spatiotemporal patterns of pre columbian people in amazonia a current goal among many scientific disciplines is to incorporate data on past human land use and climate change into current global climate and vegetation models here we used existing archaeological and paleoecological data to provide a spatiotemporal reconstruction of human history in greater amazonia over the holocene we used an ensemble distribution model based on a database of georeferenced 14 c dated material and environmental factors to predict the changes in spatial distributions of past human occupation sites we ran these models for the precultivation 13 000 6000 yr ago early cultivation 6000 2500 yr ago and late cultivation 2500 500 yr ago periods the ensemble models suggest that people mostly inhabited the peripheral areas of greater amazonia and the eastern sections of the main amazon river and its larger tributaries during the precultivation period human populations began growing exponentially through the early cultivation period and people spread and expanded into the interior forests and along river channels in western amazonia populations continued growing through the late cultivation period in these same regions our results suggest that many forests particularly in the peripheral regions and riverine locations may still be in recovery from disturbances that have occurred repeatedly through the holocene

variability of el nino southern oscillation activity at millennial timescales during the holocene epoch the variability of el nino southern oscillation enso during the holocene epoch in particular on millennial timescales is poorly understood palaeoclimate studies have documented enso variability for selected intervals in the holocene but most records are either too short or insufficiently resolved to investigate variability on millennial scales1 2 3 here we present a record of sedimentation in laguna pallcacocha southern ecuador which is strongly influenced by enso variability and covers the past 12 000 years continuously we find that changes on a timescale of 2 8 years which we attribute to warm enso events become more frequent over the holocene until about 1 200 years ago and then decline towards the present periods of relatively high and low enso activity alternating at a timescale of about 2 000 years are superimposed on this long term trend we attribute the long term trend to orbitally induced changes in insolation and suggest internal enso dynamics as a possible cause of the millennial variability however the millennial oscillation will need to be confirmed in other enso proxy records

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

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

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

north atlantic forcing of amazonian precipitation during the last ice age the last glacial period was marked by rapid reorganizations of oceanic and atmospheric circulation speleothem records from the amazon basin suggest that precipitation variability was linked to these events the last glacial period was marked by multiple abrupt reorganizations of ocean and atmosphere circulation1 on thousand year timescales slowing of the atlantic meridional overturning circulation was associated with cooling in the high northern latitudes whereas strengthened circulation was linked to northern warming1 2 in the tropics these millennial scale events were primarily reflected in altered patterns of precipitation3 these hydrologic fluctuations induced ecological changes in the atlantic seaboard and the high andes2 but less is known about the amazon basin here we reconstruct precipitation over amazonian ecuador over the past 94 000 years using a 18o record from speleothems collected in santiago cave in western amazonia we interpret the variability of the 18o record as changes in the source and amount of precipitation with the exception of the period between 40 000 and 17 000 years ago abrupt high frequency changes coincide with shifts in north atlantic circulation indicating a high latitude influence on amazonian precipitation over millennial timescales on longer timescales the record shows a relationship to precessional changes in the earth s orbit in light of the lack of extreme aridity in our records we conclude that ecosystems in western amazonia have not experienced prolonged drying over the past 94 000 years

integrated palaeoecology and archaeology a powerful approach for understanding pre columbian amazonia the old paradigm that amazonia s tropical ecosystems prevented cultural development beyond small scale shifting agricultural economies that had little environmental impact no longer holds true for much of amazonia a diversity of archaeological evidence including terra preta soils raised fields causeways large habitation mounds geometric earthworks and megalithic monuments all point to considerable cultural complexity and environmental impacts however uncertainty remains over the chronology of these cultures their diet and economy and the scale of environmental impact and land use associated with them here we argue that a cross disciplinary approach closely coupling palaeoecology and archaeology can potentially resolve these uncertainties we show how with careful site selection pairing small and large lakes close proximity to archaeological sites transects of soil pits and choice of techniques e g pollen phytoliths starch grains charcoal stable isotopes these two disciplines can be successfully integrated to provide a powerful tool for investigating the relationship between pre columbian cultures and their environment

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

the use of summed radiocarbon probability distributions in archaeology a review of methods abstract using a database of australian archaeological radiocarbon dates n 2996 this paper explores three key methodological issues associated with the use of summed probability plots of radiocarbon data 1 the minimum sample size needed for a statistically reliable plot 2 the effect of radiocarbon calibration on the structure of these plots and 3 the application of a taphonomic correction to such time series data the results identify several protocols recommended as best practice when using summed probability plots 1 a minimum sample size of 500 radiocarbon dates should be used and the sample size and the mean of the standard deviations of the radiocarbon dates t in the sample should both be reported 2 a moving average trendline of 500 800 years should be used to offset the effects of the calibration process and 3 surovell et al correcting temporal frequency distributions for taphonomic bias journal of archaeological science 36 2009 1715 1724 is explored with modifications and temporal limits

holocene fires in the northern amazon basin abstract the ubiquitous occurrence of charcoal in the forest soils of the upper rio negro region of colombia and venezuela indicates the presence of frequent and widespread fires in the amazon basin possibly associated with extremely dry periods or human disturbances charcoal ranged from 3 12 to 24 76 mg cm 3 in the upper 50 cm of soil and was more abundant in oxisols and ultisols than in other soil types charcoal dates range from 6260 yr b p to the present several dates coincide with dry phases recorded during the holocene ceramic shards were found at several sites and thermoluminescence analysis indicates that their ages range from 3750 to 460 yr b p the age of charcoal and shards confirms that this region has been subjected to fire and human disturbances during the past 6000 yr

genesis of the kapuskasing ontario migmatitic mafic granulites by dehydration melting of amphibolite the importance of quartz to reaction progress migmatitic granulite grade mafic gneisses make up a significant part of the kapuskasing structural zone ksz ontario although they contain a common mineral assemblage hornblende hbl plagioclase pl diopside di garnet grt quartz qtz titanite ttn the mafic gneisses show wide variations in modal mineralogy from hornblende rich to diopside garnet rich varieties and all gradations between up to 25 vol segregated plagioclase quartz rich trondhjemitic leucosome tdh is intimately associated with the mafic gneiss occurring in a continuum of patches veins and transecting dykes at scales ranging from decimetres to micrometres the texture and composition of the leucosome combined with p t estimates for the host rocks above the solidus suggest it represents crystallized trondhjemitic melt quartz is mainly restricted to the segregated leucosomes but more rarely occurs in a variety of interstitial textures in the mafic gneiss suggesting that it crystallized from a melt phase rather than having been present as a solid phase at peak metamorphic conditions modal and textural data indicate a reaction relationship of the form hbl pl qtz grt di ttn leucosome tdh implying that the granulite forming process involved dehydration melting of an amphibolite protolith pressure temperature estimates from grt di pl qtz geothermobarometry are 9 kbar and 685 735 c however based on experimental studies of dehydration melting of amphibolite we estimate that peak conditions were closer to 11 kbar 850 c mass balance analysis using the technique of singular value decomposition and reaction space analysis were used to quantify the reaction and to determine the controls on reaction progress the following mass balance provides a model for the natural reaction 1 00 hbl 0 92 pl 3 76 qtz 1 14 grt 1 54 di 0 21 ttn 1 49 tdh 0 14 pg 0 39 fe 1mg 0 33 nasica 1al 1where pg is a pargasite like exchange in all model mass balances tested quartz is a reactant with a large coefficient we argue that the abundance of quartz in the amphibolite protolith was the primary control on the differing extents of reaction observed mineral compositional variation exerted a secondary control on reaction progress with fe richer layers containing an richer plagioclase and more actinolitic amphibole reacting earliest i e at lowest temperatures comparison of the calculated amount of melt produced in the gneisses with that now observed implies expulsion of 5 30 of the melt these volumes are similar to those predicted from ree modelling of archaean tonalities and trondhjemites from a garnet amphibolite source suggesting that the ksz mafic gneisses may be representative of partially depleted source rocks for trondhjemite tonalite generation

geochemistry and petrology of the jequie granulitic complex brazil an archean basement complex the jequie granulitic complex is part of the extensive high grade metamorphic terrain located within the sao francisco craton of northeastern brazil some jequie rocks appear to have been formed in the middle archean 3 1 ga from preexisting sialic crust the dominant mineral composition of these rocks is quartz microcline plagioclase hyperstene and occurs over an extensive area 2 000 km2

vegetational history of a site in the central amazon basin derived from phytolith and charcoal records from natural soils abstract analysis of phytoliths and macroscopic charcoal from natural soils near manaus brazil shows that the central amazonian terra firme forest has not been stable during the holocene phytolith taxonomic composition and radiocarbon ages indicate that a closed forest has continuously existed in the area since at least 4600 yr b p but that forest composition and species abundance changed significantly sometime during the holocene epoch there is no evidence that the vegetation experienced swidden cultivation the distribution and dates of the charcoal in the soils indicate that forest fires between 1795 and 550 yr b p burned an area of about 200 hectares vegetational changes and fires appear to be a result of climatic drying that may have affected large areas of the amazon basin over the past 5000 to 7000 years soil phytolith analysis is a promising tool for the study of environmental history in the humid tropics and may reduce the previous dependence on lake and swamp sediments at least for the holocene epoch

fossil dripwater in stalagmites reveals holocene temperature and rainfall variation in amazonia abstract most proxy records used for reconstruction of holocene climate of amazonia are unable to quantitatively distinguish between the effect of temperature and rainfall amounts we present a new isotope technique applied to a 13 500 yr stalagmite archive from peruvian amazonia by analysing the coupled isotope composition of fossil dripwater trapped in stalagmite fluid inclusions and that of the calcite hosting the fluid inclusions we were able to calculate independent paleotemperatures and rainfall amounts this stalagmite record shows that holocene climate variation was controlled by orbitally forced southward migration of the inter tropical convergence zone while temperature remained constant isotope variation of rainwater reflected in fluid inclusion water 18o composition suggests a 15 30 increase in convective rainfall through the holocene a comparison of the low land peruvian fluid inclusion record with the high andean huascaran ice core record shows a constant 12 offset of 18o curves for the holocene suggesting that andean vertical temperature gradients lapse rates did not vary much over the last 9000 years during the younger dryas interval however the offset of 18o values was much higher than in the holocene this may be attributed to a relative drop in air temperatures in the highlands higher lapse rate caused by long distance teleconnections to climate perturbations in the north atlantic in a wider perspective fluid inclusion isotope analysis drastically improves paleotemperature reconstructions based on speleothem calcite 18o data because it provides the 18o value of drip water through time which is usually the most important unknown in paleotemperature equations

sm nd isotopic investigation of neoproterozoic and cretaceous igneous rocks from southern brazil a study of magmatic processes abstract nd evolutionary paths for diversified igneous suites from southern brazil are here re evaluated using published results we interpret the end paths considering the secondary fractionation of 147sm 144nd due to major petrogenetic processes the inclusion of nd isotopes and geochemical data for precambrian and mesozoic basic rocks allow improving the discussion on the subcontinental lithosphere beneath southern brazil late neoproterozoic rocks mostly granitoids are exposed in two regions of the southern brazilian shield an eastern collisional belt and a western foreland the latter included two geotectonic domains amalgamated at this time the sao gabriel arc 900 700 ma and the taquarembo cratonic block magma genesis mainly involved mixture of crustal and incompatible element enriched mantle components both with a long residence time continental segments are the neoarchaean paleoproterozoic lower crust ca 2 55 ga in the western foreland and paleoproterozoic neoproterozoic recycled crust 2 1 0 8 ga in the collisional belt granitoids with a single crustal derivation are limited in the southern brazilian shield mixing processes are well registered in the western foreland where the re enriched old mantle was probably mixed with a 900 700 ma old subducted lithosphere and a 2 55 ga old lower crust the contribution of the latter increased from the early 605 580 ma to the later 575 550 ma neoproterozoic events which may be due either to crustal thickening or to delamination of the lithosphere magma sources were diversified in the 660 630 ma collisional belt initially they involved the mixing between two components with similar nd isotopic ratios a 2 1 0 8 ga old recycled crust and a subduction processed old mantle regional heating and abundant production of granitic melts with diversified contribution of enriched mantle components mark the end of the collisional period at 630 580 ma we can also attribute this to the delamination of the lithosphere so that the same geodynamic process may explain the magmatism in the whole shield at the end of the dom feliciano orogeny mesozoic rocks include flood basalts from the cretaceous parana province and sub coeval alkalic suites multiple processes of metasomatism affected the lithospheric mantle resulting in some complexity but they mainly register two enriched mantle components both generated during neoarchaean paleoproterozoic events one end member has a more pronounced subduction signature the other one probably resulted from the re enrichment of the first component at the end of the camboriu collisional orogeny 2 0 ga

decadal changes in the southern hemisphere sea surface temperature in association with el nino southern oscillation and southern annular mode the spatial structure of el nino southern oscillation enso and the southern annular mode sam which are the two most important climate modes affecting sea surface temperature sst variability in the southern hemisphere sh appear to have changed since 1999 the characteristic features of the enso and sam related atmospheric and oceanic variability in the sh are compared between two sub periods 1979 1998 and 1999 2012 using cyclostationary empirical orthogonal function analysis during the earlier period of 1979 1998 the enso is characterized by conventional eastern pacific type in which the signals in the sh constitute the pacific south america teleconnection pattern in contrast due to a shift of the active center of enso to the central pacific in the later period 1999 2012 atmospheric circulation and sst variability over the sh significantly vary moreover the sam related sst variability also shows remarkable differences before and after 1998 1999 this difference is primarily attributed to differences in the non annular spatial component of the sam between the two periods due to the changes in the spatial structure of the sam as well as those of the enso sst variability in the sh displays a marked change between the two periods detailed descriptions of the decadal changes of the sh sst in terms of interaction in the oceanic and atmospheric variability are presented along with the possible implications of this change

slash burn and churn landscape history and crop cultivation in pre columbian amazonia abstract palaeoecologists argue that the vegetation of amazonia has changed markedly since the late pleistocene and show that forest fires have been increasingly more common since the beginning of the holocene because these occurrences coincide with the onset of human colonisation of the region students of amazonia discuss the extent to which it s holocene fire record reflects climatically related and or anthropogenic causes one factor that complicates inferences is the overall heterogeneity of the basin s vegetation during drier moments of the holocene and or within less humid sub regions of the basin it is difficult to conclude whether fire signals reflect more frequent natural fires or more intense burning by human communities another factor is that pre columbian livelihoods were not homogenously distributed throughout the region and changed over the course of the holocene this paper seeks to help establish an anthropogenic baseline against which holocene evidence for burning can be assessed to this end it reviews the role that slash and burn cultivation has had in discussions about pre columbian amazonia discusses how this account has been modified by recent research on pre columbian anthropogenic soils and queries the extent to which the regionally heterogeneous timing of crop domestication and cultivation may have influenced the fire history of pre columbian amazonia

zoning patterns in orthopyroxene and garnet in granulites implications for geothermometry compositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the minto terrane of northern quebec the compositional maps provide clear evidence of late exchange of fe fe mg after ca in garnet and al in orthopyroxene had been quenched in the extent of late fe mg exchange was controlled by neighbouring minerals with negligible fe mg gradients against plagioclase and quartz and substantial gradients against exchangeable fe mg minerals cores of grains in contact with exchangeable fe mg neighbours are progressively more reset in fe fe mg as grain size decreases whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in fe fe mg than cores of the largest grains gradients of ca in garnet and of al in orthopyroxene in grains of uniform fe fe mg preserve a high temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular fe mg diffusion in garnet and orthopyroxene was rapid enough to homogenize fe fe mg the transition from efficient intergranular exchange at relatively high temperatures to local fe mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock possibly an internally generated dehydration melt phase implications for geothermometry of granulites include the following numerical values are particular to this study 1 core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in fe fe mg relative to the most refractory compositions by an amount equivalent to 120d c 2 fe mg exchange thermometry using even the most refractory fe fe mg compositions may not record peak granulite conditions possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock and 3 temperature sensitive net transfer equilibria involving al solubility in orthopyroxene yield temperatures up to 150d c higher than maximum fe mg exchange temperatures even in grains with flat fe fe mg compositional profiles making them a better means of estimating peak granulite temperatures than fe mg exchange thermometry

deepest exposed crust of brazil geochemistry of paleoproterozoic depleted santa maria chico granulites abstract the deepest crustal processes of brazil are investigated in the santa maria chico granulitic complex of rio grande do sul state depth of ca 30 km is inferred from garnet bearing mafic granulite geobarometry for the deformation of the bimodal basic acid sequence of late archean age this low k tholeiitic magmatism occurred in an island arc environment which accreted to the atlantica supercontinent in a collisional episode at ca 2 1 ga during the transamazonian cycle although this magmatism is strongly depleted in lithophile elements its geochemistry is still magmatic and shows that the metamorphism did not disturb the original distribution of the elements in the host rocks significantly the associated trondhjemites could have been the differentiated portions of the basic magmas but more likely were the result of the partial melting of the garnet bearing mafic residue this suite of high grade rocks is one of several protoliths for the generation of the abundant k granites of the 600 ma brasiliano cycle from southern brazil this high grade rock association is rather comparable to the lewisian complex of scotland

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

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

the relationship of pacific deep tropical convection to the winter and springtime extratropical atmospheric circulation of the south pacific in el nino events during some el nino events rossby wave trains rwt are observed to strongly modulate the seasonal atmospheric circulation of the south pacific extratropics in the austral winter and spring here it is shown that seasonally intensified deep tropical convection is confined to the pacific intertropical convergence zone itcz close to the dateline in those events with strong rwt modulation but not in other cases in other cases deep convection either withdraws from the itcz or the itcz strongly interacts with the south pacific convergence zone spcz lying at 5 10degreess in both cases this points to a weakening of the local tropical hadley circulation that may be crucial for rossby wave generation it is also found that the expected rwt response in the high latitudes can fail to occur even when sst in the central tropical pacific are very high

charnockite geobarometers based on coexisting garnet pyroxene plagioclase quartz numerous geothermometers based on compositions of coexisting minerals have been calibrated experimentally or theoretically during the past 10 yr in contrast few continuous reaction geobarometers exist which have had wide application one based on the reaction of cordierite to garnet sillimanite and quartz is limited by a relatively narrow pressure range of application and also by conflicting calibrations1 2 because the mixing relations of mg and fe and the role of h2o in cordierite3 are not understood another successful barometer uses the assemblage garnet plagioclase al2sio5 and quartz4 5 continuous geobarometers based on the reaction of orthofer rosilite to fayalite plus quartz6 and on the aluminium content of enstatite in equilibrium with garnet7 have been used to a lesser extent the former barometer is limited by rarity of the requisite assemblage and the latter by uncertain experimental calibration for crustal rocks we calibrate here two mineralogical geobarometers from measured thermodynamic data the participating minerals pyroxenes garnet plagioclase and quartz are frequently associated in high grade metamorphic rocks calculation of pressures for several terrains show that the geobarometers yield reasonable and consistent results for the entire range of crustal pressures

volcanic cycles and setting in the neoproterozoic iii to ordovician camaqua basin succession in southern brazil characteristics of post collisional magmatism the camaqua basin comprises a volcano sedimentary succession located in southernmost brazil and represents a molasse basin formed at the post collisional stage of the brasiliano pan african orogenic cycle in the neoproterozoic iii to ordovician period this basin is one of the most well preserved ancient volcano sedimentary sequences undeformed and unmetamorphic in the world dominantly developed on a continental setting under subaerial conditions it is composed of five major stratigraphic units four of them with a distinct volcanic character from the bottom to the top as 1 marica 2 bom jardim 3 acampamento velho 4 santa barbara and 5 guaritas allogroups a concise sight of geochemical and isotopic rock data is presented as well as stratigraphic correlation and description of rock structures and textures that lead to the identification of their genetic processes the aim of this paper indicating a relation with a coeval plutonism and volcanism that evolved from high k calc alkaline to shoshonitic and ended with a silica saturated sodic alkaline magmatism with a crustal component represented by peraluminous granites volcanic deposits from bottom to top are made mostly of volcanogenic sedimentary deposits succeeded by basic to intermediate lava and pyroclastic flows of shoshonitic affinity followed by intermediate and acid lava flows and ignimbrites of sodic alkaline affinity the last volcanic event is represented by basalt pahoehoe flows probably of mildly alkaline sodic affinity

ages u pb shrimp and la icpms and stratigraphic evolution of the neoproterozoic volcano sedimentary successions from the extensional camaqua basin southern brazil abstract during the ediacaran southern brazil was the site of multiple episodes of volcanism and sedimentation which are best preserved in the 3000 km2 camaqua basin the interlayered sedimentary and volcanic rocks record tectonic events and paleoenvironmental changes in a more than 10 km thick succession in this contribution we report new u pb and sm nd geochronological constraints for the 605 to 580 ma bom jardim group the 570 ma acampamento velho formation and a newly recognized 544 ma volcanism depositional patterns of these units reveal the transition from a restricted fault bounded basin into a wide shallow basin the expansion of the basin and diminished subsidence rates are demonstrated by increasing areal distribution and compressed isopachs and increasing onlap of sediments onto the basement to the west the sm nd isotopic composition of the volcanic rocks indicates mixed sources including crustal rocks from the adjacent basement both neoproterozoic and paleoproterozoic sources are indicated for the western part of the basin whereas only the older paleoproterozoic signature can be discerned in the eastern part of the basin

juvenile accretion at 750 700 ma in southern brazil a revista economic geology nao autoriza a publicacao de seus artigos em repositorios institucionais

petrology and geochemistry of the neoproterozoic guaxupe granulite facies terrain southeastern brazil abstract geochemical and oxygen isotopic data on the neoproterozoic granulite facies rocks enderbites charnockite and charnokitic augen gneisses of guaxupe brazil are assembled geochemical discrimination function calculations show an igneous origin for majority of the rock types in the area major and trace element data especially ree are similar to tonalite and granodiorite oxygen isotope composition for charnockitic gneisses and mafic granulites yield 18o values in the range of 8 8 to 11 and fall in the range of quartzofeldspathic rocks of medium to high grade terrains elsewhere a small scale variation of 18o values compatible with the chemical data imply the preservation of pre metamorphic oxygen isotope compositions minimum peak pressure temperature conditions are 8 5 kbar and 850 c the high grade metamorphism did not appear to have altered significantly the geochemical and oxygen isotope compositions of the protoliths approximately 2 5 kbar of pressure decrease accompanied by 200 c cooling has been recorded for a portion of the cooling path the p t path is consistent with a single event namely the formation and uplift of the guaxupenappe in the neoproterozoic brasiliano cycle