niobium enriched basalts from the wabigoon subprovince canada evidence for adakitic metasomatism above an archean subduction zone abstract late archean niobium enriched basalts from the central sturgeon lake assemblage and neepawa group of the western wabigoon subprovince have mantle normalized nb la between 0 8 and 1 3 and zr y between 4 and 7 they are compositionally similar to basalts attributed to adakite metasomatism of mantle wedge regions in cenozoic subduction zones sajona et al j petrol 37 1996 693 726 in detail their sc ree systematics suggest the archean basalts were generated above the garnet stability field an association with adakite like volcanic rocks an absence of komatiites and the arc like attributes of their host sequences suggest a subduction related origin for the basalts if current models of adakite and niobium enriched basalt genesis are valid then additional examples of these rocks should be relatively common in other archean greenstone belts

double flood basalts and plume head separation at the 660 kilometer discontinuity several of the world s flood basalt provinces display two distinct times of major eruptions separated by between 20 million and 90 million years these double flood basalts may occur because a starting mantle plume head can separate from its trailing conduit upon passing the interface between the upper mantle and the lower mantle this detached plume head eventually triggers the first flood basalt event the remaining conduit forms a new plume head which causes the second eruptive event the second plume head is predicted to arrive at the lithosphere at least 10 million years after the first plume head in general agreement with observations regarding double flood basalts

the southern margin of the caribbean plate in venezuela tectono magmatic setting of the ophiolitic units and kinematic evolution abstract the southern caribbean plate margin in venezuela consists of a w e elongated deformed belt composed of several tectonic units dismembered along the northern part of the south america continental plate since the late cretaceous the present review based on petrology and tectono magmatic significance of each unit makes it possible to define the main geotectonic elements and to reconstruct the paleogeographic domains from late jurassic to tertiary a mid ocean ridge basalt morb proto caribbean oceanic basin loma de hierro unit b oceanic plateau dutch and venezuelan islands basement c rifted continental margin cordillera de la costa and caucagua el tinaco units with within plate tholeiitic wpth magmatism d an intra oceanic subduction zone represented by island arc tholeiitic iat magmatism villa de cura and dos hermanas units of early cretaceous age e an early cretaceous ocean continent subduction trench filled by melange franja costera f a new intra oceanic subduction zone represented by the tonalitic arc magmatism of late cretaceous age dutch and venezuelan islands regional tectonic constraints and coherent kinematic reconstruction suggest an original near mid america location of the jurassic cretaceous proto caribbean oceanic realm from early to late cretaceous one sub continental subduction with melanges franja costera unit and two main stages of intra oceanic arc magmatism are recorded in the so called eo caribbean phases the first consists of generally metamorphosed and deformed volcano plutonic sequences with iat affinity villa de cura and dos hermanas units probably in relation to a southeastward dipping subduction the second is mainly represented by generally unmetamorphosed tonalitic intrusives cutting the oceanic plateau in the dutch and venezuelan islands and related to the new intra oceanic subduction with reverse lithospheric sinking the latter probably marked the onset of the aves lesser antilles arc system in the late cretaceous

geochronology geochemistry and tectonic evolution of the western and central cordilleras of colombia abstract autochthonous rocks of the pre cretaceous continental margin of nw south america the tahami terrane are juxtaposed against a series of para autochthonous rock units that assembled during the early cretaceous allochthonous oceanic crust of the caribbean large igneous province collided with and accreted onto the margin during the late cretaceous we present the first regional scale dataset of zircon u pb la icp ms ages for intrusive and metamorphic rocks of the autochthonous tahami terrane early cretaceous igneous para autochthonous rocks and accreted oceanic crust the u pb zircon data are complemented by multiphase 40ar 39ar crystallization and cooling ages the geochronological data are combined with whole rock major oxide trace element and ree data acquired from the same units to constrain the tectonic origin of the rock units and terranes exposed in the western cordillera cauca patia valley and the central cordillera of colombia the tahami terrane includes lower paleozoic orthogneisses 440 ma that may have erupted during the active margin stage of the rheic ocean basement gneisses were intruded by permian continental arc granites during the final assembly of pangea triassic sedimentary rocks were subsequently deposited in rift basins and partially melted during high t metamorphism associated with rifting of western pangea during 240 220 ma continental arc magmatism during 180 145 ma is preserved along the whole length of the central cordillera and was followed by an early cretaceous out board step of the arc axis and the inception of the quebradagrande arc that fringed the continental margin back stepping of the arc axis may have been caused by the collision of buoyant seamounts which were coeval with plateau rocks exposed in the nicoya peninsular of costa rica rapid westward drift of south america closed the quebradagrande basin in the late aptian and caused medium high p t metamorphic rocks of the arquia complex to exhume and obduct onto the continental margin subduction beneath hot spot derived rocks of the caribbean plateau 100 92 ma formed an intra oceanic arc 92 75 ma which collectively comprise the late cretaceous caribbean large igneous province the remnant ocean basin located between south america and the caribbean large igneous province was partly consumed via continental subduction forming the large antioquia batholith the caribbean large igneous province collided and accreted to south america during 75 70 ma along the cauca almaguer fault resulting in the cessation of both arcs and the paleocene onset of subduction beneath the accreted oceanic crust

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

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

mesozoic oceanic terrane in the central andes of colombia a suspect terrane exists along the western flank of the central cordillera in the colombian andes it consists of sea floor of late jurassic or early cretaceous age that was accreted onto the continental edge at about 125 130 ma along the romeral fault zone during a pause between the jurassic and cretaceous cycles of plutonism

discovery of lower cretaceous synmetamorphic thrust tectonics in french lesser antilles la desirade island guadeloupe implications for caribbean geodynamics located east of guadeloupe the island of la desirade exhibits the oldest rocks of the lesser antilles arc and the eastern caribbean plate an old magmatic basement is composed of 1 late jurassic ophiolitic complex with meta basaltic pillow lavas and interbedded radiolarites 2 acid igneous complex comprising meta quartz diorite and meta rhyolitic lavas flows and 3 meta diabasic microdioritic dyke swarm complex we present and discuss the discovery of synmetamorphic thrust tectonics in this island based on detailed structural analysis we evidenced two main compressive events the first event d1 is characterized by pervasive folding associated with thrust development in the northeastern part of the island a major thrust fault the grand abaque thrust has been discovered which displaces for several kilometers the acid igneous complex onto the ophiolitic unit the second deformation event d2 is featured by a main pervasive cleavage s2 observed at the regional scale associated with upright folding f2 and by the development of two sets of conjugated steeply dipping strike slip shear zones dextral 130 n and sinistral 20 n striking respectively microstructural observations indicate that greenschist facies metamorphism is clearly contemporaneous with the development of d1 and d2 superimposed tectonic structures ar ar geochronology was performed on two bulk of adularia minerals sampled in the northeast volcanic complex within a deformed zone related to a d2 event these analyses yield well defined and concordant plateau ages at 106 2 1 7 ma and 107 2 1 8 ma thus shortening and thickening tectonics occurred during lower cretaceous albian in the available framework of caribbean geodynamics the synmetamorphic thrust tectonics we discovered in the french lesser antilles is related to the collision between an aptian albian oceanic plateau and the andean cordilleran east dipping subduction zone our results support subduction polarity reversal during lower cretaceous

mantle plumes and continental tectonics abstract it is now recognised that plumes originate from a thermal boundary layer deep in the mantle a new plume consists of a large head with a diameter of approximately 800 to 1200 km and it is followed by a smaller tail which has a diameter of 100 to 200 km at the top of its ascent the head flattens to form a disk 2000 to 2500 km in diameter that may melt to produce a continental flood basalt or oceanic plateau the tail melts to produce chains of volcanic activity such as the hawaiian emperor chain the ascent of a new plume leads to surface uplift of 500 to 1000 m over a length scale of 2000 km this places the lithosphere under tension but the horizontal forces produced do not result in runaway extension unless the continental lithosphere is already anomalously hot and therefore weak if the lithosphere is cold and strong slow extension leading to a new spreading ridge may postdate plume arrival by 10 to 40 m y the time difference being the time taken to conduct heat from the plume to the overlying lithosphere plume heads may control the timing and position of new ocean ridges but are not the fundamental force that drives plate tectonics the heat from plume tails may weaken the overlying plate and pin the position of spreading the ascent of a mantle plume can also have an important influence on subduction large oceanic plateaus such as the ontong java plateau are more buoyant than normal oceanic crust because they have a thicker basaltic layer and because they are underlain by a 200 km thick layer of hot material from a mantle plume head as a consequence oceanic plateaus are more difficult to subduct than normal oceanic crust this can lead to jamming of plates at subduction zones resulting in a flip in the sense of subduction as appears to have happened in the caribbean region or to obduction of the plateau finally when a plume head is emplaced beneath continental crust heat released from the plume may lead to melting of the overlying crust the granites associated with many greenstone belts and intercontinental magmatic provinces such as the paleozoic lachlan fold belt of eastern australia and the middle proterozoic anorogenic granites of the central and southwestern united states may have formed in this way

detrital zircon u pb geochronology of southern guerrero and western mixteca arc successions southern mexico new insights for the tectonic evolution of southwestern north america during the late mesozoic late jurassic cretaceous arc related volcaniclastic rocks from the southern guerrero and western mixteca terranes of mexico were analyzed by u pb detrital zircon geochronology laser ablation multicollector inductively coupled plasma mass spectroscopy to place constraints on the depositional history and provenance of the rocks pre middle jurassic basement rocks and sandstone from the upper cretaceous mexcala formation were also analyzed to defi ne the origin and provenance of the prevolcanic substratum and the time of accretion of guerrero composite terrane sequences data from the taxco taxco viejo teloloapan and arcelia assemblages indicate that the youngest 129 141 ma zircon fraction in each sequence was derived from local volcanic sources whereas older populations ca 247 317 365 459 530 617 712 878 947 964 1112 1188 1350 1420 1842 1929 2126 2439 and 2709 3438 ma show sediment infl ux from varied sources most likely through grain recycling the major zircon clusters in these sequences match the populations recorded in the nearby acatlan complex in contrast the huetamo sample is dominated by lower cretaceous ca 126 ma zircons of local volcanic provenance and the zihuatanejo sample contains zircon clusters ca 259 ca 579 and ca 947 1162 ma comparable to major populations recorded in the underlying arteaga complex a sample from the middle triassic middle jurassic arteaga complex at tzitzio contains zircon clusters ca 202 247 ca 424 ca 600 ca 971 and ca 2877 ma consistent with an ultimate derivation from both north american and south american sources the sample from the las ollas suite contains comparable zircon populations ca 376 475 ca 575 ca 988 1141 and ca 2642 2724 ma and it is interpreted to be part of the prevolcanic basement in contrast the youngest zircon cluster ca 105 ma in the mexcala formation coincides with the major volcanic events in the taxco taxco viejo teloloapan and arcelia assemblages whereas the older clusters ca 600 ca 953 ca 1215 ca 1913 and ca 2656 2859 ma broadly match the major populations recorded in rocks from the acatlan complex these new data combined with available geochemical and isotopic data indicate that the taxco taxco viejo arc assemblage developed on continental crust the acatlan complex is the most plausible candidate the teloloapan and arcelia arc assemblages were developed on oceanic crust as offshore arcs facing the acatlan complex the zihuatanejo terrane assemblages were developed on the arteaga complex and evidence no infl uence from the acatlan complex this suggests that these assemblages were formed farther away or in a restricted basin the guerrero composite and mixteca arc successions are coeval with the alisitos arc of northern mexico and in part with the nevada and klamath ranges of the southwestern united states and with the arc series from the greater and lesser antilles and northern south america data indicate that during late mesozoic time southwestern north america was a site of intensive volcanism in a complex arc trench system similar to that of the east pacifi c our data are consistent with a diachronic accretion of the guerrero composite terrane sequences beginning during late cenomanian time with the amalgamation of the teloloapan and probably the arcelia assemblages and fi nishing at the end of cretaceous time with the accretion of the zihuatanejo terrane assemblages

scratching the surface estimating dimensions of large igneous provinces a study of five major basaltic provinces including oceanic plateaus volcanic passive margins and continental flood basalts shows that they are voluminous constructions of extrusive igneous rock underlain by intrusive rock crustal thickness ranges from 20 to 40 km and lower crust is characterized by high 7 0 7 6 km s seismic velocities volumes and emplacement rates derived for two oceanic plateaus the ontong java and kerguelen broken ridge reveal short lived pulses of increased global crustal production and suggest an origin involving the lower mantle the ontong java rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid ocean ridge system despite the importance off large igneous provinces in studies of mantle dynamics and the global environment scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features

geochemistry of the mabujina complex central cuba implications on the cuban cretaceous arc rocks the margins of the caribbean plate are marked by cretaceous island arc basalts associated with accreted fragments of the cretaceous colombian caribbean oceanic plateau in cuba the cretaceous volcanic island arc rocks are in fault contact with the mabujina complex interpreted as an oceanic jurassic to early cretaceous arc basement with local island arc rocks the cuban cretaceous island arc consists of early to late cretaceous volcanic series associated with limestones while the pre albian arc rocks consist of tholeiitic basalts and rhyolites the post albian volcanic series is characterized by calc alkaline andesites the cretaceous lavas have sr and nd isotopic compositions similar to the intraoceanic arcs and the pb isotopic initial ratios are close to the east pacific rise mid ocean ridge basalt field according to our data the mabujina arc rocks are tholeiites and calc alkaline basalts developed in a jurassic and or early cretaceous intraoceanic island arc their nd sr and pb isotopic compositions indicate that they derive from a depleted mantle source contaminated by sediments this subduction magmatism is not related to the classic early cretaceous caribbean tholeiitic series but is similar to the late jurassic to early cretaceous guerrero arc terrane from mexico and may represent its southernmost extension thus the different tectonic units of central cuba cannot be easily correlated with those of hispaniola our data also indicate that two different island arcs were tectonically juxtaposed in central cuba the classical lower and upper cretaceous suites of the greater antilles arc and a jurassic to early cretaceous island arc suite with a pacific provenance

plume mantle source heterogeneity through time insights from the duarte complex hispaniola northeastern caribbean 1 located in the cordillera central of hispaniola the duarte complex offers an opportunity to study an on land fragment of the caribbean colombian oceanic plateau geochemical sr nd isotope and 40ar 39ar radiometric age data combined with detailed mapping have shown that the duarte complex includes two lithostratigraphic units composed of four geochemical groups of metavolcanic rocks group ia low ti high mg basalts and group ib high ti picrites and primitive high mg basalts occur interlayered in the lowermost levels of the lower unit group ii light rare earth element lree enriched picrites ferropicrites and high mg basalts forms the main lava sequence of the lower unit and group iii lree enriched fe ti basalts is present exclusively in the upper unit nd isotope and incompatible trace element patterns are diverse in the duarte complex metavolcanics and are consistent with mantle sources related to a heterogeneous plume mantle melt modeling suggests that an early extensive melting 10 20 of shallow mantle resulted in the formation of relatively depleted group i and ii magmas whereas the late more enriched group iii magmas were the product of deeper low degree 96 ma hence an early cretaceous phase of the caribbean colombian oceanic plateau construction is recorded in hispaniola

the petrology and geochemistry of cretaceous to recent volcanism in colombia the magmatic history of an accretionary plate margin the pacific margin of colombia records a complex history of subduction related magmatism from at least cretaceous times cretaceous lavas from the western cordillera and part of the central cordillera of the colombian andes exhibit some chemical characteristics typical of island arc tholeiites including low lil element abundances and flat to light depleted ree patterns but also have chemical and geological affinities with deposits from marginal basins and with transitional type ocean floor basalts an appraisal of the geological and chemical evidence leads to the conclusion that these rocks were erupted in an oceanic environment in close proximity to the continent a suite of basic rocks from the coastal cordillera of upper cretaceous to tertiary age has been interpreted elsewhere as abyssal tholeiites following accretion of the cretaceous sequences on to cratonic s america volcanism continued in the upper miocene with the eruption of basalts and basaltic andesites on the eastern flanks of the western cordillera this tholeiitic suite is lree and lil element enriched compared with the cretaceous lavas and represents the trenchward portion of an ensialic island arc sequence subduction related magmatism continued from the miocene with the formation of composite calc alkaline strato volcanoes mostly along the axis of the central cordillera these miocene to recent lavas are dominantly andesitic with strong lil and lree enrichment and 87 sr 86 sr ratios of 0 7042 0 7047

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

petrology and geotectonic development of the western ecuadorian andes the basic igneous complex abstract the basic igneous complex bic consists of several volcanic formations which form the ecuadorian western cordillera and coastal cordillera the rocks differ significantly from other cordillerian successions and form an isolated tectnic block the upper cretaceous to lower tertiary volcanic series of the ecuadorian basic igneous complex are of island arc type according to their petrography and geochemistry an outer tholeiitic island arc series parts of the pinon formation can be distinguished from a central calc alkalic series depleted in potassium macuchi celica and silante formations the chontal yunguilla and zapotillo formations as well as the upper cayo formation guayaquil cherts are interpreted to represent back arc sediments and sediments of small intra arc basins within a largely submarine island arc the cayo formation consists almost entirely of volcaniclastic and epiclastic rocks deposited on the oceanward side of the island arc which was situated close to the south american continent volcanic activity continued into early eocene times as revealed by k ar dating of the cayo and silante formations 52 ma a small back arc basin was filled with the sediments of the yunguilla formation similar to present day volcanically active island arcs e g aleutians and kuriles kamchatka the bic island arc was mostly underlain by oceanic crust although parts of it built on continental crust celica formation southern ecuador in the early tertiary the island arc was accreted to the south american continent cessation of volcanic activity and accretion to the continent may be explained by increased drift velocities of the lithospheric plates accompanied by a change in the geometry and stress in the subduction zone

pore water evolution in oilfield sandstones constraints from oxygen isotope microanalyses of quartz cement abstract oxygen isotope microanalyses of authigenic quartz in combination with temperatures of quartz precipitation constrained by fluid inclusion microthermometry and burial history modelling are employed to trace the origin and evolution of pore waters in three distinct reservoirs of the brae formation in the miller and kingfisher fields north sea oxygen isotope ratios of quartz cements were measured in situ in nine sandstone thin sections with a cameca ims 4f ion microprobe in conjunction with quartz cement paragenesis in the reservoirs constrained from textural and cathodoluminescence cl microscopy studies pore water evolution was reconstructed from the time of deposition of the sandstones in the upper jurassic until the present cl photomicrographs of quartz overgrowths in the brae formation sandstones show three cement zones a b and c which can be related to different oxygen isotope compositions 1 the earliest and thinnest zone a homogeneous cl pattern with probable 18 o values between 23 and 26 direct measurements were not possible precipitated in the sandstones at temperatures 18 o values between 15 and 18 precipitated in the sandstones most likely between 70 and 90 c 3 the third zone c homogeneous cl pattern and directly measured 18 o values between 16 and 22 precipitated in the sandstones most likely at temperatures 90 c calculated oxygen isotope compositions of pore waters show that zone a quartz cements and enclosing concretionary calcite precipitated from a meteoric type fluid 7 during shallow burial 18 o 7 to a more 18 o enriched fluid 18 o 4 as burial continued to 3 0 km data from zone c quartz cements are consistent with further fluid evolution from 18 o 4 to basinal type fluids with 18 o similar to the present day formation water oxygen isotope composition 0 6 at 4 0 km burial a similar pore water evolution can be derived for all three reservoirs studied indicating that hydrogeologic evolution was similar across sandstones of the whole brae formation the quartz cement zones observed in the brae formation sandstones and the pore water history derived for the area studied is analogous to published petrographic and pore water evolution data from the nearby brent group reservoirs and from reservoirs located in the haltenbanken area on the atlantic margin offshore norway considering quartz cement is a major porosity occluding phase in many reservoir sandstones and because pore waters both dissolve quartz and carry the dissolved silica to cementation sites the data presented are valuable for improving the understanding and prediction of reservoir quality development in sandstones globally

petrochemistry and tectonic significance of cretaceous island arc rocks cordillera oriental dominican republic abstract cretaceous island arc rocks of the caribbean island arc system have been exposed by cenozoic faulting in the cordillera oriental in eastern hispaniola high k 2 o intermediate to felsic volcanic rocks loma la vega volcanics are interbedded with marine epiclastic sedimentary rocks and tuffs las guajabas tuffs and unconformably overlie pre aptian sedimentary rocks low k 2 o volcanic rocks guamira volcanics and a granodioritic to tonalitic intrusion el valle pluton the petrology and geochemistry of these units in conjunction with regional stratigraphic data are used to speculate on the tectonics of the newly developing caribbean island arc system during early and late cretaceous time the loma la vega volcanics are characterized by the presence of large phenocrysts of sanidine and minor amounts of clinopyroxene opaque oxides and rare leucite in a devitrified matrix of chlorite and clay although the volcanic rocks have undergone some low temperature alteration metamorphism which redistributed some major elements and large ion litho phile trace elements the high field strength elements rare earth elements and radiogenic isotopes appear to have been minimally affected based on abundances of the relatively immobile elements trace element enrichment patterns and isotopic compositions the loma la vega volcanics are considered part of the high k calc alkaline ca or shoshonitic island arc volcanic series in contrast pre aptian early cretaceous volcanic and plutonic rocks of the underlying los ranchos formation have chemical characteristics similar to rocks in the island arc tholeiitic or primitive island arc pia series that form coeval and along strike sections of the early cretaceous caribbean island arc in other parts of present day hispaniola cuba puerto rico and the virgin islands an abrupt and regional change in composition from island arc tholeiites to high k calc alkaline rocks is coincident with a hypothesized reversal in subduction polarity in pre aptian time as inferred from previously published tectonic models polarity reversal may have been triggered by attempted subduction of the caribbean oceanic plateau beneath this segment of the caribbean island arc the observed magmatic and tectonic effects of the proposed cretaceous caribbean arc reversal are similar to the better documented neogene subduction reversal event in the solomon islands arc in the southwest pacific

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

missing history 16 71 ma of the galapagos hotspot implications for the tectonic and biological evolution of the americas we present the results of volcanological geochemical and geochronological studies of volcanic rocks from malpelo island on the nazca plate 15 8 17 3 ma belonging to the gala pagos hotspot tracks and igneous complexes 20 8 71 3 ma along the pacific margin of costa rica and panama the igneous complexes consist of accreted portions of ocean island and seamount volcanoes and aseismic ridges representing the missing primarily subducted history of the gala pagos hotspot the age and geochemical data directly link the gala pagos hotspot tracks on the pacific ocean floor to the caribbean large igneous province ca 72 95 ma confirming a pacific origin for the caribbean oceanic plateau from the gala pagos hotspot we propose that emplacement of this oceanic plateau between the americas and interaction of the gala pagos hotspot tracks with the central american arc played a fundamental role in the formation of land bridges between the americas in late cretaceous paleocene and pliocene holocene time the land bridges allowed the exchange of terrestrial faunas e g dinosaurs mastodons saber tooth cats and ground sloths between the americas and served as barriers for the exchange of marine organisms between the central pacific ocean and the caribbean sea and the central atlantic ocean

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

lip reading recognizing oceanic plateaux in the geological record basaltic oceanic plateaux are important features in the geological record not only do they record ancient mantle plume activity but they also are believed to be important building blocks in the formation of the continental crust in this paper we review the salient features of two cretaceous oceanic plateaux the ontong java and the caribbean colombian thick sequences of predominantly homogeneous basalt the occurrence of high mgo basalt including komatiites and an apparent absence of sheeted dyke complexes in addition pyroclastic deposits may be scarce we then explore ways of distinguishing plateaux from basaltic sequences erupted in different tectonomagmatic settings continental flood basalt provinces island arcs back arc basins ocean islands and mid ocean ridges using these criteria potential archaean and proterozoic oceanic plateaux are reviewed and identified finally we explore how these remnant oceanic plateaux became incorporated into the continents by reviewing the proposed accretion mechanisms for the cretaceous caribbean colombian oceanic plateau on the basis of evidence from south america and the tonalites of the southern caribbean island of aruba

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

geological history of the cretaceous ophiolitic complexes of northwestern south america colombian andes abstract the western cordillera of colombia was formed by intense alpine type nappe forming folding and thrusting the cretaceous 80 120 ma b p tholeiitic material of the western cordilleran nappes has been obducted onto the paleozoic and precambrian polymetamorphic micaschists and gneiss of the central cordillera near yarumal the antioquia batholith 60 80 ma b p intrudes both obducted cretaceous oceanic material and the polymetamorphic basement rock of the central cordillera therefore nappe emplacement and obduction onto the central cordillera occurred during late senonian to early paleocene the nappes travelled from northwest to southeast so that the highest unit the rio calima nappe therefore has the most northwestern source whereas the lowest units originated from a more southeastward direction sedimentological analysis of the volcanoclastic and sandy turbidite material from each unit suggests a marginal marine environment during cretaceous times the opening of this marginal sea from now on called the colombia marginal basin probably originated by detachment of a block from the south american continent related to the farallon south america plate convergence in the popayan area southern colombia the central cordilleran basement exhibits glaucophane schist facies metamorphism this high pressure low temperature metamorphism is of early cretaceous 125 ma b p age and is related to an undated metaophiolitic complex the ophiolitic material originating from the western cordilleran is thrust over both the blueschist belt and the metaophiolitic complex these data suggest that the occidente colombiano suffered at least two phases of ophiolitic obduction during mesozoic time

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

high mg extrusive rocks from the romeral zone ophiolites in the southewestern colombian andes abstract in the southwestern colombian andes high mg extrusive rocks occur within the mesozoic late cretaceous ophiolitic fragments from the romeral fault zone a major suture between the continental domain of the central cordillera and the oceanic domain of the western cordillera the high mg rocks include picrites and picritic basalts and are associated in places with peridotites showing transitional to extrusive textures plagioclase free picrites characterized by quench textures have been found in two localities and occur as pillow lava rio boloblanco or as clasts of monogenic and polygenic volcanic breccias el encenillo they show similar olivine phyric micro spinifex textures with groundmass consisting of hopper and plate olivine or ca poor pyroxene at el encenillo skeletal clinopyroxene and glass hyaloporphyritic texture occurs in the rio boloblanco picrites at pillow rims chemically these picrites are characterized by mgo contents of 20 and are distinguished by the content of tio 2 and incompatible elements in a high ti type rio boloblanco and a very high ti type el encenillo the latter particularly remarkable for the high contents of ti zr and nb which give evidence of an alkaline tendency phase chemistry is consistently different between the two picrite types but comparably high al contents up to 14 of skeletal pyroxenes suggest that the magmas were erupted and cooled in similar conditions olivine phyric picritic rocks characterized by microlitic non quenched groundmasses occur as clasts of volcanic breccias at el encenillo picritic basalts and are present with comparatively high frequency within major los azules and minor upper chapungo valley ophiolitic fragments from the romeral zone either as lavas and volcanigenic breccias these rocks are mostly richer in olivine than the quench textured picrites contain modal plagioclase and relatively low al pyroxenes as regards chemistry a high ti and very low al group los azules upper chapungo valley and a very high ti group el encenillo can also be distinguished within these picrites a comparison with the ultramafic volcanic rocks from gorgona island in the nearby pacific domain off colombia among which the unique phanerozoic komatiites were discovered indicates close similarities between the rio boloblanco picrites and some gorgona island komatiites olivine phyric komatiites with micro spinifex groundmass regarding their petrochemical features it is suggested that they record a unique event related to the same tectonic environment and consequently that they belong to the same structural unit developed in the late cretaceous and extending from continental colombia to the pacific domain this is in contrast with previous subdivisions of the ophiolitic complexes from southwestern colombia into distinct age groups progressively younger from the central cordillera toward the pacific ocean

late jurassic oceanic crust and upper cretaceous caribbean plateau picritic basalts exposed in the duarte igneous complex hispaniola abstract four distinct rock units have been recognized near el aguacate in the janico juncalito la vega area of the duarte complex dominican republic 1 serpentinites crosscut by numerous diabasic dikes 2 basalts interbedded with late jurassic ribbon cherts 3 picrites and ankaramites relatively enriched in incompatible trace elements and 4 amphibolites and gneissic amphibolites chemically similar to oceanic plateau basalts similar ar ar ages of late magmatic amphibole from a picrite and hornblende from an amphibolite documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty decl

the formation and fate of large oceanic igneous provinces abstract large igneous provinces are conspicuous features of late phanerozoic geology and include continental flood basal rifted continental margin volcanic sequences and oceanic plateaus the latter are formed in an environment which typically recycles back into the mantle on a time scale of 200 m y but because comparisons have recently been made between oceanic plateaus and precambrian greenstone belt sequences new questions arise about their formation their fate and their preservation here we review some critical aspects of three oceanic plateaus ontong java kerguelen and the caribbean colombian abducted plateau and comment on their make up and the factors governing their preservation with particular relevance to ancient terranes many large igneous provinces can be linked to mantle plumes where plumes ascend beneath spreading ridges their energy is transformed into a large melt volume producing over thickened plateau crust where the spreading rate is low in relation to magma supply the plateau may become subaerial e g iceland but with fast spreading the plateau remains submarine thicker lithosphere may result in plume incubation before magma extrusion and there are many intermediate situations where plumes could readily break through thin lithosphere oceanic or continental because magma supply exceeds extension rate plateaus may be characterised by thick sequences of flows and sills rather than the sheeted dykes typical of phanerozoic ophiolites precambrian greenstones could represent imbricated oceanic plateaus or plumes penetrating thin continental lithosphere the initial high temperature and the buoyant nature of the depleted refractory keel of plateaus contributes to their preservation relative to normal oceanic crust when they collide with active margins they choke the subduction zone causing subduction flip or backstep and the development of extensive talc alkaline arc volcanism on top of the plateau sequences however after x 100 m y they are potentially negatively buoyant so if fluids become available to promote tmnsfotmation of the deeper zones to eclogite they may be able to spontaneously subduct

the andean ophiolitic megastructures on the buga buenaventura transverse western cordillera valle colombia abstract a structural study of the buga buenaventura transverse in the central part of the western cordillera of colombia has shown the presence of three structural units which from bottom to top and from west to east are the rio dagua unit the rio calima unit and the loboguerrero window unit all three units comprise strata between 120 and 80 m y old overlying a basement of green rocks showing the characteristics of submarine flows the bolivar ultrabasic and basic massif is geographically linked to the rio calima unit in which green rocks predominate and is separated from it by a tectonic contact the upper part of the massif on the other hand shows high temperature metamorphic rocks formed during its emplacement the rio dagua unit shows two tectonic phases with isoclinal folding and development of schistosity the first phase is contemporaneous with low grade metamorphism equivalent to lower greenschist facies conditions the loboguerrero window unit shows a large recumbent fold oriented towards the southeast deformation in the rio calima unit is weaker and appears to correspond to a higher structural level than in the two other units as the attitudes of the s1 schistosity in the rio dagua unit and of the shear zones located at the green rocks sediments contact in this same unit are similar to that of the overthrust at the base of the rio calima unit we are led to postulate that the overthrust belongs to the first phase of deformation as also does the recumbent fold in the loboguerrero window unit we are thus led to propose a southeastward direction of emplacement for the nappes in the western cordillera of colombia in conclusion the authors extend their observations and propose a new structural interpretation of the occidente colombiano

peeling oceanic crust in subduction zones thin slabs of oceanic pillow basalt formed at ridge axes with associated pelagic sediments are common features in ancient accretionary complexes estimates of the average thicknesses of scraped off basaltic sections of oceanic crust in accretionary prisms vary from a few tens of metres to 300 m with a mean thickness of 80 100 m in the complexes studied the permeability contrast within the uppermost part of the oceanic crust is an important factor governing the thickness of delaminated oceanic crust this mechanism is an effective means of separating oceanic crust altered at low temperatures from the remainder of the downgoing slab and thus controlling geochemical budgets in subduction zones

isotopic evidence for the origin of the manihiki and ontong java oceanic plateaus abstract pb nd and sr isotopic results for lavas of the cretaceous ontong java and manihiki oceanic plateaus fall well within the modern day oceanic island or hotspot field the data provide no evidence of old continental basements but indicate a major involvement of kerguelen type or em i like mantle in the sources of both plateaus which appear to have probably been formed at least in part by hotspots however the presently active hotspots that pacific plate reconstructions suggest might have been possible plateau sources lack kerguelen type isotopic compositions either these hotspots did not participate in the formation of the two plateaus or if they did kerguelen type material must have been volumetrically much more important early in their existence two hypotheses for the origins of these plateaus which involve hotspot sources are consistent with the sparse available geochemical geochronological and geophysical data the first holds that the plateaus formed cataclysmically in association with surfacing plume heads the second posits a relatively steady but robust hotspot at or near a ridge crest and requires a much longer period of formation a near ridge origin appears to be indicated by evidence that most of the pacific plateaus were built largely on relatively young ocean crust however we suggest that a near ridge origin is also compatible with the plume head concept in that plume heads appear very likely to become associated with spreading axes through their influence on rift propagation which should be substantially greater than for ordinary hotspots in either case the lack of hotspot tracks seamount chains attached to the two plateaus would be a consequence of ridge migration or rift propagation in a near ridge setting

is the lower duarte igneous complex hispaniola a remnant of the caribbean plume generated oceanic plateau eleven samples of metapicrites metaankaramites diabases and cumulates of the lower duarte complex in central hispaniola were analyzed for major trace element and nd sr isotopic compositions t

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

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

ophiolite complex from la tetilla southwestern colombia south america a disrupted ophiolitic complex of cretaceous age from the romeral fault zone in the southwestern colombian andes occurs in the la tetilla area near popayan it represents a peculiar association of low ti and high ti tholeiitic basalts among the ophiolitic complexes from the romeral zone it includes 1 a main sequence with incipient metamorphism under prehnite pumpellyite to greenschist facies conditions consisting in ascending order of cumulus wehrlite and gabbro massive gabbro and basaltic and doleritic lavas and breccias the basalts are characterized by the crystallization of clinopyroxene prior to plagioclase cpx phyric basalt group and have a primitive low ti tholeiite composition mg numbers between 70 6 and 65 4 the clinopyroxene chemistry is consistent with this magmatic affinity the plutonic rocks can be related to a similar low ti tholeiitic magma 2 a sequence of pervasively metamorphosed basalts with greenschist facies metamorphism of oceanic type structurally underlies the mai

regional provenance from southwestern colombia fore arc and intra arc basins implications for middle to late miocene orogeny in the northern andes andean orogenic processes controlled the spatial and temporal distribution of the magmatic and sedimentary record this contribution integrates new u pb zircon ages heavy mineral analyses and biostratigraphic constraints from the neogene sedimentary record of the fore arc and intra arc basins and volcano plutonic rocks of southwestern colombia to reconstruct these orogenic processes the results reveal continuous arc magmatism since the late oligocene with a major post middle miocene magmatic peak and exhumation when integrated with other geological constraints the tectonic evolution of the margin includes eocene oligocene oblique convergence with limited magmatic activity followed by the initiation of a late oligocene early miocene arc that migrated to the east in the middle miocene when it experienced a major increase in magmatic activity crustal deformation exhumation and thickening this orogenic evolution is related to the shallowing of the slab dip due to the subduction of the neogene nazca plate

the trace element composition of the lavas and dikes from a 3 km vertical section through the lava pile of eastern iceland the tertiary lava succession of eastern iceland dips at shallow angles towards and beneath the axial quaternary volcanic zone a 3 km vertical section through part of this tertiary pile studied during the iceland research drilling project contains flows ranging in composition from basalt to icelandite this succession can be divided into three distinct stratigraphic groups upper and lower groups 1 7 and 0 6 km thick with zr y ratios of about 4 5 and a 0 7 km thick middle group with a zr y ratio of about 6 5 the middle group also has a significantly higher ce yb ratio and higher elemental sr abundances the data on within group variation are equally compatible with high level fractionation of a plagioclase olivine clinopyroxene assemblage with accessory apatite and opaque minerals or with open system fractionation models dikes occur throughout the 3 km section and are comparable in trace element composition to the lavas of the upper group the dikes are part of a n s trending swarm originating from the breiddalur volcanic center some 15 km to the south these dikes were injected laterally northward into the reydarfjordur lava pile at the time of the eruption of the upper group lavas the lower two major lava groups may be related to similar but unidentified discrete volcanic centers perhaps buried downdip the new data on this deeper section corroborate earlier studies which indicated that on a gross scale the eastern iceland lava pile becomes less mafic with depth this vertical change occurs because each major volcanic center erupted more silicic lavas close to the center and more magnesium rich compositions toward the periphery the formation of the lava pile occurred by tilting toward the spreading center and overlapping repetition of such major units which resulted in the more mafic edges of the lava groups forming the topographically higher parts of the crustal section

evaluating highly resolved paleoclimate records in the frequency domain for multidecadal scale climate variability 1 do the chronological methods used in the construction of paleoclimate records influence the results of the frequency analysis applied to them we explore this phenomenon using the dongge cave speleothem record u series chronology with variable time steps t and the el malpais tree ring index cross dating of ring width series interpolation of the dongge cave record to a constant t resulted in the suppression of periodicities 20 years altering the red noise model used for significance testing frequency analysis of temporal subsets of the el malpais tree ring index revealed that concentrations of variance varied with the number of ring width series frequency analyses of these records identified significant periodicities some common to both 25 and 69 years cross wavelet analysis which examines periodicities in the time domain revealed that coherency between these records occurs intermittently we found the chronology methods can influence the ability of frequency analysis to detect periodicities and tests for coherency

tectonic evolution of the caribbean the development of the caribbean is discussed in terms of modern tectonic theory the nature of the site on which the caribbean formed is examined and the development of the rifted margins of the caribbean is described constraints on caribbean evolution from the relative motions of north and south america are briefly examined and the caribbean oceanic plateau is discussed the great island arc system of the caribbean is addressed in detail emphasizing the way the great arc of the caribbean was segmented the role of central america in caribbean history is briefly considered

overview and plumbing system implications of monogenetic volcanism in the northernmost andes volcanic province abstract monogenetic volcanic fields are commonly related to rifts and or intraplate tectonic settings however they are also less commonly recognised in subduction zones including both front and back arc volcanoes e g monogenetic volcanoes associated with the llaima volcano in chile or the san agustin volcanic field in colombia here we describe monogenetic volcanic fields associated with subduction related polygenetic volcanism in the northernmost part of the andes northern volcanic zone nvz 2 s to 4 30 n these fields are associated with the main axis of the quaternary volcanoes they are linked to the polygenetic san diego cerro machin volcano tectonic province 140 km long scvtp in colombia the chain that hosts the iconic nevado del ruiz volcano presently three monogenetic volcanic fields with a typical calc alkaline signature have been identified on both sides of this province from south to north they are 1 pijaos monogenetic volcanic field pmvf 2 villamaria termales monogenetic volcanic field vtmvf and 3 samana monogenetic volcanic field smvf pmvf is located 25 km south of cerro machin volcano the southernmost active volcano of the scvtp this field was formed by at least two eruptions with both effusive and explosive eruptive styles three cones and a maar are recognised previous work has defined the volcanoes as basaltic andesitic in composition which we recognised as the most mafic expression mgo 10 11 wt in the whole scvtp its source is related to the same but deeper magma that feeds the volcanoes in the scvtp stratigraphic relationships show that the volcanoes are younger than the underlying alluvial and volcaniclastic ibague fan 5 km of the axis of the province this field is made up of at least 14 volcanoes aligned with the active villamaria termales fault system as previous work has recognised the volcanism has been mainly effusive represented by lava domes and some lava flows since the 1980s studies have framed these andesitic to dacitic volcanoes in the context of the history of the nevado del ruiz volcano it is inferred that the magmatic source is a shallow magma reservoir underneath the scvtp stratigraphic and field relationships have shown that the last eruption occurred

adakite like volcanism of ecuador lower crust magmatic evolution and recycling in the northern andes of ecuador a broad quaternary volcanic arc with significant across arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes quaternary volcanic centers occur from west to east along the western cordillera frontal arc in the inter andean depression and along the eastern cordillera main arc and in the sub andean zone back arc the adakite like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt mantle interactions or of lower crustal melting fractional crystallization and assimilation processes in this paper we present petrographic geochemical and isotopic sr nd pb data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers pululagua pichincha ilalo chacana sumaco forming a nw se transect at about 0 latitude and encompassing the frontal pululagua pichincha main ilalo chacana and back arc sumaco chains all rocks display typical subduction related geochemical signatures such as nb and ta negative anomalies and lile enrichment they show a relative depletion of fluid mobile elements and a general increase in incompatible elements from the front to the back arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab we observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene and or amphibole bearing cumulates and of magma mixing at all frontal and main arc volcanic centers within each volcanic center rocks display correlations between evolution indices and radiogenic isotopes although absolute variations of radiogenic isotopes are small and their values are overall rather primitive e g end 1 5 to 6 87sr 86sr 0 7040 0 70435 rare earth element patterns are characterized by variably fractionated light to heavy ree la ybn 5 7 34 and by the absence of eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation we interpret the petrographic geochemical and isotopic data as indicating open system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower to mid crustal levels as well as by assimilation of mafic lower crust and or its partial melts thus we propose that the adakite like signatures of ecuadorian rocks e g high sr y and la yb values are primarily the result of lower to mid crustal processing of mantle derived melts rather than of slab melts and slab melt mantle interactions the isotopic signatures of the least evolved adakite like rocks of the active and recent volcanoes are the same as those of tertiary normal calc alkaline magmatic rocks of ecuador suggesting that the source of the magma did not change through time what changed was the depth of magmatic evolution probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic carnegie ridge

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