geochemistry of silicic magmas in the macolod corridor sw luzon philippines evidence of distinct mantle derived crustal sources for silicic magmas silicic volcanic deposits 65 wt sio2 which occur as domes lavas and pyroclastic deposits are relatively abundant in the macolod corridor sw luzon philippines at makiling stratovolcano silicic domes occur along the margins of the volcano and are chemically similar to the silicic lavas that comprise part of the volcano pyroclastic flows are associated with the laguna de bay caldera and these are chemically distinct from the domes and lavas at makiling stratovolcano as a whole samples from the laguna de bay caldera contain lower concentrations of mgo and higher concentrations of fe2o3 t than the samples from domes and lavas the laguna de bay samples are more enriched in incompatible trace elements the silicic rocks from the domes makiling volcano and laguna de bay caldera all contain high alkalis and high k2o na2o ratios melting experiments of primitive basalts and andesites demonstrate that it is difficult to produce high k2o na2o silicic magmas by fractional crystallization or partial melting of a low k2o na2o source however recent melting experiments sisson et al contrib mineral petrol 148 635 661 2005 demonstrate that extreme fractional crystallization or partial melting of k rich basalts can produce these silicic magmas our model for the generation of the silicic magmas in the macolod corridor requires partial melting of mantle derived evolved moderate to k rich crystallized calc alkaline magmas that ponded and crystallized in the mid crust major and trace element variations along with oxygen isotopes and ages of the deposits are consistent with this model

early mesozoic southward subduction history of the mongol okhotsk oceanic plate evidence from geochronology and geochemistry of early mesozoic intrusive rocks in the erguna massif ne china abstract in this paper we present new zircon u pb ages hf isotope data and whole rock major and trace element data for early mesozoic intrusive rocks in the erguna massif of ne china and we use these data to constrain the history of southward subduction of the mongol okhotsk oceanic plate and its influence on ne china as a whole the zircon u pb dating indicates that early mesozoic magmatic activity in the erguna massif can be subdivided into four stages at 246 ma 225 ma 205 ma and 185 ma the 246 ma intrusive rocks comprise a suite of high k calc alkaline diorites quartz diorites granodiorites monzogranites and syenogranites with i type affinities the 225 ma intrusive rocks consist of gabbro diorites and granitoids and they constitute a bimodal igneous association the 205 ma intrusive rocks are dominated by calc alkaline i type granitoids that are accompanied by subordinate intermediate mafic rocks the 185 ma intrusive rocks are dominated by i type granitoids accompanied by minor amounts of a types these early mesozoic granitoids mainly originated by partial melting of a depleted and heterogeneous lower crust whereas the coeval mafic rocks were probably derived from partial melting of a depleted mantle modified by subduction related fluids the rock associations and their geochemical features indicate that the 246 ma 205 ma and 185 ma intrusive rocks formed in an active continental margin setting related to the southward subduction of the mongol okhotsk oceanic plate the 225 ma bimodal igneous rock association formed within an extensional environment in a pause during the subduction process of the mongol okhotsk oceanic plate every magmatic stage has its own corresponding set of porphyry deposits in the southeast of the mongol okhotsk suture belt taking all this into account we conclude the following 1 during the early mesozoic the mongol okhotsk oceanic plate was subducted towards the south beneath the erguna massif but with a pause in subduction at 225 ma and 2 the southward subduction of the mongol okhotsk oceanic plate not only caused the intense magmatic activity but was also favorable to the formation of porphyry deposits

petrology of mariana trough and lau basin basalts the crust of actively opening back arc basins has close chemical and mineralogic similarities to normal mid ocean ridge basalts n morb but some subtle but distinctive differences exist these are largely due to differences in volatiles fe oxidation ratio abundances of large ion lithophile elements lil and some high field strength hfs elements isotope data for sr pb nd he and o are variable but broadly are morb like we present data contrasting mariana trough mtb and lau basin basalts lbb showing how they differ in certain trace element and isotopic signatures and compare both to n morb relative to n morb and lbb the mtb are enriched in alkalis alkaline earths light rare earth elements ree h2o and fe fe even for primitive samples while having morb type he 3 4 7 8 to 8 4 ra sr 87 86 mean 0 70293 and nd e 10 2 the lau basin is zoned with a central area having n morb characteristics for h2o lil ree nd e 8 1 to 9 0 and o but a depletion in hfs elements sr is more radiogenic mean 87 86 0 70347 and 3he 4he 3 4 7 2 to 11 0 ra is high relative to n morb the lau basin has a zone bordering the adjacent island arcs which is like mtb in both back arc basins relatively primitive unfractionated basalts with mg 100 mg fe 62 are found which resemble postulated primitive n morb calculated parental magmas obtained by adding xenocrystic olivine plot near or on 3 and 4 phase boundaries at 1 1 5 gpa for the ol di si system low p fractional crystallization and magma mixing explain most of the observed rock types possible explanations for differences between mtb and lbb include differences in the extent of previous melting episodes of their mantle sources and effects of source metasomatism through dehydration and partial melting of subducted lithosphere the long and complex history of mariana trough mantle sources led to a metasomatized mantle which gives rise to the distinctive mtb in contrast the lau basin is dominated by basalts derived from a source depleted in volatiles lil and hfs elements a previous melting history is implied mixing with a mantle component enriched in 87sr and 3he may be due to mantle counterflow driven by the subduction process

temporal association of arc continent collision progressive magma contamination in arc volcanism and formation of gold rich massive sulphide deposits on wetar island banda arc whole rock 87sr 86sr and 18o analyses of volcanic rocks and 3he 4he analyses of sulphides and sulphates from mineralized rocks on wetar indonesia indicate a variable contribution of assimilated crustal material or sediment sourced from the subducted australian craton to the south these new data support the idea of progressive source contamination with precisely dated events showing that wetar island hosts the most extreme examples of crustal assimilation in the region the increased continental contamination occurs during the pliocene zanclian to piacenzian during distinct magmatic events between 5 and 4 ma and at 2 4 ma when 87sr 86sr ratios in unaltered lavas with whole rock 18o values between 5 7 and 9 6 increase from 0 707484 to extreme radiogenic values of 0 711656 the earlier of these magmatic events is important in the generation of the hydrothermal systems responsible for the mineralization recorded on wetar samples from this yield radiogenic 3he 4he ratios between 0 5 and 1 4 r ra similar to the data from volcanic rocks on nearby romang the later magmatic event coincides with the arrival of the australian continental margin at the subduction zone along the banda arc progressive incorporation of continental sourced components into the source region below the wetar island edifice coincides with the formation of gold rich volcanogenic massive sulphide deposits hosted within the contaminated volcanic pile

low δ 18 o tephra from a compositionally zoned magma body fisher caldera unimak island aleutians abstract we present the results of an oxygen isotope study of phenocrysts in pumice clasts and ash layers produced by the 9100 yr bp composite dacite basaltic andesite climactic eruption that formed fisher caldera in the eastern aleutians products of the eruption represent a low 18o magma with 18o plagioclase 4 79 0 24 and clinopyroxene 3 81 0 23 corresponding to equilibrium at magmatic temperatures dacitic and overlying basaltic andesitic tephra of the climactic eruption subsequent intracaldera basaltic to andesitic lavas and a cumulate inclusion are similarly low in 18o other analyzed lavas and pyroclastics of unimak island and the lower alaska peninsula as well as precaldera fisher basalt have normal 18o magmatic values 5 5 we propose a model in which prior to 9100 yr bp normal mantle derived basaltic magma coalesced in a large shallow precaldera magma chamber during late wisconsin glaciation lowering of magmatic 18o resulted then from long term assimilation of 5 10 of syn glacial hydrothermally altered country rocks differentiation of basaltic magma was concurrent with this assimilation and produced low 18o fisher dacites cumulates and post caldera crystal richer lavas we propose the use of 18o values of phenocrysts especially alteration resistant pyroxene in tephra as a tool for tephrochronological and tephrostratigraphic correlation distinctly low 18o values are useful in identification of the fisher ash in the eastern aleutians and in the lower alaska peninsula

permian bimodal volcanism in the zhangguangcai range of eastern heilongjiang province ne china zircon u pb hf isotopes and geochemical evidence abstract in this paper we report on zircon u pb dating hf isotopes major and trace elements and sr nd isotope data with the aim of constraining the petrogenesis and regional tectonic evolution of late paleozoic volcanic rocks in the zhangguangcai range of eastern heilongjiang province ne china located in the eastern segment of the central asian orogenic belt caob between the siberian and north china cratons the late paleozoic volcanic rocks in the zhangguangcai range are composed mainly of basalt basaltic andesite rhyolite and minor dacite the results of la icp ms zircon u pb dating for two basaltic rocks three rhyolites and one dacite indicate that they formed in the early permian ca 292 ma the mafic rocks have sio2 50 13 53 80 wt k2o 0 98 2 28 wt mg 0 51 0 71 cr 144 541 ppm ni 74 260 ppm 87sr 86sr i 0 7044 and end t 4 28 whereas the felsic rocks have sio2 69 12 77 98 wt k2o 3 09 5 33 wt mg 0 17 0 36 87sr 86sr i 0 7032 and end t 4 32 these data are typical of bimodal volcanism the mafic volcanic rocks are characterized by a strong enrichment in large ion lithophile elements liles such as rb ba sr and pb depletion in high field strength elements hfses such as nb ta and ti depletion in heavy rare earth elements hrees and weak negative eu anomalies eu eu 0 88 0 94 on the other hand the felsic rocks show a strong depletion in nb ta sr p and ti enrichment in th u and k and relatively large negative eu anomalies eu eu 0 28 0 95 taken together these data suggest that the mafic magma was derived from the partial melting of a depleted lithospheric mantle modified by subducted slab derived fluids and that the felsic magma originated by partial melting of newly accreted crust the early permian bimodal volcanic rocks together with the coeval a type granites indicate an extensional environment similar to a back arc basin such a setting was possibly related to subduction of the paleo asian oceanic plate beneath the jiamusi and songnen zhangguangcai range massifs

neoarchean granitoid gneisses in eastern hebei north china craton revisited abstract neoarchean granitoid gneisses are widely distributed throughout eastern hebei eastern north china craton which are dominated by deformed and metamorphosed tonalite trondhjemite granodiorite ttg diorite and granite this study presents the results of systematic zircon u pb geochronological whole rock geochemical and sm nd isotopic analyses of the neoarchean granitoid gneisses in eastern hebei these data provide insights into the archean paleoproterozoic multiple tectonothermal events as well as the petrogenesis of these gneisses zircon u pb isotopic dating indicate that the granitoid gneisses were contemporaneously emplaced between 2546 10 and 2510 10 ma reflecting a giant neoarchean igneous event throughout eastern hebei subsequently these rocks were subjected to regional amphibolite facies metamorphism at 2 48 2 45 ga the close spatial and temporal relationships in terms of magmatism and metamorphism at ca 2 5 ga suggest a uniform tectonothermal evolution of eastern hebei the granitic gneisses have mainly originated from the partial melting of juvenile metamorphosed greywackes with minor involvement of basalts the large geochemical and isotopic variations within the dioritic and ttg gneisses both provide evidence for the mixing of mafic and felsic magmas coupled with fractional crystallization however the chemical differences between the dioritic and ttg gneisses might result from individual mafic magma sources viz basaltic and high mg melts the mafic magma may also form the basalt or komatiite within the greenstone belt or evolve via fractional crystallization prior to the magma mixing large scale granitoid activities were possibly related to mafic magma underplating the combined geochronological geochemical and geological data support an archean proto mantle plume model for a geodynamic interpretation of the eastern north china craton during the neoarchean

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

oxygen isotope trajectories of crystallizing melts insights from modeling and the plutonic record elevated oxygen isotope values in igneous rocks are often used to fingerprint supracrustal alteration or assimilation of material that once resided near the surface of the earth the 18 o value of a melt however can also increase through closed system fractional crystallization in order to quantify the change in melt 18 o due to crystallization we develop a detailed closed system fractional crystallization mass balance model and apply it to six experimentally and naturally determined liquid lines of descent llds which cover nearly complete crystallization intervals melt fractions of 1 to 0 1 the studied llds vary from anhydrous tholeiitic basalts to hydrous high k and calc alkaline basalts and are characterized by distinct melt temperature sio 2 trajectories as well as crystallizing phase relationships our model results demonstrate that melt fraction temperature sio 2 relationships of crystallizing melts which are strongly a function of magmatic water content will control the specific 18 o path of a crystallizing melt hydrous melts typical of subduction zones undergo larger increases in 18 o during early stages of crystallization due to their lower magmatic temperatures greater initial increases in sio 2 content and high temperature stability of low 18 o phases such as oxides amphibole and anorthitic plagioclase versus albite conversely relatively dry tholeiitic melts only experience significant increases in 18 o at degrees of crystallization greater than 80 total calculated increases in melt 18 o of 1 0 to 1 5 can be attributed to crystallization from 50 to 70 wt sio 2 for modeled closed system crystallizing melt compositions as an example application we compare our closed system model results to oxygen isotope mineral data from two natural plutonic sequences a relatively dry tholeiitic sequence from the upper and upper main zones uumz of the bushveld complex south africa and a high k hydrous sequence from the arc related dariv igneous complex mongolia these two sequences were chosen as their major and trace element compositions appear to have been predominantly controlled by closed system fractional crystallization and their llds have been modeled in detail we calculated equilibrium melt 18 o values using the measured mineral 18 o values and calculated mineral melt fractionation factors increases of 2 3 and 1 1 5 in the equilibrium melts are observed for the dariv igneous complex and the uumz of the bushveld complex respectively closed system fractional crystallization model results reproduce the 1 increase observed in the equilibrium melt 18 o for the bushveld uumz whereas for the dariv igneous complex assimilation of high 18 o material is necessary to account for the increase in melt 18 o values assimilation of evolved supracrustal material is also confirmed with sr and nd isotope analyses of clinopyroxene from the sequence beginning with a range of mantle derived basalt 18 o values of 5 7 pristine mantle to 7 0 heavily subduction influenced mantle our model results demonstrated that high silica melts i e granites with 18 o of up to 8 5 can be produced through fractional crystallization alone lastly we model the zircon melt 18 o fractionations of different llds emphasizing their dependence on the specific sio 2 t relationships of a given crystallizing melt wet relatively cool granitic melts will have larger zircon melt fractionations potentially by 1 5 compared to hot dry granites therefore it is critical to constrain zircon melt fractionations specific to a system of interest when using zircon 18 o values to calculate melt 18 o

mercury isotope fractionation in waters and sediments of the murray brook mine watershed new brunswick canada tracing mercury contamination and transformation abstract mercury isotope fractionation was investigated in different environmental compartments of a mining impacted ecosystem the watershed of the murray brook deposit in northern new brunswick canada mercury contaminated ground water surface water as well as sediment and samples of suspended particulate matter were collected alongside gossan creek downstream of mine tailings and analyzed for mercury isotopic composition while leachates from the tailings and ground water samples showed a uniform and similar mercury isotopic signature confirming the predominant role of the mining residues in the contamination of the surrounding environment samples collected from the surface flow system provided evidence of significant mass dependent fractionation of mercury with distance from the source as dissolved mercury contents steadily decreased from 31 1 to 1 4 g hg l 1 with distance from the headwaters of the stream 202hg in the creek water progressively increased resulting in a fractionation of 0 9 over 1 6 km as a result mercury isotopic ratios in the creek appeared to closely follow a rayleigh model assuming a constant fractionation factor reactant product of 1 00038 stream bed sediments also demonstrated an enrichment of heavy isotopes along the creek with an isotopic shift of 1 over the distance of gossan creek 3100 m although mercury losses from the water column result partly from partitioning to the sediments measured mercury isotopic ratios differences between dissolved and solid phases were likely small and this study did not observe significant fractionation due to partitioning processes results rather suggested that fractionation observed in the surface water system of gossan creek is most likely the result of a combined effect of reduction and volatilisation processes the lighter isotopes being preferentially reduced and evaporated from the water column additional sediment samples collected along reaches of eighteen mile brook and the upsalquitch river that receive water from gossan creek and from tributaries unaffected by the mine contamination were analyzed for mercury isotopic composition data obtained from those sediments clearly demonstrated that mercury derived from mining activities is isotopically different from mercury naturally present in the area measured isotopic differences can ultimately be employed to trace sources of mercury notwithstanding potential fractionation caused by natural transformation processes

timing and nature of the xinlin xiguitu ocean constraints from ophiolitic gabbros in the northern great xing an range eastern central asian orogenic belt jifeng ophiolitic melange ultramafic rocks meta basalts and gabbros crops out in the northern segment of the great xing an range the eastern segment of the central asian orogenic belt which marks the closure of the xinlin xiguitu ocean associated with the collision between the erguna block and xing an block in order to investigate the formation age and magma source of the jifeng ophiolitic melange the gabbros from newly discovered the jifeng ophiolitic melange are studied with zircon u pb ages whole rock geochemistry and zircon hf isotopes zircon u pb dating from the ophiolitic gabbros yields u pb age of 647 5 3 ma which may represent the formation age of the ophiolitic melange the gabbros display low sio2 tio2 k2o contents high na2o lree contents and indistinctive ree fractionation la yb n 1 97 2 98 it shows an e morb like affinity while the element concentrations of the jifeng samples are lower than that of e morb more importantly nb displays negative anomaly in comparison with th which shows a transitional ssz type ophiolite signature moreover the e hf t values of 647 ma zircons in the gabbros range from 8 4 to 13 4 and the corresponding hf single stage ages t dm1 are between 687 and 902 ma which is obviously older than the crystallization age of 647 ma these geochemical features can be explained as melts from the partial melting of a depleted mantle source meta somatized by fluids derived from a subducted slab accordingly we conclude that the jifeng ophiolitic melange is probably related to transitional ssz type ophiolite and developed in an intra oceanic subduction which indicates that an ocean the xinlin xiguitu ocean existed between the erguna block and xing an block the ocean s formation might be no later than the neoproterozoic 647 ma and it was closed in the late cambrian because of the collision between the erguna block and xing an block

oxygen isotope composition of a section of lower oceanic crust odp hole 735b 1 the oxygen and hydrogen isotope compositions of vein minerals and 18o values of 64 whole rocks from the deeper kilometer of the 1 5 km lower crustal section of odp hole 735b in the sw indian ocean were analyzed in order to investigate seawater penetration into and reaction with the lower oceanic crust vein amphiboles have d values of 35 to 65 indicating formation from seawater fluids and hydrothermal fluids containing a component of low d magmatic fluid crystal plastic shear zones reacted with 18o enriched fluids at high temperatures and have 18o values of 6 0 7 0 indistinguishable from surrounding rocks reaction with seawater dominated fluids over a range of temperatures 350 700 c led to local 18o depleted rocks but much of the section exhibits 18o enrichment 18o up to 7 6 as the result of static alteration of plagioclase over a range of temperatures 250 to 600 c and fluid compositions chloritic veins and associated alteration by 18o enriched fluids occurred at temperatures of 150 250 c and late carbonates and fe oxyhydroxides formed from cold 10 seawater associated with open fault related fractures bulk rock 18o values of the lower 1 km range from 3 7 to 10 2 but are mostly 5 6 7 6 and the section has a mean bulk 18o of 6 5 1 0 in contrast rocks from the shallower 500 m exhibit net 18o depletion 5 1 1 0 reflecting high temperature deformation and seawater hydrothermal alteration stakes 1991 the weighted average 18o for the entire gabbro section of hole 735b is 6 0 exhibiting little or no change compared to unaltered crust although tectonism at slow spreading rates may allow exposure of lower crust to greater amounts of low temperature alteration and 18o enrichment there is no evidence for a consistent difference in the bulk 18o of the plutonic sections of oceanic crust formed at fast versus slow spreading rates

crustal differentiation processes at krakatau volcano indonesia anak krakatau is a basaltic andesite cone that has grown following the famous caldera forming 1883 eruption of krakatau it breached sea level in 1927 and since the 1950s has been growing at an average rate of similar to 8 cm a week we present new major and trace element data combined with whole rock delta o 18 sr and nd isotope data for 1883 1993 and 2002 krakatau eruptive products and the surrounding crust bombs erupted from anak krakatau during 2002 contain frothy metasedimentary and plutonic xenoliths that show variable degrees of thermal metamorphism plastic deformation and partial melting contact metamorphic minerals such as cordierite and tridymite in metasedimentary xenoliths are consistent with high temperature metamorphism and incorporation at mid to upper crustal depth energy constrained assimilation and fractional crystallization modelling of whole rock data suggests that the anak krakatau magmas have a genetic relationship with the 1883 eruption products the geochemical impact of crustal contaminants on whole rock compositions is apparently small and we conclude that low levels of assimilation of a quartzo feldspathic sediment are recorded in anak krakatau magmas plagioclase phenocrysts from the 2002 eruption exhibit disequilibrium textures and complex compositional zoning however and are also isotopically variable with a total range in sr 87 sr 86 of 0 center dot 7043 0 center dot 7048 as determined by in situ laser ablation inductively coupled plasma mass spectrometry this suggests that although shallow crustal assimilation appears to have had a limited effect on whole rock chemistry a complex late stage differentiation history is recorded within the magma s cargo of crystals and xenoliths

crustal contamination and sulfide immiscibility history of the permian huangshannan magmatic ni cu sulfide deposit east tianshan nw china abstract the huangshannan mafic ultramafic intrusion is a permian ni cu sulfide bearing intrusion in the southern margin of the central asian orogenic belt the intrusion consists of an ultramafic unit which is composed of lherzolite and olivine websterite and a mafic unit which is composed of olivine gabbronorite gabbronorite and leuco gabbronorite this intrusion was formed by two separate pulses of magma a more primitive magma for the early ultramafic unit and a more evolved magma for the late mafic unit u pb isotope geochronology of zircon from the mafic unit yields an age of 278 2 ma according to its olivine and cr rich spinel compositions the estimated parental magma of lherzolite for the huangshannan intrusion has 12 4 wt mgo indicating picritic affinity fractional crystallization modeling results and the presence of rounded sulfide inclusions in an olivine crystal fo 86 7 indicate that sulfide immiscibility was achieved at the beginning of olivine fractionation co magmatic zircon crystals from gabbronorite have a 18o value close to 6 5 which is 1 2 higher than the typical mantle value and suggests significant crustal contamination 20 the positive ehf t values of co magmatic zircon which vary from 9 2 to 15 3 and positive whole rock end t values which vary from 4 7 to 7 8 also indicate that the parental magma was derived from a depleted mantle source and contaminated by 5 20 juvenile arc crust and then by 5 upper crustal materials however modeling results of sulfur content at sulfide saturation reveal that such a large amount of crustal contamination is not sufficient to trigger sulfide saturation in the parental magma which strongly suggests that external sulfur addition probably during contamination has played a critical role in causing sulfide immiscibility furthermore the arc magmatism geochemical signatures of the huangshannan intrusion such as significant nb and ta depletion relative to la and low ca contents in olivine are interpreted as the result of slab derived fluid modification during previous subduction we speculate that the contamination of juvenile arc crust may have occurred in the lower crust because of picritic magma underplating and that partial melting in the mantle was triggered by the impingement of a mantle plume or lithosphere delamination in a post subduction environment

paleozoic intrusive rocks from the dunhuang tectonic belt nw china constraints on the tectonic evolution of the southernmost central asian orogenic belt abstract the dunhuang tectonic belt dtb is of great importance for understanding the tectonic evolution of the southernmost central asian orogenic belt caob in this study the temporal spatial distribution petrogenesis and tectonic setting of the paleozoic representative intrusive rocks from the dtb were systematically investigated to discuss crustal evolution history and tectonic regime of the dtb during paleozoic our results reveal that the paleozoic magmatism within the dtb can be broadly divided into two distinct episodes of early paleozoic and late paleozoic the early paleozoic intrusive rocks represented by a suite metaluminous slight peraluminous and medium to high k calc alkaline i type granitoids crystallized at silurian ca 430 410 ma are predominantly distributed along the northern part of the dtb they were probably produced with mineral assemblage of eclogite or garnet amphibole rutile in the residue and were derived from magma mixing source of depleted mantle materials with various proportions of archean mesoproterozoic continental crust the late paleozoic intrusive rocks can be further subdivided into two stages of late devonian stage ca 370 360 ma and middle carboniferous stage ca 335 315 ma the former stage is predominated by metaluminous to slight peraluminous and low k tholeiite to high k calc alkaline i type granitic rocks distributed in the central part of the dtb they were also generated with mineral assemblage of amphibolite to eclogite facies in the residue and originated from magma source of depleted mantle materials mixed with different degrees of old continental crust the later stage is represented by adakite and alkali rich granite exposed in the southern part of the dtb the alkali rich granites studied in this paper were possibly produced with mineral assemblage of granulite facies in the residue and were generated by partial melting of thickened lower continental crust zircon hf isotopes and field distribution of those paleozoic intrusive rocks reveal that both the silurian and the late devonian magmatic activities predominantly represent crustal growth processes in the dtb accompanied by different degrees of reworking of pre existing continental crust however the middle carboniferous ca 335 315 ma magmatic activity reflects a crustal reworking process the silurian and late devonian intrusive rocks were most likely formed in the arc related subduction zones whereas the middle carboniferous intrusive rocks were possibly formed in a transitional tectonic setting from compression to extension representing the final stage of paleozoic orogeny in the dtb these paleozoic magmatic rocks further suggest that the dtb has reactivated from a stable block to an orogen and undergone two episodes the early paleozoic and the late paleozoic of orogeny during paleozoic it represents a paleozoic accretionary orogen of the southernmost margin of the caob between the tarim craton and north china craton and tectonically extends northward to the beishan orogen and westward to the eastern south tianshan belt

reconstructing the oxygen isotope composition of late cambrian and cretaceous hydrothermal vent fluid oxygen isotope analyses 18o of 16 quartz epidote pairs from late cambrian betts cove and mings bight newfoundland ordovician thetford mines quebec canada and cretaceous troodos cyprus ophiolites are used to calculate the 18o of the hydrothermal fluids from which they crystallized we combine these with 3 quartz fluid inclusion measurements and 3 quartz magnetite measurements from the cambrian ophiolites to explore how the range in the 18o of submarine hydrothermal vent fluid has varied between the late cambrian cretaceous and today the range of calculated 18o values of vent fluid 4 to 7 4 is larger than that of modern seafloor hydrothermal vent fluid 0 to 4 we employ two numerical models to ascertain whether this range is most consistent with changes in paleo seawater 18o or with changes in the reactive flow path in ancient hydrothermal systems a static calculation of the vent fluid oxygen isotope composition as a function of the water rock ratio suggests that in an ocean with a lower 18o than today the range of vent fluid 18o should be larger our data however show little evidence that the 18o of the ocean was much lower than the global ice free value of 1 2 a dual porosity model for reactive flow through fractured and porous media is used to model the relative evolution of the 87sr 86sr and 18o of vent fluid in contact with rock our 87sr 86sr and 18o for cretaceous epidotes suggest the strontium concentration of the cretaceous oceans may have been much higher than at present the 87sr 86sr and 18o data from cambrian epidotes are strikingly different from the younger samples and are difficult to model unless fluid rock interaction in the cambrian hydrothermal systems was substantially different it is also possible that some of the quartz epidote veins have been reset by obduction related metamorphism our data suggest that the high calcium to sulfate ratio in early and cretaceous seawater may have affected the degree of strontium isotope exchange causing hydrothermal fluids to have 87sr 86sr closer to that of seawater than in modern systems

a mainly crustal origin for tonalitic granitoid rocks superior province canada implications for late archean tectonomagmatic processes the central wabigoon subprovince of the superior province like introduction most plutonic domains within archean cratons is dominated by over the last decade information fundamental to undergranitoid rocks of the tonalite trondhjemite granodiorite ttg standing and resolving many questions regarding arseries heterogeneous 2 74 throughout archean time documentation of alterative ga tonalite gneiss crust by younger tonalite magmas was probably petrogenetic models for archean ttg or ttd suites is an important process unlike the model of ttgs representing direct of particular relevance to archean tectonic processes see partial melts of subducting slabs in an arc setting this model de wit 1998 hamilton 1998 smithies 2000 as well requires no direct tie to subduction careful re evaluation of the as processes of archean continental crustal growth and ttg classification is required for it can mistakingly pigeon hole recycling e g ireland et al 1994 de wit 1998 hamilton temporally and genetically unrelated rocks and perhaps assume an 1998 henry et al 1998 2000 nutman et al 1999 incorrect petrogenetic or tectonic model an important role of crustal as in most archean shield terrains plutonic domains recycling processes in central wabigoon tonalite petrogenesis is in of the western superior province typically consist mainly keeping with evidence that supports substantial rates of continental of ttg or ttd compositions but also include more recycling as far back as the earliest archean mafic and more evolved granitoid compositions thurs

assimilation of sediments embedded in the oceanic arc crust myth or reality abstract arc magmas are commonly assumed to form by melting of sub arc mantle that has been variably enriched by a component from the subducted slab although most magmas that reach the surface are not primitive the impact of assimilation of the arc crust is often ignored with the consequence that trace element and isotopic compositions are commonly attributed only to varying contributions from different components present in the mantle this jeopardises the integrity of mass balance recycling calculations here we use sr and o isotope data in minerals from a suite of volcanic rocks from st lucia lesser antilles arc to show that assimilation of oceanic arc basement can be significant analysis of 87sr 86sr in single plagioclase phenocrysts from four soufriere volcanic complex svc st lucia hand samples with similar composition 87sr 86sr 0 7089 0 7091 reveals crystal isotopic heterogeneity among hand samples ranging from 0 7083 to 0 7094 with up to 0 0008 difference within a single hand sample o 18 measurements in the svc crystals show extreme variation beyond the mantle range with 7 5 to 11 1 for plagioclase n 19 10 6 to 11 8 for quartz n 10 9 4 to 9 8 for amphibole n 2 and 9 to 9 5 for pyroxene n 3 while older lavas pre soufriere volcanic complex with less radiogenic whole rock sr composition 87sr 86sr 0 7041 0 7062 display values closer to mantle range 6 4 to 7 9 for plagioclase n 4 and 6 to 6 8 for pyroxene n 5 we argue that the 87sr 86sr isotope disequilibrium and extreme o 18 values provide compelling evidence for assimilation of material located within the arc crust positive correlations between mineral o 18 and whole rock 87sr 86sr 143nd 144nd and 206 207 208pb 204pb shows that assimilation seems to be responsible not only for the isotopic heterogeneity observed in st lucia but also in the whole lesser antilles since st lucia encompasses almost the whole arc range of isotopic compositions this highlights the need for detailed mineral scale investigation of oceanic arc suites to quantify assimilation that could otherwise lead to misinterpretation of source composition and subduction processes

u pb zircon geochronology of plutonism in the northern peninsular ranges batholith southern california implications for the late cretaceous tectonic evolution of southern california

origin of geochemical variability by arc continent collision in the biru area southern sulawesi indonesia analyses of igneous rocks from the eocene calc alkaline and miocene geochemistry can be modelled by varying contributions potassic volcanic arc in southwest sulawesi indicate that magmas from the mantle wedge fluid from the subducted slab became more heterogeneous in their trace element and pb sr nd and the presence or absence of a subducted sedimentary isotopic signature following the collision of the buton microcontinent component white patchett 1984 gill williams with the arc at 15 ma isotopic ratios become more continental 1990 woodhead johnson 1993 kepezhinskas et al 4 my after the collisional event sr sr 0 7085 nd 1997 peate et al 1997 alves et al 1999 hoogewerff nd 0 5125 pb pb 19 2 pb pb 15 73 1999 pearce et al 1999 eiler et al 2000 despite pb pb 39 4 as the overriding plate consists of young agreement on the importance of these components there sundaland crust whereas the subducted sediment is likely to is still some dispute about the importance of an ocean have been shed from a compositionally distinct microcontinent of island basalt oib component in the wedge stolz et al australian derivation we can be certain that the continental isotopic 1990 edwards et al 1991 and the implied role of signature reflects subduction of continental material rather than residual rutile ryerson watson 1987 foley et al crustal contamination the isotopic compositions of the magmas 2000 if an arc is built upon continental crust or can be explained by the melting of a mixed mantle wedge consisting overlain by a thick layer of sediment there is a potential of fluid fluxed and sediment modified morb mantle in this model contribution from the upper plate via assimilation the maximum amount of sediment added to the mantle source is fractional crystallization afc processes which can 10 sometimes be difficult to distinguish from the signature of subducted sediment davidson 1987 thirlwall et al 1996 gasparon varne 1998 the only unambiguous way to distinguish between subducted sediment and

oxygen isotope and trace element geochemistry of zircons from porphyry copper system implications for late triassic metallogenesis within the yidun terrane southeastern tibetan plateau abstract it is intriguing to investigate the generation of the magmatic system and the geochemical signatures that distinguish the ore bearing ones from barren ones especially by examining the oxygen isotopic and trace element compositions of zircon the oxygen isotopic composition and trace element abundances of zircons from the mineralized pulang porphyries and the coeval disuga and lannitang andesites of the yidun terrane eastern tibetan plateau have been determined by secondary ion mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icp ms in this study respectively analyzed zircons have relatively high but invariant 18 o values 5 8 6 9 that are indicative of derivation from homogeneous mantle or mantle derived magmas together with contribution of mature crustal materials we propose a model for the late triassic magmatism in the yidun terrane involving magmas derived from partial melting of subduction metasomatized mantle peridotites that subsequently experienced melting assimilation storage homogenization mash processes the low calculated ti in zircon temperatures 644 c 863 c generally between 640 c and 690 c for these zircons may be caused by the lower solidus temperature for zircon in hydrous magmas and partly controlled by the crystallization of amphibole zircon grains from porphyry copper system display small negative europium anomalies mostly eu eu 0 5 contrasting with pronounced negative eu anomalies in zircons mostly eu eu magmato hydrothermal quartz vein cu mo w deposit magmato hydrothermal quartz vein mo w deposit this study suggests that zircon trace element geochemistry probably bears significant implications for elucidating the origin of the arc magmatism and guiding exploration strategies for porphyry copper deposits

paleoproterozoic metavolcanic rocks in the ji an group and constraints on the formation and evolution of the northern segment of the jiao liao ji belt china abstract in this paper we present new petrological and whole rock geochemical data for the paleoproterozoic metavolcanic rocks in the lower part of the ji an group within the northern segment of the jiao liao ji belt china as well as u pb dates and lu hf isotope data for the zircons in these rocks the new data help in understanding the original nature of these volcanic rocks as well as their tectonic setting and the formation and evolution of the belt the zircons can be divided into two groups of which one is of a typical magmatic origin whereas the other shows a metamorphic origin zircon u pb dating and lu hf isotopic results show that the magmatic zircons have peak ages at 2184 2365 2503 and 2710 ma with ehf t values ranging from 1 40 to 8 95 10 86 to 1 48 14 68 to 7 53 and 1 05 and with corresponding model ages of 2 18 2 69 2 80 3 57 2 54 3 83 and 3 22 ga respectively the metamorphic zircons show two peak ages at 1857 and 1914 ma geochemical data show that the protolith of the metavolcanic rocks consists mainly of basaltic andesite andesite and dacite along with minor basalt and rhyolite and that these rocks formed a continuous calc alkaline magmatic sequence in this sequence the intermediate basic rocks have relatively low sio2 contents are enriched in mgo feot the light rare earth elements lrees cr co and ni and depleted in heavy rare earth elements hrees and high field strength elements hfses such as nb ta and ti indicating derivation by partial melting of a depleted lithospheric mantle that had been metasomatized by fluids or melts derived from a subducted slab on the other hand the acidic rocks have high contents of sio2 and low contents of mgo and feot are enriched in alkalis ba th u and k and depleted in nb ta p and ti these geochemical characteristics together with their hf isotope data indicate the acidic rocks could have been derived from the partial melting of juvenile crust and a small amount of ancient crustal material possibly as old as late archean we consider therefore that the metavolcanic rocks in the lower part of the ji an group were formed at ca 2 18 ga as an assemblage of volcanic rocks typical of an active continental margin and that they were affected by two stage metamorphic events at 1 90 and 1 85 ga taking into account the regional geological data we consider that the evolution of the northern segment of the jiao liao ji belt was related to an arc continent collision before the middle paleoproterozoic

oxygen isotope geochemistry of oceanic arc lavas variations of oxygen isotope ratios in arc related lavas can constrain the contributions of subducted crustal igneous rocks sediments and fluids to the sub arc mantle we have measured oxygen isotope ratios in 72 arc and back arc lavas from five ocean ocean subduction zone systems using laser fluorination analyses of olivine and other phenocrysts and glass eighty percent of our samples have delta 18o values for any given phase olivine plagioclase glass or biotite within 0 2 per thousand of the average value for that phase in upper mantle peridotites and mid ocean ridge basalt morb the range for each phase is 1 0 per thousand this result contrasts with previous studies of whole rock samples which are significantly more variable even after exclusion of samples believed to be altered or fractionated by magmatic differentiation and demonstrates that most arc related lavas contain 1 2 of 18o enriched crustal oxygen from any source i e assimilation or subducted contributions elevations in delta 18o that do occur in these basic arc derived magmas relative to values most common for mantle derived lavas are associated both with enriched radiogenic isotope signatures and even more strongly with chemical indices consistent with high integrated extents of melting of their peridotite sources we interpret these relationships as evidence that melting in the sources of the high delta 18o lavas we have studied was fluxed by addition of high delta 18o aqueous fluid or perhaps a hydrous melt from the subducted slab such that sources that contain relatively large components of slab derived fluid or melt are both relatively 18o enriched and also experienced relatively large amounts of melting we have developed a quantitative model linking the amount of melting to the extents of 18o radiogenic isotope and trace element enrichment in a mantle source being fluxed by addition of aqueous fluid comparison of this model with observed variations in the geochemistry of lavas from the vanuatu fiji new caledonia region the suite of related samples showing the greatest range in delta 18o observed in this study constrains the amounts and chemical and isotopic compositions of slab derived phases in the sources of these arc related lavas assuming a delta 18o value of 20 per thousand for the slab derived fluid 0 5 1 0 wt is added to the sources of most mantle derived arc magmas the maximum amount of slab derived flux in the sources of arc magmas according to our results is 2 5 wt

pre explosive conduit conditions during the 2010 eruption of merapi volcano java indonesia the 2010 eruption is the largest explosive event at merapi volcano since 1872 the high energy of the initial 26 october explosions cannot be explained by simple gravitational collapse and the paroxysmal explosions were preceded by the growth of a lava dome not large enough to ensure significant pressurization of the system we sampled pumice from these explosive phases and determined the pre explosive depths of the pumices by combining textural analyses with glass water content measurements our results indicate that the magma expelled was tapped from depths of several kilometers such depths are much greater than those involved in the pre 2010 effusive activity we propose that the water rich magma liberated enough gas to sustain the explosivity our results imply that the explosive potential of volcanoes having dome forming effusive activity is linked to the depth from which fresh magma can be evacuated in a single explosion regardless of the evacuated volume

detrital zircon u pb lu hf and o isotopes of the wufoshan group implications for episodic crustal growth and reworking of the southern north china craton abstract the paleoproterozoic orogens of the north china craton ncc provide important windows to evaluate the crustal evolution history during early precambrian here we report results from sims zircon geochronology and oxygen isotopes as well as la icpms zircon hf isotopic analyses on detrital zircons from mesoproterozoic neoproterozoic sandstones of the wufoshan group from the southern ncc to probe the paleoarchean paleoproterozoic crustal evolution our data show episodic magmatism during the paleoarchean neoarchean and paleoproterozoic as inferred from the 207pb 206pb age range of 3514 2751 ma and the well defined 207pb 206pb weighted mean ages of 2669 15 ma 2510 8 ma 2402 39 ma 2298 36 ma 2179 10 ma 2058 16 ma 1940 6 ma 1874 7 ma and 1819 5 ma the similarity in u pb geochronology and hf isotopes of the archean detrital zircons from the wufoshan group to those from the paleoproterozoic songshan group reveal that the 2669 ma zircons record a major phase of neoarchean crustal growth and that 2510 ma zircons correspond to the latest neoarchean crustal reworking the 18o depleted isotopic features displayed by some detrital zircons are absent in those from songshan group and together with the rarity of core rim textures which were widespread in archean zircons from the songshan group we suggest that the source materials for the mesoproterozoic neoproterozoic wufoshan group are different from the archean cratonic source for the paleoproterozoic songshan group we propose that the zircons were probably sourced from the newly formed trans north china orogen tnco our paleoproterozoic zircon data provide robust evidence for continuous and complex reworking of the mesoarchean to early neoarchean crust in the southern ncc during early paleoproterozoic at ca 2402 ma 2298 ma and 2179 ma the middle paleoproterozoic ages 2058 ma 1940 ma and 1874 ma mark a partial cycle of oceanic crust subduction to continental collision during the formation of the tnco the extreme oxygen isotopic variation with the fairly restricted range in hf isotopes demonstrates that the components from both high temperature and the low temperature rock water interaction induced by the release of hydrothermal fluids from the subducted altered oceanic crust must have played a significant role during the growth of the zircons the paleoproterozoic thermal event at 1819 ma records post collisional uplifting as evidenced by an apparent negative correlation in the hf t vs 18o plot possibly inherited from the mixing of the ancient crust and the juvenile input during the oceanic subduction and continental collision integrated with the information from granulite facies metamorphic rocks in the orogenic root of the tnco our data suggest that the episodic paleoproterozoic magmatism was mainly induced by the tectonics of plate subduction and subsequent continental collision leading to the final cratonization of the ncc following the mesoproterozoic neoproterozoic sedimentation of the paleoproterozoic zircons except for the pb loss during the subsequent thermal events there was no major alteration in the hf and oxygen isotopic compositions thus the remarkably large variation of oxygen isotopes from the lower than normal mantle to the supracrustal value 18o 2 28 10 09 in the paleoproterozoic detrital zircons further demonstrates that these zircons came from the uplifted and exposed upper part of the tnco our detrital zircon data provide convincing evidence for the repeated crustal reworking in early paleoproterozoic and the onset of modern style plate tectonics in the ncc in middle paleoproterozoic

oxygen isotopes of an early archaean layered ultramafic body southern west greenland implications for magma source and post intrusion history a large number of metamorphosed ultramafic bodies are found in the region sw of the isua greenstone belt in southern west greenland enclosed in archaean banded tonalitic gneisses these enclaves of dunite and harzburgite range in size from a few to several hundred meters the ujarassuit nunat layered body occurs as a large enclave 800 m 100 m entrained within tonalitic gneisses and preserves primary igneous layering and textures this enclave has been subjected to amphibolite grade metamorphism but for the most part the magmatic mineralogy remains intact separated minerals have been analysed for 18o by co2 laser fluorination 18o values range from 4 49 to 4 89 for olivine and from 5 70 to 5 86 for orthopyroxene all chromite values are 2 5 closure temperatures recorded for olivine orthopyroxene pairs vary from 700 to 900 c much lower than expected for an ultramafic magma widespread late magmatic re equilibration has resulted in significant decreases in 18o for chromites and small changes of 0 3 in olivine but orthopyroxene values show little variation from typical magmatic signatures the lowering of 18o in chromite is associated with significant fe enrichment thermometry suggests that the re equilibration took place at 680 750 c with equilibration between plagioclase and amphibole in residual gabbro anorthosite melt around 700 c hydration possibly associated with this late melt stage resulted in further changes to chromite chemistry formation of secondary amphibole phlogopite and chlorite and may have further enhanced 18o depletion in disseminated chromites all these events are related to late magmatic processes only the later metamorphic mineral ruby corundum present in amphibole harzburgite adjacent to an intrusive gneiss vein clearly indicates an external oxygen input to this system olivines from an unlayered dunite body 3 5 km away from ujarassuit nunat have higher 18o values around 5 25 typical of more recent high temperature mantle peridotite nodules and suggesting that mantle oxygen composition at 3 8 ga was already similar to the present composition

uncoupled o and hf isotopic systems in zircon from the contrasting granite suites of the new england orogen eastern australia implications for studies of phanerozoic magma genesis abstract the permo triassic granites of the new england orogen eastern australia were emplaced into a volcanic arc complex accreted to the eastern gondwana margin in the late devonian or early carboniferous zircon u pb dating shows that the s type hillgrove 297 ma and bundarra 287 ma supersuites predated intrusion of the i type moonbi supersuite 250 ma by up to 50 ma the high 18ozrn of the s type granites 10 0 11 5 and range of u pb ages 370 300 ma and 18ozrn 5 10 of their inherited zircon cores show that their source rocks were predominantly weathered carboniferous volcaniclastics the youngest deposited

geochronology geochemistry and deformation history of late jurassic early cretaceous intrusive rocks in the erguna massif ne china constraints on the late mesozoic tectonic evolution of the mongol okhotsk orogenic belt abstract this paper presents new zircon and sphene u pb ages biotite and hornblende 40 ar 39 ar ages hf isotopic data and geochemical data for five mesozoic plutons in the erguna massif of ne china these data are used to constrain the late mesozoic tectonic evolution of the mongol okhotsk orogenic belt this new dating when combined with previously published ages indicates that the late jurassic early cretaceous j 3 k 1 intrusive rocks can be subdivided into three stages that represent periods of magmatism during the late jurassic 155 ma early early cretaceous 137 ma and late early cretaceous 123 ma in addition the rocks have undergone later deformation recorded by peak ages of 137 and 123 ma the late jurassic and early early cretaceous intrusive rocks in the study area are dominantly syenogranites and are either a type granites or are classified as alkaline series suggesting that they formed in an extensional environment the late early cretaceous intrusive rocks in this area are generally monzogranitic and were emplaced as dikes in an extensional environment along with coeval bimodal volcanics these data combined with the presence of regional unconformities in the northern part of hebei province and western part of liaoning province and the spatial distribution of coeval volcanic rocks in ne china suggest the late jurassic and early early cretaceous magmatisms and the early early cretaceous deformation in this area occurred in an extensional environment related to the delamination of a thickened part of the crust after closure of the mongol okhotsk ocean in comparison the late early cretaceous deformation and magmatism occurred in an extensional environment related to either delamination of the previously thickened crust related to the mongol okhotsk tectonic regime or the subduction of the paleo pacific plate or the combined influence of these two tectonic regimes

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

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

generation of normal and adakite like calc alkaline magmas in a non subductional environment an sr o h isotopic study of the apuseni mountains neogene magmatic province romania abstract neogene magmatism in the apuseni mountains of romania was active between 15 and 7 ma with an isolated final eruption at 1 6 ma several groups of magmatic rocks can be distinguished in the province on the basis of ages trace elements and isotopic compositions most are of calc alkaline affinity despite the fact that the region is located 200 km from the postulated carpathian subduction trench however adakite like calc alkaline magmas were also erupted and the youngest eruptions were of alkaline affinity magmatic rocks older than 13 5 ma are characterised by the highest 87sr 86sr 18o and d values in their phenocryst phases this early magmatism is considered to be a result of crustal melting related to the fast rotation of small crustal blocks volcanic rocks younger than 13 5 ma have lower 87sr 86sr and 18o ratios with a narrow range for pyroxene and amphibole mineral separates indicating closed system fractional crystallization a larger scatter of 87sr 86sr and 18o values is found in plagioclase phenocrysts and groundmasses indicating that a small amount of crustal assimilation also occurred the d values for hydrogen bearing phases such as amphibole biotite and groundmass indicate fractional crystallisation in a closed system and a trend of decreasing d in the source with time the magma source of normal calc alkaline adakite like magmas was not related to contemporaneous subduction but was likely due to melting of delaminated eclogitic mafic lower crust in contrast the late stage alkaline magmas were related to asthenospheric upwelling petrological analysis of the complex succession of magmas in the apuseni mountains province demonstrates that normal calc alkaline and adakite like magmatism can be generated in a non subductional environment in this case associated with an unusual extensional setting

revisiting the age and emplacement process of the huangshandong ni cu deposit in the central asian orogenic belt northwestern china implications for multiple magma extractions from a short lived staging magma chamber abstract magmatic ni cu deposits that associated with mafic ultramafic intrusions in convergent settings could form from multiple magma pulses with age difference varying from 1 ma to 10 ma years the huangshandong deposit in nw china of the southern central asian orogenic belt was formed in such a setting the intrusion is composed of four units formed by four magma pluses a gabbroic sequence phase i crystallized from an evolved magma a sheet like ultramafic body phase ii crystallized from a relatively primitive magma a dike like gabbronorite body phase iii crystallized from an evolved magma and an irregular ultramafic unit phase iv crystallized from a relatively primitive magma our zircon sims u pb dating shows that the phase i and phase iv magmas of the huangshandong deposit were emplaced at 280 3 1 9 ma and 281 4 2 2 ma respectively indicating that the four magma pulses were intruded over a very short period and cannot be distinguished within the uncertainty of 2 ma moreover the zircon crystals in the four units record identical hf t values average of 12 1 13 5 and 18o values average of 6 3 6 5 however the intrusive relationships of the four units illustrate that the huangshandong complex cannot form by in situ fractionation therefore the indistinguishable hf o isotopic values together with their contemporaneous emplacements indicate that the four magma pulses could originate from a same staging magma chamber the overall enrichment of hf t value relative to that of the depleted mantle 16 and elevation of 18o values relative to the mantle value 5 3 suggest 15 20 crustal material incorporation additionally the magmas may have been contaminated prior to their arrival at the staging magma chamber where the magma has subsequently experienced variable olivine fractionations and relatively primitive magma replenishment for most mafic ultramafic intrusions which are characterized by long duration magmatism 10 ma in convergent settings e g intrusions in the kalatongke area tulaergen intrusions xiarihamu intrusions the most important ni cu mineralization are closely related to the last stage magmatism this finding suggests that the intrusions emplaced at the late stage of subduction and post collisional period of the orogenic process may have more potential for ni cu mineralization exploration