an 40ar 39ar thermochronology of the ofoten troms region implications for terrane amalgamation and extensional collapse of the northern scandinavian caledonides fifteen 40ar 39ar cooling ages are reported for metamorphic hornblende and muscovite from far traveled terranes constituting the ofoten nappe stack of northern norway eight cooling ages on hornblende range from 425 to 394 ma and seven muscovite ages from the same or nearby outcrops as the hornblendes range from 400 to 373 ma these data are compared with 40ar 39ar ages from over a large part of the northern caledonides to evaluate regional mineral cooling patterns results indicate that 1 scandian silurian devonian metamorphism was predominant 2 most of the nappes investigated contain some vestige of pre scandian tectonism and or metamorphism 3 hornblende and muscovite cooling ages are progressively younger to the west and south which suggests a hinged to the east mineral cooling pattern and 4 a late out of sequence thrust is the only disruption of this cooling pattern synmetamorphic amalgamation of the nappes resulted from scandian a type subduction the hinged to the east mineral cooling pattern implies isostatic adjustment and exhumation of the footwall of a west dipping crustal scale extensional fault located somewhere west of the present norwegian coast during late synorogenic gravitational collapse the late out of sequence fault formed contemporaneously with uplift in the hinterland implying a kinematic and temporal connection with east directed contractional faulting in the foreland

the petrology of a large intra oceanic silicic eruption the sandy bay tephra kermadec arc southwest pacific abstract the pumiceous pyroclastic deposits known as the sandy bay tuff on macauley island in the kermadec group represent a medium scale silicic eruption in an oceanic subduction setting the sandy bay eruption occurred about 6310 yr bp forming a large submarine caldera in the summit of the mainly submarine and essentially basaltic macauley volcano the sandy bay magma contained

southern kermadec submarine caldera arc volcanoes sw pacific caldera formation by effusive and pyroclastic eruption abstract three newly identified submarine caldera volcanoes brothers healy and rumble ii west of the southern kermadec arc within water depths 80 and low density 0 3 0 5 g cm 3 erupted pumiceous clasts these observations are interpreted as evidence of subaqueously quenched pyroclastic eruption s sheathed by ambient water flashing to steam discharging some 5 km3 of pyroclastic material from healy volcano caldera collapse is inferred to be penecontemporeous with the pyroclastic eruption s although it is possible formation as an explosive crater occurred but we consider the latter unlikely given the volume of erupted material and water depth extrapolation of these observations to other submarine arcs indicates pyroclastic eruption of silicic magmas and syn eruptive caldera collapse is possible within water depths of 500 1000 m

submarine silicic caldera at the front of the izu bonin arc japan voluminous seafloor eruptions of rhyolite pumice myojin knoll caldera a submarine rhyolitic center 400 km south of tokyo is one of nine silicic calderas along the northern 600 km of the izu bonin ogasawara arc and the first anywhere to receive detailed submersible based study the caldera slightly smaller than the crater lake structure in oregon is 6 7 km in diameter its inner walls are 500 900 m high and it has a remarkably flat floor at 1400 m below sea level mbsl the caldera collapse volume is 18 km 3 suggesting that more than 40 km 3 of pumiceous tephra may have been erupted at the time the caldera formed precaldera seafloor eruptions built a broad volcanic edifice consisting of overlapping composite volcanoes made of rhyolitic lavas shallow intrusions and a variety of volcaniclastic deposits including thick accumulations of rhyolitic pumice erupted at 900 500 mbsl the caldera forming eruption produced a 150 200 m deposit of nonwelded fines depleted pumice that resembles a colossal layer of popcorn at the top of the caldera wall freshly erupted pumice behaved as sinkers or floaters depending on the environment in which it cooled the pumice clasts deposited proximally and exposed in the caldera wall were likely quenched in eruption columns that remained below sea level this pumice ingested seawater and sank as gases filling its vesicles cooled particularly as steam in its vesicles condensed to liquid water some eruption columns may have broken through the sea surface and entered the air especially during vigorous phases of the caldera forming eruption these pumices had the opportunity to ingest air as they cooled becoming floaters as they fell back to the sea these could have been carried distally on the sea surface by the combined effects of ocean currents and wind the age of the caldera is unknown but it may be as young as several thousand years its magmatic system at depth retains sufficient heat to sustain an actively growing intracaldera kuroko type polymetallic sulfide deposit rich in gold and silver and topped by chimneys emitting fluids as hot as 278 c sufficient time has elapsed however for a 250 m high postcaldera dome to grow on the caldera floor and for the caldera rim to be deeply scalloped by slumping

dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

petrogenesis and tectonic implications of late devonian arc volcanic rocks in southern beishan orogen nw china geochemical and nd sr hf isotopic constraints abstract late devonian ca 370 ma volcanic rocks provide important information about the nature of magmatism during the tectonic transition between the early and late paleozoic in the beishan orogen southern central asian orogenic belt they are predominantly an andesitic dacitic rhyolitic assemblage characterized by alkali contents ranging from slightly calcic to slightly alkaline the rhyolitic rocks are generally ferroan whereas the andesitic rocks are magnesian these volcanic rocks exhibit similar trace element characteristics to those of continental arcs strongly negative e nd t values 2 8 to 3 6 and high sr isotopic compositions initial 87 sr 86 sr 0 7036 0 7108 suggest that they are mainly derived from an ancient crust however the positive zircon e hf t values 1 4 to 16 4 support the role of juvenile components in their genesis indicating the significant input of new mantle derived magmas these characteristics imply a hybrid derivation from an ancient crustal source with the addition of juvenile materials during magma genesis or perhaps heterogeneous contamination or hybridization during magma emplacement combined with the regional geology our results indicate that the late devonian magmatism resulted from a southward retreat of the subduction zone which records significant continental crustal growth in a transitional arc or an accretionary arc setting the distinct geochemical compositions especially the nd hf isotope decoupling of the dundunshan volcanic rocks imply a significant change in the geodynamic setting in the late paleozoic

andesites and dacites from daisen volcano japan partial to total remelting of an andesite magma body voluminous andesite and dacite lavas of daisen volcano sw japan contain features suggesting the reverse of normal fractionation anti fractionation in the sense that magma genesis progressed from dacite to andesite accompanied by rises in temperature a positive correlation exists between phenocryst content 0 40 vol and wt sio2 61 67 phenocryst rich dacites contain hornblende and plagioclase that are generally unaltered clear and euhedral however phenocryst poor rocks contain sieve textured plagioclase resorbed plagioclase and opacite in which hornblendes are pseudomorphed some daisen rocks contain two coexisting pyroxenes many orthopyroxene phenocrysts from two pyroxene lavas have high ca overgrowth rims up to 50 mm a feature consistent with crystallization from a higher temperature magma than the core rim compositions are similar from phenocryst to phenocryst in individual samples temperatures of 800 900 c are obtained from the cores whereas temperatures of 1000 1100 c are indicated for the rims lavas ranging from aphyric andesite 61 wt sio2 to phenocryst rich dacite 67 wt sio2 have similar sr sr 0 7045 0 7052 and nd nd 0 5127 0 5128 isotopic variability within daisen volcano is likely to be mantle derived reflecting isotopic variability within the magma source region associated with a single mantle diapir the daisen andesites and dacites have the same trace element signatures as the associated basalts and were probably derived from primary magmas at the same general depth 60 km our interpretation is that mantle derived hydrous magnesian andesite generated in the same mantle diapir as coexisting basalt magma may be parental to the hydrous calc alkaline magmas in daisen volcano we suggest a two stage process involving mid crustal solidification of bodies of this calcalkaline magma followed by varying degrees of partial melting from body to body to produce the magmatic trends and phenocryst zoning patterns observed the heat required for this melting according to our model was supplied by the intermittent rise of subjacent basaltic magma

formation of distinct granitic magma batches by partial melting of hybrid lower crust in the izu arc collision zone central japan the miocene kofu granitic complex kgc occurs in the izu collision zone where the izu bonin mariana ibm arc has been colliding with the honshu arc since the middle miocene the kgc includes rocks ranging in compositions from biotite bearing granite the shosenkyo and mizugaki plutons and hornblende biotitebearing granodiorite tonalite quartz diorite and granite the shiodaira sanpo hirose and sasago plutons to hornblende bearing tonalite and trondhjemite the ashigawa tonogi pluton indicating that it was constructed from multiple intrusions of magma with different bulk chemistry the sr isotopic compositions corrected to sensitive high resolution ion microprobe shrimp zircon ages sri suggest that the primary magmas of each pluton were formed by anatexis of mixed lower crustal sources involving both juvenile basalt of the ibm arc and shimanto sedimentary rocks of the honshu arc after the primary magmas had formed the individual plutons evolved by crystal fractionation processes without significant crustal assimilation or additional mantle contribution shrimp zircon u pb ages in the kgc range from 16 8 to 10 6ma and overlap the resumption of magmatic activity in the ibmand honshu arcs at c 17ma and the onset of ibm arc honshu arc collision at c 15ma the age of the granite plutons is closely related to the episodic activity of arc magmatism and distinct granitic magma batches could be formed by lower crustal anatexis induced by intrusion of underplated mantle derived arc magmas based on pressures determined with the al in hornblende geobarometer the kgc magmas intruded into the middle crust thus the kgc could represent an example of the middle crust layer indicated throughout the ibm arc by 6 0 6 5 km s seismic velocities this granitic middle crust layer acted buoyantly during the ibm arc honshu arc collision leading to accretion of buoyant ibmarc middle crust to the honshu arc

a kuroko type polymetallic sulfide deposit in a submarine silicic caldera manned submersible studies have delineated a large and actively growing kuroko type volcanogenic massive sulfide deposit 400 kilometers south of tokyo in myojin knoll submarine caldera the sulfide body is located on the caldera floor at a depth of 1210 to 1360 meters has an area of 400 by 400 by 30 meters and is notably rich in gold and silver the discovery of a large kuroko type polymetallic sulfide deposit in this arc front caldera raises the possibility that the numerous unexplored submarine silicic calderas elsewhere might have similar deposits

remobilization of andesite magma by intrusion of mafic magma at the soufriere hills volcano montserrat west indies the 1995 1999 eruption of the soufriere hills volcano montintroduction serrat has produced a crystal rich andesite containing quenchthe soufriere hills volcano montserrat located in the textured mafic inclusions which show evidence of having been molten lesser antilles island arc in the west indies began to when incorporated into the host magma individual crystals in the erupt on 18 july 1995 after about 350 years of dormancy andesite record diverse histories amphibole phenocrysts vary from young et al 1998b the eruption was preceded by a pristine and unaltered to strongly oxidized and pseudomorphed by period of seismic swarms which began in january 1992 anhydrous reaction products plagioclase phenocrysts are commonly previous earthquake crises have occurred on montserrat reverse zoned often with dusty sieve textures reverse zoned rims are in 1896 1897 1932 1935 and 1966 1967 powell 1938 also common on orthopyroxene phenocrysts pyroxene geothermometry shepherd et al 1971 initial phreatic and phregives an average temperature of 858 20 c for orthopyroxene atomagmatic activity was followed by extrusion of an phenocryst cores whereas reverse zoned rims record temperatures andesitic lava dome in november 1995 pyroclastic flows from about 880 to 1050 c the heterogeneity in mineral rim generated by gravitational collapse of the unstable dome compositions zoning patterns and textures is interpreted as reflecting have been the dominant form of activity throughout the non uniform reheating and remobilization of the resident magma eruption but periods of explosive activity producing body by intrusion of hotter mafic magma convective remobilization pumice flows and falls have also occurred robertson et results in mixing together of phenocrysts that have experienced al 1998 young et al 1998b dome growth ceased different thermal histories depending on proximity to the intruding suddenly in march 1998 and further activity up to may mafic magma the low temperature and high crystallinity are 1999 has mainly involved gravitational collapses of the interpreted as reflecting the presence of a cool highly crystalline dome and periods of ash venting with some small scale magma body beneath the soufriere hills volcano the petrological vulcanian explosive events observations in combination with data on seismicity extrusion rate the soufriere hills volcano is the only active centre and so2 fluxes indicate that the current eruption was triggered by on montserrat the volcano is a composite of at least recent influx of hot mafic magma five andesitic lava domes flanked by pyroclastic deposits rea 1974 identified four other volcanic centres all

origin of felsic volcanism in the izu arc intra arc rift an intra arc rift iar is developed behind the volcanic front in the izu arc japan bimodal volcanism represented by basalt and rhyolite lavas and hydrothermal activity is active in the iar the constituent minerals in the rhyolite lavas are mainly plagioclase and quartz whereas mafic minerals are rare and are mainly orthopyroxene without any hydrous minerals such as amphibole and biotite both the phenocryst and groundmass minerals have felsic affinities with a narrow compositional range the petrological and bulk chemical characteristics are similar to those of melts from some partial melting experiments that also yield dry rhyolite melts the hydrous mineral free narrow mineral compositions and low al2o3 affinities of the iar rhyolites are produced from basaltic middle crust under anhydrous low temperature melting conditions the iar basalt lavas display prominent across arc variation with depleted elemental compositions in the volcanic front side and enriched compositions in the rear arc side the across arc variation reflects gradual change in the slab derived components as demonstrated by decreasing ba zr and th zr values to the rear arc side rhyolite lavas exhibit different across arc variations in either the fluid mobile elements or the immobile elements such as nb zr la yb and chondrite normalized rare earth element patterns reflecting that the felsic magmas had different source the preexisting arc crust formed during an earlier stage of arc evolution most probably during the oligocene prior to spreading of the shikoku back arc basin the lack of systematic across arc variation in the iar rhyolites and their dry shallow crustal melting origin combines to suggest re melting of preexisting oligocene middle crust by heat from the young basaltic magmatism

volcanology and eruptive styles of barren island an active mafic stratovolcano in the andaman sea ne indian ocean barren island india is a relatively little studied little known active volcano in the andaman sea and the northernmost active volcano of the great indonesian arc the volcano is built of prehistoric possibly late pleistocene lava flows dominantly basalt and basaltic andesite with minor andesite intercalated with volcaniclastic deposits tuff breccias and ash beds deposited by pyroclastic falls and surges which are exposed along a roughly circular caldera wall there are indications of a complete phreatomagmatic tephra ring around the exposed base of the volcano a polygenetic cinder cone has existed at the centre of the caldera and produced basalt basaltic andesite aa and blocky aa lava flows as well as tephra during historic eruptions 1787 1832 and three recent eruptions 1991 1994 95 2005 06 the recent aa flows include a toothpaste aa flow with tilted and overturned crustal slabs carried atop an aa core as well as locally developed tumuli like elliptical uplifts having corrugated crusts based on various evidence we infer that it belongs to either the 1991 or the 1994 95 eruptions the volcano has recently 2008 begun yet another eruption so far only of tephra we make significantly different interpretations of several features of the volcano than previous workers this study of the volcanology and eruptive styles of the barren island volcano lays the ground for detailed geochemical isotopic and petrogenetic work and provides clues to what the volcano can be expected to do in the future

lithostratigraphic hydrothermal and tectonic setting of the boundary volcanogenic massive sulfide deposit newfoundland appalachians canada formation by subseafloor replacement in a cambrian rifted arc the boundary volcanogenic massive sulfide vms deposit 0 5 mt 3 5 cu 4 zn 1 pb 34 0 g t ag in the tally pond group central newfoundland represents one of the best preserved subseafloor replacement style vms deposits in the northern appalachian orogen the deposit is hosted within a late cambrian 510 ma volcanic sequence consisting predominantly of rhyolitic flows and associated volcaniclastic rocks footwall strata are dominated by rhyolitic lapilli tuff tuffs lesser rhyolite flows and in situ rhyolite breccias the hanging wall consists of massive quartz bearing flow banded lobe and breccia facies rhyolite the deposit occurs at and below the contact between these two units and comprises pyrite chalcopyrite and lesser sphalerite and other minerals massive mineralization contains abundant clasts of the surrounding host rocks including chlorite sericite quartz altered rhyolite lapilli and ash hydrothermally altered rocks consist of variably intense chlorite with lesser sericite and quartz chlorite alteration occurs in a discordant geometry likely representing hydrothermal upflow zones and chlorite sericite quartz occur as blankets that are parallel to the volcanic stratigraphy likely representing alteration associated with replacement the hanging wall rhyolite flows also contain moderate to intense pervasive quartz and sericite alteration both the hanging wall and footwall are characterized by strong na2o sr depletions k2o mgo fe2o3 ba enrichments high alteration index values e g ba sr chlorite carbonate pyrite index ccpi alteration index ai and enrichments in base metals and volatile metals e g cu zn pb hg the presence of abundant remnant wall rock and host rock clasts within the ore intricate sulfide replacement of laminated porous tuff and sand dikes replacement fronts in host lithofacies and intense alteration in both the footwall and hanging wall where the hanging wall is preserved are all features consistent with formation of the bulk of the boundary deposit via subseafloor replacement the deposit likely formed as a result of cooling of metal bearing hydrothermal fluids mixing with ambient seawater and pore water entrained seawater within the volcanic rocks at a permeability interface between young unlithified highly permeable footwall volcaniclastic rocks and relatively impermeable hanging wall rhyolitic flows in our model this permeability boundary was an important feature that promoted subseafloor replacement within the deposit immobile element systematics of rhyolitic rocks from the boundary deposit lack major differences in primary petrochemistry between hanging wall and footwall strata all rocks from the deposit area are subalkalic with transitional zr y ratios 2 8 4 5 la sm ratios 1 normalized to upper crust and primitive mantle normalized signatures with slightly light rare earth element lree enriched patterns with flat heavy ree hree and negative nb ti and eu anomalies these geochemical features coupled with existing nd pb isotope data zircon inheritance patterns and geologic information are consistent with rhyolitic rocks at the boundary deposit having formed by re melting of arc basement with continental crust or recycled continental crust present in the source region it is likely that the deposit and its associated rhyolitic host rocks formed within a cambrian continental or pericontinental rifted arc along the margin of ganderia within the iapetus ocean

recent remobilisation of shallow level intrusions on montserrat revealed by hydrogen isotope composition of amphiboles abstract ion probe measurements of hydrogen isotopes were made on amphiboles representing different stages of the ongoing eruption of the soufriere hills volcano montserrat the majority 80 of the andesitic amphiboles show relative intra and inter crystal d homogeneity with a mean of 38 12 consistent with known primary magmatic values the remainder 20 of amphiboles show marked d heterogeneity with a mean of 6 30 the heterogeneous amphiboles have 100 m rims with d values similar to the homogeneous crystals but core values which are significantly heavier than primary magmatic values early in the eruption 50 of amphiboles are isotopically heterogeneous but post spring 1996 all amphiboles are homogeneous we interpret these ion probe data in terms of development of a shallow andesitic magma chamber by repeated emplacement of andesitic magma in the shallow crust over at least the last 100 yr we suggest such shallow intrusions emplaced during the volcano seismic crises preceding the ongoing eruption solidified some regions interacted with hydrothermal fluids and isotopic exchange took place generating the heavy hydrogen isotope signatures in the heterogeneous amphiboles on onset of the ongoing eruption such old igneous material was remobilised with intimate mixing between the new magma batch and the old intrusions indicated by the close proximity of homogeneous and heterogeneous crystals the isotopically primary magmatic rims on the heterogeneous crystals indicate hydrogen isotope exchange over a period of a few weeks consistent with timescales of magma ascent at the soufriere hills volcano once the old igneous material was flushed out by spring 1996 the eruption proceeded by extruding material dominated by the new batch of magma this resulted in a marked increase in extrusion rate

the early miocene 25 ma volcanism in the northern kyushu palau ridge enriched mantle source injection during rifting prior to the shikoku backarc basin opening the northern kyushu palau ridge kpr remnant conjugate arc of the izu ogasawara bonin mariana ibm active arc is dominated by basalt andesite except for the komahashi daini seamount where acidic plutonic rocks of 38 ma were recovered these mafic to intermediate volcanics are produced by the rifting volcanism in the proto ibm arc associated with spreading of the shikoku basin the hfse and hree contents and ratios of these volcanics indicate enriched source mantle composition compared to recent volcanic front the lile ratios exhibit similar characteristics to reararc volcanism of the recent izu arc and some enriched volcanics exhibit high abundance of sediment melt inputs based on these observations and compilations of the published data set the replacement event of the wedge mantle under the ibm arc occurred two times the first event occurred between 45 and 38 ma with pacific type mantle being replaced by depleted indian type mantle the second event occurred between 36 and 25 ma enriched mantle flowed from reararc side the slab component during the proto ibm arc rifting was a similar characteristic to recent reararc volcanism of the izu arc and sediment melt added in a local area

wet and dry basalt magma evolution at torishima volcano izu bonin arc japan the possible role of phengite in the downgoing slab the arc front volcanoes of sumisu 31 58n 1408e andtorishima 30 58n 140 38e in the central izu bonin arc are similar in size and rise as relatively isolated edifices from the seafloor together they provide valuable along arc information about magma generation processes the volcanoes have erupted low k basalts originating from both wet and dry parental basaltic magmas low zr basalts and high zr basalts respectively based on models involving fluid immobile incompatible element ratios la sm the parental basalts appear to result from different degrees of partial melting of the same source mantle 20 and 10 for wet and dry basalt magmas respectively assuming that the wet basalts contain greater abundances of slab derived components than their dry counterparts geochemical comparison of these two basalt types permits the identification of the specific elements involved in fluid transport from the subducting slab using an extensive set of new geochemical data fromtorishima where the top of the downgoing slab is about 100 km deep we find that cs pb and sr are variably enriched in the low zr basalts which cannot be accounted for by fractional crystallization or by differences in the degree of mantle melting these elements are interpreted to be selectively concentrated in slab derived metasomatic fluids variations in k high field strength element and rare earth element concentrations are readily explained by variations in the degree of melting between the lowand high zr basalts these elements are not contained in the slab derived fluids rb and ba exhibit variable behaviour in the low zr basalts ranging from immobile similar to k to mildly enriched in some low zr basalts we suggest that the k rich mica phengite plays an important role in determining the composition of fluids released from the downgoing slab in arcfront settings where slab depth is 100 km phengite is stable and the fluids released from the slab contain little k in back arc settings however where the slab is at 100 140 km depth phengite is unstable and k rich fluids are released we conclude that cross arc variations in the k content of arc basalts are probably related to differing compositions of released fluids or melts rather than the widely held view that such variations are controlled by the degree of partial melting

seismological evidence for variable growth of crust along the izu intraoceanic arc 1 the processes that create continental crust in an intraoceanic arc setting are a matter of debate to address this issue we conducted an active source wide angle seismic study to examine along arc structural variations of the izu intraoceanic arc the data used were acquired over a 550 km long profile along the volcanic front from sagami bay to tori shima the obtained structural model showed the existence of felsic to intermediate composition middle crust with a p wave velocity vp of 6 0 6 5 km s 1 in its upper part and 6 5 6 8 km s 1 in its lower part the thickness of the middle crust varied markedly from 3 to 13 km the underlying lower crust also consisted of two layers vp of 6 8 7 2 km s 1 in the upper part and vp of 7 2 7 6 km s 1 in the lower part the upper of these layers was interpreted to consist of plutonic gabbro and the lower layer was interpreted to be mafic to ultramafic cumulates average crustal velocities calculated from our model showed remarkable lateral variation which correlated well with arc volcanism low average crustal seismic velocities 6 7 km s 1 due to thick middle crust were obtained beneath basaltic volcanoes e g o shima miyake jima hachijo jima aoga shima while higher average velocities 7 1 km s 1 were obtained beneath rhyolitic volcanoes e g nii jima kurose south hachijo caldera myoji knoll and south sumisu caldera we concluded from these observations that continental crust grows predominantly beneath the basaltic volcanoes of the izu arc and that rhyolitic volcanism may be indicative of a more juvenile stage of crustal evolution or remelting of preexisting continental crust or both

petrogenesis of volcanic rocks from saipan and rota mariana islands and implications for the evolution of nascent island arcs an 40 ar 39 ar age of 45 1 ma determined for lavas from northern saipan confirms that these high silica rhyolites erupted during the proto arc stage of volcanism in the izu bonin mariana system which is characterized elsewhere by eruption of boninitic lavas incompatible trace element concentrations and sr hf nd and pb isotope ratios for these rhyolites are transitional between those of c 48 ma boninitic lavas and post 38 ma first arc andesites and dacites from saipan and rota that have typical subduction related compositions these transitional compositions are modeled by crystal fractionation of parental tholeiitic basalt combined with assimilation of young boninitic crust a second stage of rayleigh fractionation in the upper crust is required by sio 2 concentrations that exceed 77 wt and near zero compatible element concentrations first arc magma compositions are consistent with fractionation of basalt and assimilation of crust similar in composition to the first arc magmas themselves the mantle sources of the proto arc and first arc lavas from saipan and rota are similar to those of philippine back arc basin basalts based on nd and hf isotopic compositions the pb isotope compositions of these lavas are between those of pacific sea floor basalts and jurassic and younger cherty and clay rich sediments this contrasts with the boninitic proto arc volcanic rocks from guam and deep sea drilling project sites 458 and 459 that have pb isotope compositions similar to pacific basin basalts and volcaniclastic sediments the preferred explanation for the difference in the nature of proto arc volcanism between saipan and other fore arc locations is that the crust ceased extending 3 4 myr earlier beneath saipan this was caused by a change from mantle upwelling fore arc extension and shallow melting to an environment dominated by more normal mantle wedge convection stable crust and deeper melting

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

climate variability in the spanish pyrenees during the last 30 000 yr revealed by the el portalet sequence abstract palynological sedimentological and stable isotopic analyses of carbonates and organic matter performed on the el portalet sequence 1802 m a s l 42 48 00 n 0 23 52 w reflect the paleoclimatic evolution and vegetation history in the central western spanish pyrenees over the last 30 000 yr and provide a high resolution record for the late glacial period our results confirm previous observations that deglaciation occurred earlier in the pyrenees than in northern european and alpine sites and point to a glacial readvance from 22 500 to 18 000 cal yr bp coinciding with the global last glacial maximum the patterns shown by the new high resolution pollen data from this continental sequence chronologically constrained by 13 ams 14c dates seem to correlate with the rapid climate changes recorded in greenland ice cores during the last glacial interglacial transition abrupt events observed in northern latitudes heinrich events 3 to 1 oldest and older dryas stades intra allerod cold period and 8200 cal yr bp event were also identified for the first time in a lacustrine sequence from the central western pyrenees as cold and arid periods the coherent response of the vegetation and the lake system to abrupt climate changes implies an efficient translation of climate variability from the north atlantic to mid latitudes

the setting style and role of magmatism in the formation of volcanogenic massive sulfide deposits throughout earth s history all volcanogenic massive sulfide vms hosting environments are associated with specific assemblages of mafic and felsic rocks with distinct petrochemistry petrochemical assemblages indicative of formation at anomalously high temperatures within extensional geodynamic environments in mafic dominated juvenile ophiolitic vms environments there is a preferential association with mafic rocks with boninite and low ti tholeiite mid ocean ridge basalt morb and or back arc basin basalt affinities representing forearc rifting or back arc initiation mid ocean ridges or back arc basin spreading or back arc basins respectively felsic rocks in juvenile oceanic arc environments in archean terrains are high field strength element hfse and rare earth element ree enriched in post archean juvenile oceanic arc terrains felsic rocks are commonly hfse and ree depleted and have boninite like to tholeiitic signatures in vms environments that are associated with continental crust i e continental arc and back arc and dominated by felsic volcanic and or sedimentary rocks evolved environments felsic rocks are the dominant hosts to mineralization and are generally hfse and ree enriched with calc alkalic a type and or peralkalic affinities representing continental arc rifts continental back arcs and continental back arcs to continental rifts respectively coeval mafic rocks in evolved environments have alkalic within plate ocean island basalt like and morb signatures that represent arc to back arc rift versus back arc spreading respectively the high temperature magmatic activity in vms environments is directly related to the upwelling of mafic magma beneath rifts in extensional geodynamic environments e g mid ocean ridges back arc basins and intra arc rifts underplated basaltic magma provides the heat required to drive hydrothermal circulation extensional geodynamic activity also provides accommodation space at the base of the lithosphere that allows for the underplated basalt to drive hydrothermal circulation and induce crustal melting the latter leading to the formation of vms associated rhyolites in felsic dominated and bimodal vms environments rifts also provide extensional faults and the permeability and porosity required for recharge and discharge of vms related hydrothermal fluids rifts are also critical in creating environments conducive to preservation of vms mineralization either through shielding massive sulfides from seafloor weathering and mass wasting or by creating environments conducive to the precipitation of subseafloor replacement style mineralization in sedimented rifts subvolcanic intrusions are also products of the elevated heat flow regime common to vms forming environments shallow level intrusive complexes i e within 1 3 km of the seafloor may not be the main drivers of vms related hydrothermal circulation but are likely the manifestation of deeper seated mantle derived heat i e 3 10 km depth that drives hydrothermal circulation these shallower intrusive complexes are commonly long lived i e millions of years and reflect a sustained thermally anomalous geodynamic environment such a thermally anomalous environment has the potential to drive significant hydrothermal circulation and therefore multi phase long lived subvolcanic intrusive complexes are excellent indicators of a potentially fertile vms environment the absence of intrusive complexes however does not indicate an area of low potential as they may have been moved or removed due to post vms tectonic activity in some cases shallow level intrusive systems contribute metals to the vms hydrothermal system

seismic structure of the upper mantle beneath the philippine sea from seafloor and land observation implications for mantle convection and magma genesis in the izu bonin mariana subduction zone abstract we have determined the three dimensional shear wave speed structure of the upper mantle in and around the philippine sea region using seismograms recorded by dense land based and long term broadband ocean bottom seismographic stations we used a surface wave tomography technique in which multimode phase speeds are measured and inverted for a 3 d shear wave speed structure by incorporating the effects of a finite frequency and ray bending the new ocean bottom data provided us with improved spatial resolution 300 km in the philippine sea region in the upper 120 km the shear wave speed structure is well correlated with seafloor age at depths greater than 160 km fast anomalies of the subducting pacific plate are clearly defined we also found slow speed anomalies beneath the kyushu palau ridge at depths greater than 120 km along the izu bonin ogasawara mariana arc we have detected three separate slow anomalies in the mantle wedge at depths shallower than 100 km beneath the back arc each anomaly has a width of 500 km moreover these three anomalies have a close relationship with the three groups of frontal and rear arc volcanoes having distinct sr nd and pb isotope ratios we suggest that each of the anomalies is a site of large scale flow of upper mantle into the mantle wedge and that each already contains a component from the adjacent subducting slab

field and geochemical constraints on mafic felsic interactions and processes in high level arc magma chambers an example from the halfmoon pluton new zealand the 140 ma halfmoon pluton of stewart island new zealand provides direct evidence for a number of physico chemical processes that operate at depth within active arc settings it is characterized by a sequence of mingled mafic sheets and enclaves that are preserved within intermediate felsic host rocks way up structures and textures are consistent with a younging direction to the south and allow a south dipping magma chamber stratigraphy to be constructed the similarity in modal mineralogy geochemistry and isotopic signature between the various components and their proximity in space and time indicates that the system can be considered as a single composite pluton with discrete portions active at different times interpretation of mingling structures and textures in the field combined with detailed geochemical and geochronological analyses has allowed the identification of several physical and chemical processes operating within the chamber including 1 multiple mafic magma pulses that were extensively mingled and sometimes physically mixed at the exposed level and at depth 2 the development of mineral fabrics e g aligned plagioclase and hornblende crystals and alignment of mafic enclaves caused by processes of crystal accumulation magmatic flow and compaction 3 shortening of the chamber during crystallization as a result of magmatic loading 4 fractional crystallization of the intermediate felsic host magma that was interrupted periodically by mafic magma intrusion 5 incremental assembly of the pluton from the base up 6 limited chemical transfer between the mingled mafic sheets and enclaves and the surrounding intermediate felsic host rocks variations in morphology chemistry and texture of the mingled mafic rocks relative to their position within the magmatic stratigraphy indicate that processes operating within the chamber varied in both space and time mineral assemblages chemical characteristics primitive isotopic signatures and a lack of zircon inheritance indicate that the amphibole rich calc alkaline halfmoon pluton was emplaced into a juvenile arc setting and experienced no contamination with ancient crustal materials a model of pluton construction and evolution is presented that shows that the halfmoon pluton consisted of at least two adjacent magma pods that grew incrementally by episodic replenishments of mafic magma into a magma chamber of evolving intermediate felsic composition data are consistent with a model whereby hydrous amphibole rich basaltic magmas ponded at the crust mantle interface and episodically rose injected and mingled

high silica rhyolites of niijima volcano in the northern izu bonin arc japan petrological and geochemical constraints on magma generation and supply abstract niijima volcano in the northern part of the quaternary izu bonin volcanic arc japan consists predominantly of rhyolitic volcanic units associated with minor basaltic and andesitic units most of the rhyolites are characterized by a phenocryst assemblage of orthopyroxene opx cummingtonite cum biotite bt and hornblende which are classified into four types opx cum cum cum bt and bt in eruption order the major element compositions of the rhyolites are typified by high sio2 73 78 wt and k2o 1 5 3 3 wt contents and the trace elements are indicative of a highly differentiated nature which differs significantly from rhyolites at other sites in the izu bonin arc the presence of cummingtonite phenocrysts and mineralogical and whole rock chemical compositions of the rhyolite lavas clearly indicate that these magmas were produced under low temperatures 13 units in a shallow magma system at the same time and at deeper levels basaltic and rhyolitic parental magmas mixed to produce andesitic magmas between the basaltic and rhyolitic parental magmas in some cases basaltic and andesitic magmas in deeper level ascended and mingled with shallow rhyolitic magmas before eruption the eruption of rhyolitic magmas from shallow levels might have been triggered by the injection of basaltic and andesitic magmas from deeper levels the complex magmatic system beneath niijima volcano formed within a relatively thick crustal sequence on the rear arc side of the izu bonin volcanic arc

a study of rare earth element ree sio 2 variations in felsic liquids generated by basalt fractionation and amphibolite melting a potential test for discriminating between the two different processes the origin of felsic magmas 63 sio2 in intra oceanic arc settings is still a matter of debate two very different processes are currently invoked to explain their origin these include fractional crystallization of basaltic magma and partial melting of lower crustal amphibolite because both fractionation and melting can lead to similar major element trace element and isotopic characteristics in felsic magmas such lines of evidence have been generally unsuccessful in discriminating between the two processes a commonly under appreciated aspect of rare earth element ree solid liquid partitioning behavior is that d ree for most common igneous minerals especially hornblende increase significantly with increasing liquid sio2 contents for some minerals e g hornblende and augite ree partitioning can change from incomptatible d 1 at high liquid sio2 when this behavior is incorporated into carefully constrained mass balance models for mafic basaltic amphibolite melting intermediate andesitic amphibolite melting lower or mid to upper crustal hornblende present basalt fractionation and mid to upper crustal hornblende absent basalt fractionation the following general predictions emerge for felsic magmas e g 63 to 76 sio2 partial melting of either mafic or intermediate amphibolite should regardless of the type of melting equilibrium fractional accumulated fractional yield ree abundances that remain essentially constant and then decrease or steadily decrease with increasing liquid sio2 content at high liquid sio2 contents lree abundances should be slightly enriched to slightly depleted i e c l c o 2 to 0 2 while hree abundances should be slightly depleted c l c o 1 to 0 2 lower crustal hornblende bearing basalt fractionation should yield roughly constant ree abundances with increasing liquid sio2 and exhibit only slight enrichment c l c o 1 2 mid to upper crustal hornblende bearing basalt fractionation should yield steadily increasing lree abundances but constant and then decreasing hree abundances at high liquid sio2 contents lree abundances may range from non enriched to highly enriched c l c o 1 to 5 while hree abundances are generally non enriched to only slightly enriched c l c o 1 to 2 hornblende absent basalt fractionation should yield steadily increasing ree abundances with increasing liquid sio2 contents at high sio2 contents both lree and hree are highly enriched c l c o 3 to 4 it is proposed that these model predictions constitute a viable test for determining a fractionation or amphibolite melting origin for felsic magmas in intra oceanic arc environments where continental crust is absent

missing oligocene crust of the izu bonin arc consumed or rejuvenated during collision the 50 myr old izu bonin mariana ibm arc consists mostly of oligocene middle and lower crust that underlies the upper crust these units are in turn covered by quaternary volcanic rocks seismic imaging forearc geology ocean drilling program drilling and magnetic anomalies suggest that most ibm arc crust was created in eocene oligocene times however remnants of this old crust have never been found at the northern end of the arc where it is colliding with the honshu arc izu collision zone two batholiths in this collision zone thetanzawa tonalites and the kofu granitic complex were emplaced during the miocene 4 17 ma major elements zr y rare earth element ratios and normalized abundance patterns and sr nd isotopic data indicate that these plutonic bodies are compositionally similar to the oligocene ibm volcanic rocks and that they are dissimilar to the miocene pliocene and quaternary ibm lavas and volcaniclastic rocks we suggest that the miocene plutonic rocks in the izu collision zone were derived from partially melted oligocene middle crust a model is proposed in which ibm arc middle crust in the collision zone was partially melted during the collision and then intruded into the overlying upper crust of the honshu and ibm arcs this resulted in the complete loss of chronological information related to their original source

geochemical differentiation processes for arc magma of the sengan volcanic cluster northeastern japan constrained from principal component analysis abstract in this study with a view of understanding the structure of high dimensional geochemical data and discussing the chemical processes at work in the evolution of arc magmas we employed principal component analysis pca to evaluate the compositional variations of volcanic rocks from the sengan volcanic cluster of the northeastern japan arc we analyzed the trace element compositions of various arc volcanic rocks sampled from 17 different volcanoes in a volcanic cluster the pca results demonstrated that the first three principal components accounted for 86 of the geochemical variation in the magma of the sengan region based on the relationships between the principal components and the major elements the mass balance relationships with respect to the contributions of minerals the composition of plagioclase phenocrysts geothermal gradient and seismic velocity structure in the crust the first the second and the third principal components appear to represent magma mixing crystallizations of olivine pyroxene and crystallizations of plagioclase respectively these represented 59 20 and 6 respectively of the variance in the entire compositional range indicating that magma mixing accounted for the largest variance in the geochemical variation of the arc magma our result indicated that crustal processes dominate the geochemical variation of magma in the sengan volcanic cluster

low δ18o magmas in the carboniferous intra oceanic arc central tibet implications for felsic magma generation and oceanic arc accretion abstract low 18o magmas 18ozircon pb dating indicates that they were formed simultaneously at 350 ma slightly younger than the early carboniferous ophiolites 355 ma the forearc ophiolites and volcanic rock assemblages constitute an intra oceanic subduction system evidenced by their similar sr nd isotopes sims zircon o isotope analyses indicate that the felsic rocks have 18ozircon values similar to or lower than that of the mantle the dacite and trondhjemite samples have homogeneous 18ozircon values of 5 1 0 4 2sd and 4 6 0 5 2sd respectively the tonalite samples exhibit a large range of 18ozircon values from 2 7 to 5 0 most of which are lower than the mantle like value of 5 3 0 6 2sd we suggest that the dacites and trondhjemites were produced by partial melting of unaltered lower crust whereas the low 18o tonalites originated by melting of hydrothermally altered lower crust and both were generated in the early stage of this intra oceanic arc this process resulted in a weak middle arc crust of intermediate composition by allowing decoupling of upper and lower arc crust this plays an important role in the accretion of intra oceanic arcs to continents and ultimately in the crustal growth

silicic ignimbrites within the costa rican volcanic front evidence for the formation of continental crust abstract the origin of silicic magmas greater than 65 silica in areas without old evolved continental crust has long been an enigma in oceanic arcs silicic volcanic deposits are not common which has generally been attributed to subduction related magmas rising without stalling or fractionating through a relatively thin mafic oceanic crust the volcanic arc in costa rica is built on a thick 40 km oceanic plateau and extensive silicic magmatism dominated by silicic ignimbrites has occurred in the region in the absence of old evolved continental crust the silicic deposits display the common large ion lithophile element lile enrichment and high field strength element hfse depletion observed in magmas generated by subduction processes which are not characteristic of melts generated from oceanic plateaus key trace element ratios in the ignimbrites vary with position along the volcanic front and this variation is similar to that of the volcanic front lavas ignimbrites contain unique cumulative frequency distributions of incompatible trace element ratios that demonstrate different magma batches for each unit we propose that the caribbean oceanic plateau in costa rica is being converted to continental crust in large part by the emplacement of silicic magmas in this area the origin of these silicic magmas is due to reprocessing juvenile mantle derived subduction related magmas that have ponded in the crust

tectonomagmatic development of the eocene pasevh pluton nw iran implications for the arabia eurasia collision abstract the pasveh pluton is one of a number of intrusive complexes in the northwestern part of the sanandaj sirjan zone ssz nw iran the major part of this complex is composed of high k calc alkaline granitic rocks the geochemical data indicate that the granitic rocks although compositionally similar are not cogenetic and can be divided into three groups the first group mesocratic granitoids is represented by the largest outcrop in the pasveh pluton it is mainly composed of metaluminous magnesian monzodiorite to granodiorite with abundant mme and gabbroic enclaves zircon la icp ms u pb dating yielded eocene crystallization ages ca 40 ma for this group the monzodiorite endt 2 0 to 2 5 granodiorite endt 1 5 to 1 6 mme and gabbroic enclave endt 3 3 to 4 7 have distinct nd isotopic compositions the second group is composed of eocene 39 7 0 6 ma weakly peraluminous monzogranites that have carboniferous and cretaceous inherited zircons leucocratic granitic dikes of adakitic composition comprise the third group of pasveh silicic rocks the dikes are slightly younger 37 5 0 3 ma than host rocks and cross cut the mesocratic granitoids our results indicate that the underplating of basaltic magmas related to extensional stress in the overlying iranian plate during the eocene induced partial melting of the lower crust differences in the amount of partial melting of the source rocks are likely responsible for the volume and heterogeneity of their melt compositions subsequent to melt generation it is possible that the silicic magmas mixed with mafic magmas in variable proportions to produce the hybridized rocks and enclaves the intrusion of leucogranite dikes after the emplacement of the pasveh pluton suggests that during or slightly after neo tethys subduction there was another period of magmatism that was likely related to the melting of the subducted slab

sumisu volcano izu bonin arc japan site of a silicic caldera forming eruption from a small open ocean island sumisu volcano was the site of an eruption during 30 60 ka that introduced 48 50 km3 of rhyolite tephra into the open ocean environment at the front of the izu bonin arc the resulting caldera is 8 10 km in diameter has steep inner walls 550 780 m high and a floor averaging 900 m below sea level in the course of five research cruises to the sumisu area a manned submersible two rovs a deep tow camera sled and dredge samples were used to study the caldera and surrounding areas these studies were augmented by newly acquired single channel seismic profiles and multi beam seafloor swath mapping caldera wall traverses show that pre caldera eruptions built a complex of overlapping dacitic and basaltic edifices that eventually grew above sea level to form an island about 200 m high the caldera forming eruption began on the island and probably produced a large eruption column we interpret that prodigious rates of tephra fallback overwhelmed the sumisu area forming huge rafts of floating pumice choking the nearby water column with hyperconcentrations of slowly settling tephra and generating pyroclastic gravity currents of water saturated pumice that traveled downslope along the sea floor thick compositionally similar pumice deposits encountered in odp leg 126 cores 70 km to the south could have been deposited by these gravity currents the caldera rim presently at ocean depths of 100 400 m is mantled by an extensive layer of coarse dense lithic clasts but syn caldera pumice deposits are only thin and locally preserved the paucity of syn caldera pumice could be due to the combined effects of proximal non deposition and later erosion by strong ocean currents post caldera edifice instability resulted in the collapse of a 15 sector of the eastern caldera rim and the formation of bathymetrically conspicuous wavy slump structures that disturb much of the volcano s surface

spectroscopic ftir imaging of water species in silicic volcanic glasses and melt inclusions an example from the izu bonin arc recent advances in ftir spectroscopic imaging allow images of elements or molecules such as water to be constructed on samples over a large sub mm area the imaging detector consists of 64 64 channels providing 4096 individual spectra simultaneously collected over a spectral range of 4000 900 cm 1 the wavelength range includes measurable peaks for water and carbon species making it ideal for the analysis of volcanic glasses particularly melt inclusions advantages of spectroscopic imaging include high spatial resolution 5 m allowing the analysis of melt inclusions too small to be analysed by other micro ftir techniques and the acquisition of high definition in situ images of the materials analysed the technique is applied to the analysis of water species concentrations and distribution in silicic volcanic glasses from sumisu caldera and torishima volcano in the izu bonin arc to test the application of imaging to geologic samples results obtained are found to be comparable to analyses by conventional micro ftir techniques withsimilarprecision butgreaterspatialresolution groundmassglassesfromsumisucalderadacitescontain1 2 1 4wt h2o and an obsidian sample from torishima volcano contains 0 2 wt h2o consistent with water saturation at their respective pressures of eruption melt inclusions in honeycomb plagioclase from sumisu caldera have higher water contents 1 02 1 38 wt h2o than inclusions from torishima volcano plagioclase 0 22 0 41 wt h2o despite evidence for many inclusions being trapped wholly within honeycomb plagioclase low water contents similar to those of the groundmass glasses suggest that the inclusions underwentwaterlossbydegassingand or diffusion alternatively iflowwatercontents ofthemeltinclusionsreflectthecompositionof the magma at the time of entrapment then the magma can be constrained as having originated from a water poor source despite the ambiguity in initial water contents of the magma the anhydrous mineral assemblage of all sumisu caldera and torishima silicic rocks indicates anoriginfrom awater poorsource consistent withmodelsfor partialmeltingofwater poor mafic lower mid crust ratherthan melting of wet lower mid crust or differentiation from basaltic magmas

geology alteration and lithogeochemistry of the hood volcanogenic massive sulfide vms deposits nunavut canada the hood volcanogenic massive sulfide vms deposits are hosted by the 2 68 ga amooga booga volcanic belt abvb in the northwestern archaen slave craton and consist of three deposits hood 10 41 and 41a and three occurrences 46 461 and 462 the mineralized zones consist of massive to semi massive pyrrhotite pyrite chalcopyrite sphalerite and galena hosted predominantly by felsic volcanic flows within the predominantly mafic abvb the mineralized lenses occur at different stratigraphic levels and have textural alteration and stratigraphic features consistent with formation via subseafloor replacement the felsic volcanic rocks in the hood deposits can be subdivided into groups based on immobile trace element geochemistry the main felsic types a and b are petrographically indistinguishable type a has higher high field strength element hsfe and rare earth element ree contents than type b suggesting a higher temperature of formation type a rocks also have higher nb ta values indicative of a greater mantle input in their genesis compared to type b rocks mineralization is more closely associated with type a than type b rocks the two mafic volcanic rock types previously identified in the abvb type i and type ii both occur within the hood deposits the type ii mafic group is interpreted to be the result of variable crustal contamination of type i magma the volcanic rocks of the abvb are interpreted to have formed in a continental margin arc back arc setting the genesis of these magmatic suites involved magmatic underplating and emplacement through pre existing sialic basement that resulted in crustal melting mantle crust mixing and contamination leading to the aforementioned geochemical features in both mafic and felsic suites this type of extensional tectonic environment was likely associated with high heat flow and is similar to global vms environments proximal to extending continental margins e g sturgeon lake bathurst and finlayson lake the similarities of the abvb to other slave craton greenstone belts further highlights the overall potential for greenstone hosted vms mineralization in the slave craton chlorite sericite quartz is the dominant hydrothermal alteration assemblage in the hood deposits and is typical of vms style mineralization mass change calculations illustrate that elemental changes are typical of vms environments with gains in fe2o3 mgo and base metals associated with chlorite alteration near mineralized zones k2o gains associated with sericite alteration and losses of na2o in both alteration types

origin of granites in an archean high grade terrane southern india archean deep level granites in southern india are similar geochemically to young granites from continentalmargin arc systems they exhibit light ree enriched patterns with variable but chiefly positive eu anomalies this is in striking contrast to the negative eu anomalies typical in high level archean granites in addition the deep level granites are relatively enriched in ba and sr and depleted in total ree and high field strength elements hfse one pluton the sankari granite has unusually low contents of ree and hfse most of the deep level granites appear to represent cumulates with variable amounts of trapped liquid and of minor phases resulting from fractional crystallization of a granitic parent such parental granitic magmas can be produced by batch melting of archean tonalite at middle to lower crustal depths the sankari granite requires a tonalitic source depleted in ree and hfse archean tonalites and tonalitic charnockites exhibit original igneous geochemical signatures and their average composition does not show a significant eu anomaly hence they cannot represent the positive eu anomaly complement to the negative eu anomaly high level granites our results suggest that archean deep level granites may represent this complement such granite may form in waterrich zones in the middle or lower crust and be produced in response to dehydration of the lower crust by a rising co2 rich fluid phase

silicic magmas in the izu bonin oceanic arc and implications for crustal evolution abundant rhyolitehaseruptedfromthe izu bonin marianavolca nic arc ibm arc from its earliest stage eocene to the present geochemically three types of quaternary rhyolites exist in the izu bonin arcfront and theyare closely related tovolcanotypeand crus tal structure the dominantly basaltic islands of the volcanic frontproduce small volumes of rhyolites that we call r1 the submarinecalderas of the volcanic front erupt mostly rhyolite that we call r2 seamounts knolls and pillow ridges in thebackarcextensionalzoneare mostly basaltic but also contain rhyolites that we call r3 thethickest total crust and the thickest intermediate compositionmiddle crust occurs below the dominantly basaltic volcanoes whereas the intermediate composition middle crust tends to be thin nerbeneaththesubmarinecalderas r1rhyolitesmaybederivedfromquaternary andesitic sources whereas r2 and r3 rhyolites may bederivedfromoligoceneones thehighercao al

are arc type rocks the products of magma crystallisation observations from a simple oceanic arc volcano raoul island kermadec arc sw pacific raoul island is the emergent summit of a large intra oceanic strato volcano in what is globally one of the simplest of subduction settings in this simple setting erupted magmas span the compositional range from basalt to dacite but none have the high mg numbers and high ni and cr expected of primitive mantle derived melts the lavas range from aphyric to highly porphyritic and are characterised by phenocryst assemblages dominated by plagioclase accompanied by clinopyroxene olivine orthopyroxene and spinel phenocryst core compositions and zoning profiles are remarkably uniform irrespective of total phenocryst content or geochemical composition indicating a decoupling of melt and crystal components in the system a consistent model for the raoul magmatic system is that primitive high mg magma generated in a melt column within the underlying mantle wedge is transformed into a series of derivative low mg magmas by fractional crystallisation within the lower crust low mg magma accumulates variable quantities of crystal cargo as it ascends toward the surface through a crystal mush zone these processes are essentially those that characterise continental subduction related magmatic systems but differ only in the absence of an evolved crustal component

petrogenesis of the rambler rhyolite formation controls on the ming vms deposit and geodynamic implications for the taconic seaway newfoundland appalachians canada the ming cu zn ag au volcanogenic massive sulfide vms deposit is hosted by the upper cambrian lower ordovician rambler rhyolite formation which consists of a folded northeast plunging felsic dome complex developed in the uppermost segment of the obducted supra subduction baie verte oceanic tract in the north central newfoundland appalachians the deposit is overlain by interstratified mafic volcanic flows and volcaniclastic rocks of the snooks arm group the upper 1 km of the rambler rhyolite formation consists of a coherent felsic lithofacies at its base overlain by a quartz bearing volcaniclastic dominated lithofacies units 1 2 and 1 3 geochemically these rocks are light rare earth element lree and large ion lithophile element lile enriched la yb pn 4 20 th 2 58 4 05 ppm and high field strength element hfse depleted y 5 64 6 59 ppm rhyodacite with calk alkalic affinities th yb 2 zr y 7 the last pulse of felsic volcanism unit 1 3 is intimately associated with the massive sulfide lenses and has fi and fii type rhyolite signatures high la yb ratios commonly attributed to deep 30 km crustal melting the end t values of the rhyolites range from 2 5 to 1 3 indicating the rocks were influenced by crustal material a 10 percent batch partial melt of a garnet amphibole metamorphosed normal mid oceanic ridge basalt n morb and island arc tholeiite iat crustal material can generate a melt similar to the felsic rocks hosting the ming deposit however the iat reproduces much better the absolute abundances of lile th sr hfse nb zr y and middle rare earth elements mree of the felsic rocks it is interpreted that the felsic melt came from melting of a subducted slab with iat affinity thus explaining the depth of melt generation and fi fii signatures of the felsic rocks moreover the slab derived siliceous melt may also have contributed magmatophile elements for example ag au se te sn sb hg into the hydrothermal system once it had reached near surface depths and may explain the epithermal suite element enrichment in the ming deposit this study shows that strongly fractionated felsic volcanic rocks associated with boninites such as those associated with the ming vms deposit can be important hosts for precious metal enriched vms and may be as prospective as tholeiitic felsic volcanic sequences for example fiii fiv type rhyolite low la yb ratios the base of the snooks arm group is comprised of spatially restricted sulfide bearing mafic breccia overlain by a regionally extensive sedimentary sequence end t 3 1 to 5 5 which is in turn overlain by interstratified high mg basalt end t 1 6 th enriched back arc basin basalt babb enriched mid oceanic ridge basalt e morb end t 6 6 and lree enriched low ti tholeiitic tuffs end t 0 5 four generations of mafic to intermediate dikes and sills cross cut the rambler rhyolite formation and share similar geochemical characteristics to the extrusive rocks of the snooks arm group suggesting comagmatic relationships the combination of variations in nb yb th yb and end t values within and between units suggest melts derived from depleted to enriched mantle material with melts synchronously generated by both slab metasomatized mantle wedge and upwelling back arc asthenosphere

remelting of an andesitic crust as a possible origin for rhyolitic magma in oceanic arcs an example from the izu bonin arc

tephrostratigraphy and provenance from iodp expedition 352 izu bonin arc tracing tephra sources and volumes from the oligocene to recent provenance studies of widely distributed tephras integrated within a well defined temporal framework are important to deduce systematic changes in the source scale distribution and changes in regional explosive volcanism here we establish a robust tephro chronostratigraphy for a total of 157 marine tephra layers collected during iodp expedition 352 we infer at least three major phases of highly explosive volcanism during oligocene to pleistocene time provenance analysis based on glass composition assigns 56 of the tephras to a japan source including correlations with 12 major and widespread tephra layers resulting from individual eruptions in kyushu central japan and north japan between 115 ka and 3 5 ma the remaining 101 tephras are assigned to four source regions along the izu bonin arc one of exclusively oligocene age is proximal to the bonin ridge islands two reflect eruptions within the volcanic front and back arc of the central izu bonin arc and a fourth region corresponds to the northern izu bonin arc source first order volume estimates imply eruptive magnitudes ranging from 6 3 to 7 6 for japan related eruptions and between 5 5 and 6 5 for ibm eruptions our results suggest tephras between 30 and 22 ma that show a subtly different izu bonin chemical signature compared to the recent arc after a 11 m y gap in eruption tephra supply from the izu bonin arc predominates from 15 to 5 ma and finally a subequal mixture of tephra sources from the palaeo honshu and izu bonin arcs occurs within the last 5 ma

oxygen isotope geochemistry of the lassen volcanic center california resolving crustal and mantle contributions to continental arc magmatism this study reports oxygen isotope ratios determined by laser fluorination of mineral separates mainly plagioclase from basaltic andesitic to rhyolitic composition volcanic rocks erupted from the lassen volcanic center lvc northern california plagioclase separates from nearly all rocks have o values 6 1 8 4o higher than expected for production of the magmas by partial melting of little evolved basaltic lavas erupted in the arc front and back arc regions of the southernmost cascades during the late cenozoic most lvc magmas must therefore contain high o crustal material in this regard the o values of the volcanic rocks show strong spatial patterns particularly for young rhyodacitic rocks that best represent unmodified partial melts of the continental crust rhyodacitic magmas erupted from vents located within 3 5 km of the inferred center of the lvc have consistently lower o values average 6 3o 0 1o at given sio2 contents relative to rocks erupted from distal vents 47 0 km average 7 1o 0 1o further magmas erupted from vents situated at transitional distances have intermediate values and span a larger range average 6 8o 0 2o basaltic andesitic to andesitic composition rocks show similar spatial variations although as a group the o values of these rocks are more variable and extend to higher values than the rhyodacitic rocks these features are interpreted to reflect assimilation of heterogeneous lower continental crust by mafic magmas followed by mixing or mingling with silicic magmas formed by partial melting of initially high o continental crust 9 0o increasingly hybridized by lower o 6 0o mantle derived basaltic magmas toward the center of the system mixing calculations using estimated endmember source o values imply that lvc magmas contain on a molar oxygen basis approximately 42 to 4 isotopically heavy continental crust with proportions declining in a broadly regular fashion toward the center of the lvc conversely the o values of the rhyodacitic rocks suggest that the continental crust in the melt generation zones beneath the lvc has been substantially modified by intrusion of mantlederived basaltic magmas with the degree of hybridization ranging on a molar oxygen basis from approximately 60 at distances up to 12 km from the center of the system to 97 directly beneath the focus region these results demonstrate on a relatively small scale the strong influence that intrusion of mantle derived mafic magmas can have on modifying the composition of pre existing continental crust in regions of melt production given this result similar but larger scale regional trends in magma compositions may reflect an analogous but more extensive process wherein the continental crust becomes progressively hybridized beneath frontal arc localities as a result of protracted intrusion of subduction related basaltic magmas

volcanoes of the diamante cross chain evidence for a mid crustal felsic magma body beneath the southern izu bonin mariana arc

geology petrochemistry and genesis of the bimodal lavas of osham hill saurashtra northwestern deccan traps abstract the saurashtra region in the northwestern deccan continental flood basalt province india is notable for compositionally diverse volcano plutonic complexes and abundant rhyolites and granophyres a lava flow sequence of rhyolite pitchstone basaltic andesite is exposed in osham hill in western saurashtra the osham silicic lavas are ba poor and with intermediate zr contents compared to other deccan rhyolites the osham silicic lavas are enriched in the light rare earth elements and have end t 65 ma values between 3 1 and 6 5 and initial 87sr 86sr ratios of 0 70709 0 70927 the osham basaltic andesites have initial end values between 2 2 and 1 3 and initial 87sr 86sr ratios of 0 70729 0 70887 large ion lithophile element concentrations and sr isotopic ratios may have been affected somewhat by weathering notably the sr isotopic ratios of the silicic and mafic rocks overlap however the nd isotopic data indicate that the silicic lavas are significantly more contaminated by continental lithosphere than the mafic lavas we suggest that the osham basaltic andesites were derived by olivine gabbro fractionation from low ti picritic rocks of the type found throughout saurashtra the isotopic compositions and the similar al2o3 contents of the osham silicic and mafic lavas rule out an origin of the silicic lavas by fractional crystallization of mafic liquids with or without crustal assimilation as previously proposed for some icelandic rhyolites and supported here by melts modelling the osham silicic lavas may have been derived by partial melting of hot mafic intrusions emplaced at various crustal depths due to heating by repetitively injected basalts the absence of mixing or mingling between the rhyolitic and basaltic andesite lavas of osham hill suggests that they reached the surface via separate pathways

origin of silicic magmas and the compositional gap at sumisu submarine caldera izu bonin arc japan abstract sumisu caldera volcano izu bonin arc japan is dominated by the bimodal assemblage of basalt basaltic andesite 66 wt sio2 andesites 56 61 wt sio2 are minor in volume and a compositional gap of 6 wt sio2 exists between the andesites and the dacites most andesites contain sieve textured plagioclase and some dacites have resorbed plagioclase containing many glass inclusions some orthopyroxene phenocrysts in andesites and dacites have overgrowth rims with a high ca content a feature consistent with crystallization from a higher temperature magma than their cores dacites rhyolites and glasses in the matrix and plagioclases possess compositions similar to those of partial melts produced by dehydration melting of calc alkaline andesite tanzawa tonalite at low pressure 3 kbar phenocryst assemblages of these andesites dacites and rhyolites resemble restite phase assemblages of the dehydration melting prolonged fractionation or partial melting of basalts in contrast is untenable based on petrography and fractionation calculations we suggest that the silicic magmas at sumisu caldera volcano have been produced by dehydration melting of solidified calc alkaline andesite protolith in the upper to middle crust which segregated efficiently from restite crystals in rare situations reheating and en mass remobilization permitted the same andesites to erupt as andesites 56 61 wt sio2 thus sumisu has a relatively narrow temperature difference 900 1100 c associated with a large compositional range 56 74 wt sio2

mineral disequilibrium in lavas explained by convective self mixing in open magma chambers characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features including reversely zoned crystals resorption surfaces wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix these features are often interpreted as evidence of the mixing of magmas of contrasting composition temperature and origin1 2 here however we propose that such features can also be caused by convection within a magma body with a single composition that is heated from below and cooled from above we describe petrological observations of andesite lava erupted at the soufriere hills volcano montserrat which indicate a heating event and the intermingling of crystals that have very different thermal histories we present experimental data on a representative groundmass composition of this lava which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature the concept of convective self mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions

geochronology and geochemistry of volcanic rocks of the bima formation southern lhasa subterrane tibet implications for early neo tethyan subduction abstract bima formation volcanic rocks which record the history of neo tethyan subduction are found within the central and eastern segments of the southern lhasa subterrane tibetan plateau zircon u pb dating whole rock major and trace element analysis and sr nd pb hf isotopic compositions of bima formation volcanic rocks from the central segment of the southern lhasa subterrane were used to constrain the magmatic and tectonic evolution of the lhasa terrane during the early mesozoic zircon u pb dating of five samples yielded consistent ages of 184 3 2 4 to 176 8 3 5 ma the dominant volcanic rock types within the bima formation are basalts basaltic andesites andesites and dacites which are enriched in the large ion lithophile elements e g rb sr and ba and depleted in high field strength elements e g nb ta and ti 87sr 86sr t ratios are low 0 702900 0 704146 end t and ehf t values are high and positive 4 4 to 6 9 and 9 6 to 15 7 respectively and pb isotope ratios are homogeneous initial 206pb 204pb 18 28 18 40 207pb 204pb 15 53 15 56 208pb 204pb 38 21 38 38 combining the new data with those from a previous study of bima formation volcanic rocks from the eastern segment of the southern lhasa subterrane indicates that the bima formation formed between the middle triassic and early jurassic it suggests that more widespread early mesozoic volcanic rocks in the southern margin of the lhasa terrane the basaltic rocks of the bima formation were generated by partial melting of a depleted mantle wedge metasomatized by slab derived fluids and subsequently experienced fractional crystallization without significant crustal contamination the andesitic and dacitic rocks were formed by fractional crystallization of the basaltic magma our study indicates that the bima formation volcanic rocks were generated within a continental island arc setting related to northward subduction of the neo tethyan oceanic slab during the early mesozoic

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry