contrasting arc and morb like assemblages in the paleoproterozoic flin flon belt manitoba and the role of intra arc extension in localizing volcanic hosted massive sulphide deposits the flin flon belt trans hudson orogen manitoba and saskatchewan is the largest paleoproterozoic volcanic hosted massive sulphide vms district in the world with 118 7 million tonnes mt of z

volcanism associated with extension in an oligocene miocene arc southwestern viti levu fiji the wainimala group rocks of southwestern viti levu fiji represent part of an oligocene miocene island arc some of the early volcanism in this arc took place on a substrate of eocene oligocene frontal arc crust mainly andesitic transitional calcalkaline lavas were erupted from major subaerial volcanic edifices while elsewhere tholeiitic magmas formed low lying basaltic lava fields and small felsic volcanic centres on the seafloor the intrusion of a dense bimodal basalt dacite dyke swarm into the underlying frontal arc crust indicates that significant crustal extension accompanied the eruption of the inter edifice lavas geochemical variations within the wainimala volcanic suite are thought to have been generated through fractional crystallisation processes while relatively slow ascent and mixing of magmas took place beneath the major edifices efficient shallow level fractionation and rapid ascent of magmas occurred in the areas between them the contrasting styles of volcanism may reflect the interaction of ascending magmas with pre existing heterogeneities in the frontal arc substrate

forty million years of magmatic evolution in the mariana arc the tephra glass record tephra glasses retrieved from 10 deep sea drilling project dsdp cores around the mariana arc system make up a remarkable record of explosive volcanism in the marianas over the past 40 m y major element compositions for approximately 1800 tephra glasses from basalt through rhyolite are reported and used to examine the nature and history of this activity three maxima of volcanic explosivity presumably related to especially vigorous volcanism in the mariana arc are identified with the biggest maximum at around 18 11 ma and two other maxima at 35 24 and 6 0 ma the two younger maxima are contemporaneous with peaks in explosive volcanism observed for other western pacific arcs explosive arc volcanism has been predominantly tholeiitic since shortly after arc inception no boninitic glasses were found the tephra glasses belong to the low to medium k suite except during an enigmatic phase of medium to high k explosive volcanism during the late miocene 11 7 ma even though mariana tephra glasses are largely similar in composition to mariana arc lavas the tephra show a much larger compositional range than that found for subaerial mariana arc lavas which totally lack dacitic to rhyolitic volcanic products the tephra glasses define a bimodal population in terms of silica content with a pronounced minimum or daly gap around 65 66 sio2 mariana tephra glasses are fractionated with the least evolved glass being fe rich and having a magnesium number mg 100mg mg fe 2 of 55 basaltic tephra glasses contain 16 a1203 average 14 3 a1203 and are not high alumina basalts this contrasts with the observation that modern mariana arc lavas contain 15 21 average 17 4 a1203 the high alumina basalt lavas of the mariana arc probably reflect plagioclase accumulation and not liquid compositions all tephra glasses plot near low pressure cotectics and reaction curves on the subprojection olivine clinopyroxene quartz mafic and felsic samples define distinct trends the mafic trend reflects fractional crystallization of mantle derived basaltic magma whereas the felsic trend may be due either to anatexis of mariana arc crust or to fractionation of mafic melts the tephra glass data reinforce the model that the magmatic evolution of the mariana arc has been dominated by low pressure fractionation perhaps accompanied by anatexis episodic changes in melting regime to generate miocene potassic tephra may be related to changing mantle sources and processes related to episodes of back arc basin spreading these episodic changes are superimposed on a long term increase in potassium that reflects progressive metasomatism of the mantle source long term increases in k20 contents for mariana arc magmas inferred from the tephra glass record are 0 004 wt m y mafic 0 011 wt m y intermediate and 0 023 wt m y felsic

the guerrero suspect terrane western mexico and coeval arc terranes the greater antilles and the western cordillera of colombia a late mesozoic intra oceanic arc accreted to cratonal america during the cretaceous the guerrero suspect terrane composed of late jurassic early cretaceous sequences extends from baja california to acapulco and is considered to be coeval with the late mesozoic igneous and sedimentary arc sequences of the greater antilles the west indies venezuela and the western cordillera of colombia these sequences represent the remnants of an arc which accreted to the north american and northern south american cratons at the end of the cretaceous in western mexico the arc sequences built on continental crust consist of high k calc alkaline basalts andesites and rhyolites enriched in lree with abundant siliceous pyroclastic rocks interbedded either with aptian albian reefal limestones or red beds they do not show magmatic changes during the arc development in contrast the arc sequences built on oceanic crust show an evolution with time arc activity began with the development of depleted low k tholeiitic mafic suite guanajuato igneous sequence followed first by mature tholeiitic basalts and then by calc alkaline olivine basalts interbedded with micritic limestones and radiolarian oozes of early cretaceous age at the end of the arc growth during aptian albian times calc alkaline pillow basalts and and esites poured out in the volcanic front while shoshonitic olivine basalts extruded in the back arc the tholeiitic and shoshonitic mafic rocks as well as the calc alkaline lavas are mildly enriched in lree y and nb and show high nd ratios typical of oceanic arcs in contrast the calc alkaline mafic suite enriched in lree y and nb exhibits lower nd ratios suggesting that it was derived by the partial melting of a mantle source contaminated either by paleozoic subducted sediments or old source enrichments oib the cretaceous arc rocks of the greater antilles interbedded with and or capped by aptian albian limestones the cretaceous andesites of northern colombia the cretaceous tholeiitic and calc alkaline volcanic rocks of venezuela and the cretaceous volcano plutonic arc assemblage of tobago share a similar magmatic evolution with the western mexican oceanic arc the tholeiitic plutono volcanic assemblage of tobago depleted in lree and characterized by high nd values is similar to the guanajuato volcano plutonic sequence of mexico considered to represent the pristine stage of the arc the mature tholeiitic sequences exposed in the proto caribbean arc show flat to moderately enriched lree patterns like those of the guerrero terrane however felsic plutonic and volcanic rocks prevail in the caribbean calc alkaline suites accompanied locally by shoshonitic lavas characterize the end of arc magmatic activity in both places thus the geochemical features of the late jurassic cretaceous arc series of the guerrero terrane and the proto caribbean are consistent with the following plate tectonic model the guerrero terrane and the proto caribbean probably belonged to the same intra paleo pacific arc system the development of which was related to the subduction of oceanic basins fringing the north and northern south american cratons this subduction zone was wsw dipping while subduction was going on these magmatic arcs drifted moved closer to the north and south american cratons and finally collided with the american borderlands at different periods during the cretaceous the late mesozoic guerrero and proto caribbean arc sequences show striking similarities with the miocene calc alkaline lavas dredged from the banda ridges the north marianas seamount province and the halmahera and philippine arcs we suggest that the diverse but mostly submarine segments of this late mesozoic intra pacific arc rimmed the north and south american cratons as much as these tertiary arcs rim southeast asia

caracterisation genese et evolution geodynamique du terrain de guerrero mexique occidental the guerrero terrane western mexico is composed of late jurassic early cretaceous plutono volcanic and volcano sedimentary sequences of the alisitos teloloapan arc that accreted to the north american craton at the end of the early cretaceous the geodynamic evolution of the guerrero terrane is that of the alisitos teloloapan intraoceanic arc partly built on continental crust and partly on oceanic crust the growth of the arc was likely linked to the subduction of the arperos and olvidada basins fringing the north american borderland the subduction was dipping west south west the continent based segment of the arc which is presently exposed mainly in northwestern mexico is composed of aerial and submarine k rich calc alkaline basalts andesites and rhyolites and of siliceous pyroclastic rocks interbedded with aptian albian bioclastic carbonates or red beds bearing dinosaurus foot prints the calc alkaline basalts and andesites show light rare earth elements enriched patterns and high concentration

geochemistry and tectonic setting of two contrasting archaean felsic volcanic associations in the eastern goldfields western australia abstract archaean felsic volcanic rocks from two areas in the eastern yilgarn craton of western australia comprise dacite and rhyolite with subordinate basalt and andesite one suite from one of the areas black flag group is dominated by subaqueous dacite lava lobes and their reworked products the other suite from the other area melita and jeedamya volcanics consists of abundant rhyolitic and rhyodacitic pyroclastic deposits with less abundant dacitic lava flows both suites are intercalated with volumetrically subordinate basic and intermediate volcanic rocks black flag group dacites and rhyolites have steep ree patterns la yb cn about 40 and no eu anomaly high sr and al2o3 low nb and y hence high sr y melita and jeedamya dacites and rhyolites have flatter ree patterns la yb cn between 2 and 9 negative eu anomalies eu eu 0 4 0 6 and higher zr nb y and lower sr hence lower sr y on chemical grounds mafic rocks in both areas are genetically unrelated to felsic volcanic rocks black flag group andesites dacites and rhyolites are probably unrelated by fractional crystallization but resulted from 22 28 partial melting of amphibolite leaving a residual mineralogy of amphibole plagioclase garnet ilmenite clinopyroxene alternatively andesites may have been derived by partial melting of a metasomatised mantle source jeedamya rhyodacite could have been produced by 35 vapour absent melting of tonalite leaving a residual mineralogy of plagioclase orthopyroxene amphibole ilmenite the source would have had a slightly lree enriched pattern with a positive eu anomaly residual garnet in the source of black flag group rocks requires melting at depths greater than 35 km which is below the base of the present tectonically thinned crust the most likely source is subducted oceanic crust with black flag group rocks erupted in a back arc basin melita and jeedamya felsic volcanic rocks resulted from melting of thicker continental crust adjacent to the back arc

small scale heterogeneity of phanerozoic lower crust evidence from isotopic and geochemical systematics of mid cretaceous granulite gneisses san gabriel mountains southern california an elongate belt of mid cretaceous compositionally banded gneisses and granulites is exposed in cucamonga terrane in the southeastern foothills of the san gabriel mountains of southern california banded gneisses include mafic granulites of two geochemical types type 1 rocks are similar to high al arc basalts and andesites but have higher hfse high field strength element abundances and extremely variable lile largeion lithophile element abundances while type 2 rocks are relatively low in al and similar to alkali rich mor midocean ridge or intraplate basalts intercalated with mafic granulites are paragneisses which include felsic granulites aluminous gneisses marble and calc silicate gneisses type 1 mafic granulites and calcic trondhjemitic pegmatites also oceur as cross cutting synmetamorphic dikes or small plutons small scale heterogeneity of deep continental crust is indicated by the lithologic and isotopic diversity of intercalated ortho and paragneisses exposed in cucamonga terrane geochemical and isotopic data indicate that k rb and u depletion and sm nd fractionation were associated with biotite muscovite dehydration reactions in type 1 mafic granulites and aluminous gneisses during high grade metamorphism field relations and model initial isotopic ratios imply a wide range of protolith ages ranging from early proterozoic to phanerozoic

inception and early evolution of the ordovician macquarie arc of eastern gondwana margin zircon u pb hf evidence from the molong volcanic belt lachlan orogen abstract the ordovician macquarie arc is faulted against and surrounded by coeval gondwana derived quartz turbidites in the lachlan orogen of southeastern australia how these juvenile island arc rocks were emplaced and structurally interleaved with continental margin sequences of eastern gondwana is an ongoing debate understanding the inception of this arc is critical in building an accurate early paleozoic reconstruction for eastern gondwana however the arc s inception and early evolution are poorly constrained due to the lack of reliable geochronological data integrated field observations zircon u pb hf mineral and whole rock geochemistry are presented here for the oldest units of the molong volcanic belt which is a central strand of the dissected macquarie arc geochemistry indicates that these are calc alkaline rocks with high k locally shoshonitic to medium k affinities the stratigraphically lowest mafic volcanic and volcano sedimentary rocks of the mitchell formation have a unimodal earliest ordovician zircon u pb age of 479 8 3 8 ma with uniform depleted mantle like initial zircon ehf values of 12 to 13 the lack of pre ordovician zircons and the uniform positive initial ehf value indicate that the macquarie arc was initiated in an intra oceanic setting far from the influence of eastern gondwana the stratigraphically overlying fairbridge volcanics includes some more evolved volcano sedimentary components and has a polymodal but mostly ordovician zircon u pb age population with a youngest component at 444 3 2 4 ma overall its ordovician grains show a greater spread in initial ehf values from 14 down to 8 than the mitchell formation which suggests some continental influence at least 35 million years after the arc was initiated the lack of any significant gondwanan inheritance in early ordovician volcaniclastic rocks of the macquarie arc along with geochemical comparisons with modern island arcs suggest that the arc evolved outboard of gondwana probably as a result of steep easterly directed subduction as the macquarie arc approached eastern gondwana there would have been increasing quantities of continent derived sediment subducted beneath the arc which may account for the decreasing and more variable zircon initial ehf values and the appearance of sparse precambrian continental detritus at the arc s moribund stage the collision of the macquarie island arc with gondwana initiated the benambran orogeny and reflects an important mechanism of episodic continental growth involving the addition of juvenile oceanic crust to a continental margin which contrasts and alternates with periods of purely accretionary growth

late miocene to pliocene palaeogeography of viti levu fiji islands abstract the island of viti levu is located on the fiji platform in an area that has been tectonically active since at least the late eocene the geological history of viti levu during this time is one of the competing influence of tectonics volcanism and eustatic sea level change in shaping the preserved stratigraphic and therefore palaeogeographic record the focus of this paper is the palaeogeographic evolution of viti levu from the period following the deformation of the tuva group and all older rock units of the vityaz arc around 7 5 ma to the present initial sedimentation during following the deformational event was influenced by the sinistral oblique slip deformation associated with anticlockwise rotation of the platform this tectonically dominated period of island evolution saw the establishment of a number of sedimentary basins many of which continued to receive sediments through until at least the earliest pliocene from 6 5 ma onward volcanic activity became the main influence on the stratigraphic record although evidence for major eustatic events miocene pliocene boundary are recorded in some sequences toward the end of the early pliocene tectonic uplift of viti levu resulted in a poor geological record for this time the quaternary stratigraphic record reflects relatively minor mostly eustatic events in coastal areas around major river systems

neoarchean evolution of differing convergent margin assemblages in the wabigoon subprovince geochemical and geochronological evidence from the lake of the woods greenstone belt superior province northwestern ontario abstract the neoarchean lake of the woods greenstone belt is subdivided into three lithostratigraphic assemblages a lowermost mafic volcanic assemblage a compositionally diverse predominantly volcanic middle assemblage and a predominantly sedimentary uppermost assemblage the secular evolution and geodynamic settings of these assemblages are constrained with detailed geochronological and geochemical data the lower keewatin is the oldest assemblage with a zircon u pb age of 2738 2 0 ma it consists of mg and fe tholeiitic basalts mg number 30 65 with flat to slightly enriched rare earth elements ree la yb cn 0 9 1 6 elevated large ion lithophile elements lile and undepleted high field strength elements hfse the tholeiites do not appear to have exact modern analogs but do have some similarities to primitive arc tholeiites and back arc basin basalts suggesting an archean destructive margin setting the upper keewatin assemblage disconformably overlies the lower keewatin assemblage it consists of volcanic and sedimentary groups two geochemically distinct volcanic suites are recognized the dominant suite consists of compositionally diverse calc alkalins basalt to rhyolite felsic end members have yielded zircon u pb ages ranging from 2712 to 2723 ma the suite exhibits highly fractionated ree la yb cn 4 35 elevated lile and nb and ta depletion anomalies which are very similar to modern island arc volcanic rocks developed in proximity to continental margins a coeval suite of mafic to ultramafic volcanic rocks consists of mg tholeiite komatiitic basalt and ultramafic komatiite mg numbers 45 80 the suite exhibits depleted lree la yb cn 0 6 1 0 slightly elevated lile and undepleted to slightly enriched hfse suggesting that this geochemically primitive suite was produced by partial melting of a refractory mantle source perhaps as second stage melts of the same mantle source from which the lower keewatin assemblage basalts were previously derived the electrum assemblage unconformably overlies the upper keewatin assemblage in the northern part of the belt it consists of conglomerates sandstones and minor volcanic rocks having detrital ages indicating deposition after 2699 ma the volcanic rocks consists of andesite to rhyolite with shoshonitic to calc alkaline geochemical affinity as indicated by highly fractionated ree la yb cn 12 112 highly enriched lile and pronounced nb ta and ti depletion anomalies samples from the syenitic falcon island pluton appear to be coeval with electrum assemblage volcanics having an u pb isotopic age of 2694 ma and similar but even more alkaline chemistry consisting of highly fractionated lree and lile trace elements with very pronounced nb ta and ti depletion anomalies these late tectonic suites are geochemically similar to modern shoshonites with a continental influence formed in the late stages of a mature island arc or a continental arc setting

late jurassic to eocene geochemical evolution of volcanic rocks in puerto rico the late jurassic to eocene deformed volcanic volcaniclastic and sedimentary rocks of puerto rico are divided into three igneous provinces the southwestern central and northeastern igneous province based on the stratigraphic position approximate ages could be assigned to the flow rocks in these provinces ba nb and la sm diagrams are presented to illustrate the origin and evolution of the flow rocks the oldest rock in the southwestern province may include morb early cretaceous volcanic rocks in the central and northeastern province have low ba nb and la sm that are interpreted as an early island arc stage with none or only minor contribution of slab derived material the late cretaceous to eocene volcanic rocks have a wide range of values for the ba nb and la sm that are interpreted as the result of admixture of a variable amount of slab derived material the maricao basalt maastrichtian to eocene in the southeastern igneous province has the geochemical signature of magmas formed in an extensional setting

paleoproterozoic 1 90 1 86 ga arc volcanism in the flin flon belt trans hudson orogen canada geochemical and isotopic nd sr data are reported on paleoproterozoic 1904 1864 ma maficintermediate 1870 ma collision and subsequent intra arc deformation

petrochemistry and tectonic significance of cretaceous island arc rocks cordillera oriental dominican republic abstract cretaceous island arc rocks of the caribbean island arc system have been exposed by cenozoic faulting in the cordillera oriental in eastern hispaniola high k 2 o intermediate to felsic volcanic rocks loma la vega volcanics are interbedded with marine epiclastic sedimentary rocks and tuffs las guajabas tuffs and unconformably overlie pre aptian sedimentary rocks low k 2 o volcanic rocks guamira volcanics and a granodioritic to tonalitic intrusion el valle pluton the petrology and geochemistry of these units in conjunction with regional stratigraphic data are used to speculate on the tectonics of the newly developing caribbean island arc system during early and late cretaceous time the loma la vega volcanics are characterized by the presence of large phenocrysts of sanidine and minor amounts of clinopyroxene opaque oxides and rare leucite in a devitrified matrix of chlorite and clay although the volcanic rocks have undergone some low temperature alteration metamorphism which redistributed some major elements and large ion litho phile trace elements the high field strength elements rare earth elements and radiogenic isotopes appear to have been minimally affected based on abundances of the relatively immobile elements trace element enrichment patterns and isotopic compositions the loma la vega volcanics are considered part of the high k calc alkaline ca or shoshonitic island arc volcanic series in contrast pre aptian early cretaceous volcanic and plutonic rocks of the underlying los ranchos formation have chemical characteristics similar to rocks in the island arc tholeiitic or primitive island arc pia series that form coeval and along strike sections of the early cretaceous caribbean island arc in other parts of present day hispaniola cuba puerto rico and the virgin islands an abrupt and regional change in composition from island arc tholeiites to high k calc alkaline rocks is coincident with a hypothesized reversal in subduction polarity in pre aptian time as inferred from previously published tectonic models polarity reversal may have been triggered by attempted subduction of the caribbean oceanic plateau beneath this segment of the caribbean island arc the observed magmatic and tectonic effects of the proposed cretaceous caribbean arc reversal are similar to the better documented neogene subduction reversal event in the solomon islands arc in the southwest pacific

an archean oceanic felsic dyke swarm in a nascent arc the hunter mine group abitibi greenstone belt canada abstract the 2730 ma old hunter mine group hmg a dominantly felsic subaqueous volcanic sequence was formed during early arc construction in the abitibi greenstone belt quebec canada the western part of the hmg contains a felsic dyke swarm up to 1 5 km wide and traceable up section for 2 5 km five distinct generations were identified 1 aphanitic to feldspar phyric dykes 2 quartz feldspar phyric dykes with nd values suggest that an older sialic substrate was not involved in their genesis we infer that the felsic rocks were generated by melting of mafic oceanic crust the swarm was emplaced during nascent oceanic island arc development and was related to rifting of the arc the conformably overlying morb like basalts and basaltic komatiites of the stoughton roquemaure group used the same conduits and further indicate splitting of the arc hmg and associated parts of the abitibi greenstone belts bear a strong resemblance to modern rifted intraoceanic arcs of the western pacific

early paleozoic magmatic record from the northern margin of the tarim craton further insights on the evolution of the central asian orogenic belt abstract the southern tianshan orogenic belt stob is an important region to understand the prolonged evolution of the central asian orogenic belt caob a comprehensive study of zircon u pb ages and lu hf isotopes whole rock elements and sr nd isotopes was carried out for two intermediate mafic intrusions in the baicheng and heiyingshan areas of southwest tianshan la icp ms zircon u pb dating yielded crystallization ages of 431 3 ma mswd 0 073 n 31 and 430 3 ma mswd 0 104 n 30 for the baicheng diorites and 423 9 2 1 ma mswd 0 5 n 12 for the heiyingshan gabbros these data in combination with other late silurian ages previously reported for the intrusive suites from the stob indicate an early paleozoic magmatic event in the region in situ zircon hf isotope data on two samples from the baicheng diorites show ehf t values of 2 4 to 4 9 and 0 2 to 6 and the heiyingshan gabbros exhibit similar ehf t values from 1 to 1 8 the dioritic pluton exhibits na rich arc type geochemical and calc alkaline affinity and shows trace element patterns characterized by enrichment in large ion lithophile elements rb ba k and sr and depletion in high strength field elements nb ta ti zr and hf these features resemble those of arc type igneous rocks combined with initial 87sr 86sr ratios of 0 7047 to 0 7064 and end t of 1 6 to 5 06 we infer that the magma from which the baicheng dioritic pluton formed was sourced from a mantle wedge which was subjected to metasomatism by fluids and melts related to subducted oceanic slab and the overlying sediments the heiyingshan gabbro shows a na rich nature and tholeiitic affinity they not only display supra subduction zone affinity but also are geochemically similar to e morb possible implying a back arc basin setting the initial 87sr 86sr ratios of the heiyingshan samples range from 0 7046 to 0 7051 and end t values vary from 1 76 to 5 19 we speculate that the upwelling of the asthenospheric mantle to the supra subduction wedge facilitated by subduction roll back provided the source for the heiyingshan gabbroic rocks together with other geologic evidence we correlate the generation of the baicheng diorite with the southward subduction of the south tianshan ocean during the early paleozoic the heiyingshan gabbroic plutons were probably formed in a back arc basin in response to the southward subduction of the paleo tianshan ocean in combination with coeval arc type magmatic rocks widely exposed in the southern margin of the central tianshan block a double sided subduction model is proposed to explain the evolution of the south tianshan ocean in the silurian

rapid rotation of fiji paleomagnetic evidence and tectonic implications the timing and magnitude of rotation of fiji in relation to the tectonic evolution of the sw pacific region has been investigated by extensive 126 sites paleomagnetic sampling of late miocene and younger units generally samples yielded well defined characteristic remanence directions which indicate that the main fiji platform rotated by 135 17 5 counterclockwise commencing at or before 10 ma this very large and rapid rotation ceased abruptly at 3 ma coincident with the onset of spreading on the major n s aligned spreading centers in the north fiji basin latest miocene to pliocene paleomagnetic results indicate that viti levu the main island itself has suffered no internal disruption by localized rotations after 5 ma we present a reconstruction for the origin of fiji which differs markedly from previous models such that fiji tonga and lau form a continuous n s aligned arc and that arc reversal was initiated at or near fiji itself

a crustal model for zealandia and fiji summary existing maps of satellite free air gravity and bathymetry and the crust 2 0 global crustal structure model have been used to make new bouguer gravity anomaly and crustal thickness maps of the sw pacific region to analyse the region s crustal structure the new maps clearly outline the limits of the rifted largely submerged continent of zealandia the bouguer gravity anomalies of ocean crust large igneous provinces and backarc basins vary simply and predictably with age the bathymetric fiji platform lies in a water depth of 19 km for the entire fiji platform comparable to stretched and submerged parts of zealandia the question whether the continental crustal thickness of fiji is due to its actually being a rifted part of gondwana zealandia or due to enhanced eocene pliocene magmatism remains open but testable with the latter more likely using the new accurate 3 d crustal thickness and published radiometric dating of lavas we estimate that the magmatic portions of southwest pacific cenozoic subduction related arcs three kings tonga and vanuatu have grown at net volumes which vary from 263 to 1400 km3 km 1 with net addition rates of 17 31 km3 km 1 ma 1

pillow lava basalts with back arc morb affinity from the usagaran belt tanzania relics of orosirian ophiolites the paleoproterozoic usagaran orogenic belt in east africa is an exemplary site of preserved relics of paleoproterozoic volcanic arc magmas and subducted and displaced oceanic crust this paper describes the geochemical characteristics of displaced pillow basalts from the usagaran belt konse group our data indicate that the konse pillow basalts have tholeiitic composition and are overprinted by greenschist facies metamorphic conditions but their primary geochemical signatures are preserved by ree and fluid immobile elements the la sm n ratios 0 62 1 09 and ree patterns point to normal mid ocean ridge basalt n morb and transitional morb t morb mantle source the analysis of high valency elements and trace element patterns points to a mixed signature of morb tholeiites and island arc tholeiites with elevation of ba th u eu and sr this composition is similar to that of phanerozoic back arc suprasubduction zone ophiolites therefore usagaran belt pillow basalts are an analogue of the tethyan type suprasubduction zone ophiolite evolution and emplacement in the precambrian the emplacement of the konse pillow basalts is older than the neighbouring 1920 1870 ma volcanic arc magmas and is probably coeval with the formation of the 2000 ma yalumba eclogites with morb affinity thus the konse pillow basalts might have been displaced from their suprasubduction zone tectonic setting of igneous construction within the yalumba ocean basin around 2000 ma thematic collection this article is part of the tethyan ophiolites and tethyan seaways collection available at https www lyellcollection org cc tethyan ophiolites and tethyan seaways

magmatic evolution of late cenozoic volcanic rocks of the lau ridge fiji three main groups of lavas are exposed on islands of the lau ridge the lau volcanic group lvg 14 0 5 4 ma are predominantly andesite korobasaga volcanic group kvg 4 4 2 4 ma are predominantly basalt and mago volcanic group mvg 2 0 0 3 ma are basalt hawaiite lvg and kvg lavas are mostly medium k tholeiitic rocks with high lile hfse ratios characteristic of islands ares while mvg lavas are ne normative alkalic rocks with high lile and hfse characteristic of ocean island basalts lvg lavas have high nd 8 0 8 4 and low 87sr 86sr 0 70273 0 70349 similar to n morb whereas kvg lavas have slightly more radiogenic values nd 7 5 8 4 87sr 86sr 0 70323 0 70397 mvg lavas form an isotopically distinct group having lower nd 4 6 4 9 and 87sr 86sr ranging from 0 70347 0 70375 lvg lavas were erupted in a primary oceanic island arc vitiaz arc during the miocene basaltic lavas were derived by approximately 19 partial melting of mantle wedge peridotite with only minor subduction component andesites and dacites were produced by low pressure plagioclase pyroxene titanomagnetite dominated crystal fractionation kvg lavas were erupted during the period immediately prior to or during the initial stages of rifting in the lau basin and like lvg lavas show significant chemical differences at the northern and southern ends of the lau ridge lavas at the northern end type ii appear to be derived from a more depleted source than lvg but with a greater amount of subduction component those at the southern end type i probably came from a slightly more enriched source with less subduction component mvg basalts and hawaiites were derived from an enriched mantle with little or no subduction input the hawaiites type i could not have been derived from the basalts type ii and the two magma types must have come from different sources indicating mantle heterogeneity the lack of subduction influence indicates the mvg lavas are tectonically unrelated to the present day tonga arc and the lack of depletion indicators suggests they have tapped a different new part of the mantle wedge this may reflect introduction of sub pacific mantle through the present tonga lau subduction system

sedimentation and volcanism in an oligocene miocene intra oceanic are and fore are southwestern viti levu fiji rocks belonging to the lower oligocene to middle miocene wainimala group in southwestern viti levu can be divided into an are assemblage representing the axial part of a volcanic are and a basinal assemblage representing the trenchward flank of the are and the inner part of the fore are basin the are assemblage consists mainly of volcaniclastic rudites and lava flows with minor shallow water limestones and is intruded by mid to late miocene gabbro and tonalite stocks the basinal assemblage to the north is floored by part of an earlier eocene to lower oligocene are the lower part of this assemblage includes volcaniclastic rudites representing the dispersal aprons of major volcanic edifices to the trenchward of the are axis and basaltic lavas and fissure fed rhyolite lava domes and breceia ridges erupted in deeper water away from these centres the rhyolites are directly related to the acidic members of a dense compositionally bimodal dyke swarm that intrudes the underlying earlier are rocks recording substantial crustal extension the basinal volcanic rocks are interbedded with and overlain by a sequence of pelagic carbonates thin bedded epiclastic turbidites and acidic tuffs the basinal succession evolved from an intra are to an inner fore are setting as volcanism within the assemblage died away in the early miocene in the late early miocene increased uplift and emergence of the are massif increased the sediment yield to the fore are basin resulting in the deposition of thicker bedded coarser grained turbidites

the nadi basin neogene strike slip faulting and sedimentation in a fragmented arc western viti levu fiji the nadi basin in western viti levu is one of a number of sedimentary basins controlled by e w to ne sw faults that developed on the fiji platform in the late miocene the initial basin fill belonging to the upper miocene nadi sedimentary group consists of mainly basement derived breccias and sandstones deposited at bathyal depths by sediment gravity flows on a basin floor dissected by braided e w faults the main source area lay to the south across the actively down faulting basin margin in the latest miocene a change in provenance took place as debris spread southward from a newly developed e w chain of shoshonitic volcanoes koroimavua volcanic group some 12 km north of the southern basin margin continued normal displacement along the southern margin allowed the deposition of a thick sequence of shoshonitic sandstones at bathyal depths on the downthrown side in the earliest pliocene the onset of basin inversion was marked by shallowing to neritic depths and the onlap of sandstones and shoshonitic conglomerates southward onto pre nadi sedimentary group basement as inversion proceeded the basin fill was folded and southward directed reverse displacement took place along the southern basin margin evidence from the nadi and other late miocene basins indicates that they developed as a result of broadly e w directed sinistral strike slip probably related to a shearing collision of the melanesian border plateau with the northern margin of the fiji platform during the breakup of the formerly continuous outer melanesian arc it is suggested that prior to the establishment of the fiji fracture zone at 7 6 ma much of the left lateral displacement currently taking place along that plate boundary was accommodated by strike slip deformation within the fiji platform

geochemistry of late proterozoic basaltic rocks from southeastern new brunswick canada late proterozoic volcanic rocks of the coldbrook group 630 600 ma from the avalon zone of the appalachians in southeastern new brunswick canada are predominantly bimodal and occur in three belts fault blocks eastern central and western the basaltic rocks were emplaced in a convergent plate margin setting and range from incompatible element depleted island arc tholeiites in the eastern belt to calc alkalic types of the western belt the distinct compositional zonation of the basaltic rocks resembles the across arc variation observed in recent volcanic arc systems the trends include an increase of the lile and hfse abundances and the zr sio2 th sio2 la yb th nb th hf and zr y ratios from the eastern belt in the southeast through the central belt to the western belt in the northwest the zonation of the coldbrook group is attributed to a northwest dipping subduction zone during late proterozoic time it is suggested that some of the late proterozoic basins in the avalon zone of the northern appalachians reached a stage of oceanization

early mesozoic oceanic subduction related volcanic rocks pindos basin greece abstract small outcrops of volcanic rocks of mid triassic to early jurassic age in the deep water mesozoic pindos basin have been systematically sampled and analysed geochemically to interpret the tectonic environment in which these rocks formed associated sediments indicate a rifting environment the volcanic rocks include all the main types found in modern back arc basins shoshonites with clinopyroxenes of calc alkaline affinity occur at the basin margin minor enriched type morb contains clinopyroxene typical of morb minor basalts that resemble oib with some subduction related characteristics contain clinopyroxene typical of alkahne rocks thick andesites and pyroclastic rocks with morb like trace elements except for some lile enrichment are associated with cumulate gabbro and oceanic type plagiogranite these thick andesites and pyroclastic rocks formed at an intra backarc ocean ocean subduction zone the supra subduction zone ophiolites of the eastern margin of the pindos basin which are of similar age are probably related to the same subduction zone the overall volcanic assemblage is related to formation of a back arc basin associated with the subduction of paleo tethys the hypothesis that late triassic eariy jurassic rifting formed only small pull apart basins by transtension is inconsistent with the range of volcanic products and the evidence for significant intra back arc subduction

temporal change in chemistry of magma source under central kyushu southwest japan progressive contamination of mantle wedge volcanism related to subduction of the philippine sea phs plate began in central kyushu at 5 ma after a pause of igneous activity lasting about 10 m y it formed a large volcano tectonic depression the hohi volcanic zone hvz and has continued to the present at a decreasing eruption rate the products are largely andesite and dacite which became enriched in k with time the proportion of tholeiitic to calc alkalic rocks also increases with time calc alkalic high mg basaltic andesites ybbs were erupted in the early stage of the hvz activity 5 3 ma and high alumina basalts kjbs were erupted in the later stage 2 0 ma in contrast to the basalts in the hvz northwest kyushu basalts nwkbs have been erupted on the backarc side of the hvz since 11 ma and hence are not related to the phs plate subduction they are mainly high alkali tholeiitic to alkali basalt that shows no notable chemical change with time nwkb ybb and kjb have morb normalized incompatible element spectra that differ from each other as is well expressed in both nb and sr anomalies the patterns of kjb and nwkb are typical of those for island arc basalt iab and ocean island basalt oib respectively ybb shows a pattern intermediate between the two we suggest that the magma source beneath the hvz changed in composition from an oib type mantle to an iab type mantle as the subduction of phs plate advanced however the magma source remained fertile under northwest kyushu in order to explain the temporal change of source mantle beneath the hvz we propose a model for progressive contamination of the mantle wedge in which three processes contamination by a slab derived component subtraction of magma from the mantle and mixing of the mantle residue and slab derived component are repeated as subduction continues as long as the progressive contamination of mantle wedge proceeds its trace element composition converges at a steady state value for a short period this value does not depend on the initial composition of the mantle wedge but instead on the composition of the slab derived component the trace element composition of the magma produced in such a mantle wedge approaches that of the slab derived component with time but the major element composition is determined by the phase relations of mantle peridotite the slab derived component may be basaltic liquid that is partially melted from rutile bearing eclogite

volcanism in nascent back arc basins behind the shichito ridge and adjacent areas in the izu ogasawara arc northwest pacific evidence for mixing between e type morb and island arc magmas at the initiation of back arc rifting mineral chemistry major and trace elements and 87sr 86sr ratios are presented for 29 igneous rocks dredged from the northern portion of the izu ogasawara arc these rocks are compositionally bimodal basement gabbro and trondhjemite from the arc are extremely poor in k2o 0 05 0 19 and rb 0 48 0 62 ppm and their ree patterns and sr isotope ratios indicate that there are island arc tholeiites quaternary volcanic rocks from the present volcanic front shichito ridge active arc back arc seamounts east side inactive arc and torishima knoll between the two back arc depressions incipient back arc basins behind the active arc have the same geochemical characteristics as the above plutonic rocks though they are not as depleted in k and rb rhyolite pumice from the backarc depression is also the depleted island arc tholeiite whereas basalts from the depression have compositions that are transitional between morb and island arc tholeiites in trace element ti ni cr v y and zr and mineral chemistries the back arc depression basalts have relatively high ban cen 0 66 1 24 cen ybn 1 1 1 9 and k ba 45 105 and low 87sr 86sr 0 70302 0 70332 and ba sr 0 1 0 2 which are similar to other back arc basin basalts and e type morb but are quite unlike the depleted island arc tholeiites the diverse trace element and sr isotope compositions of basalt andesite from the back arc depressions imply the interplay between e type morb and island arc tholeiite these chemical characteristics and the relationships of ce yb n vs ba ce n and ce yb n vs 87sr 86sr suggest that the back arc depression magmas are generated by mixing of e type morb and depleted island arc tholeiite magmas geochemical characters of the associated rhyolite from the depression are compatible with partial melting of lower crust

petrogenetic evolution of felsic volcanic sequences associated with phanerozoic volcanic hosted massive sulphide systems the role of extensional geodynamics abstract felsic volcanic hosted massive sulphide deposits form in many different geotectonic environments usually associated with episodes of extension however empirical geochemical links between felsic volcanism geotectonic environment and extensional geodynamics have not been adequately documented so as to distill the fundamental relationships between these features and formation of volcanic hosted massive sulphide vhms mineralization which is important considering most vhms deposits are hosted in felsic volcanic bearing sequences using felsic geochemical data from six different vhms mineralized phanerozoic geotectonic settings it seems that the felsic geochemical data are consistent with the overall geotectonic interpretations for these areas which in turn helps in stratigraphic and geotectonic interpretations for detailed vhms exploration in extensional geodynamic settings bimodal volcanism is common compared to the continuum of compositions observed under compressional to neutral high stress regimes in calc alkaline arc building environments the empirical geochemical metallogenic vhms relationships evident in the archean felsic volcanic rocks in particular the low zr y n yb n 2 o conditions in different source areas and geotectonic environments they seem to be the predominant controls on the compositional evolution of felsic magmas in extensional settings because of the limited degree of assimilation and fractional crystallization afc affecting their compositions with rapid intrusion into the uppermost crust to positions were they achieve neutral buoyancy in the volcanic to subvolcanic environment within plate felsic or a type anorogenic type and anomalous ocean ridge or type felsic magmas that have tholeiitic affinities relatively low alkali elements are common within parts of the volcanic sequences hosting massive sulphide deposits reflecting in part a relationship to extension the high contents of high field strength elements zr ga lree hree y nb etc and other lines of evidence indicate they are relatively high temperature relatively reduced magmas probably derived from previously dehydrated granulitic or melted oceanic simatic or continental sialic crust low p granulites or amphibolites but also in some cases possibly mantle rocks such that they are significantly water undersaturated during melting i e a h 2 o 1 reduced viscosities typical of high temperature melts enable rapid intrusion to higher levels diking vs diapirism higher rates of heat advection from mantle mafic magmas into the thickened oceanic and or continental crust during lithospheric extension are needed to facilitate melting of these rock to form a type and or or type partial melts tholeiitic felsic this usually results in bimodal magmatism as magma mixing is not favored in extensional settings especially at high magma production intrusion and extrusion rates i e high heat flow this would inhibit ferromagnesian phase in particular hornblende and magnetite fractionation especially at low f o 2 in the felsic magmas that would deplete the melt in y hree as well as ore related elements particularly zn which is 2 to 3 times higher in a type and or type rocks compared to other magma types therefore the empirical felsic igneous geochemical metallogenic relationships observed by others low zr y n yb n

mid cretaceous transtension in the canadian cordillera evidence from the rocky ridge volcanics of the skeena group the age relations geochemistry and sedimentology of the rocky ridge formation of the skeena group are used to test competing tectonic reconstructions for the mid cretaceous canadian cordillera as well as the timing and location of the accretion of the insular superterrane pollen and macrofossil assemblages indicate that these intrabasinal basalts were erupted along the southern margin of the bowser basin in the early albian to early cenomanian single crystal fusion and step heating 40ar 39ar dating of hornblendes in one basalt flow from the uppermost part of the formation yielded middle cenomanian ages of 94 3 0 4 95 6 1 6 and 95 0 1 6 ma vesicular basalt flows interbedded with crystal rich tuff breccias contain evidence for hot emplacement as pyroclastic flows individual eruptive centers are identified by their proximal facies paleoflow indicators within the lava flows paleoflow indicators within interbedded volcaniclastic fluvial deposits geochemical differences and geographic isolation of volcanic deposits major and trace element geochemistry from 20 sampled lava flows indicates an alkali basalt composition for the volcanics the basalts of the northern rocky ridge volcanic center show enrichment of light rare earth and large ion lithophile elements with strong negative nb ta anomalies whereas the basalts of the southern tahtsa lake volcanic center show depletion to slight enrichment of light rare earth elements slight enrichment of large ion lithophile elements with minimal negative nb ta anomalies the geochemistry combined with paleogeographic and regional tectonic reconstruction suggests a continental arc setting with intraarc extension the presence of deeper marine facies to the west and the lack of a western sediment source in the skeena group indicate that the technically active insular superterrane was not west of the study area during mid cretaceous time thus we reconsider the omineca belt as the main axis of a mid cretaceous continental arc placing the intermontane superterrane in the intraarc to forearc position with the rocky ridge volcanics erupted along the forearc side of the omineca arc coeval regional strike slip faulting and reconstructed oblique plate convergence suggest a transtensional setting for rocky ridge intraarc extension an electronic supplement of tables a1 a2 may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 95tc03496 mid cretaceous transtension in the canadian cordillera evidence from the rocky ridge volcanics of the skeena group kari n bassett and karen l kleinspehn diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order

petrology and geochemistry of target rocks from the bosumtwi impact structure ghana and comparison with ivory coast tektites the 10 5 km diameter bosumtwi crater in ghana west africa is the most likely source crater for the ivory coast tektites as the tektites and the crater have the same age 1 07 ma and there are close similarities between the isotopic and chemical compositions of the tektites and crater rocks the crater is excavated in 2 1 2 2 ga old metasediments and metavolcanics of the birimian supergroup here we present the first integrated petrographic and geochemical study of rocks from the bosumtwi impact crater a variety of target rocks from the bosumtwi impact structure were selected to represent the major rock types that have been described before resulting in four groups shale phyllite graywacke and two different types of granites from dispersed dikes and from the so called pepiakese intrusion at the northeastern side of the crater these rocks were analyzed for their major and trace element composition and their petrographic characteristics in addition representative samples were also analyzed for their o sr and nd isotopic compositions the target rocks do not show any unambiguous evidence of shock metamorphism i e planar deformation features pdfs distinct impact characteristic shock effects pdfs were identified only in clasts within suevite derived melt fragments the compositional range of the target rocks is significantly wider than that of the ivory coast tektites but overlap the tektite compositions a best fit line for the bosumtwi crater rocks in a rb sr isotope evolution diagram yields an age of 1 98 ga and an initial 87sr 86sr ratio of 0 701 which is close to results previously obtained for granitoid intrusions in the birimian of ghana our nd isotopic data yield depleted mantle model ages ranging from 2 16 to 2 64 ga and nd values of 17 2 to 25 9 harmonic least squares hmx mixing calculations were able to reproduce the composition of ivory coast tektites from a mixture of bosumtwi country rocks that include about 70 phyllite graywacke 16 granite dike and 14 pepiakese granite the oxygen isotopic composition of the metasedimentary rocks and granite dikes 18o 11 3 13 6 and the tektites 18o 11 7 12 9 also this work agree fairly well while the pepiakese granites have lower values 18o 8 6 9 0 indicating that only minor amounts of these rocks were incorporated in the formation of the ivory coast tektites the large variation in sr and nd isotopic compositions of the target rocks do not allow the unambiguous identification of distinct endmember compositions but in both a 87sr 86sr vs 1 sr plot and an sr vs nd diagram the tektites plot within the area occupied by the metasedimentary and granitic bosumtwi crater rocks thus our data support the interpretation that the composition of the ivory coast tektites is similar to that of rocks exposed at the bosumtwi impact structure indicating formation during the same impact event

paleogeographic and paleotectonic evolution of a basement controlled archean supracrustal sequence chibougamau caopatina quebec the 7 14 km thick archean volcano sedimentary sequence in the chibougamau caopatina region ccr composed of two mafic to felsic cycles and two thick essentially epiclastic successions is divided into a southern caopatina segment scs and northern chibougamau segment ncs the sedimentary units which evolved after cycle 1 volcanic rocks in the scs and after the combined cycles 1 and 2 in the ncs document a diachronous sedimentary basin evolution this criterion has been employed to distinguish an early evolutionary stage in the south and a late evolutionary stage in the north in each segment the basin episode represents a unique extensional event volcanism preceded the major sedimentary rifting event in both segments composition of sediments and sedimentary facies correlated with the interpreted basin margin faults support the interpretation of two phases of crustal extension volcanic construction and sill intrusion were the prevalent factors governing emergence of volcanic islands in the cc

geochemistry of 1 9 ga morb and oib like basalts from the amisk collage flin flon belt canada evidence for an intra oceanic origin subaqueously erupted basalts that occur in kilometre scale allochthons within the 1 9 ga flin flon belt canada appear to have been generated at oceanic ridges and possibly oceanic plateaus remote from archean cratons the ocean floor basalts fall into two categories 1 n type resembling n morbs and mariana type back arc basin basalts depleted to flat ree patterns high zrnb variable thnb and initial nd 3 3 to 5 4 2 e type resembling transitional and plume morbs slightly enriched ree patterns lower zrnb initial nd 3 1 to 4 5 in the largest and best studied allochthon the elbow athapapuskow assemblage mixing between depleted n morb and enriched oib sources or melts coupled with variable addition of a subduction lile component can explain the chemical variations in the basalts zircon u pb dates of 1904 ma for a syn volcanic diabase sill and 1901 6 5 ma for a gabbro peridotite cumulate complex demonstrate that crystallization of the ocean floor basalts overlapped with in part eruption of the tectonically juxtaposed 1 90 1 88 ga arc volcanic rocks the elbow athapapuskow allochthon is interpreted as back arc basin crust that developed simultaneously with flin flon arc magmatism subaerially erupted basalts that chemically resemble tholeiitic oibs 8 14 mgo relative hree depletion initial nd 2 2 to 3 4 occur in tectonic contact with the elbow athapapuskow assemblage the oibs may have been generated by deeper garnet residue melting of enriched mantle tapped during extension in the elbow athapapuskow back arc basin and were possibly erupted onto a remnant arc deeper mantle melting is also indicated by the presence of the lree enriched oceanic plateau like basalts of the sandy bay assemblage the back arc 01b and plateau volcanic assemblages were jux taposed against ca 1 9 ga arc assemblages in a philippines like intraoceanic accretionary complex by 1 87 ga

early geochemical evolution of an oceanic island arc and backarc fiji and the south fiji basin the initial 25 to 35 ma of oceanic arc volcanism in fiji the early arc stage was dominated by eruption of lree depleted low k tholeiitic basalts and andesites the oldest volcanics include the highest hfse group morb like in composition plus high mg basalts and basic andesites with flat ree patterns as low as 4x chondritic concentrations these erupted either within the volcanic arc during its first 5 ma or in an adjacent backarc during its first ma of formation subsequent volcanics of the early arc stage apparently erupted onto early backarc basin crust most are intermediate between earlier extremes in hfse contents but at least one volcano produced strongly lree depleted island arc tholeiites in contrast medium k tholeiitic and calc alkaline andesites erupted during the mature arc stage parental basalts were enriched in lree but depleted in hree relative to earlier basalts due to a change in source composition counterflowing morb source asthenosphere within the mantle wedge dominated vol

the emergence and future of near surface geophysics over the past 30 years geophysical methods have assumed a much more prominent and integral role in many investigations where subsurface features have environmental and engineering importance in fact the field once referred to as environmental and engineering geophysics has broadened to include other applications e g archeology forensics and is now commonly referred to more generally as near surface geophysics it is difficult to precisely define near surface geophysics and the definition will likely depend on whom you ask however we define it as the use of geophysical methods to investigate the zone between the surface and hundreds of meters into the earth s crust applications include but are not limited to potable water management engineered infrastructure and construction site clearance gas storage natural hazard mitigation mining forensics and archaeology although the same physical principles are relevant for any target depth the high degree of near surface heterogeneity ra

zircon u pb and molybdenite re os geochronology and geochemistry of jinchang porphyry gold copper deposit ne china two phase mineralization and the tectonic setting abstract whether multiple mineralization styles in porphyry deposits are the result of one or multiple pulses of magmatism is unknown in this paper we present new age and geochemical data on the jinchang porphyry gold copper au cu deposit an important deposit in ne china and in the eastern central asian orogenic belt caob field geological study suggests that both the mineralization and ore causative magmatism at jinchang are multiphase la icp ms zircon u pb dating of the monzogranite ore host yielded 197 1 ma which is considerably older than the ore causative monzonite 115 3 ma and granodiorite 107 2 ma molybdenite re os dating on the breccia pipe hosted au cu ores yielded a well defined isochron age of 114 3 ma coeval to the monzonite emplacement the au cu mineralization was followed by another phase of crypto explosive breccia pipe hosted disseminated vein type au mineralization this au only mineralization phase was auriferous pyrite re os dated to be 103 101 ma coeval to the granodiorite emplacement geochemically the monzonite and granodiorite are enriched in liles and lrees but depleted in hfses and hrees the ore causative rocks are i type and display continental arc geochemical affinities we suggest that the monzonite and granodiorite were generated in an intracontinental extensional setting probably caused by the early cretaceous subduction roll back of the paleo pacific plate

age constraints and characteristics of subaqueous volcanic construction the archean hunter mine group abitibi greenstone belt the archean hunter mine group hmg is a 6 7 km thick south facing volcanic sequence at the southern limit of the northern volcanic zone abitibi greenstone belt the hmg is composed of 1 felsic lava flows and endogenous and exogenous domes with autoclastic breccia deposits 2 pyroclastic fire fountain products and high to low concentration volcaniclastic sediment gravity flows of autoclastic and pyroclastic origin interstratified with iron formation 3 mafic pillowed and brecciated flows 4 a prominent felsic dyke swarm and 5 abundant mafic intrusions the stratigraphy is complex as inferred by detailed volcano sedimentary facies mapping a 5 7 km wide n trending felsic dyke swarm coupled with abundant synvolcanic and synsedimentary faults chaotic breccias and mineralization suggest a central volcanic depression which is consistent with a caldera structure voluminous effusive activity and explosive fire fountaining eruptions contributed to caldera development rifting of this arc edifice is supported by the presence of felsic dykes and mafic tholeiitic sills the lithofacies and overlying komatiites flows are consistent with a deep water marine setting u pb age determinations help constrain the hmg caldera formation to 6 million years the sampled quartz feldspar phyric dykes yielded u pb zircon ages of 2728 3 4 3 4 and 2728 9 0 8 ma and together with a previously published dyke age of 2729 6 1 4 ma from the same dyke swarm a case is made that the swarm evolved over 1 million years 2728 6 2729 6 ma a lobate flow and flow breccia revealed similar ages of 2724 6 4 6 1 7 and 2727 6 4 2 2 0 ma whereas the up to 1 km thick gabbro diorite roquemaure sill precedes both felsic dykes and flows as indicated by the 2731 8 2 2 2 0 ma age the 2706 4 5 7 4 3 ma feldspar quartz phyric dykes occurred late in the history of the hmg and were not associated with subaqueous volcanic construction the 207pb 206pb ages of analyzed zircon fractions have an inherited component within error of the oldest crystallization ages obtained for the hmg complex collectively these age determinations and inheritance as well as stratigraphy support a tripartite division into a 2734 2730 ma lower formational stage a 2732 ma middle formational stage and a 2730 2728 ma upper formational stage the study shows that age constraints without detailed facies mapping may lead to erroneous conclusions on volcanic construction and that felsic volcanic dykes and flows may inherit an earlier component of their own volcanic edifice

gamilaroi terrane a devonian rifted intra oceanic island arc assemblage nsw australia the gamilaroi terrane comprises part of the new england orogen of eastern australia and contains devonian strata preserving evidence of their development in a variety of arc related environments radiolarian studies provide new and more precise constraints on stratigraphy and together with geochemistry allow development of a new model for evolution of the terrane a diverse suite of volcaniclastic sedimentary rocks intercalated with regionally extensive extrusive and high level intrusive meta felsic igneous rocks of low k and calc alkaline volcanic affinity is present these rocks constitute the oldest in situ volcanic rocks in the gamilaroi terrane and are similar to those found in modern intra oceanic volcanic island arc settings basaltic intrusions and lavas occur at higher stratigraphic levels major and trace element compositions together with relatively flat chondrite normalized ree distributions resemble those from rocks formed in modern intra oceanic island arc rift zones the close spatial association of these rock types together with an absence of continent derived detritus is consistent with interpretation of development of the gamilaroi terrane in an intra oceanic island arc which experienced local rifting this interpretation is a departure from previous models for the new england orogen which interpret the gamilaroi terrane as a forearc basin that developed above a long lived cambrian to permian west dipping subduction zone fringing the continental margin of gondwana by analogy with modern arc continent collision zones in which crust between colliding continents and island arcs is preferentially subducted beneath the approaching arc we suggest that oceanic crust intervening between gamilaroi terrane and gondwana was subducted eastwards under the western margin of the gamilaroi terrane arc during a latest devonian collision event the gamilaroi terrane was obducted onto the gondwana margin this resulted in subduction flip and subsequent development of an east facing continental margin arc system on top of the gamilaroi terrane