volcano ice sea interaction in the cerro santa marta area northwest james ross island antarctic peninsula abstract we present here the results of detailed mapping lithofacies analysis and stratigraphy of the neogene james ross island volcanic group antarctic peninsula in the cerro santa marta area northwest of james ross island in order to give constraints on the evolution of a glaciated volcanic island our field results included recognition and interpretation of seventeen volcanic and glacial lithofacies together with their vertical and lateral arrangements supported by four new unspiked k ar ages this allowed us to conclude that the construction of the volcanic pile in this area took place during two main eruptive stages eruptive stages 1 and 2 separated from the cretaceous bedrock and from each other by two major glacial unconformities u1 and u2 the u1 unconformity is related to antarctic peninsula ice sheet expansion during the late miocene before 6 2 ma and deposition of glacial lithofacies in a glaciomarine setting following this glacial advance eruptive stage 1 6 2 4 6 ma volcanism started with subaerial extrusion of lava flows from an unrecognized vent north of the study area with eruptions later fed from vent s centered at cerro santa marta volcano where cinder cone deposits and a volcanic conduit lava lake are preserved these lava flows fed an extensive 7 km long hyaloclastite delta system that was probably emplaced in a shallow marine environment a second unconformity u2 was related to expansion of a local ice cap centered on james ross island which truncated all the eruptive units of eruptive stage 1 concomitant with glacier advance renewed volcanic activity eruptive stage 2 started after 4 6 ma and volcanic products were fed again by cerro santa marta vents we infer that glaciovolcanic eruptions occurred under a moderately thin 300 m glacier in good agreement with previous estimates of paleo ice thickness for the james ross island area during the pliocene

direct dating of mid crustal shear zones with synkinematic allanite new in situ u th pb geochronological approaches applied to the mont blanc massif dating the timing of motion on crustal shear zones is of tremendous importance for understanding the assembly of orogenic terranes this objective is achieved in this paper by combining petrological and structural observations with novel developments in in situ u th pb geochronology of allanite a greenschist facies shear zone within the mont blanc massif is documented allanite is synkinematic and belongs to the mylonitic assemblage la icp ms u th pb isotope analyses of allanite reveal high contents and highly radiogenic isotopic compositions of the common pb component the use of measured pb isotope compositions of associated minerals feldspars and chlorite is critical for accurate common pb correction and provides a powerful mechanism for linking allanite growth to the metamorphic assemblage a mean 208pb 232th age of 29 44 0 95 ma is accordingly taken for synkinematic allanite crystallisation under greenschist facies conditions this age reflects the timing of the mont blanc underthrusting below the penninic front and highlights the potential of directly dating deformation with allanite

apatite feldspar u pb thermochronometer a reliable mid range 450 c diffusion controlled system the u pb systematics of apatite are commonly dominated by simple volume diffusion without secondary growth and recrystallization that can affect u pb systematics of sphene monazite and zircon although apatite is a relatively low u mineral both u and pb are lattice bound and thus the u pb systematics are not controlled by micro inclusions as appears to be the case for other low u minerals such as garnet and staurolite closure temperature estimates from both experimental and empirical studies range from 425 c to as high as 500 c for typical diffusion radii 25 m and cooling rates 2 100 c ma respectively this range can be exploited to constrain cooling and exhumation histories of both igneous and metamorphic rocks in addition the closure temperature for pb diffusion in apatite fills a gap between that for pb diffusion in sphene 650 c and diffusion of ar in hornblende 500 c muscovite 350 c and biotite 300 325 c apatite is a common accessory phase in many rock types and u pb apatite dating can be a reliable and powerful thermochronometer as it combines mid range cooling ages with the strength of the u pb system in testing for isotopic closure precisions of u pb apatite dates can range from 1 2 to as low as 0 3 for proterozoic or archean dates the main limitation to accuracy of u pb apatite dates is a moderate to low ratio of u to pb and corresponding dependence of concordia coordinates on the choice of initial pb isotopic composition for proterozoic aged samples with measured 206pb 204pb of 100 the range of geologically reasonable initial pb isotopic compositions can shift the calculated u pb age by 60 ma fortunately even though the diffusion rate of pb in alkali feldspar is less than in apatite it appears that the isotopic composition of pb in coexisting feldspar is a good approximation of the initial isotopic composition in apatite for many geologic settings examples are given from an archean terrane in se wyoming that was unroofed and cooled during proterozoic uplift and from proterozoic terranes in sw us that have contrasting thermal histories

airborne gravity reveals interior of antarctic volcano abstract understanding antarctic volcanoes is important as they provide a window on magmatic and tectonic processes of the antarctic plate and contain datable records of ice sheet changes we present the results from the first detailed airborne radar and gravity surveys across james ross island northern antarctic peninsula which is dominated by mt haddington an ice covered miocene recent alkaline stratovolcano the surveys provide new insights into the subsurface structure of the volcano and hence its development which are unavailable from the surface geology alone we show that mt haddington is associated with a significant negative bouguer gravity anomaly 26 mgal which suggests that there has not been significant pooling and solidification of a dense shallow level mafic magma chamber during the growth of the volcano over at least the past 6 m y which is consistent with independent geochemical evidence simple flexural isostatic models cannot explain the localised negative bouguer anomaly 3d modelling techniques show that the negative anomaly is best explained by a shallow low density intra crustal body with its top close to or at the surface although comparable gravity anomalies are commonly associated with large 20 km ash filled calderas as seen at yellowstone or toba there is no geological evidence on james ross island for a similar structure we therefore propose that the james ross island volcanic edifice subsided into the thick underlying pile of relatively soft jurassic and cretaceous sediments which were displaced by low density hyaloclastite breccia the type of deformation envisaged is similar to that associated with concepciou or iwaki volcanoes in south america although mt haddington is much larger

global positioning system measurements of ground deformation caused by magma intrusion and lava discharge the 1990 1995 eruption at unzendake volcano kyushu japan abstract global positioning system gps measurements made around unzendake volcano kyushu japan since january 1991 have detected ground deformation caused by magma intrusion and lava discharge in the intermittent phreatic and phreatomagmatic eruption stage the ground was inflating after growth of the lava dome and following frequent pyroclastic flows at unzendake volcano the ground began deflating these ground deformations are explained by the inflation and deflation of a mogi s source model a point source model located about 6 km west of the active crater at a depth of 11 km at an aseismic region the observed horizontal displacement vectors pointed radially away from the estimated pressure source during inflation and pointed to the pressure source during deflation the horizontal displacements at the reference gps station calculated from contraction of the estimated pressure source coincide well with the actual horizontal displacements observed from other gps baseline systems these observations validate our estimates for the pressure source based on the relation between the deformation volume of the ground surface and the discharged volume of the lava it is estimated that during the eruption there was magma supply from the deeper portion as well as magma discharge at the crater magma is estimated to be supplied to the reservoir at an average rate of 1 1 105 m3 day magma intrusion began in december 1989 at the latest and continued for 1 9 103 days

melt generation during rifting in the north sea in a forthcoming paper mckenzie and o nions1 invert rare earth element concentrations in mid ocean ridge basalts to calculate the variation of melt fraction with depth in the asthenospheric mantle beneath ocean ridges they find that melting persists to greater depths about 80 km than would be predicted by parameterizations of melting experiments2 about 50 km this result has important implications for predicting the volume and composition of melt produced from asthenosphere of normal potential temperature during continental rifting the north sea rift is a good location to test models for melt generation because the physical conditions of rifting are well constrained3 8 using the distribution of melt fraction with depth derived in ref 1 we calculate the volume and composition of melt produced from the asthenosphere in the most stretched part of the north sea the forties region when this melt is mixed in approximately equal proportions with an alkaline melt similar to that produced from metasomatized lithosphere by very small amounts of stretching on the rift flanks it accounts for the volume and the elemental and isotopic composition of the basalts in the most extended region

geochemical and sr nd isotopic constraints on the mantle source of neoproterozoic mafic dikes of the rifted eastern laurentian margin north central appalachians usa abstract abundant and widely distributed unmetamorphosed mafic dikes intrude mesoproterozoic rocks of the new jersey highlands the age of the dikes is imprecisely known but interpreted to fall between 615 and 576 ma which is consistent with the range of ages of mafic dikes from labrador and nova scotia south to pennsylvania that were emplaced along the rifted eastern laurentian margin new jersey highlands dikes are a few cm to 18 m wide and have lengths of as much as several km they have sharp largely discordant contacts against enclosing mesoproterozoic rocks and aphanitic chilled margins that grade into coarser grained interiors columnar joints are present locally and suggest emplacement at a shallow crustal level geochemical compositions of the dikes range from alkalic to less common tholeiitic basalt having generally high tio2 p2o5 zr nb y and la yb and low mgo cr and ni tio2 contents define high ti and low ti dikes that differ in high field strength elements hfse and light rare earth elements lree but overlap in abundances of most other elements dike magma evolved in an ascending mantle plume of oib like asthenosphere from enriched higher tio2 compositions to more depleted lower tio2 compositions subtle differences in the dike compositions are due to variations in the amount of partial melting within the plume and the depth of melt segregation sr nd isotope values of both dike compositions overlap and are characterized by end t of 1 5 to 3 8 and initial 87sr 86sr ranging from 0 7032 to 0 7077 higher sr isotope ratios are interpreted as resulting from local interaction of the dike magma with heterogeneous high 87sr 86sr lithospheric mantle having emi or emii like geochemical characteristics dikes form tabular structures that have long segments striking an average of n44 e and short segments striking about east west their regional geometries form right stepping rhomb shaped patterns due to emplacement into rift related dilational fractures likely formed through a combination of southeast directed extension and strike slip shear stresses geochemical compositions of the dikes are the same regardless of their structural trend or location implying they formed during a single magmatic event they along with other mafic dikes in the north central appalachians were emplaced in a within plate tectonic setting along the rifted margin of eastern laurentia prior to opening of the iapetus ocean

chemical weathering in antarctica an example of igneous rock particles in big lachman lake sediments james ross island jezerni sedimenty velkeho lachmanova jezera ostrov jamese rosse antarktida dominuji jilovite a prachovite castice r zneho petrografickeho slo eni ty jsou sopecneho p vodu a jsou odvozeni p eva n z mendelova souvrstvi intenzita chemickeho zv travani vulkanick ch castic v sedimentech byla vyjad ena chemick m indexem alterace cia kter popisuje p em nu ivc na jilove mineraly pozorovane mineralni asociace montmorillonitu illitu a kaolinitu pravd podobn pochazela z p em ny sopecneho skla a ivc na jilove mineraly mira zv travani vyjad ena pomoci cia z lakustrialnich sediment je podobna jako u spodnojursk ch na ti bohatach vulkanick ch hornin souvrstvi mount poster a st ednojursk ch na ti bohat ch vulkanick ch hornin ze souvrstvi mapple z antarktickeho poloostrova nami studovane sedimenty ukazuji dobrou shodu se slo enim st ednojursk ch ryolit ze souvrstvi mapple a naopak se castecn lisi od slo eni kenozoick ch bazalt vulkanicke skupiny ostrova jamese rosse zdrojov material z jezernich sediment pochazi z antarktickeho poloostrova a byl pravd podobn transportovan p es kanal prince gustava mal mi ledovci v pr b hu neogenu a kvarteru

reply to comment on onset of the movement along the ailao shan red river shear zone constraint from 40 ar 39 ar dating results for nam dinh area northern vietnam by wang et al 2000 journal of asian earth sciences 18 281 292

cretaceous alkaline intra plate magmatism in the ecuadorian oriente basin geochemical geochronological and tectonic evidence abstract small volumes of cretaceous alkaline basaltic magmas have been identified in the sedimentary infill of the ecuadorian oriente foreland basin they are characterized by a restricted range of compositional variation low lile hfse ratios and sr nd isotope values within the range of oceanic island basalts oib reflection seismic data show that a pre existing nne ssw triassic and jurassic rift controls the location and occurrence of these alkaline eruptive sites radiometric ages 40ar 39ar incremental heating method and the biostratigraphic record of their surrounding sediments indicate a nne ssw systematic age variation for the emplacement of this alkaline volcanism from albian 110 5 2 ma in the northern part of the oriente basin to campanian 82 2 2 0 ma in the west central part the geochemical geochronological and tectonic evidences suggest that asthenospheric mantle has upwelled and migrated to the ssw into the region underlying the pre existing triassic and jurassic rift thin spot we propose that subduction was abandoned subsequent to the accretion of allochthonous terranes onto the ecuadorian and colombian margin in the latest jurassic earliest cretaceous causing the relict slab material corresponding to the eastwards directed leading plate to roll back unmodified asthenospheric mantle migrated into the region previously occupied by the slab this resulted in partial melting and the release of magmatic material to the surface in the northern part of the oriente basin since at least aptian times then magmatism migrated along the ssw trending central wrench corridor of the oriente basin during the upper cretaceous probably as a consequence of the lateral propagation of the transpressive inversion of the triassic jurassic rift eventually the late cretaceous east dipping andean subduction system was renewed farther west and the development of the compressional retro foreland oriente basin system halted the cretaceous alkaline magmatic activity

effects of ridge crest trench interaction on antarctic phoenix spreading forces on a young subducting plate precise measurements of spreading rates on marine magnetic profiles collected to the west of the antarctic peninsula have enabled some consideration of the forces governing plate motion since antarctic phoenix motion has been controlled by the local rather than global force balance over the last 35 m y the total effective driving force per unit length of trench is calculated to have ranged between 2 6 and 3 6 1012 n m which is much less than is commonly thought necessary to support subduction conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench the low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed driving forces show a strong correlation with observed spreading rates which indicates that resistive forces were largely velocity dependent fluid migration up the subduction zone may elevate temperatures in and around the shear zone reducing resistive forces below the levels required by purely conductive models changes in convergence rate may affect the depths of both the brittle ductile deformation boundary and the basalt eclogite phase change causing a negative feedback which would appear as a velocity dependent resistive force the different driving forces acting on the ne and sw parts of the phoenix plate as a consequence of older oceanic lithosphere at the trench in the ne caused antarctic phoenix spreading to take place about a near pole to the sw since 21 m y ago and ultimately resulted in disruption of the phoenix plate about 9 m y ago spreading rates decreased abruptly about 6 m y ago probably because of e w compression across the long transform faults bounding the phoenix plate however spreading on the last three segments of the antarctic phoenix ridge continued at least until 4 m y ago either spreading stopped progressively from sw to ne or the final stage took place about a very near pole to the sw a magnetic quiet zone extends up to 95 km from the margin between the tula fracture zone and the north anvers fracture zone and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision the absence of a magnetic quiet zone associated with the most recent ridge crest trench collisions suggests a change in sedimentary regime during the late miocene anomalously fast apparent spreading rates between 23 and 21 m y ago are thought to indicate an error in this part of the magnetic reversal time scale

large volume silicic volcanism along the proto pacific margin of gondwana lithological and stratigraphical investigations from the antarctic peninsula jurassic magmatism in western gondwana produced the most voluminous episode of continental volcanism in the phanerozoic era during the early to middle jurassic some 2 5 3 million km3 of dominantly basalt and to a lesser extent rhyolite were erupted onto a supercontinent in the early stages of break up the major silicic portion of the gondwana magmatic province is exposed in patagonian south america the volcanic rocks of patagonia have been collectively termed the chon aike province and constitute one of the world s most voluminous silicic provinces the volcanic rocks are predominantly pyroclastic dominated by ignimbrite units of rhyolite composition volcanic rocks crop out sporadically across much of the once contiguous antarctic peninsula and are considered to form an extension of the chon aike province a continuation of the province to include the antarctic peninsula would extend its strike length along the active pacific margin by c 2000 km volcanic rocks exposed along the east coast of the antarctic peninsula defined here as the mapple formation are also dominated by rhyolitic ignimbrite flows with individual units up to 80 m in thickness and a total thickness of c 1 km the ignimbrites vary in degree of welding from high grade rheomorphic ignimbrites with parataxitic textures to unwelded lithic rich ignimbrites rhyolite lava flows air fall horizons debris flow deposits and epiclastic deposits are volumetrically minor occurring as interbedded units within the ignimbrite succession the lithology and stratigraphy of the jurassic volcanic rocks of the mapple formation are presented and comparisons are made to the chon aike province a consistent stratigraphy of permo triassic metasedimentary rocks unconformably overlain by terrestrial mudstone siltstone sequences which are in turn conformably overlain by largely silicic subaerial volcanic rocks is present at several localities along the antarctic peninsula and at localities in the chon aike province precise zircon u pb middle jurassic ages exist for two volcanic formations from the antarctic peninsula and a middle lower jurassic age has been suggested for the underlying sedimentary formations based on fossil flora analysis the antarctic peninsula chronostratigraphy coupled with lithological similarities indicate a close relationship to those sequences of the chon aike province

the south american retroarc foreland system the development of the bauru basin in the back bulge province abstract the aim of this research is to understand the tectonic setting of the bauru basin this basin in central eastern south america has been classified as intracratonic but the basin fill geometry the involved subsidence mechanisms and the age of the deposits are poorly understood in this work the ranges of the fossil taxa are analyzed and ages are proposed for the lithostratigraphic units isopach maps were used to reconstruct the stratigraphic intervals of the basin fill the stratigraphy of the bauru basin is compared with that of the adjacent basins and the data are integrated with the available information on south american geodynamics the fossil record indicates that sediment accumulated from the cenomanian to early paleocene beginning after the mochica phase of the andean orogeny the basin fill geometry demonstrates migration of the depocenter through time which occurred simultaneously with migration of the andean basin and immediately after the orogenic events of the peruvian phase we propose that the bauru basin is a component of a retroarc foreland system developed during the early stages of the andean evolution and that it was developed in the back bulge province of this system the andean basin constitutes the foredeep depozone of this foreland system including the potosi oriente acre and maranon basins in addition the upper cretaceous of the parecis and solimoes basins were likely also developed in the back bulge province the thickness of the bauru accumulation indicates that other mechanisms might have overlapped the flexural subsidence in this back bulge province

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

contrasting responses of rb sr systematics to regional and contact metamorphism laramie mountains wyoming usa archean supracrustal sequences of pelitic quartzitic calcareous and mafic compositions in the central laramie mountains wyoming have been affected by two metamorphic events a 1 78 ga amphibolite grade regional metamorphism and a 1 43 ga contact metamorphism resulting from the intrusion of the laramie anorthosite complex lac rb sr whole rock isotopic data from both outside and within the lac contact aureole define a linear array that lies along a 1 78 ga isochron this date has been independently established as the time of amphibolite facies regional metamorphism associated with collision of the archean wyoming province and the proterozoic colorado province along the cheyenne belt the rb sr isotopic data require that sr was redistributed during regional metamorphism on a scale of at least tens of metres although within the 2 km wide aureole of lac the pelitic rocks were thermally metamorphosed at temperatures greater than 800 c none of the whole rock rb sr data from samples within the lac aureole show evidence of resetting at 1 43 ga it is interpreted that the regional metamorphism involved fluid transport which facilitated sr isotopic resetting whereas the contact metamorphism occurred in a relatively dry environment in which isotopic mobility was restricted to centimetre scale or less rb sr data for biotite feldspar and whole rock from a regional metamorphosed pelitic schist give an isochron age of 1450 40 ma which is interpreted as a cooling age resulting from crustal uplift rb sr data for biotite quartz feldspar and whole rock from a pelitic schist affected by contact metamorphism give an isochron age of 1420 43 ma the time of isotopic re equilibration in response either to crustal uplift or to both contact metamorphism and crustal uplift this study demonstrates that although the response of isotopic systems to metamorphism is complex isotopic data provide insight into metamorphic processes that is difficult to obtain by other means

antarctic peninsula continental magnesian andesites indicators of ridge trench interaction during gondwana break up subduction of young hot oceanic lithosphere and a major episode of lithospheric extension have been considered as key tectonomagmatic factors in the genesis of setouchi type continental magnesian andesites we present data on a new discovery of such andesites from the northern part of the antarctic peninsula and propose that subduction of newly created oceanic lithosphere occurred along the proto pacific margin of antarctica during the middle jurassic continental magnesian andesite magmatism was contemporaneous with the early stages of gondwana break up and we suggest that subduction of young hot oceanic lithosphere and ridge trench interaction may have contributed to the disintegration of gondwana by thermally weakening the lithosphere and developing a broad linear extensional zone along the proto pacific margin

titanite rutile thermochronometry across the boundary between the archaean craton in karelia and the belomorian mobile belt eastern baltic shield abstract u pb isotopic dating has been carried out on titanites and rutiles from the karelian protocraton the belomorian mobile belt and the intervening junction zone these are some of the principal archaean crustal units in the baltic shield which have undergone regeneration to various degrees during the palaeoproterozoic palaeoproterozoic resetting of u pb titanite ages was complete in the belomorian belt and almost complete in the junction zone while it hardly affected the karelian protocraton in the latter major crustal cooling occurred at 2 71 2 69 ga after a major igneous event at 2 74 2 72 ga but a tectonothermal event at 2 65 2 64 ga was less comprehensive in the belomorian belt a northeastern marginal zone immediately underlying the collisional thrusting suture of the lapland kola orogen has somewhat higher titanite ages of ca 1 94 1 87 ga than the central zone where these ages range between 1 87 and 1 82 ga comparison between the titanite and rutile u pb ages suggests a postorogenic cooling rate between 2 and 4 ma in these parts of the belt the neoarchaean junction zone between the karelian and belomorian provinces was a zone of particularly intense tectonic magmatic and hydrothermal activity during or after the palaeoproterozoic lapland kola orogeny dominant newly grown titanites in that zone have ages as young as 1 78 1 75 ga and the age differences between the titanite and rutile u pb ages are substantially smaller than elsewhere

u pb sphene dating of metamorphism the importance of sphene growth in the contact aureole of the red mountain pluton laramie mountains wyoming the relative importance of thermal diffusion versus new growth or recrystallization on u pb isotopic data from sphene is assessed through a study of amphibolites and granite gneisses within the contact aureole of the red mountain pluton laramie anorthosite complex wyoming samples were collected along a traverse approximately perpendicular to the margin of the intrusion over a distance of 0 13 to 2 65 km from the contact the 207pb 206pb ages of sphene from amphibolite samples range between 1 43 ga the intrusive age of the red mountain pluton to 1 78 ga the age of regional metamorphism the 207pb 206pb ages of sphene in rocks metamorphosed above 700 c are within error of the intrusive age of the pluton and appear to have resulted from diffusional resetting of preexisting sphene and the metamorphic growth of additional sphene at 1 43 ga at greater distance from the contact the 207pb 206pb ages range from 1 45 to 1 72 ga this 300 million year spread in ages is interpreted to result from two periods of sphene growth one produced during regional metamorphism at 1 78 ga and another generation of newly grown or recrystallized sphene that formed during contact metamorphism at 1 43 ga these two age populations may be identified on the basis of petrographic textures the morphologies and color differences of grain separates as well as by the u pb systematics in rocks metamorphosed to temperatures less than 700 c sphene growth was the dominant process controlling the response of the u pb isotope system to contact metamorphism sphene grew well outside the zone of obvious contact metamorphism the u pb sphene ages were reset by diffusion only at high temperatures supporting the experimentally determined closure temperature estimates for the u pb system in sphene of around 650 c cherniak 1993 this study demonstrates that u pb ages of sphene can be used to date metamorphism not only in areas with a simple geologic history such as igneous intrusion or single metamorphic or deformational events but also to date multiple events so long as different generations of sphene can be identified and separated

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

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 tectono magmatic episodes as revealed by shrimp u pb geochronology and microanalysis of zircon and titanite from the central okcheon belt korea abstract the south central okcheon belt of the southern korean peninsula experienced polyphase granitic magmatism and deformation during the mesozoic this study presents integrated results of ion microprobe u pb geochronology and microtextural and microchemical analyses of zircon and titanite from pre tectonic baegnok and cheongsan and post tectonic boeun and sokrisan plutons with respect to the cheongsan shear zone the baegnok granodiorite sample yielded coherent zircon and titanite 206pb 238u ages of 225 6 1 8 and 223 9 4 4 ma indicating a rapid early cooling of the pluton the cheongsan k feldspar megacryst bearing biotite granite sample yielded a coeval zircon 206pb 238u age of 224 7 1 8 ma on the other hand the titanite grains from this rock are categorized into two age populations corresponding to the late triassic 214 3 7 9 ma and the late cretaceous 96 2 8 2 ma growth the older population is clearly of igneous origin considering partly preserved oscillatory zoning in back scattered electron images therefore the age indicates contrasting early cooling paths of the two pre tectonic plutons the younger population is interpreted to be the product of superimposed metasomatic reactions involving plagioclase ilmenite rutile epidote and calcite the two populations show different patterns in their atomic al fe ratios zircons from the boeun and sokrisan plutons yielded 206pb 238u ages of 172 2 2 1 and 94 2 2 1 ma respectively with the latter being indistinguishable from the younger titanite age of the cheongsan pluton the shrimp u pb age of syn deformational fine grained titanite grains lying on sigmoidal foliations in a mylonite from the cheongsan shear zone has a large error 165 47 ma due to their low u pb ratios possibly affected by post tectonic magmatic activities this study highlights the importance of detailed microanalysis prior to the isotopic dating of samples that have experienced complex tectono magmatic episodes

short paper the significance of mid palaeozoic basement in graham land antarctic peninsula a mid palaeozoic basement to the antarctic peninsula is confirmed by a geochronological study of the gneissose country rock of the mesozoic magmatic arc of eastern graham land rb sr whole rock ages of 410 15 and 426 12 ma from samples of orthogneiss indicate a previously unrecognized episode of granitic magmatism in the antarctic peninsula during the silurian sm nd data on garnet whole rock pairs indicate that subsequent amphibolite facies metamorphism occurred in late carboniferous times probably as a result of plutonism

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

melting of heterogeneous mantle in a slab window environment examples from the middle tertiary masset basalts queen charlotte islands british columbia voluminous basalts dominate a middle tertiary bimodal volcanic assemblage in the queen charlotte islands british columbia canada these moderately enriched mid ocean ridge basalt morb like magmas were associated with significant crustal extension related to the subduction of the pacific farallon ridge and the initiation of a slab window environment which persisted for 35 ma these asthenospheric melts were derived from a low degree of melting of a heterogeneous peridotite source mainly in the spinel stability field as inferred from fractional melting inversion calculations the basaltic magmas in turn gave rise to both tholeiitic and calc alkaline volcanic strata up to 3 5 km thick in local grabens the tholeiitic basalts resemble morb with rare earth element ree patterns ranging from slightly depleted to enriched in light ree lree the calc alkaline basaltic andesites are enriched in lree but are not strongly depleted in nb the rocks have overlapping nd sr and pb isotopic compositions sim

geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the famous region bulk rock major and trace element variations in selected basalts from the famous area in conjunction with a detailed study of the chemical compositions of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationship among the various lava types the distribution of nio in olivine and cr spinel phenocrysts distinguishes the picritic basalts plagioclase phyric basalts and plagioclase pyroxene basalts from the olivine basalts for a given mg mg fe2 atomic ratio of the mineral the nio content of these phenocrysts in the former three basalt types is low relative to that in the phenocrysts in the olivine basalts the zr nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase pyroxene basalts and in addition distinguishes the picritic basalts from the other basalt types these differences indicate that the different magma groups could not have been processed through the same magma chamber and preclude any direct inter relationship via open or closed system fractional crystallization

combined sr nd pb and li isotope geochemistry of alkaline lavas from northern james ross island antarctic peninsula and implications for back arc magma formation abstract we present a comprehensive geochemical data set for a suite of back arc alkaline volcanic rocks from james ross island volcanic group jrivg antarctic peninsula the elemental and isotopic sr nd pb and li composition of these cenozoic basalts emplaced east of the antarctic peninsula is different from the compositions of the fore arc alkaline volcanic rocks in southern shetlands and nearby bransfield strait the variability in elemental and isotopic composition is not consistent with the jrivg derivation from a single mantle source but rather it suggests that the magma was mainly derived from a depleted mantle with subordinate oib like enriched mantle component em ii the isotopic data are consistent with mantle melting during extension and possible roll back of the subducted lithosphere of the antarctic plate magma contamination by triassic early tertiary clastic sediments deposited in the back arc basin was only localized and affected li isotopic composition in two of the samples while most of the basalts show very little variation in 7 li values as anticipated for mantle driven li isotopic composition these variations are difficult to resolve with radiogenic isotope systematics but li isotopes may prove sensitive in tracking complex geochemical processes acting through the oceanic crust pile including hydrothermal leaching and seawater equilibration

a preliminary assessment of the hydrocarbon potential of the larsen basin antarctica abstract the larsen basin on the northwest margin of the weddell sea formed as a mesozoic ensialic basin during gondwana breakup deposition was either in half grabens on the extending weddell sea margin or in a restricted back arc basin at the northern end of this basin 5 6 km of sedimentary rock crop out on james ross island exposing elements of a large potential hydrocarbon system aeromagnetic and outcrop data suggest that the basin structure inferred from james ross island can be recognised at least as far south as 70 s upper jurassic anoxic marine strata deposited prior to the main phase of arc development form a rich potential source t o c up to 3 5 with both marine and terrestrial kerogens arc derived volcaniclastic sediments of barremian oligocene age form a regressive megasequence basal strata represent slope apron and rudaceous submarine fan deposits proximal to the margin fan conglomerates form lenticular bodies hundreds of metres thick and tens of kilometres across enveloped in slope apron mudstones late cretaceous fault reactivation and uplift led to dramatic shallowing of the basin with deposition of shelf facies although there are many potentially attractive reservoir targets there may be problems of pore occlusion due to the abundant labile volcanic grains however there is evidence of more quartzose sandstone towards the top of the section and inferentially toward the basin centre in the larsen basin there is moderate potential for oil generated from upper jurassic source rocks and reservoired in cretaceous and tertiary sandstones and conglomerates in large stratigraphic or structural traps caused by partial basin inversion during deposition

geochemistry and origins of ueno and on take basaltic to andesitic rocks 3 ma produced by distinct contributions of subduction components central japan abstract ueno basaltic rocks 2 7 1 6 ma and on take volcanic rocks 0 75 ma crop out in a restricted area in central japan and were analyzed for major and trace elements and sr and nd isotopes the ueno rocks were erupted as numerous monogenetic volcanoes largely of basaltic compositions whereas the on take lavas form a composite stratovolcano dominated by andesites the ueno basalts are primarily geochemical mixtures of two components 1 sub alkalic common basalts generated from the lithospheric subcontinental mantle with 87sr 86sr 0 706 end 2 and k nb 1000 and 2 rare within plate basalts wpb of alkali basaltic composition with 87sr 86sr 0 705 end 5 k nb 180 subordinate ueno andesites were formed by assimilation fractional crystallization processes within the lower crust having 87sr 86sr 0 709 end 8 but without appreciable assimilation in the upper crust on the other hand the on take basalts have 87sr 86sr 0 705 0 0005 end 2 1 k nb 1200 and geochemical signatures characteristic of continental arc basalts the on take andesites appear to have been contaminated frequently by upper crustal materials it is proposed that the injection of a wpb type diapir yielded partial melting of the subcontinental mantle forming the common ueno basalt magmas the arc type basaltic magmatism which was probably the result of subduction of the pacific plate occurred at 1 ma after the ueno volcanism addition of slab derived components with 87sr 86sr 2300 modified the subcontinental mantle forming the source of on take basalts

dating deposition and low grade metamorphism by in situ upb geochronology of titanite in the paleoproterozoic timeball hill formation southern africa abstract titanite coexists with zircon in a felsic tuff in the paleoproterozoic timeball hill formation southern africa two generations of titanite are preserved euhedral brown crystals with apatite inclusions and colorless matrix filling cement the brown titanite has elevated u th and fe and low al consistent with a magmatic origin whereas the colorless titanite has higher al and f contents suggestive of a metamorphic paragenesis in situ shrimp geochronology of zircons extracted from a polished thin section of the tuff bed suffer from inheritance and pb loss and yield an imprecise date 2259 35 ma whereas coexisting brown titanites give a weighted mean 207pb 206pb age of 2278 7 ma this date is consistent with u pb zircon ages for tuffs in the same stratigraphic unit from a nearby drill hole the brown titanite is interpreted to be magmatic and to provide a reliable estimate for the age of deposition in situ u pb dating of matrix filling titanite cement from the same stratigraphic interval yields an age of 2145 12 ma which corresponds with previous estimates for a low grade tectonothermal event in southern africa our results demonstrate that titanite is a versatile chronometer that can be used to constrain depositional ages and those of low grade metamorphism thus increasing the number of sedimentary rocks that can be dated and elucidating the low temperature geological history of depositional basins

role of strike slip faulting in the tectonic evolution of the antarctic peninsula the antarctic peninsula mesozoic magmatic arc has had a long history of dextral strike slip deformation the deformation was initially associated with the development of a wide accretionary complex by the migration of fore arc slivers and the formation and inversion of a thick fore arc basin succession it also formed an important component within major shear zones in the arc and may have controlled the formation of sedimentary basins in the back arc region although some transcurrent motion within the fore arc region was related to a component of oblique subduction the main movement occurred during the breakup of gondwanaland and the formation of a major transtensional rift system a new reconstruction for this part of gondwanaland is presented taking this transcurrent motion into consideration

δd and δ18o variations of the magmatic system beneath deception island volcano antarctica implications for magma ascent and eruption forecasting abstract geochemistry of volatiles in active volcanoes provides insights into the magmatic processes and evolution at depth such as magma evolution and degassing which can be implemented into volcanic hazards assessment deception island is one of the most active volcanoes in antarctica with more than twenty explosive eruptions documented over the past two centuries hydrogen and oxygen isotopic variations in the volatiles trapped in the deception island rocks glass and melt inclusions in phenocrysts provide essential information on the mechanisms controlling the eruptive history in this volcanic suite thus understanding the petrological and related isotopic variations in the island has the potential to foresee the possible occurrence and its main eruptive features of a future eruption the new isotopic results of this study combined with exhaustive petrologic knowledge reveal in deception island i fast ascent and quenching of most magmas preserving pre eruptive magmatic signal of water contents and isotopic ratios with local modification by rehydration due to glass exposition to seawater and by meteoric and fumarolic waters ii a plumbing system s variable with time and currently dominated by closed system degassing leading to explosive eruptions iii control on the interactions of ascending magmas with the surface waters producing hydrovolcanic activity throughout the two main fault systems in deception island and at different magma stagnation depths these results can be considered in further studies of volcanic monitoring to improve the capability to interpret geophysical data and signals recorded during volcanic unrest episodes and hence forecast volcanic eruptions and related hazards

electron probe ultrachron microchronometry of metamorphic monazite unraveling the timing of polyphase thermotectonism in the easternmost wyoming craton black hills south dakota a metapelite from the easternmost wyoming craton black hills south dakota has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800 1700 ma assembly of southern laurentia three deformational fabrics are recognized in oriented thin sections an ene trending s 1 fabric preserved as oblique inclusion trails in garnet porphyroblasts a nnw trending s 2 fabric preserved as microlithons in the rock matrix and a flattening fabric s 3 which transposed s 1 s 2 and dominates the matrix a complex monazite porphyroblast has been analyzed in situ with the electron microprobe ultrachron to constrain the timing of s 1 s 3 fabric formation associated with monazite growth the core of this grain uniquely preserves the s 1 s 2 fabrics as sigmoidal inclusion trails the mean total pb age of this domain is 1750 10 ma all dates reported at 95 confidence n 39 spots which is equivalent to the published 2 0 7 pb 2 0 6 pb age for the same domain these results validate the total pb dating method in general and the ultrachron in particular for reliable age determination in low th monazite and are interpreted as 1750 ma minimum ages for the s 1 s 2 fabrics and sequential d 1 d 2 collisional events that imposed them n directed arc accretion and e w continental collision respectively a higher th y rim of this same rosetta grain truncates the s 1 s 2 sigmoid and is associated with resorption textures in garnet porphyroblasts coupled release of y and an s 3 fabric that pervasively overprinted s 1 s 2 in the rock matrix the mean ultrachron date of this domain is 1692 5 ma n 17 spots which is slightly younger that the published isotopic age for all monazite rims combined these results support a 1715 1690 ma timeframe for localized doming d 3 related to granite magmatism the onset of which has been dated independently at 1715 3 ma the timing of post d 3 cooling through 350 and 300 c is constrained by 4 0 ar 3 9 ar dates of 1610 and 1480 ma obtained for separates of d 3 matrix muscovite and biotite respectively which are interpreted as closure ages this study shows that fabrics in poly deformed rocks can be dated by linking monazite spot ages to key microtextures further the results of this micrometer scale study enhance previous knowledge of local thermotectonism black hills and regional terrane assembly laurentia

in situ la icpms trace elements and u pb analysis of titanite from the mesozoic ruanjiawan w cu mo skarn deposit daye district china abstract in this paper we present u pb ages and trace element compositions of titanite from the ruanjiawan w cu mo skarn deposit in the daye district eastern china to constrain the magmatic and hydrothermal history in this deposit and provide a better understanding of the u pb geochronology and trace element geochemistry of titanite that have been subjected to post crystallization hydrothermal alteration titanite from the mineralized skarn the ore related quartz diorite stock and a diabase dike intruding this stock were analyzed using laser ablation inductively coupled plasma mass spectrometry la icpms titanite grains from the quartz diorite and diabase dike typically coexist with hydrothermal minerals such as epidote sericite chlorite pyrite and calcite and display irregular or patchy zoning these grains have low lree hree and high th u and lu hf ratios coupled with negative eu and positive ce anomalies the textural and compositional data indicate that titanite from the quartz diorite has been overprinted by hydrothermal fluids after being crystallized from magmas titanite grains from the mineralized skarn are texturally equilibrated with retrograde skarn minerals including actinolite quartz calcite and epidote demonstrating that these grains were formed directly from hydrothermal fluids responsible for the mineralization compared to the varieties from the quartz diorite stock and diabase dike titanite grains from the mineralized skarn have much lower ree contents and lree hree th u and lu hf ratios they have a weighted mean 206 pb 238 u age of 142 2 ma mswd 0 7 2 in agreement with a zircon u pb age of 144 1 ma mswd 0 3 2 of the quartz diorite and thus interpreted as formation age of the ruanjiawan w cu mo deposit titanite grains from the ore related quartz diorite have a concordant u pb age of 132 2 ma mswd 0 5 2 which is 10 12 ma younger than the zircon u pb age of the same sample and thus interpreted as the time of a hydrothermal overprint after their crystallization this hydrothermal overprint was most likely related to the emplacement of the diabase dike that has a zircon u pb age of 133 1 ma and a titanite u pb age of 131 2 ma the geochronological results thus reveal two hydrothermal events in the ruanjiawan deposit an early one forming the wu cu mo ores related to the emplacement of the quartz diorite stock and a later one causing alteration of the quartz diorite and its titanite due to emplacement of diabase dike it is suggested that titanite is much more susceptible to hydrothermal alteration than zircon results from this study also highlight the utilization of trace element compositions in discriminating titanite of magmatic and hydrothermal origins facilitating a more reasonable interpretation of the titanite u pb ages

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

geochemistry and tectonic setting of alkaline volcanic rocks in the antarctic peninsula a review abstract the numerous miocene recent alkaline volcanic outcrops in the antarctic peninsula form a substantial volcanic province the least well known part of a major belt of alkaline volcanism that extends between south america and new zealand the outcrops consists mainly of aa and pahoehoe lavas and hyaloclastites which locally contain accidental nodules of spinel lherzolite and other mantle derived lithologies the province is predominantly basaltic with two major differentiation lineages 1 a sodic series of olivine and alkali basalt hawaiite mugearite trachy phonolite and trachyte and 2 a relatively potassic highly undersaturated series of basanite tephrite and phono tephrite all the lavas show varying effects of fractionation by crystallization of olivine and clinopyroxene joined by plagioclase in the hawaiites to trachytes fractional crystallization can probably explain most of the chemical variation observed within each outcrop but variable partial melting is necessary to account for the differences in incompatible element enrichment between the two series and between the individual outcrops the degree of partial melting may not have exceeded 3 as is the case for many other alkaline magmas the volcanism is an intraplate phenomenon but there is no correlation in timing between the cessation of subduction and the inception of alkaline volcanism the activity cannot be related to the passage of the coupled pacific antarctic plate over a stationary mantle hot spot although the precise causal relationship with tectonic setting is unknown regional extension was a prerequisite for giving the magmas rapid access to the surface

dating the longevity of ductile shear zones insight from 40ar 39ar in situ analyses we attempt to improve temporal constraints on the longevity and the termination of ductile shear zones by performing texturally controlled in situ 40ar 39ar analyses of pre kinematic muscovite biotite and k feldspars of syn kinematic phengite and k feldspar and of post kinematic phengite within the same samples of sinistral shear zones from the western tauern window eastern alps additionally two samples were dated by the rb sr method microsampling relative sequences of mineral formation based on microstructural cross cutting relationships were confirmed by in situ 40ar 39ar analyses showing that syn kinematic minerals are in general younger than pre kinematic minerals and older or of equal age than the post kinematic minerals of the same sample from the rim to the core of the western tauern window syn kinematic phengite and k feldspar reveal a set of formation ages varying between 33 and 15 ma for the northernmost and peripheral shear zone ahorn shear zone between 24 and 12 ma for the intermediate shear zone network tuxer shear zones and between 20 and 7 ma for the southernmost and central shear zone greiner shear zone the age variation of syn kinematic phengite and k feldspar analyses is larger than the analytical error of each age obtained in addition isochron calculations of the syn kinematic minerals reveal atmospheric like 40ar 36ar intercepts therefore the obtained age values of the syn kinematic minerals are interpreted as formation ages which date increments of a long lasting deformation period the time range of deformation of each shear zone system is bracketed by the oldest and youngest formation ages of syn kinematic phengite and k feldspar post kinematic phengite laths show the youngest formation ages and overlap with the youngest syn kinematic formation ages this relationship indicates that post kinematic growth occurred immediately after syn kinematic mineral formation at the end of ductile sinistral shear hence the termination of deformation is dated by the ages of these post kinematic phengite blasts pre kinematic minerals are characterized by break down and exsolution reactions and their age values are heterogeneous and often affected by the presence of extraneous ar these age values are usually older than but sometimes overlapping with ages of the syn kinematic minerals using the temporal constraints obtained by the ages of pre syn and post kinematic minerals we could assess partly overlapping time intervals of syn kinematic mineral formation of 19 myr 33 15 ma in the ahorn shear zone 13 myr 24 12 ma in the tuxer shear zones and 14 myr 20 7 ma in the greiner shear zone this indicates successive localization and propagation of ductile shear zones in the western tauern window from lower metamorphic sites at the rim towards higher metamorphic sites in the center

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

subduction of diverging plates and the principles of slab window formation consumption of an ocean basin by subduction commonly brings a sea floor spreading ridge toward a deep sea trench if plate divergence and convergence continue after the ridge intersects the subduction zone a slab window forms between the subducted parts of the diverging oceanic plates producing anomalous thermal physical and chemical effects in the surrounding asthenospheric mantle in turn these conditions alter the tectonic and magmatic evolution of the overriding plate usually disturbing ordinary fore arc and arc regimes differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate anomalous magmatism from fore arc to back arc may be accompanied by fore arc metamorphism strike slip faulting uplift extension and in extreme cases rifting the shape and size of the window are controlled mainly by the pre subduction ridge transform trench configuration slab dip angles and vectors of plate convergence subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates subducted transform faults continue to be active usually as oblique slip faults until the plates separate as transform faults subduct they become longer on the plate which occupies the acute angle between ridge and trench and shorter on the other plate trains of isolated windows produced by subduction of a segmented ridge transform system progressively expand during descent commonly merging together to form a composite slab window oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows one at each site of ridge trench intersection above a slab window arc volcanism diminishes and may be replaced by volcanism of mid ocean ridge or rift affinity the change in chemical character reflects various processes including elevated heat flow decreasing hydration of the upper mantle juxtaposition of supra and sub slab mantle reservoris asthenospheric upwelling and melting of the trailing plate edges if the slab window migrates the anomalous magmatic regime may be replaced by renewed arc volcanism identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications

cordilleran slab windows the geometry and geologic implications of subducted spreading ridges are topics that have bedeviled earth scientists ever since the recognition of plate tectonics as a consequence of subduction of the kula farallon and east pacific rises slab windows formed and migrated beneath the north american cordillera the probable shape and extent of these windows which represent the asthenosphere filled gaps between two separating subducting oceanic plates are depicted from the late cretaceous to the present possible effects of the existence and migration of slab windows on the cordillera at various times include cessation of arc volcanism and replacement by rift or plate edge volcanism lithospheric uplift attenuation and extension and increased intensity of compressional tectonism eocene extensional tectonism and alkaline magmatism in southern british columbia and the northwestern united states were facilitated by slab window development

temperature time paths in the high p manicouagan imbricate zone eastern grenville province abstract the manicouagan imbricate zone miz eastern grenville province is a series of lithotectonic units assembled from different crustal levels during the grenvillian orogeny structural evidence along with u pb dating suggests that two separate stages of metamorphism occurred at ca 1050 and 1020 ma respectively the first stage is characterised by high pressure and temperature high pt metamorphism up to eclogite facies with a range of maximum recorded pressures between 14 and 20 kbar consistent with variations in burial depth maximum t conditions recorded from apparently different structural levels are rather similar 800 900 c suggesting high heat flow during eclogite facies metamorphism evidence of high temperatures 700 800 c recorded during the early stage of tectonic exhumation indicate near isothermal decompression the second stage is a high t event coeval with the emplacement of syn metamorphic granite and is recorded by rocks in the upper most tectonic slice boundary zone in the south east miz this event may also be responsible for amphibolite facies overprinting across the rest of the miz using the u pb and pt data sets along with published diffusion rates for pb in monazite titanite and rutile tt paths have been modelled for several of the lithotectonic units in most of the units suggested cooling rates appear to have been relatively fast during initial exhuamtion but show a prolonged period of slower cooling 5 10 c ma for 30 40 ma apparently at odds with the steep near isothermal pt paths preservation of eclogitic textures and preservation of garnet zoning in the highest grade rocks on the other hand in the boundary zone the recorded cooling rates are much faster 20 c ma indicative of tectonic exhumation this phase of rapid cooling in the boundary zone corresponds in time with the slow cooling rates at 1030 1000 ma in the rest of the miz thus thrusting of the boundary zone over the rest of the miz appears to have caused a thermal overprint which was superimposed on the tt paths taken as a whole the tt paths for the miz are interpreted as representing two separate metamorphic events which were both relatively short in duration

subduction products according to experimental prediction knowledge of the thermal structure and processes in subduction zones is so uncertain that our more extensive knowledge of source materials and phase relationships remains insufficient for prediction of magmatic products the phase relationships do provide important constraints for testing geophysical and petrogenetic models especially when considered along with geochemical constraints physical questions such as the behavior of partially molten rocks are intimately related to the compositions of magmas from source to eruption site tentative predictions and conclusions include 1 andesite is not a primary magma from oceanic crust 2 it is unlikely that liquids from subducted oceanic crust yield andesites by fractionation except by melting of amphibolite in an especially warm subducted crust 3 it is more likely that hydrous siliceous magmas from the crust leak into the overlying mantle 4 andesite magmas can be generated in peridotite only under exceptional circumstances 5 h2o undersaturated basic magmas from peridotite modified by aqueous fluids or hydrous magma may yield andesite by fractionation 6 phlogopite peridotite could escape melting and yield alkalic magmas at deeper levels 7 andesite magmas could be produced in continental crust only by extreme heating through underplating of basalts or by mixing of basalt and rhyolite derived from crust most of these conclusions need revision if convection in the asthenosphere wedge raises the temperature of the subducted slab to 1250 c at 100 km as proposed by marsh 1979a this demonstrates the dependence of experimental predictions on geophysical models

mantle flow and dynamic topography associated with slab window opening insights from laboratory models abstract we present dynamically self consistent mantle scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics mantle flow and associated dynamic topography over geological time scales the adopted setup consists of a two layer linearly viscous system simulating the subduction of a fixed plate of silicone lithosphere under negative buoyancy in a viscous layer of glucose syrup mantle our experimental setting is also characterized by a constant width rectangular window located at the center of a laterally confined slab modeling the case of the interaction of a trench parallel spreading ridge with a wide subduction zone we found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab on the contrary slab induced mantle circulation quantified both in the vertical and horizontal sections using the feature tracking image analysis technique is strongly modified in particular rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components with the importance of each evolving according to kinematic stages mantle coming from the oceanic domain floods through the slab window indenting the supra slab mantle zone and producing its deformation without any mixing between mantle portions the opening of the slab window and the upwelling of sub slab mantle produce a regional scale non isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca 1 and 5 km in nature assuming that our modeling results can be representative of the natural behavior of subduction zones we compared them to the tectonics and volcanism of the patagonian subduction zone we found that the anomalous backarc volcanism that has been developing since the middle miocene could result from the lateral flow of sub slab mantle and that part of the patagonian uplift could be dynamically supported

eocene tectonometamorphism on serifos western cyclades deduced from zircon depth profiling geochronology and mica thermochronology abstract phengite bearing metasedimentary sequences preserving relict glaucophane from serifos western aegean are indicative of high pressure conditions during cycladic blueschist metamorphism depth profiling and conventional sims u pb geochronology and o isotope analysis on zircon from highly strained orthogneisses at the base of the sequence reveal xenocrystic zircon with varying degrees of recrystallization one end member population has needle like or prismatic morphologies is internally oscillatory zoned with little to no rim material created during recrystallization these zircons yield triassic to carboniferous ages elevated th u and 18 o mostly ranging between 4 and 11 in marked contrast a separate zircon population has a spongy structure created by complete recrystallization of the pre existing crystal these zircons possess low th u and flat hree patterns and yield eocene ages with 18 o 7 chondrite normalized ree profiles are extremely depleted within this spectrum other zircons show variable states of recrystallization and mixed ages regression of u pb data from three mylonitic orthogneiss samples that exhibit near complete recrystallization and similar age depth profiles yields c 40 ma ages these new data are the first geochronological constraints that successfully demonstrate an eocene metamorphic event in the western cyclades and new 40 ar 39 ar phengitic mica ages of 38 32 ma presented here suggest the metamorphic wedge was exhumed into the shallow crust shortly thereafter

timing of deformation phases within the south finland shear zone sw finland abstract in undetermined the palaeoproterozoic svecofennian crust in southern and central fennoscandia was established about 1 8 ga ago after a prolonged history of accretion and intrusion during late stages of the svecofennian orogeny deformation was partitioned into several crustal scale shear zones in present day finland sweden and estonia one such major ductile deformation zone the south finland shear zone sfsz extends for almost 200 km through the a over circle land archipelago in southwestern finland and further along the southern and southwestern coast of finland this more than a kilometer wide transpressional zone appears to have been repeatedly reactivated the deformation started with a period of regional ductile dextral shearing of igneous rocks producing striped granodioritic and tonalitic gneisses the ductile phases are locally overprinted and followed by ductile to semi ductile deformation evidenced by mylonite zones of variable width the last stage of tectonic activity along the shear zone is recorded by pseudotachylites within this study we dated zircons sims u pb and titanites id tims u pb from eight rock samples and two pseudotachylite whole rock samples ar 40 ar 39 in order to reconstruct the deformation and re activation history of the shear zone the results suggest that the medium grained gneisses underwent three distinct deformation phases separated by time intervals without regional deformation the ductile deformation within the study area initiated at similar to 1 85 ga a second more intensive deformation phase existed around 1 83 ga by which the shear zone was already well developed finally the last ductile event is recorded by similar to 1 79 ga metamorphic titanites in relatively granoblastic granitoid gneisses that nevertheless already display protomylonitic textures suggesting the initiation of large scale mylonitisation around or soon after this time the age of a pseudotachylite sample and hence the brittle deformation is bracketed between 1 78 and 1 58 ga based on the age of pegmatites cut by pseudotachylites as well as 40ar 39ar minimum ages for the pseudotachylite respectively the data imply that the rocks within the study area entered the ductile brittle transition zone due to rapid cooling and exhumation of the crust after similar to 1 79 ga less

u pb ages of apatite in the western tauern window eastern alps tracing the onset of collision related exhumation in the european plate we performed single spot u pb analyses of zircons and apatites from eighteen samples distributed along three sections crossing the western tauern window eastern alps zircon analyses yield concordant ages of similar to 295 ma dating the variscan emplacement of the zentralgneiss batholith common pb corrected apatite u pb ages show two age groups the first is characterized by apparent pb 206 u 238 ages scattering between similar to 300 and 40 ma and a second having a relatively uniform cluster of age values around 30 ma we applied an age extractor algorithm tuffzirc isoplot to all apatite analyses of each sample to calculate median ages from the younger age cluster and asymmetric 2 sigma errors as their uncertainties which are mostly in the range of similar to 10 these median ages are interpreted to date cooling below similar to 450 500 degrees c during alpine orogeny after the thermal climax of barrovian metamorphism in the western tauern window cooling ages give a consistent pattern over the study area of similar to 31 29 ma along each section younger cooling ages were obtained in the center compared to its margins the eastern section shows older cooling ages of similar to 36 34 ma whereas the western section younger cooling ages of similar to 26 25 ma the reliability of both age trends 1 younging towards the centers of the sections and 2 younging towards the west across all three sections were verified by six independent two sample student s t tests spatially the cooling age pattern resembles those observed from geochronometers having lower closure temperatures similar to 375 110 degrees c that are largely related to doming of the tauern window cooling rates of five samples where additional fission track ages are available yielded uniform results and were averaged to 15 3 1 9 k myr 2 sigma linear extrapolation using the specific cooling rates and the estimated maximum temperature t max indicates a time range of similar to 37 34 ma interpreted to date the thermal climax of barrovian metamorphism in the western tauern window

trench proximal volcanism following ridge crest trench collision along the antarctic peninsula on the basis of multichannel seismic reflection magnetics and bathymetry data a pleistocene holocene 0 1ma volcanic seamount has been identified at a location very close to the shelf edge off the west coast of the antarctic peninsula the seamount overlies the subducted trace of a transform fracture zone which divides segments of the margin along which collision took place 3 1 and 6 0 ma ago new trace element and isotopic data from dredged samples demonstrate that the seamount appears to be predominantly formed of relatively primitive undersaturated alkali basalt the basalts have geochemical signatures that are indistinguishable from ocean island basalts and some continental alkali basalts e g th ta 1 0 1 5 rb nb 0 5 87sr 86sr 0 70265 0 7028 143nd 144nd approximately 0 5129 and are totally lacking in any evidence for interaction with subduction enriched mantle geochemical similarities with other more trench distal postsubduction alkalic basalts along the antarctic peninsula are striking strongly implying that all the postsubduction basalts were derived from a chemically similar asthenospheric source region the basalts were most likely to have been generated as a result of the formation of a slab window beneath the antarctic peninsula following ridge crest trench collision subduction component free subslab asthenosphere upwelled into the incipient void left by the continued sinking of the leading plate following collision and decompressional melting resulted this type of trench proximal volcanism following ridge crest trench collision differs from that in other locations where calc alkaline volcanism persisted or ophiolite obduction occurred

direct dating of deformation u pb age of syndeformational sphene growth in the proterozoic laramie peak shear zone in this paper we show that deformation can be dated by combining mesoscopic and microscopic structural observations with an understanding of metamorphic mineral reactions and u pb ages of newly grown sphene titanite this approach can be used on a variety of rock types that have been deformed at a wide range of metamorphic conditions in an example from the proterozoic laramie peak shear zone of southeastern wyoming a single period of syntectonic sphene growth in sheared mafic dikes is documented both by a strong spatial relationship between deformation and metamorphism and by sphene microtextures u pb analyses of sphene separates give overlapping concordant or nearly concordant ages with a weighted mean of the 207 pb 206 pb ages of 1763 7 ma we interpret this direct age of deformation as evidence that the laramie peak shear zone records the cratonic response to the 1 78 1 74 ga cheyenne belt collisional event

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

geochronologic constraints on the tectonic evolution and exhumation of nanga parbat western himalaya syntaxis revisited abstract we examine the timing of deformation and exhumation of the nanga parbat haramosh massif in the western syntaxis of the himalaya this study presents geochronologic and thermochronologic data obtained from basement shear zone and intrusive units within the massif to reveal the cenozoic tectonic evolution of the massif and to document the extent of the plio pleistocene tectonic activity these results combined with structural and petrologic observations indicate that the western himalayan syntaxis was tectonically active for a protracted length of time and that the deformational succession was punctuated by three episodes since the beginning of the india asia collision 1 the western syntaxial indian plate rocks future nanga parbat underwent metamorphism melting and deformation during the initial collision of india with asia and the associated island arc the result of which is recorded as eocene to early miocene metamorphic and magmatic ages and oligocene cooling preserved in the indian co

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

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

basement complexes in the wasatch fault utah provide new limits on crustal accretion new and reinterpreted isotopic data for crystalline rocks ex posed in the wasatch range require a reevaluation of precambrian crustal boundaries in utah crystalline rocks of the santaquin com plex underwent metamorphism prior to ca 1670 ma consistent with sr and nd isotope data mafic to intermediate rocks have major element trace element and isotope ratios indicative of derivation in an arc accreted to the archean craton in proterozoic time requiring the crustal suture to be north of the santaquin complex farther north the farmington canyon complex has been considered ar chean based on published nd model ages and discordant u pb zir con ages however nd model ages and zircons could be inherited from sedimentary protoliths u pb and electron microprobe ages of monazite have a mode at 1650 to 1700 ma concordant with the santaquin complex and lack inheritance we propose that the farmington canyon complex was first cratonized from archean derived sediments in the proterozoic requiring a crustal suture to be north of it as well accretion ages of arc terranes in southeastern wyoming are 60 100 m y older than in utah thus a serious reevaluation of basement architecture in utah is needed and a pre viously unrecognized temporal complexity of accretion is indicated

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

protolith ages and timing of deformation in the eastern marginal part of the sveconorwegian orogen southwestern sweden abstract u pb zircon and titanite ages from variably metamorphosed igneous rocks in the eastern marginal part of the sveconorwegian orogen eastern segment north of lake vanern sweden suggest that these rocks belong to the younger c 1 71 1 65 ga igneous suite of the c 1 85 1 65 ga transscandinavian igneous belt tib two porphyritic meta granites and one isotropic monzonite yield u pb zircon ages of 1661 27 1674 7 and 1674 7 ma respectively these ages support earlier interpretations that the eastern segment north of lake vanern contains tectonically reworked tib rocks and that there is no major lithological discontinuity either across the sveconorwegian frontal deformation zone or within the eastern segment igneous brown coloured titanites from five samples yield similar u pb ages as the zircons in contrast colourless and oblate shaped titanites from the western penetratively deformed parts of the eastern segment give concordant ages of 976 4 and 956 6 ma further to the east similar titanite from both a penetratively deformed meta granite and a ductile shear zone within a porphyritic quartz monzonite are strongly discordant close to late sveconorwegian lower intercept ages this type of titanite is interpreted to be syntectonic and formed during the upper greenschist east to amphibolite west facies fabric forming event in the eastern segment since the brown igneous titanites and the zircons share a common igneous origin and the single type of metamorphic titanites are either concordant or strongly discordant towards late sveconorwegian intercept ages it is inferred that no major tectonothermal event occurred between the time of tib emplacement and the late sveconorwegian tectonothermal reworking pre sveconorwegian palaeo to mesoproterozoic deformational and metamorphic events gothian which have been inferred west of and further south within the eastern segment appear to be absent in the northern part of this crustal unit

analysis of james ross island volcanic complex and sedimentary basin based on high resolution aeromagnetic data abstract high resolution aeromagnetic surveys provide a geophysical tool to help image the subsurface structure of volcanoes and their tectonic framework here we interpret high resolution aeromagnetic data and models for james ross archipelago and surrounding regions to provide a new perspective on neogene magmatism emplaced along the eastern margin of the antarctic peninsula based on the analysis and modelling of magnetic anomalies we were able to image the subglacial extent of miocene to recent alkaline rocks of the james ross island volcanic group and map tectonic structures that appear to have exerted important controls on neogene magmatism high amplitude linear anomalies detected over mount haddington stratovolcano were modelled as caused by subvertical feeder bodies extending to a depth of at least 3 km these feeder bodies may have been emplaced along a n s oriented volcano tectonic rift zone we also identified several effusive subglacial centres and imaged two concentric magnetic arcs which we related to neogene volcanic and subvolcanic lineaments likely controlled by mid cretaceous strike slip fault belts and associated deformation zones the regional magnetic quiet zone that encompasses james ross island is caused by the thick sedimentary infill of the larsen basin and a low amplitude magnetic high within the basin is inferred to reflect a basement push up structure associated with strike slip faulting along the eastern edge of the antarctic peninsula in the offshore regions magnetic anomalies southwest of tabarin peninsula and west of vega island may reflect recent volcanic structures that have yet to emerge from the sea floor

geology geochronology and tectonic setting of late cenozoic volcanism along the southwestern gulf of mexico the eastern alkaline province revisited a nnw trending belt of alkaline mafic volcanic fields parallels the gulf of mexico from the u s border southward to veracruz state in eastern mexico previous studies grouped this volcanism into the so called beastern alkaline provinceq eap and suggested that it resulted from gulf parallel extensional faulting migrating from north to south from oligocene to present on the basis of new geologic studies forty nine unspiked k ar and two 40 ar 39 ar ages we propose a new geodynamic model for the volcanism along the southwestern gulf of mexico we studied in detail four of the six recognized fields of mafic alkaline volcanism in veracruz state 1 the lavas flows of tlanchinol area 7 3 5 7 ma 2 the alamo monogenetic field and sierra de tantima 7 6 6 6 ma 3 the poza rica and metlatoyuca lava flows 1 6 1 3 ma and 4 the chiconquiaco palma sola area 6 9 3 2 ma other two mafic volcanic fields may represent the continuation of alkaline volcanism to the southeast the middle miocene lavas at anegada high offshore port of veracruz and the middle to late miocene volcanism at the los tuxtlas the existence of major neogene extensional faults parallel to the gulf of mexico i e n s to nnw sse proposed in previous works was not confirmed by our geological studies elongation of volcanic necks vent alignment and faults mapped by subsurface data trend dominantly ne to ene and nw to nnw these directions are parallel to transform and normal faults that formed during the late jurassic opening of the gulf of mexico ascent of mafic magmas was likely facilitated and controlled by the existence of these pre existing basement structures

structural significance of l tectonites in the eastern central laramie mountains wyoming abstract the formation of l tectonites is little understood and scarcely studied however it is probably an important part of plastic deformation in the crust to improve our understanding of this strain phenomenon i present a detailed case study of a kilometer wide domain of l tectonites developed in and around the 2 05 ga boy scout camp granodiorite bscg in the laramie mountains wyoming detailed mapping and structural analyses allow for the reconstruction of the structural setting of this domain of apparent constrictional strain elongation lineations plunge moderately to the south southwest and lie parallel with both the local fold hinge lines and regional fold axes whereas poles to foliation generally cluster in the northwest quadrant roughly defining fold axial surfaces map scale folds are west northwest vergent but at the outcrop and thin section scales there is no evidence for a significant component of simple shear reconstruction of the orientation of contacts in and around the bscg i

the scotia sea gateway no outlet for pacific mantle abstract the scotia sea in the south atlantic holds a prominent position in geodynamics because it has been proposed as a potential outlet of asthenosphere from under the shrinking pacific into the mantle beneath the opening atlantic shear wave splitting and geochemical studies have previously tested this hypothesis here we take a different approach by calculating present day dynamic topography of the region in search for a systematic trend in dynamic topography decreasing from west to east in response to a flow related pressure gradient in the sublithospheric mantle to this end we reconstruct the kinematic history of the scotia sea which is characterized by complex back arc spreading processes active on a range of time scales our plate reconstructions allow us to derive an oceanic age grid and to calculate the associated residual dynamically maintained topography of the scotia sea by comparing present day isostatically corrected topography with that predicted from our reconstruction the results provide no indication for a systematic trend in dynamic topography and we conclude that the material needed to supply the growing subatlantic mantle must be derived from elsewhere