age and origin of the ardal dike complex sw norway false isochrons incomplete mixing and the origin of caledonian granites in basement nappes 1 granitic dikes intruding the upper thrust sheets of the jotun nappe complex southwest norway previously assigned a sveconorwegian age based on rb sr data are redated to 427 1 ma by u pb the erroneous rb sr age can be explained by mixing of magmas with different isotopic compositions similar geochemical signatures evidence of sr mixing inherited proterozoic zircons and synkinematic modes of emplacement are also seen in silurian granites in the lifjorden complex as well as in the lindas nappe and the totland unit of the bergen arc complex we propose that these granites are related and derived from anatexis of rb poor sediments of baltica affinity overridden by the nappes during scandian thrusting this interpretation is compatible with the presence of the granites in nappes of both oceanic and of continental affinities and their absence in the autochthonous basement

geochemistry of the granitic rocks from the kwandonkaya complex northern nigeria abstract the kwandonkaya complex forms one of the most prominent and typical ring structures in the centre of jos plateau state nigeria lithologically it is composed of amphibole and biotite granites emplaced within archean basement gneisses and migmatites the chemical and mineralogical characteristics of this complex suggest that it was derived from crustal source materials trace elements are high in rb li f sn nb w and low in sr and ba compared to the world average for rocks of similar composition indicating a high degree of fractionation however the k rb ratios and petrographic evidence indicate that recrystallization may have redistributed the elements after the initial solidification of the rocks particularly in the biotite granites and that the distribution of tin may have been initially controlled by crystal silicate melt equilibra tin probably preferentially entered the aqueous phase during the late stages of solidification it was the interaction between this phase and early formed minerals that led to greisenization and tin tungsten mineralization although total rock chemical composition show no marked trends the composition of the biotites and hornblende define a trend in the compositional triangle fe3 fe2 mg when the biotites are related to the experimentally studied ternary system kfe33 alsi3o12 h 1 kfe32 alsi3o10 oh 2 kmg3alsi3o10 oh 2 and coupled with the estimated position of biotite solid solutions for different oxygen buffers the trend suggests that oxygen fugacities in the magmas during biotite formation were slightly higher than those defined by the fe2sio4 sio2 fe3o4 buffer the ratio of ferric to ferrous iron in pairs of the minerals from the kwandonkaya complex is the same for both hornblende and biotite and suggests crystallization in a closed but buffered environment

metamorphic field gradients in the central alps metamorphic field gradients were determined across the entire amphibolite grade central alps c 50 100 km p t were calculated from 116 samples acquired from our own field work from samples provided to us by others and from rocks with mineral compositions described in the literature only fluid conserved equilibria were used to determine p t the use of an internally consistent thermodynamic database and mineral solid solution models makes the results robust and reduces relative errors the results are presented in contour maps temperature increases from 500 to 550 c along the limit of amphibolite grade metamorphism in the north and west to c 675 c toward the south at the insubric line near the town of bellinzona maximum recorded pressures of c 7 kbar are in a central region c 20 km north of the insubric line and decrease both to the north 5 5 kbar and south 4 5 kbar the p t results indicate that there is a relatively large area that reached conditions in the sillimanite stability field but developed neither sillimanite nor fibrolite this is interpreted as a result of kinetic constraints on nucleation and growth because of the small amounts of thermal overstep 40 c of the kyanite sillimanite phase boundary comparison of p t conditions with carbonate isograds in the region indicate that fluids present during metamorphism were not dominated by a homogeneous external source examination of the two dimensional distribution of pressure and temperature in the context of thermal and tectonic models indicates that two thermal pulses aected the central alps during the tertiary in the second heat aected only the southern parts of the area and overprinted the previously established p t gradients

basement hosted quartz barite sulfide veins in the french alps a record of alpine tectonic fluid expulsion in the external crystalline massifs structural fluid inclusion and isotope s and sr evidences quartz barite sulfide veins hosted by a late variscan monzonite in the polymetamorphic basement of the aiguilles rouges massif western alps france have been the subject of a combined structural microthermometric fluid inclusion and sulfur and strontium isotope study the mineralized veins have a maximum age of 40 ma and were emplaced along vertical tension fractures formed or possibly reopened during the mesoalpine thrust phase two mineralization stages have been recognized a major stage of quartz barite veins with minor and irregularly distributed sulfides and sulfosalts and a minor crosscutting stage of quartz stibnite veins fluid inclusion microthermometry homogeneous 87 sr 86 sr ratios of the barite and the texture and mineralogical composition of the polymetallic quartz barite veins indicate that they were formed mainly due to the cooling of a single aqueous fluid with a salinity around 9 wt percent nacl equiv contemporaneous pressure variations cannot be excluded sulfur isotope thermometry yields formation temperatures between 255 degrees and 270 degrees c and combined with the fluid inclusion isochores they indicate pressures of 0 45 to 1 55 kbars corresponding to a depth of 3 6 super 2 3 sub 1 9 km assuming lithostatic conditions sulfur in the fluid was dominated by h 2 s which was oxidized at the ore site due to cooling of the fluid the vein barites are enriched in 87 sr with respect to the host rocks and indicate that the strontium and the barium were leached from deeper lithologies after the emplacement of the quartz barite veins the hydrothermal system was opened toward shallower structural levels allowing the ingress of meteoric water the late quartz stibnite veins are associated with a distinct and warmer co 2 bearing fluid we conclude that the h 2 o nacl ore bearing fluid originated at a depth of about 15 km by dewatering of mesozoic sedimentary rocks during the collisional thrust mesoalpine phase subhorizontal thrust planes between the basement crystalline rocks and overlying sedimentary units inhibited mixing or regeneration of the system with superficial fluids subvertical fluid migration was restricted to faults in the polymetamorphic basement late alpine extension tectonics allowed the upward migration of deep co 2 bearing fluids and also favored the influx of surface fluids that had remained isolated from the h 2 o nacl hydrothermal system

partitioning of sr ba rb y and lree between alkali feldspar and peraluminous silicic magma samples from well known rhyolitic systems in the western usa were analyzed to determine trace element partition coefficients between sanidine and coexisting melt both d sr and d eu correlate positively with an concentration in sanidine barium not surprisingly shows more correlation with or in sanidine all three show negative correlations with whole rock al2o3 partition coefficients for ree show a decreasing trend with strong positive d eu d lree correlate negatively with the whole rock al2o3 in peraluminous rocks al2o3 in magma seems to exert a major influence on partition coefficients of trace elements with increasing al2o3 concentration the melt composition becomes increasingly similar to that of feldspar incompatible trace elements rb y nb and zr in sanidine show positive trends whereas compatible trace elements sr eu and ba show negative correlations when plotted against whole rock al2o3 lrees which are expected to substitute for ca in the sanidine structure behave like sr and eu because trace element partitioning is influenced by both crystal and magma compositions multiple regression analysis has been used to predict trace element partition coefficients the equations will assist workers in the selection of partition coefficients used in petrogenetic models of silicic systems

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

ages and geodynamic settings of xilamulun mo cu metallogenic belt in the northern part of the north china craton abstract the xilamulun belt along the northern part of the north china craton is located in eastern segment of the central asian orogenic belt and has great economic potential for mo cu mineralization more than ten medium to large ore deposits have been discovered in this region in the recent years the major types of mineralization type include porphyry chehugou mo cu kulitou mo cu xiaodonggou mo and jiguanshan mo quartz vein nianzigou mo xinjing mo epithermal hongshanzi mo u and alteration assemblage liulingou mo the timing of mineralization was previously thought to be yanshanian 208 290 ma however indosinian 260 208 ma ages for intrusions and mineralization have been recognized in recent years based on geochronologic data and regional geological evidence it is suggested that the mineralization in the xilamulun belt was formed during multiple events the mineralization processes are related to a post collisional extension stage 258 210 ma with the generation of the porphyry molybdenum copper deposit a tectonic stress transformation from ns to ew 185 150 ma that gave rise to vein or porphyry molybdenum deposit and a lithospheric thinning stage 140 110 ma with porphyry molybdenum deposit

experimental partitioning of rb cs sr and ba between alkali feldspar and peraluminous melt hydrous partial melting experiments performed between 650 and 750 c at 200 mpa h20 on synthetic metapelite compositions quartz albite muscovite biotite mineral mixtures doped with ba sr rb and cs yielded alkali feldspar crystals with a wide range of compositions in equilibrium at their rims with peraluminous melt measured partition coefficients for normally trace lithophile elements between feldspar and melt d m fsp gl m ba sr rb cs do not depend on either temperature or bulk composition of melt for the compositions studied values of d sr fsp gl are between 10 and 14 and appear to be independent of the albite and orthoclase contents of the feldspar crystals in contrast values of d ba fsp gl and d rb fsp glare strongly dependent on the orthoclase content of feldspar relationships that can be expressed by the following linear equations d ba fsp gl 0 07 0 25 orthoclase and d rb fsp gl 0 03 0 0 1 orthoclase where orthoclase is in mole percent these equations reproduce the range of previously reported values for d ba and d rb determined on natural and synthetic samples a single partition coefficient for cs was also determined at d cs fsp gl 0 13 these data can be used in conjunction with recently published partition coefficients for muscovite biotite and plagioclase feldspars blundy and wood 1991 icenhower and london 1995 to model quantitatively the trace element signatures of peraluminous magmas during anatexis and crystallization depending on bulk composition and pressuretemperature trajectories during episodes of partial melting ba and sr and to a lesser extent rb are retained at the source while cs and li are enriched in the first melts these results explain why peraluminous granitic magmas with s type sedimentary sources carry a distinctive enrichment in li and cs

the chemistry of barium anomalies in the berisal complex simplon region switzerland the petrographic and geochemical features of two zoisite celsian gneiss outcrops from the berisal complex characterised by a syn kinematic mineral assemblage that contains celsian and barium white micas and a maximum whole rock bao content of 8 36 wt are described the outcrops are enclosed in a larger body of garnet bearing two mica augen gneiss which has intrusive contacts with the surrounding garnet biotite muscovite paragneiss and also contains small outcrops of two mica clinozoisite gneiss the zoisite celsian gneisses are strongly enriched in all alkaline earth elements are depleted in the alkali elements and have high zr hf and nb ta ratios compared with the surrounding gneisses the zoisite celsian two mica clinozoisite and garnet bearing two mica augen gneisses have al cnk molar ratios 1 and the zoisite celsian gneisses are also enriched in zr y and nb chondrite normalised patterns for the rare earth elements ree show light ree enrichment with a negative eu anomaly on the basis of field and geochemical data it is argued that the barium anomalies in the berisal complex are the result of igneous fractionation of barium into anorthosite dominated cognate inclusions within a larger volume of calc alkaline peraluminous melt on the basis of u pb zircon ages a conservative estimate for the age of magma crystallisation is placed at 460 10 ma and thus was related to significant late ordovician granitoid magmatism in gondwana derived microcontinents during collision between avalonia cadomia and gondwana

trace element partitioning between alkali feldspar and peralkalic quartz trachyte to rhyolite magma part i systematics of trace element partitioning new alkali feldspar felsic magma trace element partition coefficients d values for rb sr ba eu y zr nb ga zn trivalent ree be cs hf pb th and u for 30 samples of peralkalic quartz trachyte and rhyolite are presented d values of incompatible elements vary systematically with melt polymerization parameters increasing with whole rock silica but decreasing in rocks with higher agpaitic indices a i mol na k al d values for sr and ba evaluated to be accurate vary systematically with crystal chemistry probably substituting for na in the ca poor alkali feldspar phases apparent d values for sr and ba from pre quaternary systems are fraught with contamination and analytical errors respectively and should be used with caution d eu decreases exponentially with a i ranging from compatible in weakly peralkalic a i 1 2 rocks these regular variations strongly suggest that partition coefficients for these elements may be predicted accurately if whole rock and crystal chemical parameters are known

gondwana derived microcontinents the constituents of the variscan and alpine collisional orogens abstract the european variscan and alpine mountain chains are collisional orogens and are built up of pre variscan building blocks which in most cases originated at the gondwana margin such pre variscan elements were part of a pre ordovician archipelago like continental ribbon in the former eastern prolongation of avalonia and their present day distribution resulted from juxtaposition through variscan and or alpine tectonic evolution the well known nomenclatures applied to these mountain chains are the mirror of variscan resp alpine organization it is the aim of this paper to present a terminology taking into account their pre variscan evolution at the gondwana margin they may contain relics of volcanic islands with pieces of cadomian crust relics of volcanic arc settings and accretionary wedges which were separated from gondwana by initial stages of rheic ocean opening after a short lived ordovician orogenic event and amalgamation of these elements at the gondwanan margin the still continuing gondwana directed subduction triggered the formation of ordovician al rich granitoids and the latest ordovician opening of palaeo tethys an example from the alps external massifs illustrates the gradual reworking of gondwana derived pre variscan elements during the variscan and alpine tertiary orogenic cycles

melt composition control of zr hf fractionation in magmatic processes zircon zrsio4 and hafnon hfsio4 solubilities in water saturated granitic melts have been determined as a function of melt composition at 800 and 1035 c at 200 mpa the solubilities of zircon and hafnon in metaluminous or peraluminous melts are orders of magnitude lower than in strongly peralkaline melt moreover the molar ratio of zircon and hafnon solubility is a function of melt composition although the solubilities are nearly identical in peralkaline melts zircon on a molar basis is up to five times more soluble than hafnon in peraluminous melts accordingly calculated partition coefficients of zr and hf between zircon and melt are nearly equal for the peralkaline melts whereas for metaluminous and peraluminous melts dhf dzr for zircon is 0 5 to 0 2 consequently zircon fractionation will strongly decrease zr hf in some granites whereas it has little effect on the zr hf ratio in alkaline melts or similar depolymerized melt compositions the ratio of the molar solubilities of zircon and hafnon for a given melt composition temperature and pressure is proportional to the hf zr activity coefficient ratio in the melt the data imply that this ratio is nearly constant and probably close to unity for a wide range of peralkaline and similar depolymerized melts however it changes by a factor of two to five over a relatively small interval of melt compositions when a nearly fully polymerized melt structure is approached for most ferromagnesian minerals in equilibrium with a depolymerized melt dhf dzr typical values of dhf dzr range from 1 5 to 2 5 for clinopyroxene amphibole and titanite because of the change in the hf zr activity ratio in the melt the relative fractionation of zr and hf by these minerals will disappear or even be reversed when the melt composition approaches that of a metaluminous or peraluminous granite it is thus not surprising that fractional crystallization of such granitic magmas leads to a decrease in zr hf whereas fractional crystallization of depolymerized melts tends to increase zr hf there is no need to invoke fluid metasomatism to explain these effects results demonstrate that for ions with identical charge and nearly identical radius crystal chemistry does not alone determine relative compatibilities rather the effect of changing activity coefficients in the melt may be comparable to or even larger than elastic strain effects in the crystal lattice

neogene lateral extension during convergence in the central alps evidence from interrelated faulting and backfolding around the simplonpass switzerland abstract the simplon fault zone occurs at the western end of the lepontine metamorphic dome in the central alps and represents a major low angle 25 dip normal fault associated with wsw ene directed lateral extension movement began in the early miocene and continued until at least 3 ma accumulating a total vertical throw of around 10 km corresponding to 24 km displacement parallel to the fault this displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic detachment fault the simplon line in the central region simplon village to zwischbergen some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone due to the higher exposed structural level the importance of the cataclastic component increases towards the northwest in the northern steep belt around the simplonpass retrograde greenschist facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with sse vergent large scale backfolding representing nnw sse shortening and crustal thickening the combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen allowing lateral escape of continental material normal faulting and the resultant lateral escape was not just superficial it extended to depths of at least 15 km and probably to depths exceeding 25 km the overall geometry and kinematics of the simplon fault zone and in particular the interplay between folding and faulting is very similar to that described from the metamorphic core complexes and detachment faults of the cordillera of the southwestern usa within the central alps extension was associated with rapid exhumation of the lepontine metamorphic dome as evidenced by very rapid cooling rates determined from mineral ages in the footwall similar to the cenozoic tectonics of the himalayas and tibet orogen parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction more limited extension of the same age and orientation is known from a large area of the european foreland greater extension in the mountain belt compared with the foreland was accommodated by major strike slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern alps of haute provence france coeval folding and orogen parallel extension such as at the simplonpass were developed in a dextral transpressive regime due to oblique wnw ese convergence across the alps during the neogene

columbite solubility in granitic melts consequences for the enrichment and fractionation of nb and ta in the earth s crust the behaviour of niobium and tantalum in magmatic processes has been investigated by conducting mnnb2o6 and mnta2o6 solubility experiments in nominally dry to water saturated peralkaline aluminium saturation index a s i 0 64 to peraluminous a s i 1 22 granitic melts at 800 to 1035 c and 800 to 5000 bars the attainment of equilibrium is demonstrated by the concurrence of the solubility products from dissolution crystallization mn doped and nb or ta doped experiments at the same pressure and temperature the solubility products of mnnb2o6 ksp nb and mnta2o6 ksp ta at 800 c and 2 kbar both increase dramatically with alkali contents in water saturated peralkaline melts they range from 1 2 10 4 and 2 6 10 4 mol2 kg2 respectively in subaluminous melt a s i 1 02 to 202 10 4 and 255 10 4 mol2 kg2 respectively in peralkaline melt a s i 0 64 this increase from the subaluminous composition can be explained by five non bridging oxygens being required for each excess atom of nb5 or ta5 that is dissolved into the melt the ksp nb and ksp ta also increase weakly with al content in peraluminous melts ranging up to 1 7 10 4 and 4 6 10 4 mol2 kg2 respectively in the a s i 1 22 composition columbite tantalite solubilities in subaluminous and peraluminous melts a s i 1 02 and 1 22 are strongly temperature dependent increasing by a factor of 10 to 20 from 800 to 1035 c by contrast columbite tantalite solubility in the peralkaline composition a s i 0 64 is only weakly temperature dependent increasing by a factor of less than 3 over the same temperature range similarly ksp nb and ksp ta increase by more than two orders of magnitude with the first 3 wt h2o added to the a s i 1 02 and 1 22 compositions whereas there is no detectable change in solubility for the a s i 0 64 composition over the same range of water contents solubilities are only slightly dependent on pressure over the range 800 to 5000 bars the data for water saturated sub and peraluminous granites have been extrapolated to 600 c conditions at which pegmatites and highly evolved granites may crystallize using a melt concentration of 0 05 wt mno 70 to 100 ppm nb or 500 to 1400 ppm ta are required for manganocolumbite and manganotantalite saturation respectively the solubility data are also used to model the fractionation of nb and ta between rutile and silicate melts predicted rutile melt partition coefficients increase by about two orders of magnitude from peralkaline to peraluminous granitic compositions it is demonstrated that the nb2o5 ta2o5 activity coefficient ratio in the melt phase depends on melt composition this ratio is estimated to decrease by a factor of 4 to 5 from andesitic to peraluminous granitic melt compositions accordingly all the relevant accessory phases in subaluminous to peraluminous granites are predicted to incorporate nb preferentially over ta this explains the enrichment of ta over nb observed in highly fractionated granitic rocks and in the continental crust in general

physical and chemical properties of the epidote minerals an introduction epidote minerals are known since the 18th century but at that time the greenish to dark colored varieties were termed actinolite or schorl and not distinguished from the minerals to which these names apply today hauy defined the mineral species and introduced the name epidote in 1801 whereas werner in 1805 used the term pistacite quoted from hintze 1897 epidote is derived from greek epidosis to increase because the base of the rhombohedral prism has one side larger than the other and pistacite refers to its green color all references for names after luschen 1979 blackburn and dennen 1997 weinschenk 1896 proposed the name clinozoisite from its monoclinic symmetry and zoisite like composition for those monoclinic members of the epidote family that are fe poor optically positive and have low refractive indices and birefringence zoisite was probably confused with tremolite until the beginning of the 19th century in 1804 siegmund zois baron von edelstein 1747 1819 an austrian sponsor of mineral collections found and described a new mineral in a handspecimen from the saualpe mountains in carinthia that was named zoisite by werner hauy 1822 interpreted zoisite as a variety of epidote and included it in his epidote spezies weiss 1820 presented a theory of the epidote system and also discussed crystal morphological features of the epidote minerals weiss 1828 rammelsberg 1856 studied the relationship between epidote and zoisite and presented a compilation of chemical analyses of zoisite he already noticed that the relative concentrations of di tri and tetravalent cations are identical in zoisite and epidote but that the fe content in zoisite about 2 3 5 wt fe2o3 is generally less than in epidote about 9 12 wt fe2o3 piemontite see bonazzi and menchetti 2004 was probably first described in 1758 by cronstedt

precambrian tholeiitic dacitic rock suites and cambrian ultramafic rocks in the pennine nappe system of the alps evidence from sm nd isotopes and rare earth elements major element trace element and sm nd isotope analyses were made of polymetamorphic hornblendefelses plagioclase amphibolites and banded amphibolites from the berisal complex in the simplon area italy switzerland to determine their age origin and genetic relationships in light of major and rare earth element data the hornblendefelses are inferred to have originally been pyroxene rich cumulates the plagioclase amphibolites and the dark layers of the banded amphibolites to have been tholeiitic basalts and the light layers dacites the sm nd isotope data yield isochron ages of 475 81 ma for the hornblendefelses 1 018 59 ma for the plagioclase amphibolites and 1 071 43 ma for the banded amphibolites the 1 ga magmatic event is the oldest one ever found in the crystalline basement of the pennine nappes the sm nd isotope data support the consanguinity of the tholeiitic dark layers and the dacitic light layers of the banded amphibolites with the tholeiitic plagioclase amphibolites and the ultramafic hornblendefelses

neogene kinematics of the central and western alps evidence from fission track dating zircon and apatite fission track analysis has established the continuation of a zone of rapid change in mineral ages extending from the simplon fault zone in the simplon alps through the rhone valley and along the southeastern side of the mont blanc and belledonne massifs apatite fission track ages to the northwest of this zone generally range from 1 4 to 6 ma whereas those to the southeast range from 6 to 13 ma corresponding values for zircon are 11 13 and 16 26 ma respectively the zone corresponds to the position of the frontal pennine thrust in the mont blanc belledonne region this structure a prominent feature on recent deep seismic profiles may have been reactivated in the neogene with an important normal fault component this is suggested by apatite and zircon fission track ages in the footwall that are younger than those in the hanging wall and by the observed elimination of the lower pennine or simplon nappes along this fault zone in the rhone valley mont blanc belledonne region resulting in the juxtaposition of north penninic and helvetic ultra helvetic nappes at the current erosion level the distribution of fission track ages reflects this movement and the related neogene exhumation of the mont blanc and belledonne external massifs which has continued until less than 1 4 m y ago i e it is almost certainly still active and has tilted the overlying nappe sequence

k ar dating of illitic clays in jurassic nugget sandstone and timing of petroleum migration in wyoming overthrust belt abstract authigenic illite is a prominent pore fill in the nugget sandstone the main reservoir rock of most fields in the southwest wyoming overthrust belt illite a good k ar clock was dated from several well samples all from the absaroka thrust sheet this includes a producing well in the clear creek field where seven samples traverse the gas oil and water zones the ages of the clear creek suite are virtually concordant at 110 2 m y assuming hydrocarbon emplacement would have arrested authigenesis in the oil and gas zones the similarity of ages from the hydrocarbon zones with the water zone indicates hydrocarbon emplacement was post 110 m y ago middle cretaceous ages obtained from the other absaroka sheet nugget samples fall in the narrow range of 102 to 120 y b p this indicates illite authigenesis was a relatively short lived event for the nugget in the absaroka sheet the wyoming idaho utah overthrust belt involves several thrust sheets each of which was emplaced over its foreland sequentially from west to east over a time spanning tens of millions of years we attribute the mid cretaceous illite growth in the absaroka sheet to burial conditions established when that part of the nugget was thrust upon by the crawford sheet the burial was accomplished both tectonically and by synorogenically derived sediment if true our illite dates imply a somewhat older age for the crawford sheet than previously interpreted we attribute the post illite hydrocarbon emplacement in the absaroka nugget as a result of thrusting of the absaroka sheet on top of its foreland containing middle cretaceous petroleum source bed shales these beds were thermally matured wh n buried by the emplacement of the absaroka sheet and its derived sediment in the late cretaceous end of article last page 414

geochemistry and genesis of the lengenbach pb zn as tl ba mineralisation binn valley switzerland the lengenbach pb zn as tl ba mineralisation is located in triassic dolostones of the penninic zone in the swiss alps where alpine metamorphism reached upper greenschist to lower amphibolite grade geochemical data are used to constrain the origin of this unique occurrence two metamorphic redox environments are present the as iii rich zone is controlled by barite pyrite while the reduced zone contains graphite or pyrrhotitepyrite and formally zerovalent as the as iii rich zone is characterised by a mineral assemblage consistent withfo2 in the stability field of barite pyrite an as pb tl rich sulphide melt coexisted with a hydrothermal fluid at 300 c in this zone mineralised dolostones are anomalous in as pb ag tl hg zn ba cd fe cu mo u v b ga cr and possibly sri and au in order of decreasing enrichment as pb and zn are present in the 0 1 to 1 range tl and ag reach several hundred ppm uraninite is concentrated in silicate rich bands and yields a late alpine u pb age of 18 5 0 5 ma pb and s isotopic variations are interpreted by metamorphic overprinting and re equilibration within an isochemically metamorphosed mineralisation hydrothermal sulphides are more strongly affected by uranogenic pb than massive pb as sulphides representing a former sulphide melt the least overprinted mineralisation is characterised by206pb 204pb 18 44 18 56 207pb 204pb 15 60 15 75 208pb 204pb 38 44 38 84 and 34s sulphide 25 2 s isotopic variations are largely a result of sulphide sulphate re equilibration yielding temperatures of 450 30 c 87sr 86sr ratios of mineralised samples are lower than or equal to host dolostones precluding major infiltration of basement derived fluids during alpine metamorphism the sr source 87sr 86sr close to 0 708 probably was seawater with a radiogenic detrital mineral component the genesis of the unique lengenbach mineralisation is interpreted as the result of isochemical metamorphic overprinting of a carbonate hosted stratiform sulphide mineralisation well crystallised sulphide minerals in fissures and druses formed during retrograde cooling of a sulphide melt in equilibrium with a hydrothermal fluid the primary mineralisation was probably formed at or close below the sea floor and fed by sulphide poor hydrothermal fluids sulphide was largely derived from seawater by open system bacterial sulphate reduction u v and mo may be seawater derived

origin of archean anorthosites evidence from the bad vermilion lake anorthosite complex ontario the bad vermilion lake anorthosite complex 2 700 m y is exposed over an area of about 100 km2 near rainy lake ontario as is typical of other archean anorthosites it is composed of coarse 1 30 cm across equidimensional euhedral to subhedral calcic an80 plagioclase in a finer grained mafic matrix the amount of mafic matrix in individual samples ranges from none to about 70 by volume the complex has been variably metamorphosed to greenschist facies zoisite chlorite and hornblende are abundant but primary plagioclase is preserved in many places