the metagabbros orthogneisses and paragneisses of the connemara complex western ireland although now largely disappeared within the 400 ma old galway granite the syntectonic connemara metagabbro gneiss complex which crystallized at 490 ma arenig is the largest intrusion into the dalradian rocks it is part of widespread cambro ordovician volcanism and plutonic injection into continental crust in the appalachian caledonide orogenic belt being synchronous in crystallization with the widely obducted 490 ma ophiolites within the orogeny the peridotites and hornblende gabbros were injected broken up and shouldered aside by quartz diorite granodiorite granite and trondhjemite gneisses and it is shown that these orthogneisses are not the in situ differentiation products of the gabbro magma the hornblende in the metagabbros is mainly igneous but is partly metamorphic and replaces olivine fo 75 92 orthopyroxene en 65 92 clinopyroxene wo 48 5 en 50 39 fs 5 14 and plagioclase typically an 80 92 range an 50 96 an aureole at least 10 km wide in the sillimanite zone was imposed and cordierite garnet sillimanite dalradian hornfelses were partially melted at temperatures up to 1000 o c near to the gabbros at 4 5 1 kbar corresponding to c 18 km depth this suggests that the complex forms part of the root of a now eroded batholith of calc alkaline affinities the gabbros corresponding to the transition zone to a basic bottom which is often supposed to underlie many batholiths within the orthogneisses are sheets of paragneiss formed from dalradian metasediments which have been folded up from the floor and down into sides of the magma chamber which process could be illustrative of the means by which basic magma can intimately interact with crustal material the 18 o 16 o d h nd and sr isotope evidence and the existence of inherited zircons in the gneisses indicate that the gabbro and gneiss magmas were significantly contaminated with crustal possibly dalradian material both before and after intrusion with the lowest initial 87 sr 86 sr at 490 ma being 0 708 even in samples far removed from metasediment the metagabbros are tholeiitic to high alumina basalt in character but the gneisses have distinct calc alkaline geochemistry the chemistry igneous hornblende and highly calcic plagioclase match many arc plutons in the western usa the syntectonic nature and the ubiquitous gneissic fabric in the quartz diorites trondhjemites and granites is a consequence of early movements which were probably in part connected with the separation of the connemara massif and which culminated in slicing off the largely inverted complex and carrying it southwards on the mannin thrust which underlies the whole of connemara the complex is thus a fragment of a now disrupted cambro ordovician continental magmatic arc or continent island arc environment which can be traced along the se edge of the laurentian continent

a giant deep seated slope deformation in the italian alps studied by paleoseismological and morphometric techniques this work proves the existence of a large deep seated gravitational slope deformation dgsd in a hilly region of the southwestern alps whereas dgsd are usually linked with high relief energy in mountain environments moreover we describe the usefulness of applying paleoseismological techniques by means of trench excavation to date and understand the deformation history and genesis of recent morphostructures and we found evidence of causative relationships between dgsd and surface landslides the studied dgsd of mt croce della tola mt scincina italy is the largest of the western alps it is 7 km long and involves a minimum area of 16 km2 with a volume 3 5 km3 probably extending further ne beneath lake maggiore surface several parallel scarps representing the surface expression of slip planes affect the upper part of the slope whereas the lower part presents a convex profile dgsd at mt croce della tola started in the last interglacial period 120 40 ka bp and the following glacial phase was not able to significantly modify the general slope geometry post glacial deformation also occurred especially at mt scincina after 25 ka bp post glacial and active surface landslides developed on the convex lower part of the slope suggesting that they resulted from instability due to the new profile assumed by the slopes during the deep seated deformation the occurrence of mt scincina dgsd with a very low topographic gradient is interpreted as an effect induced by on other large dgsd

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

petrology and geochemistry of a dredged clinopyroxenite dolerite basal complex from the jan mayen volcanic province norwegian greenland sea abstract serpentinised plagioclase olivine clinopyroxenite and dolerite fragments were dredged from the western jan mayen fracture zone near its intersection with the kolbeinsey ridge according to major and trace element analyses of the rocks and minerals a the clinopyroxenite represents cumulates which crystallised from a jan mayen like alkaline magma at an intermediate basaltic stage in the uppermost mantle or lower crust and b the dolerite fragments constitute an oceanic within plate tholeiitic series with alkaline affinities these rocks have been subjected to serpentinization low medium grade metamorphism metasomatism brecciation and veining part of which probably took place during fracture zone tectonism by analogy with the geological development of the canary islands they are considered to form part of a layered basal complex to the alkaline jan mayen volcanic province

the variscan calc alkalic plutonism of western corsica mineralogy and major and trace element geochemistry abstract petrographic and structural observations on the calc alkalic plutonism of western corsica revealed the existence of several successively emplaced units associated with large basic bodies the present mineralogical and geochemical study deals with the genesis evolution and relationships of these different units basic plutonism is represented by three genetically linked types of rock norites and troctolites with cumulate textures characterized by low ree contents and either no eu anomaly or a positive eu anomaly gabbros with enriched lree relatively to hree patterns probably close to an initial basaltic liquid and diorites ranging up to charnockites which represent liquids evolved to varying degrees mainly by fractional crystallization trace element data and studies on the evolution of pyroxene pairs demonstrate the consanguinity of these calc alkaline basic rocks which are derived from a high alumina basaltic melt the various granitoids granodiorites monzogranites and leucocratic monzogranites i e adamellites have distinct evolution trends as shown by the composition of their mafic minerals and by trace element distributions they cannot be considered as being derivatives of the basic suite and they cannot be related by a common fractionation sequence rather they represent distinctive batches of crustal anatexis in addition hybridization phenomena with the basic melt are noticed in granodiorites the particular problem of the low la yb eu eu and the high u th cs leucocratic monzogranites is discussed in detail in addition to more conventional trace element diagrams the simultaneous statistical treatment of all the geochemical data by correspondence factor analysis is shown to be a very use tool in distinguishing between the different units and to classify the elements according to their geochemical properties

timescales of orogeny jurassic construction of the klamath mountains an electronic supplement of this material may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 94ycj2454 timescales of orogeny jurassic construction of the klamath mountains b r hacker m m donato c g barnes m o mcwilliams and w g ernst diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order classical interpretations of orogeny were based on relatively imprecise biostratigraphic and isotopic age determinations that necessitated grouping apparently related features that may in reality have been greatly diachronous isotopic age techniques now have the precision required to resolve the timing of orogenic events on a scale much smaller than that of entire mountain belts forty five new 40ar 39ar ages from the klamath mountains illuminate the deformation metamorphism magmatism and sedimentation involved in the jurassic construction of that orogen leading to a new level of understanding regarding how preserved orogenic features relate to ancient plate tectonic processes the new geochronologic relationships show that many jurassic units of the klamath mountains had 200 ma or older volcanoplutonic basement subsequent formation of a large 170 ma arc was followed by contractional collapse of the arc collision with a spreading ridge may have led to large scale nw se extension in the central and northern klamaths from 167 to 155 ma coincident with the crystallization of voluminous plutonic and volcanic suites marked cooling of a large region of the central klamath mountains to below 350 c at 150 ma may have occurred as the igneous belt was extinguished by subduction of colder material at deeper structural levels these data demonstrate that the klamath mountains and perhaps other similar orogens were constructed during areally and temporally variant episodes of contraction extension and magmatism that do not fit classical definitions of orogeny

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

pge mineralization in andesite explosive breccias associated with the poperechny iron manganese deposit lesser khingan far east russia whole rock geochemical 190pt 4he isotopic and mineralogical evidence abstract we report in this paper an unusual occurrence of platinum group minerals in evolved explosive breccia associated with the poperechny iron manganese deposit lesser khingan range far east russia pgms in andesite breccia are represented by fe pt solid solutions 85 and pgm mostly os ir ru solid solutions sulfides and sulfarsenides 15 textural and compositional variations in pgm assemblages suggest that pt fe and os ir ru solid solutions as well as erlichmanite laurite series sulfides were formed during high temperature fractionation of mantle derived mafic parental melt similar to alaskan type complexes and were entrained in the evolved andesitic melt during its emplacement in the crust pd pt plumbostannide and copper gold solid solutions reflect late magmatic re crystallization and metasomatism early cretaceous 125 ma age of ferroplatinum in the explosive breccia suggests that pgm bearing ultramafic material could have been sampled during regional slab window tectonics related to the late mesozoic subduction of izanagi plate along southern margin of the north asian continent

the influence of water on the petrogenesis of subductionrelated igneous rocks the presence of dissolved water can significantly change the sequence in which different minerals crystallize from a silicate magma1 3 this will alter the compositional path followed by residual liquids the liquid line of descent and modify the types and compositions of crystals that accumulate at the site of cooling the sharp decrease in the solubility of water in silicate melts with decreasing pressure makes direct measurements of pre eruptive concentrations in magmas difficult because much of the dissolved water boils off before eruption here we report experimental results on the crystallization of basalts that contain substantial dissolved water up to 6 wt and show how the results can be used to infer the amount of dissolved h2o involved in the petrogenesis of lavas that preserve a record of their liquid lines of descent or the accumulated minerals left behind after solidification the presence of abundant magmatic water is a signature of subduction zone volcanism4 and can be used to associate a suite of magmas or igneous cumulates with a convergent margin back arc or interarc tectonic setting

the generation of high sr y plutons following late jurassic arc arc collision blue mountains province ne oregon abstract high sr y plutons sr y 40 occupy large areas in ancient and modern orogenic belts yet considerable controversy exists regarding mechanisms of their generation the tectonic settings in which they form and their relationship to contractional deformation through time in the blue mountains province ne oregon a suite of late jurassic 148 145 ma high sr y plutons intrude middle jurassic 162 157 ma low sr y 600 ppm and sr y values 40 and positive initial epsilon hf values 10 5 to 7 8 2 weighted average these geochemical and isotopic results are consistent with geochemical models suggesting derivation from partial melting of island arc crust in the presence of a plagioclase poor to absent clinopyroxene hornblende garnet bearing source depths 35 40 km we propose that the transition from low sr y to high sr y magmatism resulted from orogenic thickening of island arc crust in the dixie butte area during late jurassic arc arc collision between the olds ferry and wallowa island arcs at 159 154 ma this fundamental change in crustal structure influenced post orogenic magmatism and resulted in a relatively brief 3 myr 148 145 ma episode of high sr y magmatism other high sr y plutons occur throughout the us sector of the western north american cordillera e g salmon river suture zone klamath mountains peninsular ranges and closely follow major contractional events involving arc arc and arc continent collisions

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

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

neoproterozoic arc related mafic intrusions along the northern margin of south china implications for the accretion of rodinia abstract south china has been considered as part of the rodinian supercontinent during neoproterozoic time although its paleogeographic position within this supercontinent is still a matter of debate the wangjiangshan and bijigou complexes along the northern margin of south china are among the largest mafic intrusions in china new shrimp zircon u pb results indicate that these two intrusions have crystallization ages of 820 ma and 780 ma respectively enrichment of large ion lithophile and light rare earth elements and depletion of high field strength elements in these intrusions suggest derivation from an active arc along a continental margin this interpretation of these intrusions as part of a continental arc assemblage is in contrast with the previous view that they were products of a neoproterozoic mantle plume that initiated the breakup of rodinia the presence of a neoproterozoic magmatic arc suggests that rodinian oceanic lithosphere was subducted beneath the present northern margin of south

petrological evolution of the crust and mantle in its simplest form crust mantle evolution can be thought of as a study of the processes by which material is lost from the mantle and forms new crust the overall goal is achievement of mass balance between the average composition of material known to have been added to the crust and the estimated average composition of the entire crust attainment of this goal for the earth has been elusive primarily because the materials available for study have generally been involved in several stages of evolution including fractionation within both the crust and mantle and recycling of material from crust to mantle thereby complicating the interpretation of available data although the overall composition of the continental crust generally has been modelled as andesitic taylor and mclennan 1981a the present upper continental crust is granodioritic and requires a more mafic lower crust it has been proposed taylor and mclennan 1981a that originally the archean crust had an andesitic composition throughout its volume and that partial melting in the lower crust developed more felsic melts that rose to enrich the upper crust leaving a more mafic residuum of lower crust this concept has been challenged by weaver and tarney 1981a who argue that the granulites of presumed lower crustal origin do not have the correct compositions for this model if one considers more recent additions to the crust from the mantle over the past several hundred million years it appears that these latest additions are basaltic with very little if any granitic additions in fact barker 1981 in his introduction to a special issue of jgr on granites and rhyolites states to date no one has demonstrated that such magmas may form in the mantle indeed much experimental and geochemical work indicates that such siliceous liquids cannot originate there therefore such magmas must be generated in the crust in island arc environments where the average composition of added material normally is thought to be andesitic a anderson 1982 has shown that the parent melt is basaltic not andesitic in addition kay 1980 has shown that the k content of the newly added material is less than the average k content of the previously existing crust in the oceanic crust the additions from the mantle are basaltic and according to o hara 1982 probably are derived from very mafic picritic melts as parents continental flood basalts which have also added material from mantle to crust also are clearly derived from basaltic parental melts basaltic volcanism study project 1981 thus the evidence for the nature of material subtracted from the mantle and added to the crust for the past several hundred million or perhaps a billion years points to very mafic melts this contrasts significantly with the overall andesitic composition of continental crust it implies furthermore that if the overall archean crustal composition was andesitic as estimated by taylor and mclennan 1981a then additions to the crust have become more mafic with time a concept in direct conflict with most models in which secular trends are from mafic to felsic with time veizer and jansen 1979 and mclennan et al 198o a question which is beginning to appear either explicitly or implicitly is whether crustal volcanic rocks or the crust as a whole really represents the parent melt that formed in the mantle a anderson 1982 o hara 1982 and nisbet and walker 1982

a nd and sr isotopic study of the trinity peridotite implications for mantle evolution field evidence indicates that the trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center a sm nd mineral isochron for a plagioclase lherzolite yields an age t 427 32 ma and initial e nd 10 4 0 4 which is distinctly higher than that expected for typical depleted mantle at this time this age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzolite p t field this time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere the sm nd model age of the plagioclase lherzolite total rock is t chur nd 3 4 ae this suggests that the trinity peridotite was derived from a mantle that was depleted rather early in earth history the peridotite contains many generations of pyroxenite dikes and some microgabbro dikes we report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite a microgabbro dike yields a sm nd mineral isochron age of t 435 21 ma and e nd 6 7 0 3 a pyroxenite dike yields an initial e nd 7 3 0 4 the initial e nd values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite demonstrating that they are not genetically related rb sr data do not give any coherent pattern however some bounds can be put on initial sr values of e sr 21 for the plagioclase lherzolite and e sr 8 7 for the microgabbro dike it is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone major and trace elements and sm nd isotopic data indicate a multiple stage history for the trinity peridotite a small melt fraction was extracted from an undepleted source 3 4 ae or more ago to produce the proto lherzolite a large fraction of melt 12 to 23 was extracted from the proto lherzolite to produce the present rock the lherzolite was then crosscut by dikes from average depleted mantle 0 44 ae ago the data are compatible with the depleted mantle source being formed very early in earth history although most available data indicate that the depleted upper mantle has been relatively well stirred through time the trinity data suggest that very ancient nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid ocean ridge basalts as well as the oceanic lithosphere which they intrude

temporal variation of mineralogy and petrology in cognate gabbroic enclaves at arenal volcano costa rica gabbroic enclaves ejected during the current eruption phase a 1 and during the latest prehistoric eruption phase a 2 of arenal volcano show systematic variations in texture mineralogy and composition as a function of host rock chemistry and timing of eruption the most differentiated enclaves occur in the more differentiated a 2 lavas enclaves in the a 1 volcanics are consistently less evolved within the current a 1 eruption the most mafic enclaves are amphibole bearing rocks that were erupted during the first 2 3 years of activity 1968 1970 these enclaves occur in the most differentiated a 1 volcanics and are not in equilibrium with their host rocks they crystallized from a hydrous melt that was slightly more mafic than anything erupted during the current cycle we interpret the enclaves as sidewall crystallization products of a melt possibly a high alumina basalt that was immediately parental to the a 1 lavas enclaves that occur in a 1 rocks erupted after 1970 and all of the a 2 enclaves are amphibole free and less mafic than the early a 1 enclaves their chemistry suggests that they formed during the early to intermediate crystallization of their host lavas none of the enclaves contain minerals that might have equilibrated with a primary mantle derived melt geothermometry is consistent with geochemistry with amphibole bearing a 1 enclaves yielding the highest pyroxene temperatures ave 1090 c and a 2 enclaves the lowest ave 1030 c geobarometry suggests mid to upper crustal depths for the crystallization of all enclaves the enclaves are cognate and reflect pre eruptive crystallization of arenal magmas they record evolution from a hydrous basaltic magma to the drier basaltic andesites that characterize the current eruption volatiles appear to have been lost due to depressurization during the slow ascent of the magmas through the upper levels of the crust following the initial explosive eruption volatile loss and depressurization resulted in the destabilization and the progressive resorption of amphibole the a 2 lavas may represent the long term fractionation products of basaltic andesite magmas similar in composition to the a 1 lavas anorthitic plagioclase commonly thought of as a phase stabilized by high ca na and high water pressure continued to crystallize in a system with relatively low ca na and which had dehydrated and or depressurized to the point at which amphibole was no longer stable this suggests that compositional characteristics other than high ca na or high water content may have stabilized the anorthite in the basaltic and basaltic andesite melts at arenal we speculate that the high alumina content of the arenal magmas may be the stabilizing factor

the aurora volcanic field california nevada oxygen fugacity constraints on the development of andesitic magma the aurora volcanic field located along the northeastern margin of mono lake in the western great basin has erupted a diverse suite of high k and shoshonitic lava types with 48 to 76 wt sio2 o ver the last 3 6 million years there is no correlation between the age and composition of the lavas three quarters of the volcanic field consists of evolved 4 wt mgo basaltic andesite and andesite lava cones and flows the majority of which contain sparse euhedral phenocrysts that are normally zoned there is no evidence of mixed hybrid magmas the average eruption rate over this time period was 200 m 3 ykm 2 yyear which is typical of con tinental arcs and an order of magnitude lower than that for the slow spreading mid atlantic ridge all of the au rora lavas display a trace element signature common to subduction related magmas as exemplified by baynb ratios between 52 and 151 pre eruptive water contents ranged from 1 5 wt in plagioclase rich two pyroxene andesites to 6 wt in a single hornblende lamprophyre and several biotite hornblende andesites calculated oxygen fugacities fall within 0 4 and 12 4 log units of the ni nio buffer the aurora potassic suite follows a classic calc alkaline trend in a plot of feo t ymgo vs sio 2 and displays linear decreasing trends in feo t and tio2 with sio2 content suggesting a prominent role for fe ti oxides during differentiation however develop ment of the calc alkaline trend through fractional crys tallization of titanomagnetite would have caused the residual liquid to become so depleted in ferric iron that its oxygen fugacity would have fallen several log units below that of the ni nio buffer nor can fractionation of hornblende be invoked since it has the same effect as titanomagnetite in depleting the residual liquid in ferric iron together with a thermal stability limit that is lower than the eruption temperatures of several andesites 1040 10808c derived from two pyroxene ther mometry unless some progressive oxidation process occurs fractionation of titanomagnetite or hornblende cannot explain a calc alkaline trend in which all erupted lavas have oxygen fugacites the ni nio buffer in con trast to fractional crystallization closed system equi librium crystallization will produce residual liquids with an oxygen fugacity that is similar to that of the initial melt however the eruption of nearly aphryic lavas ar gues against tapping from a magma chamber during equilibrium crystallization a process that requires crys tals to remain in contact with the liquid a preferred model involves the accumulation of basaltic magmas at the mantle crust interface which solidify and are later remelted during repeated intrusion of basalt as an end member case closed system equilibrium crystallization of a basalt followed by equilibrium partial melting of the gabbro will produce a calc alkaline evolved liquid namely high sio 2 and low feo t ymgo with a relative fo2 corrected for the effect of changing temperature that is similar to that of the initial basalt differentiation of the aurora magmas by repeated partial melting of previous underplates in the lower crust rather than by crystal fractionation in large stable magma chambers is consistent with the low eruption rate at the aurora vol canic field

native metals and intermetallic compounds in subduction related ultramafic rocks from the stanovoy mobile belt russian far east implications for redox heterogeneity in subduction zones abstract plutonic associations in orogenic belts are potent indicators of multi stage fractionation of primitive arc magmas and growth of island arc crust under evolving redox conditions in ancient subduction zones ildeus lucha ultramafic mafic complex ilc was emplaced at 232 233 ma within the mesozoic stanovoy convergent margin and subsequently underwent multi stage hydrothermal alteration and greenschist to amphibolite facies metamorphism 140 ma followed by adakite and k lamprophyre magmatism 114 117 ma dunite wehrlite pyroxenite and gabbro in the ilc are composed of olivine clinopyroxene orthopyroxene plagioclase and late magmatic amphibole mineral compositions in the ilc are typical of island arc plutonic complexes characterized by early crystallization of orthopyroxene plagioclase intercumulus and presence of magmatic amphibole ultramafic mafic rocks display high field strength hfs element depletions coupled with large ion lithophile lil element enrichments indicating formation via crystal fractionation of mafic primary magma derived from a subduction related mantle source some ultramafic rocks from the ilc contain native metals w pt au ag zn bi and intermetallic compounds cu au ag pt rh pd cu sn zn etc included in silicate minerals or observed as discrete phases in the finer grained silicate oxide sulfide matrix textural evidence and association with either primary magmatic or secondary metasomatic or metamorphic mineral phases suggest either magmatic high temperature or metasomatic low temperature origin of metals and metallic compounds the following models of their formation can be proposed on the basis of the presented data a crystallization w and possibly pt from metal rich arc magmas under unusually reduced conditions in subduction related lithosphere b formation from magmatic sulfides during pervasive serpentinization and release of abiogenic hydrocarbons and c precipitation from chlorine rich saline aqueous fluids associated with collision related metasomatism of the stanovoy island arc crust

assimilation of peridotite in zoned calc alkaline plutonic complexes evidence from the big jim complex washington cascades the big jim complex is a concentrically zoned ultramafic to felsic plutonic complex which intruded the pelitic chiwaukum schist most of the major plutonic rock types from websterite through hornblendite gabbronorite hornblende gabbro and diorite to granodiorite enclose harzburgite and metaperidotite xenoliths similar to foliated metaperidotite lenses included in the chiwaukum schist the larger xenoliths preserve tectonite fabrics all have mg s mole fraction mgo mgo feo from 0 90 to 0 89 the same as those of chiwaukum metaperidotites and distinctly different from undeformed big jim dunite mg s 0 84 to 0 82 and websterite 0 82 to 0 78 contact relations indicate widespread stepwise replacement of harzburgite by pyroxenite hornblendite gabbro and diorite thermodynamic modelling using an expanded regular solution model for silicate liquids ghiorso 1985 ghiorso and carmichael 1985 predicts that reaction between olivine fo90 and a liquid with the composition of big jim diorite 1 5 wt h2o at 1 100 c and 3 kb would produce websterite mg s 0 75 to 0 81 and dunite 0 79 to 0 82 this process is exothermic and results in a negative change in volume since it increases total solid mass under conditions of decreasing temperature modelled crystal fractionation with assimilation of olivine reproduces important features of the chemical variation observed in the big jim complex where crystal fractionation alone fails the big jim complex has affinities with other ultramafic to felsic plutonic complexes such as the bear mountain complex snoke et al 1981 1982 and the emigrant gap complex james 1971 the latter have wehrlite and clinopyroxenite rather than websterite but both have concentric zoning with olivine bearing rock types surrounded by successively more felsic pyroxenite gabbro and diorite in general concentrically zoned complexes of this type may form where magma reacts with mantle derived wall rock or ultramafic cumulates assimilation of peridotite in fractionating magma may be important in subduction related magmatic arcs

mineral chemistry and crystallization conditions of the artfjallet gabbro and dolerites central swedish caledonides abstract on the basis of plagioclase zoning and other considerations the compositions of the silicate minerals in the artfjallet gabbro are judged to be close to their original magmatic compositions they can be used therefore to estimate the crystallization conditions pyroxene temperatures are 1280 1210 c for picrites 1240 1180 c for olivine gabbros and 1180 1050 c for norites olivine spinel augite thermometry yields ca 1200 c for the picrites comparison with data from melting experiments on basalts suggests that the artfjallet gabbro is one of the few intrusions where pyroxene compositions still record magmatic temperatures the pressure is constrained between 6 and 10 kbar by the country rock metamorphism and the presence of plagioclase in the gabbro no combination of the anorthite ca tschermakite an cats and forsterite enstatite reaction formulations gives a correct pressure most yielding overestimates the an cats reaction is unreliable especially for low alumina clinopyroxenes because of the small cats activities and consequent large errors 5 10 kbar during crystallization of the gabbro the silica activity probably was nearly constant and the oxygen fugacity near the qfm buffer for the dolerites which occur in and are related to the gabbro temperatures are considered to reflect their liquidus solidus interval pyroxene ca 1200 950 c brown hornblende 1025 975 c and biotite roughly 1000 900 c in quartz dolerites temperatures for hornblende 820 660 c and biotite ca 800 c are lower the primary water content of the gabbro and dolerite magmas is estimated to have been between 0 5 and 1 5 wt

la icp ms analyses on coral growth increments reveal heavy winter rain in the eastern mediterranean at 9 ma abstract sediment particles incorporated into coral skeletons reflect variation in composition and amount of suspended material in ambient water during coral growth they can be used to identify periods of enhanced storm frequency and associated freshwater discharge tortonian late miocene porites corals from crete aegean sea eastern mediterranean show pronounced annual density bands in x ray photographs 18o compositional variability reflects the annual banding equivalent with a 7 c annual sea surface temperature sst cycle over a seven year period fine sediment particles are concentrated in layers with skeletal porosity parallel to growth increments variations in the chemical composition of coral skeletons can result from changes in the environment therefore variations in element concentrations with a spatial resolution of 500 m along a transect perpendicular to growth increments were measured using laser ablation inductively coupled plasma mass spectrometry la icp ms complementary x ray diffraction x ray fluorescence and scanning electron microscope analytical techniques were applied in order to characterize the sediment particles besides sr ca and u ca variability indicating sst seasonality the alternation of layers of pure aragonite and layers of aragonite coated with sediment particles e g kaolinite montmorillonite quartz is reflected in systematic variations in major and trace element concentrations this element pattern results from a seasonal environmental mechanism where periodically enhanced input of sediment particles into the coral reef environment causes high concentrations of non lattice bound elements e g al mn mg potential sources for the non lattice bound elements i e elements related to the clay minerals such as airborne saharan dust nile derived sediment suspension as well as volcanic ash fall can be excluded on the basis of chemical mineralogical or geological evidence in contrast normalized element patterns indicate that the clay minerals represent weathering products of local rocks e g ophiolites which at present crop out on crete in mountainous areas and formed islands exposed to erosion during the late miocene high concentrations of the non lattice bound elements correlate with low ssts of the winter months the discrepancy in sst estimations between 18o and sr ca seasonality suggests that fresh water originating from intense winter rain caused a reduction of the annual 18o amplitudes and transported suspended material to the near shore reef thus we relate the seasonality in non lattice bound element concentrations to climate variations in the eastern mediterranean region during the late miocene our results indicate that the present day mediterranean type climate with a strong seasonality of precipitation and heavy winter rainfall events may have occurred at least temporarily during the late miocene at 9 ma in the eastern mediterranean

the petrology and geochemistry of granitic gneisses from the east arunta inlier central australia implications for proterozoic crustal development the entia gneiss complex is an early to middle proterozoic inlier composed of upper amphibolite facies orthogneisses and paragneisses in the eastern end of the arunta block in central australia it is separated from overlying structurally distinct compositionally different and probably younger gneisses of the harts range complex by a subhorizontal detachment zone this paper discusses the petrology of four major granitic gneiss bodies one of these the bruna gneiss has intruded this detachment zone the other three are components of the structurally underlying entia gneiss complex the entia gneiss complex has a distinctive subhorizontally layered character with recumbent folds and flat lying schistosity the granitic gneisses that are components of this complex appear to have intruded an actively deforming terrain and are folded sheet like bodies the orthogneisses of the entia complex range from gabbro to true granite in composition and the granites are i type on both geochemical and petrographic criteria they are significantly different from the relatively alkali rich granites of equivalent age which occur in some of the other northern australian proterozoic blocks and are most like the granites of the modern cordilleran belts particularly in the association of diorite and gabbro with granodiorite it is suggested they are formed in narrow collisional belts formed as a result of closure of intracratonic rift basins the high surface area to volume ratio of these granites which is a consequence of the deformational environment into which they were intruded has resulted in considerable wall rock exchange in the more differentiated end members of the suites this has led to contamination by a net gain of k2o rb and ba and loss of relatively compatible elements including the ree y nb and tio2 compared with the syn tectonic entia granites the bruna granite gneiss is post tectonic and shows geochemical affinities with a type or anorogenic palaeozoic granites most of this granite is confined to the detachment zone but the very latest phases also intrude the harts range cover sequence an important conclusion of this study is that detachment zones of the type described from this area of the eastern arunta block exercise strong control on plutonism into the early to middle proterozoic crust as long as the very mobile style of early proterozoic tectonics continues the intracrustal dislocation zone confines syn tectonic granitic magmatism to deeper levels

pervasive assimilation of carbonate and silicate rocks in the hortavaer igneous complex north central norway the intrusive complex at hortav r represents a magma transfer zone in which multiple pulses of gabbroic and dioritic magmas evolved along fe and alkali enrichment trends extreme alkali enrichment resulted in nepheline normative and sparse nepheline bearing monzodioritic and monzonitic rocks more evolved monzonitic and syenitic rocks are silica saturated and in some cases quartz bearing previous and current research recognized an abundance of clinopyroxene and other ca rich phases such as scapolite grossular rich garnet and igneous textured calcite among the mafic and intermediate rocks even the most pyroxene rich samples contain low sc concentrations which suggests early intense fractionation of clinopyroxene these features and the alkali enrichment are consistent with assimilation of carbonate rich host rocks carbon isotope ratios of the igneous textured calcite indicate an origin of the carbon from host rocks rich in calcite consistent with assimilation however low end values 3 4 to 10 2 and moderate initial 87 sr 86 sr values 0 7052 to 0 7099 indicate the need for assimilation of quartzofeldspathic rocks as well models of combined assimilation and fractional crystallization indicate that assimilation of simple end members either carbonate or silicate cannot explain the entire data set instead variable proportions of carbonate and silicate materials were assimilated with the most pronounced assimilation effects in the mafic rocks the reasons for variable degrees of assimilation are as yet uncertain it is possible that assimilation of calc silicate rocks with variable carbonate silicate proportions resulted in the range of observed compositions however the importance of carbonate assimilation in mafic rocks compared to felsic ones suggests that assimilation of carbonates was predominant at high temperature and or mafic magma compositions and assimilation of silicates was predominant at lower temperature and or felsic magma compositions we suggest that the ability of the mafic magma to dissolve higher proportions of carbonate contaminants is the result of the magma s ability to form clinopyroxene as a product of assimilation in any case extensive carbonate assimilation was possible because co2 escaped from the system d 2004 elsevier b v all rights reserved

geochronology and geochemistry of early permian mafic ultramafic complexes in the beishan area xinjiang nw china implications for late paleozoic tectonic evolution of the southern altaids abstract the hongshishan and pobei mafic ultramafic complexes outcrop in the beishan area in the southernmost altaids they consist of olivine gabbros dunites and pyroxenites zircons from olivine gabbros from the hongshishan complex yield a u pb age for emplacement of 281 8 2 6 ma the olivine gabbros have low sio 2 47 08 48 66 tio 2 0 14 0 29 mno 0 07 0 09 and k 2 o 0 06 0 09 but high mg values 81 t fe 2 o 3 4 93 6 29 the hongshishan and pobei mafic ultramafic complexes are characterized by negative anomalies in high field strength elements zr hf nb ta which we interpret as arc related signatures they also have spikes in large ion lithophile fluid mobile elements ba u pb sr and high mobile immobile element ratios i e primitive mantle normalized sr nd ba la 1 which are attributed to an aqueous fluid component in the source zircon hf isotopes indicate that the source magma was derived either from a depleted mantle with some crustal contamination or from an enriched mantle these data are interpreted to indicate that the hongshishan and pobei mafic ultramafic complexes were emplaced in the early permian subduction related environment which suggests that the paleo asian ocean in the southernmost part of altaids did not close until after early permian

petrology and geochemistry of mafic ultramafic fragments from the aguablanca ni cu ore breccia southwest spain aguablanca southwest spain is the first economic ni cu pge deposit found in southern europe two features make it an unusual example of magmatic sulfide ore it is related to the development of an andean type continental magmatic arc and it is hosted by a subvertical magmatic breccia the structural style and the geodynamic context of the deposit contrast with most plutonic ni cu pge deposits elsewhere which occur at specific levels of layered mafic intrusions in rift environments the ni cu deposit is hosted by the aguablanca intrusion a mafic body composed of gabbronorite and minor quartz diorite gabbro and norite sulfides are concentrated in a gabbronorite matrix along a subvertical dip of 70 80 n funnel like magmatic breccia that contains barren or slightly mineralized ultramafic mafic cumulate fragments modal compositions of the fragments reflect a wide variety of rock types including peridotite hornblende rich werhlite dunite and hornblende rich harzburgite pyroxenite ortho and clinopyroxenite gabbro gabbro gabbronorite and hornblende gabbro and anorthosite the primary silicate assemblage includes olivine fo91 fo79 orthopyroxene mg no 0 85 0 73 clinopyroxene mg no 0 93 0 62 plagioclase an99 an38 amphibole mg no 0 87 0 68 and phlogopite mg no 0 89 0 64 the wide range of rock types and the fe enrichment trends in the primary ferromagnesian silicates suggest magmatic differentiation processes from the parent melts with the fragments representing different stages of cumulate formation the ore bearing breccia contains both semimassive and disseminated sulfides in the gabbronorite matrix textures vary between meso and orthocumulate and the rock forming magmatic silicates are orthopyroxene mg no 0 83 0 74 clinopyroxene mg no 0 89 0 78 plagioclase an50 an77 and intercumulus amphibole mg no 0 86 0 70 phlogopite 0 84 0 69 and minor quartz the gabbronorite in the matrix of the breccia is petrographically and chemically very similar to that of the unmineralized parts of the main aguablanca intrusion and exhibits a similar differentiation trend suggesting that the matrix of the ore bearing breccia and the unmineralized rocks belong to a same magmatic suite the local presence of mafic ultramafic fragments in the barren aguablanca intrusion supports this suggestion the presence of highly ni depleted olivine whole rock cu zr ratios below 1 and the local occurrence of disseminations of magmatic sulfides in the peridotite fragments point to sulfide segregation before and or during the formation of the peridotite cumulates mantle normalized incompatible trace element patterns of the fragments along with published sulfur isotope data are consistent with crustal contamination suggesting that addition of crustal sulfur from pyrite bearing black slates led to sulfide saturation these results support a model in which sulfides segregated and settled during the differentiation of an unexposed mafic ultramafic complex now sampled as fragments in the breccia whereas the overlying silicate magma most probably fed by successive fresh magma injections underwent fractional crystallization giving rise to this cumulate sequence the emplacement of the ore breccia took place at temperatures above the monosulfide solid solution mss solidus but below the olivine and pyroxene solidus likely owing to the explosive injection of a new pulse of magma into the chamber which mingled with the sulfide liquid and disrupted the overlying cumulate sequence as a consequence fragments reached their current position in the breccia injected along with the sulfide and the silicate melts which subsequently formed the sulfide rich gabbronorite