a new occurrence of lazulite from the main central thrust in kumaun himalaya india fluid inclusion epma and raman spectroscopy focusing on lazulite in a highly tectonized zone

stable isotope geochemistry and phase equilibria of coesite bearing whiteschists dora maira massif western alps peak metamorphic temperatures for the coesite pyrope bearing whiteschists from the dora maira massif western alps were determined with oxygen isotope thermometry the 18o smow values of the quartz after coesite 18o 8 1 to 8 6 n 6 phengite 6 2 to 6 4 n 3 kyanite 6 1 n 2 garnet 5 5 to 5 8 n 9 ellenbergerite 6 3 n 1 and rutile 3 3 to 3 6 n 3 reflect isotopic equilibrium temperature estimates based on quartz garnet rutile fractionation are 700 750 c minimum pressures are 31 32 kb based on the pressure sensitive reaction pyrope coesite kyanite enstatite in order to stabilize pyrope and coesite by the temperature sensitive dehydration reaction talc kyanite pyrope coesite h2o the a h2o must be reduced to 0 4 0 75 at 700 750 c the reduced a h2o cannot be due to dilution by co2 as pyrope is not stable at x co2 0 02 t 750 c p 30 kb in the absence of a more exotic fluid diluent e g ch4 or n2 a melt phase is required granite solidus temperatures are 680 c 30 kb at a h2o 1 0 and are calculated to be 70 c higher at a h2o 0 7 consistent with this hypothesis kyanite jadeite quartz bands may represent a relict melt phase peak p t f h2o estimates for the whiteschist are 34 2 kb 700 750 c and 0 4 0 75 the oxygen isotope fractionation between quartz 18o 11 6 and garnet 18o 8 7 in the surrounding orthognesiss is identical to that in the coesitebearing unit suggesting that the two units shared a common final metamorphic history hydrogen isotope measurements were made on primary talc and phengite d smow 27 to 32 on secondary talc and chlorite rite after pyrope d 39 to 44 and on the surrounding biotite d 64 and phengite d 44 gneiss all phases appear to be in nearequilibrium the very high d values for the primary hydrous phases is consistent with an initial oceanicderived connate fluid source the fluid source for the retrograde talc chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization the similar d but dissimilar 18o values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium but not oxygen isotope equilibrium the unusual hydrogen and oxygen isotope compositions of the coesite bearing unit can be explained as the result of metasomatism from slab derived fluids at depth

migration paths and precipitation mechanisms of ore forming fluids at the shuiyindong carlin type gold deposit guizhou china abstract shuiyindong is one of the largest and highest grade stratabound carlin type gold deposits in china this paper reports on the results of petrographic studies electron microprobe analyses empa of arsenian pyrite and the mass transfer during mineralization and alteration and it presents the deposit scale distributions of au as sb hg tl and trace elements in a representative cross section across the shuiyindong carlin type gold deposit guizhou province the main objectives were to identify the precipitation mechanisms of minerals or elements from fluids and the migration paths of ore forming fluids petrographic and empa studies indicate that gold in the primary ores is mainly hosted by arsenian pyrite mass transfer associated with alteration and mineralization shows that au as sb hg tl and s were significantly added to all mineralized rocks fe 2 o 3 and sio 2 were immobile in the main orebodies that are hosted in bioclastic limestone and cao na 2 o sr and li were removed from country rocks the relations between fe and s indicate that the sedimentary rocks at the shuiyindong deposit contain more iron than is needed to combine with all of their contained sulfur to form pyrite this suggests that sulfidation and decarbonation were the principal mechanism of gold precipitation at the shuiyindong deposit hg sb and as commonly formed sulfide minerals such as stibnite realgar and orpiment in late stage quartz calcite veins or absorbed by organic matter in argillite fluid cooling presumably led to depositions of stibnite realgar and orpiment in late stage quartz calcite veins organic matter likely served as a reductant in argillite for the ore fluids causing the precipitation of as sb hg and s as well as au deposit scale distributions of gold and other relevant elements reflect the passage of fluids through the rocks rock strata and structures allowed the ore forming fluids to migrate horizontally along the unconformity surface of the middle upper permian converge on the high position of an anticline and then ascend into the overlying strata along the anticlinal axis the distributions of the major and trace elements show that elements that accompanied the ore forming fluids include au as sb hg tl and s and that na 2 o and li were exhausted in the longtan formation at the anticlinal core during gold mineralization the enrichment of co cr and ni in the longtan formation at the anticlinal core might be associated with deformation that formed the anticline or with gold mineralization different host rocks were preferentially mineralized by different elements the bioclastic limestone is commonly enriched in au whereas the argillite is preferentially enriched in as hg sb and tl the zonation of ore forming elements in the deposit appears to be sb tl as hg au hg as from bottom to top enrichment of au as sb hg and tl provides useful guidance for the exploration for carlin type gold deposits in guizhou anomalies of as and hg in soil or stream sediment might be an important clue and these elements can be used as indicator elements ore forming fluids migrated along the unconformity surface of the middle upper permian and the anticlinal axis so these are favorable sites for exploration for carlin type gold deposits in guizhou

fluid rock interaction during subsolidus microtextural development of alkali granite as exemplified by the saertielieke pluton ulungur of the northern xinjiang china three lithological groups i medium grained with magmatogenic arfvedsonite ii medium grained with secondary arfvedsonite and iii fine grained with magmatogenic arfvedsonite are identified in the saertielieke alkali granite pluton ulungur of the northern xinjiang china a weak negative correlation between the 18o values of alkali feldspar and quartz separates from each group and the distinctly lower 18o values of alkali feldspar separates from groups i and ii than those from group iii are interpreted in terms of superimposed closed system and open system isotope exchange a small amount of locally exsolved magmatic fluid is involved in the development of the perthitic texture in alkali feldspar at 400 c that results in a volume increase and hence causes quartz deformation the microtextural changes promote the closed system oxygen isotope exchange between quartz and alkali feldspar that causes a dispersion in the quartz 18o values however the distinctly lower 18o values of alkali feldspar and secondary arfvedsonite coupled with their microtextural characteristics indicate that meteoric derived water plays an important role in the further development of alkali feldspar exsolution texture at t 400 c and directly causes secondary arfvedsonite formation the estimated relative exchange rates kquartz kfeldspar karfvedsonite of 10 100 1 for groups i and iii and 10 100 100 for group ii suggest that alkali feldspar quartz and secondary arfvedsonite have exchanged with meteoric derived water mainly via dissolution reprecipitation whereas magmatogenic arfvedsonite has exchanged via diffusion

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

40ar 39ar dating of microgram feldspar grains from the paired feldspathic achondrites gra 06128 and 06129 abstract 40 ar 39 ar ages of single feldspar grains from the paired meteorites graves nunatak 06128 gra8 8 grains and 06129 gra9 26 grains are presented plateau ages 70 of the 39 ar released ranged from 4000 to 4600 ma with an average 1 uncertainty of 90 ma the most precise ages obtained were 4267 17 ma for a grain from gra8 and 4437 19 ma and 4321 18 ma for two grains from gra9 isotope correlation diagrams yield less precise ages ranging from 3800 to 5200 ma with an average 1 uncertainty of 250 ma they indicate a negligible trapped component plateau ages integrated total fusion ages and isochron ages are internally concordant at the 95 confidence level the distribution of the plateau ages for gra9 is bimodal with peaks at 4400 and 4300 ma in contrast the plateau age distribution for gra8 peaks at about 4260 ma with broad wings extending toward younger and older ages to explain the distributions of grain ages we prefer a scenario that includes a major post formation event about 4400 ma ago and a later melt intrusion event that heated gra8 more than some parts of gra9

a high resolution record of multiple diagenetic events ultraviolet laser microprobe ar ar analysis of zoned k feldspar overgrowths abstract high spatial resolution uv laser ar ar analyses were performed in situ on k feldspar overgrowths from sandstone hosted pb zn bodies from central sweden two different populations of ages were detected precambrian cambrian 528 10 to 567 8 ma and late silurian 403 6 to 453 8 ma these were both detected in single pore spaces with the older population adjacent to the detrital feldspar whilst the younger population was always distal to the detrital grain this younging of authigenic k feldspar ages with distance from the detrital grain margin is discussed in terms of disturbance to the argon system within an authigenic overgrowth that is of a single age a the overgrowths are precambrian cambrian in age and the late silurian population might be an artefact of argon loss due to partial resetting b the overgrowths are late silurian and the precambrian cambrian population might have resulted from excess argon uptake and c the overgrowths are late silurian but the precambrian cambrian population is an experimental artefact arising from the analysis of overlapping detrital authigenic feldspars diffusion modelling shows that the partial resetting scenario is a very unlikely cause for the observed age variation because it requires a very particular arrangement of sub grain sizes excess argon was not detected in either the k feldspar or quartz the data are best interpreted as the ages of two discrete diagenetic events an early burial diagenesis and a later tectonically induced fluid flow event relating to the collapse of the caledonian orogen further the use of 38arcl 39ark as a proxy for cl k ratio highlights the physical boundaries between feldspar populations where all other petrographic sem and cl analyses have failed to do so this study shows that high spatial resolution uv laserprobe ar ar analysis is a powerful tool for extracting age information from complex hydrothermal minerals as a means to investigating fluid flow events

factors in the preservation of coesite the importance of fluid infiltration the survival of coesite in ultrahigh pressure uhp rocks is most commonly attributed to rapid exhumation continuous cooling during uplift and inclusion in strong phases that can sustain a high internal over pressure during decompression exceptions to all of these criteria exist perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite we used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of uhp rocks in all cases oh concentrations in coesite are below the detection limit of 100 ppm h2o the silica phases surrounding coesite however show varying amounts of h2o this is most spectacularly observed in pyrope quartzites from the dora maira massif that contain at least three phases of silica replacing coesite also distinguished by varying color of cathodoluminescence cl palisade textured quartz 100 ppm h2o red violet cl mosaic quartz which is actually chalcedony up to 0 4 wt h2o yellow brown cl and a rare highly hydrated silica phase interpreted to be opal 7 wt h2o dark blue cl very similar signatures are observed in a grospydite xenolith from the roberts victor kimberlite the quartz replacing coesite in other uhp samples studied contains on the order of 500 ppm h2o or less and most measurements are under the detection limit of our technique we infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase the formation of hydrous silica phases such as chalcedony and opal however must take place at much lower temperatures after cracking of the host phase which allows external fluids to infiltrate delay of fluid infiltration to low temperatures where kinetics are slow even in the presence of water is the most critical factor in the preservation of coesite

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

tectonic and petrogenetic implications of major and rare earth element chemistry of troodos glasses cyprus chilled pillow margins dike margins and interpillow hyaloclastites from the extrusive sequences of the troodos ophiolite cyprus reveal two compositional groups of glasses 1 andesites to dacitic andesites containing plagioclase ca rich pyroxene and ca poor pyroxene as phenocrysts occur in a stratigraphically low position and 2 basaltic andesites containing olivine and ca rich pyroxene phenocrysts occur in an upper position in the lava sequence the glasses contain between 52 and 62 wt sio2 and have mg numbers between 65 and 27 the troodos glasses are very similar to basaltic andesite or boninitic glasses from the mariana trench and together these two groups define a trend intermediate between typical calcalkaline and tholeiite evolutions the troodos glasses are inferred by difference to contain 3 4 5 wt h2o which is taken to be of primary origin and to reflect the ocean depth of extrusion and quenching based on normative projections in the basalt phase diagram there are indications that elevated water pressures during formation of the phenocryst assemblages may have shifted the isobaric pseudo quaternary peritectic towards silica quantitative crystal fractionation models fail to explain the observed chemical variation it is suggested that the chemical variation may result from high pressure melts equilibrated to lower pressure 2 and 3 phase cotectic relations some mariana and the troodos glasses have similar ree patterns with relatively low abundances depletion in lree and flattening in the hree region the close similarities between the troodos and the mariana glasses point to an identical tectonic and petrogenetic control on their origin

pressures and temperatures in precambrian metamorphic rocks modelp t estimates from nearly 100 precambrian rocks have been derived by comparing published descriptions of metamorphic mineral assemblages to a standard set of mineral stability data pressures temperatures and metamorphic geotherms show large variations nevertheless modelp t data reveal two distinct secular trends average metamorphic geotherms have declined through time and average metamorphic pressures have increased through time metamorphic geotherms and pressures from archean rocks average 54 c km and 4 1 kbar respectively corresponding figures for early proterozoic terranes are 47 c km and 4 3 kbar and for late proterozoic terranes 35 c km and 6 kbar these secular trends may reflect gradual cooling and thickening of the earth s crust at least in orogenic belts there are exceptions to both trends found in localities where mineral assemblages indicate high pressures 6 12 kbar and low metamorphic geotherms 20 30 c km these assemblages formed in archean crust that at least locally was thicker than 20 km during metamorphism

magmatic and hydrothermal chronology of the giant rio blanco porphyry copper deposit central chile implications of an integrated u pb and 40ar 39ar database the history of hypabyssal intrusion and hydrothermal activity in the northeastern and central parts of the be hemothian sensu clark 1993 rio blanco los bronces porphyry copper molybdenum deposit is clarified on the basis of integrated u pb and 40ar 39ar geochronology isotope dilution thermal ion mass spectrometry id tims u pb dates for zircon separates and id tims and sensitive high resolution ion microprobe shrimp dates for single zircon grains in pre syn and late mineralization volcanic and intrusive host rocks in the rio blanco don luis and sur sur mining sectors provide a temporal framework for interpretation of incremental heating and spot fusion 40ar 39ar dates for respectively magmatic biotite and hydrothermal biotite muscovite and orthoclase the ore deposit is hosted in part by 16 77 0 25 to 17 20 0 05 2 ma andesitic volcanic strata of the farellones formation but the major host rocks are units of the san francisco batholith including the 11 96 0 40 ma rio blanco granodiorite mine terminology the 8 40 0 23 ma cascada granodiorite and the 8 16 0 45 ma diorite hypabyssal dacitic intrusions late porphyries emplaced into the batholith yield 206pb 238u id tims dates ranging from 6 32 0 09 ma quartz monzonite porphyry through 5 84 0 03 ma feldspar porphyry to 5 23 0 07 ma don luis porphyry the late mineralization rio blanco dacite plug yields a shrimp zircon age of 4 92 0 09 ma the 40ar 39ar plateau ages for phenocrystic biotites in quartz monzonite porphyry feldspar porphyry and don luis porphyry as well as the preore diorite range only from 5 12 0 07 to 4 57 0 06 ma all are significantly younger than the corresponding zircons and exhibit no correlation with intrusive sequence the 40ar 39ar ages for hydrothermal biotite and orthoclase veins within the san francisco batholith units fall in a narrow interval from 5 32 0 27 to 4 59 0 11 ma hydrothermal sericites muscovite one associated with chalcopyrite yielded spot fusion ages of 4 40 0 15 ma rio blanco granodiorite hosted and 4 37 0 06 ma don luis porphyry hosted comparison with the id tims and shrimp zircon ages indicates that most of the 40ar 39ar ages even 95 percent plateaus do not record initial magmatic cooling or hydrothermal alteration mineralization events evidence for quasipervasive reheating to at least 300 c by successive intrusions published re os ages for two molybdenite samples range from 5 4 to 6 3 ma and overlap extensively with the zircon u pb ages for the late porphyries they imply that cu mo mineralization overlapped temporally with the emplacement of at least quartz monzonite porphyry and feldspar porphyry units of the late porphyry suite and was therefore contemporaneous with the rise of dacitic melts to subvolcanic levels hydrothermal activity is inferred to have continued until 4 37 0 06 ma following intrusion of the don luis porphyry and the early stages of emplacement of the rio blanco dacite plug complex hypogene cu mo mineralization therefore probably persisted for 2 m y the geochronologic data do not resolve whether ore formation was continuous or episodic but the observed crosscutting relationships between intensely altered and mineralized country rocks and less altered and mineralized late porphyry bodies support a model in which the ascent of metal rich brines from an unexposed zone of the parental magma chamber was periodically stimulated by magma perturbation and hypabyssal intrusion

geochemistry of a type granites in the huangshaping polymetallic deposit south hunan china implications for granite evolution and associated mineralization abstract the huangshaping granites in hunan province south china were investigated for their geochemical characteristics three types of granites have been petrographically identified quartz porphyry granophyre and granite porphyry whole rock geochemistry suggests that the huangshaping granites especially the granite porphyry exhibit typical a type granite characteristics with their enrichment in si rb u th and nb and significant depletion in ba sr ti eu and p based on the al y and zr contents as well as the ree patterns of the rocks investigated the quartz porphyry and the granophyre are classified as a1 type alkaline granites whereas the granite porphyry is considered as a2 type aluminous granite whole rock and quartz feldspar o isotope data yields a wide range of 18osmow values 11 09 26 32 the granites are characterized by high radiogenic pb isotopic composition the present day whole rock pb isotopic ratios are 206pb 204pb 18 706 19 155 207pb 204pb 15 616 15 711 and 208pb 204pb 38 734 39 296 combining the o pb isotope compositions with major trace and ree geochemistry and regional geology characteristics the huangshaping granites were determined to resemble within plate granites that were mainly derived from a felsic infracrustal source related to continental extension the magma source of the quartz porphyry and the granophyre may have been generated from deeper depths and then ascended rapidly with limited water content and low oxygen fugacity which contributed to cu pb and zn mineralization on the other hand the magma that generated the granite porphyry may have ascended relatively slower and experienced pronounced crystal fractionation upper crustal basement rock contamination assimilation and wall rock interaction producing the sn and w rich granite porphyry this study reveals the crustal extension process and associated magmatic metallogenic activities during 180 150 ma in south hunan

the 40ar 39ar laser analysis of k feldspar constraints on the uplift history of the grenville province in ontario and new york 1 a comprehensive geochronologic database describes a period of late extension across the metasedimentary belt mb of the grenville province in northeastern north america because extension continued through much of the young unroofing history of the region these data do not bracket the timing of final extension across all shear zones and do not constrain the timing of final juxtaposition of terranes analysis of k feldspar in the mb by the 40ar 39ar method can give information on the very latest perturbations in the area since k feldspars have multiple low closure temperatures ranging from about 150 to 300 c these multiple diffusion domains require temperature time modeling to fully interpret argon release spectra this modeling requires precise knowledge of the temperature of degassing of each step which usually requires the use of a resistance furnace for sample analysis we establish a first order relationship between laser power and temperature which can be used for multidiffusion domain modeling of k feldspars comparison of samples analyzed by both methods reveals virtually no difference between the systems supporting the validity of k feldspar analyses by laser step heating our data suggest that using the laser for k feldspars can give results that are geologically reasonable precise and easier to collect in this case these data are then applied to the cooling history of the north american grenville province our k feldspar analyses show that the latest extensional motion along shear zones in the mb is after 900 ma and the region is uplifting as a uniform block by 780 ma

californian blueschists subduction and the significance of tectonostratigraphic terranes glaucophane and related schists are present as tectonic fragments in ophiolitic suture zones and as discrete lithotectonic belts along the accreted mesozoic tertiary californian margin occurrences include parts of the klamath mountains the western sierran foothills the coast ranges faulted marginal segments of the mojave desert the transverse ranges and the southern california borderland these high pressure low temperature blueschist assemblages reflect the thermal regime of subduction zone environments considerable underflow accompanied drifting and the assembly of far traveled tectonostratigraphic terranes as documented by sea floor magnetic anomaly patterns and age relationships of the oceanic crust capped lithosphere the eastern limbs of paleo pacific plates especially the farallon cocos have been extensively or completely overridden by the westward encroaching north american plate 7000 km since early cretaceous time and nearly 10 000 km since jurassic time subduction is attested to by remnant high pressure mineral assemblages scattered throughout california by construction of related roughly contemporaneous calc alkaline volcanic plutonic belts and forearc basin deposits and by the stranding of ophiolitic complexes although substantial northward drift transported exotic oceanic and continental materials to the growing californian crust and caused extensive dislocation of the post paleozoic continental margin much of the plate motion evidently involved a large component of convergence and eastward underflow terrane shuffling has complicated the picture but the dominant mechanism of continental growth at the californian margin during mesozoic and paleogene time was subduction

geotherms evolution of the lithosphere and plate tectonics abstract for the purpose of calculating precambrian geotherms modern average geotherms need to be defined curves calculated from estimated plate sections are close to those of clark and ringwood 1964 thicknesses of lithospheric plates at any time in the past can be estimated from fossil geotherms because subduction of oceanic plates depends upon their density average densities of precambrian plates should place important constraints on the beginning of subduction this method however only indicates that in all probability subduction did not begin before 1000 ma ago geological data suggest a date of about 800 or 900 ma ago formation of archean continents would have proceeded from partial melting of basaltic crust and the end of that phase of earth s evolution would be a thermally controlled feedback mechanism lowering geothermal gradients in sub continental mantle the beginning of subduction was also thermally controlled and probably accompanied the increasing stability of eclogite in the upper mantle

hydrothermal and unexpected diagenetic alteration in permian shales of the lodeve epigenetic u deposit of southern france traced by k ar illite and k feldspar dating abstract mineralogical and chemical analyses combined with k ar dating of illite and k feldspars were carried out on bulk samples and different fractions from permian shales around the lodeve u deposit to identify diagenetic and hydrothermal alteration episodes and their timing authigenic 2m1 and 1m illite with k ar ages between 270 and 245 ma are the predominant mineral types in the shales they are of early diagenetic origin relative to deposition and formed apparently directly from pyroclastic materials in a lagoonal to deltaic environment under semi arid conditions albite and a first generation of k feldspar developed by a replacement of analcime the k feldspar growing only in the former lagoonal environment that favored also early diagenetic dolomitization at the expense of calcite the hydrothermally altered shales next to the u deposit contain authigenic quartz fe chlorite neoformed illite and a second generation k feldspar with k ar ages beginning at about 220 ma triassic although jurassic and cretaceous ages were also obtained this younger age spectrum jurassic cretaceous is close to previous u pb isotope data from the u deposit probably reflecting several episodes of u mobilization therefore the hydrothermal activity extended episodically from triassic to cretaceous time within the u deposit the hydrothermal temperatures attained locally 220 c as the k ar system of illite from early permian diagenesis is partially reset triassic hot fluids ascended along faults into the northern area of the lodeve basin during an initial rifting of the nearby western tethys ocean further fluid pulses were recorded at lodeve during the opening of the north atlantic and separation of europe from africa jurassic cretaceous which also caused the peak hydrothermal activities in central and western europe as the lodeve area is located between two independently developing oceans it could have been supplied with hot fluids for a long period of time

experimental investigation of the alluaudite triphylite assemblage and development of the na in triphylite geothermometer applications to natural pegmatite phosphates in order to assess the stability of the primary alluaudite triphylite assemblage we performed hydrothermal experiments between 400 and 800 c starting from the lina2mn x fe 3 2 fe3 po4 4 compositions x 1 054 1 502 1 745 that represent the ideal compositions of the alluaudite triphylite assemblages from the kibingo rwanda hagendorf sud germany and buranga rwanda pegmatites respectively the pressure was maintained at 1 kbar and the oxygen fugacity was controlled by the ni nio buffer the results of these experiments show that the alluaudite triphylite assemblage crystallizes at 400 and 500 c while the association alluaudite triphylite maricite appears at 600 and 700 c the limit between these two domains at ca 550 c corresponds to the maximum temperature that can be reached by the alluaudite triphylite assemblages in granitic pegmatites because maricite has never been observed in such geological environments at 800 c the formation of the x phase triphylite assemblage indicates a strong reduction of the bulk composition according to the reaction 0 5lim2 po4 triphylite 3na2m2 2 fe3 po4 3 alluaudite 1 5h2o 4 5nam2 po4 maricite li0 5na1 5m5 2 po4 4 x phase h3po4 0 75o2 m2 fe2 mn secondary ion mass spectrometry sims was used at our knowledge for the first time to measure li in all the li bearing phosphates a specific methodological procedure was developed with the ion microprobe to get accurate li2o data over a wide concentration range spanning from few ppm li up to 11 wt li2o our sims analyses of the synthesized phosphates indicate that the li contents of alluaudites maricites and x phase increase progressively with temperature while the li content of triphylite type phosphates decreases due to the li na substitution the na exchange equilibrium between triphylite type phosphates and alluaudite is correlated with the temperature according to the equation ln x na tri x na all 7 0 7 103 t 5 4 9 this equation can be used to estimate the crystallization temperature of triphylite alluaudite assemblages independently of the oxygen fugacity

40 ar 39 ar geochronology and stable isotope geochemistry of late hercynian intrusions from north eastern iberia with implications for argon loss in k feldspar the cooling history of hercynian calc alkalic post kinematic plutonic intrusions of the montnegre massif ne spain has been determined by 40ar 39ar analysis of two hornblendes four biotites and eight k feldspars kfs the hornblendes have 40ar 39ar total fusion ages of 291 3 ma and define the magmatic cooling of the basic and oldest structural intrusions the biotites from the acid and intermediate rocks have 40ar 39ar plateau ages of 285 3 ma which date cooling of the intrusions through argon closure in biotite the 40ar 39ar ages of the k feldspars vary widely ranging from 276 191 ma a correlation between k feldspar 40ar 39ar total fusion age and several other features such as structural state microstructures indicated by obliquity and to some degree optically visible perthites is consistent with post crystallisation partial argon loss in the k feldspars the d values of the biotites also correlate with age and chlorite contents but this is not so for the 18o values of either feldspar or quartz we infer that most microtextural changes occurred during cooling of the batholith but a possibly mesozoic late disturbing hydrothermal event of weak intensity and with only minor fluid circulation must have occurred this event provoked significant argon loss in the most structurally complex k feldspars and is recorded in the hydrogen but not oxygen isotope data

an island arc origin for the canyon mountain ophiolite complex eastern oregon u s a abstract the canyon mountain ophiolite oregon is exceptional in lacking sheeted dikes basaltic pillow lavas and sediments that are characteristic of many other ophiolites instead the uppermost portion of the complex consists of a significant volume of plagiogranites which in addition to minor basalts intrude a large section of keratophyres believed to be of volcanic origin the trend of intrusive rocks and of bedding in the keratophyres is mostly parallel to layering in the underlying gabbroic cumulates and to contacts between units in the remainder of the ophiolite it is suggested that the plagiogranites basalts and keratophyres comprise a sill complex both the plagiogranites and the keratophyres are similar respectively to low k2o plutonic and extrusive rocks of island arcs the mineralogy and penetrative deformation structures of the ultramafic and some of the gabbroic rocks of the ophiolite indicate greater depth of formation related to magmatism and diapirism above a benioff zone radiometric age dates of plagiogranites confine the minimum age of the complex to the early permian the canyon mountain ophiolite may thus be correlative with other fragments of a lower permian arc terrane throughout northeastern oregon which were chaotically mixed during renewed subduction in middle to late triassic time

volcanic glass compositions of the troodos ophiolite cyprus fresh volcanic glass is preserved throughout the extrusive section of the troodos ophiolite which indicates that the lavas have not been pervasively metamorphosed glass compositions reveal the existence of two major magma suites apparently corresponding to distinct stratigraphic intervals the basal 400 500 m of the sequence consists of an andesite dacite rhyolite assemblage containing abundant hyaloclastites the remainder of the section comprises a basalt basaltic andesite assemblage with high mgo and low tio 2 and total iron the lower sequence is interpreted as an evolved arc tholeiite suite the upper has some similarities to boninitic lavas the close association in time and space of these two suites is similar to that observed in the mariana and bonin arcs and suggests that all of the troodos lavas were erupted in a subduction zone environment most probably in an incipient arc or fore arc

40ar 39ar ages in deformed potassium feldspar evidence of microstructural control on ar isotope systematics detailed field and microstructural studies have been combined with high spatial resolution ultraviolet laser 40ar 39ar dating of naturally deformed k feldspar to investigate the direct relationship between deformation related microstructure and ar isotope systematics the sample studied is a 1 000 ma torridonian arkose from skye scotland that contains detrital feldspars previously metamorphosed at amphibolite facies conditions 1 700 ma the sample was subsequently deformed 430 ma ago during caledonian orogenesis the form and distribution of deformation induced microstructures within three different feldspar clasts has been mapped using atomic number contrast and orientation contrast imaging at a range of scales to identify intragrain variations in composition and lattice orientation these variations have been related to thin section and regional structural data to provide a well constrained deformation history for the feldspar clasts one hundred and forty three in situ 40ar 39ar analyses measured using ultraviolet laser ablation record a range of apparent ages 317 1030 ma the k feldspar showing the least strain records the greatest range of apparent ages from 420 1 030 ma with the oldest apparent ages being found close to the centre of the feldspar away from fractures and the detrital grain boundary the most deformed k feldspar yields the youngest apparent ages 317 453 ma but there is no spatial relationship between apparent age and the detrital grain boundary within this feldspar the oldest apparent ages are recorded from orientation domain boundaries and fracture surfaces where an excess or trapped 40ar component resides orientation contrast images at a similar scale to the ar analyses illustrate a significant deformation related microstructural difference between the feldspars and we conclude that deformation plays a significant role in controlling ar systematics of feldspars at both the inter and intragrain scales even at relatively low bulk strains the data show that ar loss and trapping within the deformed k feldspars reflects the presence of a deformation induced population of small diffusion domains in combination with short circuit diffusion along deformation induced defects the complex history of microstructures induced in the k feldspars during their cooling alteration erosion and sedimentation do not appear to be as significant as deformation induced microstructures in controlling the distribution of apparent ages at the grain scale

reorganization of geological sciences and particularly of metamorphic geology by the advent of plate tectonics a personal view abstract plate tectonics having being formulated in geophysics in the 1960 s was introduced into and caused reorganization of geological sciences in about 1968 1975 an anticipated system of reorganized geology was formulated around 1970 by a few pioneering geologists with a strong background of global scale regional geology pre existing dutch school tectonics with an explicit uniformitarian tendency played an important role in the beginning development and acceptance of plate tectonic viewpoints global scale views in metamorphic geology that had begun prior to the advent of plate tectonics were developed into tectonic models of regional metamorphism as well as discussions of the p t t paths and of secular changes in the type of regional metamorphism

high precision 40ar 39ar dating of two consecutive hydrothermal events in the chuquicamata porphyry copper system chile 40ar 39ar data have been used to resolve the relatively high temperature 350 c potassic alteration event at chuquicamata from the lower temperature 350 c quartz sericite alteration phase four k feldspars from the potassic zone yielded plateau large domain ages in the range 32 9 33 8 ma mean 33 4 0 3 ma the fact that apparent ages of co existing biotites span a similar range suggests that there was very rapid cooling through feldspar and biotite closure temperatures especially if these minerals closed at substantially different temperatures multi domain diffusion modelling of the feldspar data suggests that at this time cooling was rapid ca 60 80 my through closure temperatures of 330 440 c as these closure temperatures are substantially higher than the ones normally assumed for biotite i e about 300 c they and or the calculated cooling rates may be unreliable four sericites from the quartz sericite zone yielded variably discordant age spectra the least discordant of these has a plateau at 31 1 0 3 ma and this is interpreted as the time of this alteration event some k feldspars in the quartz sericite zone were completely overprinted at this later time and most of the other ones studied show partial overprinting overprinting appears to decrease as distance from the central quartz sericite zone increases in the more remote areas effects of the later event appear constrained to narrow fault controlled alteration zones the data are consistent with a conceptual model proposed by zentilli et al zentilli m leiva g rojas j graves m c 1994a the chuquicamata system revisited extended abstract society of economic geologists symposium copper deposits of the andes new developments concepcion chile october 17 18 1994 proceedings 2 1647 1651 wherein potassic alteration by late magmatic fluids occurred at depth prior to rapid exhumation of the system quartz sericite alteration perhaps caused by a new pulse of porphyry intrusion at depth occurred some 2 3 my later principally in the core of the deposit although alteration did propagate outwards through the action of hydrothermal fluids passing through a brittle relatively shallow fracture system

phase relations in the mgo p2o5 h2o system and the stability of phosphoellenbergerite petrological implications the polymorphic relations for mg3 po4 2 and mg2po4oh have been determined by reversed experiments in the temperature pressure t p range 500 1100 c 2 30 kbar the phase transition between the low pressure phase farringtonite and mg3 po4 2 ii the mg analogue of sarcopside is very pressure dependent and was tightly bracketed between 625 c 7 kbar and 850 c 9 kbar the high temperature high pressure polymorph mg3 po4 2 iii is stable above 1050 c at 10 kbar and above 900 c at 30 kbar the low pressure stability of farringtonite is in keeping with its occurrence in meteorites the presence of iron stabilizes the sarcopside type phase towards lower p from the five mg2po4oh polymorphs only althausite holtedahlite mg2po4oh the hydroxyl analogue of wagnerite and mg2po4oh were encountered relatively speaking holtedahlite is the low temperature phase 600 c mg2po4oh the high temperature low pressure phase and mg2po4oh the high temperature high pressure phase with an intervening stability field for althausite which extends from about 3 kbar at 500 c to about 12 kbar at 800 c althausite and holtedahlite are to be expected in f free natural systems under most geological conditions however wagnerite is the most common mg phosphate mineral implying that fluorine has a major effect in stabilizing the wagnerite structure coexisting althausite and holtedahlite from modum s norway show that minor fluorine is strongly partitioned into althausite kdf oh 4 and that holtedahlite may incorporate up to 4 wt sio2 synthetic phosphoellenbergerite has a composition close to mg0 9 0 1 2mg12p8o38h8 4 it is a high pressure phase which breaks down to mg2po4oh mg3 po4 2 h2o below 8 5 kbar at 650 c 22 5 kbar at 900 c and 30 kbar at 975 c the stability field of the phosphate end member of the ellenbergerite series extends therefore to much lower p and higher t than that of the silicate end members stable above 27 kbar and below ca 725 c thus the si p ratio of intermediate members of the series has a great barometric potential especially in the si buffering assemblage with clinochlore talc kyanite rutile h2o application to zoned ellenbergerite crystals included in the dora maira pyrope megablasts western alps reveals that growth zoning is preserved at t as high as 700 725 c however the record of attainment of the highest t and or of decreasing p through p rich rims 1 to 2 si pfu is only possible in the presence of an additional phosphate phase oh bearing or even oh dominant wagnerite in these rocks otherwise the trace amounts of p in the system remain sequestered in the core of si rich crystals 5 to 8 si pfu and can no longer react

the stability of primary alluaudites in granitic pegmatites an experimental investigation of the na 2 mn 2 2 x fe 1 2 x po 4 3 system in order to assess the geothermometric potential of the na2 mn2 2xfe1 2x po4 3 system x 0 1 which represents the compositions of natural weakly oxidized alluaudites we performed hydrothermal experiments between 400 and 800 c at 1 kbar under an oxygen fugacity f o2 controlled by the ni nio nno fe2o3 fe3o4 hm cu2o cuo ct and fe fe3o4 mi buffers when f o2 is controlled by nno single phase alluaudites crystallize at 400 and 500 c whereas the association alluaudite maricite appears between 500 and 700 c the limit between these two fields corresponds to the maximum temperature that can be reached by alluaudites in granitic pegmatites because maricite has never been observed in these geological environments because alluaudites are very sensitive to variations of oxygen fugacity the field of hagendorfite na2mnfe2 fe3 po4 3 has been positioned in the f o2 t diagram and provides a tool that can be used to estimate the oxygen fugacity conditions that prevailed in granitic pegmatites during the crystallization of this phosphate

multistage reaction textures in xenolithic high mgal granulites at anakapalle eastern ghats belt india examples of contact polymetamorphism and infiltration driven metasomatism high mgal rocks occur as xenoliths up to 2 m in diameter in mafic granulites at a newly discovered locality near anakapalle following an early phase of deformation ultrahigh temperature uht metamorphism and near isothermal decompression the rocks were intruded in a lit par lit manner by felsic melts charnockite which caused local scale metasomatism a subsequent deformation produced isoclinal folds and the distinct gneissic foliation of the charnockite still at granulite facies conditions the sequence of multiphase reaction textures in the high mgal xenoliths reflects the changes of physico chemical conditions during the polyphase evolution of the terrane uht metamorphism stage 1 1000 c c 10 kbar is documented by relics of extremely coarse grained domains with the assemblage orthopyroxene opx 1 garnet grt 1 sapphirine spr 1 spinel spl 1 rutile rt a subsequent phase of near isothermal decompression in the order of 1 2 kbar stage 2 resulted in extensive replacement of grt1 and opx1 megacrysts by lamellar opx2 spr2 symplectites the intrusion of felsic melt stage 3 led to the development of a narrow metasomatic black wall reaction zone bt sil plg3 opx2 3 rt at the immediate contact of the xenoliths and in melt infiltration zones to the partial replacement of opx2 spr2 symplectites by biotite and sillimanite and or plg3 mainly at the expense of orthopyroxene with concomitant coarsening of the intergrowth texture the subsequent deformation stage 4 further modified the symplectite textures through polygonization recrystallization and grain size coarsening the deformation was followed by a period of cooling and decompression stage 5 c 800 c 4 7 kbar as indicated by local growth of late garnet grt5 at the expense of opx spr plg domains at static conditions recently published isotope data suggest that the multistage evolution of the high mgal granulites at anakapalle followed a discontinuous p t trajectory that may be related to heating of the crust through magmatic accretion culminating in deep crustal uht metamorphism at 1 4 ga stage 1 fast uplift of the uht granulites into mid crustal levels as a consequence of extensional tectonics stage 2 emplacement of felsic magmas in the grenvillian at c 1 ga stage 3 resulting in reheating of the crust to high t conditions followed by a phase of compressional tectonics stage 4 and a period of cooling to the stable geotherm stage 5 still in the grenvillian

excess argon in melt inclusions in zero age anorthoclase feldspar from mt erebus antarctica as revealed by the method abstract historically erupted 1984 anorthoclase phenocrysts from mt erebus yield k ar and 40 ar 40 ar apparent ages as old as 700 ka indicating the presence of excess argon 40 ar 39 ar furnace step heating results from anorthoclase reveal a positive correlation between the cl k ratio and apparent age because chlorine up to 1700 ppm is present in melt inclusions but not in the anorthoclase crystal lattice this correlation suggests that the excess argon is associated with melt inclusions trapped within the anorthoclase during rapid crystal growth confirmation of the source of excess argon comes from step heating experiments on multiple anorthoclase aliquots separated from two phenocrysts and one glass aliquot prepared from the matrix of a volcanic bomb the anorthoclase phenocrysts were crushed and hf etched to yield aliquots containing 30 10 and 1 melt inclusions the step heated anorthoclase with 30 and 10 melt inclusions yielded the highest cl k ratios and apparent integrated ages of 640 30 ka and 179 16 ka respectively the anorthoclase with i melt inclusions yielded significantly lower cl k ratios and apparent integrated ages of 48 8 ka and 50 30 ka the step heated volcanic glass yielded the least variable cl k ratios and a total gas age of 101 16 ka argon released from the anorthoclase and the trapped melt inclusions can be distinguished by differences in their degassing behavior allowing geologically more reasonable ages to be obtained melt inclusions exposed on the exterior of anorthoclase grains principally degas during furnace extraction at temperatures less than 1200 c inclusions entirely within anorthoclase grains principally degas at temperatures greater than 1200 c when incongruent melting of the anorthoclase allows melt inclusion hosted excess argon to escape anorthoclase aliquots prepared with less than 1 inclusions can be fitted with a plateau for heating steps below 1200 c to yield ages as young as 8 2 ka whereas steps above 1200 c yield ages in excess of 100 ka however anorthoclase aliquots containing 10 30 melt inclusions yield ages in excess of 200 ka for heating steps below 1200 c minimizing the effects of the excess argon from melt inclusions relies on sample preparation and step heating fine crushing and treatment with hydrofluoric acid removes many of the larger melt inclusions small melt inclusions which remain within the anorthoclase degas primarily at temperatures above 1200 c temperatures below 1200 c yield the most accurate ages attempts at post analytically correcting for the chlorine correlated excess argon are hindered by the variations in 40 ar e cl within and between samples elevated 40 ar e cl ratios in bubbles within the melt inclusions as deduced from in vacuo crushing experiments are the most likely cause for some or all of a sample s total 40 ar e cl variation in addition relative solubilities of argon and chlorine within phonolitic melts may be partly responsible for variations in 40 ar e cl

the effects of deformation induced microstructures on intragrain 40ar 39ar ages in potassium feldspar a combination of forescattered scanning electron microscopy and both ultraviolet uv laser and cycle stepped heating 40 ar 39 ar dating techniques have been applied to a single deformed grain of potassium feldspar to investigate the relationship between microstructure and 40 ar 39 ar isotope systematics the form and distribution of deformation induced microstructures have been mapped by using intragrain variations in lattice orientation these variations are related to thin section and regional structural data to provide a well constrained deformation history for the feldspar grain thirty uv laser 40 ar 39 ar analyses show age variations that correlate with the internal microstructure stepped heating data provide further evidence of this relationship our approach allows the recognition of different ar reservoirs within the deformed feldspar and identifies microstructural controls on ar migration such a relationship has important implications for the understanding and interpretation of 40 ar 39 ar geochronological data and the subsequent use of such data to constrain the temperature vs time evolution of deformed rocks

intercrystalline cation partitioning between minerals of the triplite zwieselite magniotriplite and the triphylite lithiophilite series in granitic pegmatites minerals of the triphylite lithiophilite li fe mn po4 and the triplite zwieselite magniotriplite series mn fe mg 2po4f occur in the late stage period of pegmatite evolution unfortunately neither are the genetic relationships between these phosphates fully understood nor are thermodynamic data known consequently phosphate associations and assemblages from 8 granitic pegmatites clementine ii rubicon ii and iii and tsaobismund namibia hagendorf sud and rabenstein germany valmy france viitaniemi finland have been tested for compositional zoning and intercrystalline partitioning of main elements by electron microprobe techniques although the selected pegmatites display varying degrees of fractionation and the intergrowth textures indicate different genetic relationships between the phosphates the plots of mole fractions xfe fe fe mn mg ca xmn mn fe mn mg ca and xmg mg fe mn mg ca can be fitted relatively well with smooth curves in roozeboom diagrams their deviations from symmetrical distribution curves are mainly dependent upon xmg or xca and upon non ideal solutions surprisingly small differences between the partition coefficients were detected for intergrowths of different origin however the partitioning of shared components among coexisting phases is clearly dependent upon the conditions of formation compositional zoning is observed only when both fe mn phosphates are intergrown mutually or with other fe mn mg mineral solid solutios thus the zoning does not seem to be due to continuous crystallization but to later diffusion processes the triplite structure has preference for mn mg and ca while fe prefers minerals of the triphylite series a quantification of main element fractionation between minerals of the triphylite and the triplite series is possible in the cases where diffusion can be excluded for the fe fe mn ratios of core compositions an equation with a high correlation coefficient r 0 988 was determined fe fe mn tr fe fe mn li 2 737 1 737 fe fe mn li tr triplite series li triphylite series consequently the fe fe mn ratio of the triplite series can now also be used in the interpretation of pegmatite evolution just like that of the triphylite series which has been successfully applied in the past

age of mineralization by basinal fluids at the el soldado manto type copper deposit chile 40ar 39ar geochronology of k feldspar abstract manto type copper deposits of the southern central andes are strata bound epigenetic mineral concentrations hosted by bimodal volcanic and volcaniclastic rock suites they are economically important but since they are not obviously related to intrusions their genesis has been controversial to some extent because it has proven difficult to date the ore forming event and distinguish it from the age of low grade regional metamorphism el soldado the largest manto type copper deposit in central chile is hosted by marine volcanic and subvolcanic intrusive rocks of the lower cretaceous extensional arc to discriminate between regional metamorphism and mineralization we dated 11 samples of k feldspar by the stepwise heating 40 ar 39 ar method hydrothermally precipitated k feldspar associated with copper sulfides precipitated at temperatures of 300 c and range in age from 100 5 1 5 to 106 1 1 1 ma mean 103 1 6 ma n 8 syn mineralization k feldspar gave an age of 102 5 2 0 which is interpreted to be the age of mineralization k feldspar unrelated to ore yields older ages between 109 4 1 1 and 112 2 ma mean 110 3 1 4 ma n 3 is variably reset by the younger ca 103 ma mineralization episode the above ages are interpreted to represent the age of alkali redistribution and crystallization due to burial metamorphism of the host sequence fission track dating of apatite in the host rocks yields an age of ca 90 ma indicating fast cooling of the system to less than ca 100 c after mineralization time temperature modeling of fission track length data are compatible with initial fast exhumation followed by slow exhumation and cooling to surface temperatures high levels of atmospheric ar were detected in all k feldspar samples and are tentatively attributed to connate meteoric fluids incorporated into the basinal metamorphic mineralizing fluids

metamorphism and tectonics of the himalaya metamorphism connected with the main growth and deformation stages of the himalaya ranges from mid cretaceous to the quaternary northward subduction gave rise in the early to mid cretaceous to an island are complex with greenschists amphibolites granulites and blueschists in the w kohistan ladakh and an andean type margin with greenschist amphibolite grade metamorphism in the e india and tibet ophiolitic nappes were thrust southwards over the indian continental margin in the palaeogene during the final stages of closure of the southern arm of tethys the indus zangbo suture bifurcates westwards and between the two sutures lies the kohistan ladakh island arc which was converted to an andean type arc in the palaeocene eocene the karakorum range underwent northward subduction at least during the late cretaceous to give rise to a complex and prominent calc alkaline batholith post collisional southward thrusting of crustal slabs over the indian continental margin took place in the early miocene with production of high grade metamorphism inverted isograds and crustal melt granites later southward thrusting in the pliocene to quaternary gave rise to localized low grade recrystallization against the thrusts

physical constraints on the migration of immiscible fluids through partially molten silicates with special reference to magmatic sulfide ores the migration of dense fe rich immiscible liquids through partially molten silicates is of critical importance to several issues including the formation of magmatic sulfide ore deposits and the formation of metallic cores in terrestrial planets the migration of immiscible dense liquids can be considered to occur in three distinct regimes dictated by the relative sizes of the dense liquid droplets and the crystals making up the silicate crystal mush or partially melted silicate rock if dense microdroplets are smaller than the diameter of the narrowest portions of pore throats between silicate crystals then downward motion is possible the maximum rate of downward settling in loosely packed crystal mushes occurs when the droplet size is approximately one tenth the crystal size as long as the silicate melt between crystals remains microdroplets are capable of migrating distances of hundreds to thousands of meters vertically through crystal mushes contained in cooling dikes with typical grain sizes stranding of larger droplets similar in size to the crystals results from capillary forces preventing droplet deformation as they attempt to pass into pore throats narrower than themselves only in very coarse grained mushes with grain sizes greater than about 2 cm can droplets the size of intergranular pores migrate downwards on the other hand coalescence of many microdroplets into pre existing stranded droplets may generate connected net textured domains of the dense liquid that are many times larger than the grain size of the mush when the vertical height of the connected domain is great enough the pressure gradient inside the dense phase exceeds the capillary force impeding downward motion through narrow pore throats and the immiscible phase is able to move down along vertically oriented networks these networks should not suffer stranding until they reach a solid layer or the base of the intrusion we suggest that in most cases sulfide droplets formed by fractional segregation in large magma bodies should come to rest at the top of the crystal pile whereas sulfides introduced to magmas by assimilation of fine grained sulfidic sediments should be capable of passing downward through crystal mushes to form massive or net textured accumulations at the bottom of the partially molten zone

fluid expulsion from overpressured basins implications for pb zn mineralisation and dolomitisation of the east midlands platform northern england abstract our understanding of burial diagenesis within carbonates is often limited by poor constraints on available fluid volumes and geochemistry however regional stratigraphic and burial history data are often readily available using these data to develop numerical models which couple sedimentological and hydrological basin evolution we estimate the volumes and geochemistry of fluids that were available to drive dolomitisation and pb ore genesis within the carboniferous derbyshire platform of northern england current conceptual models of these processes invoke tectonic release of burial induced overpressure developed within adjacent dinantian basins as a drive for reactive fluid flow to the platform our simulations show that compaction driven flow may lead to a supply of fluids that is more complex in its temporal evolution than may be expected spatial variations in the rate of fluid expulsion from different sediments lead to a staggered delivery of fluids from different sources rapid fluid expulsion within deeper sediments leads to a downwards decreasing pressure gradient that subsequently draws down fluid from within overlying sediments thus early fluid supply to the platform is sourced from the deep basins while later fluid supply descends from above the platform as well as from the sides we suggest that such a flow development may have important implications for the relative timing and distribution of a sequence of diagenetic products within the platform this hypothesis is tested using volume estimates from our simulations we conclude that although this staggered fluid supply model may be applicable generically it appears that it is only valid for explaining mineralisation in the derbyshire platform fluid volumes supplied to the platform are insufficient to explain dolomitisation our simulations are supported by a sensitivity analysis that identifies that compaction driven flow in this system is strongly controlled by the rate of burial and sediment permeabilities within the compacting basins

oxygen isotope heterogeneities and diffusion profile in composite metamorphic magmatic garnets from the pyrenees unusually large 1 8 o heterogeneities 4 within single crystals are reported in garnets from dioritic migmatites in the pyrenees these heterogeneities together with contrasting ca and p zoning allow the identification of different growth zones garnet cores with high 1 8 o values 12 14 are relatively poor in ca 7 9 mol grs and rich in p 400 900 ppm p 2 o 5 in contrast garnet rims with lower 1 8 o values 7 12 o are richer in ca 12 14 grs and poorer in p 100 200 ppm these growth zones can be ascribed to a metamorphic event followed by crustal partial melting and contamination by magmas from the mantle high 1 8 o intra crystalline contrasts result from mineral growth in an open magmatic system involving the interaction of partial melts with distinct 1 8 o signatures at the garnet core rim interface compositional profiles in major divalent cations are consistent with the relaxation of an initial sharp step in ca fe and mg by ca fe mg interdiffusion at the same interface an o isotope profile is documented the analogy of ca and o isotope profiles suggests that the 1 8 o distribution may also result from a diffusion process in this particular case temperature garnet composition oxygen fugacity o diffusion appears to be of the same order of magnitude as ca fe mg interdiffusion considering a duration of 10 ma for the plutono metamorphic event in the pyrenees ca and o diffusivities in the range 10 2 2 m 2 s at 850 c are retrieved from the measured profiles like ca o diffusion in garnet at magmatic temperatures 850 900 c is both slow enough to preserve large 1 8 o heterogeneities and fast enough to generate relaxation profiles

troodos extrusive series of cyprus a comparison with oceanic crust the extrusive section on the northern flank of the troodos ophiolite is 1 000 to 1 500 m thick and consists of about 12 lithologic units of pillowed and massive lava and breccia some pillowed units which average 100 to 200 m in thickness vary systematically from lava tubes up to 6 m in diameter at the base through common pillows less than 1 m in diameter to small pillows and extrusive breccias at the top these assemblages appear to be individual volcanoes suggesting episodic volcanic accretion the upper 100 to 200 m of the section is pervasively altered and oxidized but fresh volcanic glass is preserved throughout most of the extrusive section the lavas are subdivided on geochemical evidence into a lower differentiated andesite to dacitic andesite series resembling evolved island arc tholeiites and an upper series of picrite mafic basalt and basaltic andesite very low in tio2 k2o p2o5 nb rb and ba the highly depleted basalt and basaltic andesite resemble lavas from ocean crust near island arcs lateral continuity of lithologic units interpreted as individual volcanoes and the relatively constant dips of the extrusive rocks suggest that tilting and rotation have not played major roles in the accretion process the occurrence of fractionated lavas at the base and primitive lavas at the top suggests a broad zone of accretion located above a subduction zone

fluid origin and migration of the huangshaping w mo polymetallic deposit south china geochemistry and 40ar 39ar geochronology of hydrothermal k feldspars abstract the huangshaping deposit is a world class w mo pb zn cu polymetallic deposit that formed during the jurassic magmatic event in the central nanling region south china in order to reveal the three dimensional development of mineralization and alteration in this complicated fluid rock system four typical hydrothermal k feldspar samples were collected from potassic alteration zones at different elevation levels around main w mo ore bodies followed by precise trace element analysis and 40 ar 39 ar dating the results show that these k feldspars have a pronounced lanthanide tetrad effect with high contents of ga ta nb u y and hree but low concentrations of ba sr eu and zr suggesting a hydrothermal origin additionally correlation between the ca cl k ratios and the apparent ages suggest that metasomatic fluids played an important role in the formation of the hydrothermal k feldspars the plateau 40 ar 39 ar ages which represent the timing of potassic alteration vary from 152 9 0 5 to 154 3 0 6 ma spatially showing younger trends from north to south and from shallow to deep these dates correspond to the previous published molybdenite re os data variations in different elevation levels which record the mineralization event with relatively older ages ranging from 153 8 to 159 4 ma combined with the spatial relation analysis among samples the fluid migration rates associated with the mineralization and alteration are calculated at 10 100 m ma 1 thus we propose a fluid downward migration model for the huangshaping polymetallic deposit

40ar 39ar thermochronometry of k feldspar from the ktb borehole germany abstract the great depth and relatively steep geothermal gradient 29 c km of the ktb deep crustal borehole located in the bavarian region of germany provides the basis for our study of diffusive argon loss in k feldspar the observed borehole temperature at 9 km is 265 c which reaches well into the argon closure interval for k feldspar 125 350 c four microcline and two adularia samples distributed relatively evenly down the 9 1 km deep borehole were analyzed by the 40ar 39ar method the thermal histories of the shallow samples are consistent with existing thermochronologic data from the borehole after correction for chlorine correlated excess 40ar the deepest two samples result in age spectra that record zero apparent ages over the first few percent of 39ar released which is predicted by diffusion theory where a range of diffusion volumes are present inverse modeling of the argon kinetics of these two deep samples reveals that the present elevated geothermal gradient in the borehole has existed for only the last approximately one million years recent volcanism within 30 km of the borehole is likely the source of this present day thermal pulse

whiteschists and orthoamphibole cordierite rocks and the p t t path of the modum complex south norway abstract this paper describes the second known occurrence of whiteschist from norway whiteschists and orthoamphibole cordierite rocks which can both be described in the m f ash system are found in the modum complex together with albitites in contact zones between metagabbros and supracrustals they are formed due to extensive hydrothermal activity bulk rock and mineral chemistry define a bimodal fe mg distribution talc kyanite schists and fe poor orthoamphibole cordierite rocks form a leucocratic group and fe rich orthoamphibole cordierite rocks a melanocratic group the break down of an early orthoamphibole cordierite paragenesis to a later talc kyanite paragenesis indicates a cooling path at isobaric conditions or under increasing pressure the temperature pressure regime is of the order of 600 750 c and 6 8 kb reaction textures with talc albite kyanite forming pseudomorphs after gedrite indicate excess albite these nonisochemical reactions indicate the presence of an active na bearing fluid at deep levels in the crust

is the vourinos complex an island arc ophiolite abstract a geochemical study has been undertaken on the vourinos ophiolites northern greece a complex long known for its unusual characteristics such as an environment of acidic rocks and a calc alkaline chemical affinity the nd sr isotopic ratios and the hf th and ta th ratios are indicative of an island arc origin for vourinos as opposed to the mid oceanic ridge origin inferred for other ophiolites such as inzecca corsica other data on trace elements confirm that the cumulative suite and the lavas originated from the same magma through a simple fractional crystallization process and show that this magma would have formed through partial melting of an already highly depleted material it is thus possible to distinguish ophiolites with morb characteristics from island arc ophiolites such as the vourinos complex the existence of the latter type imposing new constraints on the possible tectonic processes for emplacement

ellenbergerite a new high pressure mg al ti zr silicate with a novel structure based on face sharing octahedra ellenbergerite occurs as purple millimetre size grains associated with talc kyanite clinochlore rutile and zircon in composite inclusions within decimetre large pyrope crystals 90 98 mole percent end member in the quartzite layer of the dora maira massif western alps from which coesite has been recently reported chopin 1984 it is hexagonal a 12 255 8 c 4 932 4 a z 1 space group p63 mohs hardness 6 5 dmes 3 15 dcal 3 10 no cleavage uniaxial negative and vividly pleochroic colourless e colourless to deep lilac with colour zoning the intensely coloured variety has 1 6789 5 e 1 670 1 microprobe analysis yields sio2 39 1 p2o5 0 45 al2o3 25 1 tio2 4 0 mgo 22 2 feo 0 20 sum 99 05 wt including h2o 8 0 coulometrically the formula calculated on a o28 oh 10 basis implying 7 5 wt h2o is mg6 71 fe0 03 ti0 61 al6 00 si7 92 p0 08 o28 oh 10 the colour zoning is due to nearly complete ti zr substitution in addition ellenbergerite may contain more than 8 wt p2o5 with strictly correlated changes of si mg al and ti zr contents over 80 of which represent the sial pmg substitution

multiple mesozoic mineralization events in south china an introduction to the thematic issue mesozoic mineral deposits in south china include world class deposits of w sn and sb and those that provide the major sources of ta cu hg as tl pb zn au and ag for the entire country these deposits can be classified into polymetallic hydrothermal systems closely related to felsic intrusive rocks sn w mo granites cu porphyries polymetallic and fe skarns and polymetallic vein deposits and low temperature hydrothermal systems with no direct connection to igneous activities mvt deposits epithermal au and sb deposits recent studies have shown that they formed in the triassic indosinian jurassic cretaceous early yanshanian and cretaceous late yanshanian stages indosinian deposits include major mvt pb zn ag deposits and granite related w sn deposits early yanshanian deposits are low temperature sb au and high temperature w sn and cu porphyry types many late yanshanian deposits are low temperature au as sb hg and u deposits and also include high temperature w sn polymetallic deposits the formation of these deposits is linked with a specific tectonothermal evolution and igneous activities this special issue brings together some of the latest information in eight papers that deal with the origins and tectonic environments of mineral deposits formed in these stages we anticipate that this issue will stimulate more interests in these ore deposits in south china

zircon morphology geochronology and trace element geochemistry of the granites from the huangshaping polymetallic deposit south china implications for the magmatic evolution and mineralization processes abstract the huangshaping polymetallic deposit is a large scale deposit located in the central nanling region south china different types of mineralization have been classified in the area which are attributed to three types of granitoids from the composite granitic pluton quartz porphyry granophyre and granite porphyry in order to study the magmatic genesis and evolutionary process as well as constrain the crystallization and mineralization events 100 zircon separates from different intrusive stages of the pluton have been analyzed for trace element contents and u pb isotopic concentrations the results show that zircons from the huangshaping granitoids are enriched in th u hf ti ree and other trace elements indicating hydrothermal fluid alteration and transformation additionally there is a large variation between different types of granitoids in terms of trace element concentrations compared to the zircons from the quartz porphyry and the granophyre zircons from the granite porphyry are more enriched in th u hf ti and p but strongly depleted in eu this suggests that the magma that formed the granite porphyry possessed higher temperatures possibly experienced stronger crystal fractionation and originated from a more evolved complex magmatic source region with higher degree of crustal maturity combined with characteristics of crystal morphology and internal texture hydrothermal zircons were distinguished from magmatic zircons in each granitoid to reveal mineralization processes the u pb geochronology results show that there are two major stages of mineralization 190 ma weak cu pb zn mineralization related to the quartz porphyry and the granophyre and 155 ma strong w sn mo pb zn mineralization associated with the granite porphyry compositions of these hydrothermal zircons suggest that the earlier stage mineralization could be close to single relatively cold reduced and quickly ascended fluids whereas the later stage might be related to hot oxidized strongly fractionated and highly evolved mixed fluids this study also indicates that hydrothermal zircons from typical composite granitic plutons could be a good tool to reveal the multi stage magmatic and mineralization process in middle to late jurassic south china