lajishankou ophiolite complex implications for paleozoic multiple accretionary and collisional events in the south qilian belt the lajishan ophiolite complex in the qilian orogen is one of several ophiolites situated between the qaidam and north china blocks that record episodic closure of the proto tethyan ocean detailed field relations and geochemical and geochronological studies are critical to unraveling the tectonic processes responsible for an extensive period of intraoceanic subduction that produced juvenile ophiolite island arc terranes which were obducted onto continental margins during ocean closure the lajishankou ophiolite complex crops out along the northern margin of the south qilian belt and was thrust over a neoproterozoic ordovician passive margin sequence that was deposited upon the proterozoic central qilian block the mafic rocks in lajishankou ophiolite complex are the most abundant slices and can be categorized into three distinct groups based on petrological geochemical and geochronological characteristics massive island arc tholeiites 509 ma back arc dolerite dykes and 491 ma pillow basaltic and dolerite slices that are of seamount origin in a back arc basin these results together with spatial relationships indicate that the cambrian island arc rocks ophiolite complex and accretionary complex developed between 530 and 480 ma as a single intraoceanic arc basin system as a result of south directed subduction of the proto tethyan ocean prior to early ordovician obduction of this system onto the central qilian block final continental amalgamation involved continental collision of the central qilian block with the qaidam block during the late ordovician this model solves the long lasting discussion on the emplacement of the lajishan ophiolite and contributes to an improved understanding of multiple accretionary and collisional processes in the qilian orogen

intermediate to felsic middle crust in the accreted talkeetna arc the alaska peninsula and kodiak island alaska an analogue for low velocity middle crust in modern arcs 1 seismic profiles of several modern arcs have identified thick low velocity midcrustal layers vp 6 0 6 5 km s that are interpreted to represent intermediate to felsic plutonic crust the presence of this silicic crust is surprising given the mafic composition of most primitive mantle melts and could have important implications for the chemical evolution and bulk composition of arcs however direct studies of the middle crust are limited by the restricted plutonic exposures in modern arcs the accreted talkeetna arc south central alaska exposes a faulted crustal section from residual subarc mantle to subaerial volcanic rocks of a jurassic intraoceanic arc and is an ideal place to study the intrusive middle crust previous research on the arc which has provided insight into a range of arc processes has principally focused on western exposures of the arc in the chugach mountains we present new u pb zircon dates radiogenic isotope data and whole rock geochemical analyses that provide the first high precision data on large intermediate to felsic plutonic exposures on kodiak island and the alaska peninsula a single chemical abrasion thermal ionization mass spectrometry analysis from the afognak pluton yielded an age of 212 87 0 19 ma indicating that the plutonic exposures on kodiak island represent the earliest preserved record of talkeetna arc magmatism nine new dates from the extensive jurassic batholith on the alaska peninsula range from 183 5 to 164 1 ma and require a northward shift in the talkeetna arc magmatic axis following initial emplacement of the kodiak plutons paralleling the development of arc magmatism in the chugach and talkeetna mountains radiogenic isotope data from the alaska peninsula and the kodiak archipelago range from nd t 5 2 to 9 0 and 87sr 86srint 0 703515 to 0 703947 and are similar to age corrected data from modern intraoceanic arcs suggesting that the evolved alaska peninsula plutons formed by extensive differentiation of arc basalts with little or no involvement of preexisting crustal material the whole rock geochemical data and calculated seismic velocities suggest that the alaska peninsula represents an analogue for the low velocity middle crust observed in modern arcs the continuous temporal record and extensive exposure of intermediate to felsic plutonic rocks in the talkeetna arc indicate that evolved magmas are generated by repetitive or steady state processes and play a fundamental role in the growth and evolution of intraoceanic arcs

the supra ophiolitic sedimentary cover of the asbestos ophiolite quebec canada first geochemical evidence of transition from oceanic to continental sediment flux abstract the 473 5 3 myr asbestos ophiolite complex of the quebec appalachians was formed in a forearc basin and obducted on a margin of laurentia 460 myr ago the complex together with its sedimentary cover is well exposed at burbank hill 130 km sw of quebec city where eight distinct lithologies have been identified 1 pyroxenites and wehrlites with minor dunitic layers 2 fractured gabbros 3 breccia containing gabbro and diabase fragments 4 polygenic conglomerates 5 red mudstone chert sandstone succession 6 tuffs with intercalated grey chert 7 greenish grey mudstones and 8 slates and sandstones of the saint daniel melange the gabbroic and mafic volcanic rocks show the characteristics of arc tholeiites tio2 0 8 0 85 wt and boninites tio2 45 la sm n 1 9 3 3 these rock types also occur as mafic fragments in the polygenic conglomerate the mafic fragments crystals and rocks within the conglomerate and the ophiolitic sandstones also show the same greenschist facies metamorphism as the mafic igneous rocks of the complex the conglomerate was probably formed after the forearc crust was fragmented by pre obduction normal faults these fault scarps would have promoted the erosion of the oceanic crust leading to the accumulation of diverse detritus in grabens the sedimentary lithologies which overlie the conglomerate exhibit a gradual increase in continent derived material la sm n 5 and la yb 20 and a decrease in ophiolitic material moving upward in the stratigraphic sequence the sedimentary rocks at the bottom of the sequence group 3 are mostly composed of ophiolite material with only 20 of crustal material they also have a high content of cr and mgo with values of 350 480 ppm and 5 7 8 respectively the rocks of the uppermost part of the pile group 1 which contain up to 80 crustal material have low abundances of cr and mgo 30 100 ppm and 2 7 4 5 wt continental detritus on the top of the asbestos ophiolite suggests the complex formed near the laurentian continental margin and that the eroded continental material had access to the basin where the ophiolite was formed this is consistent with the obduction of the asbestos and thetford mines ophiolites on the laurentian margin shortly after their formation the proximity of thick laurentian continental crust near the trench could explain why the subduction zone blocked up soon after the ophiolites formation

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

heterogeneous strain regime at the west of the ogasawara plateau in the western pacific ocean from inversion of earthquake focal mechanisms abstract the ogasawara plateau has a major control on the geology of the region between the izu bonin and mariana subduction zones this region is important for understanding active collisional zones we attempt to illustrate the stress field in the wadati benioff zone of this region through seismic estimations 49 events m 3 5 are chosen for waveform inversion by the isola software to distinguish the stress field according to available focal mechanism solutions the heterogeneous stress field parameters are determined for several depth ranges based on the epicentral distribution and seismic stress field the subduction system at the west of the ogasawara plateau can be divided into three segments separated by 26 5 n and 24 n at depths of 0 50 km in the outer trench slope the slab is characterized by a varying stress filed the best fit stress model with horizontal 3 and vertical 1 is related to the plate bending and preexisting faults for the central segment the solutions present a compressional stress regime in the outer trench slope without preexisting faults induced by the resistance of the buoyant plateau the solutions for the depth range of 0 50 km in the forearc region 1 coincides with the plate convergence and 3 dips steeply to the west in the northern and central segments controlled by the friction effect and convergence of the plates however this depth range in the southern segment shows an extensional stress range associated with subduction of older slab segment relatively low rate of plate convergence and the morphological adjustment of the slab between different segments at depths of 50 200 km the inhomogeneous stress field exhibits a complicated extensional stress system with strike slip focal mechanisms at depths of 200 400 km the seismicity presents a pronounced seismic gap and the stress field cannot be determined the seismic gap may be attributed to the missing or tear of the subducting slab which is caused by the subduction of the buoyant ogasawara plateau the mechanism solutions are characterized by an inhomogeneous strain regime below 400 km driven by gravity and strong contortion of the slab and resistance to penetration of the 660 km discontinuity in contrast there are little earthquakes below 400 km in the southern region of the ogasawara plateau

nd and sr isotopic compositions of lavas from the northern mariana and southern volcano arcs implications for the origin of island arc melts nd and sr isotopic data are reported for lavas from 23 submarine and 3 subaerial volcanoes in the northern mariana and southern volcano arcs values of end range from 2 4 to 9 5 whereas 87sr 86sr ranges from 0 70319 to 0 70392 these vary systematically between and sometimes within arc segments the nd and sr isotopic compositions fall in the field of ocean island basalt oib and extend along the mantle array lavas from the volcano arc mariana central island province and the southern part of the northern seamount province have end to 10 and 87sr 86sr 0 7032 to 0 7039 these are often slightly displaced toward higher 87sr 86sr at similar end in contrast those lavas from the northern part of the mariana northern seamount province as far north as iwo jima show oib isotopic characteristics with end and 87sr 86sr 0 7035 to 0 7039 plots of 87sr 86sr and end versus ba la and la yb n support a model in which melts from the mariana and volcano arcs are derived by mixing of oib type mantle or melts therefrom and a metasomatized morb type mantle or melts therefrom an alternate interpretation is that anomalous trends on the plots of nd and sr isotopic composition versus incompatible element ratios found in some s nsp lavas suggest that the addition of a sedimentary component may be locally superimposed on the two component mixing of mantle end members

double ridge subduction recorded in the shimanto accretionary complex japan and plate reconstruction combining a quantitative model relating ridge subduction and radiolarian biostratigraphical data from the shimanto belt including accreted midocean ridge basalt the plate configuration for the western north pacific since 83 ma can be reconstructed the kula north new guinea and north new guinea pacific ridges passed along the japan are assuming a constant plate motion the half spreading rates angles at which ridges entered the trench convergent rates and angles and migration rates of triple junctions can be calculated

transformation of juvenile izu bonin mariana oceanic arc into mature continental crust an example from the neogene izu collision zone granitoid plutons central japan abstract granitic rocks sensu lato are major constituents of upper continental crust recent reviews reveal that the average composition of phanerozoic upper continental crust is granodioritic although oceanic arcs are regarded as a site producing continental crust material in an oceanic setting intermediate to felsic igneous rocks occurring in modern oceanic arcs are dominantly tonalitic to trondhjemitic in composition and have lower incompatible element contents than the average upper continental crust therefore juvenile oceanic arcs require additional processes in order to get transformed into mature continental crust enriched in incompatible elements neogene granitoid plutons are widely exposed in the izu collision zone in central japan where the northern end of the izu bonin mariana ibm arc juvenile oceanic arc has been colliding with the honshu arc mature island arc since middle miocene the plutons in this area are composed of various types of granitoids ranging from tonalite to trondhjemite granodiorite monzogranite and granite three main granitoid plutons are distributed in this area tanzawa plutonic complex kofu granitic complex and kaikomagatake granitoid pluton tanzawa plutonic complex is dominantly composed of tonalite and trondhjemite and characterized by low concentration of incompatible elements and shows geochemical similarity with modern juvenile oceanic arcs in contrast kofu granitic complex and kaikomagatake granitoid pluton consists mainly of granodiorite monzogranite and granite and their incompatible element abundances are comparable to the average upper continental crust previous petrogenetic studies on these plutons suggested that 1 the tanzawa plutonic complex formed by lower crustal anatexis of juvenile basaltic rocks occurring in the ibm arc 2 the kofu granitic complex formed by anatexis of hybrid lower crust comprising of both basaltic rocks of the ibm arc and metasedimentary rocks of the honshu arc and 3 the kaikomagatake granitoid pluton formed by anatexis of hybrid lower crust consisting of k rich rear arc crust of the ibm arc and metasedimentary rocks of the honshu arc these studies collectively suggest that the chemical diversity within the izu collision zone granitoid plutons reflects the chemical variation of basaltic sources i e across arc chemical variation in the ibm arc as well as variable contribution of the metasedimentary component in the source region the petrogenetic models of the izu collision zone granitoid plutons suggest that collision with another mature arc continent hybrid lower crust formation and subsequent hybrid source anatexis are required for juvenile oceanic arcs to produce granitoid magmas with enriched compositions the izu collision zone granitoid plutons provide an exceptional example of the collision induced transformation from a juvenile oceanic arc to the mature continental crust

lithium boron and lead isotope and trace element systematics of quaternary basaltic volcanic rocks in northeastern japan mineralogical controls on slab derived fluid composition abstract across arc li b pb isotope systematics together with trace element geochemistry and phase relationships in subducting slabs are used to understand the petrogenesis of lavas from the northeastern japan arc pb nb ratios and pb isotope compositions of the arc lavas decrease with depth to slab whereas li y u nb th nb and la sm show an upward profile across the arc with peaks at 180 km depth to the wadati benioff zone wbz the peaks in these profiles may reflect the influence of lawsonite breakdown in the slab at 180 km depth b nb ratios of the arc lavas do not show clear across arc variation and the 11b values show a concave upward across arc profile with minimum values at around 180 km in depth of the wbz these results possibly reflect the decomposition of tourmaline at 180 km our results suggest that accessory minerals and minor phases in the subducting slab play important roles in determining the chemical compositions of fluid released by dehydration and the resulting trace element and isotope composition of arc lavas li isotope compositions of the lavas are morb like and do not show systematic across arc variation contrasting with izu arc lavas which show maximum 7li values at the volcanic front and a systematic decrease towards the back arc the strikingly different results for the northeastern japan arc and the izu arc are likely to be caused by differing extents of li isotope fractionation due to differing subduction zone physical characteristics such as thermal structure of the subducting slab and subduction angle thus not only chemical characteristics of the subducting slab but also the physical as well as chemical characteristics of the subduction zone could affect the li isotope compositions of subducting slabs producing heterogeneity in the li isotope composition of the mantle

explosive deep water basalt in the sumisu backarc rift eruption of 1 million year old tholeiitic basalt 1800 meters below sea level 18 megapascals in a backarc rift behind the bonin arc produced a scoriaceous breccia similar in some respects to that formed during subaerial eruptions explosion of the magma is thought to have produced frothy agglutinate which welded either on the sea floor or in a submarine eruption column the resulting 135 meter thick pyroclastic deposit has paleomagnetic inclinations that are random at a scale of 2 5 meters high magmatic water content which is about 1 3 percent by weight after vesiculation contributed to the explosivity

subduction hinge migration along the izu bonin mariana arc abstract hinge migration or rollback is thought to have a significant influence on the tectonics of trench arc backarc systems where the backarc region is actively opening the hinge is usually either stationary or retreating in a mantle hotspot frame of reference the izu bonin mariana system represents a highly anomalous case in which the hinge is reportedly advancing behind the retreating eastern edge of the overriding philippine sea plate we have re evaluated the motion of the subduction hinge using the best available estimates of the motion of the philippine sea plate and estimated maximum rates of extension within the izu bonin arc if the plate motions have not changed appreciably the subduction hinge of the izu bonin trench has advanced over the last 5 ma the approximate average rate of hinge advance is 0 6 mm a 1 at the north end of the izu bonin arc increasing to 24 30 mm a 1 at 27 n the rate of advance of the mariana hinge at 18 n is 10 20 mm a 1 the dip of the subducted slab is strongly correlated with the motion of the philippine sea plate suggesting that the rapid retreat of the overriding plate affects mantle flow we have found a tentative hinge advance mechanism in a simple newtonian viscous corner flow model though the model has severe geometrical and rheological limitations it suggests that the retreat of the overriding plate gives rise to excess uplift pressures acting on the top surface of the subducted slab if the hinge is not assumed to be fixed the excess pressure will tend to rotate the slab arcward steepening the slab and advancing the subduction hinge thus the westward motion of the philippine sea plate may explain both the advance of the subduction hinge and the observed relationship between the motion of the philippine sea plate and the dip of the subducted slab along the izu bonin mariana trench

pb isotope variations in hydrogenetic fe mn crusts from the izu bonin fore arc abstract fe mn crusts were recovered from the western escarpment of the bonin ridge in the izu bonin fore arc region dive site 824 28 612 n 141 803 e at water depths of c 2900 m using the shinkai 6500 submersible during cruise yk 04 05 major and trace element data and xrd mineralogy indicate that the crusts are hydrogenetic in origin we present profiles of variations in pb isotope composition measured in situ by laser ablation mc icp ms across two of the crusts the isotopic variations are systematic and can be matched up between the two crusts indicating similar growth rates the crust pb isotope composition rules out any local source for pb from within the izu bonin mariana arc system either from hydrothermal activity or through leaching of volcanic detritus input of a globally well mixed volcanic pb component either from aerosols or as an absorbed component on aeolian dust has been proposed as a mechanism to explain the pb isotope composition of central pacific deep water however the izu bonin crusts are displaced to lower 206 pb 204 pb and higher 208 pb 204 pb which requires an additional pb source one possibility is that as water is advected from the south outboard of the luzon ryukyu honshu arc system it is progressively polluted by pb derived by weathering and erosion of these young island arc volcanic systems using a constant co flux model growth rates are estimated at 7 13 mm ma which would suggest that these crusts provide a record of changes in the composition of deep water in the izu bonin fore arc region of the western pacific ocean over the last 4 8 ma over this interval the main feature has been a progressive decrease in 207 pb 206 pb 0 843 to 0 839 and 208 pb 206 pb 2 088 to 2 080 with time the interior parts have compositions similar to those of crusts from the izu bonin fore arc while the rims have compositions similar to crusts from the central western pacific

multiple ash layers in late quaternary sediments from the central indian basin abstract we have investigated three sediment cores collected from water depths 5000 m along the transect 76 30 e in close proximity to a fracture zone in the central indian basin cib the cores yielded five volcanic horizons of which four have visual and dispersed shards rhyolitic glass shards of bubble wall platy angular and blocky types were retrieved from various stratigraphic horizons in the cores the abundance of glass shards composition of bulk sediments and 230thexcess ages of the host sediments were used to distinguish the volcanic horizons of the four volcanic horizons three are now newly reported and correspond to ages of 85 107 109 and 142 146 ka while the fourth horizon is of 70 75 ka by using trace element ratios and cr and nb based normative calculations cryptotephra has been identified for the first time from the cib sediment the cryptotephra forms the fifth ash horizon and is of 34 ka a comparison with the published data on volcanic tephra in and around the indian ocean indicate the shard rich horizon srh of 70 75 ka to resemble the younger toba tuffs ytt while the other volcanic horizons that were deposited during different time periods do not correlate with any known marine or terrestrial records these tephra layers have produced a tephrostratigraphic framework across the tectonically and volcanically complex regions of the cib due to the lack of terrestrial equivalents of these tephra it is hypothesized that the newly found volcanic horizons may have been derived from submarine volcanic eruptions multiple layers of submarine volcaniclastic deposits found at water depths as great as 5300 m reaffirm the growing belief that submarine phreatomagmatic eruptions are much more common in the intraplate region of the indian ocean than previously reported

origin and geodynamic implication of the dupal isotopic anomaly in volcanic rocks from the philippine island arcs newly acquired sr nd and pb isotope data combined with other recently available data in the literature clearly show that philippine arc volcanic rocks do not have a true dupal isotopic anomaly instead most of the volcanic rocks must have come from a mantle source with a dupal like indian ocean to enriched ocean island basalt composition this mantle source is present beneath the philippine sea plate and the south china sea plate on either side of the philippine arc systems the isotopic signature of philippine arc volcanics is most probably inherited from the previous southern locations of the different geologic terranes that compose the modern philippine island arcs the new data also show that the isotopic signature of philippine arc volcanics similar to volcanic rocks from other western pacific island arc back arc settings is mainly a function of the mantle source and was only recently modified by a component from subducted sediments and crust

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

strike slip tectonics within the northernmost philippine sea plate in an arc continent collisional setting abstract the geological processes in the northernmost philippine sea plate which is bounded by the suruga and sagami troughs are a typical example of an active collision zone we attempt to illustrate the stress field through seismic estimations and geodetic analysis and propose the kinematic mode of the northernmost tip of the philippine sea plate seven events m 4 0 are chosen for waveform inversion by the isola software to distinguish the stress field in particular six of the chosen events which exhibit strike slip motion are distributed in the eastern area where few focal mechanisms have been reported by previous studies according to the available focal mechanisms strike slip faults with similar p and t axes are widely distributed in the study area the stress inversion suggests that the northern area is characterized by a nw se compression and a ne sw extension stress regime although some spatial differences exist as indicated by an analysis of the geodesy epicenters focal mechanisms gravity anomalies and velocity structure the deformation in the northernmost tip is mainly accommodated by several conjugate strike slip fault systems with steep dips that center on the izu volcanic line generally the maximum principal stress of the kinematics is derived from the collision between the philippine sea plate and central japan because of the different subduction angles rates and directions of the down going plate diverging slab pull forces along the suruga and sagami troughs may be causing the ne nne extension in most of the areas that are bounded by the two troughs the extension propagates southwards along the izu volcanic line and reaches the area adjacent to miyake jima

petrogenesis of late paleozoic tholeiitic nb enriched calc alkaline and adakitic rocks in southwestern mongolia implications for intra oceanic arc evolution abstract intra oceanic subduction has been taken as one of the most important contributors to the crustal growth of the immense central asian orogenic belt caob however the detailed geological processes of the development and growth of numerous island arcs still remain unclear in order to explore the development history of representative ancient island arcs we have conducted geochronological and geochemical studies on an intrusive rock association in southwestern mongolia the studied rock association comprises a mafic intrusive complex consisting of norite and diabase porphyry diabase and diorite dykes new zircon geochronological data and whole rock geochemical compositions reveal sequential magmatic events of an island arc development the mafic intrusive complex has an emplacement age of 356 ma old geochemically its two components of norite and diabase porphyry have a common tholeiitic character but show striking differences in elemental contents the norites are characterized by relatively higher mgo 7 25 16 1 wt lower tio 2 0 54 0 93 wt and relatively lower nb contents 0 99 6 39 ppm than those of the nb enriched nb 9 ppm diabase porphyry these characters together with variable nb la ratios suggest that the parental magma of the norites was derived from a refractory mantle source metasomatized by slab derived fluids and or melts whereas the generation of the nb enriched diabase porphyry was associated with the mantle metasomatism by slab derived melts the 335 ma diabase dykes have negative nb ta ti anomalies and elevated lree and lile contents similar to calc alkaline subduction related magmas which together with high zircon e hf t values 10 6 to 15 8 indicate a derivation from a mantle wedge metasomatized by slab related fluids the 290 ma dioritic dykes show the highest sio 2 53 1 58 3 wt and total alkali k 2 o na 2 o 5 84 7 77 wt contents and their distinctively high sr y 56 1 97 9 ratios and low yb the timing and petrogenesis of the basic intermediate rock association may supply valuable information on tectonic evolution of an intra oceanic subduction accretion system of the caob the coexistence of the early carboniferous tholeiitic nb enriched mafic intrusive complex and boninite as well as picrite may represent an immature intra oceanic island arc the middle carboniferous calc alkaline diabase dykes were probably associated with the relatively mature island arc the permian adakitic rocks was likely produced by partial melting of a delaminated lower crust in an extensional regime when numerous subduction accretion complexes were likely amalgamated together and docked on the siberian active margin

lead isotope study of the late archean lac des iles palladium deposit canada enrichment of platinum group elements by ponded sulfide melt lead isotopic compositions of mineral separates are presented from the mineralized zones roby twilight and high grade zones of the lac des iles palladium deposit approximately 2 69 ga and tonalitic country rocks approximately 2 727 to 2 775 ga in the wabigoon subprovince of the western superior province of canada plagioclase separates show pb isotope ratios similar to those of the late archean depleted mantle sulfide minerals coexisting with plagioclase have a more radiogenic pb isotope composition but very low concentrations of u 0 2 ppb u and th 0 01 ppb compared to pb 50 ppb the isotope data are consistent with a model involving the incorporation of radiogenic pb from country rocks to sulfide melt near the base of the magma chamber or along the conduit of the parental magmas the proposed interpretation is also supported by decreasing cu pd ratios from earlier barren to late fertile gabbro intrusions in the complex the ponding sulfide melt became enriched in platinum group elements as it was interacting with a large magma volume during the evolution of the lac des iles igneous complex

across arc variation of lava chemistry in the izu bonin arc identification of subduction components abstract in order to understand the role of the subducted lithosphere in producing the geochemical characteristics of arc magmas major and trace element along with sr and nd isotope compositions have been determined for quaternary volcanic rocks from the izu bonin intra oceanic arc 87 sr 86 sr and 143 nd 144 nd ratios decrease away from the volcanic front of this arc and lie on mixing lines between the assumed isotopic compositions of fluid phases mainly derived from the basalt layer of the subducted lithosphere and upper mantle materials in the sub arc wedge this across arc variation can be explained through a simple sequence of processes involving initial release of fluid phases from the subducted oceanic crust to produce hydrous peridotite at the base of the mantle wedge this hydrous peridotite is dragged downward with the slab and releases a second stage metasomatizing fluid beneath the volcanic arc the higher concentrations of both sr and nd in the fluid beneath the volcanic front than those beneath the back arc side may be a possible cause of the observed across arc variation in sr nd isotopic ratios the difference in compositions of fluid phases is attributed to the different hydrous phases which decompose in the hydrous peridotite layer amphibole beneath the volcanic front and phlogopite beneath the back arc side of the volcanic arc the mineralogically controlled fluid addition may also be responsible for the across arc variation in rb k and rb zr ratios increasing away from the volcanic front

sea surface temperature record of a late cretaceous tropical southern tethys upwelling system abstract late cretaceous sea surface temperatures ssts were reconstructed based on tex 86 data from the unique high productivity upwelling system in the southern tethys margin israel ssts were determined from two oil shale sequences of the santonian early maastrichtian 85 to 68 ma in southern efe syncline and central shefela basin israel paleo latitude 8 15 n these two sampling sites represent the inner and outer belts of the upwelling system our tex 86 data indicate a significant long term sst cooling trend from 36 0 to 29 3 c during the late santonian and the early campanian in the southern tethys margin consistent with the opening of the equatorial atlantic gateway eag based on our data from the equatorial region and literature data from the polar region we reassessed the equator to pole sst gradient to 22 c this value is closer to the modern ocean equator to pole sst gradient 30 c than suggested from previous studies furthermore the sst data suggests the presence of cooler surface water in the inner belt 27 7 c caused by the upwelling system and warmer surface water 29 2 c further seaward in the outer belt

the systematics of boron isotopes in izu arc front volcanic rocks the process of fluid release from the subducting slab beneath the izu arc volcanic front izu vf was examined by measuring b concentrations and b isotope ratios in the neogene fallout tephra odp site 782a both were measured by secondary ion mass spectrometry in a subset of matrix glasses and glassy plagioclase hosted melt inclusions selected from material previously analyzed for major and trace elements glasses and radiogenic isotopes sr nd pb bulk tephra these tephra glasses have high b abundances 10 60 ppm and heavy 11b values 4 5 to 12 0 extending the previously reported range for izu vf rocks 11b 7 0 to 7 3 the glasses show striking negative correlations of 11b with large ion lithophile element lile nb ratios these correlations cannot be explained by mixing two separate slab fluids originating from the subducting sediment and the subducting basaltic crust respectively model a two alternative models models b and c are proposed model b proposes that the inverse correlations are inherited from altered oceanic crust aoc which shows a systematic decrease of b and lile with increasing depth from basaltic layer 2a to layer 3 paralleled by an increase in 11b from 1 to 10 to 24 in this model the contribution of sedimentary b is insignificant 4 of b in the izu vf rocks model c explains the correlation as a mixture of a low 11b 1 composite slab fluid a mixture of metasediment and metabasalt derived fluids with a metasomatized mantle wedge containing elevated b 1 2 ppm and heavy 11b 14 the mantle wedge was likely metasomatized by 11b rich fluids beneath the outer forearc and subsequently down dragged to arc front depths by the descending slab pb b isotope systematics indicate that at arc front depths 53 of the b in the izu vf is derived from the wedge this implies that the heavy 11b values of izu vf rocks are largely a result of fluid fractionation and do not reflect variations in slab source provenance i e subducting sediment vs basaltic crust since the b content of the peridotite at the outer forearc 7 58 ppm b mean 24 16 ppm is much higher than beneath the arc front 1 2 ppm b the hydrated mantle wedge must have released a b rich fluid on its downward path this wedge flux can explain 1 the across arc decrease in b and 11b e g izu kuriles without requiring a progressive decrease in fluid flux from the subducting slab and 2 the thermal structure of volcanic arcs as reflected in the b and 11b variations of volcanic arc rocks

pre 3750 ma supracrustal rocks from the nuvvuagittuq supracrustal belt northern quebec geochemistry and u pb ion microprobe zircon geochronology guided by high resolution mapping 1 50 scale was used to define a minimum age of ca 3750 ma for supracrustal rocks from the nuvvuagittuq supracrustal belt nsb in the northern superior province quebec canada mineralogy and geochemistry of critical field relationships preserved at the porpoise cove locality describe a supracrustal succession of mafic amphibolites and ultramafic rocks finely banded quartz magnetite units with intermixed coarse grained ferruginous quartz pyroxene rocks and quartz biotite schists that superficially resemble polymict metaconglomerates with large up to 10 cm deformed polycrystalline quartz and mafic clasts all units in the mapped outcrop have sharp lithological contacts narrow dominantly trondhjemitic orthogneiss sheets locally preserve intrusive contact relationships to the supracrustals the total extent of the supracrustal enclave is 8k m 2 it is strongly deformed and the full deformation history appears to be shared by all units with later modifications from leucogranitoid intrusions the quartz biotite schists record complex zircon growth at 3500 and 2800 ma interpreted to reflect metamorphic events zircons separated from orthogneisses in the enclave including one sheet that transects a banded quartz pyroxene magnetite unit yield magmatic 207 pb 206 pb ages of ca 3750 ma these ages are slightly younger than earlier provisional reports for an nsb orthogneiss from porpoise cove the nuvvuagittuq supracrustals are the oldest rocks thus far reported for the minto block they overlap in age with the ca 3 70 3 83 ga isua supracrustal belt and akilia association rocks in west greenland and they represent an important new area for exploration of the early earth

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

petrogenesis and tectonic setting of volcanic rocks in the xiaoshan and waifangshan areas along the southern margin of the north china craton constraints from bulk rock geochemistry and sr nd isotopic composition abstract as part of the xiong er volcanic belt along the southern margin of the north china craton volcanic rocks in the xiaoshan and waifangshan areas have a compositional range from the basaltic andesite andesite dacite to rhyolite which display consistent variation trends in terms of their major and trace elements and sr nd isotopic compositions the variable yb contents with nearly constant la yb and tb yb ratios of volcanic rocks in two areas suggest that the fractional crystallization may have played an important role in the differentiation from the basaltic andesite through andesite and dacite to rhyolite the volcanic rocks in these two areas are characterized by the lile and lree enrichments and negative hfse anomalies implying hydrous melting of a mantle wedge in a subduction zone variable sr nd ratios of the basaltic andesite and andesite are interpreted as a result of the fluid addition from a subducting slab non radiogenic nd isotopic compositions as well as high zr y and nb y ratios suggest that the volcanic rocks in these areas were derived from an enriched mantle source on the other hand the volcanic rocks of the basaltic andesite and andesite possess markedly higher fe ti and hfse concentrations than those of typical intra oceanic arcs implying that the mantle source from which the volcanic rocks were derived was metasomatised by siliceous melts during the archean to paleoproterozoic subduction collision in the trans north china orogen these data suggest that in the paleo mesoproterozoic the southern margin of the north china craton was most likely an andean type continental arc in which slab dehydration not only induced the melting of a pre existing metasomatised mantle source but also released lile enriched fluids into the mantle source masking the inherent hfse enriched characteristics of the volcanic rocks along the southern margin of the craton the results of this study indicate that the north china craton like many other continental components e g north america greenland baltica amazonia australia etc of the supercontinent columbia nuna also underwent a subduction related outgrowth along its southern margin during the paleo mesoproterozoic time

rapid timescales of differentiation and evidence for crustal contamination at intra oceanic arcs geochemical and u th ra sr nd isotopic constraints from lopevi volcano vanuatu sw pacific abstract the extent and geochemical impact of crustal contamination during magmatic evolution in intra oceanic subduction zone settings is often assumed to be of minimal significance but remains poorly constrained acquiring such information is crucial if meaningful timescales of magma generation and crustal residence beneath volcanoes are to be determined high mgo basalts and differentiates erupted over the last 100 years at lopevi volcano display a strong negative correlation between 87sr 86sr isotope ratio and indices of differentiation e g sio2 even though these lavas have a restricted sr nd isotopic range compared to other vanuatu arc lavas this trend provides compelling evidence for the interaction of rising mafic magmas with primitive sub arc crust and so this volcano affords an excellent opportunity to investigate and ascertain timescales of crustal interaction using u series data quantitative geochemical modelling of whole rock trace element ratios 87sr 86sr isotope compositions and u series data shows that assimilation of a relatively small degree partial melt of 380 ka mafic oceanic crust similar to pacific or indian morb in 87sr 86sr isotopic composition during fractional crystallisation of magma exerts major control on the 230th 232th and 226ra 230th activity ratios of the lavas the incorporation of higher 230th 232th and lower 226ra 230th assimilated material drives compositions closer to secular equilibrium than simple closed system differentiation reducing calculated apparent timescales of closed system differentiation from th isotope composition 104 105 by orders of magnitude modelling suggests that assimilation occurs extremely rapidly at lopevi with timescales for magma generation differentiation and eruption on the order of 102 years

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

the flaw in the crustal zircon archive mixed hf isotope signatures record progressive contamination of late stage liquid in mafic ultramafic layered intrusions

experimental constraints on the effects of pressure and h 2 o on the fractional crystallization of high mg island arc basalt h2o undersaturated melting experiments of synthesized basalt sio2 50 7 wt mgo 8 3 wt mg 60 were conducted at fo2 corresponding to nno 1 and nno 1 to clarify the effects of pressure 2 7 kbar and h2o on fractional crystallization in island arcs h2o content was ranged from nominally anhydrous to 4 4 wt differentiation trends namely the liquid lines of descent change sensitively according to pressure h2o relations tholeiitic differentiation trends are reproduced with h2o 2 wt in primary magma with such quantities of h2o fractional crystallization is controlled by olivine plagioclase at 2 kbar increasing the pressure from 2 to 4 kbar induces early crystallization of orthopyroxene instead of olivine and therefore sio2 enrichment in the residual melts is suppressed increasing h2o 2 wt in primary magma stabilizes clinopyroxene relative to orthopyroxene and or magnetite although the phase relations and proportions strongly depend on fo2 and h2o content differentiation trends are always calc alkaline

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

triassic depleted lithospheric mantle underneath the paleozoic chinese altai orogen evidence from morb like basalts abstract whole rock geochemical and sr nd pb hf isotopic analyses were conducted on the triassic ashele post orogenic basalts from the chinese altai the tholeiitic ashele basalts possess moderate na2o 2 46 4 52 wt but extremely low k2o 0 06 wt contents and are depleted in light rare earth elements lree their la sm n and la yb n ratios range from 0 78 to 0 88 and from 0 72 to 0 84 respectively which together with their low initial 87sr 86sr 0 7048 0 7052 high end t 4 9 to 5 6 and ehf t 14 3 to 16 3 values indicate that the magmas were derived from a depleted mantle source their 206pb 204pb and 207pb 204pb ratios 17 919 18 125 and 15 507 15 522 respectively also support this interpretation on the other hand the ashele basalts are enriched in cs th and u but depleted in rb ba nb and ta possibly due to slight mantle metasomatism by subduction related fluids because there was no record for large scale thermal activity in the deep mantle during the triassic and geological evidence demonstrates reactivation of the erqis fault we propose that such a tectonic reactivation affected the lithospheric mantle and resulted in its partial melting and subsequent generation of the ashele basalts this study shows that a depleted lithospheric mantle not an ancient continental lithosphere was underneath the chinese altai in the triassic which was possibly accreted during the paleozoic orogeny therefore the lithospheric mantle in this area today was possibly evolved from such a depleted mantle modified by consecutive partial melting after the paleozoic orogeny

isotopic and trace element constraints on mixing and melting models of marginal basin volcanism bransfield strait antarctica bransfield strait is a narrow marginal basin separating the south shetland islands from the northern end of the antarctic peninsula quaternary volcanism occurs in the strait as subaerial and submarine volcanoes aligned on the inferred axis of rifting and as two subaerial off axis volcanoes on the northern margin of the strait the bransfield strait lavas are similar to published analyses from other marginal basins ranging from basalts and basaltic andesites to trachytes they exhibit moderate enrichments in alkali and alkaline earth elements relative to high field strength elements that are typical of many back arc basin basalts the seamount basalts have trace element chemistry similar to enriched mid ocean ridge basalts e morb but with variously higher alkali and alkaline earth element concentrations and frequently lower nb low pressure fractional crystallization accounts for most of the compositional variation within individual volcanoes but it does not explain intervolcano differences even though the volcanoes are closely spaced and presently or recently active melting of depleted mantle mixed with 0 5 2 crust or sediment explains the isotopic and trace element variations not accounted for by magma chamber processes the off axis volcanoes are the products of less partial melting than the on axis volcanoes one of the off axis volcanoes also contains more of an enriched component in its source some of the lavas have rb sr too low to account for their 87sr 86sr perhaps due the loss of rb during mantle metasomatism or interaction between slab derived fluids and the mantle source of the basalts strontium neodymium and 207pb 204pb isotopic signatures remained constant during the transition from tertiary island arc volcanism to quaternary marginal basin volcanism but 206pb 204pb increased high cen smn of basalts from some volcanoes requires residual garnet in the source thus the young 15 25 ma at the trench subducted slab is either deeper than 60 km beneath the volcanoes or it has fractured and is no longer a coherent barrier to melts generated below it

the xiong er volcanic belt at the southern margin of the north china craton petrographic and geochemical evidence for its outboard position in the paleo mesoproterozoic columbia supercontinent abstract the xiong er volcanic belt covering an area of more than 60 000 km 2 along the southern margin of the north china craton has long been considered an intra continental rift zone and recently interpreted as part of a large igneous province formed by a mantle plume that led to the breakup of the paleo mesoproterozoic supercontinent columbia however such interpretations cannot be accommodated by lithology mineralogy geochemistry and geochronology of the volcanic rocks in the belt lithologically the xiong er volcanic belt is dominated by basaltic andesite and andesite with minor dacite and rhyolite different from rock associations related to continental rifts or mantle plumes which are generally bimodal and dominated by mafic components however they are remarkably similar to those rock associations in modern continental margin arcs in some of the basaltic andesites and andesites amphibole is a common phenocryst phase suggesting the involvement of h 2 o rich fluids in the petrogenesis of the xiong er volcanic rocks geochemically the xiong er volcanic rocks fall in the calc alkaline series and in most tectono magmatic discrimination diagrams the majority of the xiong er volcanic rocks show affinities to magmatic arcs in the primitive mantle normalized trace element diagrams the xiong er volcanic rocks show enrichments in the lile and lree and negative nb ta ti anomalies similar to arc related volcanic rocks produced by the hydrous melting of metasomatized mantle wedge nd isotope compositions of the xiong er volcanic rocks suggest that 5 15 older crust has been transferred into the upper lithospheric mantle by subduction related recycling during archean to paleoproterozoic time available shrimp and la icp ms u pb zircon age data indicate that the xiong er volcanic rocks erupted intermittently over a protracted interval from 1 78 ga through 1 76 1 75 ga and 1 65 ga to 1 45 ga though the major phase of the volcanism occurred at 1 78 1 75 ga such multiple and intermittent volcanism is inconsistent with a mantle plume driven rifting event but is not uncommon in ancient and existing continental margin arcs taken together the xiong er volcanic belt was most likely a paleo mesoproterozoic continental magmatic arc that formed at the southern margin of the north china craton similar paleo mesoproterozoic continental magmatic arcs were also present at the southern and southeastern margins of laurentia the southern margin of baltica the northwestern margin of amonzonia and the southern and eastern margins of the north australia craton which are considered to represent subduction related episodic outbuilding on the continental margins of the paleo mesoproterozoic supercontinent columbia therefore in any configuration of the supercontinent columbia the southern margin of the north china craton could not have been connected to any other continental block as proposed in a recent configuration but must have faced an open ocean whose lithosphere was subducted beneath the southern margin of the north china craton

volcanic evolution of the south sandwich volcanic arc south atlantic from multibeam bathymetry abstract new multibeam bathymetry data are presented for the south sandwich intra oceanic arc which occupies the small sandwich plate in the south atlantic and is widely considered to be a simple end member in the range of intra oceanic arc types the images show for the first time the distribution of submarine volcanic tectonic and erosional depositional features along the whole length of the 540 km long volcanic arc allowing systematic investigation of along arc variations the data confirm that the volcanic arc has a simple structure composed of large volcanoes which form a well defined volcanic front but with three parallel cross cutting seamount chains extending 38 60 km from near the volcanic front into the rear arc there is no evidence for intra arc rifting or extinct volcanic lines topographic evidence for faulting is generally absent except near the northern and southern plate boundaries most of the volcanic arc appears to be built on ocean crust formed at the associated back arc spreading centre as previously proposed from magnetic data but the southern part of the arc appears to be underlain by older arc or continental crust whose west facing rifted margin facing the back arc basin is defined by the new bathymetry the new survey shows nine main volcanic edifices along the volcanic front and ca 20 main seamounts the main volcanoes form largely glaciated islands with summits 3 0 3 5 km above base levels which are 2500 3000 m deep in the north and shallower at 2000 2500 m deep in the south some of the component seamounts are interpreted to have been active since the last glacial maximum and so are approximately contemporaneous with the volcanic front volcanism seven calderas all either submarine or ice filled have been identified adventure volcano a newly discovered submarine volcanic front caldera volcano is described for the first time all but one of the calderas are situated on summits of large volcanoes in the southern part of the arc and most are associated with current or historic volcanic or hydrothermal activity shallow shelves around the islands are generally 1 10 km wide submerged banks up to 1100 m deep are interpreted as subsided erosional surfaces seamounts and emergent volcanoes experienced a range of mass wasting processes including by landsliding and smaller mass flows

the volcanoes of an oceanic arc from origin to destruction a case from the northern luzon arc abstract volcanoes were created grew uplifted became dormant or extinct and were accreted as part of continents during continuous arc continent collision volcanic rocks in eastern taiwan s coastal range cr are part of the northern luzon arc an oceanic island arc produced by the subduction of the south china sea plate beneath the philippine sea plate igneous rocks are characterized by intrusive bodies lava and pyroclastic flows and volcaniclastic rocks with minor tephra deposits based on volcanic facies associations sr nd isotopic geochemistry and the geography of the region four volcanoes were identified in the cr yuemei chimei chengkuangao and tuluanshan near vent facies associations show different degrees of erosion in the volcanic edifices for chimei chengkuangao and tuluanshan yuemei lacks near vent rocks implying that yuemei s main volcanic body may have been subducted at the ryukyu trench with the northward motion of the philippine sea plate these data suggest a hypothesis for the evolution of volcanism and geomorphology during arc growth and ensuing arc continent collision in the northern luzon arc which suggests that these volcanoes were formed from the seafloor emerging as islands during arc volcanism they then became dormant or extinct during collision and finally were uplifted and accreted by additional collision the oldest volcano yuemei may have already been subducted into the ryukyu trench

isotopic characteristics of subduction fluids in an intra oceanic setting izu bonin arc japan new radiogenic isotope and trace element data are presented for the volcanic sequences along 600 km of the active izu bonin arc the oligocene izu arc and their associated rift basins as with many intra oceanic island arcs the pliocene recent izu bonin frontal arc lavas are highly depleted in zr nb and the rare earth elements relative to typical mid ocean ridge basalt morb indicating that the mantle wedge source has undergone a previous episode of melting ratios between these elements such as nb zr and la sm as well as 143nd 144nd do not vary significantly along the length of the frontal arc these parameters suggest that each of the arc volcanoes is derived from similar melt fractions of the mantle wedge however ba zr ba rb and 87sr 86sr increase along the frontal arc to the north this leads us to propose that a variable enrichment in ba and radiogenic sr is superimposed on the mantle wedge sr nd and pb nd isotope variation indicate that both sr and pb become more radiogenic after fluid addition however pb isotope ratios do not correlate with increases in pb concentration or ratios such as ba zr and nb pb in other words the pb isotopic composition of the arc lavas appears to be independent of the amount of pb introduced by subduction fluids into the mantle source this buffering of pb isotopes along the frontal arc means that the isotopic composition of the lavas is indistinguishable from that of the fluid isotopic mixing models presented for the arc are only illustrative of the many plausible combinations of components and quantities despite this we are able to determine that the mantle wedge has isotopic characteristics similar to indian ocean morb and that the subduction fluid solute is primarily derived from subducted oceanic basalt with a 2 contribution from subducted sediment lavas in the oligocene izu arc and fore arc basin were derived from a mantle wedge of similar composition to the active arc despite levels of pb enrichment comparable to those of the modern arc the pb isotopes of the oligocene volcanics indicate a lower sediment input into the melting region

magmatic evolution of the alboran region the role of subduction in forming the western mediterranean and causing the messinian salinity crisis the magmatic evolution of the alboran region westernmost mediterranean contains important clues for improving our understanding of the origin of mediterranean style back arc basins and the desiccation of the mediterranean sea in the messinian we use new laser 40ar 39ar age and geochemical major and trace element and o sr nd pb isotope data from igneous rocks from southern spain the alboran sea and northern morocco to reconstruct the magmatic evolution of the westernmost mediterranean since the eocene lower oligocene dikes near malaga 33 6 0 6 ma and middle to upper miocene volcanic rocks from the alboran sea area 6 57 0 04 to 11 8 0 4 ma can be subdivided into two groups 1 lree depleted relative to n morb primarily tholeiitic series and 2 lree enriched primarily calc alkaline series volcanic rocks both groups are generally enriched in fluid mobile elements e g rb th u k and pb relative to fluid immobile elements e g nb ta lree the lree depleted group has 143nd 144nd 0 5128 0 5130 isotope ratios similar to atlantic morb but higher 87sr 86sr 0 7046 0 7100 in contrast the lree enriched group has less radiogenic nd 0 5121 0 5126 and tend to more radiogenic sr 0 7066 0 7205 isotopic composition pb isotope ratios are surprisingly uniform and have compositions similar to marine sediments analyses of mineral separates show that mafic melts with relatively low 18o 5 6 7 2 had high 87sr 86sr 0 7048 0 7088 7 4 10 6 14 1 and 8 4 40 0 49 3 modeling of the trace elements and sr nd pb o isotopic compositions provides compelling evidence for the contamination of the mantle source with hydrous fluids melts which can be explained through subduction of oceanic lithosphere beneath the alboran basin but not through detachment delamination of lithospheric mantle we present a geodynamic model that reconstructs the late eocene to quaternary evolution of the western mediterranean through westward roll back of subducted tethys oceanic lithosphere slab roll back resulted in a large scale reorganization of the western mediterranean paleogeography causing the closure of marine gateways linking the atlantic ocean to the mediterranean sea isolation of the mediterranean sea led to its desiccation causing the messinian salinity crisis

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

regional slaty cleavage formation and fold axis rotation by re use and reactivation of pre existing foliations the fiery creek slate belt north queensland abstract a deformation history comprised of six separate deformation events of differing intensity has affected the rocks of the south palmer river region of the the hodgkinson province north queensland within this region a zone of pervasive slaty cleavage herein termed the fiery creek slate belt has developed as a result of the superposition of fabrics formed during several of these events the most important processes in the formation of this composite cleavage were the re use and reactivation of the favourably oriented steep n s trending s2 foliation by the intense fourth deformation event d4 this produced micro meso and macroscopic folds in an originally shallow s3 foliation produced during the intervening d3 deformation with an axial planar s2 s4 foliation the d4 stretching lineation l44 plunges subvertically to steeply north and indicates that shear during d4 was oriented steeply north south in the fiery creek slate belt d2 fold axes are interpreted to have formed in much shallower orientations than their present moderately n s plunging to subvertical orientations we consider this to be a result of d4 shear which caused variable degrees of rotation of d2 fold axes toward the d4 stretching lineation due to subparallelism of the bulk shortening directions of the d2 and d4 events near total destruction of the pre d4 foliations during slaty cleavage formation has produced a misleading impression of a simple deformation history there is no relationship between metamorphic grade and intensity of slaty cleavage development

long distance magma transport from arc volcanoes inferred from the submarine eruptive fissures offshore izu oshima volcano izu bonin arc abstract long distance lateral magma transport away from volcanic centers in island arcs is emerging as a common phenomenon where the regional stress regime is favorable it should also be recognized as an important factor in the construction and growth of island arcs and a potential trigger for devastating eruptions in this contribution we report on recent investigations into the magma dynamics of izu oshima volcano an active basaltic volcano with an extensive fissure system izu oshima is flanked by numerous subparallel nw se trending submarine ridges extending up to 22 km to the nw and the se from the central vent during a recent submersible survey we have identified that these ridges are fissures which erupted basaltic spatter and lava flows furthermore lavas are petrographically similar along each ridge while there are noticeable differences between ridges the subparallel ridges are observed to transect a series of seamounts the izu tobu monogenetic volcanoes which are dispersed across this area of the rear arc however there are consistent petrographic and chemical differences between these seamounts and the ridges indicating that they have different magma sources yet they are essentially bounding each other in dive tracks the most appropriate scenario for their development is one where the izu tobu volcanoes are fed by an in situ underlying source while the nw se ridges are fed by lateral magma transport from izu oshima magma erupted from each ridge is of a consistent geochemistry along its length but has experienced crystal fractionation and some plagioclase accumulation compositions of the ridges are also very similar to lavas from the subaerial cones that can be traced down the flanks of izu oshima this implies that pairs of subaerial cones and submarine ridges represent the locus of magma transport events away from the storage system beneath izu oshima hence magma from this crustal reservoir moved upward to feed the on edifice cones as well as laterally to supply submarine fissures and dykes to a distance of 22 km

resolving the conundrum in origin of the manipur ophiolite complex indo myanmar range constraints from nd isotopic ratios and elemental concentrations in serpentinized peridotite abstract major trace and rare earth element ree abundances along with nd isotopic ratios are measured in serpentinized peridotite samples from the manipur ophiolite complex moc indo myanmar range imr an attempt is made to resolve the conundrum caused by apparently contradictory petrogenetic models and tectonic settings reported in literature nd isotope ratios reported for the first time range from 7 6 to 10 4 in epsilon units in the whole rock samples of serpentinized harzburgite and lherzolite variation of la yb with e ndt is consistent with the progressive addition of a subduction derived fluid to a depleted mantle source literature data from the mafic extrusive intrusive and ultramafic rocks together with new data generated during this study can be explained by a two stage petrogenetic model non modal dynamic melting in the mantle wedge subsequent to the mixing of fluids derived from the subducted slab explains the observed range of elemental abundances in the moc mafic and ultramafic rocks this model also explains the observed variability of nb and th in the mafic rocks without invoking different sources for the different types of rocks taking into account field petrographic and geochemical evidences we propose that moc comprises a buoyant fore arc mantle wedge system along with the subducted slab that was obducted during the terminal stage of subduction of the neotethys below the burmese plate

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

the magmatic plumbing of the submarine hachijo nw volcanic chain hachijojima japan long distance magma transport 1 recent geophysical observations on basaltic composite volcanoes in izu bonin arc reveal the process of long distance lateral magma transport within arc crust to clarify a long distance magma transport system of the basaltic arc volcano from geological and petrological aspects we investigated 20 km long submarine volcanic chains hachijo nw chain and hachijo kojima chain and cones on the northeastern slope ne edifices as well as subaerial satellite cones nested hachijo nishiyama volcano in the northern izu arc front basalts from hachijo nw chain have more primitive composition than those from other edifices the composition of the hachijo nw chain basalts is controlled by fractional crystallization while plagioclase accumulation occurred in ne edifices and subaerial satellite cones trace element and isotopic characteristics indicate that the same basaltic primary magma is involved in all sections of the volcano this leads us to consider that magma was transported long distances between the nishiyama volcano and the hachijo nw chain primitive magma was laterally transported nnw for at least 20 km in the middle to lower crust 10 20 km deep from nishiyama volcano with only minimal crustal level modifications and formed hachijo nw chain on the other hand magmas experienced crystal fractionation and accumulation at shallow magma chamber beneath nishiyama volcano seems to have been transported in a short distance 5 km and formed ne trending edifices and subaerial satellite cones the long distance magma transport seems to be controlled by a regional extensional stress regime while short distance transport may be controlled by local stress regime affected by load of main volcanic edifice

variation in the mantle sources of the northern izu arc with time and space constraints from high precision pb isotopes abstract we present new ages and geochemical data for back arc lavas from the northern izu bonin arc 33 35 n including high precision double spike pb isotope measurements the northern part of the izu bonin arc is distinct from the rest of the arc as it lacks active rifting behind the volcanic front but it does have quaternary volcanoes e g niijima however in common with the rest of the arc the northern section has back arc seamount chains and ne sw volcanic ridges 40ar 39ar dating of volcanic rocks has revealed that quaternary volcanism is limited to within 40 km of the volcanic front miocene and pliocene volcanism extended as far as 120 km west of the current volcanic front along the back arc seamounts and ridges the chemical characteristics of back arc volcanism are significantly different in the pliocene quaternary compared to the miocene opx cpx andesite and hornblende andesite are dominant in miocene volcanic centres while pliocene and quaternary centres are characterized by basalt and rhyolite miocene volcanic centres show a marked correlation between th ce and pb and nd isotopes generally these lavas have higher 7 4 and lower 143nd 144nd with increasing th ce in contrast the pliocene and quaternary lavas show little if any th enrichment relative to potential mantle sources and no correlation with isotopes these correlations suggest that partial melt of sediment from the subducting slab was an important component in the miocene whereas the pliocene quaternary volcanic centres show little evidence of sediment melt and are restricted to a contribution of fluid from altered oceanic crust and fluid from sediment quaternary volcanoes at similar distances from the volcanic front are calculated to have similar compositions and amounts of slab derived fluid in their sources however on pb pb isotope plots they lie closer to the nhrl towards south i e 8 4 decreases towards south almost parallel but distinct trends on pb pb plots imply differing mantle sources depending on the northward position within the arc this variation in mantle wedge composition is seen to continue into the central part of the arc ishizuka o taylor r n milton j a nesbitt r w fluid mantle interaction in an intra oceanic arc constraints from high precision pb isotopes earth planet sci lett 211 2003a 221 236 the observed along arc isotopic variation can be explained by the presence of two mantle components a morb source observed in the back arc basins of the philippine sea plate e g hickey vargas r origin of the indian ocean type isotopic signature in basalts from philippine sea plate spreading centers an assessment of local versus large scale processes j geophys res 103 1998a 20963 20979 and a more pacific morb like source in the northern part of the izu bonin arc miocene to pliocene volcanism was active over a much wider back arc region than present and these lavas show the greatest influence of sediment melt generated at slab depths of 250 km however in the quaternary active volcanism was constrained to the vicinity of the volcanic front this quaternary magmatism can be related to the release of fluid from the slab at relatively shallow depths 110 200 km these spatial and temporal chemical variations could be related to either the change of the angle of subducting pacific plate and or changes in the regional stress regime e g resulting from the collision of the izu bonin arc with the honshu arc or in association with initiation of rifting in the south

chlorine isotope variations across the izu bonin mariana arc chlorine isotope ratios were determined for volcanic gas geothermal well ash and lava samples along the izu bonin mariana volcanic front serpentinite clasts and muds from serpentine seamounts conical south chamorro torishima basalts from the guguan cross chain and sediments from ocean drilling program odp sites 800 801 802 and 1149 there is no systematic variation in 37 cl values along the volcanic front in either gas or ash samples in contrast distinct variations occur across the arc implying variations in the fluid source at different depths within the subduction zone serpentinite clasts and serpentine muds from the seamounts tap a source of 30 km depth and have 37 cl values of structurally bound chloride of 0 4 0 4 n 24 identical to most seafloor serpentinites suggesting a serpentinite chrysotile and or lizardite to antigorite transition fluid source tapping deeper levels of the subduction zone 115 130 km depth volcanic gases and ashes have 37 cl values averaging 1 1 1 0 n 29 precisely overlapping the range measured in sediments from odp cores 1 1 0 7 n 11 and limited altered oceanic crust aoc both sediments and aoc are possible cl sources in the volcanic front the guguan cross chain basalts come from the greatest depths and have an average 37 cl value of 0 2 0 2 n 3 suggesting a second serpentine derived source in this case from antigorite breakdown at 200 km depth

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago

arc volcanism in an extensional regime at the initiation of subduction a geochemical study of hahajima bonin islands japan hahajima represents an eocene forearc volcanic centre produced during the initiation of subduction within an intra oceanic domain of the western pacific the eocene oligocene izu bonin arc forearc is envisaged as a broad region of volcanism generated in an extensional tectonic regime the forearc is likely to have consisted of a series of small rift basins punctuated and bordered by coherent volcanic edifices hahajima represents one such edifice at the southern termination of the chichijima mukojima rift basin major element trace element and isotopic data are presented for representative volcanic rocks from hahajima the volcanic sequence comprises porphyritic basalt and andesite breccias and flows intercalated with tuffaceous sandstones all lavas contain high proportions of plagioclase phenocrysts 20 36 and comparatively low mafic mineral contents the andesite suite is typified by the presence of magnetite and orthopyroxene phenocrysts and pseudomorphs after olivine basalts are dominated by an assemblage of plagioclase clinopyroxene and olivine nodules and xenocrysts typical of the andesites are found in the basalts and vice versa suggesting an intimate relationship between the two magmas during volcanism the basaltic suite follows a tholeiitic fractionation path iron enrichment while the andesites are more representative of calc alkaline magma evolution these trends are found to be clearer when corrections are made for the excess proportions of plagioclase in the whole rock trace element differences between the suites can not be reconciled by a straightforward crystallization model but can be related to a similar parental magma if the shallow level magma evolution involves a combination of open and closed system fractionation processes comparisons of volcanic suites within and between each site sampled along the izu bonin forearc indicate that each eruptive centre has an individual trace element signature this suggests that variations within each site can be explained by melting and or shallow level fractionation processes while differences between localities are related to heterogeneities in the composition trace element and isotopic enrichment and thermal characteristics of the source the hahajima volcanic rocks contrast with the more recent arc magmas of the izu bonin system in that they share some features such as high mg and elevated zr y with the contemporaneous boninitic lavas