sequence stratigraphy of the punta ballena member of the jama formation early pleistocene ecuador insights from integrated sedimentologic taphonomic and paleoecologic analysis of molluscan shell concentrations abstract early pleistocene strata are well exposed along the uplifted eastern margin of the esmeraldas caraquez basin particularly on the coastal cliff west of the city of jama a type locality for shallow marine quaternary sedimentary exposures on north central ecuador here at least 180 m of sediments accumulated in inner shelf to subaerial settings are present jama formation this paper is concerned with the middle stratigraphic unit of the jama formation the punta ballena member pbm integrated sedimentologic paleontologic and stratigraphic observations reveal that the pbm is composed of eight disconformity bounded depositional sequences hiatal shell concentrations which formed as a consequence of landward and seaward stratal attenuation are associated with sequence bounding and intrasequence surfaces together with sedimentologic characteristics these fossiliferous levels are important indicators of facies architecture and therefore are useful tools for interpreting sequence stratigraphy according to their taphonomic features and taxonomic composition the hiatal shell concentrations occurring within the pbm may be grouped into two main types each of which occupies a predictable stratigraphic position within the sequences current wave winnowed beds at the cycle base and community shell concentrations in a roughly mid cycle position current wave winnowed beds commonly underlain by thalassinoides burrows that penetrate to depths of as much as 1 5 m below ravinement surfaces are transported skeletal concentrations and formed through physical processes in shallow high energy environments they comprise a time averaged mixture of specimens having different ecologies and states of preservation and accumulated mainly under conditions of total passing of siliciclastic sediment i e terrigenous sediments are delivered to the site as suspended load but owing to the high environmental energy failed to accumulate community shell concentrations occur in sparsely fossiliferous intervals and result from reduced net sedimentation within low energy relatively deeper settings they are composed of matrix supported loosely clustered life oriented and little disturbed fossil assemblages but are packed in excess of likely living densities and are visually dominated by successive generations of the infaunal bivalve raeta undulata which colonized a slowly aggrading muddy seafloor the sedimentological and paleontological evidence presented here not only supports the results of previous studies which demonstrate the value of condensed shell concentrations in the identification of stratigraphically significant surfaces but also provides adds to the distinctive features of these shell beds

age and sources of gold mineralization in the marmato mining district nw colombia a miocene pliocene epizonal gold deposit abstract epigenetic gold mineralization occurs in the marmato mining district within the calima terrain of the setentrional andes colombia regional rocks associated with this mineralization include graphite and chlorite schists of the arquia complex metamorphosed during the cretaceous miocene sandstones shales and conglomerates of the amaga formation as well as pyroclastic rocks clasts of basalt andesites and mafic lavas and subvolcanic andesitic dacitic bodies of the combia formation 9 to 6 ma the subvolcanic marmato stock hosts mesothermal and epithermal low sulfidation au ag ores in the form of distensional veins stockwork and quartz veinlets within brecciated zones ore minerals are pyrite sphalerite and galena with subordinate chalcopyrite arsenopyrite pyrrhotite argentite and native gold electrum sericitized plagioclase from a porphyry dacite yielded a k ar age of 5 6 0 6 ma interpreted as the age of ore deposition this is in close agreement with the age of reactivation of the cauca romeral fault system 5 6 0 4 ma which bounds the calima terrain a porphyry andesite dacite 6 7 0 1 ma hosting the au ag veins shows a measured 87sr 86sr between 0 70440 and 0 70460 end between 2 2 and 3 2 and 206pb 204pb 207pb 204pb and 208pb 204pb ratios of 18 964 to 19 028 15 561 to 15 570 and 38 640 to 38 745 respectively the 87sr 86sr and end values of rocks from the arquia group range from 0 70431 to 0 73511 and 12 91 to 10 0 respectively whereas the corresponding pb isotopic ratios 206pb 204pb 207pb 204pb and 208pb 204pb range from 18 948 to 19 652 15 564 to 15 702 and 38 640 to 38 885 respectively 87sr 86sr and end values obtained on sulfides from the gold quartz veins which occur at shallow and intermediate levels range from 0 70500 to 0 71210 and from 1 11 to 2 40 in the deepest veins end values lie between 1 25 and 3 28 and the 87sr 86sr of calcite and pyrite fall between 0 70444 and 0 70930 the 206pb 204pb 207pb 204pb and 208pb 204pb ratios of all mineralization are in the ranges 18 970 to 19 258 15 605 to 15 726 and 38 813 to 39 208 respectively carbonates have an average 87sr 86sr ratio of 0 70445 which is within the range of values measured in the host dacite the sr isotopic data indicate that carbonic fluids have a restricted hydrothermal circulation within the host igneous body while the sr pb and nd isotopic compositions of the sulfides suggest that the fluids not only circulated within the marmato stock but also throughout the arquia complex inferring that these rocks offer a potential target for mineral exploration based on geological and geochronological evidence the epizonal marmato gold ores formed during the miocene to pliocene as a result of cooling of the marmato stock and reactivation along a crustal scale fault zone related to thermal processes in an accretionary oceanic continental plate orogen

a note on the iugs reclassification of the high mg and picritic volcanic rocks the purpose of this short note is to offer some comments and observations on the new iugs classification of high mg rocks le bas 2000 particularly its use of the terms komatiite and picrite with no reference to texture or the original usage of these terms at the outset of this discussion we would like to place on record that we are in agreement with the need to redefine the nomenclature of high mg rocks there are however important issues relating to both the use of the term komatiite for all extrusive volcanic rocks with 18 wt mgo and the definition and use of the term picrite neither of which are discussed by le bas

a model for the simulation of combined major and trace element liquid lines of descent abstract variations of major and trace element partition coefficients as functions of temperature and composition have severely limited the range of composition for quantitative modeling of the differentiation of natural magmatic systems application of a two lattice melt model greatly reduces compositional dependence for trace elements this allows the temperature dependence of isobaric trace element partitioning to be evaluated independently thus making it possible to model an expanded range of conditions and compositions the first goal of this investigation was to construct a model to simulate major and trace element systematics of natural differentiation processes by combining trace element partitioning with a major element phase equilibrium model nielsen 1985b the second goal was the application of this model to the simulation of magma chamber recharge assimilation eruption and fractional crystallization these simulations indicate that the proportions of minerals fractionating from an open system are not necessarily the same as those in a closed system recharge or assimilation will drive a magma composition off the cotectic along which it is evolving however fractionation will drive the liquid back towards the cotectic in a recharged magma chamber as the recharge flux increases the fractionating mineral proportions will be increasingly biased towards those phases which appear first from the primitive magma the bulk partition coefficient for an open system can be considerably different 5 10 or more for some elements compared to a closed system paired assimilation and fractional crystallization are often cited as the mechanisms responsible for the origin of high al basalts and andesites however the addition of an al rich assimilant into a fractionating basaltic magma will increase the proportion of plagioclase to the extent that al in the derived liquids will be effectively buffered or even decrease the assimilation of forsteritic olivine by reaction keleman and ghiorso 1986 increases the proportions of pyroxene and olivine thus reducing the fe enrichment and al depletion characteristic of low pressure basalt fractionation

oceanic plateau formation a cause of mass extinction and black shale deposition around the cenomanian turonian boundary the cenomanian turonian boundary 90 4 ma represents a major period of worldwide environmental disturbance the physical manifestations of this are elevated atmospheric and oceanic temperatures a significant sea level transgression and a period of widespread anoxia leading to the formation of oceanic black shales and the extinction of 26 of all genera elevated 13 c values and enrichment of trace elements in cenomanian turonian boundary sediments combined with a reduction in 87 sr 86 sr also imply a severe environmental perturbation at this time oceanic crustal production rates reached their highest level of the last 100 million years this was principally caused by extensive melting of hot mantle plumes at the base of the oceanic lithosphere and the development of vast areas up to 1 10 6 km 2 of thickened oceanic crust in the pacific and indian oceans the anomalous volcanism associated with the formation of these oceanic plateaux may have been responsible for the environmental disturbances c 90 ma these eruptions would also have resulted in the emission of large quantities of co 2 into the atmosphere leading to global warming additionally the emission of so 2 h 2 s co 2 and halogens into the oceans would have made seawater more acidic resulting in the dissolution of carbonate and further release of co 2 this run away greenhouse effect was probably put into reverse by the decline of the anomalous volcanic activity and by increased co 2 driven productivity in oceanic surface waters leading to increased organic carbon burial black shale deposition anoxia and mass extinction in the ocean basins

tectonic evolution of the caribbean and northwestern south america the case for accretion of two late cretaceous oceanic plateaus it is widely accepted that the thickened oceanic crust of the caribbean plate its basaltic accreted margins and accreted mafic terranes in northwestern south america represent the remnants of a single ca 90 ma oceanic plateau we review geologic geochemical and paleomagnetic evidence that suggests that the caribbean colombian oceanic plateau in fact represents the remnants of two different oceanic plateaus both dated as ca 90 ma the first of these plateaus the caribbean plateau formed ca 90 ma in the vicinity of the present day galapagos hotspot northeastward movement of the farallon plate meant that this plateau collided with the proto caribbean arc and northwestern south america 10 m y after the plateau s main phase of formation paleomagnetic evidence suggests that the second of these plateaus the gorgona plateau formed at 268 308s possibly at the site of the present day sala y gomez hotspot over the next 45 m y this plateau was carried progressively northeastward on the farallon plate and collided in the middle eocene with the proto andean subduction zone in northwestern south america the recognition of a second ca 90 ma pacific oceanic plateau strengthens the link between plateau formation and global oceanic anoxic events

origin and cretaceous tectonic history of the coastal ecuadorian forearc between 1 n and 3 s paleomagnetic radiometric and fossil evidence the presence of at least one block pallatanga block derived from the caribbean colombian oceanic plateau has been proposed in the western cordillera of ecuador new 40ar 39ar dating of the basement of the coastal blocks 90 87 ma and fossil ages in the overlying calentura fm 89 84 ma argue for late turonian coniacian extrusion of the basement of the pinon block paleomagnetic inclinations indicate that the mafic basement of the pinon and san lorenzo blocks extruded at equatorial low southern latitudes no subsequent significant latitudinal drift can be observed during the cretaceous rapid changes in paleomagnetic declination between 73 and 70 ma in both the pinon and the san lorenzo blocks indicate that the large igneous province which originally hosted the coastal blocks experienced a major tectonic event during late campanian times similar late turonian coniacian basement ages and late campanian tectonic events are reported in the pallatanga block of the western cordillera we propose that the pinon san lorenzo and pallatanga blocks all derive from a common large igneous province the caribbean colombian oceanic plateau and were incorporated into the south american margin by collision during the late campanian 73 70 ma

evolution of the late pleistocene mojanda fuya fuya volcanic complex ecuador by progressive adakitic involvement in mantle magma sources the mojanda fuya fuya volcanic complex consists of two nearby volcanoes mojanda and fuya fuya the older one mojanda volcano 0 6 to 0 2 ma was first constructed by andesites and high silica andesites forming a large stratovolcano lower mojanda this edifice was capped by a basaltic andesite and andesitic cone upper mojanda which collapsed later to form a 3 km wide summit caldera after large phreatomagmatic eruptions the lower fuya fuya edifice was constructed by the extrusion of viscous si rich andesitic lavas and dacitic domes and the emission of a thick sequence of pyroclastic flow and fallout deposits which include two voluminous rhyolitic layers an intermediate construction phase at fuya fuya is represented by a mainly effusive cone andesitic in composition san bartolo edifice the construction of which was interrupted by a major sector collapse in the late pleistocene finally a complex of thick siliceous lavas and domes was emplaced within the avalanche amphitheatre forming the upper fuya fuya volcanic centre this paper shows that the general evolution from an effusive to an explosive eruptive style is related to a progressive adakitic contribution to the magma source although all the rocks of the complex are included in the medium k field of continental arcs the fuya fuya suite 61 75 wt sio2 shows depletion in y and hree and high sr y and la yb values compared to the less silicic mojanda suite 55 66 5 wt sio2 the mojanda calc alkaline suite was generated by partial melting of an adakite metasomatised mantle source that left a residue with 2 garnet followed by fractional crystallization of dominant plagioclase pyroxene olivine at shallow intra crustal depths for fuya fuya geochemical and mineralogical data suggest either 1 partial melting of a similar metasomatised mantle with more garnet in the residue 4 followed by fractional crystallization involving plagioclase amphibole and pyroxene or 2 mixing of mafic mantle derived magma from the mojanda suite and slab melts followed by the same fractional crystallization process

an autochthonous geological model for the eastern andes of ecuador abstract we describe a traverse across the cordillera real and sub andean zone of ecuador poorly known areas with very little detailed mapping and very little age control the spine of the cordillera comprises deeply eroded triassic and jurassic plutons the roots of a major arc emplaced into probable palaeozoic pelites and metamorphosed volcanic rocks the w flank comprises a jurassic submarine basaltic andesitic volcanic sequence which grades up into mixed jurassic cretaceous volcanic and sedimentary rocks of the inter andean valley the sub andean zone on the e flank of the cordillera comprises a newly recognized cretaceous basin of cleaved mudrocks quartz arenites and limestones east of the syndepositional cosanga fault the cretaceous basin thins into a condensed sequence that is indistinguishable from the rocks of the adjacent hydrocarbon bearing oriente basin the principal penetrative deformation of the cordillera real was probably latest cretaceous palaeocene it telescoped the magmatic belts but shortening was largely partitioned into the pelites between plutons the plutons suffered inhomogenous deformation some portions completely escaped tectonism the pelites conserve two foliations the earliest comprises slaty cleavage formed under low or sub greenschist conditions the later is a strong schistosity defined by new mica growth it largely transposed and obliterated the first both foliations may have developed during a single progressive deformation we find inappropriate recent terrane models for the cordillera real and sub andean zone of ecuador instead we find remarkable similarities from one side of the cordillera to the other including a common structural history in place of sutures we find mostly intrusive contacts between major plutons and pelites triassic to cretaceous events occurred on the autochthonous western edge of the archaean guyana shield the latest cretaceous paleocene deformation is interpreted as the progressive collision of an oceanic terrane s with the south american continent young fault movements have subsequently juxtaposed different structural levels through the cordillera real orogen

age and geochemistry of the beata ridge primary formation during the main phase 89 ma of the caribbean large igneous province highlights at least parts of the beata ridge formed during the main clip stage at 95 83 ma sampling of numerous volcanic rocks indicates a broad extrusive magmatic event depleted and enriched geochemical signatures point to a heterogeneous mantle source geochemical heterogeneities can occur on a small scale of tens of kilometers abstract the caribbean large igneous province clip a cretaceous oceanic flood basalt province presumably formed at the initiation of the galapagos hotspot during the m81 cruise of the german r v meteor we sampled the beata ridge a prominent submarine structure in the caribbean sea belonging to the clip the ridge offers the opportunity to directly sample basement sequences of the central submarine part of the clip complementing numerous studies of accreted clip sequences exposed on land around the margins of this lip the majority of the recovered beata ridge samples are volcanic implying that at least parts of the beata ridge were formed during a large extrusive event in contrast to previous assumptions that the structure is primarily composed of intrusive rocks several stratigraphically controlled profiles were sampled along the western slope of the beata ridge using the remotely operated vehicle rov kiel 6000 and revealed variously alternating sequences of magmatic rocks lavas pillow breccias tuffs and gabbros and sediment plains we report new 40ar 39ar age and geochemical major and trace element sr nd hf pb isotope data for the recovered magmatic samples although the 40ar 39ar analyses display disturbed age spectra they suggest an age range of 92 4 76 9 ma thus our age data show for the first time that the beata ridge also formed during the main magmatic stage of the clip 95 83 ma previous studies suggested that the beata ridge was formed during a second lower volume magmatic phase of the clip 81 71 ma possibly related to decompression melting during an extensional phase in the caribbean most samples display relatively flat chondrite normalized rare earth element ree patterns commonly observed throughout the clip but light ree enriched and depleted compositions are also present the occurrence of enriched and depleted incompatible element and radiogenic isotope signatures implies a heterogeneous mantle source region as is observed for other lips worldwide since a high degree of geochemical variability is observed over short stratigraphic intervals within the rov profiles melt homogenization did not operate as effectively as commonly assumed for lips instead the plume head probably preserved some domains of enriched and depleted components whereas most of the melts during the main stage have intermediate compositions with flat ree patterns representing mixtures of the enriched and depleted components

new geological and geochronological data from the colombian andes continental growth by multiple accretion the regional geology of colombia consists of a composite palaeozoic to cenozoic sedimentary basin overlying precambrian shield rocks which are exposed in the eastern cordillera magdalena valley and at least part of the extensive llanos plains a palaeozoic metamorphic schist belt with abundant post metamorphic igneous rocks comprises the central cordillera a mesozoic volcanic suite extends from the western flank of the central cordillera and accounts for the bulk of the western cordillera a cenozoic sedimentary sequence probably overlying a late cretaceous oceanic crust and fore arc complex forms the pacific coastal plain the continental edge of central and southern colombia is interpreted as a composite margin made up of successively accreted oceanic island arc related sequences of palaeozoic early mesozoic and late cretaceous ages the present day cycle which began in the miocene is not associated with island arc volcanism

the contribution of geochemistry to ancient harbor geoarcheology the example of ostia antica abstract once trapped in ancient harbor basins sediments form environmental archives that have been widely studied by geoarcheologists in recent decades especially to help reconstruct fluvio marine landscapes of the last millennia in some cases classic environmental markers cannot be used for this purpose either because of their scarcity in the sedimentary deposits or because analytical costs limit the resolution that can be achieved in order to remedy these shortcomings and to complement the more commonly used proxies elemental and isotopic geochemistry has been added to the geoarcheological toolkit here we show how to read the evolution of the paleo environmental dynamics in the water column of ostia antica rome s first maritime harbor using the geochemical and isotopic record of a 3000 year old sediment core drilled in the ancient harbor basin a comparison of the results obtained from ostia antica with those of other ancient mediterranean harbors reveals the nature of the main environmental processes operating during the formation of sedimentary deposits in harbor basins from this comparative approach it appears that the respective weight of each control factor is dependent on the coastal geomorphological context of the sites where the harbors were established since the discovery of the harbor of ostia antica in 2014 this method has provided the means for the first time to identify two distinct harbor basin regimes an initial marine dominated regime from the middle of the 4th c bc to the 3rd c bc and a later freshwater dominated regime up to the 2nd c bc more generally we observe the effects of the dynamics of the deltaic progradation of the tiber which very early on was subject to a hydro climatic component on the processes of alluviation of the harbor basin additionally and also for the first time in harbor geoarcheology pb isotope compositions measured specifically on uncontaminated sediments demonstrate their utility for both identifying the geological sources of the sediments of the tiber delta and discriminating finer from larger particles the present study further provides an opportunity to test the validity of two hypotheses recently put forward 1 that a series of three tsunamis is recorded in the harbor silts and 2 that an initial lagoon type harbor was constructed at ostia antica which later evolved into a fluvial harbor neither of these hypotheses are supported by the present data

distinguishing between in situ and accretionary growth of continents along active margins abstract active continental margins represent both sites of crustal loss by tectonic erosion and gain by juvenile magmatism and oceanic arc plateau accretion estimating the gain component depends on distinguishing between growth of margins by either accretion of oceanic crust or juvenile magmatic addition during extension distinguishing between these allochthonous and autochthonous models is frequently ambiguous due to difficulties in interpreting the tectonic origin of mafic rocks we present a detailed geochronological and isotopic study constraining the evolution of the continental margin of northwest south america during 190 113 ma semi continuous slab rollback resulted in crustal thinning juvenile magmatism and ultimately the removal of continental slivers negating previous hypotheses that the margin evolved by the accretion of an island arc however basic crustal growth estimates along with comparisons between rocks exposed in northern peru and the northern andes imply that uneven losses of continental crust occurred along northwest south america for mesozoic exposures and that the accretion of the caribbean large igneous province clip onto nw south america probably played a crucial role in maintaining and preserving mesozoic continental crust north of the huancabamba deflection

allochthonous terranes in northwestern ecuador abstract formations and lithostratigraphic units of the ecuadorian northwestern cordillera can be divided into three major groups the first group comprises lower cretaceous e morb basalts conformably overlain by tholeiitic to boninitic components of an ensimatic island arc the second group consists mainly of tholeiitic to calc alkaline volcanic and volcano sedimentary island arc deposits unconformably on top of the first group and of eocene to early oligocene age the third group is represented by calc alkaline continental volcanics ranging from late oligocene to recent two main tectonic events occur during early eocene and middle oligocene times the first event can be interpreted as an accretion of a slab of oceanic basalt with its overlying arc material the suture zone is marked by slices of morb type material delineating the southward extension of the colombian cauca patia fault zone the second event is related to a major rearrangement of plate tectonic regimes in the eastern pacific during oligocene times its nature is most probably transpressional accompanied by thrusting

geochemistry and tectonic setting of pre collision cretaceous and paleogene volcanic rocks of ecuador pre collision cretaceous lower tertiary volcanic rocks of western ecuador from the coastal area pinon and san lorenzo formations and western cordillera macuchi and celica formations consist of three rock suites the first suite includes tholeiites of ocean floor affinities that have distinct light ree depleted patterns and low la nb la hf and th hf ratios the island arc tholeiites of the second suite have lower contents of ti zr hf and nb the third suite comprises calc alkalic rocks mainly continental margin andesites and dacites the continental volcanic arc rocks crop out in an area east of the tholeiitic rocks the boundary between the two areas is the major suture separating continental south america from accreted terrenes to the west a model is suggested for the geodynamic evolution of what is now western ecuador during the late cretaceous it includes two east dipping subduction zones of turonian to santonian ages and the accretion of the oceanic island arc to the continent

the early cretaceous san juan plutonic suite ecuador a magma chamber in an oceanic plateau sections through an oceanic plateau are preserved in tectonic slices in the western cordillera of ecuador south america the san juan section is a sequence of mafic ultramafic cumulates to establish that these plutonic rocks formed in an oceanic plateau setting we have developed criteria that discriminate intrusions of oceanic plateaus from those of other tectonic settings the mineralogy and crystallization sequence of the cumulates are similar to those of intra plate magmas clinopyroxene predominates throughout and orthopyroxene is only a minor component rocks of intermediate composition are absent and hornblende is restricted to the uppermost massive gabbros within the sequence the ultramafic cumulates are very depleted in light rare earth elements lree whereas the gabbros have flat or slightly enriched lree patterns the composition of the basaltic liquid in equilibrium with the peridotite calculated using olivine compositions and ree contents of clinopyroxene contains between 16 and 8

oceanic plateau model for continental crustal growth in the archaean a case study from the kostomuksha greenstone belt nw baltic shield abstract field studies combined with chemical and isotope data indicate that the kostomuksha greenstone belt in the nw baltic shield consists of two lithotectonic terranes one mafic igneous and the other sedimentary separated by a major shear zone the former contains submarine komatiite basalt lavas and volcaniclastic lithologies and the latter is composed of shelf type rocks and bif komatiitic and basaltic samples yield sm nd and pb pb isochron ages of 2843 39 and 2813 78 ma respectively their trace element compositions resemble those of recent pacific oceanic flood basalts with primitive mantle normalized nb th of 1 5 2 1 and nb la of 1 0 1 5 this is in sharp contrast with island arc and most continental magmas which are characterized by nb th la n 1 calculated initial nd isotope compositions nd t 2 8 to 3 4 plot close to an evolution line previously inferred for major orogens momo which is also consistent with the compositions of recent oceanic plateaux the high liquidus temperatures of the komatiite magmas 1550 c and their al depleted nature require an unusually hot 1770 c mantle source for the lavas 200 c hotter than the ambient mantle at 2 8 ga and are consistent with their formation in a deep mantle plume in equilibrium with residual garnet this plume had the thermal potential to produce oceanic crust with an average thickness of 30 km underlain by a permanently buoyant refractory lithospheric mantle keel nb u ratios in the komatiites and basalts calculated on the basis of th u pb relationships range from 35 to 47 and are thus similar to those observed in modern morb and oib this implies that some magma source regions of the kostomuksha lavas have undergone a degree of continental material extraction comparable with those found in the modern mantle the mafic terrane is interpreted as a remnant of the upper crustal part of an archaean oceanic plateau when the newly formed plateau reached the active continental margin its upper part collided with the sedimentary terrane but was too buoyant to subduct as a result the volcanic section of the plateau was imbricated and obducted thus becoming a new segment of continental crust the deeper zones were delaminated and tectonically underplated or subducted

interactions between volcanism and geodynamics in the southern termination of the ecuadorian arc abstract this study focuses on the construction and evolution through time of volcanic edifices located in the southern part of the ecuadorian arc with the objective to contribute to a better understanding of the interactions between magmatism slab geometry and the activity of tectonic faults our new groundmass k ar ages obtained for a dozen volcanoes from the southern quaternary arc are rather young without ages older than 800 ka and highlight an increasing volcanic activity between 300 and 100 ka these new temporal constraints suggest that a southward migration of the ecuadorian arc occurred during the last 600 ka we interpret this evolution as the result of major and recent geodynamic reorganization in ecuador related to the activation of lithospheric faults and to the flexure of the slab at depth following the inland prolongation of the grijalva fracture zone both phenomena could have been induced by the oblique subduction of the nazca plate towards the convex margin of the south american continent and the coupling of these two plates along the wadati benioff zone progressively intensified by the carnegie ridge subduction southward migration of the slab flexure at depth could have changed pressure and temperature conditions favoring the partial melting of the mantle wedge and magma genesis thereby inducing a southward migration of the ecuadorian arc volcanoes these results contribute therefore to a better knowledge of the current arc dynamics in the northern andes and provide insights into the use of geochronological data applied to volcanic rocks for studies of past tectonic activity in ecuador

cenozoic stages of forearc evolution following the accretion of a sliver from the late cretaceous caribbean large igneous province sw ecuador nw peru

accreted oceanic terranes in ecuador southern edge of the caribbean plate the western part of ecuador is made of several oceanic terranes which comprise two oceanic plateaus of early 120 ma and late cretaceous age 90 ma respectively the older oceanic plateau was accreted to the andean margin in the late campanian 75 ma fragments of the turonian coniacian plateau were accreted to the ecuadorian margin in the late maastrichtian 68 ma guaranda terrane and late paleocene 58 ma pinon naranjal terrane the guaranda terrane received either fine grained oceanic sediments of coniacian maastrichtian age or island arc back arc volcanic suites of middle campanian middle maastrichtian age the pinon naranjal terrane recorded a comparable history completed in the maastrichtian paleocene either by pelagic cherts or by island arc products macuchi arc the late cretaceous plateau of ecuador is interpreted as part of the caribbean oceanic plateau cop because their evolutions are comparable if so the cop was not formed by the galapagos hotspot but on the farallon oceanic plate south of ecuador and close to the south american margin the cop belonged to the farallon plate until a subduction zone separated both plates in the middle campanian giving way to a well developed mid campanian mid maastrichtian island arc accretion in the late maastrichtian triggered a change in the subduction system and the development of a new arc system of late maastrichtian late paleocene age which crosscut the south america cop plate boundary the last accretion occurred in the late paleocene

variability of nb u and th la in 3 0 to 2 7 ga superior province ocean plateau basalts implications for the timing of continental growth and lithosphere recycling extensive volcanic sequences of tholeiitic basalts having near flat ree patterns and spatially associated with komatiites occur in many archean superior province greenstone belts they are considered to be fragments of intraoceanic volcanic plateaus derived from a plume basalts from the 2 9 3 0 ga lumby lake greenstone belt carefully screened for minimum alteration have variable nb u ratios of 36 to 58 least altered basalts from the abitibi and wawa greenstone belts also have variable nb u ratios of 25 to 50 and 28 to 42 respectively compared to an average value of 47 for modern ocean basalts in the abitibi suite nb u correlates positively with nb lapm but negatively with th lapm alteration can be ruled out as the cause of nb u variation as there are no correlations of nb u with loi or eu eu and nb u correlates with nb th in all three suites of basalts numerous lines of evidence indicate that crustal contamination can be eliminated as the cause of nb u variability especially for samples with nb u 36 high nb u ratios can be explained by recycling ocean crust processed through a subduction zone high nb u and nb lapm low th lapm into the mantle source of the basalts whereas low nb u ratios can be accounted for by recycling complementary subarc mantle lithosphere or continental crust low nb u and nb lapm high th lapm into the mantle source for the entire population of basalts th lapm ratios generally 1 may result from recycling the residue of slab derived archean type tonalites having high th la ratios nb u ratios as high as the 47 10 range of modern ocean basalts have been found in a 2 7 ga volcanic belt of the yilgarn craton taken together the results signify very early growth of the continental crust rather than episodic growth over several ga

pre orogenic volcanic assemblages and structure in the western cordillera of ecuador between l 40 s and 2 20 s a major reverse fault with a separation of at least a few thousand meters is exposed in the western cordillera of ecuador near 2 s ree and other incompatible elements patterns show i that the rocks exposed in the upthrust block to the east of the fault aremorb that had not yet been identified and ii that a volcanic island arc assemblage extensively outcrops in the downthrust block west of the fault

geometrical control of subduction related magmatism the mesozoic and cenozoic plutonic history of western colombia radiometric age data from western colombia combined with geological evidence allow five main plutonic episodes ranging in age from triassic to tertiary to be recognized it is suggested that variation in the convergence angle of the oceanic plate relative to the continental edge was an important factor affecting the timing and spatial distribution of plutonic activity on a regional scale major breaks in activity are probably best attributed to either low angle parallel convergence or periods of accretion along the convergent margin

cretaceous and tertiary terrane accretion in the cordillera occidental of the andes of ecuador new field geochronological geochemical and biostratigraphical data indicate that the central and northern parts of the cordillera occidental of the andes of ecuador comprise two terranes the older pallatanga terrane consists of an early to late cretaceous oceanic plateau suite late cretaceous marine turbidites derived from an unknown basaltic to andesitic volcanic source and a tectonic melange of probable late cretaceous age the younger macuchi terrane consists of a volcanosedimentary island arc sequence derived from a basaltic to andesitic source a previously unidentified regionally important dextral shear zone named the chimbo toachi shear zone separates the two terranes regional evidence suggests that the pallatanga terrane was accreted to the continental margin the already accreted cordillera real in campanian times producing a tectonic melange in the suture zone the macuchi terrane was accreted to the pallatanga terrane along the chimbo toachi shear zone during the late eocene probably in a dextral shear regime the correlation of cretaceous rocks and accretionary events in the cordillera occidental of ecuador and colombia remains problematical but the late eocene event is recognised along the northern andean margin

quaternary sanukitoid like andesites generated by intracrustal processes chacana caldera complex ecuador implications for archean sanukitoids high mg diorites enriched in incompatible elements and their extrusive equivalents are rare subduction related rock types that have been found in modern arc settings and in late archean sequences where they are associated with trondhjemite tonalite granodiorite ttg suites archean rocks with these geochemical features are known as sanukitoids and despite their limited abundance are considered to be the indicators of the onset of modern plate tectonics because of their similarities to modern subduction related high mg andesites and diorites understanding the genesis of sanukitoid rocks is thus an essential step towards understanding crustal growth processes the accepted petrogenetic models for modern enriched high mg andesites and their archean equivalents the sanukitoids consist of metasomatic enrichment of the mantle wedge by slab components and its subsequent partial melting or the modification of siliceous slab components through continuous reaction with mantle peridotite during their ascent through the mantle wedge we present new data on the petrography mineral chemistry and whole rock geochemistry major and trace elements and sr nd pb isotopes of andesitic rocks from an 30 ka lava flow yuyos flow from the chacana caldera complex eastern cordillera of ecuador these rocks show a remarkable geochemical affinity with archean sanukitoids including high magnesium numbers 0 58 0 63 accompanied by high contents of incompatible elements e g th 17 23 ppm u 6 7 5 ppm ba 1600 1800 ppm sr 1430 1565 ppm la 74 94 ppm additionally the sanukitoid like andesites of yuyos are associated with predominant silica rich adakite like andesites which are widespread throughout the quaternary arc of ecuador this makes the quaternary ecuadorian magmatic province a close equivalent of the archean ttg sanukitoid association the bulk rock geochemistry petrography and mineral chemistry data indicate that the sanukitoid like features of the andesites of the yuyos flow derive from intracrustal recycling of the felsic intermediate to mafic ultramafic roots of the quaternary volcanic arc of ecuador by normal mantle derived basaltic magmas with the geochemical characteristics of continental arc basalts or high alumina basalts in view of the similarities between the yuyos andesites and archean sanukitoids in terms of geochemistry and lithological association we suggest that genetic models should consider the possibility of intracrustal recycling as a process responsible for the peculiar signatures of both archean sanukitoids and modern enriched high mg andesites

the geology and mesozoic collisional history of the cordillera real ecuador abstract the geology of the metamorphic rocks of the cordillera real of ecuador is described in terms of five informal lithotectonic divisions we deduce that during the mesozoic repeated accretionary events occurred and that dextral transpression has been of fundamental importance in determining the tectonic evolution of this part of the northern andes the oldest event recognised of probable late triassic age may be related to the break up of western gondwana and generated a regional belt of s type plutons during the jurassic major calc alkaline batholiths were intruded following this in latest jurassic to early cretaceous time a volcano sedimentary terrane of possible oceanic or marginal basin origin the alao division and the most westerly gneissic chaucha arenillas terrane were accreted to continental south america the accretion of the oceanic western cordillera took place in latest cretaceous to earliest tertiary time this latter event coincided with widespread thermal disturbance as evidenced by the large number of young k ar mineral ages recorded from the cordillera real

the nature and provenance of accreted oceanic terranes in western ecuador geochemical and tectonic constraints western ecuador consists of a complex tectonic melange of oceanic terranes accreted to the continental margin from late cretaceous to eocene time new geochemical data from these accreted terranes arising from a 5 year british geological survey mapping programme indicate that they comprise rocks from a variety of oceanic tectonic settings from thickened and relatively unsubductable oceanic plateau basalts through island arc tholeiites with occasional more calc alkaline lavas to back arc basin basalt sequences this study has enabled us to construct a new geodynamic model for the cretaceous tertiary evolution of the northern andes and has placed important new constraints on the extent of oceanic plateau sequences in colombia and around the caribbean the age and nature of sediments combined with evidence for the age of peak metamorphism suggests that a prolonged 15 20 ma accretionary event occurred in late cretaceous time and involved the collision of an oceanic plateau represented by the pallatanga unit with the continental margin this accreted unit can be correlated with similar oceanic plateau sequences from the western cordillera of colombia and those within and around the caribbean region the naranjal and macuchi island arcs and the associated la portada back arc basin developed along the accreted margin from late campanian to eocene time and these arcs accreted to the continental margin along with oceanic plateau material represented by the pinon unit and pedernales esmeraldas sequences during eocene time the development of island arcs which separate the two accretionary events implies that the most westerly coastal oceanic plateau sequences both in ecuador pinon and pedernales esmeraldas and in colombia gorgona and serrania de baudo cannot belong to the caribbean colombian oceanic plateau ccop it therefore appears that at least two different oceanic plateaux are preserved within the accreted oceanic terranes of the northern andes it is possible that the ccop formed over the galapagos hotspot as previously proposed but the more westerly coastal plateau was derived from a more southerly hotspot source region such as sala y gomez in the se pacific

the geological history of northwestern south america from pangaea to the early collision of the caribbean large igneous province 290 75ma abstract northwestern south america preserves a record of the assembly of western pangaea its disassembly and initiation of the far western tethys wilson cycle subsequent pacific margin magmatism and ocean plateau continent interaction since the late cretaceous numerous models have been presented for various time slices although they are based on either spatially restricted datasets or dates that are inaccurate estimates of the time of crystallisation here we review a very large quantity of geochronological geochemical thermochronological sedimentological and palaeomagnetic data that collectively provide tight constraints for geological models these data have been collected over a trench pacific parallel distance of 1500 km colombia and ecuador and reveal important temporal trends in rifting and subduction the temporal framework for our model constraints are obtained from robust concordant zircon u pb ages of magmatic rocks during 290 75 ma the late cretaceous thermal history of the margin

provenance of the upper cretaceous to upper eocene clastic sediments of the western cordillera of ecuador geodynamic implications abstract the late cretaceous eocene clastic deposits of the western cordillera of ecuador record significant changes in the source areas grain size and location of the depocenters related to the accretion of oceanic terranes that constitute the present day western cordillera and coast major changes in the source areas occurred in the late maastrichtian and late middle eocene they are interpreted as corresponding to the accretion of the guaranda and macuchi oceanic terranes respectively major increases in the grain sizes occurred in the late maastrichtian late paleocene and late middle eocene and seem to coincide with the accretion of the guaranda pinon and macuchi terranes respectively the increasing occurrence of plutonic or metamorphic fragments and the westward shift of the depositional areas through the paleocene upper eocene interval indicate an increasing uplift and erosion of the cordillera real continuous although jerky uplift of the latter during the maastrichtian eocene period supports the idea that the accreted oceanic material contributed to the crustal thickening and relief creation of the ecuadorian andes

mafic pegmatites intruding oceanic plateau gabbros and ultramafic cumulates from bolivar colombia evidence for a wet mantle plume

modification of an oceanic plateau aruba dutch caribbean implications for the generation of continental crust the generation of the continental crust may be connected to mantle plume activity however the nature of this link and the processes involved are not well constrained an obstacle to understanding relationships between plume related mafic material and associated silicic rocks is that later tectonic movements are liable to obscure the original relationships particularly in ancient greenstone belts studies of younger analogous regions may help to clarify these relationships on the island of aruba in the southern caribbean a sequence of partly deformed mafic volcanic rocks intruded by a predominantly tonalitic batholith is exposed the mafic lavas show geochemical and isotopic affinities with other basaltic picritic and komatiitic rocks that crop out elsewhere in the caribbean these are well documented as belonging to an 88 91 ma plume related oceanic plateau which is allochthonous with respect to the americas and is thought to have been formed in the pacific region the 85 to 82 ma tonalitic rocks share some geochemical characteristics high sr and ba low nb and y with archaean tonalite trondhjemite granodiorite ttg suites field relationships suggest that deformation of the plateau sequence possibly related to collision with a subduction zone was synchronous with intrusion of the aruba batholith new incremental heating 40ar 39ar dates combined with existing palaeontological evidence show that cooling of the batholith occurred shortly after eruption of the plateau basalt sequence sr nd isotopic data for both rock suites are uniform 87sr 86sri 0 7035 endi 7 whereas pb isotopes are more variable plateau sequence 206pb 204pb 18 6 19 1 207pb 204pb 15 54 15 60 208pb 204pb 38 3 38 75 aruba batholith 206pb 204pb 18 4 18 9 207pb 204pb 15 51 15 56 208pb 204pb 38 0 38 5 this suggests that there has been a minor sedimentary input into the source region of the batholith however the limited time interval between basaltic and tonalitic magmatism makes a normal subduction related origin for the tonalites improbable instead models involving derivation of tonalite from partial melting of the plateau sequence or alternatively genesis in an unusual subduction zone environment are investigated

subducted lithospheric boundary tomographically imaged beneath arc continent collision in eastern indonesia this work was funded by the national science foundation nsf grant ear 1250214 as well as dikti grant 127 sp2h ptnbh drpm 2018

cretaceous basaltic terranes in western colombia elemental chronological and sr nd isotopic constraints on petrogenesis accreted terranes comprising mid to late cretaceous picrites basalts and dolerites occur in three north south trending belts in western colombia in the central cordillera western cordillera and along the pacific coast the geochemistry of these rocks is consistent with an oceanic plateau plume related origin and they most probably formed in the pacific as part of the caribbean oceanic plateau these igneous rocks display small but significant inter cordillera variations being younger and more depleted in incompatible trace element ratios and with more positive end values to the west the igneous rocks of the pacific coast serrania de baudo are dated at 73 78 ma 40ar 39ar and those of the western cordillera at 90 ma whereas the volcanics of the central cordillera are believed to be older than 100 ma most of the igneous rocks are basaltic and it is suggested that they have fractionated from picritic primary magmas generated by partial melting within a hot mantle plume variable and positive end values reveal that the plume must have been heterogeneous originating from a mantle source with a long term history of depletion partial melt modelling suggests that the composition of the basalts requires at least some input from a mantle source region containing garnet and that the extent of partial melting required to reproduce the composition of the erupted basalts is of the order of 20 mixing of melts from different depths either in the mantle melting column or during fractionation in lithospheric magma chambers can explain the relative homogeneity of basaltic lavas erupted to form this and other oceanic plateaux the caribbean colombian oceanic plateau may have formed at an oceanic spreading centre and valuable comparisons can be made between iceland and the caribbean colombian plateau

multiple plume events in the genesis of the peri caribbean cretaceous oceanic plateau province the oceanic crust fragments exposed in central america in north western south america and in the caribbean islands have been considered to represent accreted remnants of the caribbean colombian oceanic plateau ccop on the basis of trace element and nd sr and pb isotopic compositions we infer that cumulate rocks basalts and diabases from coastal ecuador have a different source than the basalts from the dominican republic the latter suite includes the 86 ma basalts of the duarte complex which are light rare earth element ree enriched and display relative to normal mid ocean ridge basalts nmorb moderate enrichments in large ion lithophile elements together with high nb ta pb and low th contents moreover they exhibit a rather restricted range of nd and pb isotopic ratios consistent with their derivation from an ocean island type mantle source the composition of which includes the himu high 238u 204pb component characteristic of the galapagos hotspot in contrast the 123 ma ecuadorian oceanic rocks have flat ree patterns and relative to nmorb are depleted in zr hf th and u moreover they show a wide range of nd and pb isotopic ratios intermediate between those of ocean island basalts and nmorb it is unlikely on geochemical grounds that the plume source of the ecuadorian fragments was similar to that of the galapagos in addition because of the nne motion of the farallon plate during the early cretaceous the ecuadorian oceanic plateau fragments could not have been derived from the galapagos hotspot but were likely formed at a ridge centered or near ridge hotspot somewhere in the se pacific

along strike variations in the thermal and tectonic response of the continental ecuadorian andes to the collision with heterogeneous oceanic crust oblique to strike geological segmentation in the andean chain has been previously recognised at various scales and is commonly attributed to changes in the convergence vectors of the oceanic and continental plates as well as the upperplate expressions of differing along strike subducted slab age strength and composition we present new white mica and biotite 40 ar 39 ar and zircon and apatite fission track data from several traverses across the cordillera real of ecuador in the northern andes that reveal distinct along strike differences in the timing of accelerated crustal cooling during the cenozoic the data record elevated cooling rates from temperatures of v380 c during v65 55 and v43 30 ma from all sampled regions of the cordillera real and at v15 ma and since v9 ma in the northern cordillera real each cooling period was probably driven by exhumation in response to the accretion and subduction of heterogeneous oceanic crust elevated cooling rates of up to v30 20 c myr were initiated during the palaeocene and eocene early oligocene along the entire contemporaneous margin of ecuador and were driven by the accretion of the oceanic pallatanga terrane and pin on macuchi block respectively onto northwestern south america both of these geological provinces originated at the southern parts of the leading and trailing boundaries of the caribbean plateau and accreted onto the margin during the approximately northeastward migration of the plateau into its current position within ecuador the development of higher topography and elevated cooling rates of up to 50 c myr at v15 ma and since v9 ma are restricted to the region north of 1 30ps and is situated above the postulated subducted flatslab section of the aseismic carnegie ridge plate convergence rate calculations suggest the carnegie ridge collided with the ecuador trench at v15 ma which caused the pre existing coastal provinces to displace to the northeast subsequently driving extension and marine ingressions in southern ecuador and compression and uplift in northern ecuador fl 2001 elsevier science b v all rights reserved

partitioning of oblique convergence in the northern andes subduction zone migration history and the present day boundary of the north andean sliver in ecuador along the ecuadorian margin oblique subduction induces deformation of the overriding continental plate for the last 15 ma both exhumation and tectonic history of ecuador suggest that the northeastward motion of the north andean sliver nas was accompanied by an eastward migration of its eastern boundary and successive progressively narrowing restraining bends here we present geologic data earthquake epicenters focal mechanisms gps results and a revised active fault map consistent with this new kinematic model all data sets concur to demonstrate that active continental deformation is presently localized along a single major fault system connecting fault segments from the gulf of guayaquil to the eastern andean cordillera although secondary faults are recognized within the cordillera they accommodate a negligible fraction of relative motion compared to the main fault system the eastern limit is then concentrated rather than distributed as first proposed marking a sharp boundary between the nas the inca sliver and the subandean domain overthrusting the south american craton the nas limit follows a northeast striking right lateral transpressional strike slip system from the gulf of guayaquil isla puna to the andean cordillera and with the north south striking transpressive faults along the eastern andes eastward migration of the restraining belt since the pliocene abandonment of the sutures and reactivation of north south striking ancient fault zones lead to the final development of a major tectonic boundary south and east of the nas favoring its extrusion as a continental sliver accommodating the oblique convergence of the nazca oceanic plate toward south america

permo triassic anatexis continental rifting and the disassembly of western pangaea crustal anatectites are frequently observed along ocean continent active margins although their origins are disputed with interpretations varying between rift related and collisional we report geochemical isotopic and geochronological data that define an 1500 km long belt of s type meta granites along the andes of colombia and ecuador which formed during 275 223 ma these are accompanied by amphibolitized tholeiitic basaltic dykes that yield concordant zircon u pb dates ranging between 240 and 223 ma a model is presented which places these rocks within a compressive permian arc setting that existed during the amalgamation of westernmost pangaea anatexis and mafic intrusion during 240 223 ma are interpreted to have occurred during continental rifting which culminated in the formation of oceanic crust and initiated the break up of western pangaea compression during 275 240 ma generated small volumes of crustal melting rifting during 240 225 ma was characterized by basaltic underplating the intrusion of tholeiitic basalts and a peak in crustal melting tholeiitic intrusions during 225 216 ma isotopically resemble depleted mantle and yield no evidence for contamination by continental crust and we assign this period to the onset of continental drift dissected ophiolitic sequences in northern colombia yield zircon u pb dates of 216 ma the permo triassic margin of ecuador and colombia exhibits close temporal faunal and geochemical similarities with various crustal blocks that form the basement to parts of mexico and thus these may represent the relict conjugate margin to nw gondwana the magmatic record of the early disassembly of pangaea spans 20 ma 240 216 ma and the duration of rifting and rift drift transition is similar to that documented in cretaceous tertiary rift settings such as the west iberia newfoundland conjugate margins and the taupo lau havre system where rifting and continental disassembly also occurred over periods lasting 20 ma

the petrogenesis of gorgona komatiites picrites and basalts new field petrographic and geochemical constraints gorgona island colombia is remarkable not only because it contains the only phanerozoic komatiites but also because it has mafic to ultramafic lavas with a wide range of compositions from moderately enriched to extremely depleted relative to bulk earth the komatiite flows are in many respects similar to archaean komatiites they formed from mgo rich 18 liquids and have upper spinifex zones and lower cumulate zones the cumulate zones of archaean komatiites contain many solid grains in contrast more than 90 of the olivine in the gorgona cumulates is highly skeletal this combined with the fact that the gorgona cumulate zones are thinner than those in archaean komatiites suggests that the komatiite magma became strongly superheated en route to the surface the komatiites have trace element contents intermediate between those of the basalts and the ultramafic tuffs some basalts have isotope compositions indicative of long term enrichment in incompatible elements whereas other basalts and ultramafic volcanics have isotopic signatures that imply corresponding depletion it is apparent that the plume source region of the gorgona magmas was markedly heterogeneous with at least two source components contributing to the observed variation in composition this heterogeneity may have resulted from the incorporation of different components into the plume source or it may be the result of complex melting and melt extraction processes during the ascent of a heterogeneous plume despite earlier suggestions that there may have been a significant age gap between depleted komatiite and basalt flows and the enriched basal new 4oar 39ar dating of basalts and gabbros are more consistent with all being generated at 87 ma during formation of the caribbean colombian plateau possibly at the galapagos hotspot