petrology mineral chemistry and geochemistry of late triassic ni cu ore bearing mafic ultramafic intrusions hongqiling northeastern china petrogenesis and tectonic implications the hongqiling magmatic ni cu sulfide deposit situated on the southern margin of the eastern central asian orogenic belt caob is composed of over 30 mafic ultramafic intrusions these ore beari

alaskan type nature and pge mineralization of the wuxing mafic ultramafic complex in eastern part of the central asian orogenic belt abstract the wuxing mafic ultramafic complex is located in the eastern part of the central asian orogenic belt caob it consists of hornblende olivine clinopyroxenite hornblende clinopyroxenite clinopyroxenite gabbro and diorite hornblende occurs in all the rock types of the complex and is pargasite in composition clinopyroxene is diopsidic in composition with high cao and low tio2 contents the flat ree and sub parallel trace element patterns of both clinopyroxene and hornblende suggest that these rocks were formed by fractional crystallization from common magma with no significant crustal contamination enrichment in large ion lithophile elements relative to high field strength elements of the silicate minerals together with the variable sulfur isotope compositions 34s 2 55 11 01 of the sulfides indicate the involvement of subducted oceanic materials into the mantle source these petrological mineralogical and geochemical features strongly suggest the alaskan type nature of the wuxing complex the hydrous parental magma of the wuxing complex was derived from partial melting of the mantle wedge which had been metasomatized by subducted melts fluids the wuxing complex is a unique pt pd rich ni cu sulfide deposit in the caob the ore minerals which are dominated by pyrite pyrrhotite pentlandite chalcopyrite and platinum group mineral pgm mainly occur in clinopyroxenite and hornblende clinopyroxenite the sulfide ores are predominantly present as net textured and disseminated textures pentlandite and chalcopyrite associated with pyrrhotite commonly occur in the marginal zone of the pyrrhotite grains the pgms occur as interstitial grains at the contact between base metal sulfides and silicate minerals or along the crack of pyrrhotite or clinopyroxene or as inclusions in pyrrhotite the occurrences of sulfides and pgms in the wuxing complex together with sulfur isotope compositions and pge patterns reveal that the pge enrichment of the alaskan type complexes can be attributed to the high degree partial melting of the mantle source sulfur involvement in the source and pge transportation of hydrous melts

sims zircon u pb geochronology and sr nd isotopes of ni cu bearing mafic ultramafic intrusions in eastern tianshan and beishan in correlation with flood basalts in tarim basin nw china constraints on a ca 280 ma mantle plume zircon sims u pb dating of the poshi hongshishan bijiashan and huangshan ni cu bearing and xiangshan ni cu ti fe bearing mafic ultramafic intrusions in the eastern tianshan and beishan rift yields a relatively restricted range of 278 6 ma to 284 0 ma the histogram of compiled age data of basalts in the tarim basin and mafic ultramafic intrusions in the eastern tianshan and beishan rift has a peak of 280 ma which probably represents the time of mantle plume activity the basalts have lower end t values in the range of 9 2 1 7 and mg of 2 wt indicating that they were generated directly from a peripheral zone of the mantle plume by low degree of melting the mafic ultramafic intrusions have higher end t of 1 3 11 2 and mg of 33 90 and lower tio2

850 790 ma bimodal volcanic and intrusive rocks in northern zhejiang south china a major episode of continental rift magmatism during the breakup of rodinia abstract shrimp u pb zircon ages and geochemical and nd isotopic data are reported for mid neoproterozoic igneous rocks in northern zhejiang province south china the shenwu dolerite dykes the daolinshan granite diabase complex and the shangshu bimodal basalt rhyolite volcanic rocks were dated at 849 7 ma 794 9 ma and 792 5 ma respectively all the studied mafic rocks are tholeiitic in compositions and exhibit overall moderate enrichments in most incompatible trace elements resembling intraplate basaltic rocks in continental rifts the 850 ma shenwu dolerites are the earliest anorogenic basaltic rocks identified so far after the ca 1 1 0 9 ga sibao orogeny but seem to predate the neoproterozoic rifting our results are clearly at variance with the alternative tectonic model that an active continental margin existed along the southern margin of the yangtze block until ca 800 ma the intensive 790 ma bimodal volcanic and plutonic rocks part of a major phase of mid neoproterozoic rifting are concurrent with the widespread mid neoproterozoic intraplate magmatism throughout south china they are most likely related to the proposed mantle superplume that led to the breakup of the supercontinent rodinia

petrology of the motaghairat mafic ultramafic complex eastern desert egypt a high mg post collisional extension related layered intrusion abstract the geodynamic settings of the precambrian mafic ultramafic complexes in the eastern desert of egypt have important bearing on understanding the geotectonic evolution of the arabian nubian shield we present a detailed petrological study on a layered mafic ultramafic intrusion that is located at the contact between the precambrian continental crust and the miocene red sea oceanic crust the motaghairat layered intrusion consists of basal lherzolite orthopyroxenite troctolite olivine gabbro and anorthosite on the top variations in modal mineralogy and mineral chemistry along with the chemical composition of these units suggest their derivation from a common high mg tholeiitic parent melt through fractional crystallization processes the parental magma was derived from a metasomatised mantle source the primitive mantle normalized patterns of the calculated melts exhibit enrichment in u relative to th and ba relative lree which indicate that the enriched lithospheric mantle source was metasomatised by fluids derived from a subducted oceanic crust rather than by a sediment melt geological and petrological evidences suggest that the layered motaghairat intrusion was emplaced during post orogenic extension following subduction break off and lithospheric delamination after the collision between the amalgamated island arc terranes and the saharan metacraton the heat source required to melt the metasomatised lithospheric mantle was derived from the upwelling of hot asthenosphere after the subduction break off

petrogenesis and tectonic implications of late carboniferous a type granites and gabbronorites in nw iran geochronological and geochemical constraints abstract carboniferous igneous rocks constitute volumetrically minor components of iranian crust but preserve important information about the magmatic and tectonic history of sw asia ghushchi granites and gabbronorites in nw iran comprise a bimodal magmatic suite that intruded ediacaran cambrian gneiss and are good representatives of carboniferous igneous activity precise sims u pb zircon ages indicate that the gabbronorites and granites were emplaced synchronously at 320 ma ghushchi granites show a type magmatic affinities with typical enrichments in alkalis ga zr nb and y depletion in sr and p and fractionated ree patterns showing strong negative eu anomalies the gabbronorites are enriched in lrees nb ta and other incompatible trace elements and are similar in geochemistry to oib type rocks granites and gabbronorites have similar e nd t 1 3 to 3 4 and 0 1 to 4 4 respectively and zircon e hf t 1 7 to 6 2 and 0 94 to 6 5 respectively the similar variation in bulk rock e nd t and zircon e hf t values and radiometric ages for the granites and gabbronorites indicate a genetic relationship between mafic and felsic magmas either a crystal fractionation or silicate liquid immiscibility process further work is needed to resolve petrogenetic details the compositional characteristics of the bimodal ghushchi complex are most consistent with magmatic activity in an extensional tectonic environment this extension may have occurred during rifting of cadomian fragments away from northern gondwana during early phases of neotethys opening

alaskan type kedanshan intrusion central inner mongolia china superimposed subduction between the mongol okhotsk and paleo pacific oceans in the jurassic abstract the xing an inner mongolia accretionary belt in the eastern central asian orogenic belt caob was produced by the subduction of three oceanic plates the paleo asian mongol okhotsk and paleo pacific oceanic plates the interactions between these plates remain unclear here we report an alaskan type ultramafic mafic intrusion in the kedanshan area central inner mongolia china the main lithologies of this intrusion include cumulate dunite pyroxene peridotite olivine pyroxenite and cumulate gabbro with late gabbroic anorthositic dykes minerals and whole rock compositional variations display characteristics of an arc cumulate trend alaskan type through fractional crystallization of mg rich and hydrous basaltic magma associated with oceanic subduction zircons from two gabbro samples yield early jurassic ages of 193 6 ma and 179 4 ma respectively we conclude that this ultramafic mafic complex is an accumulated intrusion from an arc related high mg magma chamber above a supra subduction zone considering the ages location and tectonic setting of the complex we suggest that it was most likely generated by melting of a large and triangle shaped mantle wedge during superimposed subduction between the mongol okhotsk and the paleo pacific oceanic plates in the jurassic

origin of permian gabbroic intrusions in the southern margin of the altai orogenic belt a possible link to the permian tarim mantle plume abstract zircon u pb dating and hf isotopic compositions whole rock and mineral geochemistry as well as field and petrographic observations are reported for the permian gabbroic intrusions in the southern margin of the altai orogenic belt similar zircon ages 280 ma of these intrusions to that of the permian tarim mantle plume suggest a possible link to the tarim plume olivine grains from the intrusions have fo contents ranging from 69 to 81 and nio from 0 05 to 0 21 wt indicating the crystallization from evolved mafic magmas clinopyroxene grains from the intrusions have 0 3 to 1 5 wt tio 2 and display a rift to arc like affinity the intrusions are tholeiitic and have slightly lree depleted to lree enriched chondrite normalized rare earth element ree patterns zircons from the intrusions have e hf t values higher than 8 0 whereas the whole rock e nd t values range from 2 7 to 7 0 rocks from the intrusions have nb la of 0 2 to 0 8 consistent with the derivation from a depleted sub lithospheric mantle with variable degrees of crustal contamination the presence of primary hornblende and or biotite in the rocks indicates metasomatism by slab derived fluids melts in the mantle source we propose that upwelling of asthenospheric mantle in the permian tarim mantle plume along the irtish suture zone resulted in partial melting of the sub continental lithospheric mantle based on a comparison between the ni cu sulfide bearing kalatongke complex and un mineralized intrusions we suggest that ni cu sulfide mineralized intrusions in the altai orogenic belt are characterized by the enrichments of lrees and liles and by the presence of primary water bearing minerals

chemical overprinting of magmatism by weathering a practical method for evaluating the degree of chemical weathering of granitoids abstract quantitative determination of the degree of chemical weathering of rocks is a fundamental task in environmental and engineering geology and many weathering indices based on whole rock chemistry have been proposed however most classical indices are of limited application to granitoids in a wide area because these lithotypes generally exhibit wide chemical variation arising from their petrogenesis the chemical evolution produced during rock weathering therefore overprints pre existing magmatic chemical variation this problem can cause many classical weathering indices to yield misleading results this study proposes a method that compensates for the influence of petrogenesis on calculation of the weathering index the method is based on a bivariate plot of the magmatic chemical variation mcv in granitoids and the degree of chemical weathering dcw the mcv axis must be based on an element that reflects magmatic processes and is also relatively immobile during rock weathering in this study tio 2 contents are utilized for the mcv the dcw axis is fundamentally defined by the ratios of more mobile to less mobile elements during weathering and hence many classical indices can be applied the improved value of the degree of chemical weathering dcw i for a weathered rock is derived by dcw i s mcv cv mcv 1 dcw 1 where mcv 1 is the measured composition e g tio 2 content of the weathered rock dcw 1 denotes the ratios of more mobile to less mobile elements of the weathered rock the s parameter is the slope of the least square linear regression for fresh granitoids in the mcv dcw relationship mcv cv is a correction factor which is given by the average point on the mcv axis e g average tio 2 of the fresh rocks this method is useful for evaluating the degree of weathering of various granitoids and enhances the practical application of many weathering indices

erosion by rivers and transport pathways in the ocean a provenance tool using 40ar 39ar incremental heating on fine grained sediment measured from 39 ar and 37 ar respectively are consistent with typical values for k and ca bearing minerals calculations show that the bulk mineralogy is reflected in the outgassing k ca spectra and identify plagioclase as the dominant mineral contributing to the plateau defining portion of the age spectra a second model predicts bulk sediment ages from integrated bedrock cooling age area estimates in order to examine whether bulk sediment plateau ages are representative of the average cooling age of rocks from a given river basin calculated and observed ages are notably similar in three river basins when topographic and lithologic effects are accounted for overall this technique shows considerable promise not only in tracking individual terrigenous sources in the marine realm but also for understanding processes such as erosion and sediment transport in terrestrial systems

magmatic unmixing in spinel from late precambrian concentrically zoned mafic ultramafic intrusions eastern desert egypt abstract spinel is widespread in the ultramafic core rocks of zoned late precambrian mafic ultramafic complexes from the eastern desert of egypt these complexes gabbro akarem genina gharbia and abu hamamid are precambrian analogues of alaskan type complexes they are not metamorphosed although weakly altered each intrusion is composed of a predotite core enveloped by pyroxenites and gabbros at the margin silicate mineralogy and chemistry suggest formation by crystal fractionation from a hydrous magma relatively high cr 2 o 3 contents are recorded in pyroxenes up to 1 1 wt and amphiboles up to 1 4 wt from the three plutons the chrome spinel crystallized at different stages of melt evolution as early cumulus inclusions in olivine inclusions in pyroxenes and amphiboles and late magmatic intercumulus phase the intercumulus chrome spinel is homogenous with narrow range of chemical composition mainly fe 3 rich spinel spinel inclusions in clinopyroxene and amphibole reveal a wide range of al 27 44 wt al 2 o 3 and mg 6 13 wt mgo contents and are commonly zoned the different chemistries of those spinels reflect various stages of melt evolution and re equilibration with the host minerals the early cumulus chrome spinel reveals a complex unmixing structures and compositions three types of unmixed spinels are recognized crystallographically oriented irregular and complete separation unmixing products are al rich type i and fe 3 rich type ii spinels with an extensive solid solution between the two end members the compositions of the unmixed spinels define a miscibility gap with respect to cr al fe 3 extending from the fe 3 al join towards the cr corner spinel unmixing occurs in response to cooling and the increase in oxidation state the chemistry and grain size of the initial spinel and the cooling rate control the type of unmixing and the chemistry of the final products causes of spinel unmixing during late magmatic stage are analogous to those in metamorphosed complexes the chemistry of the unmixed spinels is completely different from the initial spinel composition and is not useful in petrogenetic interpretations spinels from oxidized magmas are likely to re equilibrate during cooling and are not good tools for genetic considerations

source and tectonic implications of tonalite trondhjemite magmatism in the klamath mountains in the klamath mountains voluminous tonalite trondhjemite magmatism was characteristic of a short period of time from about 144 to 136 ma early cretaceous it occurred about 5 to 10 m y after the 165 to 159 ma josephine ophiolite was thrust beneath older parts of the province during the nevadan orogeny thrusting from 155 to 148 ma the magmatism also corresponds to a period of slow or no subduction most of the plutons crop out in the south central klamath mountains in california but one occurs in oregon at the northern end of the province compositionally extended members of the suite consist of precursor gabbroic to dioritic rocks followed by later more voluminous tonalitic and trondhjemitic intrusions most plutons consist almost entirely of tonalite and trondhjemite poorly defined concentric zoning is common tonalitic rocks are typically of the low al type but trondhjemites are generally of the high al type even those that occur in the same pluton as low al tonalite the suite is characterized by low abundances of k2o rb zr and heavy rare earth elements sr contents are generally moderate 450 ppm by comparison with sr rich arc lavas interpreted to be slab melts up to 2000 ppm initial 87sr 86sr 18o and nd are typical of mantle derived magmas or of crustally derived magmas with a metabasic source compositional variation within plutons can be modeled by variable degrees of partial melting of a heterogeneous metabasaltic source transitional mid ocean ridge to island arc basalt but not by fractional crystallyzation of a basaltic parent melting models require a residual assemblage of clinopyroxene garnet plagioclase amphibole residual plagioclase suggests a deep crustal origin rather than melting of a subducted slab such models are consistent with the metabasic part of the josephine ophiolite as the source because the josephine ophiolite was at low t during nevadan thrusting an external heat source was probably necessary to achieve significant degrees of melting heat was probably extracted from mantle derived basaltic melts which were parental to the mafic precursors of the tonalite trondhjemite suite thus under appropriate tectonic and thermal conditions heterogeneous mafic crustal rocks can melt to form both low and high al tonalitic and trondhjemitic magmas slab melting is not necessary

u pb ages and hf o isotopes of zircons from late paleozoic mafic ultramafic units in the southern central asian orogenic belt tectonic implications and evidence for an early permian mantle plume abstract u pb age and hf o isotope data of zircons from the mafic ultramafic complexes and their related rhyolites and granites in the eastern tianshan and beishan regions were obtained by sims and la icpms the hf o isotopic composition and hf model age of the zircons are closely related to their tectonic setting and formation age the zircons from the mafic ultramafic complexes in the bogeda haerlike and jueluotage belts have higher ehf t 8 17 and lower 18o 4 6 whereas most of the zircons in the gabbros granites and rhyolite from the middle tianshan massif and beishan rift display lower ehf t 0 8 and higher 18o 5 8 the positive ehf t and relatively lower 18o values suggest that these mafic ultramafic complexes were derived from depleted mantle which was subjected to subduction related modification processes the hf isotopic composition of zircons in the granites has revealed that the growth of juvenile crust was also very significant in the southern margin of the central asian orogenic belt during the paleozoic the hf model ages of the analyzed zircons together with the regional geology suggest that the beishan area had a northward subduction possibly from ca 900 ma to ca 400 ma whereas the eastern tianshan had a south directed subduction most likely from ca 600 ma to ca 310 ma additionally zircons with ca 280 ma u pb ages display wider and more scattered ehf t and 18o variations than the relatively older and younger ones which further support the early permian mantle plume model

petrological geochemical and geochronological constraints on the origin of the xiadong ural alaskan type complex in nw china and tectonic implication for the evolution of southern central asian orogenic belt abstract the xiadong mafic ultramafic complex is located in the central tianshan terrane the southern part of the central asian orogenic belt it is composed of dunite hornblende clinopyroxenite hornblendite and hornblende gabbro several dykes of hornblendite hornblende gabbro and gabbroic diorite cut the main body these rocks are characterized by adcumulate textures the dunites contain high fo 92 3 96 6 olivines with rare orthopyroxene and plagioclase hornblende chromite and magnetite are common phases in all of the rock types geochemically the rocks of the xiadong complex display high mgo contents and extremely low trace element abundances although the rocks of the main body show higher mgo contents and lower trace element abundance compared to the dykes both are characterized by flat ree patterns and enrichment of lile relative to hsfe these petrological mineralogical and geochemical features indicate that the xiadong complex is a typical ural alaskan type complex the cross cutting and intrusive relationships of the rock units and their distinct geochemical features suggest that the main body represents a different pulse from the intrusive dyke the positive whole rock e nd t values together with variable 87 sr 86 sr i ratios show a depleted mantle source metasomatized by subduction related material zircon u pb dating on one hornblende gabbro from the main body of the complex yields a u pb 206 pb 238 u age of 479 ma and zircons from three samples of the dykes yield ages of 477 379 and 313 ma all the zircons have positive e hf t and slightly higher 18 o than normal mantle and their hf o isotopic values are correlated to their ages the older samples are comparable to those from the permian mafic ultramafic complexes in the beishan terrane which were derived from a mantle source metasomatized by subduction of the south tianshan ocean while the younger samples are similar to those from the paleozoic complexes in the eastern tianshan which record the subduction features of the junggar ocean these results suggest that the central tianshan was a continental arc from at least the ordovician to the carboniferous the lithospheric mantle beneath the central tianshan was depleted in sr nd hf isotopic compositions and was metasomatized by melts and or fluids from the subduction of the south tianshan and junggar oceans in paleozoic the formation of the xiadong ural alaskan type complex was the product of arc magmatism via high degree melting of the lithospheric mantle beneath the southern central asian orogenic belt

translithospheric mantle diapirism geological evidence and numerical modelling of the kondyor zoned ultramafic complex russian far east we report new structural microstructural petrological and major and trace element data on ultramafic rocks from the kondyor zoned ultramafic complex in far east russia the ultramafic rocks are subdivided into three subconcentric lithologies from core to rim 1 a metasomatic domain where generally phlogopite rich dykes pervasively intrude dunite 2 a main dunite core 3 a pyroxenite rim the ultramafic rocks have nearly vertical contacts with the surrounding archaean basement gneisses quartzites and marbles and hornfelsed riphean sediments the hornfelsed sediments show a relatively steep 60 outward dipping layering which rapidly flattens to horizontal away from the inner contact although the riphean sediments define a dome like structure the inward shallow dipping foliation of the dunites indicates a synformal structure detailed petro structural investigations indicate that the kondyor dunites were deformed by solid state flow under asthenospheric mantle conditions the outward textural change from coarse to fine grained equigranular dunite and the outward increasing abundance of subgrains and recrystallized olivine grains suggest dynamic recrystallization while fluid circulation was channelized within the core metasomatic zone with a decreasing melt fraction from core to rim and also suggest that solid state deformation induced grain size reduction towards the cooling border of the kondyor massif based on their geochemistry the dunites are interpreted as mantle rocks strongly affected by reaction with melts similar to the jurassiccretaceous aldan shield lamproites rim pyroxenites were formed by a melt consuming peritectic reaction implying the existence of at least a small conductive thermal gradient around the dunite body while the latter was still at near solidus temperature conditions this suggests that the zoned structure of kondyor was initiated at mantle depths most probably within the subcontinental lithosphere upon cooling the lamproitic melts were progressively focused in the central part of the massif and drained into vein conduits where they reacted with the wall rock dunite two dimensional numerical modelling based on finite differences with a marker in cell technique incorporates temperature dependent rheologies for both molten and non molten host rocks the modelling consolidates the structural petrological and geochemical interpretations which show that the dunites represent the synformal flat lying apex of an asthenospheric mantle diapir triggered by fluid pressure channelized in the core which nearly reached the earths surface we conclude that translithospheric mantle diapirism is an important mode of mass transfer in the earth

genesis of the wuxing pt pd rich cu ni sulfide deposit in the eastern central asian orogenic belt evidences from geochronology elemental geochemistry and sr nd hf isotopic data the wuxing pt pd rich cu ni sulfide deposit in heilongjiang province northeast china is located to the northeast of the dunhua mishan fracture of the eastern central asian orogenic belt the mafi

the petrology of poas volcano lavas basalt andesite relationship and their petrogenesis within the magmatic arc of costa rica abstract basalt andesite and dacite from poas volcano exhibit a distinct petrogenetic evolution the basalts al 2 o 3 16 17 4 k 2 o 1 43 1 61 are hypersthene normative tholeiites with phenocrysts of labradorite olivine and two pyroxenes orthopyroxene en 67 56 is generally rimmed by clinopyroxene fs 10 18 wo 39 48 in all basalts and in a few andesites pyroxene phenocrysts have al and ti rich salite rims that surround cores of orthopyroxene or augite having lower values the al z values of tetrahedral site occupied by al for these augites are above 6 thus defining on the al z vs tio 2 diagram an intermediate trend between arc related cumulates and rift related tholeiites and ophiolite layered cumulates moreover the presence of olivines with coronae of wormy intergrowths of orthopyroxene and ti magnetite similar to those occurring in metagabbros of the granulite facies and characteristic of arenal gabbroic xenoliths together with anorthite megacrysts an 94 91 suggest a possible contamination origin for these basalts sr isotope ratios for the basalts are between 0 703816 and 0 703849 whereas andesites and dacites have lower ratios 0 703676 0 703715 these relationships together with the mineral compositions suggest that the basalts may have formed from basaltic andesites by contamination with olivine gabbros and anorthosites of the lower crust pearce element ratio analysis confirms that the basalts had a different origin from the rest of the suite and that the dacite may be derived by crystal fractionation of the less silicic basaltic andesite with removal mainly of clinopyroxene and plagioclase

origin of pge depleted ni cu sulfide mineralization in the triassic hongqiling no 7 orthopyroxenite intrusion central asian orogenic belt northeastern china the 216 ma hongqiling no 7 intrusion is located in the southern margin of the central asian orogenic belt northeastern china it was the second largest ni producer after jinchuan in china the intrusion crops out in an area of 0 013 km2 on the surface it intruded gneisses and marbles of the paleozoic hulan formation the intrusion is mainly composed of orthopyroxenite 95 vol with minor harzburgite and norite over 90 vol of the intrusion contains disseminated net textured or massive sulfides the bulk sulfide ores recalculated to 100 sulfide have low average pge concentrations 5 6 ppb os 3 1 ppb ir 4 7 ppb ru 2 2 ppb rh 31 ppb pt and 11 ppb pd and high cu pd ratios 3 105 3 107 consistent with the compositions of sulfide liquids segregated from pge depleted magma olivine composition fo 86 mol indicates a fractionated parental magma back calculation shows that the parental magma was possibly derived by olivine fractionation from a primary magma containing 16 wt mgo numerical modeling indicates that pge depletion in the parental magma of the hongqiling no 7 intrusion was due to previous sulfide segregation in the staging chamber whole rock data show light rare earth element ree enrichments negative nb ta anomalies and positive nd t 216 ma values from 3 to 5 which can be explained by mixing of a mantle derived magma with a granitic melt formed by magma underplating in the crust the os t 216 ma values of sulfide ores vary from 50 to 187 the highly variable os t values coupled with similar nd t values indicate preferential assimilation of crustal sulfides during magma rock interaction based on the observations we propose that the parental magma of the hongqiling no 7 intrusion formed by contamination of mantle derived s undersaturated magma by crustally derived granitic melt together with olivine crystallization in a staging chamber the contaminated magma became pge depleted due to sulfide segregation in response to crustal contamination the pge depleted magma became s undersaturated again during ascent due to the negative effect of pressure on the maximum solubility of sulfur in magma upon arrival near the present level of the hongqiling no 7 intrusion a second event of sulfide segregation took place due to addition of crustal sulfides from immediate country rocks the immiscible sulfide liquids and orthopyroxene crystallizing from the magma were retained in a dynamic conduit to form the ore bearing intrusion while much of the magma left the conduit to form some of the nearby sulfide poor intrusions we suggest future exploration in the region to focus on olivine bearing and orthopyroxene dominant intrusions with ages similar to that of the hongqiling no 7 intrusion

roots of an archean volcanic arc complex the lac des iles area in ontario canada abstract the lac des iles area is located in the wabigoon subprovince an archean granite greenstone terrain in the superior province in ontario canada the area consists of several unmetamorphosed plutons of biotite and hornblende tonalite and mafic and ultramafic intrusions which are emplaced into an older tonalitic gneiss the mafic ultramafic bodies are concentric in shape and consist of periodotite pyroxenite gabbro and hornblende gabbro cumulates and diorite the largest intrusion the lac des iles complex consists of several mafic and ultramafic centers an internal sm nd isochron from the southern ultramafic complex gives an intrusion age of 2736 28 ma mswd 1 5 nd t 1 2 this age is also assumed to be representative of the time of emplacement of the other mafic and ultramafic bodies as well as the unmetamorphosed tonalite plutons the tonalitic gneiss represents older continental crust and had an nd value of 1 at this time the nd t values of the felsic and mafic ultramafic suites are positive ranging from 0 to 2 this range can be explained if primary magmas derived from a depleted mantle reservoir assimilated continental crust such as the tonalite gneiss during their ascent this implies that the tonalite magmas in the lac des iles area represent mixtures of the tonalite country rock and mantle magmas which formed by partial melting of depleted mantle or of mafic rocks underplated beneath the continental lithosphere the morphology and the rock assemblages of the mafic and ultramafic intrusions in the lac des iles area resemble alaskan type intrusions which are common components of phanerozoic magmatic arcs associated with subduction zones in addition the crystallization sequence in the mafic ultramafic intrusions suggests that they are derived from a parental magma with high al basalt affinities the mafic ultramafic rocks are depleted in high field strength elements but enriched in large ion lithophile elements and the tonalites belong to calc alkaline rock series displaying fractionation trends towards trondhjemitic compositions thus the archean lac des iles area and the root zones of phanerozoic active continental margins share many geological geochemical and petrological features which would suggest that the lac des iles area formed in an andean type continental margin

examination of the deep levels of an island arc evidence from the tonsina ultramafic mafic assemblage tonsina alaska the tonsina ultramafic and mafic assemblage located in the northern chugach mountains along the trans alaska crustal transect tact is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc it is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the peninsular terrane along the northern fringe of the chugach mountains in southern alaska these masses the roots of a mid jurassic island arc have been uplifted along the north dipping border ranges fault system and crop out sporadically for 700 km from kodiak island to eastern alaska the tonsina ultramafic mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite 1 km thick overlain by a narrow zone of websterite and clinopyroxenite above these ultramafic rocks is a 5 to 6 km thick layered gabbro that is garnet bearing at its base textural relationships and chemical trends throughout the series of dunite pyroxenite garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous tholeiitic parent magma at deep levels i e at the base of a mature island arc before being emplaced along the border ranges fault the rocks were crystallized from a melt at pressures between 9 5 and 11 kbar 28 to 33 km depth and equilibrated at temperatures of 800 to 90o c based on phase relationships in the cumulus assemblages pyroxene thermobarometry and olivine spinel thermometry transitional sequences from ultramafic to mafic rocks are characterized by increasingly al rich pyroxenes and spinels or garnet and the absence of olivine plagioclase assemblages deep level fractionation of al poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of al in the evolving magma as crystallization continued the phases recorded this alumina concentration and the resultant gabbros are al rich this observed trend supports the theory of high alumina basalt generation by fractional crystallization of olivine and pyroxene comparison of the tonsina rocks with aleutian lower crustal xenoliths as well as other exposures of arc related lower crustal magma chambers in japan pakistan and western north america indicates that this process is a common mechanism for island arc magma evolution at depth

a detrital record of lower oceanic crust exhumation within a miocene slow spreading ridge macquarie island southern ocean uplift exhumation and denudation of the lower oceanic crust are recorded by sedimentary rocks of macquarie island 54 30 s 158 54 e which were deposited within the slow spreading proto macquarie spreading ridge between ca 9 and 12 ma measured stratigraphic sections typically contain basal basaltic breccia lithofacies that are overlain by a thick sequence of enriched mid ocean ridge basalt e morb with thin intercalations of gabbroic sedimentary lithofacies basaltic detritus has zeolite to lower greenschist metamorphic grades typical of the upper oceanic crust and gabbroic detritus has upper greenschist to amphibolite metamorphic grades typical of the lower oceanic crust breccia clast counts and sedimentary structures indicate that basaltic lithofacies were locally derived from the footwalls of adjacent spreading related faults sedimentary structures detrital clinopyroxene major and trace element geochemistry and 206 pb 238 u zircon geochronology indicate that the gabbroic lithofacies were more distally derived from a paleogene aged tholeiitic morb source detrital zircon populations of ca 27 and ca 33 ma correspond to oceanic magnetic anomalies 8o and 13o respectively and exclude ca 8 5 ma gabbroic rocks of macquarie island as a potential source geodynamic reconstructions show that anomaly 8o crust from the southeast indian ridge was juxtaposed against the active proto macquarie spreading ridge when sedimentary rocks of macquarie island were deposited and was a likely source for the gabbroic lithofacies the proto macquarie spreading ridge and southeast indian ridge were connected by the jurru long offset transform which has undergone significant transpression since 27 ma this transpression formed a bathymetric transverse ridge that was composed of structurally isolated blocks of heterogeneously aged paleogene source crust which provided the source for macquarie island9s gabbroic sedimentary lithofacies

evidence against subduction related magmatism for the jiaoziyan gabbro northern dabie shan china the largest gabbroic intrusion jiaoziyan in the northern orthogneiss of the dabie shan was investigated to test whether such intrusions represent arc related or collision related magma as postulated in recent regional tectonic models the jiaoziyan intrusions comprise a gabbro monzogabbro suite and are locally characterized by cumulus textured anhydrous silicates petrologic indications against arc magmatism include 1 coexisting olivine plagioclase compositions beyond the documented range of arc gabbros and 2 a compositional trend of al ti ratios in the gabbroic clinopyroxenes that is significantly different from subduction related gabbros worldwide in contrast the jiaoziyan rocks are mineralogically similar to rift gabbros geochronologic and geometric arguments against collision related magmatism i e as a consequence of slab breakoff include the following 1 crystallization of the jiaoziyan mafic intrusion postdated the triassic 245 240 ma subduction and peak orogenic metamorphism by 100 m y and 2 the rocks of the dabie cretaceous magmatic suite including jiaoziyan are on the footwall plate rather than on the hanging wall of the triassic subduction zone framework and do not show a linear distribution these observations argue that the voluminous early cretaceous magmatism in the northern orthogneiss is geodynamically unrelated to the qinling dabie orogeny but was caused by extensional tectonism related to regional magmatic underplating beneath the thinned dabie crust

chemical evolution of intercumulus liquid as recorded in plagioclase overgrowth rims from the skaergaard intrusion the intercumulus liquid of a crystal mush fills pore spaces and typically solidifies to form overgrowths on cumulus grains and poikilitic post cumulus minerals if the liquid is immobile solidification produces zoned intercumulus minerals as a result of progressive fractionation of the residual liquid convection within the mush results in buffering of the liquid composition and thus limits mineral zonation for fully solidified cumulates fossil changes in liquid composition or porosity are difficult to identify however detailed study of immobile minor components of plagioclase overgrowth rims can provide information about the progressive solidification of intercumulus material ti contents of plagioclase overgrowths in samples from the lowermost parts of the skaergaard intrusion show strong variations with anorthite content with decreasing xan ti concentrations first rise and then fall consistent with changing tio2 contents of the intercumulus liquid during solidification tio2 in plagioclase decreases sharply at an55 reflecting local saturation of fe ti oxides ti in clinopyroxene oikocrysts also falls rimward but zoning in faster diffusing species fe mg is limited other than slight reverse zones that may occur on the plagioclase margins xan falls continuously during crystallization the reverse zoning is interpreted as the result of compaction driven dissolution and reprecipitation of plagioclase the continual decrease in xan is exploited together with back scattered electron images of the cumulates to produce calibrated images showing regions of progressive crystallization this allows the regions crystallizing at each stage of solidification to be visualized these images show that the final remnants of interstitial melt were present in triangular pockets and as thin grain boundary melt films this approach can provide information about the progressive reduction of porosity during cumulate solidification

petrogenesis and ore genesis of the lengshuiqing magmatic sulfide deposit in southwest china constraints from chalcophile elements pge se and sr nd os s isotopes the lengshuiqing magmatic ni cu ore deposit is hosted in four small mid neoproterozoic mafic ultramafic intrusions in the western margin of the yangtze craton in china the ore bearing intrusions were emplaced into early neoproterozoic metamorphosed volcanic sedimentary rocks granites and diorites the intrusions are dominated by lherzolite olivine websterite websterite and gabbro with sulfide mineralization associated with ultramafic rocks in the lower parts of the intrusions the forsterite contents of olivine crystals in the lengshuiqing ore bearing intrusions are up to 86 mol indicating that their parental magma experienced moderate degrees of fractional crystallization olivine crystals in the intrusions are depleted in ca

petrology and tectonic significance of gabbros tonalites shoshonites and anorthosites in a late paleozoic arc root complex in the wrangellia terrane southern alaska plutonic rocks intrusive into the late paleozoic tetelna formation of southern alaska are the underpinnings of the late paleozoic skolai arc of the wrangellia terrane there are four groups of intrusive rocks within the skolai arc 1 gabbro diorite plutons that contain gabbroic to anorthositic cumulates along with a differentiated series of gabbros and diorites of basaltic to andesitic composition 2 silicic intrusions including tonalite granodiorite and granite 3 monzonitic to syenitic plutonic rocks of the ahtell complex and related dikes and sills 4 fault bounded bytownite anorthosite of uncertain age and association these anorthosites may be related to post skolai nikolai greenstone magmatism the silicic rocks yield discordant u pb zircon ages of 290 320 ma early to late pennsylvanian relative age relations suggest that the oldest intrusive rocks are the gabbro diorite plutons the youngest are the monzonitic rocks and that the silicic rocks span this range the gabbro diorite pluto

the early permian mafic ultramafic complexes in the beishan terrane nw china alaskan type intrusives or rift cumulates abstract the petrogenesis and geodynamic setting of the early permian mafic ultramafic complexes in the beishan terrane nw china have important bearing on the prospects of ni cu pge sulfide and ti fe oxide deposits as well as in understanding the history of evolution of the southern central asian orogenic belt caob here we present results from a detailed study on a representative suit of a mafic ultramafic rocks from the bijiashan complex in the beishan terrane the complex is composed of dunite troctolite olivine gabbro and gabbro without clinopyroxenite or hornblendite in addition to olivine and clinopyroxene orthopyroxene and plagioclase are also present in all these rock types in the absence of chromite or primary hornblende the minerals display marked compositional variations with the fo content in olivines ranging from 66 to 83 and the clinopyroxenes ranging from diopside to augite the whole rock geochemistry shows good correlations among the major elements and the trace elements are characterized by flat ree patterns with eu positive anomalies and hfse depletion relative to lile these features are comparable to other mafic ultramafic complexes in the beishan and eastern tianshan terranes but obviously deviate from the typical features of alaskan type intrusives the present study suggests that the beishan complexes were formed in a post orogenic extensional environment with potential for ni cu mineralization rather than in arc related setting with alaskan type pge mineralization our study confirms that the beishan area does not conform to an arc related setting but was located within a rift setting in permian probably related to permian mantle plume event suggesting that the subduction of the paleo asian ocean had culminated by this time and the southern caob witnessed a post orogenic extensional regime in late paleozoic

simultaneous measurement of sulfur and lead isotopes in sulfides using nanosecond laser ablation coupled with two multi collector inductively coupled plasma mass spectrometers abstract we herein report the coupling of a nanosecond laser ablation system with a large scale multi collector inductively coupled plasma mass spectrometer nu1700 mc icpms np 1700 and a conventional nu plasma ii mc icpms np ii for the simultaneous laser ablation and determination of in situ s and pb isotopic compositions of sulfide minerals we found that the required aerosol distribution between the two spectrometers depended on the pb content of the sample for example for a sulfide containing 100 3000 ppm pb the aerosol was distributed between the np 1700 and the np ii spectrometers in a 1 1 ratio while for lead contents 3000 and

devonian alaskan type ultramafic mafic intrusions and silicic igneous rocks along the southern altai orogen implications on the phanerozoic continental growth of the altai orogen of the central asian orogenic belt abstract in this contribution we have carried out an integrated study of the petrography geochronology geochemistry and mineral chemistry for the devonian alaskan type ultramafic mafic complexes and granitoids widely distributed along the southern altai orogen in order to provide a better understanding of the continental growth process and the regional tectonic evolution la icpms zircon u pb dating of three mafic samples one acidic volcanic rock sample and one granitic sample yielded the concordia ages of ca 400 ma olivine chemical compositions indicate these mafic intrusions crystallized from evolved mafic magmas and compositions of the clinopyroxene and hornblende show typical arc affinities whole rock compositions reveal their tholeiitic signature with enriched lrees and liles and intensive depleted hfses nb ta zr and hf leading to low hfse lree ratios nb la 0 1 0 4 isotopically these mafic intrusions have positive zircon e hf t 7 6 to 14 3 mostly 10 and positive whole rock e nd t 0 8 to 5 3 values which suggest that their parental magmas were derived from a time integrated depleted mantle source these features together with the presence of primary water bearing minerals biotite and or hornblende suggest that their primitive magmas were derived from a time integrated depleted lithosphere mantle which were metasomatized by fluids released from subduction slab these findings in combination with coeval voluminous granitoids and arc signature volcanic rocks indicate that the underplating of mafic magmas at the continental arc root leads to the partial melting of the continental crust it is inferred that mafic magmatic underplating in the altai orogenic belt significantly contributed to the phanerozoic vertical continental crust growth in the altai orogen

experimental constraints on the origin of the 1991 pinatubo dacite crystallisation and mantle interaction experiments have been performed on the 1991 pinatubo dacite to constrain its petrogenesis in the dacite h2o system at 1000 mpa magnetite and either clinopyroxene or amphibole are the liquidus phases no garnet is observed at this pressure dacite mantle interaction at 920 mpa produces massive orthopyroxene crystallisation in addition to amphibole phologopite amphibole crystallising in dacite at 1000 mpa faithfully reproduces aluminium rich hornblende preserved in the cores of amphibole phenocrysts of the 1991 dacite suggesting a high pressure stage of dacite crystallisation under high melt h2o contents 10 wt and low temperatures 950c altogether plagioclase amphibole and melt inclusion compositions suggest that the pinatubo dacite was water rich oxidized and not much hotter than 900c when emplaced in the shallow reservoir in which most phenocrysts precipitated until the onset of the 1991 eruptions the strongly fractionated ree pattern demands garnet somewhere during dacite genesis which in turn requires high pressure derivation partial melting of subducted oceanic crust gives melts unlike the pinatubo dacite interaction of these slab melts with sub arc peridotite is unable to produce such type of dacite nor is a direct mantle origin conceivable on the basis of mantle dacite interaction experimental results dehydration melting of underplated basalts requires temperatures too high to match with petrological constraints and does not allow production of dacite melts having the low feo mgo high h2o ni and cr contents typical of pinatubo like magmas on the face of chemical and phase equilibrium constraints the most plausible origin of the pinatubo dacite is via high pressure fractionation in the upper mantle of an hydrous oxidised primitive basalt that crystallises amphibole and garnet upon cooling as shown by recent phase equilibrium work dacite melts so produced are directly expelled from the uppermost mantle or lower crust to shallow level storage reservoirs from which they erupt occasionally following injection of h2o poor mantle derived basalts that can ascent without massive crystallisation unlike their hydrous counterparts parental to dacite magmas that rest at mantle depths dacites such as the pinatubo one may thus witness particular h2o rich conditions of the sub arc mantle rather than the melting of a young and hot subducting oceanic plate

controlled electromagnetic sources for measuring electrical conductivity beneath the oceans 1 forward problem and model study exact closed form expressions for the electromagnetic induction fields produced by vertical and horizontal current sources in the conducting ocean overlying a one dimensional earth are derived from the maxwell equations numerical methods for the evaluation of the solutions are given including correction for the finite size of real sources simple models of the electrical conductivity structure of the ocean crust and lithosphere are deduced from geologic petrologic and laboratory data and their electromagnetic response is modeled horizontal electric dipole sources produce much larger field amplitudes than their vertical counterparts for a given frequency and range and the horizontal electric field offers superior received signal performance reflections of electromagnetic waves from the sea surface and thermocline must be considered for low enough frequencies or long ranges estimates of the ambient noise level from natural electromagnetic sources in the frequency range 0 01 10 hz are presented the ability of controlled sources to determine features of the conductivity of the ocean crust and upper mantle especially low conductivity zones is demonstrated if the mantle conductivity is low enough horizontal ranges of 50 km and conductivity estimates to over 20 km depth can be achieved

discrimination of ophiolitic from nonophiolitic ultramafic mafic allochthons in orogenic belts by the al ti ratio in clinopyroxene in mafic igneous suites typical of anorogenic igneous provinces mid ocean ridges back arc basins continental rifts and hotspots clinopyroxenes of tholeiitic and silica undersaturated rocks incorporate al in clinopyroxene chiefly via the charge coupled substitution vimgivsi2 vitiival2 as the catial2o6 molecule this substitution is promoted by high titania activity relative to silica in the melt and hence is particularly characteristic of silica undersaturated mafic rocks mafic igneous suites of convergent plate margins i e arc tholeiites and calc alkalic gabbros and basalts characteristically incorporate al in clinopyroxene largely as the cafe3 alsio6 molecule the vimgivsi vife3 ival substitution is promoted by relatively high fugacities of h2o and o2 a characteristic feature of arc axis magmas recognition of cumulates that are complementary to compositions of eruptive rocks in arcs is crucial to development of valid petrogenetic models for arc magmas plots of tetrahedrally coordinated al vs octahedral ti in augite of gabbroic and ultramafic cumulates show that augite data arrays for arc related cumulates have a trend of substantially higher al ti in clinopyroxene than do those from rift related tholeiites the al ti ratio in clinopyroxene may be utilized to discriminate ophiolitic from nonophiolitic ultramafic mafic allochthons in orogenic belts

applicability of 1d and 2 5d marine controlled source electromagnetic modelling we present two and a half dimensional 2 5d and three dimensional 3d integral equation modelling of the marine controlled source electromagnetic method we implement 2 5d modelling using a point source and a two dimensional reservoir and compare the results with point source responses from one dimensional and three dimensional reservoir models these methods are based on an electric field domain integral equation formulation we show how the 2 5d method performs in terms of both accuracy and computing speed with different configurations we compare the results from 1d 2 5d and 3d modelling for a symmetrically placed reservoir and the in line acquisition configuration as a function of different reservoir sizes in the cross line direction thickness and for different frequencies and depths depending on the model s parameters 2 5d modelling can be considered as an accurate and fast method for marine controlled source electromagnetic acquisition optimization and interpretation if the thickness of the reservoir is less than one fifth of the skin depth of the embedding and if the depth of the reservoir is two or more times the skin depth of the embedding the largest amplitude difference between two dimensional and three dimensional reservoirs is less than 10 when the source is above the centre of the reservoir in this paper we discuss supporting examples with different configurations where the 2 5d results lead to an optimistic detection estimate phase differences between 2 5d and 3d modelling are even smaller and the 2 5d solution can be used to assess the ability to detect the reservoir with a given acquisition configuration

the feasibility of reservoir monitoring using time lapse marine csem monitoring changes in hydrocarbon reservoir geometry and pore fluid properties that occur during production is a critical part of estimating extraction efficiency and quantifying remaining reserves we examine the applicability of the marine controlled source electromagnetic csem method to the reservoir monitoring problem by analyzing representative 2d models these studies show that csem responses exhibit small but measureable changes that are characteristic of reservoir depletion geometry with lateral flooding producing a concave up depletion anomaly curve and bottom flooding producing a concave down depletion anomaly curve lateral flooding is also revealed by the spatial temporal variation of the csem anomaly where the edge of the response anomaly closely tracks the retreating edge of the flooding reservoir measureable changes in csem responses are observed when 10 of the resistive reservoir is replaced by conductive pore fluids however to avoid corrupting the relatively small signal changes associated with depletion the acquisition geometry must be maintained to a fraction of a percent accuracy additional factors such as unknown nearby seafloor inhomogeneities and variable seawater conductivity can mask depletion anomalies if not accounted for during repeat monitoring measurements although addressing these factors may be challenging using current exploration csem practices straightforward solutions such as permanent monuments for seafloor receivers and transmitters are available and suggest the method could be utilized with present day technology

sulfide saturation in mafic magmas is external sulfur required for magmatic ni cu pge ore genesis the importance of externally derived sulfur in the genesis of sulfide rich magmatic ni cu platinum group element pge deposits remains a key yet unresolved issue calculations utilizing a variety of mafic magma types indicate that in general sulfide saturation by fractional crystallization occurs after ni has been depleted due to olivine crystallization cu and pge rich layers may form during relatively later stages of closed system crystallization but unless the collection of the cotectic proportion of immiscible sulfide is extremely efficient the mass of sulfide is too small to produce an economic deposit we show that there are numerous processes that may lead to early sulfide saturation in mafic ultramafic magmas contamination of mantle derived magmas by siliceous country rocks or their partial melts will lower the sulfur content needed to induce sulfide liquid saturation typically by amounts ranging from 200 to 700 ppm the mixing of magmas particularly if the result is to lower the liquidus temperature of the mixed magma may also lower the sulfur content needed to attain sulfide saturation by similar amounts an increase in magma fo2 related to the addition of volatiles such as h2o and co2 is less effective in decreasing the sulfur concentration needed to achieve sulfide liquid saturation contamination processes that lead to an increase in the activity of sio2 in the melt and hence may promote orthopyroxene rather than olivine crystallization aid in generating relative ni enrichment in remaining liquid as a result of the lower dni mineral melt value of orthopyroxene relative to olivine although contamination and magma mixing may produce early sulfide saturation without the addition of externally derived sulfur ni rich sulfide deposits can form in such cases only from large volume open systems where the efficiency of sulfide collection is high no matter what the liquidus minerals may be without the addition of country rock derived sulfur the mass of sulfide necessary to generate economic ni cu pge concentrations requires efficient sulfide collection from large but not necessarily unrealistic volumes of magma small deposits 4 30 mt of sulfide may form from the collection of cotectic proportions of sulfide from less than 50 km3 of magma larger deposits such as those at noril sk could involve more than 200 km3 of magma this volume of magma is not unreasonable particularly in rift plume related settings despite such possibilities sulfur isotope data clearly indicate that externally derived sulfur has been involved in the formation of many large deposits and that collection of mantle derived sulfide in sufficient quantities to produce orebodies is a rare process we propose that magmatic ni cu pge sulfide ore formation normally requires significant sulfide supersaturation and that the addition of sulfur derived from xenoliths is the most viable mechanism for producing sulfide well above the cotectic proportion in mafic ultramafic magmas

a mafic intrusion of arc affinity in a post orogenic extensional setting a case study from ganluogou gabbro in the northern yidun arc belt eastern tibetan plateau abstract generally arc related or subduction related mafic magmas are formed during or slightly postdate subduction and characterized by depletion in high field strength elements hfses relative to the large ion lithophile elements liles and light rare earth elements lrees combining with mineral chemistry and sr nd isotopes these geochemical characteristics were usually used to define an arc setting especially for some ancient arcs that had been strongly modified by later tectonic activities however we report an exceptional case from the northern part of the triassic yidun arc belt eastern tibetan plateau the ganluogou gabbro 152 ma occurs as several intrusive bodies its mineral assemblage is olivine chrysolite plagioclase anorthite clinopyroxene diopside amphibole edenite and pargasite and phlogopite whole rock geochemistry shows low sio 2 42 87 46 99 wt total rear earth elements ree 22 8 28 4 ppm na 2 o k 2 o 0 92 1 34 wt and high al 2 o 3 mgo and feo contents it has small variations of initial 87 sr 86 sr 87 sr 86 sr i 0 7053 0 7055 and e nd t values 4 8 to 1 8 all the samples exhibit enrichment in liles including th and u but strongly depleted in hfses including nb ta zr and hf for the mineral chemistry there are two type amphiboles amp i show higher v sc cr sr nb and zr contents but lower th and u contents than those of amp ii their ree patterns range from convex shape without eu anomaly to lree enriched pattern with weak positive eu anomaly we suggest that amp i was crystallized from a liquid that was mainly buffered by olivine clinopyroxene and plagioclase while amp ii crystallized from later melt that was mainly buffered by olivine based on clinopyroxene chemistry compositions of coexisting olivine and plagioclase and whole rock sr nd isotopes the parental magma of the ganluogou gabbro is interpreted as a tholeiitic arc affinity magma which might be derived from an n morb mantle that had been metasomatised by slab derived melts in the late triassic 237 206 ma thus the ganluogou gabbro provides an example that magmas exhibiting arc affinity could in fact be formed in a post orogenic extensional setting

the gaojiacun intrusion rift or subduction related comment on revisiting the yanbian terrane implications for neoproterozoic tectonic evolution of the western yangtze block south china by li et al 2006 precambrian res 151 2006 14 30

submarine and superimposed contact metamorphic oxygen isotopic exchange in an oceanic arc sawyers bar area central klamath mountains california usa new bulk rock oxygen isotope data indicate a complicated history of fluid rock interactions in the upper few kilometers of a basaltic arc attending its mesozoic accretion to the western margin of north america folded multiply recrystallized weakly metasomatized mafic volcanics and interstratified sediments of the sawyers bar terrane an eastern segment of the western triassic and paleozoic belt were investigated the following scenario can now be reconstructed 1 island arc tholeiites iats ocean island basalts oibs and distal turbidites were deposited in a subsea environment during permian and early mesozoic time 170 245 ma basaltic rocks underwent low temperature alteration by seawater recrystallization occurred at 100 200 c and 1 kbar alkali exchange and variable mg enrichment were accompanied by increases in bulk rock 18o values of the greenstones from 6 to approximately 10 o preceeding initial stages of island arc formation 2 middle jurassic 165 170 ma suturing of the seaward oceanic arc structurally beneath a landward 227 ma blueschist terrane resulted in regional deformation and greenschist facies metamorphism pervasive overprinting took place without important chemical or isotopic exchange under conditions of 300 425 c 3 1 kbar 3 granitoid plutons emplaced during late middle jurassic time 160 165 ma heated adjacent wallrocks to 500 600 c at pressures of approximately 2 3 kbar thermal upgrading resulted in devolatilization of isotopically heavy metasediments and in the exchange of high 18o fluids with intercalated greenstones 18o values in iat and oib metavolcanics increased from 9 to 10 along axial portions of ns trending folds to more than 15 o where metabasalts are intimately interlayered with the metasediments construction of this sector of the continental margin through the assembly of a weakly deformed and polymetamorphosed oceanic arc was attended by a sequence of aqueous fluid rock isotopic exchanges reflecting the p t time evolution of the complex

a new marine controlled source electromagnetic receiver with an acoustic telemetry modem and arm folding mechanism we developed a new marine controlled source electromagnetic receiver for detecting methane hydrate zones and oil and gas reservoirs on the seafloor which is not imaged well by seismic reflection surveys to determine the seafloor structure the electromagnetic receiver should have low noise power consumption clock drift error and operating costs while being highly reliable because no suitable receiver was available in our laboratory we developed a new marine controlled source electromagnetic receiver with these characteristics the receiver is equipped with acoustic telemetry modem and an arm folding mechanism to facilitate deployment and recovering operations to demonstrate the applicability of our new receiver we carried out a field experiment offshore of guangzhou in the south china sea where methane hydrates have been discovered we successfully obtained controlled source electromagnetic data along a profile about 13 km long all six new receivers were recovered and high quality electromagnetic data were obtained relatively high apparent resistivity values were detected the results of the offshore field experiment support the claim that the electromagnetic data obtained using the new receiver are of sufficient quality for the survey target

la icp ms zircon u pb age of longwang gabbro of shanmen silver deposit in siping city and its geological significance in the mining area of the large size shanmen silver deposit in siping of southern jilin province ni cu sulfide mineralization was also found besides ag au mineralization however no reliable isotopic dating result was available for the deposit based on the interior structure of zircons from the ore bearing longwang gabrro intrusion and the results of cl analysis the authors performed in situ zircon u pb dating by means of la icp ms and as a result obtained reliable age data the results show that the gabbro intrusion was formed at 225 1 ma mswd 2 7 consistent with the age of the no 1 intrusion in the hongqiling cu ni sulfide deposit of central jilin province the comparability of the ages between the shanmen silver deposit in southern jilin and the hongqiling cu ni sulfide deposit in central jilin provides the evidence for the previous argument that both dunhua mishan and yilan yitong faults are northern extensions of the well known tanlu fault zone and that the two types of mineralization belong to different stages of tanlu fault evolution it is concluded that the two types of mineralization probably have the same derivation which could serve as an reliable guide to the exploration of these two types of mineralization in this area

tectonic setting and significance of 2 3 2 1ga magmatic events in the trans north china orogen new constraints from the yanmenguan mafic ultramafic intrusion in the hengshan wutai fuping area abstract a detailed study of the 2 2 ga yanmenguan mafic ultramafic intrusion in the hengshan area is presented with the aim of clarifying the tectonic history of the trans north china orogen in the north china craton during the period 2 3 2 1 ga shrimp u pb dating of magmatic zircons from a gabbro indicates emplacement at 2193 ma the whole rock and mineral geochemistry of this intrusion are akin to alaskan type complexes and are typical of subduction related magmas the parental magma likely originated from the depleted mantle wedge above a subduction zone and experienced fractional crystallization of olivine and clinopyroxene the continental arc represented by the mafic ultramafic intrusions and contemporary granitic and volcanic rocks together with the luliang back arc basin constitute a marginal arc back arc basin system that evolved between 2 3 and 2 1 ga based on geochronological datasets and available data on magmatism metamorphism and structural deformation a comprehensive new tectonic model for the evolution of the tnco during the neoarchean to paleoproterozoic is constructed 1 2 5 ga intra oceanic arc back arc basin system and its accretion at the end of the neoarchean 2 a long hiatus in magmatism and tectonism between 2 5 and 2 3 ga 3 2 3 2 1 ga marginal arc back arc basin magmatism developed in the accreted hengshan wutai fuping arc terranes and the luliang terrane 4 2 1 ga collision resulting in closure of the 2 3 2 1 ga luliang back arc basin accretion of the luliang hengshan wutai fuping arc terranes and their attachment to the western block of the north china craton 5 the intrusion of 1970 1910 ma post collisional mafic dykes in an extensional event 6 final collision between the western and eastern blocks between 1 9 and 1 8 ga resulting in extensive magmatism tectonism and metamorphism and lastly 7 post 1 8 ga crustal extension

geochemistry and geochronology of meta igneous rocks from the tokat massif north central turkey implications for tethyan reconstructions located in the eastern pontides of the sakarya zone in north central turkey the tokat massif records the closure of both the paleo tethyan karakaya complex and neo tethyan ocean basins meta igneous samples collected from the region were studied to determine their sources and ages we find significant geochemical differences between metagabbros of the karakaya and neo tethyan units in terms of their trace elements neo tethyan rocks are consistent with generation in an island arc setting whereas karakaya assemblages were likely generated in an oceanic spreading center environment karakaya metagabbros also contain glaucophane consistent with subduction subsequent to formation small 2 50 m zircon and baddeleyite grains from four karakaya metagabbros were dated in thin section using an ion microprobe the results demonstrate the reliability of the method to directly constrain the tectonomagmatic history of these types of assemblages the rocks yield late permian early triassic 238u 206pb crystallization ages of 258 14 ma 1 zircon and 254 8 ma 1 baddeleyite and an early cretaceous minimum metamorphic age of 137 8 ma 1 zircon some zircon grains and baddeleyite grains with zircon overgrowths yield early to middle jurassic ages here we present a model in which metamorphism and deformation in this region occurred during northward subduction and closure of a paleo tethyan ocean basin and accretion of the karakaya units to the laurasian continental margin this was followed by the onset of closure of the neo tethys during the campanian paleocene and accretion of island arc units to the tokat region

a review of the application of multiple s isotopes to magmatic ni cu pge deposits and the significance of spatially variable δ33s values the determination of 33 s values in conjunction with 34 s values provides an effective means of evaluating the importance of s derivation from archean country rocks in magmatic ni cu pge ore genesis particularly where 34 s values of potential s source rocks are near 0 and similar to most s of mantle derivation however 33 s values of sulfides in some archean rocks may also be near 0 care should be taken in the evaluation of s derivation from archean rocks based solely on this parameter although anomalous 33 s values are found primarily in archean rocks rare occurrences of nonzero 33 s values have been identified in sulfides from younger rocks and hence knowledge of the multiple s isotope composition of potential contaminants is essential for an accurate assessment of s contamination of mantle derived magmas restricted ranges in 33 s values may be characteristic of particular archean formations and when used in conjunction with mixing calculations may fingerprint a distinct s source rock spatial variability in 33 s values within different types of mineralized systems may be caused by the emplacement of isotopically distinct magma pulses by locally variable mixing proportions and the addition of secondary sulfide to the primary assemblage in the case of conduit type deposits in the midcontinent rift system three component mixing calculations show that both high and low 33 s values in some units that are similar to end member values in the archean country rocks must be a function of assimilation of primarily archean s even though proterozoic carbonaceous sedimentary country rocks contain in excess of 1 wt s and are particularly viable s source rocks

continental margin tectonics forearc processes recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of u s research from the period 1987 1990 topics addressed include the geometry of accretionary prisms coulomb wedge taper and vertical motion in response to tectonic processes offscraping vs underplating or subduction the response to oblique convergence fluids in forearc settings the thermal framework and the effects of fluid advection and serpentinite seamounts also included is a comprehensive bibliography for the period

geochronological petrological and geochemical studies of the daxueshan magmatic ni cu sulfide deposit in the tethyan orogenic belt southwest china

early cretaceous granitoids in the southern pamir implications for the meso tethys evolution of the pamir plateau abstract although it is generally accepted that the pamir was formed by long term terrane drifting and accretion to the south margin of the asia the tectonic evolution during the mesozoic remains unclear the voluminous cretaceous intrusions in the southern pamir karakorum provide key information for understanding the mesozoic evolution in this region in this contribution we report detailed petrography ages mineral geochemistry whole rock elemental and sr nd geochemistry and in situ zircon lu hf isotopic compositions of the early cretaceous granitoids from the chinese wakhan corridor southern pamir these granitoids are composed primarily of granodiorite and minor monzogranite and host dioritic enclaves the granitoids were emplaced at 107 102 ma based on their zircon u pb ages they have 63 73 wt sio2 mg between 39 and 53 and acnk mostly lower than 1 1 and are i type granites the granitoids display arc like geochemical signatures characterized by lree enriched patterns variable negative eu anomalies lan ybn 11 38 23 27 eu eu 0 36 0 74 and a significant nb ta trough nb la 0 24 0 41 isotopically the granitoids have negative whole rock end t 8 73 7 35 and zircon ehf t 17 7 5 3 values the coeval dioritic enclaves ca 104 ma contain 57 63 wt sio2 and have high mg 46 57 low cr 6 86 79 50 ppm and ni 2 88 10 00 ppm these enclaves exhibit arc like trace element patterns nb la 0 34 0 43 with variable negative eu anomaly eu eu 0 52 0 87 uniform whole rock end t 4 74 4 73 and a large range of zircon ehf t values 13 3 3 7 in line with their rock association and geochemistry we suggest that the dioritic enclaves were derived from melting of a phlogopite bearing mantle source modified by subduction related process followed by fractional crystallization and later assimilation by granitic hosts these early cretaceous granitoids were most likely generated by partial melting of the precambrian lower crust with variable involvement of mafic magma parental to the dioritic enclaves our new data combined with the previous studies reveal that the widespread early cretaceous magmatism in the southern pamir karakorum were most likely generated by flat slab subduction of the meso tethys ocean during the cretaceous

comment on revisiting the yanbian terrane implications for neoproterozoic tectonic evolution of the western yangtze block south china precambrian res 151 2006 14 30

mapping thin resistors and hydrocarbons with marine em methods insights from 1d modeling the use of marine controlled source electromagnetic em csem sounding to detect thin resistive layers at depths below the seafloor has been exploited recently to assess the resistivity of potential hydrocarbon reservoirs before drilling we examine the sensitivity of the csem method to such layers with forward and inverse modeling in one and three dimensions the 3d modeling demonstrates that if both source and receivers are over a tabular 3d target 1d modeling predicts the observed response to very high accuracy experimental design can thus be based on 1d analysis in which hundreds of range and frequency combinations can be computed to find the optimal survey parameters for a given target structure modeling in three dimensions shows that the vertical electric field response is largest over the edges of a 3d target the 3d modeling also suggests that a target body needs to have a diameter twice the burial depth to be reliably seen by csem sounding a simple air wave model energy propagating from source to receiver via the atmosphere allows the effects of the target layer and atmosphere to be separated and shows where sensitivity to the target is diminished or lost because of finite water depth as a function of range frequency and seafloor resistivity unlike dc resistivity sounding the marine csem method is not completely t equivalent and in principle can resolve resistivity and thickness separately smooth inversion provides an estimate of the method s resolving power and highlights the fact that although the radial csem fields contain most of the sensitivity to the thin resistive target inverted alone they produce only increasing resistivity with depth inclusion of the radial mode csem data forces the recovery of the thin resistor but magnetotelluric data can be used more effectively to achieve the same result

petrogenesis and tectonic implications of the early jurassic fe ti oxide bearing xialan mafic intrusion in se china constraints from zircon hf o isotopes mineral compositions and whole rock geochemistry abstract abundant jurassic bimodal igneous rocks are present in the nanling region southeastern china their relationship with the tectonic evolution of southeastern china in the jurassic is still a matter of debate the 194 ma xialan gabbroic intrusion is the oldest mafic intrusion in the jurassic nanling igneous belt discovered to date the intrusion also hosts a significant fe ti oxide deposit thus this intrusion is important for the studies of fundamental controls on the early jurassic basaltic magmatism in the region and on the fe ti oxide mineralization in this type of intrusion in this paper we report hf o isotopic compositions of zircon for the intrusion and the stratigraphic variations of whole rock and important mineral compositions in the intrusion based on variations in mineral assemblages and the compositions of cumulus minerals the xialan intrusion is divided into four cyclic units i to iv from the base to the top our results indicate that each unit represents a new input of magma with composition more primitive than the resident magma the contents of fo in olivine which occurs in the base of units i and ii are 66 mol this indicates that the parental magma for the intrusion is highly fractionated extensive fractional crystallization and density driving crystal sorting appear to have played a critical role in the formation of important fe ti oxide layers in the upper parts of units i and ii clinopyroxene trace element analyses indicate that the parental magma for the xialan intrusion is characterized by pronounced negative nb ta anomalies some of the zircon crystals from the xialan intrusion have e hf t 10 and 18 o between 5 2 and 5 8 this together with the depleted sr nd isotopic compositions in whole rocks reported previously by others indicates a dominant asthenospheric mantle source with significant contribution of the overlying sclm the e hf t and 18 o values of zircon from the intrusion are negatively correlated consistent with contamination with crustal materials during magma ascent and final emplacement mixing calculation based on these data shows that 2 o 3 tio 2 compositions of clinopyroxene from the xialan intrusion are consistent with those of clinopyroxene from mafic ultramafic intrusions in continental rift zones based on the results from this study and previous studies by others we propose that the basaltic magmatism in the early jurassic in the nanling region was related to continental rifting and asthenosphere upwelling we interpret that the paleo pacific plate flat subducted during triassic and then the subducted paleo pacific plate became delaminated and foundered at ca 195 ma the early jurassic mafic ultramafic intrusions syenites and bimodal volcanic rocks represent a rift related magmatism in response to the upwelling of asthenospheric mantle originating from different degrees of mixing of magma from partial melting of the asthenospheric mantle and overlying sclm due to the break up of subducted flat slab beneath se china continent

mineral chemistry petrogenesis and tectonic setting of the wateranga layered intrusion southeast queensland australia the wateranga layered mafic intrusion 28 km2 in area 500 m thick is a tholeiitic undeformed unmetamorphosed permo triassic layered gabbroic pluton intruded into the late carboniferous goodnight beds of the goodnight block in southeast queensland the intrusion mainly consists of gabbro and norite associated with subordinate amounts of troctolite anorthosite and orthopyroxenite and rare picrite olivine gabbro is the dominant rock type of the intrusion fractionation followed a tholeiitic trend with iron enrichment in the liquid petrographic mineral chemical and whole rock geochemical data have been used to divide the intrusion into lower middle and upper zones which are interpreted as reflecting magma chamber replenishment the observed changes in the crystallization order between the zones reveal that a single parental magma is inadequate to explain the data the common differentiation indices such as an content of plagioclase mg s of olivine clinopyroxene orthopyroxene and whole ro

petrogenesis of the pt pd mineralized jinbaoshan ultramafic intrusion in the permian emeishan large igneous province sw china the jinbaoshan ultramafic intrusion is a sheet like body with a thick wehrlite unit in the center and thin pyroxenite units at the margins pge are enriched in several disseminated sulfide zones in the intrusion olivine from the intrusion has low fo and depleted ni contents compared to olivine from coeval emeishan picrites whole rock major and trace element concentrations suggest that the jinbaoshan wehrlites originally contained 30 trapped liquid the total amount of sulfide in the rocks exceeds that which could have been dissolved in the trapped liquid the jinbaoshan wehrlites are interpreted to represent residual assemblages formed by dissolution of plagioclase by passing magma no clear evidence of crustal contamination is indicated by s nd and os isotopes we envision that sulfide saturation occurred at depth due to olivine and chromite crystallization immiscible sulfide droplets were transported to the jinbaoshan conduit where they accumulated and reacted with magma successively passing through the conduit to achieve high pge concentrations

crustal evolution of island arc ultramafic magma galmoenan pyroxenite dunite plutonic complex koryak highland far east russia alaskan type platinum bearing plutons and potassium enriched mafic to ultramafic volcanic rocks are temporally and spatially associated within the late cretaceous paleocene achalvayam valaginskii intra oceanic palaeo arc system allochthonousy present in the kogak highland and kamchatka peninsula far east russia the compositions of the parental magmas to the alaskan type complexes are estimated using the galmoenan platonic complex as an example this complex composed of dunites pyroxenites and minor gabbros is the largest 20 km3 in the system and the best studied owing to associated platinum placer deposits the compositions of the principal mineral phases in the galmoenan intrusive rocks olivine fo 79 92 clinopyroxene 1 3 5 wt al2o3 0 1 0 5 wt tio2 and cr spinel 5 15 wt al2o3 and 0 3 0 7 wt tio2 are typical of liquidus assemblages in primitive island arc magmas in intra oceanic settings and closely resemble the mineral compositions in the achaivayam valaginskii ultramafic volcanic rocks the temporal and spatial association of intrusive and extrusive units and the similarity of their mineral compositions suggest that both suites were formed from similar parental magmas ne composition of the parental magma for the galmoenan platonic rocks is estimated using previously reported data for the achaivayam valaginskii ultramafic volcanic rocks and phenocryst hosted melt inclusions quantitative simulation of crystallization of the parental magma in the galmoenan magma chamber shows that the compositions of the cumulate units are best modelled by fractional cryallization with periodic magma replenishment the model calculations reproduce well the observed mineral assemblages and the trace element abundances in clinopyroxene based upon the estimated composition of the parental magmas and their mantle source we consider that fluxing of a highly refractory mantle wedge similar to the source of boninites by chlorine rich aqueous fluids is primarily responsible for both high degrees of partial melting and the geochemical characteristics of the magmas including their enrichment in platinum group elements