along arc lateral variation of rb sr and k ar ages of cretaceous granitic rocks in southwest japan cretaceous granitic rocks were emplaced over a distance of 700 km along arc in southwest japan rb sr and k ar ages of a major group of these granitic rocks with ilmenite series ore mineralogy were examined rb sr whole rock ages of 92 8 4 0 ma and rb sr and k ar biotite ages of 80 88 ma were obtained on one group of these granitic rocks from kamo sera area of central hiroshima prefecture the k ar ages of various minerals combined with the rb sr whole rock age give a smooth cooling curve which suggests a 5 to 10 ma time lag between intrusion and cooling at 300 c for the cretaceous granitic rocks the rb sr whole rock and rb sr k ar biotite ages of these granitic rocks become younger eastward along the southwest japan arc and the time lag between the two systems remains constant at 5 to 10 ma over the entire area the along arc age variation does not support the genetical relationship of the cretaceous granitoids with steady state subduction the cretaceous granitic province at the eastern margin of eurasian continent was at least partly formed by an episodic event such as ridge subduction

paleomagnetism of coastal california and baja california alternatives to large scale northward transport paleomagnetic data from the santa lucia orocopia sloa and baja borderland bba allochthons of coastal california and baja california have been interpreted to indicate up to 2500 km of post mid cretaceous northward transport of these regions with respect to interior north america however with neogene strike slip offsets taken into account geological interpretations correlate basement rocks of the coastal allochthons with continental basement rocks directly across the san andreas and related fault systems we have examined paleomagnetic data from sloa and bba and conclude that apparent discordances can be explained without large scale pre neogene tectonic transport three major observations are fundamental to this analysis 1 paleolatitudes derived from volcanic rocks of the jurassic eugenia formation of bba and coast range ophiolite of sloa are concordant when compared to revised jurassic reference paleomagnetic poles from interior north america 2 isotopic and paleobarometric data from the peninsula ranges batholith in southern california indicate that the batholith has been tilted northeast side up by an amount that can account for discordant paleomagnetic directions observed in plutonic rocks of the batholith without large scale northward transport 3 literal interpretation of the paleolatitudes determined from paleomagnetic directions in upper cretaceous and paleogene marine sedimentary rocks of sloa and bba requires north then south then north transport and a complex motion history between the two allochthons however concordant paleolatitudes are indicated by some sedimentary rocks while coeval or younger sedimentary rocks of the same allochthon have discordant paleolatitudes coupled with recent documentations of compaction shallowing of paleomagnetic inclination in other marine sedimentary rocks these inconsistencies suggest that paleolatitudes derived from most of the marine sedimentary rocks of sloa and bba are biased towards low paleolatitudes by compaction shallowing

paleomagnetism of the upper cretaceous nanaimo group southwestern canadian cordillera the baja b c model has the insular superterrane and related entities of the canadian cordillera subject to 3000 km of northward displacement with respect to cratonic north america from 90 to 50 ma the upper cretaceous nanaimo group on and about vancouver island british columbia is a prime target to test the model paleomagnetically because of its locality and age we have widely sampled the basin 67 sites from seven islands spread over 150 km santonian to maastrichtian age most samples have low unblocking temperatures 450 c and coercivities 10 mt and strong present field contamination forcing us to reject three quarters of the collection beds are insufficiently tilted to provide a conclusive fold test and we see evidence of relative vertical axis rotations however inclination only analysis indicates pretilting remanence is preserved for many samples both polarities are observed and reversals correlate well to paleontological data proving that primary remanence is observed the mea

petrology and geochemistry of mafic rocks in the acasta gneiss complex implications for the oldest mafic rocks and their origin abstract the acasta gneiss complex located in the western part of the slave province canada is widely recognized as the oldest eoarchean terrane in addition to felsic gneisses with the ages of 3 6 4 0 ga minor mafic rocks occur as rounded to elliptical enclaves and inclusions within the felsic gneisses despite serving as potential sources of geochemical information on the hadean mantle the mafic rocks have received less attention in previous studies thus we conducted a comprehensive geological petrological and geochemical investigation on the acasta mafic rocks to constrain their petrogenesis and geodynamic setting the mafic rocks comprise massive to weakly foliated amphibolite garnet amphibolite and hornblendite with variable abundances of hornblende plagioclase chlorite and quartz and subordinate clinopyroxene garnet and cummingtonite they commonly underwent high grade metamorphic recrystallization under amphibolite to upper amphibolite facies conditions the observed variations in mineral assemblages abundances and compositions reflect large differences in whole rock compositions likely caused by crustal anatexis during the eoarchean thermal events responsible for the generation of the surrounding felsic gneisses infiltration or extraction of felsic melts formed due to partial melting of precursor rocks can account for an overall negative correlation between al 2 o 3 and mgo contents and variable enrichments in the incompatible elements despite the widespread influence of anatexis on the geochemistry of acasta mafic rocks we identified the precursor compositions of the least modified amphibolites as basaltic magmas they are characterized by intermediate al 2 o 3 and mgo contents on the observed array and by near chondritic patterns for incompatible trace elements except for slightly negative nb and ta anomalies we considered two scenarios to explain the origin of eoarchean basaltic rocks with nb ta anomalies 1 generation of nb ta deficient basaltic magma in a suprasubduction setting analogous to modern arcs derived magmas and 2 generation of nb ta deficient basaltic magma from the melting of a nb ta deficient primitive mantle possible if the core contains significant proportions of the earth s nb and ta budget although the operation of plate tectonics and the presence of subduction zones at the end of hadean may be an attractive explanation for the observed nb ta depletions the chondritic relative proportions of other immobile trace elements for acasta mafic rocks leave open the possibility of their formation from an nb ta deficient primitive mantle

helium and neon isotopes in the imnaha basalt columbia river basalt group evidence for a yellowstone plume source abstract helium neon and argon isotopic compositions were measured in two flows of the columbia river flood basalt the imnaha basalt has a 3 he 4 he ratio of 11 4 times atmospheric and 20 ne 22 ne and 21 ne 22 ne ratios characteristic of a plume component the measured 3 he 4 he is a lower limit due to possible preferential 3 he loss and or addition of radiogenic 4 he a wanapum basalt flow erupted approximately 2 ma later in the waning stages of volcanism has more morb like noble gases the he nd and sr isotopic compositions of these lavas suggest that the columbia river basalts were derived from the yellowstone plume head which contained both high helium plume material and entrained depleted mantle as the eruptions progressed the plume component in the melting region was gradually diluted or replaced

continental flood basalts episodic magmatism above long lived hotspots abstract the eruption of continental flood basalt cfb may reflect episodic magmatism above long lived mantle plumes the iceland and yellowstone hotspots have generated successive cfb provinces large intrusive complexes anomalous uplift basin formation and rifting events and linear volcanic chains dating back 120 and 70 ma respectively amagmatic intervals occurred 1 when ascent of plumes to shallow levels was impeded by impenetrable lithosphere resulting in sub lithospheric ponding of plume mantle and 2 in response to dispersion by subducting oceanic lithosphere in convergent margin settings by comparison with the plume eruptive potential of typical oceanic hotspots it is apparent that preservation of only a small portion of plume mantle ponded during an amagmatic interval is necessary to account for large volume of cfbs thermal erosion lithospheric attenuation translation of ponded hotspot mantle to the base of thinner penetrable lithosphere and passage of plume mantle through slab windows in subducting oceanic lithosphere led to subsequent breakthrough and eruption of cfb since both mantle plume and plate tectonic processes have been operating since the archean it seems likely that the migration of continents over hotspots with attendant magmatic and tectonic consequences is a common occurrence in the geological record

northward motion of the whitehorse trough paleomagnetic evidence from the upper cretaceous carmacks group reply the upper cretaceous carmacks group 70 4 2 4 ma comprises gently dipping basaltic and andesitic lava flows overlying volcaniclastic deposits of the intermontane belt in the whitehorse trough the sampling area is in southern yukon and northern british columbia it lies west of the tintina northern rocky mountain trench fault and teslin suture zone and east of the denali shakwak fault volcanic sections were sampled in three regions spread over 300 km providing the first paleomagnetic data from pre tertiary volcanic rocks in the northern canadian cordillera alternating field and thermal demagnetization revealed stable magnetization for 18 of the 27 sites collected the overall mean direction d 166 7 i 71 4 k 53 95 4 8 n 18 sites is pre folding and is most probably primary latest cretaceous this gives a paleopole at 109 4 e 82 1 n k 21 a95 7 8 a critical evaluation of north american cratonic data yields a reference paleopole for the latest cretaceous at 185 8 e 77

origin of high magnesian andesites in the setouchi volcanic belt southwest japan ii melting phase relations at high pressures melting phase relations of an augite olivine high magnesian andesite and an augite olivine basalt from the miocene setouchi volcanic belt in southwest japan have been studied under water saturated water undersaturated and under anhydrous conditions both the andesite and the basalt are characterized by low feo mgo ratios 0 86 and 0 76 in weight respectively and qualify as primary magmas derived from the upper mantle the andesite melt coexists with olivine orthopyroxene and clinopyroxene at 15 kbar and 1030 c under water saturated conditions and at 10 kbar and 1070 c under water undersaturated conditions 7 wt h2o in the melt the basalt melt also coexists with the above three phases at 11 kbar and 1305 c under anhydrous conditions and at 15 kbar and 1205 c in the presence of 4 wt water present studies indicate that high magnesian andesite magmas may be produced even under water undersaturated conditions by partial melting of mantle peridotite it is suggested that two types of high magnesian andesites in the setouchi volcanic belt augite olivine and bronzite olivine andesites were produced by different degrees of partial melting augite olivine andesite magmas whose mantle residual is lherzolite were formed by lower degrees of partial melting than bronzite olivine andesite magmas which coexist with harzburgite the basalt magmas which were often extruded in close proximity to the high magnesian andesite magmas are not partial melting products of a mantle peridotite which had previously melted to yield high magnesian andesite magmas

paleomagnetism and geobarometry of the big creek batholith suggests that the yukon tanana terrane has been a parautochthon since early jurassic abstract the 190 5 ma big creek hornblende syenite and monzogabbro batholith has an outcrop area of 400 km2 within the yukon tanana terrane ytt in the west central part of the yukon territory of canada paleomagnetic analysis of 245 specimens from 21 sites isolates a characteristic remanent magnetization residing principally in magnetite and subordinately in hematite the unit mean direction for the batholith from 16 normal polarity and two reverse polarity sites is d 305 6 i 72 1 95 3 9 al in hornblende geothermobarometry at six locations yields emplacement depths of 16 2 km three arguments suggest that the characteristic remanent magnetization was acquired on exhumation at 180 ma more significantly the pole position indicates no northward translation but counterclockwise rotation implying that the ytt formed close to its present position on the jurassic margin of the north american craton this implies in turn that the allochthonous terranes of the intermontane and coast belts of the western cordillera impacted onto and overrode the ytt upon accretion during late cretaceous to middle eocene time

age of emplacement and basement character of the cache creek terrane as constrained by new isotopic and geochemical data new u pb and rb sr isotopic and major and trace element geochemical data are reported for late triassic to eocene granite bodies that intrude the cache creek and stikine terranes in the atlin bennett area of northwestern british columbia the u pb isotopic age data help constrain closure of the cache creek ocean and obduction of the cache creek terrane to before 172 ma middle jurassic low 87sr 86sr initial ratios 0 7037 0 7046 and lack of evidence for inheritance in zircons suggest that rocks underlying the cache creek terrane are largely primitive in nature with derivation of intrusive rocks predominantly from unevolved sources a genetic link between late triassic granitic plutons and stuhini arc volcanics of stikinia is supported by trace and rare earth element data which indicate generation of magma in a volcanic arc setting geochemical patterns for postaccretion middle jurassic and late cretaceous to early tertiary age plutons are similar to those of the late triassic indicating that middle ju

early paleozoic high mg diorite granodiorite in the eastern kunlun orogen western china response to continental collision and slab break off abstract phanerozoic high mg diorites and granodiorites are known to be related to diverse subduction styles at convergent margin settings but have not been linked to collision processes the santonggou complex in the eastern kunlun orogen is composed of a dominant granodiorite intruded by small stocks and dykes of porphyritic granodiorites and diorites which are all characterized by high mgo and ni but low yb and y contents mafic microgranular enclaves mmes occur in the granodiorite and the porphyritic granodiorites la icpms u pb analyses of zircon from the granodiorite porphyritic granodiorite and porphyritic diorite yield similar ages of 432 2 ma 432 2 ma and 427 3 ma respectively plagioclase in the granodiorite exhibits complex zonations with resorbed and patchy cores plagioclase and hornblende phenocrysts in the porphyritic granodiorites and diorites also show textural and compositional zonations the highest e nd t value of 0 6 in mmes suggests asthenosphere derived mafic magma these lines of mineralogical and geochemical evidence together with results from geochemical modeling suggest that the porphyritic diorites were derived from mantle metasomatized slab melts the porphyritic granodiorites and the granodiorite were generated by simultaneous assimilation and fractional crystallization of the magma approximated by the porphyritic diorites plus mixing of asthenosphere derived mafic and evolved granodioritic magmas pressure estimates from the al in hornblende geobarometer suggest that the granodiorite had been uplifted to near the surface before the emplacement of the porphyritic granodiorites partial melting of the slab upwelling of the asthenosphere and rapid orogenic uplift at 430 ma suggest that the cessation of subduction and the initiation of collision as well as slab break off occurred almost simultaneously in the eastern kunlun orogen

geometric test for late cretaceous paleogene intracontinental transform faulting in the canadian cordillera the tintina trench northern rocky mountain trench tt nrmt fault zone and the fraser river straight creek fr sc fault zone are separate en echelon concentric small circle fault segments of a composite intracontinental transform fault zone more than 2500 km long that cuts diagonally across the canadian cordillera from the outboard part of a tectonic collage of accreted foreign terranes in the south into the north american preaccretionary continental margin in the north most of the 450 km of right hand slip on the tt nrmt fault zone was transformed southward during the late cretaceous and paleocene into oblique convergence in the southern canadian rockies the remainder probably comprising less than 100 km was transformed southwestward during early and middle eocene time via a zone of distributed shear and east west crustal stretching into right hand slip on the en echelon fr sc fault zone these interpretations based on regional systematic mapping of geologic structures are in conflict with interpretations of paleomagnetic measurements that call for more than 1000 km of post mid cretaceous right hand displacement along the general locus of the tt nrmt fault zone involving foreign terranes and the parts of north america to which they had been accreted this paradox must be resolved

subduction modified mantle derived triassic high mg andesites in the sanjiang tethys eastern tibet abstract andesites in the yangla district of the sanjiang tethys eastern tibet are characterized by relatively high mgo and with rich large ion lithophile elements liles indicating similarities to typical high mg andesites formed in continental collisional zones zircon u pb age of 232 1 ma for these andesites shows that they occurred synchronously with the regionally widespread late middle triassic granitoids and bimodal volcanic rocks primitive mantle normalized spidergrams for these andesites show depletion in high field strength elements hfses with sr nd pb isotopic compositions that are comparable to both arc and morb like high mg rocks generated from subduction modified sources modelling results suggest that an input of 20 sediment derived melts into mantle derived melts could be possibly responsible for the sr nd isotopic characteristic of these andesites morb and arc like compositions likely indicate a metasomatized mantle wedge modified by subduction processes during the closure of the paleo tethys ocean and subsequent continental collision it is speculated that the late middle to late triassic intra continental re activation associated with regional tectonic collapse and thermal relaxation triggered the melting of this previously subduction modified mantle leading to the formation of the high mg andesites in the yangla district

post triassic para autochthoneity of the yukon tanana terrane paleomagnetism of the early cretaceous quiet lake batholith

the cordilleran ribbon continent of north america the north american cordilleran orogen is the result of a two stage process a triassic jurassic accretion within panthalassa forming saybia a composite ribbon continent and b late cretaceous collision of saybia with north america this model requires that a large portion of the continental foreland of the orogen is exotic the exotic continental component of saybia cassiar platform is distinguished from the autochthon on the basis of its a triassic eurasian fauna b involvement in a major late triassic early jurassic orogenic event and c young in part grenvillian basement and mantle a mid cretaceous magmatic arc records west dipping subduction beneath the east margin of saybia the related accretionary prism consists of imbricated shale chert and deep water limestones the medial basin and overlies an isotopically juvenile mantle domain carbonatite complexes delineate the cryptic suture separating saybia and the autochthon paleomagnetic and paleobotanical data place saybia 2000 km to the south relative to the autochthon at 80 ma late cretaceous thrust belt development records transpression between the north moving ribbon continent and the autochthon pinning against the okhotsk chukotka arc in siberia buckled saybia giving rise to the alaskan promontory

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

lithospheric structure of the yukon northern canadian cordillera obtained from magnetotelluric data 1 two goals of lithoprobe s geoscientific studies in the phanerozoic accretionary cordillera of western north america were to define the subsurface geometries of the terranes and to infer the physical conditions of the crust these questions were addressed in canada s southern cordillera a decade ago and have more recently been addressed in the northern cordillera of which one component of the new studies is magnetotelluric mt profiling from ancestral north american rocks to the coast we present a resistivity cross section and its interpretation of the northern cordillera derived from modeling data from 42 mt sites along a 470 km long ne sw profile beneath the coast belt southwestern end of the profile a deep crustal low resistivity layer dips inland we interpret the crustal part of this conductor as being due to metasedimentary rocks emplaced and metamorphosed during paleocene kula plate subduction a strong lateral transition in lithospheric mantle resistivity exists below the intermontane belt that is spatially coincident with changes in chemical and isotopic characteristics of tertiary to recent alkaline lavas suggesting that isotopically enriched lithosphere related to the coast belt basalts extends partly beneath the intermontane belt the unusually high lower crustal resistivity in the intermontane and omineca belts similar in value to the resistivity found in the unextended part of central british columbia excludes the presence of fluids or conducting metasediments finally our resistivity model displays strong lateral variation of the middle and lower crust between different terranes within the same belt as a result of the complex structural evolution of the lithosphere index terms 0925 exploration geophysics magnetic and electrical methods 1515 geomagnetism and paleomagnetism geomagnetic induction 8110 tectonophysics continental tectonics general 0905 8120 tectonophysics dynamics of lithosphere and mantle general keywords magnetotelluric accreted terranes northern canadian cordillera

a paleomagnetic test for northward tectonic transport of the eastern selwyn basin northern canadian cordillera data from the mid cretaceous ragged pluton ragged pluton is a member of the tungsten plutonic suite in the eastern selwyn basin this circular massive 97 ma granitic pluton is 5 km in diameter it intrudes deformed and metamorphosed clastic strata of the neoproterozoic to lower cambrian vampire formation thermal and alternating field step demagnetization data for 216 specimens from 21 plutonic sites isolated a stable characteristic remanent magnetization chrm direction at 20 sites of declination d 325 9 inclination i 80 4 95 2 7 k 142 thermal demagnetization and saturation remanence analysis show that the chrm is carried by both single domain pyrrhotite and magnetite ragged pluton s paleopole at 73 9 n latitude 191 4 e longitude dp 5 0 dm 5 2 is concordant with its co eval 97 ma north american reference paleopole showing at 95 confidence that the pluton has not been significantly rotated or translated relative to north america this paleomagnetic result favours tectonic models in which the eastern selwyn basin is

terrane wrecks coupled oroclines and paleomagnetic inclination anomalies abstract three major orogenic belts that are host to coupled oroclines terrane wrecks are likewise characterized by or directly associated with paleomagnetic inclination anomalies 1 the north american cordillera 2 the western european variscan belt and 3 the eastern european alpine system implied paleolatitudinal discrepancies in each orogen are of similar magnitude to the translation required by orogen parallel shortening during oroclinal formation the alaskan coupled oroclines accommodated 3000 km of orogen parallel shortening in the north american cordillera where primary remanent paleomagnetic inclinations in late cretaceous rocks require the bulk of the belt to have been translated 2000 to 3000 km northward relative to cratonic north america the iberian coupled oroclines accommodated 1100 km of orogen parallel shortening in the western european variscan belt within which mid paleozoic paleomagnetic inclinations are shallow with respect to stable gondwana the implied existence of a 3500 km wide paleotethys ocean between the variscan autochthon and the north gondwana margin is seemingly at odds with paleontological constraints the carpathian balkan oroclines accommodated 1000 km of orogen parallel shortening in the eastern alpine system directly north northeast of an eastern mediterranean region where primary remanent inclinations in late oligocene to miocene rocks require northward translation between 500 and 1000 km palinspastic restoration of the alaskan and carpathian balkan oroclines fully accounts for the translations recorded by the cordilleran and eastern mediterranean inclination anomalies respectively restoration of the iberian coupled oroclines confirms the existence of paleotethys but only accounts for one third of the paleolatitudinal discrepancy recorded by the variscan inclination anomaly fully resolving the variscan inclination anomaly requires 1 a paleomagnetic data set of greater temporal and spatial extent 2 refinement of the geometric and geological continuity of the variscan belt across western europe

melting of a subducting slab and production of high mg andesite magmas unusual magmatism in sw japan at 13 15 ma characteristic high mg andesite magmas were produced in the sw japan arc at 13 15 ma that was synchronous with the commencement of subduction of a very young 11 m y lithosphere of the shikoku basin numerical simulation suggests that temperature at the surface of such a young subducting plate is high enough for partial melting both of the subducting sediments and oceanic crust at the beginning of the subduction high mg andesite magmas were likely to be produced by interaction between silicic slab melts and the overlying mantle wedge hma magmas may be commonly produced in the archean subduction zones under relatively high mantle temperature conditions contributing to making continental crusts

paleomagnetism and tectonic significance of albian and cenomanian turbidites ochoco basin mitchell inlier central oregon 1 understanding continental growth and convergent margin dynamics associated with terrane accretion and modification of the cordilleran margin of north america is prevented by conflicts in paleogeographic models for major terranes oceanic plates and the north american margin we present new paleomagnetic data suggesting that the blue mountains superterrane located at an inboard portion of the cordilleran margin has undergone substantial northward translation and clockwise rotation relative to north america since mid cretaceous time positive baked contact conglomerate and fold tests provide evidence that the magnetization of these rocks dates from their deposition these results yield a mean direction of d 10 8 i 58 5 k 127 95 4 1 and n 11 comparison of magnetic inclinations and magnetic fabrics shows no evidence for inclination error a paleolatitude of 39 2 4 8 4 3 n is obtained indicating a paleolatitude anomaly of 15 9 4 1 northward translation of 1760 460 km and cw rotation of 37 7 3 with respect to north america since circa 93 ma application of an ad hoc 5 inclination error yields translation of 1200 460 km through comparison with other units in the western cordillera we infer that the mitchell inlier was located adjacent to the sierra nevada arc during mid cretaceous time and that the blue mountains province may correlate with the intermontane superterrane in british columbia other interpretations such as large inclination errors or an independent displacement history for the mitchell inlier are possible but unlikely more work is needed to fully test these hypotheses and resolve major questions of cordilleran tectonics

the early paleozoic huangtupo vms cu zn deposit in kalatag eastern tianshan implications from geochemistry and zircon u pb geochronology of volcanic host rocks abstract the tianshan in china forms the southern part of the central asian orogenic belt caob and also hosts important polymetallic resources here we investigate the huangtupo medium size copper zinc deposit located in the central part of the kalatag copper polymetallic belt with a view to evaluate the nature and genesis of the early paleozoic volcanogenic massive sulfide vms cu zn deposits this deposit is correlated to intermediate felsic volcano sedimentary sequences of the early paleozoic daliugou formation which are mainly composed of andesite dacite and rhyolite with minor basalt as well as their pyroclastic equivalents the ore bodies are hosted in the contact zone between andesitic volcanic breccia and tuff representing an eruptive hiatus between the two volcanic cycles with massive sulphide lens at the top and stockwork zone at lower part of the daliugou formation we report new zircon u pb data major and trace element geochemistry and sr nd isotopes from the andesite dacite and andesitic tuff in the huangtupo deposit zircon from the dacite and the andesite occurring above the ore bodies yield and age of 440 1 1 2 ma and 439 7 0 95 ma and that from the andesitic tuff in footwall shows and age of 442 0 1 1 ma all samples show typical geochemical signatures of subduction related calc alkaline magmas with slight enrichment in lile and lree but depletion in nb and ti content relative to primitive mantle the andesites are characterized by high mgo 7 6 10 8 wt contents and mg values 64 72 with low sr y and la yb ratios belonging to sanukite both dacite and tuff have low mgo

on the occurrence and implications of jurassic primary continental boninite like melts in the zagros orogen ultramafic rocks ranging from pyroxenites to hornblendites are associated with granitoids of the aligoodarz intrusive complex in the central sanandaj sirjan zone representing the mesozoic continental arc segment of the zagros orogen as inferred from the ultramafic whole rock composition and the most primitive clinopyroxene composition in pyroxenites the geochemical signature of primary melt is significantly different from that of the continental arc basalts in particular primary melt is characterized by extremely low concentrations of incompatible elements and high concentrations of mg and refractory elements typical of boninites amphibole is a late crystallizing mineral in these rocks and is in textural and chemical disequilibrium with olivine orthopyroxene clinopyroxene amphibole crystallized from a liquid underwent differentiation through a process of melt rock reaction in particular early differentiated boninitic cumulates reacted with later melts with a strong crustal signature similar to aligoodarz granodiorite u pb zircon geochronology from ultramafic rocks and surrounding quartz diorite yield similar ages and indicate that they are coeval with aligoodarz granitoids ca 165 170 ma however the occurrence of a marked negative eu anomaly in zircon from the ultramafic rocks which is absent in the boninitic primary melt indicates that zircons crystallized from the infiltrating melt and in turn date the timing of melt infiltration the interaction between ultramafic cumulates and infiltrated melt has generated a new liquid compositionally similar to high mg andesites and to the quartz diorites hosting the ultramafic cumulates the scenario that better account for the genesis of boninitic melts in the sanandaj sirjan zone is partial melting of a depleted mantle wedge in response to the onset of neotethys subduction according to this hypothesis middle jurassic calc alkaline magmatism in the sanandaj sirjan zone represents the mature stage of arc magmatism postdating boninite generation by about 10 ma

evaluation of the baja controversy using paleomagnetic and faunal data plume magmatism and piercing points abstract the controversy over late cretaceous locations of baja alaska baja bc stikinia and western quesnellia centres around whether a they were within 1000 km of their present positions or b located 1000 5000 km south along the western coast of laurentia to be transported northwards by the kula plate during the late cretaceous and cenozoic the latter is supported by paleomagnetic data including data that pass a fold contact or reversal test whereas the former is based upon correlation of strata across faults although various critical tests have been proposed and tried interpretations are equivocal e g alternative interpretations for the source of detrital zircons in baja bc we propose that the presence of plume magmatism provides unique vertical piercing points thus correlation of the 70 ma plume magmatism in northern stikinia with the yellowstone hotspot locates stikinia 2000 km to the south and 1000 km off the western coast of laurentia on the other hand the absence of 90 70 ma yellowstone plume magmatism in the presently adjacent baja bc but the presence of 60 47 ma yellowstone plume spreading ridge magmatism in southern baja bc southern vancouver island and coast ranges of washington and oregon implies that it moved at least 1200 km northwards between 70 and 60 ma i e a rate of 12 cm year the spreading ridge may be correlated with the kula farallon ridge furthermore the presence of middle late triassic plume magmatism in wrangellia and the peninsula and alexander terranes suggests 2000 km of relative dextral movement between baja alaska and baja bc applying these conclusions to pre 90 ma paths of baja alaska bc across the pacific ocean suggests that it had three components 1 northeasterly between 230 and 180 ma 2 eastsoutheasterly between 180 and 140 ma and 3 northeasterly between 140 and 90 ma whereas the latter component is consistent with published terrane tracks the 180 140 ma component is at odds suggesting that this latter segment be reassessed the weight of evidence in favour of correlating geological piercing points across the gulf of california appears to outweigh paleomagnetic evidence for 1000 2500 km of apparent northward displacement of baja california in the cretaceous and early tertiary which may be explained by 15 20 tilting of fault blocks recorded by 40 ar 39 ar cooling ages thus we conclude that whereas baja alaska baja bc stikina and western quesnellia were transported between 1000 and 5000 km northwards along the western margin of laurentia during the late cretaceous and cenozoic baja california has not moved more than that required to open the gulf of california

anisotropy of magnetic susceptibility studies in tertiary ridge parallel dykes iceland tertiary margin normal aishihik dykes yukon and proterozoic kenora kabetogama composite dykes minnesota and ontario abstract mafic dykes of different ages were collected from three different tectonic settings and analyzed using anisotropy of magnetic susceptibility ams as a proxy for magmatic flow during intrusion in iceland ridge parallel basaltic dykes were sampled on each side of the active tectonic boundary the dykes are k max orientation indicating vertical emplacement the eocene aishihik dyke swarm intrudes the yukon tanana terrane in the yukon province canada over an area 200 by 60 km these dykes were intruded normal to the accretionary margin are porphyritic andesites and have an intermediate geochemical signature based on major and trace element analyses ten dykes were sampled and 111 cores analyzed using ams and the dykes preserve a vertical k max orientation indicating intrusion was vertical through 30 km of continental crust the 2 06 ga kenora kabetogama dykes in northern minnesota and western ontario crosscut a variety of archean terranes thickness 50 km in a radiating pattern the unmetamorphosed basaltic dykes are 1 120 m wide 10 110 km in length are vertical in orientation and can be grouped as either being single intrusion or multiple intrusion composite dykes ams data preserve a vertical k max orientation for the southerly locations 2 dykes n 53 and horizontal k max for the remainder to the northwest 15 dykes n 194 maximum magnetic susceptibility axes 4 dykes n 92 for composite dykes are scattered and yield inconsistent flow directions with regard to the dyke margin almost all of our results are normal in that the magnetic foliation the plane containing k max and k int normal to k min is parallel to the dyke planes which gives us confidence that the magnetic lineations i e k max orientations are parallel to magmatic flow

petrology of tholeiitic lavas from tanna island new hebrides importance of cumulative processes in island arc magmatism tanna one of the southernmost islands of the new hebrides volcanic arc is made of late pliocene to recent island arc tholeiitic basalts and andesites with sio2 contents ranging from 45 to 57 these lavas are highly porphyritic 30 50 in volume phenocrysts of plagioclase are the most abundant together with olivine and clinopyroxene the groundmass contain plagioclase augite olivine magnetite and glass pigeonite tridymite sanidine and rarely biotite may also occur the olivines and clinopyroxenes show an iron enrichment from the cores of phenocrysts to their rims and the groundmass crystals but their compositional variations are not correlated with the mg fe ratio of bulk host rocks the most fe rich compositions being found in mg rich lavas plagioclase compositions range from an95 to an60 in the basalts and an60 to an50 in the andesites but within each group they are not correlated with sio2 or na2o contents of host lavas consequently the bulk major element compositions of tanna volcanic rocks cannot be considered as primarily controlled by crystal separation from successive liquids the oxyde sio2 variations diagrams and the modal compositions and mineral chemistry show that crystal accumulation is the predominant mechanism accounting for bulk rock compositions however this does not exclude fractional crystallization the variation of the calculated groundmass mineralogy strongly suggest the occurrence of crystal removal mainly clinopyroxene and magnetite

the yukon tanana terrane part of north america at 215 ma from paleomagnetism of the taylor mountain batholith alaska abstract the massive unmetamorphosed 212 ma taylor mountain batholith underlies 648 km2 of the yukon tanana terrane ytt in east central alaska and intrudes klippen thrust sheets of the fortymile river assemblage and the chicken metamorphic complex structural evidence indicates that the batholith is neither tilted nor rotated about a vertical axis paleomagnetic analysis of 295 specimens from 24 sites in the batholith and cross cutting mafic dikes isolates a stable primary characteristic remanent magnetization in magnetite with unblocking temperatures of 500 to 585 c that places the batholith s paleopole at 61 4 n 102 2 e n 17 95 5 2 on the north american apparent polar wander path apwp at 215 5 ma 1 the taylor mountain paleopole alone and all ytt paleopoles together show that the ytt has been a para autochthonous part of the north american margin since the late triassic further the intermontane belt imb paleopoles are distinct from the north american and ytt paleopoles prior to the mid eocene and provide two apwp segments one imb segment includes the quesnel cache creek eastern coast belt and northern stikine terranes and the second more speculative imb segment includes the central and southwestern stikine and methow terranes this analysis supports tectonic models for the northern cordillera in which the imb terranes were obducted onto north america during the late cretaceous to mid eocene co eval with the laramide orogeny

paleomagnetism of the flores volcanics vancouver island in place by eocene time the eocene flores volcanics of the insular belt of southwestern vancouver island have u pb zircon ages of 51 50 ma and a mean direction of magnetization d i of 349 8 69 6 12 collecting sites

jurassic location of the yukon tanana terrane from palaeomagnetism of the folded mississippian tatlmain batholith and ragged stock

late cretaceous age of the hutshi mount nansen and carmacks groups southwestern yukon territory and northwestern british columbia volcanic rocks of the hutshi mount nansen and carmacks groups occur in the southwestern yukon where they unconformably overlie the yukon crystalline terrane and deformed strata of the whitehorse trough the volcanic rocks are faulted and tilted locally altered and largely postorogenic the more basic carmacks volcanics locally overlie intermediate to acid rocks of the mount nansen group but are mostly in isolated exposures northwest of the hutshi and mount nansen volcanics hutshi mount nansen volcanics of the miners range are porphyritic partly vesicular calc alkaline andesite flows and flow breccias intruded by calc alkaline alkali rich rhyolite and two feldspar andesite dikes a low greenschist metamorphic overprint affects most rocks carmacks volcanics near carmacks are flows epiclastic breccias and sintered tuffs interbedded with immature volcanic sandstone one analysed breccia clast is calc alkaline andesite but the flows are potassic basalt trachybasalt and tristanite recent assignme

the big flush paleomagnetic signature of a 70 ma regional hydrothermal event in displaced rocks of the northern canadian cordillera the 70 ma carmacks group a subaerial volcanic succession which once covered much of central southwest yukon has a paleomagnetic remanent direction which passes the fold test and the reversal test a new collection of 13 sites combined with 13 sites from a previous study renders a pole 088 6 e 78 4 n a95 7 8 which is far sided with respect to the pole for cratonic north america and implies a displacement from the south of 1900 700 km late triassic mandanna member red beds and early jurassic nordenskild formation tuffs deformed in the late jurassic fail the fold test and conglomerate test but pass a contact test with eocene dykes the postdeformational remanent direction is identical to that isolated from the carmacks group the magnetic signature contained in these older formations is probably an overprint produced by an extensive hydrothermal system active during carmacks extrusion geological work indicates that the carmacks group is plume related given its paleomagnetic latitude and geological nature we hypothesize that the carmacks group is a displaced segment of the yellowstone hot spot track and the hydrothermal system which remagnetized the older rocks was established by mantle upwelling below the region resume le groupe de carmacks go de 70 ma est une succession volcanique subaorienne qui couvrait jadis la majeure partie du centre sud ouest du yukon il possde une direction paloomagnotique romanente certifioe par le test du pli et le test des inversions une nouveau groupe de 13 sites combino 13 sites diune otude antorieure fournit un pule 088 6 e 78 4 n a95 7 8 qui est considorablement oloigno par rapport au pule du craton de liamorique du nord et qui implique un doplacement partir du sud de 1900 700 km les couches rouges du membre de mandanna du trias tardif et les tufs du formation de nordenskild du jurassique prococe qui subirent une doformation durant le jurassique tardif ont ochouo le test du pli et le test de conglomorat mais ils ont passo avec succs le test de contact avec les dykes de li ocne la direction romanente post tectonique est identique celle isoloe du groupe de carmacks la signature magnotique qui caractorise ces formations plus anciennes est probablement une roaimantation produite par un systme hydrothermal otendu qui fut actif durant liextrusion de carmacks liotude goologique rovle que le groupe de carmacks est apparento un panache tant donno sa latitude paloomagnotique et sa nature goologique nous avanaons lihypothse que le groupe de carmacks reprosente un segment doplaco du traco du point chaud de yellowstone et que le systme hydrothermal qui a roaimanto les roches les plus anciennes est ds liascension du manteau sous la rogion otudioe

major dextral transcurrent displacements along the northern rocky mountain trench and related lineaments in north central british columbia the northern rocky mountain trench and a number of other prominent lineaments along and east of the eastern margin of the intermontane belt mark faults along which dextral transcurrent movements have been dominant offsets of shelf to off shelf facies boundaries in lower paleozoic rocks indicate a cumulative displacement of at least 750 km and probably 900 km within the system of faults related to those in the northern rocky mountain and the tintina trenches farther west another system of faults appears to offset plutons and stratigraphic assemblages along the eastern margin of the intermontane belt by as much as 300 km these faults including the kutcho and the pinchi connect in part with the teslin suture zone in yukon territory and probably with the fraser river straight creek fault zone in southern british columbia although dextral transcurrent faulting may have taken place between the middle jurassic and early cenozoic the most convincing evidence points to middle cretaceous and particularly to early cenozoic eocene displacements the eocene movements were temporally related to plutonism volcanism lamprophyre dike emplacement high heat flow sedimentation in grabens and rapid uplift of northwesterly trending elongate ranges climactic episodes of granite emplacement particularly in and near the northern omineca crystalline belt at 100 m y 70 m y and 50 m y ago may have been facilitated by changes from dominantly compressional to dominantly transcurrent and related tensional strain

discordant paleomagnetic poles from the canadian coast plutonic complex regional tilt rather than large scale displacement stratigraphic petrologic and isotopic data indicate that parts of the coast plutonic complex and the north cascade range have been tilted northeast side up by angles of approximately 30 degree about north northwest trending axes these tilts can account for discordant paleomagnetic directions observed in mid cretaceous plutons from these regions without large scale displacement relative to north america

deciphering shallow paleomagnetic inclinations 2 implications from late cretaceous strata overlapping the insular intermontane superterrane boundary in the southern canadian cordillera 1 significant discrepancies exist between paleomagnetic and geologic estimates of the timing and magnitude of terrane displacement in the southern canadian cordillera lithostratigraphic palynological geochronologic geochemical and structural data to which we add paleomagnetic data demonstrate that upper cretaceous strata in churn creek are laterally equivalent facies of the silverquick powell creek spc succession these rocks therefore comprise a single overlap sequence linking the insular and intermontane superterranes by late albian cenomanian time 95 ma contradicting previous assertions based solely on paleomagnetic interpretation that major 2000 km latitudinal displacement separated the two regions the churn creek succession retains primary magnetic remanence as demonstrated by positive fold conglomerate and contact tests the sediments do not hold the anisotropy signature of compaction shallowed inclinations the mean inclination of all spc sites from both mount tatlow and churn creek is 55 6 2 3 n 36 sites corresponding to a paleolatitude of 36 1 2 4 this reflects northward translation of the composite insular intermontane superterrane of 3050 450 km between 85 and 50 ma the silverquick powell creek succession at churn creek unconformably overlies the northern end of the spences bridge group a late early cretaceous volcanic arc assemblage on the intermontane superterrane paleomagnetic data in the companion paper place it 1050 450 km south of its current position between 100 and 105 ma integration of these new data sets require that the insular and intermontane superterranes formed a single enormous crustal block that underwent complex episodic south north translation between circa 105 and 50 ma the timing rate and vectors of translation are problematic in terms of geologic constraints along the continental margin

yellowstone in yukon the late cretaceous carmacks group the late cretaceous carmacks group a thick subaerial volcanic succession that once covered much of southwest yukon was deposited on an uplifted terrane and is divisible into a lower fragmental unit and an upper flood basalt unit coeval hydrothermal activity resulted in widespread alteration and gold mineralization the lavas are shoshonites enriched in large ion lithophile and light rare earth elements but depleted in high field strength elements ankaramitic absarokite flows in the upper carmacks group range up to 15 wt mgo requiring a high liquidus temperature 1400 c at 1 bar dry high k 2 o contents 3 of these magnesian lavas indicate that the potassic character of the volcanic suite was established in the mantle although previously interpreted as subduction related the carmacks group was erupted during a cordilleran wide magmatic lull and lacks coeval calc alkalic batholiths the lavas are petrologically similar to plume related eocene to pliocene potassic lavas of the western united states new paleomagnetic collections combined with previous work place the carmacks group 17 2 6 5 1900 700 km south of its present position relative to the craton during deposition near the paleolocation of the yellowstone hotspot the spatial coincidence similarity of tectonic setting and lithologic similarity of the carmacks group and yellowstone volcanic successions suggest that the carmacks group is the 70 ma effusion of the yellowstone hotspot subsequent northward displacement of the carmacks group is attributed to coupling with the kula plate correlation of the carmacks group and the yellowstone hotspot fixes the paleolatitude and the paleolongitude of the terranes of the northern intermontane belt at 70 ma

additional k ar isotopic dates for the carmacks group upper cretaceous west central yukon three whole rock k ar analyses on andesites from the indian river area west central yukon yielded dates of 69 0 1 7 65 2 1 7 and 64 8 1 7 ma these confirm earlier dates indicating the carmacks group is late cretaceous in age a single date of 51 4 3 9 ma was obtained from a tuff in the sixty mile river area west central yukon suggesting an eocene age for these deposits however this date is considered equivocal because it is anomalous when compared with the other dates from the area

aluminum dependent trace element partitioning in clinopyroxene as technical advances have dramatically increased our ability to analyze trace elements the need for more reliable data on the compositional dependence of trace element partitioning between minerals and melt has become increasingly important the late cretaceous carmacks group of south central yukon comprises a succession of primitive high mg ankaramitic lavas characterized by shoshonitic chemical affinities and containing large complexly zoned clinopyroxene phenocrysts the compositional zonation of the clinopyroxene phenocrysts is characterized by relatively fe rich mg mg mg fe 0 85 but mottled cores surrounded by mantles of cyclically zoned clinopyroxene whose mg varies repeatedly between 0 9 and 0 80 these cyclically zoned clinopyroxene mantles appear to record the repeated influx and mixing of batches of primitive with more evolved magma in a deep sub crustal 1 2 gpa magma chamber s laser ablation icp ms was used to analyze the trace element variation in these zoned clinopyroxenes the results indicate more than a threefold variation in the absolute concentrations of th zr rare earth elements ree and y within individual clinopyroxene phenocrysts with no apparent change in the degree of ree or high field strength element hfse fractionation the variation in absolute abundances of trace elements correlates closely with the major element composition of the clinopyroxene with the most enriched clinopyroxene having the lowest mg and highest al contents the problem is that the amount of crystal fractionation required to explain the major element variation 20 in these clinopyroxene phenocrysts cannot explain the increase in the abundance of the incompatible trace elements which would require more than 70 crystal fractionation if constant partition coefficients are assumed the anomalous increase in incompatible trace elements appears to reflect an increase in their partition coefficients with increasing aliv in the clinopyroxene with an increase in al2o3 from 1 5 to 4 0 wt during 20 crystal fractionation over a temperature decrease of 100 c being associated with more that a threefold increase in the partition coefficients of th zr ree and y the magnitude of these increases may indicate that the substitution of these trace elements into clinopyroxene is better modeled in some natural systems by a local charge balance model rather than the distributed charge model that better replicates the results of annealed experiments these findings indicate that the effect of al on the partition coefficients of incompatible trace elements in clinopyroxene may be under appreciated in natural magmatic systems and that the application of experimentally determined clinopyroxene partition coefficients to natural systems must be done with caution

melting experiments on lherzolite klb 1 under hydrous conditions and generation of high magnesian andesitic melts the partial melting of natural lherzolite klb 1 at 1 gpa was studied for both h2o undersaturated and saturated conditions the quenched glasses obtained at 1000 and 1050 c are andesitic in composition having 54 4 60 3 wt sio2 whereas partial melts formed at temperatures above 1100 c have basaltic compositions this suggests that high magnesian andesites can be formed by direct partial melting of the hydrous upper mantle only at relatively low temperatures these andesitic melts together with those obtained at 1100 c contain more than 3 wt h2o and have compositions characteristic of hydrous melts high sio2 and al2o3 and low feo mgo and cao the results of melting studies on klb 1 show a strong increase in the degree of melting when h2o is added to lherzolite at a given temperature

u pb zircon dating geochemical and sr nd hf isotopic compositions of early indosinian intrusive rocks in west qinling central china petrogenesis and tectonic implications the qinling dabie sulu orogenic belt is the junction between the north and south china blocks which resulted from the final amalgamation of china continents during the indosinian indosinian granitoids are widespread in the qinling orogen and their geneses can thus constrain the evolution of china continent we carried out a combined u pb zircon dating and geochemical study for the shuangpengxi granodiorite pluton and the xiekeng diorite granodiorite pluton in the middle part of the west qinling orogen u pb zircon dating shows that the magma crystallization ages of 242 3 ma for the shuangpengxi pluton and 244 242 ma for the xiekeng pluton geochemical and sr nd hf isotopic compositions reveal that the magma of the shuangpengxi granodiorite was derived from partial melting of crustal materials the xiekeng diorites can be divided into high al diorite and high mg diorite both of them resulted from partial melting of enriched lithospheric mantle but their mantle source had been modified by previous slab derived melt the high al diorite was formed by fractional crystallization of olivine pyroxene and or preferential accumulation of plagioclase and the high mg diorite was formed by fractional crystallization of olivine and or preferential accumulation of pyroxene the xiekeng granodioritic porphyry was formed by mixing of crust derived and mantle derived melts we propose that the early indosinian magmatism resulted from break off of subducted oceanic slab after collision the slab break off model can well explain the linear distribution of the early indosinian plutons and rapid crustal uplift during the middle triassic in the west qinling

northward motion of the whitehorse trough paleomagnetic evidence from the upper cretaceous carmacks group discussion discordant paleomagnetic directions in mid cretaceous plutons of the coast plutonic complex and north cascades have been interpreted as indicating mid cretaceous paleolatitudes 2000 2500 km south relative to north america beck et al 1981 irving et al 1985 umhoefer 1987 but this magnitude of northward transport contradicts interpreted pre midcretaceous links between the north american miogeocline and westerly parts of the cordillera price and charmichael 1986 because paleohorizontal cannot be directly determined in plutonic rocks the discordant paleomagnetic directions may also be explained by tilting of the plutons about a horizontal axis primarily because of the consistent nature of the paleomagnetic discordance over a large region beck et al 1981 and irving et al 1985 have preferred the interpretation of northwards transport however butler et al 1989 recently pointed out geological evidence indicating that widespread northeast side up tilting may have affected the western portion of the coast plutonic complex and the north cascades resolution of this debate is of obvious importance to understanding the tectonic evolution of the canadian cordillera marquis and globerman reported results of a paleomagnetic study of the upper cretaceous carmacks group in the whitehorse trough the carmacks group is composed of gently dipping basaltic and andesitic lava flows that have been dated at 70 4 2 4 ma these rocks lie west of the tintina northern rocky mountain trench fault and the teslin suture zone because paleohorizontal can be unambiguously determined for these volcanic rocks the paleomagnetic data presented by marquis and globerman are potentially very important in resolving the tilt versus translation question marquis and globerman collected paleomagnetic samples in three regions spread over 300 km based on detailed laboratory studies they successfully isolated characteristic remanent magnetization chrm from 18 of 27 paleomagnetic sites flows the acquired paleomagnetic data pass a fold test of stability and contain sites of both normal and reversed polarity and magnetic properties indicate that the chrm is a primary thermoremanent magnetization the observed angular dispersion of virtual geomagnetic poles vgp s indicates some degree of required sampling of geomagnetic secular variation marquis and globerman determined a paleomagnetic pole from the carmacks group at 82 n 109 e with 95 confidence region ag5 of radius 7 8 using a 70 ma north american reference paleomagnetic pole of 77 7 n 185 8 e ag5 7 7 and the methods of beck 1980 as modified by demarest

geochemistry of holocene volcanic rocks associated with ridge subduction in baja california mexico although the geochemistry of magmatism associated with the subduction of oceanic lithosphere is well understood1 6 the geochemical signature of lavas produced shortly after subduction of an oceanic spreading centre has not been characterized the baja california peninsula in mexico provides an ideal scenario to study the latter process geophysical models indicate that in response to oblique collision of the ancestral east pacific rise epr with the western seaboard of north america at 29 myr subduction of the farallon plate was succeeded by the progressive southwards development of a transform fault system7 9 at 12 5 myr a considerable length of the epr was simultaneously subducted beneath baja thus terminating subduction processes along this segment10 by 3 5 myr the locus of transcurrent faulting had switched from the west of baja to within the developing gulf of california coupling baja to the pacific plate11 13 we present here data for holocene volcanic rocks from baja california norte which show that post ridge subduction volcanism has a distinctive geochemistry these results have important consequences for the interpretation of ancient orogenic belts

origin of high magnesian andesites in the setouchi volcanic belt southwest japan i petrographical and chemical characteristics high magnesian andesites of middle miocene age occur in southwest japan forming an obvious volcanic belt these andesites have low feo mgo ratios 0 546 0 931 and are rich in ni 101 312 ppm co 30 0 45 1 ppm and cr 208 756 ppm they are relatively aphyric phenocrysts 10 vol and the phenocrysts of magnesian olivine fo88 are in equilibrium with the host high magnesian andesite magmas on the basis of the fe mg exchange partitioning these features suggest that the high magnesian andesites are not differentiated or accumulative they appear to represent primary andesites generated in the upper mantle these southwest japanese high magnesian andesites are rich in incompatible elements and show light rare earth enrichment relative to boninites suggesting that the former is derived from a less depleted mantle source than the latter

large scale fractures related to inception of the yellowstone hotspot during middle miocene time western north america was subject to flood basalt volcanism dike swarm injection and broad scale fracturing and folding of the crust we propose a simple model to account for these events and for a regional pattern of geologic and geophysical features aeromagnetic maps reveal some of the most important elements of this pattern which are several narrow arcuate anomalies here referred to as the northern nevada rifts these rifts extend hundreds of kilometers across nevada and are likely caused by highly magnetic middle miocene mafic dikes with the aid of filtering techniques the anomalies can be traced into oregon together with other geologic features such as fold axes dike swarms and faults they produce a spoke like pattern fanning over 2208 of arc that converges toward a point near the oregon idaho border lat 448n a possible cause for this pattern is a point source of stress at the base of the crust related to the formation of the yellowstone hotspot the spoke like pattern however does not persist at large distances from the emerging hotspot several hundred kilometers to the south the northern nevada rifts deviate significantly 308 from a radial trend we show that a simple model imposing a point source of stress at the base of the crust and a regional stress field aligned with the presumed middle miocene stress direction fits the observed fracture pattern it thus accounts for both the radial pattern present near the nascent hotspot and the far field pattern due to regional stresses

intra oceanic subduction shaped the assembly of cordilleran north america the western quarter of north america consists of accreted terranes crustal blocks added over the past 200 million years but the reason for this is unclear the widely accepted explanation posits that the oceanic farallon plate acted as a conveyor belt sweeping terranes into the continental margin while subducting under it here we show that this hypothesis which fails to explain many terrane complexities is also inconsistent with new tomographic images of lower mantle slabs and with their locations relative to plate reconstructions we offer a reinterpretation of north american palaeogeography and test it quantitatively collision events are clearly recorded by slab geometry and can be time calibrated and reconciled with plate reconstructions and surface geology the seas west of cretaceous north america must have resembled today s western pacific strung with island arcs all proto pacific plates initially subducted into almost stationary intra oceanic trenches and accumulated below as massive vertical slab walls above the slabs long lived volcanic archipelagos and subduction complexes grew crustal accretion occurred when north america overrode the archipelagos causing major episodes of cordilleran mountain building a new explanation for the origin of the accreted terranes that form the mountainous cordillera of western north america is proposed and tested stationary intra oceanic subduction deposited massive slab walls in the mantle and grew volcanic archipelagos at the surface which were overridden by and accreted to north america during cretaceous times the generally accepted account of how the geology of western north america developed involves the oceanic farallon plate acting as a conveyor belt sweeping crustal terranes onto the continental margin as it moves beneath the north american plate this model fails to explain many aspects of this accreted terrane and here karin sigloch and mitchell mihalynuk present an alternative model based on new tomographic images of lower mantle slabs and modelling of deep mantle structure in the region they propose that during the cretaceous period there were a series of island arcs west of north america beneath which proto pacific plates subducted into almost stationary trenches long lived volcanic archipelagos and subduction complexes grew above the slabs and then accreted onto north america as it overrode the archipelagos causing episodes of cordilleran mountain building

the 3d geophysical investigation of a middle cretaceous to paleocene regional decollement in the cordillera of northern canada and alaska

petrogenesis of the middle triassic erenhot granitoid batholith in central inner mongolia northern china with tectonic implication for the triassic mo mineralization in the eastern central asian orogenic belt abstract voluminous post orogenic magmatism holds a key to monitoring important geodynamic processes and crustal evolution within an extensional accretionary orogen the middle triassic erenhot granitoid batholith from central inner mongolia northern china is tectonically located in the eastern central asian orogenic belt this batholith consists of monzogranite syenogranite alkali feldspar granite and albite granite with minor mafic to felsic enclaves and dykes the formation of this composite batholith involved distinct mafic and felsic magma end members concomitant episodic mafic felsic interaction and intra felsic magma evolution mafic diabasic and high mg intermediate dioritic dykes show enrichment in large ion lithophile elements depletion in high field strength elements and heterogeneous isotopic compositions indicative of a metasomatized lithospheric mantle source inherited from prior oceanic slab subduction the monzogranites and aplites have arc like elemental features and variable isotopic compositions isr t 0 706132 end t 0 4 zircon ehf t 2 8 to 7 4 zircon 18o 5 32 to 6 74 clearly suggesting that the felsic magma end member of a dual heritage was derived from dominant newly underplated mafic crust and subordinate ancient lower crust hybridization processes between mantle derived mafic magma and crust derived felsic magma result in the formation of monzonitic enclaves isr t 0 704850 end t 0 1 with an isotopic resemblance to the host granites fractional crystallization of the monzogranitic magmas endows the differentiated syenogranites and alkali feldspar granites with highly evolved elemental characteristics we further deduce that the magmatic immiscibility between k rich and na rich residual melts in a deep reservoir produces albite granites with distinct deformation imprints variable k2o contents and wide range of crystallization temperatures combining with coeval regional igneous episodes as well as concomitant sedimentary and structural events in the northern china mongolia tract the triassic erenhot composite batholith can be considered as post orogenic magmatic products possibly within a coupled geodynamic scenario between lithospheric dripping and concurrent gravitational collapse therefore the documentation of this large scale triassic post orogenic magmatism could not only lead to the characterization of the eventual closure of the paleo asian ocean but also capture a snapshot into the tectonic background for the widespread porphyry type mo mineralization in the eastern central asian orogenic belt during early mesozoic

eocene 51 ma end to northward translation of the coast plutonic complex paleomagnetism and k ar dating of the white pass dikes abstract the white pass mafic dikes are located in the coast belt inboard of the coast shear zone denali fault system east of skagway alaska astride the alaska british columbia border the dikes intrude eocene felsic plutons in the coast plutonic complex three k ar analyses date the dikes at 51 9 0 8 ma biotite 50 1 1 0 ma and 45 0 0 8 ma both whole rock structural analysis on these vertical dikes indicates that the study area has not been tilted significantly since dike emplacement conventional alternating field and thermal step demagnetization and saturation remanence paleomagnetic methods were used to analyse 407 specimens from 27 dike and five host rock sites three contact tests show that the specimens retain a primary thermoremanent magnetization trm carried in pseudosingle to multidomain magnetite although some magnetization in a few dikes is carried by pyrrhotite the dual polarity trm is mostly reversed and the 24 accepted dike sites give a pole position of 245 7 e 76 3 n dp 4 2 dm 4 4 indicating a paleolatitude discordance of 8 4 south and a clockwise rotation of 40 9 with respect to the north american craton these data along with other paleomagnetic data on eocene rocks indicate that the allochthonous eastern coast and intermontane belt terranes had halted their northward translation by about 51 ma but continued to be rotated clockwise by an average of 24 10 thereafter probably on underlying sole faults

early cretaceous high mg diorites in the yanji area northeastern china petrogenesis and tectonic implications abstract mesozoic granitic rocks are widely distributed in northeast ne china however high mg dioritic rocks are considerably rare here we report a newly recognized high mg diorite the xintun diorite in the yanji area ne china to constrain its origin and implications for the tectonic evolution of eastern asian continental margin zircon u pb dating yields a crystallization age of 128 1 ma for the xintun diorite the diorites are characterized by high mgo 4 4 6 6 wt cr 119 239 ppm ba 419 514 ppm and sr 649 747 ppm contents and mg values 59 64 but low feototal mgo ratios 1 2 1 4 with geochemical features similar to those of sanukitic high mg andesites hmas they show moderate radiogenic sr isr 0 7047 0 7050 and nd end 0 3 1 1 with high la sm ratios which are indicative of contributions from sediment components the mineral assemblage of euhedral hornblende magnetite and titanite implies a water rich and oxidized signature for their primitive magmas these features suggest that the xintun high mg diorites were probably formed via partial melting of the subducting sediments and subsequent interaction of mantle peridotites with both melts and aqueous fluids geochemical modeling reveals that hornblende dominated fractional crystallization under water sufficient conditions enabled the evolved magmas to acquire adakitic signatures we believe that the paleo pacific subduction beneath eastern asian continental margin caused large scale back arc extension of ne china in the early cretaceous and consequently induced the asthenospheric flow toward the mantle wedge reheating subducting sediments enough to cause melting therefore the occurrence of the xintun high mg diorites signifies the onset of extensive back arc extension of eastern asian continental margin at ca 128 ma

paleomagnetic and geobarometric study of the mid cretaceous whitehorse pluton yukon territory this is the first of several lithoprobe paleomagnetic studies underway to examine geotectonic motions in the northern canadian cordillera except for one controversial study estimates for terranes underlying the intermontane belt in the yukon have been extrapolated from studies in alaska southern british columbia and the northwestern united states the whitehorse pluton is a large unmetamorphosed and undeformed tonalitic body of mid cretaceous age 112 ma that was intruded into sedimentary units of the whitehorse trough in the stikinia terrane geothermobarometric estimates for eight sites around the pluton indicate that postmagnetization tilting has been negligible since cooling through the hornblende crystallization temperature and that the pluton is a high level intrusion paleomagnetic measurements for 22 of 24 sites in the pluton yield a well defined characteristic remanent magnetization chrm direction that is steeply down and northwards the chrm direction gives a paleopole of 285 5 e 81 7 n

timing of the late cretaceous ignimbrite flare up at the eastern margin of the eurasian plate new zircon u pb ages from the aioi arima koto region of sw japan abstract the kinki district in the inner zone of southwest japan is characterized by late cretaceous volcanic rocks of the aioi and arima groups and the koto rhyolites from west to east these rocks are dominated by pyroclastic flow deposits related to caldera forming events here we present new laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb ages for the aioi group four isolated caldera filling deposits the lower middle and upper parts of the arima group and one intrusive rock the kashihara quartz gabbro and the lower and upper parts of the koto rhyolites including the oldest volcanic unit within these rhyolites these analyses yielded ages of ca 86 82 ma for the aioi group ca 83 81 ma for the arima group 78 6 ma for the kashihara quartz gabbro and ca 74 73 ma for the koto rhyolites the aioi group represents a cluster of calderas and the ages obtained for individual units in this group differ from the ages of adjacent units the arima group and the koto rhyolites both consist of pyroclastic flow deposits associated with caldera forming events and the ages of these rocks are all the same within error this suggests that the aioi group represents a series of individual caldera forming eruptions that are distinct from the arima group and the koto rhyolites which formed during a single stage of caldera formation the u pb ages presented here indicate that the late cretaceous caldera forming eruptions in the study area occurred at intervals of 1 myr and represent individual events that lasted for

mineralogy geochemistry and petrogenesis of kurile island arc basalts whole rock major and trace element and mineral chemical data are presented for basaltic rocks from the main evolutionary stages of the kurile island arc nw pacific an outer inactive arc contains a cretaceous lower tertiary sequence of tholeiitic calcalkaline and shoshonitic basalts the main arc miocene quaternary is dominated by weakly tholeiitic with lesser alkalic basalts the mineralogy of kuriles basalts is characterised by an rich plagioclases a continuous transition from chromites to titanomagnetites pyroxenes with low fe3 contents and without strong fe enrichment abundance of groundmass pigeonites and the absence of amphiboles there is an increase in k2o contents both along arc northwards and towards the reararc side the basalts show an exceptionally wide but continuous range of k2o contents 0 1 4 7 which correlate with other lil element contents tholeiitic basalts with low lil element contents la yb and th u but high k rb p2o5 la and zr nb were derived from depleted lherzolitic mantle which had suffered fluid metasomatism by k rb cs sr ba pb and h2o only alkali basalts are also thought to be derived from depleted mantle but melt metasomatism involved addition of all lil elements to a garnet lherzolite mantle the kuriles basalts and their mantle sources range continuously between these two end member compositions the metasomatic fluids melts were probably released by early dehydration and later melting within subducted oceanic lithosphere though the process is not adequately constrained

new u pb age constraints on latest cretaceous magmatism and associated mineralization in the fawnie range nechako plateau central british columbia new u pb ages and k ar dates primarily for rocks proximal to mineral occurrences in the fawnie range of central british columbia document latest cretaceous ca 74 66 ma continental arc igneous activity and date associated base and precious metal mineralization u pb ages of ca 73 69 ma for the capoose pluton and hypabyssal to extrusive garnet rhyolites at the capoose prospect demonstrate a latest cretaceous age for mineralization and a likely plutonic source for mineralizing fluids a u pb age of ca 67 ma for a late mineralized felsic dyke and two k ar dates ca 70 and 68 ma for hornfelsed jurassic volcanic rocks at the blackwater davidson prospect constrain a latest cretaceous age for mineralization a u pb age of ca 74 ma for a fine grained diorite sill that cuts a significant epithermal gold vein at the tsacha prospect places a minimum age on mineralization at this probable jura cretaceous deposit and documents latest cretaceous magmatism latest cretaceous k ar dates are reported for an andes

reconstructing the ancestral yellowstone plume from accreted seamounts and its relationship to flat slab subduction recent geodynamic analyses have emphasized the relationship between modern flat slab subduction zones and the overriding of buoyant oceanic crust although most models for the evolution of the late mesozoic cenozoic laramide orogeny in the southwestern united states involve flat slab subduction the mechanisms proposed are controversial an examination of the geological evolution of the 60 50 ma crescent terrane of the coast ranges indicates that it was formed in a shallowing upward loihi type oceanic setting culminating in the eruption of subaerial lavas plate reconstructions indicate that the crescent terrane was emplaced into ca 20 ma crust and the presence of subaerial lavas implies an uplift due to the plume of ca 4 2 km which we use to calculate a minimum buoyancy flux of 1 1 mg s 1 similar to that of the modern yellowstone plume published paleomagnetic data indicate that the crescent terrane was formed at a paleolatitude similar to that of the yellowstone plume the crescent seamount was accreted within 5 my of the cessation of plume magmatism plate reconstructions indicate that it would have originated about 750 km to the west of the north american plate margin if it developed above a fixed yellowstone plume and are therefore consistent with the recorded very short interval between its formation and tectonic emplacement we interpret the crescent terrane as due to the ancestral yellowstone plume such a plume would have generated an elongate swell and related plateau that would have been overridden by the north american margin taken together the relationship between flat slab and overriding of oceanic plateau in laramide times would have been analogous to the relationship between modern andean flat slab subduction zones and the juan fernandez and nazca oceanic plateaus d 2003 elsevier science b v all rights reserved

influence of overlapping decollements syntectonic sedimentation and structural inheritance in the evolution of a contractional system the central kuqa fold and thrust belt tian shan mountains nw china

tectonic setting of eocene boninite magmatism in the izu bonin mariana forearc abstract middle eocene boninites were simultaneously generated over a large region during the early history of the izu bonin mariana ibm arc however widespread boninite magmatism is not recognised in younger subduction zones of similar dimensions this suggests that an additional tectonic or thermal factor influenced the generation of the ibm boninite suite regional uplift ocean island basalt style magmatism and high heat flow also characterised the northern philippine sea plate psp at the start of the middle eocene these features are similar to those observed in large volume basaltic volcanic provinces such as the early tertiary north atlantic and suggest that the ibm boninite suite may have been produced because there was already a thermal anomaly in the mantle beneath the psp the reconstructed middle eocene location of the ibm arc and west philippine basin lies close to the present day manus basin where petrological and geochemical evidence indicate the presence of a mantle plume a calculated plume track linking these locations through time also passes close to the eauripik rise an aseismic ridge on the caroline plate during the oligocene and miocene therefore we propose that a thermal anomaly or mantle plume influenced the magmatic and tectonic development of the western pacific from the middle eocene until the present day

large scale coast parallel displacements in the cordillera a granitic resolution to a paleomagnetic dilemma abstract resolution of the paleomagnetic dilemma the discrepancy between large paleomagnetically determined dextral displacement of outboard portions of the northern cordillera and much smaller offsets implied by mapping and stratigraphic correlations is fundamental to understanding the tectonic evolution of the cordillera this paper presents structural orientation data from the middle cretaceous dawson range batholith of west central yukon and its wallrocks and suggests that some of the missing displacement may be found in intrusions the elongate northwest trending batholith has a margin parallel foliation a sub horizontal stretching lineation and records syn intrusive dextral shearing in country rocks adjacent to the batholith north trending lineations are deflected clockwise into near parallelism with the batholith s margins lineations from wallrock screens within the batholith are all aligned parallel with the batholith s long axis the big creek strike slip fault forms the north margin of the batholith and accommodated a minimum of 20 km of dextral slip these observations imply that the batholith invaded an active dextral shear zone accommodated shearing while crystallizing and focused post crystallization fault development the batholith is conservatively estimated to have accommodated 45 km of syn intrusive shearing collectively middle cretaceous intrusions of the northern cordillera may account for 400 km of previously unrecognized dextral displacement

the eocene southern vancouver island orocline a response to seamount accretion and the cause of fold and thrust belt and extensional basin formation abstract a deflection of the fault controlled southwestern coastline of vancouver island suggests the presence of a minor orocline with a southern crustal block south of barkley sound alberni inlet rotated 20 counterclockwise relative to a northern fixed crustal block about a pole of rotation located northeast of port alberni in this paper two models of orocline development one of pure block rotation and one of pure bending are proposed the predictions of these models are tested against available geological maps structural orientation data identified regions of extension and contraction and paleomagnetic data structural orientation and paleomagnetic data are consistent with 18 of post late cretaceous counter clockwise rotation of the southern crustal block relative to the northern fixed crustal block a southward increase in the magnitude of rotation evident in the structural orientation data argues for a model of bending both bending and block rotation models predict the development of a zone of contraction along the northeast margin of the southern crustal block coincident with the location of the eocene cowichan fold and thrust belt that diminishes northward toward the pole of rotation as predicted the fold and thrust belt is characterized by a northerly decrease in the amount of shortening from 30 at the south end of the thrust belt to 0 shortening north of port alberni the northerly decrease in shortening is complemented by a north to south change in structural style from cylindrical to conical folds and finally to planar undeformed strata the model of block rotation predicts the presence of a zone of extension extending southwest from the zone of rotation coincident with the location of eocene extensional structures within barkley sound and with horst and graben structures in the offshore eocene to miocene tofino basin extension is less than predicted by a model of pure block rotation and suggests that much of the oroclinal rotation was accommodated by bending timing constraints indicate that orocline development was coeval with and resulted from the eocene accretion of seamounts of the crescent terrane these findings demonstrate that oroclinal orogeny the buckling of a linear crustal beam about vertical axes of rotation can significantly impact the geometry structure and character of an orogenic belt even where the buckles are minor

high mg diorites derived from sanukitic hma magmas kyushu island southwest japan arc evidence from clinopyroxene and whole rock compositions abstract high mg diorites that have similar whole rock composition to high mg andesites hmas should not be simply interpreted as rocks solidified from the hma magmas because the high mg diorites may be mafic cumulates derived from a different magma from the hmas the hmas contain unique clinopyroxenes with higher mg and si than those of other sub alkaline series igneous rocks the mg and si are controlled by the source magma composition rather than its crystallized conditions such as pressure and temperature the chemical composition of clinopyroxenes would present important information for the investigation of the source of high mg diorites we considered the source of early cretaceous high mg diorites on kyushu island southwest japan arc based on their clinopyroxene and whole rock compositions the clinopyroxenes have similar chemical characteristics to those in hmas rather than those in other sub alkaline rocks moreover the whole rock compositions are equivalent to the sanukitic hma and do not show features of mafic cumulates this indicates that the high mg diorites solidified from sanukitic hma magmas it is generally believed that the sanukitic hma magmas involve the subduction of a young and or hot oceanic slab was situated in their genesis therefore the occurrence of the high mg diorites suggests that kyushu was situated in the tectonic setting of young and or hot slab subduction in the early cretaceous

paleomagnetism and age of three canadian rocky mountain diatremes paleomagnetic results and age estimates derived from them arc presented for three diatremes using as a basis of comparison the combined apparent polar wander apw path for north america and europe of van der voo the cross diatreme of the front ranges of the canadian rocky mountains has yielded a radiometric age of 241 ma earliest triassic and is hosted by the flat lying pennsylvanian tunnel mountain formation it has normal polarity magnetization and yields a paleopole correctly placed according to its radiometric age on the apw path the blackpool diatreme for which no radiometric age is available which is located in the main ranges of the rocky mountains is known to be post late ordovician because it is hosted by rocks of that age it also has magnetization of normal polarity and yields a paleopole that when calculated with respect to present horizontal is coincident with the latest cretaceous to paleocene paleopole for north america the paleopole when calculated with respect to bedding

the late ordovician granitoids in the east kunlun orogenic belt northwestern china petrogenesis and constraints for tectonic evolution of the proto tethys ocean petrology whole rock geochemistry in situ zircon u pb ages and lu hf isotopic data are reported for granodiorite and monzonitic granite samples from the harizha region in the eastern section of the east kunlun orogenic belt the results of the la icp ms zircon dating indicate that the granodiorite and monzonitic granite were emplaced at 447 9 3 0 ma and 443 8 3 5 ma respectively i e in the late ordovician early paleozoic periods the granodiorite is characterized by relatively high mgo 4 52 5 91 wt sr 541 712 ppm cr 218 357 ppm ni 56 7 80 7 ppm and v 66 8 94 7 ppm with low y 10 40 13 60 ppm and yb 1 05 1 45 contents this results in elevated mg 65 68 sr y 39 78 58 84 and la yb n 10 21 18 30 ratios common features for adakitic high mg andesite hma the granodiorite is metaluminous to weakly peraluminous and high k calc alkaline in composition in addition the granodiorite is enriched in light rare earth elements lree and large ion lithophile elements liles rb k sr and pb u and nd and depleted in high field strength elements hfses nb ta zr hf ti and p the zircon grains from the granodiorites have ehf t values of 5 5 to 4 7 with two stage hf model ages tdm2 ranging from 0 92 to 1 71 ga these features suggest that the granodiorites were formed by mixing processes of crustal melts and magma from a metasomatized mantle in a subduction setting the monzonitic granite possesses high normative corundum values 2 34 4 10 wt is strongly peraluminous a cnk ratios of 1 16 1 32 with minor mineral muscovite with high k calc alkaline affinity furthermore enrichments in large ion lithophile elements rb k and pb u th and nd depletions in high field strength elements nb ta and ti and ba and p and mainly right inclined ree patterns with negative eu anomalies exhibits an s type granite affinity the magmatic zircons from the monzonitic granite yielded ehf t values between 4 9 and 2 7 corresponding to two stage hf model ages of 1 60 1 73 ga therefore this study suggests that the monzonitic granite magma was derived from crustal material metagreywacke in a syn collision setting subsequently by combining these findings with the previous research results this study propose that a transverse diachronism closure model which entailed an earlier on the sides later in the middle in an east west direction in the east kunlun orogenic belt for the proto tethys ocean s evolution process

petrogenesis of early permian sanukitoids from west junggar northwest china implications for late paleozoic crustal growth in central asia abstract sanukitoids and their equivalents are rare subduction related rock types that have been found in modern arc settings and in late archean sequences the investigation of sanukitoids is of particular importance to a better understanding of crust forming processes and continental growth in this paper we report zircon u pb and ar ar ages and major element trace element and sr nd hf isotope data for the bieluagaxi dioritic pluton and dikes from the southern part of the west junggar nw china these rocks formed in the early permian and show a remarkable geochemical affinity with cenozoic sanukitoids of the setouchi volcanic belt of sw japan with high mg 48 73 values cr 54 539 ppm ni 21 197 ppm contents and th la 0 15 0 37 ratios and low sr y ratios 16 27 and sr 263 442 ppm contents they may be generated by the partial melting of subducting sediments and subsequent melt mantle interaction additionally the sanukitoids are also widespread in the karamay baogutu area of west junggar accompanied by high sr average 713 ppm contents and sr y 50 130 ratios and low y 6 9 12 6 ppm contents the difference in petrochemical characteristics between the baogutu karamay and bieluagaxi sanukitoids can be explained by the difference in depth of initial melting origin composition and fractional crystallization the baogutu karamay sanukitoids were probably formed under eclogitic conditions while the bieluagaxi sanukitoids were at a shallower depth moreover the compositional similarity between continental crust forming rocks and the bieluagaxi sanukitoids suggests that the sanukitoids genesis could be closely related to the process of continental crust formation the late carboniferous early permian sanukitoids in the west junggar may be an indicator of anomalous thermal activity ridge subduction may play a crucial role in the evolution and growth of the continental crust in central asia

u pb ages of zircons from mesozoic intrusive rocks in the yanbian area jilin province ne china transition of the paleo asian oceanic regime to the circum pacific tectonic regime abstract geology in the yanbian area ne china is a key to understanding the relationship between the paleo pacific tectonic regime and the older paleo asian oceanic regime especially the transition between the regimes during the mesozoic a compilation of 131 u pb zircon ages including 18 obtained in the present study shows that the late paleozoic to mesozoic magmatism in the yanbian area took place from 285 to 105 ma mainly 205 160 ma and that there was a peak of activity at 175 ma and a hiatus from the late jurassic to early cretaceous 150 130 ma provisionally we subdivide the magmatic activity into four dominant stages middle permian to middle triassic 280 240 ma late triassic 230 205 ma early to middle jurassic 205 160 ma and early cretaceous 130 105 ma middle permian intrusive rocks including pyroxenite gabbro diorite granodiorite tonalite and monzogranite are interpreted to have been emplaced along an andean style continental margin in association with the subduction of the paleo asian oceanic plate the late permian middle triassic intrusive rocks are mainly quartz monzonite monzogranite and syenogranite and are considered to have been the result of syn orogenic post collisional activities the late triassic magmatism involved not only the emplacement of mafic ultramafic intrusive rocks but also coeval i and a type granitoids indicating an extensional environment after the final closure of the paleo asian ocean since the jurassic the tectonic setting has been dominated by subduction of the paleo pacific oceanic plate previously published data for the yanbian area and age data from surrounding regions show that the entire region from the yanbian area to the korean peninsula and the japanese islands experienced peaks in igneous activities that ranged from 200 to 165 ma during the jurassic and 120 to 110 ma and 90 to 65 ma during the cretaceous the jurassic intrusive rocks were emplaced mainly in the west and cretaceous intrusive rocks in the east in other words there is a southeastwards younging of mesozoic intrusive rocks that appears to have been closely associated with the evolution of the paleo pacific ocean taking all this into account we conclude that the transition from the paleo asian oceanic regime to the circum pacific tectonic regime took place during the late triassic to early jurassic

melting experiments on a high magnesian andesite abstract melting experiments were conducted on a high magnesian bronzite olivine andesite teraga ike andesite which is considered to be a primary andesite the high magnesian andesite magma is in equilibrium with both olivine and orthopyroxene at about 15 5 kbar and 1080 c under h2o saturated conditions and at lower pressure and higher temperature under h2o undersaturated conditions this suggests that high magnesian andesites could be generated by the partial melting of upper mantle peridotite containing a small amount of h2o

geochemistry of high mg andesites from cape vogel papua new guinea abstract a detailed geochemical investigation of high mg andesites from cape vogel papua new guinea shows that these rocks are chemically as well as mineralogically unusual they are nonetheless important to our understanding of the earth s mantle since they represent primary magmas possibly derived in an island arc environment two groups of high mg andesites are recognized at cape vogel type c characterized by a concave ree pattern la yb n and zr nb 35 and type e with la yb n of 3 7 and zr nb 19 the origin of the two groups and their unique trace element patterns is attributed to a complex series of enrichment events superimposed on a previously depleted mantle source in general the enrichment events do not seem similar to those represented in island arc volcanics while some similarities are noted for oceanic ridge and island settings a marked depletion in ti relative to zr and y amongst high mg andesites is a unique facet of their geochemistry trends in the major and trace element geochemistry of type e high mg andesites were modelled orthopyroxene fractionation and or accumulation can explain much of the variation although decoupling of major and selected trace elements is noted source heterogeneity may also be a factor the origin of these rocks and their tectonic setting remains enigmatic however enough geochemical and petrographic evidence is available to suggest they are distinct from most low ti basalts associated with ophiolites

tectonic setting and geochronology of the sarsuk au polymetallic deposit in xinjiang nw china constraints from pyrite re os zircon u pb dating and hf isotopes abstract the sarsuk au polymetallic deposit is located in the ashele volcano sedimentary basin of the south altay orogenic belt aob northwest china within the deposit rhyolite porphyry and diabase are widespread and emplaced into the strata the orebodies of the sarsuk deposit are hosted by the rhyolite porphyry while diabase cuts through the rhyolite porphyry and part of the orebody rhyolite porphyry 1 yields a zircon u pb age of 384 1 2 ma mswd 0 69 and the diabases yield a zircon u pb age of 380 0 74 ma mswd 3 1 four auriferous pyrite samples from vein sulfide ores and one pyrite sample from massive ores display a re os isochron age of 383 11 ma mswd 1 2 these data show that the bimodal devonian magmatism and rhyolite porphyry hosted ore bodies emplaced in a narrow interval during 384 380 ma diabases have the geochemical characteristics of tholeiitic basalts with relatively high mg values 49 4 61 4 cr and ni a slight positive eu anomaly eu 0 98 1 12 and relatively low concentrations of sr and la yb n rhyolite porphyries have similar characteristics to those of sanukitoide in terms of high mg values 47 6 55 7 an obvious negative eu anomaly eu 0 33 0 35 and enrichments of large ion lithophile elements and light rare earth elements the aforementioned chemical characteristics indicate that the sarsuk deposit was located in the arc environment of a subduction zone during the early to middle devonian hf isotope analyses of the diabases exhibited ehf t values of 4 86 to 8 96 the two stage hf model age tdm2 was 805 1064 ma hf isotope analyses of the rhyolite porphyry exhibited a ehf t value range of 1 78 to 8 49 and the two stage hf model age tdm2 was 449 528 ma the original diabase and rhyolite porphyry magmas were derived from a mixing of the depleted mantle and juvenile crust material the pyrite re os system indicated that crust derivedand s rich sediments also contributed to the ore formation accordingly based on the results of this study and a review of the regional magmatic framework we can conclude that the sarsuk deposit was formed at 383 ma in an island arc subduction environment furthermore we establish that the magmatic rocks and ore are co genetic

rethinking yellowstone in yukon and baja british columbia paleomagnetism of the late cretaceous swede dome stock northern canadian cordillera 1 paleomagnetic results obtained from the 69 8 ma swede dome stock emplaced into the pericratonic yukon tanana terrane ytt west of dawson yukon indicate a minimal northward translation of 360 575 km and a clockwise rotation of 20 23 with respect to north america in accord with the displacement of 425 km along the nearby tintina fault coeval carmacks group volcanics in contrast have previously indicated 1900 km northward translation and minimal rotation and have been proposed to be a displaced manifestation of the fixed yellowstone hot spot thus providing an independent estimate of the late cretaceous paleoposition of the baja british columbia terranes of western north america a compilation of recent mid and late cretaceous paleomagnetic results from the ytt and intermontane belt imb terranes reveals that the carmacks group paleomagnetic result is anomalous the ytt and imb terranes which form the bulk of baja british columbia appear instead to have experienced 1000 km northward translation with respect to north america since 70 ma consistent with geological estimates of motion along plausible accommodating faults in addition past mobility of the hawaii emperor hot spot implies that the pacific hot spots may have experienced 1200 km of southward motion from 81 to 47 ma if so then the paleoposition of the yellowstone hot spot if it existed at 70 ma would likely have produced the carmacks magmatism 1000 km north of yellowstone s current latitude consistent with a moderate 1000 km post 70 ma northward translation of the ytt and imb terranes to their present position in north america

eocene rotation of the coast plutonic complex and intermontane belt paleomagnetism of eocene plutons along the klondike highway paleomagnetic results are reported for the 59 ma skagway 54 ma fraser 53 ma summit lake and 48 ma clifton felsic plutons of the eastern coast plutonic complex cpc that outcrop along the south klondike highway in alaska and british columbia thermal and alternating field step demagnetizing methods yielded stable characteristic remanent magnetization chrm directions for all 29 sites of normal reversed and mixed polarity the chrm resides in single or pseudosingle domain magnetite and or pyrrhotite that is shown to be primary by contact tests with the 47 ma vertical white pass mafic dikes paleopoles from six 56 to 50 ma mean 52 2 ma intermontane belt yukon tanana terrane imb ytt units that cannot be explained by tectonic tilt are compared with nine clustered 59 to 46 ma mean 52 4 ma eastern cpc paleopoles both paleopole populations show nonsignificant poleward northward translation relative to north america imb ytt 3 7 5 3 n cpc 4 3 6 4 s overall 1 2 4 9 s ind

geophysical investigation and reconstruction of lithospheric structure and its control on geology structure and mineralization in the cordillera of northern canada and eastern alaska a reconstruction of the tintina fault is applied to regional geophysical and topographic data facilitating the definition of west trending lineaments within the lower crust and or mantle lithosphere oblique to the nw trending structure of the cordilleran terranes the lineaments which exhibit a range of geophysical and geological signatures are interpreted to be related to the liard transfer zone continuous to the denali fault that divided lower and upper plates during late proterozoic cambrian rifting of the laurentian margin three dimensional gravity models show a density increase in the lower crust and mantle lithosphere to the north the transfer zone also divides bimodal mantle xenolith suites to the south from unimodal suites to the north these conclusions suggest that extended north american basement related to laurentian margin rifting that would have brought mantle lithosphere rocks to a shallow depth continuously underlies a thin carapace of accreted terranes in western yukon and eastern alaska the interpreted continuity of north american basement reaffirms that if oroclinal bending of the intermontane terranes occurred then it was prior to its emplacement upon the rifted basement examination of the spatial relationships between mineral occurrences and postaccretionary cretaceous lithospheric lineaments from their manifestation in geophysical geological and topographic data suggest that the late proterozoic lineaments influenced mesozoic mineralization through influence on the development of the shallow crustal structure intrusion and exhumation and erosion

new 42 ma cratonic north american paleomagnetic pole from the yukon underscores another cordilleran paleomagnetism geology conundrum eocene posttectonic plutons of the beaver river alkalic complex in southeastern yukon intruded devonian mississippian and triassic sandstones in the foothills of the canadian cordillera a paleomagnetic collection of 27 sites from three separate plutons produced 326 specimens that were analyzed using alternating field and thermal step demagnetization methods the a component characteristic remanent magnetization chrm resides in magnetite with normal polarity in the 42 6 0 8 ma beaver river pluton reversed polarity in the 42 1 0 7 ma larson creek east pluton and both polarities in the 41 3 0 4 ma larson creek west pluton corresponding with magnetic polarity chrons 20n 19r and the boundary between chron 19r and 18n respectively the chrms of the plutons are indistinguishable 2 with a mean for the 42 0 0 5 ma complex of d 158 8 i 73 1 n 21 sites 95 3 0 k 116 8 a positive paleomagnetic contact test shows the a component to be primary and the poorly isolated b component

petrogenesis of the early silurian dashuang high mg basalt andesite dacite in eastern south china origin from a palaeosubduction modified mantle the early paleozoic kwangsian event the first compressive orogeny after the rodinia assemblage in the south china block scb is characterized by the angular unconformity between the devonian and pre devonian sequences and widespread granitoids and small volume mafic rocks however the mechanism triggering this event is highly debated and the associated mantle source is poorly known our recent investigations identified the occurrence of early silurian high mg basalt andesite dacite in the dashuang area along the northwestern yunkai domain in the scb this study presents a set of new zircon u pb geochronological whole rock geochemical and sr nd isotopic data for these volcanic rocks they yield weighted mean 206 pb 238 u ages of c 430 443 ma similar to the age peaks of early middle paleozoic magmatism and metamorphism in the scb these volcanic rocks have high mgo cr and ni contents and mg numbers they also exhibit arc like geochemical affinities with crustal like sr nd isotopic compositions similar to the neoproterozoic high mg basalt andesite in the eastern scb such signatures combined with other geological observations suggest that the dashuang high mg rocks were derived from an ancient probably early neoproterozoic mantle wedge source the early paleozoic intra continental reactivation in response to late orogenic extension triggered the melting of this palaeosubduction modified mantle which led to the formation of these volcanic rocks

paleomagnetic and geobarometric study of the late cretaceous mount lorne stock southern yukon territory the 75 ma mount lorne monzodiorite stock has been studied paleomagnetically to estimate the tectonic motion of the northern canadian cordillera since the late cretaceous the stock is one of sever

petrogenesis of the devonian high mg rock association and its tectonic implication for the chinese altai orogenic belt nw china abstract the chinese altai is a key region to decipher the accretionary history of the gigantic caob one of the largest accretionary orogenic belts on the earth however tectonic implication of the widely distributed paleozoic magmatic rocks remain debated in this study a suite of igneous rocks including high mg dacite of the kangbutiebao formation and the associated gneissic granodiorites have been studied for their petrogenesis and tectonic implication the high mg dacite is dated at 400 2 ma which broadly resembles the emplacement age of the high mg gneissic granodiorites of 406 9 ma possibly suggesting that they were generated in the same tectono magmatic event both rocks have predominant zircon inheritances of 500 ma which is consistent with the age of predominant zircon population for the widespread habahe sedimentary sequence indicating that their precursor magmas were probably mainly derived from the habahe sediments the high mg rock association has similar ree and trace element patterns to those of the habahe sediments and they plot close to the field of the habahe sediments in th ta versus la ta diagram their high a cnk ratios 1 33 2 05 imply magma source dominated by sedimentary rocks similarly their high la sm ratios also suggest a greater contribution of sediments rather than altered oceanic crust in the magma generation however the high mg values 51 of the studied rocks argue against derivation purely from remelting of the habahe sediments in order to explain the high mg values of these rocks the participation of mantle derived melt into the precursor magma is considered zircons from this rock association mostly yield e hf t values from 0 85 to 9 71 supporting the involvement of juvenile materials in the magma generation samples from the high mg rock association fall in the same field as the sanukitoids in tio 2 versus mg and sr y versus y diagrams and they plot in the fractional crystallization trend of differentiated sanukitoids in harker diagram possibly suggesting a petrogenetic process like the typical sanukitoid magma elsewhere this study shows that similar to the sanukitoids in the setouchi area our high mg rock association also involved melting of subducted sediments in a hot subduction regime combined with previous work a ridge subduction regime is considered to interpret the early devonian tectonic evolution of the chinese altai