paleoproterozoic tectonic transition from collision to extension in the eastern cathaysia block south china evidence from geochemistry zircon u pb geochronology and nd hf isotopes of a granite charnockite suite in southwestern zhejiang abstract the badu complex and associated paleoproterozoic granitoids are among the oldest known rocks in the cathaysia block in south china the paleoproterozoic units of the badu complex are dominantly composed of metapelitic rocks and meta greywackes here we report la ss icp ms laser ablation split stream inductively coupled plasma mass spectrometry zircon u pb data from a newly discovered garnet bearing granite which show an emplacement age of 1929 15 ma and metamorphism at 1872 34 ma we also report u pb ages of 1886 16 ma 1858 7 ma 1848 11 ma from a gneissic granodiorite and two charnockites respectively the garnet bearing granite is peraluminous with a cnk range from 1 1 to 1 3 the rock shows relatively high sio2 k2o and rb contents and low total ree sr cao and ferromagnesian components typical of leucogranites the whole rock nd two stage model age tdm2 nd of this rock is ca 2 7 ga zircon hf crustal model ages tdmc hf peak at about 2 7 ga and abundant inherited zircons occur with u pb ages in the range of 2044 to 2803 ma evidences from zircon u pb age and hf isotope compositions whole rock nd isotopes and whole rock major and trace elements suggest a metasedimentary protolith and that the garnet bearing granite leucogranite was derived by partial melting in a thickened crust at about 1 93 ga the gneissic granodiorite and charnockites show indistinguishable major and trace element features as well as zircon hf and whole rock nd isotope compositions indicating that they were generated from the same source rocks the gneissic granodiorite and charnockites are ferroan calc alkalic and metaluminous with a cnk range from 0 80 to 0 98 they display relatively low sio2 contents and ga al ratios suggesting their a type affinity their zircon tdmc hf age peak is 2 9 ga and whole rock tdm2 nd ages range from 2 8 to 2 9 ga these high temperature rocks were generated possibly through the partial melting of ancient amphibolites 2 9 ga with heat input from asthenosphere upwelling and magmatic underplating in an extensional setting at about 1 85 1 88 ga combined with data from previous studies we suggest three episodes of major crustal growth in this region at 1 8 1 9 ga 2 7 ga and 2 9 ga during the 1 8 1 9 ga event extensive reworking also took place from the 1 93 ga syn collision leucogranite garnet bearing granite the 1 88 1 85 ga post collision a type granites and charnockites the granite charnockite suite and the1 85 1 77 ga intraplate basaltic rocks li 1997 and xiang et al 2008 in the region a paleoproterozoic orogenic cycle can be deciphered in the cathaysia block the rapid transition of tectonic regime from collision to post collisional extension can be correlated to the assembly of the supercontinent columbia

multiple crust reworking in the french armorican variscan belt implication for the genesis of uranium fertile leucogranites muscovite peraluminous granites mpgs form by partial melting of the continental crust and can be related to metalliferous deposits such as tin tungsten and uranium u metal enrichment in mpgs commonly results from fractional crystallization but the metal contents of the source play a major role for their fertility between ca 320 and 300 ma late carboniferous the french armorican variscan belt was intruded by numerous u fertile mpgs that contain inherited zircon grains with a wide range of ages from archean to carboniferous u pb and hf isotopic data of zircon grains from brioverian to carboniferous sediments cambrian to early carboniferous granitoids and late carboniferous mpgs indicate that the crust of the armorican massif is made up by detritus mainly derived from the west african craton 3500 1600 ma tdm 3 8 2 3 ga grenvillian belt 1200 900 ma tdm 2 7 1 2 ga and avalonian cadomian belt 800 550 ma tdm 2 5 0 8 ga and that the crust was affected by magmatic events at 510 470 ma tdm 1 6 0 6 ga 410 330 ma tdm 1 6 1 ga and 320 300 ma furthermore they reveal that the late carboniferous mpgs were mainly formed by partial melting of brioverian sediments with cambro ordovician and devonian carboniferous granitoids which are all genetically linked with each other and characterized by th u 4 the new data suggest that the u fertile mpgs result from multiple reworking of u rich brioverian sediments deposited ca 550 ma ago on the northern margin of gondwana and partially molten during several paleozoic events causing a successive increase in u content in the middle upper crust

outcrop to subsurface cycle correlation in the milankovitch frequency band middle cretaceous central italy time series analysis of the cenomanian through aptian interval in well logs from offshore adriatic boreholes facilitates a cycle by cycle correlation with measured sections of equivalent age in umbria central italy the cycle by cycle match can be demonstrated between filtered spontaneous potential logs and filtered bed thickness data but is unequivocal only at wavelengths of greater than 10 metres which are believed to represent a 2 myr eccentricity signal at shorter wavelengths tool resolution becomes a limiting factor as a result of low sedimentation rates the 100 41 and 21 kyr milankovitch signals all appear to be aliased to longer wavelengths in the sonic and gamma ray log records and tend to blend with the 400 kyr signal this is especially true in the aptian albian interval attempts to filter and correlate on the bases of these shorter wavelengths were therefore unsuccessful accepting a 2 myr duration for each cycle permits an estimation of less than 7 myr to be made for the duration of the cenomanian stage

precambrian u pb zircon ages in eclogites and garnet pyroxenites from south brittany france an old oceanic crust in the west european hercynian belt abstract u pb zircon ages have been determined for two eclogites from the vendee and for two garnet pyroxenites from the baie d audierne in an episodic pb loss model the two discordia would give upper intercept ages around 1300 1250 ma and lower intercepts ages of 436 384 ma two interpretations are proposed 1 the 1250 1300 ma ages may reflect an unspecified upper mantle event or process the paleozoic ages correspond to the tectonic emplacement of an eclogitic mantle fragment into the continental crust 2 the protolith may have been extracted from the upper mantle 1250 1300 ma ago and stored in a crustal environment until it was metamorphosed under high pressure conditions around 400 ma ago this latter model is favoured by available geologic and isotopic data consequently we propose that a 1300 ma old oceanic crust was tectonicly incorporated into a sialic basement during the proterozoic this mixture was subsequently subducted during the paleozoic

orbital forcing and sedimentary sequences abstract the variation of eccentricity is connected with the most celebrated attempts to determine a limited portion of geologic time in the elaboration of the theory of the ice age that bears his name croll correlated two important epochs of glaciation with epochs of eccentricity computed to have occurred 100 000 and 210 000 years ago as the analysis of the glacial history progresses these correlations will eventually be established or disproved and should they be established it is possible that similar connections may be made between events far more remote gilbert 1900 p 15 16 but the precessional motion pulses steadily on through the ages like the swing of a frictionless pendulum its throb may or may not be caught by the geological process which obtains in a particular province and in a particular era but whenever the conditions are favorable and the connection is made the record should reflect the persistence and the regularity of the inciting rhythm gilbert 1900 p 18

the ancenis terrane an exotic duplex in the hercynian belt of armorica western france abstract the ancenis terrane bordering the loire france consists of precambrian brioverian basement and three paleozoic sedimentary sequences marine ordovician to devonian shelf sediments lower carboniferous synorogenic intermontane sediments the culm and coal bearing namurian deposits the terrane is completely bound by faults and its geology is in marked contrast with that of its neighbours new data and observations along the terrane s southern boundary show that the ordovician devonian sequence occupies a transpressional shear zone and can be described as a tectonic melange the culm outcrop previously interpreted as a sinistral pull apart structure is reinterpreted as a lower carboniferous dextral pull apart and it is shown that carboniferous transpression and thrusting remodelled the pull apart basin into its present mica fish shape with its remarkable concentric structure and within a plate tectonic setting of mega dextral displacements along the northern boundary of gondwana the ancenis terrane can be interpreted as an exotic duplex

paleomagnetic and ams evidence for a variscan ductile clockwise rotation of the ile de groix blueschists south brittany france consequence on the late hercynian structural pattern of westernmost europe abstract the terrigenous sedimentary rocks and blueschists of the ile de groix france and surrounding sea floor display a complex metamorphic history comprising three major episodes of deformation that took place between 420 and 294 ma paleomagnetic measurements show that three components of magnetization can be isolated the low unblocking temperature component isolated below 300 c is close to the present magnetic dipole field the high component which reaches 580 c and even 680 c in some places corresponds to pre variscan magnetic directions already obtained for armorica since the groix island has always been part of the armorican microplate the corresponding magnetic inclinations can be used for dating tectonic events by comparing with the armorican apparent polar wander path apwp l1 lineations associated with sheath folds are contemporaneous with remagnetization that took place between 336 and 359 ma this is in good agreement with published radiogenic data the anisotropy of the magnetic susceptibility study shows that l1 lineations are the magnetic markers of tectonic rotation part of the dextral d2 ductile rotation of l1 around a vertical axis occurred around 310 ma paleomagnetic data also suggest that the clockwise rotation started earlier near the close of blueschist metamorphic event the clockwise rotation increases towards the southern end of groix where it trends 80 an early magnetization at 420 ma is regarded as the age of the initial emplacement of the klippe of groix restoration of the initial shape of groix island shows that the l1 fabric was broadly east west oriented before its impingement by an unevenness of the south armorican suture zone

origin of granitic magma by crustal remobilisation rb sr and pb pb geochronology and isotope geochemistry of the late archaean qorqut granite complex of southern west greenland abstract rb sr and pb pb whole rock isochrons on the qorqut granite complex yield ages of 2530 30 myr initial 87 sr 86 sr 0 7081 0 0008 and 2580 80 myr respectively a model relating initial sr and pb isotopic compositions of the qorqut granites to the sr and pb isotopic compositions of the amitsoq gneisses ca 3700 myr and nuk gneisses ca 2900 myr at 2550 myr ago as well as sr and pb contents of the gneiss units suggests that between 40 and 50 of the qorqut granite magma was generated by partial melting of amitsoq gneisses and the remainder by partial melting of nuk gneisses

astronomical frequencies for pre quaternary palaeoclimate studies the sensitivity of the earth s orbital frequencies involved in the astronomical theory of palaeoclimates to the earth moon distance and consequently to the length of the day and to the dynamical ellipticity of the earth has been investigated for the last billion years the shortening of the earth moon distance and of the length of the day back in time induces a shortening of the fundamental astronomical periods for the obliquity and the precession a shortening which is larger for the longer periods these periods of precession and obliquity influenced by the changing earth moon distance length of the day and dynamical ellipticity of the earth have been recomputed for the last 440 million years

major bounding surfaces in aeolian sandstones a climatic model large areas of fixed vegetated dunes occur in the sahel and southern margin of the sahara these dunes were active during late pleistocene times and have been stabilized as a result of a change to more humid less windy conditions in the holocene the stabilized surfaces which are in part of erosional origin are of regional extent within aeolian sand sequences such surfaces form the highest order bounding surfaces since regional bounding surfaces of this type occur in all the world s major deserts it is suggested that they should also be present in some ancient aeolian sandstone sequences

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

sr z sbnd nd isotopic record of multi stage interactions between mantle derived magmas and crustal components in a collision context the ultramafic granitoid association from vivero hercynian belt nw spain late stage epizonal calc alkaline granitoids of the hercynian belt are often associated with coeval more basic rocks such granitoids have been the focus of special attention when studying the role of mantle derived magmas in granitoid genesis these studies conclude that basic magmas are involved in granitoid production but their contribution to the chemical composition of granitoids is often controversial the vivero massif nw spain includes calc alkaline granitoids and coeval ultramafic and mafic rocks but these granitoids are syntectonic and intruded into middle crustal levels during an early stage scattered major and trace element chemistry together with heterogeneous sr nd isotopic results preclude an origin of this ultramafic to granitoid association by any closed system fractionation process ultramafic rocks include peridotites pyroxenites and homblendites with endi from 3 5 to 1 3 and 87sr86sr i ratio from 0 7059 to 0 7080 they are interpreted as cumulates formed by in situ fractional crystallization from contaminated basaltic magmas particularly by interaction with 87sr enriched fluids mafic rocks comprise biotite amphibole diorites quartz diorites and tonalites with ndi from 3 8 to 0 0 and 87sr86sr i ratio from 0 7058 to 0 7073 they represent either cumulates or more evolved magmas of calc alkaline affinity these rocks display an inverse correlation between the degree of differentiation and the degree of contamination inferred from isotope data this points to wall rock assimilation by thermal erosion during ascent of basic magmas up to the middle crust and supports the participation of mantle sources with positive end and 87sr86sr i 0 7058 granitoids range from amphibole biotite tonalites through biotite granodiorites to monzonitic granites they show an isotopic trend which diverges from that of ultramafic and mafic rocks endi 6 0 to 1 9 decreases while 87sr86sr i ratio 0 7061 to 0 7082 increases from tonalites to monzonitic granites reflecting an increasing contribution of crustal components in more differentiated rocks assimilation and fractional crystallization mixing included account for such an evolution contamination of basic magmas by this process occurring at deeper crustal level produced hybrid tonalites and related mafic magmatic enclaves only monzonitic granites are interpreted as pure crustal melts their hree fractionated patterns gd yb 4 5 suggest equilibrium with a garnet rich residuum partial melting of moderate time integrated rb sr protolith such as greywackes and or meta igneous rocks of intermediate acid composition could account for the isotopic composition of these monozogranites this example shows that at least some calc alkaline granitoids of collisional setting are not produced by pure intra crustal melting they are formed through a complex multi stage hybridization process involving mantle derived magmas crustal components and several concomitant magmatic processes

precadomian relicts in the armorican massif their age and role in the evolution of the western and central european cadomian hercynian belt abstract well dated precambrian is mostly developed in the north of the armorican massif in which area voluminous cadomian magmatism is dated at between 650 and 550 ma much older relicts occur at cap de la hague in guernsey and in the tregor and are also found in the northern continental margin of the iberian peninsula in all these occurrences whole rock systems have been opened so that the true ages cannot be determined by the rb sr whole rock isochron method four u pb zircon ages are between 1 8 and 2 ga in guernsey icart orthogneisses in tregor port beni trebeurden morguignen orthogneisses there is no evidence from strontium isotopes that these isolated and scattered relicts have a wide extension or that such ancient continental crust played an important role in magma genesis from 650 ma to 270 ma ago on the contrary the evolution of initial 87sr 86sr ratios with time shows that the observed mid and west european continental crust is probably not older than 700 ma the increase of the initial 87sr 86sr ratios of the magmas with time suggests that after its formation in cadomian times this segment of continental crust evolved virtually as a closed system and hercynian magmatism arose principally from re melting of relatively young sialic components

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

partial melting of crustal rocks abstract shales and graywackes were first metamorphosed at 650 c and then partially melted at 700 and 750 c at 2 4 6 and 8 kilobars in the presence of 0 75m nacl 0 45m kcl and 0 225m cacl2 0 750m nacl solutions in experiments with shales kk na ratio in the decreases with increasing pressure at 650 and 700 c however at 750 c this ratio is equal to 0 5 at all pressures investigated this suggests that melts at 700 c and at 2 to 8 kilobars pressure may be affected metasomatically whereas melts at 700 c and in the same pressure range will not melt composition produced in the shale kcl nacl experiments is granite at 2 4 and 6 kilobars pressure whereas the melt compositions in the shale cacl2 nacl experiments range from quartz monzonite 2 5 kilobars to granodiorite above 5 kilobars experiments with graywacke kcl nacl produced melts of trondhjemite composition at 2 2 5 4 6 and 7 5 kilobars these results indicate that partial melting of crustal rocks such as metamorphosed shales and graywackes in the deeper parts of the crust can produce large volumes of granitic magmas ranging in composition from true granite to trondhjemite to quartz monzonite and granodiorite

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

metallogenesis of the french part of the variscan orogen part ii time space relationships between u au and snw ore deposition and geodynamic events mineralogical and upb data abstract mineralogical geochemical structural and isotopic u pb studies were carried out on representative uranium and gold deposits from the french part of the variscan orogen results are compared with complementary data from the literature in order to propose a reconstruction of the metallogenesis in the variscan terranes with a particular emphasis on u deposits the data show that the early metal concentrations or pre enrichments in the terranes are affected by successive metallogenic stages which caused significant metal transport and deposition thus the significant geodynamic events which affected the variscan terranes were responsible for multistage metal deposition or alteration of the early concentrations and for the superimposed features of the metal deposits u pb geochronological results complemented by a compilation of geochronological data on u and pb zn ba f deposits show that three major stages of u deposition took place in western europe in the periods 325 300 ma 290 260 ma and 190 170 ma in addition small uranium mineralization events have been dated at 425 ma 90 110 ma and as tertiary about 40 ma the permo stephanian stage is associated with late brittle deformation associated with major shear zone movements and constitutes the major stage of u concentration the jurassic phase has a prominent part in the hydrothermal remobilization of uranium and in f ba pb zn deposition throughout western europe especially in the eastern part of the massif central other stages correspond to subordinate uranium migration within variscan terranes during earlier or later geodynamic phases the silurian metamorphism in vendee and the mesozoic to tertiary tectonic movements related to the pyrenean or alpine orogenies in the south eastern part of the massif central and in the alps data available on au and sn w as sb deposits have been considered for comparison with the u metallogenesis stages related to the deposition of such metals display significant similarities with those related to uranium deposition they occurred mostly during visean to permian events related to the intrusion of the late visean sn w and late carboniferous sn w li f granitoids and to the late brittle deformation stages which affected the hercynian basement in the vicinity of major shear zones au as sb however the significant discrepancies between the physical chemical conditions of solubilization transport and deposition of the metal led to distinct deposit location independently of the metal source location within the terranes

stratigraphic correlation of presumed milankovitch cycles in the blue lias hettangian to earliest sinemurian england simple digital filtering methods allow the correlation of patterns of metre scale cycles between outcrop sections and well logs through the hettangian to earliest sinemurian part of the lower lias in southern england the matching of cyclic patterns is sufficiently confident to allow the identification of a significant break in the blue lias section at lyme regis dorset the standard ammonite zones of the hettangian stage are shown to be of quantifiably unequal duration

amalgamated accumulations resulting from climatic and eustatic changes akchar erg mauritania the akchar erg of the sahara of western mauritania shows a morphology and stratigraphy that can be recognized as the amalgamation of late pleistocene and holocene deposits that reflect eustatic and climatic events mapping trenching and dating by 14c methods and artefacts show that the prominent complex linear dunes draas of the akchar erg are actually composite features showing at least three constructional and two destructional phases the constructional phases are represented by three convex up layers i a modern veneer moulded into superimposed crescentic dunes which partially mantle the larger linear bedforms ii a middle partly root turbated sand deposited sometime during the last 4000 years and iii a core of linear dune sand formed during the last glacial period 13 000 20 000 yr bp which today shows relict relief intense root turbation and pedogenesis these constructional phases are separated by super bounding surfaces that coincide with erg destructional phases surface 2 bounds the middle aeolian sand and is marked by a lag surface of small granules surface 1 is a very prominent surface with an abundance of neolithic artefacts and represents stabilization of the linear dunes during the humid interglacial period 4000 11 000 yr bp interdraa deposits originated during the interglacial period and consist of continental lacustrine limestones and sandstones humic sands deposited in marshes and sabkhas on the coast the sabkhas originated during interglacial highstand of sea level when interdraa areas were marine embayments and subsequently dried during regression the draa and interdraa sequences therefore in spite of being adjacent facies actually represent different events and were not formed simultaneously the upwind sand sheet margin of the akchar erg shows exposures of the middle and core aeolian sands which were previously protected from deflation by vegetation being progressively cannibilized in the current phase of erg construction and revealing a crystalline basement rock in this proximal area conditions are not favourable for the incorporation of these aeolian accumulations into the stratigraphic record

nd and sr isotopic characterization of the wrangellia terrane and implications for crustal growth of the canadian cordillera nd and sr isotopic data are reported from samples of most of the major lithologic units of the wrangellia terrane exposed on vancouver island and the queen charlotte islands initial epsilon nd values range from 1 0 to 7 3 and initial 87 sr 86 sr ratios range from 0 70323 to 0 70481 these ratios are similar to those of modern island arcs and demonstrate the unevolved juvenile nature of this terrane these data indicate that wrangellia probably resided in an intra oceanic environment until its accretion to north america nd crustal residence ages for clastic sedimentary rocks of wrangellia mean t cr 0 77 pm 0 33 ga t dm 0 57 pm 0 35 ga are much closer to their stratigraphic ages than is the case for the ages of many other phanerozoic sedimentary rocks thus providing strong evidence for phanerozoic crustal addition to north america early proterozoic crustal components cannot comprise more than about 6 of wrangellia therefore the terrane consists of 94 new mantle d

petrogenesis of garnet two mica granites in the ruby mountains nevada garnet two mica granites of jurassic 160 3 m y and cretaceous 83 1 3 m y ages intrude amphibolite facies metasedimentary rocks of precambrian age and lower paleozoic sedimentary rocks in the northern part of the ruby mountains nevada high initial 87sr 86sr and high values of 18o for minerals suggest a dominantly sedimentary source for the magmas total rare earth contents are low and less fractionated cen ybn 10 than granitic rocks of similar composition in the sierra nevada and show a negative eu anomaly eu eu 0 57 these data are consistent with melting in the stability field of plagioclase with little contribution to the melt from refractory minerals such as zircon sphene and apatite minor euhedral garnets are manganese rich and magnesium poor al60sp34py3 6gr2 8 compared to garnet al79sp6py13gr2 in the intruded metamorphic rocks and show an increase in manganese and a decrease in calcium in the outer few microns fractionation of magnesium iron between garnet and biotite and of sodium potassium between k feldspar and plagioclase in the granites suggest submagmatic equilibration temperatures in the range of 365 to 505 c whereas oxygen isotope equilibration temperatures for two granites are higher at 480 and 570 c the compositions of the granites plot near minimum melting compositions in the water saturated quartz albite orthoclase system comparison of muscovite plus quartz stability the water saturated granite solidus and experimental garnet melt equilibria suggests pressures of crystallization no lower than about 3 5 kbar this pressure is compatible with that estimated from garnet plagioclase sillimanite quartz equilibria for the intruded metasedimentary rocks

geochemical comparison between himalayan and hercynian leucogranites abstract isotopic sr pb nd o ree and trace element data from three himalayan nepal and six hercynian brittany france leucogranites are compared for the himalayan granites makulu mustang and especially manaslu the 87sr 86sr versus 87rb 86sr scatter diagrams and hence variability of the initial 87sr 86sr ratios reflect heterogeneity of the source materials these ratios are very high 0 730 0 770 for manaslu the very high 207pb 204pb ratios 15 8 at the corresponding 206pb 204pb ratio of 18 8 the very low 143nd 144nd ratios i nd 13 to 17 and the very high 18o 16o ratios 12 confirm the crustal origin with the paragneisses of the tibetan slab formation i as the probable parent source for the six hercynian granites with an individual rb sr isochron established for many of them the initial 87sr 86sr ratios range from 0 706 to 0 717 the lead neodymium and oxygen isotope compositions confirm their crustal source of presumably palaeozoic age but not specifically identified the rare earth patterns and thorium contents are comparable to those for himalayan granites the similarities of the geochemical signatures isotopes and trace elements of the himalayan and hercynian leucogranites suggest that the petrogenetic processes were comparable and that the himalayan continental collision model can be applied to the hercynian the depletion in ree and th implies either interaction of the granitic liquids with fluid or more probably early precipitation of monazite the increase in sm nd in the granites compared to their parent materials implies that the neodymium model ages of primary mantle extraction are maximum rather than minimum ages in contrast to the hercynian leucogranites the extremely radiogenic sr of the himalayan leucogranites reflects the very long crustal residence time before the melting event the apparent uniformity of the initial 87sr 86sr ratios of the individual hercynian granites may be related to an aging effect where the straight line fitting improves with increase in age of the pluton

the southern part of the armorican orogeny a result of crustal shortening related to reactivation of a pre hercynian mafic belt during carboniferous time abstract this synthesis of detailed geological and geophysical information relates deep crustal structure to deformation and the emplacement of granites in the south armorican massif and the adjacent continental shelf near the continental margin a zone of geophysical anomalies is regarded as a nearly complete line of suture between a southern continent and america a major mafic body in the trace of the supposed suture in interpreted as a relic of probable oceanic crust which has apparently acted as a deflector of regional strain it is proposed that late in the hercynian collision history during late carboniferous time compressional strain acted across the mafic body and can be directly related to generation of younger regional structures in the metasediments and to localization of granite emplacement

late pervasive crustal scale extension in the south armorican hercynian belt vendee france the coast of vendee in the southern hercynian armorican massif shows a complete barrovian metamorphic section from upper crustal low grade metasediments down to high grade partially molten rocks the bottom of the pile is intruded by numerous synkinematic granitic dykes the pattern of the dyke array shows that syn emplacement deformation combined coaxial sub vertical shortening and sub horizontal shearing strain estimates using dykes and deformed metamorphic isogrades suggest a maximum amount of bulk shortening in the range of 80 results are consistent with extensional tectonics mainly accommodated by large pervasive thinning rather than by strain localisation along a detachment zone q 2003 elsevier ltd all rights reserved

cyclostratigraphic reasoning and orbital time calibration cyclostratigraphy postulates that the stratigraphic record cyclically encodes the periodic orbital forcing of terrestrial insolation thus providing a time calibration regular cycles are sought using spectral analysis of lithological data series but there are inherent ambiguities in this method it may detect more cyclicities than conventional orbital forcing allows but only those with the closest correspondence to estimated orbital frequencies are used for time calibration irregular cycles are subjectively defined in terms of non rhythmic repetitions of facies the calibrations assume that they record the spatially distorted sedimentary effects of orbitally forced periodicity in insolation the null hypothesis that such non rhythmic repetitions are autogenic rather than orbitally forced cannot however be rejected both types of cyclicity conform to an expected universe where orbital forcing is reliably and recognisably encoded in the stratigraphic record neither form of cyclicity rules out the presence of hiatuses thus even if orbital in origin neither can provide dependably refined orbitally scaled time calibration

importance of melt fraction and source rock composition in crustal genesis the example of two granitic suites of northern portugal in the montalegre area northern portugal two varieties of syn to late tectonic hercynian aluminous granitoids are distinguished 1 1 muscovite rich two mica granites forming many successively emplaced bodies with little chemical variation the chemical characteristics of these units suggest partial melting of various crustal rocks under distinct p t and water saturation conditions there has been no homogenization between the different melt batches the rocks contain very few restitic minerals segregation processes may have operated either in the source rock or in an early magmatic stage 2 2 a cordierite biotite and muscovite bearing granitic body its mineralogical composition is extremely heterogeneous and it contains many xenoliths the genesis of this granite results essentially from partial melting of peraluminous orthogneisses the melt fraction was higher than the rheological critical melt percentage rcmp during anatexis the rigid framework of the source rock broke down leading to migration of both melt and residual minerals towards upper crustal levels the heterogeneity of the resulting granite is mostly related to an incomplete separation of residual crystals and melt the existence of biotite and cordierite rich samples is the result of accumulation of residual minerals during the magma emplacement this study emphasizes the role of the source rock composition which controls the degree of partial melting at given p t and water saturation conditions during crustal melting the strong geochemical differences between the two kinds of granitoids are related to the near absence of residual minerals in the muscovite rich granites and to the presence of residual minerals in the cordierite bearing granite it is suggested that the muscovite rich granites are mostly low melt fraction rcmp the presence of many residual crystals alkali feldspars and plagioclases as well as dark minerals in the cordierite granite implies a high viscosity of the melts and therefore water undersaturated melting conditions

a new model for the hercynian orogen of gondwanan france and iberia abstract the hercynian orogen of gondwanan france and iberia is a collage of disparate terranes separated by faults plate reconstructions indicate that the orogen developed during a massive dextral transpression as laurentia slid along the boundary of gondwana and as laurentia baltica and east avalonia rotated clockwise to form pangea it is proposed therefore that the collage is the result of this transpression and that it is an amalgamation of displaced tectonostratigraphic terranes which represent the dismembering of the gondwanan shelf with its stable platform ordovician devonian sedimentary sequence and the neighbouring rheic ocean specifically it is proposed that 1 the slice of the gondwanan shelf that now forms north brittany was displaced dextrally from nw africa around iberia by more than 2000 km 2 the ophiolites oceanic material and volcanic arcs of south armorica and the massif central represent slices of the rheic ocean displaced dextrally along the gondwana margin and shuffled in amongst slices of gondwanan shelf 3 the convergent component of the transpression is represented by thrusts throughout the region the leon terrane of france and ophiolite bearing klippe in nw iberia the ibero armorican arc was formed by wrapping the mobile dextral shear belts about a rigid iberian basement block the dextral porto tomar shear zone represents the more important shearing in iberia and the iberian sinistral shears are interpreted as bookshelf type structures subsidiary to the dextral shearing

neoproterozoic shear zone tectonics within the icartian basement of guernsey and sark channel islands the channel islands of guernsey and sark are amongst the few localities within the neoproterozoic cadomian orogenic belt where palaeoproterozoic basement is exposed basement units collectively referred to as icartian comprise orthogneisses e g the c 2000 ma icart granite gneiss metasediments and amphibolites on guernsey the protolith to the icart granite gneiss was intruded into metasediments already deformed during a d 1 deformation event both were variably deformed during a d 2 event within an approximately north south trending steeply dipping oblique dextral shear zone on sark metasediments and amphibolites carry a d 1 fabric not necessarily correlatable with that on guernsey which was deformed during d 2 to form recumbent gently inclined folds and tectonic fabrics which locally carry top to the south kinematic indicators early cadomian quartz diorites which intruded the icartian basement carry variably developed magmatic and moderate temperature 400 550 c ls solid state fabrics which are similarly oriented to d 2 basement structures this indicates that the quartz diorites were emplaced and deformed during a protracted d 2 event published 40 ar 39 ar amphibole cooling ages of c 600 ma from guernsey and sark quartz diorites and a host icartian amphibolite are consistent with reworking of basement units during the cadomian orogeny the contrasting kinematic pattern during cadomian deformation implies that a flat lying shear zone on sark may have accommodated oblique dextral strike slip movement on guernsey with an overall southerly direction of tectonic transport deformation most likely occurred in the hanging wall of a major south dipping neoproterozoic subduction zone

the rapid formation of sputnik planitia early in pluto s history modelling suggests that the icy region on pluto known as sputnik planitia formed shortly after charon did and has since been stable with its latitude corresponding to a minimum in annual solar illumination and its longitude determined by tidal forces from charon douglas hamilton et al present modelling data that imply that pluto s sputnik planitia basin may be an ice cap rather than the product of an impact their data suggest that the feature formed shortly after pluto s largest satellite charon did and has since been stable with its latitude corresponding to a minimum in annual solar illumination over its orbital precession period of about a million years and its longitude determined by tidal forces from charon this is one of four papers on the geology of sputnik planitia in this issue of nature in news views amy barr puts these latest contributions into context pluto s sputnik planitia is a bright roughly circular feature that resembles a polar ice cap it is approximately 1 000 kilometres across and is centred on a latitude of 25 degrees north and a longitude of 175 degrees almost directly opposite the side of pluto that always faces charon as a result of tidal locking1 one explanation for its location includes the formation of a basin in a giant impact with subsequent upwelling of a dense interior ocean2 once the basin was established ice would naturally have accumulated there3 then provided that the basin was a positive gravity anomaly with or without the ocean true polar wander could have moved the feature towards the pluto charon tidal axis on the far side of pluto from charon2 4 here we report modelling that shows that ice quickly accumulates on pluto near latitudes of 30 degrees north and south even in the absence of a basin because averaged over its orbital period those are pluto s coldest regions within a million years of charon s formation ice deposits on pluto concentrate into a single cap centred near a latitude of 30 degrees owing to the runaway albedo effect this accumulation of ice causes a positive gravity signature that locks as pluto s rotation slows to a longitude directly opposite charon once locked charon raises a permanent tidal bulge on pluto which greatly enhances the gravity signature of the ice cap meanwhile the weight of the ice in sputnik planitia causes the crust under it to slump creating its own basin as has happened on earth in greenland5 even if the feature is now a modest negative gravity anomaly it remains locked in place because of the permanent tidal bulge raised by charon any movement of the feature away from 30 degrees latitude is countered by the preferential recondensation of ices near the coldest extremities of the cap therefore our modelling suggests that sputnik planitia formed shortly after charon did and has been stable albeit gradually losing volume over the age of the solar system

origin of kaolinization in brittany nw france with emphasis on deposits over granite stable isotopes o h constraints abstract in the the armorican massif nw france the formation of kaolinite in surficial conditions is demonstrated by the existence of alteration profiles enriched in kaolinite at the top of metamorphic and sedimentary formations for some of the largest deposits located in granitic massifs the origin of kaolinization hydrothermal or supergene is debatable because these deposits are often spatially associated with networks of quartz veins o and h isotopic compositions of kaolinitic rocks are very similar in all types of deposits from alteration profiles from sedimentary deposits and from those after granites and are consistent with a supergene origin 18okaol 20 2 1 dkaol 55 to 67 in the pl meur variscan leucogranite some of the kaolinite rocks located in the vicinity of massive quartz veins may contain a minor hydrothermal component on the basis of slightly lower bulk isotopic compositions some of the late quartz veins with peculiar shapes draperies or wires made of coalescent submillimetric euhedral quartz may be evidence of this hydrothermal event at temperatures of the order of 100 150 c 18oqtz 23 1 we conclude that in the light of the present data kaolinite of the armorican massif is essentially of supergene origin the local association of kaolinite with quartz veins in deposits after granites is likely due to an increase of permeability induced by the former event of hydrothermal alteration

leucogranites of the himalaya karakoram implications for magmatic evolution within collisional belts and the study of collision related leucogranite petrogenesis abstract two types of leucogranite have been identified in a collisional setting in the himalaya type a e g manaslu high himalaya are syn tectonic crustal melts related to hot over cold thrusting and subsequent fluid fluxing type b e g baltoro karakoram is related to lower crustal melting during following crustal thickening and thermal re equilibration followed by crystal fractionation for type b melting is triggered promoted in the lower crust by input of heat and material from the sub continental lithospheric mantle which is contemporaneously melting due to removal of the slab input and downward displacement both these granite types can be explained in terms of recent thermal models for continental collision and so might be expected to be found in other orogenic terranes these models also show that extension and thinning will occur as a result of the crust becoming gravitationally unstable due to thickening some time after collision thinning will lead to upwelling and melting of asthenosphere which could then intrude the base of the crust and promote further melting this could explain the common occurrence of a type granitoids as final plutonic events in orogenic terranes following intrusion of the collision related type a and type b granites existing genetic classification schemes only classify granites in terms of source type or broad environmental affinity e g collisional to further subdivide such broad environmental fields it is considered that processes and a general evolutionary model must be used source type is not suitable due to the number of source components that are often involved and the overlap of these components from one environment to another one or two selected features of a granite are not however sufficiently exclusive to identify different petrogenetic histories we propose that an evaluation of field style mineralogy isotope and elemental geochemistry should be used to subdivide the collisional granite field into type a and type b or manaslu and baltoro types respectively

rare earth elements in the bimodal mount peyton batholith evidence of crustal anatexis by mantle derived magma the mount peyton batholith of central newfoundland is one of many siluro devonian acadian plutons of the appalachians with contrasting mafic and silicic compositions the distributions of rare earth and other trace elements in 18 representative samples indicate that the contrasting rock types are not genetically related by any crystal liquid fractionation process rather they support earlier suggestions that the granitic melt was generated by crustal melting due to intrusion of the mantle derived mafic magma and the two evolved independently through processes involving some crystal fractionation and contamination with country rocks during ascent

sequence stratigraphy and orbital forcing in permian rotliegend desert deposits a discussion rotliegend sequence stratigraphy in the southern permian basin suggests climatically forced fluctuations in the level of an expanding playa lake system the associated facies changes are abrupt and define essentially isochronous sequence boundaries spectral analyses of gamma ray logs have indicated that individual sequences display internal cyclicity consistent with orbital milankovitch forcing confirmatory results have been reported from the spectral analysis of field logs from the continental permian brodick beds of the isle of arran these studies claim that since the cycles are orbitally forced they have chronostratigraphic significance however they are based on dubious assumptions concerning the unvarying rate and continuity of deposition of the sequences in question in the case of the southern permian basin the assumption is applied to c 50 m units but the result is an implausible chronostratigraphic model for the brodick beds the assumption is applied to a 675 m succession and there is a crucial ambiguity in the relationship between the lithostratigraphic cycles and the permian orbital cycles the studies do not provide persuasive evidence of the consistent encoding of milankovitch orbital cycles in permian desert deposits

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

subsidence of the paris basin and its bearing on the late variscan lithosphere evolution a comparison between plate and chablis models abstract in this study two different models for the lithospheric thermal evolution are used to predict the long term subsidence of the paris basin the plate or chablis models have a constant temperature or constant heat flow at the base of the lithosphere these boundary conditions yield a different thermal relaxation time for a given equilibrium lithospheric thickness the final thermal steady state does not depend on the boundary condition fixed at the base of the lithosphere the thermal evolution of the lithosphere is computed taking into account temperature and pressure dependent thermal characteristics metamorphism in the crust top crustal erosion and phase transition in the mantle spinel garnet the long term tectonic subsidence of the paris basin constrains the mesozoic thermal evolution of the lithosphere and the amplitude of the initial thermal anomaly the chablis model including the effects of spinel garnet phase transition predicts present day lithospheric thickness in better agreement with observation than the plate model the chablis model involves a basal heat transfer of 37 mw m2 similar to the value beneath oceanic plates the long term subsidence of the paris basin obviously results from the decay of a thermal anomaly initiated during late variscan times we first assume that at 300 ma the lithosphere was less than 150 km thick and the thickened crust started returning to its present thickness 35 km by uniform lithospheric extension the subsidence data can be explained by short stephano autunian as well as long stephano triassic lasting extension these hypotheses both implicitly refer to extensional collapse of the variscan belt however the evolution of the variscan belt from 350 to 300 ma may have involved almost no crustal thickening but an anomalously thin lithosphere ca 55 km the corresponding thermal evolution of the lithosphere is computed using the chablis model with an initially high heat transfer of 61 mw m2 at the base of the base of the lithosphere at 280 ma this heat transfer is tuned to 40 mw m2 leading to lithospheric cooling from 280 to 30 ma which can also explain the paris basin subsidence for the short lasting extension hypothesis the late variscan lithospheric evolution predicted with the chablis model yields paleogeography magmatism and retrograde p t t paths in good agreement with those reported from areas around the paris basin conversely the plate model does not allow mantle melting during the end of carboniferous and permian times the long lasting extension hypothesis does not explain the paleogeographic evolution the timing of magmatism and retrograde p t paths showing isothermal decompression the hypothesis with a strong lithospheric delamination cannot be ruled out however it is less plausible than the extensional collapse hypothesis

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

the oxygen isotope composition of hercynian granites and pre hercynian gneisses from the schwarzwald sw germany the oxygen isotope composition of 56 hercynian granites and 42 pre hercynian gneisses has been investigated in addition some mineral 18o data and 5 d values of whole rocks have been obtained the granites from the n schwarzwald show in general relatively uniform 18o values between 11 5 and 13 5 those from the s schwarzwald are less uniform and lighter in 18o and range from 2 3 to 11 5 the gneisses from the pre hercynian basement exhibit more or less the same variation and range from 1 7 to 10 4 18o values 6 only occur in the s schwarzwald and obviously indicate hydrothermal interactions of meteoric waters which probably took place after the emplacement and solidification of the granites and which equally affected granites and gneisses due to the nearly identical 18o 16o ratios of s schwarzwald granites and gneisses it is proposed that such gneisses in the pre hercynian basement qualify as the precursor rocks of the s schwarzwald granites whereas for the n schwarzwald granites crustal rocks with heavier 18o values unknown from the present surface have to be postulated this distribution is also reflected on a 87sr 86sr 18o 16o diagram

geochemical investigations of the origin of the manaslu leucogranite himalaya nepal the himalayan orogeny is characterized by an intraplate subduction zone the thrusting of the tibetan slab over the foreland is probably responsible for the origin of the manaslu type leucogranites this geochemical study tries to examine the crustal origin of the granite to verify whether the paragneisses of the tibetan slab are the most probable parent material and to determine the subsequent petrogenetic processes the rb sr systematics suggest that 1 1 the scatter of data points in the 87sr86srvs 87rb86sr diagram may be due to the heterogeneity of the initial 87sr86sr ratios 2 2 the initial isotopic ratios are very high 0 730 0 760 and are in agreement with those of the gneisses of the tibetan slab at the time of the melting pb isotopic ratios are homogeneous they are also consistent with a crustal prehistory the ree and th contents are unexpectedly low in comparison with the much higher contents of the parent material monazite left in the residue of melting or more probably extracted in an early stage of the magmatic history may explain the observed ree and th abundances alternatively field observations show that the emplacement of the granite has been accompanied by a very strong hydrothermal activity in consequence the low ree and th contents of the granite could also be due to the loss by solution during prolonged fluid circulation throughout the granitic melt and the sr and pb isotope distribution to the effect of late to post magmatic fluids the isotopic character of which was determined by the underlying tibetan slab

uranium mobilization from the variscan questembert syntectonic granite during fluid rock interaction at depth leaching of uranium fertile granites represents a major source of uranium as uraninite is easily dissolved in oxygenated aqueous solutions this phenomenon is well documented at surface conditions but remains poorly documented for granites at depth in this study we propose that surface derived oxidized hydrothermal fluids leached uranium from uraninite in the questembert peraluminous granite at temperatures greater than 70 to 160 c this variscan synkinematic granite is characterized by widespread and pervasive development of vertical and permeable c s structures these structures likely facilitated the infiltration of oxidized hydrothermal fluids from the surface their circulation at depth and the subsequent fluid rock interaction in the granite published oxygen isotope data shows that it has undergone subsolidus fluid rock interaction dated between 312 and 303 ma by 40ar 39ar analyses on muscovite these interactions were responsible for the concomitant decrease of the feldspar 18o values together with uranium leaching mass balance calculations suggest that this hydrothermal event could have liberated several hundred thousand tonnes of uranium from the questembert granite the liberated uranium may have been dispersed in early permian intramountainous basins and therefore disseminated over large areas this study emphasizes that the efficiency of uranium leaching may be directly related to the extent of subvertical structure development in granites emplaced along strike slip shear zones which allow for downward infiltration of oxidized surface derived fluids a specific and systematic sampling is required to better constrain the proposed model

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

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

late proterozoic zircon ages from a basic ultrabasic complex a possible cadomian orogenic complex in the hercynian belt of western europe the basic ultrabasic complex of belle isie en terre northwestern part of the armorican massif france has been dated at 602 4 1 m y by the u pb zircon method the calc alkaline trend of the metabasic rocks and the geologic context suggest that this ophiolite like complex is related to a back arc orogenic setting similar to the geodynamic environment that has been proposed for pan african calc alkaline ophiolite like complexes of late proterozoic age around the west african craton our results are compatible with the closing in late proterozoic time of a channel ocean north of the basement and formed during the cadomian orogeny rather than with possible caledonian or hercynian subduction