early detection of first time slope failures using acoustic emission measurements large scale physical modelling early warning systems for slope instability need to alert users of accelerating slope deformation behaviour to enable safety critical decisions to be made this study shows that acoustic emission

granites and the geodynamic history of the neoproterozoic brasilia belt central brazil a review abstract recent field and geochronological studies have demonstrated the importance of granitic magmatism in the evolution of the neoproterozoic brasilia belt in central brazil this is an orogenic belt developed in response to the convergence between the amazon sao francisco congo and parana continental blocks the presence of neoproterozoic juvenile arc rocks and syn collisional peraluminous granites challenged previous intracontinental evolution models for the belt the granitoid intrusions reviewed in this paper record the different stages of evolution of the orogen and their field and isotopic characteristics can be used to reconstruct the tectonic history of the belt the main field and isotopic characteristics of four granite suites associated with the brasilia belt are reviewed i 1 77 1 58 ga old rift related a type granite intrusions ii ca 0 8 0 7 syn collisional granitoids iii arc metatonalites and metagranodiorites ca 0 9 to 0 63 ga and iv bimodal post orogenic suite ranging in age from ca 0 59 to 0 48 ga these rocks suggest that during most of the neoproterozoic the western margin of the sao francisco continent faced a large oceanic basin where subduction and oceanic lithosphere consumption started at ca 0 9 ga roughly coeval with the initial stages of the break up of rodinia final ocean closure happened at ca 0 63 0 60 ga with crustal thickening uplift and erosion post orogenic extension related magmatism took place between ca 0 6 and 0 5 ga and was partially contemporaneous with the deposition of the paraguay and tucavaca sedimentary successions resulting from the rifting event related to the break up of laurentia from southwestern gondwana

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

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

early silurian glaciation along the western margin of gondwana peru bolivia and northern argentina palaeogeographic and geodynamic setting the record of the ordovician silurian transition within the southern peru bolivia palaeozoic basin extending across peru bolivia and northern argentina is characterized by a diamictite bearing unit which overlies several different ordovician units and underlies mid silurian shales we review the lithostratigraphy sedimentology and biostratigraphy of this unit revising stratigraphic relationships and several ideas and interpretations previously proposed analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and environments involved during its deposition turbidite and shale interbeds within the diamictites indicate a deep marine environment with interbedded mud flows debris flows slumps and large displaced slabs providing evidence for sediment instability and resedimentation chitinozoan biostratigraphy indicates a llandovery age in contrast with previous proposals of a hirnantian age and suggests the need for more detailed sedimentologic and biostratigraphic studies and a reassessment of this diamictite unit in relation to adjacent regions in south america the resedimented character of the deposit explains some of the ordovician fauna previously described which must be considered as recycled from underlying units in situ fauna needs to be reassessed as it may be endemic diachronic and or indicative of migrations within gondwana glacially faceted and striated clasts as well as large granitoid boulders within the resedimented materials provide evidence for glaciation of the source area and are interpreted as recycled from former glacigenic deposits the evidence found in the central andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late llandovery telychian the precise age of glaciation in this part of gondwana cannot be confirmed do to a lack of true tillites tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western gondwana during the late ordovician and early silurian the new data imply that the ordovician silurian boundary was not preserved in some areas of the basin and that an erosional hiatus disconformity with variable regional significance is present between different ordovician units and the diamictite unit late ordovician late caradoc early ashgill euxinic black shales locally present in the central altiplano and eastern cordillera of bolivia may represent the youngest ordovician unit in the region and more research should be focused on them paleogeographic reconstruction of the early silurian peru bolivia basin depicts the extension of the diamictite unit southwards into northern argentina and western paraguay connecting with the parana basin through the asuncion arch to the north it continued into northern peru and ecuador connecting with the silurian record in colombia and venezuela and to the northeast into brazil solimoes and amazonas basins the sedimentary record of glaciation in the peru bolivia basin was due to the development of local ice fields to the west of the basin related with tectonism and resulting reliefs along the active margin of gondwana and probably did not coincide with the development of the north african late ashgillian ice cap

testing thrust tectonic models at mountain fronts where has the displacement gone the alternative relationships that can exist between a mountain front and the adjacent foreland basin have been recognized for many years however seismic reflection data from such areas are commonly of poor quality and therefore structural models may contain large uncertainties in view of scientific and commercial interest in mountain belts we have reviewed the methods for discriminating between alternative interpretations using a case study from the montagna dei fiori in the central apennines italy in this area mesozoic and tertiary carbonate sediments are juxtaposed with a foredeep basin containing up to 7 km of messinian and plio pleistocene siliciclastic sediments a new structural model for this area demonstrates how the structures in this area form a kinematically closed system in which displacement is transferred from the thrust belt to blind structures beneath the present day foreland growth strata show that pliocene shortening was initially rapid 15 mm a 1 followed by slower rates during the final stages of deformation variations in structural elevation indicate a component of basement involvement during thrusting and this is further supported by magnetic modelling the results illustrate the interaction of thin and thick skinned structures in the central apennines and the methods for discriminating between alternative structural models

present knowledge of the magmatic evolution of the eastern cordillera of peru abstract the studies which have been carried out in the eastern cordillera of peru over the past 20 years prove the existence of at least three orogenic cycles the late precambrian the hercynian and the andean each one accompanied by a more or less abundant magmatism 1 the precambrian leaving aside the prasinites possibly derived from synsedimentary volcanites precambrian magmatism in the huanuco region consists of a meta igneous ultramafic to mafic association serpentinites meta gabbros meta diorites syntectonic meta tonalites and post tectonic dioritic and granitic intrusive bodies 2 the hercynian 550 to 220 m y from the cambrian to the upper devonian the existence of a synsedimentary magmatism is known syntectonic granites were emplaced during the eohercynian phase but the major part of magmatism is of a late permian to early trias age and is characterized by the intrusion of granitoids and by volcanism of calc alkaline tendency it appears that the nepheline syenite of macusani may belong to a terminal episode of this magmatic period 3 during the andean evolution there is considerably less magmatism in the eastern cordillera than during the preceding cycles plutonism which is essentially acid is only well represented in central peru volcanic rocks are more abundant and more widely distributed knowledge of the precambrian is still insufficient to enable us to place the magmatism of this orogenic belt in a geodynamic context it seems that the hercynian magmatism is related to extension episodes affecting a continental crust on the other hand it is probable that the andean magmatism of the eastern cordillera is linked at least indirectly to the presence of an active convergent margin to the west of peru dating from the mesozoic

the apparent polar wander path for south america during the permian triassic abstract many of the controversies that arise in global reconstructions for permian triassic time could be resolved by taking into account the large latitudinal and counter clockwise movement of gondwana during that interval of time the proper trace of the apparent polar wander curve should differentiate one position for the early permian another position for the late permian and yet another for the triassic by doing so and comparing the apparent polar wander paths apwp of south america and africa it is easy to see that both curves have the same shape therefore it is possible to arrive at a good fit between them which previous analyses were unable to achieve this new proposed permian triassic track of the apwp reveals a hook not hitherto recognized that should be accounted for in global reconstructions

zircon and whole rock nd pb isotopic provenance of middle and upper ordovician siliciclastic rocks argentine precordillera graptolite bearing middle and upper ordovician siliciclastic facies of the argentine precordillera fold thrust belt record the disintegration of a longlived cambro mid ordovician carbonate platform into a series of tectonically partitioned basins a combination of stratigraphic petrographic u pb detrital zircon and nd pb whole rock isotopic data provide evidence for a variety of clastic sediment sources four upper ordovician quartzo lithic sandstones collected in the eastern and central precordillera yield complex u pb zircon age spectra dominated by 1ae05 1ae10 ga zircons secondary populations of 1ae22 1ae30 and 1ae46 ga rare 2ae2 and 1ae8 ga zircons and a minor population 2 of concordant zircons in the 600 700 ma range archaean age grains comprise 1 of all zircons analysed from these rocks in contrast a feldspathic arenite from the middle ordovician estancia san isidro formation of the central precordillera has two well defined peaks at 1ae41 and 1ae43 ga with no grains in the 600 1200 ma range and none older than 1ae70 ga the zircon age spectrum in this unit is similar to that of a middle cambrian quartz arenite from the la laja formation suggesting that local basement rocks were a regional source of ca 1ae4 ga detrital zircons in the precordillera terrane from the cambrian onwards the lack of grains younger than 600 ma in upper ordovician units reinforces petrographic data indicating that ordovician volcanic arc sources did not supply significant material directly to these sedimentary basins nd isotopic data n 32 for middle and upper ordovician graptolitic shales from six localities define a poorly mixed signal end 450 ma 9ae 6t o 4ae5 that becomes more regionally homogenized in upper ordovician rocks 6ae 2 1 ae0 tdm 1ae51 0ae15 ga n 17 a trend reinforced by the u pb detrital zircon data it is concluded that proximal recycled orogenic sources dominated the siliciclastic sediment supply for these basins consistent with rapid unroofing of the precordillera terrane platform succession and basement starting in mid ordovician time common pb data for middle and upper ordovician shales from the western and eastern precordillera n 15 provide evidence for a minor 30 component that was likely derived from a high l u pb terrane

30 million years of permian volcanism recorded in the choiyoi igneous province w argentina and their source for younger ash fall deposits in the parana basin shrimp u pb zircon geochronology evidence abstract we present four shrimp u pb zircon ages for the choiyoi igneous province from the san rafael block central western argentina dated samples come from the yacimiento los reyunos formation 281 4 2 5 ma of the cochico group lower choiyoi section andesitic breccias dacitic to rhyolitic ignimbrites and continental conglomerates agua de los burros formation 264 8 2 3 ma and 264 5 3 0 ma and cerro carrizalito formation 251 9 2 7 ma upper choiyoi section rhyolitic ignimbrites and pyroclastic flows spanning the entire permian succession of the choiyoi igneous province a single zircon from the el imperial formation that is overlain unconformably by the choiyoi succession yielded an early permian age 297 2 5 3 ma while the main detrital zircon population indicated an ordovician age 453 7 8 1 ma the new data establishes a more precise permian age artinskian lopingian for the section studied spanning 30 ma of volcanic activity volcanological observations for the choiyoi succession support the occurrence of explosive eruptions of plinian to ultraplinian magnitudes capable of injecting enormous volumes of tephra in the troposphere stratosphere the new shrimp ages indicate contemporaneity between the choyoi succession and the upper part of the parana basin late paleozoic section from the irati up to the rio do rasto formations encompassing about 24 ma geochemical data show a general congruence in compositional and tectonic settings between the volcanics and parana basin permian ash fall derived layers of bentonites thickness and granulometry of ash fall layers broadly fit into the depletion curve versus distance from the remote source vent of ultraplinian eruptions thus we consider that the choiyoi igneous province was the source of ash fall deposits in the upper permian section of the parana basin data presented here allow a more consistent correlation between tectono volcanic permian events along the paleo pacific margin of southwestern gondwana and the geological evolution of neighboring paleozoic foreland basins in south america and africa

geomechanical model test for stability analysis of high arch dam based on small blocks masonry technique abstract rock mass deformation and strength property simulation plays a key role in the rupture tests of arch dam geomechanical model subject to the limit of geometry scale usually the simulation of tectonic discontinuities such as faults disturbed belts crush belts and weak interlayer were ignored as safety buffer which may yield overstated safety factors in this paper small block masonry technique and low strength binder invented make the simultaneous simulation of the rock mass and tectonic discontinuities available which is more coincident with real engineering conditions based on the material test of blocks and binder the small blocks masonry technique has been successfully applied in the rupture test of xiluodu arch dam with the assistance of high accuracy monitoring and measuring system the stress strain and displacement distribution of the dam abutments and rock foundations are obtained by analyzing these monitoring results the failure process mode and mechanism of the dam body and abutments are derived for further identification of weakness at last the comparative analysis with other arch dam geomechanical rupture test results yields the evaluation of the dam s global stability and necessary reinforcement measures

ordovician back arc foreland and ocloyic thrust belt development on the western gondwana margin as a response to precordillera terrane accretion 1 the famatina belt southern central andes records a circa 470 m y shortening episode ocloyic orogeny affecting the peri gondwanan back arc basin in response to the accretion of the precordillera terrane collision created distinct features across and along the margin some of which persisted into the present andean structure from east to west the ocloyic deformation is recorded across the telescoped region of famatina represented by 1 an east vergent fold thrust belt evftb involving a major angular unconformity between early and middle ordovician rocks 2 a highly deformed volcano sedimentary belt and 3 an east vergent ductile shear zone affecting famatinian granites axis attitudes of ordovician refolded anticlines in the evftb show west east tectonic transport in agreement with kinematics of ocloyic high strain belts indicating shortening orthogonal to the present pacific margin eastward displacement of the ordovician volcanic arc at 27 s latitude together with a documented sinistral slip zone across the proto andean margin suggests tectonic indentation

terrane processes at the margins of gondwana the process of terrane accretion is vital to the understanding of the formation of continental crust accretionary orogens affect over half of the globe and have a distinctively different evolution to wilson type orogens it is increasingly evident that accretionary orogenesis has played a significant role in the formation of the continents the pacific margin of gondwana preserves a major orogenic belt termed here the australides which was an active site of terrane accretion from neoproterozoic to late mesozoic times and comparable in scale to the rockies from mexico to alaska or the variscan appalachian orogeny the new zealand sector of this orogenic belt was one of the birthplaces of terrane theory and the australide orogeny overall continues to be an important testing ground for terrane studies this volume summarizes the history and principles of terrane theory and presents 16 new works that review and synthesize the current state of knowledge for the gondwana margin from australia through new zealand and antarctica to south america examining the evolution of the whole gondwana margin through time

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

neoproterozoic backarc basin sensitive high resolution ion microprobe u pb and sm nd isotopic evidence from the eastern pampean ranges argentina the eastern pampean ranges comprise high grade supracrustal sequences with linear belts of mafic ultramafic bodies representing ophiolite remnants new u pb and nd isotopic data suggest that the tectonic evolution of the pampean ranges started ca 640 ma with the deposition of supracrustal sequences in a backarc basin between a neoproterozoic magmatic arc to the east and the pampia terrane to the west ophiolite remnants of this backarc basin yielded a whole rock isochron indicating the age of 647 77 ma 2 and nd initial time t of 5 2 sensitive high resolution ion microprobe u pb data for detrital metasediments show provenance patterns with two main age populations the older 1 1 0 9 ga and the younger between ca 0 7 and 0 6 ga the neoproterozoic population is relatively more abundant in sediments of the easternmost units of the eastern pampean ranges and becomes less abundant toward the west depleted mantle ages show a similar pattern with ages generally increasing from east 1 42 ga to west 1 76 ga suggesting the presence of neo proterozoic sources to the east of the ranges the provenance data do not support previous evolution models for the eastern pampean ranges according to which the supracrustal sequence represents the passive margin of the rio de la plata craton early cambrian collision and high grade metamorphism mark the final stages of evolution of the belt and were shortly followed by calc alkaline metaluminous and peraluminous granitic magmatism ca 530 514 ma the results suggest that the geological evolution of the eastern pampean ranges took place between ca 640 and 514 ma coeval with other brasiliano orogens in brazil e g the paraguay and araguaia fold belts

0 3 byr of drainage stability along the palaeozoic palaeo pacific gondwana margin a detrital zircon study the palaeo pacific margin of gondwana in the present day south central andes is marked by tectonic activity related to subduction and terrane accretion we present detrital zircon u pb data encompassing the palaeozoic era in northern chile and northwestern argentina cathodoluminescence images reveal dominantly magmatic zircon barely affected by abrasion and displaying only one growth phase the main age clusters for these zircon grains are ediacaran to palaeozoic with an additional peak at 1 3 0 9 ga and they can be correlated with grenvillian age and the brasiliano pampean and famatinian orogenies the zircon data reveal main transport from the nearby ordovician famatinian arc and related rocks the silurian sandstone units are more comparable with cambrian units with brasiliano and transamazonian ages 2 2 1 9 ga being more common because the silurian deposits were situated within or east of the extinct famatinian arc hence the arc acted as a transport barrier throughout palaeozoic time the complete suite of zircon ages does not record the accretions of exotic terranes or the palaeozoic glacial periods we conclude that the transport system along the palaeo pacific margin of gondwana remained stable for c 0 3 byr and that provenance data do not necessarily reflect the interior of a continent hence inherited geomorphological features must be taken into account when detrital mineral ages are interpreted supplementary material u pb data cl images and detailed geological maps are available at www geolsoc org uk sup18796

bentonite geochronology marine geochemistry and the great ordovician biodiversification event gobe abstract attribution of ordovician climate forcing to explosive volcanism and the potential global importance of volcanism in ordovician biodiversification suggest the necessity of evaluating the relationships between k bentonite deposition and increasingly high resolution records of marine biogeochemical change globally ordovician strata preserve an extensive record of explosive volcanism including the widely recognized lower to middle ordovician famatina k bentonite suite in argentina and the upper ordovician millbrig deicke kinnekulle suite of north america and europe here we present high resolution id tims u pb zircon ages of k bentonites from measured sections of the san juan formation talacasto and cerro la chilca section of the argentine precordillera k bentonites from the argentine precordillera provide stratigraphically consistent i e younging upward ages that range from 473 45 0 70 ma to 469 53 0 62 ma and constrain the age of a low magnitude 2 globally recorded negative carbon isotope excursion evaluation of the timing of k bentonite deposition in the argentina precordillera relative to marine biostratigraphic and biogeochemical records provides insight into relationships between explosive volcanism and regional to global environmental change from a regional standpoint these ages provide critical direct evidence for a dapingian to earliest darriwilian age of the upper san juan formation at sampled localities these ages are consistent with carbon isotope data suggesting that the san juan formation in the region of its type section is coeval with only the base of the often correlated table head group of western newfoundland this data thus highlights the difficulties in using regional biostratigraphic data particularly within erosionally truncated or otherwise diachronous units to define the timeframe of carbon isotope chemostratigraphy new geochronological data also indicate that a discrete negative carbon isotope excursion within the san juan and table head formations is correlative to a globally recognized pre mdice negative excursion and indicates that this aspect of the marine carbon isotope record can be used as a discrete chronologic marker san juan formation bentonites however cannot be discretely correlated with observed environmentally significant changes in the middle ordovician marine geochemical records of carbon sulfur strontium or sea surface temperature these results suggest that the extent of volcanism represented by the famatina bentonite suite was insufficient to affect global surface environments and that the relationship between explosive volcanism and environmental change may not be straightforward as previously suggested

spatial and glacial interglacial variations in provenance of the chinese loess plateau the chinese loess plateau clp covers an extensive area over 440 000 km2 and provides an unprecedented terrestrial record of neogene climate however it is still unclear whether the provenance of these loess deposits is uniform or contains spatial and temporal differences here this is addressed by comparing detrital zircon age spectra of typical loess and paleosol samples from three distant sites located at the western middle and southeastern parts of the clp our results reveal that the zircon age spectra not only change between loess and paleosol layers but also vary from the western to the eastern clp at least during the last glacial cycle the discrepancies of the zircon age spectra among different sites suggest that the loess provenance of clp is heterogeneous and spatially variable although it has been suggested that the mineralogical elemental and isotopic compositions of loess deposits on clp are highly homogenous spatially and in glacial interglacial cycles

u pb shrimp zircon dating of grenvillian metamorphism in western sierras pampeanas argentina correlation with the arequipa antofalla craton and constraints on the extent of the precordillera terrane abstract metamorphism of grenvillian age ca 1 2 ga u pb zircon dating is recognized for the first time in the western sierras pampeanas sierra de maz conditions reached granulite facies ca 780 c and ca 780 mpa comparing geochronological and petrological characteristics with other outcrops of mesoproterozoic basement particularly in the northern and central arequipa antofalla craton we suggest that these regions were part of a single continental crustal block from mesoproterozoic times and thus autochthonous or parautochthonous to gondwana

the south american ancestry of the north patagonian massif geochronological evidence for an autochthonous origin new u pb and 40 ar 39 ar age data from deformed and undeformed granitoids of the north patagonian massif establish the presence of early cambrian and widespread ordovician magmatism in northern patagonia these data suggest that the pampean cambrian and famatinian ordovician magmatic belts of the sierras pampeanas are continuous into patagonia shrimp u pb age spectra from detrital zircons of cambro ordovician metasedimentary rocks show patterns very similar to those from equivalent units of the pampia block over 500 km farther north these results suggest that the north patagonian massif was likely part of the south american margin of gondwana in the early palaeozoic and strongly argue in favour of an authochtonous or para autochthonous origin for this block

crustal recycling of metamorphic basement late palaeozoic granitoids of northern chile 22 s implications for the composition of the andean crust upper palaeozoic silicic magmatism is widespread in the central introduction andes but its origin is poorly constrained we investigated wholethe magmatic arc in the puna altiplano plateau of the rock chemical and isotopic composition of upper palaeozoic grancentral andes rests on a continental crust of a maximum itoids and their early palaeozoic high grade country rocks in thickness of 70 km crustal thickening is mainly atthe chilean coastal cordillera and precordillera at 22 s in tributed to shortening of the crust and not to juvenile comparison with an upper cretaceous granitoid the age of the magmatic additions from the subduction zone see review late palaeozoic granitoids from a rb sr isochron of 300 ma by allmendinger et al 1997 this implies a major is consistent with k ar cooling ages of hornblende and biotite contribution of pre mesozoic material to the present similar major and trace element patterns as well as nd and pb thick crust because mesozoic juvenile additions to the isotopic composition of upper palaeozoic granitoids and gneisses crust are focused in the mesozoic magmatic arcs mainly point to a source of the granitoids that is similar to the gneisses at in the chilean coastal cordillera fig 1 e g scheuber outcrop sr isotope ratios of the upper palaeozoic granitoids are et al 1994 lucassen et al 1996a which is not involved less radiogenic than those in many of the gneisses we propose a in the crustal thickening the last major reorganization of stratification of the early palaeozoic crust with a rb deficient the crust before the cenozoic was in the early palaeozoic granulitic mid lower crust resulting in less radiogenic sr compared orogeny between 500 ma the age of peak metawith the upper crust based on the interpretation of the p t t morphism and 400 ma the age of the final exhumation history and isotopic composition of the lower palaeozoic meta damm et al 1990 1994 lucassen et al 1996b unmorphic basement and of the isotopic composition of the late published data 1998 becchio et al 1997 the widepalaeozoic granitoids and younger magmatic rocks nd isotopic spread upper palaeozoic granitoid magmatism starting composition is identical in lower and upper crust and in crustal at 300 ma was assigned to arc magmatism e g brown melts from the late palaeozoic to recent the cretaceous granitoid 1991 bahlburg herve 1997 or to melting of the evolved from partial melts of a mantle derived source with concrust at a passive margin breitkreuz zeil 1984 and siderable contamination by the old crustal component the crust references therein the metamorphic history of the prethat formed in the early palaeozoic is the major source of material mesozoic crust was studied in detail before lucassen et for the cenozoic tectonic thickening of the andean crust al 1999a unpublished data 1998 summary of previous

provenance analysis and tectonic setting of the ordovician clastic deposits in the southern puna basin nw argentina provenance studies on early to middle ordovician clastic formations of the southern puna basin in north western argentina indicate that the sedimentary detritus is generally composed of reworked crustal material tremadocquartzrich turbidites tolar chico formation mean composition q t 89 f7 l4 are followed by volcaniclastic rocks and greywackes tolillar formation mean q t 33 f42 l25 these are in turn overlain by volcaniclastic deposits mean qt24 f30 l46 of the diablo formation late arenig early llanvirn that are intercalated by lava flows all units were deformed in the ocloyic orogeny during the middle and late ordovician sandstones of the tolar chico formation are characterized by th sc ratios 1 la sc ratios 10 whereas associated fine grained wackes show slightly lower values for both ratios lree light rare earth elements enrichment of the arenites is 50 chondrite eu eu values are between 0 72 and 0 92 and flat hree heavy rare earth elements patterns indicate a derivation from mostly felsic rocks of typical upper crustal composition the e n d t s e d values scatter around 11 to 9 the calculated nd t d m residence ages vary between 1 8 and 2 0 ga indicating contribution by a palaeoproterozoic crustal component the th sc and la sc ratios of the tolillar formation are lower than those of the tolar chico formation normalized ree rare earth elements patterns display a similar shape to paas post archaean average australian shale but with higher abundances of hrees eu eu values range between 0 44 and 1 17 where the higher values reflect the abundance of plagioclase and feldspar bearing volcanic lithoclasts average e n d t s e d values are less negative at 5 1 and nd t d m are lower at 1 6 ga this is consistent with characteristics of regional rocks of upper continental crust composition which most probably represent the sources of the studied detritus the rocks of the diablo formation have the lowest th sc and la sc ratios lower lree abundances than the average continental crust and are slightly enriched in hrees eu eu values are between 0 63 and 1 17 the nd isotopes e n d t s e d 3 to 1 t d m 1 2 ga indicate that one source component was less fractionated than both the underlying early ordovician and the overlying middle ordovician units synsedimentary vulcanites in the diablo formation show the same isotopic composition our data indicate that the sedimentary detritus is generally composed of reworked crustal material but that the diablo formation appears to contain 80 of a less fractionated component derived from a contemporaneous continental volcanic arc there are no data indicating an exotic detrital source or the accretion of an exotic block at this part of the gondwana margin during the ordovician

geodynamic evolution and tectonostratigraphic terranes of northwestern argentina and northern chile in ordovician time gondwana in the area of northwestern argentina and northern chile had a west facing active margin the evolution of this margin culminated in the ocloyic orogeny at the end of ordovician time this orogeny was caused by the collision of the allochthonous arequipa antofalla terrane with this margin the early paleozoic evolution of northwestern argentina and northern chile contrasts markedly with the accretionary history of central argentina and central chile where the precordillera and chilenia terranes docked in the late ordovician and late devonian periods respectively an inspection of the available stratigraphic and geochronological data on sedimentary volcanic and plutonic units of the southern central andes of northern chile and northwestern argentina reveals a lull in magmatic and metamorphic activity lasting for 100 m y from early silurian to early late carboniferous time this is interpreted as corresponding to a tectonic scenario in which the present andean margin was a passive margin of gondwana this passive margin developed in response to the rifting off of a part of the arequipa antofalla terrane the present location of this block is unknown late carboniferous time marks the renewed onset of subduction initiating the andean plate tectonic setting still prevalent today recently proposed models explain the late ordovician orogeny by the collision of laurentia with western south america during laurentia s clockwise motion around south america and away from its position in the neoproterozoic supercontinent in its present form this hypothesis is difficult to reconcile with the paleozoic tectonostratigraphic evolution of the southern central andean region

the proto andean margin of gondwana an introduction a basic background is presented for the discussion of the early palaeozoic geology of western argentina covered by this book this includes the definition and terminology of orogenic cycles on this part of the gondwana margin represented by the eastern sierras pampeanas the pampean orogeny early cambrian relates to an intense but short lived period of terrane collision predating the rifting of the precordillera terrane from laurentia the famatinian cycle is predominantly represented by intense subductionrelated magmatism of early middle ordovician age developed on the continental margin of gondwana during the rifting and drifting of the precordillera terrane the grenvillian basement of the latter is further exemplified by a new rb sr whole rock isochron age of 1021 12 ma for orthogneisses from the western sierras pampeanas a mid ordovician granite in this area dated at 481 6 ma by u pb ion microprobe data may be related to rifting while the precordillera terrane was still attached to laurentia a divergence of opinion is pointed out between some authors in this book who favour mid ordovician collision of the precordillera with gondwana and others who place it much latter in silurian or devonian times

correlation of ordovician diamictites from argentina and south africa using detrital zircon dating abstract the results of detrital zircon u pb geochronology suggest a maximum depositional age of 485 7 2 ma for a glacial diamictite from the sierra del volcan in eastern argentina tandilia system earlier interpretations associated the deposit with the neoproterozoic snowball earth9 hypothesis the data allow direct correlation for the first time between early palaeozoic deposits in both south america and south africa connecting the glacial deposits from southern bolivia to central argentina with those in south africa pakhuis formation on the basis of these results a new distribution map of glacial cover corresponding to the hirnantian stage can be developed supplementary material sampling and analytical techniques data a simplified map and bse images are available at http www geolsoc org uk sup18384

the gondwana connections of northern patagonia a multidisciplinary study u pb sensitive high resolution ion microprobe geochronology hf and o isotopes in zircon sr and nd isotopes in whole rocks as well as major and trace element geochemistry has been carried out on granitoid samples from the area west of valcheta north patagonian massif argentina these confirm the cambrian age of the tardugno granodiorite 528 4 ma and the late permian age of granites in the central part of the yaminue complex 250 ma together with petrological and structural information for the area we consider a previously suggested idea that the cambrian and ordovician granites of northeastern patagonia represent continuation of the pampean and famatinian orogenic belts of the sierras pampeanas respectively our interpretation does not support the hypothesis that patagonia was accreted in late palaeozoic times as a far travelled terrane originating in the central transantarctic mountains and the arguments for and against this idea are reviewed a parautochthonous origin is preferred with no major ocean closure between the north patagonian massif and the sierra de la ventana fold belt supplementary material u pb shrimp analytical data geochemical analyses and sample global positioning system locations are available at www geolsoc org uk sup18722

remagnetization and chemical alteration of sedimentary rocks remagnetization is a common phenomenon in rocks and developing a greater understanding of its mechanisms has several benefits acquisition of a secondary magnetization is usually tangible evidence of a diagenetic or thermal event which can be dated using palaeomagnetic techniques this is important because the timing of diagenetic and thermal events is commonly difficult to determine remagnetization can also obscure primary magnetizations and a better understanding of remagnetization could improve our ability to uncover primary magnetizations many chemical remagnetization mechanisms have been proposed including those associated with chemical alteration by a number of different fluids e g orogenic weathering mineralizing hydrocarbons and burial diagenetic processes e g clay diagenesis maturation of organic matter this book contains case studies and review articles that focus on remagnetization chemical remagnetization mechanisms and magnetic changes associated with chemical alteration by hydrocarbons

provenance variations in the late paleozoic accretionary complex of central chile as indicated by detrital zircons abstract we present detrital zircon u pb shrimp age patterns for the central segment 34 42 s of an extensive accretionary complex along coastal chile together with ages for some relevant igneous rocks the complex consists of a basally accreted high pressure low temperature western series outboard of a frontally accreted eastern series that was overprinted by high temperature low pressure metamorphism eleven new shrimp detrital zircon age patterns have been obtained for meta turbidites from the central 34 42 s segment of the accretionary complex four from previously undated metamorphic complexes and associated intrusive rocks from the main andean cordillera and three from igneous rocks in argentina that were considered as possible sediment source areas there are no mesozoic detrital zircons in the accretionary rocks early paleozoic zircons are an essential component of the provenance and grenville age zircons and isolated grains as old as 3 ga occur in most rocks although much less commonly in the western series of the southern sector in the northernmost sector 34 38 30 s proterozoic zircon grains constitute more than 50 of the detrital spectra in contrast with less than 10 in the southern sector 39 42 s the youngest igneous detrital zircons in both the northern western 307 ma and eastern series 345 ma are considered to closely date sedimentation of the protoliths both oxygen and lu hf isotopic analyses of a selection of permian to neoproterozoic detrital zircon grains indicate that the respective igneous source rocks had significant crustal contributions the results suggest that early paleozoic orogenic belts pampean and famatinian containing material recycled from cratonic areas of south america supplied detritus to this part of the paleo pacific coast in contrast in the southern exposures of the western series studied here permian detrital zircons 253 295 ma dominate indicating much younger deposition the northern sector has scarce early to middle devonian detrital zircons prominent south of 39 s the sedimentary protolith of the northern sector was probably deposited in a passive margin setting starved of devonian achalian detritus by a topographic barrier formed by the precordillera and possibly chilenia terranes devonian subduction related metamorphic and plutonic rocks developed south of 39 s beyond the possible southern limit of chilenia where sedimentation of accretionary rocks continued until permian times

structural evolution of a composite middle to lower crustal section the sierra de pie de palo northwest argentina 1 the sierra de pie de palo of northwest argentina preserves middle to lower crustal metamorphic rocks that were penetratively deformed during ordovician accretion of the precordillera terrane to the gondwana margin new structural petrologic and geochronologic data from a 40 km structural transect reveals that the sierra de pie de palo preserves a middle to lower crustal ductile thrust complex consisting of individual structural units and not an intact ophiolite and cover sequence top to the west thrusting occurred intermittently on discrete ductile shear zones from 515 to 417 ma and generally propagated toward the foreland with progressive deformation ordovician crustal shortening and peak metamorphic temperatures in the central portion of the sierra de pie de palo were synchronous with retro arc shortening and magmatic flare up within the famatina arc accretion of the precordillera terrane resulted in the end of arc flare up and the onset of synconvergent extension by 439 ma continued synextensional to postextensional convergence was accommodated along progressively lower grade shear zones following terrane accretion and the establishment of a new plate margin west of the precordillera terrane the results support models of cordilleran orogens that link voluminous arc magmatism to periods of regional shortening the deformation metamorphic and magmatic history within the sierra de pie de palo is consistent with models placing the region adjacent to the famatina margin in the middle cambrian and not as basement to the precordillera terrane

the pacific gondwana margin in the late neoproterozoic early paleozoic detrital zircon u pb ages from metasediments in northwest argentina reveal their maximum age provenance and tectonic setting abstract u pb detrital zircon ages are reported from puncoviscana formation late neoproterozoic early cambrian and meson group late cambrian greywackes of northwest argentina to constrain provenance and depositional environment the new data are combined with previously published detrital zircon ages to show that puncoviscana formation age patterns contain two broad groups late mesoproterozoic early neoproterozoic 1150 850 ma and late neoproterozoic early cambrian 650 520 ma with their relative proportions varying inversely with youngest component age the 1150 850 ma age components are dominant in greywackes with oldest late neoproterozoic components 600 ma the former diminish considerably when late neoproteozoic components become dominant and younger to 520 ma a northernmost greywacke sample from purmamarca jujuy is distinctive whilst its zircon age pattern partly resembles other puncoviscana formation samples it contains no cambrian late neoproterozoic ages the youngest ages being early neoproterozoic this may reflect an early neoproterozoic passive margin depocentre for the formation or an older early neoproterozoic succession within it which may predate the brasiliano orogeny in brazil the youngest age components c 520 ma in a greywacke from rancagua cachi salta province dominate an almost unimodal pattern suggestive of contemporary volcanic sources at a late early cambrian depocentre detrital zircon age patterns of the meson group lizoite formation have major cambrian latest neoproterozoic components resembling those of the puncoviscana formation but its mesoproterozoic component is diminished and there are no significant age components of this age small youngest components at c 500 ma suggest a maximum late cambrian stratigraphic age the puncoviscana formation detrital zircon patterns suggest a provenance in a continental hinterland having a stabilised extensive late mesoproterozoic orogen with minor paleoproterozoic and archean precursors and a more variable late neoproterozoic orogen containing an evolving sequence of less extensive subcomponents a direct relationship with the brazilian shield is suggested with sediment supplies originating within active margin orogens of the interior and collisional orogens at the suture between african and south american cratons but ultimate deposition in passive margin environments of western gondwanaland

time constraints on the tectonic evolution of the eastern sierras pampeanas central argentina the application of the shrimp u pb dating technique to zircon and monazite of different rock types of the sierras de cordoba provides an important insight into the metamorphic history of the basement domains additional constraints on the pampean metamorphic episode were gained by pb pb stepwise leaching pbsl experiments on two titanite and garnet separates results indicate that the metamorphic history recorded by crd free gneisses m2 started in the latest neoproterozoic earliest cambrian 553 and 543 ma followed by the m4 metamorphism at 530 ma that is documented in the diatexites zircon ages of 492 ma in the san carlos massif correlate partly with rather low th u ratios 0 1 suggesting their growth by metamorphic fluids this age is even younger than the pbsl titanite ages of 506 ma it is suggested that the fluid alteration relates to the beginning of the famatinien metamorphic cycle in the neighbouring sierra de san luis and has not affected the titanite ages the ptt evolution can be correlated with the plate tectonic processes responsible for the formation of the pampean orogene i e the accretion of the pampean basement to the rio de la plata craton m2 and the later collision of the western pampean basement with the pampean basement

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

geomechanical model test for failure and stability analysis of high arch dam based on acoustic emission technique abstract the internal damage of the model during the geomechanical model test is difficult to be detected the failure and instability process of the arch dam are always accompanied by a large number of acoustic emission ae signals which is related to the number of cracks in the model ae technique was applied in the geomechanical model test of mengdigou arch dam to analyze its failure process and overall stability noise filtering was conducted based on spectrum analysis of ae signal waveform parameter analysis was performed after noise filtering the overall stability of the dam was evaluated by three characteristic overload safety factors the crack initiation factor k 1 nonlinear deformation factor k 2 and ultimate load factor k 3 which were studied by ae parameter analysis the failure process of mengdigou arch dam was analyzed by the statistical analysis of ae hits and the wave velocity variation before and after test ae analysis results were compared with the displacement and video monitoring results at the same time to verify the correctness of the evaluation method based on ae technique the results showed that the failure process of the dam and foundation was well described by the ae technique and the three safety factors of mengdigou arch dam were defined more clearly and as for mengdigou arch dam the fracture occurred at the dam heel area in the early stage of loading and the abutment of right bank was better than the left the foundation damage was not serious after test and the overall instability was determined by instability of dam based on ae technique the three overload safety factors was determined more clearly k1 1 5 2 0 k2 4 5 5 0 and k3 11 11 5 which provided an important reference for the mengdigou arch dam construction

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

corrigendum to early gondwanan connection for the argentine precordillera terrane earth planet sci lett 205 2003 349 359

dehydroxylation of omphacite of eclogite from the dabie sulu dehydroxylation of omphacites in eclogites from uhp dabie sulu terrain has been investigated using a combined method of petrological observations mineral chemistry heat treatment mossbauer and micro ftir spectroscopy progressive heating of omphacite in air up to 1100 c results in complex multi stage processes involving oxidation and dehydrogenation at different temperatures during heating fe2 at the m1 site is oxidized to fe3 accompanied by a decrease in the intensity of the 3460 cm 1 oh band this reaction can be described as fe2 m1 oh fe3 m1 o2 o3 1 2h2 and most likely represents loss of h bonded to o1 adjacent to the m1 site at higher temperatures continued oxidation results in further dehydroxylation due to h loss from oh bands 3530 cm 1 and 3630 cm 1 these bands represent h defects with higher thermal stability slower diffusivity in this case oxidation of fe2 provides a means for charge balancing h loss over the whole crystal structure rather than locally oh bands at 3530 and 3630 cm 1 are consistent with h incorporation onto o2 with vibration of oh dipoles towards adjacent tetrahedral vacancies rapid loss of h during annealing at even low temperatures implies that significant h loss from eclogites occurs during exhumation furthermore changes in relative oh band intensity are expected during exhumation due to different thermal behaviour of h defects in omphacite as such the ratio of oh bands in omphacite spectra could be used to deduce exhumation conditions

lower carboniferous post orogenic granites in central eastern sierra de velasco sierras pampeanas argentina u pb monazite geochronology geochemistry and sr nd isotopes the central eastern part of the sierra de velasco sierras pampeanas nw argentina is formed by the large huaco 40 30 km and sanagasta 25 15 km granite massifs and the small la chinchilla stock 2 2 km the larger granites intrude into ordovician metagranitoids and crosscut devonian mylonitic shear zones whereas the small stock sharply intrudes into the huaco granite the two voluminous granites are biotitic muscovitic and biotitic porphyritic syeno to monzogranites they contain small and rounded tonalitic and quartz dioritic mafic microgranular enclaves the small stock is an equigranular zinnwaldite and fluorite bearing monzogranite the studied granites are silica rich sio2 70 potassium rich k2o 4 ferroan alkali calcic to slightly calk alkalic and moderately to weakly peraluminous a cnk 1 06 1 18 huaco granite 1 01 1 09 sanagasta granite 1 05 1 06 la chinchilla stock they have moderate to strong enrichments in several lil li rb cs and hfs nb ta y th u elements and low sr ba and eu contents u pb monazite age determinations indicate lower carboniferous crystallization ages 350 358 ma for the huaco granite 352 7 1 4 ma for the sanagasta granite and 344 5 1 4 ma for the la chinchilla stock the larger granites have similar nd values between 2 1 and 4 3 whereas the younger stock has higher nd of 0 6 to 1 4 roughly comparable to the values obtained for the carboniferous san blas granite 1 4 to 1 7 located in the north of the sierra the huaco and sanagasta granites have a mainly crustal source but with some participation of a more primitive possibly mantle derived component the main crustal component can be attributed to ordovician peraluminous metagranitoids the la chinchilla stock derives from a more primitive source suggesting an increase with time in the participation of the primitive component during magma genesis the studied granites were generated during a post orogenic period in a within plate setting possibly as a response to the collapse of the previous famatinian orogen extension of the crust and mantle upwelling they are part of the group of middle devonian lower carboniferous granites of the sierras pampeanas the distribution and u pb ages of these granites suggests a northward arc parallel migration of this mainly post orogenic magmatism with time

deformation of gondwana margin turbidites during the pampean orogeny north central argentina basement uplifts in the eastern sierras pampeanas of north central argentina expose sedimentary rocks that originated on the neoproterozoic to early cambrian margin of gondwana for over 1000 km along strike a series of transects through unmetamorphosed sections of the vendian aged puncoviscana formation and metasedimentary equivalent rocks from deeper structural levels demonstrates that an early stage compaction cleavage has been folded into chevron structures with a concomitant axial planar tectonic cleavage prior to erosion and deposition of the overlying middle to late cambrian meson group changes in structural style are shown to be gradational all mesoscopic ductile fold structures in the region can be explained with one major deformation episode late stage andean brittle folds and thrusts are more noticeable in the northern sections but do not significantly alter the overall structural style or geometry of the basement rocks a static metamorphic overprint in the ordovician with associated granitic intrusions only affects fabrics and mesoscale structures adjacent to plutons published age constraints indicate an overlap between the timing of sedimentary deposition and that of structural and metamorphic events at depth we develop a tectonic model for the pampean orogeny that involves the buttressing of off scraped sedimentary rocks above a subducting slab beneath the gondwana margin the collision of outboard terranes did not occur in this area until at least the mid ordovician famatinian orogeny

history of crustal growth and recycling at the pacific convergent margin of south america at latitudes 29 36 s revealed by a u pb and lu hf isotope study of detrital zircon from late paleozoic accretionary systems detrital zircon provides a powerful archive of continental growth and recycling processes we have tested this by a combined laser ablation icp ms u pb and lu hf analysis of homogeneous growth domains in detrital zircon from late paleozoic coastal accretionary systems in central chile and the collisional guarguaraz complex in w argentina because detritus from a large part of w gondwana is present here the data delineate the crustal evolution of southern south america at its paleopacific margin consistent with known data in the source regions zircon in the guarguaraz complex mainly displays an u pb age cluster at 0 93 1 46 ga similar to zircon in sediments of the adjacent allochthonous cuyania terrane by contrast zircon from the coastal accretionary systems shows a mixed provenance age clusters at 363 722 ma are typical for zircon grown during the braziliano pampean famatinian and post famatinian orogenic episodes east of cuyania an age spectrum at 1 00 1 39 ga is interpreted as a mixture of zircon from cuyania and several sources further east minor age clusters between 1 46 and 3 20 ga suggest recycling of material from cratons within w gondwana the youngest age cluster 294 346 ma in the coastal accretionary prisms reflects a so far unknown local magmatic event also represented by rhyolite and leucogranite pebbles it sets time marks for the accretion history maximum depositional ages of most accreted metasediments are middle to upper carboniferous a change of the accretion mode occurred before 308 ma when also a concomitant retrowedge basin formed initial hf isotope compositions reveal at least three juvenile crust forming periods in southern south america characterised by three major periods of juvenile magma production at 2 7 3 4 ga 1 9 2 3 ga and 0 8 1 5 ga the 176hf 177hf of mesoproterozoic zircon from the coastal accretionary systems is consistent with extensive crustal recycling and addition of some juvenile mantle derived magma while that of zircon from the guarguaraz complex has a largely juvenile crustal signature zircon with pampean famatinian and braziliano ages 660 ma originated from recycled crust of variable age which is however mainly mesoproterozoic by contrast the carboniferous magmatic event shows less variable and more radiogenic 176hf 177hf pointing to a mean early neoproterozoic crustal residence this zircon is unlikely to have crystallized from melts of metasediments of the accretionary systems but probably derived from a more juvenile crust in their backstop system

early carboniferous sub to mid alkaline magmatism in the eastern sierras pampeanas nw argentina a record of crustal growth by the incorporation of mantle derived material in an extensional setting a recently discovered granitic intrusion at cerro la gloria in western sierra de famatina nw argentina is representative of sub to mid alkaline carboniferous magmatism in the region the main rock type consists of microcline quartz and plagioclase with amphibole magnetite ilmenite biotite epidote zircon allanite and sphene as accessory minerals we report a u pb zircon shrimp age for the pluton of 349 3 ma mswd 1 1 i e tournaisian whole rock chemical composition and nd isotope analyses are compatible with an origin by melting of older mafic material in the lower crust endt between 0 58 and 0 46 and tdm values of about 1 1 ga the pluton is intruded by penecontemporaneous to late alkaline mafic dykes that are classified as back arc basalts coeval early carboniferous a type granites occur farther east in the sierras pampeanas probably generated during lithospheric stretching overall the early carboniferous granitic rocks show a west to east mineralogical and isotopic zonation indicating that magma genesis involved a greater contribution of juvenile material of mantle character to the west based on the observed patterns of geochronology geochemistry and field relationships we suggest that a type magma genesis in the eastern sierras pampeanas was linked to an andean type margin where the lithospheric mantle played a role in its generation

high precision u pb calibration of carboniferous glaciation and climate history paganzo group nw argentina the duration and geographic extent of carboniferous glacial events in southern gondwana remain poorly constrained despite recent evidence for a more dynamic glacial history than previously considered we report 10 high precision 2 0 1 u pb ages for the permian carboniferous paganzo group nw argentina that redefi ne the chronostratigraphy of the late paleozoic paganzo and rio blanco basins and signifi cantly refi ne the timing of glacial events and climate shifts in the western region of southern gondwana radiometric calibration of the paganzo group indicates three pulses of carboniferous glaciation in the mid visean the late serpukhovian to earliest bashkirian and between the latest bashkirian to early moscovian an abrupt shift in depositional style from high sinuosity single storied fl uvial deposits and clay rich paleosols to low sinuosity multi storied feldspathic fluvial deposits inter calated with eolianites and calcic paleosols is constrained to the latest moscovian and earliest kasimovian these constraints indicate a relatively abrupt climate shift from humid subhumid to nonseasonal semiarid regional climate conditions that occurred signifi cantly earlier than previously inferred early permian this period of high latitude aridity was contemporaneous with a shift to dryland depositional environments and a major vegetation regime shift documented throughout the pangean paleotropics in the pennsylvanian

u pb th pb and ar ar geochronology from the southern sierras pampeanas argentina implications for the palaeozoic tectonic evolution of the western gondwana margin

age constraints on the tectonic evolution and provenance of the pie de palo complex cuyania composite terrane and the famatinian orogeny in the sierra de pie de palo san juan argentina abstract new u pb age determinations confirm earlier interpretations that the strongly deformed and metamorphosed mafic and intermediate igneous rocks of the pie de palo complex represent a mesoproterozoic fragment of suprasubduction zone oceanic crust a gabbroic pegmatite interpreted to have formed during arc rifting or subsequent back arc spreading yielded a u pb age of 1204 5 3 4 7 ma highly tectonized ultramafic mafic cumulates occurring at the structural base of the pie de palo complex and previously interpreted to represent remnants of a primitive arc phase prior to rifting and back arc spreading could not be dated but should be older than 1204 ma if these inferences are correct tabular sill like bodies of leucogabbro diorite and calc alkaline tonalite granodiorite sills yielded ages of 1174 43 and 1169 8 7 ma respectively they may represent a younger more evolved arc phase established after arc rifting or a younger tectonically unrelated mesoproterozoic arc shrimp analysis of metamorphic zircon rims with low th u ratios in vvl 110 gave a 206pb 238u age of 455 10 ma similar to lower intercept dates determined by discordia lines combined these data indicate that the bulk of the amphibolite facies metamorphism present in the pie de palo complex was generated during the famatinian orogeny analysis of six single detrital zircon grains in a metasedimentary quartzofeldspathic garnet mica schist tectonically interleaved with the igneous rocks of the pie de palo complex and tentatively correlated with the difunta correa metasedimentary sequence of other workers yielded three age populations 1150 1160 ma 1050 1080 ma and 665 ma indicating that these sedimentary rocks were deposited during the late neoproterozoic or early paleozoic in addition they confirm structural evidence that intercalation of rocks of the pie de palo complex with isolated slivers of these sedimentary rocks is due to tectonic imbrications these ages are also consistent with a laurentian provenance and earlier interpretations that these rocks once represented a sedimentary cover to the pie de palo complex the zircon population of 1050 1080 ma could be derived from grenville age felsic plutons identified elsewhere in the pie de palo complex by other workers however no evidence has been found in our samples for a grenville age orogenic event invoked previously to explain accretion of the oceanic pie de palo complex to laurentia prior to the late neoproterozoic early cambrian rifting and drift of cuyania

provenance of late palaeozoic metasediments of the sw south american gondwana margin a combined u pb and hf isotope study of single detrital zircons combined u pb and lu hf isotope measurements of single detrital zircon grains in carboniferous metasediments from patagonia delineate the source areas of the sediments the detritus represented by four metasandstone samples was deposited prior to onset of subduction in late carboniferous time along the south patagonian proto pacific gondwana margin a broad series of detrital zircon age peaks 0 35 0 7 ga 0 9 1 5 ga and a large spread 0 3 3 5 ga in the age spectra require numerous sources a fifth metasediment was deposited after the onset of subduction this syncollisional sample shows two distinct u pb age peaks at c 290 ma and 305 ma this points to a few sources only patagonia west antarctica initial hf isotope compositions of selected u pb dated zircons from the carboniferous metasediments reveal zircon protoliths originating from both recycled crust and juvenile sources ehf t 0 4 3 5ga 14 to 12 a comparison with crustal compositions of possible source areas indicates that the detritus mainly originated from the interior of gondwana extra andean patagonia the argentine sierra de la ventana southernmost africa east antarctica as well as northern chile and northwestern argentina the sediment transportation paths are consistent with an autochthonous palaeogeographical position of patagonia with respect to gondwana in carboniferous time

neoproterozoic a type magmatism in the western sierras pampeanas argentina evidence for rodinia break up along a proto iapetus rift a type orthogneisses of mid neoproterozoic age 774 6 ma u pb shrimp zircon age are reported for the first time from the grenvillian basement of the western sierras pampeanas in argentina these anorogenic meta igneous rocks represent the latest event of rodinia break up so far recognized in grenvillian basement exposures across andean south america moreover they compare well with a type granitoids and volcanic rocks along the appalachian margin of laurentia blue ridge thus adding to former evidence that the western sierras pampeanas grenvillian basement was left on the conjugate rifted margin of eastern laurentia during rodinia break up and the consequent opening of the iapetus ocean

k bentonites in the argentine precordillera contemporaneous with rhyolite volcanism in the famatinian arc new u pb radiometric dates for k bentonite horizons within the lower cambrian to middle ordovician platform carbonates from the precordillera terrane of nw argentina provide further constraints on models for the allochthonous or parautochthonous accretion of this terrane two k bentonite layers from the talacasto section yield indistinguishable sensitive high resolution ion microprobe shrimp u pb zircon dates of 469 5 3 2 ma and 470 1 3 3 ma respectively these are within uncertainty of the u pb shrimp zircon date of 468 3 3 4 ma for a porphyritic rhyolite from the famatinian magmatic arc sierra de las planchadas near rio chaschuil geochemical and isotope data also demonstrate the similarity of the k bentonite and chaschuil rhyolite parent magmas thus it is highly probable that the famatinian arc volcanoes provided the ash for the k bentonite horizons suggesting proximity to the precordillera terrane during the deposition of the lower cambrian to middle ordovician platform carbonates this implication supports a mid ordovician collision model but could also be compatible with a parautochthonous model for docking of the precordillera terrane by movement along the pacific margin of gondwana rather than across the iapetus ocean