evolution of the eastern volcanic ridge of the canary islands based on new kar data the results of 64 new k ar age determinations together with 32 previously published ages show that after a period of erosion of the basal complex miocene volcanic activity started around 20 ma in fuerteventura and 15 ma in lanzarote forming a tabular succession of basaltic lavas and pyroclastics with a few salic dykes and plugs this series includes five separate volcanic edifices each one with its own eruptive history in fuerteventura several miocene eruptive cycles have been identified in the central edifice one around 20 17 ma followed by two others centred around 15 and 13 ma in the southern edifice the maximum of activity took place around 16 14 ma whereas in the northern one the main activity occurred between 14 and 12 ma in lanzarote a first cycle of activity took place in the southern edifice between 15 5 and 14 1 ma followed by another between 13 6 and 12 3 ma in the northern edifice three pulses occurred 10 2 8 3 6 6 5 3 and 3 9 3 8 ma an important temporal gap greater in fuerteventura than in lanzarote separates series i from the plio quaternary series ii iii and iv formed by multi vent basaltic emissions in fuerteventura the following eruptive cycles have been identified 5 2 9 2 4 1 8 1 7 0 8 0 4 and 0 1 ma in lanzarote the activity was fairly continuous from 2 7 ma to historic times with a maximum in the lower pleistocene eruptive rates in the series i edifices were on the average 0 1 0 01 km3 ka comparable but slightly smaller than in similar edifices in tenerife and la gomera but much lower than in gran canaria average post miocene eruptive rates were about 0 013 0 027 km3 ka in lanzarote and 0 003 0 007 km3 ka in fuerteventura all these volcanic edifices show a similar general sequence fissural eruptions erosion multi vent volcanism repeated at different periods in different parts of the eastern islands of the canaries the model of growth of the series i edifices is comparable to those in tenerife and la gomera long periods of activity sometimes greater than 6 m y with pulses separated by gaps however salic and intermediate differentiates frequent in tenerife and la gomera are very scarce in these islands the fuerteventura lanzarote ridge shows a decrease in volcanic activity with time and also a certain ssw nne polarity in the temporal development of volcanism

the tectonically controlled emplacement of a vertically sheeted gabbro pyroxenite intrusion feeder zone of an ocean island volcano fuerteventura canary islands abstract the miocene px1 gabbro pyroxenite pluton fuerteventura canary islands is a 3 5 5 5 km shallow level intrusion 0 15 0 2 gpa and 1100 1120 c interpreted as the feeder zone to an ocean island volcano it displays a vertical magmatic banding expressed in five 50 to 100 metre wide nne ssw trending alkaline gabbro sequences alternating with pyroxenites this emplacement geometry was controlled by brittle to ductile shear zones generated by a regional e w extensional tectonic setting that affected fuerteventura during the miocene at a smaller scale the px1 gabbro and pyroxenite bands consist of metre thick differentiation units which suggest emplacement by periodic injection of magma pulses as vertical dykes that amalgamated similarly to a sub volcanic sheeted dyke complex individual dykes underwent internal differentiation following a solidification front parallel to the dyke edges this solidification front may have been favoured by a significant lateral horizontal thermal gradient expressed by the vertical banding in the gabbros the fractionation asymmetry within individual dykes and the migmatisation of the wall rocks pyroxenitic layers result from the fractionation and accumulation of clinopyroxene olivine plagioclase crystals from a mildly alkaline basaltic liquid they are interpreted as truncated differentiation sequences from which residual melts were extracted at various stages of their chemical evolution by subsequent dyke intrusions either next to or within the crystallising unit compaction and squeezing of the crystal mush is ascribed to the incoming and inflating magma pulses the expelled interstitial liquid was likely collected and erupted along with the magma flowing through the newly injected dykes clinopyroxene mineral orientation as evidenced by ebsd and micro x ray tomography investigations displays a marked pure shear component supporting the interpretation of the role of compaction in the generation of the pyroxenites conversely gabbro sequences underwent minor melt extraction and are believed to represent crystallised coalesced magma batches emplaced at lower rates at the end of eruptive cycles clinopyroxene orientations in gabbros record a simple shear component suggesting syn magmatic deformation parallel to observed nne ssw trending shear zones induced by the regional tensional stress field this emplacement model implies a crystallisation time of 1 to 5 years for individual dykes consistent with px1 emplacement over less than 0 5 my a minimum amount of approximately 150 km3 of magma is needed to generate the pluton part of it having been erupted through the central volcanic centre of fuerteventura if the regional extensional tectonic regime controls the px1 feeder zone initiation and overall geometry rates and volumes of magma depend on other source related factors high injection rates are likely to induce intrusion growth rates larger than could be accommodated by the regional extension in this case dyke intrusion by propagation of a weak tip combined with the inability of magma to circulate through previously emplaced and crystallised dykes could result in an increase of non lithostatic pressure on previously emplaced mushy dyke walls thus generating strong pure shear compaction within the pluton feeder zone and interstitial melt expulsion these compaction dominated processes are recorded by the cumulitic pyroxenite bands

trace element distribution among rock forming minerals from metamorphosed to partially molten basic igneous rocks in a contact aureole fuerteventura canaries abstract low pressure partial melting of basanitic and ankaramitic dykes gave rise to unusual zebra like migmatites in the contact aureole of a layered pyroxenite gabbro intrusion in the root zone of an ocean island basal complex fuerteventura canary islands these migmatites are characterised by a dense network of closely spaced millimetre wide leucocratic segregations their mineralogy consists of plagioclase an 32 36 diopside biotite oxides magnetite ilmenite amphibole dominated by plagioclase in the leucosome and diopside in the melanosome the melanosome is almost completely recrystallised with the preservation of large relict igneous diopside phenocrysts in dyke centres comparison of whole rock and mineral major and trace element data allowed us to assess the redistribution of elements between different mineral phases and generations during contact metamorphism and partial melting dykes within and outside the thermal aureole behaved like closed chemical systems nevertheless zr hf y and rees were internally redistributed as deduced by comparing the trace element contents of the various diopside generations neocrystallised diopside in the melanosome leucosome and as epitaxial phenocryst rims from the migmatite zone are all enriched in zr hf y and rees compared to relict phenocrysts this has been assigned to the liberation of trace elements on the breakdown of enriched primary minerals kaersutite and sphene on entering the thermal aureole major and trace element compositions of minerals in migmatite melanosomes and leucosomes are almost identical pointing to a syn or post solidus reequilibration on the cooling of the migmatite terrain i e mineral melt equilibria were reset to mineral mineral equilibria

petrology of amphibolite facies mafic and ultramafic rocks from the catalina schist southern california metasomatism and migmatization in a subduction zone metamorphic setting the catalina schist of southern california is a subduction zone metamorphic terrane it consists of three tectonic units of amphibolite high p greenschist and blueschist facies rocks that are structurally juxtaposed across faults forming an apparent inverted metamorphic gradient migmatitic and non migmatitic metabasite blocks surrounded by a meta ultramafic matrix comprise the upper part of the catalina amphibolite unit fluid rock interaction at high p high t conditions caused partial melting of migmatitic blocks metasomatic exchange between metabasite blocks and ultramafic rocks infiltration of silica into ultramafic rocks and loss of an albitic component from nonmigmatitic clinopyroxene bearing metabasite blocks partial melting took place at an estimated p 8 11 kbar and t 640 750 c at high h2o activity the melting reaction probably involved plagioclase quartz trondhjemitic melts were produced and are preserved as leucocratic regions in migmatitic blocks and as pegmatitic dikes that cut ultramafic rocks the metasomatic and melting processes reflected in these rocks could be analogous to those proposed for fluid and melt transfer of components from a subducting slab to the mantle wedge aqueous fluids rather than melts seem to have accomplished the bulk of mass transfer within the mafic and ultramafic complex

low pressure water assisted anatexis of basic dykes in a contact metamorphic aureole fuerteventura canary islands oxygen isotope evidence for a meteoric fluid origin migmatites produced by low pressure anatexis of basic dykes are found in a contact metamorphic aureole around a pyroxenite gabbro intrusion px2 on fuerteventura dykes outside and inside the aureole record interaction with meteoric water with low or negative 18o whole rock values 0 2 to 3 4 decreasing towards the contact recrystallised plagioclase diopside biotite and oxides from within the aureole show a similar evolution with lowest 18o values 2 8 4 2 4 4 and 7 6 respectively in the migmatite zone close to the intrusion relict clinopyroxene phenocrysts preserved in all dykes retain typically magmatic 18o values up to the anatectic zone where the values are lower and more heterogeneous low 18o values decreasing towards the intrusion can be ascribed to the advection of meteoric water during magma emplacement with increasing fluid rock ratios higher dyke intensities towards the intrusion acting as fluid pathways and higher temperatures promoting increasing exchange during recrystallisation

macedonio melloni and the foundation of the vesuvius observatory abstract the story of the exciting years in the middle of last century when the osservatorio vesuviano and other important scientific institutions were established by the bourbon government has been reconstructed on the basis of original documents macedonio melloni a physicist from parma was appointed director and charged with the choice of site and instrumentation the construction of the observatory was completed in 1848 and only a few months later melloni was dismissed as a director for political reasons

shear zones as a result of intraplate tectonics in oceanic crust the example of the basal complex of fuerteventura canary islands ductile shear zones affecting igneous rocks pyroxenites syenites gabbros and carbonatites belonging to the basal complex of the island of fuerteventura are described here they develop typical mylonites with their cortege of structures and crystallographic fabrics an evolution from brittle to ductile conditions has been recognized and interpreted as being due to the ascent of alkaline magmas and related fluids and subsequent emplacement of weak rocks carbonatites syenites along the shear zones local metamorphic changes favour the ductile behaviour via a reaction enhanced softening mechanism the shear zones are arranged in a nearly orthorhombic pattern kinematic criteria suggest an extensional tectonic scenario with a bulk irrotational nonplane deformation the long axis of the finite deformation ellipsoid is e w and horizontal the available data allow us to establish this deformation as being from the late oligocene to the early miocene this is tentatively related to the plate tectonics evolution and mantle anomalies in the northwestern corner of the african plate

volcanic complexes in the eastern ridge of the canary islands the miocene activity of the island of fuerteventura abstract fuerteventura has been since early stages of its growth the result of three different adjacent large volcanic complexes southern central and northern the definition of these volcanic complexes and their respective growing episodes is based on volcano stratigraphic morphological and structural criteria particularly radial dyke swarms each complex has its own prolonged history that might be longer than 10 m y during that time several periods of activity alternating with gaps accompanied by important erosion took place the evolution of each volcanic complex has been partially independent but all the three are affected by at least three miocene tectonic phases that controlled considerably their activity the volcanic complexes are deeply eroded and partially submerged in the core of the northern and the central volcanic complexes there is a set of submarine and plutonic rocks intensely traversed by a dyke swarm known as the basal complex the basal complex has been interpreted in different ways but all previous authors have considered it to be prior to the subaerial shield stage of the island here we advance the idea that the basal complex represent the submarine growing stage of the volcanic complexes and the hypabyssal roots plutons and dykes of their successive subaerial growing episodes two seamounts situated nearby southwest of the island might be interpreted as remains of two other major volcanoes these two volcanoes together with those forming the present emerged island of fuerteventura and finally those of famara and los ajaches situated further north on lanzarote constitute a chain of volcanoes located along a lineation which is subparallel to the northwestern african coastline and which may relate to early atlantic spreading trends in the area

melt segregation in the continental crust before melt will segregate from its restite and source there must be both a driving force and space into which the melt can move the continental crust is anisotropic and in most cases is subject to deviatoric stresses during melting under these conditions melt segregation begins when an interconnected network of melt forms and the source becomes permeable melt is driven by pressure gradients from its restite toward dilatant sites set up in the heterogeneously deforming source the source is continually drained of melt and so need never contain more melt than that required to maintain permeability 26 where the solid matrix breaks down and diapirism can occur

cumulate recycling and crustal evolution in the bay of islands ophiolite the lower crustal section of the north arm mountain massif of the bay of islands ophiolite is composed of interlayered gabbroic and ultramafic cumulates despite pervasive ridge related deformation key contact relationships are preserved in low strain domains or late kinematic intrusions angular and crenulated gabbroic enclaves in ultramafic rocks and ultramafic dikes in gabbroic rocks imply that many of the ultramafic rocks intruded into older gabbroic hosts olivine gabbro gabbro gabbro norite norite and fe oxide amphibole gabbros the ultramafic intrusions are composed of dunite wehrlite harzburgite lherzolite and olivine websterite clino to ortho pyroxenite reaction rims line many peridotitic gabbroic contacts the voluminous pyroxenites of the lower crust are petrologically identical to in situ pyroxenite reaction rims and are thought to represent partially digested gabbros fine grained harzburgitic sills crosscut the foliation of wehrlitic host rocks with orthopyroxenite reaction rims

features of crustal structure under the canary islands abstract the data and interpretation of a seismic refraction project carried out in the canary islands in 1977 and 1979 are presented the moho is reached at about 15 km under fuerteventura gran canaria and tenerife whereas under lanzarote a value of 11 km has been found the velocity at the surface varies even within one island from 2 8 to 4 4 km s there are variations in the velocity of the main crustal layer under the volcanic deposits a continuous velocity depth function seems to be more appropriate for the island of tenerife contrary to fuerteventura lanzarote and gran canaria which seem to have a layered structure there is evidence for pronounced structural differences between lanzarote and fuerteventura the mantle velocity has a value of about 8 0 km s under the western islands whereas a very low value of 7 4 km s has been found under lanzarote and fuerteventura despite the clear structural differences between the islands our results leave little doubt about the oceanic character of all the islands the differences between the eastern and western group may be due to the islands being situated close to the african continental margin the seismic velocities under the volcanic deposits do not support the idea of a common basement for all the islands they rather suggest independent volcanic edifices finally a comparison of the structure of the crust under the canary islands with that under other volcanic islands reveals remarkable similarities

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

thermal model of successive dike injections and implications for the development of intraplate volcanoes abstract temperatures in the root zones of volcanoes play a critical role in the development and persistence of shallow level magmatic reservoirs in the crust here we present a 1d thermal model allowing evaluation of the thermal impact of magma travelling in conduits to the surface on the root zone of a volcano this thermal model has been developed to better understand the formation of a vertical intrusion located in the root zone of a dismembered miocene volcano on fuerteventura canary archipelago this intrusion named px1 constitutes an almost pure amalgamation of dikes of either clinopyroxenitic or gabbroic composition both types of dikes display cumulate textures and are interpreted as resulting from the protracted crystallization of a mafic magma the formation of clinopyroxenitic in contrast to gabbroic dikes requires that the residual melt was extracted at high temperature 1050 to avoid plagioclase crystallization simulations of multiple dike injections show that the temperature in the root zone increases significantly with the addition of dikes but the maximum temperature reached in the system depends on the duration of magma flow in the conduits and the time interval between dike injections i e repose period active flow is the critical parameter that distinguishes instantaneous dike injection from a magmatic conduit without significant magma flow 1 month high temperature conditions 1000 c cannot be maintained in the pluton unless dikes are very thick and the repose period is extremely small on the other hand magma flow times of one to several months combined with short time intervals between dike injections persistent high temperature in the vicinity of magma conduits limits the differentiation of melts in transit to the surface providing a potential explanation for why lavas of mafic to intermediate composition predominate in intraplate volcanoes such as fuerteventura or fogo island cape verde archipelago in extreme cases when temperatures over 1000 1050 c in the central part of the feeding zone are maintained for years the remaining magma in the conduit does not solidify but is preserved in a mushy state new pulses of magma would not be able to cross this zone but would rather amalgamate in the incipient magma reservoir the present model differs from previous models of sill intrusion in that magmas do not need to pond at depth to create a reservoir but merely supply heat while travelling to the surface depending on the time interval between dike injections and the duration of magma flow through the crust magma rising in vertical conduits could directly feed the volcanic edifice or could lead to the formation of magma reservoirs this process may explain why some volcanoes erupt mafic or differentiated magmas during distinct periods of activity

metamorphism in oceanic layer 3 gorringe bank eastern atlantic gorringe bank is an anomalously high structure of the eastern part of the north atlantic which was known to be composed of mantle derived peridotites layer 4 and gabbros layer 3 during the submersible cruise cyagor ii in 1981 the contact between layer 4 and layer 3 was observed on mount gettysburg and interpreted as tectonic the overlying series of gabbro was extensively sampled on both mounts composing the bank gettysburg and ormonde coarse grained to pegmatoid clinopyroxene gabbros predominate and are associated with differentiated rocks ferrogabbros and diorites cumulate gabbros are missing the gabbroic section sampled is therefore interpreted as the upper part of the plutonic section most samples were strongly recrystallized during two distinct events metamorphism occurred close to the ridge axis from interaction of a seawater derived fluid with still hot gabbros high temperature shear zones favoured fluid circulation but the water rock ratio estimated from the sodium input was very small in undeformed rocks 1 the low w r ratio explains the strong evolution of the fluid phase and therefore some particular compositions of secondary minerals low temperature alteration occurred when the gabbros were tectonically emplaced close to the sea bottom

melt segregation in the continental crust distribution and movement of melt in anatectic rocks the grain and outcrop scale distribution of melt has been mapped in anatectic rocks from regional and contact metamorphic environments and used to infer melt movement paths at the grain scale anatectic melt is pervasively distributed in the grain boundaries and in small pools consequently most melt is located parallel to the principal fabric in the rock typically a foliation short branched arrays of linked melt bearing grain boundaries connect melt depleted parts of the matrix to diffuse zones of melt accumulation protoleucosomes where magmatic flow and alignment of euhedral crystals grown from the melt developed the distribution of melt leucosome and residual rocks normally melanocratic in outcrop provides different but complementary information the residual rocks show where the melt came from and the leucosomes preserve some of the channels through which the melt moved or sites where it pooled different stages of the melt segregation process are recorded in the leucosome melanosome arrays regions where melting and segregation had just begun when crystallization occurred are characterized by short arrays of thin branching leucosomes with little melanosome a more advanced stage of melting and segregation is marked by the development of residual rocks around extensive branched leucosome arrays generally oriented along the foliation or melting layer places where melting had stopped or slowed down before crystallization began are marked by a high ratio of melanosome to leucosome because most of the melt has drained away very few leucosomes remain to mark the melt escape path this is common in melt depleted granulite terranes many migmatites contain abundant leucosomes oriented parallel to the foliation mostly these represent places where foliation planes dilated and melt drained from the matrix via the branched grain boundary and larger branched melt channel leucosome arrays collected melt collected in the foliation planes was partially or fully expelled later when discordant leucosomes formed leucosomes or veins oriented at high angles to the foliation layering formed last and commonly lack melanocratic borders hence they were not involved in draining the matrix of the melting layer discordant leucosomes represent the channels through which melt flowed out of the melting layer

ree bearing minerals in a ti rich vein from the adamello contact aureole italy zirconolite aeschynite ce titanite and apatite have been found as minor or accessory minerals in a ti rich tio2 2 1 4 5 wt hydrothermal vein occurring in dolomite marbles at the contact with a tonalite intrusion of the tertiary adamello batholith northern italy the vein consists of four distinct mineral zones comprising from margin to center 1 forsterite calcite 2 pargasite calcite titanite sulfides 3 phlogopite calcite titanite sulfides and 4 titanian clinohumite spinel calcite sulfides zirconolite occurs in two vein zones only in the phlogopite zone it is invariably anhedral often corroded and exhibits complex chemical zonation patterns in the titanian clinohumite zone zirconolite is idiomorphic and characterized by a pronounced discontinous chemical zoning but shows no evidence of corrosion the considerable compositional variation observed for zirconolite in wt ree2o3 0 74 16 8 uo2 0 59 24 0 tho2 0 67 17 1 is due to the zoning and may be attributed to four major substitutions described by the exchange vectors 1 th u mg fe2 ca 1 ti 1 2 ree al ca 1 ti 1 3 ree fe2 nb ta ca 1 ti 1 4 hf zr 1

the impact of the 1991 mt pinatubo eruption on deep sea foraminiferal communities a model for the cretaceous tertiary k t boundary the ash layer from the 1991 mt pinatubo eruption provided a natural laboratory to assess mass mortality and the rates of recovery of deep marine ecosystems after a catastrophic event and to draw a parallel with the environmental impact of the k t boundary event on deep sea foraminiferal communities the first recolonization assemblages of benthic foraminifers after the mt pinatubo ash fall exhibit surprising similarities with benthic foraminiferal communities established following the k t boundary event i e comparatively low abundance and diversity absence of suspension feeders dominance of mobile infaunal taxa and a high proportion of agglutinated foraminifers the first foraminifers to recolonize the deep sea after both catastrophic events were opportunistic forms with high capability for rapid dispersal however major differences are evident in the timing and duration of the faunal transitions post eruption assemblages with high proportions of epibenthic suspension feeders became established after a short initial recolonization phase of approx 6 7 years on the mt pinatubo ash layer in the k t sections comparatively stable new benthic foraminiferal assemblages including suspension feeders only became established several thousand years after the event delayed recovery of the pelagic ecosystem and poor deep water ventilation probably slowed down the recovery of the benthic community structure after the k t boundary c 2005 elsevier b v all rights reserved

introduction to special section mechanisms and consequences of melt segregation from crustal protoliths much of petrological effort has been concentrated on attempts to discern define and interpret regularly recurring patterns of igneous and metamorphic petrogenesis i do not wish to deny the existence of discernable order in petrological phenomena but every geological event is unique if we concentrate too heavily on discernment of order and pattern may we perhaps overlook or underrate the unique quality of each igneous or metamorphic episode possibly uniqueness may have as great significance as conformity to pattern when we attempt to fit the phenomena of petrology into the broad framework of geology francis j turner presidential address to the mineralogical society of america november 11 1969

geochronology of some canarian dike swarms contribution to the volcano tectonic evolution of the archipelago we present new conventional k ar and 39ar 40ar age data and geometric observations on dike swarms which intrude the islands of la gomera tenerife and fuerteventura canary archipelago the younger dated dikes of la gomera 5 4 5 2 ma were emplaced by a radial fissural structure independently superimposed on an older one dated between 10 5 and 7 0 ma the dated dike swarms of both the anaga and teno peninsula of tenerife are contemporaneous 5 8 3 6 ma while ages of 0 43 ma were measured on two dikes intruding the pedro gil ridge the dikes of the basal complex of fuerteventura give well defined 39ar 40ar plateaus when they are situated far away from basic and ultrabasic plutons and disturbed 39ar 40ar age spectra when they are situated in the vicinity of the plutons these disturbances are probably due to a thermal metamorphism the results of both the conventional k ar and 39ar 40ar stepwise heating methods show the existence of an important magmatic phase between 24 and 17 ma the dike swarms of the basal complex are partly contemporaneous with other dike systems intruding the basaltic series i of fuster et al 1968a this phase seems to correspond to the final stages of activity on the main nne ssw volcano tectonic axis following this a volcanic edifice was built on the nw part of jandia peninsula mainly between 15 4 and 14 2 ma the similarity of the ages obtained on dikes and lava flows shows the importance of fissural volcanism in these islands this study shows that dating of dike swarms reveals the existence of major eruptive phases an important part of the products of which have been eroded brief episodes of fissural volcanic activity sometimes probably as short as 200 000 years may be correlated with the migration of the lithosphere over a fixed hot spot the north south 3 5 ma old present and the nne ssw 24 17 ma old oriented dike swarms of la palma staudigel et al in prep and fuerteventura may represent the regional paleostress orientation plane 1 2 which affected the lithosphere during their intrusion although other phenomena may control the orientation of dikes these directions are concordant with the known contemporaneous nw african tectonics

the dyke swarms of the amanay massif fuerteventura canary islands spain abstract the sheeted dyke swarms of the amanay massif in fuerteventura exhibit variable trends but generally strike n45 e main and n110 subsidiary on the basis of petrographic studies the dykes are classified as ankaramitic basalts trachybasalts hawaiites and lamprophyres camptonites this compositional variability is attributed to different extents of partial melting of the mantle and to differences in the degree of fractional crystallization we present a detailed account of field characteristics mineralogy and petrography of the sheeted dyke complex that intruded the sedimentary sequence plutonic and submarine volcanic formations

oxygen and hydrogen isotope studies on the basal complexes of the canary islands implications on the conditions of their genesis 18o 16o and d h ratios have been measured in rocks and mineral separates from the basal complexes of fuerteventura gomera and la palma these complexes comprise alkali plutonic rocks ranging from olivine gabbros to syenites and dikes of various mineralogy cutting them all metamorphosed under greenschist conditions k ar determinations gave ages from 48 to 17 my the 18o s and d s of these rocks range from 1 4 to 11 and 113 to 45 respectively with a majority of rocks depleted in heavy isotopes relative to normal deep seated values these values do not correspond to those found in ophiolitic sequences which for a comparable temperature range give minimum 18o s 3 these values are explained in terms of water rock interactions involving limited amounts of meteoric waters from high elevations this implies the existence of a very important volcanic edifice over fuerteventura in the miocene oligocene period with elevations between 2 500 and a maximum of more than 4 000 m

generation of plagiogranite by amphibolite anatexis in oceanic shear zones field geochronological and rare earth element ree evidence obtained from the fournier oceanic fragment of the canadian appalachians indicates that its plagiogranite component was generated by the anatexis of amphibolite and not by the fractionation of basic magma we propose that the process occurred in two stages first gabbro was plastically deformed at high temperature to form dry low angle shear zones that subsequently evolved to amphibolite via the addition of light ree enriched hydrothermal solutions second the amphibolite underwent partial melting during shear to yield a migmatite comprising bands of plagiogranite alternating with amphibolite restite the plagiogranite locally coalesced to form pods dikes and lenses that injected the surrounding undeformed gabbro we attribute the development of the fournier plagiogranite to dynamothermal processes occurring in proximity to a spreading center due to asthenosphere induced shear within ocean layer 3 this serves as an important illustration of the dynamic nature of metamorphism and melting that can occur in the ocean crust

an experimental study on the shallow level migmatization of ferrogabbros from the fuerteventura basal complex canary islands abstract spectacular shallow level migmatization of ferrogabbroic rocks occurs in a metamorphic contact aureole of a gabbroic pluton of the tierra mala massif tm on fuerteventura canary islands in order to improve our knowledge of the low pressure melting behavior of gabbroic rocks and to constrain the conditions of migmatization of the tm gabbros we performed partial melting experiments on a natural ferrogabbro which is assumed as protolith of the migmatites the experiments were performed in an internally heated pressure vessel ihpv at 200 mpa 930 1150 c at relatively oxidizing conditions distinct amounts of water were added to the charge from 930 to 1000 c the observed experimental phases are plagioclase an60 70 clinopyroxene amphibole titanian magnesiohastingsites two fe ti oxides and a basaltic k poor melt above 1000 c amphibole is no longer stable the first melts are very rich in normative plagioclase 70 wt this indicates that at the beginning of partial melting plagioclase is the major phase which is consumed to produce melt in the experiments plagioclase is stable up to high temperatures 1060 c showing increasing an content with temperature this is not compatible with the natural migmatites in which an rich plagioclase is absent in the melanosomes while amphibole is stable our results show that the partial melting of the natural rocks cannot be regarded as an in situ process that occurred in a closed system considerable amounts of alkalis probably transported by water rich fluids derived from the mafic pluton underplating the tm gabbro were necessary to drive the melting reaction out of the stability range of plagioclase a partial melting experiment with a migmatite gabbro showing typical in situ textures as starting material supports this assumption crystallization experiments performed at 1000 c on a glass of the fused ferrogabbro with different water contents added to the charge show that generally high water activities could be achieved crystallization of amphibole independently of the bulk water content even in a system with very low initial bulk water content 0 3 wt increasing water contents produce plagioclase richer in an reduces the modal proportion of plagioclase in the crystallizing assemblage and extends the melt fraction high melt fractions of 30 wt could only be observed in systems with high bulk water contents 2 wt this indicates that the migmatites were generated under water rich conditions probably water saturated since those migmatites which are characterized as in situ formations show generally high amounts of leucosomes 30 wt

syntectonic generation and segregation of tonalitic melts from amphibolite dikes in the lower crust striding athabasca mylonite zone northern saskatchewan vapor absent melting in a swarm of amphibolite dikes in the lower crust has produced segregations of tonalitic and trondhjemitic composition in a variety of textures and structures that dramatically illustrate the mechanisms of melt segregation during deformation the 3 2 ga chipman dikes intrude the mylonitized chipman tonalite within the striding athabasca mylonite zone of northern saskatchewan dike emplacement spans a major sinistral transpressive ductile deformation and granulite facies metamorphic event older dikes are intensely folded and sheared younger dikes are undeformed major and trace element analyses indicate that the dikes are tholeiitic basalts similar in composition to midocean ridge basalt thermobarometry finds conditions of 750 850 c 1 0 gpa the youngest most pristine dikes contain hornblende and plagioclase with minor clinopyroxene and garnet older migmatitic dikes have tonalitic to trondhjemitic segregations spatially associated with garnet crystals where small the segregations occupy tails or strain shadows on every garnet crystal where garnets and segregations are large leucosomes form an interconnected network that extends into the host tonalite tonalitic pools probably of dike origin collect in boudin necks and fold hinges the chipman dikes are interpreted to have been emplaced solidified and partially melted during ductile shearing in the lowermost crust perhaps near the base of an archean island arc they appear to offer an exceptional view of magma genesis where underplated mantle derived mafic magmas provide not only heat but also a component of material to new felsic magmas deformation is essential to the process allowing access of mafic magma into the lower crust and facilitating the segregation of the new felsic melts from their source rocks