relief inversion triggered by subduction of an active spreading ridge evidence from glacial morphology in central patagonia this study examines the response of the central patagonian cordillera to the subduction of the active south chile ridge and the development of the asthenospheric slab window beneath patagonia since the pliocene we investigated the eastern part of the cordillera where the glacio volcanic piedmont is topographically inverted glacial morphology and associated deposits capping the top of these piedmonts reveal that during the pliocene after 3 ma glaciers were flowing to the east over a regional surface connecting the western cordillera and the palaeo piedmont n160 trending normal faulting along the western edge of the inverted plateau disrupted the geomorphic surface after 3 ma at a regional scale extensional tectonics is responsible for a relief inversion with a minimum value of 800 m the timing of extensional tectonics is contemporaneous with the subduction of the spreading axis and the development of the slab window beneath central patagonia

in situ investigations of allanite hydrothermal alteration examples from calc alkaline and anorogenic granites of corsica southeast france allanite is with monazite the main repository for light rare earth elements ree in the continental crust and can be used in u th pb geochronology this mineral has been shown to be prone to alteration the geochemical exchanges occurring between allanite and hydrothermal fluids were explored using backscattered scanning electron microscopy electron microprobe and laser ablation inductively coupled plasma mass spectrometry la icp ms the alteration mechanisms found show the restricted role of metamictization and correspond to the allanite to epidote transformation or to the leaching of the allanite a crystallographic sites hence the ree are mostly removed from this phase during fluid mineral interactions although a heavy rare earth rich fluid may imprint its geochemical signature on the allanite altered zones it appears that even if this ree holder presents evidence of alteration the bulk mineral and whole rock ree and sm nd isotope signatures were not significantly affected in the cases studied alteration results mostly in the input of common lead thus restricting the usefulness of allanite for u th pb dating when alteration occurs finally the alteration mechanisms found here confirm the much more limited flexibility of silicates crystalline lattice as compared to phosphates

crustal structure off norway 62 to 70 north abstract extensive geophysical surveys have been undertaken on the volcanic passive continental margin offshore norway between 62 and 70 n during the last 25 years three main margin segments have been identified the lofoten vesteralen margin the voring margin and the more margin the main features of the margins are prominent marginal highs including seaward dipping reflector sequences and an up to 22 km thick volcanic and transitional crust prominent escarpments the voring plateau escarpment and the faeroe shetland escarpment and up to 12 km deep post jurassic sedimentary basins east of the escarpments velocity depth solutions from about 250 sonobuoys expanding spread profiles and refraction profiles have been compiled and contoured isovelocity horizon contour maps and velocity transects outline a crust which broadly thickens from an oceanic crust with a normal oceanic type velocity structure to a ca 35 km thick continental crust with a continental velocity structure beneath the norwegian coast anomalous features include local crustal thickening below the more and vering marginal highs and high velocity bodies in the lower crust in the extension of the precambrian lofoten vesteralen archipelago the free air seasat derived gravity anomalies show a good correlation with the high velocity bodies and show prominent ne trending highs from the rockall plateau porcupine plateau region over the more voring and lofoten vesteralen margins to the southwestern barents sea

paleomagnetism and 40ar 39ar chronology of lavas from meseta del lago buenos aires patagonia 1 as part of a larger project investigating paleosecular variation in south america 36 paleomagnetic and geochronology sites were sampled on meseta del lago buenos aires 47 s 289 e 200 km east of the active plate boundary in southern patagonia basaltic lavas sampled range from late miocene to late pleistocene yet most of the individual lavas are younger than 3 3 ma thirty eight isochron ages determined via 40ar 39ar incremental heating experiments frame a detailed stratigraphy the isochron ages range from 66 9 4 1 ka for scoria cones and a youthful lava flow in the rio pinturas valley to 10 12 1 35 ma for thick plateau forming lavas exposed along the se edge of the meseta the last 3 3 myr is characterized by seven episodes of volcanism at ca 3 2 3 0 ma 2 4 ma 1 7 ma 1 35 ma 1 0 ma 750 ka 430 330 ka and 110 ka the bulk of lavas forming the surface of the meseta erupted in the last 1 7 my all sites have stable magnetization and after step demagnetization using either thermal or alternating field techniques yield characteristic directions held by magnetite and or titanomagnetite ten sites have distinct transitional directions defined by pole latitudes 55 and associated ages indicate possible connections to known reversals within the matuyama chron including the onset of the jaramillo subchron 1 016 0 01 ma the cobb mountain subchron 1 25 0 03 ma the ontong java 1 event 1 37 0 03 ma and the termination of the olduvai subchron 1 72 0 02 ma remaining sites are divided into normal 14 sites and reversed polarity 12 sites at 95 confidence mean normal and reversed directions overlap the mean direction for 26 sites is i 63 0 d 3 4 95 5 4 which is indistinguishable from the expected geocentric axial dipole paleosecular variation measured by the dispersion of virtual geomagnetic poles about the rotation axis is higher than expected at 20 0 model g using data for the past 5 myr predicts a dispersion of 17 this discrepancy may be due to true dispersion of the field in southern south america or it may be an artifact of inadequate sampling

vp vs ratio along the voring margin ne atlantic derived from obs data implications on lithology and stress field a total of 13 regional ocean bottom seismograph obs profiles with an accumulated length of 2207 km acquired on the voring margin ne atlantic have been travel time modelled with regards to s waves the vp vs ratios are found to decrease with depth through the tertiary layers which is attributed to increased compaction and consolidation of the rocks the vp vs ratio in the intra campanian to mid campanian layer 1 75 1 8 in the central voring basin is significantly lower than for the layers above and beneath suggesting higher sand shale ratio this layer was confirmed by drilling to represent a layer of sandstone this mid cretaceous anomaly is also present in the northern voring basin as well as on the southern lofoten margin further north the vp vs ratio in the extrusive rocks on the voring plateau is estimated to be 1 85 conformable with mafic basaltic rocks landward of the continent ocean transition cot the vp vs ratio in the layer beneath the volcanics is estimated to be 1 67 1 75 these low values suggest that this layer represents sedimentary rocks and that the sand shale ratio might be relatively high here the vp vs ratio in the crystalline basement is estimated to be 1 67 1 75 in the basin and on the landward part of the voring plateau indicating the presence of granitic granodioritic continental crust in the lower crust the vp vs ratio in the basin decreases uniformly from southwest to northeast from 1 85 1 9 to 1 68 1 73 suggesting a gradual change from mafic gabbroic to felsic granodioritic lower crust significant 3 5 azimuthal s wave anisotropy is observed for several sedimentary layers as well as in the lower crust all these observations can be explained by invoking the presence of liquid filled microcracks aligned vertically along the direction of the present day maximum compressive stress nw se

subduction of the south chile active spreading ridge a 17 ma to 3 ma magmatic record in central patagonia western edge of meseta del lago buenos aires argentina abstract the chile triple junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent the mlba plateau meseta del lago buenos aires is one of the neogene alkali basaltic plateaus located in the back arc region of the andean cordillera at the latitude of the current chile triple junction the genesis of mlba can be related with successive opening of slabs windows beneath patagonia within the subducting nazca plate itself and between the nazca and antarctic plates detailed 40ar 39ar dating and geochemical analysis of bimodal magmatism from the western flank of the mlba show major changes in the back arc magmatism which occurred between 14 5 ma and 12 5 ma with the transition from calc alkaline lavas cerro plomo to alkaline lavas mlba in relation with slab window opening in a second step at 4 3 ma alkaline felsic intrusions were emplaced in the western flank of the mlba coevally with the mlba basalts with which they are genetically related these late oib like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting nazca plate during the opening of the south chile spreading ridge related slab window these basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones the close association of bimodal magmatism with extensional tectonic features in the western mlba is a strong support to the model of patagonian collapse event proposed to have taken place between 5 and 3 ma as a consequence of the presence of the asthenospheric window scr 1 segment of south chile ridge below the mlba area

allanite and other ree rich epidote group minerals epidote group minerals rich in rare earth elements ree in particular allanite are common accessory phases in igneous metamorphic metasomatic and sedimentary rocks small amounts of ree are present in most epidote group minerals but in allanite and the related minerals dissakisite ferriallanite dollaseite khristovite and androsite the ree are essential structural constituents an important characteristic of ree rich epidote group minerals is that their octahedrally coordinated m sites contain major amounts of divalent cations this paper summarizes literature data for these minerals and discusses their chemistry occurrence phase relations and petrologic and geologic significance the chapter emphasizes allanite because it is the most common and best studied of the ree rich epidote group minerals epidote group minerals contain isolated silicon tetrahedra and corner sharing groups of two tetrahedra and are thus assigned to the disilicate or sorosilicate structural family for a detailed description of the structure see franz and liebscher 2004 the epidote group structural formula is a2m3 sio4 si2o7 o f oh or in a simplified form a2m3si3o11 o f oh in which a ca sr pb2 mn2 th ree3 and u and m al fe3 fe2 mn3 mn2 mg cr3 and v3 deer et al 1986 there are two structurally different a sites a 1 and a 2 with different coordination numbers and there are three different m sites m 1 m 2 and m 3 which are all octahedrally coordinated ueda 1955 dollase 1971 in epidote group minerals trivalent ree are accommodated in the a sites which in endmember epidote both contain ca the incorporation of ree3 is commonly charge balanced by a divalent cation fe2 mn2 mg substituted for a trivalent one in the m sites table 1

cenozoic and late mesozoic stratigraphy and subsidence history of the norwegian margin the stratigraphy of the passive continental margin off norway is mapped utilizing both deep sea drilling project dsdp data at five sites on the outer margin and multichannel seismic mcs data collected by lamont doherty geological observatory distinct units in the acoustic stratigraphy are tied to biostratigraphic and lithostratigraphic units in the drill data and mapped throughout most of the margin stratigraphy of the pre lower eocene section not encountered in drilling is inferred from depositional patterns and correlation with the continental shelf sequences the stratigraphy of which has been developed independently from detailed surveys the margin can be divided into three physiographic provinces two broad shelf and slope regions separated by the jan mayen fracture zone and the steep narrow shelf and slope off the lofoten islands each experienced a separate cenozoic subsidence history the voring plateau is partly underlain by oceanic crust it subsided nearly 1 000 m less than expected from thermal contraction but it did so following the development of a relatively deep water environment in the paleocene north of the voring plateau the margin may have subsided a few hundred metres more than thermal studies had predicted there is no evidence for a phase of extensional deformation associated with the inception of spreading the youngest faulted level occurs at the late jurassic level preceding sea floor spreading by nearly 100 ma and deformation in the cretaceous section is characterized by large uplift structures that may be of halokinetic origin the physiographic provinces as well as the cenozoic subsidence patterns along the margin appear to be related to the style and extent of mesozoic basin development and tectonics along the margin

geochronology and geochemistry of the triassic bimodal volcanic rocks and coeval a type granites of the olzit area middle mongolia implications for the tectonic evolution of mongol okhotsk ocean abstract the olzit volcanism in middle mongolia comprises a bimodal suite of basalts and peralkaline rhyolites adjacent to the main mongolia lineament the basalts are characterized by enrichment in lile and lree and depletion in hfse with typical sr nd isotopic signatures end t 2 50 to 0 38 and 87sr 86sr i 0 7058 0 7063 indicating they were likely derived from partial melting of an enriched lithospheric mantle modified by subducted slab derived fluids the rhyolites show a close affinity to a type granites with enrichment in lile and lree and depletion in nb ta and ti they also show a significant negative eu anomaly and have end t values ranging from 0 50 to 1 38 and initial 87sr 86sr ratios ranging from 0 7022 to 0 7200 suggesting the rhyolites stem from partial melting of crustal rocks rather than fractional crystallization of the basaltic melt the rhyolite porphyry yields a shrimp zircon u pb age of 207 2 ma mswd 1 42 indicating the bimodal volcanic suite formed in the late triassic the miarolitic per alkaline granite and biotite bearing granite which are associated with the bimodal volcanic rocks show typical a type granitic geochemical affinity with end t 0 89 0 91 and 87sr 86sr i 0 7021 0 7043 indicating they are likely generated by partial melting of crustal rocks similar to the rhyolitic end member of bimodal suite the miarolitic per alkaline granite and biotite bearing granite yielded shrimp zircon u pb ages of 209 2 ma mswd 0 91 and 213 3 ma mswd 1 65 respectively which are nearly coeval with the age of the bimodal volcanic suites in view of the new geochemical and chronological data in this study we suggest the olzit late triassic bimodal volcanic rocks together with the coeval a type granites represent a back arc basin extensional environment which probably related to the roll back of mongol okhotsk oceanic plate during the southward subduction under the central mongolia microcontinent

temporal evolution of a long lived syenitic centre the kangerlussuaq alkaline complex east greenland abstract we present new mineral and whole rock compositions and sr nd hf pb o h isotope data on samples from the kangerlussuaq alkaline complex 1000 km2 in central east greenland part of the north atlantic igneous province this complex mainly consists of the kangerlussuaq intrusion but includes at least 13 separate satellite intrusions emplaced in the uppermost crust close to the unconformity between archaean gneisses and overlying palaeogene flood basalts the complex is divided into i older satellite intrusions 55 53 ma composed of multiple syenites and granites and minor gabbros and peridotites ii the voluminous kangerlussuaq intrusion 50 ma which displays a gradual transition from quartz syenites nordmarkites at the margin to nepheline syenites foyaites in the centre and iii younger satellite intrusions 47 45 ma of minor syenites granites and diorites concentrated southeast of the kangerlussuaq intrusion the complex displays a temporal evolution in which sio2 decreases 74 56 wt and total alkalis 6 16 wt amphibole na k content 206pb 204pbmeas endi and ehfi 3 to 11 increase from the older intrusions through the nepheline syenites this is followed by a reversal to higher silica 62 73 wt and lower total alkalis 9 12 wt amphibole na k content 206pb 204pbmeas endi and ehfi 13 to 2 in the younger satellite intrusions temporal changes in the location of magma plumbing systems and in magma production rates played a profound role in controlling silica content alkalinity and degree of crustal contamination during development of the complex phonolitic magma was only generated after prolonged magmatism had shielded the conduits from interaction with country rock the parental magmas were probably basanitic to alkali olivine basaltic in composition the older satellite intrusions and the kangerlussuaq intrusion have low 18o magma values 1 to 6 compared to the younger more crustally contaminated satellite intrusions 4 5 to 7 it appears that the magmas only had sufficient over pressure to intrude the basalt cover when larger volumes of less contaminated trachyte magma were produced resulting in the generation of low 18o magmas due to dehydration of hydrothermally altered basalt xenoliths

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

3 d seismic structure of the source area of the 1993 latur india earthquake and its implications for rupture nucleations abstract the latur earthquake mw 6 1 of 29 september 1993 is a rare stable continental region scr earthquake that occurred on a previously unknown blind fault in this study we determined detailed three dimensional 3 d p and s wave velocity vp vs and poisson s ratio structures by inverting the first p and s wave high quality arrival time data from 142 aftershocks that were recorded by a network of temporary seismic stations the source zone of the latur earthquake shows strong lateral heterogeneities in vp vs and structures extending in a volume of about 90 90 15 km3 the mainshock occurred within but near the boundary of a low vp high vs and low zone this suggests that the structural asperities at the mainshock hypocenter are associated with a partially fluid saturated fractured rock in a previously unknown source zone with intersecting fault surfaces this might have triggered the 1993 latur mainshock and its aftershock sequence our results are in good agreement with other geophysical studies that suggest high conductivity and high concentration of radiogenic helium gas beneath the source zone of the latur earthquake our study provides an additional evidence for the presence of fluid related anomaly at the hidden source zone of the latur earthquake in the scr and helps us understand the genesis of damaging earthquakes in the scr of the world

extensional tectonics during late cretaceous evolution of the southern central andes evidence from the chilean main range at 35 s abstract the study of late cretaceous earliest paleogene series in the chilean andes at 35 s provides new constraints on the coeval evolution of the area and the southern central andes these correspond to the continental series of brcu and the overlying plan de los yeuques formation pyf zircon age determinations reveal that these represent an almost continuous record of deposition respectively from the cenomanian early campanian to the campanian danian according to paleogeographic reconstructions the brcu series corresponds to the coeval foreland basin deposits in a near arc position which was westerly bounded by the earliest late cretaceous orogen detrital zircon age spectra for this reveal only a cretaceous component thus indicating a depocenter disconnected to that represented by coeval synorogenic deposits in argentina the volcanic rocks of the pyf record an eastern migration of the magmatic loci into the area at least since 80 ma its outcrops display a series of syn sedimentary normal faults indicating extensional conditions throughout its deposition its volcanic rocks conform an apparent bimodal suite characterized by arc like affinities low pressure conditions for the corresponding magmas and juvenile isotopic signatures for all the compositional spectra comparison and modeling of such compositional characteristics indicate a derivation from parental magmas similar to those currently sourcing the southern segment of the andean southern volcanic zone 37 42 s altogether these characteristics suggest an extensional intra arc basin setting for the deposition of this unit probably framed by a normal to thin crust of 30 35 km the extensional conditions observed for this unit are also recorded in numerous late cretaceous arc deposits along the andean margin thus indicating that this corresponds to a regional scale feature this evidence records a change from late cretaceous from compressional to extensional conditions in the arc area which agrees well with major plate tectonic configuration proposed for this time span in the andean margin

geochronology petrogenesis and tectonic implications of permian felsic rocks of the central great xing an range ne china the hegenshan heihe suture hhs is an important suture zone between the xing an and songliao xilinhot blocks in the eastern central asian orogenic belt caob many studies have examined the northern and southern parts of this suture zone but the middle segment remains under researched and controversial this paper presents new research into permian felsic rocks along the middle segment of the hhs including alkaline granites rhyolites and granodiorites zircon u pb dating of these rocks yielded precise permian ages of 300 4 2 5 292 8 2 7 and 293 1 2 2 ma the alkaline granites and the rhyolites are a type rocks that were derived from lower crustal protoliths with the melting of these protoliths caused by asthenospheric upwelling associated with lithospheric delamination in comparison the granodiorites are i type granites were generated from a mantle derived magmatic source the petrogenesis of these granites also involved the assimilation of sedimentary rocks and fractional crystallization this permian magmatism occurred during post collisional extension following the amalgamation of the xing an and songliao xilinhot blocks

igneous history of the andean cordillera and patagonian plateau around latitude 46 degrees s from the middle jurassic onwards persistent igneous activity in the southern andes around 46 s was controlled by easterly dipping subduction along the pacific margin cogenetic plutonic rocks belonging to the patagonian batholith and calc alkaline volcanics ranging from basaltic andesites to rhyolitic tuffs and ignimbrites are the principal products erosion of the primary volcanics has led at various times to the development of thick volcaniclastic sequences for example in the cretaceous lower tertiary divisadero formation the coyhaique region marks the northerly extension of a narrow back arc basin in which the marine neocomian successions accumulated volcaniclastics from the island arc which presumably lay to the west are intercalated with the sediments although the marine basin was short lived a mildly extensional back arc regime may have existed through much of mesozoic recent times widespread basalt rhyolite volcanism on the eastern side of the cordillera seems to have been associated with this tectonic environment remnants of the patagonian basalt plateau at latitude 45 47 s extend from the argentine chile frontier to lago colhue huapi four principal age and compositional groups have been distinguished in the lavas i the oldest which are about 80 ma occur in sections at senguerr and morro negro they are almost exclusively tholeiitic but show some calc alkaline affinities and resemble in other respects basalts from marginal basins ii the second group 57 43 ma occur in the lower part of the chile chico section with a compositional spread from olivine tholeiites through alkali basalts to one occurrence of a basanite iii the upper part of the main plateau sequence where the flows are in the range 25 9 ma are dominantly of alkali basalt composition iv post plateau flows from small cinder cones on the surface of the plateau range in age from ca 4 ma to 0 2 ma or less they are mostly highly undersaturated basanites with occasional leucite basanites enriched in incompatible elements a few of the earlier tholeiites with calc alkali traits may have been closely associated with subduction or marginal basin processes the younger lavas are more alkalic intraplate types generated in the remote back arc extensional zone

polyphase rift evolution of the voring margin mid norway constraints from forward tectonostratigraphic modeling the mid norwegian voring margin bounding the northern atlantic underwent several rift episodes after the caledonian orogeny permo triassic extension concentrated at the eastern inner margin followed by jurassic cretaceous extension at the center of the margin finally more westward the last rift episode led to continental breakup in the earliest eocene although the general architecture of the margin may be clear the timing and distribution of particularly the oldest rift structures are poorly constrained we use two dimensional forward tectonostratigraphic modeling for better quantitative understanding of the tectonic evolution of the margin two regional profiles crossing the margin have been analyzed modeling of the two profiles integrates the basin shape and fill as well as the underlying crustal structure of the margin the modeling allows us 1 to quantify the distribution and timing of extension of subsequent rift episodes 2 to predict the basin and crustal architecture at deep levels and 3 to test different interpretations of the seismic data the level of lithospheric necking during basin formation is constrained by basement topography and moho depth and lies within the middle crust 20 km furthermore we investigate the migration of the rift axes and the elastic lithospheric strength during the different rift episodes locations of previous extension are not reactivated during renewed rifting instead renewed rifting occurs preferentially in regions where the lithosphere has experienced less extension

basin structure on the norwegian margin from analysis of digitally recorded sonobuoys during r v robert d conrad cruise 21 a large number of sonobuoys were recorded in the voring more and adjacent basins at the margin off norway the digital sonobuoy data have been transformed into the domain of intercept time tau and ray parameter p and inverted by the tau sum method to obtain better definition of the near seafloor velocities using also the precritical energy a move out correction has been applied to the tau p data conventional analyses of the x t data were also performed in general the various types of analyses complement each other and should be used interactively together with forward modeling to obtain the optimal velocity depth function the sonobuoy results have outlined in a regional sense the pre cenozoic sedimentary basin at the margin this basin attains a maximum sediment thickness of 8 10 km of which only the upper half is defined by seismic reflection profiling at the outer margin the marginal escarpments form major discontinuities in the lateral velocity distribution the survey has proven that sonobuoy profiles recorded during standard seismic reflection profiling in major marginal basins may provide important new information particularly about the deeper parts of the basins that is not directly available from the reflection profiles

petrogenesis of late paleoproterozoic liangcheng charnockites and s type granites in the central northern margin of the north china craton implications for ridge subduction abstract the 1930 1890 ma garnet bearing peraluminous liangcheng granitoids are mainly distributed in the fengzhen belt in the north china craton ncc they consist of early porphyritic coarse grained charnockites magmatic orthopyroxene granitic rocks and late medium to coarse grained s type crustal melt granites the charnockites are distinguished from the s type granites by their higher tio 2 al 2 o 3 feo t mgo cao and trace element contents especially the heavy rare earth elements ree the charnockites show varied light ree enrichments la yb n 1 92 18 0 negative eu anomalies eu eu 0 2 0 9 and negative anomalies in high field strength elements hfses e g nb ta ti and in sr compared with the neighbouring elements in spidergrams their 87 sr 86 sr t t 1900 ma ratios vary from 0 704 to 0 707 with one at 0 74 and nd t t 1900 ma values range from 4 to 1 in contrast the liangcheng s type granites show strong light ree enrichments la yb n 13 3 230 positive eu anomalies eu eu 1 1 3 6 negative anomalies in hfses e g nb ta ti and negative or positive anomalies in sr compared with neighbouring elements in spidergrams their 87 sr 86 sr t t 1900 ma ratios vary from 0 703 to 0 712 and nd t t 1900 ma values range from 3 to 1 both the charnockites and s type granites originated by melting of in situ meta sediments with significant charnockites or little s type granites amounts of additional melts from the parental magma of the adjacent xuwujia gabbronorites high mg 19 23 wt samples of the xuwujia gabbronorites have whole rock re contents of 0 16 1 93 ppb os contents of 1 6 9 8 ppb 187 re 188 os ratios of 0 08 1 76 and 187 os 188 os ratios of 0 1168 0 1186 with one exception at 0 1845 these values are comparable with those of typical komatiites and agree with a depleted source for these rocks the charnockites have received 10 30 wt or even larger amounts of melt contribution from the xuwujia gabbronorites whereas the s type granites received only few melts from the gabbronorites and they possibly had lower melting temperature than the charnockites we consider that the charnockites and s type granites had surface counterparts represented by two types of rhyolitic to dacitic volcanics in the halaqin succession which also contains basalts to basaltic andesites that are geochemically comparable to the xuwujia gabbronorites or the charnockites some andesites from their field relations and geochemical and thermal characteristics we propose that the peraluminous charnockites and s type granites were most likely generated by ridge subduction at 1930 ma prior to the final cratonization of the ncc at 1850 ma and that this ridge subduction was also responsible for the regional 1950 1880 ma gabbronorites charnockites s type granites volcanics as well as the 1930 ma ultrahigh temperature metamorphism in the ncc

southern patagonian plateau basalts and deformation backarc testimony of ridge collisions abstract the distribution and volume of tertiary patagonian plateau basalts and the evolution of the patagonian fold and thrust belt between 46 and 49 s appear to be closely tied with ridge trench interactions along the continental margin to the west eocene plateau basalts south of 43 s are associated with a gap in the eocene arc and can be related to a proposed collision of the aluk farallon ridge against the trench at this time eruptions of late miocene to recent plateau basalts between 46 and 49 s can be related to time transgressive slab windows in the underlying mantle generated by the collision of segments of the chile ridge at 10 14 ma and 6 ma the principal deformation in the patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with late miocene ridge collision comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment almost all eocene to recent plateau basalts between 46 and 49 s have oib like ocean island basalt trace element signatures showing that the mantle source region changed little with time plateau basalts with more arc like signatures appear to be contaminated by continental crust relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows

mesozoic cenozoic denudation history of the patagonian andes southern chile and its correlation to different subduction processes fission track ft analysis is applied to assess the mesozoic and cenozoic thermal and denudational history of the patagonian andes between 44 and 51 s and the geologic and geomorphic response of late cenozoic subduction of the active chile rise mid oceanic spreading center on the overriding plate seventy two ft ages from 43 samples are presented zircon ft ages indicate fast post intrusion cooling of cretaceous parts of the patagonian batholith and previously unreported miocene magmatic rocks south of 48 s metamorphic basement rocks to the east of the batholith are constrained as having been deposited and metamorphosed in the early carboniferous and late permian apatite ft data reveal initiation of accelerated cooling and denudation at ca 30 ma at the western margin of southern continental south america followed by an up to 200 km eastward migration of the locus of maximum denudation that ceased at ca 12 8 ma at the position of the present day main topographic divide this migration is proposed to be related to either coeval eastward migration of the retroarc deformation the effects of subduction erosion in the overriding plate at the peru chile trench or less likely shallowing of the angle of subduction east of the divide 3 km of denudation has occurred since the late cretaceous enhanced denudation is interpreted to be the result of increased tectonic uplift driven by a large increase in convergence rates at ca 28 26 ma that triggered orographically enhanced precipitation on the west side of the patagonian andes allowing increased erosion by fluvial incision and mass transport processes the actual process of spreading center subduction had remarkably little influence on denudation in the upper plate and indeed coincides with a slowdown in denudation

pliocene extensional tectonics in the eastern central patagonian cordillera geochronological constraints and new field evidence recent field work and review of radiometric data obtained from neogene lavas and plutonic rocks exposed in the eastern central patagonian cordillera 46 48os which overlie subducted segments of the south chile ridge suggest important late miocene to pleistocene morphological changes in relation to base level variations and or tectonic events we present new field observations from a region south of lago general carrerabuenos aires between the main cordillera and the meseta del lago buenos aires demonstrating that normal faulting controlled valley incisions and occurred during lava emplacement at 5 4 ma and after 3 ma we also show that the 12 ma basaltic flows of the meseta del lago buenos aires 2000 m a s l have been subjected to deep incision with younger lavas dated at 1 2 ma partially filling the valleys these incisions are thought to reflect progressive eastward tilting of the entire meseta our new observations together with additional features from central patagonia strongly suggest that tectonic events led to a regional widespread morphological change after 5 ma the coincidence in time and space between the subduction of segments of the south chile ridge at 6 and 3 ma causing opening of a slab window and strong base level variations in the studied area suggests a cause and effect relationship in central patagonia compressional tectonics ended well before extensional events reported here causes of uplift and further extension are probably completely disconnected the uplift is purely tectonic in origin and occurred prior to the subduction of the south chile ridge extension should be a consequence of this subduction

modelling shear waves in obs data from the voring basin northern norway by 2 d ray tracing three component recordings from an array of five ocean bottom seismographs in the northwestern part of the voring basin have been used to obtain a 2 d shear wave s wave velocity depth model the shear waves are identified by means of travel time differences compared to the compressional p waves and by analyzing their particle motions the model has been obtained by kinematic travel time ray tracing modelling of the obs horizontal components

reply to discussion on the eocene bimodal piranshahr massif of the sanadaj sirjan zone west iran a marker of the end of collision in the zagros orogen journal vol 166 2009 pp 53 69 we thank mark b allen for giving us the opportunity to strengthen our case for the 41 ma piranshahr massif postdating the collision of the arabian and iranian plates in the sanandaj sirjan zone in the zagros orogen mazhari et al 2009 because of his suggestion that the piranshahr massif is not a reliable tectonic marker and that based on regional reasoning the collision must have been considerable younger we are permitted to elaborate on the reasoning behind and provide further evidence for the interpretation of our new geochemical and geochronological data according to allen in our paper the main argument presented for a tectonic setting post dating initial collision is the a2 type chemistry of the felsic rocks within the massif by comparison with other a2 type granitoids reviewed by eby 1990 1992 it is concluded the the intrusions were emplaced some 10 20 ma after initial collision apparently our interpretation cannot be correct as many a type granitoids have now been described from subduction zone settings i e before collision these include those studied by zhao et al 2008 and espinoza et al 2008 it appears that

neogene to quaternary tectonic evolution of the patagonian andes at the latitude of the chile triple junction abstract we conducted a preliminary structural and tectonic analysis of the region north and south of the lake general carrera buenos aires lgcba southern patagonian andes chile and argentina in order to study the effects of the subduction of the active south chile spreading ridge scr on the development of the tectonic front of the patagonian andes at the latitude of the chile triple junction ctj here one segment of the scr entered the trench at 6 ma and the corresponding slab window is now located below the study area we emphasize the segmentation of the tectonic front which can be divided into four successive oblique segments a to d trending mainly n170 and right laterally offset this geometrical pattern is consistent with a transpressive stress regime in relation to oblique subduction the four segments of the tectonic front are separated and locally deformed by n170 oriented more recent sinistral strike slip fault zones kinematic analysis of minor fault and striaes confirms the abundance of strike slip faults and suggests rotation of the stress tensor after at least 9 ma a consequence of the subduction of the scr beneath the south american plate at the latitude of the lgcba is a two step history 1 pre ridge subduction oblique to the western margin of the south america plate showing contractional regime with its main axis trending ne sw responsible for dextral transpressive deformation and 2 post ridge subduction more orthogonal convergent regime recent uplift occurred later starting at 3 5 ma and could still be active neotectonic activity has been evaluated in the region from preliminary analysis of river patterns and fluvial lacustrine terraces anomalies in stream development can be found although evidences for recent to active faulting can be shown effects of syn glacial and post glacial related processes cannot be ruled out the evolution of this region during the late cenozoic can be divided into three distinct periods 1 20 15 ma deposition of foreland continental detrital sediments rio zeballos group followed by a major contractional phase leading to the development of the main thrust front sometimes between 15 and 10 9 ma 2 9 4 ma erosion peneplanation and emplacement of voluminous plateau basalts coeval with fluvial glacial sedimentation and 3 3 0 ma renewed orogenic activity uplift and disruption of former erosional surfaces in relation to activation of strike slip fault zones and north trending thrusts onset of the major contractional event in the region of study can be correlated with the initiation of the subduction of the chile ridge at 14 15 ma in southernmost patagonia inducing a different stress regime in the frontal arc region

rock lithology and porosity determination from shear and compressional wave velocity data examined in this study are previously published laboratory shear s and compressional p wave velocity measurements on water saturated sandstone calcareous sandstone dolomite and limestone cores as well as laboratory porosity measurements on the sandstone and limestone cores sandstone and limestone porosities range from 092 to 299 and from 006 to 229 respectively differential pressure was varied from 500 to 6000 psi corresponding to approximate burial depths from 290 to 3460 m respectively sandstone limestone and dolomite are effectively separated by poisson s ratio or equivalently by the ratio of p to s wave velocity separation of sandstone and limestone appears to result from the difference in of the matrix material namely quartz 056 and calcite 316 respectively an empirical function 1 v a b was fit by regression analysis to sandstone and limestone velocity vs and vp versus porosity values at each differential pressure in this equation a and b are const

crustal structure and evolution of the southern voring basin and voring transform margin ne atlantic abstract the dominantly passive volcanic voring and more margins ne atlantic are separated by the 200 km long voring transform margin vtm the southern voring basin and the vtm have been studied by use of four regional ocean bottom seismograph obs profiles combined by gravity modelling the models demonstrate a complex pattern of magmatism along the transform margin the distribution of magmatism seems to be related to the existence and trend of a lower crustal 8 km s body interpreted as eclogitized rocks present in the southern voring basin early tertiary breakup related magmatic leakage across the continent ocean transition cot appears to be facilitated where this layer is absent these results support earlier workers who have concluded that the jan mayen fracture zone originated from a caledonian zone of weakness we propose that partly eclogitized rocks were uplifted into the lower crust close to this zone during the caledonian orogeny and that this body acted as a barrier to magma emplacement during the late cretaceous early eocene phase of rifting breakup the eclogitized terrain also appears to have caused northeastward channeling of the late cretaceous early tertiary intrusions within the voring basin an up to 10 km thick pre cretaceous sedimentary basin in the southern voring basin may be genetically related to the ns trending late paleozoic and mesozoic rift basins in north east greenland

geochemical constraints on the origin of bimodal magmatism at the okinawa trough an incipient back arc basin abstract the magmatism at the axial zone of the middle okinawa trough a young continental back arc basin comprises a bimodal basaltic rhyolitic suite accompanied by minor intermediate rocks we report major and trace element and sr nd isotopie data for the intermediate to silicic suites to provide constraints on their petrogenesis the rhyolites recovered as lava and pumice fall into three geochemical groups type 1 2 and 3 rhyolites type 1 rhyolites have 87sr 86sr 0 7040 0 7042 and 143nd 144nd 0 5128 0 5129 identical to those of associated basalts and are characterized by highly fractionated ree patterns petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt in contrast type 2 rhyolites and andesite have slightly higher 87sr 86sr 0 7044 0 7047 but similar 143nd 144nd 0 5128 compared to those of the basalts the compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization afc processes of the basalt magma quantitative analysis suggests assimilation fractional crystallization ma mc ratios of 0 05 hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present we emphasize that mechanical extension in this part of the okinawa trough involves gabbro ic lower crust that resulted from fractionation of mantle derived basaltic magmas type 3 rhyolite occurs only as pumice which makes its derivation questionable this rhyolite has major and trace element compositions and sr nd isotopie ratios which suggests that it may be derived from volcanic activity on the southern ryukyu volcanic front and arrived in the okinawa trough by drifting on the kuroshio current

40ar 39ar and k ar chronology of pleistocene glaciations in patagonia during the pleistocene east of lago buenos aires argentina at 46 5 s at least 19 terminal moraines were deposited as piedmont glaciers from the patagonian ice cap advanced onto the semi arid high plains adjacent to the southern andes exceptional preservation of these deposits offers a rare opportunity to document ice cap fluctuations during the last 1 2 m y 4 0 ar 3 9 ar incremental heating and unspiked k ar experiments on four basaltic lava flows interbedded with the moraines provide a chronologic framework for the entire glacial sequence the 4 0 ar 3 9 ar isochron ages of three lavas that overlie till 90 km east of the cordillera at lago buenos aires and another 120 km from the andes along rio gallegos at 51 8 s that underlies till strongly suggest that the ice cap reached its greatest eastward extent ca 1100 ka at least six moraines were deposited within the 256 k y period bracketed by basaltic eruptions at 1016 10 ka and 760 14 ka similarly six younger more proximal moraines were deposited during an 651 k y period bracketed by an underlying 760 14 ka basalt and the 109 3 ka cerro volcan basalt flow that buried all six moraines coupled with in situ cosmogenic surface exposure ages of moraine boulders the 109 ka age of cerro volcan implies that moraines deposited during the penultimate local glaciation correspond to marine oxygen isotope stage 6 further westward toward lago buenos aires six additional moraines younger than the cerro volcan basalt flow occur surface exposure dating of boulders on these moraines combined with the 1 4 c age of overlying varved lacustrine sediment indicates deposition during the last glacial maximum lgm 23 16 ka although antarctic dust records signal an important patagonian glaciation at 60 40 ka moraines corresponding to marine oxygen isotope stage 4 are not preserved at lago buenos aires apparently these were overrun and obliterated by the younger ice advance at 23 ka notwithstanding the overall pattern of glaciation in patagonia is one of diminishing eastward extent of ice during successive glacial advances over the past 1 m y we hypothesize that tectonically driven uplift of the patagonian andes which began in the pliocene yet continued into the quaternary in part due to subduction of the chile rise spreading center during the past 2 m y maximized the ice accumulation area and ice extent by 1 1 ma subsequent deep glacial erosion has reduced the accumulation area resulting in less extensive glaciers over time

early cretaceous morb type basalt and a type rhyolite in northern tibet evidence for ridge subduction in the bangong nujiang tethyan ocean abstract new zircon u pb ages major and trace element data and hf isotopic compositions are presented for bimodal volcanic rocks of the zhaga formation zf in the western middle segment of the bangong nujiang suture zone bnsz northern tibet the genesis of these rocks is described and implications for late stage evolution of the bangong nujiang tethyan ocean bnto are considered detailed studies show that the zf bimodal rocks which occur as layers within a typical bathyal to abyssal flysch deposit comprise morb type basalt that formed at a mid ocean ridge and low k calc alkaline a type rhyolite derived from juvenile crust the combination of morb type basalt calc alkaline a type rhyolite and bathyal to abyssal flysch deposits in the zf leads us to propose that they formed as a result of ridge subduction the a type zf rhyolites yield la icp ms zircon u pb ages of 118 112 ma indicating formation during the early cretaceous data from the present study combined with regional geological data indicate that the bnto underwent conversion from ocean opening to ocean closure during the late jurassic early cretaceous the eastern segment of the bnto closed during this period while the western and western middle segments were still at least partially open and active during the early cretaceous accompanied by ridge subduction within the bangong nujiang tethyan ocean

crustal structure and transform margin development south of svalbard based on ocean bottom seismometer data abstract the barents sea is located in the northwestern corner of the eurasian continent where the crustal terrain was assembled in the caledonian orogeny during late ordovician and silurian times the western barents sea margin developed primarily as a transform margin during the early tertiary in the northwestern part south of svalbard multichannel reflection seismic lines have poor resolution below the permian sequence and the early post orogenic development is not well known here in 1998 an ocean bottom seismometer obs survey was collected southwest to southeast of the svalbard archipelago one profile was shot across the continental transform margin south of svalbard which is presented here p wave modeling of the obs profile indicates a caledonian suture in the continental basement south of svalbard also proposed previously based on a deep seismic reflection line coincident with the obs profile the suture zone is associated with a small crustal root and westward dipping mantle reflectivity and it marks a boundary between two different crystalline basement terrains the western terrain has low 6 2 6 45 km s 1 p wave velocities while the eastern has higher 6 3 6 9 km s 1 velocities gravity modeling agrees with this as an increased density is needed in the eastern block the s wave data predict a quartz rich lithology compatible with felsic gneiss to granite within and west of the suture zone and an intermediate lithological composition to the east a geological model assuming westward dipping caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust as well as the observed structuring due to the transform margin setting the tectonic thinning of the continental block during opening of the norwegian greenland sea is restricted to the outer 35 km of the continental block and the continent ocean boundary cob can be located to within 5 km in our data distinct from the outer high commonly observed on transform margins the upper part of the continental crust at the margin is dominated by two large rotated down faulted blocks with throws of 2 3 km on each fault apparently formed during the transform margin development analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile and that the downfaulting dominates the whole margin segment from spitsbergen to bjornoya south of bjornoya the faulting leaves the continental margin to terminate as a graben 75 km south of the island adjacent to the continental margin there is no clear oceanic layer 2 seismic signature however the top basement velocity of 6 55 km s 1 is significantly lower than the high 7 km s 1 velocity reported earlier from expanding spread profiles esps and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7 8 km thick sedimentary overburden

timing and origin of deformation along the patagonian fold and thrust belt the andean orogeny in the patagonian cordillera of southern south america reflects the consequences of the mesozoic and cenozoic subduction of an oceanic plate beneath the south american continental margin the geological evolution of the region has been influenced by the eocene collision and subduction of the farallon aluk ridge and the miocene recent subduction of the chile ridge another aspect of plate interaction during this period was two intervals of rapid plate convergence one at 50 42 ma and the other at 25 10 ma between the south american and the oceanic plates it has been proposed that the collision of the chile ridge with the trench was responsible for the development at least in part of the patagonian fold and thrust belt this belt extends for more than 1000 km along the eastern foothills of the southern andes between 46 and 54 s along the southwestern rim of the austral basin the interpretation of a link between subduction of the ridge and formation of the fold and thrust belt is based on assumed time coincidences between contractional tectonism and the collision of ridge segments during middle and late miocene times the main tertiary contractional events in the patagonian fold and thrust belt took place during latest cretaceous palaeocene eocene and during miocene times although the timing of deformation is still poorly constrained the evidence currently available suggests that there is little or no relationship between the timing of the fold and thrust belt and the collision of ridge segments most if not all of the contractional tectonism pre dated the latest episodes of ridge collision collision of a ridge crest with the continental margin has been active for the past 14 to 15 million years contrary to the suggestion of a relationship between ridge subduction and compression the main result of this collision has been fast uplift and extensional tectonism the initiation of the patagonian fold and thrust belt in latest cretaceous or early tertiary times coincided with a fundamental change in the tectonic evolution of the austral basin throughout the cretaceous most of this basin subsided as a broad backarc continental shelf only in latest cretaceous times and coinciding with the initiation of the fold and thrust belt the basin underwent a transition to a retro arc foreland basin this change to an asymmetrically subsiding foreland basin with an associated foreland fold and thrust belt was related to uplift of the andean orogenic belt in the west

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

andean foothills structures in northern magallanes basin argentina the foothills of the northern segment of the magallanes basin between 47 and 49 s latitudes record the development of a complex fold and thrust belt in the patagonian cordillera during the late miocene upper paleozoic sedimentary and metasedimentary rocks and thick sequences of lower cretaceous deposits with well defined source and reservoir rocks are deformed by the andean events the structure is characterized by a triangle zone and a series of related underthrusts which define a nonemergent backthrust system at the mountain front differences in the degree of deformation from south to north reveal the evolution of the triangle zone the stratigraphy of the molasse deposits constrains the beginning of the deformation at least to the early tertiary although the final structure was developed during the late miocene these deformed deposits are covered by extensive late miocene horizontal retro arc basalts analysis of the evolving subduction zone located west of and beneath the study area shows that the formation of a subsequent volcanic gap in the magmatic arc and the development of extensive retro arc basalts are related to the collision of a segment of the chile ridge during the late miocene the final deformation and uplift of the patagonian cordillera fold and thrust belt at these latitudes are closely linked to this collision

a morphotectonic analysis of central patagonian cordillera negative inversion of the andean belt over a buried spreading center 1 we use digital elevation models and satellite imagery data completed with field observations to develop an integrated morphotectonic analysis of the anomalous topography of the central cordillera of patagonia between 46 s and 48 s in a region overlying subducted segments of the chile spreading ridge this region shows a number of peculiar features not expected in a normal subduction related belt located away from a buried spreading center first it is characterized by overall short wavelength topography and highly contrasted reliefs typical of deeply incised young mountain belts but surprisingly also by an average very low elevation of 900 m second the cordillera lacks central high reliefs instead there is a central depressed domain with average low slope gradients including the deepest point of the study area 300 m below sea level in the bottom of lake general carrera buenos aires this depressed area is flanked by western and eastern regions of higher elevation having opposite general slope dips to the east the central patagonian cordillera is now separated from the flat plains of the pampean domain representing its former piedmont and several elevated basaltic plateaus mesetas by 160 n 180 n trending morphotectonic corridors and the western border scarps of the mesetas these scarps correspond to neotectonic features controlled by recent vertical faults mostly active after 3 ma remnants of a former peneplain the avellanos surface are widespread within the entire cordilleran region from geological evidence this peneplain formed between 15 and 4 ma and was connected originally with the mesetas domain and now is dissected by these corridors it is shown that a negative inversion related to extensional and strike slip faulting occurred in the frontal region of the cordillera at 3 ma synchronously with the emplacement of magmatic products timing of the extensional deformation in central patagonia is concomitant with the migration at depth of different segments of the south chile ridge which entered the trench at 6 and 3 ma respectively and the consequent opening of a slab window allows asthenospheric hot mantle to reach lithospheric regions finally migration of segments of the chile spreading ridge since 6 ma triggered regional uplift of the cordillera followed by limited extension in relation with increased thermal weakening of the crust

habitat of hydrocarbons on the norwegian continental shelf eds a m spenceret al norwegian petroleum society graham and trotman 1986 isbn 0 86010 833 3 price 65 00

the south patagonian batholith 150 my of granite magmatism on a plate margin a new database of 70 u pb zircon ages mostly determined by shrimp indicates that the south patagonian batholith resulted from the amalgamation of subduction related plutons from the late jurassic to the neogene construction of the batholith began with a voluminous previously undetected late jurassic bimodal body mainly composed of leucogranite with some gabbro emplaced along its present eastern margin within a restricted time span 157 to 145 ma this episode is at least in part coeval with voluminous rhyolitic ignimbrites of the tobifera formation deposited in the deep rocas verdes basin east of the batholith this was the last of several southwestward migrating silicic volcanic episodes in patagonia that commenced in an early jurassic extensional tectonic regime the quasi oceanic mafic floor of the basin was also contemporaneous with this late jurassic batholithic event as indicated by mutually cross cutting field relationships changes in subduction parameters then triggered the generation of earliest cretaceous plutons cretaceous 1 144 137 ma west of the late jurassic ones a westward shift that culminated at 136 127 ma cretaceous 2 along the present western margin of the batholith most mid to late cretaceous cretaceous 3 126 75 ma and paleogene 67 40 ma granitoids are represented by geographically restricted plutons mainly emplaced between the previously established margins of the batholith and mostly in the far south no associated volcanic rocks of similar age are known at present in this area during the final neogene stage of plutonism 25 15 ma a recurrence of coeval volcanism is recognized within and east of the batholith typical endt values for the granitoids vary from strongly negative 5 in the late jurassic to progressively higher values for cretaceous 1 4 cretaceous 2 0 7 cretaceous 3 2 and the paleogene 5 followed by lower and more variable ones in the neogene 1 to 5 these variations may reflect different modes of pluton emplacement large crustal magma chambers developed in the early stages late jurassic to cretaceous 1 leading to widespread emplacement of plutons with a crustal signature whereas the cretaceous 2 cretaceous 3 and palaeogene parts of the batholith resulted from incremental assembly of small plutons generated at greater depths and with higher endt this does not in itself justify the idea of a reduction in crustal character due to progressive exhaustion of fusible material in the crust through which the magmas passed

seismic noise tomography in the chile ridge subduction region summary we used cross correlation of ambient seismic noise recorded in the chile triple junction ctj region to estimate interstation surface wave time domain green s functions and then inverted traveltimes to obtain crustal surface wave velocity models interstation distances within the chile ridge subduction project crsp temporary seismic network ranged from 40 to 100 km we selected 365 d and cross correlated and stacked 24 hr of vertical component data at 38 stations pairs resulting in nominally 703 traveltimes along assumed straight interstation paths velocities in 2 d cells of 30 km 30 km were calculated using a linear least squares inversion of the rayleigh wave group velocity traveltimes furthermore we performed a rayleigh wave group velocity dispersion analysis to estimate the sensitivity of different period waves at depth and to calculate a 3 d shear velocity model of the patagonian crust the process was applied to cross correlation pairs determined in two period bands 5 10 s corresponding to shallow crustal velocities down to approximately 10 km depth and 10 20 s for velocities down to around 20 km our results show that cell velocities correlate well with known geological features we find high crustal velocities where the patagonian batholith outcrops or is likely present at depth and low velocities correlate with the active volcanic arc of the southern volcanic zone and the subducted chile ridge in taitao peninsula where thermal activity of hot springs is present high velocities in the mountainous portions of the southeastern study area appear to correlate with outcropping older metamorphic units low velocity in the east correlate with sequences of volcaniclastic deposits

alkaline syenites in eastern cathaysia south china link to permian triassic transtension two alkaline syenite plutons the tieshan and yangfang plutons have recently been recognized within ne trending fault zones in eastern cathaysia south china the rocks are very enriched in k2o 6 28 9 39 wt rare earth elements ree particularly light ree and large ion lithophile elements but are relatively low in high field strength elements isotopically they are characterized by high initial 87sr 86sr 0 7093 to 0 7123 and low nd t values 5 64 to 10 63 the geochemical data suggest that the alkaline syenites most likely formed via fractional crystallization of enriched mantle derived magmas sensitive high resolution ion microprobe zircon u pb dating indicates that these two intrusions have late permian 254 4 ma and early triassic 242 4 ma crystallization ages respectively our data suggest that a tectonic regime dominated by transtension probably existed from at least the latest permian into the triassic and was responsible for the formation of the tieshan and yangfang alkaline syenites when combined with previous paleomagnetic structural and sedimentology data we suggest that the transtension along the ne trending strike slip fault zones was related to oblique subduction of the pacific plate underneath south china

petrogenesis of the eocene and mio pliocene alkaline basaltic magmatism in meseta chile chico southern patagonia chile evidence for the participation of two slab windows the meseta chile chico mcc 46 48s is the westernmost exposure of eocene lower basaltic sequence lbs 55 40 ma k ar ages and mio pliocene upper basaltic sequence ubs 16 4 ma k ar ages flood basalt volcanism in patagonia the mcc is located south of the lago general carrera buenos aires lgcba southeast from the present day chile triple junction ctj east of the actual volcanic gap between southern south volcanic zone and austral volcanic zone ssvz and avz respectively and just above the inferred location of the south chile ridge segment subducted at 6 ma scr 1 erupted products consist of mainly ne normative olivine basalt with minor hy normative tholeiites basalt trachybasalt and basanite mcc lavas are alkaline 42 7 53 1 wt sio2 3 8 wt na2o k2o and relatively primitive ni 133 360 ppm cr 161 193 ppm co 35 72 ppm 4 16 5 mgo wt they have a marked oib like signature as shown by their isotopic compositions 87 sr 86 sro 0 70311 0 70414 and qnd 4 7 5 1 and their incompatible trace elements ratios ba la 10 20 la nb 0 46 1 09 ce pb 15 52 27 5 sr lab25 reflecting deep mantle origin ubs primitive lavas have characteristics similar to those of the eocene lbs basalts while ubs intermediate lavas show geochemical imprints la nbn1 sr lan25 low ce pb nb u compatible with contamination by arc slab derived and or crustal components we propose that the genesis and extrusion of

stratigraphic and structural evolution of the tertiary cosmelli basin and its relationship to the chile triple junction at the taitao peninsula in southern chile 46 5 s an active mid ocean ridge is being subducted under the south american continent continental crust south of the current triple junction has experienced subduction of three ridge segments since the mid miocene a series of mid late tertiary sedimentary basins lies inboard of the triple junction the cosmelli basin lies in the region which has experienced the earlier ridge subduction events within the cosmelli basin fill abrupt facies dislocations give clear evidence of marked base level changes four sequence boundaries are delineated with an upward increase in degree of facies dislocation which is believed to reflect increasing magnitudes of base level fall the lower part of the basin fill is folded and then thrust eastward as a series of imbricate slices while the overlying greater thickness of fluvial sediments is only gently tilted westwards this geometry indicates that the early basin fill was deforming due to contractional tectonics while the later basin fill was being deposited these processes controlled accommodation space and thus stratigraphic architecture during much of the basin life this complex basin history plus the presence of basaltic sheet intrusions may reflect the underlying slab history as successive ridge segments were subducted

seismic anisotropy inferred from wide angle reflections off lofoten norway indicative of shear aligned minerals in the upper mantle abstract data from six three component ocean bottom seismographs situated on a 175 km long profile on the continental shelf off lofoten northern norway have been modelled using 2 d seismic ray tracing and p and s wave velocity models for the upper mantle are presented a dipping structure about 10 km below the moho causes strong p and s reflections as well as p to s conversions this upper mantle reflection might be related to a possible seaward dipping master fault and or the presence of partially hydrated peridotite the modelling of these arrivals reveals significant seismic anisotropy in the upper mantle the maximum p wave anisotropy is inferred to be in excess of 15 the maximum s wave anisotropy in the order of 5 10 and the axis of the fast velocity is inferred to be inclined at least 45 from the horizontal the strong anisotropy for p waves indicates that its cause is related to alignment of minerals we postulate that the inferred steep inclination of anisotropic minerals might be a result of shearing along faults and fractures during post caledonian extensional episodes it must be emphasized however that it cannot be excluded that more complicated anisotropic models with other geological implications can also explain the observations satisfactorily the study demonstrates the importance of incorporating s waves in analysis of seismic data because the strong p wave anisotropy would not have been discovered without the constraints given by the s waves

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

crustal transect from the north atlantic knipovich ridge to the svalbard margin west of hornsund the crustal structure along a 312 km transect stretching from the axial mountains of the north atlantic knipovich ridge to the continental shelf of svalbard has been obtained using seismic reflection data and wide angle obs data the resulting seismic vp and vs models are further constrained by a 2 d gravity model the principal objective of this study is to describe and resolve the physical and compositional properties of the crust in order to understand the processes and creation of oceanic crust in this extremely slow spreading counterpart of the north atlantic ridge systems vp is estimated to be 3 50 6 05 km s for the upper oceanic crust oceanic layer 2 with a marked increase away from the ridge the measured vp of 6 55 6 95 km s for oceanic layer 3a and 7 10 7 25 km s for layer 3b both with a vp vs ratio of 1 81 except for slightly higher values at the ridge axis does not allow a clear distinction between gabbro and mantle derived peridotite 10 40 serpentized the thickness of the oceanic crust varies a lot along the transect from the minimum of 5 6 km to a maximum of 8 1 km the mean thickness of 6 7 km for the oceanic crust is well above the average thickness for slow spreading ridges 10 mm year half spreading rate the areas of increased thickness could be explained by large magma production rates found in the zones of axial highs at the ridge axis which also have generated the off axial highs adjacent the ridge we suggest that these axial and off axial highs along the ridge control the lithological composition of the oceanic crust this approach suggests normal gabbroic oceanic crust to be found in the areas bound by the active magma segments the axial and off axial highs and mantle derived peridotite outside these zone

petrogenesis of early cretaceous alkaline dolerite dykes in the dabie orogen china constraints on the timing of lithospheric thinning abstract the dabie orogen in central china has long been proposed to transform from triassic compression related to northward deep subduction to late jurassic to early cretaceous extension involving marked crustal thinning and doming processes zircon u pb ages mineral chemistry and whole rock geochemistry of alkaline dykes intruding early cretaceous intermediate mafic ultramafic complexes are presented herein to provide evidence for the timing of lithospheric thinning and extension these dykes of dominantly dolerites with minor monzodiorites have zircon u pb ages of 128 3 129 7 ma and are characterized by sr nd isotopic ratios around the enriched mantle i the dolerites can be divided into two groups with the first group having lower incompatible elements but more enriched sr nd isotopes than the second group equilibrium partial melting and isotopic mixing modeling show that the dolerites were first generated from partial melting of a heterogeneous enriched mantle at different degrees and then experienced fractional crystallization and crustal assimilation systematic normative minerals trace elements and isotopic contrasts of early cretaceous gabbros 125 130 ma dolerite dykes of this study and early cretaceous mafic dykes 111 115 ma indicate that the source regions for the dolerites are deeper than those for the host gabbros therefore the lithospheric mantle thickening probably continued until the formation of the dolerites and the lithospheric mantle thinning occurred significantly later than previously proposed the thickness of the lithosphere was more than the depth range for the source region of alkaline basalts at 125 130 ma but changed to less than that for the source region of tholeiitic basalts before 115 ma suggesting a minimum decrease of 20 km within 10 m y this rate of thinning can be explained only by delamination of voluminous crustal root with the underlying lithospheric mantle between 125 and 115 ma

continent ocean transition on the voring plateau ne atlantic derived from densely sampled ocean bottom seismometer data 1 a 180 km long seismic wide angle profile was acquired across the voring plateau ne atlantic using a tuned air gun array and three component ocean bottom seismometers deployed with 5 km spacing the seismic p wave data have been modeled by ray tracing inversion and the model has been constrained by s wave and gravity modeling the data and modeling have allowed us to depict the crustal structure and nature of the continent ocean transition cot in a classical volcanic margin case the p wave velocity near the top of the main crustal layer is estimated to 6 0 km s landward of the 25 km wide cot the seaward increase to 6 5 km s in the cot is conformable with heavily intruded continental crust within this zone farther seaward the velocity increase to 6 9 km s in the same layer suggests the presence of oceanic crust the abundant magmatism landward of and within the cot is primarily observed as extrusives forming wedges of seaward dipping reflectors and mafic lower crustal intrusions underplating the maximum thickness of the oceanic crust is measured to 23 5 km at the seaward termination of the cot

crustal origin of early proterozoic syenites in the congo craton ntem complex south cameroon two petrographically different syenite suites can be distinguished in the mostly archean ntem complex of cameroon a suite of two pyroxene syenites and a suite of clinopyroxene syenites the syenites occur as discrete intrusions surrounded by archean gneisses based on structural relationships the syenites are of late to post orogenic origin the pb pb zircon evaporation ages indicate that the syenites intruded at ca 2 3 ga and thus postdate significantly the archean episode of crust formation and differentiation that generated the surrounding gneiss terrane the isotope signatures and the trace elements provide unambiguous evidence that the two syenite suites are of crustal origin the syenite suites do not show simple chemical trends have highly variable trace element contents and display only a small variation in their major element contents particularly in their sio2 contents the isotope characteristics including 143nd 144nd 87sr 86sr and common pb point to a heterogeneous source or multiple sources for the syenites the sr isotope compositions calculated for the time of intrusion vary over a wide range from an unrealistically low 87sr 86sr ratio of 0 690 to high values of 0 713 in contrast the initial 143nd 144nd ratios are more homogenous with end 2320 from 5 8 to 7 5 the pb isotopes from leached k feldspars indicate that the sources of the syenites had a history of u depletion and th u enrichment the syenites show the same isotope characteristics as the surrounding gneiss terrane thus the deep continental crust of the ntem complex itself is the most likely source for the syenite melts this conclusion is also supported by the heterogeneity of the trace elements in the syenites that exclude a homogenous source in addition inherited zircons from the syenites yield pb pb evaporation ages that are consistent with ages obtained for zircons extracted from the surrounding gneisses in contrast to many other syenites worldwide the syenites in the ntem complex do not contain juvenile mantle material but are the sole products of crustal reworking thus the syenites did not contribute to the crustal growth in the ntem complex however this conclusion may not preclude the possibility that the syenites were generated in the deep crust during a time of addition of mantle material to the continental crust the cause for the generation of these large syenite bodies in the ntem complex is most likely an episode of deep seated thermal activity that was so far not known from this part of the congo craton

ree fractionation and nd isotope disequilibrium during crustal anatexis constraints from himalayan leucogranites abstract the processes controlling rare earth element ree behaviour during crustal anatexis have profound implications not only for ree abundances of crustally derived granites but also for their model nd ages for typical protoliths of anatectic granites such as are exposed in metapelites from the high himalayan crystalline series hhcs a high proportion 90 of bulk rock lree and mree are sited in the accessory phases apatite and monazite and of hree in garnet and apatite mass balance constraints indicate that observed ree abundances of himalayan leucogranites cannot be generated from ree deficient reactant phases in their metasedimentary protoliths muscovite biotite plagioclase and quartz but must result from the dissolution of accessory phases ree and th abundances in anatectic melts are modelled by combining empirical expressions for monazite apatite and zircon solubilities as a function of temperature melt composition and melt water content with analysed accessory phase ree abundances this approach which allows the contribution to the melt from each accessory phase to be identified successfully predicts himalayan leucogranite trace element abundances the lree and th concentrations of a metapelite derived melt are buffered by monazite stability although apatite dissolution contributes up to 10 of lree to the melt together with 50 of the mree and 90 of the hree zircon contributes a negligible proportion of ree to the melt during anatexis dissolution of monazite and apatite in the melt results in 1 increasing ree abundances and lree hree ratios with increasing temperature of melting 2 a negative eu anomaly in the melt 3 high sm nd ratios in the melt relative to its metasedimentary protolith and 4 nd isotope disequilibrium between the melt and its protolith all of these features are observed in the ree geochemistry of himalayan leucogranites the combined effects of sm nd fractionation and nd isotope disequilibrium during anatexis imply that model nd ages calculated for anatectic granites derived from apatite and monazite bearing protoliths should be treated with considerable caution

neogene patagonian plateau lavas continental magmas associated with ridge collision at the chile triple junction extensive neogene patagonian plateau lavas 46 5 to 49 5 s southeast of the modern chile triple junction can be related to opening of asthenospheric slab windows associated with collisions of chile rise segments with the chile trench at 12 ma and 6 ma support comes from 26 new total fusion whole rock 40ar 39ar ages and geochemical data from back arc plateau lavas in most localities plateau lava sequences consist of voluminous tholeiitic main plateau flows overlain by less voluminous 2 to 5 million year younger alkalic postplateau flows northeast of where the ridge collided at 12 ma most lavas are syncollisional or postcollisional in age with eruptions of both sequences migrating northeastward at 50 to 70 km ma plateau lavas have ages from 12 to 7 ma in the western back arc and from 5 to 2 ma farther to the northeast trace element and isotopic data indicate main plateau lavas formed as larger percentage melts of a garnet bearing oceanic island basalt oib like mantle than postplateau lavas the highest percentage melts erupted in the western and central plateaus in a migrating slab window model main plateau lavas can be explained as melts that formed as upwelling subslab asthenosphere which flowed around the trailing edge of the descending nazca plate and then interacted with subduction altered asthenospheric wedge and continental lithosphere alkaline postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window thermal problems of high pressure melt generation of anhydrous mantle can be explained by volatiles h2o and co2 introduced by the subduction process into slab window source region s an oib like rather than a mid ocean ridge basalt morb like source region and the lack of magmatism northeast of where ridge collision occurred at 13 to 14 ma can be explained by entrainment of weak plume s or regional variations in an ambient oib like asthenosphere

a petrogenetic study of anorogenic felsic magmatism in the cretaceous paresis ring complex namibia evidence for mixing of crust and mantle derived components abstract paresis is one of a group of cretaceous ring complexes extending from the coast some 350 km ne across the damara belt it consists of over 90 rhyolites and comendites with subordinate intrusions of quartz syenite and alkali feldspar syenite these felsic units are accompanied by very minor amounts of silica undersaturated basalt phonolite and lamprophyre located near the edge of the congo craton paresis is the farthest inland of any felsic complex in the cretaceous ring complex group the other ring complexes in the vicinity consist of carbonatites and undersaturated alkaline rocks e g okorusu kalkfeld etaneno ondurakorume geochemical and isotopic data provide evidence for both mantle and crustal components in the sources of the paresis magmas the alkaline basalts phonolite and lamprophyre have overlapping sr and nd isotopic initial ratios which plot within the mantle array close to bulk earth values e nd 0 9 to 2 8 and 87 sr 86 sr i 0 7042 0 7054 and may indicate a mantle plume component the rhyolite units comprise metaluminous feldspar rhyolites and peraluminous more differentiated quartz feldspar rhyolites both units show prominent negative nb and ta anomalies on mantle normalized multielement diagrams and have extremely nonradiogenic nd ratios e nd 21 and sr initial ratios of 0 7117 0 7138 these isotopic values suggest a crustal origin from pre damara early precambrian gneisses and granitoids which are exposed in the congo craton and related inliers in northern namibia the comendites are peralkaline highly differentiated rocks in contrast to the rhyolites comendites lack mantle normalized nb and ta anomalies have higher hfse and lree contents extreme negative eu anomalies and e nd values of 11 like the comendites alkali feldspar syenites are peralkaline lack nb and ta anomalies have negative eu anomalies and high hfse concentrations their e nd values are 6 5 to 8 the quartz syenites overlap with the peralkaline units in isotopic composition but they are chemically very different they are metaluminous have a low degree of differentiation and have prominent negative mantle normalized nb and ta anomalies like the rhyolites the sr nd isotopic composition and incompatible element ratios of the comendites quartz syenites and alkali feldspar syenites are intermediate between those of the rhyolites and basalts precluding a simple origin by basalt fractionation or crustal melting instead the data indicate that the comendite and syenites formed from hybrid magmas with both crustal and mantle derived components the metaluminous quartz syenites can be modelled by assimilation of pre damara crust by basaltic magma the peralkaline nature and trace element characteristics of comendite and alkali feldspar syenites on the other hand require an alkaline and hfse enriched basic endmember whose fractionated equivalent may be the paresis phonolite

three dimensional density model of the nazca plate and the andean continental margin 1 we forward modeled the bouguer anomaly in a region encompassing the pacific ocean 85 w and the andean margin 60 w between northern peru 5 s and patagonia 45 s the three dimensional density model that reproduces the gravity field is a continental scale representation of density structure to 410 km depth that characterizes the mantle and crust of the oceanic nazca plate subducted slab and continental margin with a minimum number of bodies we predefined the density of each body after studying the dependency of density on composition of crustal and mantle materials and pressure temperature conditions appropriate for the andean setting a database of independent geophysical information constrains the geometry of the top of the subducted slab locally the moho of the oceanic and continental crusts and indirectly the lithosphere asthenosphere boundary underneath the continental plate other boundaries notably the intracrustal density discontinuity separating upper from lower crust below the continent were not constrained and their geometry is the result of fitting the observed and calculated bouguer anomaly during forward modeling this contribution presents the model to the andean geoscientific community and contains some tools like a sensitivity analysis that helps potential users of the model to interpret its results we describe and discuss some of these results in order to illustrate the application of the model to the study of a wide range of phenomena e g modification of oceanic plate structure by hot spots shape of the subducted slab thermal structure of the continental lithosphere compensation mechanism and formation of orogenic relieve causes of andean segmentation

k ar dating young rhyolitic rocks a case study of the sierra la primavera jalisco mexico this study demonstrates the feasibility of dating by the k ar method an eruptive sequence of silicic volcanic rocks with ages close to the limits of conventional 14 c dating forty six conventional k ar analyses of sanidine and glass from rhyolites of the sierra la primavera mexico yielded dates of 5 to 150 ka thousand years one sigma errors range from 3 to 39 ka although the samples contain small amounts of extraneous argon most of the dates are consistent with relative ages determined in the field the exceptions are dates on immediately postcaldera eruptive units which are older than the dates for the caldera producing ash flow tuff a similar result has been obtained on dating tuffs and lavas of some other pleistocene calderas this effect is attributed to incomplete outgassing of the larger amount of extraneous argon dissolved in magma that occupied a position deeper in the chamber than the ash flow tuff magma and that erupted immediately after caldera collapse we present two new error equations one a simple extension of the commonly used equation demonstrates the importance of including a term for the uncertainty in the spectrometer mass discrimination factor in the error equation the second equation is a more rigorous formulation of the propagation of errors in the calculation of the age it yields meaningful error estimates for all proportions of radiogenic 40 ar and because it expresses the error in absolute rather than relative terms it can be used to put an upper limit on the age of a sample when no radiogenic 40 ar has been resolved

the serra da graciosa a type granites and syenites southern brazil part 2 petrographic and mineralogical evolution of the alkaline and aluminous associations abstract the serra da graciosa granites and syenites comprise five distinct plutons in the brasiliano pan african graciosa a type province southern brazil six petrographic series can be identified in these plutons 1 alkaline series 1 composed of amphibole bearing alkali feldspar syenites with varied mafic mineralogy and quartz contents from alkali feldspar syenites with calcic amphibole clinopyroxene olivine and allanite to alkali feldspar quartz syenites with sodic calcic amphibole and chevkinite perrierite and to alkali feldspar granites with sodic amphibole 2 alkaline series 2 characterized by amphibole bearing alkali feldspar granites with limited modal variations but amphibole compositions also varying from calcic to sodic 3 alkaline series 3 of limited occurrence which includes alkali feldspar syenites with olivine and clinopyroxene and no amphibole 4 aluminous series 1 of widespread occurrence with various petrographic facies of biotite granites with amphibole 5 aluminous series 2 characterized by alkali feldspar granites with biotite and only minor amphibole 6 monzodiorites typically with biotite calcic amphibole and augitic clinopyroxene partially mingled with granitic magmas the mafic minerals present are in general fe rich with correspondingly low mg and al contents in alkaline series 1 amphiboles crystallized in progressively more oxidizing and alkaline conditions while in alkaline series 2 the initial conditions were somewhat more oxidizing and shifted to reducing in the final stages in aluminous series 1 and aluminous series 2 amphiboles are calcic and comparatively homogeneous the amphiboles in the monzodioritic rocks while also homogeneous are more mg rich and show compositions quite distinct from the calcic varieties in the other associations and this is also the case for clinopyroxene mg in biotite decreases from the monzodioritic rocks to aluminous series 1 and further to aluminous series 2 contrasting evolution of the various associations suggests that several coeval magmatic series are present in the serra da graciosa granites

shear wave velocity structure of the shallow sediments in the bohai sea from an ocean bottom seismometer survey abstractto investigate the seismic velocity structure of the shallow sediments in the bohai sea of china we conducted a shear wave velocity inversion of the surface wave dispersion data from a survey of 12 ocean bottom seismometers obss and 377 shots of a 9000 in3 air gun with obs station spacing of approximately 5 km and air gun shot spacing of approximately 190 m high quality scholte wave data were recorded by the obss within 0 4 5 km offset we retrieved the scholte wave phase velocity dispersion for the fundamental mode and first overtone in the frequency band of 0 9 3 0 hz with the phase shift method and inverted for the shear wave velocity structure of the shallow sediments with a damped iterative least squares algorithm pseudo 2d shear wave velocity profiles with a depth of approximately 400 m revealed coherent features of relatively weak lateral velocity variation we also estimated the uncertainty in shear wave velocity structure based on the pseudo 2d profiles from six trial inversions wi

partial melting of slab window margins genesis of adakitic and non adakitic magmas abstract when a mid ocean spreading ridge subducts it typically splits apart at depth to form two tapered slab edges separated by asthenospheric mantle within a slab window we examine the fate of the slab edges using simplified slab window geometries specific thermal parameters and assumptions regarding shear stress and slab hydration six fundamental zones of slab anatexis are identified and classified according to expected melt and restite compositions non adakitic melts of granodioritic to tonalitic composition are generated along the plate edges at depths of 5 65 km whereas adakitic melts form proximal to the edges at depths of 25 90 km as anatexis proceeds the subducted crust is converted to a migmatite of slab melt and eclogitic restite much of the migmatite may shear away from the slab and become incorporated into the mantle the melts will rise and leave behind fragments of restite within mantle peridotite if the peridotite is part of the overriding plate then the restite fragments may become long term residents of the continental lithospheric mantle however if the restite becomes entrained in the asthenosphere it may undergo upwelling and melting or flow away as ductile streaks to become long term mantle heterogeneities slab windows are thereby identified as important sites for slab melting and geochemical replenishment of the mantle

crustal composition of the more margin and compilation of a conjugate atlantic margin transect highlights the basement at the mid norwegian more margin is dominantly felsic in composition a lower crustal body is interpreted as a mixture of continental blocks and eclogite the thickness of the outer lower crustal body is twice as thick on the east greenland margin the thinning during this first phase of post caledonian extension was highest for proto norway abstract the inner part of the volcanic passive more margin mid norway expresses an unusual abrupt thinning from high onshore topography with a thick crust to an offshore basin with thin crystalline crust previous p wave modeling of wide angle seismic data revealed the presence of a high velocity 7 7 8 0 km s body in the lower crust in this transitional region these velocities are too high to be readily interpreted as early cenozoic intrusions a model often invoked to explain lower crustal high velocity bodies in the region we present a vp vs model derived from the modeling of wide angle seismic data acquired by use of ocean bottom seismograph horizontal components the modeling suggests dominantly felsic composition of the crust an average vp vs value for the lower crustal body is modeled at 1 77 which is compatible with a mixture of continental blocks and caledonian eclogites the results are compiled with earlier results into a transect extending from onshore norway to onshore greenland back stripping of the transect to early cenozoic indicates asymmetric conjugate magmatism related to the continental break up further back stripping to the time when most of the caledonian mountain range had collapsed indicates that the thinning during the first phase of extension was about 25 higher for proto norway than proto greenland

a regional shear wave velocity model in the central voring basin n norway using three component ocean bottom seismographs abstract a regional ocean bottom seismometer obs dataset was acquired in the central and northern part of the voring basin n norway in 1992 thirty three component obss were deployed along seven regional profiles and the shear waves recorded on the horizontal components have been modelled by use of 2 d ray tracing utilizing an established p wave model in the shallow sediments 1 4 km depth mean vp vs ratios of 2 1 2 2 and 2 3 2 5 are inferred along the nw se and ne sw trending profiles respectively the relatively high mean vp vs ratios in this interval are probably related to unconsolidated sediments in the uppermost layers the azimuthal variation in the vp vs ratios with the fast direction nw se indicates a shear wave anisotropy of 5 10 in this interval consistent with observations further north in the lofoten area in the 5 10 km depth interval high velocity sill intrusions in the sediments are mapped by the p wave modelling these high velocity intrusions act as efficient conversion interfaces confirmed by the shear wave modelling a regional decrease in the vp vs ratios to 1 7 1 8 are observed at 4 6 km depth possibly related to sand rich sediments in the middle cretaceous the upper crust is clearly mapped by the shear waves and the inferred vp vs ratio of 1 75 confirms that the crust in the voring basin is of continental origin an increase of the vp vs ratio 1 8 1 85 in the lower crust indicates a change in its composition possibly explained by a mixing of continental crust and magmatic underplating

the chon aike province of patagonia and related rocks in west antarctica a silicic large igneous province the field occurrence age classification and geochemistry of the mesozoic volcanic rocks of patagonia and west antarctica are reviewed using published and new information dominated by rhyolitic ignimbrites which form a bimodal association with minor mafic and intermediate lavas these constitute one of the largest silicic igneous provinces known equivalent in size to many mafic lips diachronism is recognized between the early middle jurassic volcanism of eastern patagonia marifil and chon aike formations and the middle jurassic earliest cretaceous volcanism of the andean cordillera el quemado ibanez and tobifera formations this is accompanied by a change in geochemical characteristics from relatively high zr and nb types in the east to subalkaline arc related rocks in the west although the predominance of rhyolites remains a constant factor all of the associated mafic rocks are well fractionated compared to direct mantle derivatives petrogenetic models favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidification and remelting the formation of large amounts of intracrustal silicic melt acted as a density barrier against the further rise of mafic magmas which are thus rare in the province

high precision u pb zircon dating of the chalten plutonic complex cerro fitz roy patagonia and its relationship to arc migration in the southernmost andes 1 we report new high precision u pb ages and geochemical data from the chalten plutonic complex to better understand the link between magmatism and tectonics in southern patagonia this small intrusion located in the back arc region east of the patagonian batholith provides important insights on the role of arc migration and subduction erosion the chalten plutonic complex consists of a suite of calc alkaline gabbroic to granitic rocks which were emplaced over 530 kyr between 16 90 0 05 ma and 16 37 0 02 ma a synthesis of age and geochemical data from other intrusions in patagonia reveals a striking similarities between the chalten plutonic complex and the neogene intrusions of the batholith and differences to other back arc intrusions such as torres del paine b a distinct e w trend of calc alkaline magmatic activity between 20 and 17 ma we propose that this trend reflects the eastward migration of the magmatic arc and the consistent age pattern between the subduction segments north and south of the chile triple junction suggests a causal relation with a period of fast subduction of the farallon nazca plate during the early miocene previously proposed flat slab models are not consistent with the present location and morphology of the southern patagonian batholith we advocate alternatively that migration of the magmatic arc is caused by subduction erosion due to the increasing subduction velocities during the early miocene

petrogenesis of syenite granite suites from the bryansky complex transbaikalia russia implications for the origin of a type granitoid magmas abstract two syenite granite suites metaluminous and peralkaline that form the bryansky complex in transbaikalia russia have been studied with the aim to constrain the existing models of a type granitoid magma generation the bryansky complex is a large intrusive body of about 1600 km2 emplaced in the central part of the mongolian transbaikalian granitoid belt which extends for more than 2000 km and is 200 300 km wide the belt comprises about 350 a type granitoid plutons and numerous volcanic fields u pb and rb sr isotope dating revealed that all the intrusive rocks of the complex and closely associated comendites were emplaced within a narrow time span 279 285 ma the isotope characteristics are rather similar for all main rock types the metaluminous suite has a 87sr 86sr t value of 0 7050 0 001 end t from 1 9 to 3 0 and the peralkaline suite has 87sr 86sr t 0 7053 0 0008 end t 2 1 and 2 4 comendites and trachyandesites have similar end t values from 2 2 to 3 5 but a slightly higher 87sr 86sr t value of 0 7062 0 0002 the systematic change in chemical and mineralogical composition from syenitic to granitic rocks in both suites and the similar isotopic ratios suggest that the granites were formed by fractional crystallization of the syenite magmas several lines of evidence suggest that metaluminous syenite is the parental magma for the whole bryansky complex study of melt inclusions in quartz phenocrysts from the peralkaline granite and in pyroxene from the nordmarkite indicates that fractional crystallization has resulted in significant f enrichment in the granitic magma up to 1 5 1 7 wt the syenite magmas crystallized at rather high temperature 940 c whereas the near liquidus temperature of the peralkaline granite was lower 760 790 c very high homogenization temperatures of the melt inclusions in quartz phenocrysts from comendites 1000 1100 c suggest that the alkali rich silicic magma formed at a depth of 50 60 km far exceeding the normal crust thickness the sr nd isotope data advocate the main role of mantle derived material in the source region from which the alkali rich syenitic and granitic magmas were produced

crust mantle interaction in the evolution of the ilimaussaq complex south greenland nd isotopic studies sm nd isotopic compositions were determined for the peralkaline ilimaussaq complex of the gardar province of southern greenland the majority of the samples in the agpaitic and augite syenitic units have near chondritic initial nd 0 whereas a few samples trend towards nd values as low as 6 at the time of intrusion 1143 ma this latter value from a sample taken from the margin of the complex lying on the evolutionary trend for ketilidian country rock granitoids suggests that large scale contamination took place only at the margins of the complex the similarity of the nd isotopic compositions of the augite syenite and agpaitic units suggests that their parental magmas were derived from the same reservoir a comparison of the nd with existing sr and hf isotopic data for the complex suggests an origin by combined assimilation fractionation processes assimilation fractional crystallization modeling of the isotopic compositions indicates that the ilimaussaq magmas could have formed through fractional crystallization of a basaltic melt while assimilating granitic crust the model requires initially higher assimilation rates from basalt to augite syenite composition with subsequent decreasing assimilation rates from augite syenite to agpaitic compositions alkali granites which formed after the intrusion of the augite syenites have isotopic compositions intermediate between those of the augite syenites and the surrounding ketilidian basement this implies even greater amounts of assimilation and is interpreted as evidence for an origin through fractionation of a basaltic or augite syenite magma with concurrent assimilation of ketilidian crust

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

magmatic expression of tectonic transition from oceanic subduction to continental collision insights from the middle triassic rhyolites of the north qiangtang block abstract the tectonic transition from subduction to collision is a fundamental process during orogenesis yet the magmatic expression of this transition and related deep geodynamic processes remain unclear this study focuses on a newly identified volcanic belt within the moyun zaduo sulu area of the north qiangtang block and presents new zircon u pb data that indicate that this belt formed during the middle triassic 247 241 ma a time characterized by a regional transition from subduction to collisional tectonism the volcanic belt is located to the south of a permian to early triassic arc and is dominated by high k calc alkaline and peraluminous rhyolites these rhyolites have low mg nb ta and eu values contain low contents of sr have high rb sr and whole rock end t values and show positive zircon ehf t values all of which suggest that they formed from magmas generated by the dehydration melting of juvenile crustal material the migration of middle triassic volcanism in this region was most likely caused by rollback of the subducting longmucuo shuanghu tethyan oceanic slab combining our new data with previously published results of numerical modeling of subduction collisional processes and regional data from north central tibet yields insights into the magmatic expressions and related deep geodynamics of the transition from oceanic subduction to continental collision this combination of data also suggests that variations in oxygen fugacity can be used as a proxy for the discrimination of magmatism related to subduction the transition from subduction to collision and collisional tectonism