niobium enriched basalts from the wabigoon subprovince canada evidence for adakitic metasomatism above an archean subduction zone abstract late archean niobium enriched basalts from the central sturgeon lake assemblage and neepawa group of the western wabigoon subprovince have mantle normalized nb la between 0 8 and 1 3 and zr y between 4 and 7 they are compositionally similar to basalts attributed to adakite metasomatism of mantle wedge regions in cenozoic subduction zones sajona et al j petrol 37 1996 693 726 in detail their sc ree systematics suggest the archean basalts were generated above the garnet stability field an association with adakite like volcanic rocks an absence of komatiites and the arc like attributes of their host sequences suggest a subduction related origin for the basalts if current models of adakite and niobium enriched basalt genesis are valid then additional examples of these rocks should be relatively common in other archean greenstone belts

geochemistry and geochronology of paleozoic intrusions in the nalati narati area in western tianshan xinjiang china implications for paleozoic tectonic evolution abstract the nalati narati area in the chinese western tianshan is characterized by abundant paleozoic intrusions including granites diorites and gabbros they are not only indicators of the interaction between crust and mantle but also useful clues for tracing the tectonic history of the tianshan orogen most early paleozoic granitoids biotite monzonitic granites and muscovite granites of this study are from the yili block the biotite monzonitic granites mixed source derived i type granites have a zircon u pb age of 497 0 5 9 ma indicating the time of the subduction of the terskey oceanic crust in the late cambrian the 427 2 5 7 ma zircon u pb age of the s type muscovite granites let us interpret that these granites may have been formed during the crust thickening process after the collision between the yili block and the nalati block in western tianshan the late paleozoic biotite granites muscovite granites quartz diorites biotite monzonitic granites granodiorites and alkali granites have respectively a la icpms zircon u pb age of 371 8 6 ma 357 2 7 5 ma 313 9 2 5 ma and 296 9 2 4 ma the biotite granites display i type geochemical features and are considered to have been derived from a lower continental crust source the muscovite granites have a pronounced s type affinity and are considered to have been formed by the partial melting of thickened continental crust after the collision between the central tianshan belt and the south tianshan belt the quartz diorites are adakite like and have an i type affinity which are considered to have been formed by partial melting of a delaminated lower crust in a post orogenic extension setting the granodiorites also show some typical geochemical features of adakite their formation is considered to be related to the lower crustal delamination and the ascending of asthenosphere after the collision event the alkali granites show an a type granite affinity they may have been formed in a within plate tectonic setting and possibly related to rifting as a consequence of extension the gabbros can be divided into two groups based on their geochemical characteristics and ages the older gabbros 415 429 ma with lower tio 2 contents are formed in an arc related setting the younger ones 340 318 ma with higher tio 2 content are formed in an intracontinental setting and influenced by crustal contamination magmatic activities recorded by paleozoic intrusions can constrain the tectonic history of the tianshan region from subduction of oceanic crust to intraplate extensional collapse and to rifting and can also corroborate that the final closure of the south tianshan ocean occurred in the late devonian early carboniferous time

middle miocene to present plate tectonic history of the southern central american volcanic arc abstract new mid miocene to present plate tectonic reconstructions of the southern central american volcanic arc cava reveal that the inception of cocos ridge subduction began no earlier than 3 ma and possibly as late as 2 ma the cocos ridge has been displaced from the malpelo ridge to the southeast since 9 ma along the panama fracture zone pfz system ambiguous pfz and coiba fracture zone cfz interaction since 9 ma precludes conclusively establishing the age of initial cocos ridge subduction detailed reconstructions based on magnetic anomalies offshore reveal several other variations in subduction parameters beneath southern central america that preceded subduction of the cocos ridge including southeastward migration of the nazca cocos caribbean triple junction along the middle america trench mat from 12 ma to present and subduction of 2 km high scarps both parallel and perpendicular to the trench from 6 to 1 ma the timing of changes in subduction processes has commonly been determined by and correlated with geologic changes in the upper plate however reliable 40 ar 39 ar dating of these events has become available only recently abstr programs geol soc am 2002 these new dates better constrain the magmatic and structural history of southern costa rica observations from this data set include a gap in the volcanic record from 11 to 6 ma which coincides temporally with emplacement of most plutons in southern costa rica normal arc volcanism ceased after 3 5 ma in southern costa rica and pliocene mostly 1 5 ma adakite volcanism was widely distributed from central panama to southern costa rica though volumetrically insignificant this new data reveals that many geologic phenomena commonly attributed to subduction and underplating of the buoyant cocos ridge in fact precede inception of cocos ridge subduction and seem to correlate more favorably in time with earlier tectonic events adakite volcanic activity corresponds in space and time with the subduction of a large scarp associated with a tectonic boundary off southern panama regional unconformities and an 11 6 ma gap in arc volcanism match temporally with oblique subduction of the nazca plate beneath central and southern costa rica cessation of volcanic activity low temperature cooling of plutons in the cordillera de talamanca ct and rapid increases in sedimentation in the fore arc and back arc basins coincide with passage of the nazca cocos caribbean triple junction and initiation of subduction of rough crust associated with cocos nazca rifting 3 5 ma closely followed by initial subduction of the cocos ridge 2 3 ma none of the aforementioned geologic events occurred at a time that would allow for underplating by the cocos ridge rather they are probably related to complex interactions with subduction of complicated plates offshore all of the aforementioned events indicate that the southern central american subduction system has been in flux since at least 12 ma

age and origin of earliest adakitic like magmatism in panama implications for the tectonic evolution of the panamanian magmatic arc system abstract 40 20 ma marks a fundamental interval in the evolution of the 70 0 ma panamanian magmatic arc system during this period there is no evidence of panamanian magmatic arc activity to the east of the panama canal basin while to the west and in localized regions to the east of the panama canal basin a phase of intrusive only activity is recorded fundamentally geochemical and geochronological evidence presented herein indicate that this intrusive activity was predominantly adakitic like and becomes younger from west to east along an approximately w e striking lineament granodiorites of the petaquilla batholith western panama yield lam icp ms 206 pb 238 u zircon ages of 29 0 0 7 0 6 ma 28 5 0 7 0 5 ma 28 3 0 5 0 4 ma and 26 2 0 5 0 9 ma to the east of the panama canal basin zircons from a hypabyssal diorite of the mainly intermediate maje subvolcanic suite cedes a mean 206 pb 238 u age of 18 9 0 4 ma relative to other 70 5 ma panamanian magmatic arc lavas and intrusives maje and petaquilla intrusives yield adakitic like major and trace element abundances e g 15 wt al 2 o3 generally 3 5 wt na 2 o 400 ppm sr 120 and strongly fractionated hree patterns these 30 26 ma petaquilla and 19 ma maje suites are also compositionally similar to a subvolcanic suite of rare circa 25 ma adakitic like andesitic intrusives which occur within the panama canal basin midway between petaquilla and maje and at the same approximate latitude as petaquilla and maje collectively the geochemical and geochronological data for the adakitic like intrusives arc consistent with formation via partial melting of lowermost mafic crust above a sub horizontal slab tear that propagated from the west petaquilla to the east maje between 30 and 19 ma our new tectonic model postulates that collision between the panamanian magmatic arc system and an indentor e g a tract of thickened buoyant oceanic crust or plateau occurred at about 40 ma a time of which coincides with the initiation of left lateral offset of the early i e 70 40 ma arc system this collision resulted in the shutdown of the early arc system possible steepening of the subducting farallon slab and ultimately slab break off and the phase of mainly adakitic like intrusive activity subsequent to slab removal by 20 ma ne dipping subduction jumped to the south and initiated production of the later i e post 20 ma arc system soon thereafter

geodynamic evolution of southern costa rica related to low angle subduction of the cocos ridge constraints from thermochronology abstract the late tertiary shallow subduction of the cocos ridge under the caribbean plate controlled the evolution of the cordillera de talamanca in southeast costa rica which is a mountain range that consists mainly of granitoids formed in a volcanic arc setting fission track thermochronology using zircon and apatite as well as 40ar 39ar and rb sr age data of amphibole and biotite in granitoid rocks constrain the thermal history of the cordillera de talamanca and the age of onset of subduction of the cocos ridge shallow intrusion of granitoid melts resulted in fast and isobaric cooling a weighted mean zircon fission track age 13 analyses and rb sr biotite ages of about 10 ma suggest rapid cooling and give minimum ages for granitoid emplacement in some cases 40ar 39ar and rb sr apparent ages of amphibole and biotite are younger than the zircon fission track ages which can be attributed to partial resetting by hydrothermal alteration apatite fission track ages range from 4 8 to 1 7 ma but show no correlation with the 3090 m elevation span over which they were sampled the apatite ages seem to indicate rapid exhumation caused by tectonic and isostatic processes the combination of the apatite fission track ages with subduction parameters of the cocos plate such as subduction angle plate convergence rate and distance of the cordillera de talamanca to the trench implies that the cocos ridge entered the middle america trench between 5 5 and 3 5 ma

rocks of ages propagation of hot rock cookery in western north america cook stone technology s old world roots were established by 30 000 b p and reappeared in the new world by 10 000 b p after millennia of direct fire cooking hot rock cookery which is necessary for foods that require prolonged cooking facilitated land use intensification by affording greater utilization of nutrients in available foods on a given landscape this technology gradually diversified during the early holocene in western north america by 4000 b p its initial intensification was underway final intensification began by 2000 b p and typically peaked during the last 1500 years propagation of hot rock cookery exemplifies pre columbian food crises and signals carbohydrate revolutions wherein more high cost foods feed growing populations as modeled cook stone griddles earth ovens and steaming pits with rock heating elements are more costly facilities insofar as fuel is used to heat rocks that in turn extend cooking time and temperature more expensive still is stone boiling given that fuel heats rocks that in turn heat water that cooks the food even more expensive in terms of energy expended is the manufacture of heating elements in the form of stone ceramic and metal cooking containers all of which afford further evidence of land use intensification

country rock contamination of magmas associated with the baogutu porphyry cu deposit xinjiang china abstract the baogutu porphyry cu deposit located in the west junggar region xinjiang china has the reduced mineralization features baogutu is associated with a late carboniferous intrusive complex that was emplaced into lower carboniferous volcano sedimentary sequences the complex consists of a main stage diorite stock and late stage diorite porphyry dikes the former hosts the most of the cu mineralization the main stage stock has a wide range of composition from minor gabbro through dominant diorite to tonalite porphyry these are all inhomogeneous over a few meters with no distinct boundary between the gabbro diorite and tonalite porphyry some felsic microgranular enclaves and heterogeneous phenocrysts occur in these rocks the latter is a single mineral with a range of compositions the elemental and sr nd compositions of the tonalite porphyry have a transitional character from the gabbro and diorite to the country rocks these findings suggest possible country rock contamination mineral composition data suggest that the primary magma at baogutu is oxidized i type magma the gabbro and diorite have high positive e nd t 3 3 to 6 0 and low initial 87 sr 87 sr ratios 0 7035 0 7038 and are characterized by moderately fractionated rare earth element ree patterns pronounced nb depletion and weak negative eu anomalies these observations suggest that the baogutu magmas were derived from partial melting of a metasomatized mantle wedge and underwent significant country rock contamination after emplacement the contamination process involved the felsic components and reduced materials e g carbonaceous sediment which caused the chemical variation of the baogutu rocks and the reduced mineralization features in the baogutu deposit thus contamination played an important role in the formation of the host rocks and associated mineralization at baogutu

ireteba pluton eldorado mountains nevada late deep source peraluminous magmatism in the cordilleran interior abstract the ireteba pluton is a 66 ma documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape mathrm biotite pm mathrm muscovite pm mathrm garnet end document granite emplaced at the eastern edge of the cordilleran plutonic belt in southeastern nevada in common with other cordilleran peraluminous granites its mineralogy major element chemistry isotopic composition and abundant proterozoic zircon inheritance document crustal origin distinctive trace element chemistry field relations and inherited components furth

a pseudo adakite derived from partial melting of tonalitic to granodioritic crust kyushu southwest japan arc abstract cretaceous peraluminous granitic intrusions occur in kyushu in the southwest japan arc as batholiths or stocks accompanied by metaluminous tonalite to granodiorite the peraluminous intrusions are classed as adakitic based on sr y y ppm relationships but they contain abundant k feldspar a phase not common in typical adakites to consider their significance in the arc system we investigated the tsutsugatake stock as representative of the intrusions exposed in central kyushu the tsutsugatake rocks are mainly garnet bearing two mica leuco granitoids their sr y ratios and rare earth element concentrations are appropriate for adakite but k 2 o na 2 o ratios 0 53 1 43 and al 2 o 3 13 7 15 0 wt mgo 0 20 0 55 wt sr 207 462 ppm cr 12 ppm and ni 8 ppm contents differ from those of typical adakite geochemical modeling shows that the petrogenesis of the tsutsugatake stock can be explained by partial melting of arc type tonalite or adakitic granodiorite under middle to lower crustal conditions leaving amphibole and plagioclase as residual phases the differences between the tsutsugatake stock and classical adakites lie in their source rocks and melting conditions the tsutsugatake source was not basaltic and the melting pressure was insufficient to stabilize garnet the tsutsugatake rocks are here designated pseudo adakites such pseudo adakites may be found in both modern and ancient arcs and would play an important role in crustal evolution as a member of the recycled constituent

bimodal back arc alkaline magmatism after ridge subduction pliocene felsic rocks from central patagonia 47 s abstract volumetrically minor microsyenites alkali microgranite and related trachytic dykes intrude early pliocene oib like alkali basaltic and basanitic flows of the meseta del lago buenos aires in central patagonia 47 s 71 30 w and occur together with scarce trachytic lava flows whole rock k ar ages between 3 98 and 3 08 ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post plateau basaltic sequence that they intrude during a bimodal mafic felsic magmatic episode devoid of intermediate compositions chemically these rocks have a1 type granitoid affinities and are characterized by high silica and alkali contents 60 68 wt sio2 8 7 10 8 wt na2o k2o major and trace elements patterns evidencing evolution by low pressure fractional crystallization and sr and nd isotopic signatures similar to those of coeval basalts 87sr 86sr o 0 70488 0 70571 143nd 144nd o 0 512603 0 512645 nevertheless some of them have the most radiogenic sr values ever reported for a magmatic rock in the meseta and even in the whole neogene patagonian plateau lavas province 87sr 86sr o 0 70556 0 70571 143nd 144nd o 0 512603 0 512608 in addition very high contents of strongly incompatible elements in the most evolved rocks together with sr isotopic ratios higher than those of coeval basalts suggest the occurrence of open system magmatic processes continuous fractional crystallization from a primitive basaltic source similar to post plateau coeval basalts towards alkali granites combined with small rates of assimilation of host jurassic tuffs afc in a shallow magmatic reservoir best explains the geochemical and petrographic features of the felsic rocks therefore a1 type magmatic rocks can be generated by open system crystallization of deep asthenospheric melts in back arc tectonic settings in central patagonia these 3 4 ma old alkaline intrusions occur aligned along a n160 170 trending lineament the zeballos fault zone stacking the morphotectonic front of one segment of the patagonian cordillera intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area related to major regional tectonics occurring in possible relation with the collision of one segment of the chile spreading ridge with the trench

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

petrogenesis of a voluminous quaternary adakitic volcano the case of baru volcano the origin of adakite magmas remains controversial because initially the term adakite had petrogenetic significance implying an origin by direct melting of the eclogitized subducting oceanic crust many models have been produced for their origin and until now there has not been a straightforward method to discriminate between these models in a given adakite suite here we use detailed chronological and geochemical studies of selected adakitic edifices that allows for the determination of the magmatic output rate parameter qe which has been correlated with the rates of magma generation deep within subduction zones by providing temporal and eruption rate estimates we provide constraints on the possible petrogenetic processes involved in the generation of adakite like signatures adakite magmas derived from the melting of the subducting slab should be volumetrically insignificant when compared to the adakite like magmas produced by typical arc magma generation processes in this study we use this observation and the extraordinary stratigraphic exposure from miocene to present in an adakitic volcano in panama and to study the temporal and chemical variation in erupted magmas to estimate rates of magma generation detailed chemical and geochronological analyses of baru volcano indicate that the volcanic edifice was constructed in its entirety during the quaternary and magmas display adakite like features such as steep rare earth elements patterns pronounced depletions in the heavy rare earth elements low y high sr and high sr y the magmatic output rates qe that we have calculated show that compared to other typical adakitic volcanoes most of the volcanic edifice of baru volcano was constructed extremely rapidly 213 k a and in time frames that are similar to typical arc volcanoes the observed chemical and mineralogical variation coupled with the high magma production rates indicate that baru volcano is more representative of a typical arc volcano than a small volume melt of the subducting oceanic crust the technique we outline may have broader application in determining the petrogenetic conditions of other adakite suites

early silurian to early carboniferous ridge subduction in nw junggar evidence from geochronological geochemical and sr nd hf isotopic data on alkali granites and adakites abstract the central asian orogenic belt caob evolved through a long lived orogeny involving multiple episodes of subduction and accretion marking a major phase of continental growth during the paleozoic the northern part of the western junggar region nw junggar offers a window into these processes particularly to constrain the timing of closure of the paleo asian ocean here we report geochemical geochronological and isotopic data from k feldspar granites and adakitic rocks from the nw junggar region zircon u pb ages suggest that the granites were emplaced during early silurian to the early carboniferous 434 328 ma the granites show geochemical characteristics similar to those of a type granites with high sio2 71 13 76 72 wt na2o k2o 8 00 9 59 wt and al2o3 12 28 14 08 wt but depleted sr nb ta and eu they display moderate to high positive end t and ehf t values 4 26 8 21 and 7 69 14 60 respectively and young nd and hf model ages t2dm nd 489 740 ma and t2dm hf 471 845 ma suggesting magma derivation through partial melting of lower crust in the boshchekul chingiz and zharma saur arcs the adakites are characterized by high sr content 406 5 751 6 ppm and low y 13 8 16 4 ppm and yb 1 5 1 8 ppm content yielding relatively high sr y ratios 25 38 49 41 similar to those of modern adakites they have high positive end t and ehf t values 7 85 8 25 and 13 23 15 97 respectively and young nd and hf model ages t2dm nd 429 535 ma and t2dm hf 355 550 ma indicating that their source magmas were likely derived from partial melting of the oceanic crust beneath the boshchekul chingiz arc the petrogenesis and distribution of the a type granites and adakites as well as the tectonic architecture of the region suggest that a ridge subduction event might have occurred during the early silurian to early carboniferous in combination with previous studies in the chinese altai we suggest a two sided ridge subduction model for the junggar altai region

nd isotope systematics of 2 7 ga adakites magnesian andesites and arc basalts superior province evidence for shallow crustal recycling at archean subduction zones abstract an association of adakite magnesian andesite ma and nb enriched basalt neb volcanic flows which erupted within normal intra oceanic arc tholeiitic to calc alkaline basalts has recently been documented in 2 7 ga wawa greenstone belts large positive initial nd values 1 95 to 2 45 of the adakites signify that their basaltic precursors with a short crustal residence were derived from a long term depleted mantle source it is likely that the adakites represent the melts of subducted late archean oceanic crust initial nd values in the ma 0 14 to 1 68 nb enriched basalts and andesites neba 1 11 to 2 05 and normal intra oceanic arc tholeiitic to calc alkaline basalts and andesites 1 44 to 2 44 overlap with but extend to lower values than the adakites large tightly clustered nd values of the adakites together with th ce and ce yb systematics of the arc basalts that rule out sediment melting place the enriched source in the sub arc mantle accordingly isotopic data for the ma neba and normal arc basalts can be explained by melting of an isotopically heterogeneous sub arc mantle that had been variably enriched by recycling of continental material into the shallow mantle in late archean subduction zones up to 200 ma prior to the 2 7 ga arc if the late archean wawa adakites ma and basalts were generated by similar geodynamic processes as their counterparts in cenozoic arcs involving subduction of young and or hot ocean lithosphere then it is likely that late archean oceanic crust and arc crust were also created and destroyed by modern plate tectonic like geodynamic processes this study suggests that crustal recycling through subduction zone processes played an important role for the generation of heterogeneity in the archean upper mantle in addition the results of this study indicate that the nd isotope compositions of archean arc and plume derived volcanic rocks are not very distinct whereas phanerozoic plumes and intra oceanic arcs tend to have different nd isotopic compositions

an adakitic pluton on kyushu island southwest japan arc abstract an adakitic pluton is designated the shiraishino granodiorite ca 15 km2 in outcrop area on kyushu island southwestern japanese arc sr y and rare earth element concentrations and crystallization characteristics of the pluton are similar to those of adakites geochemical and tectonic considerations show that the pluton was derived from partial melting of a subducted slab with minor contamination of high 87sr 86sr material and subsequent fractional crystallization of plagioclase hornblende compiled sr and y data indicate that most cretaceous granitic rocks of southwestern japan generated by partial melting of mafic lower crust adakitic plutons derived by slab melting occur rarely and have older ages it is concluded that plutonism in the southwestern japanese arc during cretaceous time was began with the emplacement of small adakitic intrusions and progressed to more abandant granitic rocks of the type derived from the mafic lower crust

boron geochemistry of the central american volcanic arc constraints on the genesis of subduction related magmas boron contents were measured in representative quaternary lavas from the central american volcanic arc to evaluate along strike variations in subduction processes despite the significant range in b concentrations 2 37 ppm in the mafic lavas b la ratios vary in a systematic fashion along the arc higher values 1 are typical between guatemala and northern costa rica whereas low values most 0 5 typify central costa rica and western panama b la is highly correlated with 10be 9be r2 0 94 excluding one sample and appears to be a useful indicator of subduction contributions to the magma sources if enrichments of both b and 10be are proportional to the flux of subducted sediment along strike variations in b la suggest at least a twofold variation in this flux with maximum values below western nicaragua and minimum values below costa rica and western panama where the cocos ridge is being subducted these data may also reflect significant differences in thermal state of the descending slab which in turn differentially affects release patterns of fluids and fluid mobile trace elements and possibly melting processes beneath different parts of the arc the following scenario is suggested to explain the geochemical results beneath the northwestern part of the arc steep subduction of older relatively cold slab favors more efficient subduction of fluid components to depths beneath the volcanic front the released fluids carry fluid mobile elements to the overlying mantle which upon melting produces calcalkalic magmas shallow subduction of warmer slab beneath the southeastern part of the arc favors shallow release of fluids and limits fluid related metasomatism of sub arc mantle beneath the volcanic front under such conditions b la and ba la ratios in the sub arc mantle vary little from values seen in oceanic island basalts magmas in this part of the arc nevertheless display the highest la yb and lowest ba la and b la ratios which are consistent with prior light rare earth element enrichment in the source significantly lower degrees of melting or a combination thereof because some of the largest volcanoes occur in costa rica and magma flux there is nearly an order of magnitude higher than elsewhere in the arc source enrichment is considered to be the more plausible explanation it is proposed that quaternary magma production below costa rica involved lithospheric sources containing trapped or stored melt components but this enrichment process is unlikely to have involved typical arc magmas or subduction fluids because we see no b enrichment

high and low k granites and adakites at a palaeoproterozoic plate boundary in northwestern australia the lamboo complex is one of several palaeoproterozoic terrains in northern australia that is intruded by high k i type granites the oldest granites in the complex consist of the high k 1865 1850 ma paperbark supersuite with initial e nd values between 2 7 and 4 0 tonalite sheets of the c 1850 ma dougalls suite have high al 2 o 3 na 2 o sr sr y and la yb and low y and hree and initial e nd values between 1 4 and 0 7 the rocks have compositions similar to adakites that form by melting of mafic rocks in the deep crust with garnet in the residue granites of the 1835 1805 ma sally downs supersuite range from high k types through to crustally derived adakites all with initial e nd values between 0 1 and 1 2 the two supersuites and the dougalls suite were intruded into different tectonic settings previously all of the granites in the lamboo complex were interpreted as partial melts of a chemically and isotopically uniform underplate beneath a rifted archaean craton however their spatial and temporal distribution and their varied compositions are best explained by large scale horizontal plate motions and subduction of oceanic crust

late carboniferous adakitic granodiorites in the qiongkusitai area western tianshan nw china implications for partial melting of lower crust in the southern central asian orogenic belt abstract carboniferous granitic rocks are wide spread in the western tianshan but their tectonic contexts remain controversial we present zircon u pb age major element trace element and in situ zircon hf isotopic data for the qiongkusitai granodiorites from the southern part of the yili block in the western tianshan the granodiorites were emplaced in the late carboniferous ca 314 ma and are characterized by high sio 2 62 1 65 7 wt high sr 376 485 ppm low y 13 7 17 7 ppm and low yb 1 34 1 81 ppm with sr y ratios of 23 34 such characteristics imply adakitic affinities their low concentrations of cr 3 85 6 75 ppm co 7 63 10 7 ppm and ni 2 94 6 80 ppm suggest little if any interaction with a mantle source compared to the thickened lower crustal derived adakitic rocks the relatively low nb ta ratios of the qiongkusitai adakitic granodiorites indicate that they were probably generated by partial melting of amphibolite facies rocks but not eclogite facies rocks the qiongkusitai adakitic granodiorites have positive zircon e hf t values of 0 7 to 5 1 and t dm2 of 1 28 1 68 ga and they are interpreted as being derived from remelting of amphibolite facies mesoproterozoic basement with an input of juvenile material late carboniferous magmatism in the western tianshan was possibly triggered by asthenospheric upwelling as a result of the roll back of the subducted southern tianshan oceanic lithosphere

geochemical evidence for a lithospheric source for magmas from los humeros caldera puebla mexico abstract this study reports new geochemical and sr nd pb radiogenic isotope data on hy normative quaternary basaltic to rhyolitic volcanic rocks as well as two samples of mesozoic to palaeozoic intrusive igneous rocks from los humeros caldera lhc located in the eastern part of the mexican volcanic belt mvb the isotopic ratios of mafic basaltic rocks range as follows 87 sr 86 sr 0 703350 0 703387 143 nd 144 nd 0 512867 0 512888 206 pb 204 pb 18 849 18 866 207 pb 204 pb 15 593 15 598 and 208 pb 204 pb 38 490 38 491 these values are similar to the rift related mafic rocks from the volcanic front of the central mvb for comparison the entire suite from basalt b to rhyolite r in the lhc shows the following ranges 87 sr 86 sr 0 703350 0 704678 143 nd 144 nd 0 512702 0 512888 206 pb 204 pb 18 710 18 866 207 pb 204 pb 15 593 15 631 and 208 pb 204 pb 38 444 38 594 the basaltic rocks from los humeros do not show the characteristic negative nb anomaly with respect to ba and ce as is typical of such magmas in volcanic arcs the available geochemical and isotopic evidence does not support the generation of the lhc mafic magmas by direct slab melting nor by indirect fluid transport to the mantle participation of the subducted cocos plate instead they could be generated in the upper mantle from a lithospheric source the evolved andesitic to rhyolitic magmas could have been originated from such mafic magmas through assimilation coupled with extensive fractional crystallisation two different compositions representing lower lc and upper crusts uc as well as a hypothetical crust ha were tested as plausible assimilants for the assimilation coupled with fractional crystallisation process

evidence for middle eocene and younger land emergence in central panama implications for isthmus closure the rise of the isthmus of panama linked to a number of climatic paleoceanographic and biological events has been studied mostly from indirect often distal geochemical and biotic evidence we have upgraded existing geologic mapping in central panama with more than 2000 field stations over 40 petrographic analyses and more than 30 new geochronological and thermochronological analyses this data set suggests that the isthmus was an uninterrupted chain above sea level from late eocene until at least late miocene times the basement complex of central panama is a folded faulted 3 km thick arc sequence intruded by granitoid bodies and onlapped by mildly deformed upper eocene and oligocene strata six u pb zircon ages in the granitoids along with published geochronological data reveal intense late paleocene to middle eocene magmatism 58 39 ma a temporary cessation of magmatic activity between 38 and 27 ma and renewed magmatism between 25 and 15 ma in a position 75 km south of the former magmatic axis thermochronological analyses in zircon eight u th he ages and in apatite crystals four u th he ages and nine fission track ages obtained from a subset of 58 54 ma granitoid bodies record a concordant lutetian age 47 42 ma cooling from 200 c to 70 c in 5 m y and cooling below 40 c between 12 and 9 ma cooling is linked to exhumation by an angular unconformity that separates the deformed basement complex below from mildly deformed upper eocene to oligocene terrestrial to shallow marine strata above exhumation and erosion of the basement complex are independently confirmed by lower miocene strata that have a detrital zircon signature that closely follows the central panama basement complex age distribution these results greatly restrict the width and depth of the strait separating southern central america from south america and challenge the widely accepted notion that the central american seaway closed in late pliocene time when the ice age began

late pleistocene to holocene activity at bakening volcano and surrounding monogenetic centers kamchatka volcanic geology and geochemical evolution the different roles of variable mantle sources and intra crustal differentiation processes at bakening volcano kamchatka and contemporaneous basaltic monogenetic centers are studied using major and trace elements and isotopic data three suites of volcanic activity are recognized 1 plateau basalts of lower pleistocene age 2 andesites and dacites of the bakening volcano the new bakening volcano dacitic centers nearby and 3 contemporaneous basaltic cinder cones erupted along subduction zone parallel n s faults age data show that the last eruptions in the bakening area occurred only 600 1200 years ago suggesting the volcano is potentially active major element variations and petrographic observations provides evidence for a fractionation assemblage of olivine clinopyroxene plagioclase magnetite within the basaltic suite the fractionation in the andesites and dacites is dominated by amphibole clinopyroxene orthopyroxene and plagioclase plus minor amounts of magnetite and apatite the youngest cpx opx andesites of bakening main volcano deviate from that trend their source was probably formed by mixing of basaltic magmas into the silicic magma chamber of the bakening volcano overall trace element patterns as well as the sr nd pb isotopic compositions are quite similar in all rocks despite large differences in their chemical composition from basalt to rhyodacite in detail however the andesite dacites of the central bakening volcano show a stronger enrichment in the more incompatible elements and depletion in hree compared to the monogenetic basaltic centers this results in a crossing of the ree pattern for the two suites the decrease in the hrees can be explained by amphibole fractionation a slab component is less likely because it would result in fractionation of the hree from each other which is not observed the higher relative amounts of lile in the dacitic and the large scatter in the basaltic rocks must be the result of a variable source enrichment by slab derived fluids overprinting a variable depleted mantle wedge the plateau basalts are less depleted in hfse and show a more fractionated hree pattern these lavas could either result from a slab component or the addition of an oib type enriched mantle in their source q 2000 elsevier science b v all rights reserved

very shallow melting of oceanic crust during spreading ridge subduction origin of near trench quaternary volcanism at the chile triple junction 2nd 2 9 to 3 8 and d 18 o values 6 4 to 6 9 respectively consistent with mixed magma sources that include a morb type component and sediments we propose that the high si dacites are derived from the hydrous melting of a mixture of morb and sediments at high temperatures 800 900 c under low pressures 0 8 gpa the low si dacites originate from the melting of a similar source under higher pressures consistent with depths of 25 45 km two scenarios accounting for the near trench position of these latter rocks are envisioned the first invokes rapid tectonic erosion and changes in sedimentary wedge geometry the second one postulates that parts of the slab are subducted rapidly to depths of 20 30 km right under the trench index terms 3640 mineralogy and petrology igneous petrology 8120 tectonophysics dynamics of lithosphere and mantle general 9360 information related to geographic region south america keywords slab melting chile trench ridge subduction forearc magmatism geochemistry calc alkaline magmatism

petrogenesis of late mesozoic granitoids and coeval mafic rocks from the jiurui district in the middle lower yangtze metallogenic belt of eastern china geochemical and sr nd pb hf isotopic evidence abstract large scale cu au mineralization is associated with the late mesozoic granitoids in the jiurui district of the middle lower yangtze mineralization belt in eastern china to constrain the petrogenesis of these granitoids and coeval mafic rocks a detailed geochemical and sr nd pb hf isotopic study was performed the jiurui granitoids are made up primarily of granodiorite porphyry and quartz diorite porphyry these granitoids are characterized by sio 2 and k 2 o contents of 58 8 wt to 68 6 wt and 1 9 wt to 5 7 wt respectively these granitoids show relatively high mgo contents 1 0 wt to 3 1 wt average 2 1 wt and high mg values 39 to 70 average 54 the jiurui granitoids are enriched in light rare earth elements lree large ion lithophile elements lile and compatible trace elements cr ni and v but are relatively depleted in nb ta y and yb these rocks show a negligible negative eu anomaly eu eu 0 76 1 13 average 0 91 and nearly no negative sr anomaly the whole rock initial 87 sr 86 sr ratios range from 0 7060 to 0 7092 and the e nd t values vary from 5 4 to 2 0 the granitoids show radiogenic pb isotopic ratios with values of 206 pb 204 pb 17 93 18 21 207 pb 204 pb 15 55 15 58 and 208 pb 204 pb 38 16 38 56 for the plagioclases the zircon hf isotopic compositions show e hf t values from 11 8 to 2 4 the coeval mafic rocks consist of lamprophyre diabase and fine grained mafic dyke these rocks are characterized by sio 2 contents ranging from 47 6 wt to 54 8 wt with a negative eu anomaly and a positive to negative sr anomaly the whole rock initial 87 sr 86 sr ratios range from 0 7059 to 0 7071 and the e nd t values vary from 3 8 to 1 4 by comparing the geochemical and isotopic compositions of the jiurui granitoids and the coeval mafic rocks we conclude that the granitoids are similar to adakites that were likely related to the delamination processes and the coeval mafic rocks may have originated directly from enriched lithospheric mantle the thickened lower continental crust and underlying enriched lithospheric mantle may have been simultaneously delaminated into the asthenospheric mantle this process led to the partial melting of the thickened lower crust and the enriched lithospheric mantle and the melts experienced an interaction with the asthenospheric mantle during ascension mafic melts could be generated from the enriched lithospheric mantle which was heated by the upwelling asthenospheric mantle the geochemical and isotopic compositions of mafic rocks were most likely controlled by the afc process a geochemical and isotopic comparison of the granitoids in the jiurui district with those of two other districts e g angui and tongling in the same mineralization belt indicates that a significant crust mantle interaction played a crucial role in their petrogeneses and metallogeneses from the southwest to northeast in this metallogenic belt the proportion of mantle component that was involved in the granitic magma shows a decreasing trend from the jiurui to the angui and tongling districts it is suggested that the juirui district has the highest potential for a significant amount of cu au accumulation because the mantle is an important supply for these metals

significance of slab derived partial melts and aqueous fluids for the genesis of tholeiitic and calc alkaline island arc basalts evidence from mt arayat philippines abstract mt arayat an inactive stratovolcano is located behind the volcanic front of the bataan segment of the taiwan luzon arc in the central valley of luzon the philippines volcanism in this arc segment results from subduction of south china sea basin crust along the manila trench eruption products of mt arayat comprise pyroclastics and porphyritic basalt and andesite lavas of high k calc alkaline affinity compared to typical oceanic island arc volcanics the basalts display slightly elevated abundances of ta and nb relative to zr and hf and of ta nb zr hf and light rare earth elements lree s relative to y and heavy rare earths hree s their ree distribution suggests mixing between high hree low lree hree and low hree high lree hree components the latter suggesting the presence of residual garnet in the source region of mt arayat basalts enrichment of lree s relative to hree s is typical of calc alkaline and high k calc alkaline island arc basalts but is usually missing or small in island arc tholeiites lree enrichment is often attributed to a metasomatizing aqueous fluid generated during dehydration of the subducted slab for this model to be applicable extremely low abundances in the fluid of agents like f oh and or co2 3 which form hard complexes with the ree s are a prerequisite due to the temperature dependence of the eu 3 eu 2 redox potential divalent eu predominates in aqueous fluids of low complex forming capacity in the subduction zone environment whereas the other ree s with the possible exception of yb are still trivalent a lree enriched aqueous fluid generated by slab dehydration should therfore display a positive eu anomaly however lree enriched island arc basalts generally do not show positive eu anomalies implying that this lree enrichment is unlikely to result from ree transfer by an aqueous fluid instead a model is favoured which assumes contributions from both slab derived aqueous fluids and partial melts in the source region for island arc tholeiites the mantle wedge is only affected by a lile enriched aqueous fluid whereas the source region for calc alkaline and high k calc alkaline basalts is additionally metasomatized by small volumes of melt generated by small degrees of partial melting of the subducted slab with garnet as a residual phase these lree enriched partial melts may react with the depleted peridotite of the mantle wedge to produce domains of refertilized garnet peridotite subsequent partial melting produces calc alkaline melts whose trace element compositions display signatures of both depleted peridotite and more fertile garnet peridotite

mesozoic high ba sr granitoids from north china geochemical characteristics and geological implications mesozoic magmatism occurred extensively in the north china block ncb and the dabie sulu orogen dso post dating the north south china collision resulting in abundant intrusive and volcanic rocks ranging from basic to acidic compositions the intermediate acidic intrusive rocks can be grouped into two types namely high ba sr granitoids and low ba sr granitoids that both have distinct geochemical characteristics the high ba sr granitoids are similar in most of the incompatible trace element systematics to the associated basic rocks which probably originated from melting of subcontinental lithospheric mantle indicating significant mantle contributions to them geochemical similarities are observed between the basic rocks from the ncb and dso implying a regional scale magma generating mechanism and that mantle enrichment beneath the dso was independent from the triassic deep continental subduction in the region we therefore interpret that the mesozoic magmatism resulted from delamination of the ancient lithospheric mantle beneath the eastern part of north china

late miocene to recent plate tectonic history of the southern central america convergent margin new plate reconstructions constrain the tectonic evolution of the subducting cocos and nazca plates across the southern central american subduction zone from late miocene to recent because of the strong relationships between lower and upper caribbean plate dynamics along this margin these constraints have wide ranging implications for the timing and growth of upper plate deformation and volcanism in southern central america the reconstructions outline three important events in the neogene history of this margin 1 the coeval development of the panama triple junction with the initiation of oblique subduction of the nazca plate at 8 5 ma 2 the initiation of seamount and rough crust subduction beginning at 3 4 ma and 3 cocos ridge subduction from 2 to 3 ma a comparison of these events with independent geologic geomorphic volcanic and stratigraphic data sets reveals that the timing rates and origin of subducting crust directly impacted the neogene growth of upper plate deformation and volcanism in southern central america these analyses constrain the timing geometry and causes of a number of significant tectonic and volcanic processes including rapid plio quaternary arc fore arc contraction due to cocos ridge subduction the detachment of the panama microplate at 1 3 ma and the late miocene cessation of mantle wedge derived volcanism across 300 km of the subduction zone

an experimentally based model for the origin of tonalitic and trondhjemitic melts abstract the results of melting experiments have been used to establish equations relating melt composition to temperature t pressure p and water content of the source y h2o for tonalitic and trondhjemitic melts produced by partial melting of average archean tholeiite aat a normative classification diagram for these rocks has been translated into a t p y h2o diagram showing the required conditions of partial melting tonalites form at high temperatures low pressures and high water contents while trondhjemites form at lower temperatures higher pressures and lower water contents as pressure increases the k content increases but the melts never achieve the composition of a granodiorite or granite a method for estimating the conditions for the formation of natural tonalites and trondhjemites is proposed the method has been successfully tested against other experimental studies and natural rocks where the temperature pressure conditions have been constrained by traditional geochemical methods

evolution of the early paleozoic hongguleleng balkybey ocean evidence from the hebukesaier ophiolitic melange in the northern west junggar nw china abstract the hebukesaier ophiolitic melange is located in west junggar northwest china based on detailed field mapping we identified three rock assemblages in the melange ophiolite seamount and rhyolite porphyry the ophiolite comprises mainly metaperidotites pyroxenites gabbros dolerites diabase dike plagiogranites basalts and cherts two gabbro and one plagiogranite samples have ages of 512 9 ma 505 5 ma and 502 5 ma respectively which constrain the formation time of the oceanic crust the geochemical characteristics of the basaltic rocks gabbros and basalts suggest that the ophiolite formed by intrusion of mid ocean ridge melts the ocean island basalts occur as tectonic blocks within the melanges that coexist closely with marble siliceous mudstone and chert with formation characteristics of a seamount the early silurian rhyolite porphyry samples 435 2 ma are characterized by high al2o3 14 30 15 31 wt sr y 22 26 78 42 and la yb n 20 27 24 24 similar to those of late silurian early devonian adakitic rocks 420 411 ma from the xiemisitai mountains furthermore the rhyolite porphyry samples display high positive ehf t values and low mg values as well as low concentrations of compatible elements cr co and ni suggesting that the rhyolite porphyry originated from partial melting of lower juvenile crust the rhyolite porphyry displays spatio temporal characteristics similar to those of the post collision plutons 434 405 ma of the xiemisitai mountains the rhyolite porphyry intruded the ophiolite in the melange implying that the hongguleleng balkybey ocean closed prior to the early silurian our results combined with other geological evidence indicate that the northern west junggar underwent ocean basin spreading intra oceanic subduction arc arc collision the closure of the ocean and post collisional extension during the middle cambrian early devonian

the nature of magmatism at palinpinon geothermal field negros island philippines implications for geothermal activity and regional tectonics the palinpinon geothermal field negros island philippines is a high temperature liquid dominated geothermal system in an active island arc volcanic setting this paper presents a regional context for the palinpinon geology discusses the petrogenetic evolution of magmatism in the district and assesses the genetic relationships between intrusion and geothermal circulation the oldest rock formation the lower puhagan volcanic formation middle miocene is part of a volcanic sequence that is traceable throughout the visayas region and is related to subduction of the sulu sea oceanic basin in a southeasterly direction beneath the sulu arc late miocene to early pliocene times mark a period of regional subsidence and marine sedimentation a thick sequence of calcareous sediments okoy formation was deposited during this period magmatism in early pliocene to recent times coincided with commencement of subduction at the negros sulu arc this produced basaltic andesites and andesites belonging to the southern negros and cuernos volcanic formations during this time the puhagan dikes and the nasuji pluton intruded middle miocene late miocene and early late pliocene formations based on radiogenic 40ar 39ar dating of hornblende the puhagan dikes are 4 1 4 2 ma and the nasuji pluton 0 3 0 7 ma this age difference confirms these intrusions are not genetically related the early pliocene age of the puhagan dikes also confirms they are not the heat source for the current geothermal system and that a much younger intrusion is situated beyond drill depths igneous rock formations in southern negros are the products of regional island arc magmatism with medium k calcalkaline basaltic to dacitic compositions their adakitic affinity implies that the melting of subducted oceanic basalt has influenced magmatism in this region considering the regional tectonic history the most likely scenarios for the generation of slab melts are 1 during the middle miocene by the melting of relatively young less than 20 ma oceanic crust 2 during early pliocene times by the initiation of subduction along the negros sulu trench and 3 during late pliocene times by the melting of young less than 10 20 ma oceanic crust the adakitic composition of the magmas at palinpinon has promoted the formation of a porphyry copper style magmatic hydrothermal system that is comparable to mineralised porphyry deposits elsewhere in the philippines

oldest volcanic hosted submarine iron ores in south china evidence from zircon u pb geochronology and geochemistry of the paleoproterozoic dahongshan iron deposit abstract the dahongshan iron deposit is hosted in the paleoproterozoic submarine metavolcanic rocks of the dahongshan group in the yangtze block south china la icp ms dating of hydrothermal zircon grains from the genetically associated albitite and dolomite albitite show ca 2008 ma ages that are consistent with the zircon ages from the host metavolcanic rocks ca 2012 ma and postdated the post ore diabase dike ca 1724 ma marking the dahongshan iron deposit as the oldest submarine volcanic hosted deposit so far as known the ore hosting metavolcanic rocks in the dahongshan deposit have low ni 9 1 77 4 ppm cr 1 0 63 0 ppm and co contents 5 6 62 9 ppm suggesting the fractionation of olivine clinopyroxene and plagioclase within the magma chamber the major and trace element features of the alkaline to tholeiitic metavolcanic rocks are consistent with high degree partial melting of the mantle wedge metasomatized by melts enriched in high field strength elements hfses which were derived from the subducted slab in volcanic arc setting based on an evaluation of the morphology of orebody ore fabrics petrology and melt fluid inclusions as well as the geochemical characteristics of the major ore mineral magnetite we correlate the iron mineralization in the dahongshan deposit with hydrothermal process induced by the high temperature high salinity and fe rich brines derived through magmatic exsolution the similar characteristic of ce and eu anomalies of the dahongshan iron deposit and banded iron formations bifs suggest that the dahongshan deposit was formed in reducing environment although the two types of iron ores were generated through distinct processes with hydrothermal processes dominating for the submarine volcanic hosted iron deposits whereas the bifs were formed through chemical precipitation

petrogenesis of early carboniferous adakitic dikes sawur region northern west junggar nw china implications for geodynamic evolution abstract adakitic dikes are widespread in the sawur region northern west junggar northwestern china zircon u pb analyses of the dikes have yielded consistent ages of ca 334 ma the dikes are characterized by relatively high sio 2 55 5 61 8 wt al 2 o 3 14 5 16 3 wt and sr 468 1005 ppm and low y 10 1 14 1 ppm and yb 0 93 1 39 ppm contents with high sr y 34 74 ratios and slight eu anomalies which are analogous to those of slab derived adakites in addition the dikes are relatively mgo rich 1 75 3 57 wt mg 44 56 with high th yb ratios and positive end 334 ma 6 2 to 6 5 and ehf 334 ma 11 3 to 15 8 values the data suggest that the dikes were generated by partial melting of subducted oceanic crust and overlying sediments however these early carboniferous adakitic rocks and associated i type granites in the northern part of west junggar are petrochemically distinct from the late carboniferous adakitic plutons from the southern part of west junggar the latter are associated with charnockites a type granites tholeiites and nb enriched alkaline basalts as well as magnesian dikes which were generated by ridge subduction during the late carboniferous the distinct lithological association of the early carboniferous adakites is consistent with flat slab subduction which was characterized by anomalous heating of the slab and consequential partial melting to generate adakitic magma after generation of the early carboniferous adakites the subducting slab cooled reverted to a more normal subduction angle in the late early carboniferous and was associated with more typical i type granite arc magmatism

late silurian early devonian adakitic granodiorite a type and i type granites in nw junggar nw china partial melting of mafic lower crust and implications for slab roll back abstract late silurian early devonian magmatism of the nw junggar region in the central asian orogenic belt provides a critical geological record that is important for unraveling regional tectonic history and constraining geodynamic processes in this study we report results of zircon u pb ages and systematic geochemical data for late silurian early devonian largely granitic rocks in nw junggar aiming to constrain their emplacement ages origin and geodynamic significance the magmatism consists of a variety of mafic to felsic intrusions and volcanic rocks e g adakitic granodiorite k feldspar granite syenitic granite gabbro and rhyrolite u pb zircon ages suggest that the granitoids and gabbros were emplaced in the late silurian early devonian 420 405 ma adakitic granodiorites are calc alkaline characterized by high sr 407 532 ppm low y 12 2 14 7 ppm yb 1 53 1 77 ppm cr mostly

partial melting of subducting oceanic crust the conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two dimensional thermal models of subduction zones for porosities of 1 containing h20 the amount of partial melt generated at the wet basaltic solidus is limited to 100 mpa can be maintained by rocks close to or above their melting temperatures in the absence of high shear stresses substantial melting of the oceanic crust will only occur during subduction of very young 5 ma oceanic lithosphere partial melting of hydrated basalt amphibolites derived from the mid ocean ridge has been proposed e g 1 3 as being responsible for the generation of certain recent high al andesitic to dacitic volcanic rocks adakites three of these volcanic suites mount st helens southern chile and panama occur in volcanic arcs where oceanic crust 25 ma is being subducted at rates of 1 3 cm yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments however oceanic lithosphere is not currently being subducted beneath baja and new guinea where recent adakites are also present suggesting that some adakite magmas may form by water undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately

quantifying arc migration and the role of forearc subduction erosion in the central aleutians abstract the active aleutian arc is currently classified as an accreting margin as most of the arc has a well developed frontal accretionary prism that formed in the plio pleistocene we focus on the central aleutian island arc where the arc massif was built to near its current size by the late eocene and has since seen limited growth and endured episodic erosion prior to the development of the accretionary prism as a result the volcanic front has migrated northwards tens of kilometers since inception along with previous mapping dating and geochemical data sixteen new 40ar 39ar dates and new geochemical data for volcanic rocks on several central aleutian islands supplement recent geochronologic data from six other islands in this region and facilitate an attempt to understand how and where aleutian arc volcanism has migrated with time long term average arc migration rates range from 0 6 to 5 km ma in three distinct locations the arc migration rate increases to the west and is greatest over the last five million years as the arc migrated from a 7 5 ma volcanic front to its current location arc migration and erosion during this most recent period may have been expedited by either the subduction of the kula ridge which ceased spreading in the early eocene or clockwise rotation of the pacific plate in the late miocene some central aleutian lavas and plutonic rocks have elevated la yb sm yb and sr yb most of which were emplaced during times of arc migration or in the waning stages of a peak in magmatic activity the strongly fractionated trace element patterns are similar to the basaltic andesites to dacites from the western aleutian seafloor which are interpreted to be the result of residual garnet during partial melting of eclogite in the subducting oceanic crust however the existing subduction parameters coupled with the timing of the central aleutian adakite like lavas favors a model in which significant subduction erosion of the mafic forearc including erosion and subsequent melting of the trapped kula plate i e morb like oceanic crust as well as early aleutian arc basalts in the mid crust may be partly responsible for the eclogitic component observed in central aleutian magmas

coeval potassic and sodic calc alkaline series in the post collisional hercynian tanncherfi intrusive complex northeastern morocco geochemical isotopic and geochronological evidence abstract the post collisional late hercynian tanncherfi intrusive complex tic is part of a widespread intrusive episode in the moroccan meseta the complex contains a wide range of rock types from monzogabbros to monzogranites two distinct magmatic series are recognized 1 a potassic shoshonitic series consisting of monzogabbros quartz monzonites and monzogranites and 2 a sodic granodioritic series represented by quartz monzodiorites and granodiorites all the tanncherfi plutonic rocks display similar spider diagram profiles with lile and lree enrichment and nb ta ti depletion which are typical of subduction related magmas combined major trace element compositions and sr nd isotopic results indicate that the two series have been derived from a lile and lree enriched continental lithospheric mantle source under different partial melting and or depth conditions intrusion of the tanncherfi rocks was not temporally related to subduction and the enrichment of their source is likely to be linked to preceding subduction events the two series evolved by fractional crystallization of clinopyroxene plagioclase hornblende biotite k feldspar and accessories fe ti oxide minerals titanite apatite and zircon for the potassic series while the sodic series combined fractional crystallization with assimilation of felsic magmas with lower sr isotopic ratio than the more mafic term of the series the quartz monzodiorite the intrusion of the potassic magmas 344 6 ma marks a major change in the tectonic regime of eastern meseta these magmas intruded during post thickening uplift and extension both probably favored by convective thinning of the lithosphere this model provides a reasonable mechanism for the genesis of other hercynian intrusive complexes in morocco

ridge collision slab window formation and the flux of pacific asthenosphere into the caribbean realm mantle wedge derived arc volcanism ceased in southern costa rica after ca 8 ma because of subduction of the aseismic cocos ridge beneath the central american arc and the subsequent opening of a slab window geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5 8 and 2 ma identify a source derived from the galapagos plume the presence of this source is explained by an influx of pacific upper mantle into the caribbean mantle wedge through a slab window where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted cocos ridge by using geochemical and isotopic signatures we trace this upper mantle flow beneath central america from southern costa rica northward at a rate of 40 mm yr

zircon u pb ages and geochemistry of the huai an ttg gneisses terrane petrogenesis and implications for 2 5ga crustal growth in the north china craton abstract the huai an gneiss terrane is a typical archean terrane within the trans north china orogen of the north china craton ncc and is composed of tonalite trondhjemite granodiorite ttg suite and dioritic granulite facies gneisses precise shrimpii and cameca ims1280 zircon u pb ages demonstrate that the huai an gneisses formed around 2 55 2 50 ga and underwent granulite facies metamorphism at 1 85 1 80 ga the ttg gneisses are compositionally equivalent to high sio2 adakites with high sio2 and al2o3 concentrations strongly fractionated rare earth element ree patterns la yb n 10 54 with positive or negligible eu anomalies eu eu 0 9 3 4 low heavy ree hree and y concentrations high sr 435 938 ppm concentrations and high sr y ratios 48 261 high mg numbers 33 60 and high compatible element concentrations e g cr 11 183 ppm and ni 11 82 ppm indicate that the magmas that formed these gneisses were derived from interaction between adakitic melts and mantle material the dioritic gneisses have compositions similar to high mg diorites or sanukitoids with high al2o3 14 46 17 61 wt mgo 3 0 6 4 wt cr 100 304 ppm and ni 57 109 ppm concentrations and fractionated ree patterns with weak negative or no eu anomalies eu eu 0 8 1 0 all the ttg and dioritic gneiss samples analysed during this study have negative nb ta and ti anomalies compared to neighbouring elements in spider diagrams with the majority having high positive nd t and hf t values close to values of the depleted mantle these geochemical characteristics strongly suggest that protoliths of the huai an ttg and dioritic gneisses originated in a juvenile crust forming event at 2 55 2 50 ga through partial melting of subducted oceanic crustal material these magmas also assimilated mantle peridotite material during their ascent this indicates that 2 5 ga mantle derived magmatism led to significant crustal growth in the north china craton and was probably associated with subduction

genetic link between emi and emii an adakite connection abstract geochemical modeling of the origin of enriched mantle i emi and enriched mantle ii emii is conducted from the perspective of adakite production for the model the average composition of adakites is re estimated from published data for eighteen trace elements although the concentrations determined for highly incompatible elements are very high about 100 times of primitive mantle these high concentrations can be explained by melting of oceanic crust without sediment contribution the compiled data further suggest that the mantle slab melt reaction would play a major role in the production of basic adakites in addition crystal fractionation in the magma chamber should produce additional chemical variations in adakites in particular for acidic adakites to examine the effect of chemical variations on the isotopic composition of recycled adakites broad correlations between trace elements and sio2 concentrations and the melts program are employed the results suggest that recycling of a basic adakite sio2 55 can account for emi isotopic signatures with storage times of about 2 0 gyr the isotopic compositions of less basic adakites and their evolved magmas shift towards emii values with increasing sio2 concentrations in particular evolved acidic adakite can yield emii isotopic signatures these lines of evidence suggest that the recycling of adakites at various stages of evolution can conceivably produce the entire isotopic range between emi and emii reservoirs consequently adakite recycling via sediment subduction or subduction erosion can account for the origins of emi and emii reservoirs in this context residual garnet under high pressure and plagioclase fractionation at low pressure might play an essential role in producing the chemical variations among adakites that ultimately govern the isotopic compositions of these geochemical reservoirs

seamount ridge and transform subduction in southern central america understanding the factors that control subduction zone processes is a first order goal in the study of convergent margins in southern central america a growing body of research reveals strong links between the character of the subducting slab and the mechanics of important processes that include subduction erosion fluid flow deformation and seismogenesis in this paper i evaluate the role that seamount ridge and transform subduction have in the development of upper plate deformation and volcanism by summarizing previous work across a 500 km long region of central america where each of these three scenarios are present along strike the data show that the subduction of short wavelength bathymetry e g seamounts and faults on the seafloor produces short wavelength deformation that persists for relatively short timescales 104 105 years whereas the subduction of longer wavelength bathymetry e g the aseismic cocos ridge results in longer wavelength deformation that endures over a longer time scale 106 years the timing and distribution of upper plate deformation are consistent with subhorizontal cocos ridge subduction driving upper plate deformation and the increased crustal thickness 20 km of the subducting cocos ridge is likely one of the most important factors in the production of upper plate contraction and crustal thickening the data illustrate a fundamental connection between lower plate properties and upper plate deformation and highlight the profound influence that bathymetry and crustal thickness have in the localization and kinematics of upper plate strain and volcanism in middle america

the genesis of the ores and intrusions at the yuhai cu mo deposit in eastern tianshan nw china constraints from geology geochronology geochemistry and hf isotope systematics abstract the yuhai porphyry cu mo deposit is located in the eastern tianshan orogenic belt of the southern central asian orogen belt being an economically important porphyry cu deposit in nw china the deposit comprises sixteen buried orebodies that are predominantly associated with dioritic and granodioritic intrusions and are structurally controlled by roughly ne trending faults la icp ms zircon u pb dating yielded crystallization ages of 441 6 2 5 ma mswd 0 03 n 24 for diorite and 430 4 2 9 ma mswd 0 04 n 19 and 430 3 2 6 ma mswd 0 09 n 24 for granodiorite in situ zircon hf isotope data on a diorite sample show e hf t values from 8 7 to 18 6 and two granodiorite samples exhibit similar e hf t values from 12 6 to 19 6 and 12 6 to 18 9 respectively the dioritic and granodioritic intrusions belong to a low k tholeiite series and are relatively enriched in large ion lithophile elements k ba pb and sr and are depleted in high field strength elements th nb ta and ti moreover these intrusions have high sio 2 al 2 o 3 and mgo contents low na 2 o p 2 o 5 and tio 2 contents low nb ta ratios and slightly positive eu anomalies re os dating of molybdenite intergrowth with chalcopyrite yielded a well constrained 187 re 187 os isochron age of 351 7 2 9 ma mswd 1 5 with a weighted average age of 355 7 2 4 ma mswd 0 69 ma indicating that the yuhai cu mo deposit is younger than the intrusion of the diorite and granodiorite combined with the regional geological history and above mentioned data we suggest that the yuhai intrusions were most likely derived from the partial melting of mantle components that were previously metasomatized by slab melts formed by the northward subduction of the ancient tianshan ocean plate beneath the dananhu tousuquan island arc during the silurian to carboniferous under the subduction related tectonic setting the metasomatized mantle magma was emplaced into the shallow crust and induced the formation of the early carboniferous yuhai cu mo deposit and the hydrothermal fluids of enriched sulfides probably played an important role in the cu mo mineralization

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

petrology of the rabaul caldera area papua new guinea abstract the volcanic rocks erupted in the rabaul area mostly belong to a calc alkaline association here designated the main series they range from high alumina basalt minimum sio2 48 4 through medium k basaltic andesite to high k andesite dacite and rhyodacite maximum sio2 69 3 rare sodic rhyolites sio2 73 75 are mineralogically and geochemically distinct from the main series volcanics the rhyolites contain quartz and hornblende whereas the main series volcanics have a quartzfree anhydrous mineralogy even in the most siliceous rhyodacites amphibole and biotite which occur rarely in dacite are deuteric major and trace elements in the main series rocks show coherent continuous variation trends against sio2 rhyolite analyses consistently depart from these trends particularly for the incompatible elements k rb ba zr y and ree leastsquares mass balance calculations for major elements and rayleigh fractionation computations for trace elements show that the main series magma compositions can be modelled stepwise as products of a pure crystal fractionation process rhyolite cannot be derived from main series magma by crystal fractionation geochemical scatter in the main series may be caused by co genetic magma mixing as seen in the malaguna pyroclastics and latlat pyroclastics units hybrid mixing between main series magma and rhyolite is rare but may account for some anomalous medium k dacites such as the kulau ignimbrite derivation of parental high alumina basalt by partial melting in the mantle wedge is assumed while rhyolite which has an inappropriate geochemistry for a partial melt from subducted lithosphere is thought to be a product of partial melting of high alumina basalt underplating the crust volcanic hazard is related to magma composition with dacite sio2 65 and rhyolite capable of producing ignimbrite being the most dangerous changes in the style of rabaul volcanicity throughout the life of the caldera are not well documented but progressive weakening of the faulted substructure may now be allowing more primitive basalts and less fractionated intermediate magmas to reach the surface more freely

zircon u pb ages trace elements and nd hf isotopic geochemistry of guyang sanukitoids and related rocks implications for the archean crustal evolution of the yinshan block north china craton abstract the yinshan block is a unique coherent neoarchean basement terrane of the western block of the north china craton in this paper three archean plutons namely the dajitu banshentu and haolaigou plutons have been investigated based on the field observations the major difference among these plutons is that many ultramafic mafic enclaves are enclosed in the dajitu and banshentu plutons whereas none occurs in the haolaigou pluton geochemically the dajitu and banshentu plutons are characterized by low sio2 contents 56 67 wt high mgo 1 9 7 1 wt cr 10 947 ppm ni 2 274 ppm and mg 44 64 contents negative nb ta and ti anomalies high large ion lithophile elements liles with no or slightly negative eu anomalies similar to a typical archean sanukitoid new geochronological data indicate that the dajitu and banshentu plutons were emplaced at 2523 7 ma and 2480 20 ma with inherited ages of 2606 19 ma and 2647 19 ma respectively in contrast the haolaigou pluton shows high sio2 68 71 wt mgo mg 46 55 cr and ni contents negative nb ta and ti anomalies and high liles with strongly positive eu anomalies high sr and ba concentrations and sr y and la yb ratios and low y and hrees abundances it was emplaced at 2466 15 ma with an inherited age of 2638 21 ma the whole rock nd t and zircon hf t values of the haolaigou pluton are positive the whole rock tdm nd and zircon tdm1 hf values range between 2 6 and 2 8 ga with a peak at 2 7 ga indicating that the melt was generated by the partial melting of a subducted slab interacting with the overlying mantle wedge the dajitu pluton has nd and hf isotope characteristics identical to those of the haolaigou pluton implying that the two plutons have similar sources it should be noted however that the dajitu pluton displays sanukitoid characteristics which are the result of interaction with ultramafic mafic enclaves the whole rock nd t and zircon hf t values of the banshentu pluton are mostly negative the whole rock tdm nd and zircon tdm1 hf values of the pluton range between 2 7 and 2 9 ga with a peak at 2 8 ga indicating that the pluton was generated from the melting of a lower crust the sanukitoid characteristics of the pluton resulted from interactions with an ultramafic enclave taken together these results with the geochronological geochemical data for other archean rocks in this area suggest that the yinshan block experienced significant crustal growth at 2 8 2 6 ga and further crustal reworking that led to strong magmatism at 2 5 ga

early cretaceous subduction related adakite like rocks of the gangdese belt southern tibet products of slab melting and subsequent melt peridotite interaction abstract the limited geochronology and geochemistry data available for the early cretaceous igneous rocks of the southern gangdese belt southern tibet has resulted in the proposal of conflicting geodynamic models for the generation of the widespread cretaceous igneous rocks in the middle and northern parts of the belt to explore this issue we present shrimp u pb zircon data and geochemical and sr nd pb hf isotopic data for the mamen andesites from the southern margin of the gangdese belt the mamen andesites emplaced at 136 5 ma are sodic na2o k2o 1 2 2 3 and have geochemical characteristics typical of adakites i e high al2o3 high la yb ratios and sr contents low y and hree contents and positive eu anomalies except for high cr ni and mgo contents the andesites have initial 87sr 86sr t ratios of 0 70413 0 70513 positive end t values of 3 7 5 8 and 206pb 204pb t ratios of 18 37 18 51 207pb 204pb t ratios of 15 59 15 65 and 208pb 204pb t ratios of 38 43 38 72 in situ hf isotopic analyses of zircons that had previously been dated by shrimp yielded positive initial ehf t values ranging from 11 0 to 15 5 a model calculation using trace element and sr nd pb isotopic data indicates that several percent of subducted sediment is required to generate the mamen andesites which were derived via the partial melting of subducted neo tethyan slab morb sediment fluid and subsequently hybridized by peridotite in the mantle wedge our data indicate that the neo tethyan oceanic crust was subducted northward beneath the gangdese belt during the early cretaceous at a high angle our results are inconsistent with a tectonic model that advocates the low angle or flat slab subduction of neo tethyan oceanic crust in generating the widespread cretaceous magmatism recorded in the gangdese belt

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

adakites the key to understanding lile depletion in granulites abstract for more than 20 years it has been argued that granulites of the lower continental crust are depleted in fluid mobile elements such as cs rb u and th either because they were removed by melting of the lower crust or through dehydration we argue that there is little evidence for a simple relationship between granulite facies metamorphism and element depletion and propose that for felsic orthogneisses the depletion may be a primary feature of crust generation processes modern adakites show a range of fluid mobile element ratios and adakites from the austral volcanic zone avz chile show fluid mobile element ratios very similar to those found in lower crustal felsic granulites this suite of adakites has a strong slab derived component we propose that modern adakites of the type found in the austral volcanic zone are analogous in their genesis to lower crustal felsic granulites and that their mafic precursors were already depleted in fluid mobile trace elements prior to melting an incremental melting model in which fluid mobile elements are released from an eclogitic slab prior to partial melting can explain the fluid mobile element behaviour of both modern adakites and lower crustal felsic granulites

arc continent collision and orocline formation closing of the central american seaway closure of the central american seaway was a local tectonic event with potentially global biotic and environmental repercussions we report geochronological six u pb la icp ms zircon ages and geochemical 19 xrf and icp ms analyses data from the isthmus of panama that allow definition of a distinctive succession of plateau sequences to subduction related protoarc to arc volcaniclastic rocks intruded by late cretaceous to middle eocene intermediate plutonic rocks 67 6 1 4 ma to 41 1 0 7 ma paleomagnetic analyses 24 sites 192 cores in this same belt reveal large counterclockwise vertical axis rotations 70 9 6 7 and moderate clockwise rotations between 40 4 1 and 56 2 11 1 on either side of an east west trending fault at the apex of the isthmus rio gatun fault consistent with isthmus curvature an oligocene miocene arc crosscuts the older deformed and segmented arc sequences and shows no significant vertical axis rotation or deformation there are three main stages of deformation 1 left lateral strike slip offset of the arc 100 km and counterclockwise vertical axis rotation of western arc segments between 38 and 28 ma 2 clockwise rotation of central arc segments between 28 and 25 ma and 3 orocline tightening after 25 ma when this reconstruction is placed in a global plate tectonic framework and published exhumation data is added the central american seaway disappears at 15 ma suggesting that by the time of northern hemisphere glaciation deep water circulation had long been severed in central america

adakitic granites within the ne jiangxi ophiolites south china geochemical and nd isotopic evidence geochemical and nd isotopic data are reported for the ca 970 ma albite granites within the neoproterozoic ne jiangxi ophiolite south china these granites are high in al2o3 15 17 na2o 7 3 8 5 mg 43 62 sr 383 920 ppm sr y 166 444 and y yb 12 14 but low in k2o 0 2 0 6 tio2 0 3 p2o5 0 1 furthermore they display consistently high morb like initial end values of 4 6 to 6 7 at ca 970 ma these geochemcial and isotopic characters are similar to those of the modern adakites compared with the modern adakites the xiwan albite granites are depleted in middle to heavy ree with y yb ratio of 12 14 which can be attributed to residual amphibole and garnet in the source geological and geochemical features suggested that the granites were generated by low degrees of partial melting of subducted spilitized oceanic crust at pressures high enough to stabilize garnet and amphibole these are the first adakitic granites identified within the ne jiangxi ophiolite their presence shows that granitic rocks that intrude ophiolites can be generated not only during the processes of sea floor spreading oceanic crust shearing and obduction of fragmented oceanic lithosphere as previously thought but also during the process of subduction of oceanic lithosphere the shrimp u pb zircon age of 968 23 ma for the xiwan adakitic granite geochimica 23 1994 117 should be re interpreted as the timing of subduction of oceanic crust rather than as the age of oceanic crustal formation as previously interpreted

miocene to late quaternary patagonian basalts 46 47 s geochronometric and geochemical evidence for slab tearing due to active spreading ridge subduction miocene to quaternary large basaltic plateaus occur in the back arc domain of the andean chain in patagonia they are thought to result from the ascent of subslab asthenospheric magmas through slab windows generated from subducted segments of the south chile ridge scr we have investigated three volcanic centres from the lago general carrera buenos aires area 46 47 s located above the inferred position of the slab window corresponding to a segment subducted 6 ma ago 1 the quaternary rio murta transitional basalts display major trace elements and sr and nd isotopic features similar to those of oceanic basalts from the scr and from the chile triple junction near taitao peninsula e g 87sr 86sr o 0 70396 0 70346 and end 5 5 3 0 we consider them as derived from the melting of a chile ridge asthenospheric mantle source containing a weak subduction component 2 the plio quaternary 1 and tio2 2 wt these intermediate basalts differ from their strictly alkalic equivalents by having lower high field strength element hfse and higher end up to 5 4 these features are consistent with their derivation from an enriched mantle source contaminated by ca 10 rutile bearing restite of altered oceanic crust the petrogenesis of the studied mio pliocene basalts from mlba and mcc is consistent with contributions of the subslab asthenosphere the south american subcontinental lithospheric mantle and the subducted pacific oceanic crust to their sources however their chronology of emplacement is not consistent with an ascent through an asthenospheric window opened as a consequence of the subduction of segment scr 1 which entered the trench at 6 ma indeed magmatic activity was already important between 12 and 8 ma in mlba and mcc as well as in southernmost plateaus i e 6 ma before the subduction of the scr 1 segment we propose a geodynamic model in which oib and intermediate magmas derived from deep subslab asthenospheric mantle did uprise through a tear in the slab which formed when the southernmost segments of the scr collided with the chile trench around 15 ma during their ascent they interacted with the patagonian supraslab mantle and locally with slivers of subducted pacific oceanic crust that contributed to the geochemical signature of the intermediate basalts