cadomian subduction collision and variscan transpression in the badajoz cordoba shear belt southwest spain a discussion on the age of the main tectonometamorphic events

an experimental investigation of the behavior of amphibole in the system na 2 o mgo al 2 o 3 sio 2 h 2 o at high pressures the behavior of an amphibole in the system na2o mgo al2o3 sio2 h2o and its relationships with sodium micas have been examined at high pressures a sodium mica of composition intermediate between wonesite and preiswerkite is produced from low pressures to at least 28 kbar at 750 c from differing bulk compositions in water excess experiments minor amphibole has been found in water excess runs but a c2 m amphibole approaching glaucophane composition exists only in water undersaturated systems this amphibole coexists with quartz in runs on glaucophane composition from 18 kbar to 24 kbar at 700 c in this pressure range the amphibole is displaced from ideal glaucophane composition by substitution of naaaliv for si and mgm4mgm2 for nam4alm2 above 24 kbar the a site of this amphibole empties leading to the coexistence of amphibole jadeite quartz between 24 kbar and 28 kbar above 28 kbar talc forms while the modal amount of amphibole decreases tight buffering of the amphibole to ideal glaucophane composition does not occur and small amounts of naalsi 1 substitution are found in the talc this is reflected modally in the persistence of quartz with talc jadeite and amphibole amphibole and quartz continue to decrease in modal proportions with rising pressures but remain in the run products of synthesis experiments to at least 35 kbar at 750 c

the malpica lamego line a major crustal scale shear zone in the variscan belt of iberia abstract the malpica lamego line mll is a deformation zone in the variscan belt of nw iberia nw spain and n portugal that runs parallel to the chain for at least 275 km bearing i type granodiorite plutons along most of its length the mll affects previous structures by which high pressure and ophiolitic rocks were exhumed and emplaced on the iberian plate during earlier deformation phases correlation and reconstruction of the stratigraphy of these sheets or tectonic units at both sides of the shear zone allows a preliminary estimate of the accumulated vertical and horizontal offsets after the tectonic activity of the fault the value of the separations of crustal scale proportions reaches a maximum 15 km of vertical offset that decreases gradually to the south the structural record found in the rocks indicates a strike slip regime that in general does not fit the geometry of the offsets we suggest that the mll went through two different stages during the same orogenic cycle a first dip slip episode a reverse faulting event overprinted by a later strike slip reactivation

provenance constraints from detrital zircon u pb ages in the nw iberian massif implications for palaeozoic plate configuration and variscan evolution detrital zircons from pre orogenic upper ordovician to devonian low grade metasedimentary rocks have been dated by laser ablation inductively coupled plasma mass spectrometry la icp ms to 1 constrain the variations of zircon age populations within the autochthonous sequences of nw iberia 2 compare u pb detrital zircon ages with previous data 3 test the hypothesis of the armorica microplate as a peri gondwanan terrane separated from gondwana the similarity in the ages of detrital zircons found in the palaeozoic samples studied here to those published for the upper proterozoic lower cambrian and lower ordovician sediments of nw iberia argues against the separation of armorica and points to a common source area for all of the palaeozoic detrital formations of nw iberia the west african craton and the surrounding pan african belts la icp ms u pb dating of zircons from a synorogenic flysch preserved in klippen in the core of a syncline establishes a maximum depositional age for this deposit as early namurian this age is also a maximum age limit for thrusting and refolding in this part of the iberian massif correlation of the zircon age populations with published ages confirms the link between the emplacement of the allochthonous complexes of nw iberia and synorogenic sedimentation

p t evolution of eclogites from the agualada unit ordenes complex northwest iberian massif spain implications for crustal subduction abstract eclogite lenses in the agualada unit western ordenes complex spain contain the peak mineral assemblage garnet prograde rim alm 48 mol prp 30 mol omphacite jd max 36 mol quartz rutile and rare zoisite which equilibrated at t 700 c and p 12 14 kbar garnet shows discontinuous growth zoning with a pyrope poor intermediate zone alm 51 mol prp 10 mol between a core zone where pyrope is slightly higher alm 46 mol prp 16 mol and areas just inward from the rims where the maximum pyrope contents alm 48 mol prp 30 mol are recorded in atoll interiors garnet contains inclusions of a first generation of omphacite jd max 40 mol this omphacite is replaced in the matrix by a second generation jd max 36 mol with higher fe fe mg ratio the compositions of garnet and omphacite suggest a complex syneclogitic tectonothermal evolution for the agualada unit characterized by 1 eclogite facies metamorphism t 585 c p 12 13 kbar followed by 2 cooling during a slight decompression t 500 c p 11 12 kbar and 3 a final increase in p and t to form the garnet rim matrix omphacite mineral assemblage the agualada unit is part of a subduction complex which affected the gondwana margin at the beginning of the variscan cycle the p t evolution of the agualada eclogites is closely related to the structural evolution of the accretionary complex and the whole orogenic wedge the cooling event recorded by the agualada eclogites may have resulted from the accretion of a new colder crustal slice under the unit whereas the final progradation reflects the emplacement of the agualada unit directly under the mantle wedge this evolution fits well with previously presented the retical models both for the tectonothermal evolution of accretionary complexes and for the dynamic evolution of orogenic wedges p t paths such as the one for the agualada unit eclogites probably reflect a prolonged structural evolution although evidently rarely preserved such paths are probably the rule rather than the exception during plate convergence

impact vaporization of planetesimal cores in the late stages of planet formation differentiated planetesimals may have delivered iron rich material to earth in giant impacts at the end of accretion impact experiments suggest that the planetesimals iron cores vaporized aiding dispersal and mixing into earth s mantle

high pressure phengite decomposition in the weissenstein eclogite munchberger gneiss massif germany the petrography mineral chemistry and petrogenesis of a sample from the weissenstein eclogite bavaria germany has been investigated the total mineral assemblage comprises garnet clinopyroxenei ii quartz amphibolei ii rutile phengite epidote allanite plagioclase biotite apatite pumpellyite titanite sphene zircon alkali feldspar and calcite textural observations combined with geothermobarometry fe mg distribution between clinopyroxene garnet and phengite garnet jadeite content of omphacite si content of phengite and an content of plagioclase provide indications of two different stages in the metamorphic evolution of the rock the main phengitequartz eclogite mineral equilibration occurred at minimum p 13 17kbar minimum t 620 50 c the retrograde symplectite stage clinopyroxeneii amphiboleii biotite plagioclase occurred at ptotal between 12 and 8 5 kbar

opposite p t t paths of hercynian metamorphism between the upper units of the cabo ortegal complex and their substratum northwest of the iberian massif abstract in the cabo ortegal region two metamorphic groups can be distinguished which have followed opposite p t paths in their hercynian metamorphism the upper group comprises the three uppermost units of the allochthonous complex with mafic and related lithologies of cabo ortegal the lower metamorphic group includes the two basal units of the complex and the lithological sequence outcropping in the ollo de sapo anticlinorium the lithologies of the upper group were affected by high p and medium high t eo hercynian metamorphism during a subduction episode related to the onset of the hercynian orogenic cycle as a result of such metamorphism the basic rocks were transformed into b type eclogites and also into high p mafic granulites the advance of the hercynian collision produced the progressive uplift of the subducted lithologies and finally their emplacement as large thrust sheets over the lower metamorphic group this final evolution is clearly reflected in the p t t paths of the high p lithologies which show pronounced decompressions with only minor cooling the evolution of the lower metamorphic group does not reflect high p events the characteristic regime is a barrovian type medium p metamorphism showing p t t paths with a progressive compression during heating at least partially the burial of the rock association was due to the overthrusting of a thick allochthonous pile this is represented by the upper metamorphic group

correlation of the nappe stack in the ibero armorican arc across the bay of biscay a joint french spanish project a correlation between allochthonous units exposed in the nw iberian massif and the southern armorican massif is carried out based on lithological associations structural position age and geochemistry of protoliths and tectonometamorphic evolution the units on both sides of the bay of biscay are grouped into upper middle and lower allochthons whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the parautochthon the lower allochthon represents a fragment of the outermost edge of gondwana that underwent continental subduction shortly after the closure of a palaeozoic ocean which in turn is represented by the middle allochthon the latter consists of supra subduction ophiolites and metasedimentary sequences alternating with basic mid ocean ridge basalt morb type volcanics with inheritances suggesting the proximity of a continental domain seafloor spreading began at the cambro ordovician boundary and oceanic crust was still formed during the late devonian covering the lifetime of the rheic ocean which is possibly represented by the middle allochthon the opening of the oceanic domain was related to pulling apart the peri gondwanan continental magmatic arc which is represented by the upper allochthon

turbulent mixing of metal and silicate during planet accretion and interpretation of the hf w chronometer in the current view of planet formation the final assembly of the earth involved giant collisions between proto planets 1000 km radius with the moon formed as a result of one such impact at this stage the colliding bodies had likely differentiated into a metallic core surrounded by a silicate mantle during the moon forming impact nearly all metal sank into the earth s core we investigate to what extent large self gravitating iron cores can mix with surrounding silicate and how this influences the short lived chronometer hf w used to infer the age of the moon we present fluid dynamical models of turbulent mixing in fully liquid systems attempting to place constraints on the degree of mixing erosion of sinking cores driven by rayleigh taylor instability does lead to intimate mixing and equilibration but large blobs 10 km diameter do not emulsify entirely emulsification is enhanced if most of the accreting metal cores deform into thin structures during descent through the earth s mantle yet only 1 20 of earth s core would equilibrate with silicate during earth s accretion the initial speed of the impactor is of little importance we proceed to evaluate the mixing potential for shear instabilities where silicate entrainment across vertical walls causes mixing the turbulent structure indicates that vortices remain at the largest scale and do not mix to centimeter length scale where diffusion operates and isotopes can equilibrate thus incomplete emulsification and equilibration of accreting iron cores is likely to occur the extent of metal silicate equilibration provides key information for interpretation of siderophile budgets and the timing of core formation using the hf w chronometer the time scale of core formation derived from the hf w chronometer is usually tied to the last major metal silicate re equilibration believed to coincide with time of the moon forming impact however we show that large cores have limited ability to reset the hf w system in the silicate earth excess 182 w in bulk silicate earth is more sensitive to early core formation processes than to radiogenic ingrowth after the last giant impact

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

fluid dynamics of local martian magma oceans the evolution of a melt region produced by a large impact during mars formation is addressed while some impact induced melt is redistributed during crater excavation sufficiently large impacts much larger than basin forming impacts generate an intact melt region which is retained beneath the excavation zone i e a local magma ocean local magma ocean evolution depends on the effective rheology controlling large scale deformation of the solid part of the planet mechanism of crystallization and melt region size within the uncertainties of various parameters two scenarios are possible for sufficiently weak rheology or large melt region size evolution is characterized by rapid extrusion and formation of a global magma ocean for sufficiently strong rheology or small melt region size in situ crystallization to a partially molten solid state occurs prior to isostatic adjustment subsequent to in situ crystallization local magma ocean evolution depends on melt region size and efficiency of lateral redistribution compared to bulk conductive cooling for large melt regions lateral spreading occurs via plastic deformation and results in an asymmetric global partial melt layer for small melt region size viscous spreading viscous can result in bulk cooling below the solidus prior to formation of a global layer a hypothesis for the origin of the hemispherical crustal dichotomy and tharsis rise is suggested the dichotomy is associated with a global partial melt layer produced by evolution of a large local magma ocean after dichotomy formation evolution of a second smaller local magma ocean is related to tharsis development

high pressure metamorphism in mafic schist from northern vermont the only confirmed occurrence of glaucophane and omphacite in the appalachian mountains of new england is in mafic schist from tillotson peak 44 48 72 33 north central vermont mineral assemblages observed all with epidote sphene magnetite phengitic muscovite carbonate calcite or dolomite apatite sulfide are 1 actinolite glaucophane chlorite garnet quartz 2 actinolite glaucophane chlorite albite garnet 3 actinolite glaucophane omphacite garnet 4 glaucophane chlorite garnet quartz 5 glaucophane omphacite quartz 6 albite glaucophane chlorite quartz and 7 albite paragonite chlorite quartz electron microprobe investigations show that plagioclase chlorite sphene white mica carbonate and magnetite grains are relatively homogeneous however amphibole pyroxene epidote and garnet grains are zoned indicating incomplete equilibration and changing physical conditions during metamorphism glaucophane in assemblage 6 has the general formula rim composition na 1 9 m4 ca 0 1 mg 1 8fe 1 2 2 al 1 0 vi fe 0 3 3 si 7 9 al 0 1 iv o 22 oh 2 glaucophane in assemblages 1 to 5 is somewhat poorer in the glaucophane endmember and richer in actinolite ca rich amphibole grains are zoned extensively with barroisite cores and actinolite rims omphacite grains in assemblages 3 and 5 have the general formula rim composition na 0 5 ca 0 5 mg 0 4 fe 0 1 2 al 0 4 vi fe 0 1 3 si 2o 6 in spite of the significant zoning the partitioning of elements among the minerals at their grain margins suggests that all seven assemblages above may coexist at the same physical conditions of metamorphism in mafic rocks with a relatively small range in bulk composition correlation of mineral compositions and assemblages with mineral stability data suggests high pressure facies series metamorphism at about 9 2 kb and 450 100 c with p h2o p total it is proposed that the metamorphism occurred during the ordovician

tectonic evolution of a continental subduction exhumation channel variscan structure of the basal allochthonous units in nw spain a regional study starting from detailed geological mapping has been carried out in the malpica tui complex of galicia in nw spain the complex is formed by two units representing pieces of the external edge of gondwana subducted and exhumed during the variscan collision the study shows that synsubduction and early synexhumation structures in continental subduction channels tends to be obscured and even erased once exhumation is complete detailed structural analysis matched with the knowledge of the history and available data for other galician basal units have elucidated the major structures developed during the subduction exhumation process the results include evidence of the plate convergence causing early variscan continental subduction of the gondwana margin subduction was followed by exhumation driven by ductile thrusting within the subduction channel which in turn provoked crustal duplication in the subducted slab and modified the initial tectonometamorphic architecture of the subduction wedge the next step was accretion to the adjacent continental domains placing the subduction wedge on top of unsubducted parts of the gondwana margin via ductile thrusting thrusting was preceded by progressive propagation of a train of recumbent folds toward the foreland that affected the previous structural stack subsequent transference of oceanic rheic and peri gondwanan terranes to the gondwana margin took place by out of sequence thrusting followed by crustal extensional collapse and strike slip tectonics

quartz c axis texture mapping of a variscan regional foliation malpica tui unit nw spain abstract mineral assemblages developed under high pressure and low to intermediate temperature conditions h p li t are found in eclogites orthogneisses and metasediments of the malpica tui unit mtu either defining or surrounded by a regional foliation that was mostly re equilibrated under amphibolite to greenschist facies conditions this regional tectonic foliation tracking the exhumation path of the mtu in the context of the variscan collision in sw europe has been studied using the microstructural record in quartz from the different rock types most of the quartz c axis patterns show two distinct asymmetries contained in perpendicular planes which are related to two different non coaxial components of the finite fabric i e the regional foliation and the lineation the asymmetry of deformation inferred from quartz c axis textures suggests that fabric accumulation during exhumation occurred under 3 d general flow departing strongly from the simple shear type this has significant tectonic implications for the interpretation of regional stretching lineations which in the present geodynamic setting of irregular collisional tectonics are not parallel to the direction of general tectonic transport and ultimately to the plate convergence direction

high pressure micro inclusions and development of an inverted metamorphic gradient in the santiago schists ordenes complex nw iberian massif spain evidence of subduction and syncollisional decompression the santiago schists are located in the basal unit of the ordenes complex one of the allochthonous complexes outcropping in the inner part of the hercynian belt in the north west of the iberian peninsula their tectonothermal evolution is characterized by the development of an eo hercynian metamorphic episode c 374 ma of high p low to intermediate t the mineral assemblage of the high p episode is preserved as a very thin si s1 foliation included in albite porphyroblasts being composed of albite garnet i white mica 1 chlorite 1 epidote quartz rutile ilmenite the equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 10 c and 14 7 0 7 kbar probably minimum pressure the later evolution syn d2 of the schists defines a decompressive and slightly prograde p t path which reached its thermal peak at c 525 10 c and 7 kbar decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient so that the zones of garnet ii biotite with an upper subzone with chloritoid and staurolite developed from bottom to top of the formation the estimated p t path for the santiago schists suggests that the basal unit probably a fragment of the gondwana continental margin was uplifted immediately after its subduction at the beginning of the hercynian orogeny it also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension probably related to the continued subduction of the gondwana continental margin and the contemporaneous development of compensatory extension above it the inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin when it came into contact with the upper parts of the schists along a detachment probably of extensional character the general metamorphic evolution of the santiago schists with the development of high p assemblages with garnet prior to decompressive and prograde parageneses with biotite is unusual in the context of the european hercynian belt and shows a close similarity to the tectonothermal evolution of several high p low to intermediate t circum pacific belts

detrital zircon and tectonostratigraphy of the parautochthon under the morais complex ne portugal implications for the variscan accretionary history of the iberian massif zircon geochronology using laser ablation inductively coupled plasma mass spectrometry combined with a new detailed geological map and a stratigraphic study of the parautochthon at the eastern rim of the morais allochthonous complex has provided new insights into the evolution of the iberian variscan belt the data confirmed that the parautochthon is composed of two structural units the higher and more deformed called the upper parautochthon consists of recumbently folded pre variscan low grade metasediments deposited in the northern gondwana passive margin during the opening of the rheic ocean it represents the variscan tectonic duplication of the nw iberian autochthonous sedimentary sequence triggered by the advance of a stack of allochthonous units formed previously in an accretionary prism below this unit the lower parautochthon is a less pervasively deformed tectonic slice including two lithostratigraphic units travanca and vila cha formations composed of culm type synorogenic low grade metasediments deposited in a tectonic trench located between the accretionary prism and a peripheral bulge with detrital zircon age populations pointing to a late devonian early carboniferous age these sediments were subsequently detached from the autochthon along mechanically weak silurian carbonaceous slates the provenance study on the detrital zircons implies that this basin was fed mainly from the active margin supplementary material the complete u pb age results obtained by la icp ms analysis are available at http www geolsoc org uk sup18781

metamorphic amphiboles composition and coexistence amphiboles have long been key players in metamorphic petrology the concept of facies was developed for rocks of basaltic composition eskola 1920 1939 fyfe et al 1958 and within this concept amphiboles had a leading role the first appearance of amphiboles the presence of amphiboles of certain compositions or last appearance of amphiboles that are replaced by pyroxene bearing assemblages mark a number of major facies boundaries the transition from greenschist to amphibolite facies is marked in part by hornblende replacing actinolite and the granulite facies is defined by the breakdown of hornblende to orthopyroxene bearing assemblages the blueschist facies is defined by the appearance of glaucophane while the transition from blueschist facies to eclogite facies conditions is indicated by the breakdown of glaucophane to omphacitic pyroxene bearing assemblages references to amphibole names actinolite glaucophane or rocks characterized by certain amphiboles amphibolite hornblende figure prominently in six facies names turner 1981 liou et al 1985 oh et al 1988 evans 1990 peacock 1993 a large part of the literature that is devoted to metamorphic petrology of amphiboles concerns reactions which produce amphiboles from more hydrated mafic igneous rocks or lead to the destruction of amphiboles as they break down to form anhydrous or nearly anhydrous mineral assemblages consequently amphiboles can be thought of as occupying the wide intermediate portion of p t xfluid space in which metamorphic rocks are found the number of citations of the published amphibole nomenclature serves as a yardstick meterstick of the interest in amphiboles and a minimum estimate of the amount of literature devoted to this mineral two versions of the nomenclature of amphiboles have appeared 1978 and 1997 leake 1978 leake et al 1997 and they have been cited a over 2300 times in the past 28 years this works out to a minimum

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

experiments on fragmentation and thermo chemical exchanges during planetary core formation abstract the initial thermo chemical state of telluric planets was largely controlled by mixing following the collision of differentiated proto planets up to now most models of planet formation simply assume that the iron core of the impactors immediately broke up to form an iron rain within a large scale magma ocean leading to the rapid equilibration of the whole metal with the whole mantle only recent studies have focused on resolving the fluid mechanics of the problem with the aim to define more relevant diffusion advection models of thermal and chemical exchanges within and between the two fluids furthermore the influence of the viscosity ratio on this dynamical process is generally neglected whilst it is known to play a role in the breakup of the initial iron diapirs and in the shape of the resulting droplets here we report the results of analog laboratory experiments matching the dynamical regime of the geophysical configuration high speed video recording allows us to describe and characterize the fluid dynamics of the system and temperature measurements allow us to quantify the diffusive exchanges integrated during the fall of the liquid metal we find that the early representation of this flow as an iron rain is far from the experimental results the equilibration coefficient at a given depth depends both on the initial size of the metal diapir and on the viscosity of the ambient fluid whereas the falling speed is only controlled by the initial size various scalings for the diffusive exchanges coming from the literature are tested we find good agreement with the turbulent thermal model developed by deguen et al 2014

turbulent metal silicate mixing fragmentation and equilibration in magma oceans abstract much of the earth was built by high energy impacts of planetesimals and embryos many of these impactors already differentiated with metallic cores of their own geochemical data provide critical information on the timing of accretion and the prevailing physical conditions but their interpretation depends critically on the degree of metal silicate chemical equilibration during core mantle differentiation which is poorly constrained efficient equilibration requires that the large volumes of iron derived from impactor cores mix with molten silicates down to scales small enough to allow fast metal silicate mass transfer here we use fluid dynamics experiments to show that large metal blobs falling in a magma ocean mix with the molten silicate through turbulent entrainment with fragmentation into droplets eventually resulting from the entrainment process in our experiments fragmentation of the dense fluid occurs after falling a distance equal to 3 4 times its initial diameter at which point a sizable volume of ambient fluid has already been entrained and mixed with the dense falling fluid contrary to previous assumptions we demonstrate that fragmentation of the metallic phase into droplets may not be required for efficient equilibration turbulent mixing by drastically increasing the metal silicate interfacial area may result in fast equilibration even before fragmentation efficient re equilibration is predicted for impactors of size small compared to the magma ocean depth in contrast much less re equilibration is predicted for large impacts in situations where the impactor core diameter approaches the magma ocean thickness

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

laboratory experiments on the breakup of liquid metal diapirs the validity of the iron rain scenario i e the widely accepted model for the dynamics of iron sedimentation through a magma ocean during the latest stage of the earth s accretion is explored via a suite of laboratory experiments liquid gallium and mixtures of water and glycerol are used as analogs of the iron and the molten silicate respectively this allows us to investigate the effects of the viscosity ratio between iron and silicate and to reproduce the relevant effects of surface tension on the fragmentation dynamics while the classical iron rain scenario considers a population of purely spherical drops with a single characteristic radius that fall towards the bottom of the magma ocean at a unique velocity without any further change our experiments exhibit a variety of stable shapes for liquid metal drops a large distribution of sizes and velocities and an intense internal dynamics within the cloud with the superimposition of further fragmentations and merging events our results demonstrate that rich and complex dynamics occur in models of molten metal diapir physics further we hypothesize that the inclusion of such flows into state of the art thermochemical equilibration models will generate a similarly broad array of complex and likely novel behaviors

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

tectonic evolution of variscan iberia gondwana laurussia collision revisited an integrated interpretation of the late paleozoic structural and geochronological record of the iberian massif is presented and discussed under the perspective of a gondwana laurussia collision giving way to the variscan orogen compressional and extensional structures developed during the building of the variscan orogenic crust of iberia are linked together into major tectonic events operating at lithosphere scale a review of the tectonometamorphic and magmatic evolution of the iberian massif reveals backs and forths in the overall convergence between gondwana and laurussia during the amalgamation of pangea in late paleozoic times stages dominated by lithosphere compression are characterized by subduction both oceanic and continental development of magmatic arcs over and under thrusting of continental lithosphere and folding variscan convergence resulted in the eventual transference of a large allochthonous set of peri gondwanan terranes the iberian allochthon onto the gondwana mainland the iberian allochthon bears the imprint of previous interaction between gondwana and laurussia including their juxtaposition after the closure of the rheic ocean in lower devonian times stages governed by lithosphere extension are featured by the opening of two short lived oceanic basins that dissected previous variscan orogenic crust first in the lower middle devonian following the closure of the rheic ocean and then in the early carboniferous following the emplacement of the peri gondwanan allochthon an additional major intra orogenic extensional event in the early middle carboniferous dismembered the iberian allochthon into individual thrust stacks separated by extensional faults and domes lateral tectonics played an important role through the variscan orogenesis especially during the creation of new tectonic blocks separated by intracontinental strike slip shear zones in the late stages of continental convergence

genesis and solvus relations of submicroscopically intergrown paragonite and phengite in a blueschist from northern california electron microbeam techniques have been used to examine submicroscopically intergrown paragonite phengite and chlorite from the south fork mountain schist of the franciscan terrane of northern california which was subjected to blueschist facies metamorphism the sample also contains quartz albite lawsonite and rutile the subassemblage albite lawsonite rutile requires metamorphic conditions on the low temperature side of the equilibrium albite lawsonite rutile paragonite sphene quartz h2o t 200 c and p 7 4 kbars based on thermodynamic data of holland and powell 1990 the white micas appear to be optically homogeneous but back scattered electron images can distinguish two different micas by their slight difference in contrast electron microprobe analyses empa of micas show na na k ranging from 0 2 to 0 8 the two micas are resolved by transmission electron microscopy tem as packets of phengite and paragonite that range from 20 to several hundred nm in thickness the compositions determined by analytical electron microscopy aem constrain the limbs of the phengite paragonite solvus to values of na na k 0 02 and 0 97 representing less mutual solid solution than ever reported by empa the textural relations imply that the sheet silicates were derived from reactions between fluids and detrital clays and that they are in an intermediate stage of textural development we caution that microprobe analyses of apparently homogeneous sheet silicates may yield erroneous data and lead to faulty conclusions using phengite barometry and paragonite muscovite thermometry especially in fine grained rocks that formed at relatively low temperatures

differentiated planetesimal impacts into a terrestrial magma ocean fate of the iron core abstract the abundance of moderately siderophile elements iron loving e g co ni in the earth s mantle is 10 to 100 times larger than predicted by chemical equilibrium between silicate melt and iron at low pressure but it does match expectation for equilibrium at high pressure and temperature recent studies of differentiated planetesimal impacts assume that planetesimal cores survive the impact intact as concentrated masses that passively settle from a zero initial velocity and undergo turbulent entrainment in a global magma ocean under these conditions cores greater than 10 km in diameter do not fully mix without a sufficiently deep magma ocean we have performed hydrocode simulations that revise this assumption and yield a clearer picture of the impact process for differentiated planetesimals possessing iron cores with radius 100 km that impact into magma oceans the impact process strips away the silicate mantle of the planetesimal and then stretches the iron core dispersing the liquid iron into a much larger volume of the underlying liquid silicate mantle lagrangian tracer particles track the initially intact iron core as the impact stretches and disperses the core the final displacement distance of initially closest tracer pairs gives a metric of core stretching the statistics of stretching imply mixing that separates the iron core into sheets ligaments and smaller fragments on a scale of 10 km or less the impact dispersed core fragments undergo further mixing through turbulent entrainment as the molten iron fragments rain through the magma ocean and settle deeper into the planet our results thus support the idea that iron in the cores of even large differentiated planetesimals can chemically equilibrate deep in a terrestrial magma ocean

3d analysis of an ordovician igneous ensemble a complex magmatic structure hidden in a polydeformed allochthonous variscan unit the basal units of the allochthonous complexes of nw iberia represent a fragment of the external edge of gondwana subducted and subsequently exhumed during the variscan collision the structural analysis carried out in orthogneissic massifs of the southern part of one of these the malpica tui unit reveals the generation of recumbent folds and associated axial planar foliation during their exhumation these folds nucleated in irregular igneous bodies that were initially deformed during the subduction event and show east to southeast vergence down plunge projection of surface data and a series of regularly spaced cross sections have been used to build 3d models of the two main bodies of orthogneiss of calc alkaline and peralkaline compositions respectively the first is presently a lens shaped body folded in a recumbent syncline whereas the peralkaline gneiss also affected by a train of asymmetric recumbent folds in the south exhibits in the north a fold like structure which is interpreted as inherited from its primary geometry that of a ring dike pluton

experiments on turbulent metal silicate mixing in a magma ocean we compare the results of laboratory fluid dynamics experiments with theoretical models of turbulent entrainment in two phase buoyant plumes and thermals to estimate the amount of mixing between the iron core of a planetesimal or planetary embryo and the molten silicates in a magma pool formed by its impact with a growing planet during accretion we demonstrate experimentally that turbulent plumes in immiscible fluids fragment into droplets and that the envelope of the droplet distribution is indistinguishable from the envelope of a turbulent plume in a single phase fluid under the same dynamical conditions implying that turbulent entrainment concepts are applicable to immiscible fluids mixing a series of experiments are made on the time evolution of a two phase turbulent thermal representing a planetesimal core falling under its own negative buoyancy in a spherical fluid representing an impact produced magma pool two stages of mixing are observed in these experiments the first corresponding to turbulent entrainment during free fall of the thermal the second corresponding to entrainment in turbulent gravity currents that form after the thermal reaches the base of the magma pool we demonstrate that the amount of mixing between metals and silicate liquids in a magma pool depends on the rouse number of the metal fragments and increases rapidly with the radius of the proto planet we derive a similarity model for metal silicate mixing in a spherical magma pool assuming a constant entrainment coefficient for the turbulent liquid metal phase this model supports our experimental finding that a small metal core of an impactor will become diluted i e mix within a deep magma pool but a large impacting core will not fully dilute before reaching the base of the magma pool

hercynian blueschist metamorphism in north portugal tectonothermal implications aegirine jadeite clinopyroxene 60 mol jadeite locally occurs within blueschists of the lower allochthon exposed in the tras os montes region of northern portugal peak conditions attained during blueschist facies metamorphism are estimated to have been c 420 c and 11 kbar porphyroblastic white mica paragonite phengite within the blueschist assemblage records a 36ar 40ar versus 39ar 40ar isotope correlation age of 329 4 1 6 ma in view of the relatively low t nature of the metamorphism the c 330 ma age is interpreted to date closely the high p recrystallization this tectonothermal activity is interpreted to have resulted from structural emplacement of a previously assembled crystalline nappe complex upper allochthon ophiolite nappe onto iberian protoliths of the lower allochthon during terminal stages of the hercynian orogeny

blueschist and eclogite in mylonitic allochthons ross river and watson lake areas southeastern yukon two new occurrences of eclogite associated with mylonitic rocks are documented the first near ross river is in a quartz rich metasedimentary host that includes glaucophane schist the host forms part of a belt of high pressure rocks more than 50 km long and several kilometres wide between ross river and faro in the second occurrence north of watson lake eclogite is associated with mafic and ultramafic rocks occurring in a klippe more than 100 km2 in area that rests on unmetamorphosed triassic rocks of the north american miogeocline geothermobarometry shows that the eclogites were metamorphosed at 10 15 kb 1 kb 100 mpa between 470 and 750 c these peak conditions are comparable to those for other eclogites in the yukon and adjacent alaska the age of metamorphism and mylonitization is triassic or older the high pressure rocks occur at the leading edge of the most inboard accreted terrane along its boundary with north american miogeoclinal strata over a length of several hundred kilometres t

geochemistry and tectonostratigraphy of the basal allochthonous units of sw iberia evora massif portugal keys to the reconstruction of pre pangean paleogeography in southern europe abstract the basal allochthonous units of nw and sw iberia are members of an intra gondwana suture zone that spreads across the iberian massif and was formed during the collision of gondwana and laurussia in the late paleozoic this suture zone is made of allochthonous terranes and is currently preserved as a tectonically dismembered ensemble a multi proxy analysis is applied to the basal allochthonous units of iberia to test their affinity and potential usage for tracing a suture zone a comparison of the lithostratigraphy tectonometamorphic evolution geochronology and geochemical characteristics of the ediacaran series of these units reveals striking affinities they derive from rather similar immature sedimentary successions deposited along the same continental margin and in relation to a cadomian magmatic arc sm nd systematics indicates that the isotopic sources are among the oldest of the iberian massif ca 2 15 1 5 ga suggesting a very strong contribution from the west african craton these ediacaran series were affected by high p and low to medium t metamorphism blueschist to eclogite facies during the late devonian ca 370 ma they occur below allochthonous ophiolitic sequences and on top of autochthonous or parautochthonous domains lacking of high p and low to medium t devonian metamorphism i e tectonically sandwiched between lithosphere scale thrusts the combination of all these characteristics makes these particular ediacaran series different from the rest of the terranes of the iberian massif such singularity could be useful for tracing more occurrences of the same suture zone along the variscan orogen particularly in cases where its preservation and recognition may be cryptic it also contributes to improve the paleogeographic reconstruction of the margin of gondwana during the ediacaran

allochthonous terranes involved in the variscan suture of nw iberia a review of their origin and tectonothermal evolution abstract nw iberia includes a rather complete section of a variscan suture where different terranes with continental or oceanic affinities appear with clear structural relationships three groups of terranes namely upper ophiolitic and basal units and a frontal tectonic melange appear in galicia in cabo ortegal ordenes and malpica tui complexes they constitute a huge allochthonous pile thrust over the iberian parautochthonous and autochthonous domains which represent the section of the gondwanan margin that escaped continental subduction during the variscan cycle considering the allochthonous character of the nappe pile and the strong deformation associated to the variscan collision there are problems to identify the original tectonic setting of the terranes and thence it is difficult to reconstruct the paleogeographic context during the variscan and pre variscan times in detail key features to perform any model for the variscan convergence should consider the existence of two different high p metamorphic events dated at c 400 and 370 ma respectively separated in time for the generation of mafic ultramafic sequences at c 395 ma which constitute the most common ophiolites described in the variscan suture some dynamic models developed in nw iberia have important problems to explain the observed tectonothermal evolution especially the older high p metamorphic event and the exhumation of deeply subducted transitional type sections on the other hand the recently discovered participation of an older continental crust in the generation of different protoliths of the middle devonian ophiolites makes difficult the interpretation of these ophiolites in relation to open wide oceanic domains this paper describes the distribution structure lithologies geochronology and chemical composition of the terranes involved in the variscan suture of nw spain the scope of this description ranges from detailed regional aspects to the discussion of the development of the variscan orogen in the context of the assembly of pangea a two stage collisional model affecting a wide gondwanan platform may explain most of the evidences in nw iberia the generation of a long pull apart basin probably occurred after the first collision where the devonian ophiolites were formed these ophiolites have been in general described in the context of the rheic ocean but according to the new existing data they do not seem related to this ocean but rather they were formed after its closure

early ordovician continental break up in variscan europe ndsr isotope and trace element evidence from bimodal igneous associations of the southern massif central france abstract the high grade marvejols group and the low grade albigeois cevennes sedimentary sequence contain bimodal igneous rocks of early ordovician age which are representative of a widespread thermal event in the european variscides comparison of their nd sr isotope and trace element characteristics provides additional evidence for their origin in an ensialic extensional setting as an alternative to the back arc model proposed by previous authors we propose that these associations record a continental break up episode unrelated to contemporaneous subduction in this model the widespread early paleozoic bimodal magmatism marks the birth of a mid european oceanic arm we propose that the break up was controlled by both transtensional processes and mantle plume activity high grade associations such as the marvejols group and low grade bimodal associations such as the albigeois cevennes might be regarded as representing opposite rifted passive margins the northern marvejols type margin was involved in a late ordovician silurian subduction like process with hp ht metamorphism in contrast the southern albigeois cevennes margin remained largely unaffected possibly as part of the overriding plate of the subduction zone pseudo calc alkaline signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break up episodes in this case negative nb anomalies are produced by the addition of crustal components enriched in th and lree relative to nb this inferred alternative origin of nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data

singular equilibria in paragonite blueschists amphibolites and eclogites chemographic constraints from high pressure mafic rocks of the introduction nevado filabride complex betic rif mountain belts se spain the relevance of singular equilibria has been largely and from worldwide high pressure terrains have been used to unravel recognized in chemical systems of interest for the earth phase relationships between amphibole garnet clinopyroxene and sciences after the early experiments of konovalov on albite in the system na2o cao feo mgo al2o3 sio2 h2o azeotropic or congruent transformations ricci 1951 ncfmash at quartz paragonite clinozoisiteand waterexhaustively discussed the fundamentals of singularities saturated conditions extensive analysis of element partitioning two types of singularities were recognized the first between amphibole and clinopyroxene suggests that the fe mg called type m and most relevant is associated with azeopartition coefficient is low at low temperature and high pressure tropic points and the second called type k is related to conditions high temperature blueschists low temperature eclogites the intersection of solvi with compositional loops as and a progressive increase is achieved by an increase in temperature occurs in the alkali feldspar system at low pressures or by a decrease in pressure or by a decrease in the sodium content morse 1970 examples and discussion of both type m of the clinopyroxene mass balance analysis of paragonite eclogites and type k singular points in igneous systems have been and amphibolites suggests that at least two singular reactions are given by morse 1980 e g see chapters 16 and 18 the possible in ncfmash clinopyroxene paragonite quartz widespread occurrence of singularities in igneous systems amphibole albite clinozoisite water garnet absent and is obviously related to the extremely large compositional amphibole clinozoisite garnet clinopyroxene paragonite range of silicate liquids the well known concept of quartz water albite absent albite absent singular equithe thermal barrier or thermal divide implies the librium is relevant for the blueschist eclogite transition whereas occurrence of singular equilibria and their importance garnet absent equilibrium may control the eclogite to amphibolite in the petrogenesis of granites e g modelled in the binary transition a complex arrangement of divariant reactions singular system kalsilite silica lindsley 1966 or basalts e g in reactions and conjugate non degenerate reactions arises in topologies the binary system forsterite silica or in the ternary system at the blueschist eclogite amphibolite transitions schreinemakers forsterite silica diopside or forsterite silica anorthite analysis in p t diagrams and mixed variable t xm xc and presnall et al 1979 has been widely discussed p xm xc diagrams suggest that complex pressure temperature in contrast the significance and implications of singular paths are not mandatory if a recurrent appearance and disappearance point nets at subsolidus conditions have been largely of similar phase assemblages is observed in paragonite bearing mafic undervalued although korzhinskii 1963 analysed azeorocks the p t location of the singular curve net is discussed in tropic transformations in subsolidus open systems only the context of nevado filabride complex mafic rocks recently have applications to metamorphic assemblages

heat partitioning in metal silicate plumes during earth differentiation abstract we present an analytical model for core formation that includes both metal diapirs and liquid conduits and produces a superheated core we start from the standard model of earth formation consisting of accretion by planetesimal or planetary embryo impacts large enough to create global or local magma oceans our model consists of two main fluid dynamical structures 1 large liquid metal diapirs and 2 narrow conduits with liquid silicate and small liquid metal droplets we assume that transport of metal from a near surface magma ocean to the growing core occurs within these structures the release of gravitational potential energy from the descent of the metal is converted to heat through viscous dissipation which is broadly distributed around the large liquid metal diapirs but localized in the conduit around the smaller metal droplets this model gives an average core temperature during accretion and a post accretion average core temperature for a broad range of impact histories and chemical quilibration assumptions we consider three types of earth accretion histories with impacts that are evenly spaced over 30 ma these include 1 equal sized impacts 2 equal sized impacts with a final mars sized impact and 3 exponentially decreasing impact sizes with a final mars sized impact our preferred model is based on core mantle chemical equilibrium estimates that correspond to a fractionation f the ratio of metal in the diapir to the total metal content in each impact of f 0 64 and accretion history type 3 this model produces a post accretion average core temperature that ranges from 5930 k to 5700 k for 8 to 90 impacts respectively the efficiency of the metal silicate conduits defined as the ratio of the heating of the core by the conduits to their potential energy released has an approximate value of 0 59 for our preferred model the residual post accretion structures include a basal magma ocean and a network of mantle conduits that may dictate the style of hadean tectonics

the metamorphic evolution of the muxia finisterre region galice nw spain during the hercynian orogenesis mineral chemistry and crystal growth deformation relationships in metapelites orthogneises and metabasic rocks from the muxia finisterre region show that this area has been affected by plurifacial regional metamorphism during the hercynian orogenesis paragenetic evolution seems to be related to a change of the metamorphic gradient towards relatively lower ps conditions in the mesozone while this character is less evident in catazonal rocks in both cases there was a continuous increase in t and decrease in ie657 02

compositional and thermal equilibration of particles drops and diapirs in geophysical flows core formation crystal melt separation mingling of immiscible magmas and diapirism are fundamental geological processes that involve differential motions driven by gravity diffusion modifies the compo sition or and temperature of the considered phases while they travel solid particles liquid drops and viscous diapirs equilibrate while sinking rising through their surroundings with a time scale that depends on the physics of the flow and the material properties in particular the internal circulation within a liquid drop or a diapir favors the diffusive exchange at the interface to evaluate time scales of chemical thermal equilibration between a material falling rising through a deformable medium we propose analytical laws that can be used at multiple scales they depend mostly on the non dimensional peclet and reynolds numbers and are consistent with numerical simulations we show that equilibration between a particle drop or diapir and its host needs to be considered in light of the flow structure complexity it is of fundamental importance to identify the dynamic regime of the flow and take into account the role of the inner circulation within drops and diapirs as well as inertia that reduces the thickness of boundary layers and enhances exchange through the interface the scaling laws are applied to predict nickel equilibration between metals and silicates that occurs within 130 m of fall in about 4 minutes during the metal rain stage of the earth s core formation for a mafic blob 10 cm diameter sinking into a felsic melt trace element equilibration would occur over 4500 m and in about 3 years

a non cylindrical model for the northwest iberian allochthonous terranes and their equivalents in the hercynian belt of western europe abstract a case is made for a non cylindrical development of the european hercynian belt far travelled allochthonous terranes in the belt consist of a mixture of units including catazonal units ophiolites and units derived from the continental margin of gondwana they often depict a metamorphic history of intermediate to high pressure type and caledonian to eo hercynian age and are thought to have been put together in the early stages of the orogeny through a process partly obductive directed onto the continental margin of gondwana considerations upon the disposition of the terranes their relationships with the paleogeography of the gondwana realm and the structures in them suggest that their emplacement took place in an oblique fashion at a small angle to the margin of the continent the proposed allochthon could be related to the development of an island arc or a wedge like subplate in the southern part of the baltica plate later convergence between gondwana and laurussia involving transcurrent mostly dextral movements combined with tectonic escape phenomena would account for the present complexity of the belt and for the narrow strip the allochthonous terranes delineate in westernmost europe

sm nd isotope geochemistry and tectonic setting of the metasedimentary rocks from the basal allochthonous units of nw iberia variscan suture galicia the basal units of the allochthonous complexes of nw iberia are formed by thick metasedimentary rock sequences intruded by granitoids ranging in composition from calc alkaline c 493 ma to minor alkaline peralkaline massifs c 475 470 ma and mafic rocks the granitoids were transformed into variably deformed othogneisses and the associated mafic rocks were transformed into amphibolites blueschists and eclogites during eo variscan high p metamorphism dated at c 370 ma two different superimposed metasedimentary rock sequences can be distinguished the lower sequence maximum depositional age at c 560 ma is mainly composed of metagreywackes while the upper sequence maximum depositional age at c 500 ma consists of mica schists and other minor types major and trace element geochemistry of the metagreywackes of the lower sequence suggests that they were generated in relation to a peri gondwanan arc system built on the thinned continental margin although some chemical transition to passive margin greywackes is also observed this sedimentary sequence was probably deposited in an ediacaran early cambrian back arc setting or retro arc setting closer to the thinned platform of the continental margin the geochemical features of the sedimentary rocks of the upper sequence suggest some affinity with passive margin sediments they were probably deposited closer to the continental domain and to certain distance from the most active zones of the magmatic arc the nd model ages of 23 analysed samples are paleoproterozoic and range between 1782 ma and 2223 ma average value 1919 ma the nd model ages are slightly younger in the upper sequence than in the lower sequence but altogether they define a single population and therefore the two metasedimentary rock sequences can be clearly related sedimentation probably took place within the same basin located in the continental platform of gondwana the main source areas of these sedimentary rocks did not change during the late neoproterozoic and cambrian times the nd model ages are very old and they seem to be compatible with paleoproterozoic or archean source areas with only minor participation of younger sources probably represented by intrusive cadomian pan african granitoids

variscan exhumation of a subducted paleozoic continental margin the basal units of the ordenes complex galicia nw spain a structural and metamorphic study was carried out in the basal units of the ordenes complex in spain thought to represent a subducted part of the paleozoic margin of gondwana according to their metamorphic evolution this part of the margin was subducted at the onset of the variscan orogeny becoming part of an accretionary complex developed below a colliding element built previously variations in the pt conditions of the first high pressure metamorphic event along the units indicate a polarity of the subduction to the west subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units whereas the convergence continued recumbent folds and thrusts developed along with successive normal detachments compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading the unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted units

an experimental investigation of the mechanical behaviour of an unsaturated gneiss residual soil the paper presents compression and triaxial tests conducted under controlled suction in horizons b and c of a residual soil gneissic rock profile from south east brazil horizon b found at the surface consists of a 2 m thick reddish lateritic clayey soil layer whereas horizon c is a saprolitic soil layer that reaches great depths the block samples representing horizons b and c were manually taken from depths of 1 m and 5 m respectively the test programme carried out consisted of isotropic and anisotropic compression tests as well as drained and constant water content triaxial tests tests were performed under saturated and unsaturated conditions at controlled suction strength parameters c and were shown to increase with matric suction the critical state parameters were shown to vary with suction which also expanded the yield curve data obtained may be useful for the development of soil constitutive models for unsaturated soils

direct numerical simulation of an iron rain in the magma ocean core formation in terrestrial planets is a complex process possibly involving several mechanisms this paper presents a direct numerical simulation of one of these the separation of an emulsion of metal in a magma ocean the model using a fully lagrangian approach called the moving particle semi implicit method solves the equations of fluid dynamics including a proper treatment of surface tension it allows investigation of the balances controlling the distribution of drop size and velocity in both two and three dimensional situations a scaling analysis where buoyancy is balanced by both surface tension and inertia correctly predicts the average values in these quantities the full calculation gives an average drop radius of 1 5 cm falling at a velocity of about 30 cm s 1 analysis of the full distribution remains interesting and shows that a significant part of the smallest droplets is entrained upward by the return flow in molten silicate and might be entrained by succeeding thermal convection in addition we investigate the conversion of gravitational energy into viscous heating and the thermal equilibration between both phases we find that viscous heating is essentially produced at the surface of iron drops and that thermal equilibration is dominated by advection scaling thermal diffusion to chemical diffusion leads to the estimation that the latter would happen in less than 100 m in the magma ocean

diachronous variscan tectonothermal activity in the nw iberian massif evidence from 40ar 39ar dating of regional fabrics abstract multigrain concentrates of hornblende and muscovite together with whole rock slate phyllite samples have been dated 27 analyses using 40ar 39ar incremental release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwestern sectors of the variscan iberian massif hornblende concentrates from amphibolites in the allochthonous ordenes complex yield plateau isotope correlation ages of 425 ma and 377 ma muscovite concentrates and whole rock slate phyllite from this and the cabo ortegal complex yield plateau ages which range from 367 ma to 295 ma analyses of similar material from the relative autochthon yield plateau ages between 359 ma and 275 ma muscovite concentrates from three late to post kinematic granitic stocks yield plateau ages between 309 ma and 274 ma at least seven of the 40ar 39ar analyses from metamorphic rocks record variable thermal rejuvenation of intracrystalline argon systems associated with emplacement of proximal granitic stocks the remaining analyses may be used to constrain the local age of various variscan tectonothermal events the oldest fabric ages are recorded in allochthonous units whereas the youngest fabric ages occur along the boundary between internal and external zones middle devonian ages are recorded in the allochthon and suggest a chronological continuity with deformational events in the relative autochthon where variscan deformation initiated in the upper devonian and diachronously prograded eastward the first deformational events recorded in the limit within the internal and external zones occured ca 20 25 ma later lower namurian variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge however the entire orogen remained tectonically active with different structural features forming at different times and at different places an average propagation rate of ca 5 km m y is suggested by consideration of a 20 25 ma difference in correlative fabric ages and present separations

on the relationship between kilometer scale sheath folds ductile thrusts and minor structures in the basal high pressure units of the cabo ortegal complex nw spain abstract large sheath folds in the basal high pressure nappes of the cabo ortegal complex are described and a kinematic interpretation provided the principal penetrative and map structures relate to regional d2 deformation which produced foliations s2 bearing mineral and stretching lineations l2 and several types of folds a type sheath like and folded folds the latter structures are subparallel to the trend of the orogen their attitude suggests that the units involved shared a common tectonic evolution during progressive ductile deformation of an anisotropic medium reconstruction of major geological structures was accomplished through projection of map scale features onto the ductile flow plane and the plane perpendicular to the ductile flow direction the structures reconstructed illustrate their development in the deeper structural levels of an orogenic channel subjected to high pressure metamorphism during the early phases of the hercynian orogeny in nw iberia we argue that orogen normal tectonic displacements of up to a few hundreds of kilometers represent the minor components of the transpression with possibly thousands of kilometers along strike dextral displacement between the intervening plates during subduction collision

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

magmatism and early variscan continental subduction in the northern gondwana margin recorded in zircons from the basal units of galicia nw spain in situ uranium lead dating la sf icpms and sims and lu hf isotope analyses la mc icp ms of zircon from eclogite facies rocks from the basal units of the variscan belt in galicia constrain their magmatic and metamorphic evolution and give some clues about the nature and origin of the involved basement the samples studied are two felsic gneisses two eclogites and one eclogitic gneiss of intermediate composition metatonalite oscillatory zoned zircon cores from the felsic samples gave a main clustering of u pb ages at 493 2 and 494 2 ma and some older ages that represent inherited cores zircon grains from the intermediate and one of the mafi c rocks show no inherited cores and yielded ages of 494 3 and 498 6 ma respectively interpreted as time of protolith crystallization variably developed homogeneous zircon rims in one felsic gneiss yielded an age of 372 3 ma and very tiny zircons of one eclogite gave 350 2 ma both of which we interpret as metamorphic ages the new age data demonstrate that the calc alkaline magmatic suite described in the basal unit is ca 20 ma older than the alkaline to peralkaline plutonic suite of the same unit dated at 472 2 ma rodriguez et al 2007 and thus probably represents a distinct geologic event overgrowth rims are interpreted as metamorphic on the basis of their lu hf and th u ratios the 372 3 age is considered as dating the high pressure high p metamorphism and is essentially in agreement with previous ar ar and rb sr data this high p metamorphism marks the initial early variscan subduction of the gondwana margin the inherited zircon ages and hf isotopic composition of zircons point to a considerable input of crustal material with west african craton provenance to the felsic magma

upper crust reworking during gravitational collapse the bembibre pico sacro detachment system nw iberia abstract the kinematics of the basal allochthon in the sw of the ordenes complex is analysed to constraint its evolution during collisional and postcollisional stages of the variscan orogeny two distinct sequences have been identified in the basal allochthon of this sector the upper and lower sequences in close correlation with the subdivision of the basal allochthon in the malpica tui complex three main tectonic episodes have been established in the basal allochthon a high pressure event related to continental subduction a mesozonal event of regional exhumation by thrusting and recumbent folding and a regional scale extensional episode that resulted in the development of the bembibre pico sacro detachment system the bembibre pico sacro system rejuvenated pre existing shear zones whose weakness favoured the nucleation of the detachments shear zones associated with the detachments overprinted the previous tectonic fabrics under conditions ranging from lower amphibolite to greenschist facies and with heterogeneously distributed ductile to brittle deformation the detachment system is coeval with late orogenic collapse and widespread magmatism and represents its upper crustal expression it correlates with mid and lower crustal flow and the development of gneiss domes at depth in such a way that the basal allochthon can be considered a rheological boundary between the more rigid allochthonous sequences above and the more viscous schistose domain and autochthon below

variscan oceans ages origins and geodynamic implications inferred from geochemical and radiometric data abstract a review of the magmatic associations providing evidence for the existence of extensional regimes during the palaeozoic evolution of variscan europe is presented the available geochronological data allow recognition of two major rifting events one in the early ordovician and the other in the middle late devonian scarce relicts of ophiolitic complexes indicate that both extensional episodes reached a true ocean spreading stage it is suggested that the variscan belt resulted from a complex two stage collision process through successive suturing in the silurian and the early carboniferous of these oceanic domains the superposition of two distinct opening closure cycles involving geographically separate oceanic basins may partly account for the unusual width of the variscan orogen

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

heat partitioning in terrestrial planets during core formation by negative diapirism article i nfo we investigate the gravitational heat partitioning between silicate and metallic phases during core formation by negative diapirism in terrestrial planets we model the dynamic evolution of an iron diapir sinking through a solid silicate proto mantle our calculations include viscous heating and strong temperature and composition dependence of viscosity we show that the diapir mean temperature evolves towards an asymptotic value td which depends on the efficiency of viscous heating and heat transfer between the diapir and its surroundings and on the viscous rheology we derived a simple analytic model that captures the thermal evolution observed in our numerical experiments this model can be applied to determine the heat distribution within terrestrial planets with a large number of diapirs during their growth and early differentiation we show that the partitioning of core formation heating during negative diapirism depends strongly on the size distribution of the iron diapirs a large number of small sinking iron diapirs favors metal silicate heat exchange and leads to a relatively hot core which allows a sustainable dynamo as on earth a small number of large diapirs have the opposite effect and could correspond to a mars like planet with an early dynamo that is no longer active in all cases negative diapirism tends to leave the lowermost mantle significantly hotter than its upper part which could trigger large amounts of melting in the early deep mantle

a re evaluation of metal diapir breakup and equilibration in terrestrial magma oceans abstract due to mechanisms such as impact heating early atmospheric thermal blanketing and radioactive heating the presence of at least one global magma ocean stage in the early histories of terrestrial planets seems unavoidable in such a context a key question to constrain the early thermo chemical evolution of the earth is how much iron diapirs provided by differentiated impactors emulsified during their sinking towards the bottom of an early magma ocean in the past years several workers have focused on this question using however various approaches and making different assumptions while most studies favor rapid breakup and equilibration of iron bodies during their sinking through the magma ocean recent work suggests that iron bodies of size comparable or greater than a few tens of kilometers may preserve most of their initial volume as they reach the bottom of a magma ocean therefore leading to metal silicate disequilibrium to clarify the discrepancies and the differences among studies i have conducted a series of numerical simulations and theoretical calculations to derive the conditions and the timing for the breakup of metal diapirs of any size sinking through a silicate magma ocean with a large range of plausible viscosity values the obtained breakup criterion is used to derive stable diapir sizes and their ability to equilibrate with the surrounding silicates i show that for plausible magma ocean viscosities diapirs with initial radii smaller than the thickness of a magma ocean rapidly break up into stable diapir sizes smaller than 0 2 m at which metal silicate equilibration is rapidly achieved