from migmatite to magma crustal melting and generation of granite in the camboriu complex south brazil abstract partial melting of continental crust and granite genesis were investigated in the camboriu region south brazil structural geology and geochemistry were used in order to characterize the link between the high ba sr itapema granite ig and the syntectonic melting of orthogneisses and amphibolites of the camboriu complex cc the ig is a sheet like intrusive body of biotite monzogranite to granodiorite with large amount of migmatitic amphibolite and gneiss xenoliths and a well developed magmatic foliation concordant with the main gneissic banding discordant m thick granitic dykes which crosscut the migmatitic sequence and transport magma and xenoliths to feed the ig main body mark the transition from the migmatitic to the magmatic system within the complex whole rock geochemical data reveal important contamination and assimilation processes of xenoliths during ig magmatic flow which give rise to different facies and geochemical signatures the generation of the ig is an example of a continuous exposure from source to sink formed through a continuous and efficient deformation driven melting extraction transport system generated during coeval folding and anatexis of the cc metamorphic sequence

late cenozoic transpressional ductile deformation north of the nazca south america antarctica triple junction abstract the southern andes plate boundary zone records a protracted history of bulk transpressional deformation during the cenozoic which has been causally related to either oblique subduction or ridge collision however few structural and chronological studies of regional deformation are available to support one hypothesis or the other we address along and across strike variations in the nature and timing of plate boundary deformation to better understand the cenozoic tectonics of the southern andes two east west structural transects were mapped at puyuhuapi and aysen immediately north of the nazca south america antarctica triple junction at puyuhuapi 44 s north south striking high angle contractional and strike slip ductile shear zones developed from plutons coexist with moderately dipping dextral oblique shear zones in the wallrocks in aysen 45 46 top to the southwest oblique thrusting predominates to the west of the cenozoic magmatic arc whereas dextral strike slip shear zones develop within it new 40ar 39ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike slip oblique slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen similar ages were obtained on both high strain pelitic schists with dextral strike slip kinematics 4 4 0 3 ma laser on muscovite biotite aggregates aysen transect 45 s and on mylonitic plutonic rocks with contractional deformation 3 8 0 2 to 4 2 0 2 ma fine grained recrystallized biotite puyuhuapi transect oblique slip dextral reverse kinematics of uncertain age is documented at the canal costa shear zone 45 s and at the queulat shear zone at 44 s published dates for the undeformed protholiths suggest both shear zones are likely late miocene or pliocene coeval with contractional and strike slip shear zones farther north coeval strike slip oblique slip and contractional deformation on ductile shear zones of the southern andes suggest different degrees of along and across strike deformation partitioning of bulk transpressional deformation the long term dextral transpressional regime appears to be driven by oblique subduction the short term deformation is in turn controlled by ridge collision from 6 ma to present day this is indicated by most deformation ages and by a southward increase in the contractional component of deformation oblique slip to contractional shear zones at both western and eastern margins of the miocene belt of the patagonian batholith define a large scale pop up structure by which deeper levels of the crust have been differentially exhumed since the pliocene at a rate in excess of 1 7 mm year

zircon u pb shrimp dating of a neoproterozoic overprint in paleoproterozoic granitic gneissic terranes southern brazil the neoproterozoic florianopolis batholith provides the major clue for unraveling the pre brasiliano cycle evolution of santa catarina state southern brazil because it contains septa of basement units such as the camboriu and aguas mornas complexes reactivated during the younger brasiliano orogeny at about 600 ma we establish precise u pb zircon shrimp ages and nd isotopic evolution of four key samples from the septa the zircon population displays complex internal structures in three of the four dated samples the complex crystals have well preserved magmatic cores and altered rim domains the core populations were dated at about 2180 ma aguas mornas complex about 2160 ma camboriu complex amphibolite xenolith and 2000 ma camboriu complex orthogneiss whereas the ages of the measured rims and altered domains are about 590 ma in all three samples on the other hand the presidente nereu tonalite has a simpler zircon population without core rim separation and yields magmatic ages about 2200 ma the nd isotopic analyses of the three samples emphasize the polycyclic evolution of the batholith which is much more complex than previously realized the evolution is marked by two successive paleoproterozoic accretionary orogenic events at about 2200 2175 ma and by a crustal melting event at about 2000 ma which had an archaean source sm nd depleted mantle model age tdm 2860 ma these orogenic events were followed 1 4 billion years later by a neoproterozoic hydrothermal metamorphic overprint at about 590 ma this time gap permits us to identify a long intracratonic period the atlantica supercontinent the unraveling of the proterozoic evolution and the precise characterization of the basement remnants show that the paleoproterozoic was a crust building orogenic interval and was the major source of the much later neoproterozoic granites the integrated use of back scattered electron and cathodoluminescence imaging with shrimp spot dating and nd isotopes provide a clear understanding of the timing of crustal generation and deformation in southern brazil including strong evidence for a long lasting supercontinent cycle

diachronic collision slab break off and long term high thermal flux in the brasiliano pan african orogeny implications for the geodynamic evolution of the mantiqueira province abstract the mantiqueira province in se brazil and uruguay is a complex brasiliano pan african orogenic province that has recently been the topic of conflicting geodynamic models focusing the possibility of multiple collision events during the ediacaran ordovician assembly of the western gondwana supercontinent a thorough compilation of recently obtained high resolution u pb geochronology and geochemistry in the mantiqueira province provided the following results a considering the entire orogenic system the pre syn and post orogenic periods range from 670 595 ma 620 550 ma and 560 490 ma respectively and are coeval with the intrusion of 3 distinct magmatic sequences within each geographic segment of this large orogenic province evolving from i calc alkaline arc related magmatism to ii syn orogenic anatectic magmatism coeval with the metamorphic climax to iii calc alkaline alkaline including shoshonitic and a type affinity magmatism that reveal significant lower crust and mantle contribution b progressively younger ages are present from south to north irrespective of the orogenic period considered implying that the different geographic sectors were from south to north diachronically amalgamated into the mantiqueira province results provide important evidence for the existence of a single diachronic collision event at 630 590 ma followed by the thermal climax at 615 560 ma slab break off asthenospheric upwelling and mantle underplating that sustained long term high heat flux conditions until 540 490 ma when post tectonic calc alkaline alkaline granitoids probably admixed with small amounts of slab melts formed at depth intruded the middle low crust magmatic and metamorphic sequences during the final stages of thermal and orogenic collapse of the mantiqueira province belts

space time relation of post collisional granitic magmatism in santa catarina southern brazil u pb la mc icp ms zircon geochronology of coeval mafic felsic magmatism related to the major gercino shear zone abstract la mc icp ms u pb zircon dating was performed on syntectonic early post collisional granitic and associated mafic rocks that are intrusive in the brusque metamorphic complex and in the florianopolis batholith major tectonic domains separated by the neoproterozoic major gercino shear zone mgsz in south brazil the inferred ages of magmatic crystallization are consistent with field relationships and show that the syntectonic granites from both domains are similar with ages around 630 620 ma for high k calc alkaline metaluminous granites and ca 610 ma for slightly peraluminous granites although ca 650 ma inherited zircon components are identified in granites from both domains important contrasts on the crustal architecture in each domain are revealed by the patterns of zircon inheritance indicating different crustal sources for the granites in each domain the granites from the southern domain florianopolis batholith have essentially neoproterozoic 650 700 ma and 900 950 ma inheritance with a single 2 0 2 2 ga inherited age obtained in the peraluminous mariscal granite in the northern brusque metamorphic complex the metaluminous rio pequeno granite and associated mafic rocks have scarce inherited cores with ages around 1 65 ga whereas the slightly peraluminous serra dos macacos granite has abundant paleoproterozoic 1 8 2 2 ga and archean 2 9 3 4 ga inherited zircons our results are consistent with the hypothesis that the mgsz separates domains with distinct geologic evolution however the contemporaneity of 630 610 ma granitic magmatism with similar structural and geochemical patterns on both sides of this major shear zone indicates that these domains were already part of a single continental mass at 630 ma reinforcing the post collisional character of these granites

deformation and geochronology of syntectonic granitoids emplaced in the major gercino shear zone southeastern south america abstract the major gercino shear zone is one of the ne sw lineaments that separate the neoproterozoic dom feliciano belt of brazil and uruguay into two different domains a northwestern supracrustal domain from a southeastern granitoid domain the shear zone striking ne is composed of protomylonites to ultramylonites with mainly dextral kinematic indicators in santa catarina state southern brazil the shear zone is composed of two mylonite belts the mylonites have mineral orientations produced under greenschist facies conditions at a high strain rate strong flattening and coaxial deformation indicate the transpressive character while the role of pure shear is emphasized by the orientation of the mylonite belts in relation to the inferred stress field component the quartz microstructures point out that different dynamic recrystallization regimes and crystal plasticity were the dominant mechanisms of deformation during the mylonitization process additionally the fabrics suggest that the glide systems are activated for deformation conditions compatible with the metamorphism in the middle greenschist facies elongated granitoid intrusions belonging to two petrographically geochemically and isotopically distinct rock associations occur between the two mylonite belts the structures observed in the granites result from a deformation range from magmatic to solid state conditions points to a continuum of magma straining during and just after its crystallization conventional u pb analysis of multi crystal zircon fractions yielded essentially identical ages of 609 16 ma and 614 2 ma for the two granitic associations and constrain the transpressive phase of the shear zone k ar ages of biotites between 585 and 560 ma record the slow cooling and uplift of the intrusions some k ar ages of micas in regional mylonites are similar suggesting that thermo tectonic activity was intense up to this time probably related to the agglutination of the granite belt to the supracrustal belt nw of the mgsz

the mesoproterozoic supercontinent atlantica in the brazilian shield review of geological and u pb zircon and sm nd isotopic evidence abstract supercontinent atlantica was formed in south america by the orogenies of the trans amazonian cycle namely the accretionary encantadas orogeny 2250 2100 ma and the collisional camboriu orogeny 2100 2000 ma large belts 500 to more than1000 km long formed during the trans amazonian cycle in the three major cratons of south america in the northern part of the amazon craton in the sao francisco craton and in the rio de la plata craton however other large 1000 km segments of archean crust remained little deformed in the cratonic interior of supercontinent atlantica such as a major portion of the tocantins province in central brazil in addition smaller fragments up to 100 km of trans amazonian cycle continental crust are present inside the neoproterozoic brasiliano cycle orogenic belts in the eastern half of extra andean south america in contrast the dominant volume about 90 of neoproterozoic belts was formed by reworking of archean and paleoproterozoic crust zircon u pb geochronology associated with nd isotopes indicates the widespread presence of supercontinent atlantica crust in south america and sensitive high mass resolution ion microprobe investigation of zircons indicates that crust in southern brazil was formed mostly in the trans amazonian cycle zircon geochronology also reveals that supercontinent atlantica was preceded by an intra cratonic period lasting about 300 my and was succeeded by an intra cratonic period of about 1000 to 1300 my many of the trans amazonian cycle belts and crustal fragments may have been accreted to south america during the neoproterozoic the demonstration that they all were once part of supercontinent atlantica must await further multi technique investigations

intrusion level of granitic massifs along the hercynian belt balancing the eroded crust abstract hercynian granitoid intrusions form a long 3200 km belt comparable in size to other batholiths in the world six massifs have been selected which encompass cabeza de araya extremadura spain guitiriz galicia spain pontivy and mortagne brittany france la marche massif central france and fichtelgebirge bavaria germany detailed gravity surveys over these massifs and subsequent inversion provide their shape at depth correlation of the deeper zones with internal structures determine the place of the root zones the shape of the massifs is examined along the strike of the chain the emplacement of individual massifs is controlled by local tectonics most granites are not deeply rooted but one massif cabeza de araya spain shows a root zone presently as deep as 14 km most have about half of their volume in the first 3 km below the present surface estimates of the magma volume transferred result in 1500 km 3 issued from one specific feeder yielding a total of 70 000 km 3 of magma intruded all along the chain the depth of emplacement of the granitic massifs does not show any significant trend along the strike of the chain the shallower massifs in the french massif central correspond to more deeply eroded areas in the center of the chain their root zone as well as the change in the dip of the walls are presently observed at depths ranging between 4 and 6 km in hercynian granites both variations are interpreted as being related to the brittle ductile transition at the time of emplacement gross thermal considerations place the transition at its former place during magma emplacement indicating that the upper crust has not been eroded by more than 6 8 km this estimate severely contrasts with models involving a doubled crust

u pb zircon ages from volcanic and sedimentary rocks of the ediacaran bas draâ inlier anti atlas morocco chronostratigraphic and provenance implications abstract u pb la icp ms analyses on zircon extracted from felsic ignimbrites and sandstones of the bas dra succession bds constrain the post pan african evolution in the anti atlas aa at the northern margin of gondwana the 2000 m thick ediacaran bds consists of four volcanic units including i rhyolitic ignimbrite sheets ao i ii high silica andesitic and rhyolitic lavas ao ii iii silica poor lava and pyroclastics ao iii and iv rhyolitic lava dome complexes ao iv ao iii is dominated by basaltic and andesitic lava and it contains at least three sandstone lenses the bds is capped by an angular unconformity which is defined by sedimentary rocks of the adoudou formation of the taroudant group the u pb data set 99 zircon grains with 90 110 concordance from the ignimbrite yields an age of 565 6 ma 564 8 5 6 ma interpreted as the crystallization age which represents the early volcanic activity in the bds for the upper bds maximum ages of ca 552 and ca 541 ma are indicated by u pb ages of detrital zircons extracted from sandstone these age constraints confine the bds to the upper ouarzazate supergroup most detrital zircon ages three sandstone samples with 367 zircon grains analyzed fall in the range of ca 606 541 ma this main ediacaran peak is interpreted to have been derived from the post pan african magmatic complexes surrounding the west african craton wac secondary age peaks at ca 652 ma and ca 655 ma are inferred to originate from early to late pan african magmatism of the anti atlas region due to the complete absence of pre neoproterozoic zircon ages our data set is interpreted to indicate a large voluminous volcanic episode that covered the palaeoproterozoic and cryogenian basement or the river system was confined to the bds basin facies and provenance analyses of bds sandstones indicate that the detrital input was mostly locally derived with a minor amount from an external source in addition large peaks of older zircon ages are recognized only on the lower sandstone lens bd 86 aoiii thus the drainage system was large within the lower part and became more localized at the end of bds evolution the maximum depositional age for the zircons separated from the uppermost ao iii sandstone lens 541 ma shifts the precambrian cambrian boundary down from the lower adoudou formation as proposed by previous studies to the uppermost ouarzazate supergroup or to the hiatus between both units the volcanic activity within the bds is linked to a transtensional regime that operated along the gondwana margin during the ediacaran the bds volcanic activity is contemporaneous with the widespread volcanic volcanoclastic and siliciclastic rocks of upper ouarzazate supergroup

tectonics structural evolution and caldera formation on vulcano island aeolian archipelago southern tyrrhenian sea abstract a detailed structural study and a review of the eruptive history of the island of vulcano aeolian arc southern tyrrhenian sea have been carried out with the purpose of understanding the evolution of both volcanic and tectonic structures the island is composed of four main volcanoes which date from about 120 ka to historical times two calderas occur in the central sector of the island the time space evolution of the volcanism indicates a shifting of the activity from the southeastern sectors towards the northwest two main systems of nw se trending right lateral strike slip faults affect the island ne sw and n s trending normal faults are also present geophysical investigation indicate the nw se trending strike slip faults are likely to be the surface expression of a main regional crustal structure this system of discontinuity is related with the stress field acting in the southern sector of the aeolian archipelago normal n s and ne sw oriented faults represent extensional fractures the ne sw trending structures are related to a local stress field the results of the kinematic analysis as well as the geometric relationships between the faults and the constant subsidence velocities are consistent with the opening of a pull apart basin volcanological and geochronological data are also consistent with the proposed model in the general deformative style affecting the island the calderas can be explained as the consequence of different opening phases of a pull apart basin

relationship between tectonism and volcanism along the great sumatran fault zone deduced by spot image analyses abstract satellite images provide evidence for numerous stepovers pull apart grabens and volcanic structures along the nw trending right lateral great sumatran fault zone high resolution spot images permit us to analyze the relationship between volcanic and tectonic structures this analysis reveals that releasing stepovers and pull aparts are ephemeral structures along the great sumatran fault zone and that the geometry of the strike slip fault evolves permanently through time the interpretative structural maps obtained from spot image analysis reveal that the formation of huge peculiarly shaped volcanic calderas has occurred in large releasing stepover fault zones and that the bounding faults of rectangular pull apart basins are analogous to the circular ring faults of calderas in particular detailed study of the segments at the southernmost 150 km long termination of the great sumatran fault zone from northwest of ranau lake to semangka bay allows us to describe the development of the ranau collapse caldera this caldera is located within a pull apart basin bounded by a stepover its peculiar rectangular shape and its relatively large size about 200 km 2 are controlled by the evolution of the extinct ranau releasing stepover similarly the toba elliptical caldera one of the largest volcanic caldera in the world is elongated parallel to the present trace of the great sumatran fault and appears to be related to a releasing stepover associated with a wide pull apart basin that is not active at present

structural evolution and tectonic setting of the porongos belt southern brazil the sw ne striking porongos belt located between juvenile neoproterozoic rocks in the west and the dom feliciano belt characterized by intense reworking of older crust in the east comprises a greenschist to amphibolite facies metavolcano metasedimentary succession porongos sequence of unknown age with some exposures of palaeoproterozoic gneisses encantadas gneisses high temperature ductile deformation of the basement gneisses comprises at least two magmatic events followed by three deformational phases including folding and shearing dt1 dt3 and can be attributed to the palaeoproterozoic trans amazonian orogeny the deformation of the porongos sequence occurred during the neoproterozoic brasiliano orogeny and comprises four ductile deformation phases db1 db4 including two phases of isoclinal folding associated with shearing recorded in mylonitic layers followed by closed nw vergent folding and thrusting leading to formation of a thrust stack uplift of the basement and formation of late tectonic sedimentary basins occurred as a result of semi ductile to brittle block faulting in a sinistral strike slip regime the porongos sequence can be subdivided into a southeastern and a northwestern part trace element analyses as well as sm nd and rb sr geochemical data indicate partial melting and significant contamination by old continental crust for the metavolcanic rocks the metavolcanic rocks show end t 780 ma values of 20 64 and 21 72 northwestern units and 6 87 southeastern sequence the metasedimentary rocks were derived from late palaeoproterozoic to archaean sources and the data indicate different sources for the northwestern and southeastern rock units of the porongos sequence end t 780 ma are 6 25 and 6 85 in the southeastern units with tdm model ages between 1734 and 1954 ma and vary between 14 72 and 17 96 in the northwestern parts which have tdm model ages between 2346 and 2710 ma high 87 sr 86 sr t values between 0 7064 and 0 7286 confirm reworking of older crust isotopic signatures of the porongos sequence do not show indications for a significant contribution from a neoproterozoic juvenile source a passive margin or continental rift environment is suggested for the tectonic setting of the porongos belt which is compatible with both deposition of shallow marine to deep marine sediments and stretching of continental crust leading to volcanism which is characterized by significant contamination by old continental crust

fracture sealing in zircon as evaluated by electron microprobe analyses and back scattered electron imaging abstract the internal structure of zircon is commonly revealed by back scattered electron bse images grains from a 750 700 ma old talc alkaline mafic metatonalite from southernmost brazil show relict igneous faces irregular and diffuse metamorphic overgrowths and fracture sealing sealing of fractures is observed as light grey patches in bse images epma profiles show strong but differential increases in hf and u in the sealed fractures unlike the common assertion in the literature that fractures are only pathways of leaching sealing of fractures occurs near borders of grains as well as in their cores and is not visible during standard petrographic examination thus careful bse image evaluation is needed before either in situ or whole grain isotopic determinations are made

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

evolution of the major gercino shear zone in the dom feliciano belt south brazil and implications for the assembly of southwestern gondwana the dom feliciano belt developed during the brasiliano pan african orogenic cycle due to the tectonic interaction between the rio de la plata congo and kalahari cratons along with the amalgamation of smaller continental fragments together with its prolongations to the south the major gercino shear zone constitutes one of the main lineaments of the orogenic system establishing a more than 1000 km long ne trending tectonic boundary between a granitic batholith and a metavolcano sedimentary association based on combined field structural microstructural and textural data together with new and published geochronological data a refined model for the geological evolution of the major gercino shear zone is presented regional nw verging low dipping structures were generated between 650 and 615 ma preceding the formation of the shear zone this was followed by the main deformation phase corresponding to pure shear dominated dextral strike slip interpreted do be controlled by regional transpression during oblique convergence of the continental terranes this stage lasted until ca 585 ma and was coeval with the continuous emplacement of granitic magmatism along the structure strain partitioning and localization led to the development of mylonitic belts along the intrusion borders mostly under greenschist facies metamorphic conditions late stage ductile deformation along the structure was active during the cooling of the intrusions until ca 550 ma while the deformation front of the orogenic system migrated to counterpart orogenic belts in the african continent after tectonic stabilization the major gercino shear zone recorded episodic brittle reactivation possibly related to the phanerozoic evolution of the active margins and intracratonic basins in gondwana 2018 springer verlag gmbh germany part of springer nature

geochemistry of the active azufre planchon peteroa volcanic complex chile 35 15 s evidence for multiple sources and processes in a cordilleran arc magmatic system

contrasting nature of deformation along an intra arc shear zone the liquine ofqui fault zone southern chilean andes abstract the liquine ofquifault zone lofz cuts the patagonian batholith and the modern volcanic arc of southern chile for ca 1000 km the rock fabric along three transects of the lofz combined with new and published geochronological data suggest marked differences in the nature and timing of deformation along strike in the liquine transect 39 s a 1 km wide northeast striking subvertical mylonitic zone shows north plunging stretching lineations this mylonitic zone has been juxtaposed by high angle reverse faulting against a nearly undeformed miocene granitoid metamorphic assemblages and microstructures in the mylonites indicate greenschist facies conditions and sinistral reverse displacement deformation pre dates a 100 2 ma undeformed porphyritic dike hornblende 40ar 39ar mean age in the reloncavi transect 41 42 s deformation in cretaceous and miocene plutons is predominantly brittle kinematic analysis of two fault populations yields compressional and dextral strike slip stress regimes interpreted as late miocene in age in the hornopiren transect 42 43 s a 4 km wide mylonitic zone developed in plutons and wallrocks shows subhorizontal stretching lineations and dextral displacement a single fault population overprinting the mylonites supports a dextral strike slip stress regime available u pb k ar and 40ar 39ar dates on deformed cenozoic plutons and wallrock range from 9 to 13 ma on hornblende and from 6 to 3 ma on biotite microstructures and mineral assemblages indicate that the youngest ductile fabrics in the plutons formed at greenschist facies similar to the biotite ar closure temperature subparallel magmatic and solid state fabrics combined with geochronology suggest that dextral displacement was continuous during emplacement and cooling of the plutons dextral oblique subduction of the nazca plate beneath western south america has driven long term intra arc deformation at the southern andes plate boundary zone ridge collision in turn has favored dextral displacement at the leading edge of the continent since the pliocene

u pb and 40ar 39ar geochronology of the baiyunshan gneiss central guangdong south china constraints on the timing of early palaeozoic and mesozoic tectonothermal events in the wuyun wuyi yunkai orogen composite sensitive high resolution ion microprobe shrimp u pb zircon and 40 ar 39 ar step heating biotite hornblende ages are used to provide constraints on the timing and origin of the felsic gneissic rocks in the baiyunshan mountains region and to elucidate their tectonothermal history shrimp dating and cl imaging of zircons give magmatic zircon crystallization ages between late ordovician and early silurian c 453 5 ma 446 ma and 439 ma for three representative felsic gneisses suggesting that most of the baiyunshan gneiss cannot represent basement rocks of the cathaysia block as previously thought including the present age information a synthesis of available age data for regional wuyun wuyi yunkai events reflects the emergence of orogen wide magmatism that could be syn orogenic and have occurred mainly between 460 and 420 ma in the south china block inherited zircons are abundant with ages clustering at late mesoproterozoic 1189 1017 ma and middle mesoproterozoic 772 ma which reveals that the baiyunshan orthogneiss samples a crustal basement containing significant igneous or recycled components related to the rodinia amalgamation and break up a shrimp date of 212 12 ma from a white rim of zircon provides evidence for metamorphic overprinting of an indosinian tectonothermal event on the baiyunshan gneiss incremental heating experiments with six biotite samples and one hornblende sample from a variety of metamorphic rocks yielded two distinct 40 ar 39 ar age groups 150 155 and 94 98 ma the older ages are similar to zircon u pb dates for widespread granitic intrusions in central guangdong we attribute them to late jurassic magmatism induced thermal resetting of the biotite k ar system on the other hand the younger age group is interpreted to record either cooling through the biotite closure temperature of 300 350 c or a second resetting of biotite ar isotopes at c 94 98 ma due to contemporaneous magmatic activity our present age data suggest that the maofengshan orthogneiss was exhumed to 8 to 10 km crustal levels at c 150 ma whereas the eastward components of gneissic rock masses appear to have passed upward through the same crustal depth synchronously or later by c 94 ma exhumation of middle crustal level rocks in the study area since c 155 ma is roughly coeval with exhumation of gneissic rocks from elsewhere in the wuyun orogen suggesting a large scale mechanism for the exhumation pulse related to the yanshanian extensional tectonic regime

relations between surface faulting and granite intrusions in analogue models of strike slip deformation analogue models investigating the emplacement of granitic magmas at upper crustal levels during strike slip deformation are presented in this paper variations in parameters such as ratio between horizontal displacement dv and vertical magma upraising iv and the number of feeding points were considered during the experiments model results show that a strong interaction between surface faulting and granite intrusions develops during deformation on one side magma intrusion influences the sequence of structures development and the final fault pattern on the other side magma emplacement is strongly influenced by the syn intrusion lateral displacement dt which in turn reflects variations in dv and iv for high dt values i e high displacement velocity or low injection rate the importance of the horizontal displacement with respect to the vertical magma upraising results in asymmetric and drop shaped plutons with a sheared tail elongating in the sense of the strike slip displacement in this case the intrusion long axis tracks the long axis of the strain ellipsoid for low dt values i e low displacement velocity or high injection rate the strike slip displacement is strongly subordinated to the vertical magma upraising and pluton emplacement is controlled by development of tensional shears at the surface in these conditions the long axis of the magmatic bodies lies in the quadrants of compression of the strike slip deformation and intrusions are apparently dragged in an opposite sense to that imposed by the transcurrent deformation

the link between volcanism and tectonics in the southern volcanic zone of the chilean andes a review abstract the nature of the interplay between tectonics and volcanism is a major question in continental margin tectonics the southern andes volcanic zone svz located at the obliquely convergent nazca south america plate margin between 33 s and 46 s offers a unique opportunity to address this question because of along strike changes in crustal thickness tectonic style and well constrained long term and short term kinematic history the complex interaction between tectonic and magmatic processes is evidenced by both the architecture and geochemical signature of volcanic systems main first order factors accounting for the along strike variations in the nature and composition of volcanism are crustal thickness and the existence of a major intra arc fault system the liquine ofqui fault zone lofz second order factors include the local nature of the volcanic arc basement two main categories of volcano tectonic associations have been identified according to the spatial distribution and internal organization of individual volcanoes and clusters of volcanoes with respect to both the overall strike of the volcanic arc and the first and second order active inactive basement faults kinematically coupled associations those that are directly related to the current dextral transpressional tectonic regime include ne trending volcanic alignments of stratovolcanoes and or monogenetic cones kinematically uncoupled associations in which coupling with the current regime is not required include stratovolcanoes built on top of ancient reverse and strike slip faults of the volcanic arc basement and monogenetic cones lying along master faults of the lofz whereas mostly primitive magmas are found in the first category more evolved terms can occur genetically associated with the second category we propose a simplified three dimensional model for the magma plumbing system along and across the svz that integrates kinematically coupled and kinematically uncoupled volcano tectonic associations within the framework of first and second order geological and structural constraints

postmagmatic cooling and late cenozoic denudation of the north patagonian batholith in the los lagos region of chile 41 42 15 s zircon and apatite fission track ft thermochronology was applied to investigate the history of cooling and denudation of the southern andes between 41 and 42 15 s in relation to the late cenozoic activity of the liquine ofqui fault zone lofz and the northward migration of the chile triple junction ctj fifty six zircon and 51 apatite ft ages plus 37 apatite confined track length distributions were obtained mainly from plutonic rocks of the north patagonian batholith npb in the main andean cordillera apatite ft ages and track lengths indicate a stage of rapid cooling at 5 3 ma along both sides of the lofz whereas older miocene ages with monotonous cooling histories were obtained further away from the fault zircon ft ages range from cretaceous to pliocene with marked differences observed along and across the lofz three different types of temperature time histories characterise the post magmatic cooling of the npb in the region deep intrusions with moderate and steady cooling rates intrusions in the upper crust with very slow cooling rates following a stage of initial rapid cooling and rapidly cooled and exhumed shallow intrusions the latter with younger ages towards the fault zone the most prominent denudation episode along the lofz is late miocene to pliocene coeval with plate tectonic reconstructions for the arrival and subduction of the chile rise beneath the taitao peninsula

multiple subduction components in the mantle wedge evidence from eruptive centers in the central southern volcanic zone chile in the andean central southern volcanic zone basalts from small eruptive centers near the large composite center volcan villarrica are poor in fluid mobile elements such as b cs rb k pb ba and u compared with concurrently erupted villarrica basalts new 1 0 be and u series isotopic data for these centers show that fluid mobile element poor small eruptive center basalts have small 1 0 be 9 be ratios 1 6 1 9 10 1 1 and 2 3 8 u 2 3 0 th activity ratios near 1 0 whereas basalts from villarrica show 2 3 8 u enrichment and larger 1 0 be 9 be ratios 4 0 6 4 10 1 1 these results suggest that small eruptive center basalts include materials derived from the subducted lithosphere that were stored in the mantle wedge for 350 k y to 3 m y that these materials are poor in fluid mobile elements may reflect fluid expulsion during solidification or their formation in an initially hotter subduction setting in contrast the composite center basalts sample materials rich in fluid mobile elements that were recently transferred into the mantle wedge from the subducted lithosphere the results confirm that mantle wedges in subduction zones include subducted materials added to the wedge over both long and short time scales

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

geochemistry and crustal evolution of volcano sedimentary successions and orthogneisses in the sao gabriel block southernmost brazil relics of neoproterozoic magmatic arcs abstract precambrian metaplutonic rocks of the sao gabriel block in southernmost brazil comprise juvenile neoproterozoic calc alkaline gneisses cambai complex the connection with associated ultra mafic metavolcanic and metasedimentary rocks palma group is not well established the whole complex was deformed during the brasiliano orogenic cycle both metasedimentary and metavolcanic rocks as well as metaplutonic rocks of the cambai complex have been sampled for geochemical analyses in order to get constraints on the tectonic setting of these rocks and to establish a tectonic model for the sao gabriel block and its role during the assembly of west gondwana the major element compositions of the igneous rocks palma group and cambai complex indicate a subalkaline character most orthogneisses have a calc alkaline chemistry many metavolcanic rocks of the palma group show signatures of low k tholeiitic volcanic arc basalts trace element data especially ti zr y nb of most igneous samples from both the lower palma group and the cambai complex indicate origin at plate margins i e in a subduction zone environment this is corroborated by relative enrichment in lree low contents of nb and other high field strength elements and enrichment in lile like rb ba and th the data indicate the possible existence of two suites an oceanic island arc and a continental arc or active continental margin however some ultramafic samples of the lower palma group in the western sao gabriel block indicate the existence of another volcanic suite with intra plate character which possibly represents relics of oceanic island basalts oib trace element data indicate contributions from andesitic to mixed felsic and basic arc sources for the metasedimentary rocks the patterns of chondrite and n morb normalized spider diagrams resemble the patterns of the igneous rocks i e lile and lree enrichment and hfs depletion the geochemical signatures of most igneous and metasedimentary samples and their low 87sr 86sr t ratios suggest only minor contribution of old continental crust a geotectonic model for the sao gabriel block comprises east ward subduction and following accretion of an intra oceanic island arc to the eastern border of the rio de la plata craton at ca 880 ma and westward subduction beneath the newly formed active continental margin between ca 750 and 700 ma the sao gabriel block represents relics of an early brasiliano oceanic basin between the rio de la plata and kalahari cratons this ocean to the east of the rio de la plata craton might be traced to the north and could possibly be linked with neoproterozoic juvenile oceanic crust in the western brasilia belt goias magmatic arc

tracking the sources and the evolution of the late neoproterozoic granitic intrusions in the brusque group dom feliciano belt south brazil la icp ms and shrimp geochronology coupled to hf isotopic analysis abstract the central domain of the neoproterozoic dom feliciano belt in the brazilian state of santa catarina is mainly composed of a metavolcano sedimentary sequence the brusque group and its crystalline basement such as the camboriu complex this association was intruded in its late deformational stage by voluminous granitic magmatism related to the metamorphic climax of the area the intrusions share a distinct crustal affinity and are divided into three suites from oldest to youngest as indicated by field criteria sao joao batista valsungana and nova trento we present seven new la icp ms and two shrimp u pb zircon ages coupled with in situ analysis of hf isotopes and use them to discuss the magmatic sources and evolution of the intrusions and propose an evolutionary model for this plutonic association intrusion mostly took place at 615 585 ma but the field stratigraphy is not necessarily reflected by u pb ages which establish considerable overlaps between intrusions from different suites inherited zircon crystals are common and reflect a long geological history that matches the geochronological record of the brusque group and the camboriu complex hf ratios are remarkably widespread but strengthen the characteristic crustal signature of the magmatism and partially overlap with whole rock isotopic data for the same rocks we propose that the intrusions were caused by a regional heating event within the central domain of the dom feliciano belt in santa catarina with limited direct mantellic contribution and more significant indirect thermal contribution through the voluminous magmatism of the recently juxtaposed florianopolis batholith this triggered large scale partial melting of the local continental crustal and formed the studied granites the long lasting magmatic event promoted the protracted generation and emplacement of numerous distinct intrusions synchronically leading to contrasts between the field and u pb stratigraphy the suites varied geochemical and isotopic characteristics probably reflect a combination of the heterogeneities of their source rocks with the impact of different magmatic processes during granitogenesis and emplacement while the valsungana and nova trento suites were probably generated within a deeper crustal level and underwent some degree of magmatic interaction the peraluminous sao joao batista suite has an upper crust signature that indicates the assimilation of metasedimentary rocks

mesozoic to quaternary continental margin dynamics in south central chile 36 42 s the apatite and zircon fission track perspective zircon and apatite fission track data provide constraints on the exhumation history fault activity and thermal evolution of the south central chilean active continental margin 36 s 42 s which we use to assess the tectonic and geomorphic response of the margin to the andean subduction regime several domains with different exhumation histories are identified the coastal cordillera is characterized by uniform and coherent exhumation between late triassic 200 ma and late miocene times with surprisingly slow average rates of 0 03 0 04 mm a thermal anomalies related to late cretaceous and early miocene magmatism have regionally modified fission track age patterns the upper cretaceous thermal overprint is of previously unrecognized significance and extent in the coastal cordillera south of 39 s with the exception of a local but distinct pliocene to recent exhumation period in the high relief cordillera nahuelbuta segment between 37 s and 38 s cenozoic overall exhumation in the coastal cordillera was very slow the sedimentary record shows that uplift and subsidence here was episodic with low amplitudes and durations this rules out large scale long term cenozoic accretion trench parallel tilting and tectonic erosion processes in the forearc the main andean cordillera shows markedly greater long term exhumation rates than the coastal cordillera and at 39 s a steep exhumation gradient to the south long term average pliocene to recent exhumation rates of 1 to 2 mm a in the liquine area 39 45 s are almost an order of magnitude more rapid than average paleogene to recent exhumation near lonquimay 38 30 s and farther north while no imprint of the intra arc liquine ofqui fault zone on the exhumation pattern is evident long term exhumation rates decrease from the crest of the andes toward the western foothills exhumation gradients correlate with climatic gradients suggesting a causal link to the variable intensity of late miocene to pleistocene glacial erosion

emplacement of volcanic vents and geodynamics of central anatolia turkey abstract observations on synthetic aperture radar sar scenes of the european remote sensing ers satellite and digital elevation models dems complemented by field structural analysis permit a new understanding of relationships between tectonics and volcanism since the late miocene 10 ma in central anatolia volcanic edifices form elongate stratovolcanoes linear clusters and volcanic ridges they indicate emplacement on tension fractures and tail crack or horsetail features for instance the kara dag volcano is rooted on a tail crack which accommodates a horizontal left lateral throw component at a fault termination caldera complexes of cappadocia are associated with horsetail fault patterns the emplacement of volcanoes also benefits from larger scale tectonic structures the erciyes dag volcano is localized by the sultan saz releasing bend which opens along the sinistral strike slip ecemis fault deformation has been analysed from tension fractures which are perpendicular to the direction of extension and from field structural analysis on a regional scale the tectonic regime responsible for the distribution of volcanic vents in this area of convergence and lateral extrusion is not compression but extension the central taurus range is the thermally uplifted shoulder of the adana cilicia basin which is related to lithosphere thinning westward movements in the northwestern part of the studied area are influenced by the active back arc aegean extension situated to the west farther to the south the direction of motion turns southwest and south under the influence of the opening of the adana cilicia basin we interpreted that extension in the central anatolian plateau is related to crustal blocks moving above sub horizontal detachment surfaces located in the lower crust this is based on several facts the tuz golu fault zone is a within crust detachment the tuz golu basin does not affect the whole lithosphere because otherwise it would have been bordered by thermally uplifted shoulders movements change trend within a small 50 km region

the liquine ofqui fault zone a long lived intra arc fault system in southern chile abstract plate reconstructions for the cenozoic document relatively steady right oblique subduction of the farallon nazca plate beneath the chilean continental margin the kinematic signature recorded along the intra arc liquine ofqui fault zone lofz a major feature of the southern andes may help to understand the way in which the nazca south america slip vector has been partitioned into strain and displacement along and across the continental margin the lofz consists of two nne trending right stepping straight lineaments a strike slip duplex at the right step and curved features which splay off the straight lineaments toward the northwest the lofz runs mostly through heterogeneously deformed cenozoic plutonic rock of the north patagonian batholith and patchy metamorphic wall rock early cenozoic volcano sedimentary rocks and dyke swarms found in close spatial association with the strike slip duplex are believed to have developed in strike slip related basins quaternary volcanoes are aligned parallel to the lofz both ductile and brittle kinematic indicators within centimeter to meter wide high strain zones document late cenozoic dextral shear deformation contrasting left lateral deformation recorded on older and wider mylonitic zones suggests that the lofz may be a long lived shear zone that accommodated continental scale deformation arising from the farallon nazca south america plate convergence a block rotation pattern as indicated by paleomagnetic data is consistent with the geometry and cenozoic kinematics of the lofz

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

space geodetic estimation of the nazca south america euler vector abstract gps data from four sites in the nazca plate easter island galapagos robinson crusoe and san felix islands and from five sites in the stable core of the south american plate enabled us to estimate the euler vector of the nazca plate with respect to south america the observed velocities of easter island 6 6 cm yr at 102 3 galapagos 5 1 cm yr at 90 0 robinson crusoe 6 6 cm yr at 80 1 and san felix 6 0 cm yr at 82 1 are significantly slower than the global plate model nuvel 1a velocites for those four sites our estimated euler pole is located at 48 8 n 91 7 w with a rate of 0 59 m y the nuvel 1a and earlier space geodetic studies give rotation rates that are 20 faster

seismological aspects of the 1988 1989 lonquimay chile volcanic eruption abstract more than 1600 earthquakes were recorded in a four month period on portable seismographic stations installed in the lonquimay area immediately after the initiation of the volcanic eruption of december 25 1988 hypocentral parameters were calculated on a subset of 150 events showing clear p and s arrivals seismic activity was restricted to a north south elongated region that included the main crater of the lonquimay volcano the new navidad crater lies along the eastern margin of the area of seismicity most of the hypocenters are located at depths shallower than 6 km and none have depths exceeding 10 km the largest earthquakes of the sequence took place on dec 28 m 4 7 jan 12 13 m 4 8 and 5 0 and feb 24 m 5 3 the number of events per day varied it peaked on dec 27 with more than 260 events and more than 200 events were recorded on jan 13 both of which corresponded to the days of large earthquakes the relation of large to small events given by the b value of the magnitude frequency distribution log n a bm is typical of volcanic sequences b 1 7 and much larger than the equivalent for tectonic environments the rate of decay of the number of aftershocks of each large earthquake is proportional to t p with p increasing from 0 63 to 1 08 as the eruption progressed in time initial low values of p indicate a slow stress release in the region larger absolute values of p are comparable with tectonic environments and might be indicators of the last stages of the eruption process the total seismic energy release is 9 6 10 19 ergs about five orders of magnitude less than the estimated thermal energy the spatial distribution of earthquakes suggests that the sources of stress change associated with the eruption are located under and possibly to the south of the main crater of the lonquimay volcano and that the evacuation channels of magma occupied a zone of weakness in the region of the new crater

volcanism in a compressional andean setting a structural and geochronological study of tromen volcano neuquen province argentina we document evidence for growth of an active volcano in a compressional andean setting our data are surface structures and 39ar 40ar ages of volcanic products on tromen volcano tromen is an active back arc volcano in the andean foothills of neuquen province argentina its volcanic products are unconformable upon mesozoic strata of the neuquen basin the volcano straddles a n s trending pop up which formed during e w shortening the main underlying structures are eastward verging thrusts their traces curve around the eastern foot of the volcano minor folds and faults also occur in the volcanic cover of tromen as a result of e w shortening new 39ar 40ar ages for these volcanic rocks are younger than 2 27 0 10 ma and show that tromen has been active almost continuously from the late pliocene to the holocene we conclude that volcanism and thrusting have been coeval and that magma must have reached the surface in a tectonic setting of horizontal compression our results have wider implications for magmatic processes in such settings

how local stress fields prevent volcanic eruptions recent studies show that dike injections in composite volcanoes and rift zones are much more common than dike fed eruptions although this observation is of fundamental importance in volcanic hazard studies and for understanding the formation and structure of the ocean crust the mechanisms that prevent dikes from reaching the surface are still poorly understood here we present new numerical models of magma chambers hosted by crustal layers of contrasting mechanical properties such as stiff lava flows and soft pyroclastic rocks the numerical models vary in sophistication from simple 4 layer models to a complex and realistic 30 layer model in the latter the layer thicknesses approach those of individual lava flows and pyroclastic flows as observed in the field in iceland and elsewhere more specifically for a magma chamber with a top at the depth of 1 5 km as is common in iceland each layer in the 30 layer model has a thickness of 50 m this is similar to the thicknesses of many pahoehoe lava flows and pyroclastic hyaloclastite and sedimentary units in iceland tilting of the lava pile is a general feature of volcanic areas to take this into account we also present several numerical models with dipping layers where in accordance with observations the dips of the layers increase with depth in the lava pile all the numerical models for thick and thin layers as well as for tilted layers yield some common results first that during magma chamber inflation local stresses in the stiff layers encourage dike propagation while those in the soft layers tend to arrest dike propagation and prevent volcanic eruptions second that the local stresses in the stiff layers may trigger seismogenic faulting while the soft layers remain seismically quiet the models with the dipping layers indicate that the general tilting of the lava pile may result in shifting of volcanic activity at the surface the shifting is because the local stresses that develop above a magma chamber located in a tilted lava pile control the pathways of injected dikes the direction of shifting is in the dip direction of the tilted lava pile and the amount of shifting increases with increasing dip all the results suggest that for an eruption to occur the stress conditions along the potential pathway of a dike must be basically homogeneous

neotectonics in the foothills of the southernmost central andes 37 38 s evidence of strike slip displacement along the antinir copahue fault zone 1 the antinir copahue fault zone acfz is the eastern orogenic front of the andes between 38 and 37 s it is formed by an east vergent fan of high angle dextral transpressive and transtensive faults which invert a paleogene intra arc rift system in an out of sequence order with respect to the cretaceous to miocene fold and thrust belt 3 1 1 7 ma volcanic rocks are folded and fractured through this belt and recent indicators of fault activity in unconsolidated deposits suggest an ongoing deformation in spite of the absence of substantial shallow seismicity associated with the orogenic front neotectonic studies show the existence of active faults in the present mountain front the low shallow seismicity could be linked to the high volumes of retroarc derived volcanic rocks erupted through this fault system during pliocene and quaternary times this thermally weakened basement accommodates the strain of the antinir copahue fault zone absorbing the present convergence between the south america and nazca plates

combined sr o isotope relationships and petrogenesis of andean volcanics of south america active andesitic type volcanism in the southern andes lat 42 36 s is characterized by low sr and o isotopic compositions which fall in the range 87sr 86sr 0 7036 0 7043 18o 5 2 6 8 in contrast volcanic centres in the central andes lat 24 21 s are significantly enriched in both 87sr and 18o we present the first combined sr and o isotope study of the southern volcanic zone svz in chile crustal thickness 40 km 1 and of the southern part of the central volcanic zone cvz in the borderlands of bolivia chile and argentina crustal thickness 60 70 km 1 only one of 13 volcanic centres studied lascar exhibits a correlated increase in both 87sr and 18o parallel with the basalt andesite rhyolite compositional change indicative of contamination by upper crust through a fractional crystallization assimilation mechanism the other centres of the cvz for which there is an overall sr o correlation document the variable contamination of mantle derived magmas at depth in the lower crust which had been thickened by mesozoic and earlier plutonism

first seismic record for intra arc strike slip tectonics along the liquine ofqui fault zone at the obliquely convergent plate margin of the southern andes a temporal seismic network recorded local seismicity along a 130 km long segment of the transpressional dextral strike slip liquine ofqui fault zone lofz in southern chile seventy five shallow crustal events with magnitudes up to m tief w 3 8 and depths shallower than 25 km were observed in an 11 month period mainly occurring in different clusters those clusters are spatially related to the lofz to the volcanoes chaiten michinmahuida and corcovado and to active faulting on secondary faults further activity along the lofz is indicated by individual events located in direct vicinity of the surface expression of the lofz focal mechanisms were calculated using deviatoric moment tensor inversion of body wave amplitude spectra which mostly yield strike slip mechanisms indicating a ne sw direction of the p axis for the lofz at this latitude the seismic activity reveals the present day activity of the fault zone the recent m tief w 6 2 event near puerto aysen southern chile at 45 4 s on april 21 2007 shows that the lofz is also capable of producing large magnitude earthquakes and therefore imposing significant seismic hazard to this region

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

internal structures of zircons from archaean granites from the darling range batholith implications for zircon stability and the interpretation of zircon u pb ages internal structures in zircons from granitoids from the late archaean darling range batholith show secondary features revealed by hf etching which record reconstitution of the zircons and modification of the distribution of trace elements during post crystallisation cooling of the granitoid zircons from the granites commonly contain unzoned to weakly zoned cores surrounded by rims showing oscillatory zoning which has been modified by recrystallisation the most striking feature is the development of high trace element concentration areas found in zircons from a number of granites these structures range from enhanced trace element concentrations in primary zones to a single accumulation of most trace elements in one band about half way between the outer edge and the centre of the zircon in any zircon the extent of the concentration of trace elements towards the formation of a single trace element band appears to be inversely related to the fading and broadening of primary oscillatory zones in the outer rim this suggests that the trace element bands formed by migration of trace elements from the outer primary zones to new concentration sites on an inner set of primary zones this explanation is supported by the formation of multiple curved trace element bands that transgress primary zoning and the determination of younger shrimp ages on depleted zircon outer rims compared to remnant primary oscillatory zoned areas of the zircon and unzoned centres also observed in some granite zircons is a finely convoluted zoning which overprints oscillatory zoning in parts of a zoned zircon and in rare cases occurs throughout the zircon this structure is explained in terms of secondary migration and reconcentration of trace elements in curved bands all structures can be transgressed by generally rounded lobes and patches of low u weakly nebulously zoned zircon this is interpreted as a late stage interaction between the zircon and fluids formed during cooling and crystallisation of the granitoid resulting in recrystallisation of affected parts of the zircon with accompanying loss of trace elements from the zircon

sr and nd isotopic and trace element compositions of quaternary volcanic centers of the southern andes isotopic compositions of samples from six quaternary volcanoes located in the northern and southern extremities of the southern volcanic zone svz 33 46 s of the andes and from four centers in the austral volcanic zone avz 49 54 s range for 87sr 86sr from 0 70280 to 0 70591 and for 143nd 144nd from 0 51314 to 0 51255 the ranges are significantly greater than previously reported from the southern andes but are different from the isotopic compositions of volcanoes in the central and northern andes basalts and basaltic andesites from three centers just north of the chile rise trench triple junction have 87sr 86sr 143nd 144nd la yb ba la and hf lu that lie within the relatively restricted ranges of the basic magmas erupted from the volcanic centers as far north as 35 s in the svz of the andes the trace element and sr and nd isotopic characteristics of these magmas may be explained by source region contamination of subarc asthenosphere with contaminants derived from subducted pelagic sediments and seawater altered basalts by dehydration of subducted oceanic lithosphere in the northern extremity of the svz between 33 and 34 s basaltic andesites and andesites have higher 87sr 86sr rb cs and hf lu and lower 143nd 144nd than basalts and basaltic andesites erupted farther south in the svz which suggests involvement of components derived from the continental crust in the avz the most primitive sample high mg andesite from the southernmost volcanic center in the andes 54 s has sr and nd isotopic compositions and kr b and ba la similar to morb the high la yb of this sample suggests formation by small degrees of partial melting of subducted morb with garnet as a residue samples from centers farther north in the avz show a regionally regular northward increase in sio2 k2o rb ba ba la and 87sr 86sr and decrease in mgo sr kr b rb cs and 143nd 144nd suggesting increasingly greater degrees of fractional crystallization and associated intra crustal contamination

crust magma interactions and the evolution of arc magmas the san pedro pellado volcanic complex southern chilean andes field petrographic and trace element data indicate that volcanic rocks of the quaternary san pedro pellado complex lat 36 s chile have incorporated a silicic component that may be derived from the crust isotope ratios of sr nd and pb vary little among the volcanic rocks this is due primarily to the limited isotopic contrast between the mesozoic tertiary crust and the young magmas isotope and element compositions of san pedro pellado rocks are similar to those reported from other localities in the southern volcanic zone of the chilean andes we suggest that crustal assimilation is a significant process in the genesis of differentiated volcanic rocks in the southern volcanic zone the san pedro pellado example illustrates the importance of considering all types of evidence not just isotopic data in the evaluation of crustal contributions to arc magmas

west gondwana amalgamation based on detrital zircon ages from neoproterozoic ribeira and dom feliciano belts of south america and comparison with coeval sequences from sw africa

crustal sources involved in continental arc magmatism a case study of volcan mocho choshuenco southern chile volcan mocho choshuenco a pleistocene holocene basaltic andesite to dacite shield volcano located at lat 40 s in the southern volcanic zone svz of the andes is built on the thinnest svz continental crust 30 km which consists largely of paleozoic sedimentary and metamorphic rocks and mesozoic plutons sr pb nd and o isotopic ratios of mocho choshuenco lavas 87 sr 86 sr 0 70399 to 0 70422 206 pb 204 pb 18 57 to 18 61 207 pb 204 pb 15 60 to 15 63 208 pb 204 pb 38 45 to 38 57 143 nd 144 nd 0 512787 to 0 512881 18 o 5 8 to 6 8 standard mean ocean water are similar to those of other svz centers and exhibit no correlation with indices of differentiation however incompatible trace element systematics indicate that evolved lavas have been contaminated by crustal plutonic rocks by 5 to 15 rb ba k ba rb la and k la ratios increase with increasing sio 2 content as a result of assimilation of orogenic calc alkalic plutonic rocks enriched in rb and k relative to ba and la xenoliths of plutonic rocks recovered from mocho choshuenco lavas are slightly more radiogenic in 87 sr 86 sr less radiogenic in 143 nd 144 nd and comparable in pb isotopic ratios and they have variable 18 o contents 87 sr 86 sr 0 70482 to 0 70557 206 pb 204 pb 18 60 to 18 64 207 pb 204 pb 15 60 to 15 62 208 pb 204 pb 38 53 to 38 56 143 nd 144 nd 0 512686 18 o 3 3 to 5 5 smow contamination of mocho choshuenco parental magmas by crustal melts with small isotopic contrast imposed little variation on isotopic heterogeneities existing in the parental compositions these heterogeneities probably were produced during magma crust interaction at the crust mantle boundary or in the lower crust

kinematic variations across eastern cordillera at 24 s central andes tectonic and magmatic implications abstract the eastern cordillera central andes 24 s consists of a basement involved thrust system resulting from miocene quaternary eastward migrating compression separating the puna plateau from the santa barbara system foreland the inferred tertiary strains arising from shortening in the eastern cordillera and santa barbara system are similar higher than in the puna slip data collected on the major n s trending faults of eastern cordillera show a westward progression from dip slip contraction to dextral and sinistral motions this consistently with established tectonic models may result from partitioning due to the oblique mio quaternary underthrusting of the brazilian shield north of 24 s this strain partitioning has three main implications 1 as the dextral and sinistral shear in the eastern cordillera are 62 and 29 of the compressive strain respectively the eastern cordillera results more strained than santa barbara system foreland contrary to previous estimates 2 the partitioning in the eastern cordillera may find its counterpart in that to the west of the central andes giving a possible structural symmetry to the central andes 3 the easternmost n s strike slip structures in the eastern cordillera coincide with the easternmost mio pliocene magmatic centres in the central andes at 24 s provided that further to the east the crust is partially molten the absence of magmatic centres may be explained by the presence of pure compressive structures in this portion of the eastern cordillera

multiple accretion at the eastern margin of the rio de la plata craton the prolonged brasiliano orogeny in southernmost brazil the neoproterozoic eoplalaeozoic brasiliano orogeny at the eastern margin of the rio de la plata craton in southernmost brazil and uruguay comprises a complex tectonic history over 300 million years the southern brazilian shield consists of a number of tectono stratigraphic units and terranes the sao gabriel block in the west is characterized by c 760 690 ma supracrustal rocks and calc alkaline orthogneisses including relics of older c 880 ma old igneous rocks both igneous and metasedimentary rocks have positive end t values and neoproterozoic tdm model ages they formed in magmatic arc settings with only minor input of older crustal sources a trondhjemite from the sao gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation d3 yield an u pb zircon age la icp ms of 701 10 ma giving the approximate minimum age of the sao gabriel accretionary event the encantadas block further east containing the supracrustal porongos belt and the pelotas batholith is in contrast characterized by reworking of neoarchean to palaeoproterozoic crust the 789 7 ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the porongos belt provides a time marker for the basin formation zircons of a sample from tonalitic gneisses constituting the palaeoproterozoic basement of the porongos belt form a cluster at 2 234 28 ma interpreted as the tonalite crystallization age zircon rims show ages of 2 100 2 000 ma interpreted as related to a palaeoproterozoic metamorphic event the porongos basin formed on thinned continental crust in an extensional or transtensional regime between c 800 700 ma the absence of input from neoproterozoic juvenile sources into the porongos basin strongly indicates that the encantadas and sao gabriel blocks were separated terranes that became juxtaposed next to each other during the brasiliano accretional events the tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the rio de la plata craton i accretion of an intra oceanic arc at c 880 ma passinho event and ii accretion of the 760 700 ma cambai vila nova magmatic arc sao gabriel event the latter event also includes the collision of the encantadas block with the rio de la plata craton to the west collision and crustal thickening was followed by sinistral shear along sw ne trending orogen parallel crustal scale shear zones that can be traced from southern brazil to uruguay and have been active between 660 and 590 ma voluminous granitic magmatism in the pelotas batholith spatially related to shear zones is interpreted as late to post orogenic magmatism possibly assisted by lithospheric delamination it marks the transition to the post orogenic molasse stage localized deformation by reactivation of preexisting shear zones continued until c 530 ma and can be assigned to final stages of the amalgamation of west gondwana

comment on provenance of the arroyo del soldado group ediacaran to cambrian uruguay implications for the palaeogeographic evolution of southwestern gondwana by blanco et al precambrian res 171 2009 57 73 thus it is truly tectonics which governs stratigraphy and thetwobranchesofthegeologicalsciencesareinseparable astruc tural geologists who is not a stratigrapher is only a geometer not a geologist for he reasons about abstract surfaces and vol umes emptied of their history and a stratigrapher who neverconcerned himself with tectonics would produce only a deadstratigraphy gignoux 1950 p 3 the paper provenance of the arroyo del soldado group ediacaran to cambrian uruguay implications for the palaeogeo graphicevolutionofsouthwesterngondwana blancoetal 2009 presented new u pb shrimp sm nd and geochemical data ofsome units of arroyo del soldado group asg in order to betterconstraining the evolution of sedimentary protoliths and there fore to justify a rift tectonic setting for deposition of those units unfortunately the data presented are incapable of answering thetectonic evolution of neoproterozoic lower paleozoic brasilianounits located at the east of the uruguayan territory we point outthattheauthors interpretationisnotsubstantiatedonthedata thescarcity of geological and isotopic data obtained for dom felicianobelt dfb determinesawidespanofgeotectonicmodels blancoetal 2009 use force andadapttheavailableisotopicdatatosustaintheir tectonic model other plausible models can also explain the

new evidence for protolith ages of lewisian granulites northwest scotland the granulite facies scourian rocks in the central region of the lewisian complex of northwest scotland play an important role in the understanding of the development of mid archean high grade gneiss complexes whereas the later largely proterozoic history of the scourian has been clarified in a detailed u pb study unraveling the archean ages of the protoliths using conventional isotope techniques has proved impossible in order to advance our understanding of the deep crustal processes responsible for these granulites it is essential to establish the age s of accretion of the protoliths and their subsequent tectonothermal history a combined cathodoluminescence cl and sensitive high mass resolution ion microprobe shrimp single zircon study has revealed a hitherto unrecognized morphological complexity in zircons from the type localities of the granulites use of cl allowed identification of relict oscillatory igneous zoning metamorphic overgrowths and irregular areas of recrystallization from the shrimp data an age of ca 2960 ma is inferred for the gneiss protoliths which were altered considerably during an important metamorphic event ca 2490 ma

kinematic constraints on intra arc shear and strain partitioning in the southern andes between 38 s and 42 s latitude 1 on the basis of structural field work fault kinematic analysis and the analysis of digital imagery we describe the northern part of the southern andean intra arc liquine ofqui fault zone lofz as an sc like fault zone system accommodating part of the nazca south american plate convergence obliquity kinematic modeling suggests that the lofz accommodated 124 24 21 km of dextral displacement between 40 s and 42 s and 67 13 11 km between 38 s and 40 s since the pliocene associated vertical axis rotations are 31 4 clockwise and 9 1 counterclockwise along synthetic and antithetic faults respectively mean pliocene to recent shear rates along the lofz decrease northward from 32 6 mm yr to 13 3 mm yr compatible with partitioning of half of the convergence obliquity into the intra arc zone north of 40 s and complete partitioning to the south the displacement gradient along the intra arc zone results in margin parallel shortening of the fore arc

the role of xenoliths and flow segregation in the genesis and evolution of the paleoproterozoic itapema granite a crustally derived magma of shoshonitic affinity from southern brazil abstract the paleoproterozoic itapema granite ig is a large granitic body that intrudes orthogneisses and amphibolites altogether representing part of the basement for neoproterozoic granitic magmatism in southern brazil it is composed of fine to medium grained hornblende biotite monzogranites to granodiorites that host abundant country rock xenoliths and irregularly distributed mafic aggregates interaction between magmatic flow and low flow velocity zones related to xenolith rich portions resulted in flow segregation and mineral fractionation as the main mechanisms that led to compositional and textural diversity expressed also in geochemical trends the geometry and distribution of flow foliation suggest that the itapema granite is a subhorizontal sheet like intrusion and that its emplacement was controlled by thrusting under upper amphibolite facies conditions many geochemical characteristics of the itapema granite are similar to those of shoshonitic granitoids however lower alkali contents lack of basic or intermediate associated rocks and intimate temporal association with high grade gneisses and migmatites suggest that it was crustally derived its origin is compatible with melting of amphibolitic orthogneisses derived from continental tholeiitic or shoshonitic sequences under high geothermal gradient conditions where temperatures reached at least 850 c

sm nd isotope geochemistry of metamorphic volcano sedimentary successions in the sao gabriel block southernmost brazil evidence for the existence of juvenile neoproterozoic oceanic crust to the east of the rio de la plata craton abstract juvenile neoproterozoic dioritic tonalitic trondhjemitic and granodioritic gneisses in the sao gabriel block southern brazil have been identified by geochronologic studies age proposals for associated ultra mafic metavolcanic and metasedimentary rocks however range from archean to neoproterozoic whole rock sm nd analyses presented here support a neoproterozoic age for these rocks tdm model ages of the ultra mafic metavolcanic rocks range between 0 65 and 1 35 ga with nd t positive values between 3 16 and 6 87 tdm model ages of metasedimentary and metavolcanoclastic rocks vary between 0 77 and 1 19 ga with nd t values between 1 2 and 6 23 tonalitic calc alkaline gneisses show nd t values of 4 34 and 6 3 and tdm model ages of 0 89 and 0 72 ga respectively a late kinematic granite santa zelia granite display slightly negative nd t values 1 6 and a higher tdm model age of about 1 4 ga these data support the existence of meso neoproterozoic juvenile oceanic crust and island arc rocks during the brasiliano orogenic events the main source rocks of the metasedimentary units are previously formed juvenile rocks the data also indicate minor assimilation of older crustal material and or contamination of the melts by radiogenic nd released from older rocks on the subducting slab existence of widespread old sialic crust in the subduction zone environment however can be ruled out indicating important orogenic accretion between 0 9 and 0 7 ga a geotectonic model for the sao gabriel block and the eastern margin of the rio de la plata craton comprises eastward subduction and following accretion of an intra oceanic island arc between 0 9 and 0 8 ga and a subsequent westward subduction with formation of an active continental margin at the eastern margin of the rio de la plata craton between 0 8 and 0 7 ga we postulate that the juvenile rocks of sao gabriel block represent relics of a neoproterozoic ocean between the rio de la plata craton and a continental block encantadas block possibly derived from the kalahari craton subduction and arc accretion began roughly coeval with the initial stages of the break up of rodinia 0 9 ga and indicate a peripheric rio de la plata craton in relation to the rodinia supercontinent with evolution from a passive margin to an active margin in the beginning of the neoproterozoic brasiliano orogenic events

incipient axial collapse of the main cordillera and strain partitioning gradient between the central and patagonian andes lago laja chile 1 lago laja is a late quaternary volcanic dammed lake located near the drainage divide of the south central andes field observations lake reflection seismic profiles bathymetry and remote sensing data reveal an active fault system that runs parallel to the volcanic arc along the axis of the main cordillera the lago laja fault system llfs normal faults of this extensional system cut late pleistocene volcanics 2 7 mm yr since 7 1 ka the llfs accounts for 0 7 of arc normal extension at an average minimum rate of 1 2 mm yr and strain rate of 10 14 s 1 seismites and surface ruptures evidence m 6 paleoearthquakes the main cordillera at 37 s is a large scale pop up structure uplifted by thrusting along its foothills in this light we interpret extension in the axial and highest part of the andes as incipient synorogenic gravitational collapse in response to uplift and crustal thickening thermal weakening due to elevated heat flow and postglacial lithospheric rebound and unbending have probably contributed to the arc limited collapse and holocene acceleration of deformation rates the lack of significant strike slip offsets along the llfs as well as along both foothills thrust systems at 37 s contrasts with the intra arc dextral fault zone south of 38 s regional structural data indicates that north of 38 s diffusely distributed strain reflects low partitioning of oblique subduction while to the south deformation is localized in a discrete strike slip fault zone along the volcanic arc reflecting a higher degree of partitioning we relate this strain partitioning gradient to favorable fault orientations in the fore arc north of the arauco peninsula a major seismotectonic boundary

oblique opening and noncoaxial emplacement of the jurassic independence dike swarm california abstract several features of the independence dike swarm ids indicate that dikes opened obliquely to their walls in a sinistral sense the ids is largely latest jurassic in age 148 ma and has been traced for more than 600 km along its nw se strike from east central to southern california field and petrographic observations from the ids in the sierra nevada indicate that i some and perhaps all of the dikes initially opened perpendicular to their margins and then were sinistrally sheared and ii sinistral displacement occurred during and shortly after dike injection dike emplacement overlapped in time with the formation of wall rock mylonite zones which accommodated shear varying in sense from sinistral to west side down reverse we suggest that the ids intruded dilatant fractures within a regional sinistral shear system the swarm is located within or along the eastern flank of the cordilleran subduction related magmatic arc the ids and associated wall rock deformation may record partitioning into the magmatic arc of the sinistral component of strongly oblique late jurassic subduction

topography effect at the critical sv wave incidence possible explanation of damage pattern by the whittier narrows california earthquake of 1 october 1987 abstract buildings damaged by the whittier narrows california earthquake of 1 october 1987 were concentrated in several areas most of which were located 8 to 10 km from the epicenter corresponding to the critical incidence for sv waves and were near topographic irregularities such as the northern part of whittier just south of puente hills the purpose of this study is to show the possibility that this anomalous damage pattern is due to the amplification by the topographic irregularity when sv waves are near critical incidence first we examined the accelerograms obtained at the usgs station 7215 bright ave nearest to downtown whittier and conclude that the dominantly eastwest motion observed at the station is due to the influence of the building in which accelerograms were recorded we next demonstrate that strong ground motions recorded by 12 stations in the epicentral area show a polarization direction pattern that is primarily sv type which is consistent with a simple thrust fault located at the hypocenter then we calculated the response of a two dimensional hill with the height 0 3 km and the width 2 4 km to 1 a plane sv wave with a nearly critical angle of incidence 2 a horizontal line force 3 a haskell type 2d dislocation source and 4 a bouchon type 2d multiple crack source the results show that the amplification due to the hill relative to the flat surface is more than 1 5 for all the source models since this amplification is nearly independent of the source type and spectrum we conclude that the combined effect of the topographic irregularity and critically incident sv waves might be responsible for the concentration of damage observed during the whittier narrows earthquake

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

compressional and transpressional stress pattern for pliocene and quaternary brittle deformation in fore arc and intra arc zones andes of central and southern chile kinematic analysis of fault slip data for stress determination was carried out on late miocene to quaternary rocks from the fore arc and intra arc regions of the chilean andes between 33 and 46 south latitudes studies of neogene and quaternary infilling the central depression as well as plutonic rocks of the north patagonian batholith along the liquine ofqui fault zone have revealed various compressional and or transpressional states of stress in the pliocene the maximum compressional stress 1 was generally oriented east west during the quaternary the deformation was partitioned into two coeval distinctive states of stress in the fore arc zone the state of stress was compressional with 1 oriented in a n s to nne ssw direction in the intra arc zone the state of stress was transpressional with 1 striking ne sw along the coast in one site 37 30 s the quaternary strain deformation is extensional with an e w direction which can be explained by a co seismic crustal bending readjustment

on the nature of buttressing in margin parallel strike slip fault systems margin parallel strike slip faults commonly develop along convergent plate margins where an oceanic plate subducts obliquely beneath a continental plate such faults detach a sliver of continental crust from the leading edge of the overriding plate fault motion then causes the sliver to move parallel to the continental margin in the direction of the transverse component of relative plate motion however few slivers move as rapidly as predicted by simple analyses of the forces acting on them this discrepancy must be due principally to the buttress effect resistance to displacement of the sliver arising from a space problem at its leading edge more simply a sliver can move only if it has somewhere to go buttresses can be geometrical as when the continental margin changes trend abruptly or they can arise from changes in the physical properties of the overriding plate or in other factors such as subduction angle and angle of obliquity that help determine whether sliver motion will occur a strongly buttressed sliver will move only insofar as the buttress can be overcome obvious ways in which a buttress can be overcome are by thickening the crust or widening the sliver examples of both are known crustal thickening entails elevation increase sliver motion is opposed by both gravity and friction widening the sliver is a more complex process that results in areas of extension and compression forming within the sliver as well as a characteristic pattern of overlapping curved fault segments this latter response may be the cause of curvature in many strike slip fault systems

eruptive stratigraphy of the tatara san pedro complex 36 s southern volcanic zone chilean andes reconstruction method and implications for magma evolution at long lived arc volcanic centers

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

magma interaction processes in syn extensional granitoids the tertiary menderes metamorphic core complex western turkey abstract western turkey which forms the eastern part of the aegean region was subjected to continental extension that led to formation of metamorphic core complexes and associated syn extensional granitoids this study deals with petrogenesis of the syn extensional early miocene alacamdag ag and middle miocene salihli sg granitoids and associated mafic microgranular enclaves mme in order to better understand the time progressive evolution of the mantle sources beneath the extended continental crust in western turkey ag and sg granitoids consist of undeformed and ductility deformed granitoids together with abundant mmes they are calc alkaline to high k calc alkaline rocks that are metaluminous to slightly peraluminous mg of ag host rocks is slightly lower than that of sg host rocks ag host rocks have higher na 2 o ba rb rb la and lower al 2 o 3 cao mgo tio 2 zr y values than those of the sg host rocks ag and sg host rocks differ from those of mmes with their lower al 2 o 3 cao mgo fe 2 o 3 tio 2 sr v mg and higher sio 2 values mmes are intermediate corresponding to monzonite monzodiorite in ag and diorite in sg compositions and are more mafic with respect to their host rocks in the primitive mantle pm normalized trace element patterns host rock and mme samples have similar trace element patterns all of these rocks are enriched in large ion lithophile elements liles cs rb ba th k and sr and strongly depleted high field strength elements hfses ta nb p ti compared to the primitive mantle the isotope ratios of the syn extensional ag and sg rocks display increasing radiogenic strontium and decreasing radiogenic neodymium isotopic values for the ag and sg host rocks and mme samples are 87 sr 86 sr ag host 0 708835 0 710206 and e nd t ag host 5 36 to 7 36 87 sr 86 sr ag mme 0 709107 0 709801 and e nd t ag mme 5 36 to 7 36 87 sr 86 sr ag mme 0 709107 0 709801 and e nd t ag mme 5 55 to 6 51 87 sr 86 sr sg host 0 712200 0 712408 and e nd t sg host 8 03 to 8 61 87 sr 86 sr sg mme 0 712028 0 712351 and e nd t sg mme 7 57 to 8 48 syn extensional granitoids in western turkey were mainly affected by crustal contamination fractional crystallization and magma mixing mingling mm processes magma mixing mingling partial mixing appear to have larger affects on the compositional range of the magmas than those generated by partial melting crustal contamination and fractional crystallization mafic and felsic magmas forming syn extensional granitoids have also undergone metasomatism chemical equilibrium and diffusional exchange processes during cooling syn extensional granitoids have been derived from a hybrid magma that originated from mixing of coeval lower crustal derived felsic magma and lithospheric mantle derived mafic magmas during extensional processes

melt collecting structures and the formation of extraction dykes during syntectonic anatexis of the camboriu complex south brazil abstract the camboriu complex metamorphic sequence comprises orthogneisses and amphibolites with widespread melting features generated at upper amphibolite facies conditions regional to mesoscale structures are shown to control generation collection and extraction of melts as partial melting starts it is locally focused as layer parallel leucosomes in banded gneisses and isolated pockets of in situ leucosomes in massive amphibolites the further tightening of cm to m scale upright folds and conjugate shear planes oblique to axial planar position trigger the coalescence and migration of melts first along the main banding then along axial planes of folds and finally giving rise to a network where melt is collected the melt network eventually acquires enough mobility to form m wide extraction dykes roughly parallel to axial planes of folds capable of transferring the products of migmatization to upper crustal levels to feed granitic plutons the orientation and geometry of pure shear dominated structures in the camboriu complex are in accordance with deformation partitioning between this and adjancent shear zones which is expected for the transpressive character of the neoproterozoic southern brazilian shear belt

is there a close spatial relationship between faults and plutons abstract using spatial relationships between individual plutons and faults to support genetic correlations between faulting and magmatism is meaningless since even random magmatic or tectonic processes will result in some plutons adjacent to faults our initial analyses of populations of faults and carboniferous plutons in the armorican massif france and faults and alleghanian plutons in the southern appalachians usa indicate that plutons have broad distributions with respect to faults but with a tendency for plutons to occur away from faults maxima of integrated pluton areas occur at 1 4 appalachians and 1 2 armorican of the distance between the average fault spacing in these orogens although there is a great need for statistical evaluations of relationships between populations of igneous bodies and structures in a wide variety of settings and crustal depths our initial studies suggest that faults do not preferentially channel magma during ascent or emplacement and that these are relatively unfocused processes within orogenic belts

magma emplacement in a thrust ramp anticline the gavorrano granite northern apennines italy 1 magma emplacement in the upper crust is often associated with crustal extension whereby space for intrusions is made by dilation along transtensive and strike slip faults the lenticular shape of several intrusions indicates that a further mechanism for magma intrusion is laccolith emplacement with roof uplift of the overburden additionally melts exploit rheological discontinuities e g sedimentary layering during their ascent we present an emplacement model for intrusion of the shallow level gavorrano granite northern apennine italy which is located at the core of an open anticline the shape of the intrusion and structural features of the host rocks are indicative of a synkinematic emplacement in a growing thrust anticline space was provided by the opening of dilatation zones at the core of anticline as a consequence of different amounts of translation between hanging wall and footwall units which were separated by evaporitic rocks these evaporitic units acted as major decollement layers analog models provide results in good agreement with the structural setting of the anticline and emphasize the possibility that melts filled the dilatation zone with the decollement layers further facilitating intrusion