deep earthquakes in subducting slabs hosted in highly anisotropic rock fabric analysis of deep subduction zone earthquakes those at depths greater than 60 km reveals the physical and chemical properties of a descending oceanic lithosphere at mantle depths over the past five decades it has been observed that a large fraction of deep earthquakes has non double couple non dc seismic radiation patterns in contrast shallow earthquakes tend to have dc radiation patterns due to mechanisms of shear faulting these observations have been used to argue that deep earthquakes rupture differently from shallow earthquakes here we show that the observed global distribution of non dc deep earthquakes could be caused by shear faulting mechanisms but in a highly anisotropic laminated rock fabric that surrounds the deep earthquakes within subducted slabs for intermediate depth earthquakes 60 300 km we found a large shear wave anisotropy of 25 possibly caused by laminated fabric or aligned melt pockets oriented parallel to the slab interface which provides new supporting evidence for the metamorphic dehydration reactions in slabs however at deep focus depths 300 km the putative metastable phase change mechanism alone cannot explain the seismic anisotropy instead our results and those from recent experiments suggest materials such as magnesite or perhaps carbonatite melt may play a role in generating deep focus earthquakes radiation patterns for deep earthquakes could be a result of shear faulting mechanisms similar to those for shallow earthquakes but in highly anisotropic rock fabric suggest seismic analyses

geodynamic evolution of the mauritanide bassaride and rokelide orogens west africa abstract the mauritanide bassaride and rokelide orogens occur along the western edge of the west african craton these record a polyphase tectonothermal evolution including pan african i c 650 ma and pan african ii c 550 ma events together with local hercynian late paleozoic overprinting pan african i activity is most penetratively recorded in the bassarides and resulted from late proterozoic collision of a western continental structural block pan african ii orogenesis increases in intensity from the southern mauritanides through the bassarides and dominates the rokelides this tectonothermal activity appears to reflect collision of the west african and guyanean cratons during assembly of gondwana hercynian activity is concentrated along the margin of a western continental block which underwent relative eastward translation during collision of gondwana and laurentia this resulted in extensive thrusting of intracontinental foreland sequences external nappes and more ductile imbrications of pre deformed and metamorphosed late proterozoic rift sequences and western calc alkaline igneous successions internal nappes

sedimentary provenance and palaeoenvironment of the baixo araguaia supergroup constraints on the palaeogeographical evolution of the araguaia belt and assembly of west gondwana

constraining the hydration of the subducting nazca plate beneath northern chile using subduction zone guided waves abstract guided wave dispersion is observed from earthquakes at 180 280 km depth recorded at stations in the fore arc of northern chile where the 44 ma nazca plate subducts beneath south america characteristic p wave dispersion is observed at several stations in the chilean fore arc with high frequency energy 5 hz arriving up to 3 s after low frequency

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

upb geochronology of the central ribeira belt brazil and implications for the evolution of the brazilian orogeny abstract the ribeira belt in southeastern brazil was generated during the brazilian orogeny the south american equivalent of the pan african orogeny 700 450 ma the central sector of the belt is characterised by a series of northwest vergent crustal scale thrust stacks emplaced under amphibolite grade conditions major shear zones separate four lithotectonic domains andrel ndia to the northwest juiz de fora paraiba and coastal domains analyses of zircon monazite and titanite from a variety of rock types from the four domains lead to the following conclusions all domains record an important tectono thermal event at 590 565 ma represented by partial melting granitoid intrusion and remobilisation of older basement gneisses and corresponding to thrusting and development of dextral shear zones also recorded are older metamorphic ages 611 604 ma of unknown significance only the paraiba and coastal domains in the south record a younger metamorphic event m2 at 535 520 ma most likely associated with another episode of thrusting and shear zone development the absence of m2 metamorphism in the andrel ndia and juiz de fora domains can be explained by post 535 520 ma emplacement of the paraiba domain late to post tectonic activity is represented by 503 492 ma metamorphism precursors of the basement gneisses are 2185 2134 ma old rocks generated during the transamazonian orogeny some of which contain a significant archean component with minimum ages of 2981 2846 ma the ages presented above define specific metamorphic events for the brazilian orogeny for the first time in southeastern brazil in addition together with previously published data they indicate that the basement rocks found in the allochthonous domains of the central ribeira belt represent an extension of the units present in the southern sao francisco craton and indicate that the foreland was more extensive than the presently defined craton

tectonic evolution of south america during the late proterozoic abstract this paper discusses the evolution and the late proterozoic assembly of the south american continent i e the west gondwana under the light of global tectonics the northern portion of the continent the amazonian craton is a pre brasiliano domain it may be a fragment derived from laurentia this domain behaved as a large continental plate which was accreted to the central and southeastern portions of the continent brasiliano domain the brasiliano domain had a more complex history and composition during the brasiliano cycle middle proterozoic to early paleozoic in the brasiliano domain several late proterozoic continental plates have been identified sao luiz west africa congo kasai sao francisco rio de la plata arequipa antofalla which acted as forelands to the brasiliano orogenic belts that surround and amalgamate these plates smaller fragments of pre brasiliano continental lithosphere are common in the interior of the brasiliano orogenic belts and played diversified roles microplates microcontinents structural highs some of them have acted as true backlands hinterlands flanked by arc magmatism two principal types of brasiliano orogenic belts are recognized between the continental plates a marginal orogenies to the late proterozoic plates with pelitic carbonatic rocks and discrete volcanism which were thrust by collision onto the margin of the plates and which seem to represent final products of previous passive margin sequences b distal orogenics internal belts within the continental masses sometimes a branching system of orogenics that contain varied clastic and minor carbonate sedimentary rocks accompanied by important bimodal volcanism and calc alkaline plutonism for these orogenies a greater variety of basinal scenarios and tectonic settings may be visualized the amalgamation of the continent during the late proterozoic i e the formation of the brasiliano structures indicates the existence of practically synchronous multilateral stresses within lithospheric plates this can be produced by complementary adjustment of the continental plates as a response to major global events elsewhere analogous to modern continental collisions two large and important tectonic zones may be candidates to such a scenario the hijaz mozambique zone site of connection of east and west gondwana and the transbrasiliano lineament which cuts across south america and extends to north africa following the zone of connection between pre brasiliano and brasiliano domains other small ocean openings and closings inside the various brasiliano areas dom feliciano gariep for instance were minor events and complementary to the other two major tectonic zones mentioned above

reviewing the puzzling intracontinental termination of the aracuai west congo orogenic belt and its implications for orogenic development abstract palinspastic reconstructions suggest that the late proterozoic cambrian brasiliano pan african orogenic belt in southeast brazil and west congo terminated northwards into an embayment within the sao francisco congo cratonic unit the orogenic shortening that created the aracuai west congo orogen in this embayment has been explained by tightening of the horseshoe shaped sao francisco congo craton in a fashion referred to as nutcracker tectonics we show that this model is incompatible with the general orogenic evolution proposed in recent literature which involves 1 50 m y of subduction of oceanic crust and associated arc formation followed by 2 collisional orogeny and crustal thickening quantitative considerations show that the original nutcracker model is too rigid to explain even the second crustal thickening part let alone any long pre collisional history to soften the model we suggest that the so called sao francisco congo bridge was broken by a 150 km wide orogenic corridor along the current african atlantic margin this corridor adds sufficient mobility to the system to explain the orogenic thickening of the crust to 60 65 km however even with this additional softening the confined nature of this orogen is incompatible with prolonged arc development we therefore suggest that oceanic crust was nonexistent or very limited in the macaubas basin and reject the widely published model involving 50 m y of subduction of oceanic crust and related arc development instead we find strong support for a hot intracontinental orogen model in the currently available p t geochronologic petrographic and structural data in this model extensive melting and flow of the middle crust is likely to have caused spreading of the upper crust in an orogenic setting that was created by collisions along the n w and s margins of the sao francisco craton from 630 ma

holocene provenance shift of suspended particulate matter in the amazon river basin abstract the strontium sr neodymium nd and lead pb isotope signatures of suspended particulate matter spm in rivers reflect the radiogenic isotope signatures of the rivers drainage basin these signatures are not significantly affected by weathering transport or depositional cycles but document the sedimentary contributions of the respective sources we report new sr nd and pb isotope ratios and element concentrations of modern spm from the brazilian amazon river basin and document the past evolution of the basin by analyzing radiogenic isotopes of a marine sediment core from the slope off french guiana archiving the last 40 kyr of amazon river spm and the holocene section of sediment cores raised between the amazon river mouth and the slope off french guiana the composition of modern spm confirms two main source areas the andes and the cratonic shield in the marine sediment core notable changes occurred during the second phase of heinrich stadial 1 i e increased proportion of shield rivers spm and during the last deglaciation i e increased proportion of madeira river spm together with elsewhere constant source contributions furthermore we report a prominent offset in sr and nd isotopic composition between the average core value end 11 7 0 9 2sd 87sr 86sr 0 7229 0 0016 2sd and the average modern amazon river spm signal end 10 5 0 5 2sd 87sr 86sr 0 7213 0 0036 2sd we suggest that a permanent change in the amazon river basin sediment supply during the late holocene to a more andean dominated spm was responsible for the offset

constraints on subduction geodynamics from seismic anisotropy 1 much progress has been made over the past several decades in delineating the structure of subducting slabs but several key aspects of their dynamics remain poorly constrained major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology slab hydration and dehydration mechanical coupling between slabs and the ambient mantle the geometry of mantle flow above and beneath slabs and the interactions between slabs and deep discontinuities such as the core mantle boundary observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy when mantle rocks are deformed a preferred orientation of individual mineral crystals or materials such as partial melt often develops resulting in the directional dependence of seismic wave speeds measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems here i review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge slab and sub slab anisotropy i also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations i evaluate different models in light of constraints from seismology geodynamics and mineral physics finally i discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress particularly in frontier areas such as understanding slab dynamics in the deep mantle

plutonism deformation and metamorphism in the proterozoic flin flon greenstone belt canada limits on timing provided by the single zircon pb evaporation technique the authors have used the single zircon pb evaporation technique to determine the age of granitoids in the western flin flon domain of the trans hudson orogen in canada and to constrain the timing of molasse sedimentation deformation metamorphism and mesothermal gold mineralization the plutons were intruded between 1 860 and 1 834 ma about 30 m y after volcanism and provide further evidence of a major period of intrusive activity throughout the trans hudson orogen at that time the older plutons 1 860 to 1 848 ma are synchronous with p2 deformation and the early stages of peak metamorphism whereas the younger plutons are synchronous with or postdate the p3 deformation event molasse sedimentary units are deformed and metamorphosed thus deposition must have begun at about 1 860 ma mesothermal gold mineralization postdates all plutons and may be related to high grade peak metamorphism in adjacent terranes at about 1 815 ma this study also provides further support for the use of the pb evaporation technique as a suitable method for dating single zircons and indicates that the sup 208 pb sup 206 pb ratio of zircons may be a useful tool in detecting the presence and source of xenocrystic zircons

timing of terrane accretion in the neoproterozoic eopaleozoic ribeira orogen se brazil abstract new id tims u pb geochronological data of the oriental terrane combined with detailed geological data of other terranes support the presented tectonic evolutionary model for the central segment of the neoproterozoic ribeira belt southeastern brazil that can be subdivided in four major tectonic units a the occidental terrane regarded as the reworked margin of the sao francisco craton to the northwest b the paraiba do sul klippe c the oriental terrane comprising arc complexes and d the cabo frio terrane to the southeast the oriental terrane and the cabo frio terrane were progressively accreted to the reworked continental margin of the sao francisco paleoplate the occidental terrane contains pre 1 8 ga basement slivers interlayered with neoproterozoic passive margin successions andrel ndia group the oriental terrane focus of the work was firstly accreted to the sao francisco margin at ca 580 ma it displays three structural domains thrusted to nw toward the reworked sao francisco margin from west to east a the cambuci domain that is interpreted as the fore arc association b the costeiro domain that contains a distal passive margin succession intruded by the ca 790 and 635 620 ma rio negro arc complexes and c the italva domain that comprises marbles psamitic gneisses and amphibolites of ca 840 and is regarded both as the proximal passive margin of the oriental terrane or a back arc basin syn to late collisional granitoids and three metamorphic episodes ca 605 580 and 550 ma indicate a complex history for the collision i they were also described for the kaoko damara and west congo only the youngest one pan african belts located at occidental africa and are supposed to record the main episode of convergence in western gondwana a second collisional stage at ca 530 ma accreted the cabo frio terrane to the oriental terrane and is possibly related with the closure of a back arc basin located between the oriental terrane and kalahari craton that remained open until the ordovician the effects of the collision ii were also detected at the oriental terrane finally late to post tectonic granites marked the tectonic collapse of the orogenic building between 513 480 ma

the history of a continent from upb ages of zircons from orinoco river sand and smnd isotopes in orinoco basin river sediments abstract we report shrimp u pb ages of 49 zircons from a sand sample from the lower orinoco river venezuela and nd model ages of the fine sediment load from the main river and tributaries the u pb ages reflect individual magmatic or metamorphic events the sm nd model ages reflect average crustal residence ages of the sediment sources together they allow delineation of the crust formation history of the basement precursors of the sediments the u pb ages range from 2 83 to 0 15 ga and most are concordant or nearly so discrete age groupings occur at 2 8 2 1 and 1 1 ga the oldest group contains only three samples but is isolated from its closest neighbors by a 600 ma age gap larger age groupings at 2 1 and 1 1 ga make up about a third and a quarter of the total number of analyses respectively the remaining analyses scatter along concordia and most are younger than 1 6 ga the 2 8 and 2 1 ga ages correspond to periods of crust formation of the imataca and trans amazonian provinces of the guyana shield respectively and record intervals of short but intensive continental growth these ages coincide with 2 9 and 2 1 ga nd model ages of sediments from tributaries draining the archean and proterozoic provinces of the guyana shield respectively indicating that the u pb ages record the geological history of the crystalline basement of the orinoco basin zircons with ages corresponding to the major orogenies of the north atlantic continents the superior at 2 7 ga and hudsonian at 1 7 1 9 ga were not found in the orinoco sample the age distribution may indicate that south and north america were separated throughout their history nd model ages of sediments from the lower orinoco river and andean tributaries are 1 9 ga broadly within the range displayed by major rivers and dusts this age does not coincide with known thermal events in the region and reflects mixing of sources with different crust formation ages the igneous and metamorphic history of these sources as recorded by the detrital zircons is that of the orinoco basin basement this implies that despite evidence of fast sedimentary recycling global similarities in nd crustal residence ages and the probability of cross continent mixing through continental drift the sedimentary material carried by individual rivers is mainly derived from the crystalline basement in the basin the global semblance in nd isotope ratios in major river sediments and atmospheric dusts results from the averaging effect of large scale sampling of the continents which are heterogeneous in age on smaller regional scales a large portion of the continental crust in the orinoco basin formed during the trans amazonian orogeny at 2 0 2 1 ga and smaller portions formed both earlier at 2 8 ga and later after 1 6 ga these observations which are consistent with the relative sizes of crustal age provinces in the orinoco basin indicate that sediments from the lower orinoco and andean tributaries contain 25 35 of material added to the crust since trans amazonian times nd model ages of these sediments underestimate the average crust formation age of the basement of the orinoco basin by only about 10 if this relationship holds in other river basins then nd model ages of major rivers and wind blown particulates indicate that the mean age of the continental crust is 1 9 2 ga

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

thermochemistry and phase equilibria of hydrous phases in the system mgo sio2 h2o implications for volatile transport to the mantle calorimetric phase equilibrium and in situ equation of state data for antigorite talc and dense hydrous magnesium silicate dhms phases have been obtained to construct a self consistent thermodynamic data set that ensures internal consistency well behaved extrapolation and physically reasonable values of h s v k k and for talc antigorite and hydrous phase a used in conjunction with temperature profiles of subducting oceanic lithosphere the calculated phase equilibria show that there is no barrier to subducting substantial amounts of water to depth of 400 600 km in colder slabs since the slab can remain in the stability field of hydrous phases throughout its descent once water is brought to these depths it can probably diffuse into hotter mantle regions via hydrous mg2sio4 or by other mechanisms indicating that the transition zone is unlikely to be dry

lower crustal melting and the role of open system processes in the genesis of syn orogenic quartz diorite granite leucogranite associations constraints from sr nd o isotopes from the bandombaai complex namibia abstract the bandombaai complex southern kaoko belt namibia consists of three main intrusive rock types including metaluminous hornblende and sphene bearing quartz diorites allanite bearing granodiorites and granites and peraluminous garnet and muscovite bearing leucogranites intrusion of the quartz diorites is constrained by a u pb zircon age of 540 3 ma quartz diorites granodiorites and granites display heterogeneous initial nd and o isotope compositions end 540 ma 6 3 to 19 8 18o 9 0 11 6 but rather low and uniform initial sr isotope compositions 87sr 86srinitial 0 70794 0 70982 two leucogranites and one aplite have higher initial 87sr 86sr ratios 0 70828 0 71559 but similar initial end 11 9 to 15 8 and oxygen isotope values 10 5 12 9 the geochemical and isotopic characteristics of the bandombaai complex are distinct from other granitoids of the kaoko belt and the central zone of the damara orogen our study suggests that the quartz diorites of the bandombaai complex are generated by melting of heterogeneous mafic lower crust based on a comparison with results from amphibolite dehydration melting experiments a lower crustal garnet and amphibole bearing metabasalt probably enriched in k2o is a likely source rock for the quartz diorites the granodiorites granites show low rb sr 1 and are most likely generated by biotite dehydration melting of heterogeneous felsic lower crust all segments of the lower crust underwent partial melting during the pan african orogeny at a time 540 ma when the middle crust of the central damara orogen also underwent high t medium p regional metamorphism and melting geochemical and isotope data from the bandombaai complex suggest that the pan african orogeny in this part of the orogen was not a major crust forming episode instead even the most primitive rock types of the region the quartz diorites represent recycled lower crustal material

u pb geochronology of the southern brasilia belt se brazil sedimentary provenance neoproterozoic orogeny and assembly of west gondwana abstract the brasilia belt borders the western margin of the sao francisco craton and records the history of ocean opening and closing related to the formation of west gondwana this study reports new u pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the sao francisco craton and date the orogenic events leading to the amalgamation of west gondwana ages of detrital zircons by id tims and la mc icpms were obtained from metasedimentary units of the passive margin of the sao francisco craton from the main tectonic domains of the belt the internal allochthons araxa group in the araxa and passos nappes the external allochthons canastra group serra da boa esperanca metasedimentary sequence and andrel ndia group and the autochthonous or cratonic domain andrel ndia group the patterns of provenance ages for these units are uniform and are characterised as follows archean paleoproterozoic ages 3 4 3 3 3 1 2 7 and 2 5 2 4 ga paleoproterozoic ages attributed to the transamazonian event 2 3 1 9 ga with a peak at ca 2 15 ga and to the ca 1 75 ga espinhaco rifting of the sao francisco craton ages between 1 6 and 1 2 ga with a peak at 1 3 ga revealing an unexpected variety of mesoproterozoic sources still undetected in the sao francisco craton and ages between 0 9 and 1 0 ga related to the rifting event that led to the individualisation of the sao francisco paleo continent and formation of its passive margins an amphibolite intercalation in the araxa group yields a rutile age of ca 0 9 ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin detrital zircons from the autochthonous and parautochthonous andrel ndia group deposited on the southern margin of the sao francisco craton yielded a provenance pattern similar to that of the allochthonous units this result implies that 1 6 1 2 ga source rocks must be present in the sao francisco craton they could be located either in the cratonic area which is mostly covered by the neoproterozoic epicontinental deposits of the bambui group or in the outer paleo continental margin buried under the allochthonous units of the brasilia belt crustal melting and generation of syntectonic crustal granites and migmatisation at ca 630 ma mark the orogenic event that started with westward subduction of the sao francisco plate and ended with continental collision against the parana block and goias terrane continuing collision led to the exhumation and cooling of the araxa and passos metamorphic nappes as indicated by monazite ages of ca 605 ma and mark the final stages of tectonometamorphic activity in the southern brasilia belt whilst continent continent collision was proceeding on the western margin of the sao francisco craton along the southern brasilia belt eastward subduction in the east was generating the 634 599 ma rio negro magmatic arc which collided with the eastern sao francisco margin at 595 560 ma much later than in the brasilia belt thus the tectonic effects of the ribeira belt reached the southernmost sector of the brasilia belt creating a zone of superposition the thermal front of this event affected the proximal andrel ndia group at ca 588 ma as indicated by monazite age the participation of the amazonian craton in the assembly of western gondwana occurred at 545 500 ma in the paraguay belt and ca 500 ma in the araguaia belt this together with the results presented in this work lead to the conclusion that the collision between the parana block and goias terrane with the sao francisco craton along the brasilia belt preceded the accretion of the amazonian craton by 50 100 million years

anisotropy in the subducting slab observations from philippine sea plate events in taiwan in the southernmost ryukyu subduction zone slab guiding behavior from intermediate depth earthquakes is well documented with a low frequency 2 hz first p arrival followed by sustained high frequency 3 10 hz wave trains such waves developed by propagating along a long path within the slab are expected to have high sensitivity to anisotropy within the slab we determine shear wave splitting parameters from 178 intraplate events that are deeper than 100 km the possible slab anisotropy associated polarization pattern shows the fast direction at n65 e and delay time of 0 13 0 45 s this is stronger than the previously documented crust effect 0 1 s similar to the mantle wedge effect 0 28 s in average but weaker than the upper mantle effect 1 3 s in average in taiwan the fast axis reflects the fossil spreading direction of philippine sea plate with minor clockwise rotation due to the collision to eurasian plate

role of magma pressure tectonic stress and crystallization progress in the emplacement of syntectonic granites the a type estrela granite complex carajas mineral province brazil the archaean syntectonic a type estrela granite complex carajas mineral province brazil consists of three plutons emplaced in a greenstone sequence under low pressure conditions 180 p 310 mpa it is composed mainly of annite ferropargasite hedenbergite and ilmenite bearing monzogranites the contact aureole is affected by a subvertical penetrative schistosity conformable with the limits of the plutons meso to microstructures and mineral reactions in the granites indicate that deformation occurred in a continuum from above solidus to low t subsolidus conditions two distinct planar structures are observed i a concentrical primary foliation s0 corresponding to rhythmic isomodal phase layering associated with a faint grain shape fabric it is horizontal in the centre and vertical towards the edges of the plutons and ii a steep to subvertical foliation s1 associated with the deformation of s0 and accompanied with emplacement of synplutonic dykes and veins of leucocratic granites and pegmatites emplacement differentiation and consolidation of the estrela granite complex are considered to result from a continuous evolution under decreasing temperatures in a single stage strained crust transpression with two main periods 1 the first period is controlled by body forces and it corresponds to inflation with magma ponding as long as the rheology is melt dominated magma pressure is the critical parameter and almost no strain is recorded with decreasing t magmas crystallize and differentiate leading to a concentrical magmatic phase layering the growing magmatic bodies are mechanically decoupled from the country rocks and their evolution depends on internal magma chamber processes 2 for higher amount of crystallization residual melt fraction f 0 5 the role of magma pressure becomes insignificant establishment of a continuous crystal framework leads to the coupling of plutons with their surroundings and deformation in response to tectonic stress most of the strain is recorded during this period which starts from the rigid percolation threshold and extends to subsolidus low grade conditions this leads to deformation of the partially crystallized volume and redistribution of fluid enriched differentiated melts the amount of crystallization through the rheological thresholds appears as the critical parameter determining the transition from magma controlled processes inflation and differentiation of the magma chamber with development of a phase layering to tectonic controlled processes deformation of the phase layering and redistribution of residual melts this accounts for the fact that syntectonic plutons commonly display intermingled boudinaged layers with distinct modal compositions and in some cases well preserved rhythmic layering

the rokelides of west africa pan african aulacogen or back arc basin abstract a reassessment of the stratigraphy structure and metamorphism of the pan african rokelides tectono thermal belt in senegal guinea bissau guinea sierra leone and liberia indicates that the belt developed not as a plate margin collision orogen but as either an aulacogen or a back arc basin oblique to the main trend of the west african fold belt the main southern part of the late proterozoic to mid phanerozoic west african fold belt trends southwestwards via the koulountou belt of senegal and guinea bissau into south america where relics of pan african age occur within the andean cordilleras

u pb geochronology of archean magmatism and basement reactivation in the carajas area amazon shield brazil abstract the archean carajas subprovince is a metallogenic district bif bauxite cu ni mn au located in the easternmost amazonian craton in brazil the main conclusions from a u pb study of the itacaiunas supergroup and a locality in the xingu complex are the following the xingu basement complex the oldest unit underwent migmatization at 2859 2 ma and also grew new zircon at 2851 4 ma the luanga mafic intrusion is 2763 6 ma old and coeval with the 2759 2 ma felsic volcanics of the grao para group amphibolites from the salobo group contain 2851 ma zircons were metamorphosed 2761 3 ma and intruded by granitic veins ca 2758 ma at the time of grao para magmatism and also grew new zircon at 2732 2742 ma the data indicate that the salobo group is part of the xingu complex and not of the itacaiunas supergroup as heretofore believed the pojuca group possibly coeval with the salobo group was metamorphosed at 2732 3 ma a late archean reactivation event of regional extent is marked by titanite growth at 2581 5 ma in the mirim area intrusion of the old salobo granite at 2573 2 ma new zircon growth in salobo group amphibolites at 2555 4 3 ma and monazite growth in a metamorphosed iron formation at 2551 2 ma in addition k metasomatism associated with shear zones formed titanite in salobo group amphibolites at 2497 5 ma and new titanite at 2519 5 ma in the xingu complex events in the 2581 2497 ma interval are probably associated with the latest activity of a wnw ese sinistral shear system which could also be responsible for the establishment of a transtensional basin filled by the volcano sedimentary sequence of the grao para bahia and rio fresco groups finally undeformed proterozoic granitoid intrusions were dated at 1883 2 ma cigano 1883 5 2 ma musa 1880 2 ma carajas and 1874 2 ma pojuca

th u pb isotopic systems and internal structures of complex zircons from an enderbite from the pium complex carajas province brazil evidence for the ages of granulite facies metamorphism and the protolith of the enderbite abstract shrimp u pb micro analyses are reported for parts of complex zircons from a granulite facies enderbite from the pium complex in the carajas metallogenic province of the amazon craton of brazil zircons from the granulite consist of two types of grains a those with oscillatory zoned cores surrounded by rims composed of two or more curved overlapping nebulously zoned domains distinguished by differences in cathodoluminescence intensity and b grains which are composed entirely of such domains and have no obvious cores the shrimp u pb age of 3002 14 ma determined for oscillatory zoned zircon cores is interpreted as dating the parent rock of the enderbite this age is resolved from the u pb age of 2859 9 ma determined on zircon rims which is interpreted as a time point within the evolving granulite facies metamorphism shrimp u pb ages of cores and rim type zircon are generally concordant to slightly discordant where uranium contents are less than about 820 ppm but once this value is exceeded the zircon becomes strongly discordant suggesting that a threshold value of radiation damage exists above which zircon stability with respect to pb loss is sharply reduced the results provide the first substantial age determination of the granulite metamorphism in the carajas province the zircon u pb ages have not been influenced by the transamazonian metamorphism dated at ca 2 0 1 85 ga from ar ar and rb sr mineral ages

mid mantle anisotropy in subduction zones and deep water transport the earth s transition zone has until recently been assumed to be seismically isotropic increasingly however evidence suggests that ordering of material over seismic wavelengths occurs there but it is unclear what causes this we use the method of source side shear wave splitting to examine the anisotropy surrounding earthquakes deeper than 200 km in slabs around the globe we find significant amounts of splitting 2 4 s confirming that the transition zone is anisotropic here however there is no decrease in the amount of splitting with depth as would be the case for a metastable tongue of olivine which thins with depth suggesting this is not the cause the amount of splitting does not appear to be consistent with processes in the ambient mantle such as lattice preferred orientation development in wadsleyite ringwoodite or mgsio3 perovskite we invert for the orientation of several mechanisms subject to uncertainties in mineralogy and deformation and the best fit is given by updip flattening in a style of anisotropy common to hydrous phases and layered inclusions we suggest that highly anisotropic hydrous phases or hydrated layering is a likely cause of anisotropy within the slab implying significant water transport from the surface down to at least 660 km depth

large scale trench normal mantle flow beneath central south america abstract we investigate the anisotropic properties of the fore arc region of the central andean margin between 17 25 s by analyzing shear wave splitting from teleseismic and local earthquakes from the nazca slab with partly over ten years of recording time the data set is uniquely suited to address the long standing debate about the mantle flow field at the south american margin and in particular whether the flow field beneath the slab is parallel or perpendicular to the trench our measurements suggest two anisotropic layers located within the crust and mantle beneath the stations respectively the teleseismic measurements show a moderate change of fast polarizations from north to south along the trench ranging from parallel to subparallel to the absolute plate motion and are oriented mostly perpendicular to the trench shear wave splitting measurements from local earthquakes show fast polarizations roughly aligned trench parallel but exhibit short scale variations which are indicative of a relatively shallow origin comparisons between fast polarization directions from local earthquakes and the strike of the local fault systems yield a good agreement to infer the parameters of the lower anisotropic layer we employ an inversion of the teleseismic waveforms based on two layer models where the anisotropy of the upper crustal layer is constrained by the results from the local splitting the waveform inversion yields a mantle layer that is best characterized by a fast axis parallel to the absolute plate motion which is more or less perpendicular to the trench this orientation is likely caused by a combination of the fossil crystallographic preferred orientation of olivine within the slab and entrained mantle flow beneath the slab the anisotropy within the crust of the overriding continental plate is explained by the shape preferred orientation of micro cracks in relation to local fault zones which are oriented parallel to the overall strike of the andean range our results do not provide any evidence for a significant contribution of trench parallel mantle flow beneath the subducting slab

syntectonic granite emplacement at different structural levels the closepet granite south india abstract the closepet granite in south india is a large 400 km long but only 30 km wide elongate late archaean granitic body structural levels from deep crust to upper levels crop out as evidenced by a shallowing of paleo depths from north to south all along the closepet granite this allows the study of the emplacement of the same granitic body at various crustal levels four zones have been identified a root zone where magmas are collected in active shear zones a transfer zone featuring large scale magma ascent and crystal liquid partitioning in the granitic mush a gap where the mush was filtered allowing only the liquids to rise shallow intrusions filled with this liquid the closepet granite was emplaced syntectonically field work and anisotropy of magnetic susceptibility allowed documentation of steep foliations with subhorizontal lineations both in the root and transfer zones and in the shallow intrusions remote sensing evidenced a network of shear zones bounding the closepet granite in the porphyritic root and transfer zones magmas cooled slowly thus developing strong fabrics during large scale dextral shearing ascent of residual liquids amidst the crystallizing solid framework was not recorded in the fabrics however these liquids were channelised through the gap and infilled the homogeneous shallow intrusions where rapid cooling only permitted the development of feint although wholly consistent fabrics

brasiliano crustal structure and the tectonic setting of the parnaiba basin of ne brazil results of a deep seismic reflection profile a 1430 km deep crustal seismic reflection profile of the parnaiba basin shows an asymmetric structured western margin and a gently dipping eastern margin the 3 km thick phanerozoic sedimentary section overlies a pronounced planar regional unconformity that crosses three precambrian blocks with differing seismic facies the amazonian araguaia block the parnaiba block and the borborema block the blocks are separated by steep crustal scale boundaries across which seismic facies change abruptly in the west the ophiolitic metasedimentary rocks of the araguaia group overlie the amazonian craton both craton and metasediments terminate eastward against a steep east dipping fault zone defining the amazonian araguaia block and parnaiba block boundary reactivation of this neoproterozoic margin in the late triassic and late jurassic early cretaceous folded and elevated basement and basin over 2 km a second crustal boundary defines the eastern margin of the parnaiba block with the neoproterozoic borborema block this boundary is interpreted as the extension of the transbrasiliano shear zone these data demonstrate that the basement of the parnaiba basin was formed during brasiliano orogenesis by west directed collision related thrusting succeeded by lateral accretion along steep crustal scale boundaries after formation of a post brasiliano peneplain the parnaiba basin developed seamlessly across three very different crustal blocks and appears to have been significantly larger than its present outline no extensive underlying rift system is evident suggesting that basement structure had little to do with basin formation but that episodic reactivation of the boundary zones and basement fabric has controlled the structuring and preservation of the basin

tearing up rodinia the neoproterozoic palaeogeography of south american cratonic fragments rodinia was initially defined as a long lived supercontinent that assembled all the continental fragments around laurentia and remained stable from 1000 up to 750 ma nonetheless recent work has cast doubt on the rodinia palaeogeography and even on the timing of its assembly and break up the geochronological and palaeomagnetic databases accumulated for south america and africa in the last decade show that most of these continental fragments were not part of rodinia a wide brasiliano ocean separated most of the south american and african cratons from the laurentia amazonia rio apa west africa margin this ocean was closed between 940 and 630 ma along the pampean paraguay araguaia pharusian mobile belts moreover accretion along the south american and african platforms was a diachronous and long lived process that involved several intra oceanic and continental magmatic arcs and microcontinents this evolution started at around 1000 ma and ended at around 520 ma with the final assembly of gondwana

inversion for anisotropy from non double couple components of moment tensors 1 an inversion method for retrieving seismic anisotropy from non double couple components of seismic moment tensors is presented the method requires a set of highly accurate moment tensors of earthquakes that occurred in a homogeneous anisotropic focal area on differently oriented faults in contrast to standard methods retrieving anisotropy from travel times or from shear wave splitting which yield an overall anisotropy averaged along a whole ray path the presented method yields a local value of anisotropy just in the focal area the method is robust being able to retrieve the orientation as well as strength of anisotropy even for low anisotropy symmetries as for orthorhombic symmetry the method can utilize the moment tensors constrained to have zero trace but using unconstrained moment tensors is advantageous the method is applied to retrieving anisotropy in the tonga subduction zone using moment tensors of deep focus earthquakes reported in the harvard centroid moment tensor catalog the inversion is complemented by tests on synthetic data to assess its stability and the accuracy of the results the inversion indicates that the subduction zone is anisotropic with orthorhombic symmetry the orientation of the intra slab anisotropy is defined by axes azimuth dip a1 320 54 a2 121 38 and a3 223 81 the errors in the azimuth and dip are about 5 the first and second axes lie along the downdip motion of the slab and along the normal to the slab respectively the strength of the p s1 and s2 anisotropy is of 7 3 1 5 13 4 2 5 and 12 6 3 5 respectively the errors of anisotropy strength are only rough estimates which reflect random but not systematic errors in the moment tensors used in the inversion the values for anisotropy strength in the slab are remarkably higher than those observed in the surrounding mantle the symmetry axes of anisotropy coincide with the principal stress directions in the slab this manifests a primary impact of stress on anisotropy formation the retrieved anisotropy in the slab can serve as an additional constraint on its structure and mineralogical composition

deep focus earthquakes and recycling of water into the earth s mantle for more than 50 years observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic at these depths rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep focus seismicity could originate from dehydration and high pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle thus seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet

the araguaia fold belt brazil a reactivated brasiliano pan african cycle 550 ma geosuture abstract the araguaia tocantins geosuture which separates the araguaia fold belt afb from the archean amazonian craton was active in the late middle proterozoic the baixo araguaia supergroup was deposited consisting of the estrondo group lower quartzites with intercalated schists xambioa formation schists and canto da vazante formation upper feldspathic schists and the tocantins group consisting of the couto magalhaes formation phyllites quartzites slates limestones and metacherts and pequizeiro formation upper chlorite schists and associated mafic ultramafic bodies the deformational history includes four regional phases of deformation within this supracrustal sequence recumbent folds with vergence to the west refolding with a n s trend an intense crenulation episode and late thrusting from east to west metamorphism is of intermediate or intermediate high pressure type with garnet biotite chlorite and sericite isograds succeeded by a slightly or non metamorphosed zone from east to west rocks surrounding sparse gneissic cored domes contain isograds of staurolite kyanite and fibrolite these isograds are believed to be associated with the 1100 ma uruacuano event the brasiliano orogeny strongly affected the afb with displacements due to transcurrent reactivation of great and old faults of the basement slight folding in the supracrustal sequence intrusion of small granite bodies and development of domes with associated normal faults the area underlain by the estrondo group was uplifted at this time causing the deposition of the rio das barreiras polymictic conglomerate of the central area k ar and rb sr analyses date this thermo tectonic event at 550 100 ma the archean basement is exposed in the cores of domes as a granite gneiss association the colmeia complex which shows thermo tectonic features that may be interpreted as polycyclic imprints jequie transamazonian uruacuano and brasiliano events

pb loss patterns in zircons from a high grade metamorphic terrain as revealed by different dating methods upb and pbpb ages for igneous and metamorphic zircons from northern sri lanka abstract we report u pb and 207 pb 206 pb ages for multigrain fractions single grains and grain domains in zircons from highgrade igneous and sedimentary rocks of the vavuniya charnockite province vcp in northern sri lanka using the ionmicroprobe shrimp a new vapour digestion technique vdt and evaporation charnockitic enderbitic and mangeritic gneisses are derived from i type tonalitic syenitic granodioritic and granitic precursors for which the u pb systematics of zircons as determined by shrimp suggest crystallization ages between 1000 and 1100 ma many translucent round and multifacetted zircons in these rocks are not of metamorphic but of igneous origin and acquired their shape through partial zircon dissolution and recrystallization the large majority of zircons exhibit severe and variable pb loss some 540 580 ma ago which we ascribe to granulite grade metamorphism almost immediately followed by local retrogression most of the zircons lost pb from the entire grain with no domains remaining that reflect the original pb isotopic composition it was therefore impossible to obtain meaningful 207 pb 206 pb evaporation ages for such grains and this cautions against use of this method in zircons that experienced severe pb loss however zircons formed during high grade metamorphism apparently retain their primary pb isotopic composition and were dated successfully by the evaporation technique yielding ages between 553 26 and 560 30 ma conventional multigrain analysis and small fractions following vdt for two samples although much more precise than the shrimp data yielded only strongly discordant data points with patterns reflecting variable pb loss these data alone would not have made it possible to obtain reliable primary zircon crystallization ages 207 pb 206 pb evaporation ages for detrital zircons from two garnetiferous gneiss samples of sedimentary origin in the centre of the vcp revealed a maximum age of 1329 18 ma whereas a metaquartzite near the southern margin of the vcp contained zircons with 207 pb 206 pb ages between 1857 14 and 2543 14 ma we interpret the latter rock as belonging to the highland complex which occurs adjacent to the vcp our zircon data in combination with nd model ages milisenda et al 1994 suggest the vcp to be part of the wanni complex a distinct crustal domain in northern sri lanka characterized by relatively young ages in comparison with much older ages in the adjacent highland complex

soft sediment deformation at the base of the neoproterozoic puga cap carbonate southwestern amazon craton brazil confirmation of rapid icehouse to greenhouse transition in snowball earth stratigraphic and isotopic data identify the lower 45 m of the araras group on the southwest margin of the amazon craton as a neoproterozoic platform cap carbonate deposited below wave base upon varanger glacial diamictites of the puga formation the basal beds consist of moderately deep water pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tube like structures and fenestral nonmicrobial planar laminites with tepee like features above the basal carbonates are deep water bituminous lime mudstones with alternating thin calcite crusts and lime mudstone laminae commonly disrupted by calcite crystal fans pseudomorphs after aragonite the basal contact of the puga cap exhibits soft sediment deformational structures principally load casts that are here attributed to rebound induced seismicity acting upon both recently deposited carbonate sediments and underlying unconsolidated diamictite these features constitute the first clearly recognized sedimentological evidence for the rapid change from icehouse to greenhouse conditions as postulated in the snowball earth model of neoproterozoic glaciation

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

seismic characteristics of central brazil crust and upper mantle a deep seismic refraction study 1 a two dimensional model of the brazilian central crust and upper mantle was obtained from the traveltime interpretation of deep seismic refraction data from the porangatu and cavalcante lines each approximately 300 km long when the lines were deployed they overlapped by 50 km forming an e w transect approximately 530 km long across the tocantins province and western sao francisco craton the tocantins province formed during the neoproterozoic when the sao francisco the paranapanema and the amazon cratons collided following the subduction of the former goias ocean basin average crustal vp and vp vs ratios moho topography and lateral discontinuities within crustal layers suggest that the crust beneath central brazil can be associated with major geological domains recognized at the surface the moho is an irregular interface between 36 and 44 km deep that shows evidences of first order tectonic structures the 8 05 and 8 23 km s 1 p wave velocities identify the upper mantle beneath the porangatu and cavalcante lines respectively the observed seismic features allow for the identification of 1 the crust has largely felsic composition in the studied region 2 the absence of the mafic ultramafic root beneath the goias magmatic arc and 3 block tectonics in the foreland fold and thrust belt of the northern brasilia belt during the neoproterozoic seismic data also suggested that the bouguer gravimetric discontinuities are mainly compensated by differences in mass distribution within the lithospheric mantle finally the goias tocantins seismic belt can be interpreted as a natural seismic alignment related to the neoproterozoic mantle domain

closure of the clymene ocean and formation of west gondwana in the cambrian evidence from the sierras australes of the southernmost rio de la plata craton argentina abstract the formation of gondwana took place across a series of brasiliano pan african suture zones that record late neoproterozoic to earliest paleozoic collisions between precambrian cratons in south america an internal suture zone marks the disappearance of the clymene ocean that separated the amazon craton from the sao francisco and rio de la plata cratons new geochronological data from the southern end of this massive collision zone in the sierras australes of central eastern argentina document paleoproterozoic crust and suggest an ediacaran age for the oldest sedimentary rocks these two observations extend the known limit of the rio de la plata craton at least 200 km sw of previous estimates new data also confirm the occurrence of late ediacaran to late cambrian magmatism in the sierras australes the age of these hypabyssal to volcanic rocks corresponds to igneous events in the pampean belt along the western margin of the rio de la plata craton although the shallow level magma emplacement in the sierra da ventana study area contrasts with the deeply exhumed rocks of the pampean orogeny type locality these new age data are compared with a broad compilation of geochronological age clymene collision belts to the north the paraguai and araguaia belts the close overlap of the timing of orogenesis indicates the age of clymene ocean closure in its northern reaches in the south the pampean belt was unconfined allowing continued tectonic activity and crustal accretion throughout the paleozoic

207pb 206pb ages of zircons from syn collisional i type porphyritic granites of the central ribeira belt se brazil abstract in the central segment of the ribeira belt southeast brazil several foliated porphyritic granitic bodies intrude high grade migmatitic gneisses of the andrel ndia and juiz de fora domains and embu complex results of geological geochemical and geochronological investigations of the maromba pedra selada serra do lagarto and funil porphyritic i type granites provide profound similarities except for the distinct geochemical behavior of the funil granite perhaps related to a different crustal source these granitoids show similar structural textural and mineralogical features pb evaporation of single zircons provided ages of 586 6 579 6 6 3 586 3 4 8 and 584 5 ma for the granites respectively coincident with the syn collision i episode of the central ribeira belt the intrusion of i type porphyritic granitoids coeval with the main collisional event has not often been reported in the geological literature the most common syn collisional granitic magmatism has normally an s type signature or even a slightly peraluminous i type character however the occurrence of coeval i and s type syn tectonic granites along the central ribeira belt as observed in the investigated area and discussed in this paper is noteworthy