synorogenic melting of mafic lower crust constraints from geochronology petrology and sr nd pb and o isotope geochemistry of quartz diorites damara orogen namibia quartz diorites represent the earliest ca 540 ma and most primitive plutonic rocks in the pan african damara belt and they pre date the main phase of high t regional metamorphism two suites of synorogenic quartz diorites are unusual among damaran intrusive rocks in their elemental and isotopic features comparison of the diorite compositions with melts from amphibolite dehydration melting experiments points to a garnet bearing meta tholeiite probably enriched in k2o as a likely source rock partial melting processes generated mafic ca 50 wt sio2 quartz diorites in the deep crust at temperatures of between 1 000 and 1 100 c based on comparison with experimental results and similar temperature estimates based on p2o5 solubility in mafic rocks subsequently the quartz diorites evolved by multistage polybaric differentiation processes including fractional crystallization of mainly hornblende and plagioclase and assimilation of felsic basement gneisses although their chemical characteristics high lile low hfse resemble those of other quartz diorites with calc alkaline affinities they differ in their enriched sr initial 87sr 86sr 0 70943 0 71285 nd initial e nd 9 1 to 15 2 and o 18o 6 8 8 1 isotope compositions neodymium model ages tdm that range from 1 7 to 2 2 ga and large variation in 207pb 204pb relative to 206pb 204pb indicates involvement of ancient crustal material lead 206pb 204pb 17 08 17 23 207pb 204pb 15 53 15 62 208pb 204pb 37 71 38 16 isotope compositions are strongly retarded indicating that the source underwent a pre pan african u pb fractionation and u depletion it is proposed that the quartz diorites originated by synorogenic high temperature melting of mafic lower crust this contrasts with previous suggestions favouring an origin of these rocks by melting of an enriched mantle during pan african times with characteristics modified by subduction of oceanic crust and sedimentary rocks

role of continental crust in petrogenesis of orogenic volcanic associations abstract orogenic volcanic rocks form a continuous spectrum between tholeiitic associations of island arcs and calc alkaline associations of continental margins and collision zones although these settings show a wide variation in plate convergence rate depth of scismic zone nature and age of subducted and over riding lithosphere the compositional characteristics of the volcanic association is correlated most strongly with crustal thickness tholeiitic associations dominated by basaltic rocks and with high feott mgo and low k2o for a given sio2 content are restricted to island arcs with crustal thickness less than 20 km more calc alkaline associations dominated by andesitic rocks with lower feot mgo and higher k2o for a given sio2 content occur in areas where crustal thickness exceeds 20 km these suggest that 1 fractional crystallization occurring within the continental crust may be the major mechanism responsible for variations within orogenic volcanic associations and 2 formation of calc alkaline associations may require the presence of continental crust these relationships suggest that calc alkaline magmas may be derived from more tholeiitic magmas by fractional crystallization possibly combined with crustal melting or contamination the extension of these processes might be responsible for formation of silicic ignimbrites occurring in continental areas where the crustal thickness exceeds 30 km in areas of crustal extension

relative roles of source composition fractional crystallization and crustal contamination in the petrogenesis of andean volcanic rocks there are well established differences in the chemical and isotopic characteristics of the calc alkaline basalt andesite dacite rhyolite association of the northern n v z central c v z and southern volcanic zones s v z of the south american andes volcanic rocks of the alkaline basalt trachyte association occur within and to the east of these active volcanic zones the chemical and isotopic characteristics of the n v z basaltic andesites and andesites and the s v z basalts basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere conversely the alkaline lavas are derived from basaltic parent magmas formed from mantle of within plate character recent basaltic andesites from the cerro galan volcanic centre to the se of the c v z are derived from mantle containing both subduction zone and within plate components and have experienced assimilation and fractional crystallization a f c during uprise through the continental crust the c v z basaltic andesites are derived from mantle containing subduction zone components probably accompanied by a f c within the continental crust some c v z lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing the petrogenesis of the c v z lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation fractional crystallization and magma mixing during uprise through the continental crust

early variscan i type pluton in the pre alpine basement of the western alps the ca 360 ma cogne diorite nw italy abstract located at the internal border of the grand saint bernard zone the diorite and its aureole lie on top of intensively studied alpine eclogitic units but this pluton poorly studied yet has kept locally almost undeformed the pluton intruded at 360 ma country rocks mostly composed of dark shales with na2o k2o and minor mafic intercalations of tholeiitic basalt affinity this association is characteristic of the vanoise france basement series where available age determinations suggest an early paleozoic age parts of the pluton and of its hornfels aureole that is evidenced here for the first time in the punta bioula section of valsavaranche valley nw italy have been well preserved from the alpine deformation syn emplacement hardening dehydration induced probably prevented strain enhanced alpine recrystallization magmatic rock types range continuously from subordinate mafic types at sio2 48 of hornblendite with cumulative or appinite affinities to the main body of quartz diorite to quartz monzonite sio2 up to 62 p t estimates for the pluton emplacement based on the abundance of garnet in the hornfelses using also zircon and apatite saturation thermometry and al in hornblende barometry suggest t 800 950 c and minimum p in the 0 2 0 5 gpa range with records of higher pressure conditions up to 1 2 gpa in hornblendite phlogopite cored amphibole the high k na k calc alkaline geochemistry is in line with a destructive plate margin setting based on major element data and radiogenic isotope signature end360 ma from 1 2 to 0 9 87sr 86sr360 ma from 0 7054 to 0 7063 the parental magmas are interpreted in terms of deep seated metabasaltic partial melts with limited contamination from shallower sources the low radiogenic nd content excluding a major contribution from vanoise tholeiites there is no other preserved evidence for variscan magmatism of similar age and composition in the western alps but probable analogs are known in the western and northern parts of french massif central regarding the alpine tectonics not only the age of the pluton and its host rocks instead of the permo carboniferous age previously believed but also its upper mylonitic contact suggest revisions of the alpine nappe model the cogne diorite allegedly constituted the axial part of the e verging pli en retour backfold du valsavaranche a cornerstone of popular alpine structural models in fact the alleged fold limbs as attested here by field and geochemical data do not belong to the same unit and the backfold hypothesis is unfounded

petrogenesis of ca 240 ma intermediate and felsic intrusions in the nan getan implications for crust mantle interaction and geodynamic process of the east kunlun orogen abstract the east kunlun orogen in the northern qinghai tibet plateau is an ideal region in which to investigate net crustal growth through a process of syn collisional felsic magmatism in this paper we report zircon u pb geochronology whole rock geochemistry and multiple isotope data from the nan getan quartz diorite porphyrites and syenogranites in the east kunlun orogen zircon u pb ages indicate that the quartz diorite porphyrites and syenogranites were synchronously emplaced at 240 ma the syenogranites are characterized by a high sio2 77 4 79 2 wt and differentiation index di 95 and marked depletion in sr ba and eu thereby indicating that they are highly fractionated i type granitoids these rocks yielded zircon ehf t of 4 8 to 3 0 and hf crustal model ages t dm c hf of 1565 1075 ma the quartz diorite porphyrites are characterized by a high mgo of 4 19 6 37 wt and mg of 59 61 these porphyrites have varying whole rock end t 13 2 to 10 8 206pb 204pb 240ma of 18 325 18 511 207pb 204pb 240ma of 15 608 15 630 208pb 204pb 240ma of 38 167 38 614 and 18osmow of 12 5 13 7 these signatures indicate that the nan getan syenogranites were most likely generated by an extensive differentiation of mixed parental magmas with both mantle derived melt and crustal melt triggered by intrusion or underplating by mantle derived basaltic magma the nan getan mg rich diorites were likely derived from magma mixing between melts of ancient lower crustal materials and melts of an enriched mantle source that was metasomatized by fluids during ascent in combination with the presence of the regional angular unconformity and the contemporaneous collisional metamorphism the 240 ma magmatism in the east kunlun orogen is inferred to have occurred in a compressional setting from subduction to syn collision the nan getan granitoids may have recorded the onset of collision between the bayan har songpan ganze and east kunlun terrane and net continental crust growth at 240 ma

fluid transfers at a basement cover interface part ii large scale introduction of chlorine into the basement by mesozoic basinal brines significant fracture and porosity sealing characterizes the sedimentary cover basement interface in the northwestern margin of the aquitaine basin france dolomite and calcite and sometimes fluorite barite and quartz constitute most of the fracture fillings they contain primary inclusions of brines having chlorinities in the range of 3 3 to 5 5 mol cl kg solution with total homogenization temperatures in between 65 and 130 c for quartz and slightly lower for dolomite barite fluorite and calcite crush leach analyses indicate that brines are characterized by na k ratios of 5 to 40 na li ratios of 20 to 530 and cl br ratios of 200 to 1000 which are rather typical of deep basinal brines the fluid 18o signature is estimated to be 6 6 1 8 smow for a crystallization temperature of 100 20 c between 80 and 120 c and the d value is 30 10 smow the fluid source for the fracture filling mineral is interpreted as a deep sedimentary brine expelled during a period of maximum subsidence in the aquitaine basin which migrated along the sediment cover basement interface which is characterized by high permeabilities below the toarcian shales the sealing is likely to be linked to the mixing of the brines with dilute ascending hot waters these dilute waters infiltrated from emerged zones convected and heated at depth reaching temperatures of 100 c up to 150 c on the basis of cation geothermometry extensional activity of probable cretaceous age related to the gascogne gulf rifting could be considered as the most likely cause of a significant fluid migration event at the basement cover interface all along the margins of the french massif central these processes are large scale as shown by the similarities of mineral sequences fluid types and general features of most of the f ba pb zn deposits located at the basement sedimentary cover interface the mass transfer of cl linked to this stage is significant and accounts for the early introduction of large amounts of chlorine in the granitoid microporosity this probably explains the significant chlorine concentrations of the present day fluids recovered in the granitoid aquifer

identification of an underfilled foreland basin system in the upper devonian of the central pyrenees implications for the hercynian orogeny a foreland basin succession has been identified in the frasnian of the central pyrenees this succession comprises a carbonate dominated transgressive system which recorded the cratonward migration of the foreland basin subsidence and siliciclastic depocenters which recorded the progression of the thrust fold deformation the foreland basin system has always been maintained in deep marine environments i e at an underfilled depositional state it was associated with a thrust wedge which descended toward a deep marine hinterland i e with a type of orogenic wedge usually related to subduction zones the frasnian foreland basin system differs from the one known in the carboniferous which evolved to overfilled depositional state and was associated with a thrust wedge rising toward a mountainous hinterland consequently the hercynian orogeny in the pyrenees seems to result first from a frasnian thrusting controlled by a subduction zone located north of the pyrenees and second from a carboniferous thrusting controlled by the surrection of a frontal thrust belt in the pyrenees the association of underfilled foreland basin systems and hinterland dipping thrust wedges as exemplified in the frasnian of the pyrenees can be interpreted as illustrative of the initial stages of thrust wedge growth in deep marine settings

geodynamic model for the palaeozoic crustal consolidation of western and central europe abstract the crystalline basement of western and central europe consists of a mosaic of crustal elements which were consolidated during pre grenvillian the grenvillian dalslandian morarian cadomian caledonian and hercynian orogenic cycles contemporaneous with the caledonian suturing of the precambrian laurentia greenland and fennosarmatian shields a number of gondwana derived cadomian micro cratons were accreted to the southern margin of laurasia following the late caledonian paroxysm the devonian and early carboniferous evolution of europe was dominated by continued northward subduction of the proto tethys plate at an arc trench system parallelling the southern margin of laurasia the accretion of additional gondwana derived continental fragments back arc rifting and a sinistral translation of major proportions between the laurentia greenland and the fennosarmatian sub plates the acadian and bretonian orogenies were of the pacific type the visean collision of gondwana with laurasia marked the onset of the himalayan type variscan orogeny during which collision related compressive stresses overpowered the devonian early carboniferous back arc rift systems and caused the development reactivation of a subduction zones the central armorican and saxothuringian successor basins became folded and destroyed during the latest visean whilst the variscan foredeep became scooped out in part by nappes involving the basement during the late westphalian major crustal shortening during the variscan diastrophism was accompanied by the anatectic remobilisation of subducted crustal and upper mantle material and a widespread syn and late orogenic magmatism the latest carboniferous early permian reorientation of the convergence direction between gondwana and laurasia induced the development of a complex wrench fault system transecting the variscan fold belt and an extensive post orogenic volcanism the hypotheses summarised here require confirmation by further palaeomagnetic and radiometric data and faunal analyses

evolution of high ca high sr c series basalts from grenada lesser antilles the effects of intra crustal contamination basaltic lavas from grenada lesser antilles may be divided into two series on the basis of cao mgo relations the c series roughly corresponds to the ankaramites or high sr series of the earlier literature and may be distinguished from the m series basalts by higher cao at a given mgo and by their strongly augite phyric nature compositional variation within the c series is dominated by fractional crystallization of augite and plagioclase but this was accompanied by assimilation of sedimentary material from the island arc crust the assimilation fractional crystallization process is well illustrated by an increase in 206pb 204pb from 18 6 to 19 4 with fractionation and correlated increases in nd sr zr sm and rb ba the most primitive members of the c series can not be generated from erupted m series near primary basalts but instead show evidence for an origin by olivine fractionation from high mg basalts never seen at the surface these appear to have been derived from a source similar to that of morb but enriched in 87sr and other hydrophile elements by dehydration of subducted lithosphere and in ree and radiogenic pb by a small component of subducted sediment

geochemical constraints on restite composition and unmixing in the velay anatectic granite french massif central the main anatectic granite of the velay complex is unique among major french massif central hercynian granitoids in that rather than having an entirely lower crustal source it formed by mixing between partial melts of the meta igneous lower crust and upper crustal country rock schists and orthogneisses the geochemical variations in the velay main anatectic granites cannot however be explained by mixing alone as their compositions range to lower sio2 with higher al2o3 fe2o3 and tio2 and lower na2o and cao than either end member in mixing the variations are interpreted as being due to the presence of up to 35 restite in minimum melts of country rock compositions primary restites form equilibrium assemblages represented by biotite ilmenite and surmicaceous enclaves which consist of biotite apatite zircon and almandine the main anatectic granites more rarely contain schist and gneiss enclaves quartz resisters and plagioclase restites secondary restites are mainly represented by cordierite and possibly k feldspar which formed by recrystallisation of primary biotite rich restites the unique characteristics of the velay main anatectic granites are likely to be due in part to its late formation close to the end of the hercynian orogeny the metasedimentary lower crust may have become too refractory to yield large volumes of melt following partial melting to form the other major massif central granitoids the heat necessary for partial melting at higher crustal levels was transferred from the lower crust by the intrusion of i type granites and low volume diorites from the mantle upper crustal anatexis was mainly controlled by muscovite breakdown reactions 830 to 850 c and the liberation of water due to the recrystallisation of biotite to cordierite the temperatures necessary for biotite breakdown were only achieved locally and resulted in the formation of high lree granites

a combined o sr nd and pb isotopic and trace element study of crustal contamination in central andean lavas i local geochemical variations systematic variations in o sr nd and pb isotopic ratios and in sr nd pb and rb trace element abundances in central andean andesitic lavas are analyzed in terms of combined crustal assimilation and fractional crystallization the results of integrated isotopic model calculations show that observed local isotopic and trace element variations within the arequipa and barroso volcanic groups of southern peru are best fit by a simple model involving shallow or intermediate depth crustal assimilation of charcani gneiss or an equivalent rock accompanied by dominantly plagioclase fractional crystallization a consequence of this result is that the rbsr pseudoisochrons reported for the arequipa and barroso volcanics are wholly artifacts of the combined processes of assimilation and plagioclase fractionation as are all of the pb isotopic variations measured in the same lavas already interpreted by tilton and barreiro 14 to be the result of crustal contamination isotopic and trace element compositions of the primary magmas for the arequipa and barroso volcanic series are obtained by correcting for assimilation and fractional crystallization the starting magmas are characterized over the entire range of permissible primary compositions by high87sr 86sr ratios low143nd 144nd ratios and high sr concentrations relative to normal island arc andesites these results appear to require prior contamination of the magmas but not involving plagioclase fractionation the uncertainty in pb and o isotope ratios computed for possible primary magmatic compositions is sufficiently large however that it is not possible to preclude a subcrustal origin for at least some regional variation in sr and nd isotopic ratios

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

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

ore deposits of the french massif central insight into the metallogenesis of the variscan collision belt the french massif central fmc represents the whole west european variscan wev belt in terms of both the geodynamic evolution and the metallic content thus a study of the metallogenic evolution of the fmc may elucidate the conditions that allow the mineralisation of a collision belt since recent collision belts e g the himalayas or the alps show that mineralisation does not necessarily result from the collision process the palaeozoic history of the fmc is divided into three geodynamic stages unevenly involved from the metallogenic view point the eo variscan stage cambrian to silurian was not important the meso variscan stage devonian early carboniferous was of limited importance and most of the mineralisations formed during the neo variscan stage late carboniferous early permian in addition some more mineralisation was produced during the mesozoic because of the thermal reactivation linked with the alpine orogenies

crustal evolution of the hercynian belt of western europe evidence from lower crustal granulitic xenoliths french massif central abstract granulite facies lower crustal xenoliths scavenged by the tertiary basaltic pipes of bournac and roche pointue massif central france have been analysed for major and trace elements and radiogenic sr nd isotope ratios they display wide variations in major elements rare earth elements and isotopes 87 sr 86 sr 0 7041 0 7216 nd 11 9 to 1 5 which tend to correlate with the three different rock types encountered basic and acidic meta igneous and metasediments the basic liquids are of calc alkaline affinity and cumulates are abundant there is a gap in silica content between the basic and acid meta igneous xenoliths the isotopic compositions suggest mixing of old pre existing continental crust represented by the metasediments and mantle derived basic melts which represent juvenile addition to the crust during the hercynian orogeny the basic meta igneous rocks have been contaminated during their emplacement into the crust whereas the surrounding metasediments have suffered partial fusion and contamination by the same liquid producing the hercynian granitoids which have nd and sr initial ratios identical to those of the lower crustal xenoliths the residue from melting may be represented by the acid meta igneous rocks some of which have positive eu anomalies these results indicate that an important basaltic underplating event occurred during the evolution of hercynian orogenic belt although the exact timing of the underplating requires further constraint

lead isotopic composition of hercynian granitic k feldspars constrains continental genesis lead isotope compositions have been determined in many k feldspars from granitic rocks emplaced during the variscan hercynian orogeny sensu lato around 300 myr in various parts of europe and the eastern us the ratios show a regular geographical pattern with contours arched around southern france and parallel to the atlantic coast of the us this distribution parallels predriftpalaeogeographical zones during the palaeozoic the lead isotope pattern of variscan granitic rocks is similar to that of some orogenic volcanic suites but is better explained by crustal melting than by an hercynian mantle crust material mixing the continental crust from these areas must be older than 3 000 myr large scale fluid circulation in the crust during orogenic processes apparently explains the observed geographical patterns of lead isotopes

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

feasibility of graphic determination of stress from fault slip data the equation describing 2d sections through an ellipsoid is similar in formulation to the equation describing fault slip directions in relation to the stress tensor thus enabling the conversion between them so the existing graphic method for determining the optical indicatrix or fabric ellipsoid can be used to infer stress from fault slip data if they are converted to sectional measurements after conversion fault slip data are displayed on a stereogram and then are directly used to determine the poles to the circular sections of the stress ellipsoid from which the ellipsoid is readily reconstructed this is of importance for visual appreciation in stress inversion however the graphic method is only applicable to cozonal sectional measurements and is unable to discriminate spurious data in order to make up for this deficiency a computer aided method has been developed in this paper and its feasibility illustrated by using an example

a multidisciplinary study of a syntectonic pluton close to a major lithospheric scale fault relationships between the montmarault granitic massif and the sillon houiller fault in the variscan french massif central 2 gravity aeromagnetic investigations and 3 d geologic modeling 1 new gravity and aeromagnetic investigations have been carried out to understand the emplacement mechanisms of a granitic pluton and the relationships with a nearby lithospheric scale fault this paper concerns the second part of a methodological multidisciplinary study and complements previous geochronologic and anisotropy of magnetic susceptibility ams studies on the same pluton in the northern part of the variscan french massif central fmc the montmarault massif crops out along the sillon houiller fault shf bouguer and aeromagnetic anomaly maps imply thickening of the pluton along the shf and suggest laccolitic spreading northwestward on the basis of petrophysical measurements direct two dimensional 2 d joint gravity and magnetic modeling has been performed along 10 cross sections in order to quantitatively constrain the 3 d pluton geometry and its relationships with surrounding geologic units these geophysical cross sections new structural information field and ams measurements and petrophysical data have been integrated into a regional 3 d geological and geophysical model altogether the results obtained from geochronology petromagnetic fabrics part 1 gravity and aeromagnetic investigations as well as 3 d modeling part 2 demonstrate that the montmarault pluton was emplaced during the namurian period along the shf our results further show that at that time in response to a nw se regional extension if the shf existed it behaved as a normal fault mylonites attesting for synmagmatic normal motion on the northeastern part of the montmarault pluton strengthen this tectonic scenario during the late carboniferous the fmc experienced ne sw extension along the shf by 80 km of brittle left lateral wrench offset this second tectonic event is well recorded in the stephanian coal basins which were formed along nw se listric brittle faults and constrain the present day shape of the montmarault pluton

geochemistry of basalts from central and southern new hebrides arc implication for their source rock composition abstract the basaltic rocks from the central and southern islands of the new hebrides aneityum tanna erromango efate emae tongoa and epi have geochemical features typical of island arc volcanics they are enriched in lile and depleted in zr hf nb and ta compared to n type morb the rocks were derived from a similar upper mantle source as n type morb but with a higher degree of partial melting in addition their source was enriched in lile k rb sr ba and lree probably by migrating hydrous fluids released during the dehydration of the subducted oceanic slab the basalts from futuna island which is located farther from the trench display characteristics typical of calc alkaline rocks the futuna basalts were generated from a different lile enriched upper mantle source it seems that this upper mantle source was modified by interaction with partial melts from the subducted oceanic lithosphere

40 ma of hydrothermal w mineralization during the variscan orogenic evolution of the french massif central revealed by u pb dating of wolframite we present u pb thermal ionization mass spectrometer tims ages of wolframite from several granite related hydrothermal w sn deposits in the french massif central fmc located in the internal zone of the variscan belt the studied wolframite samples are characterized by variable u and pb contents typically 10 ppm and show significant variations in their radiogenic pb isotopic compositions the obtained u pb ages define three distinct geochronological groups related to three contrasting geodynamic settings i visean to namurian mineralization 333 327 ma coeval with syn orogenic compression and emplacement of large peraluminous leucogranites ca 335 325 ma ii namurian to westphalian mineralization 317 315 ma synchronous with the onset of late orogenic extension and emplacement of syn tectonic granites ca 315 310 ma and iii stephanian to permian mineralization 298 274 ma formed during post orogenic extension contemporaneous with the permian volcanism in the entire variscan belt the youngest ages 276 274 ma likely reflect the reopening of the u pb isotopic system after wolframite crystallization and may correspond to late hydrothermal alteration e g ferberitization our results demonstrate that w sn mineralization in the fmc formed during at least three distinct hydrothermal events in different tectono metamorphic settings over a time range of 40 ma

magma mixing in the genesis of hercynian calc alkaline granitoids an integrated petrographic and geochemical study of the sazava intrusion central bohemian pluton czech republic the devonian early carboniferous 354 1 3 5 ma conventional zircon u pb age sazava intrusion biotite amphibole quartz diorite tonalite and granodiorite of the central bohemian pluton cbp associated with bodies of olivine pyroxene amphibole gabbro gabbrodiorite quartz diorite and rare hornblendite gives an opportunity for a comparative study of a rather shallow calc alkaline magma mixing zone at two levels separated by a vertical difference of approximately 1 km the deeper section p ibram displays the direct evidence for the existence of a long lived periodically tapped and replenished floored magma chamber masli the contacts between the subhorizontal sheet like basic bodies and the surrounding commonly cumulus rich sazava granitoid are arcuate and cut by a series of veins and ascending pipes shallow dipping swarms of strongly elongated and flow aligned mafic microgranular enclaves mme concordant with the contacts of the basic bodies are commonplace the higher level teletin section shows relatively independent basic intrusions some of them distinctly hybrid in character and mainly of quartz dioritic composition surrounded by relatively homogeneous nearly cumulus free sazava tonalite rich in texturally variable mme larger quartz microdiorite bodies and the mme both interpreted as hybrids contain varying proportions of highly heterogeneous plagioclase megacrysts with complex zoning particularly well shown by cathodoluminescence cl most often the megacrysts have cores of labradorite anorthite partly resorbed and overgrown by andesine rims but some are strongly brecciated and fragments have been annealed by rim growth also characteristic are long prisms of apatite oikocrysts of quartz and k feldspar and zoned amphibole the latter has brown pargasite and magnesiohastingsite cores resorbed and overgrown by magnesiohornblende compositionally similar to the amphibole in the sazava tonalite the brown cores are probably relics of a higher p t stage with temperature exceeding 900 c plots of major element analyses of the teletin quartz microdiorite and the mme fall close to the line joining the compositions of the evolved sazava granodiorite tonalite and the associated gabbros the estimated minimal proportions of the acidic end member range from ca 15 to 20 for the quartz microdiorite and ca 40 to 60 for the enclaves field relationships macroscopic and mineralogical features and whole rock geochemistry are consistent with a hybrid origin of the quartz microdiorite masses and enclaves unzoned calcic plagioclase cores are interpreted as having crystallized from a basic magma prior to its injection into a high level acidic magma chamber the sudden decrease in ca activity and introduction of water due to magma mixing with tonalitic melt led to their resorption and brecciation quartz ocelli that are abundant locally in some basic bodies are most likely xenocrysts introduced from the felsic magma in the basic melt they were resorbed and became a substrate for heterogeneous nucleation of the amphibole or pyroxene coronas strong undercooling of the basic magma resulted in the growth of long prismatic apatite together with dendritic boxy cellular plagioclase both types of plagioclase cores were subsequently overgrown by more sodic mantles at the same time small unzoned plagioclase crystals nucleated in the groundmass following thermal re adjustment crystallization of the hybrid magma ended with considerably slower growth of quartz and k feldspar oikocrysts

devonian mississippian i type granitoids in the western carpathians a subduction related hybrid magmatism abstract sims zircon u pb dating of i type granitoids from four variscan crystalline basement outcrops in the western carpathians tribec nizke tatry cierna hora and slovenske rudohorie mts implies that they originated between 367 and 353 ma therefore they belong to the earliest variscan i type granitoids the oldest devonian age at ca 367 364 ma is obtained from an enclave bearing tonalite and associated dikes of the tribec mts several granodiorites from the nizke tatry vepor and cierna hora mts 353 357 ma show younger early carboniferous ages these data require a re assessment of older models that explained the genesis of i type granitoids in the western carpathians based on the assumption of younger middle to upper carboniferous ages the i type granite massifs of the western carpathians most probably originated in an arc related environment within the galatian superterrane an assemblage of gondwana derived fragments the early age of i type magmatism in the western carpathians marks the beginning of a north dipping subduction of the paleotethys ocean under ibero ligerian and intra alpine terranes we suggest a term proto tatricum for that part of the galatian superterrane where devonian mississippian i type granitoids were emplaced now the granitoids are incorporated as a part of crystalline basement into the alpine tatric and veporic units within present west carpathian mountain chain

wave transformation on a coral reef wave transformation of regular waves was measured in a laboratory model of a fringing reef with a steep face and an outer reef top slope gradually decreasing in the landward direction data was obtained for various wave conditions and water levels a nonlinearity parameter fco g1 25ho 0 5t2 5 hc 1 75 based upon one proposed by swart and loubser 1979 is proposed as a suitable parameter for classifying wave transformation regimes on this reef in particular when fco 150 waves plunge on the reef edge and the amount of wave energy reaching a shore or structure is small 16 when fin co 100 waves spill on the reef top but the greater part of their energy is transmitted over the reef top the maximum values of the wave height to water depth ratio on the reef top were found to be consistent with nelson s analyses for laboratory and field data which indicate that the maximum stable wave height to depth ratio h d on a horizontal bottom never exceeds 0 55 for shallow water waves fc 500 the experimental data confirms that the maximum value of h d decreases when fc decreases but that it also increases when the bottom slope increases

early variscan magmatism along the southern margin of laurasia geochemical and geochronological evidence from the biga peninsula nw turkey massive fine grained metavolcanic rocks of the camlica metamorphic unit exposed in the biga peninsula northwestern anatolia have provided new carboniferous ages and arc related calc alkaline petrogenesis constraints suggesting that the biga peninsula was possibly involved in the variscan orogeny the metavolcanic rocks are mainly composed of metalava and metatuff and have the composition of andesite chondrite normalized ree patterns from these rocks are fractionated lan ybn 2 2 to 8 9 europium anomalies are slightly variable eu eu 0 6 to 0 7 and generally negative average eu eu 0 68 the metavolcanic rocks have a distinct negative nb anomaly and negative sr hf ba and zr anomalies these large negative anomalies indicate crustal involvement in their derivation tectonic discrimination diagrams show that all metavolcanic rocks formed within a volcanic arc setting zircon ages la icp ms of two samples yield 333 5 2 7 and 334 0 4 8 ma these ages are interpreted to be the time of protolith crystallization this volcanic episode in the biga peninsula correlates with other variscan age and style of magmatism and by association with a collisional event leading to the amalgamation of tectonic units during the variscan contractional orogenic event carboniferous calc alkaline magmatism in the sakarya zone is ascribed to arc magmatism as a result of northward subduction of paleo tethys under the laurasian margin geochemical and u pb zircon data indicate that the sakarya zone is strikingly similar to that of the armorican terranes in central europe the biga peninsula shows a connection between the sakarya zone and the armorican terranes

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

modelling diverse processes in the petrogenesis of a composite batholith the central bohemian pluton central european hercynides the multiple intrusions making up the central bohemian pluton in the central european hercynides have petrographic and geochemical features consistent with the presence of four main granitoid suites major element trace element and sr nd isotopic compositions are used to model their petrogenesis partial melting of metabasic locks or of a chur like mantle source are interpreted to have produced melts parental to the most primitive calc alkaline sazava suite interaction of basic with more acidic magmas followed by extensive amphibole plagioclase dominated fractionation accounts for the production of trondhjemites alternatively the trondhjemites correspond to small degree melts of a metabasic source afc assimilation fractional crystallization modelling with a paragneiss as a contaminant and increasing d nd values simulates the characteristics of the blatna suite closed system fractionation of strongly enriched mantle derived magmas or their interaction with leucogranitic melts is deduced for the petrogenesis of the shoshonitic certovo bremeno suite partial melting of a metasedimentary source followed by k feldspar dominated fractionation accounts for the granites of rite ricany suite the progression from relatively primitive calc alkaline granitoids towards evolved k rich calc alkaline and shoshonitic rocks is interpreted to reflect the increasing enriched mantle input in the petrogenesis of the later suites the evidence for hercynian subduction is equivocal and the mantle enrichment could have been significantly older

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

collision vs subduction related magmatism two contrasting ways of granite formation and implications for crustal growth earth s continental crust is dominantly made of buoyant felsic igneous material granitoids that were ultimately extracted from the mantle as a result of earth s differentiation since felsic melts are not in chemical equilibrium with the mantle they can originate either from melting of older crustal lithologies or from differentiation of a primitive mantle melt only the latter case will contribute to crustal growth to understand the mechanisms of continental crust growth and differentiation through time it is therefore necessary to unravel the respective contribution of these two different mechanisms in the genesis of granitoid suites in modern earth granitoids are chiefly generated in convergent plate boundaries subduction and collision this paper examines the granitic suites in a late collision environment the variscan french massif central fmc and compares them with the suites found in an oceanic arc we therefore describe and compare two end members sites of granite generation in the fmc several main types of granites are described muscovite and cordierite bearing peraluminous granites resp mpg and cpg contain large amounts of inherited zircons and their chemistry demonstrates that their sources were older crustal material resp metasediments and metaigneous on the other hand potassic calc alkaline granites kcg associated to potassic diorites vaugnerites do not contain inherited zircons and ultimately derive from the vaugnerites the vaugnerites in turns form by partial melting of a mantle contaminated by the regional crust therefore although they are isotopically similar to the crust the kcg are net contributors to crustal growth thus we conclude that although late orogenic settings are dominated by crustal melting and recycling they may be sites of net crustal growth even though this is not visible from isotopes only in contrast arc granitoids are purely or almost purely mantle derived however the preservation potential of arcs is much smaller than the preservation of late orogenic domains such that at the scale of a whole orogenic belt late orogenic magmatism is probably as important as arc magmatism finally we speculate that the situation may have been similar in the archaean or even more skewed towards late orogenic sites or similar environments dominated by melting of a altered mafic protocrust owing to the hotter mantle and less stable subductions during that period

palaeozoic granitoids from the french massif central age and origin studied by 87rb87sr system abstract according to geochronological results granitoids of the french massif central fall into two main chronological groups 1 a cambro silurian group 545 410 ma occurring all over the massif central later metamorphosed into orthogneisses during the devonian metamorphism 400 370 ma 2 an upper devonian carboniferous group 360 280 ma including non or weakly deformed granites the oldest 360 340 ma localized in the north of the massif central and the youngest 340 285 ma outcropping all over the massif geochemistry of rb sr and isotopic evolution diagram give the following results strontium initial ratios for orthogneisses increase with time from 0 704 545 ma to 0 710 410 ma excluding a purely mantellic origin of the magmas strontium initial ratios for granites show that their magmas cannot result from a remelting of orthogneisses the isotopic evolution of the leucogranitic family defines a trend originating 370 ma i e during the main metamorphic phase the source rocks of the early palaeozoic granitoids contain old zircons 1 5 2 0 ga probably originating from an earlier continental crust but the evidence of this crust is not pointed out by strontium isotopes

feasibility of afc models for the petrogenesis of calc alkaline magma series one of the most popular petrogenetic concepts is that of simultaneous assimilation and fractional crystallisation afc the equations governing these processes are well known and widely applied the power and attraction of the model are that with the minimum of constraints petrogenetic schemes can be erected for essentially any group of rocks these models are usually based on the observed variations in a small number of trace elements and or isotopic systems in this paper we have generated well constrained afc models for the variscan calc alkaline querigut complex in the french pyrenees our models are based on geological constraints on the identities of the parent magma and candidate assimilants and the sm nd and rb sr isotope systematics of the rocks the modelling is successful in mimicking the isotope array for the actual rocks however the results are massively inconsistent with those produced using the major and trace element concentrations our major conclusion is that afc modelling should be undertaken only with robust initial constraints on the compositions of the parent magma and the proposed assimilant with due respect for crystal liquid partition coefficients in so far as they are known the results should then be accepted only if the isotope based model parameters are consistent with those derived from major and trace element variations

neodymium isotopic composition of quaternary island arc lavas from indonesia abstract 143 nd 144 nd ratios measured in quaternary lavas from java and the banda arc of indonesia range from 0 51242 to 0 51280 and exhibit an inverse correlation with 87 sr 86 sr isotopically the indonesian samples resemble andean rather than island arc lavas the samples from java plot either within or adjacent to the mantle array towards higher 87 sr 86 sr ratios samples from the banda arc and the anomalous calc alkaline volcano papandajan are characterized by relatively low 143 nd 144 nd and high 87 sr 86 sr ratios these characteristics are consistent with the interpretation that subducted terrigenous material was involved in the genesis of these lavas furthermore the banda arc samples appear to lie on a mixing line between isotopic compositions characteristic of the mantle and upper continental crust a high k trachyte from the alkaline volcano muriah java has isotopic characteristics of the mantle 143 nd 144 nd 0 51270 87 sr 86 sr 0 70424 which implies that the extreme enrichment in large ion lithophile elements in its source must have occurred only shortly before its formation the inferred 143 nd 144 nd ratio of the unmodified mantle beneath java and the banda arc is lower than that observed in mid ocean ridge basalt which may have important implications for a better understanding of the geochemical structure of the mantle

geochemical comparison between minettes and kersantites from the western european hercynian orogen trace element and pbsrnd isotope constraints on their origin trace element and isotopic characteristics of late carboniferous to early permian minettes and kersantites have been determined these lamprophyres have been sampled throughout the western european hercynian orogen from brittany to the west to schwarzwald to the east in spite of sharp petrological differences reflected by mineralogy and major element geochemistry minettes and kersantites exhibit close identity with respect to trace element and isotopic features these features comprise enrichment in incompatible elements highcs rb and lowce pb ratios ta and ti relative depletion high abundance in transition elements and highni mg ratios pb isotope ratios are undistinguishable from those measured on hercynian continental crust initial143nd 144nd ratios are between0 5120 ei 5 and0 5122 ei 1 for minettes and kersantites whereas initial87sr 86sr ratios vary between 0 7055 0 710 for minettes and 0 707 0 708 for kersantites no simple mixing relations are visible on rbsr and smnd isochron diagrams the exceptional homogeneity of these geochemical characteristics along a 1000 km traverse does not allow for an hypothesis of enrichment through upper level assimilation and thus leads to propose that these rocks originated through melting of a mantle enriched by recycling of crustal material

the isotope systematics of a juvenile intraplate volcano pb nd and sr isotope ratios of basalts from loihi seamount hawaii abstract sr nd and pb isotope ratios for a representative suite of 15 basanites alkali basalts transitional basalts and tholeiites from loihi seamount hawaii display unusually large variations for a single volcano but lie within known ranges for hawaiian basalts nd isotope ratios in alkali basalts show the largest relative variation 0 51291 0 51305 and include the nearly constant tholeiite value 0 51297 pb isotope ratios show similarly large ranges for tholeiites and alkali basalts and continue tatsumoto s 31 loa trend towards higher 206pb 204pb ratios resulting in a substantial overlap with the kea trend 206pb 204pb ratios for loihi and other volcanoes along the loa and kea trends 31 are observed to correlate with the age of the underlying lithosphere suggesting lithosphere involvement in the formation of hawaiian tholeiites loihi lavas display no correlation of nd sr or pb isotope ratios with major element compositions or eruptive age in contrast with observations of some other hawaiian volcanoes 38 isotope data for loihi as well as average values for hawaiian volcanoes are not adequately explained by previously proposed two end member models new models for the origin and the development of hawaiian volcanoes must include mixing of at least three geochemically distinct source regions and allow for the involvement of heterogeneous oceanic lithosphere

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

sr nd al36 1997 99 122 hercynian granodiorites and monzogranites massif central france variations in sr nd and pb initial isotope ratios of three large hercynian granitoid massifs of the french massif central are consistent with formation of the granitoids by partial melting of the lower crust the nature of the lower crust of the massif central is known from the granulite facies lower crustal xenoliths brought to the surface by tertiary alkaline volcanics studies of the xenoliths have revealed that during the hercynian orogeny 360 290 ma ago the pre existing metasedimentary and acid metaigneous lower crust was intruded by basic magmas causing anatexis the mantle derived basic magmas mixed with melts from the pre existing lower crust forming the hercynian granitoids the peraluminous gueret and margeride granitoids which are very similar in their major and trace element compositions have 87sr 86sri 0 7089 to 0 7121 and ndi 7 5 to 3 9 values which indicate that they have a high sedimentary component considered to have been derived from lower crustal metasedimentary rocks however the granitoids differ significantly in their initial pb isotope compositions with the gueret granodiorites being less radiogenic indicating the presence of a slightly different crustal source component for the gueret massif in contrast monzogranites from the millevaches massif are less peraluminous and have higher la ybn and lower 87sr 86sri values 0 7064 to 0 7109 these features together with the relatively radiogenic pb isotope compositions of the millevaches granitoids suggest derivation from the acid metaigneous lower crust

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

crustal contamination in northern andean volcanics late cenozoic andesitic rocks of the colombian andes are characterized by small but systematic and well correlated variations in 87 sr 86 sr 143 nd 144 nd and 18 o that reflect significant crustal contamination the range in isotopic ratios measured in northern andean andesites is most pronounced for oxygen relatively much more subtle for sr and nd pb isotopic data so diagnostic of crustal contamination in central andean lavas exhibit minimal variation in the northern andean volcanics trace element trends and phenocryst populations suggest dominantly mafic mineral fractionation although plagioclase fractionation may be significant in later stage differentiation an assimilation fractional crystallization model is constructed to match measured isotopic data the results of that modelling indicate that the andesitic magmas of galeras and ruiz volcanoes may have assimilated up to 10 20 of crustal material correlated variations in measured isotopic ratios can be modelled satisfactorily by assuming a crustal contaminant of composition 18 o 10 per mil 87 sr 86 sr 0 710 0 712 sr 90ppm and 143 nd 144 nd 0 5120 0 5122 nd 20 30 ppm we estimate that the composition of the primary magma prior to crustal contamination was in the range 18 o 6 5 per mil 87 sr 86 sr 0 704 and 143 nd 144 nd 0 51285 values that may suggest an additional crustal component was present in the primary magma if we assume the added crustal component was derived from partial melting of subducted continental sedimentary material and or oceanic crust source contamination then model calculations suggest that crustal material comprises less than 10 to 15 wt of the primary magma this model dependent effect of source contamination is to shift isotopic ratios to the right on the sr nd plot see also hawkesworth et al 1977 whereas crustal contamination of the magma during ascent through the andean crust appears to have produced isotopic trajectories that cut downward and through the mantle array

fluid transfers at the basement cover interface part i subsurface recycling of trace carbonate from granitoid basement rocks france basement rocks usually contain trace amounts of disseminated carbonate minerals and sometimes also fractures veins filled with carbonates which may have a variety of origins extensive drilling has been performed of a granitoid body vienne western part of the french massif central overlain by a carbonate sedimentary sequence a situation which is representative of most of the variscan basement in western europe these unique samples provide the opportunity to investigate the behaviour of basement trace carbonate across the basin basement interface and the mass transfers throughout a series of fluid migration ranging from high ca 450 c to low down to 50 c temperature conditions study of quartz and carbonate found in fracture infillings reveals three major stages of fluid circulation namely 1 high to medium temperature late hercynian fluids which circulated in fractures developed during basement uplift 2 medium temperature mesozoic basinal brines migration laterally along the basement cover contact 3 late infiltration of diagenetic marine waters in the upper part of the granitoid body a majority of analyzed vein carbonates from the three stages displayed a rather restricted range of c isotopic compositions 13c between 14 and 9 pdb whatever the stage in which they were deposited and despite large variations of o isotopic ratios 18o in the range 3 to 30 smow carbon in pervasively altered rocks displays the same features and is distributed among the two broad groups defined using o isotopic ratios in veins and fluid inclusions data namely the hercynian and the mesozoic carbonates isotopic data argue that carbon was introduced as carbonates in the early stages of the retrograde hercynian metamorphism and was systematically reworked by subsequent fluids without any significant carbon introduction from external sources the only exception corresponds to the stage iii calcite veins which are located in the upper sections of the cores at depths lower than 350 m beneath the basement cover boundary and are the only witnesses of element transfer from the sediments towards the basement the present data show that trapping of carbon by ca rich plutonic rocks during their cooling history confers on these rocks an efficient self sealing capacity during later fracturing and fluid flow because carbonates possess high solubility and rapid dissolution precipitation kinetics fluid circulation occurred not only in the macroscopic system of fractures but also within some volumes of the granitic matrix in order to scavenge and redistribute the early granitic trace carbonate in the event of poly cyclic carbonate redistribution the use of carbon isotopic composition to unravel the fluid origins must be exercised with caution as early c could be easily redistributed during later fluid migration episodes

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

the moldanubian zone in the french massif central vosges schwarzwald and bohemian massif revisited differences and similarities in order to portray the main differences and similarities between the northeastern variscan segments french massif central fmc vosges black forest and bohemian massif bm we review their crustal scale architectures the specific rock associations and lithotectonic sequences as well as the ages of the main magmatic and metamorphic events this review demonstrates significant differences between the moldanubian domains in thebmand the fmc on this basis we propose distinguishing between the eastern and western moldanubian zones while the vosges black forest mountains are an intermediate section between the bm and the fmc the observed differences are the result of first the presence in the french segment of an early large scale accretionary system prior to the main variscan collision and second the duration of saxothuringian armorican subduction which generated long lived magmatic arc and back arc systems in the bohemian segment while the magmatic activity in the fmc was comparably short lived

fingerprinting sources of orogenic plutonic rocks from variscan belt with lithium isotopes and possible link to subduction related origin of some a type granites lithium li elemental and isotope data are presented for a suite of plutonic rocks i s a and ss type coexisting mafic bodies from the western carpathians slovakia that were generated throughout the complete variscan orogenic cycle i type granites show a limited range of 7li contrasting with the large variations found in s type granites and orthogneisses of distinct ages this contradicts previous reports of predominantly light li s type granites sourcing metasedimentary lithologies and isotopically heavier i type granites derived from meta igneous sources an almost exclusively heavy li isotope signature 7li 4 7 found in four out of five a type granites rules out several commonly accepted petrogenetic scenarios such as remelting of granulitic residue after formation of i type granitic melts anatexis of calc alkaline meta igneous crust or extensive closed system fractional crystallization of mantle derived magmas instead it most probably reflects a derivation of a type granites from a mantle wedge modified by slab derived fluids during an early variscan or pan african subduction episode in contrast the distinct and lower 7li values 1 2 in the contemporaneous ss type granites can be related to their likely pelitic parentage mafic rocks gabbros and diorites associated with several occurrences of granites are uniformly li rich and isotopically light 0 5 precluding a direct derivation from the mantle these signatures testify to their cumulate origin whereby kinetic effects may be a viable explanation for the light li isotope compositions associated with diffusive redistribution of li between mantle derived mafic melts and acid magmas this corroborates recent studies on faster diffusion of 6li compared with 7li in natural systems taken together the presumed dichotomy between the sources and processes leading to generation of s and i type granitic magmas does not seem to be reflected by li isotope signatures in a simple and globally valid manner interpretation of li isotope compositions thus needs to be paralleled by other available information on petrology whole rock geochemistry and the magmatic context