evolution of mg fe ratios in late variscan plutonic rocks from the external crystalline massifs of the alps france italy switzerland late variscan plutonic bodies are widespread in the external raumer et al 1993 menot et al 1994 from south to crystalline massifs of the alps argentera pelvoux belledonne north they are the argentera pelvoux belledonne grandes rousses mont blanc aiguilles rouges aar gotthard grandes rousses mont blanc and aiguilles rouges they can be classified on the basis of their mg fe mg ratio massifs western alps and the aar and gotthard massifs and mafic mineral content expressed by the b fe mg central alps fig 1 they appear as dome like structi parameter together with available ages of emplacement this tures of crystalline basement surrounded by a mostly classification highlights the existence of two plutonic suites one mesozoic sedimentary cover as a whole they define an early visean 330 340 ma and highly magnesian the other arcuate structure possibly inherited from the variscan later mainly stephanian 295 305 ma and more ferriferous orogeny bogdanoff et al 1991 this evolution from a magnesian plutonism to a more ferriferous during the variscan orogeny the ecm were part of one which also occurs in other variscan massifs e g corsica the helvetic moldanubian terrane of the internal zone might be accounted for by a combination of factors related to the of the variscides von raumer neubauer 1993 nature of the source of the magmas the physical and chemical precambrian metamorphic units and precambrian to conditions of melting and the late variscan geodynamic setting palaeozoic sediments interlayered with volcanic and as a basis for these considerations a comprehensive review is ultramafic rocks display in most areas a polymetamorphic presented of all the external crystalline massifs and their history and underwent a highly complex pre variscan late variscan intrusions and variscan evolution although they share a number of similarities the ecm each have also specific characteristics von raumer et al 1993 in addition to orthogneisses deriving from early palaeozoic plutonic and volcanic protoliths bussy von

on analysis and prediction of tides on the west coast of great britain summary long period tidal records from eight ports on the west coast of great britain are analysed using both the harmonic method and the response function time series up to 9 5 yr were used so that fine structure of certain regions of high energy in the tidal residual spectrum may be examined this also assisted in accurate separation of tides from surges according to the currently accepted formulation of m1 its composition is not compatible with its actual composition in the real tide and as such has no useful purpose to serve in computation of predictions constituents sa ssa ma and mb are adversly affected by weather and long records are essential for their accurate separation it has been observed that annual perturbations of s2 similar to those confirmed in recent years for m2 also exist but these are not readily recognisable because their speeds are the same as those of constituents t2 and r2 these perturbations being highly sensitive to meteorological forces are mainly responsible for cusps or humps in the vicinity of major tidal constituents in tidal residual spectra the relationship of radiational and gravitational tides is found to be in very good agreement with theoretical results in summer months non predictable variance reduces to about half of the annual variance reflecting the regional weather conditions of summer and winter

the crystalline units of the high himalayas in the lahul zanskar region northwest india metamorphic tectonic history and geochronology of the collided and imbricated indian plate in the high himalayan belt of northwest india crustal thickening linked to palaeogene collision between india and eurasia has led to the formation of two main crystalline tectonic units separated by the syn metamorphic miyar thrust the high himalayan crystallines sensu stricto hhc at the bottom and the kade unit at the top these units are structurally interposed between the underlying lesser himalaya and the very low grade sediments of the tibetan nappes they consist of paragneisses orthogneisses minor metabasics and chiefly in the hhc leucogranites the hhc registers a polyphase metamorphism with two main stages designated as m1 and m2 a metamorphic zonation with high temperature recrystallization and migmatization at middle structural levels and medium temperature assemblages at upper and lower levels in contrast the kade unit underwent a low temperature metamorphism rb sr and u th pb isotope data point to derivation of the orthogneisses from early palaeozoic granitoids while the leucogranites formed by anatexis of the hhc rocks and were probably emplaced during miocene time most of the complicated metamorphic setting is related to polyphase tectonic stacking of the hhc with the cooler kade unit and lesser himalaya during the himalayan history however a few inconsistencies exist for a purely himalayan age of some ml assemblages of the hhc as regards the crustal derived leucogranites the formation of a first generation mixed with quartzo feldspathic leucosomes was possibly linked to melt lubricated shear zones which favoured rapid crustal displacements at upper levels they intruded during stage m2 and the latest movements along the syn metamorphic miyar thrust but before juxtaposition of the tibetan nappes along the late metamorphic zanskar fault

u pb ages and morphologies of zircon in microgranitoid enclaves and peraluminous host granite evidence for magma mingling granites of the s type wilson s promontory batholith lachlan fold belt australia contain zircons which are euhedral and relatively large their age is 395 ma which can be considered as the best available estimate of the crystallysation age of the granites contrary to their dominance in other s type granites of the lachlan fold belt very few zircon cores give inherited ages varying between 500 and 1700 ma microgranitoid enclaves contained within the granites contain a zircon population that is dominated by relatively small anhedral or elongated crystals these give ages that are indistinguishable from the crystallisation age of the granite some enclaves which are characterised by the presence of megacrysts contain a proportion of larger euhedral zircons these zircons give inherited ages similar to the zircons from the granitic host rocks the data are in agreement with a magma mingling origin for the microgranitoid enclaves the large euhedral zircons are interpreted to have been introduced into the enclave magma during a hybridisation event which also introduced quartz and plagioclase megacrysts into the magma the relatively high proportion of inherited cores within the large zircon population of the enclaves is related to the timing of mixing between enclave and host magma this mixing event took place before the majority of the magmatic zircons crystallised in the granitic magma the small anhedral zircons within the enclaves crystallised during quenching of the globules of enclave magma against the cooler granitic magma

biomarkers of modern plants and soils from xinglong mountain in the transitional area between the tibetan and loess plateaus abstract organic compounds found in living plants and modern soils in different bioclimatic areas are important in understanding the paleoenvironmental implications of the organic matter signals from geological records using gc ms techniques a series of biomarkers mainly including n alkanes and n alkan 2 ones were identified from the typical plants e g broadleaf deciduous trees and conifers and the soils collected from xinglong mountain a transitional area between tibetan and loess plateaus the c 15 c 33 n alkane homologues were present in all samples typically c 27 or c 29 n alkanes were most abundant in tree samples and c 31 was most abundant in grasses samples meanwhile the soil samples were mainly dominated by c 29 and c 31 homologues the n alkan 2 ones showed a strong odd over even predominance of carbon numbers c 23 c 25 or c 29 n alkan 2 one homologues were most abundant in plant samples while c 27 or c 29 was abundant in soils samples the lipid biomarker distribution patterns of all the plants and soil samples suggest that the vegetation is a primary source of organic matter for the soils and that reprocessing of microbes and physicochemical reactions in the soil played an important role in the degradation of organic matter research has advanced knowledge about transformation of biomarkers in the plants organic matter microbes forest system it also can help reconstruct the process of plant successions recorded by the lipids

improving the resolution of single grain u pb dating by use of zircon extracted from feldspar application to the variscan magmatic cycle in the central alps abstract the usual discordant u pb zircon age patterns for intrusives in orogenic belts impede and exact determination of the time of emplacement they are the product of multiple overprinting and of the presence of inherited zircon dating of monophase zircons crystallized during a single event may yield higher age resolution and results that are easier to interpret during magma crystallization zircon is generated over a range of ptx conditions whereas rock forming minerals grow under more limited conditions and their relative ages can be determined petrographically zircons formed during earlier stages of magma crystallization and included in such mineral constituents are shielded from subsequent reactions outside the carrier mineral following partial melting of the protolith or xenolith assimilation newly crystallizing rock forming minerals may include zircon xenocrysts as individual grains using this insight and to facilitate the analysis of monophase zircons included in petrogenetically defined constituent minerals we have introduced additional steps in the mineral separation procedure we found the effort worthwhile in that the magmatic zircons extracted from plagioclase yielded the most concordant intrusive ages in contrast to zircons from microcline or from the interstices also inherited zircons occurring as individual grains in magmatic feldspar appear to have preserved some rudimentary morphology and a less disturbed radiogenic pb composition which allows us to distinguish provenance from discrete protoliths we were thus able to resolve small but structurally supported age differences between two groups of variscan intrusives in the central swiss alps we obtained 299 4 1 2 ma for the crystallization age of the granite gneiss in the gotthard massif with inherited zircons that suggest generation from late proterozoic 735 ma and mid proterozoic 1760 ma protoliths the massive leucocratic granites on the margin of the granite gneiss yielded an intrusive age of 294 3 1 1 ma with an inherited grain implying a source of caledonian vintage

zircon in amphibolites from naxos aegean sea greece origin significance and tectonic setting we report u pb zircon ages of c 700 550 ma 262 220 ma 47 38 ma and 15 14 ma from amphibolites on naxos island in the aegean extensional province of greece the zircon has complex internal structures based on cathodoluminescence response zoning and crosscutting relationships a minimum of four zircon growth stages are identified inherited core magmatic core inner metamorphic rim and an outer metamorphic rim trace element compositions of the amphibolites suggest igneous differentiation and crustal assimilation zircon solubility as a function of saturation temperatures zr content and melt composition indicates that the zircon did not originally crystallize in the mafic bodies but was inherited from felsic precursor rocks and subsequently assimilated into the mafic intrusives during emplacement zircon inheritance is corroborated by the complex xenocrystic nature of the zircon in one sample ages of c 700 550 ma and 262 220 ma are assigned to inherited zircon available geochemical data suggest that the 15 14 ma metamorphic rims grew in situ in the amphibolites corresponding to a high grade metamorphic event at this time however the geochemical data cannot conclusively establish if the c 40 ma zircon rims also grew in situ or whether they were inherited along with the xenocrystic cores two scenarios for emplacement of the mafic intrusives are discussed i intrusion during late triassic to jurassic ocean basin development of the aegean realm in which case the 40 ma zircon rims would have grown in situ and ii emplacement in the miocene as a result mafic underplating during large scale extension in this case only the 15 14 ma metamorphic outer rims would have formed in situ in the amphibolitic host rocks

u pb zircon dating of granulite metamorphism in the sludyanskiy complex eastern siberia abstract conventional multigrain and single grain zircon u pb ages were obtained for syn and post metamorphic granitoids from the sludyanskiy complex in the south western baikal region regarded as a classical precambrian terrain in the eastern siberia single grains of magmatic zircon from the syn metamorphic hypersthene bearing biotite trondhjemite and two pyroxene trondhjemite have yielded concordant u pb ages of 488 5 0 6 ma and 488 0 5 ma concordant u pb data for zircons of metamorphic origin in the syn metamorphic two pyroxene trondhjemite yield an age for the peak of the granulite facies metamorphism in the sludyanskiy complex as 477 6 2 ma the pattern of multigrain magmatic zircon samples from syn metamorphic granitoids reflects significant pb loss during the metamorphic episode this came to an end before 471 1 2 ma as demonstrated by concordant zircon data for the intrusion of post metamorphic amphibole two pyroxene quartz syenite our zircon data establish that the granulite facies reworking of the sludyanskiy complex took place during the palaeozoic and not during the paleoproterozoic as was previously suggested the age span between the intrusions of syn metamorphic and post metamorphic granitoids constrains the duration of the metamorphic episode to about 20 ma

geochronological constraints on the emplacement history of an anorthosite rapakivi granite suite u pb zircon and baddeleyite study of the korosten complex ukraine u pb zircon baddeleyite ages obtained for the korosten anorthosite rapakivi granite complex ukrainian shield suggest that different magmatic phases were emplaced during a period of ca 30 million years as a series of distinct igneous episodes the earliest 1789 1 2 0 ma anorthosites were followed by 1781 3 3 2 ma dykes of plagiogranite porphyries the emplacement of a major rapakivi granite phase took place at 1767 4 2 2 ma and was followed by emplacement of layered intrusions of anorthosites gabbronorites diabases and ultrabasic rocks between 1761 and 1758 ma the minimum duration of magmatism of about 30 million years the 6 15 million years interval between igneous pulses and alternation of discrete episodes of basic and felsie magmatism are common features of major anorthositemangerite charnockite rapakivi granite complexes temporal distribution of igneous activity in the korosten complex shows that the gabbro anorthosites and the granites are not comagmatic although they are possibly cogenetic and that at least four portions of granitic and basic magmas were generated during a relatively long period of at least 30 million years the time gap of about 20 25 million years between early basic and later and more voluminous granitic magmatism characteristic of the korosten pluton wiborg and salmi batholiths probably reflects the duration of extensional processes before the generation of large volumes of magma in the lower crust

crystallization and age of zircon from corsican ophiolitic albitites consequences for oceanic expansion in jurassic times abstract in corsica albitites and albite granites are considered to be the latest products of an ophiolitic magma two albitites have been selected for a study of the morphology typology and age of their zircons applying for the first time the u pb method to western tethyan ophiolites both samples come from the rospigliani series possibly formed in a transform fault zone the first one shows a cumulate texture whereas the second has features indicative of more rapid cooling zircons have precipitated at the end of the fractional crystallization process after a strong enrichment in alkali and incompatible elements in a typological diagram they plot at high temperature and high agpaicity indexes these abundant large euhedral to subhedral zircons sometimes with fine zoning may also contain inclusions of granitic composition which testifies to k fractionation in ophiolitic magmas the u pb age of 161 3myr is in the range of the ages obtained for alpine and apennines ophiolites all these ophiolitic remnants belonging to the liguro piedmont basin may represent a portion of the central atlantic ocean created during middle and upper jurassic times

hornblende k ar ages and the climax of tertiary metamorphism in the lepontine alps south central switzerland an old problem reassessed abstract twenty five new k ar ages for amphiboles from the tertiary barrovian type metamorphic terrain of the central alps were determined these results lead to the conclusion that the meso alpine metamorphism reached its peak condition and maximum expanse about 26 3 m y ago along a profile of increasing grade of alpine metamorphism in both basement and cover rocks the following observations can be made 1 pre alpine hornblendes in the zone of greenschist facies show ages ranging from 639 to 152 m y the implied open system behavior may be related to the observed mineralogical instability of these hornblendes during the alpine overprint in contrast two greenschist facies hornblendes which grew in pennine mesozoic rocks during the alpine metamorphism yield ages of 23 2 and 29 3 m y 2 on both sides of the boundary marking the first occurrence of alpine staurolite pre or early alpine hornblende k ar ages are not fully reset unless the rocks were completely recrystallized for one rock specimen which clearly exhibits two generations of hornblende an age two times greater was obtained for the smaller crystals oriented parallel to the foliation than for the structurally younger hornblende porphyroblasts which overgrew the preexisting texture 3 in the high grade area of alpine metamorphism only young alpine ages between 23 and 28 m y were found for hornblendes which grew syn to postkinematically with respect to the last penetrative deformation one amphibolitic inclusion not affected by this deformation yielded a significantly older hornblende age of 32 1 m y these results suggest that temperature controlled ar diffusion is not the main factor governing the k ar system of metamorphic hornblendes we conclude that these hornblende ages should not be interpreted as cooling ages from a structurally ill defined earlier event instead they indicate that the high grade metamorphism responsible for the well known isograd pattern in the lepontine area persisted until some 23 m y ago

two high precision u pb zircon ages for the sudbury nickel irruptive two independent u pb zircon ages were measured using norite samples from the north and south ranges of the sudbury nickel irruptive ages of and ma were obtained with a probability of fit within experimental error of 51 and 57 respectively previous determinations by the rb sr whole rock method gave ages between about 1700 and 2000 ma with errors of the order of 100 ma the difference in precision is inherent in the two methods zircons from the norite are highly radiogenic and almost concordant whereas rb sr whole rock ages are determined by very small increments in radiogenic 87sr the two zircon ages have completely different discordia so that all simple models for lead loss by continuous diffusion can be excluded

combined high precision chronometry and geochemical tracing using accessory minerals applied to the central alpine bergell intrusion central europe abstract the behaviour of nd u th and pb isotopes in granitic rocks is usually controlled by accessory minerals in this study an approach is adopted which uses the initial 143 nd 144 nd as a geochemical tracer together with high precision u th pb geochronology for accessory phases and their respective whole rock this combination of techniques is applied to zircon allanite sphene apatite and thorite from both a tonalite and a granodiorite from the tertiary alpine bergell intrusion inherited isotopic signatures have been identified in bulk zircon fractions by inverse discordance in the u pb system in the case of the granodiorite the initial nd in the zircon is 1 77 units lower than the bulk rock this result indicates that nd isotopes in zircon may serve as a useful tool for tracing crustal precursors in silicic rocks all other accessories yield an initial nd equal to their respective whole rock and u th pb ages equal to or lower than the respective intrusion ages this indicates either the absence of inherited cores or that such cores have exchanged their nd and pb in the case of apatite also sr by diffusion where available experimental mineral melt equilibria and diffusion models are consistent with this behaviour u pb geochronology of carefully selected core free zircons yields a precise intrusion age of 31 88 0 09 ma for the tonalite in contrast granodioritic zircons are partially reset due to radiation damage caused by high u concentrations however an alternative means of dating young intrusions is to use 232th 208pb ages of allanite in the case of allanite the presence of a large excess of 206pb derived from 230th indicates that no total pb loss has occurred since crystallization and the nd isotopes indicate the absence of inheritance using both the allanite age and a 238u 206pb age for sphene an intrusion age of 30 13 0 17 ma is indicated for the granodiorite taken together with an allanite th pb age for the tonalite these results strongly support the high closure temperatures for the allanites studied here 700 800 c at high post magmatic cooling rates all other accessory mineral ages are reset to values similar to hornblende 39ar 40ar ages and reflect the effect of post magmatic cooling superimposed on post metamorphic cooling

an eifelian u pb zircon date for the enagh tuff bed from the old red sandstone of the munster basin in nw iveragh sw ireland the first geochronological data from the minister basin sw ireland contradict the accepted biostratigraphical maximum age late givetian early frasnian of the oldest alluvium in the basin depocentre magmatic zircons from the enagh tuff bed a sub plinian air fall tuff within fluviatile old red sandstone of nw iveragh give a pb pb isotope age of 384 9 0 7 ma this confirms middle devonian probably eifelian sedimentation and allows possible early devonian basin initiation associated second cycle gravels suggest a pre 385 ma minimum age for the caherbla group of the dingle basin correlation with alluvium containing tetrapod trackways indicates a pre 385 ma age for the trace maker

organic geochemistry of late cenozoic sediments from the subtropical south atlantic ocean abstract organic matter has been characterized in samples of pleistocene pliocene and miocene sediments from seven deep sea drilling project sites in the subtropical south atlantic ocean organic carbon concentrations average 0 3 for most samples and n alkanoic acid n alkanol and alkane biomarkers indicate extensive microbial reworking of organic matter in these organic carbon lean sediments samples from the easternmost parts of the south atlantic contain an average of 4 1 organic carbon and reflect the high productivity associated with the benguela current lipid biomarkers show less microbial reworking in these sediments eolian transport of land derived hydrocarbons is evident at most of these oceanic locations

comparative study of sterols in shelf and slope sediments off northeastern taiwan abstract bottom sediments and suspended matter from the shelf and slope areas off northeastern taiwan were analyzed for sterol n alkanol and n alkane compositions the algal sterols cholesterol ratios mean 1 were 1 00 0 31 n 7 for the shelf sediments 3 29 0 61 n 7 for the slope sediments and 1 54 0 19 n 3 for the suspended matter the much higher proportion of cholesterol in the shelf sediments is most likely derived from mollusks in the relict sediments an additional sterol source for the shelf sediments probably is suspended matter in the water column although the shelf has no apparent sedimentation results from phytol and n alkanols suggest recent inputs of lipids from the water column to the shelf the stanol stenol ratios are lower for the older shelf sediments than for the younger slope sediments

a comparison of sedimentary aliphatic hydrocarbon distribution between the southern okinawa trough and a nearby river with high sediment discharge twelve surface sediments from the southern okinawa trough ot and nine surface sediments from a nearby river the lanyang river lr with high sediment discharge were analyzed for comparison of their aliphatic hydrocarbon distributions performing cluster analysis on all hydrocarbon data of lr and ot sediments showed that the two areas had a similarity level of only 0 15 meaning that they were quite dissimilar the average ratio of terrigenous to aquatic n alkanes was 0 99 for lr sediments and 9 64 for ot sediments indicating that the concentrations of n alkanes in lr and ot sediments were quite different furthermore the mean pristane phytane ratios for lr and ot sediments were 1 01 and 2 57 respectively the difference between them was significant student s t test at the 99 significance level the carbon preference index cpi of c25ec33 n alkanes averaged 3 26 range 2 16e4 59 for lr sediments and 2 92 range 2 35e5 24 for ot sediments no significant difference was found between the two cpi averages student s t test at the 99 confidence level however higher plant n alkanes generally maximized at c29 for lr sediments but maximized at c31 for all ot sediments strongly indicating significant differences in the origins of the hydrocarbons in these two areas all present results appear to suggest that lr sediments are not a major hydrocarbon source for ot sediments in addition there was no positive linear correlation between diploptene hop 22 29 ene and terrestrial higher plant n alkanes for lr and ot sediments

the new nb p aplite at reinhardsrieth a keystone in the lateral and depth zonations of the hagendorf pleystein pegmatite field se germany abstract the hagendorf pleystein pegmatite field hppf is the largest pegmatite concentration in central europe which has been intensively studied for its mineralogy mainly at hagendorf south pegmatite stock it was not until recently that the lateral and depth zonations of the hppf could be clarified when the newly discovered tabular aplite at reinhardsrieth aplite has been found at transition into the so called barren zone devoid of rare metals thereby the theory of the derivation of the hppf felsic mobilizates as a result of a successive differentiation of a pegmatitic melt from the flossenburg granite has to be discarded from the structural and lithological point of view stock like well zoned pegmatites e g hagendorf south developed in the hinge zone whereas tabular aplites such as reinhardsrieth evolved along the limb zone of strongly folded basement rocks in the latter case the accommodation space was smaller and the cooling of the felsic mobilizates much faster giving rise to fine grained aplitic rather than coarse grained felsic mobilizates chilled margin effect during the incipient stages of the evolution in the hppf at reinhardsrieth a stronger impact of granitization can be determined than at the neighboring aplites and pegmatites based upon the ree fractionation a tripartite subdivision may be established for the various pegmatites and aplites in the hppf 1 barren zone minor amounts of lree in a system otherwise dominated by feldspar and quartz 2 marginal zone mainly fe al phosphates plus columbite fe 3 central zone fe zn li phosphates plus columbite fe the afore mentioned zonation shows an e w trend and a bilateral symmetric arrangement of the various areas there are significant indications of an oxidizing facies to substitute for a more reducing facies at the transition from the central into the marginal zone of the hppf the key mineral for the primary pegmatite mineralization in the marginal and central zones is columbite fe its mn fe ratios plotted vs the ta nb ratios allow for an assessment to what extent metamorphic and magmatic processes have contributed to the built up of the pegmatites and aplites in the hppf even the most primitive nigrine hosted columbites fit well into this pattern forming a lithochemical halo in the roof rocks of the pegmatites the heavy mineral aggregated of rutile ilmenite nigrine giving host to the columbite as armored relicts got released from the basement rock on top of the pegmatites into the stream sediments where they are operative as an ore guide the morphology of zircon in the stream sediments has been utilized as an economic ore indicator the zr silicate is also indicative of the temperature of formation and closely supports the tripartite zonation established above manganiferous apatite is a good depth of emplacement indicator for each aplite and pegmatite respectively the higher the feo mno content of apatite the shallower the depth of emplacement of its host pegmatite or aplite vivianite in turn marks the onset of the hydrothermal alteration of the pegmatites and aplites and its mn contents relate to the depth where alteration has taken place or emplacement of the secondary phosphates the higher the mn content the shallower the depth of alteration peculiar element ratios were selected to design some key diagrams so as to corroborate the field based mineralogical and geological results in terms of zonation of genetic economic geology as well as to cater for applied economic geology as proximity indicators and ore guides when exploring for rare element pegmatite

u pb geochronologic constraints on the crustal evolution of the long range inlier newfoundland the long range mountains of newfoundland expose the northeastern most basement inlier of the appalachian orogen u pb results for two samples of basement gneiss reveal crustal formation ages of 1466 10 ma western brook pond charnockite and 1530 8 ma cat arm road gneiss indicating an affinity with pinwarian magmatism 1510 1450 ma in labrador a third sample of basement gneiss from the cat arm road dam site yielded a minimum formation age of 1631 ma these basement gneisses were intruded by granite plutons during two periods of grenvillian magmatism group i at 1032 1022 ma and group ii at 993 985 ma group i intrusions include the 1032 0 1 5 ma lomond river granite and the 1022 0 2 0 ma lake michel igneous suite group ii intrusions include the 999 4 ma potato hill charnockite 993 7 ma horse chops granite 984 9 1 6 ma cloud river granite and may include the apsy granite the metamorphic history of the long range inlier is complex and three discrete proterozoic metamorphic events can

the evolution of the polymetamorphic basement in the central alps unravelled by precise u pb zircon dating the u pb age determinations of zircon and rutile from the aar massif reveal a complex evolution of the central alpine basement the oldest components are found in zircons of metasediments which bear cores of archean age the u pb age of discordant prismatic zircons of the same rocks ranges between 580 and 680 ma an age that is typical for pan african metamorphism the zircons are interpreted as pan african detritus with archean inheritance the provenance region of the pan african zircons is assumed to be a terrane of gondwana affinity i e the w african craton or the pentevrian microplate the caledonian metamorphism left a pervasive structural imprint in amphibolite facies on the rocks of the aar massif it is dated at 456 2 and 445 ma by zircons of a layered migmatitic gneiss and a migmatitic leucosome respectively both occurring in the northernmost zones of the massif hercynian metamorphism never exceeded greenschist facies conditions and is recorded by zircon in a garnet amphibolite and by rutile in a meta psammite that yield an age of 330 ma both zircon and rutile are considered to be products of retrograde mineral reactions and therefore do not date the peak conditions of hercynian metamorphism the gastern granite at the western end of the aar massif is a contaminated granite that intruded at 303 4 ma contemporaneously with the wide spread late hercynian post collisional i type magmatism the study demonstrates the potential of isotope dilution u pb dating of single grains and microfractions in deciphering complex evolutionary histories of polymetamorphic terrains

kinematics and age of syn intrusive detachment faulting in the southern alps evidence for early permian crustal extension and implications for the pangea a versus b controversy permian basin formation and magmatism in the southern alps of italy have been interpreted as expressions of a wsw ene trending dextral megashear zone transforming early permian pangea b into late permian pangea a between 285 and 265 ma in an alternative model basin formation and magmatism resulted from n s crustal extension to characterize permian tectonics we studied the grassi detachment fault a low angle extensional fault in the central southern alps the footwall forms a metamorphic core complex affected by upward increasing top to the southeast mylonitization two granitoid intrusions occur in the core complex the synmylonitic val biandino quartz diorite and the postmylonitic valle san biagio granite u pb zircon dating yielded crystallization ages of 289 1 4 5 ma for the former and 286 8 4 9 ma for the latter consequently detachment related mylonitic shearing took place during the early permian and ended at 288 ma but kinematically coherent brittle faulting continued considering 30 anticlockwise rotation of the southern alps since early permian the extension direction of the grassi detachment fault was originally n s even though a dextral continental wrench system has long been regarded as a viable model at regional scale the local kinematic evidence is inconsistent with this and rather supports n s extensional tectonics based on a compilation of 200 u pb zircon ages we discuss the evolution and tectonic framework of late carboniferous to permian magmatism in the alps

diagenesis of free and bound lipids in terrestrial detritus deposited in a lacustrine sediment recent sediments from an oligotrophic lake loch clair having uniform organic input for 2000 yr show changes in lipid abundance and composition with increasing depth attributed to diagenesis stability of free lipids decreases in the order n alkanes alkan 2 ones sterols n alkanoic acids n alkanols n alkenoic acids diagenetic loss of shorter chain homologues is complete within 400 yr stabilisation of bound relative to free lipids increases the proportion of the former with sediment age and reduces loss of shorter chain bound homologues in an eutrophic lake cross mere shorter chain free and bound sedimentary lipids show increased abundance compared with loch clair the relative importance of higher input of shorter chain lipids derived from aquatic detritus and slower initial diagenesis due to the anoxic hypolimnion as causative factors for this difference between lake types cannot yet be assessed

kinetics of crystallization of igneous rocks the geochemistry of igneous rocks is discussed with the primary objectives of bringing together the theories underlying the kinetics of crystallization of igneous rocks and illustrating the use of these theories in understanding experimental and observational data the primary purpose of the chapter is to introduce current thinking about the kinetics of igneous rocks and to provide a basis for understanding other work a basic assumption made in the discussion is that the rate of any chemical reaction including the crystallization of igneous rocks is zero at equilibrium and proceeds at a finite rate only at a finite deviation from equilibrium as such an understanding of the processes operating in igneous rocks requires an understanding of how deviation from equilibrium affects the rates and mechanisms of the processes occurring during crystallization these processes are detailed with special emphasis given to nucleation and crystal growth jmt

the biogeochemistry of ellesmere lake u k i source correlation of leaf wax inputs to the sedimentary lipid record abstract analyses of the free lipid distributions from ellesmere lake sediments taken from two depths and comparison with leaf wax lipid distributions from six deciduous tree species around the lake were made in order to assess lipid sources close correlations of aliphatic lipid distributions in the carbon number range c 20 c 50 were seen c 28 and c 29 sterols were observed in both sediment and leaf waxes but c 27 sterols were only present in the sediments and were thus assigned a nonleaf wax source unsaturated steroidal ketones were observed both in the sediment and in certain leaf waxes and were assigned specific sources alkan 2 ones were not present in the leaf waxes but were present in the sediment samples with an almost identical distribution to the sediment n alkanes indicating a microbially mediated source c 20 c 30 n aldehydes which have not previously been widely reported in free lipid distributions of freshwater lacustrine sediments were prominent in the surface sediment and in the leaf waxes examined

timing of high pressure metamorphism in the yukon tanana terrane canadian cordillera constraints from u pb zircon dating of eclogite from the teslin tectonic zone zircon from eclogite near last peak in the teslin tectonic zone yielded a u pb isotopic age of 269 2 ma 2 the first precise age for such a rock in the yukon tanana terrane of the canadian cordillera both the morphology and geochemistry of the eclogitic zircons indicate a metamorphic origin and the u pb age therefore constrains the timing of peak high pressure metamorphism in this rock the u pb age demonstrates for the first time that an early permian high pressure metamorphic event occurred in rocks now making up the teslin tectonic zone and possibly elsewhere in the yukon tanana terrane this u pb age provides a new geochronologic pin in the evolution of the yukon tanana terrane prior to its mesozoic accretion to the north american continental margin and combined with recent 40ar 39ar muscovite data indicates that high pressure metamorphism at this time was a relatively short lived event

improved accuracy of u pb zircon dating by selection of more concordant fractions using a high gradient magnetic separation technique abstract a loop of soft iron wire or a paper clip or a ferromagnetic grid mounted between the poles of an electromagnet picks up and allows further magnetic separation of zircons previously found to be non magnetic on a frantz isodynamic separator tests on previously analysed samples indicate that most such zircons that are fairly discordant say 10 can be picked up and isolated from associated grains that are more concordant tests on new samples indicate that even when most grains can be picked up the last few percent of the sample contain less uranium and are more concordant than the bulk sample the degree of discordance is the dominant factor affecting the uncertainty of u pb zircon ages both because of the error amplification in projections and because the assumption of a simple two stage system may not be valid only by eliminating or reducing discordance can errors approaching the uncertainty in a single analysis say 2 m y for 2700 m y rocks be achieved rutile normally concentrated with zircon as non magnetic has been successfully removed from a small amount of low uranium zircon using the high intensity separation technique

hf isotope systematics in granitoids from the central and southern alps first initial hf isotopic compositions for samples from the alpine domain are presented and discussed the results are mainly based on zircons and a few whole rocks with ages between 30 and 450 ma of those so far analyzed the present day hf isotopic compositions of zircons from non metamorphic and metamorphic granitoid rocks vary between 0 2824 and 0 2829 zircon populations with concordant u pb ages have much higher initial 176hf 177hf than inversely discordant populations which have been contaminated with older zircons containing less radiogenic hf correlated nd hf crustal residence ages have been found involving model parameters of ehf end f lu hf f sm nd 1 6 for the depleted mantle and f lu hf f sm nd 1 2 for elemental fractionations in the crust the model implies 176lu 177hf of 0 017 for the bulk crust it is suggested that the granitoid rocks are the result of mixing of subcontinental mantle derived magmas with 1 7 ga old recycled and partially molten crustal material the continental mantle component mass ratio values for the granitoids range between 0 3 and 2

early archaean component 3 5 ga within a 3 05 ga orthogneiss from northern nigeria upb zircon evidence abstract conventional u pb analyses on selected single zircon grains and on zircon fractions from a granodioritic orthogneiss at kaduna northern nigeria show that the nigerian basement contains archaean components that have not been recognized until now a distinct u rich zircon group records an age of 3040 60 ma which is interpreted as the age of emplacement and crystallization of the granodioritic magma the occurrence of a magmatic event at ca 3000 ma is confirmed by the addition of core free abraded grains 3050 23 ma these ages are identical to the less precise value 2980 90 ma given by zircon fractions these results also demonstrate that zircons underwent pb loss during the pan african orogeny at 612 23 ma which is well supported by an identical age on a single monazite grain 618 8 ma moreover u pb analyses of abraded u poor core bearing crystals clearly identify the presence of source material of early archaean age 3500 ma the oldest age for crust initiating processes ever found in this area in contrast to earlier rb sr geochronology no evidence for an eburnean 2000 200 ma or kibaran 1100 200 ma orogeny is present in the u pb systems of these zircons

lipids in suspended matter from the human disturbed lanyang river northeastern taiwan due to anthropogenic disturbance such as mass road construction the sediment yield in the lanyang river northeastern taiwan has been reported to have increased substantially since 1976 water samples were collected from the river on three occasions and total suspended matter tsm was analyzed for aliphatic hydrocarbons and alkanols sterols results show that the distribution of aliphatic hydrocarbons in the tsm is rather complicated during periods of normal flow but is relatively simple during high flow and after a period of an extraordinarily high daily mean discharge carbon preference index cpi values of n alkanes c25 c33 are comparatively low ranging from 1 14 to 2 35 with an average of 1 62 when compared with a control station cpi 5 88 because there has been no anthropogenic fossil fuel contamination in the study area in the past the present results indicate a predominant hydrocarbon contribution from the old lipids derived from the bedrock argillite slate and meta sandstone due to massive road construction in 1975 1980 with a small contribution from modern vegetation by using a two end member assumption the percentage of old hydrocarbons in the tsm was estimated to be about 87 the presence of a c22n alkanol maximum is attributed mainly to bacterial contribution higher plants are considered a major source of the campesterol stigmasterol and sitosterol in the tsm

uranium lead system in fragments of a single zircon grain since the demonstration that u pb analyses of zircon fractions1 from a few grains up to several thousand grains or fragments of grains2 or single zircon grains3 4 yield reliable radiometric age data this method has been widely applied to igneous metamorphic and sedimentary rocks the present investigation of u pb systems for a suite of distinct fragments of a single zircon grain from a 1 093 2118 myr old syenite of the pikes peak batholith colorado usa demonstrates that uranium is heterogeneously distributed in this zircon crystal most parts of the grain suffered various degrees of lead loss the others show a closed system behaviour we show that degrees of lead loss are not correlated with uranium concentrations of the fragments

zircon inheritance in an igneous rock suite from the southern adamello batholith italian alps zircons from a suite of basic to acidic calcalkaline igneous rocks from the southern adamello batholith s ab southern alps n italy display complex u pb isotopic patterns which are mainly due to the presence of variable amounts of isotopically heterogeneous inherited radiogenic pb and to minor postmagmatic loss of pb inherited pb is mainly composed of 1 a 1100 ma pb component located in zircons devoid of visible cores and 2 a 450 ma component associated with conspicuous bubble rich turbid cores in concordia representation the data points conform to lower intercept ages of 40 ma a linear fit of three samples devoid of visible cores from the granodiorite defines an intrusion age of 39 3 ma u pb systematics of zircon in particular u content and crystal morphology are clearly related zircons of type g1 which form relatively late in the zircon crystallization sequence consistently show the highest u contents in each zircon population these late zircons however are not devoid of inherited radiogenic lead in a population from a granodiorite cores are randomly distributed throughout the morphological spectrum the presence of old inherited zircon components in all investigated samples furnishes proof for involvement of crustal material in the genesis of the s ab rocks samples characterized by crustal nd and sr values usually show enhanced zircon inheritance inheritance varies with differentiation and reaches a maximum for intermediate to acidic members these compositions show the highest zr saturation temperatures calculated for the rock spectrum studied textural relations between zircon and major phases indicate that the magmas of the leucocratic rocks were saturated with zr at an early stage of crystallization on the other hand zr solubilities and textural relations consistently show that melts of basic to intermediate rocks were not saturated with zr extension of the zr solubility model to mineral melt mixtures of tonalitic bulk composition demonstrates that zr solubility in the residual melt is drastically reduced by crystallization of plagioclase and amphibole survival of xenocrystic zircons in the mafic to intermediate rocks of the s ab can best be explained in terms of dissolution kinetics since temperature and h2o content of these zr undersaturated melts were favourable for relatively rapid zircon dissolution inherited zircons in particular trace element rich unstable cores cannot have been exposed to such conditions over extended time periods therefore the tonalitic or more basic magmas of the s ab cannot have been derived from crustal sources by slow processes such as burial metamorphism

morphology versus upb systematics in zircon a high resolution isotopic study of a zircon population from a variscan dike in the central alps upb isotopic measurements on individual zircon crystals combined with morphological analyses permit the identification of three distinct components within the zircon population of the saedelhorn diorite a variscan dike from the western gotthard central alps switzerland i 94 of the grains in the zircon population are elongate crystals with pronounced skeletal morphology indicative of rapid growth from a supercooled melt ii 5 of the population consist of turbid mostly subhedral zircons frequently showing d type morphology classification according to pupin and turco 1 and elevated uranium contents compared to the skeletal variety single crystal and multi grain upb isotopic data of group i and ii zircons define an intrusion age of 293 5 4 m y for the dike iii rare transparent zircon crystals 1 of the zircon population yield apparent upb ages in the range of 370 490 m y and display morphological and isotopic characteristics closely resembling those of a caledonian orthogneiss intruded by the dike this implies presence of assimilated wall rock components in the macroscopically homogeneous dike sample a comparison of the data obtained by conventional analysis of multi grain zircon fractions and those obtained by grain by grain analysis demonstrates that age resolution is considerably improved by single crystal upb dating furthermore quantitative identification of zircon components assimilated by the ascending magma along its path to the present level of exposure is feasible by the latter technique since it is likely that such zircon grains are common to a broad variety of magma types valuable information on age and composition of crustal layers not accessible to direct observation may readily become available by application of precise micro analytical techniques low initial143nd 144nd end 2 7 at the time of intrusion of the saedelhorn dike requires the magma to be derived partially or totally from a crustal source for this crustal precursor a model age of 1050 m y tdm is obtained indicating that proterozoic crust was involved in the petrogenesis of the variscan intrusives of the gotthard area

u th pb systematics of morphologically characterized zircon and allanite a high resolution isotopic study of the alpine rensen pluton northern italy abstract the rensen pluton is a typical member of the tertiary calc alkaline plutonic suite structurally connected to the periadriatic lineament it intrudes the austroalpine polymetamorphic basement of the eastern alps southwest of the tauern window northern italy zircons and allanites from tonalitic quartz dioritic and monzogranitic lithologies were studied systematically on the basis of refined grain by grain microanalytical techniques high resolution isotope analyses show that morphology and u pb systematics of zircon are highly correlated with magma evolution zircons from the main tonalitic part of the intrusion display a gradual increase of u content from early stage s type morphology to late stage g1 type morphology most of the zircon crystals include premagmatic seeds derived from old crustal sources unevolved s type zircons contain inherited components of late proterozoic early paleozoic apparent age whereas late stage g1 type zircons show inheritance of early proterozoic apparent age both types of inherited components are present in zircons from a fine grained tonalitic domain located at the center of the pluton which shows restitic structures the crustal affinity of this rock is corroborated by its relatively low e nd value 6 0 as compared to values obtained for the main tonalite 3 9 and the quartz diorite 3 5 the accuracy of cainozoic zircon ages is affected by initial radioactive disequilibrium in the u pb decay chains particularly uncertain is the fractionation behavior of protactinium which could be enriched in zircon due to the high stability of the pa 5 oxidation state because the th pb system is devoid of long lived intermediate radioactive nuclides the best approximation to the emplacement age of the rensen pluton is given by allanite th pb ages of 31 70 0 25ma 95 c l for the main tonalite and by a slightly younger age of 31 09 0 32ma for quartz diorite