short paper terrane rotations in the irish caledonides palaeozoic palaeomagnetic data from scotland and ireland have been compared on a terrane model and early caledonide clockwise rotation of a terrane through 65o 30 connemara massif and later anticlockwise rotation of up to 50 in the vicinity of clew bay have been deduced these are the firfirst estimates of terrane rotations in the european part of the caledonian appalachian orogen terrane analysis of orogenic belts is currently a major field of study to a large extent this has been stimulated by studies in the north american cordillera where large rotations and translations have been determined palaeomagnetically see beck et al 1986 and references therein terrane analysis may be applied to older orogenic belts such as the caledonides which have been particularly intensively studied previously published palaeomagnetic data from connemara morris tanner 1977 and nw galway morris et al 1973 had indicated local block rotations but furnished few constraints on either their age and magnitude or on the identities of the blocks this paper relates the currently available palaeomagnetic dataseto a terrane model of w ireland hutton dewey 1986 see fig 1 new data new palaeomagnetic data from the irish caledonides comprise the following connemara gabbro robertson 1988 the dalradian rocks of the connemara massif are unique in the british caledonides in lying to the south of a caledonian ophiolite and also south of a thick sequence of ordovician shales and volcanics of low metamorphic grade in clew bay and the south mayo trough respectively the structural

the role of hybridization in the genesis of hercynian granitodis in the gredos massif spain inferences from sr nd isotopes the gredos massif is one the better exposed granitoid complexes of the iberian massif it is composed mainly of peraluminous granitoids with subordinate basic and ultrabasic complexes the massif also contains mega enclaves of migmatites with which the granitoids show transitional contacts two major magmatic associations have been distinguished in this study 1 one comprises the granitoids with microgranular enclaves the enclaves and basic rocks 2 the other is formed by leucogranites intrusive into the former series and free of microgranular enclaves field relationships and microstructures indicate that the rocks of the first series are related by a dominant hybridization process the sr nd isotopic study reveals that this process is complex relating different end members of mantle and crustal affinities and occurred around 295 ma ago late with respect to the main deformation phases of the hercynian orogeny the granitoids with microgranular enclaves gme are part of an overall mixing trend involving palaeozoic mantle derived magma and melts of older crustal material amphibole bearing gme in general contain greater proportions of the mantle derived component than the cordierite bearing gme the actual mixing processes took place on a variety of scales sometimes between melts which were themselves hybrids on a local scale this hybridization process can be modelled by simple binary mixing as documented in the case of a composite dyke the isotopic signatures of the basic rocks are probably to a large degree the result of interaction with crustal melts though additionally the presence of an enriched mantle source cannot be elmininated microgranular enclaves and their immediate hosts have differing initial sr and nd isotopic signatures indicating that isotopic equilibrium was not attained this suggests that the enclaves did not reside in their final granitic melt for long before cooling of the whole system the enclaves are considered to have been derived from basaltic melts which had fractionated and hybridised to varying degrees late stage peraluminous leucogranites have similar initial nd isotopic compositions to the evolved gme a crustal source with a radically different nd isotopic composition or age does not need to be invoked in their petrogenesis

petrogenesis of hercynian leucogranites from the southern armorican massif contribution of ree and isotopic sr nd pb and o geochemical data to the study of source rock characteristics and ages abstract ree geochemistry and isotopic pb sr nd o compositions have been studied in up to nine separate leucogranite massifs from three structurally adjacent zones in south brittany ages decrease from 340 ma in the north to 300 ma in the south whereas initial 87 sr 86 sr ratios increase from 0 706 to 0 710 0 717 ree abundances are relatively low la n 20 80 chondrites with a strong enrichment of lree relative to hree la n lu n 13 4 26 6 pb isotope ratios are homogeneous for a given massif but heterogeneous among massifs and plot very close to a normal terrestrial lead growth curve at 300 ma ago 18 o 16 o ratios are high and relatively uniform at 10 7 to 12 8 mean 11 9 u pb studies on zircon indicate the presence of an inherited radiogenic pb component and that the sediment dominated source region had been metamorphosed during the siluro devonian ligerian negativee nd t values imply that the source region had been enriched in lree prior to the melting episode based on the combined data large scale partial melting of crustal material dominantly of sedimentary origin is proposed the model age of the source rocks is not very old 600 700 to 1200 ma the pb and sr isotopic variations observed among different massifs is probably the result of age differences in the source materials of the different leucogranites

geochemical characterization and origin of granitoids from the south bohemian batholith in lower austria major and 31 minor elements have been determined in 39 large samples of variscan granitoids from 6 plutons or intrusions from the south bohemian batholith rastenberg weinsberg mauthausen schrems eisgarn and gebharts the granitoids are mainly granites but also diorites tonalites trondhjemites granodiorites average concentrations of ba th u la ce pb nd sr and k in the weinsberg mauthausen and schrems granites exceed those in average felsic i and s type granites by factors ranging between 2 1 and 1 3 the granites melts formed at waterundersaturated conditions and intruded at 10 to 15 km depth during late tectonic and post tectonic phases of the variscan orogeny about 330 to 300 ma ago hydrothermal or low temperature alteration is excluded for the majority of samples from a study of oxygen isotopes the thickness of the plutons is estimated at about 6 km from heat balance constraints by analogy with experimental partial melting three different sources of the granitoids can be identified and chemically characterized 1 the trondhjemites tonalites and diorites in the early rastenberg pluton are products of 15 to 40 melting respectively of a mafic partly amphibolitic lower crust redwitzites from the west bohemian massif which are comparable in age partly resemble the rastenberg rocks the mafic sources of the rastenberg granitoids and redwitzites are crustally contaminated as reflected in their sr nd isotopes 2 the very large syn tectonic weinsberg pluton was formed from about 30 partial melting of a tonalitic lower crust at 800 to 850 c its low proportion of ca 10 restite has a ferrodioritic composition the post tectonic fine grained mauthausen and schrems granites which tend to a granodioritic mode are very low in restite and are also products of melting of a tonalitic source 3 the youngest leuco granite the eisgarn pluton high in si p li rb cs u 87sr 86sr and low in ca sr ba reflects a pelitic source the change from mafic to tonalitic to pelitic source composition for the granitoid sequence may indicate that the depth of melt formation decreased with time the concentration of heavy rare earth elements decreased from weinsberg to eisgarn granites which indicates an increasing proportion of garnet in the source the orogenic heat conformable with a heat flow of about 100 mwm 2 was provided by mafic intrusions an alternative would be a drastic increase of the crustal thickness which cannot be recognized by barometry of the associated metamorphic rocks exposed metamorphic country rocks occur in higher amphibolite facies indicating about 5 kbar pressure mafic intrusions contain gabbros kleinzwettl or have formed quartz diorites gebharts the latter being contaminated by granitic melts from partial melting of the wall rocks mash process largescale contamination by crustal materials can be observed in 18o and in sr nd isotopes the major mafic activity was probably caused by depression of solidus temperatures in the mantle wedge above a subduction zone where water was available from dehydration of subducted ocean crust this water initiated partial melting of ultramafic rocks and metasomatism in the uppermost mantle above the level of melting the water also mobilized highly incompatible elements ba th u la ce pb nd sr and k from the uppermost mantle and transported them into the lower crust indicators of a nearby subduction or collision zone of late variscan age in addition to the specific association of granitoidal rocks are abundant upper mantle tectonites an alternate or additional source of metasomatic fluids may have been dehydration of lower crustal rocks during variscan high grade metamorphism

geological significance of 40ar 39ar mica dates across a mid crustal continental plate margin connemara grampian orogeny irish caledonides and implications for the evolution of lithospheric collisions1 the connemara region is a world class example of a regional scale high temperature metamorphic terrain its rock record documents formation of a bi vergent orogenic wedge and associated calkalkali

s type granite generation and emplacement during a regional switch from extensional to contractional deformation central iberian zone iberian autochthonous domain variscan orogeny zircon grains extracted from s type granites of the meda escalhao penedono massif central iberian zone variscan orogen constrain the timing of emplacement and provide information about potential magma sources simple and composite zircon grains from three samples of s type granite were analyzed by la icp ms new u pb data indicate that granites crystallized in the bashkirian 318 7 4 8 ma overlapping the proposed age range of ca 321 317 ma of the nearby s type granitic rocks of the carrazeda de anciaes lamego and ucanha vilar massifs the timing of emplacement of such s type granites seems to coincide with the waning stages of activity of a d2 extensional shear zone i e pinhel shear zone developed in metamorphic conditions that reached partial melting and anatexis ca 321 317 ma dykes of two mica granites resembling diatexite migmatite are concordant and discordant to the compositional layering and s2 main foliation of the high grade metamorphic rocks of the pinhel shear zone much of the planar fabric in these dykes was formed during magmatic crystallization and subsequent solid state deformation field relationships suggest contemporaneity between the ca 319 317 ma old magmatism of the study area and the switch from late d2 extensional deformation to early d3 contractional deformation inherited zircon cores are well preserved in these late d2 early d3 s type granite plutons u pb ages of inherited zircon cores range from ca 2576 to ca 421 ma the spectra of inherited cores overlap closely the range of detrital and magmatic zircon grains displayed by the ediacaran to silurian metasedimentary and metaigneous rocks of the iberian autochthonous and parautochthonous domains this is evidence of a genetic relationship between s type granites and the host metamorphic rocks there is no substantial evidence for the addition of mantle derived material in the genesis of these late d2 early d3 s type granitic rocks the end arrays of heterogeneous crustal anatectic melts may be just inherited from the source probably reflecting mixing of a range of crustal materials with different ages and primary isotopic signatures the generation of the bashkirian s type granites has been dominated by continental crust recycling rather than the addition of new material from mantle sources

the moldanubian granitoid plutons of austria chemical and isotopic studies bearing on their environmental setting abstract the late palaeozoic moldanubian plutonic terrain of upper austria consists of granitoids that range from s type to i type in composition although many units show transitional compositional characteristics the austrian terrain displays some of the features of continent margin main arc batholiths the expanded compositional range monzodiorite to leucogranite of the rock types the high sr i types and the low to intermediate 87 sr 86 sr initials 0 705 0 708 of these i types however there are also important differences the austrian i types are low cao and high k2o units and transitional and s type granitoids are volumetrically dominant in the austrian terrain there is a scarcity of gabbros and an absence of rocks with positive nd initials the austrian terrain is dominated by granitoids of crustal or predominantly crustal origin we suggest that these formed in an inner continental arc setting rather than a main arc and can be correlated with those of the schwarzwald f r g together these are interpreted as components of a moldanubian inner continent plutonic belt

a deep seismic sounding investigation of lithospheric heterogeneity and anisotropy beneath the iberian peninsula abstract the dimensions of the iberian peninsula the facility of firing large charges in the surrounding waters the well known and relatively uniform geology and prior knowledge of the crustal structure resulted in it being chosen as a study area for the investigation of the seismic structure of the lower lithosphere via refraction wide angle reflection seismic profiling the iberian lithosphere heterogeneity and anisotropy experiment iliha with a star shaped arrangement of six long range dss profiles was carried out in october 1989 the models derived from a first interpretation of the recorded data are presented three crustal profiles cover the same western and central part of the hercynian massif as the mantle profiles the resulting interpretations all include a middle as well as a lower crustal layer above the mantle the velocities of the layers in all three models are similar however the layer depths vary beneath the profiles interpretation of the mantle derived data suggests a layered lower lithosphere one reversed line and an intersecting unreversed line indicate the layering penetrates to at least 90 km depth the homogeneity of these layers contrasts strongly with the heterogeneous hercynian surface geology velocities derived from reflected data from the deep layers suggest the constituent materials are either anisotropic or that the layers suffer a slight regional dip

the neotectonics of the tibetan plateau part of the continuing northward convergence between india and eurasia is probably taken up by strike slip displacement with tibet as a whole moving eastward away from the collision zone and undergoing east west extension molnar and tapponnier 1975 ni and york 1978 on the basis of our interpretation of landsat images we propose that strike slip faulting occurs throughout the tibetan plateau such that its north south cross section is being shortened with little contemporary folding or thrusting

the latest post variscan fluids in the spanish central system evidence from fluid inclusion and stable isotope data abstract the spanish central system has been subjected to repeated fluid incursions which were responsible for a variety of mineralizing episodes including w sn cu zn pb as ag f ba and barren quartz veins these hydrothermal fluids occurred over a 200 ma time period and the latest hydrothermal event is recorded in barren quartz veins this study is a multidisciplinary approach leading to the characterization of the hydrothermal fluids preserved in barren quartz veins which are spatially but not temporally related to hercynian upper crustal granites the veins were dated by the 39 ar 40 ar method and the fluids were examined using petrographic microthermometric chemical and isotopic methods fluid inclusions in barren quartz veins indicate that two fluids were related to this hydrothermal event the main part of the quartz veins were formed from an early low salinity 2 o nacl fluid this fluid was trapped at around 270 25 c and 0 1 1 kbar under sublithostatic to hydrostatic conditions 18 o 9 to 2 and d 70 to 34 5 values indicate a meteoric origin for water with significant water rock interactions the latest h 2 o nacl cacl 2 fluid is found in two types of fluid inclusions a primary liquid vapour type 16 24 wt nacl and 1 12 wt cacl 2 and secondary hypersaline type 7 15 wt nacl and 21 27 wt cacl 2 significant li concentrations in this fluid were confirmed this late ca bearing fluid formed quartz crystals in the central part of the veins and was trapped at 70 140 c at a maximum pressure of 0 5 kbar the low 18 o 20 to 6 and d 137 to 116 values suggest a meteoric origin for this fluid however its high salinity probably requires a source from triassic evaporite basins located in the ne tip of the spanish central system anomalously low isotopic values have been previously reported from kaolinites of lower cretaceous age anomalous climatic conditions during the cretaceous appear to be the main reason to explain this very negative meteoric water strong isotopic depletion in meteoric water has been observed in modern areas with monsoonal climates the hydrothermal evolution of barren quartz veins in the spanish central system is comparable to other hydrothermal post variscan events in central and south western europe related to the opening of the north atlantic during cretaceous time

petrogenesis of malaysian granitoids in the southeast asian tin belt part 1 geochemical and sr nd isotopic characteristics the malaysian granitoids of the southeast asian tin belt have been traditionally divided into a permian to late triassic i type dominated arc related eastern province indochina terrane and a late triassic s type dominated collision related main range province sibumasu terrane separated by the bentong raub paleo tethyan suture that closed in the late triassic the present study however shows that this model is oversimplified and that the direct application of chappell and white s 1974 i and s type classification cannot account for many of the characteristics shared by malaysian granitoids despite being commonly hornblende bearing as is typical for i type granites the roof zones of the eastern province granites are hornblende free in addition the main range province granitoids contain insignificant primary muscovite and are dominated by biotite granites mineralogically similar to many of the plutons of the eastern province in general the malaysian granitoids from both provinces are more enriched in high field strength elements than typical cordilleran i and s type granitoids the mineralogy and geochemistry of the eastern province granitoids and their relationship with contemporaneous volcanics confirm their i type nature the bulk liquid lines of descent of both granitic provinces largely overlap with one another sr nd isotopic data further demonstrate that the malaysian granitoids especially those of the main range were hybridized melts derived from two end member source regions one of which is isotopically similar to the kontum orthoamphibolites and the other akin to the kontum paragneisses of the indochina block however there are differences in the source rocks for the two provinces and it is suggested in this paper that these are related to differing proportions of igneous and sedimentary protoliths the incorporation of sedimentary sourced melts in the eastern province is insignificant which allowed the granites in this belt to maintain their i type nature the presence of minor primary tin mineralization in the eastern province compared to the much more significant tin endowment in the main range is considered to reflect the incorporation of a smaller proportion of sedimentary protolith in the melt products of the former

granulite facies metamorphism in the anatectic complex of toledo spain late hercynian tectonic evolution by crustal extension the anatectic complex of toledo consists of a heterogeneous suite of high grade metamorphic rocks mainly granulitic migmatites of pelitic composition and orthogneisses and different types of syn orogenic granitoids with minor associated basic rocks none of the metamorphic rock types records evidence of the early prograde history and only the peak and retrograde p t conditions can be estimated the intrusion of the syn orogenic granitoids is either slightly before or synchronous with the metamorphic climax these rock types thus can be used to make p t estimates of the peak and retrograde conditions microtextural analysis of reaction textures in conjunction with a petrogenetic grid has enabled construction of an incomplete clockwise p t path that is characterized by an isothermal decompression after or during the metamorphic peak temperatures the peak estimates from different geo thermobarometers are 800 50 c and 4 6 kbar the highest temperatures recorded in this part of the iberian hercynian belt it is proposed that this high t low p metamorphism is at least partially related to an extensional process possibly due to gravitational collapse of previously thickened crust

a strontium neodymium and oxygen isotope study of hydrothermal metamorphism and crustal anatexis in the trois seigneurs massif pyrenees france nd sr and o isotope analyses have been made on metamorphic and igneous rocks and minerals from a 310 340 ma hercynian age metamorphic terrane in the pyrenees france lower paleozoic shales and phyllites have 87sr 86sr values of 0 707 0 717 at 310 ma but model values at 310 ma of 0 709 0 736 based on assumed depositional age of 450 ma and an initial 87sr 86sr 0 707 on a regional scale 87sr 86sr was homogenized to about 0 713 to 0 717 in the higher grade pelitic schists during metamorphism much of this 87sr 86sr exchange occurred at very low grades below the biotite isograd but significant changes also accompanied the 18o lowering of the phyllites 13 to 16 during their transformation to andalusite and sillimanite grade schists 18o 11 to 12 all of these effects are attributed to pervasive interactions with hydrothermal fluids wickham and taylor 1985 the data also show that a syn metamorphic plutonic complex dominated by a biotite granite body was derived by mixing of a relatively mafic magmatic end member 87sr 86sr 0 7025 0 7050 and 18o 7 5 to 8 0 with two metasedimentary sources both having 87sr 86sr 0 715 and 18o 10 0 to 12 0 but with one being more homogeneous than the other the more homogeneous component and the mantle derived magmatic end member dominate at low structural levels within the complex the less homogeneous end member that dominates at high levels is clearly derived from the local paleozoic pelitic schists a rb sr age of 330 20 ma was obtained on hornblende from a deep level within the complex which fixes this age for the regional metamorphism as well although a post metamorphic granodiorite magma body at trois seigneurs also displays heterogeneities in 18o and 87sr 86sr and thus does not give a clear cut rb sr isochron the data are consistent with an emplacement age between 260 and 310 ma similar to ages of other late granodiorites in the pyrenees 143nd 144nd is very uniform within the hercynian crust both at trois seigneurs nd 3 to 7 and elsewhere in the pyrenees almost all igneous lithologies have depleted mantle mid proterozoic model ages consistent with efficient recycling of crustal material following original crustal accretion in this area at about 1600 ma or earlier rb sr mineral ages exhibit a complex cooling history reflecting late hercynian and mesozoic thermal events our results show that profound homogenization of the 87sr 86sr and 18o 16o ratios of large volumes of the crust can occur during regional metamorphism and crustal anatexis particularly in regions undergoing extensional tectonics such processes can significantly modify the isotopic compositions of the protoliths of granitic magmas this may explain why many peraluminous hercynian granitoids of western europe have anomalously low 87sr 86sr initial values compared to their probable sedimentary parent rocks

contrasting chemical and isotopic signatures from neoproterozoic metasedimentary rocks in the central iberian zone spain of pre variscan europe implications for terrane analysis and early ordovician magmatic belts neoproterozoic metasedimentary units from central spain and the northern central iberian zone ciz show distinctive geochemical signatures with respect to the schist greywacke complex which is dominant in the southwestern part of the ciz significant differences occur in major and trace element composition and initial nd isotope ratios which cannot be explained in terms of metamorphic grade and or anatexis in some layers of these metasedimentary sequences nd540 values from metapelites of central spain range from 5 6 to 10 4 whereas those from the schist greywacke complex span a narrower and higher range of 0 8 to 3 1 this difference suggests different source regions the presence of two contrasting early ordovician magmatic belts in both subzones reinforces the idea of different precambrian basement terranes in the ciz these data are crucial for pre variscan paleogeographic reconstructions of the westernmost european terranes and for checking the validity of current subdivisions of the iberian massif

the geochemistry of phosphorus in granite rocks and the effect of aluminium phosphorus has been shown to have a profound influence on the evolution of silicate melt systems it also controls the geochemical behavior of rare earths uranium thorium strontium and other elements through the relationships between melt and accessory phosphates based on experimental data harrison and watson 1984 proposed a model of saturation behavior of apatite in crustal melts as a function of temperature and silica content subaluminous granites fit in well with such a model but departures are evident in the case of peraluminous granites a systematic study of the distribution of p2o5 in granitoids from all over the world shows that there are two modes to describe phosphorus enrichment in peraluminous granitic rocks 1 the hoyos type trend which is parallel to the one in harrison and watson s hw model but with higher p2o5 concentrations at the same silica contents and 2 the pedrobernardo type trend implying an increase in phosphorus in the most sio2 rich differentiates of strongly peraluminous low ca granites pedrobernardo type leucogranites are characterized by the presence of rare phosphates or p rich feld spars since they have excess p2o5 over cao to form normative apatite we here propose the term perphosphorous granites high phosphorus contents appear to be due to elevated apatite solubility in peraluminous granitic melts rather than to high amounts of restitic apatite both the hoyos and pedrobernardo type departures from the hw model can be corrected by using the expression p2o5corrected p2o5hw efactor where p2o5hw is the prediction according to the hw model and factor is asi 1 6429t asi being the al2o3 cao na2o k2o molar ratio we suggest that such departures could be caused by a decrease in the ca activity in the melt as a result of increased peraluminousity the pedrobernardo type trend is a limiting case in which ca activity in the melt is nearly zero and calcium instead of phosphorus becomes the essential structural constituent esc of the apatite when this happens phosphorus starts behaving as an incompatible element that is concentrated in late fluids high p2o5 contents are more characteristic of s than other granite types no significant departures from the hw model have been found for i a or m type granites in central iberia granites associated with u sn and w mineralizations usually have high phosphorus contents values of p2o50 5 wt at sio270 wt are good indicators of economic mineralization

rb sr k ar and fission track ages for granites from penang island west malaysia an interpretation model for rb sr whole rock and for actual and experimental mica data penang island represents the northwestern extension of the western magmatic belt of peninsular malaysia thirty one samples of highly evolved biotite and biotite muscovite granites were used in an integrated study to unravel the complex magmatic tectonic and cooling histories of these rocks highly distorted rb sr whole rock age patterns are evident these are attributed to the partial post magmatic sr homogenization within the granite batholith which led to the rotation of isochrons towards younger ages and higher 87 86 sr intercepts the recognition of this mechanism allowed the establishment of a new rb sr interpretation model the intrusion ages of the granites can be extrapolated based on the evolutionary trend of the initial 87 86 sr including the data of bignell and snelling three episodes of granite emplacement at 307 8 ma 251 7 ma and 211 2 ma are suggested for penang and the nw main range the late triassic intrusive induced a hydrothermal conductive convection system which affected all the granites it is considered to be responsible for the rb sr whole rock age distortion the rb sr and k ar biotite age resetting and the textural and mineralogical changes in the granites the duration of the hydrothermal convections deduced from the rb sr whole rock ages is about 6 ma and 20 ma in the northern and southern parts of penang respectively fast regional cooling to 350 50 c within a time span of 1 3 ma is recognized for the late triassic feringgi intrusive from the mica ages followed by a generally slow cooling rate of about 1 c ma fission track ages in addition indicate blockwise uplift along the n s and nw se tending faults thus resulting in the exposure of deeper crustal levels in southern and eastern penang a change in the tensional regime since oligocene miocene accompanied by a southwest tilting of the island is indicated by the fission track apatite ages variable sometimes younger k ar respectively rb sr biotite ages mainly depend on the degree of hydrothermal overprint at different crustal levels an increase of the reaction surface by grain size reduction influences rb sr and k ar mica ages in similar ways as has been demonstrated by experimental data

sr nd isotopic constraints on the petrogenesis of the central bohemian pluton czech republic the sr nd isotopic data for selected granitoids of the central bohemian pluton show a broad negative correlation with the total range of 87sr 86sr 330 0 7051 0 7129 and nd330 0 2 to 8 9 the older intrusions have more depleted sr nd compositions and calc alkaline geochemistry sazava suite whereas the younger intrusions shift towards k rich calc alkaline blatna suite and shoshonitic rocks icany and certovo b emeno suites with more evolved isotopic signatures the distribution of the data is interpreted as reflecting a diversity of sources and processes rather than a single progressive crustal contamination trend the sazava suite could have originated by partial melting of metabasites or of a mantle source with an isotopic composition close to bulk earth or by hybridization of crustally derived tonalitic and mantle derived magmas variation within the blatna suite is modelled by mixing between a moderately enriched 87sr 86sr 330 0 708 nd330 3 mantle component with either an isotopically evolved metasedimentary component or with more evolved magmas of the suite the icany suite was most probably produced by partial melting of peraluminous lithologies possibly of the adjacent moldanubian unit the certovo b emeno suite evolved from strongly enriched mantle derived magmas 87sr 86sr 330 0 7128 end330 7 either through closed system fractional crystallization or interaction with magma corresponding to leucogranites of the central bohemian pluton

siluro devonian palaeomagnetism terrane emplacement and rotation in the caledonides of western ireland summary the palaeomagnetism of a silurian succession lough mask formation lavas and sediments and intrusive rocks kilbride dolerites and andesites and microgranodiorites killary harbour area in the connemara mayo segment of the irish caledonides is described magnetizations are related to the regional tectonic history with the aid of fold conglomerate and contact tests they describe sequences of shallow to intermediate inclinations in both the eastern and western sectors of this caledonian inlier the shallow components appear to pre date siluro devonian folding while the intermediate ones post date this episode the change in palaeofield inclination is identical to the transition described by contemporaneous deep level silurian to shallow and surface level lower devonian bodies in the british and scandinavian caledonides but declinations are rotated consistently to the west the lower middle silurian vector is rotated clockwise by c 100 this difference is progressively reduced to c 50 by the time of the siluro devonian d2 folding showing that this fold generation now oriented e w developed in parallelism with contemporaneous folds on the ne strike continuation of the caledonides post folding remanences continue to deviate in a clockwise sense from the remainder of the caledonides showing that rotation continued into middle upper devonian times and finally ceased during carboniferous times the integrated effect of this rotation is recognized in ordovician magnetizations from the dalradian metamorphic terrane of south connemara it can be explained by block rotations within a zone of distributed deformation according to the mckenzie jackson model and illustrated by many neovolcanic zones palaeomagnetism defines a tectonic regime commencing with rotations of c 3 myr and lasting from mid silurian to carboniferous times western ireland is sited within a closure gap along the iapetus suture extending from ireland to newfoundland and these terranes were apparently emplaced by progressive elimination of this gap during sinistral transpression across the caledonian orogen

trace element and isotopic sr nd pb o arguments for a mid crustal origin of pan african garnet bearing s type granites from the damara orogen namibia geochronological data major and trace element abundances nd and sr isotope ratios 18o whole rock values and pb isotope ratios from leached feldspars are presented for garnet bearing granites locality at oetmoed and outcrop 10 km north of omaruru from the damara belt namibia for the granites from outcrop 10 km n omaruru reversely discordant u pb monazite data give 207pb 235u ages of 511 2 ma and 517 2 ma similar to previously published estimates for the time of regional high grade metamorphism in the central zone based on textural and compositional variations garnets from these granites are inferred to be refractory residues from partial melting in the deep crust because p t estimates from these xenocrystic garnets are significantly higher 800 c 9 10 kbar than regional estimates 700 c 5 kbar the monazite ages are interpreted to date the peak of regional metamorphism in the source of the granites sm nd garnet whole rock ages are between 500 and 490 ma indicating the age of extraction of the granites from their deep crustal sources for the granites from oetmoed both sm nd and pb pb ages obtained on igneous garnets range from 500 to 490 ma these ages are interpreted as emplacement ages and are significantly younger than the previously proposed age of 520 ma for these granites based on rb sr whole rock age determinations major and trace element compositions indicate that the granites are moderately to strongly peraluminous s type granites high initial 87sr 86sr ratios 0 716 high 18o values of 13 8 negative initial end values between 4 and 7 and evolved pb isotope ratios indicate formation of the granites by anatexis of mid crustal rocks similar to the exposed metapelites into which they intruded the large range of pb isotope ratios and the lack of correlation between pb isotope ratios and nd and sr isotope ratios indicate heterogeneity of the involved crustal rocks evidence for the involvement of isotopically highly evolved lower crust is scarce and the influence of a depleted mantle component is unlikely the crustal heating events that produced these granites might have been caused by crustal thickening and thrusting of crustal sheets enriched in heat producing elements very limited fluxing of volatiles from underthrust low to medium grade metasedimentary rocks may have also been a factor in promoting partial melting furthermore delamination of the lithospheric mantle and uprise of hot mantle could have caused localized high t regions the presence of coeval a type granites at oetmoed that have been derived at least in part from a mantle source supports this model

recycled metaigneous crustal sources for s and i type variscan granitoids from the spanish central system batholith constraints from hf isotope zircon composition abstract the spanish central system scs is one of the largest granite batholiths in the european variscan belt zircons from five granitic intrusions from the eastern scs have been separated and analysed for hf isotopes by laser ablation mc icpms in order to evaluate the nature of granitic magma source two samples are cordierite bearing s type granites the rest are amphibole bearing i type monzogranites hf isotope composition of zircons from these granites defines a narrow range of negative hf values 1 1 to 5 8 typical of crustal protoliths the within sample variation of zircon hf isotope composition is slightly above the analytical uncertainty 1 5 units suggesting the mixing of different magma batches during pluton assembly the s and i type granites display the same range of initial hf values that is in agreement with previous data showing that both scs granite types have similar sr nd o pb isotope signatures these isotopic data suggest that they derived from similar sources while the variation in peraluminosity could be related to different partial melting conditions the presence of a lower ordovician inherited zircon population 478 462 ma at least in i type granites suggests the involvement of cambro ordovician orthogneisses as components in the source of some scs granites inherited zircon population of similar age has also been identified in the scs lower crustal granulites the hf isotope composition of the analysed granite zircons is within that of zircons from the above granulites moreover some evidence of a juvenile input at 560 595 ma is recorded by hf isotope data in zircons from both the scs granites and granulite xenoliths hf isotope signature in conjunction with other chemical isotopic sr nd o pb and geochronological evidences suggests that scs granites were probably derived from metaigneous lower crustal sources the variscan magmatism in central spain is dominantly a crustal reworking event

generation and emplacement of shear related highly mobile crustal melts the synkinematic leucogranites from the variscan tormes dome western spain the tormes dome consists of s type granites that intruded into ordovician augen gneisses and neoproterozoic lower cambrian metapelites metagreywackes at different extents of migmatization s type granites are mainly equigranular two mica granites occurring as 1 enclave laden subvertical feeder dykes 2 small external sill like bodies with size and shape relations indicative for self similar pluton growth and 3 as large pluton bodies emplaced at higher levels than the external ones these magmas were highly mobile as it is inferred from the high contents of fluxing components the disintegration and alignment of pelitic xenoliths in feeder dykes and at the bottom of some sill like bodies field relations relate this 311 ma magmatism u pb monazite to the regional shearing of the d3 variscan event partial melting modeling and the relatively high estimated liquidus temperatures indicate biotite dehydration partial melting 800 840 c and 400 650 mpa rather than water fluxed melting implying that there was no partial melting triggered by externally derived fluids in the shear zones instead the subvertical shear zones favored extraction of melts that formed during the regional migmatization event around 320 ma nd isotope variation among the granites might reflect disequilibrium partial melting or different protoliths mass balance and trace element partial melting modeling strongly suggest two kinds of fertile crustal protoliths augen gneisses and metapelites slight compositional variation among the leucogranites does not reflect different extent of protolith melting but is related to a small amount of fractional crystallization 13 for the equigranular granites which is generally more pronounced in shallower batholitic leucogranites than in the small and homogeneous sill like bodies the lower extent of fractional crystallization and the higher pressure emplacement conditions of the sill like bodies support a more restricted movement through the crust than for batholitic leucogranites

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

geochronological elemental and sr nd hf o isotopic constraints on the petrogenesis of the triassic post collisional granitic rocks in nw thailand and its paleotethyan implications abstract new u pb geochronological petrologic elemental and sr nd hf o isotopic data for the granites from the inthanon and sukhothai zones in nw thailand in conjunction with correlations with sw china are presented to constrain the age and position of the paleotethys ocean in this region and the associated assembly of southeast asia the geochronological data show that the granitic rocks in the inthanon and sukhothai zones herein named group 1 and group 2 granites respectively yield similar crystallization ages of 230 200 ma group 1 samples are characterized by monzogranite and granite with i and s type geochemical affinity and group 2 samples by i type monzogranite and granodiorite they have generally similar chondrite normalized ree and pm normalized multi element patterns but distinct sr nd hf o isotopic compositions group 1 samples have slightly higher initial 87sr 86sr ratios 0 7111 0 7293 but lower end t values 11 1 to 14 1 than those of group 2 samples 87sr 86sr i 0 7073 0 7278 and end t 8 3 to 11 0 group 1 samples show the lower ehf t values 5 4 to 18 2 older tdm 1 62 2 40 ga and higher 18o values 7 95 to 9 94 than those of group 2 samples ehf t of 11 1 to 4 80 tdm of 0 96 1 95 ga and 18o of 4 95 to 7 98 for the triassic crystallization zircons these geochemical signatures are similar to the kwangsian and indosinian granites in the south china and indochina blocks but distinct from those of the gangdese i type granite and sibumasu paleozoic granite our data suggest that group 1 samples mainly originated from the early paleozoic supracrustal rocks containing metapelite and metavolcanic components which had previously experienced the surface weathering group 2 samples were derived from a hybridized source of an old metamorphic and a newly underplated mafic component synthesis of our data with available regional observations indicates that the inthanon zone represents the main suture zone of the eastern paleotethyan ocean in nw thailand and links with the changning menglian suture zone in sw yunnan sw china in nw thailand a switch from the eastward subduction of the paleotethyan oceanic plate to the collision of the sibumasu with indochina blocks occurred at 237 ma and syn and post collisional time being at 237 230 ma and 200 230 ma respectively the late triassic granites in the inthanon and sukhothai zones are representative of the post collisional magmatic products

oroclinal bending in the caledonides of western ireland a mid palaeozoic feature controlled by a pre existing structural grain abstract the caledonides of the west of ireland mark a segment of the caledonian appalachian orogen where the structural grain swings from the ne sw trend that typifies the caledonides of northern britain to an e w orientation the origin of this arcuate structure has previously been proposed to be either primary reflecting the original geometry of promontories and embayments in the laurentian margin in this sector of the orogen or secondary as evidenced by palaeomagnetically determined clockwise rotations in silurian rocks in the region we report the results of a palaeomagnetic investigation of the early to middle ordovician mweelrea ignimbrites llanvirn from an accreted taconic arc the south mayo trough in that part of the western irish caledonides that has an e w trend after removal of a low temperature component of magnetisation l interpreted to represent a viscous component of the earth s present field a high temperature component of magnetisation h was isolated at fifteen sites in four ignimbrite flows h passes a palaeomagnetic fold test yielding a tilt corrected mean remanence direction d 132 6 i 29 1 95 17 9 which corresponds to a palaeopole at 36 4 e 9 9 s the pole is in reasonable agreement with established early to middle ordovician reference poles for laurentia and indicates that no significant rotation occurred between the e w south mayo trough and the laurentian margin after accretion and therefore that the curvature of this segment of the orogen is likely to be primary this contrasts with other results from silurian sequences to the south which record significant post silurian clockwise rotations examination of all reliable palaeomagnetic data from the region indicates that the clockwise rotations are restricted to the rocks south of the south mayo trough we propose that the rotations in the south are linked with the superposition of the pre existing structural grain on the younger rock units during late silurian and early devonian sinistral transpression

reply to comments on nd sr isotopic compositions of lower crustal xenoliths evidence for the origin of mid tertiary felsic volcanics in mexico by k l cameron and j v robinson cameron and robinson question the meaning of model nd ages obtained by ruiz et al 1988a for xenoliths from many localities on the easternmost edge of the sierra madre occidental smo this is a legitimate concern because the age of the xenoliths has direct bearing on the genesis of the voluminous silicic ignimbrites of the smo ruiz et al 1988b the source of these volcanics is the underlying and most important issue cameron and co workers have to be congratulated for their pioneering work on the volcanics of the smo besides the careful field work and k ar dating by f w mcdowell s group at texas summarized in mcdowell and clabaugh 1979 and p e damon at arizona i e damon et al 1981 very little petrochemical data existed for this large volcanic province until cameron started working on the problem of the origin of the silicic volcanics in the late 70 s after detailed field and petrologic studies in the batopilas area bagby 1979 cameron et al 1980 lanphere et al 1980 cameron and hanson 1982 cameron and co workers concluded that most of the smo volcanic felsic in chihuahua was produced by crystal fractionation of mantle derived basalt with little or no crustal assimilation contamination or any other crustal interaction in a more recconnaissance type study of volcanic rocks from various places of the sierra madre occidental in northern mexico see figure 1 of cameron and cameron 1985 it was concluded that the parental mantle material needed to produce the silicic volcanic rocks of the smo had sr and nd isotopic values near chur as cameron and co workers admitted their studies were hampered by a complete lack of knowledge of the mexican lower crust the suggestion was however that the lower crust and upper mantle would be sufficiently different in their isotopic and some chemical compositions and therefore that any interaction between mantle derived and the lower crustal materials would be evident in the final chemical and isotopic compositions of the

evolution of the western european continental crust implications from nd and pb isotopes in iberian sediments abstract this paper presents smz sbnd nd and geochemical data on fine grained sediments of the iberian massif spain together with new and published uz sbnd pb results of selected sedimentary rocks profiles from two major zones cantabrian and central iberian zone have been investigated both profiles exhibit almost complete stratigraphic successions from uppermost precambrian to lower carboniferous paleozoic shallow depth platform sediments are essentially unmetamorphosed the precambrian syn orogenic succession underwent a metamorphic overprint up to biotite zone 147 sm 144 nd ratios range from 0 0988 to 0 1457 and nd contents from 6 to 112 ppm nd today values vary from 16 5 to 7 6 pb contents from 1 7 to 97 ppm samples with high nt t 3 are found to be low in 2 330ma t dm are 1 92 ga cantabrian zone and 1 96 ga central iberian zone as juvenile cadomian crust is present in the iberian massif the oldest components have to be significantly older than 2 ga within the european variscides comparably old t dm were so far only described from the armorican massif france an active margin of the gondwana continent is the most probable provenance of the uppermost precambrian strata as indicated by their cadomian age syn orogenic facies and juvenile components precambrian as well as samples from the precambrian cambrian boundary gave nt t values 4 8 significantly higher than average paleozoic rocks contributions of juvenile cadomian crust to the syn orogenic sediments must be in the order of 30 50 to fit the nd and pb data there is no evidence for any addition of mafic material according to major or trace elements or to sm nd ratios lower cambrian samples are already below 7 3 in nd t thus compared to the total pre existing crust contribution of juvenile crust was very small nd t values of almost all samples ranging in stratigraphic age from 515 to 410 ma are strikingly negative 12 9 to 10 1 and thus indicate the absence of newly differentiated continental crust in the provenance s only the tremadocian of the southern central iberian zone gave high nt t values 4 0 to 1 0 again there is no evidence for addition of mafic material to the strata as precambrian and tremadocian nt t peaks are separated by the lowermost cambrian of nt t 7 juvenile crust may have been added related to the pre ordovician unconformity sardic phase during the middle paleozoic 410 330 ma nt t values increase from 12 to 7 indicating a continuous contribution of juvenile continental crust of 20 as major terranes of the iberian massif were not amalgamated until late paleozoic it can be assumed that the respective juvenile crust was formed within the regime of one or several small subduction zones nd isotope data of this study compared with published results of brittany france suggest that the medio north armoricancentro iberian domain persisted from uppermost precambrian to the variscan

zr lree rich minerals in residual peraluminous granulites another factor in the origin of low zr lree granitic melts there is a significant enrichment in some trace elements in the major residual minerals of peraluminous granulite xenoliths from the lower crust those trace elements are released from the breakdown of accessory phases at high t granulite facies conditions n850 c around 10 35 of zr is hosted in granulite rutile and garnet whereas the entire lree eu budget is controlled by feldspar the zr and ree compatible behaviour of the major granulite phases combined with the scarcity of accessory phases which are mostly included in major granulite minerals leads to a disequilibrium in accessory dissolution in the peraluminous partial melts thus the melt extracts less zr and lree and consequently generates the false impression of having lower t when applying current accessory phase dissolution models

oroclinal bending in the caledonides of western ireland new palaeomagnetic data from the caledonides of western ireland indicate that the silurian rocks of south mayo underwent oroclinal bending following folding in siluro devonian time bending was accommodated on faults cutting the silurian sequence and was driven by strike slip motion across the antrim galway line a recently recognized major curvilinear lineament we propose that the silurian rocks in the east of the region suffered up to 30 clockwise rotation increasing towards the west to approximately 70 this newly inferred differential rotation of the silurian rocks about a sub vertical axis implies that the underlying dalradian rocks of the connemara massif to the south must also have undergone clockwise rotation in late silurian early devonian time this is consistent with published palaeomagnetic data from the connemara gabbro and accounts for the swing in strike of the caledonian orogen in this part of western ireland from this we deduce that the swing in strike of the caledonian rocks elsewhere on the west coast of ireland might have a similar explanation

isotopic and trace element arguments for the lower crustal origin of hercynian granitoids and pre hercynian orthogneisses massif central france abstract the cadomian hercynian basement of the massif central france contains both pre metamorphic orthogneisses 600 410 ma and post metamorphic undeformed granitoids 360 275 ma major trace element and sr nd isotopic analyses of these intrusions indicate slight differences in source material for orthogneisses and two types of underformed granitoid a granodiorite monzogranites and leucogranites orthogneisses have moderately enriched ree patterns la yb 5 3 13 3 variable hree contents ybn 4 2 25 and strong negative eu anomalies eu eu 0 23 0 36 their tnd values are low 5 8 to 1 7 indicating a strong crustal component in the source but tsr values are also generally low 25 to 100 and overlap the field of age corrected isotopic values for lower crustal granulite xenoliths from the massif central the source of the orthogneisses is therefore considered to be dominated by a lower crustal meta igneous component with additional input from a lower crustal metasedimentary source granodiorites monzogranites tend to show strong lree enrichment la yb 11 26 with small eu anomalies eu eu 0 54 0 75 and ybn values between 6 and 8 5 these granitoids form the least evolved end members of a series trending to highly evolved leucogranites with similar la yb ratios 11 36 but with generally larger eu anomalies eu eu 0 24 0 62 and lower ybn values 3 8 initial isotopic ratios of the leucogranites tnd 7 9 to 4 5 and tsr usually 100 mostly fall within the field of age corrected lower crustal metasedimentary granulite xenoliths from the region and it is proposed that these leucogranites were formed by partial melting of lower crustal metasediments granodiorites and monzogranites have tnd between 6 5 and 3 4 and tsr values

sources of hercynian granitoids from the french massif central inferences from nd isotopes and consequences for crustal evolution the french massif central is characterized by a great abundance and variety of granitoids emplaced during hercynian times 360 280 ma distinct nd i and 87sr86sr i are not associated with particular petrographic types however lower nd i values 8 5 occur among the younger leucogranites from the western part of the massif intermediate nd i 6 correlate with aluminous monzogranites and higher values 6 to 3 with calc alkaline granodiorites in the eastern massif central nd i are homogeneous at the regional scale in the western massif central the leucogranites and the monzogranite granodiorites display different trends of nd i with emplacement ages from these results extreme models involving a depleted mantle or an old archean or early proterozoic source may be ruled out the trend of increasing nd i with decreasing age observed in the monzogranite granodiorite group suggests either an increasing contribution of a mantle component or the progressive melting of more mafic more refractory crustal sources or both the reverse trend displayed by the leucogranites might be explained in terms of the upward migration of partial melting across an isotopically zoned metasedimentary pile the large amount of granites may be the result of favourable processes during the collision tectonics combined with the unusual abundance of fertile materials enriched in water and heat producing elements in the pre carboniferous crust

crustal generation of the himalayan leucogranites abstract detailed studies of the himalayan two mica leucogranites such as the manaslu pluton indicate that they have very uniform mineralogical petrological and structural characteristics one can relate their occurrence to the thickest zones of the underlying tibetan slab in these zones migmatization attains its greatest development and vertical extension the granite is emplaced at first along the main disharmonic plane above the main central thrust mct at the top of the tibetan slab infrastructure ductile deformation of the granite is variable the granite being syn to late kinematic with regard to the functioning of the mct major elements are very homogeneous except for na and k implying that p t conditions of melting were relatively uniform the melted material was of a similar composition over a vast volume and the percentage of melting was small 10 15 trace elements are highly variable some are characteristic for very evolved material ta rb cs u or show the link with the tibetan slab ba sr whilst others are problematic th ree ree and th abundances being much less in the granite than in the tibetan slab imply that they have been extracted during one of the main stages of formation possibly by monazite radiogenic pb sr nd and stable o isotopes are consistent with the origin of the granite from the tibetan slab however the heterogeneous sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution h isotope data indicate that magmatic compositions of the main body of the manaslu granite have been preserved late or post magmatic alterations are extremely local in the main pluton to the north another belt of two mica granites occurs whose characteristics are very similar to the high himalaya belt they were probably generated in a similar way during this recent intracontinental evolution

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

middle permian early triassic magmatism in the western pontides nw turkey geodynamic significance for the evolution of the paleo tethys abstract permian triassic magmatic rocks are widely exposed in different tectonic terranes and continental blocks in the aegean region this study is focussed on the istanbul zone and the istranca strandja massif which are located in the western pontides of nw turkey the kirklareli and the tepecik plutons intruded into the metamorphic basement rocks of the istranca massif and the sancaktepe pluton is emplaced into the istanbul paleozoic sedimentary sequences we present new geochemical data for these plutons in order to interpret their petrogenesis and time of emplacement and use this information for geodynamic interpretation the zircon u pb crystallization ages are 268 3 2 1 ma for the kirklareli pluton 249 4 1 5 ma for the tepecik pluton and 257 3 1 5 ma and 253 7 1 75 ma for the the sancaktepe pluton they are high k calc alkaline to shoshonitic granitoids with aluminum saturation index asi values between 0 9 and 1 3 which define them as peraluminous and slightly metaluminous 87sr 86sr i values for the studied plutons range between 0 701123 and 0 707704 143nd 144nd i ratios calculated for crystallization ages are between 0 512052 and 0 512431 and end i values vary from 4 71 to 2 22 tdm model ages range between 0 83 and 1 41 ga these plutons are enriched in liles and lree and depleted in hfses with negative eu anomalies indicating that the melts were derived from mafic crustal sources our new data suggest that the kirklareli pluton was generated in a subduction related magmatic arc and the highly fractionated sancaktepe and tepecik plutons were formed in a back arc setting in response to the northward subduction of the paleo tethys ocean during the mid permian early triassic

u pb and rb sr geochronology of magmatism and metamorphism in the dalradian of connemara western ireland new geochronological data which clarify the timing of syn orogenic magmatism and regional metamorphism in the connemara dalradian are presented u pb zircon data on four intermediate to acid foliated magmatic rocks show important inherited components but the most concordant fractions demonstrate that major magmatism continued until 465 ma whereas the earliest basic magmatism has been dated previously at 490 ma a fine grained fabric cutting granite contains discordant zircons which also appear to be 465 ma old arc magmatism in connemara therefore spanned a period of at least 25 ma recent u pb data on titanite from central connemara which gave a peak metamorphic age of 478 ma are supplemented by u pb data on titanite and monazite from metamorphic veins in the east of connemara which indicate that low p high t regional metamorphism continued there to 465 ma i e at least 10 ma later than in the central region dated previously new rb sr data on muscovites from coarse grained segregations in different structural settings range from 475 to 435 ma in part this range probably also reflects differences in age from west to east with three ages close to 455 ma from the eastern area which is also the site of the lowest pressure metamorphism thermal modelling indicates that at any one locality the duration of metamorphism was probably as little as 1 2 ma the new dates emphasize the complexity in the spatial and temporal distribution of high level regional metamorphism caused by magmatic activity the relatively simple overall distribution of mineral appearance isograds revealed by regional mapping masks the complexity of a prolonged but punctuated metamorphic history related to multiple intrusions primarily in the southern part of connemara the later stages of magmatic activity followed progressive uplift and erosion after the onset of magmatism and were localized in the eastern part of the region

li na metasomatism related to i type granite magmatism a case study of the highly fractionated la pedriza pluton iberian variscan belt abstract this study describes rare albitites associated with highly fractionated i type granitic melts showing a mineral paragenesis that resembles those of high temperature metasomatized rocks fenites associated with alkaline and carbonatitic intrusions the studied albitites occur as lenticular bodies on the meter scale hosted in peraluminous granites and are scattered in a narrow sector next to the peraluminous and highly fractionated i type la pedriza leucogranite lpl pluton in the spanish central system albitites show sharp contacts and preserve textural features of the host granites i e isovolumetric metasomatism these rocks consist of a variably albitized magmatic feldspar framework 70 85 modal and interstitial domains where singular na li fe3 rich amphiboles and pyroxenes crystallized with albite or quartz titanite andradite grossular garnet mg li rich mica magnetite fluorite and apatite are metasomatic accessory minerals mass balance indicates enrichments of na2o feo mgo mno li f be and some hfse high field strength elements such as zn sc and sn and lile large ion lithophile elements depletion u pb data from metasomatic titanite yield 307 10 ma which is coeval with the lpl and within the range of the regional granite emplacement about 309 298 ma the lpl also shows strong compositional affinities f li na sn and zn enrichment with albitites similar sr nd d isotopic signatures between albitites and host granites suggest that magmatic fluids might be dominant during metasomatism however the heterogeneity of 18o which shifts toward lower values in the more distant outcrops suggests contribution of meteoric in origin low salinity waters during prograde metasomatism p t fo2 conditions are estimated at 0 2 0 3 gpa 645 700 c and relatively oxidized conditions we propose that na hfse flux rich residual melts of the lpl expelled two immiscible hot liquids an alkaline low density liquid and a high density hydrosilicate liquid hsl the low density fluid dissolved magmatic quartz and provided albitization of the magmatic feldspar framework by a diffusive mechanism quartz dissolution created microchannels for infiltration of the relatively low viscosity hsl forming tubular channels this fluid was enriched in network modifiers li na f be and cations ca al released after albitization and it precipitated in these microchannels to form the new metasomatic paragenesis quartz albite uncommon li rich mafic silicates and accessory minerals

water saturated and undersaturated melting of metaluminous and peraluminous crustal compositions at 10 kb evidence for the origin of silicic magmas in the taupo volcanic zone new zealand and other occurrences

geochemical characterization and isotopic age of caradocian magmatism in the northeastern iberian peninsula insights into the late ordovician evolution of the northern gondwana margin this paper presents new geochronological geochemical and isotopic data for late ordovician magmatism 455 2 ma from the variscan massifs in the northeastern iberian peninsula this magmatism is mainly composed of aluminous and metaluminous plutonic rocks and a sequence of calc alkaline ignimbrites and epiclastic volcanic rocks which were deformed and metamorphosed during the variscan orogeny the metaplutonic rocks are located in the lower part of a neoproterozoic early cambrian metasedimentary sequence in the canigo massif eastern pyrenees aluminous orthogneisses have isotopic signatures that indicate a crustal origin whereas metaluminous orthogneisses originate from a mixture of juvenile and crustal melts the metavolcanic rocks define a calc alkaline ignimbritic sequence of crustal origin located in the lower part of the late ordovician sequence in the les gavarres massif catalan coastal ranges tdm ages for all the studied late ordovician rocks range from mesoproterozoic to paleoproterozoic 1 3 ga 1 5 ga 1 7 ga and 2 2 ga suggesting a neoproterozoic crust with paleoproterozoic components and an arc signature as the most probable source for the crustal melts the isotopic similarities between the studied samples and other early ordovician magmatic rocks of the central and western iberian massif suggest repeated melt extractions from a common basement as the source for the early and late ordovician magmatism in the iberian realm however the studied massifs are more closely linked to other massifs from the mediterranean realm the most probable tectonic setting for the emplacement of the late ordovician volcanic and plutonic bodies is that of an extensional regime which postdates a previous ordovician contractional event and the opening of the rheic ocean

petrology of basement dominated terranes ii contrasting isotopic sr nd pb and o signatures of basement derived granites and constraints on the source region of granite damara orogen namibia abstract major and trace element and nd sr o and pb isotope data from granites of the high grade central part of the damara orogen khan and outlet gorge areas namibia indicate a dominantly lower crustal origin based on their appearance in field the granites can be divided into red granites white granites and grey granodiorites red granites and some of the white granites from the khan area are isotopically evolved initial e nd 12 5 to 18 9 and were likely derived from metaigneous sources with late archaean to early proterozoic crustal residence ages other white granites are less evolved initial e nd 6 5 to 8 8 and were likely derived from metasedimentary sources that are similar to the country rock metapelites grey granodiorites from the khan and outlet gorge area are also isotopically evolved initial e nd 9 9 to 13 1 but are derived from metaigneous sources with younger late proterozoic crustal residence ages major and trace element data do not support closed system fractional crystallization processes for all samples however some chemical features i e decreasing rare earth element ree abundances with increasing sio 2 underline the importance of crystal fractionation processes for each distinct magmatic pulse isotope data do not support mixing of different crust derived melts or assimilation of crustal rocks by a mafic magma instead highly evolved pb isotope compositions strongly negative e nd values and radiogenic sr isotope compositions argue for an undepleted basement as a potential source the most likely model involves mainly partial melting of different basement rocks of archaean to proterozoic age at different levels within the crust only some granites could be derived by melting of metasedimentary rocks of the pan african cover sequence the consistency of the chemical data with a crustal anatectic origin and the range in radiometric ages suggests that they intruded simultaneously with crustal thickening however some of them may have been emplaced during extensional tectonics c 40 50 ma later than the main period of crustal thickening the heating events that promoted melting of fertile deep crustal rocks might have been caused either by the inferred high heat productivity together with crustal thickening during the main periods of orogeny or by delamination of mantle lithosphere during the final extensional stages of the pan african orogeny

late triassic post collisional granites related to paleotethyan evolution in northwestern lao pdr geochronological and geochemical evidence abstract the late paleozoic to early mesozoic granites exposed in northwestern lao pdr provide important constraints on the tectonic evolution of the eastern paleotethyan ocean and regional correlation with the giant granitic belt in southeast asia new geochronological data show that the granites have late triassic zircon u pb ages of 231 220 ma they are dominated by monzogranite and biotite granite with an i type geochemical affinity these granites are enriched in lrees and liles and depleted in hfses the geochemical variations for these granites indicate the fractional crystallization of plagioclase k feldspar biotite apatite and fe ti oxides their initial 87sr 86sr ratios range from 0 7021 to 0 7105 and end t values from 1 6 to 7 3 zircon in situ ehf t values are in range of 6 0 5 3 peaks at 2 8 and 2 5 respectively hf model ages of 0 67 1 64 ga peaks at 0 83 ga and 1 45 ga respectively and 18o values of 6 6 8 0 similar to the late triassic granitoids from the eastern province in southeast asia these granites originated from a mixed source of ancient metamorphic rocks with juvenile mafic crust the late triassic granites in northwestern lao pdr formed in a post collisional setting in response to the thickened crustal collapse during the assembly of the sibumasu with indochina blocks these granites can southerly link with the eastern granite province of the eastern paleotethyan domain in southeast asia

the architecture of the european mediterranean lithosphere a synthesis of the re os evidence rhenium depletion model ages t rd of sulfides in peridotite xenoliths from the subcontinental mantle beneath central spain the calatrava volcanic field reveal that episodes of mantle magmatism and or metasomatism in the iberia microplate were linked to crustal growth events mainly during supercontinent assembly and or breakup at ca 1 8 1 1 0 9 0 6 and 0 3 ga a synthesis of available in situ and whole rock os isotope data on mantle derived peridotites shows that this type of mantle maximum t rd of ca 1 8 ga is widespread in the subcontinental mantle of europe and africa outboard from the betics maghrebides appenines front in contrast the mantle enclosed within the alpine domain records t rd as old as 2 6 ga revealing a previously unrecognized archean domain or domains in the central and western mediterranean our observations indicate that ancient fragments of subcontinental lithospheric mantle have played an important role in the development of the present architecture of the mediterranean lithosphere

geochemical and isotopic disequilibrium in crustal melting an insight from the anatectic granitoids from toledo spain in the hercynian anatectic complex of toledo act the anatectic granitoids include leucogranitic leucosomes leucogranitic massifs and restite rich granites they show a broad range of initial sr and nd isotope ratios from 0 711 to 0 720 and 0 51164 to 0 51203 respectively which clearly indicate the absence of isotopic homogenization in the melts broadly the ranges reflect the isotopic variation of the metapelitic protoliths if crustal melting occurs under water undersaturated conditions as is the case of the act the generated melts do not isotopically and chemically equilibrate with the granulitic residuum the preservation of heterogeneities could arise through a number of processes 1 duration of the process in which the presence of melts with disequilibrium features and the high solid content of several of the granites in the act point to a very short lived magmatic system 2 limited diffusion in which poorly segregated and volatile undersaturated granitoids have a very restricted opportunity for isotopic and chemical homogenization and 3 magma dynamics in which the low melting rates the high restite and low water contents and the silica rich composition of these granites together with the small volume and high viscosity values of the melts seriously restrict the physical processes leading to homogenization

origin of peraluminous granites and granodiorites iberian massif spain an experimental test of granite petrogenesis the discrimination between potential source materials involved in the genesis of iberian granites and granodiorites as well as the role of mantle crust inter actions are examined using constraints imposed by melting experiments melting assimilation experiments and sr nd isotope systematics the sr nd isotope re lationships indicate the existence of diaerent genetic trends in which juvenile mantle materials are involved by diaerent mechanisms 1 a source trend traced by a particular evolution of the pre hercynian basement and indicating mantle participation at the time of sedimen tation 2 a set of magmatic trends traced by gabbro tonalite enclave granodiorite associations implying the incorporation of new mantle material at the time of granite generation these relationships strongly support a pure crustal origin for the peraluminous leucogranites derived from partial melting of crustal protoliths and a hybrid origin for the peraluminous granodiorites these granodiorites are the most abundant granitic rocks of the central iberian zone ciz of the iberian massif implying that processes of hybridisation by assimilation and or magma mixing played an important role in granitoid production during the hercynian orogeny these hypotheses have been tested by means of melting and assimilation experiments melting experiments in the range 800 900 c and at pressures of 3 6 10 and 15 kbar indicate that 1 several potential source materials such as bt ms gneisses and metagreywackes are suitable for the production of peraluminous leucogranite melts 2 the melt compositions are always leucogranitic re gardless of pressure 3 pressure exerts a strong influ ence on the fertility of the source experiments at 3 kbar produce more than 20 vol of melt compared with less than 5 vol of melt produced at 10 and 15 kbar and at the same temperature the melting assimilation experi ments carried out at 1000 c and 4 7 and 10 kbar and using a proportion of 50 gabbro and 50 gneiss give high melt proportions more than 50 vol and noritic residues these melts have the composition of le ucogranodiorites and overlap with part of the compo sitional range of peraluminous granodiorites of the iberian massif the generation of more mafic granodiorites may be explained by the incorporation of some residual orthopyroxene to the granodiorite mag mas the low solubility of fe mg prevents the gen eration of granodiorite melts with more than 3 wt of mgo feo at all crustal pressures the large volumes of peraluminous hybrid granodiorites produced by assimilation of crustal rocks by mantle magmas imply that an important episode of crustal growth took place during the late palaeozoic hercynian orogeny in the iberian massif

mantle and crustal sources in the genesis of late hercynian granitoids nw portugal geochemical and sr nd isotopic constraints abstract large volumes of granitoids were emplaced in the hercynian central iberian zone during the last ductile deformation phase d3 300 320 ma the biotite rich granitoids are the most abundant 1 syn d3 granodiorites monzogranites 313 319 ma with calc alkaline and aluminopotassic affinities 2 late d3 granodiorites monzogranites 306 311 ma related to subalkaline and aluminopotassic series these granitoids are associated with coeval gabbro norite to granodiorite bodies and or mafic microgranular enclaves both granitoids and basic intermediate rocks show petrological geochemical and isotopic evidence of interaction between felsic and mafic magmas the mantle derived melts represented by shoshonitic gabbro norites were probably derived from an enriched and isotopically homogeneous source sr i 0 7049 to 0 7053 e nd 2 1 to 2 5 in some syn and late d3 plutons there are evidences of essentially crustal granites represented by moderately peraluminous monzogranites of aluminopotassic affinity they have similar nd model ages 1 4 ga but different isotopic compositions sr i 0 7089 to 0 7106 e nd 5 6 to 6 8 revealing a heterogeneous crust potential protoliths are metasedimentary immature sediments and or felsic meta igneous lower crust materials large amounts of hybrid magmas were generated by the interaction of these coeval mantle and crust derived liquids giving rise to slightly peraluminous monzogranites granodiorites of calc alkaline and subalkaline affinities which display more depleted isotopic compositions than the crustal end members sr i 0 7064 to 0 7085 e nd 4 4 to 6 2 petrogenetic processes involving mingling and or mixing and fractional crystallization at variable degrees in multiple reservoirs are suggested a major crustal growth event occurred in late hercynian times 305 320 ma related to the input of juvenile mantle magmas and leading to the genesis of composite calc alkaline and subalkaline plutons largely represented in the central iberian zone

crustal origin of hercynian peraluminous granitic batholiths of central spain petrological geochemical and isotopic sr nd constraints abstract in central spain it is possible to distinguish two main types of peraluminous late hercynian granites the pi types and the ps types the distinction between both types is made on the basis of differences in peraluminosity ps types are more peraluminous than pi types and also on the appearance of some characteristic mafic minerals ps types have biotite cordierite and monazite as the most typical accessory phase whereas pi types have biotite amphibole in the less evolved facies and allanite as the accessory mineral both granite types have similar trace element ratios and initial sr nd and pb isotopic signatures initial 87sr 86sr ratios of both types exhibit a large range from 0 7073 to 0 7193 whereas initial end varies in a restricted range from 5 4 to 6 6 the scarce associated basic rocks do not play a significant role in the chemical variability of these peraluminous granites which follow low pressure crystal fractionation trends from granodiorite monzogranite parental magmas mixing and afc modelling of sr and nd isotopic data reveal an unrealistically high mantle contribution based on major and trace element and isotopic data an orthogneissic protolith for both granitic series is proposed nevertheless none of the metamorphic country rocks of central spain has the appropriate sr isotopic composition to satisfy the origin of these granitic rocks and so it is argued that progressive isotopic re equilibration of crustal material during the granulization of the lower hercynian crust together with the possibility of isotopic disequilibrium during melting as has been demonstrated in migmatitic terranes in nearby areas may explain the isotopic differences between the granites and the high level metamorphic country rocks

crust and mantle contributions to granite genesis an example from the variscan batholith of corsica france studied by trace element and ndsro isotope systematics the corsican batholith formed during the variscan orogeny was studied with particular emphasis on the respective roles played by crustal and mantle derived material in granite genesis major and trace element data identify the various types of magma and their genetic relationships the magmatic processes that gave rise to the observed rocks and the primitive liquids o sr nd isotope systematics constrain the origin of primitive liquids trace element models provide data on the various source materials and their possible geodynamic setting the evolution of two main calc alkaline plutonic associations is compared an older high mg k association u1 with both ultrapotassic mafic and silicic rocks the latter ranging from monzonite to leucosyenogranite and a younger calc alkaline composite association u2 involving a mafic cumulate sequence and granodiorite to leucomonzo granite both types of granite are accompanied by mafic units that were non cogenetic with surrounding granitic melts trace element calculations indicate that a primitive liquid of monzodioritic composition gave rise to the u1 mg k granite by fractional crystallization amphibole and titanite playing a major role which with increasing crystallization gave decreasing ree contents without a strong negative eu anomaly the u2 calc alkaline suite was formed by fractional crystallization involving feldspar and lree rich minerals i e monazite from a liquid of monzogranitic composition field and petrographical data identify magma mingling in both u1 and u2 between mafic and silicic rocks but geochemical data only indicate how magma mixing processes led to u2 granodiorite geochemical modelling shows that a single protolith of calculated graywacke composition yielded the two associations under different melting conditions the high mg k monzodiorite melt was formed after partial melting of a 30 non modal batch of granulite facies metamorphic protolith low ph2o but the calc alkaline monzogranite was formed by the same process in an amphibolite facies source of similar composition higher ph2o the primitive granite magmas of both associations show the same crustal characteristics i e sri 0 706 0 707 nd t 4 3 to 2 2 and 18o 7 to 8 this provides evidence that composition of both u1 and u2 granite melts probably was controlled by physico chemical fusion rather than protolith composition parameters the u1 ultrapotassic mafic rock is thought to be of mantle origin i e a deep source containing phlogopite garnet zone refining led to a significant increase of incompatible trace element contents during ascent of the magma mantle crust interaction lowered the la yb and 143nd144nd ratios but the 87sr86sr ratio increased nevertheless interaction with associated u1 mg k granite was not important and the crustal component is thought to be different from the associated granitic magma in u2 the mafic cumulate sequence clearly of mantle origin has e morb characteristics and seems the result of 10 non modal partial melting of a heterogeneous mantle of spinel or amphibole peridotite without garnet some interaction occurred with the associated granitic melt the e morb character of the mafic rock indicates that a large part of the batholith and at least the u2 magmatic association was generated under post collisional extensional conditions

presence of palaeoproterozoic and archean components in the granulite facies rocks of central iberia the hf isotopic evidence abstract hf isotope analyses in previously dated zircon crystals from four variscan granulite facies rocks from central iberia have been studied to examine crustal evolution processes two samples are from exposed migmatite terranes the other two are granulite xenoliths from lower crustal levels hf model ages show that lower crustal granulites display a bigger contribution of archean components than exposed high grade rocks 14 of the zircon population instead of 6 reinforcing ideas of preservation of ancient crustal material in the lower crust the distribution of age hf isotope data from zircon suggests two juvenile crust formation events in central iberia the main one during the neoproterozoic 540 620 ma and a possible second mesoproterozoic event around 1 0 1 2 ga magmatism appears in the central iberian zone exclusively related to that younger juvenile addition hf data of granulite zircons agrees with recent palaeogeographical reconstruction models which present central iberia as a peri gondwanan terrane located between the west african craton and the arabian shield far from the amazonia microplate

residence and redistribution of ree y zr th and u during granulite facies metamorphism behaviour of accessory and major phases in peraluminous granulites of central spain accessory minerals are thought to play a key role in controlling the behaviour of certain trace elements such as ree y zr th and u during crustal melting processes under high grade metamorphic conditions although this is probably the case at middle crustal levels when a comparison is made with granulite facies lower crustal levels differences are seen in trace element behaviour between accessory minerals and some major phases such a comparison can be made in central spain where two granulite facies terranes have equilibrated under slightly different metamorphic conditions and where lower crustal xenoliths are also found differences in texture and chemical composition between accessory phases found in leucosomes and leucogranites and those of melanosomes and protholiths indicate that most of the accessory minerals in melt rich migmatites are newly crystallized this implies that an important redistribution of trace elements occurs during the early stages of granulite facies metamorphism in addition the textural position of the accessory minerals with respect to the major phases is crucial in the redistribution of trace elements when melting proceeds via biotite dehydration melting reactions in granulitic xenoliths from lower crustal levels the situation seems to be different as major minerals show high concentration of certain trace elements the distribution of which is thus controlled by reactions involving final consumption of al ti phlogopite a marked redistribution of hree y zr between garnet and xenotime where present and zircon but also of lree between feldspars k feldspar and plagioclase and monazite is suggested

anatexis and metamorphism in tectonically thickened continental crust exemplified by the sevier hinterland western north america the generation of granitoid magmas by partial melting of crustal rocks during continental thickening events is well documented in many geological provinces throughout the world including the late mesozoic sevier belt of western north america we present a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening where the only mantle contribution is the normal conductive supply of heat through the base of the lithosphere i e advection of mass and energy are excluded we distinguish between formation of migmatites and generation of mobile granitoid magmas and examine the temporal and spatial relationships between these two distinct anatectic processes between anatexis and regional deformation and between anatexis and metamorphism a fundamental conclusion is that if protoliths rich in hydrous minerals are present regional anatexis is the end product of classical barrovian metamorphism in thickened continental crust even in the absence of a free water rich fluid phase barrovian metamorphic facies series are predicted with thickening ratios maximum crustal thickness attained initial crustal thickness as low as 1 3 but mobile granitoid magmas are not formed if this ratio is less than approximately 1 5 above these lower bounds barrovian metamorphism and anatectic granitoid magmatism occur independently of the magnitude of thickening and of the way in which thickening is accomplished both processes are sensitive to a diminished heat supply lowering either moho heat flow or crustal radioactive heat production results in blueschist eclogite metamorphism and inhibits the formation of mobile granitoid magmas we model anatexis under fluid absent conditions and show that with such a constraint migmatization is always a syn kinematic process relative to the crustal thickening event whereas generation of mobile granitoid magmas is in most cases post kinematic relative to crustal thickening but can be syn kinematic if thickening takes more than approximately 50 myr the typical time intervals for melting are consistent with geological observations mobile granitoid magmas are predicted by most of our models within approximately 10 myr of the end of the crustal thickening event this incubation period results primarily from the temperature increase required for the dehydration melting reactions capable of producing large melt fractions to occur the energetic requirements of anatexis are relatively minor compared to conductive crustal thermal budgets as shown by the fact that once the necessary p t conditions are attained melting reactions are completed within time intervals on the order of 1 myr i e 1 2 orders of magnitude smaller than the characteristic time scales of the tectonic processes involved in crustal thickening

as ag sulphide veins in the spanish central system further evidence for a hydrothermal event of permian age the spanish central system scs has been subjected to repeated deformation and fluid flow events which have produced both sulphide bearing and barren vein systems although several hydrothermal episodes took place between 300 and 100 ma fluid circulation during the permian was especially important giving rise to a range of different types of deposits this study presents a multidisciplinary approach leading to the characterisation of the chemistry and age of the hydrothermal fluids that produced the as ag mineralised stockwork of mo nica mine bustaviejo madrid fluid inclusion data indicate the presence of two different fluids an initial ore stage i formed from a low to moderate salinity 3 8 wt eq nacl h2o nacl co2 ch4 fluid at minimum trapping temperature of 350 15 oc and 0 3 kbar a second h2o nacl fluid is found in three types of fluid inclusions a high temperature and low salinity type 340 20 oc 0 8 3 1 wt eq nacl also associated to ore stage i a moderate temperature and very low salinity type 160 255 oc 0 1 5 wt eq nacl represented in ore stage iii and a very low temperature and hypersaline type 60 70 oc 30 35 wt nacl unrelated to the mineralising stages and clearly postdating the previous types 40ar 39ar dating on muscovite from the early as fe stage i has provided an age of 286 4 ma synchronous with the late emplacement phases of la cabrera plutonic massif 288 5 ma and with other permian hydrothermal events like sn w skarns and w sn sulphide veins 18o of water in equilibrium with stage i quartz 5 3 7 7x d of water in equilibrium with coexisting muscovite 16 0 to 2 0 and sulphide 34s 1 5 3 6 values are compatible with waters that leached metamorphic rocks the dominant mechanism of the as ag deposition was mixing and dilution processes between aqueous carbonic and aqueous fluids for stage i as fe and nearly isobaric cooling processes for stages ii zn cu sn and iii pb ag the origin and hydrothermal evolution of as ag veins is comparable to other hydrothermal permian events in the spanish central system

lower carboniferous post orogenic granites in central eastern sierra de velasco sierras pampeanas argentina u pb monazite geochronology geochemistry and sr nd isotopes the central eastern part of the sierra de velasco sierras pampeanas nw argentina is formed by the large huaco 40 30 km and sanagasta 25 15 km granite massifs and the small la chinchilla stock 2 2 km the larger granites intrude into ordovician metagranitoids and crosscut devonian mylonitic shear zones whereas the small stock sharply intrudes into the huaco granite the two voluminous granites are biotitic muscovitic and biotitic porphyritic syeno to monzogranites they contain small and rounded tonalitic and quartz dioritic mafic microgranular enclaves the small stock is an equigranular zinnwaldite and fluorite bearing monzogranite the studied granites are silica rich sio2 70 potassium rich k2o 4 ferroan alkali calcic to slightly calk alkalic and moderately to weakly peraluminous a cnk 1 06 1 18 huaco granite 1 01 1 09 sanagasta granite 1 05 1 06 la chinchilla stock they have moderate to strong enrichments in several lil li rb cs and hfs nb ta y th u elements and low sr ba and eu contents u pb monazite age determinations indicate lower carboniferous crystallization ages 350 358 ma for the huaco granite 352 7 1 4 ma for the sanagasta granite and 344 5 1 4 ma for the la chinchilla stock the larger granites have similar nd values between 2 1 and 4 3 whereas the younger stock has higher nd of 0 6 to 1 4 roughly comparable to the values obtained for the carboniferous san blas granite 1 4 to 1 7 located in the north of the sierra the huaco and sanagasta granites have a mainly crustal source but with some participation of a more primitive possibly mantle derived component the main crustal component can be attributed to ordovician peraluminous metagranitoids the la chinchilla stock derives from a more primitive source suggesting an increase with time in the participation of the primitive component during magma genesis the studied granites were generated during a post orogenic period in a within plate setting possibly as a response to the collapse of the previous famatinian orogen extension of the crust and mantle upwelling they are part of the group of middle devonian lower carboniferous granites of the sierras pampeanas the distribution and u pb ages of these granites suggests a northward arc parallel migration of this mainly post orogenic magmatism with time

a jurassic garnet bearing granitic pluton from ne china showing tetrad ree patterns abstract ne china is characterized by the massive distribution of phanerozoic granitoids most of them are of i and a type granites whereas s type granites are rarely documented the present work deals with the dongqing pluton a small granitic body emplaced in the southern zhangguangcai range the pluton comprises a two mica garnet granite and a garnet bearing muscovite granite the latter occurs as veins in the former the pluton shows a gradational contact with the surrounding host granites rb sr and sm nd isotope analyses on whole rocks and minerals reveal that the two types of granites were emplaced synchronously at about 160 ma the pluton was emplaced coeval with the surrounding i type granitic pluton and had a rapid cooling history it is characterized by an initial sr isotopic ratio of 0 706 slightly negative end t values 0 5 to 1 9 and young depleted mantle model ages 970 1090 ma this suggests that the parent magma originated from partial melting of relatively juvenile crust which is largely compatible with the general scenario for much of the phanerozoic granitoids emplaced in the central asian orogenic belt geochemically the granites of the dongqing pluton are peraluminous with a shand index molar ratio a cnk of 1 0 1 1 for the two mica granites and 1 2 1 3 for the garnet bearing granites all the garnet bearing granites and some of the two mica granites show tetrad ree patterns tetrad group whereas most two mica granites show normal granitic ree patterns normal group the normal group granites exhibit depletion in nb ta p and ti in spidergrams and generally weak positive eu anomalies in ree patterns by contrast the tetrad group granites manifest depletion in ba nb ta sr p and ti and significant negative eu anomalies the trace element data constrain the parental magmas to having undergone extensive magmatic differentiation during their late stage magmatic evolution intense interaction of residual melts with aqueous hydrothermal fluids resulted in the non charac charge and radius controlled trace element behavior and the tetrad effect in ree distribution patterns this in turn leads to the invalidation of the commonly used tectonic discrimination criteria derived from trace element abundances of normal granites in view of this and previous studies we conclude that there were probably no s type granites produced in ne china during the phanerozoic consequently weathered sedimentary material did not play an important role in the genesis of the strongly peraluminous granites in the zhangguangcai range

palaeomagnetism of silurian sediments in w ireland evidence for block rotation in the caledonides summary a palaeomagnetic study of mid silurian wenlock sediments of the salrock formation in nw galway yields a reliable silurian palaeomagnetic pole position for w ireland at latitude 1 6 s longitude 287 7 e dp 2 5 dm 4 6 n 27 sites a further 12 sampling sites were located in silurian redbeds on clare island s mayo where a palaeomagnetic remanence of silurian age was also identified both remanences pre date local late caledonian folding however the clare island result has an anomalous azimuth when compared with the salrock result and with published silurian results for britain the two irish results can be reconciled by anticlockwise block rotation of the clare island region through 40 50 the rotation brings both palaeomagnetic and structural trends within the two sampling regions into alignment strike slip displacement across the clew bay fault zone producing a sinistral shear couple might have enabled such a rotation the timing and geographic extent of the proposed block rotation is as yet poorly defined however a younger age limit for rotation is provided by a late devonian or carboniferous partial magnetic overprint observed throughout both study areas and from geological evidence it seems likely that rotation is confined to the clew bay fault zone the dalradian rocks of the connemara massif located to the south of the salrock district have an e w structural grain some workers have proposed clockwise rotation of the connemara massif of ca 30 at between 500 and 390 ma which would bring the structural trends in w ireland into parallelism with caledonian trends in scotland because the silurian salrock succession which rests unconformably on the dalradian basement of connemara shows no such anomaly it provides a younger age limit for the possible rotation of connemara

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

timing of deformation associated with the syn tectonic dawros currywongaun doughruagh complex nw connemara western ireland new data are presented that demonstrate the syn tectonic intrusion of a suite of mafic and ultramafic magmas into the connemara dalradian magma was intruded during the local d2 episode but immediately prior to the onset of d3 both deformation episodes affect the gabbros forming two distinct groups of fabrics formed at different stages during the history of the intrusion applying thermal considerations to this history suggest that any gap in time between the formation of these fabrics was

petrology of basement dominated terranes i regional metamorphic t t path from u pb monazite and sm nd garnet geochronology central damara orogen namibia abstract mineral chemistry combined with sm nd garnet whole rock and u pb monazite ages provides important constraints on the geological history of metapelites migmatites and granites from the high grade basement dominated part of the damara orogen namibia based on thermobarometric data the peak of regional metamorphism reached c 700 750 c and c 5 kbar u pb monazite and sm nd garnet mineral ages obtained from unmigmatized garnet and cordierite bearing metapelites indicate that these peak metamorphic conditions were reached between 525 2 and 504 3 ma some migmatites with restitic melanosomes record sm nd garnet ages of 518 3 ma indicating that high grade regional metamorphism also culminated in partial melting some other migmatites and granites give older sm nd garnet whole rock ages between 530 3 and 540 4 ma different initial nd isotope compositions of granites leucosomes and melanosomes indicate that in this case migmatization is intrusion related crust derived granites intruded at 515 2 512 3 and 509 5 ma and are probably related to the second peak of regional metamorphism the youngest granites intruded at 469 3 ma indicating that intrusion of melts is superimposed upon the regional metamorphic history age constraints for the period between c 540 and 510 ma provide substantial evidence for crustal thickening together with igneous activity whereas the high temperature event which occurred at c 470 ma must be related to extensional tectonics the concordance between the monazite and garnet ages suggest that monazite grew at high grade metamorphic conditions it is further demonstrated that combined monazite and garnet chronometry are useful in identifying the timing and nature of particular tectonometamorphic and intrusive events in polymetamorphic high grade terranes

the genesis of felsic mafic plutonic associations a sr and nd isotopic study of the hercynian braga granitoid massif northern portugal abstract the late tectonic braga plutonic complex in the centro iberian zone north portugal was emplaced during the hercynian orogeny within a silurian sedimentary sequence and displays an acid basic association which consists of three well defined intrusions the biotite dominant braga granite minor bodies of gabbro to granodiorite composition and the mildly peraluminous gonca leucogranite these three plutonic suites present field relationships indicating a synchronous magmatic emplacement for which a rb sr age of 310 10 ma is obtained the distribution of the three plutonic units along regular curves in major and trace element diagrams suggests that the different units can be genetically related however the rb sr and sm nd isotopic results do not indicate a unique homogeneous source and a simple fractional crystallization process the gabbros have chemical characters high k ba sr and light rare earth elements and low nb ti and zr contents and isotopic compositions sri 0 70497 and nd 2 5 which suggest an alkaline magmatic affinity of shoshonitic type they are probably derived from an enriched mantle source in contrast the peraluminous gonca leucogranite sri 0 70933 and nd 6 8 is more likely the result of crustal anatexis the braga granite and the evolved members of the basic series have an intermediate isotopic composition sri 0 70532 to 0 70733 and nd 3 0 to 6 2 which can be interpreted in terms of an hybridization process between the two previous end members the chemical and sr nd isotopic compositions of the braga plutonic series can be explained by an assimilation fractional crystallization afc model between a mantle derived magma equivalent to the gabbros and a crust derived magma the gonca granite

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

origin of cordierite bearing monzogranites from the southern central iberian zone inferences from the zoned sierra bermeja pluton extremadura spain abstract voluminous peraluminous biotite biotite cordierite and cordierite muscovite bearing granitic plutons were emplaced in the southern part of the central iberian zone iberian massif during the waning stages of the variscan orogeny the role of the mantle and the nature of the crustal sources involved in the generation of these remarkable granites are still a matter of discussion to address these issues a detailed study of the sierra bermeja pluton a 60 km2 cordierite bearing monzogranite intrusion representative of the regional nisa alburquerque los pedroches magmatic alignment has been carried out this pluton was formed by an inward sequence of three distinct but near contemporaneous magma pulses outer unit ou middle unit mu and inner unit iu the nested pluton is cut by a number of syn plutonic vaugneritic dykes while lamprophyres were distinctly later the age of the pluton has been established at ca 306 ma 309 3 to 304 3 ma by la icp ms u pb dating of zircon and monazite the peraluminosity a cnk 1 01 1 33 and p2o5 contents 0 19 0 35 wt of the monzogranitic units increase inwards ou mu iu this is accompanied by an overall decrease in tio2 mgo cao and feot and most trace elements including ree the three units display similarly low 87sr 86sr306ma ratios 0 7033 0 7046 but decreasing end306ma ou 2 7 mu 3 8 iu 7 5 to 4 7 vaugneritic rocks have homogeneous composition with similar 87sr 86sr306ma ratios 0 7043 to those of the monzogranites but higher end306ma 1 1 values whole rock geochemical data imply a partial melting of mainly cambro ordovician felsic metaigneous rocks ou with increasing proportion of neoproterozoic lower cambrian metasediments of the schist greywacke complex mu that formed almost exclusively the source of the iu thermobarometric data suggest that melting processes progressed upward through the geological record and magma emplacement took place at bt ap mnz xtm the present results could be extensible to other cordierite bearing granites of the southern central iberian zone although forthcoming experimental work and detailed comparisons are required to confirm such extrapolation

the quartz diorites of limousin elemental and isotopic evidence for devono carboniferous subduction in the hercynian belt of the french massif central a chain of late devonian early carboniferous intrusions on the western margin of the french massif central the limousin tonalite belt consists of quartz diorites which are unusual among french hercynian granitoids in their low silica contents 53 68 wt sio2 and abundant hornblende they represent the earliest and most primitive plutonism post dating the main phase of hercynian metamorphism and have chemical characteristics strongly resembling andesitic lavas of calc alkaline affinity they differ from other massif central hercynian granitoids in their low 87sr86sr360 0 7047 0 7059 and high nd360 0 7 to 1 8 their initial pb composition was rather homogeneous 206pb204pb360 18 15 18 38 207pb204pb360 15 57 15 62 208pb204pb360 38 06 38 26 and their 207pb204pbi values are significantly lower than those of other hercynian granitoids the limousin quartz diorites are considered to have originated from an enriched mantle with characteristics modified by subduction of oceanic crust and terrigenous sediments the primitive quartz dioritic magmas were then contaminated in the lower continental crust by an afc process involving acid metaigneous or metasedimentary lithologies the occurrence of such plutons in the earliest phase of hercynian plutonism in the massif central is probably related to minor subduction at an active continental margin

late hercynian extensional and transcurrent tectonics in central iberia abstract at the end of the hercynian orogeny the central iberian crust which had been thickened by nappe tectonics and heated weakened overthickened by massive granitic intrusions was subjected to extensional tectonics through lithospherescale low angle detachment systems thus releasing the gravitational potential inherent in thickened crust these detachment faults evolved by tectonic denudation isostatic rebound and upward arching to define an extensionaltardi hercynian eth basin and range type province with upper plate listric high angle normal faults delimiting tilted blocks and asymmetric troughs occasionally filled with permian sediments and volcanics n s oriented transfer faults and synform antiform pairs with denuded metamorphic core complexes in the antiforms and local compressional features in the synforms postdating the gravitational collapse of the thickened crust the residual late hercynian e w oriented compressional stresses triggered a ductile transcurrent tardi hercynian dtth event this is expressed by two major shear zones and by the extrusion and deformation of porphyry dyke swarms along the previously thinned and weakened antiformal trends finally interplate stresses related to a continental scale shear zone gave rise to a brittle transcurrent tardi hercynian btth event with different sets of brittle faults indicating a n s oriented compression the present geological features are the result of these events overprinted by the alpine reactivation of the entire complex

petrogenesis of late variscan high k alkali calcic granitoids and calc alkalic lamprophyres the aber ildut north ouessant complex armorican massif france the aber ildut north ouessant variscan granitoid complex in the armorican massif is an example of high k alkali calcic zoned pluton c 304 ma in age a first magmatic batch intruded through a northern ew trending sinistral transcurrent shear zone before injecting southwards as a huge horizontal zoned sill with moderately peraluminous muscovite free granitoids in the north and strongly peraluminous muscovite bearing leucogranites to the south the second magmatic stage resulted in the intrusion of a large two mica leucogranitic body from a root zone along the same shear zone prior to the end of crystallization of the first injection finally ultrapotassic dykes including calc alkalic lamprophyres leucominettes intrude the complex the strongly peraluminous granites are interpreted as pure melting products of crustal clay rich pelitic material all the other petrographic types including leucominettes are thought to result from mixing of crustal melts and mantle derived mafic liquids highly silicic tourmaline bearing leucogranites are significantly affected by a tetrad effect in relation with ree complexing behavior aber ildut north ouessant granitoids probably formed during the variscan late orogenic stage associated with exhumation and lithospheric thinning at low pressure and by advective heating of a 50 km thick crust from hot metasomatized asthenosphere

meta igneous origin of hercynian peraluminous granites in n w french massif central implications for crustal history reconstructions seventy samples of hercynian peraluminous granites gueret millevaches and saint sylvestre massifs and metamorphic units of the limousin area were analysed for rb sr and sm nd the source rocks of the peraluminous granites can be found in the metamorphic rocks of limousin among them meta igneous rocks were largely predominent over meta sedimentary rocks in the source of the three granites millevaches and gueret granites were generated by the partial melting of rocks comprising meta volcanics and meta sediments whereas the saint sylvestre granite was produced exclusively by the melting of late precambrian granites this leads to confusing t dm nd values the confusion being amplified by the segregation of monazite during the petrogenetic evolution of the peraluminous granites which leads to dramatic fractionation in sm nd ratios the data of the present study tend to demonstrate that peraluminous granites do not give a good representation of isotopic mean crustal estimates late precambrian time seems however to have been a period of extensive crustal generation in western europe

geochronological and geochemical constraints on the petrogenesis of middle paleozoic kwangsian massive granites in the eastern south china block abstract to achieve a better understanding of the kwangsian orogenic event of the eastern south china block this paper documents a set of new zircon u pb geochronological and hf isotopic data and whole rock elemental and sr nd isotopic analytical results for the representative massive granite intrusions across the jiangshan shaoxing fault the studied samples are classified into two groups representing the rocks from the cathaysia block to the east of the jiangshan shaoxing fault group 1 and those from the eastern yangtze block between the anhua luocheng and jiangshan shaoxing faults group 2 the group 1 samples gave the zircon u pb ages of 405 454 ma and e hf t values of 3 6 to 15 2 with hf model ages of 1 6 2 4 ga group 2 yielded the zircon u pb ages of 400 432 ma and e hf t values of 0 2 to 12 7 with hf model ages of 1 3 2 2 ga geochemically the group 1 samples a cnk 1 02 1 43 have relatively lower al 2 o 3 mgo cao p 2 o 5 and e nd t but higher k 2 o na 2 o than those of group 2 a cnk 0 93 1 44 both groups show similar chondrite normalized patterns of rare earth elements with eu eu values of 0 15 0 92 and strongly negative ba sr nb p and ti anomalies in primitive mantle normalized spider diagrams their e nd t values range from 11 1 to 8 0 for group 1 and 8 9 to 5 0 for group 2 generally similar to those of precambrian paragneiss and contemporaneous gneissoid granites in the eastern south china block our geochronological results indicate that the kwangsian massive granites in the eastern south china block were crystallized between 400 ma and 454 ma synchronous to the kwangsian gneissoid granites along the wugong and wuyi baiyun yunkai domains in the eastern south china block the synthesis of these whole rock geochemical and in situ zircon hf isotopic data suggests that both the group 1 and 2 granites across the jiangshan shaoxing fault were predominantly derived from a crustal source with some proportional metapelitic and metaigneous components from the precambrian basement the input of juvenile mantle derived magma is insignificant taking into account other available evidences it is proposed that the jiangshan shaoxing fault might be a pre existing boundary between the yangtze and cathaysia blocks which was reactivated during the middle paleozoic kwangsian orogenic event the massive granitic magmatism was probably resulted from the doubly crustal thickening and subsequent isostatic readjustment in an intracontinental tectonic regime

distinct sources for syntectonic variscan granitoids insights from the aguiar da beira region central portugal abstract the variscan syntectonic granitoid plutons from the aguiar da beira region central portugal were emplaced into metasediments of late proterozoic early cambrian age during the last variscan ductile tectonic event d 3 which is related to dextral and sinistral shearing the older intrusion with a u pb id tims zircon age of 321 8 2 0 ma consists of porphyritic biotite granodiorite granite with transitional i s type geochemical signature relatively low 87 sr 86 sr 322 ratios 0 7070 0 7074 end 322 values of 3 9 to 4 6 and whole rock and zircon 18 o values of 10 6 and 8 0 respectively by contrast the younger intrusion is an s type muscovite biotite leucogranite emplaced at 317 0 1 1 ma showing more radiogenic 87 sr 86 sr 317 0 7104 0 7146 lower end 317 values of 7 7 to 8 7 and higher 18 o wr 11 3 and 18 o zr 9 5 the combined isotopic and geochemical evidence supports a lower crustal origin for the biotite granodiorite granite involving the anatexis of lower crustal metaigneous protoliths and possible hybridization with mantle derived magmas a shallower origin at mid crustal levels from pure crustal derivation through moderate degrees of partial melting of proterozoic cambrian metasediments is proposed instead for the muscovite biotite leucogranite

nature and composition of the lower continental crust in central spain and the granulite granite linkage inferences from granulitic xenoliths xenolith bearing alkaline ultrabasic dykes were intruded into the of the hercynian basement of the spanish central system in early mesozoic times the suite of lower crustal xenoliths in the dykes divided into three groups felsic peraluminous granulites metapelitic granulites and charnockitic granulites the felsic granulites form 95 of the total volume of the xenoliths whereas the charnockitic and metapelitic granulites are much less abundant 0 01 5 respectively thermobarometric calculations based on mineral paragenesis indicate equilibration conditions around 850 950 c 7 11 kbar thus the xenoliths represent lower continental crustal material superimposed on this high t high p assemblage is a high t low p paragenesis represented mainly by kelyphitic coronas reflecting re equilibration during transport in the clearly restitic mineral assemblages with up to 50 garnet and 37 sillimanite major and trace element modelling supports the idea that the late hercynian peraluminous granites of central spain represent liquids in equilibrium with restitic material of similar composition to the studied lower crustal xenoliths 87sr 86sr and end of the felsic xenoliths calculated at an average hercynian age of 300 ma are in the range 0 706 0 712 and 1 4 to 8 2 respectively these values match the isotopic composition of the outcropping late hercynian granites the sr isotopic composition of the xenoliths is lower than that of the outcropping mid crustal lithologies orthogneisses pelites a major contribution from the lower crust to the source of hercynian granites greatly reduces the necessity of invoking a large mantle contribution in models of granite petrogenesis the felsic nature of the lower continental crust in central spain contrasts with the more mafic lower crustal composition estimated in other european hercynian areas suggesting a non underplated crust in this region of the hercynian orogenic belt