age of amphibolitic metamorphism in the ophiolitic unit of the morais allochthon portugal implications for early hercynian orogenesis in the iberian massif structurally lower portions of the allochthonous morais complex are comprised of an ophiolitic unit exposed in two contrasting structural units separated by a ductile thrust fault the lower unit is dominated by variably foliated amphibolite whereas the upper unit is largely comprised of metaperidotite and metagabbro most of the ophiolitic unit has recrystallized synkinematically under amphibolite facies metamorphic conditions c 500 c and 5 kbar amphiboles formed during this metamorphism are typically magnesio hornblende an amphibole concentrate from the lower structural unit records a 39 ar 40 ar plateau age of 397 3 4 ma an amphibole concentrate from the upper structural unit displays an internally discordant 39 ar 40 ar age spectrum corresponding to a total gas age of 431 1 27 3 ma gas fractions evolved from each amphibole concentrate at intermediate and high experimental temperatures are characterized by similar apparent k ca ratios and yield well defined 36 ar 40 ar v 39 ar 40 ar isotope correlations which result in similar ages of 384 2 5 2 and 392 4 7 3 ma these are considered to be more reliable than the plateau ages and are interpreted as dating the last cooling through those temperatures required for intracrystalline retention of argon within constituent amphibole grains the associated tectonothermal event represented a first stage of hercynian orogenesis in the iberian massif it was probably related to oceanic closure and associated obduction of granulitic eclogitic massifs which at present structurally overlie the ophiolitic unit

new u pb ages for early ordovician magmatism in central spain felsic orthogneisses occur widely in the ollo de sapo domain of the central iberian zone most of which were previously considered to represent a precambrian basement to the palaeozoic sequences of this domain however new u pb dating across the berzosa riaza shear zone sierra de guadarrama central spain indicates an early ordovician age for the most representative types of orthogneisses in the eastern part of the ollo de sapo domain dated rocks include the volcaniclastic cardoso gneiss 480 2 ma from the low medium grade hanging wall the riaza gneiss 468 8 16 ma mylonitic granite in the berzosa riaza shear zone three types of leucogneiss buitrago gneiss 488 8 10 ma megacrystic granite 482 11 14 ma aplitic vein and 482 8 9 ma gneissic leucogranite and the la morcuera granitic augen gneiss 477 4 ma in the high grade footwall the new age of the cardoso gneiss brackets to the mid late arenig the so called early ordovician sardic unconformity characteristic of the central iberian zone these new ages suggest that the broadly coeval volcanism and plutonism were closely associated with the sardic events and that these orthogneisses were part of a felsic magmatic belt which extended along the ollo de sapo domain of the central iberian zone this magmatic belt is interpreted to have been active during the early ordovician break up of the peri gondwanan margin of the iapetus ocean

structural assemblage of high pressure mantle and crustal rocks in a subduction channel cabo ortegal nw spain 1 high pressure metamorphic fold nappes and ductile thrusts occur in the cabo ortegal upper allochthons they involve arc root lithospheric mantle peridotites high pressure volcanic arc granulites normal mid oceanic ridge basalt eclogites and shallow crustal volcanosedimentary high pressure gneiss units confluence of units from such disparate tectonic settings occurred at an eo hercynian convergent plate margin their internal deformation juxtaposition and high pressure synmetamorphic reworking resulted from kinematically simple but intense and polyphase ductile deformations fold nappes exhibit a remarkable sheath like form and contain mineral and stretching lineations defined by high pressure mineral assemblages subparallel to fold axes we argue that these structures outline the internal organization of high pressure sheets at deep tectonic realms of an oblique subduction collision orogenic channel

tracing crustal evolution in the nw iberian peninsula through the rbsr and upb systematics of palaeozoic granitoids a review abstract the abundant granitoids in the palaeozoic basement of the northwestern iberian peninsula include 470 440 ma old suites early middle ordovician now present as orthogneisses and several variscan suites the age data presently available suggest that the latter have been generated in three separate time intervals 330 320 ma ago late visean early namurian 310 300 ma ago westphalian and 290 280 ma ago late stephanian autunian rb sr whole rock and u pb zircon systematics along with field and chemical evidence testify to the generation of the early and late palaeozoic granitoids from lower palaeozoic and precambrian maximum age 1 7 1 6 ga sedimentary sequences respectively the source rocks contained zircons derived from a late archaean early proterozoic continental crust that probably underlies the whole of western europe although for the greater part modified beyond recognition by younger crustal reworking

the three dimensional upb method generalized models and implications for upb two stage systematics four forms of the three dimensional u pb method established so far are physically generalized within the framework of a u pb two stage evolution such a generalization provides an explicit insight into the principle of conventional u pb discordia dating its implications for both u pb geochronology and pb isotopic geology are illustrated for whole rock systems when applying the three dimensional data processing method to the experimentally determined results on the u and pb concentrations and pb isotopic composition of a given suite of cogenetic samples one cannot only obtain a unique solution to timing of two episodic events i e ages t1 and t2 directly but also shed some light on the feature of the source region of initial pb incorporated at time t1 and on the nature of u pb differentiation at the two episodic events common pb for correction in the discordia dating is clarified to be responsible for the initial pb at t1 rather than t2 such a clarification would have an impact on the conventional u pb geochronology that makes assumptions about the isotopic composition of common pb in calculating the discordias the three dimensional u pb method has advantages over the two dimensional ones u pb discordias or pb pb isochron for treating the results of u pb isotopic measurements on a suite of cogenetic samples because it takes into account three variables instead of two variables in data processing and thus enables straightforward calculation of ages responsible for two episodic events without a priori assumptions this is a particular advantage for treating the u pb isotopic data of whole rock samples and therefore for discovering possible low temperature u pb differentiation responsible for terrane uplift

short duration regional metamorphic event recorded in a variscan subduction channel malpica tui eclogites nw iberia an isotopic resetting event is disclosed at ca 350 ma based on rb sr isotopic analyses whole rock garnet clinopyroxene and phengite fractions of eclogites with coronitic garnet and planolinear eclogites from the malpica tui allochthonous complex nw iberia the eclogites were buried deformed recrystallized and subsequently exhumed in a late devonian subduction channel lu hf and sm nd isotopes date a ca 375 ma metamorphic peak of the subduction exhumation cycle that occurred under 2 6 gpa and 610 640 c and was overprinted by the 350 ma event which occurred still under high pressure conditions the latter resetting is interpreted as a very short duration regional metamorphic phenomenon recorded by high pressure rocks in the west european variscan belt at time scales shorter than those characteristic of orogenic lithosphere in subduction collision cycles

late archaean early proterozoic source ages of zircons in rocks from the paleozoic orogen of western galicia nw spain abstract u pb data are reported for nine suites of zircons and three monazites from the paleozoic orogen in western galicia nw spain one paragneiss and six orthogneisses from the early paleozoic basement and two carboniferous ca 310 ma old intrusions of two mica granite new whole rock rb sr analyses along with earlier data indicate an age of ca 470 440 ma ordovician for the emplacement of the granitic precursors of the orthogneisses monazite from the paragneiss also yields an u pb age of ca 470 ma for all nine investigated suites of zircons the u pb systematics signal the presence of a minor proportion of precambrian zircon the zircon data from two orthogneisses and the paragneiss display roughly linear arrays with upper intercepts between about 3 0 ga and 2 0 ga and lower intercepts between ca 480 and 460 ma the former are interpreted as approaching the age of the old zircon component and the latter as reflecting either the time of crystallization of new zircons from the magma orthogneisses or the time of radiogenic lead loss from the old zircons paragneiss the suites of zircons from all other investigated orthogneisses suffered isotopic disturbance posterior to the granite emplacement 470 440 ma ago in most cases leading to false discordias without geochronological significance similarly the zircons of the two investigated two mica granites do not produce meaningful discordias because of post paleozoic disturbance the monazite u pb systems of the latter granites indicate sub recent lead loss from the high initial 87 sr 86 sr ratios an involvement of precambrian continental crust material is evident in the generation of the early paleozoic suite of granites while the zircon u pb data give evidence of the presence of about 3 0 2 0 ga old late archaean early proterozoic components in the source material zircons from the oldest sedimentary rocks in the area now present as catazonal paragneisses and a likely source for the granites likewise reveal a provenance age of 3 0 2 0 ga the late archaean early proterozoic source rocks in western galicia probably formed part of an archaean continental crust underlying much of western europe but mostly modified beyond recognition by younger events

geodynamic evolution of the continental allochthonous terrane cat of the braganca nappe complex ne portugal the braganca massif in particular its continental allochthonous terrane cat is a keystone in the comprehension of the pre variscan and variscan geodynamic evolution of the eurasian plate in particular of the high grade metamorphic evolution the braganca cat comprises three types of granulites recording quite distinct metamorphic histories and paragneisses with enclosed eclogite boudins it has undergone five characterizable main deformation phases during two orogenic cycles separated by the extensive intrusion of mafic ultramafic layered bodies types 2 and 3 granulites into a pervasively deformed granulitic lower continental crust type 1 granulites geochronological data obtained on high grade metamorphic rocks from braganca and other similar rocks in the variscan fold belt have been typically interpreted in two different ways polycyclical evolution precambrian and paleozoic variscan or monocyclical evolution variscan this second hypothesis is in clear contradiction with field evidence with tectonometamorphic data and even with some geochronological data as shown in this article thus we suggest that the tectonometamorphic evolution of the braganca cat and by comparison of other similar cat in the variscan fold belt took place during two orogenic cycles one precambrian grenvillian pan african cadomian and another one paleozoic variscan

rb sr evidence for the presence of ordovician gsranites in the deformed basement of the badajoz cordoba belt sw spain a rb sr whole rock investigation of suites of samples from two orthogneiss bodies in the almendralejo area of the oporto portalegre badajoz cordoba belt points to an ordovician age for the granitic magmatism about 470 ma for the almendralejo gneiss peralkaline metagranite and about 425 ma for the ribera del fresno gneiss leucocratic subaluminous metagranite both with high initial87sr 86sr ratios this points to a post ordovician hercynian age for the metamorphism and tectonic deformation that affected the area including the gneisses and rules out the alleged cadomian or even older age

petrology structure and geotectonic implications of glaucophane bearing eclogites and related rocks from the malpica tuy mt unit galicia northwest spain abstract the malpica tuy mt unit is a narrow band of basement in galicia northwest spain composed of mica schists paragneisses and calc alkaline to peralkaline orthogneisses with minor amounts of metamorphosed basic rocks garnet omphacite glaucophane barroisite rutile phengite paragonite kyanite zoisite developed in eclogites s s and eclogitic impure sandstones garnet phengite rutile zoisite in the calc alkaline orthogeneisses and garnet phengite rutile chloritoid kyanite in the metasediments biotite breakdown in the orthogneisses and related hornfelses produced garnet with development of coronitic textures plagioclase porphyroblasts with aligned inclusions of rutile and phengite are a common feature of many paragneisses the minimum p t conditions for the equilibrium of garnet omphacite parageneses at the peak of the metamorphism were 600 c 12 2 kbar the mineral chemistry of the orthogneisses and metasediments is compatible with these conditions furthermore the structural data also suggest an in situ origin for the eclogites i e the same as the metamorphism of the enclosing rocks the crystalline area surrounding the mt unit is mainly composed of metasediments and anatectic granites the t at the peak of the metamorphism was higher than 700 c the p being of the order of 5 kbar the two domains represent different parts of the hercynian foldbelt tectonically juxtaposed the juxtaposition is the result of complex tangential tectonics three main phases of compressive deformation having been recognized a first stage of subduction of a thinned crust the materials of the mt unit and subsequent obduction development of hydrated minerals in eclogitic rocks with emplacement of these materials over an already metamorphosed crustal area is suggested the mt unit would represent a remnant of a hercynian thrust nappe

seismic evidence for active underplating below the megathrust earthquake zone in japan determining the structure of subduction zones is important for understanding mechanisms for the generation of interplate phenomena such as megathrust earthquakes the peeling off of the uppermost part of a subducting slab and accretion to the bottom of an overlying plate underplating at deep regions has been inferred from exhumed metamorphic rocks and deep seismic imaging but direct seismic evidence of this process is lacking by comparing seismic reflection profiles with microearthquake distributions in central japan we show that repeating microearthquakes occur along the bottom interface of the layer peeling off from the subducting philippine sea plate this region coincides with the location of slow slip events that may serve as signals for monitoring active underplating

geochemistry of the upper neoproterozoic and lower cambrian siliciclastic rocks and u pb dating on detrital zircons in the central iberian zone spain sandstones and shales from the upper neoproterozoic un succession in the central iberian zone ciz show parallel ree patterns and relatively restricted and similar ranges and average values of some element ratios such as al2o3 tio2 ti nb eu eu la yb n gd yb n and th u this remarkable geochemical homogeneity for related medium and fine grained rocks is unusual and strongly suggests a recycled source area however the lower cambrian lc equivalent rocks are in general terms geochemically less mature more heterogeneous and more fractionated their average ree patterns are practically coincident probably as a consequence of ree redistribution related to the reworking of sediments during a stage of sea level fall in lower cambrian times ti and zr abundances chemical index of alteration cia values and element ratios such as al2o3 tio2 k rb ti nb and rb zr can be used for discriminating purposes between the un and lc siliciclastic rocks detrital zircons from a un and a lc sandstone display morphological differences however u pb data are discordant and cannot be interpreted in a straightforward manner nevertheless age data are compatible with a model in which the continent of gondwana would have supplied zircons showing a bimodal age distribution in variable proportions to the respective sandstone units

evolution of stratigraphic sequences in multisegmented continental rift basins comparison of computer models with the basins of the east african rift system this article presents a series of numerical simulations of the stratigraphic evolution of continental rift basins we model the geomorphic and tectonic processes acting in this depositional environment which contrasts with the traditional approach of modeling the settling of sediments especially in marine clastic basins what is new in our model is that it can simulate the formation of basins with linked half grabens which is a fundamental feature of rift systems the attributes of the numerical simulations such as the overall morphology basin architecture drainage systems and stratigraphy are in good agreement with digital elevation models and reflection seismic lines of the basins of the east african rift system specifically the tanganyika and malawi rifts

origine des mineralisations de pb zn de la vallee de la benoue nigeria d apres la composition en pb des galenes et de l encaissant dans le fosse sedimentaire cretace de la benoue nigeria forme au sein du socle panafricain des indices de pb zn sont localises dans les series d ge albien a cenomanien inferieur ces mineralisations sur fractures sont considerees comme hydrothermales et formees a des temperatures relativement basses 140 c les compositions isotopiques en plomb des gites d abakaliki basse benoue et de zurak ha ute benoue respectivement encaisses dans des formations marines shales noirs et detritiques gres et arkoses montrent pour chaque gite une remarquable homogeneite prises dans leur ensemble elles presentent un caractere faiblement radiogenique pour des galenes d ge cretace situees dans un environment de croute continentale superieure dont la differenciation majeure s est produite a 600 700 ma ce caractere compare aux compositions isotopiques en plomb des roches encaissantes shales gres arkosique de feldspaths du socle panafricain et d un basalte mis en place dans le contexte d ouverture du fosse de la benoue indique une source appauvrie en uranium pour le plomb de ces mineralisations cette source correspondrait aux feldspaths issus de l erosion du socle granitique pan africain et concentres dans les series arkosiques ces series forment l essentiel de la pile sedimentaire albienne en haute benoue et sont localisees a la base des formations marines albiennes en basse benoue

geological constraints on the paleozoic tectonic evolution of tectonostratigraphic terranes in the iberian massif abstract the presence of oceanic paleozoic sequences in the iberian variscan fold belt is used to identify and separate suspected exotic terranes from a reference iberian autochthon terrane which constitutes the greater part of it in this paper the relevant data characterizing the pre accretionary evolution of the paleozoic iberian terranes are presented as well as the available evidence concerning the accretionary process itself emphasis is placed on the analysis of the sedimentary igneous and structural expression of the major events which affected the iberian terranes before during and after their amalgamation the main conclusions of such an analysis may be summarized as follows 1 a recognition of rift and passive margin stages in all the iberian massif terranes during the early paleozoic 2 b late paleozoic accretion of the exotic terranes to the iberian autochthon via a long and complex process variscan orogeny which involved initial subduction underneath the latter and eventual obduction followed by oblique collision of a promontory the iberian autochthon northwestern margin with a northwestern continental block this latter process could account for the large scale crustal imbrication shown by the northern branch of the orogen coeval with predominant strike slip faulting in the southern branch as well as with the formation of the typical arcuate shape of this orogenic belt major present uncertainties relate to the palinspastic provenance of continental suspect terranes and their correlation with similar units in other circum atlantic paleozoic orogens a discussion and speculations on this subject are dealt with in the final section of this paper

a single devonian subduction event for the hp ht metamorphism of the cabo ortegal complex within the iberian massif abstract ion microprobe data are reported for zircons from ultramafic mafic and metasedimentary high pressure hp rocks of the cabo ortegal allochthonous complex galicia nw spain the new data suggest that the protoliths of eclogites morb type and mg and al rich and mafic granulites have formed in early ordovician times 495 11 469 9 and 478 12 ma respectively involving oceanic and or continental mantle sources we have found no evidence for a hp metamorphic event at 480 ma and for the previously proposed second hp event in the silurian instead a single subduction induced pt t loop with a climax around 390 ma early devonian can be deduced from the relevant domains of zircons from the following rocks two different types of eclogites two hp paragneisses one garnet pyroxenite and one spinel harzburgite rapid cooling and exhumation minimum rates of ca 90 c ma and ca 5 8 km ma are derived for the time immediately after the peak conditions after cooling to ca 500 c slower cooling and exhumation rates ca 15 18 c ma and ca 0 18 km ma are probably artefacts due to an episode of reheating at ca 350 ma this interpretation is supported by the scatter of various mineral ages and the presence of synkinematic granitoids a subduction related acadian early devonian hp metamorphism followed by an early carboniferous collision can thus be demonstrated for the first time also for the iberian part of the european variscides

eclogite inclusions from subducted metaigneous continental crust malpica tui allochthonous complex nw spain petrofabric geochronology and calculated seismic properties this study describes the strain geometry crystal plastic deformational features isotopic age of metamorphism and calculated seismic properties of two medium temperature eclogite types from the malpica tui allochthonous complex of variscan nw iberia the eclogite types are eclogites with coronitic garnets and eclogites with a planolinear fabric both of them were buried deformed and recrystallized under maximum p and t of 2 6 gpa and 610 640 c and subsequently exhumed in a late devonian subduction channel the metamorphic peak of the subduction exhumation cycle occurred ca 375 ma ago omphacite petrofabric ties eclogites with coronitic garnet to non coaxial constrictional strain and eclogites with planolinear fabrics to non coaxial flattening strain and stretching along the lineation we also used omphacite crystallographic preferred orientations to calculate and constrain the seismic properties of the eclogites the slight variations in petrophysical properties observed are interpreted to result from variations in the strain regime recorded by pristine eclogites or from variations in the modal proportions of the constituent high pressure minerals we foresee that eclogite in subduction metamorphic complexes might be either seismically undetectable or detected as planar features with high impedance contrasts relative to their host rocks

polyphase variscan emplacement of exotic terranes morais and braganca massifs onto iberian successions evidence from 40ar 39ar mineral ages abstract the braganca and morais nappe complexes comprise internally imbricated continental tectonic units including schists deformed metagranite granulite and local eclogite and a structurally underlying ophiolite nappe high pressure assemblages in some of the continental sequences were variably retrogressed during an amphibolite facies metamorphic overprint by contrast rocks within the ophiolite nappe display prograde amphibolite facies assemblages hornblende concentrates from both tectonic units exposed in the braganca nappe complex record 40 ar 39 ar plateau and isotope correlation ages of c 385 390 ma a similar date is recorded by hornblende within mylonitic amphibolite ophiolite protolith along a ductile thrust which separates the two major nappe units muscovite within blastomylonitic metagranite lagoa augen gneiss in upper structural levels of the morais massif records 370 375 ma 40 ar 39 ar plateau ages muscovite within phyllinite developed along a ductile thrust separating the gneiss from the structurally underlying ophiolite nappe records a 375 ma age together the 40 ar 39 ar data suggest that a regionally significant early variscan amphibolite facies metamorphism occurred prior to c 385 390 ma early devonian this was associated with ductile imbrication of continental and ophiolite structural units with resultant formation of a composite nappe complex the composite nappe complex must have been maintained at relatively high structural levels throughout the remaining variscan evolution because there is no evidence for post 370 ma rejuvenation of intracrystalline argon systems in any of crystalline structural units examined this is in marked contrast to a significant late devonian c 330 ma locally high pressure metamorphism which is recorded in structurally underlying elements of the lower allochthon part of a foreland thrust complex composed of metasedimentary units with iberian protolith affinities together available chronologic constraints indicate emplacement of the previously assembled composite nappe complex into its present relative structural position at c 330 ma with resultant deformation and metamorphism of structurally underlying components of the iberian thrust complex

eclogite plagiopyrigarnite relations in the catazonal complexes of northwest spain abstract eclogites and plagiopyrigarnites in the catazonal complexes have identical major and trace element bulk compositions and very similar lattice fabrics of their constituent minerals on the other hand the chemical composition of and the element partitioning between the respective clinopyroxenes and garnets are significantly different the dimensional fabric of the respective clinopyroxenes is also different indicating dynamic crystallization for the eclogites followed by static recrystallization during their transformation to plagiopyrigarnites the eclogites are of b type facies and the plagiopyrigarnites are high pressure granulites of the clinopyroxene almandine hornblende subfacies oriented omphacite of the eclogites is gradually transformed into mosaic equigranular sodic augite and plagioclase via symplectitic intergrowths in transitional rock types indicating an increase in temperature possibly accompanied by a slight drop in pressure the inferred p t time path is supported by the temperature and pressure of coeval recrystallization established in closely associated peridotite bodies

scales of deformation partitioning during exhumation in a continental subduction channel a petrofabric study of eclogites and gneisses from nw spain malpica tui allochthonous complex

u pb ages of zircons from meta igneous and meta sedimentary rocks of the sierra de guadarrama implications for the central iberian crustal evolution u pb data of zircons from various gneisses of the eastern sierra de guadarrama central spain make it possible to reconstruct the pre hercynian and hercynian history of this area 1 upper intercept ages 2400 ma for meta igneous and 2000 ma for meta sedimentary rocks of discordias defined by data of anhedral zircon fractions document the existence of early proterozoic crust 2 lower intercept minimum ages of anhedral zircon fractions show radiogenic lead loss at about 540 ma due to a thermal event leading to volcanism and emplacement of granitoid rocks into precambrian crust growth of euhedral zircons is possibly related to this event 3 lower intercept minimum ages of about 380 ma defined by anhedral zircon fractions in meta sedimentary rocks prove an early hercynian event according to present knowledge on the metamorphism in the sierra de guadarrama this event could be explained in terms of a barrow type medium pressure metamorphism the inferred cadomian igneous event relates the geological history of central spain with that of western africa to the south and brittany to the north furthermore similarities of the crustal evolution in the area studied and other internal zones of the hercynian belt moldanubian zone french central massiv are confirmed the early hercynian event for the first time affected all the rocks of the area together the pre hercynian evolution of the two complexes is different and the present association of the basement rocks may be explained by thrusting or in terms of early hercynian nappe transport

geochemical and geochronological cross section of the deep variscan crust the cabo ortegal high pressure nappe northwestern spain the high pressure rocks of the cabo ortegal nappe and related massifs are represented by several tectonometamorphic units which underwent separate histories before regional overthrusting and amphibolite facies recrystallization at 380 ma the occurrence of final cooling is recorded by 40ar 39ar and rb sr mica ages of up to 350 ma u pb zircon dating and sm nd and rb sr isotope data together with ree and major and trace element geochemistry provide constraints on the timing of earlier tectonothermal events and the nature of the protoliths the earliest tectonothermal event occurred at 490 480 ma and corresponds to the emplacement of lree enriched magmas of probable calc alkaline affinity and their metamorphism in the granulite facies this occurred at approximately the same time as the eclogite facies metamorphism of morb like protholiths associated with basic to intermediate granulites together this suggests formation of syn accretion granulites in an active margin setting a second high grade metamorphic event is recorded at 420 ma in metasedimentary formations associated with eclogite like assemblages garnet omphacite plagioclase this event is possibly related to a collisional plate tectonic setting in a back arc basin the age and origin of the associated ultrabasic rocks remains unclear but a late pyroxenite vein has yielded an emplacement age of 390 ma

first alkaline magmatism during iberia newfoundland rifting the age and origin of alkaline rocks emplaced into the sediments of the rift related continental lusitanian basin were investigated to constrain earliest magmatic activity occurring prior to oceanic plate formation between iberia and newfoundland the u pb titanite ages are 146 5 1 6 2 sterr 145 3 1 4 and 142 3 1 0 ma and initial pb isotopic ratios of feldspars lie at 18 418 18 978 for 206pb 204pb at 15 594 15 925 for 207pb 204pb and at 37 105 39 216 for 208pb 204pb initial 87sr 86sr ratios measured in the same feldspar fractions lie at 0 705409 0 706462 this episode of magmatic activity lasting for at least 4 2 2 6 myr most likely marks a phase of maximum lithospheric thinning during which zones of weakness were created to allow deep magmas to reach the surface such zones are preferentially re activated palaeozoic faults of the iberian plate the isotope data suggest that the dominant volume of alkaline magmas was generated by partial melting of the metasomatized subcontinental iberian mantle