the complex tectonic evolution of the craton adjacent northern margin of the palaeoproterozoic ketilidian orogen southeastern greenland evidence from the geochemistry of mafic to intermediate and granitic intrusions abstract the archaean thrym complex exposed in southeastern greenland is part of the north atlantic craton its southernmost part is in a transitional zone containing archaean orthogneiss to the north and palaeoproterozoic paragneiss and granitic rocks to the south that are part of the ketilidian orogen this 50 km wide transitional zone includes ca 2830 ma orthogneiss that is similar in age to orthogneiss in the central and northern parts of the complex and ca 2740 ma orthogneiss that is synchronous with the ca 2790 2700 ma skjoldungen orogeny a significant difference between this zone and the complex to the north is the presence of greenschist facies sedimentary rocks unconformably overlying orthogneiss and intruded by ca 1820 1800 ma granitic and mafic to intermediate intrusions these intrusions are within the ca 1854 1799 ma age range of the julianehab igneous complex with suites both older and younger than ca 1830 ma that are not restricted to specific zones within the ketilidian orogen the geochemistry of the palaeoproterozoic mafic to intermediate rocks in the transitional zone shows contamination by orthogneiss from the thrym complex and emplacement within an oceanic continental transition in a within plate setting unrelated to subduction in contrast the palaeoproterozoic granites were sourced entirely from archaean orthogneiss of the thrym complex making them technically i type granites that have inherited their petrogenetic signature from the orthogneiss as a result the palaeoproterozoic granites and intermediate to mafic igneous rocks have similar chondrite normalised patterns to the orthogneiss in the archaean complex this demonstrates that discrimination diagrams cannot be assumed to be unequivocal for the rocks studied unless the geochemistry of the source rocks is also known

tectonic setting of synorogenic gold deposits of the pacific rim abstract more than 420 million oz of gold were concentrated in circum pacific synorogenic quartz lodes mainly during two periods of continental growth one along the gondwanan margin in the palaeozoic and the other in the northern pacific basin between 170 and 50 ma these ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout clastic sedimentary rock dominant terranes the auriferous veins contain only a few percent sulphide minerals have gold silver ratios typically greater than 1 1 show a distinct association with medium grade metamorphic rocks and may be associated with large scale fault zones ore fluids are consistently of low salinity and are co2 rich in the early and middle palaeozoic in the southern pacific basin a single immense turbidite sequence was added to the eastern margin of gondwanaland deformation of these rocks in southeastern australia was accompanied by deposition of at least 80 million oz of gold in the victorian sector of the lachlan fold belt mainly during the middle and late devonian lesser devonian gold accumulations characterized the more northerly parts of the gondwanan margin within the hodgkinson broken river and thomson fold belts additional lodes were emplaced in this flyschoid sequence in devonian or earlier palaeozoic times in what is now the buller terrane westland new zealand minor post devonian growth of gondwanaland included terrane collision and formation of gold bearing veins in the permian in australia s new england fold belt and in the jurassic early cretaceous in new zealand s otago schists collision and accretion of dozens of terranes for a 100 m y long period against the western margin of north america and eastern margin of eurasia led to widespread latest jurassic to eocene gold veining in the northern pacific basin in the former location late jurassic and early cretaceous veins and related placer deposits along the western margin of the sierra nevada batholith have yielded more than 100 million oz of gold additional significant ore forming events during the development of north america s cordilleran orogen included those in the klamath mountains region california in the late jurassic and early cretaceous the klondike district yukon by the early cretaceous the nome and fairbanks districts alaska and the bridge river district british columbia in the middle cretaceous and the juneau gold belt alaska in the eocene gold bearing veins deposited during the late jurassic and early cretaceous terrane collision that formed the present day russian far east have been the source for more than 130 million oz of placer gold the abundance of gold bearing quartz carbonate veins throughout the gondwanan north american and eurasian continental margins suggests the migration and concentration of large fluid volumes during continental growth such volumes could be released during orogenic heating of hydrous silicate mineral phases within accreted marine strata the common temporal association between gold veining and magmatism around the pacific rim reflects these thermal episodes melting of the lower thickened crust during arc formation slab rollback and extensional tectonism and subduction of a slab window beneath the seaward part of the forearc region can all provide the required heat for initiation of the ore forming processes

the origin and mode of emplacement of lujavrites in the ilimaussaq alkaline complex south greenland abstract lujavrites are rare meso to melanocratic agpaitic nepheline syenites that are characterized by elevated contents of elements such as li be zr ree nb th and u they are the most evolved members of the three large composite agpaitic complexes lovozero kola peninsula russia pilansberg south africa and ilimaussaq south greenland and are inferred to stem from the same deep fractionating magma sources that fed the earlier members of the complexes the composition of the melts that evolved into lujavrites is however not well known the agpaitic part of the ilimaussaq complex is divided into a roof series a floor series of cumulates and an intermediate series of lujavrites sandwiched between the two in the traditional view the lujavrites formed from residual melts left between the downward crystallizing roof series and the floor cumulates new field observations and geochemical data suggest that the floor cumulates and the main mass of lujavrites constituted a separate intrusive phase which was emplaced into the already consolidated roof series rocks largely by piecemeal stoping studies of the contact facies of the floor cumulates indicate that the initial magma of the floor cumulate lujavrite sequence was peralkaline nepheline syenitic with enhanced contents of zr hf hree y nb ta f ba and sr subsequent crystallization in a closed system resulted in the formation of the floor cumulates and lujavrites chemical analyses of dykes within and outside the complex represent stages in the magmatic evolution of the agpaitic rocks

remnants of arc related mesoarchaean oceanic crust in the tartoq group of sw greenland the tartoq group located in sw greenland consists of supracrustal rocks of mainly tholeiitic basaltic composition including pillow lavas sills dykes and gabbros as well as ultramafic rocks metamorphic grade ranges from greenschist fades to granulite facies the tartoq group crops out as a series of blocks and slivers that are imbricated with originally intrusive mesoarchaean ttg orthogneisses the supracrustal rocks form part of a se vergent fold and thrust belt consistent with the imbrication of ttg gneisses and supracrustal rocks along a convergent margin la icp ms u pb zircon dating of an intrusive ttg sheet yields a minimum age of 2986 4 ma for the tartoq group this age is consistent with mc icp ms lu hf and sm nd isotopic whole rock data for mafic samples from different blocks of the tartoq group which yield errorchron ages of 3189 65 ma and 3068 220 ma respectively the mafic supracrustal rocks of the tartoq group have chondrite normalized ree patterns with la cn sm cn of 0 67 1 96 and rather flat primitive mantle normalized multi element patterns except for scattered lile contents and generally negative nb anomalies with nb nb of 0 26 1 31 th yb varies between 0 06 and 0 47 and nb yb between 0 45 and 4 4 indicative of an arc affinity when compared to rocks from modern settings the similar geochemistry of the different lithological units together with their coeval formation as evident from trace element geochemical trends supports a co magmatic origin for the rock assemblage and their formation as imbricated relics of oceanic crust accordingly we propose that the tartoq group represents remnants of mesoarchaean oceanic crust which formed in a suprasubduction zone geodynamic environment c 2011 international association for gondwana research published by elsevier b v all rights reserved less

precise u pb ages and geochemistry of palaeoproterozoic mafic dykes from southern west greenland linking the north atlantic and the dharwar cratons palaeoproterozoic magmatism throughout the north atlantic craton nac has produced dyke swarms of diverse orientations and emplacement ages in southern west greenland previously identified swarms include the ca 2500 ma kilarsaarfik dykes ca 2215 ma boninitic norite bn dykes and the abundant ca 2040 ma kangamiut and coeval 2050 2030 ma md3 dykes additional insight into the distribution of swarms is provided by new baddeleyite u pb id tims dates interpreted as emplacement ages for six dykes the geochemistry for a total of 42 samples on 40 dykes including the dated dykes provides some constraints on their petrogenesis two e w trending dykes yield ages of 2365 2 ma and 2374 4 ma representing a hitherto unknown magmatic event we propose the name graedefjord dykes for dykes of this age in southern west greenland one ne sw trending dyke is dated at 2209 5 ma close to a previously published 2214 10 ma age for a n s trending dyke member of bn dykes three dykes yield md3 ages of 2053 2 ma e w trending 2049 6 ma n s trending and 2042 2 ma wnw ese trending ca 2 21 ga old and predominantly ne se and nne sse trending dolerites some of which are high al basalts were derived from a relatively depleted mantle and injected during a boninite dolerite magmatic event together with nearby bn dykes derived from a harzburgitic yet highly lree and lile enriched subcontinental lithospheric mantle sclm ca 2 05 ga kangamiut md3 dolerites were derived from a similar ambient asthenospheric mantle and subsequently experienced crustal assimilation during fractional crystallisation the ca 2370 ma dykes in the nac are coeval with the dharwar giant dyke swarm of the dharwar craton dc and based on this unique age match it is proposed that the dharwar and nac were neighbours in the palaeoproterozoic furthermore both blocks share 222 2 21 ga dykes and matching the trends of 2 2 2 1 and 2 37 ga swarms between the two blocks allows several plausible reconstructions c 2012 published by elsevier b v

shallow water eruption fed mafic pyroclastic deposits along a paleoproterozoic coastline kangerluluk volcano sedimentary sequence southeast greenland abstract the 200 300 m thick volcano sedimentary sequence at kangerluluk is part of the psammite zone one of four major zones which constitute the 1 8 ga ketilidian orogen in south greenland three lithofacies are emphasized in the study 1 the conglomerate sandstone 2 the volcanic and 3 the pyroclastic lithofacies the 2 40 m thick conglomerate sandstone lithofacies represents a subaerial to subaqueous fan delta deposit matrix and clast supported conglomerates are interpreted as debris flow and longitudinal gravel bar deposits erosive based conglomerate channel fills attest to stream incision trough crossbedded sandstone interpreted as lunate megaripples planar bedded sandstone indicative of upper flow regime bar top sands and small scale trough crossbeds reflecting ripples follow up section form collectively with the conglomerate 0 40 2 50 m thick fining upward sequences the sandstone dominated unit up section from the conglomerates and composed of planar and low angle crossbeds minor ripples and graded beds as well as mudstone is indicative of a lower shoreface deposit below normal wave base the clastic sedimentary rocks are suggestive of a fan delta setting the 100 200 m thick volcanic lithofacies composed of pillowed and pillow brecciated lava flows is consistent with shallow water deposition interstratification of lava flows with both conglomerate sandstone and pyroclastic lithofacies intrusion of dykes into volcaniclastic rocks and peperite formation accentuate contemporaneity between volcanism and sedimentation and is a common feature of island arcs the 1 50 m thick pyroclastic lithofacies with sharp depositional contacts to the overlying volcanic and underlying conglomerate sandstone lithofacies was emplaced in a subaqueous setting the lithofacies is divided into a planar to crossbedded tuff lapilli tuff and a bedded lapilli tuff breccia whereby both deposits are inferred to result from shallow water surtseyan type eruptions the 5 15 m thick bedded lapilli tuff breccia with abundant bomb sag structures and graded beds is considered a result of subaqueous eruptions strong enough to form an insulating steam cupola characterized by ballistically emplaced bombs that rapidly collapsed allowing for transport via mass flow processes the deposits are considered proximal to the vent the 2 50 cm thick planar to crossbedded tuff lapilli tuff featuring abundant euhedral and broken crystals of feldspar 2cm and minor pyroxene 1 cm are massive graded crossbedded and stratified the planar but laterally discontinuous beds characterized by abundant low angle scours are interpreted as low to high concentration sediment gravity flows produced directly from subaqueous tephra jets that collapsed due to massive water ingestion local breccia size pyroclasts disrupting beds are interpreted as bomb sags the mafic eruption fed surtseyan type deposits postulated to be a subaqueous counterpart of cold subaerial base surges originate from subaqueous tuff cones formed along a rugged volcanic dominated shoreline featuring high energy fan deltas

fore arc deformation and underplating at the northern hikurangi margin new zealand geophysical investigations of the northern hikurangi subduction zone northeast of new zealand image fore arc and surrounding upper lithospheric structures a seismic velocity vp field is determined from seismic wide angle data and our structural interpretation is supported by multichannel seismic reflection stratigraphy and gravity and magnetic modeling we found that the subducting hikurangi plateau carries about 2 km of sediments above a 2 km mixed layer of volcaniclastics limestone and chert the upper plateau crust is characterized by vp 4 9 6 7 km s overlying the lower crust with vp 7 1 km s gravity modeling yields a plateau thickness around 10 km the reactivated raukumara fore arc basin is 10 km deep deposited on 5 10 km thick australian crust the fore arc mantle of vp 8 km s appears unaffected by subduction hydration processes the east cape ridge fore arc high is underlain by a 3 5 km deep strongly magnetic 3 3 a m high velocity zone interpreted as part of the onshore matakaoa volcanic allochthon and or uplifted raukumara basin basement of probable oceanic crustal origin beneath the trench slope we interpret low seismic velocity high attenuation low density fore arc material as accreted and recycled suggesting that underplating and uplift destabilizes east cape ridge triggering two sided mass wasting mass balance calculations indicate that the proposed accreted and recycled material represents 25 100 of all incoming sediment and any remainder could be accounted for through erosion of older accreted material into surrounding basins we suggest that continental mass flux into the mantle at subduction zones may be significantly overestimated because crustal underplating beneath fore arc highs have not properly been accounted for

inferred palaeoproterozoic arc rifting along a consuming plate margin insights from the stratigraphy volcanology and geochemistry of the kangerluluk sequence southeast greenland the 200 to 300 m thick volcano sedimentary sequence in the kangerluluk fjord associated with penecontemporaneous and late tectonic dykes as well as a synvolcanic plutonic suite represents an integral component of the palaeoproterozoic ketilidian mobile belt south greenland the ca 1808 ma kangerluluk supracrustal sequence contains four distinct mappable lithofacies a a conglomerate sandstone lithofacies b a pyroclastic lithofacies c a volcanic lithofacies and d a peperite lithofacies the volcanic lithofacies up to 200 m thick is characterized by shallow water subaqueous brecciated and pillowed flows flows are either a feldspar phyric or b feldspar pyroxene phyric with 0 2 to 3 cm size plagioclase and 0 2 to 3 cm size pyroxene that constitute between 20 and 30 locally up to 50 of the flows mafic dykes intruded wet unconsolidated pyroclastic lithofacies resulting in the formation of peperites geochemically the volcanic and pyroclastic units represent a distinct tholeiitic magmatic suite enriched in incompatible trace elements including th la yb zr and nb and exhibiting la yb n 10 the plutonic suite and associated dykes display a calc alkaline trend with enriched lree and unfractionated flat hree patterns lan ranging between 50 and 100 la yb n ratios between 8 and 22 and negative nb and ti anomalies on the mantle normalized incompatible multi element patterns the pillowed flows lie in the continental flood basalt field on the y nb zr discrimination diagram and display a nb anomaly and k2o enrichment that collectively suggest a crustal component and or a subduction modified mantle source the geology stratigraphy of the kangerluluk area and geochemistry can be used to infer a change in magma genesis from arc to rift volcanism the 1850 to 1800 ma calc alkaline julianehab batholith represents a magmatic arc that rifted during crustal extension allowing for the ascent of mantle derived mafic magma the geochemistry of the mafic volcanic flows synvolcanic dykes and pyroclastic deposits favours a crustal component in magma genesis and offers new insights into the tectonic evolution of the ketilidian mobile belt

modeling the effect of stratification on cemented layer formation in sulfide bearing mine tailings abstract reactive transport simulations have been applied to investigate possible effects of stratification on the potential of sulfide bearing mine tailings to form protective cemented layers the simulations are based on characteristic strata found at a german tailings site including sulfide enriched heavy mineral layers mica enriched silt layers and homogeneously mixed layers the simulated secondary phases jarosite gypsum amorphous ferric arsenate amorphous fe hydroxide alunite amorphous silica and kaolinite are similar to those observed in the field using scanning electron microscope analyses of cemented layers it has been observed that the pore area becomes disconnected if the porosity is decreased to values below 15 which would indicate a strong decrease in permeability stratification was found to play a crucial role in cemented layer formation cemented layers are absent or insignificant in systems with a homogeneous distribution of fe bearing sulfides they are extensively developed in systems with a an arsenopyrite rich layer or b a mica enriched layer situated immediately below an fe sulfide enriched layer the modeling results have clearly demonstrated that the key processes operating in scenario a are very different from the key processes in scenario b in scenario a the oxidation of arsenopyrite is followed by the precipitation of amorphous ferric arsenate which can be solely responsible for significant pore reduction in scenario b the presence of a large amount of reactive aluminosilicates e g biotite and ca bearing plagioclase immediately below the fe sulfide rich layer appears to be crucial key processes are extensive formation of acid rock drainage ard followed by enhanced ph driven weathering of aluminosilicates resulting in the accumulation of secondary phases directly below the fe sulfide rich layer in both scenarios a cemented layer is formed that effectively retards the further downward movement of the oxidation front the presented details on the role of stratification in the formation of cemented layers could be considered in the construction of mining heaps as a possible measure to stimulate natural attenuation

application of quantitative mineralogy on the neutralization acid potential calculations within µm scale stratified mine tailings estimation of the long term environmental impact of mining wastes is of socio economic interest bulk analytical approaches of the neutralizing np and acid potentials ap provide in general worst case scenarios for acid rock drainage ard predictions and long term evaluation taking individual graded layers with variable reactive mineral ratios into consideration alternating zones highly contrasting in their neutralizing and acid potential can be identified and quantified using scanning electron microscope mineral liberation analyzer sem mla and energy dispersive x ray fluorescence edxrf petrographic analyst pa for the first time the bulk np and ap values increased by a factor of 2 13 and 1 37 respectively compared to the average values of the mineralogical np ap at the contrasting boundaries precipitation of pore clogging secondary phases might be enhanced these new information can be used as input parameters to model the future behavior of a non homogeneous multiplied laminated body with reactive transport modeling

the palaeoproterozoic kangerluluk gold copper mineralization southeast greenland pb and nd isotopic constraints on its timing and genesis the ketilidian mobile belt kmb in south greenland is dominated by the palaeoproterozoic julianehab batholith 1 850 1 770 ma and is bordered by supracrustal sequences the batholith is composed of granite granodiorite tonalite diorite and subordinate gabbro and appinite and formed in a juvenile magmatic arc the supracrustal rocks consist of psammites pelites and subordinate interstratified mafic volcanic rocks a rapakivi granite suite 1 755 1 735 ma intrudes them gold is an important metal in the kmb and occurs in various settings and element associations here emphasis is placed on the kangerluluk cu au mineralization in the eastern part of the mobile belt it is hosted within a remnant supracrustal succession dominated by redundant pillowed basalts structurally a syn volcanic association with gold can be distinguished from one that is clearly related to crosscutting quartz veins in between calc silicate formation in the volcanic rocks occurred contemporaneously with a pervasive sericitization bleaching of leucocratic granitoids that are intrusive into the volcanic sequence bulk rock and mineral sulfide pb isotope data are comparable in the pb isotope space the data define two groups of mineralization where one group is characterized by relatively higher 207pb 204pb at given 206pb 204pb ratios than the other pb stepwise leaching pbsl dating experiments performed on garnet and epidote associated with both groups of mineralization confirm the offset in the pb isotope data with the geochronological information from pbsl the results indicate 1 the initial gold mineralization was genetically related to metalliferous fluids associated with the emplacement of late intrusive stages of the julianehab batholith 1 800 1 770 ma and 2 local remobilization of the initial mineralization 1 700 1 750 involved a pb isotopically evolved fluid which tentatively can be correlated with the intrusion of the rapakivi granites in the area radiogenic pb was not only found in altered host rocks but also in various sulfides suggesting that fluids may have percolated through and or originated from uranium bearing graphitic schists which are abundant in the sedimentary package of the kmb lead isotopic results suggest that the rapakivi granites may have supplied heat for renewed circulation of fluids within the supracrustal rock pile

global tectonic significance of the solomon islands and ontong java plateau convergent zone abstract oceanic plateaus areas of anomalously thick oceanic crust cover about 3 of the earth s seafloor and are thought to mark the surface location of mantle plume heads hotspot tracks represent continuing magmatism associated with the remaining plume conduit or tail it is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones and therefore 1 influences the eventual composition of continental crust and 2 is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs the ontong java plateau ojp of the southwestern pacific ocean is the largest and thickest oceanic plateau on earth and the largest plateau currently converging on an island arc solomon islands for this reason this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones this volume consists of a series of four papers that summarize the results of joint us japan marine geophysical studies in 1995 and 1998 of the solomon islands ontong java plateau convergent zone marine geophysical data include single and multi channel seismic reflection ocean bottom seismometer obs refraction gravity magnetic sidescan sonar and earthquake studies objectives of this introductory paper include 1 review of the significance of oceanic plateaus as potential contributors to continental crust 2 review of the current theories on the fate of oceanic plateaus at subduction zones 3 establish the present day and neogene tectonic setting of the solomon islands ontong java plateau convergent zone 4 discuss the controversial sequence and timing of tectonic events surrounding ontong java plateau solomon arc convergence 5 present a series of tectonic reconstructions for the period 20 ma early miocene to the present day in support of our proposed timing of major tectonic events affecting the ontong java plateau solomon islands convergent zone and 6 compare the structural and deformational pattern observed in the solomon islands to ancient oceanic plateaus preserved in precambrian and phanerozoic orogenic belts our main conclusion of this study is that 80 of the crustal thickness of the ontong java plateau is subducted beneath the solomon island arc only the uppermost basaltic and sedimentary part of the crust 7 km is preserved on the overriding plate by subduction accretion processes this observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both precambrian and phanerozoic orogenic belts

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

a 40ar 39ar and u pb isotopic study of the ilimaussaq complex south greenland implications for the 40k decay constant and for the duration of magmatic activity in a peralkaline complex abstract magmatism in the gardar province south greenland is related to two main rifting events at 1280 ma and 1180 to 1140 ma little is known about the duration of the magmatic activity in a specific complex the ilimaussaq intrusion belongs to the second period of rifting and comprises an extraordinary diversity of granitic and syenitic rock types which intruded and fractionated in three successive magmatic events as the intrusion contains some of the most evolved incompatible element rich rocks on earth it was chosen for a detailed geochronological study to quantify the duration of melt production intrusion fractionation and cooling amphiboles which are abundant in all rock types including pegmatites and late magmatic veins were dated using the 40 ar 39 ar technique since the solidus temperature of the most evolved melts is below the closure temperature of amphibole and no later heating event occurred the 40 ar 39 ar ages reflect the magmatic crystallisation and can be used to determine the duration of igneous differentiation the 40 ar 39 ar plateau ages range between 1142 6 2 2 ma and 1152 3 3 7 ma using the steiger and jager steiger r h and jager e 1977 subcommision on geochronology convention on the use of decay constants in geo and cosmochronology earth planet sci lett 36 359 362 40 k decay constant these ages are younger than an u pb age of 1160 5 ma on baddeleyite from the first magma batch our results indicate that the 40 k decay constant of steiger and jager may be too high a lambda similar to that proposed by kwon et al kwon j min k bickel p and renne p r 2002 statistical methods for jointly estimating decay constant of 40 k and age of a dating standard math geol 34 457 474 is required to make the 40 ar 39 ar match the u pb age

petrogenesis of a mid proterozoic anorthosite mangerite charnockite granite amcg complex isotopic and chemical evidence from the nain plutonic suite the nain plutonic suite nps about 1 30 ga underlies some 19 000 km intruding the boundary between middle to late archean rocks in the east nain province and late archean and early proterozoic rocks in the west churchill province nd isotopic compositions in most igneous units of the nps reflect their geographic position relative to the inferred boundary between the nain and churchill provinces rare exceptions are local intrusions of olivine bearing basic rocks with highest epsilon nd 1 3 ga near 3 0 present in both eastern and western sectors of the nps anorthositic rocks have variable and significant contributions of crustal nd but notably lower i sr than granitoid rocks ferrodiorites are similar isotopically to the anorthositic group and have chemical and mineral compositions more akin to the anorthositic group than the granitoids aluminous orthopyroxene megacrysts in nps anorthositic rocks indicate crystallization pressures of 6 2 to 11 0 kbar consistent with lower cru

metallogeny of south greenland a review of geological evolution mineral occurrences and geochemical exploration data abstract south greenland has been the site of historic mining of cryolite copper graphite and gold hosts mineral deposits with gold uranium zinc niobium tantalum zirconium hafnium ree iron titanium vanadium fluorite and graphite and has additional potential for lithium beryllium phosphorus gallium and thorium data from stream sediment geochemical surveys document that south greenland is enriched in a range of these elements relative to the rest of greenland and to estimates of the upper crust composition distribution patterns for individual elements within south greenland exhibit enriched regions that are spatially related to lithological units crustal structure and known mineralisation the northern domain of south greenland includes the southernmost part of the orthogneiss dominated north atlantic craton orogenic gold mineralisation is hosted by quartz veins and hydrothermally altered rocks associated with shear zones intersecting the mesoarchaean tartoq group of mafic metavolcanic rocks geochemical exploration indicates that additional potential for gold mineralisation exists within palaeoproterozoic supracrustal rocks overlying the archaean basement rocks formed during the palaeoproterozoic ketilidian orogeny occupy a major part of south greenland and has been divided into two domains the central domain is underlain by the julianehab igneous complex forming a 100 km wide ene wsw zone centrally across south greenland intrusive and extrusive mostly felsic magmatic rocks were emplaced in two main stages 1850 1830 and 1800 1780 ma in a continental arc setting positive anomalies in aeromagnetic data indicate that mafic plutons are common in the late igneous complex intra arc mafic metavolcanic rocks contain syngenetic stratabound copper sulphide and epigenetic shear zone hosted copper silver gold mineralisation at kobberminebugt and kangerluluk whereas metasedimentary and metapyroclastic rocks contain stratabound uraninite mineralisation orthomagmatic iron titanium vanadium mineralisation is hosted by a gabbro a potential for porphyry type mineralisation related to the late intrusive stages of the julianehab igneous complex is suggested by showings with copper molybdenum and gold together with stream sediment anomalies for these elements vein type uranium mineralisation occurs in fault zones in the julianehab igneous complex related to mesoproterozoic rifting the southern domain contains an assemblage of palaeoproterozoic metasedimentary and metavolcanic rocks that underwent moderate to strong deformation peak ht lp metamorphism and partial melting with subsequent retrograde exhumation at 1790 1765 ma the supracrustal rocks contain syngenetic au as sb u and zn mineralisation in volcanic or graphite and sulphide rich sedimentary environments graphite was mined historically at two sites many stream sediment gold anomalies are located in a ne trending belt along the boundary between the early julianehab complex and the supracrustal rocks to the south they reflect a number of auriferous quartz vein occurrences including the nalunaq gold deposit hosted in a system of shear zones and probably generated as orogenic gold during ketilidian accretion the 1755 1730 ma a type ilua plutonic suite is the latest magmatic event in the ketilidian orogen the 1300 1140 ma gardar period involved continental rifting sedimentation and alkaline magmatism numerous dykes and 10 ring shaped intrusion complexes were formed across south greenland an orthomagmatic iron titanium vanadium deposit is hosted by troctolitic gabbro residual magmas and fluids resulting from extreme magmatic differentiation possibly combined with assimilation of older crust created mineral deposits including cryolite that was mined at ivigtut large low grade deposits of uranium rare earth elements zinc at kvanefjeld and tantalum niobium rare earth element zirconium at kringlerne in the ilimaussaq complex as well as tantalum niobium rare earth elements at motzfeldt so in the igaliko complex the south greenland crustal evolution records effects of mantle processes such as lithospheric extension subduction and underplating which resulted in recurrent magma emplacement in tectonically active environments as such the geology of south greenland reflects events and circumstances that are favourable to the generation and preservation of hydrothermal ore forming fluid systems during the ketilidian orogeny as well as to the development of extreme rock compositions within the gardar alkaline igneous province

anatomy and orogenic history of a paleoproterozoic accretionary belt the makkovik province labrador canada the makkovik province is a segment of a paleoproterozoic accretionary belt the makkovik ketilidian orogen that developed on the southern margin of laurentia at 1 9 1 7 ga in contrast to coeval laurentian orogenic belts that mainly resulted from collision of archean plates makkovikian ketilidian orogenesis was dominated by active margin processes including continental margin arc plutonism and juvenile terrane accretion both of which were accompanied by regional transpression in the makkovik province earliest deformation and amphibolite facies metamorphism of paleoproterozoic rift drift assemblages post hill and moran lake groups and the archean foreland nain province occurred at 1 9 ga in response to accretion of a paleoproterozoic island arc following this collision cratonward dipping subduction was established resulting in the formation of the 1895 1870 ma island harbour bay plutonic suite a calc alkaline magmatic arc built on reworked archean crust crust formation continued between ca 1

crustal structure of the ne rockall trough from wide angle seismic data modeling two wide angle seismic lines located in the northern rockall trough were acquired in may 2000 one line line e crosses the trough from the continental shelf off lewis to normal oceanic crust west of lousy bank in nw se direction the other line line d intersects with line e crosses the wyville thomson ridge in a sw ne direction and ends in the faeroe shetland basin sonobuoy data and expanding spread profiles acquired in the same area have been remodeled analysis of the seismic data using travel times and amplitudes reveals an up to 5 km thick sedimentary basin including an up to 1 5 km thick basaltic layer which is present in most of the trough further conclusions of this study are that the rockall trough is underlain by highly stretched continental crust of 13 km thickness the crust thickens to 24 km beneath lousy bank which is interpreted to be of continental nature beneath the hebrides continental shelf a three layer continental crust of 26 km is modeled an up to 12 km thick high velocity layer is observed underneath the ocean continent boundary and is interpreted as magmatic underplating resulting from excess volcanism during rifting no evidence for an underplate layer could be distinguished beneath the trough area modeling of the structure of the wyville thomson ridge revealed no existing igneous core of the ridge confirming existing theories that it is a compressional structure

geodynamic models of terrane accretion testing the fate of island arcs oceanic plateaus and continental fragments in subduction zones 1 crustal growth at convergent margins can occur by the accretion of future allochthonous terranes fats such as island arcs oceanic plateaus submarine ridges and continental fragments using geodynamic numerical experiments we demonstrate how crustal properties of fats impact the amount of fat crust that is accreted or subducted the type of accretionary process and the style of deformation on the overriding plate our results show that 1 accretion of crustal units occurs when there is a weak detachment layer within the fat 2 the depth of detachment controls the amount of crust accreted onto the overriding plate and 3 lithospheric buoyancy does not prevent fat subduction during constant convergence island arcs oceanic plateaus and continental fragments will completely subduct despite having buoyant lithospheric densities if they have rheologically strong crusts weak basal layers representing pre existing weaknesses or detachment layers will either lead to underplating of faulted blocks of fat crust to the overriding plate or collision and suturing of an unbroken fat crust our experiments show that the weak ultramafic layer found at the base of island arcs and oceanic plateaus plays a significant role in terrane accretion the different types of accretionary processes also affect deformation and uplift patterns in the overriding plate trench migration and jumping and the dip of the plate interface the resulting accreted terranes produced from our numerical experiments resemble observed accreted terranes such as the wrangellia terrane and klamath mountain terranes in the north american cordilleran belt

local glaciers record delayed peak holocene warmth in south greenland abstract local glaciers and ice caps gics respond sensitively and quickly on the scale of decades to centuries to climate variations continuous records of past fluctuations in gic size provide information on the timing and magnitude of holocene climate shifts and a longer term perspective on 21st century glacier retreat although there is broad scale agreement on millennial scale trends in holocene climate variability and fluctuations in local gics in greenland regional variations are only loosely constrained here we present three holocene proglacial lake sediment records from south greenland an area with abundant local glaciers but few holocene length paleoclimate records in addition we use geospatial analysis to model past equilibrium line altitudes elas and thereby constrain the magnitude of ablation season temperature change during the warmest and coolest periods of the holocene physical and geochemical sedimentary characteristics show that two of the proglacial lakes continued to receive glacial meltwater input until 7 3 and 7 1 ka bp the survival of local glaciers implies that south greenland remained relatively cool and that summer temperatures gradually warmed but did not warm well beyond 1 2 c above present in the early holocene in the mid holocene from 7 1 to 5 5 ka bp organic sedimentation at these two sites indicates that local glaciers became very small or more likely melted away completely the glaciers within the third lake s catchment melted away prior to 5 2 ka bp as sediments deposited earlier in the holocene could not be dated at this site we estimate that summer temperatures increased by at least 1 2 1 8 c above present by 7 3 7 1 ka bp our results are consistent with other observations that suggest a north to south gradient in the timing of holocene thermal maximum conditions with southern greenland experiencing a delayed warming relative to other regions in greenland as summer temperatures cooled in the neoglacial our records show that sustained glacier regrowth began 3 1 ka bp with glaciers in the southernmost catchment which at present receive the most precipitation in the other two catchments which host smaller glaciers in a drier environment regrowth began at 1 3 and 1 2 ka bp the timing of which is in agreement with other glacial records from the arctic atlantic region local glaciers reached their maximum late holocene extents during a cooler second phase of the little ice age lia 0 2 0 1 ka bp that we estimate was at least 0 4 0 9 c cooler than present overall these findings improve understanding of the spatio temporal dynamics of holocene glacier and climate change in greenland potentially yielding valuable information about their future response

on a gold mineralization in the precambrian tartoq group sw greenland in the precambrian tartoq group sw greenland gold occurs in three parageneses associated with arsenopyrite pyrite tennantite and chalcopyrite the gold bearing sulphide mineralizations appear as conformable quartz layers lenses and veins in greenstones of basaltic composition gold particles in the arsenopyrite pyrite layers have a true fineness of c 990 in the tennantite rich veins a fineness of c 720 and a fineness of c 500 in the chalcopyrite quartz veins the arsenopyrite pyrite layers are of submarine exhalative origin whereas the tennantite and the chalcopyrite quartz veins and lenses are of diagenetic and or metamorphic origin

source of gold in neoarchean orogenic type deposits in the north atlantic craton greenland insights for a proto source of gold in sub seafloor hydrothermal arsenopyrite in the mesoarchean abstract given that gold au mostly remained in the incipient earth mantle until ca 3 9 3 8 ga a proto source of gold may have been present in the dominantly mafic crust precursor born through first stage melting of the early earth mantle in south westernmost greenland a fragment of the north atlantic craton is characterised by greenstone belts comprising mafic volcanic and magmatic rocks and harzburgite cumulates that were emplaced at ca 3 0 ga prograde amphibolite facies metamorphic assemblages and caused local dissolution of arsenopyrite during this retrograde tectono metamorphic stage in gold rich shear zones the re os geochronometer in arsenopyrite was reset to a neoarchean age while invisible gold was liberated and deposited as free gold with 2 66 ga pyrite re os isochron ages the initial os isotope ratios of neoarchean arsenopyrite 187os 188osi 0 13 0 02 and gold bearing pyrite 0 12 0 02 overlap with the estimated 187os 188os ratio of the mesoarchean mantle 0 11 0 01 and preclude contribution of radiogenic crustal os from evolved lithologies in the accretionary arc complex but instead favour a local contribution in os from basaltic rocks and serpentinised harzburgite protoliths by metamorphic fluids thus the ca 2 66 ga lode gold mineralisation identified in the north atlantic craton may illustrate a gold endowment in shear zones in earth s stabilizing continental crust at the time of the 2 75 2 55 ga global gold event through metamorphic upgrading of bulk gold which had originally been extracted from the mesoarchean mantle and concentrated in hydrothermal arsenopyrite deposits in oceanic crust beneath the overall reduced mesoarchean ocean

origin of a crustal splay fault and its relation to the seismogenic zone and underplating at the erosional north ecuador south colombia oceanic margin 1 splay faults within accretionary complexes are commonly associated with the updip limit of the seismogenic zone prestack depth migration of a multichannel seismic line across the north ecuador south colombia oceanic margin images a crustal splay fault that correlates with the seaward limit of the rupture zone of the 1958 mw 7 7 tsunamogenic subduction earthquake the splay fault separates 5 6 6 km s velocity inner wedge basement rocks which belong to the accreted gorgona oceanic terrane from 4 to 5 km s velocity outer wedge rocks the outer wedge is dominated by basal tectonic erosion despite a 3 km thick trench fill subduction of 2 km high seamount prevented tectonic accretion and promotes basal tectonic erosion the low velocity and poorly reflective subduction channel that underlies the outer wedge is associated with the aseismic decollement thrust subduction channel fluids are expected to migrate upward along splay faults and alter outer wedge rocks conversely duplexes are interpreted to form from and above subducting sediment at 14 to 15 km depths between the overlapping seismogenic part of the splay fault and the underlying aseismic decollement coeval basal erosion of the outer wedge and underplating beneath the apex of inner wedge control the margin mass budget which comes out negative intraoceanic basement fossil listric normal faults and a rift zone inverted in a flower structure reflect the evolution of the gorgona terrane from cretaceous extension to likely eocene oblique compression the splay faults could have resulted from tectonic inversion of listric normal faults thus showing how inherited structures may promote fluid flow across margin basement and control seismogenesis

preservation of pre vegetational mixed fluvio aeolian deposits in a humid climatic setting an example from the middle proterozoic eriksfjord formation southwest greenland abstract sedimentological studies of a 30 m thick coastal cliff section within the middle proterozoic eriksfjord formation in western south greenland reveals three distinct types of fluvial sand sheet deposits that reflect perennial streams type i semi perennial streams type ii and ephemeral flash floods type iii perennial river sand sheets are characterised by co sets of medium scale trough cross beds interbedded with isolated medium and large scale high angle tabular cross beds indications of desiccation or subaerial exposure are absent semi perennial fluvial sand sheets consist predominantly of low angle cross beds interbedded with isolated sets of high angle tabular cross beds with common reactivation surfaces horizontal lamination and climbing ripple lamination form subordinate structures associated with the sand sheets are adhesion structures and 0 05 0 4 m thick sets of wind ripple lamination indicating periods of subaerial exposure and aeolian reworking high energy ephemeral flash flood sand sheets consist almost exclusively of planar parallel lamination and climbing ripple lamination with some isolated sets of low angle cross bedding scouring and internal truncation surfaces are common the three types of sand sheets are considered to reflect deposition under changing climatic conditions varying from humid to arid or semi arid aeolian deposits are preserved within the sand sheets showing characteristics of dominantly perennial flow punctuated by shorter periods of desiccation type ii while sand sheets showing features typical of arid and or semi arid flow conditions type iii contain no preserved aeolian deposits this selective preservation is interpreted to be a result of the combined effect of groundwater table level and fluvial style which in turn are inferred to have been controlled by the climatic regime the deposits show that during pre vegetational times the preservation of aeolian deposits under certain conditions may be more optimal in fluvial systems formed in a humid climate than in fluvial systems formed under semi arid or arid circumstances the occurrence of aeolian deposits within a precambrian succession of fluvial deposits therefore need not be an indication of the most arid environmental conditions

sediment and shear hosted gold mineralization of the tartoq group supracrustals southwest greenland abstract gold mineralization occurs in the archaean tartoq group supracrustals which are dominated by greenschist amphibolite facies metamorphosed mafic rocks although deformed recognizable features of the stratigraphy pillowed lavas intercalated sills and interflow sediments suggest shallow to deep sea floor deposition large intensely carbonatized structural zones cut through the greenstones subparallel to regional layering and postdating major folding they are interpreted as major shear zones and are characterized by internal ductile deformation due to high fluid pressures with brittle deformation of peripheral zones erratic high grade gold mineralization is associated with quartz pyrite lenses and veins that occur subparallel to layering within the shear zones moderate but more consistent values are found in more extensive pyrite arsenopyrite quartz layers also subparallel to layering these lenses and layers occur adjacent to or within the major carbonatized shear zones microscopically gold is found as small 10 m grains in both pyrite and arsenopyrite low but consistently anomalous grades of gold are also found in strongly contorted banded muscovite rich quart zpyrite rocks these are thought tobe thin chemically deposited interflow sediments within the greenstone pile analogous to oxide facies banded iron formation which is found nearby metamorphism has obscured critical textures but the gold was either chemically deposited along with exhalative silica and sulphides or may have been introduced later during deformation and alteration of primary bif the tartoq group rocks are similar in most respects to those of other archaean greenstone belts being distinguished only by their excellent outcrop exposure current models for the origin of archaean gold deposits are adequate to explain the tartoq gold mineralization a multistage model of gold concentration is proposed gold is initially leached from mafic volcanics and possibly underlying gneisses and redeposited at anomalous levels in chemical sediments these may be further enriched by sulphidation reactions high grade gold mineralization is typically epigenetic in style where very large volumes of fluid are channeled through shear zones undergoing brittleductile deformation

evolution of the tugtutoq central complex south greenland a high level rift axial late gardar centre abstract the tugtutoq central complex tcc comprises at least six intrusions emplaced by ring faulting and or stoping these intrusions are of syenites quartz syenites and granites with peralkaline affinities crystallized at relatively shallow depths probably as a subvolcanic complex the intrusions transect a major dyke swarm marking the axial zone of a narrow mid proterozoic continental rift the tcc appears to have formed from a succession of increasingly fractionated magma batches which are inferred to have arisen from a differentiating parent magma body near the base of the crust a culminating perthosite intrusion bla mane so syenite joined the two separate early intrusive centres to form a composite body elongate along the rift axis a rb sr whole rock isochron indicates an age of 1124 20 ma with a 87sr 86sr i of 0 70345 the magmas were relatively anhydrous although probably halogen rich producing hypersolvus rocks olivines range from fo9fa88tp3 to foofa95tp5 and pyroxenes from di35hd62ac3 to virtually pure aegirine relative to members of the preceding dyke swarm regarded as fast quenched magmas derived from the same deep crustal site of differentiation the tcc units are enriched in alkali feldspar the tcc rocks are interpreted in part as alkali feldspar rich roof facies cumulates and in part as products of salic magmas selectively depleted in fe rich ferromagnesian minerals by gravitative separation as with the salic members of the dyke swarm the tcc rocks exhibit marked negative eu anomalies the tcc is inferred to be underlain by layered meso to melanocratic syenite cumulates comparable to those seen in some of the other south greenland alkaline intrusions

from mantle to motzfeldt a genetic model for syenite hosted ta nb mineralisation abstract a genetic model for the motzfeldt tantalum niobium rich syenite in south west greenland considered to be one of the world s largest ta prospects is presented the motzfeldt primary magma formed early in regional gardar 1273 6 ma rifting isotope signatures indicate that the hf had multiple sources involving juvenile gardar hf mixed with older palaeoproterozoic or archaean hf we infer that other high field strength elements hfse similarly had multiple sources the magma differentiated in the crust and ascended before emplacement at the regional unconformity between ketilidian basement and eriksfjord supracrustals the hfse rich magmas crystallised ta rich pyrochlore which formed pyrochlore rich crystal mushes and it is these pyrochlore rich horizons rich in ta and nb that are the focus of exploration the roof zone chilled and repeated sheeting at the roof provided a complex suite of cross cutting syenite variants including pyrochlore microsyenite in a hot sheeted roof model the area was subject to hydrothermal alteration which recrystallized alkali feldspar to coarse perthite and modified the mafic minerals to hematite creating the friable and striking pink nature of the motzfeldt so centre carbon and oxygen isotope investigation of carbonate constrains fluid evolution and shows that carbonate is primarily mantle derived but late stage hydrothermal alteration moved the oxygen isotopes towards more positive values up to 21 the hydrothermal fluid was exceptionally fluorine rich and mobilised many elements including u and pb but did not transport hfse such as ta hf and nb although the u and pb content of the pyrochlore was enhanced by the fluid the hfse contents remained unchanged and therefore hf isotopes were unaffected by fluid interaction while the effect on hydrothermal alteration on the visual appearance of the rock is striking magmatic processes concentrated hfse including ta and the hydrothermal phase has not altered the grade exploration for hfse mineralisation commonly relies on airborne radiometric surveying which is particularly sensitive to the presence of u th a crucial lesson from motzfeldt is that the best target is unaltered pyrochlore which was identified less easily by radiometric survey careful petrological mineral studies are necessary before airborne survey data can be fully interpreted

geology of an archaean accretionary complex the structural record of burial and return flow in the tartoq group of south west greenland abstract this study presents structural and petrographic data from supracrustal slivers and associated ttg rocks of the mesoarchaen tartoq group in southwestern greenland that point to the burial and subsequent return flow of the mainly metamafic supracrustal and ttg rocks during subduction accretion along an obliquely convergent margin locally preserved high grade metamorphic up to lower granulite facies fabrics record the burial of the supracrustal rocks to depths 25 km during southeast directed subduction retrograde amphibolite and regionally widespread greenschist facies fabrics and structures indicate the subsequent exhumation of the rocks pro and retrograde fabrics are largely coaxial but retrograde fabrics record a reversal of shear and uniformly developed top to the east and southeast kinematics along extensional shear zones and overprinting brittle ductile fault zones extensional shearing and the concomitant retrogression are interpreted to be related to the northwest directed expulsion of the rocks in the subduction channel intrusive relationships between supracrustal slivers and ttg s leucogranites and pegmatites document the synkinematic emplacement of voluminous felsic magmas throughout this structural evolution while the structural evolution of rocks of the tartoq group shows the salient attributes of a modern convergent margin high t metamorphism pervasive ductile strains partial melting of the supracrustal rocks and syntectonic intrusion of voluminous granitoids are interpreted to reflect processes operating in a hotter archaean subduction zone slab melting and the formation of granitoids at relatively shallow depths in the subduction zone channel move the magmatic front to the fore arc region of the convergent margin the emplacement of felsic melts low strength and progressive disintegration of the downgoing slab choke subduction this has most likely resulted in only irregular and shortlived subduction disrupted by episodes of slab break off and subduction step back promoting tectonomagmatic accretion of ttg greenstones at relatively shallow levels but preventing subduction of rocks to larger depths this mode of shallow structural and magmatic accretion leads to lateral crustal growth documented by the at least 80 km wide accretionary complex of the tartoq group

geochemistry and ree minerals of nepheline syenites from the motzfeldt centre south greenland whole rock analyses including the ree are given for nineteen samples from four of the six overlapping nepheline syenite intrusions which constitute the proterozoic 1310 ma motzfeldt centre larvikite margins of one unit sm5 are apparently related to the nepheline syenites by early fractionation of ol pl followed by fractionation of ol pl cpx t amph at current levels of exposure positive eu anomalies in the larvikites might be explained by large scale incorporation of earlier pl rich cumulates low initial 875r 865r ratios imply a

structural variations of arc crusts and rifted margins in the southern izu ogasawara arc back arc system 1 we carried out a reflection refraction seismic survey across the southern izu ogasawara arc back arc system covering three arcs with different crustal ages the oldest eocene arc has middle and lower crust with high velocities of 6 4 6 6 and 6 8 7 4 km s respectively suggesting denser crustal materials the current volcanic arc has middle and lower crust with lower velocities of 5 7 6 5 and 6 7 7 1 km s suggesting advanced crustal differentiation the crust mantle transition layer with a velocity of 7 5 8 0 km s is distributed beneath the current volcanic arc and the rear arc suggesting a pool of dense materials emanating from the crust through the crustal growth these structural differences between the eocene arc and current arc indicate a difference of crustal growth based on basaltic and andesitic magmas according to known petrologic studies commonly rifted crusts have lower crusts with high velocities of over 7 0 km s and the arc back arc transition zone also has a thinner more reflective crust that may have been affected by postrifting magmatism

mid crustal partitioning and attachment during oblique convergence in an arc system palaeoproterozoic ketilidian orogen southern greenland abstract the ketilidian orogen in se greenland contains a continental magmatic arc julianehab batholith and forearc which formed in the overriding plate of a palaeoproterozoic oblique subduction boundary between c 1854 and 1795 ma during uplift and deformation of the forearc between c 1795 and 1780 ma its sedimentary sequences were invaded by arc type magmas and metamorphosed at ht lp upper amphibolite facies conditions which led to widespread anatexis and generation of abundant s type granites transpression during early deformation d 1 was partitioned into arc parallel sinistral strike slip in the magmatic arc rocks of the batholith and arc normal contraction in the sedimentary and volcaniclastic sequences of the proximal forearc psammite zone subsequently d 1 fabrics in the psammite zone were overprinted and transposed in an intense flat lying d 2 ductile shear zone with arc parallel stretching lineations and top ne kinematic indicators the kinematic significance of the d 2 shear zone is that it accommodated vertical differences in strain and displacement and permitted deformation to be partitioned differently at different levels in the crust as such it is an attachment or coupling zone that provided kinematic linkage between deformation at different crustal levels during ne transport of a forearc sliver overprinting of the upper crustal domain of d 1 strike slip partitioned transpression in the forearc during d 2 is attributed to migration of the attachment zone to a higher crustal level this migration was a consequence of an enhanced geothermal gradient in the proximal part of the forearc due to the emplacement of batholith related plutonic suites during d 2

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

crustal structure at the se greenland margin from wide angle and normal incidence seismic data abstract results from a seismic refraction and reflection line along the southeast coast of greenland give information on both the precambrian structures on the greenland continent and on the effects of the tertiary breakup of the north atlantic three seismic stations on the greenland coast recorded the airgun shots from a 279 km reflection seismic line approximately 20 km offshore the maximum offset recorded was 313 km the wide angle data show crustal thickness variation from 39 km in the south to 49 km in the north with an 8 to 17 km thick high velocity 7 5 km s layer at the base of the crust interpreted as underplating related to the opening of the north atlantic in the tertiary the boundary between the early proterozoic ketilidian orogen in the south and the archaean block to the north show little variation in seismic velocities and the reflection pattern suggests that the archaean underlies the ketilidian at depth we see no evidence that the julianehab batholith at the boundary between rocks of the ketilidian orogen and the north atlantic block is a deep structure

age and origin of early proterozoic dolerite dykes in south west greenland samples of three dolerite dykes from south west greenland give a pooled weighted mean rb sr whole rock isochron age of 2 130 65 ma initial 87sr 86sr ratios are 0 70155 0 00018 for one of the dykes naujat and 0 70277 0 00012 for the other two torssut and akinaq a fourth dyke qaqarssuaq did not yield an isochron torssut samples have significantly lower initial 207pb 204pb ratios than samples from the naujat dyke indicating that the torssut dyke has been affected by crustal contamination

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

independent ages of magmatic and hydrothermal activity in alkaline igneous rocks the motzfeldt centre gardar province south greenland the motzfeldt centre is one of four major alkaline centres belonging to the igaliko complex part of the mid proterozoic gardar province of south greenland motzfeldt comprises syenites and nepheline syenites displaying complex magmatic and subsolidus histories ta nb and ree rich zones are associated with a pervasive hydrothermal alteration process in the north and east of the complex part of a subdivision of the centre called the motzfeldt so formation msf the zircons from the msf show textures that are both magmatic and altered whereas the textures of pyrochlore have a predominantly subsolidus chemistry we compare and contrast the radiogenic isotope geochemistry of the zircon and pyrochlore to obtain independent age estimates of the magmatic and hydrothermal episodes selected analyses of magmatic zircons are concordant with an age of 1 273 6 ma altered zircons are normally discordant with one intercept at 1 261 28 ma and the other at 349 49 ma this younger age would appear on face value to date the hydrothermal event at motzfeldt to 900 ma after the magmatism however the pb pb isotope systematics from altered pyrochlore define an isochron age of 1 267 6 ma for isotopic closure of the pyrochlore following alteration during the subsolidus there is no known magmatic event in southern greenland at 350 ma and discordant points have high common lead we therefore infer that the younger intercept age is spurious the overlap between the magmatic u pb zircon and the pb pb pyrochlore age suggests that alteration of the motzfeldt centre occurred shortly after magmatism

timing of proterozoic magmatism in the gardar province southern greenland the gardar province in southern greenland is believed to have experienced three episodes of rifting all associated with the extrusion of large volumes of alkalic magma a nepheline syenite of the motzfeldt center yields a u pb apatite age of 1 35 0 01 ga an augite syenite from the ilimaussaq complex yields a sm nd pyroxene alkali feldspar magnetite age of 1 13 0 05 ga on the basis of field relationships the eriksfjord formation lavas have been interpreted as the oldest manifestation of gardar magmatism sm nd plagioclase pyroxene wholerock ages of 1 17 0 03 ga and 1 20 0 03 ga for the middle member of the eriksfjord formation however indicate that these lavas may be part of the youngest magmatic activity the available chronological data indicate that the gardar province was active for at least 200 m y from 1 35 to 1 12 ga it is difficult to reconcile this protracted history with plume models for the genesis of such alkaline flood basalt provinces

attachment formation during partitioning of oblique convergence in the ketilidian orogen south greenland subhorizontal attachment zones provide coupling between lithospheric layers in orogenic belts a mid crustal attachment zone is exposed in the palaeoproterozoic ketilidian orogen south greenland which formed as a result of north directed oblique convergence at a cordilleran type margin rifting c 2 1 ga and compressional deformation and magmatism 1850 ma on the continental margin was followed by an extended sinistral transpression from 1850 to 1730 ma now separated into three episodes or peaks of activity the first episode was focused on the back arc region and was followed by the main arc construction phase during which transpression was partitioned into strike slip and contraction components despite the longevity of this active margin system individual tectonic events took place rapidly e g development of fore arc d1 d3 and accompanying high temperature low pressure metamorphism took place over c 12 ma we explain the fore arc and batholith evolution by the upward migration of an underlying attachment structure through the upper crustal partitioned blocks this migration may be attributed to an increase in the geothermal gradient accompanied by or followed by exhumation of the mid crust the partially molten hence weak attachment zone solidified and strengthened during cooling before emplacement of the post orogenic rapakivi suite during the third distinct phase of mild sinistral transpression

agpaitic nepheline syenites a potential source of rare elements abstract the agpaitic nepheline syenites may with regard to their elevated contents of a number of rare elements be compared with the complex granite pegmatites both rock types form highly evolved magmas under closed system conditions which prevent volatiles from escaping agpaitic rocks are peralkaline nepheline syenites characterized by the presence of complex rare element minerals such as eudialyte and mosandrite instead of the simple minerals such as zircon and ilmenite uranium deposits have been found in agpaitic rocks in brazil pocos de caldas china saima and greenland ilimaussaq nb and apetite deposits in russia lovozero and khibina the type locality of agpaitic rocks the ilimaussaq complex in south greenland is characterized by elevated contents of elements such as zr ree y be li nb u th and f not only in pegmatites late veins and contact fenites but also in some of the rocks especially lujavrite and kakortokite the most important rare element containing minerals are eudialyte zr ree y nb steenstrupine u th ree rinkite nb ree y u pyrochlore and perovskite nb ta li mica li chkalovite be and villiaumite f occurrences of eudialyte rich kakortokite and lujavrite are presently being explored in the ilimaussaq complex as sources of zr ree y and nb

tectonic magmatic and metallogenic evolution of the tethyan orogen from subduction to collision abstract this paper reviews the tectonic magmatic and metallogenic history of the tethyan orogen from the carpathians to indochina focus is placed on the formation of porphyry cu mo au deposits as being the most characteristic mineral deposit type formed during both subduction and collisional processes in this region relatively little is known about the history of the paleotethys ocean which opened and closed between gondwana and eurasia in the paleozoic and few ore deposits are preserved from this period the neotethyan ocean opened in the permian early triassic as the cimmerian continental fragments the cores of turkey iran tibet and indochina rifted from the northern gondwana margin and drifted northwards these microcontinents docked with the eurasian margin at various points in the mesozoic and cenozoic and formed a complex archipelago involving several small back arc basins and remnants of the paleotethyan ocean the main neotethyan ocean and these smaller basins were largely eliminated by collision with india and africa arabia in the early eocene and early mid miocene respectively although neotethyan subduction continues beneath the hellenic arc and the makran the majority of porphyry type deposits are found in association with neotethyan subduction mainly in the mesozoic and paleogene and syn to post collisional events in the mid paleogene to neogene they are found throughout the orogen but some sections are particularly well endowed including the carpathians balkans rhodopes eastern turkey lesser caucasus nw iran se iran sw pakistan southern tibet and se tibet indochina other sections that appear barren may reflect deeper levels of erosion young sedimentary cover or lack of exploration although there may also be real reasons for low prospectivity in some areas such as minimal subduction e g the western mediterranean region or lithospheric underthrusting as proposed in western tibet over the last decade improved geochronological constraints on the timing of ore formation and key tectonic events have revealed that many porphyry deposits that were previously assumed to be subduction related are in fact broadly collision related some forming in back arc settings in advance of collision some during collision and others during post collisional processes such as orogenic collapse and or delamination of subcontinental mantle lithosphere while the formation of subduction related porphyries is quite well understood collisional metallogeny is more complex and may involve a number of different processes or sources these include melting of orogenically thickened crust previously subduction modified lithosphere including metasomatized mantle underplated mafic rocks or lower crustal arc plutons and cumulates or upwelling asthenosphere e g in response to delamination slab breakoff back arc extension or orogenic collapse the most fertile sources for syn and post collisional porphyry deposits appear to be subduction modified lithosphere because these hydrated lithologies melt at relatively low temperatures during later tectonomagmatic events and retain the oxidized and relatively metalliferous character of the original arc magmatism unusually metallically enriched lithospheric sources do not seem to be required but the amount of residual sulfide phases in these rocks may control metal ratios e g cu au in subsequent magmatic hydrothermal ore deposits relatively au rich deposits potentially form in these settings as observed in the carpathians e g rosia montan turkey kisladag copler and iran sari gunay dalli although the majority of syn and post collisional porphyries are cu mo rich and resemble normal subduction related deposits e g in the gangdese belt of southern tibet this similarity extends to the associated igneous rocks which being derived from subduction modified sources largely retain the geochemical and isotopic character of those original arc magmas while still retaining a broadly calc alkaline character these rocks may extend to mildly alkaline shoshonitic compositions and may display adakite like trace element signatures high sr y and la yb ratios reflecting melting of deep crustal garnet amphibolitic sources but they are otherwise hard to distinguish from normal subduction related magmas small post collisional mafic alkaline volcanic centers are common throughout the orogen but for the most part appear to be barren however similar rocks in other post subduction settings around the world are associated with important alkalic type porphyry and epithermal au cu deposits and the potential for discovery of such deposits in the tethyan orogen should not be overlooked

metallogenic provinces geochemical provinces and regional geology what causes large scale patterns in low density geochemical maps of the c horizon of podzols in arctic europe the 2 mm fraction of 605 samples of the c horizon of podzols collected from an area of 188 000 km2 in the european arctic was analysed for more than 40 chemical elements the results were used to construct geochemical maps which showed clear regional distribution patterns notwithstanding the very low sample density of 1 site per 300 km2 some of these patterns fit established lithological boundaries others fit lineament structures in the area and underline the relative importance of certain tectonic directions some of which have not yet been delineated on existing geological maps some mark large scale hydrothermal events and related alteration finally some are connected with prominent known ore deposits occurring in the area however several large deposits and even famous metallogenic provinces fe ni cu are hardly or not at all reflected in the regional geochemical maps in their present definitions geochemical provinces and metallogenic provinces are thus not necessarily related special geochemical features can occur at very different scales the term geochemical province is so imprecise in terms of processes leading to regional scale geochemical features that it should be avoided low density geochemistry can be used to aid the interpretation of the geological evolution of large regions it can also be used to find prospective areas within such regions

reactive transport modeling an essential tool and a new research approach for the earth sciences abstract reactive transport modeling is an essential tool for the analysis of coupled physical chemical and biological processes in earth systems and has additional potential to better integrate the results from focused fundamental research on earth materials appropriately designed models can describe the interactions of competing processes at a range of spatial and time scales and hence are critical for connecting the advancing capabilities for materials characterization at the atomic scale with the macroscopic behavior of complex earth systems reactive transport modeling has had a significant impact on the treatment of contaminant retardation in the subsurface the description of elemental and nutrient fluxes between major earth reservoirs and in the treatment of deep earth processes such as metamorphism and magma transport active topics of research include the development of pore scale and hybrid or multiple continua models to capture the scale dependence of coupled reactive transport processes frontier research questions that are only now being addressed include the effects of chemical microenvironments coupled thermal mechanical chemical processes controls on mineral fluid reaction rates in natural media and scaling of reactive transport processes from the microscopic to pore to field scale

the rapakivi granites of south greenland stable isotope characteristics of their black and white facies and the nature of their protolith abstract the rapakivi granites of south greenland were emplaced into the ketilidian orogenic belt at about 1 74 ga during an episode of extensional tectonics petrographically they can be subdivided into a roughly subequal black facies and white facies distinguished by the colour turbidity of the alkali feldspars and by the associated mineralogy which is more hydrous i e amphibole and biotite compared to olivine and pyroxene in the white facies the oxygen isotope composition of these rocks is notably homogeneous from thirteen outcrops sampled over an area extending some 200 km 100 km all alkali feldspars have an average 18 o of 10 2 0 4 this lack of variation is consistent with the consensus petrogenetic model which invokes a very well mixed two component protolith the high abundance of 18 o mean whole rock 18 o 9 4 argues that the major contribution perhaps 90 although originally mantle derived had undergone 18 o enrichment in a low temperature sedimentary cycle there is no oxygen isotopic distinction between alkali feldspars in the black and white facies which is argued to be a consequence of the relatively anhydrous nature of the melt hydrogen isotope compositions vary widely by 30 but are coherent on a hand specimen scale as demonstrated by approximately equal d of biotite and alkali feldspar it is not clear at which stage the d h heterogeneity was introduced

genesis of the tianbaoshan pb zn cu mo polymetallic deposit in eastern jilin ne china constraints from fluid inclusions and c h o s pb isotope systematics abstract the tianbaoshan pb zn cu mo deposit is located in the eastern part of jilin heilongjiang region ne china which is considered to be the eastern segment of the central asian orogenic belt field and geochronological evidences indicate that this deposit experienced three types of mineralization including hercynian skarn cryptoexplosion breccia pipe and yanshanian quartz vein the early stages of the pb zn cu mineral systems in skarn and cryptoexplosion breccia pipe are characterized by a high temperature high salinity h 2 o co 2 nacl system of hydrothermal fluids that were possibly exsolved from the hercynian wall rock granodiorite these fluids show h o isotopic compositions similar to those of typical magmatic fluids by contrast the low temperature hydrothermal fluids of the later stages are represented by low salinity nacl h 2 o systems with low h o isotopic values the skarn and cryptoexplosive breccia pipe types of pb zn cu mineralization tend to have weakly negative 34 s values of 4 0 to 0 8 mean values of 2 31 and 2 16 respectively indicating that the sulfur was sourced from the hercynian magma therefore the early stage ore forming fluids of the skarn and cryptoexplosive breccia pipe were most likely sourced from high temperature and high salinity fluids closely related to the cooling and fractional crystallization of the hercynian granodiorite while the later stages changed to nacl h 2 o meteoric water influx whereas the ore forming fluids of the quartz vein type of mo mineralization were high temperature high salinity nacl h 2 o systems that differed from those of the skarn and cryptoexplosion breccia pipe but their h o isotopic compositions also indicate a magmatic fluid the weakly enriched 34 s values of molybdenite from the quartz vein type mo mineralization 34 s 0 2 2 8 average of 1 65 are comparable with those of other mesozoic yanshanian mo deposits 34 s 0 4 4 1 with an average of 1 39 3 15 but differ significantly from those of the hercynian pb zn cu skarn and cryptoexplosion breccia pipe this indicates that the sulfur of quartz vein type of mo originated from mesozoic yanshanian magmatism and that the ore forming fluids were derived from yanshanian magmatic rocks rather than being a product of the hercynian activity the 13 c values of the fluid inclusions in quartz from the skarn cryptoexplosion breccia pipe and quartz vein types are in a narrow range of 19 5 to 9 3 similar to those of the shanxiuling group which indicates that the carbon of the three types of mineralization had the same primary origin in the shanxiuling group the lead isotope compositions of ores from the skarn and cryptoexplosion breccia pipe types of mineralization 206 pb 204 pb 18 0725 18 3627 207 pb 204 pb 15 3721 15 5694 and 208 pb 204 pb 37 5542 38 8208 overlap with those of the hercynian granodiorite and shanxiuling group marble suggesting that the lead was probably derived from a mix of two different sources the shanxiuling group and the hercynian granodiorite whereas the lead isotope compositions of ores from the quartz vein type of mo mineralization 206 pb 204 pb 18 3837 18 6949 207 pb 204 pb 15 6824 15 7293 and 208 pb 204 pb 39 1009 39 1889 are significantly higher than those of the shanxiuling group marble and the hercynian granodiorite this indicates that the lead may be a product of yanshanian magmatic activity instead of the nearby hercynian granodiorite

textural evolution of the rapakivi granites south greenland sr o and h isotopic investigations the development of rapakivi texture in feldspars from the ketilidian granitoids of south greenland has been investigated using sr o and h isotopes a low temperature signature is found in the sr and o data which seemingly contradicts some textural features that point to a magmatic origin of the plagioclase mantles around the k feldspar ovoids an origin for these mantles involving exsolution from an original alkali feldspar solid solution is proposed which involves growth of mantles over a range of conditions determined by the mobility of the exsolving sodic feldspar this mobility may be enhanced at high temperatures in the presence of melts or increased fluid pressures and at lower temperatures by the processes responsible for the transformation of k feldspar to microcline rapakivi granites with both white and dark green feldspar occur in south greenland but show no major isotopic differences although the dark alkali feldspars contain significantly more fluid equivalent fluids in the white alkali feldspars may have escaped during plagioclase exsolution

maximum entropy modeling for orogenic gold prospectivity mapping in the tangbale hatu belt western junggar china abstract the tangbale hatu belt western junggar region located in the central asian orogenic belt caob has undergone complicated accretion and collision processes during the evolution of the paleo asian ocean the geological events contributed to orogenic gold mineral systems in the region in the present study mineral systems approach was employed to evaluate critical ore forming processes such as fluid migration pathways the formation of trap zones and deposition of metals by means of translating these critical processes into mappable ore controlling variables we attempt to establish a process based quantitative evaluation model a maximum entropy maxent model was proposed to predict the potential distribution of orogenic gold deposits based on known gold deposits occurrences and ore controlling variables nine ore controlling variables including fault intersection density fault linear density the first factor score map au singularity indices au regional anomaly au local anomaly proximity to ne trending faults proximity to intrusion contacts and proximity to stratigraphic contacts were selected to identify the most potential zones endowed with gold deposits the spatial association of individual ore controlling variable with the incidence of gold deposit occurrences was investigated by response curves and the relative importance of each ore controlling variable was determined by jackknife analysis in the maxent model the results indicate that au regional anomaly derived from spectrum area s a fractal model is the most important variable followed by the first factor score map and proximity to stratigraphic contacts the model accuracy was evaluated by roc curve giving a high predictive ability auc 0 915 favorable zones for gold mineralization illustrated by the prospectivity map are spatially coincident with known gold deposits occurrences and the main mineralized trends showing that the maxent model can be efficiently employed for spatial fusion analysis of multisource geospatial data to support conceptual mineral systems modeling of geologic controls on mineral deposit occurrences

the magmatic and fluid evolution of the motzfeldt intrusion in south greenland insights into the formation of agpaitic and miaskitic rocks the 1 275 ga motzfeldt intrusive complex in the gardar failed rift province in south greenland formed from six successively intruding melt batches sm1 sm6 interpreted to be derived from a common magma source at depth five units sm1 sm5 crystallized an alkaline to peralkaline miaskitic mineral assemblage of amphibole clinopyroxene feldspar nepheline fe ti oxides zircon apatite fluorite and rarely olivine the last magmatic batch sm6 is characterized by an agpaitic mineral assemblage of aegirine nepheline alkali feldspar eudialyte and rare fluorite or sodalite coexisting mafic minerals constrain the crystallization conditions of the miaskitic rocks to about 850 6008c whereas solidus temperatures below 5008c are indicated by coexisting alkali feldspars in the agpaitic rocks oxygen fugacities during the orthomagmatic stage are below the fmq fayalite magnetite quartz buffer fmq 0 5t o 2 0 whereas late hematite provides evidence of a higher relative oxygen fugacity during late stage alteration the nd and oxygen isotope compositions of amphiboles and pyroxenes are homogeneous throughout the complex and suggest a common mantle derived magma source for all six units which is comparable with other gardar intrusions the hydrogen isotopic composition of amphiboles dd 99 to 132 indicates lowtemperature fluid rock interaction with low fluid rock ratios fluid inclusion studies indicate that h2o nacl fluids present during the magmatic stages in the miaskitic units had salinities of510 wt nacl eq calcite crystals in fluid inclusions within these rocks suggest that co2 or hco3 was an important component of the original fluid phase in contrast the agpaitic unit is characterized by a ch4 h2o naclfluid the c o h isotope compositionsofthefluid inclusions in all units are consistent with mixing between a small volume of magmatic fluid and a large volume of meteoric water the chemical evolution of the motzfeldt complex is a type example of the connection of the transition from miaskitic to agpaitic mineral assemblages with redox dependent fluid solid equilibria thetransitionfromarelatively oxidized to a relatively reduced fluid is correlated with a change from a more reduced fe 2 bearing miaskitic taramite arfvedsonite zircon sm1 sm5 to a more oxidized fe 3 bearing agpaitic assemblage aegirine eudialyte sm6 we suggest that coupled fluid solid redox equilibria involving fe 2 f e 3 c o2 and ch4 in this case were simply driven by temperature decrease and an overall increase in na k in the melt this observation sheds light on the heavily debated miaskite agpaite transition the combined temperature and compositional effect stabilized fe 3 c h 4 and enhanced the solubility of zr possibly as na zr si o complexes in the latest stage melt sm6 resulting in the crystallization ofan agpaitic mineral assemblage in terms of redox conditions during crystallization the motzfeldt rocks represent an intermediate case in the gardar province between more oxidized co2dominated intrusions such as the syenite carbonatite complex of

a new mechanism of granite emplacement intrusion in active extensional shear zones one of the principal ways in which continental crust grows is by the incorporation from deeper within the earth of large volumes of granitic magma a space problem exists1 as to how these magmas are accommodated in the crust traditionally two main emplacement mechanisms have been emphasized forceful intrusion whereby buoyancy driven magmas physically push the crust aside creating granitic diapirs and balloons and passive emplacement characterized by replacive mechanisms such as cauldron subsidence and stoping although more recent work2 3 has demonstrated that space for granites may be created within bends and offsets of large transcurrent faults the simple view of either forceful or passive still fails to account for the intrusive mechanisms of many granites here we report the discovery of a new igneous intrusion mechanism in the superbly exposed proterozoic continental crust of south greenland we have observed that rapakivi granite was intruded as large scale sheets along ductile extensional shear zones that were active during emplacement in such a process the space problem seems to be simply resolved

crust mantle interaction in the evolution of the ilimaussaq complex south greenland nd isotopic studies sm nd isotopic compositions were determined for the peralkaline ilimaussaq complex of the gardar province of southern greenland the majority of the samples in the agpaitic and augite syenitic units have near chondritic initial nd 0 whereas a few samples trend towards nd values as low as 6 at the time of intrusion 1143 ma this latter value from a sample taken from the margin of the complex lying on the evolutionary trend for ketilidian country rock granitoids suggests that large scale contamination took place only at the margins of the complex the similarity of the nd isotopic compositions of the augite syenite and agpaitic units suggests that their parental magmas were derived from the same reservoir a comparison of the nd with existing sr and hf isotopic data for the complex suggests an origin by combined assimilation fractionation processes assimilation fractional crystallization modeling of the isotopic compositions indicates that the ilimaussaq magmas could have formed through fractional crystallization of a basaltic melt while assimilating granitic crust the model requires initially higher assimilation rates from basalt to augite syenite composition with subsequent decreasing assimilation rates from augite syenite to agpaitic compositions alkali granites which formed after the intrusion of the augite syenites have isotopic compositions intermediate between those of the augite syenites and the surrounding ketilidian basement this implies even greater amounts of assimilation and is interpreted as evidence for an origin through fractionation of a basaltic or augite syenite magma with concurrent assimilation of ketilidian crust

emplacement of rapakivi granite and syenite by floor depression and roof uplift in the palaeoproterozoic ketilidian orogen south greenland in the palaeoproterozoic ketilidian orogen of south greenland tabular intrusions of the rapakivi granite suite exposed at graah fjelde lindenow fjord and qernertoq and a mesoproterozoic syenite of the gardar province exposed at paatusoq were emplaced by a combination of roof uplift and floor depression the strain associated with the emplacement of the intrusions mainly involved redistribution of mass vertically within the lithosphere space for the emplacement of the rapakivi plutons was not created during regional extension on low angle ductile shear zones as claimed by some previous workers and there is no evidence that emplacement of the intrusions coincided with extensional collapse of the orogen following crustal thickening the rapakivi granites post date peak metamorphism in the orogen by 35 46 ma and their contacts cross cut and clearly post date intense flat lying d1 d2 fabrics that formed by partitioning of deformation into arc normal and arc parallel components during oblique convergence