middle jurassic rhyolite volcanism of eastern graham land antarctic peninsula age correlations and stratigraphic relationships silicic volcanism at c 168 ma has been identified previously on the antarctic peninsula and the mapple formation which includes those volcanic rocks has been defined and documented from one area of the east coast of graham land based on age and geochemical criteria correlations have been made to the extensive chon aike province of south america which has been demonstrated to be one of the largest silicic volcanic provinces in the world rhyolitic and intermediate composition volcanic successions from six separate localities on the east coast of the antarctic peninsula are described here and are confirmed as correlatives of the mapple formation based on newly acquired geochronology and field observations they are dominantly rhyolitic crystal tuffs and or ignimbrites with ages in the interval 162 168 ma overlapping with the age of the mapple formation 167 171 ma at the type locality andesitic agglomerates are also described which are included in the same event and demonstrate the occurrence of rare intermediate volcanism which is also seen in the chon aike province a new group the graham land volcanic group is defined here and criteria are established which allow the separation of some volcanic successions out of the previously defined antarctic peninsula volcanic group which takes no account of tectonic setting eruption age or geochemistry

petrogenesis of volcanic rocks that host the world class ag pb navidad district north patagonian massif comparison with the jurassic chon aike volcanic province of patagonia argentina abstract we present the first study of the volcanic rocks of the canadon asfalto formation that host the navidad world class ag pb epithermal district located in the north patagonian massif patagonia argentina these volcanic and sedimentary rocks were deposited in a lacustrine environment during an extensional tectonic regime associated with the breakup of gondwana and represent the mafic to intermediate counterparts of the mainly silicic jurassic chon aike volcanic province lava flows surrounded by autobrecciated carapace were extruded in subaerial conditions whereas hyaloclastite and peperite facies suggest contemporaneous subaqueous volcanism and sedimentation la icpms u pb ages of zircon crystals from the volcanic units yielded middle jurassic ages of 173 9 1 9 ma and 170 8 3 ma in the navidad district volcanic rocks of the canadon asfalto formation show arc like signatures including high k basaltic andesite to high k dacite compositions rb ba and th enrichment relative to the less mobile hfs elements nb ta enrichment in light rare earth elements lree y ti depletion and high zr contents these characteristics could be explained by assimilation of crustal rocks in the jurassic magmas which is also supported by the presence of zircon xenocrysts with permian and middle upper triassic ages 281 3 ma 246 5 218 1 and 201 3 ma and quartz xenocrysts recognized in these volcanic units furthermore sr and nd isotope compositions suggest a contribution of crustal components in these middle jurassic magmas high k basaltic andesite has initial 87sr 86sr ratios of 0 70416 0 70658 and nd t values of 5 3 and 4 high k dacite and andesite have initial 87sr 86sr compositions of 0 70584 0 70601 and nd t values of 4 1 and 3 2 the range of pb isotope values 206pb 204pb 18 28 18 37 207pb 204pb 15 61 15 62 and 208pb 204pb 38 26 38 43 of navidad volcanic rocks and ore minerals suggest mixing pb sources with contributions of mantle and crust 206pb 204pb isotopic ratios of jurassic volcanic rocks of the chon aike volcanic province and sulfides of associated epithermal deposits increase with time from the volcanic event v1 188 178 ma to volcanic events v2 172 162 ma and v3 157 153 ma reflecting variations in the radiogenic pb source as volcanism was migrating towards the proto pacific margin of gondwana

a 1 78 ga large igneous province in the north china craton the xiong er volcanic province and the north china dyke swarm abstract the 1 78 ga xiong er volcanic province xvp and coeval north china giant mafic dyke swarm ncds are the most important magmatic events occurring after the amalgamation of the north china craton ncc the xvp consists of 3 7 km of extrusive volcanics and some feeder dykes sills located along the southern margin of the ncc and extending over an area 0 06 m km2 compositions vary from basalt to rhyolite but are predominantly intermediate in terms of silica content there are also minor sedimentary intercalations and pyroclastic units the sedimentary interlayers indicate an environment changing from continental facies to oceanic facies up section the xvp is characterized by fractional crystallization from an em i type mantle source and both continental arc andean type and rift environments have been proposed the ncds is widespread in the central ncc with an outcrop area 0 1 m km2 and are exposed at variable depths up to 20 km deepest in the north dyke compositions vary from basalt to andesite and dacite but are dominantly mafic and comprise two series of magmatism previous studies revealed that the ncds recorded assimilation and fractional crystallization of an em i type magma source with a minor dm contribution in the younger magmas both syn collisional and intra continental anorogenic environments have been proposed spatial and petrogenic correlations suggest a cogenetic relationship between the ncds and xvp and considered together they define a large igneous province lip of 0 1 m km2 in area and 0 1 m km3 in volume which is also notable for its continuous compositional range from mafic to felsic with no gap at intermediate compositions the petrology is explained by a common magma source that undergoes a silica poor and iron enriched fractionation trend at depth followed by a silica rich and iron poor fractionation trend in shallow level magma conduits dykes and surface lavas a mantle plume is favored as the cause of this 1 78 ga north china lip

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

large scale rhyolite peperites jurassic southern chile development of a jurassic volcano tectonic rift basin in the southern andes created a setting in which thick rhyolitic volcaniclastic sequences accumulated in submarine environments and were penetrated by hypabyssal intrusions during or shortly after deposition in the ultima esperanza district of southern chile extensive masses of peperite were produced when rhyolite magma underwent quenching disruption and commingling with wet unconsolidated sediments during intrusion at shallow levels beneath the sea floor the peperite forms discordant intrusive masses with volumes of up to several cubic kilometers in which large widely spaced coherent rhyolite feeder pods are surrounded by and grade into closely packed and dispersed peperite closely packed peperite consists of tightly fitting clasts separated by sediment filled fractures in dispersed peperite the sediment forms a matrix surrounding large masses of fractured rhyolite and smaller more widely separated rhyolite clasts evidence of in situ quench fragmentation is well preserved on both outcrop and thin section scales thin sections show that clast margins and in some cases entire small clasts underwent cooling contraction granulation releasing shards of quenched rhyolite and fragments of phenocrysts into the adjacent sediment interaction between magma and wet sediment was non explosive and involved fluidization of the host sediments creating space for the intruding magma and causing pervasive injection of highly mobile sediment along thermal contraction cracks in quench fragmented rhyolite the ability of the magma to undergo complex intermixing with large volumes of sediment with widespread preservation of in situ fragmentation textures is interpreted to reflect a relatively low magma viscosity presumably caused by retention of volatiles in the magma at the ambient pressures involved beds of redeposited peperite within the rift basin fill indicate that some of the intrusive peperite masses reached the sea floor undergoing slumping and mass flow the peperites were thus an important local source of coarse grained debris during the evolution of the basin

evolution and tectonic setting of the malani nagarparkar igneous suite a neoproterozoic silicic dominated large igneous province in nw india se pakistan abstract the neoproterozoic malani igneous suite mis in nw india along with analogous magmatic rocks from the adjoining nagarparkar region in se pakistan can be collectively classified as a silicic dominated large igneous province slip this magmatic event includes bimodal predominantly felsic volcanism granite emplacement and felsic and mafic dyke intrusions felsic rocks have typical a type affinity as indicated by high abundance of silica alkali high field strength and large ion lithophile element concentrations and low cao and mgo contents their nb negative anomalies and zr saturation parameters indicate significant crustal input and high temperature melting mafic volcanics and dykes show geochemical homogeneity and derivation from a depleted continental mantle source without any significant crustal contamination low u and th contents and no visible nb anomaly the region extending from the mount abu batholith in the east to jaswantpura in the west 2700 km2 representing a transition from the metamorphic sirohi terrane to the undeformed mis was evaluated through an integrated structural including satellite image analysis geochemical and geochronological study during the initial stage prior to 760 ma the granitic basement erinpura granites and overlying sirohi metasediments behaved in a brittle manner that led to development of linear fractures and nne trending rift structures and bimodal volcanic activity emplacement of voluminous granitic bodies in response to progressive extension of the crust is inferred during the more evolved second stage younger than 760 ma mirpur granite a representative of this younger granitic suite jalor type pink granite has yielded 753 9 ma zircon u pb crystallization age granitic plutons mark regions of crustal extension as seen in parallel alignment of plutonic bodies jaswantpura granitic belt and parallel mafic dyke swarms 340 transecting the granites structural analysis further identified an episode of crustal convergence which is documented in folding and faulting of the sindreth basin sequence and in tectonic overprint of early stage mafic rocks rifting and bimodal magmatic activity in mis is coeval with similar rock types in nagarparkar in se pakistan further traceable into the seychelles microplate and central madagascar considering the neoproterozoic paleogeography and our observations an extensional setting and an active continental margin position for mis is inferred

early cretaceous uplift in the ellsworth mountains of west antarctica apatite fission track analysis of samples covering a 4 2 kilometer vertical section from the western flank of vinson massif antarctica s highest mountain indicates that the ellsworth mountains were uplifted by 4 kilometers or more during the early cretaceous following the initial separation of east and west gondwana and accompanying the opening of the weddell sea relief of at least 1 8 kilometers has persisted in the ellsworth mountains since the early cretaceous and a maximum of 3 kilometers of uplift has occurred since that time

distinguishing similar volcanic source areas from an integrated provenance analysis implications for foreland andean basins abstract the mata amarilla formation marks the onset of the foreland stage of the austral basin which is composed of mostly nonmarine and littoral siliciclastic sediments thus providing an opportunity to study the detrital record of the late cretaceous southern foreland andes our dataset provides for the first time a comprehensive picture of the late cretaceous evolution of the austral foreland basin constituting a possible analogue to other foreland basins at the foot of the andes sandstones from the mata amarilla formation testify to variable contributions from jurassic bimodal volcanic rocks of the deseado massif and the patagonian fold and thrust belt in the context of an eastward advancing orogen sandstone petrography shows an overall feldspathic litharenite composition whereas sandstones coming from the northeast deseado massif have higher lv and lower qp proportions than samples coming from the west patagonian fold and thrust belt in the central part of the study area sandstones are characterized by higher proportions of qt associated with a greater distance and time of transport relating to its position in the austral foreland basin in spite of the increased maturity of sandstone in the central area x ray analyses permit recognition of the compositional signature of mata amarilla formation in which four clay mineral assemblages were identified s rich in smectite s k rich in smectite and kaolinite pg rich in palygorskite and i s rich in illite smectite mixed layers s assemblage evidences well crystallized smectite characteristic of volcaniclastic origin most smectite was formed during early diagenesis through alteration of labile tuffaceous material derived from the southern andes the stratigraphic variations in clay mineral assemblages reveal a strong environmental control on their distribution the transformation of smectite into illite and kaolinite is considered as product of pedogenesis whereas the presence of palygorskite indicates a coastal environment with paleosol development under poorly drained conditions

jurassic hot spring activity in a fluvial setting at la marciana patagonia argentina silicified rocks at la marciana farm deseado massif argentinean patagonia represent an ancient hot spring discharging into an active fluvial setting their fortuitous burial erosion history and minimal post depositional structural modification provide an unparalleled view of a complete exhumed late jurassic geothermal landscape and thus an opportunity to illuminate hot spring geological context and palaeoenvironmental gradients geological mapping stratigraphy and petrography revealed hydrothermal eruption events the spring vent source structural relationships dimensions of the discharge apron and hot spring facies distributions remarkable similarities to quaternary analogues from the taupo volcanic zone new zealand and yellowstone national park usa are apparent with respect to scale spatial distribution of facies and types of microbial and other palaeoenvironmentally significant fabrics

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

middle jurassic within plate granites in west antarctica and their bearing on the break up of gondwanaland five post tectonic granitic plutons isolated within the central ellsworth whitmore mountains crustal block in west antarctica form a distinctive geochemical suite all have some characteristics of s type granites and are atypical of active continental margins they range in composition from a within plate granite wpg end member with the lowest 87 sr 86 sr initial ratio 0 707 to granites with a much more marked crustal signature and high initial ratios 0 722 the granitic suite was emplaced over a restricted middle jurassic time interval at the same time as the extensive ferrar karoo tasman mafic suite and just prior to the disintegration of the supercontinent gondwanaland petrogenetic modelling suggests that the wpg end member could have been derived entirely by differentiation of the enriched mantle derived ferrar magma and the end member with the highest initial ratio by partial melting of a crustal source low initial 143 nd 144 nd ratios and proterozoic model ages are compatible with a precambrian crustal component but may alternatively as in the case of ferrar supergroup magmas reflect partial inheritance from enriched lithospheric mantle geochemically coupled to the lower crust since precambrian differentiation data from these granites are consistent with large scale underplating of mafic magma and crustal melting in response to a thermal disturbance in the gondwanaland lithosphere related in some way to break up of the supercontinent

la peligrosa caldera 47 15 s 71 40 w a key event during the jurassic ignimbrite flare up in southern patagonia argentina pyroclastic and lava vent facies from the late jurassic el quemado complex are described at the southern lake ghio in the cordillera patagonica austral based on the comprehensive study of lithology and structures the reconstruction of the volcanic architecture has been carried out four ignimbrites and one rhyolitic lava unit affected by oblique slip normal faults have been recognized the evolution of la peligrosa caldera has been modeled in three different stages 1 initial collapse consisting of a precursory downsag subsidence related to a dilatational zone which controlled the location of the caldera 2 main collapse with the emplacement of large volume crystal rich ignimbrites and megabreccias under a progressive subsidence controlled by a pull apart structure related to a transtensional regime and 3 post collapse in which lava flows and associated domes were emplaced under an oblique extensional regime the caldera records a remarkable change from transtension to oblique extension which may represent an important variation in regional deformation conditions during jurassic times la peligrosa caldera may be considered a key event to understand the eruptive mechanisms of the flare up volcanism in the chon aike silicic province 2013 elsevier b v

the toarcian oceanic anoxic event early jurassic in the neuquen basin argentina a reassessment of age and carbon isotope stratigraphy abstractthe toarcian oceanic anoxic event t oae is recorded by the presence of globally distributed marine organic carbon rich black shales and a negative carbon isotope shift with 13corg values as low as 33 interrupting an overarching positive excursion here we present new biostratigraphic data and high resolution 13corg data from two southern hemisphere localities arroyo serrucho in the north and arroyo lapa in the south of the neuquen basin argentina previous studies at these localities aimed to provide an accurate numerical age for the t oae and characterization of its carbon isotope stratigraphy the new carbon isotope data and ammonite biostratigraphy presented here from arroyo serrucho show the t oae to be recorded lower in the section than supposed by previous authors thus calling into question the published age of the t oae in this section a newly investigated exposure at arroyo lapa north shows a complex carbon isotope record with at least three high amplitude fluctuations in the h

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

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

a high resolution speleothem record of climatic variability at the allerod younger dryas transition in missouri central united states stable isotopic analysis and 234 u 230 th mass spectrometry dating of a single calcite stalagmite from onondaga cave east central missouri reveal climatic variability between 13 200 and 12 400 yr bp coincident with the aller d younger dryas transition in the gisp2 greenland ice cores n 18 o values of speleothem calcite mark decreasing temperatures between 13 200 and 13 000 yr bp while more rapidly decreasing n 13 c values record a sharp shift toward c3 vegetation at approximately 13 100 yr bp because of uncertainties in the chronological control of both this speleothem and the gisp2 record and the brevity of this speleothem record v800 years direct comparison of the two sequences is difficult the interval preserved by this speleothem may span the inter aller d cold period iacp alone both the iacp and the start of the younger dryas or only the first half of the younger dryas in any case these data demonstrate that the climate of the north american interior deteriorated significantly and rapidly during the interval associated with the aller d younger dryas transition fl 2001 elsevier science b v all rights reserved

geochemical distribution and supergene behavior of indium at the pinguino epithermal polymetallic vein system patagonia argentina abstract the pinguino deposit located in the low sulfidation epithermal metallogenic province of the deseado massif patagonia argentina represents a distinct deposit type for this region it evolved through two different mineralization events an early in bearing polymetallic event that introduced in zn pb ag cd au as cu sn w and bi represented by a complex sulfide assemblage and late ag au quartz rich veining that crosscut and overprinted the early polymetallic mineralization three in bearing mineralization stages were identified within the polymetallic event iron rich sphalerite of the second stage is the major in carrier in the deposit the spatial distribution of in within the veins shows its maximum mean values in the marta centro and ivonne norte veins in a pattern similar to the zn distribution showing a close spatial relation with the unexposed kasia intrusive complex at the vein scale and in the hypogene zone the in distribution is associated with variations in the temperature of mineralizing fluids the vertical distribution of in within the veins exhibits consistent values both below and above the oxidation level resembling the behavior of pb and sn and therefore suggesting that in behaves as an immobile element during vein weathering

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

recalibrating the breakup history of sw gondwana u pb radioisotopic age constraints from the southern cape of south africa abstract syn rift deposits often provide the only means to determine the chronology of rift initiation and evolution however the earliest syn rift packages deposited in jurassic cretaceous rift basins that formed during the breakup of sw gondwana are poorly understood because they are deeply buried beneath overlying passive margin sequences the exhumed remnants of several such rift basins are exposed in the southern cape of south africa and contain the suurberg and uitenhage groups which are predominantly continental taphrogenic fossiliferous strata interbedded with volcaniclastics here we present the first robust u pb chronostratigraphic framework for these groups by dating zircon in nine pyroclastic and five resedimented volcaniclastic deposits using laser ablation inductively coupled plasma mass spectrometry la icpms to further improve the precision and accuracy of the results we utilize chemical abrasion thermal ionisation mass spectrometry ca tims on four selected samples minimizing the effects of pb loss and further constraining depositional uncertainties we thereby show that the suurberg group was deposited rapidly during the emplacement of the karoo large igneous province in the early jurassic and likely predates the main phase of rifting whereas the uitenhage group was deposited over a prolonged 40 ma period beginning in the early jurassic and continuing into the early cretaceous the uitenhage group records two phases of rifting an initial jurassic episode that roughly coincides with the separation of east and west gondwana and is contemporaneous with widespread volcanism in sw gondwana and a subsequent period of renewed rifting during the early cretaceous opening of the south atlantic and initiation of the agulhas falkland transform this framework illustrates the complexity of long lived rift basin sedimentation and highlights the importance of high resolution chronostratigraphy when investigating and integrating the tectonic palaeogeographic and palaeontological records from the final stages of a unified sw gondwana

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

a new plumbing system framework for mantle plume related continental large igneous provinces and their mafic ultramafic intrusions abstract the magmatic components of continental large igneous provinces lips include flood basalts and their plumbing system of giant mafic dyke swarms radiating linear and the recently discovered circumferential type mafic sill provinces a lower crustal magmatic underplate mafic ultramafic m um intrusions associated silicic magmatism and associated carbonatites and kimberlites this paper proposes a new plumbing system framework for mantle plume related continental lips that incorporates all of these components and provides a context for addressing key thematic aspects such as tracking magma batches upstream and downstream and their geochemical evolution assessing the setting of m um intrusions and their economic potential interpreting deep magmatic component identified by geophysical signatures and estimating magnitudes of extrusive and intrusive components with climate change implications this plumbing system model and its associated implications needs to be tested against the rapidly improving lip record

genesis of lavas of the taupo volcanic zone north island new zealand the taupo volcanic zone forms part of the taupo hikurangi subduction system and comprises five volcanic centres tongariro taupo maroa okataina and rotorua tongariro volcanic centre is formed almost entirely of andesite while the other four centres contain predominantly rhyolitic volcanics and later fissure eruptions of high al basalt estimated total volume of each lava type are as follows 2 km3 of high al basalt 10 000 km3 of rhyolite and ignimbrite 97 4 the location of the andesites and vent alignments suggest a source from a subduction zone underlying the area however the lavas differ chemically from island arc andesites such as those of tonga in particular by having higher contents of the alkali elements light ree and sr and pb isotopes this suggests some crustal contamination and it is considered that this may occur beneath the wide accretionary prism of the subduction system amphibolite of the subduction zone will break down between 80 and 100 km and a partial melt will rise a multi stage process of magma genesis is then likely to occur high al basalts are thought to be derived from partial melting of a garnet free peridotite near the top of the mantle wedge overlying the subduction zone locations of the vents controlled largely by faults within the crust rhyolites and ignimbrites were probably derived from partial melting of mesozoic greywacke and argillite under the taupo volcanic zone initial partial melting may have been due to hydration of the base of the crust the water having come from dehydration of the downgoing slab the partial melts would rise to form granodiorite plutons and final release of the magma to form rhyolites and ignimbrites was allowed because of extension within the taupo graben dacites of the bay of plenty probably resulted from mixing of andesitic magma with small amounts of rhyolitic magma but those on the eastern side of the rotorua taupo area were more likely formed by a higher degree of partial melting of the mesozoic greywacke argillite basement this may be due to intrusion of andesite magma on this side of the taupo volcanic zone

processes controlling porosity and permeability in volcanic reservoirs from the austral and neuquen basins argentina volcanic rocks develop primary and secondary porosity and permeability depending on both their lithology and the sequence of processes involved in their formation primary processes welding deuteric crystal dissolution gas release flow fragmentation and crystal shattering may lead to high porosity and permeability the best example of which is a nonwelded ignimbrite with well developed gas pipe zones secondary processes different types of alteration however tend to decrease primary porosity however certain secondary processes such as dissolution and hydraulic fracturing may contribute to enhance total porosity and permeability these conclusions were developed through a systematic review of reservoir quality in volcanic rocks integrating lithology and process interpretation with petrophysical data the said information was taken from selected cores of volcanic rocks from the serie tobfera unit in the austral basin and the precuyano unit in the neuqun basin argentina a clear understanding of both primary and secondary processes may serve to predict the quality of volcanic reservoirs and could be used as a guide for oil and gas exploration and development in many parts of the world

tectonic evolution of the north patagonian andes 41 44 s through recognition of syntectonic strata abstract the north patagonian fold thrust belt 41 44 s is characterized by a low topography reduced crustal thickness and a broad lateral development determined by a broken foreland system in the retroarc zone this particular structural system has not been fully addressed in terms of the age and mechanisms that built this orogenic segment here new field and seismic evidence of syntectonic strata constrain the timing of the main deformational stages evaluating the prevailing crustal regime for the different mountain domains through time growth strata and progressive unconformities controlled by extensional or compressive structures were recognized in volcanic and sedimentary rocks from the cordilleran to the extra andean domain these data were used to construct a balanced cross section whose deep structure was investigated through a thermomechanical model that characterizes the upper plate rheology our results indicate two main compressive stages interrupted by an extensional relaxation period the first contractional stage in the mid cretaceous inverted jurassic lower cretaceous half graben systems reactivating the western canadon asfalto rift border 500 km away from the trench at a time of arc foreland expansion for this stage available thermochronological data reveal forearc cooling episodes and global tectonic reconstructions indicate mid ocean ridge collisions against the western edge of an upper plate with rapid trenchward displacement widespread synextensional volcanism is recognized throughout the paleogene during plate reorganization retroarc paleocene eocene flare up activity is interpreted as product of a slab rollback and fore to retroarc oligocene slab asthenospheric derived products as an expression of enhanced extension the second stage of mountain growth occurred in miocene time associated with nazca plate subduction reaching nearly the same amplitude than the first compressive stage extensional weakening of the upper plate predating the described contractional stages appears as a necessary condition for abnormal lateral propagation of deformation

upper jurassic travertine at el macanudo argentine patagonia a fossil geothermal field modified by hydrothermal silicification and acid overprinting the deseado massif hosts numerous late jurassic 150 ma fossil geothermal systems related to an extensive volcanic event developed in a diffuse extensional back arc setting detailed mapping petrography and mineralogical observations of el macanudo outcrops verify that it represents a hot spring related travertine partially replaced by silica and delineated by six sedimentary facies these are large concentric cones f1 laminated vertical columnar structures f2 porous layers f3 shrubby and irregular lamination f4 low amplitude wavy bedding f5 and mounds and breccias f6 the macanudo norte outcrop rocks constitute a silica replaced travertine sequence with development of large conical stromatolites in a deep pool or geothermally influenced shallow lacustrine environment surrounded by a subaerial travertine apron terrace whereas the macanudo sur outcrop is a subaerial travertine mound sequence structurally controlled vent areas occur in both northern f1 and southern f6 outcrops mainly located along regional nne and ene trending faults the other sedimentary units display a concentric distribution of travertine facies with respect to the interpreted vent areas the el macanudo palaeo hot spring deposit is situated in an eroded jurassic volcanic centre and records a complex evolutionary fluid history the sediments archived three different jurassic events when large and long lasting hydrothermal systems were active across the region this relative temporal sequence was formed by 1 travertine precipitation 2 development of a silica cap where early silicification was responsible for exceptional preservation of some stromatolitic fabrics and 3 acid alteration recorded by dissolution textures and clay formation and caused by a palaeo phreatic water level drop

rare earth element evidence concerning the origin of voluminous mid tertiary rhyolitic ignimbrites and related volcanic rocks sierra madre occidental chihuahua mexico abstract the mid tertiary volcanic sequence of the central sierra madre occidental in chihuahua mexico is about one kilometer thick and is composed predominantly of rhyolitic ignimbrites basaltic andesite to dacitic lavas are interbedded with the rhyolites but they are of minor volumetric importance rare earth element ree data are used to constrain a crustal anatexis model for the origin of the voluminous ignimbrites and to test a fractional crystallization model the ree patterns indicate that if the rhyolites were formed by direct crustal anatexis the residue from partial melting could contain no more than a few percent garnet or about 20 hornblende this eliminates residues with the mineralogy of amphibolite eclogite or garnet granulite but melting of a garnet free granulite source is permitted the crustal anatexis model is difficult to evaluate critically because of a lack of knowledge concerning the mid tertiary geothermal gradient and the composition of the crust beneath the sierra madre occidental in contrast the fractional crystallization model can be tested rigorously rayleigh fractionation calculations are used to closely model ree patterns in the basaltic andesite to rhyolite series the minerals involved are those occurring as phenocryst phases and the mineral proportions were generated by leastsquares major element calculations the results of the calculations are consistent with the hypothesis that the voluminous rhyolites originated by plagioclase dominated crystal fractionation

structural controls on jurassic gold mineralization and cretaceous tertiary exhumation in the foreland of the southern patagonian andes new constraints from la paloma area deseado massif argentina abstract fluid rock interaction occurring within high permeability conduits represented by fault fracture meshes is well known to control the development of hydrothermal ore deposits in this study we document the fundamental role of four sets of precursor joints reactivated as dilational faults displaying variable normal dip slip and strike slip components of motion in the formation of epithermal au ag deposits of economic interest paleostress analysis unravels how all the resulting four sets of hybrid extension shear fractures are kinematically compatible with an extensional stress field characterized by multiple permutations between the 3 and the 2 axes such permutations are interpreted to be associated with dominantly normal fault reactivation of suitably oriented early joint sets as a result of continued regional extension and by orthogonal stretching produced by differential downthrow of the hanging wall blocks of laterally terminating jurassic normal faults later fault reactivation appears to be associated with far field propagation of andean stresses marked by superposed strike slip and reverse fault slip components recording roughly e w horizontal compression this late fault reactivation could be related with a late early cretaceous shortening event known to have affected the studied sector of the deseado massif the subsequent tectonic evolution of the study area occurred within the general framework of a slow cretaceous to tertiary exhumation recorded by apatite fission track cooling ages our new thermochronometric results recording steady state cooling at a rate of c 1 c ma during the last 90 ma shed new light onto the post jurassic geodynamic evolution of the foreland of the southern patagonian andes

magmatic evolution of the austral patagonian andes abstract new geological evidence and geochronological data obtained within the patagonian cordillera between lat 47 and 49 s allow the redefinition of an andean region with different and characteristic geological evolution compared to the adjacent segments an acidic middle jurassic volcanism covered a metamorphic basement formed by low grade metasediments and metabasites of upper paleozoic age dike swarms of basic alkaline rocks were emplaced since the cretaceous in a marginal basin developed on the jurassic sialic crust the outcrop density of such bodies seems to indicate an attenuation of the sialic crust towards the south where oceanic crust of upper jurassic lower cretaceous age floored the marginal basin the cretaceous volcanic activity is well developed in the adjacent sectors but almost absent in the studied segment several magmatic pulses lead to the final emplacement of the patagonian batholith which yields its peak activity around 98 m y and presents typical geochemical variations the upper cretaceous and cenozoic magmatism are mainly exposed to the east of the cordillera and are represented by a series of alkali plateau basalts different pulses of activity have been recorded during eocene upper miocene pliocene and even during quaternary times associated with deep crustal fractures and probable upper mantle origin as evidenced by the low 87sr 86sr ratios and the ultramafic and mafic xenoliths of the lavas granitic stocks in the andean domain are post orogenic in relation to a compressive tectonic phase during miocene times the upper cenozoic calc alkaline volcanism presents a gap in this segment of the andean belt probably related with the post miocene migration of the chile rise

rates of active compressional deformation in central italy and sicily evaluation of the seismic budget historical and recent seismicity records and available source mechanisms in eastern central italy marche adriatic region in mainland southern sicily and in the tyrrhenian offshore of northern sicily show comparable deformation patterns seismotectonic considerations indicate that each of the three areas represents a broad seismogenic province of relatively homogeneous deformation on the basis of the historical earthquake catalogue the parameters of the gutenberg richter distribution have been calculated by means of a monte carlo simulation method the average moment tensors have been computed from focal mechanism data and the strain rate and velocity tensors evaluated by means of kostrov s in izv acad sci ussr phys solid earth 1 23 44 1974 relation which also considers the shape and size of the seismogenic volume the uncertainties have been systematically incorporated the results show that the three seismotectonic provinces are all undergoing shortening at seismic rates 0 3 mm year in the wsw ene direction in the eastern marche adriatic region 0 1 mm year in the n s direction in mainland southern sicily and 0 2 mm year in the nw se direction in the southern tyrrhenian zone the motion pattern in the marche adriatic and in the sicilian provinces suggests that these areas undergo active crust scale deformation along reverse shear zones in agreement with recent horizontal gps motion model and other independent evidence

jurassic to miocene k ar dates from eastern central patagonian cordillera plutons chile 45 48 s thirty nine k ar and one ar ar radiometric dates from the eastern central meso cenozoic patagonian batholith and eastern satellite plutons of the aysen region of southern chile between latitudes 45 and 48 s combined with previous dating of seven plutons have yielded six age groups 1 middle to late jurassic 2 early cretaceous 3 mid cretaceous 4 late cretaceous 5 oligocene and 6 miocene in general the cretaceous and younger ages correspond to previous reported ages for other parts of the main batholith but for the satellite plutons to the east show a wider age spectrum than the previously accepted late miocene dates these results indicate a relatively continuous late jurassic to mid cretaceous plutonism known to have been coeval with volcanic activity followed by intermittent magmatism biotite k ar dates of c 143 151 and 106 109 ma from cataclastic granitoids may be marking the time of deformation a review of all radiometric data on magmatic rocks from the region between 45 and 48 s in chile shows a gap in palaeocene ages that may correlate with a period of low angle flat slab subduction between 65 50 ma

rb sr geochronology of graham land antarctica thirty five new rb sr whole rock and mineral isochrons are reported for igneous rocks from graham land magmatism was essentially continuous but peaks of activity occurred in early jurassic early cretaceous late cretaceous and eocene times the oldest dated granite is triassic 209 3 ma volcanic rocks were erupted throughout this history and some previously supposed upper jurassic volcanic rocks must be mid cretaceous or younger a change in magma type from uniformly acid to predominantly basic or intermediate combined with westward migration of the arc may be partly responsible for previously reported transverse geochemical variations there is a marked trend of decreasing initial 87 sr 86 sr ratio of the plutonic rocks with time from c 0 707 to 0 704 although volcanic rocks tend to have higher values this trend is ascribed to the waning influence of an as yet unidentified pre magmatic crustal basement due to increasing depth of magma generation close parallels to the observed age pattern imply continuity of the andean belt through southern south america into graham land back to early mesozoic times and are not compatible with reconstructions of gondwana in which the antarctic peninsula is located w of south america

cretaceous arc volcanism of palmer land antarctic peninsula zircon u pb geochronology geochemistry distribution and field relationships abstract the cretaceous marks an episode of enhanced magmatism sedimentation and tectonic processes along the entire proto pacific convergent margin of west gondwana cretaceous magmatism across the antarctic peninsula is dominated by the intrusive lassiter coast intrusive suite which developed as episodic flare up events during the mid cretaceous at a time of increased convergence volcanic rocks of this age are poorly defined as a consequence of limited field observations and an absence of accurate geochronology recent field mapping combined with unpublished field observations has identified a region 10 000 km2 of dominantly subaerial rhyolitic pyroclastic and epiclastic successions from northern palmer land of the antarctic peninsula volcanic successions up to 1500 m in thickness consist of dominantly silicic ignimbrites lavas heterolithic breccias and lahar deposits fed by caldera forming eruptions the volcanic rocks of this region were widely considered to be early cretaceous in age based on field relationships and early geochronology new u pb zircon ages identify three distinct volcanic episodes during the late cretaceous early cenozoic at 108 ma 93 ma and 64 ma lu hf data indicate significant proportions of ancient crust in the petrogenesis of the volcanic rocks and the slightly negative ehf values are consistent with a mid position 120 km within the magmatic arc relative to more juvenile compositions close to the trench

the lower jurassic hanson formation of the transantarctic mountains implications for the antarctic sector of the gondwana plate margin the hanson formation antarctica consists of interbedded sandstones and tuffaceous rocks of early jurassic age the sandstones pebbly to medium grained range between quartzo feldspathic and volcaniclastic with some of the former being coarse grained arkoses that imply proximal sources geochronology of detrital zircons provides evidence for source rock ages whereas sandstone petrology demonstrates a mixed provenance tuffaceous strata are reworked fine to very fine grained tuffs resulting from distal plinian eruptions dated tuffs provide time constraints on the duration of volcanism the sandstones and tuffs accumulated in a rift environment geochemically the tuffs are rhyolitic in composition and the sr and nd isotope data together with the patterns on multi element diagrams suggest they were derived from a volcanic arc which is interpreted to have been located along the west antarctic gondwana margin the silicic volcanism extends the distribution and timing of magmatism in the early jurassic along that margin the early jurassic extensional regime was delimited by the plate margin region and the east antarctic craton the rift valley system along the east antarctic craton margin in which the hanson strata accumulated was the focus for subsequent emplacement of the intrusive and extrusive rocks of the lower jurassic ferrar large igneous province the early jurassic extensional rifts may have been reactivated during cretaceous cenozoic development of the west antarctic rift system

quartz latite rheoignimbrite flows of the etendeka formation north western namibia the etendeka formation of north western namibia consists of a sequence of interbedded quartz latites and tholeiitic basalts and forms part of the karoo igneous province in southern africa the age of the etendeka formation is approximately 130 135 ma the quartz latites make up a significant proportion of the stratigraphic succession 25 of the total stratigraphic thickness and form as much as 60 of the outcrop area in the southern etendeka apart from some systematic differences between pitchstones and devitrified quartz latite largely explained by alteration processes individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation geochemistry can be used as a primary criterion for the correlation of major quartz latite units over much of the southern etendeka area enabling the reconstruction of the etendeka formation stratigraphy in this region individual quartz latite units occur as voluminous 400 2600 km3 widespread up to 8800 km2 sheet like deposits typically between 40 and 300 m thick each unit consists of basal main and upper zones the main zone generally constitutes over 70 of the thickness of the unit and typically consists of texturally featureless devitrified quartz latite in contrast the basal and upper zones of the flow are characterised by flow banding pitchstone lenses and breccia with rare occurrences of pyroclastic textures the quartz latites are sparsely porphyritic 10 phenocrysts with glassy or devitrified groundmass textures the phenocrysts consist of plagioclase pyroxene titanomagnetite and rare ilmenite pyroxene geothermometry indicates high 1000 1100 c temperatures of crystallisation which coupled with the absence or primary hydrous phases indicates that the quartz latites were relatively hot h2o undersaturated magmas the quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style the local preservation of pyroclastic textures and the broad areal extent of these units lead to the conclusion that the quartz latites are high temperature rheomorphic ignimbrites i e rheoignimbrites a combination of high eruption temperature and relatively low viscosity helps to explain the often completely welded and homogeneous textures observed in most quartz latite outcrops in the etendeka area

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

contrasting evolution of calc alkalic volcanic and plutonic rocks of western chihuahua mexico two sequences of continental arc igneous rocks occur in the batopilas area of the sierra madre occidental of northwestern mexico the older includes late cretaceous to early cenozoic granitoids and the younger is a mid tertiary andesite to rhyolite series that is represented largely by voluminous ignimbrites the latter sequence contains rocks varying from 54 to 78 wt sio2 whereas the granitoids have a more restricted range in sio2 57 68 wt the major element chemistry of the two sequences is generally similar however the older granitoids have lower zr nb and higher nb y and k rb compared to volcanic rocks in addition the granitoids have generally smaller eu anomalies lower concentrations of ree and higher ce yb n 4 6 7 2 versus 3 8 4 0 than the volcanic rocks the intermediate rocks of the two sequences quartz diorites and andesites had similar petrogenetic histories and both may have originated by fractionation of mantle derived basaltic magmas in contrast the more silicic rocks granodiorites on one hand and dacites and rhyolites on the other evolved very differently the granodiorites may have originated from a crustal source leaving a residue of amphibolite or garnet granulite whereas the rhyolites were the product of protracted fractional crystallization of mantle derived melts

the bulawayan supergroup a late archaean passive margin related large igneous province in the zimbabwe craton the four principal lithostratigraphic units koodoovale manjeri reliance and zeederbergs of the widespread late archaean bulawayan supergroup in the zimbabwe craton record the early to middle stage development of a major c 250 000 km 2 volcanosedimentary basin the craton wide c 2 70 ga manjeri unit varies systematically from a tens of metres thick dominantly chemogenic basinal sediment in gradational conformable contact with the underlying several kilometres thick diachronous 2 83 2 71 ga felsic koodoovale unit in the north and west to an up to hundreds of metres thick dominantly siliciclastic retrogradational platformal sediment unconformably overlying protocratonic basement in the south and east the intermittent c 2 70 ga reliance unit consists of the flow fields of several discrete komatiitic sill flow complexes scattered throughout the craton and fed by an interlinked system of mid to upper crustal flow through feeder sills the c 2 70 ga zeederbergs unit comprises a submarine flood basalt sequence of several kilometres thickness that erupted from ene trending parallel mafic dykes preserved in the south and east the basalts form a craton wide blanket to the reliance flow fields and intervening manjeri sediments certain structural stratigraphic features rule out primary construction of the bulawayan supergroup by early lateral accretion as proposed by several previous workers instead evolution of the bulawayan basin can best be explained by vertical accretion involving 1 abundant episodic and bimodal plume related volcanism and 2 extension and rapid subsidence of continental lithosphere many aspects of the bulawayan supergroup provide important insights into archaean tectonics mantle plume activity magma generation and lithospheric plumbing

transition from back arc to foreland basin development in the southernmost andes stratigraphic record from the ultima esperanza district chile during the mesozoic evolution of the southernmost andes back arc basin formation in an extensional setting was followed by foreland basin development in a compressional setting a stratigraphic record of this tectonic transition is described from jurassic and cretaceous rock units exposed in the patagonian fold thrust belt at 51 s latitude initiation of an extensional deep marine trough in late jurassic time is documented by interstratified rhyolites and marine mudstones in the tobifera formation development of the early cretaceous rocas verdes back arc basin is recorded by zapata formation submarine slope deposits which draped the west facing passively subsiding cratonic margin of the basin the onset of andean compressional orogenesis and creation of the magallanes foreland basin is marked by the influx of coarse grained sandstone turbidites of the albian cenomanian punta barrosa formation during late cretaceous time the foreland basin configuration consisted of a narrow foredeep trough bounded by a gently sloping foreland ramp on the craton to the east this geometry reflects greater flexural subsidence of the stretched and thermally weakened passive back arc basin margin beneath the load of the obducted rocas verdes basin floor changes in depositional regime and sediment dispersal patterns in the latest cretaceous together with an eastward shift in the magallanes basin depocenter record cratonward migration of deformation in the patagonian fold thrust belt loading of the thicker cratonic lithosphere caused subsidence of the foreland ramp to form a wide flexural basin

multitrophic interactions in a geothermal setting arthropod borings actinomycetes fungi and fungal like microorganisms in a decomposing conifer wood from the jurassic of patagonia abstract a permineralized araucarioid conifer stem preserved in state of partial decay is described from the middle to late jurassic chon aike formation bahia laura group at laguna flecha negra in the deseado massif santa cruz province argentina the tree stem which comprises periderm to the secondary xylem shows evidence of interactions with xylophagous borers fungi actinomycetes and other fungal like microorganisms in the secondary phloem it displays rows of traumatic resin ducts which host abundant mycelium comprising very thin hyphae differentiated into spore producing structures comparable to actinomycete bacteria and spherical fungal like resting structures in the cortical and phloem regions there are cell groups with structural changes which appear variably degraded by fungi the secondary xylem displays decay of cellulose and lignified components at various spatial scales comparable with fungal decay patterns in modern wood further septate fungal filaments sometimes bearing clamp connections are associated with decayed regions the degraded secondary xylem shows circular to oval borings filled with frass containing degraded tracheids comparable to those bored by different wood boring beetles development of traumatic resin ducts in the phloem could have been triggered by penetration of the living stem by these xylophagous arthropods the association of the different components present suggests pathogenic and saprotrophic interactions promoting decomposition of the stem and represents a rarely documented example of multitrophic biological interactions involving several different biological groups in jurassic terrestrial paleoecosystems

geodynamic evolution of the antarctic peninsula during mesozoic times and its bearing on weddell sea history this review of the tectonic evolution of the antarctic peninsula during mesozoic times highlights four main events 1 late triassic late jurassic extension 2 late jurassic early cretaceous dextral transpression 3 early cretaceous extension and 4 mid cretaceous compression magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in early jurassic and late jurassic early cretaceous times the second of these breaks corresponded to the first compressional event there was no apparent hiatus in the magmatic record during the mid cretaceous compressional event although there was a significant change in the pattern of sedimentation in the larsen basin on the eastern margin of the weddell sea at about that time the tectonic evolution of the peninsula is compared to and puts some constraints on existing weddell sea models the late triassic late jurassic arc extension correlates with initial rifting in the weddell sea region during sinistral motion between east and west gondwana however there is no known record of large scale pre mid jurassic transcurrent deformation in the antarctic peninsula that would have been consistent with rotation of west antarctic crustal blocks in the initial rifting period the peninsula wide late jurassic early cretaceous compressional event may correlate with the major change in gondwana plate motions from e w to n s african reference frame separation of east and west gondwana this change probably resulted in formation of an ocean continent boundary along the northern margin of the weddell sea embayment and initial seafloor spreading geological data do not seem to support subduction of southwestern proto weddell sea oceanic lithosphere beneath the eastern margin of the peninsula at that time cretaceous arc extension was coeval with the initial seafloor spreading phase in the weddell sea mid cretaceous arc compression linked to a global increase in ocean floor spreading rates and a superplume event correlates with a change from ne sw to nw se spreading in the weddell sea

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

nd and pb isotope variations in the multicyclic central caldera cluster of the san juan volcanic field colorado and implications for crustal hybridization the epsilon sub nd values for six large volume 100 3000 km sup 3 ash flow tuffs and associated lavas from the multicyclic central caldera cluster of the san juan volcanic field in south central colorado are between those of proterozoic crust in the region and mantle derived basaltic magmas and the values generally become progressively higher in progressively younger tuffs and lavas the increase in the epsilon sub nd values of the tuffs from 8 0 to 6 0 with decreasing age can be modeled by assimilation and crystal fractionation of a mantle derived magma accompanied by an increase of approx 4 units in epsilon sub nd values of the assimilated crust the postulated increase in epsilon sub nd values of the crust is envisioned to have occurred by hybridization of the crust through continued injection of mantle derived magmas during the life of the magmatic system decreasing sup 206 pb sup 204 pb ratios observed in progressively younger tuffs following the initiation of caldera related volcanism cannot however be solely explained by addition of mantle derived magmas to the crust but are more likely to reflect the transfer of lower crustal pb into the upper crust as the magmatic system evolved input of large volumes 300 000 km sup 3 in the san juan volcanic field of mantle derived magmamore resulted in extensive hybridization of preexisting crust suggesting that large scale silicic volcanism involves generation of large quantities of new crust less

crustal growth of the antarctic peninsula by accretion magmatism and extension a subduction accretion model incorporating new geophysical data is presented to explain the geology of the antarctic peninsula from late palaeozoic to cenozoic time according to the model the peninsula consists of overlapping accretionary magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form the crust which contains a small proportion of sialic basement was mainly formed by accretionary and magmatic processes and modified to its present shape by extension the gondwanide orogeny for the antarctic peninsula is interpreted in terms of the accretionary processes

the effects of k metasomatism in the bahia laura volcanic complex deseado massif argentina petrologic and metallogenic consequences abstract the deseado massif is a 60 000 km2 geological region located in southern argentinean patagonia the most important geological feature of this area is an extensive plateau characterized by complex and long lived middle to upper jurassic volcanism composed of intermediate to acid volcanic rocks of the bahia laura volcanic complex this voluminous volcanism originated in a widespread extensional environment that affected all of patagonia related to the opening of the atlantic ocean the volcanic rocks are genetically related to jurassic epithermal deposits consequently the region is an important gold and silver producer with four operational mines cerro vanguardia mina martha manantial espejo and san jose and more than 50 epithermal occurrences undergoing different levels of exploration potassic metasomatism identified in the bahia laura volcanic complex is difficult to recognize in hand specimen but is characterized by k2o na2o ratios higher than 2 5 and l o i values lower than 3 wt and affects only rocks with silica contents over 68 wt the metasomatism produced an intense geochemical change in the affected rocks with significant additions of k2o rb au sb w and a minor enrichment in cs cu as and probably ag and hg strongly depleted elements are mgo cao na2o p2o5 mno sr and u whereas feot tio2 l o i pb and zn are moderately depleted finally sio2 al2o3 mo and most trace elements and ree remained almost invariant during increasing metasomatism strong similarities in lithologies and geochemical signatures observed in k metasomatized areas around the world regardless of their geological settings and fluid origins suggest that volcanic glass probably played an important role as an ion exchanger during alteration due to the high mobility of elements that occurs during k metasomatism no classification or discrimination diagram based on major elements tio2 or rb are considered to be valid for the chemical characterization of volcanic rocks based on metasomatized samples the precious and base metals enrichment depletion pattern of k metasomatized rocks is identical to the geochemical signature observed in the upper parts of low sulfidation environments from the deseado massif this geochemical signature along with spatial relations observed between k metasomatized rocks and epithermal manifestations implies that hydrothermal fluids were responsible for the metasomatism observed in the bahia laura volcanic complex

jurassic hot spring deposits of the deseado massif patagonia argentina characteristics and controls on regional distribution abstract the deseado massif santa cruz province argentinean patagonia hosts numerous middle to late jurassic age geothermal and epithermal features represented by siliceous and calcareous chemical precipitates from hot springs sinters and travertines respectively hydrothermal breccias quartz veins and widespread hydrothermal silicification they indicate pauses in explosive volcanic activity marking the final stages in the evolution of an extensive jurassic ca 178 151 ma volcanic complex set in a diffuse extensional back arc setting heralding the opening of the atlantic ocean published paleo hot spring sites for the deseado massif plus additional sites identified during our recent field studies reveal a total of 23 locations five of which were studied in detail to determine their geologic and facies associations they show structural lithologic textural and biotic similarities with miocene to recent hot spring systems from the taupo and coromandel volcanic zones new zealand as well as with modern examples from yellowstone national park u s a these comparisons aid in the definition of facies assemblages for deseado massif deposits proximal middle apron and distal siliceous sinter and travertine terraces and mounds with preservation of many types of stromatolitic fabrics that likely were controlled by formation temperature ph hydrodynamics and fluid compositions locally the mapped hot spring deposits largely occur in association with reworked volcaniclastic lacustrine and or fluvial sediments silicic to intermediate lava domes and hydrothermal mineralization all of which are related to local and regional structural lineaments moreover the numerous geothermal and significant epithermal those with published minable resources deposits of the deseado massif geological province mostly occur in four regional nnw and wnw hydrothermal structural belts northwestern northern central and southern defined here by alignment of five or more hot spring deposits and confirmed as structurally controlled by aeromagnetic data the northern and northwestern belts in particular concentrate most of the geothermal and epithermal occurrences hence jurassic hydrothermal fluid flow was strongly influenced by the most dominant and long active geological boundaries in the region the outer limits of the deseado massif horst itself

tectonic events reflected by palaeocurrents zircon geochronology and palaeobotany in the sierra baguales of chilean patagonia abstract the sierra baguales situated north of the torres del paine national park in the magallanes region of southern chile shows a well exposed stratigraphic sequence ranging from the late cretaceous to late pliocene which presents a unique opportunity to study the evolution of sedimentological styles and trends palaeoclimate changes and tectonic events during this period the depositional environment changed from a continental slope and shelf during the cenomanian campanian tres pasos formation to deltaic between the campanian maastrichtian dorotea formation and estuarine in the lutetian bartonian man aike formation during the rupelian a continental environment with meandering rivers and overbank marshes was established rio leona formation this area was flooded in the early burdigalian estancia 25 de mayo formation during the patagonian transgression but emerged again during the late burdigalian santa cruz formation measured palaeocurrent directions in this mesozoic cenozoic succession indicate source areas situated between the northeast and east southeast during the late cretaceous east southeast during the middle eocene and southwest during the early oligocene to early miocene this is confirmed by detrital zircon age populations in the different units which can be linked to probable sources of similar ages in these areas the east southeastern provenance is here identified as the antarctic peninsula or its northeastern extension which is postulated to have been attached to fuegian patagonia during the eocene the southwestern and western sources were exhumed during gradual uplift of the southern patagonian andes coinciding with a change from marine to continental conditions in the magallanes austral basin as well as a decrease in mean annual temperature and precipitation indicated by fossil leaves in the rio leona formation the rain shadow to the east of the andes thus started to develop here during the late eocene early oligocene 34 ma long before the quechua phase of andean tectonics 19 18 ma that is generally invoked for its evolution at lower latitudes

geochemical indicators of gold rich zones in the la josefina epithermal deposit deseado massif argentina abstract the gold deposit at la josefina in the deseado massif of argentina is a low sulfidation epithermal deposit with some features of the intermediate sulfidation style the au occurs in quartz veins and hydrothermal breccias hosted by acid pyroclastic rocks produced by jurassic bimodal volcanism exploration for this deposit type uses geochemical data for vectoring to mineralized rocks although a general suite of elements with anomalous concentrations around low sulfidation deposits is known that suite varies amongst individual deposits which should be studied individually the aim of this study was to determine in the la josefina deposit geochemical indicators of au rich rocks at different scales and to assess the effects of weathering on those indicators to reach these objectives a mineralized zone hosting au rich veins and a non mineralized zone hosting barren veins were compared in terms of vein mineralogy alteration minerals and geochemistry these zones were also compared with estimated element concentrations of the protolith element concentrations in the mineralized zone were then plotted against distance to the au rich veins separating the dataset into two subsets one from strongly oxidized rocks and the other from weakly oxidized rocks considering the degree of oxidation as indicative of relative degree of weathering based on alteration and vein mineralogy we interpret that the mineralized zone was hydrothermally active before during and after au mineralization while the non mineralized zone was active only before au mineralization therefore differences in element concentrations between the estimated protolith and the non mineralized zone represent geochemical changes produced before mineralization whereas differences between mineralized and non mineralized zones represent changes produced during and after mineralization we conclude that in rhyolitic to rhyodacitic pyroclastic rocks of the study area affected by predominantly argillic and silicic hydrothermal alterations high concentrations of au cu hg v co and p and low concentrations of ba are probably the best deposit scale geochemical indicators to distinguish au mineralized zones from non mineralized zones high concentrations of as mn fe pb cr ni mgo and zn as well as high values of chlorite carbonate pyrite index and low concentrations of be and cao can also distinguish mineralized from non mineralized zones high concentrations of au cu pb as hg and co indicate au rich zones independently of lithology and alteration because they were introduced into the veins and host rocks together with au at detailed scale the concentrations of 15 elements vary with distance to au rich veins therefore those elements are considered potential indicators of proximity to au rich veins of those elements au p cu pb hg ba and mgo are the strongest indicators of proximity to the au rich veins in strongly oxidized rocks some indicators of mineralized zones co mn pb zn and mgo and some indicators of proximity to au rich veins p au cu hg and ba should be used with caution or be avoided because weathering modifies their quality as indicators

late paleozoic to jurassic silicic magmatism at the gondwana margin analogy to the middle proterozoic in north america a vast region of upper paleozoic to middle jurassic 300 150 ma silicic magmatic rocks that erupted inboard of the gondwana margin is a possible phanerozoic analogue to the extensive middle proterozoic 1500 1350 ma silicic magmatic province that underlies much of the southern mid continent of north america like the north american rocks the gondwana silicic magmas appear to be melts of crust that formed about 200 300 m y earlier in the north american case this older crust formed and was accreted to the continent during a major period of crustal formation 1700 1900 ma whereas in the gondwana case the crust that melted consisted mainly of magmatic are terranes accreted to the continental margin during the paleozoic in both cases basic to intermediate magmatic rocks are extremely rare and magmatism is less abundant in regions that contain older and previously melted crust the similarities between the north american and gondwana silicic rocks suggest that both suites formed in extensional settings where basaltic magmas ponded at the base of the preheated crust caused extensive crustal melting that inhibited upward passage of the basalts in both cases silicic volcanism occurred after major assembly of a supercontinent by subduction and accretion processes and before breakup of the supercontinent by analogy with the polar wander curves for gondwana the granite rhyolite provinces may have formed during a period of very slow motion of the supercontinents relative to the poles

petrogenesis of rhyolites and trachytes from the deccan trap sr nd and pb isotope and trace element evidence trachytes and rhyolites from salsette island north of bombay have distinctive trace element and isotope features which mark them out from typical crustal melts their highly incompatible trace element and sr nd and pb isotope ratios are similar to those of the associated deccan flood basalts thus the rhyolites and trachytes are closely related to the basalts and a striking compositional gap between 50 and 65 sio2 suggests that the high sio2 rocks evolved by 10 15 partial melting followed by variable amounts of fractional crystallisation the source material could have been basalt within the deccan trap or related gabbroic rocks in deep crustal sill complexes the rhyolites yield an rb sr whole rock age of 61 5 1 9 ma with a slightly high initial 87sr 86sr 0 7085 18 it is argued that crustal extension provides a suitable regime for the generation of acid magmas by partial melting of associated basic rocks

pinguino in bearing polymetallic vein deposit deseado massif patagonia argentina characteristics of mineralization and ore forming fluids the pinguino deposit located in the low sulfidation epithermal metallogenetical province of the deseado massif patagonia argentina represents a distinct deposit type in the region it evolved through two different mineralization events an early in bearing polymetallic event that introduced in zn pb ag cd au as cu sn w and bi represented by complex sulfide mineralogy and a late ag au quartz rich vein type that crosscut and overprints the early polymetallic mineralization the indium bearing polymetallic mineralization developed in three stages an early cu au in as sn w bi stage ps1 a zn pb ag in cd sb stage ps2 and a late zn in cd ps3 indium concentrations in the polymetallic veins show a wide range 3 4 to 1 184 ppm in the highest indium values up to 1 184 ppm relate to the ps2 mineralization stage and are associated with fe rich sphalerites although significant in enrichment up to 159 ppm is also present in the ps1 paragenesis associated with sn minerals ferrokesterite and cassiterite the hydrothermal alteration associated with the polymetallic mineralization is characterized by advanced argillic alteration within the immediate vein zone and sericitic alteration enveloping the vein zone fluid inclusion studies indicate homogenisation temperatures of 308 2 327 c for ps1 and 255 312 4 c for ps2 and low to moderate salinities 2 to 5 eq wt nacl and 4 to 9 eq wt nacl respectively 34s values of sulfide minerals 0 76 to 3 61 indicate a possible magmatic source for the sulfur in the polymetallic mineralization while pb isotope ratios for the sulfides and magmatic rocks 206pb 204pb 207pb 204pb and 208pb 204pb ratios of 17 379 to 18 502 15 588 to 15 730 and 38 234 to 38 756 respectively are consistent with the possibility that the pb reservoirs for both had the same crustal source spatial relationships hydrothermal alteration styles s and pb isotopic data suggest a probable genetic relation between the polymetallic mineralization and dioritic intrusions that could have been the source of metals and hydrothermal fluids mineralization paragenesis alteration mineralogy geochemical signatures fluid inclusion data and isotopic data confirm that the in bearing polymetallic mineralization from pinguino deposit is a distinct type in comparison with the well known epithermal low sulfidation mineralization from the deseado massif

implications for paleomobility studies of the effects of quaternary volcanism on bioavailable strontium a test case in north patagonia argentina abstract strontium isotopes 87sr 86sr are used as geochemical tracers for paleomobility studies because they display predictable and stable patterns in ecosystems primarily controlled by the underlying geological regimes while bedrock geology is stable over thousands of years geomorphological processes can influence the 87sr 86sr in ecosystems over archeologically relevant timescales among these geomorphological processes the deposition and reworking of volcanic sediments over quaternary timescales are little studied but could be an important control of 87sr 86sr variations in many archeological regions north patagonia is a key archeological region to address animal and human movements and an ideal location to test the influence of quaternary volcanism on 87sr 86sr variation as it is located downwind of major volcanic centers in this study we aim to assess the main environmental and geological controls of bioavailable 87sr 86sr and to build a high resolution isoscape using a machine learning regression framework for forthcoming paleomobility studies we sampled several locations and analyzed different types of samples n 94 the ratios show a limited range of variation which is not related to the bedrock geology rather bioavailable 87sr 86sr variations display a progressive increase going eastward away from the andes following dust aerosol deposition and elevation variations r2 0 71 rmse 0 00041 we argue that this trend relates to the deposition and reworking of unradiogenic volcanic sediments by aeolian fluvial and glacial erosion during the quaternary as most of this sediment reworking occurred during glacial periods the current bioavailable 87sr 86sr variations across the study area likely represent a long term average that varied little during the holocene consequently our isoscape provides a solid base for holocene paleomobility studies in north patagonia and underlines the importance of quaternary volcanism processes for interpreting 87sr 86sr data in paleomobility studies in volcanic regions

production of jurassic rhyolite by anatexis of the lower crust of patagonia abstract the mid jurassic marifil and chon aike volcanic rocks of eastern patagonia are part of one of the largest silicic igneous provinces known rb sr geochronology indicates eruptive ages of 175 190 ma for the marifil complex mostly toarcian aalenian the majority of the rocks are isotopically uniform with initial 87sr 86sr 0 7067 0 0003 and endt 4 2 primary magmas of andesitic composition were generated by partial melting of mafic grenvillian lower crust indentified by depleted mantle model ages of 1150 1600 ma lower crustal pyroxene granulite xenoliths can be modelled as residual jurassic source although they may alternatively be co genetic cumulates the dacite rhyolite suite formed by crystal liquid fractionation processes from the primary andesites involving multistage crystallization and re melting during magma ascent with the later stages of evolution being explicable by fractional crystallization of plagioclase amphibole and accessory minerals the province represents large scale lower crustal reworking associated with the unique tectonic and igneous environment of gondwana break up

globally enhanced mercury deposition during the end pliensbachian extinction and toarcian oae a link to the karoo ferrar large igneous province the mesozoic era featured emplacement of a number of large igneous provinces lips formed by the outpouring of millions of cubic kilometres of basaltic magma the radiometric ages of several mesozoic lips coincide with the dates of oceanic anoxic events oaes as a result of these coincidences a causal link has been suggested but never conclusively proven this study explores the use of mercury as a possible direct link between the karoo ferrar lip and the coeval toarcian oae t oae mercury is emitted to the atmosphere as a trace constituent of volcanic gas and may be distributed globally before being deposited in sediments modern marine deposits show a strong linear correlation between mercury and organic matter content results presented here indicate departures from such a simple linear relationship in sediments deposited during the t oae and also during the pliensbachian toarcian transition an event that saw elevated benthic extinctions and carbon cycle perturbations prior to the t oae a number of depositional settings illustrate an increased mercury concentration in sediments that record one or both events suggesting a rise in the depositional flux of this element complications to this relationship may arise from very organic rich sediments potentially overprinting any hg toc signal whereas environments preserving negligible organic matter may leave no record of mercury deposition however the global distribution of coevally elevated hg rich levels suggests enhanced atmospheric mercury availability during the early toarcian potentially aided by the apparent affinity of hg for terrestrial organic matter although the relative importance of aquatic vs terrestrial fixation of hg in governing these enrichments remains uncertain a perturbation in atmospheric hg is most easily explained by enhanced volcanic output it is suggested that extrusive igneous activity caused increased mercury flux to the early toarcian sedimentary realm supporting the potential of this element as a proxy for ancient volcanism this interpretation is consistent with a relationship between lip formation and a perturbed carbon cycle during the pliensbachian toarcian transition and t oae the recording of these two distinct hg excursions may also indicate that the karoo ferrar lip released volatiles in temporally distinct episodes due either to multiple phases of magmatic emplacement or sporadic release of thermogenic gaseous products from intrusion of igneous material into volatile rich lithologies

the late paleozoic felsic lava dominated large igneous province in northeast germany volcanic facies analysis based on drill cores in the deep subsurface of the northeast german basin products of extensive volcanism are present that formed during the waning phase of the variscan orogeny permian carboniferous boundary large volumes 48 000 km3 of dominantly felsic magma were emplaced in the northern foreland of the variscan mountains in a terrestrial semi arid environment most of these units were inferred by previous authors to represent the products of explosive volcanic eruptions such as ignimbrites however a detailed analysis of the volcanic texture and lithofacies association shows that most units are best interpreted as coherent lavas emplaced either as extrusive units or as shallow intrusions with important implications for the reconstruction of the paleo environment this study shows that detailed textural examination of drill cores provide important observations which can be applied as criteria for the interpretation of the mode of volcanic eruptions furthermore phenocryst logging and geochemical fingerprinting based on immobile element ratios can be employed for the classification and discrimination of individual emplacement units

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

pacific microplate and the pangea supercontinent in the early to middle jurassic new biostratigraphic data based on radiolarians recovered from deep within the oceanic crustal section of ocean drilling program hole 801c in the western pacific along with existing radiometric information date this oceanic crust as late bajocian early bathonian 170 165 ma the overlying basal sediments at hole 801c are essentially identical in age middle bathonian 164 162 ma to the basal sediments at deep sea drilling program hole 534a in the central atlantic we estimate the time of formation of the pacific plate as 175 170 ma and the time of initial separation of the pangea supercontinent in the central atlantic as 190 180 ma we also identify a time of extensive subduction zone magmatism 175 159 ma at the eastern and western edges of pangea we suggest that the initial plate separation of pangea increased subduction rates at its outer margins and altered the plate boundaries in the pacific superocean leading to formation of the pacific plate

petrogenesis of the puerto eden igneous and metamorphic complex magallanes chile late jurassic syn deformational anatexis of metapelites and granitoid magma genesis a suite of schists gneisses migmatites and biotite granitoids from the puerto eden igneous and metamorphic complex peimc and biotite hornblende granitoids of the south patagonian batholith southern chile has been studied for that purpose the chemistry of minerals and the bulk rock composition of major and trace elements including rb sr and sm nd isotopes were determined mineralogical observations and geothermobarometric calculations indicate high temperature and low pressure conditions ca 600 700 c and 3 to 4 5 kbar for an event of metamorphism and partial melting of metapelites in late jurassic times previously determined by shrimp u pb zircon ages structures in schists gneisses migmatites and mylonites indicate non coaxial deformation flow during and after peak metamorphic and anatectic conditions andalusite schists and sillimanite gneisses yield initial 87sr 86sr ratios of up to 0 7134 and end150 values as low as 7 6 contemporaneous biotite granitoids and a coarse grained orthogneiss have initial 87sr 86sr ratios between 0 7073 and 0 7089 and end150 values in the range 7 6 to 4 4 this indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source thus a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids the peimc is considered as a segment of an evolving kilometre sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in late jurassic times a biotite hornblende granodiorite and a muscovite garnet leucogranite show initial 87sr 86sr ratios of 0 7048 and 0 7061 and end100 values of 2 6 and 1 8 respectively and are thus probably related to early cretaceous magmas not involved in the anatexis of the metasedimentary rocks

40ar 39ar dating of the jurassic volcanic province of patagonia migrating magmatism related to gondwana break up and subduction abstract the mesozoic large igneous province lip of patagonia southern south america which is one of the largest silicic provinces on earth has been investigated by the 40ar 39ar method twenty seven ages considered as valid including twenty plateau ages show that the volcanic activity ranging from 187 to 144 ma occurred between and contemporaneously with the initial break up of gondwana starting with the karoo antarctic tasmanian kat flood basalt province in the east and a subduction in the west the data display a regular decreasing of ages from the ene 187 ma to the wsw 144 ma along about 650 km apparently related to the tectonic structure in half grabens oriented nnw sse the good fitting of this trend with the opening of the rocas verdes sarmiento marginal basin favors a space time evolution of this continental volcanism culminating towards the ssw in a continental disruption behind the magmatic arc the observed age progression of volcanism may be the result of the variations of the physical characteristics of the subduction the spreading and thermal effect of the kat plume may have an additional effect and also could account for the unusually large volume of magma

the role of crustal melting in the formation of rhyolites constraints from sims oxygen isotope data chon aike province patagonia argentina