structural evolution of the sarandi del yi shear zone uruguay kinematics deformation conditions and tectonic significance the sarandi del yi shear zone is a crustal scale shear zone that separates the piedra alta terrane from the nico perez terrane and the dom feliciano belt in southern uruguay it represents the eastern margin of the rio de la plata craton and consequently one of the main structural features of the precambrian basement of western gondwana this shear zone first underwent dextral shearing under upper to middle amphibolite facies conditions giving rise to the reactivation of pre existing crustal fabrics in the easternmost piedra alta terrane afterwards pure shear dominated sinistral shearing with contemporaneous magmatism took place under lower amphibolite to upper greenschist facies conditions the mylonites resulting from this event were then locally reactivated by a cataclastic deformation this evolution points to strain localization under progressively retrograde conditions with time indicating that the sarandi del yi shear zone represents an example of a thinning shear zone related to the collisional to post collisional evolution of the dom feliciano belt that occurred between the meso to neoproterozoic 600 ma and late ediacaran lower cambrian times

one kilometre thick ultramylonite sierra de quilmes sierras pampeanas nw argentina abstract we describe a 1 km thick ultramylonite forming the high strain base of the 3 5 km thick el pichao shear zone in the sierra de quilmes this shear zone thrusted granulite facies migmatites onto amphibolite facies rocks during the 470 ma famatinian orogeny strain grades upwards from ultramylonites to weakly sheared migmatites across the 3 5 km zone and the mylonitic rocks define a geochemical field narrower than the protolith suggesting they underwent mixing and homogenization through shearing ultramylonites this thick are uncommon the width of a shear zone in the absence of significant compositional rheological contrasts controlling strain localization is controlled by the balance between shear heat generation and diffusion under typical crustal conditions a strain rate of 10 12 s 1 is required to form a 1 km thick ultramylonite and this is achieved when large movement velocities are imposed across the shear zone we postulate that the el pichao shear zone and its thick ultramylonite accommodated a significant fraction of convergence velocities driving the orogeny and that the wide mylonitic shear zones characteristic of the cambrian ordovician deformation of the sierras pampeanas result from the convergent movement being taken up by only a few active major shear zones

multiple migmatite events and cooling from granulite facies metamorphism within the famatina arc margin of northwest argentina the famatina margin records an orogenic cycle of convergence metamorphism magmatism and extension related to the accretion of the allochthonous precordillera terrane new structural petrologic and geochronologic data from the loma de las chacras region demonstrate two distinct episodes of lower crustal migmatization the first event preserves a counterclockwise pressure temperature path in kyanite k feldspar pelitic migmatites that resulted in lower crustal migmatization via muscovite dehydration melting at 12 kbar and 868 c at 461 1 7 ma the shape of the pressure temperature path and timing of metamorphism are similar to those of regional midcrustal granulites and suggest pervasive ordovician migmatization throughout the famatina margin one dimensional thermal modeling coupled with regional isotopic data suggests ordovician melts remained at temperatures above their solidus for 20 30 ma following peak granulite facies metamorphism throughout a time period marked by regional oblique convergence the onset of synconvergent extension occurred only after regional migmatites cooled beneath their solidus and was synchronous with the cessation of precordillera terrane accretion at 436 ma the second migmatite event was regionally localized and occurred at 700 c and 12 kbar between 411 and 407 ma via vapor saturated melting of muscovite migmatization was synchronous with extension exhumation and strike slip deformation that likely resulted from a change in the plate boundary configuration related to the convergence and collision of the chilenia terrane

the proterozoic of eastern bolivia and its relationship to the andean mobile belt abstract the previously unmapped bolivian sector of the central brazil shield 220 000 km in area has been studied during an anglo bolivian technical co operation programme the metamorphic basement comprises the lomas maneches granulitic complex the chiquitania gneissic complex and the san ignacio schist supergroup which crops out as metasedimentary belts rb sr dates on some of the granulites suggest a protolith age of 2000 ma but the outcrop pattern of the metamorphic rocks as a whole is due to the 1300 ma san ignacio orogeny which was accompanied by widespread granite emplacement and metamorphism up to granulite facies the sunsas cycle began with the deposition of the supracrustal sunsas vibosi group and closed with the development of the sunsas and aguapei mobile belts at 1000 ma the upper proterozoic brasiliano cycle is represented by a bifurcating sedimentary basin in the se of the area with a closing tectonic event of 520 ma correlation is suggested with units in the central brazil shield the bolivian belts are extrapolated east into brazil and their limits indicated to the west the belts show trends parallel to the phanerozoic andean chain and they reflect the major deflection in the chain the arica elbow furthermore when the belts are traced towards the andes their ages broadly correspond to those of the sub andean precambrian basement this leads to the proposal of an enlarged andean orogenic belt system 1000 km wide comprising a 1300 ma old proto andean orogen which has controlled the trends of subsequent mobile belts

zircon and whole rock nd pb isotopic evidence for a grenville age and a laurentian origin for the basement of the precordillera in argentina suggestions that the precordillera of argentina has an early paleozoic connection to the appalachian ouachita rifted margin of the southern u s have been based on cambro ordovician stratigraphy and faunal assemblages further support comes from comparing precambrian grenville appalachian basement with xenoliths brought up in precordil leran miocene volcanic rocks u pb zircon ages and whole rock pb and nd isotopic data from the xenoliths show that precordilleran basement along with some part of the westernmost sierras pampeanas pie de palo range to the east is essentially unique in south america and is best interpreted as being derived from laurentia a grenville age is substantiated by u pb zircon upper intersect ages of 1102 pm 6 ma for a mafic xenolith and near 1100 ma for acidic xenoliths both xenolith types contain pink doubly terminated prismatic igneous zircons with relatively high u and pb concentrations rounded colorless low u and pb zircons with overgrowths produced in a later pre

mid to late cambrian docking of the rio de la plata craton to southwestern gondwana age constraints from u pb shrimp detrital zircon ages from sierras de ambato and velasco sierras pampeanas argentina abstract the early palaeozoic stratigraphy and tectonic history of the eastern sierras pampeanas of central argentina are complicated by metamorphism and deformation resulting from the pampean 545 510 ma and famatinian 490 440 ma orogenies we report u pb sensitive high resolution ion microprobe dating of detrital zircons in two metasedimentary successions exposed at quebrada de la cebila c 28 459s 66 259w the ambato and the la cebila metamorphic complexes the ambato zircons record age peaks corresponding to pampean 530 10 ma brasiliano c 570 and c 640 ma grenville c 950 to c 1025 ma and minor neoarchaean ages similar peaks are also apparent in the la cebila sample but it additionally contains palaeoproterozoic zircons c 2 1 ga corresponding to the age of the rio de la plata craton from which they are considered to have been sourced our interpretation is that the protolith of the ambato complex was deposited prior to juxtaposition with the craton and is older than the early ordovician la cebila metamorphic complex we infer that the craton reached its current relative position in the mid to late cambrian after the main pampean tectonothermal event 530 520 ma and before deposition of the la cebila protolith and the achavil formation sierra de famatina which contain comparable detrital zircon populations

mid crustal deformation in a continental margin orogen structural evolution and timing of the famatinian orogeny nw argentina new structural data from a mid crustal segment in the eastern sierras pampeanas coupled with geochronological methods and p t estimates reveal polyphase contractional deformation and metamorphism during the famatinian orogeny over a long period of time peaks of metamorphic monazite and zircon ages are recorded at c 500 ma between 484 and 465 ma and at c 440 ma between 484 and 465 ma the region attained high temperature ht low pressure lp conditions that resulted in widespread partial melting peak at c 470 ma a contractional phase occurred during this event as suggested by syn anatectic structures as well as folding at subsolidus conditions renewed contraction under subsolidus conditions is evidenced by reverse ductile shearing and folding at c 440 ma thrusting along the la chilca shear zone caused metamorphic inversion the consistent orientation of folds in the quebrada del molle metamorphic complex el portezuelo metamorphic igneous complex and the la chilca shear zone as well as wsw directed thrusting at the later shear zone indicate uniform wsw ene directed shortening this late deformation records the ocloyic tectonic phase and brought the famatinian orogeny to an end in the late ordovician to early silurian supplementary material analytical methods methods and description of the calculated p t pseudosection diagrams bulk compositions and electron microprobe u pb and 40ar 39ar data are available at https doi org 10 6084 m9 figshare c 4697189

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

determining accurate temperature time paths from u pb thermochronology an example from the kaapvaal craton southern africa thermochronology has revolutionized our understanding of the establishment and evolution of lithospheric thermal structure however many potential benefits provided by the application of diffusion theory to thermochronology have yet to be fully exploited this study uses apatite tc 450 550 c and titanite tc 550 650 c u pb id tims thermochronology at the single to sub grain scale to separate the variable effects of volume diffusion of pb from metamorphic over growth above and below the tc of a mineral data are presented from two ca 3227 ma tonalite samples from north and south of the barberton greenstone belt bgb southern africa two distinct populations of apatite from a sample north of the bgb record fast cooling followed by metamorphic growth 10 myr later both apatite and titanite dates from south of the bgb show a strong correlation with the grain size and record 100 myr of post emplacement cooling complex core rim zoning observed in cathodoluminescence images of apatite is interpreted to reflect metamorphic overgrowth above the tc the age and topology of grain size versus date curves from titanite and apatite are used in combination with a finite difference numerical model to show that slow non linear cooling and not thermal resetting is responsible for the observed distribution the thermal histories from either side of the bgb are very different and provide unique insight into the bgb s tectonic evolution a 70 myr period of apparent stability after ca 3 2 ga terrane assembly was followed by fast exhumation south of the bgb that led to lower crustal melting and intrusion of granitic batholiths ca 3 14 3 10 ga

contrasting geochemical features of the central iberian zone shales iberian massif spain implications for the evolution of neoproterozoic lower cambrian sediments and their sources in other peri gondwanan areas abstract more than 90 samples of shales have been collected for geochemical purposes from neoproterozoic and lower cambrian sedimentary series in an extensive area of the central iberian zone ciz for similar sio2 and al2o3 abundances the neoproterozoic shales ns are characterized by relatively higher tio2 and zr abundances and by higher cr th and ti nb ratios by contrast the lower cambrian shales lcs have higher al2o3 tio2 rb zr la yb cn and th yb ratios diagrams such as al2o3 tio2 al2o3 tio2 ti nb tio2 zr tio2 zr nb and ti nb rb zr are useful to define two distinct fields that discriminate the neoproterozoic from the lower cambrian compositions these and other geochemical features suggest that these rocks were derived from different source compositions and that the lower cambrian shales lcs were chemically more fractionated than those of the neoproterozoic these distinctive chemical features are also recorded by neoproterozoic and cambrian fine grained rocks in other areas in the central iberian zone and in other zones within iberia published neodymium isotopic results consistently distinguish both compositions and together with the geochemical data support a relatively fractionated nature for the lower cambrian sediments compared to the neoproterozoic ones it is proposed that continental deposits derived from the interior orogens in north gondwana would have been the sources for neoproterozoic sediments in the iberian zones whereas the lower cambrian sediments in these zones would have been derived from older crust beneath those continental deposits thus the neoproterozoic lower cambrian stratigraphic succession in iberia reflects an inverse stratigraphy of the source area and contrasts with zones of central and western europe where equivalent sedimentary series record important coeval igneous contributions and show less fractionated geochemical characteristics finally the possible connection between the central iberian zone and the meguma zone mz in canada is discussed on the basis of certain sedimentological and geochemical features it is proposed that the sedimentary series of the central iberian zone and of the meguma zone represent respectively proximal and distal north gondwana derived sediments

early paleozoic accretionary orogenies in nw argentina growth of west gondwana abstract two early paleozoic orogenic cycles in the gondwana margin of nw argentina were driven by subduction and interrupted by collision of continental ribbons of laurentian affinity subduction started at the passive margin of gondwana possibly in response to the end of amalgamation of the supercontinent the passive margin was underlain by a hyper extended continent ocean transition and low angle subduction gave rise to a wide and hot fore arc region during the pampean orogenic cycle the arrival of the first continent ribbon ended the pampean cycle steepened the subduction further heating the fore arc and restarting arc magmatism after a magmatic lull 250 300 km trenchwards initiating the famatinian orogenic cycle this cycle started with a 30 ma long period of extension and marine sedimentation followed by the arrival of the second continent ribbon and inversion the back arc initiating a 20 ma long shortening event culminating with the shutting down of the arc this event gave rise to a 300 km wide low topography hot orogeny thus the many peculiarities of the two paleozoic orogens of nw argentina result from a subduction history that efficiently transferred heat to a 300 400 km wide belt of turbidite dominated sediments that was first part of the pampean fore arc and then part of the famatinian retroarc developed at the extended continental margin of gondwana the two orogenies with continued high heat flux lasted 110 ma giving rise to two calc alkaline arcs separated by a 300 km belt of high t low p migmatites and peraluminous granites formed by anatexis of sediments deposited on the passive margin as well as those deposited in the wide pampean fore arc these turbidite dominated sequences were metamorphosed and melted to form a continental crystalline basement and accreted to the cratonic margin adding 500 km of crystalline rocks to the margin of gondwana at the same time that a similar process was happening in eastern australia along the same continental margin interestingly this wide accretionary orogen has now become the region where the andean system developed a wide orogeny above flat slab subduction

contemporaneous plutonism and strike slip faulting a case study from the tonale fault zone north of the adamello pluton italian alps 1 a field microstructural and geochronological study of contemporaneous plutonism and strike slip faulting along the eastern tonale fault zone provides new insights into the interrelationships between magmatic emplacement contact metamorphism and shearing and it places new time constraints for the periadriatic fault system although pluton emplacement and shearing were not caused by each other they mutually interacted during contact metamorphism the character of the composite tonale fault zone varies considerably with its proximity to the northern margin of the adamello pluton from a paired greenschist facies mylonite belt northern mylonite zone and cataclastic fault zone in the west to a fault zone consisting of northern mylonite zone cataclastic fault zone and a second mylonite belt southern mylonite zone formed by contemporaneous shearing and contact metamorphism in the east ongoing strike slip motion led to telescoping and advective cooling of the deforming contact aureole new u pb zircon ages for the northernmost adamello intrusions the avio at 34 6 1 0 ma and the presanella at 32 0 2 3 ma date the onset of contact metamorphism rb sr and k ar cooling ages and new zircon fission track ages provide evidence that postintrusive cooling to below 300 c was achieved rapidly i e approximately 30 myr ago being consistent with a shallow crustal emplacement at ambient temperatures of approximately 250 c cataclastic dextral strike slip faulting continued until about 20 ma when the tonale fault zone became offset by a number of minor sinistral shears and by the giudicarie fault hence dextral strike slip motion along the periadriatic fault system east of the bergell pluton lasted from approximately 35 20 myr ago

lead incorporation during crystal growth and the misinterpretation of geochronological data from low 238u 204pb metamorphic minerals the isotopic composition of lead available for incorporation during metamorphic reactions is heterogeneous depends on the reaction history of the metamorphic rock and is commonly not accessible for measurement as the precursor minerals have been consumed during the growth of the metamorphic phases the initial lead composition has a significant effect on the age of low 238u 204pb metamorphic phases e g garnet rutile titanite staurolite vesuvianite and ilmenite using a distinct value e g leached k feldspar pb model pb rather than a geologically reasonably constrained range may result in apparently precise yet inaccurate ages since age data from metamorphic minerals are widely used to unravel the p t t d evolution of orogens inaccurate ages result in 1 incorrect timing duration of p t loops and associated with it the heat budget and mass transfer in orogens 2 arbitrary rates based on the age difference between core and rim for mineral growth p t evolution and deformation and 3 apparent sequences of isotopic closure for the u pb system of contrasting minerals

the geological and structural evolution of the long lived miocene pleistocene la hoyada volcanic complex in the geodynamic framework of the central andes argentina abstract the long lived la hoyada volcanic complex lhvc is located in the east of cordillera de san buenaventura in argentina this region is important for understanding of the geodynamic evolution of the southern puna since it is located in its southern edge where the transition between normal and low angle subduction occurs the aim of this work is to contribute to the knowledge of the volcanism of the southern puna through the analysis of the stratigraphy and geological evolution of lhvc fieldwork and image analysis have led to a new lithostratigraphic scheme that identifies lithosomes and unconformity bounded stratigraphic units defining the stratigraphic architecture of the lhvc new 40ar 39ar ages further constrain the geological evolution of this complex the resulting stratigraphic scheme combines the lithostratigraphic units into lithosomes and synthemic units with the identification of at least six superimposed eruptive centers and their products during the lhvc evolution a new stratigraphic framework for the eastern cordillera de san buenaventura composed by four supersynthems separated by unconformities that reflect periods of quiescence of the volcanic activity catastrophic destructive events and changes in the magmatic plumbing system is proposed 1 san buenaventura supersynthem ca 9 7 ma including mainly explosive rhyolitic volcanism 2 lower la hoyada supersynthem ca 7 d pyroclastic and epiclastic deposits and 4 purulla supersynthem

the ordovician las chacritas pluton sierra de humaya nw argentina origin and emplacement triggered by lateral shortening and magmatic stoping at mid crustal level field relationships and structural studies combined with in situ u th pb dating of monazite from las chacritas pluton lcp sierra de humaya provide insight into the emplacement of peraluminous magmas triggered by lateral shortening of the host rock and magmatic stoping at a mid crustal level of a retro arc zone in a convergent orogen modal and chemical compositions indicate that the lcp is composed of two main igneous units of peraluminous granitoids the predominant two mica granitoids were generated by interaction of crustal rocks with mafic or mafic derived magmas and or crystal rich magmas that entrained residual phases whereas less abundant leucocratic granitoids may have been originated by partial melting of metasedimentary rocks the calculated crystallization age of 474 4 ma is consistent with ordovician ages 477 470 ma of the high grade metamorphic rocks indicating concomitant magmatism and metamorphism during the famatinian orogeny the lcp was emplaced in the middle crust at a maximum depth of 14 5 km where attendant fracturing and ductile deformation were active field evidence shows strong temporal and spatial relationships between host rock ductile deformation and the emplacement of the pluton such as folding and strike deflection of the host rock layering and folded concordant leucocratic sheets with magmatic fabrics this suggests that material transfer processes like lateral wall rock displacement lateral shortening was a viable mechanism for the emplacement of the lcp however cross sectional restoration and field evidence such as wall rock xenoliths and intrusive truncations of the host rock foliation and fold traces suggest that magmatic stoping was a complementary mechanism to create the necessary space for the emplacement of the lcp this work supports previous studies showing that participation of multiple material transfer processes are the rule rather than the exception in the emplacement of plutons

the famatina complex nw argentina back docking of an island arc or terrane accretion early palaeozoic geodynamics at the western gondwana margin

stability of high al titanite from low pressure calcsilicates in light of fluid and host rock composition titanite of variable al and f content was found in granulite to amphibolite facies calcsilicates in central dronning maud land antarctica the highest observed al content corresponds to an xal 5 al al 1 ti of 0 53 previously such high values of xal were reported from high pressure rocks but the titanite of this study is from a lowpressure terrain the compositional variations in titanite can be described for all samples by a set of three linearly independent exchange vectors added to the catisio5 endmember titanite in most rocks these vectors are al 1f1ti21o21 t i 20 25m0 25o21oh1 and oh1f21 in one sample the ti20 25m0 25o21oh1 vector is replaced by a si20 25m0 25o21oh1 vector the actual amount of exchange along these vectors and therefore the amount of al in titanite depends on p and t on the composition of the coexisting fluid phase in terms of its h2o hf fugacity ratio and on host rock composition in terms of al2o3 tio2 activity ratio it is inferred that in suitable chemical environments high al titanite is stable over a wide p t range therefore the al content of titanite should not be used in geothermobarometry even qualitatively additionally because of the coupled substitutions al1f1ti21o21 and al1oh1ti21o21 the concentration of f in titanite is strongly dependent on the host rock chemistry this rules out the easy use of titanite as a monitor of fluid composition

mississippian volcanism in the south central andes new u pb shrimp zircon geochronology and whole rock geochemistry abstract in the northern extension of the famatina and the southern puna nw argentina prominent rhyolitic volcanic rocks traditionally referred to as ordovician are exposed resting on metamorphic basement and covered by thick late paleozoic siliciclastic successions we report new u pb shrimp ages from these rhyolites that show them to be of mississippian 348 342 ma age thus identifying a previously unknown volcanic event in this portion of western gondwana whole rock geochemistry and sr nd isotopic analyses suggest a crustal source for these rocks but with a juvenile input end t between 2 91 and 0 3 and tdm values between 1 09 and 1 1 ga this is different from the early paleozoic magmatism of western argentina where crustal recycling took place without any involvement of mantle material the carboniferous magmatism is compatible with an extensional environment developed along the terra australis accretionary orogen as a result of tectonic switching processes these rhyolites may be related to the coeval mississippian a type granites exposed to the east in the sierras pampeanas confirming the regional character of this magmatism

the mesoproterozoic maz terrane in the western sierras pampeanas argentina equivalent to the arequipa antofalla block of southern peru implications for west gondwana margin evolution the rocks of sierra de maz and sierra del espinal western sierras pampeanas represent pre famatinian ordovician basement isotope compositions nd and pb of metasedimentary rocks and shrimp u pb dating of detrital zircons combined with other geological evidence show that three parallel n s domains can be recognized the central maz domain contains pre grenvillian metasedimentary rocks deposited between 1 2 and 1 6 ga that underwent grenvillian granulite facies metamorphism and were intruded by mafic igneous rocks and massif type anorthosites metasedimentary rocks have high nd tdm ages 1 7 2 7 ga and very radiogenic pb 9 8 10 2 suggesting provenance from reworked early proterozoic or archean continental crust the domains to the east and west of the maz domain consist of three metasedimentary sequences with nd tdm ages between 1 2 and 1 6 ga and variably radiogenic pb 9 6 10 0 u pb shrimp dating of detrital zircons nd tdm model ages and comparison with other data suggest that these sequences are post grenvillian i e neoproterozoic and or early paleozoic the maz domain is interpreted as a suspect terrane similar to the northern arequipa antofalla craton that forms the basement of the central andes both underwent grenville age orogeny and were probably once continuous along the western margin of amazonia west gondwana

polyphase zircon in ultrahigh temperature granulites rogaland sw norway constraints for pb diffusion in zircon shrimp u pb ages have been obtained for zircon in granitic gneisses from the aureole of the rogaland anorthosite norite intrusive complex both from the ultrahigh temperature uht 900 c pigeonite in zone and from outside the hypersthene in isograd magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis a sample from outside the uht zone has magmatic cores with an age of 1034 7 ma 2 n 8 and 1052 5 ma 1 n 1 overgrown by m1 metamorphic rims giving ages between 1020 7 and 1007 5 ma in contrast samples from the uht zone exhibit four major age groups 1 magmatic cores yielding ages over 1500 ma 2 magmatic cores giving ages of 1034 13 ma 2 n 4 and 1056 10 ma 1 n 1 3 metamorphic overgrowths ranging in age between 1017 6 ma and 992 7 ma 1 corresponding to the regional m1 sveconorwegian granulite facies metamorphism and 4 overgrowths corresponding to m2 uht contact metamorphism giving values of 922 14 ma 2 n 6 recrystallized areas in zircon from both areas define a further age group at 974 13 ma 2 n 4 this study presents the first evidence from rogaland for new growth of zircon resulting from uht contact metamorphism more importantly it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c 950 c for at least 1 myr magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information a result inconsistent with some experimental data on pb diffusion in zircon which predict measurable pb diffusion under such conditions the implication is that resetting of zircon ages by diffusion during m2 was negligible in these dry granulite facies rocks imaging and th u y systematics indicate that the main processes affecting zircon were dissolution reprecipitation in a closed system and solid state recrystallization during and soon after m1

aluminous titanite sphene from the eclogite zone south central tauern window austria abstract highly aluminous titanite with al 2 o 3 contents up to 14 wt 50 of the caalsio 4 f oh end member occurs in siliceous dolomites from the eclogite zone south central tauern window austria 140 chemical analyses of titanites from 13 samples from the eclogite zone and adjacent tectonic units reveal that al is incorporated into titanite primarily by the mechanism ti o al f oh there is also a strong positive correlation between al and f content of these titanites indicating that the combined substitution is enhanced by the presence of f al contents of titanite vary between tectonic units with the most aluminous varieties occurring in the high pressure eclogite zone variations also occur from sample to sample within a single tectonic unit and both between and within individual grains in a single sample complex zoning with sharp chemical and optical discontinuities is observed with al 2 o 3 variations of up to 6 wt within a single sample individual grains generally have low al cores mantled by high al zones and rimmed by low al edges it is suggested that the incorporation of al into titanite is favoured by high p low t and high f activity

proterozoic early paleozoic evolution in western south america a discussion abstract the hypothesis of exotic terranes in peru bolivia argentina and chile generated discussions on the mode of transfer and extent of accretional events that may have occurred in the southern andes during the late proterozoic early paleozoic initially a tectogenesis based on autochthonous mobile fold belts was discussed following ideas emphasised the fragmentation of the supercontinent rodinia laurentia moving along the west gondwana border and colliding with the gondwana western margin the most important effect of this laurentia gondwana relationship was attributed to the argentine precordillera or cuyania terrane splitting off from laurentia and docking to gondwana in the early paleozoic in this study the most cited arguments for this laurentia precordillera relationship are discussed emphasising paleontological considerations it is shown that these arguments do not exclude a close original vicinity of the precordillera terrane to gondwana the precordillera terrane is suggested to be part of a hypothetical platform which developed between south america africa and antarctica safran platform and which was displaced to its actual position by transcurrent faults the collisional events in the sierras pampeanas ensued from strike slip movements and were responsible for the s and i type transpressional magmatism along the pampean and famatinian terranes the final result of this continent parallel movement of terrane slices is similar to that of a terrane split off from laurentia but the first named way of formation easier explains the general continuity of plate convergence at the western border of gondwana than the laurentia precordillera connection does

isotope geochemistry and geochronology of the nico perez terrane rio de la plata craton uruguay u pb shrimp zircon geochronology bulk rock geochemistry and sr nd isotopic data are herein presented in order to better constrain the tectonic evolution of the nico perez terrane npt uruguay the studies are focused on two key geological units located in the southern part of the npt namely carape complex cc and lavalleja metamorphic complex lmc the carape complex previously interpreted as part of a remnant magmatic arc formed at 1 75 ga mainly by partial melting of archean crust t dm model ages between 2 8 and 3 0 ga with strong negative e nd t values the lavalleja metamorphic complex previously interpreted as a back arc basin has volcanic units formed at 0 59 ga during the brasiliano cycle but may also contain older paleoproterozoic units the geochemistry of the lmc meta igneous rocks indicates a complex scenario two distinct signatures have been recognized one similar to morbs and the other similar oibs sm nd t dm model ages of basic rocks present a bimodal paleoproterozoic and archean distribution which is also recognized in several other units of the npt in the light of new results we conclude that the npt represents a cratonic margin setting characterized by the stacking of lithotectonic assemblages with archean paleoproterozoic and neoproterozoic age at the margin of the rio de la plata craton

petrogenesis of migmatites and leucogranites from sierra de molinos salta northwest argentina a petrologic and geochemical study abstract in sierra de molinos eastern cordillera nw argentina the relationship between different components of an anatectic system developed in high temperature and low pressure conditions we use petrological and geochemical variations to investigate the formation of granite magma from migmatites that derived from metasedimentary protoliths this provides evidence of the processes that occur during the evolution melting segregation and crystallization stages of partially molten terranes for which particular components protolith residuum and melt fractions are identified mass balance calculations are used to estimate the degree of partial melting and the degree of melt extraction in migmatites petrological and compositional data suggest that migmatites from this region are the result of open system processes and that fluid absent melting played a major role in the formation of migmatites and leucogranites the geochemical evidence suggests that the compositional variation within the anatectic rocks from molinos can be understood as a result of 1 variable degrees of melt residuum separation and 2 fractional crystallization of the anatectic melt compared to the protolith most of the migmatites have melt depleted compositions up to 0 25 of melt extraction in the same fashion most of the leucosomes represent cumulate products of fractional crystallization this means that they have lost some liquid of evolved composition and represent evidence of the melt extraction pathways and linkage to the magma ascent conduits

inherited pb components in magmatic titanite and their consequence for the interpretation of upb ages abstract the occurrence of inherited radiogenic pb components in 35 ma old magmatic titanite is reported here for the first time the about 810 ma age of the inherited components is identical to that contained in zircon from the same amphibole bearing syenite sample in both minerals the precambrian components were not totally reset during magmatic new growth at 35 ma this age pattern substantiates that the u pb chronometer in titanite is resistant to temperatures in excess of 712 c as is already known for zircon and monazite in consequence u pb ages from crystalline titanites have to be interpreted as the age of mineral formation or re crystallization rather than the time of u pb system closure during cooling of the rock

early evolution of the proto andean margin of south america from a detailed study of a 500 km transect in the sierras pampeanas central west argentina two pre silurian tectono magmatic episodes are recognized and defined each culminating in micro continental collisions against the proto andean margin of gondwana the pampean orogeny started in early cambrian time with short lived subduction indicated by ca 535 ma calc alkaline granitoids following pampean terrane collision burial to granulite facies conditions ca 9 kbar generated widespread migmatites and ca 520 ma highly peraluminous granites in the eastern sierras pampeanas after brief quiescence a second major episode the famatinian orogeny started with subduction ca 490 ma forming a wide continental arc and ensialic backarc basin this heralded the approach of laurentia to gondwana during which the precordillera terrane separated from the southern appalachian region finally colliding with gondwana in silurian devonian time

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

timing of deformation in the sarandi del yi shear zone uruguay implications for the amalgamation of western gondwana during the neoproterozoic brasiliano pan african orogeny u pb and hf zircon sensitive high resolution ion microprobe shrimp and laser ablation inductively coupled plasma mass spectrometry la icp ms ar ar hornblende and muscovite and rb sr whole rock muscovite isochron data from the mylonites of the sarandi del yi shear zone uruguay were obtained in order to assess the tectonothermal evolution of this crustal scale structure integration of these results with available kinematic structural and microstructural data of the shear zone as well as with geochronological data from the adjacent blocks allowed to constrain the onset of deformation along the shear zone at 630 625 ma during the collision of the nico perez terrane and the rio de la plata craton the shear zone underwent dextral shearing up to 596 ma under upper to middle amphibolite facies conditions which was succeeded by sinistral shearing under lower amphibolite to upper greenschist facies conditions until at least 584 ma after emplacement of the cerro caperuza granite at 570 ma the shear zone underwent only cataclastic deformation between the late ediacaran and the cambrian the sarandi del yi shear zone is thus related to the syncollisional to postcollisional evolution of the amalgamation of the rio de la plata craton and the nico perez terrane furthermore the obtained data reveal that strain partitioning and localization with time magmatism emplacement and fluid circulation are key processes affecting the isotopic systems in mylonitic belts revealing the complexity in assessing the age of deformation of long lived shear zones

from crustal thickening to orogen parallel escape the 120 myr long ht lp evolution recorded by titanite in the paleozoic famatinian backarc nw argentina

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

the ordovician sierras pampeanas puna basin connection basement thinning and basin formation in the proto andean back arc article i nfo the ordovician sierras pampeanas located in a continental back arc position at the proto andean margin of southwest gondwana experienced substantial mantle heat transfer during the ordovician famatina orogeny converting neoproterozoic and early cambrian metasediments to migmatites and granites the high grade metamorphic basement underwent intense extensional shearing during the early and middle ordovician contemporaneously up to 7000 m marine sediments were deposited in extensional back arc basins covering the pre ordovician basement extensional ordovician tectonics were more effective in mid and lower crustal migmatites than in higher levels of the crust at a depth of about 13 km the separating boundary between low strain solid upper and high strain lower migmatitic crust evolved to an intra crustal detachment the detachment zone varies in thickness but does not exceed about 500 m the formation of anatectic melt at the metamorphic peak and the resulting drop in shear strength initiated extensional tectonics which continued along localized ductile shear zones until the migmatitic crust cooled to amphibolite facies p t conditions p t d t data in combination with field evidence suggest significant ca 52 crustal thinning below the detachment corresponding to a thinning factor of 2 1 ductile thinning of the upper crust is estimated to be less than that of the lower crust and might range between 25 and 44 constituting total crustal thinning factors of 1 7 2 0 while the migmatites experienced retrograde decompression during the ordovician rocks along and above the detachment show isobaric cooling this suggests that the magnitude of upper crustal extension controls the amount of space created for sediments deposited at the surface upper crustal extension and thinning is compensated by newly deposited sediments maintaining constant pressure at detachment level thinning of the migmatitic lower crust is compensated by elevation of the crust mantle boundary the degree of mechanical coupling between migmatitic lower and solid upper crust across the detachment zone is the main factor controlling upper crustal extension basin formation and sediment thickness in the back arc basin the initiation of crustal extension in the back arc however crucially depends on the presence of anatectic melt in the middle and lower crust consumption of melt and cooling of the lower crust correlate with decreasing deposition rates in the sedimentary basins and decreasing rates of crustal extension

the early palaeozoic high grade metamorphism at the active continental margin of west gondwana in the andes nw argentina n chile the evolution of the early palaeozoic orogen of west gondwana in the cambrian to ordovician basement of the andes between 18 and 32 s is investigated for pressure and temperature conditions and age of metamorphism it is characterized by mid crust temperatures commonly above the wet granite solidus 650 c widespread felsic migmatite and rare granulite formed at pressures of ca 0 5 0 7 gpa locally 1 0 gpa these rocks represent the deepest exhumed sections of the early palaeozoic crust high pressure low temperature rocks are absent the crystallization ages compiled from the literature in combination with new data for near peak metamorphic conditions of these high grade metamorphic rocks in nw argentina and n chile are 530 500 ma and 470 420 ma both age groups are spatially overlapping radiogenic isotope signatures sr nd pb are used to characterize the early palaeozoic basement the pb and sr isotope compositions of the early palaeozoic basement indicate mixing arrays between pre palaeozoic unradiogenic and radiogenic crust crustal residence ages sm nd tdm indicate a prominent event of crust formation around 2 ga which is known continent wide this material was recycled during midproterozoic and early palaeozoic orogenies without prominent additions of new crust present in the isotope record i e accretion of compositional exotic material is absent

the crust and upper mantle of the sulu uhpm belt abstract all results from integrated geophysical investigations in the sulu region are summarized in this paper trying to reconstruct the sulu uhpm processes new seismic s wave tomographic results suggest a velocity abnormal zone occurs beneath the sulu crust revealing detailed upper mantle structures that high velocity lumps within the abnormal zone are sequentially distributed beneath the bottom of the asthenosphere these high velocity lumps might represent delaminated eclogites or residuals of the subducted oceanic plate based on integrated interpretation of the geophysical data we propose a working model for tectonic reconstruction of the sulu uhpm processes which can explain the crust and upper mantle structures of the area the involved tectonic processes are related to north eastward escaping of the sulu terrane subduction and delamination cycles of the dabie sulu oceanic plate and post orogenic lithospheric thinning and magma underplating the uhpm rocks are believed to have syn subduction delaminated down to the bottom of the asthenosphere during 245 180 ma and the delamination process seemed smooth and nearly continuous without extensive violence

timing of high grade metamorphism early palaeozoic u pb formation ages of titanite indicate long standing high t conditions at the western margin of gondwana argentina 26 29 s concordant u pb ages of c 530 510 ma and c 470 420 ma on titanite from calcsilicate orthogneiss and amphibolite rocks constrain the age of high t metamorphism in the early palaeozoic mobile belt at the western margin of proterozoic gondwana argentina 26 29 s the u pb ages document the time of titanite formation at high t conditions according to the stable mineral paragenesis and occurrence of titanite in the metamorphic fabric the presence of migmatite at all sample sites indicates temperatures were c 650 c titanite formed at similar metamorphic conditions at different times on the regional and on the outcrop scale the titanite crystals preserved their u pb isotopic signatures and chemical composition under ongoing upper amphibolite to granulite facies temperatures different thermal peaks or deformations are only detected by the different u pb ages and not by changes in the mineral paragenesis or metamorphic fabric of the samples the range of u pb ages e g in the ordovician and silurian c 470 460 440 430 420 ma is interpreted as the effect polyphase deformation with deformation enhanced recrystallization of titanite and or different thermal peaks during a long standing geographically fixed high t regime in the mid crust of a continental magmatic arc a clear correlation of the different ages with distinct tectonic events e g collision of terranes is not possible based on the present knowledge of the region

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

a 60 myr record of continental back arc differentiation through cyclic melting continental crust forms and evolves above subduction zones as a result of heat and mass transfer from the mantle below the nature and extent of this transfer remain debated although it has been recognized that arc magmatism at active continental margins can be cyclical at 50 to 1 myr timescales such cyclicity has not been recognized in the back arc here we investigate the melting of sedimentary rocks in the continental back arc of the western gondwana margin during the cambrian ordovician famatinian orogeny of northwest argentina we determine the u pb ages of zircons that formed during crustal melting and find that they range from 505 to 440 million years ago concentrated into age groups spaced by 10 15 myr this suggests multiple and cyclical melting events in the continental back arc which matches the more than 60 myr duration of the magmatic activity in the arc and demonstrates that thermal and magmatic cyclicity also extends to the continental hinterland we conclude that back arc melting reflects a long lasting pulsating cyclical heat transfer from mantle to crust that leads to thorough crustal reworking transforming sedimentary rocks into crystalline continental crust thus while magma transfer from the mantle promotes crustal growth in convergent margins cyclical melting promotes crustal maturation of the back arc melting of sedimentary rocks in the continental back arc is cyclical with peaks of magmatism every 10 to 15 million years according to zircon ages from paleozoic western gondwana margin samples

ordovician metamorphism and plutonism in the sierra de quilmes metamorphic complex implications for the tectonic setting of the northern sierras pampeanas nw argentina towards unravelling the geodynamic setting of the northern sierras pampeanas nw argentina we describe the tectono metamorphic and geochronologic evolution of sub greenschist to granulite facies metamorphic sediments granitoid plutons and pegmatites in the ordovician sierra de quilmes metamorphic complex the protoliths of the metasediments are represented by a sequence of turbidites and minor calcsilicate rocks of the neoproterozoic to cambrian puncoviscana formation the metamorphic complex consists of four zones including the 1 chlorite 2 biotite muscovite 3 garnet cordierite sillimanite and 4 orthopyroxene zones zones 3 and 4 show an increasing degree of anatexis reaching large scale diatexis in the orthopyroxene zone at p t conditions exceeding 800 c and 600 mpa at or shortly after the metamorphic peak the granitic to tonalitic cafayate pluton intruded approximately along the boundary between anatectic and non anatectic rocks retrograde near isobaric cooling of the middle crust was accompanied by non penetrative ductile shearing at granulite to amphibolite facies p t conditions evidence for significant prograde deformation is absent in the sierra de quilmes metamorphic complex monazite and titanite u pb isotopic data constrain the metamorphic peak in migmatites and calcsilicate rocks to be at or slightly prior to 470 ma retrograde amphibolite facies mineral reactions led to continuous formation of monazite and titanite during slow cooling between 470 ma and 455 ma u pb data the composite cafayate pluton intruded over a time interval of several million years between 477 ma sm nd isochron and 460 ma monazite and titanite u pb isochron followed by pegmatites a younger group of pegmatites was emplaced in the country rocks at the end of the ordovician 440 ma rb sr mineral isochrons postdating most of the retrograde shear zones resetting of the muscovite k ar and 40ar 39ar system in weakly deformed pegmatites and crystallisation of new muscovite in low grade metamorphic sediments at 400 416 ma is attributed to minor late greenschist and sub greenschist facies metamorphism and deformation the massive early ordovician heat transfer the absence of major prograde deformation and subsequent prolonged phases of non penetrative deformation in the high grade metamorphic zones during slow near isobaric cooling contradict crustal thickening and continent collision models as have been proposed for the southern sierras pampeanas we suggest continuous or stepwise extensional tectonics in a back arc or a mobile belt tectonic environment for the ordovician sierra de quilmes and the northern sierras pampeanas in general an extensional setting of the northern sierras pampeanas in the ordovician is in agreement with the coeval formation of marine extensional sediment basins in vicinity of the sierras pampeanas in northwestern argentina and southern bolivia

grenvillian massif type anorthosites in the sierras pampeanas we report the discovery of massif type anorthosites in the andean basement of the western sierras pampeanas of argentina u pb zircon dating by sensitive high resolution ion microprobe of a cogenetic gabbronorite dyke yields ages of 1070 41 ma for igneous emplacement and 431 40 ma for metamorphism these anorthosites are petrologically and geochemically comparable with those of the grenville province of laurentia palaeogeographical reconstructions of rodinia at 1 0 1 1 ga suggest that the sierras pampeanas anorthosites were part of a large anorthosite province in the late mesoproterozoic

shear zones and magma ascent a model based on a review of the tertiary magmatism in the alps 1 the alpine oligocene plutons are spatially and temporally associated with the activity of the periadriatic fault system pfs an orogen parallel crustal scale transpressive mylonitic belt excellent three dimensional exposure combined with a wealth of structural seismic petrological geochronological geochemical and paleomagnetic data collected over the last decades help to constrain the relationships between deformation ascent and emplacement of the plutons magmas were channeled from the base of the thickened continental crust into the narrow mylonitic belt of the periadriatic fault system which was used as ascent pathway to cover vertical lengths of 20 to 40 km therefore the linear alignment of the plutons at the surface is not the expression of a linear source region at depth ascent of the melts is controlled by the mylonitic foliation of the pfs which forms the only steep anisotropy continuously traversing the entire alpine crust in contrast the flow direction is not influenced by the specific kinematics of the faults final emplacement of the plutons occurred by extrusion from the periadriatic fault system into the adjacent country rocks the transition from ascent to final emplacement is favored by partitioning of transpressive deformation

the ages and tectonic setting of the faja eruptiva de la puna oriental ordovician nw argentina abstract the ordovician faja eruptiva de la puna oriental is a magmatic predominantly intrusive belt in the puna of northwestern argentina with a n s extension of ca 400 km scarce isotope geochemical ages and biostratigraphic data on some of the folded faja eruptiva country rocks assign the magmatism either to the lower and lower middle ordovician or to the latest ordovician interpretations of origin and tectonic framework of the faja eruptiva are controversial and vary between arc back arc and collisional orogenic settings we present high resolution la icp ms u pb age and hf isotope data on zircons from 10 plutonic samples covering the magmatic belt along a length of 200 km in the northern argentinian puna the xenocrystic and magmatic zircon age data have a wide spread between 2700 ma and 440 ma concordia and weighted mean age data document protracted magmatism in two phases between 480 and 460 ma and between 453 and 444 ma and constrain the time of the last intrusions at 444 3 ma and at 445 2 ma thus defining this last and main phase of intrusion at 444 ma ehf t values define a main vertical trend centered at 500 ma with ehf t values between 3 and 16 indicating significant mixing of juvenile early paleozoic melts with paleoproterozoic crustal components a second trend is formed by zircons with ages between 1 1 ga and c 500 ma and predominantly positive ehf values between 8 and 3 and originates in juvenile mantle compositions between 1 6 and 1 1 ga the spread of the zircon and hf data document that the faja eruptiva intrusives have experienced large scale contamination by the hosting crustal basement it follows that the basement of the puna is formed either by the upper proterozoic lower cambrian puncoviscana formation as an erosional product of the proterozoic orogenic belts of sw amazonia or that the puna including its puncoviscana basement is underlain by a crust shaped by these orogenies the main intrusive event at 444 ma has been linked to the ocloyic orogeny in the late ordovician the plutons intruded very likely in a sinistral strike slip regime after the main folding phase of the ocloyic orogeny had deformed the ordovician sedimentary country rocks

post collisional transition from calc alkaline to alkaline magmatism during transcurrent deformation in the southernmost dom feliciano belt braziliano pan african uruguay abstract in the southernmost dom feliciano belt of uruguay highly fractionated calc alkaline granites mildly alkaline granites shoshonitic volcanics and peralkaline intrusions and volcanics are spatially and temporal associated with the evolution of shear zones four representative magmatic unites of this diverse association were petrographic and geochemically investigated the solis de mataojo complex a medium to high k2o calc alkaline granite with signature typical of mature continental arcs and post collisional settings the maldonado granite highly fractionated calc alkaline to alkaline with characteristics that are transitional between both types of series the pan de azucar pluton with characteristics typical of post collisional alkaline granites and the las flores shoshonitic basalts geochemistry and geotectonic setting point out that slab breakoff was most likely the mechanism associated with the generation of high k calc alkaline magmas solis de mataojo and maldonado shortly after collision extension associated to the formation of molassic basins and emplacement of dolerites and basalt flows with shoshonitic affinity las flores 15and finally a shift to magmas with alkaline signatures pan de azucar simultaneous with a second transpressional phase were probably linked with lithospheric thinning through delamination this evolution took place between 615 and 575 ma according to available data contrary to previous proposals which considered this magmatism to represent the root of a continental magmatic arc a post collisional environment transitional from orogenic to anorogenic during transcurrent deformation is proposed

geochronological constraints on the evolution of the southern dom feliciano belt uruguay abstract new u pb sensitive high resolution ion microprobe k ar and ar ar data from the southernmost dom feliciano belt allow the identification of four major events orthogneisses from the punta del este terrane indicate a magmatic episode at c 770 ma and high grade metamorphism at 641 17 ma granitoid emplacement at 627 23 ma was roughly coeval with peak metamorphism volcaniclastic rocks of the las ventanas formation dated at 573 11 ma can be correlated with the peripheral foreland basin 571 8 ma transpression and coeval high k calc alkaline magmatism is recorded in the maldonado granite dated at 564 7 ma the following events are postulated 1 magmatism at 850 750 ma related to rifting 2 metamorphism and granite emplacement at 650 600 ma 3 molasse sequences and foreland basins at c 573 ma 4 late magmatism at 580 560 ma associated with transpression the data indicate that 1 the punta del este terrane could be a portion of the coastal terrane of the kaoko belt 2 granitoid emplacement at 650 600 ma in the punta del este and nico perez terranes favours westward subduction and 3 widespread post collisional synkinematic magmatism occurred in the dom feliciano and kaoko belts between 580 and 550 ma supplementary material analytical methods and data are available at http www geolsoc org uk sup18369

u pb geochronology of the grenville orogen of ontario and new york constraints on ancient crustal tectonics based on lithological structural and geophysi cal characteristics the proterozoic grenville orogen of southern ontario and new york has been divided into domains that are separated from each other by ductile shear zones in order to constrain the timing of meta morphism u pb ages were determined on metamorphic and igneous sphenes from marbles calc silicate gneisses amphibolites granitoids skarns and pegmatites in addi tion u pb ages were obtained for monazites from meta pelites and for a rutile from an amphibolite these mineral ages constrain the timing of mineral growth the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed grenville orogen based on the ages from metamorphic minerals regional and contact metamorphism occurred in the following intervals central granulite terrane