amphibolites in tectonic contact with the eoarchaean faeringehavn terrane and the meso to neoarchaean akia terrane the gold is associated with either lollingite feas arsenopyrite or pyrrhotite which belong with the amphibolite facies silicate mineral assemblages zircons were u pb dated by shrimp to constrain the timing of mineralisation the 2700 2830 ma oscillatory zoned high th u volcano sedimentary zircon as whole grains and cores are interpreted to give the maximum depositional age of the rocks hosting the gold a granite sheet barren of gold that cuts gold mineralised rocks gives an age of ca 2550 ma which is the absolute minimum age of mineralisation metamorphic zircons associated with auriferous arsenopyrite particularly close to secondary lollingite pyrrhotite and in the coexisting amphibolite facies silicate minerals were dated in situ in polished thin sections these have lower average th u and yielded dates of ca 2635 ma which is interpreted as the age of the gold bearing mineral assemblages and hence probably the mineralisation supracrustal packages containing abundant quartzo feldspathic sedimentary rocks with depositional ages of ca 2800 ma and metamorphosed at 2650 2600 ma also occur as folded tectonic intercalations along the western edge of the faeringehavn terrane south of storo moreover in the 3080 2960 ma kapisilik terrane to the north and east of storo high grade metamorphic events also occurred between 2650 and 2600 ma however the eoarchaean faeringehavn terrane to the south and east of storo shows widespread zircon growth and recrystallisation during 2720 2700 ma regional metamorphism but not at 2650 2600 ma the storo gold prospect is thus located near an important tectonic terrane boundary cryptic suture between domains with different neoarchaean metamorphic history storo mineralisation at ca 2635 ma probably occurred during deformation of this boundary shortly after it was created and at storo maybe was focussed in an area of low strain in a major antiform the general tectonic frameworks for ca 2635 ma gold mineralisation on storo and in the yilgarn craton are similar these gold provinces are compared and contrasted and the likely reason why so much less gold occurs in the nuuk region is discussed

smnd age of the fisken sset anorthosite complex west greenland abstract the fisken sset anorthosite complex of southern west greenland is a sheet like layered igneous body which has been fragmented by granitoid intrusions multiply deformed and folded metamorphosed as high as granulite facies and in some places retrogressed to lower amphibolite or greenschist facies in some localities however such as at majorqap qa va some 20 km inland from the coast these effects are minimal here metamorphic grade reached only amphibolite facies with no apparent retrogression igneous textures and structures are spectacularly preserved detailed stratigraphic sequences have been established and in some samples primary mineralogy remains we report a sm nd isotopic study on samples from this locality attempting to constrain the age of crystallization of the fisken sset complex this complex represents a useful marker unit with which the age relationships of surrounding units may be compared a five point isochron including data for whole rock samples of anorthosite metagabbro metaperidotite and separates of calcic plagioclase and mafic matrix from a coarse megacrystic leucogabbro corresponds to an age of 2 86 0 05 ga mswd 2 5 with initial end of 2 9 0 4 the sm nd age is significantly older than previous pb pb whole rock ages of the complex but is sufficiently close to the 2 8 ga high grade metamorphic event which affected the area to cause partial resetting of the sm nd system to be considered however based on the integrity of the whole rock sm nd isotopic system in similar anorthositic complexes of higher metamorphic grade and the apparent preservation of magmatic ree patterns in the analysed plagioclase megacryst and coexisting mafic matrix we conclude that 2 86 0 05 ga represents the time of crystallization of the fisken sset complex this would imply a relatively short time interval on the order of 70 ma during which anorthosite formation tonalite emplacement and high grade metamorphism took place and this must be accounted for in any tectonic model for late archean events in southern west greenland in any case the positive end value of the fisken sset complex requires that it was derived from a source with long term depletion in light ree these results add to the increasing body of data indicating the widespread presence of depleted mantle sources during the archean

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

mesoarchaean collision of kapisilik terrane 3070ma juvenile arc rocks and 3600ma isukasia terrane continental crust greenland abstract the mesoarchaean kapisilik and eoarchaean isukasia terranes in the nuuk region of southern west greenland were tectonically juxtaposed in the archaean the north of the isukasia terrane is distal from the kapisilik terrane and has only rare growth of 2690 ma metamorphic zircon and no 2980 2950 ma metamorphic zircon the southern part of the isukasia terrane lies between two 2690 ma shear zones and has locally preserved high pressure granulite facies assemblages and widespread growth of 2980 2950 ma metamorphic zircon and also sporadic growth of 2690 ma metamorphic zircon within this southern part of the isukasia terrane there is a folded klippe of mylonitised mesoarchaean detrital meta sedimentary rocks carrying 3600 and 3070 ma detrital zircons mafic and ultramafic rocks with 2970 ma metamorphic zircon overgrowths south of the isukasia terrane is the kapisilik terrane containing 3070 ma arc related volcanic rocks gabbro anorthosites and meta tonalites intruded by 2970 2960 ma granites zircons of an ivis rtoq supracrustal belt 3075 ma intermediate volcanic rock have initial hf values of 2 to 5 thus are juvenile crustal additions 3070 ma tonalites along the northern edge of the kapisilik terrane have whole rock positive initial nd values and thus are also juvenile crustal additions in contrast igneous zircons in 2960 ma granites intruded into juvenile 3075 ma supracrustal rocks of the kapisilik terrane have initial hf values of 5 to 10 and must have involved the partial melting of 3600 ma isukasia terrane rocks the integrated structural and zircon u th pb hf isotopic data show that at 2980 2950 ma the kapisilik terrane juvenile arc components collided with and over rid the isukasia terrane the southern edge of the isukasia terrane came to lie in the deep crust under the ivis rtoq supracrustal belt and melted at 2970 2960 ma to produce granites these granites derived from ancient crust rose into the upper crust where they intruded the overlying allochthonous juvenile 3075 ma ivis rtoq supracrustal belt arc assemblages the southern edge of the isukasia terrane is interpreted as an interior nappe of eoarchaean basement rocks interfolded with a klippe of mesoarchaean metasedimentary and mafic ultramafic rocks both of which are affected by 2980 2950 ma metamorphism the mixed eoarchaean mesoarchaean detrital provenance suggests that the klippe could be dismembered components of an accretionary prism or forearc crust the northern part of the isukasia terrane is interpreted as foreland free of 2980 2950 ma high grade metamorphic overprint this shows that the isukasia terrane is not a coherent block but contains ancient rocks that are parautochthonous or allochthonous to each other with contrasting later metamorphic history at 2690 ma the crustal architecture arisen from mesoarchaean collision between an older continental block and an island arc was reworked along intra crustal shear zones coeval with amphibolite facies metamorphism this reworking followed on from major terrane assembly at 2710 2700 ma in the southern part of the nuuk region when the eoarchaean faeringehavn terrane was juxtaposed with 2840 2825 ma arc rocks thus the 2980 2950 ma assembly of the isukasia and kapisilik terranes is distinct from the later 2710 2700 ma terrane assembly further south in the nuuk region

new sims u pb zircon ages from the langavat belt south harris nw scotland implications for the lewisian terrane model secondary ionization mass spectrometry sims u pb dating of zircon has been applied to rocks of the lewisian complex of south harris outer hebrides nw scotland yielding insights into the complex geological evolution of this region and enhancing our understanding of terranes in the lewisian gneiss complex results show that tonalite trondhjemite granodiorite magmatism occurred in the tarbert terrane between c 2850 and 2830 ma equivalent to similar rocks in other parts of the outer hebrides magmatism was accompanied or closely followed by high grade metamorphism at c 2830 ma the first reported occurrence of such an event at this time these rocks were affected by a subsequent high grade metamorphic event at c 2730 ma which has correlatives both in the outer hebrides and in the mainland lewisian gruinard terrane these data suggest that the terranes forming the outer hebrides and parts of the mainland lewisian were once contiguous and were fragmented by post 2730 ma events the c 1700 1660 ma ages from felsic sheets and pegmatites within the langavat shear zone may record the reassembly of these crustal fragments

chemical characteristics and genesis of the quartz feldspathic rocks in the archean crust of greenland a total of 108 samples of meta tonalites metagranodiorites granites and meta tholeiites representing groups of early to late archean age and different metamorphic history from sw and se greenland have been analyzed for ca k and 28 trace elements there is no systematic change of the chemical composition with age observable the results support petrologic experiments which suggest that tonalites and granodiorites the most abundant rocks of the archean crust are partial melting products of a mafic lower crust modelling suggest that this crust consisted of garnet amphibolite derived from a source with a bulk composition resembling a slightly enriched rather than depleted mantle the cen ybn ratio is above 10 in the majority of tonalites most samples have no eu anomaly because of a balanced contribution from the minerals of a mafic rock or a plagioclase free source the positive eu anomaly of some granodiorites and of a minor proportion of tonalites can be explained as being caused by plagioclase accumulation during differentiation or by partial melting of plagioclase rich fractions modelling with zn excludes an origin of tonalitic melts by differentiation of basaltic to dioritic magmas the archean meta diorites meta tonalites and meta granodiorites from greenland have generally lost some k and s relative to their suggested magmatic protoliths loss of rb tl pb and k and relative gain of ca sr ba and sc connected with granulitization of meta tonalites can be explained in the majority of cases by separation of about 25 percent granitic partial melt high k rb k pb zn cd and nb th ratios of granulites plus low ratios of granites are almost in balance with intermediate ratios of amphibolite facies tonalites retrogression of granulites into amphibolites was accompanied by introduction of pb tl rb ba sr and k from na cl rich brines circulating on fractures a comparison of the abundance of 24 elements characterized by different compatibility in the archean crust of greenland with the present bulk crust reflects only minor changes th nb if at all in the chemical composition of the continental crust since the archean

evolution and assembly of archean gneiss terranes in the godthabsfjord region southern west greenland structural metamorphic and isotopic evidence the godthabsfjord region of southern west greenland comprises several terranes that were assembled between 2750 and 2550 ma and folded during amphibolite facies metamorphism the terranes which are dominated by gneisses of different ages and show different preassembly metamorphic and structural histories are 1 the faeringehavn terrane containing the 3820 3600 ma amitsoq gneisses with granulite facies metamorphism at circa 3600 ma 2 the akia terrane containing the 3070 2940 ma nuk gneisses with granulite amphibolite facies metamorphism at circa 2980 ma 3 the tasiusarsuaq terrane containing circa 2900 ma gneisses with granulite facies metamorphism at circa 2800 ma and 4 the tre brodre terrane containing the 2800 2750 ma ikkattoq gneisses with amphibolite facies metamorphism at 2800 2750 ma metamorphic assemblages and structures formed prior to terrane assembly were variably overprinted during amphibolite facies metamorphism and heterogeneous strain associated with assembly recognition of the region as consisting of several terranes indicates that the anatomy of some archean high grade gneiss complexes may resemble that of orogens of proterozoic and phanerozoic age that formed as a consequence of plate tectonic processes

the structural setting and tectonic significance of the glen dessary syenite inverness shire the glen dessary syenite crops out in the steep belt of the northern highland moine it is enclosed within a psammite envelope which was previously assigned to the glenfinnan division but which is here correlated with the psammites of the loch eil division exposed to the e of the loch quoich line three episodes of deformation have been identified in the moine metasediments of the glen dessary area but only the latest d 3 is thought to have deformed the syenite imposing on it a strong steeply plunging mineral lineation the syenite is thought to occupy the core of a large intensely curvilinear d 3 synform the d 3 structures which deform the syenite can be correlated with upright structures which reworked previously flat lying already crystalline rocks recognized at loch quoich upright reworking must be younger than the glen dessary syenite dated by u pb methods at 456 5 ma two hitherto unrecorded small bodies of granitic gneiss similar to the c 1020 ma ardgour gneiss have been identified in psammites of the loch eil division near the w end of loch arkaig their presence suggests that the loch eil division has a grenville history and is part of the 9old moine9

archaean fluid assisted crustal cannibalism recorded by low δ 18 o and negative e hf t isotopic signatures of west greenland granite zircon the role of fluids during archaean intra crustal magmatism has been investigated via integrated shrimp u pb 18o and la mc icpms 176hf isotopic zircon analysis six rock samples studied are all from the nuuk region southern west greenland including two 3 69 ga granitic and trondhjemitic gneisses a 3 64 ga granitic augen gneiss a 2 82 ga granodioritic ikkattoq gneiss a migmatite with late neoarchaean neosome and a homogeneous granite of the 2 56 ga qorqut granite complex qgc all zircon grains were thoroughly imaged to facilitate analysis of magmatic growth domains within the zircon analysed there is no evidence for metamictization initial ehf zircon values n 63 are largely sub chondritic indicating the granitic host magmas were generated by the remelting of older un radiogenic crustal components zircon from some granite samples displays more than one 207pb 206pb age and correlated with 176hf 177hf compositions can trace multiple phases of remelting or recrystallization during the archaean model ages calculated using lu hf arrays for each sample indicate that the crustal parental rocks to the granites granodiorites and trondhjemites segregated from a chondrite like reservoir at an earlier time during the archaean corresponding to known formation periods of more primitive tonalite trondhjemite granodiorite ttg gneisses zircon from the 3 69 ga granite the migmatite and qgc granite contains eoarchaean cores with chondritic 176hf 177hf and mantle like 18o compositions the age and geochemical signatures from these inherited components are identical to those of surrounding tonalitic gneisses further suggesting genesis of these granites by remelting of broadly tonalitic protoliths zircon oxygen isotopic compositions n 62 over nine age populations six igneous and three inherited have weighted mean or mean 18o values ranging from 5 8 0 6 to 3 7 0 5 the 3 64 ga granitic augen gneiss sample displays the highest 18o with a mildly supra mantle composition of 5 8 0 6 inherited eoarchaean ttg derived zircon shows mantle like values igneous zircon from all other samples spanning more than a billion years of archaean time record low 18o sub mantle compositions these are the first low 18o signatures reported from archaean zircon and represent low 18o magmas formed by the remelting and metamorphism of older crustal rocks following high temperature hydrothermal alteration by meteoric water meteoric fluid ingress coupled with crustal extension associated high heat flow and intra crustal melting are a viable mechanism for the production of the low 18o granites granodiorites and trondhjemites reported here both high and low 18o magmas may have been generated in extensional environments and are distinct in composition from phanerozoic i type granitic plutonic systems which are typified by increasing 18o during intra crustal reworking this suggests that archaean magmatic processes studied here were subtly different from those operating on the modern earth and involved extensional tectonic regimes and the predominance of remelting of hydrothermally altered crystalline basement

adjacent terranes with ca 2715 and 2650 ma high pressure metamorphic assemblages in the nuuk region of the north atlantic craton southern west greenland complexities of neoarchaean collisional orogeny abstract in the gneiss complex of the nuuk region of the north atlantic craton in southern west greenland terrane juxtaposition was followed by neoarchaean folding under amphibolite facies conditions with widespread low pressure recrystallisation 5 kbar and 550 700 c the complex metamorphic overprinting requires that the p t history related to actual terrane assembly has to be extracted from very small relicts of older metamorphic assemblages combined with the u pb dating petrology and geochemistry of metamorphic zircons in the south of the region the faeringehavn terrane eoarchaean orthogneisses is tectonically overlain by a supracrustal package of amphibolites and paragneisses ca 2840 ma felsic volcano sedimentary protoliths this package is juxtaposed against a higher tectonic level represented by the tre brodre terrane 2825 ma orthogneisses and the tasiusarsuaq terrane 2920 2810 ma orthogneisses the terranes were assembled by 2710 2720 ma as shown by dating of granitic sheets intruded along the terrane boundary mylonites in the faeringehavn terrane and in the overlying 2840 ma supracrustal package relict early high pressure assemblages 12 8 kbar 700 750 c are clinopyroxene garnet plagioclase quartz hornblende in mafic rocks and garnet kyanite rutile bearing assemblages in paragneisses these are commonly replaced by lower pressure assemblages 7 5 kbar such as cordierite sillimanite garnet in paragneisses and hornblende plagioclase quartz garnet in mafic rocks in situ partial melting took place during both low and high pressure regimes metamorphic zircon in the high and low pressure assemblages yields dates of ca 2715 ma mostly with errors of the kapisilik terrane 3050 2960 ma orthogneisses and another supracrustal assemblage of amphibolites and ca 2800 ma quartzo feldspathic metasedimentary rocks are exposed north of the faeringehavn terrane and within fold cores along its western margin are bounded by folded neoarchaean mylonites the kapisilik terrane and this supracrustal assemblage include high pressure metamorphic remnants in amphibolites and metasediments metamorphic segregations with garnet clinopyroxene and kyanite respectively that formed at ca 2650 ma these remnants are overprinted by high temperature but lower pressure metamorphic events at ca 2630 2610 and 2580 ma thus remnants of early metamorphism reveal mutually exclusive ca 2715 and 2650 ma high pressure events in adjacent tectonically juxtaposed terranes repeated high pressure metamorphism in adjacent terranes is observed in younger complex collisional orogens

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

petrological and whole rock isotopic characteristics of tectonically juxtaposed archaean gneisses in the okak area of the nain province labrador relevance for terrane models in a recent model schiotte et al 1990 the archaean nain province of northern labrador is described as a tectonic collage of terranes with different origins and metamorphic histories that was assembled in the late archaean 3200 3260 ma granulite facies gneisses that are intercalated with the regional 3500 ma uivak gneisses in the northern saglek block of the nain province are interpreted as tectonic slivers of a terrane of mid archaean rocks represented by the maggo gneisses of the hopedale block 200 km farther south this terrane is thought to correlate with the akia terrane in southern west greenland in the nain province as opposed to west greenland the tectonic juxtaposition is thought to have taken place along subvertical planes there is overlap in the range of chemical compositions exhibited by the early and mid archaean orthogneisses in the okak area of the saglek block but on average the mid archaean gneisses are more mafic and less silica rich their major element chemistry exhibits much simpler evolutionary trends the ree chemistry is not related to the major element chemistry in any straightforward way in either early or mid archaean gneisses ree patterns show some correlation with the local concentration of accessory ree bearing minerals and are subject to metamorphic redistribution as late as 2560 ma in spite of this redistribution late archaean nd values can be used as a rough guide to the duration and complexity of the pre juxtaposition history nd 2560 ma of individual samples ranges from 4 4 to 7 0 in the mid archaean gneisses as opposed to values 7 7 in the early archaean gneisses the difference in metamorphic history also shows up in the whole rock pb isotopic signature the mid archaean granulite facies gneisses were juvenile intrusions the source of which can be estimated from model 1 values of 7 8 8 0 they crystallized in granulite facies sometimes between their emplacement at 3235 ma and an event of granitoid veining at 2990 ma they do not exhibit the complexities or distinctly non juvenile 207pb204pb ratios recorded from the early archaean gneisses the major volume of exposed early archaean rocks in the saglek block recrystallized under granulite facies metamorphism between 2780 and 2740 ma a suite of early archaean granitic granodioritic gneisses in granulite facies from the okak area differs from previously investigated uivak gneisses by showing evidence for larger pre metamorphic variation in upb 207pb204pb ratios that range from 14 02 to 16 30 and lower thu secondary u loss during late archaean metamorphism gave rise to a pb pb palaeoisochron a late archaean sm nd isochron with a strongly negative initial nd has been obtained on this gneiss suite the suite also exhibits other chemical peculiarities such as high cr contents a complex petrogenesis within a strongly differentiated early archaean crust is suggested there is no straightforward correlation between possible subterranes within terranes consisting of early archaean rocks in the nain province and west greenland

the initiation of strain localisation in plagioclase rich rocks insights from detailed microstructural analyses abstract in order to shed light on the cause for onset of strain localisation in plagioclase rich rocks we have performed detailed microstructural analyses on a sheared anorthosite leucogabbro using optical microscopy electron backscatter diffraction ebsd and chemical analyses the analysed sample is from an archaean unit sw greenland deformed at lower to mid crustal conditions t 675 700 c and moderate pressure the initial deformation occurred dominantly by dislocation creep and the grain size was reduced primarily by subgrain rotation recrystallisation recrystallised plagioclase grains average size 80 m are dominantly found in i clusters ii lenses and iii continuous bands subparallel to shear zone boundaries recrystallised grains in clusters and lenses display inherited crystallographic orientations their bulk crystallographic preferred orientation cpo is random however crystallographic characteristics show that parent and daughter grains have the same misorientation axes and possibly the same active slip systems recrystallised grains in continuous bands show a cpo with a single dominant active slip system 001 aligned with the structural xyz framework for these parent and daughter grains misorientation axes are random and the dominant slip system is different grain rotations of recrystallised grains are traceable back to the orientation of the adjacent porphyroclast we infer that the cause for strain localisation is recrystallisation and development of a cpo in continuous recrystallised bands microstructures in combination with misorientation and slip system analyses indicate a possible change from dislocation creep in clusters and lenses to dislocation accommodated grain boundary sliding disgbs in continuous bands this inferred shift in dominant deformation mechanism would lower the strength of the shear zone

evidence for hfse and ree mobility during calc silicate metasomatism mesoarchean 3075ma ivisaartoq greenstone belt southern west greenland abstract the mesoarchean ca 3075 ma ivisaartoq greenstone belt consists dominantly of variably deformed and metamorphosed mafic to ultramafic flows basalts to picrites in addition there are minor serpentinites gabbros clinopyroxene rich cumulates and volcaniclastic and siliciclastic sedimentary rocks primary volcanic structures such as pillows and volcanic breccia are well preserved in low strained zones the belt underwent intense metasomatic hydrothermal alteration between 3075 and 2961 ma an early stage of calc silicate metasomatism stage i resulted in the formation of epidosites in pillow cores pillow interstitials and intrusive margins of gabbroic dykes this alteration appears to have resulted from sea floor hydrothermal alteration a second stage of calc silicate metasomatism stage ii formed centimeter to meter scale layers concordant to discordant relative to the regional foliation these layers consist of diopside garnet epidote plagioclase quartz titanite vesuvianite calcite actinolite scheelite the second stage calc silicate rocks are associated with shear zones and their origin is attributed to interaction of reactive fluids with mafic and ultramafic rocks during regional metamorphism both stages of calc silicate metasomatism caused significant mobilization of major elements si mg ca fe na k and mn lile rb cs sr ba pb and lree heavy rare earth elements hree and hfse th nb ta zr ti remained relatively immobile during stage i metasomatic alteration but were variably disturbed during the stage ii metasomatism accordingly the stage ii calc silicate rocks exhibit evidence for significant mobility of th nb and ta on a local scale 1 4 m whereas zr and ti were not significantly mobilized this resulted in calc silicates with complementary trace element patterns displaying extreme negative to positive nb anomalies nb nb 0 19 7 28 transition metals ni v co cr and sc were relatively immobile during both metasomatic events amphibolites spatially related to the second stage calc silicate rocks preserve mineralogical and major element characteristics consistent with a mafic to ultramafic composition however their near primary magmatic trace element compositions were variably modified due to strong ree mobility during the stage ii metasomatic event this gave rise to various ree characteristics including lree enriched lree depleted and u shape ree patterns some of these element patterns resemble those shown by modern boninites and mid ocean ridge basalts n morb in contrast the least altered amphibolites exhibit similar trace element patterns to those displayed by the least deformed non metasomatized mafic and ultramafic pillow flows they possess relatively flat trace element patterns with negative nb ta anomalies indicating a subduction zone geochemical signature

sr and nd isotope systematics of polymetamorphic archean gneisses from southern west greenland and northern labrador sr and nd isotopic data for middle to late archean polymetamorphic felsic gneisses from localities in the nuuk area west greenland are compared and contrasted with new isotopic results for early archean amitsoq gneisses and with data for isotopically reworked kiyuktok gneisses from the saglek area labrador sr isotopic data for individual suites of felsic gneisses record the time integrated effect of variable rb sr fractionation during prograde and retrograde events as well as the effect of source inhomogeneity contrasting petrologic and sr nd isotopic characteristics are the result of differences in level of exposure caused partially by juxtaposition of terranes of different metamorphic character by movement on ductile shear zones and post shearing folding deformation sm nd systematics of felsic gneisses from nordafar ikerasakitsup akornga tinissaq and kangimut sammisoq qasigianguit define a geologically meaningless ca 3280 ma nd isochron which is the result of mixing of samples from unrela

accretion of continental crust thermal and geochemical consequences the irreversible chemical differentiation of the earth s mantle to produce sialic crust over the past 3900 ma has most probably occurred during widely separated but short lived accretion episodes these episodes involved the massive addition of juvenile sialic magma to the earth s surface thickening pre existing crust simple numerical simulations based on tectonic petrological and geochemical observations on archaean high grade orthogneiss terranes have been used to explore the metamorphic and geochemical consequences of massive thickening of sialic crust during short lived accretion episodes the location of the main sites of magmatic addition within the crust exert a profound influence on the thermal regimes geochemical differentiation of the continental crust by partial melt and vapour phase controlled processes and the development of granulite facies mineral assemblages can be integrated with the simple numerical models finally the survival of thick archaean continental crust imples the contemporaneous stabilization of thick lithospheric substructures to the newly formed continental masses

preliminary evaluation of the coalbed methane production potential and its geological controls in the weibei coalfield southeastern ordos basin china article i nfo the coalbed methane cbm geology resource and production potential in the weibei coalfield southeastern ordos basin are studied based on geological surveys and laboratory measurements the results showed that coal rank varies both laterally and vertically and changes from volatile bituminous coals in the margin to anthracites towards the basin 1 6 2 5 ro coals are composed of 60 85 vitrinite 15 40 inertinite and a trace amount of minerals methane isothermal adsorption measurements of 20 coal samples revealed that the maximum adsorption capacity on a dry and ash free basis of coals which are affected by coal rank coal maceral coal lithotype and especially to the moisture content varies from 13 91 to 29 54 m 3 t estimated gas contents range from 0 to 15 m

plate tectonics 2 5 billion years ago evidence at kolar south india the archean kolar schist belt south india is a suture zone where two gneiss terranes and at least two amphibolite terranes with distinct histories were accrted amphibolites from the eastern and western sides of the schist belt have distinct incompatible element and isotopic characteristics sugesting that their volcanic protoliths were derived from dint mantle sources the amphibolite and gneiss terranes were juxtaposed by horizontal compression and shearing between 2530 and 2420 million years ago ma along a zone marked by the kolar schist belt this history of accretion of discrete crustal terranes resembles those of phanerozoic convergent margins and thus suggests that plate tectonics operated on earth by 2500 ma

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

extrapolation of tectonic boundaries across the labrador shelf u pb geochronology of well samples near saglek fiord a northerly trending boundary between the early archean saglek block and the middle archean hopedale block extends between drill sites which respectively sampled uivak amphibolite gneiss with u pb zircon intercept ages of 3742 12 and 2752 42 ma and migmatitic lister gneiss with concordant ages of for restite and for leucosome titanite ages of ca 2508 ma are common to both rocks a nearby metapsammitic gneiss has detrital zircon and monazite ages of 2681 5 2700 4 ca 2730 and 2750 2 ma representing high grade metamorphism related to the hopedale saglek collision and metamorphic monazite of ca 2560 ma age representing metamorphism of the sediment during reactivation of the saglek hopedale suture two hundred kilometres southeast a gneissic granite records a protolith age of 3170 ma and late proterozoic pb loss near the nain makkovik boundary 1269 4 ma zircons indicate a significant extension of the nain platonic suite south of the makkovik boundary a foliated g

geodynamic setting and deformation history of an archaean terrane at mid crustal level the tasiusarsuaq terrane of southern west greenland abstract the neoarchaean tasiusarsuaq terrane is situated in the north atlantic craton approximately 50 km south of the capital nuuk and extends for 400 km to the south to the frederikshab isblink it mainly consists of granulite facies and commonly migmatitic grey gneiss narrow supracrustal belts and volumetrically minor late tectonic granites the supracrustal belts have a lithological association of picritic komatiitic and basaltic rocks together with ultramafic dykes and sills with geochemical characteristics typical of an arc type setting the grey gneiss is tonalitic to granodioritic in composition and was emplaced between ca 2 92 and 2 82 ga into crust that is probably represented by the supracrustal rocks the gneiss in turn was intruded by granite dykes at ca 2830 ma 2800 ma and by tonalitic dykes at ca 2770 ma both probably are the result of partial melting of the thickened crust late tectonic tonalitic pegmatites intruded at 2673 5 ma at amphibolite facies conditions the structural evolution from the granulite facies peak to amphibolite facies levels is characterised by three deformation stages d 1 fabrics form only relics in low strain domains of the dominant nw vergent d 2 fold and thrust structures syn tectonic granite and tonalitic dykes date the d 2 thrusting to have occurred between ca 2800 ma and 2700 ma contemporaneously with near isobaric cooling the subsequent d 3 deformation still in the amphibolite facies is marked by e w to ne sw shortening forming upright folds and near vertical strike slip shear zones at ca 2670 2630 ma the final exhumation of the terrane likely occurred first during the proterozoic and was probably mainly driven by erosion the granulites of the tasiusarsuaq terrane formed at an orogenic margin in a compressive regime a first orogenic cycle 2 8 2 7 ga here termed the tasiusarsuaq orogeny resulted in thrusting of the terrane on a rigid foreland represented by the faeringehavn terrane in the north the tasiusarsuaq granulites were further affected by a second archaean orogeny 2 67 2 58 ga the kapisilik orogeny which caused transcurrent deformation and only insignificant exhumation to mid crustal levels significantly there is no evidence that the exhumation of the granulite facies tasiusarsuaq terrane was driven by archaean extension in contrast to many modern situations where granulites are exhumed by extension related to magmatic underplating and delamination of the lithosphere the thickened tasiusarsuaq terrane was possibly stabilised due to the low density differences and equilibration by melt extrusion during granulite facies metamorphism stabilisation of the crust and lack of lithospheric delamination is therefore regarded as being more common in archaean orogens compared to younger orogens

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

the geodynamic evolution of mesoarchean anorthosite complexes inferred from the naajat kuuat complex southern west greenland layered anorthosite complexes are typical components of archean crustal domains however the geodynamic settings in which they were emplaced are still discussed as geological relationships are oft

anomalous neon and xenon in an archaean anorthosite from west greenland large excesses of129xe 131xe and132xe relative to the atmospheric xe were first reported by jeffery in an archaean anorthosite 2 9 ga from the fisken sset complex in west greenland later zadnik and jeffery observed excess21ne and22ne in the same anorthosite since the isotopic anomalies recorded in the rock may be a reflection of specific archaean geological conditions we decided to re examine the problem we studied an anorthosite from the same complex hornblende and anorthite were separated from the anorthosite and isotopic analyses were carried out on the mineral separates xe is essentially atmospheric except for minor excesses of xe produced by238u spontaneous fission in gases released from hornblende at higher temperatures in stepped heating experiments the result did not confirm earlier observations however we also observed considerable excesses of both21ne and22ne in accordance with the results of zadnik and jeffery and consistent with nucleogenic ne in many fluids in the crust the former excess is most reasonably attributable to18o n 21ne and the latter to19f n 22ne we conclude that these excesses are trapped components and that this anorthosite trapped these excess noble gases from a hydrous fluid derived from a crustal region enriched in u th and f during the 2 9 ga metamorphism

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

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

early precambrian crustal development changing styles of mafic magmatism the work of sutton watson 1951 was one of the cornerstones in the interpretation of the evolution of early precambrian archaean and early proterozoic gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted archaean cratons since then the concept of geological terranes and terrane accretion has been extended back to the archaean and different events can be distinguished by increasingly precise isotopic dating techniques there is growing evidence that with certain clear provisos the principle of uniformitarianism can be applied as equally to the early precambrian as it can through the rest of the geological record in terms of mafic magmatism there are similarities between some ancient and modern suites both intrusive and extrusive but there are also some major differences such as the presence of komatiites in the archaean and the formation of large scale layered intrusions and extensive dyke swarms at the end of the late archaean and in the early proterozoic the mantle has evolved continuously with the progressive extraction of mafic magmas but allowing for this continental tholeiites appear to have changed little with time ancient mid ocean ridge basalts are elusive because of their poor preservation record the geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present day arc systems the relative abundance of early precambrian intrusive mafic suites parallels the crustal growth curve reflecting the global crustal accretion differentiation superevent towards the close of the archaean bimodal tholeiitic noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle mafic crust continental crust system this in turn gives a possible link between the respective evolutions of archaean to recent komatiitic noritic and boninitic suites

building mesoarchaean crust upon eoarchaean roots the akia terrane west greenland constraining the source genesis and evolution of archaean felsic crust is key to understanding the growth and stabilization of cratons the akia terrane part of the north atlantic craton west greenland is comprised of meso to neoarchaean orthogneiss with associated supracrustal rocks we report zircon u pb and lu hf isotope data and whole rock geochemistry from samples of gneiss and supracrustals from the northern akia terrane including from the finnefjeld orthogneiss complex which has recently been interpreted as an impact structure isotope data record two major episodes of continental crust production at ca 3 2 and 3 0 ga minor ca 2 7 and 2 5 ga magmatic events have more evolved ehf interpreted as reworking of existing crust perhaps linked to terrane assembly felsic rocks from the finnefjeld orthogneiss complex were derived from the same source at the same time as the surrounding tonalites but from shallower melting requiring any bolide driven melting event to have occurred almost simultaneously alongside the production of the surrounding crust a simpler alternative has the finnefjeld complex and surrounding tonalite representing the coeval genesis of evolved crust over a substantial lithospheric depth hafnium isotope data from the two major mesoarchaean crust forming episodes record a contribution from older mafic eoarchaean crust invoking the involvement of an eoarchaean root in the growth of younger mesoarchaean crust puts important constraints on geodynamic models of the formation of the discrete terranes that ultimately assembled to form earth s cratons

the whole rock sm nd age for the 2825 ma ikkattoq gneisses greenland is 800 ma too young insights into archaean ttg petrogenesis abstract the ikkattoq gneisses of the archaean gneiss complex in the nuuk region southern west greenland are the orthogneiss component within the amphibolite facies tre brodre terrane they have mostly granodioritic compositions with a small amount of quartz diorite sm nd isotopic data for a quartz diorite and five granodiorite ikkattoq gneiss samples from within 5 km of the ikkattoq fjord type locality yielded a regression with a slope equivalent to 2005 52 ma mswd 0 72 regardless of the low mswd this cannot be the true age of the ikkattoq gneisses because all ikkattoq gneisses yield u pb zircon dates of c 2825 ma and they are cut by the undeformed 2560 ma qorqut granite complex this anomalously young regression age resulted instead from mixing of different nd components indicating that the ikkattoq gneisses are derived from mixed source materials taking the true age of the ikkattoq gneisses as 2825 ma from u pb zircon dating the range of initial e nd in the ikkattoq gneisses is 7 1 to 1 8 the negative initial e nd values mean that older light rare earth enriched sialic crust contributed to the igneous precursors of the ikkattoq gneisses this nd evidence for contribution of older sialic crust is supported by positive e sr values for the ikkattoq gneisses with e nd values as low as 7 1 this older crustal component has to be eoarchaean the presence of scarce quartz diorites low sio 2 high mgo suggests that ultramafic rocks upper mantle metasomatised by the passage of fluids or silicic melts were another contributing source the ikkattoq gneisses are proposed as a complex suite incorporating material derived from melting of much older sialic crust and probably upper mantle the intercalation of tectonostratigraphic terranes during collisional orogeny at c 2720 ma destroyed the architecture of this 2825 ma magmatic system and the ikkattoq gneisses now form a slice tectonically isolated from their source region in terms of trace element parameters the ikkattoq gneisses resemble phanerozoic volcanic arc granites thus an andean style arc setting for the generation of the ikkattoq gneiss precursors is possible other archaean ttg suites of the nuuk region are generally thought to represent predominantly juvenile additions to the crust in the broadest sense they do because isotopic work over the past 30 years has demonstrated that they do not represent wholesale recycling of considerably older crust however in detail within these broadly juvenile suites a contribution from older crust can be detected thus c 3000 ma type nuk gneisses from around nuuk town show a spread in e nd values down to 1 7 in this case the likely older crustal component was 3230 ma quartz diorite that occurs as enclaves in the c 3000 ma suite thus to a lesser or greater degree some meso to neoarchaean ttg suites in the nuuk region display the same internal complexities and evidence for mixed sources as modern arc suites developed near the margins of older crust

precise u pb baddeleyite ages of mafic dykes and intrusions in southern west greenland and implications for a possible reconstruction with the superior craton the archaean block of southern greenland constitutes the core of the north atlantic craton nac and is host to a large number of precambrian mafic intrusions and dyke swarms many of which are regionally extensive but poorly dated for southern west greenland we present a u pb zircon age of 2990 13 ma for the amikoq mafic ultramafic layered intrusion fiskefjord area and four baddeleyite u pb ages of precambrian dolerite dykes specifically a dyke located se of ameralik fjord is dated at 2499 2 ma similar to a previously reported ar 40 ar 39 age of a dyke in the kangamiut area for these and related intrusions of ca 2 5 ga age in southern west greenland we propose the name kilaarsarfik dykes three wnw trending dykes of the md3 swarm yield ages of 2050 2 ma 2041 3 ma and 2029 3 ma a similar u pb baddeleyite age of 2045 2 ma is also presented for a se trending dolerite iglusuataliksuak dyke in the nain province the rifted western block of the nac in labrador we speculate that the md3 dykes and age equivalent nne trending kangamiut dykes of southern west greenland together with the iglusuataliksuak dyke after closure of the labrador sea represent components of a single areally extensive radiating swarm that signaled the arrival of a mantle plume centred on what is presently the western margin of the north atlantic craton comparison of the magmatic barcodes from the nain and greenland portions of the north atlantic craton with the established record from the north eastern superior craton shows matches at 2500 ma 2214 ma 2050 2030 ma and 1960 1950 ma we use these new age constraints together with orientations of the dyke swarms to offer a preliminary reconstruction of the north atlantic craton near the north eastern margin of the superior craton during the latest archaean and early palaeoproterozoic possibly with the core zone craton of eastern canada intervening c 2010 elsevier b v all rights reserved less

the stratigraphy of a sheet of supracrustal rocks within high grade orthogneisses and its bearing on late archaean structure in southern west greenland an extensive sheet of presumed middle archaean supracrustal rocks within high grade orthogneisses in inner godthabsfjord includes a section 3 km thick with exceptional preservation of pillow structures in low k tholeiitic amphibolites unambiguously indicating way up the amphibolites are associated with pillow structured and pyroclastic komatiitic ultramafic schists dunitic harzburgitic sills a stratiform gabbro pyritous metacherts and quartzo feldspathic gneisses and schists with field and chemical characteristics suggesting they formed as acid intermediate tuffaceous sediments the sheet is tectonically overlain by older orthogneisses amitsoq with tracts and enclaves of the early archaean isua akilia supracrustal association it is underlain by younger orthogneisses and granites with tracts of older variably reworked amitsoq gneisses rocks of the higher level were emplaced on a tectonic slide which marks the upper boundary of the supracrustal sheet the original lower boundary with amitsoq gneisses in the lower level may also have been a slide which has been obscured by emplacement of younger orthogneisses composite dome in dome structures with intervening synclinal cusps were superimposed on the sub horizontal layer cake structure of the supracrustal sheet and the higher and lower levels of orthogneisses syntectonic granitic and amphibolite dykes kangilingua dykes were emplaced in parts of the synforms whilst granite emplacement and feldspar blastesis took place in the cores of some domes the dome and cusp system appears to be part of a regional series of late archaean steeply inclined to upright structures trending northeast in the godthabsfjord region in inner godthabsfjord the system evolved in response to buoyancy forces generated by injection of basaltic magmas and partial melting of orthogneisses at depth within a regime of regional compression associated perhaps with transcurrent intraplate motions

the monashee decollement at cariboo alp southern flank of the monashee complex southern british columbia canada abstract the monashee decollement is a crustal scale compressional shear zone in the southern canadian cordillera it is well exposed in alpine terrane and is imaged in the subsurface by lithoprobe seismic reflection profiles the decollement separates middle to upper crustal allochthonous assemblages from underlying middle to lower crustal rocks of the orogen development of the exposed part of the decollement at cariboo alp in the southern thor odin culmination has included formation of major isoclinal folds superposition of smaller scale folds during progressive deformation and transposition of early formed fabrics and stratigraphic boundaries into the dominant sw dipping mylonitic planar fabric hinge lines of early folds have been rotated toward the strong consistent sw plunging mineral stretching lineation as a result of ne directed shearing of fold limbs and thrust stacking of these folded sheets between a basal shear zone and overlying allochthonous assemblages an imbricate zone developed within the middle crust during peak metamorphic conditions

u pb ages of single zircons within upernavik metasedimentary rocks and regional implications for the tectonic evolution of the archaean nain province labrador detrital zircons and their postdepositional overgrowths from three units of the upernavik supracrustal association in the northern saglek block of the archaean nain province have been dated with the ion microprobe shrimp in one unit from the granulite facies area in inner saglek fiord the zircon population is dominated by early archaean grains thought to be derived from the uivak gneisses recrystallization and growth of new zircon within this metasediment took place during granulite facies metamorphism at 2761 12 ma 2 which is also a younger limit on the age of deposition in a second unit from the amphibolite facies area in outer saglek fiord detrital zircons have predominantly mid and late archaean ages the mid archaean zircons are comparable in age to the 3235 ma lister gneisses the ages of the late archaean detrital zircons 2800 2960 ma do not correspond to any known rock complex in the saglek block but plutonic rocks associated or correlative with the ca 2840 ma kanairiktok plut

the u th pb systematics of zircons from the type nuk gneisses godthabsfjord west greenland abstract u pb analyses were made on zircons separated from different lithologies within the nuk gneisses taken from godthab nuuk townsite and from the south west shore of bjorneoen size fractions of the zircons from each rock yield linear discordia which give apparent crystallisation ages for the parents of the nuk gneisses of 2890 3065 myr this spread of ages is not a result of multiple intrusion but more likely results from different degrees of diffusion and metamorphic episodic lead loss by the zircons since u pb and th pb discordance are similar loss of radiogenic lead and not u or th loss gain by radiation damage diffusion and through metamorphism is the main factor causing discordance of the u pb dates the results show that the discordia line given by the analyses of fractions of a single zircon population may not give a true crystallisation age when the rock has been strongly metamorphosed this interpretation is supported by rb sr whole rock analyses of the same rocks in which the nuk townsite rocks scatter about a reference isochron of 2840 myr while the bjorneoen samples give an isochron mswd 0 55 of 3076 27 myr and the initial ratio at 0 70205 0 00009 the zircon results do not rule out the possibility that more than one metamorphic episode affected the original nuk gneiss parents prior to 2520 myr qorqut granite intrusion a date of 2660 myr was obtained from an aplite intruded during a period of plutonic activity that included the formation of a linear belt of intense ductile deformation that extends through south western bjorneoen and godthab town

conditions of archean granulite metamorphism in the godthab fiskenaesset region southern west greenland the nordlandet peninsula akia terrane and the tasiusarsuaq terrane in southern west greenland were metamorphosed to granulite facies at 3 0 and 2 8 ga respectively temperatures of metamorphism are estimated using magnetite ilmenite garnet orthopyroxene garnet clinopyroxene and garnet biotite thermometers barometry has been carried out in the two terranes using eight different garnet barometers a uniform set of activity models for all minerals including the garnet activity model of newton et al 1986 is applied to each barometer in order to permit comparison pressure estimates using the different barometers are generally quite consistent 1 5 kbar use of the newton et al 1986 garnet activity data results in pressures similar to those obtained using other garnet activity models peak metamorphic conditions on the nordlandet peninsula are estimated to have been 800 50 c 7 9 1 0 kbar values of logfo2 are estimated to have been 1 1 to 2 0 above the quartz magnetite fayalite buffer from assemblages of magnetite ilmenite and quartz magnetite ferrosilite peak metamorphic conditions in the tasiusarsuaq terrane are estimated to have been 780 50 c 8 9 1 0 kbar estimates of fh2o and fco2 using biotite amphibile grossular anorthite and grossular scapolite equilibria are low in both terranes these results suggest that granulite metamorphism was fluid absent in both terranes and that the metamorphism in the akia terrane and possibly also in the tasiusarsuaq terrane was initiated by the injection of large volumes of magma into the lower crust