sapphirine bearing orthopyroxene kyanite sillimanite granulites from south harris nw scotland evidence for proterozoic uht metamorphism in the lewisian sapphirine bearing orthopyroxene kyanite opx ky and sillimanite opx sil granulites have been found in the lewisian complex of south harris in northwest scotland in the opx ky granulites orthopyroxene and kyanite are intergrown in a stable mineral assemblage which indicates metamorphic condition at 800 900 c 12 kbar sillimanite inclusions within orthopyroxene suggest that sillimanite formed earlier conditions are estimated at 950 30 c at 10 kbar from orthopyroxene isopleths for aluminous orthopyroxene 8 kbar according to the fmas petrogenetic grid and similar conditions were obtained by using orthopyroxene garnet geothermobarometers the two types of orthopyroxene aluminosilicate granulites indicate that the peak metamorphic conditions were over 900 c compatible with ultra high temperature metamorphism as accessory sapphirine occurs in several assemblages and with different compositions it is interpreted to be formed at different stages of the metamorphism these granulites were formed during early proterozoic high grade metamorphism due to the emplacement of the south harris igneous complex at c 2170 1870 ma and are not related to the major metamorphic episode of the badcallian inverian metamorphism at c 2700 2500 ma in the mainland lewisian

sm nd evidence for diachronous crustal accretion in the lewisian complex of northwest scotland sm nd whole rock analyses of several cogenetic rock suites from the late archaean lewisian complex of northwest scotland yield chronological data for a crustal accretion differentiation superevent cads in this terrane three layered mafic ultramafic bodies predate badcallian high grade metamorphism 2660 ma but their temporal relationship to the igneous precursors of the host quartzofeldspathic gneiss complex remains unclear regression of data from these samples yield the following achiltibuie 2850 95 ma with nd t 1 93 0 52 drumbeg 2910 55 ma with nd t 1 70 0 32 scouriemore 2670 110 ma with nd t 0 98 0 73 these data suggest that contemporaneous sub lewisian mantle had a long term light rare earth element depleted history relative to a chondritic reservoir composition three suites of quartzofeldspathic gneisses the pyroxene granulites of scourie the hornblende granulites of gruinard bay and the amphibolites of the northern region north of loch laxford have small ranges of sm nd ratios and do not yield precise regression results if these gneisses were derived from a relatively homogeneous source region perhaps represented by the mafic ultramafic bodies then sm nd model ages suggest a diachronous sequence of crustal generation decreasing in age from scourie terrane average tdm 2930 ma through gruinard bay 2860 ma to the northern region 2780 ma a diachronous sequence such as this suggests that for a cads in which separate accretion and differentiation episodes may be resolved final high grade metamorphism may not be related to magmatic crustal generation processes

lewisian crustal history from in situ sims mineral chronometry and related metamorphic textures abstract a combination of detailed petrography and in situ sims 207 pb 206 pb micro chronometry of monazite has been carried out on samples from the lewisian high grade metamorphic terrain northwest scotland two episodes of granulite facies metamorphism have been recognized the thermal maxima of which are estimated at or before 2760 12 ma 2 and at 2526 8 ma respectively pegmatites at scourie crystallised at 2480 ma the amphibolite facies metamorphism of the main laxfordian event is estimated at 1752 12 ma and the formation of kyanite and staurolite is associated with this event the oldest ages of 2760 ma have been obtained from micro inclusions of monazite in garnet it appears that garnet which has a high closure temperature for the u pb system plays an important role in the behaviour of the monazite u pb system it serves to shield monazite from reaction with the other phases and to armour monazine from diffusive pb loss or from resetting

formation of sapphirine and corundum in scapolitised and mg metasomatised gabbro terra nova 22 166 171 2010 abstract sapphirine mg3 41fe0 11al4 45 al4 55si1 45 o20 and corundum are found as inclusions 0 2 mm in cl rich scapolite me24 25 and plagioclase an20 24 in the metasomatised parts of the odegarden gabbro bamble sector south norway microtextural relationships suggest that the overall reaction plagioclase mg cl sapphirine corundum scapolite ca fe took place in two stages possibly related to infiltration of two metasomatic fronts gresen analyses support that the scapolitisation was combined with a strong mg metasomatism and removal of fe and ca on a whole rock scale the ferromagnesian minerals associated with sapphirine in the scapolite metagabbro have especially high mg ratios orthopyroxene mg 0 96 phlogopite mg 0 95 amphibole mg 0 81 0 87 clinopyroxene mg 0 95 this is much higher than in biotite mg 0 60 and clinopyroxene mg 0 78 0 81 in the unscapolitised parts of the gabbro and comparable with that found in sapphirine bearing orthoamphibole cordierite schists in the region suggesting that they also formed by metasomatism a metasomatic origin for the sapphirine has consequences for models of crustal growth and metamorphic history

shale gas potential of the lower permian gufeng formation in the western area of the lower yangtze platform china abstract china is beginning to explore unconventional gas resources in the lower yangtze platform in the last two years we conducted undiscovered hydrocarbon resource assessment to investigate the geological and geochemical characteristic of the lower permian gufeng formation which is considered as one potential exploration layers in this area four outcrops and one well were used to determine the stratigraphic correlation and 40 available samples were used for an analytical investigation because of continuous changes in the sedimentary environment the shale of the gufeng formation not only includes black shale but also calcareous mudstone silicalite siliceous mudstone and carbonaceous shale the thickness of the gufeng formation is between 0 m and 200 m and it has two small depositional centers with an sw ne strike on plane the shale in the gufeng formation shows a high average toc total organic carbon content of 3 41wt based on test of 40 samples but the tocs in different tract systems have significant differences ehst early highstand systems tract average 9 85wt lhst late highstand systems tract average 0 96wt cs condensed section average 2 61wt tst transgressive systems tract average 0 76wt the shale is in the post mature stage of thermal maturity tmax temperature of maximum pyrolysis output 339 518 c and is in the dry gas window of hydrocarbon generation mean random vitrinite reflectance value ro 1 24 1 99 clay minerals dominate the mineralogy followed by quartz minerals pyrite only is found in some samples of ehst which is consistent with the presence of less anoxic conditions and lower levels of sediment dilution during sediment deposition the ehst shale of the gufeng formation which has a high content of organic carbon tentatively could be regarded as a potential shale gas exploration target in this area however because the complex multistage tectonic background could have caused gas loss the tectonic background and the hydrocarbon preservation condition must also be considered

aluminous granulites of the archean craton of kasai zaire petrology and p t conditions aluminous granulites of the archean 2 8 ga kasai craton zaire consist of two main mineral assemblages grt opx and sil grt crd rocks the high grade metamorphic conditions as deduced from grt opx and grt opx pl qtz equilibria are 720 c 6 7 kb considering the zoning of the same minerals the slope of the p t path is estimated at 15 b c thermobarometry involving crd is consistent with those p t conditions three cordierite forming reactions have been observed petrographically grt qtz crd opx r1 grt sil crd spl r2 grt sil crd mag r3 these equilibria are continuous reactions end member reactions have slopes less than 15 b c they are decompression reactions occurring after the metamorphic climax using available thermodynamic data r3 fixes the oxygen fugacity to a value below the qfm buffer log 10f o2 17 6 at 720 c 6 7 kb and xh2ofl 0 3 in the graphite stability field the absence of graphite in the rocks shows that the end of the granulite facies metamorphism did not occur under important co 2 streaming the polymetamorphic history of this archean craton is considered 1988 oxford university press

a sm nd isotopic study of the south harris igneous complex the outer hebrides sm nd geochronology may be used to bracket the age of metamorphism in rocks which are difficult to date by other methods by coupling whole rock sm nd analyses of the principal members of the south harris igneous complex with sm nd mineral isochrons on two anothositic gabbros the age of granulite facios metamorphism has been defined whole rock analyses of three pairs of closely spaced samples of the anorthosite give consistent ages averaging 2 18 0 06 gyr but in general the data from the anorthosite do not define an isochron as a result of variable contamination of the evolving magma chamber whole rock data on the tonalite indicate that it is younger than 2 06 gyr its mean tchur age is 1 86 0 05 gyr garnet pyroxene amphibole plagioclase mineral isochrons on two anorthosite samples give identical 1 87 0 04 gyr ages which date cooling after the high pressure granulite facies metamorphism together with the tonalite whole rock data this defines the age of that metamorphism and confirms dearnley s original assignment of an early laxfordian age

proposal for a terrane based nomenclature for the lewisian gneiss complex of nw scotland the current nomenclature for the lewisian gneiss complex has evolved from lithological and structural correlations made prior to any dating initial flawed geochronological studies gave some names an apparent chronological standing but as work advanced fitting events into a coherent regional framework became increasingly difficult modern dating studies have shown that the lewisian gneiss complex was progressively assembled from disparate blocks of archaean continental crust and juvenile proterozoic arcs which satisfy the definition of terranes each terrane had its own separate accretionary and metamorphic history followed by a common history once juxtaposed against other terranes based on a new compilation of modern geochronology allied to the many detailed structural and metamorphic studies this paper proposes a new systematic terminology for the lewisian gneiss complex that is more applicable to this new tectonic framework

sm nd dating and cooling history of scourian granulites sutherland the scourian gneisses of the lewisian complex of north west scotland have been the subject of many geochronological investigations sm nd whole rock measurements suggest that the protoliths of the lewisian complex differentiated 2 92 0 05 gyr ago from an approximately chrondritic mantle1 rb sr and pb pb whole rock ages between 2 8 and 2 6 gyr reflect subsequent depletion in rb and u refs 2 3 which has generally been associated with granulite facies metamorphism zircons which are thought to have crystallized during this metamorphism have also given u pb ages of 2 66 gyr ref 4 this granulite facies metamorphism involving pressures in excess of 10 kbar and peak temperatures estimated between 820 50 c ref 5 and 1 250 c refs 6 7 affects a terrain including supracrustal rocks7 and thus implies a major tectonic event we report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in sm nd ratios garnet especially is strongly enriched in sm ref 8 data for three garnetiferous basic gneiss assemblages suggest that closure of the sm nd system occurred significantly later than the peak of the metamorphism

the palaeoproterozoic anatomy of the lewisian complex nw scotland evidence for two laxfordian tectonothermal cycles a new structural examination of palaeoproterozoic high p granulites on south harris nw scotland when integrated with previous geochronological structural and metamorphic studies on key areas of the lewisian complex suggests the existence of two distinct tectonothermal cycles within the palaeoproterozoic laxfordian event which on south harris are separated by a 100 myr hiatus in deformation the older cycle from c 1 91 to 1 85 ga records the development of an active continental margin on the archaean gneisses that dominate the complex and the subsequent onset of continent continent collision this represents the continuation of the nagssugtoqidian orogen of greenland evidence for this is concentrated in allochthonous slivers of the former active continental margin displaced during the younger cycle the younger cycle around 1 75 1 65 ga began with thrust related crustal thickening that initiated regionally extensive amphibolite facies metamorphism and ductile deformation which dominates the preserved laxfordian deformation history this may be the peripheral expression of the accretion of the malin block to the sw of the lewisian and represents the lateral continuation of the labradorian ketilidian orogen of north america supplementary material additional figures are available at http www geolsoc org uk sup18863

tectono metamorphic events in the north atlantic region in the palaeoproterozoic from the view point of high grade metamorphic rocks in the lewisian complex south harris nw scotland abstract the tectono thermal history of the lewisian complex in south harris south harris complex was inferred from its geologic and metamorphic characteristics the lithological assemblages and geochemical features of the complex suggest that its precursory rocks were composed of the subduction related accretionary complex formed in the palaeo convergent margin the complex has suffered the ultra high temperature uht metamorphism that was contemporaneous with the igneous activity to make the south harris igneous complex shic and the subsequent continent continent collisional activity a similar complex recording the geological processes of the subduction the uht metamorphism and the collision has been recognized in the lapland kola belt and new quebec in the palaeoproterozoic this suggests an assembly of micro continents to form the palaeoproterozoic supercontinent in the north atlantic region

pan african subduction collision event evidenced by high p coronas in metanorites from the agou massif southern togo the ultrabasic basic complexes from se ghana and togo to n b enin are generally considered as an unique lithotectonic unit which may decorate the suture zone of the pan african collision belt in the southern region of togo the agou massif is one of these massifs mainly consisting of granulite facies rocks metagabbronorites and metagabbros outcrop in the innermost part of the agou igneous complex aic whereas in the outermost part relict boudins of metagabbronorites are surrounded by related retrogressed rocks such as layered grt bearing amphibolites and amphibolites the latter are intruded by leucocratic to melanocratic veins including dykes of microgabbros post solidus cooling of the metagabbronorites leads to the production of ht mp granulites m1 granulite stage opx1 cpx1 pl1 ilm sil t 850 c p 8kbar following this an evolution toward high pcoronitic granulites is indicated by early cpx and grt coronite developed around opx1 and pl1 granulite 2 stage cpx2 grt2 pl2 qtz sil rt 770 c 10kbar three successive stages of retrogression are then observed from i hornblende granulite facies 700 750 c 9kbar ii to amphibolite facies 700 c 6 7kbar and iii finally greenschist facies conditions 550 500 c 3 4kbar the inferredp tpath defines an unusual anti clockwise loop characterized by a slight pressure increase according to their calc alkaline signature and their metamorphic evolution the granulites of agou are derived from deep seated basic intrusion equilibrated in the lower continental crust under mp ht conditions and never experienced an early eclogite stage this contrasts with granulites derived from eclogites that are also described in sw togo in order to explain the existence of two types of metabasites in togo we propose that the agou massif corresponds to basic magmas emplaced and cooled at the base of an active continental margin western part of the benino nigerian shield whereas the former eclogites reflect subduction of the passive margin of the west african carton during subduction process or at the transition from subduction to collision at c 610ma burial of the edge of the active margin in the subduction zone led to the formation of coronitic granulites during the pan african orogeny at c 580ma the coronitic granulites together with the eclogites were exhumed under amphibolite facies conditions 2004 elsevier b v all rights reserved

low pressure temperature stability of pyrope quartz relative to orthopyroxene kyanite a new model for aluminous orthopyroxene with vacancies determination of the stability of pyrope with quartz could provide theoretical pressure temperature p t constraints on silica saturated eclogites as well as near end member pyrope bearing terrains the lower p t stability of pyrope in the presence of quartz in the system mgo al2o3 sio2 can be modeled by the reaction 3 enstatite 2 kyanite 2 pyrope 2 quartz reaction reversals were experimentally obtained using a in diameter piston cylinder press and a 1000 t multi anvil press besides natural quartz all other starting materials including orthopyroxene pyrope and kyanite were synthesized the reaction has been bracketed at 1 50 gpa at 1040 1080 c 1 69 1 79 gpa at 1000 c 2 26 gpa at 920 940 c 2 90 gpa at 840 860 c the bracketed boundary is higher in temperature and pressure than that calculated using the newest dataset of holland et al j petrol 59 881 900 https doi org 10 1093 petrology egy048 2018 particularly at higher pressures aluminum is usually thought to be taken into orthopyroxene in the form of mg tschermak s pyroxene mgts mgal2sio6 under these experimental p t conditions in contrast the observed orthopyroxene composition indicated the involvement of a non stoichiometric component mg eskola pyroxene mges mg0 5 0 5alsi2o6 an aluminous orthopyroxene with octahedral vacancies end member thermodynamic properties for mges as well as a mixing model for the ternary enstatite mgts mges solid solution have been derived the thermodynamic calculations employing the mixing model are in good agreement with the experimental brackets the new model has potential applications to constrain p t conditions of tectonic settings such as ultrahigh pressure and ultrahigh temperature metamorphisms

association of 34s depleted pyrite layers with negative carbonate δ13c excursions at the permian triassic boundary evidence for upwelling of sulfidic deep ocean water masses 1 a marine permian triassic boundary ptb section at nhi tao vietnam contains a series of at least 9 pyritic horizons characterized by concurrent decreases in pyrite s 34spy and carbonate c isotopic compositions 13ccarb the first and largest of the events that precipitated these pyritic horizons was coincident with the late permian mass extinction while subsequent events were generally smaller and occurred at quasiperiodic intervals of 20 to 30 ka a near complete lack of organic carbon to drive bacterial sulfate reduction in sediment pore waters among other considerations argues against a diagenetic control for these relationships rather the covariant patterns documented herein are most easily explained as the product of recurrent upwelling of anoxic deep ocean waters containing 34s depleted hydrogen sulfide and 13c depleted dissolved inorganic carbon the sulfide 34s record of the study section represents a mixture of a small amount of isotopically heavy authigenic pyrite formed via in situ bacterial sulfate reduction with a generally larger quantity of isotopically light syngenetic pyrite precipitated within the water column during upwelling episodes although upwelling of toxic deepwaters has been invoked in earlier studies as a mechanism for the late permian marine mass extinction this is the first study to 1 document patterns of pyrite 13ccarb covariation that strongly support upwelling as a major process at the ptb and 2 provide evidence of multiple quasiperiodic upwelling events that may reflect reinvigoration of global ocean overturn following a prolonged interval of late permian deep ocean stagnation

two stage decompression in orthopyroxene sillimanite granulites from forefinger point enderby land antarctica implications for the evolution of the archaean napier complex magnesian metapelites of probable archaean age from forefinger point sw enderby land east antarctica contain very high temperature granulite facies mineral assemblages which include orthopyroxene 8 9 5 wt al2o3 sillimanite garnet quartz k feldspar that formed at 10 1 5 kbar and 950 50 c these assemblages are overprinted by symplectite and corona reaction textures involving sapphirine orthopyroxene 6 7 wt al2o3 cordierite and sometimes spinel at the expense of porphyroblastic garnet or earlier orthopyroxene sillimanite these textures mainly pre date the development of coarse biotite at the expense of initial mesoperthite and the subsequent formation of orthopyroxene 4 6 wt al2o3 cordierite plagioclase rinds on late biotite the early reaction textures indicate a period of near isothermal decompression at temperatures above 900 c decompression from 10 1 5 kbar to 7 8 kbar was succeeded by biotite formation at significantly lower temperatures 800 850 c and further decompression to 4 5 1 kbar at 700 800 c the later parts of this p t evolution can be ascribed to the overprinting and reworking of the forefinger point granulites by the late proterozoic c 1000 ma rayner complex metamorphism but the age and timing of the early high temperature decompression is not known it is speculated that this initial decompression is of archaean age and therefore records thinning of the crust of the napier complex following crustal thickening by tectonic or magmatic mechanisms and preceding the generally wellpreserved post deformational near isobaric cooling history of this terrain

theoretical phase relations involving cordierite and garnet revisited the influence of oxygen fugacity on the stability of sapphirine and spinel in the system mg fe al si o the theoreticalp t grid for stability relations of the phases cordierite cd sapphirine sa hypersthene hy garnet ga spinel sp sillimanite si and quartz qz of hensen 1971 has proved useful in the interpretation of metamorphic mineral assemblages formed at low oxygen fugacity both experimental data and evidence from natural rocks indicate that at high oxygen fugacity compatability relations change as a result of the enlargement of the stability field of spinel which causes a topological inversion and the stabilisation of the invariant points sa ga and cd this implies the stable existence of the univariant equilibria for f text o text 2 buffered conditions sp qz ga hy si o2 sa cd cd sp qz hy si o2 sa ga and sa sp qz hy si o2 ga cd and the divariant reaction sp qz hy si o2 sa ga cd these redox equilibria are restricted to conditions of high oxygen fugacity the proposed theoreticalp t grids for both low and high oxygen fugacity satisfactorily explain all experimental data and metamorphic mineral assemblages so far found in granulites

mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions 1 sedimentary molybdenum mo s has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic rich marine facies deposited under oxygen depleted conditions a detailed analysis of mo s total organic carbon toc covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that 1 mo s while useful in distinguishing oxic from anoxic facies is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and 2 patterns of mo s toc covariation can provide information about paleohydrographic conditions especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal these inferences are based on data from four anoxic silled basins the black sea framvaren fjord cariaco basin and saanich inlet and one upwelling zone the namibian shelf representing a spectrum of aqueous chemical conditions related to water mass restriction in the silled basin environments increasing restriction is correlated with a systematic decrease in mo s toc ratios from 45 5 for saanich inlet to 4 5 1 for the black sea this variation reflects control of mo s by mo aq which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal the basin reservoir effect the temporal dynamics of this process are revealed by high resolution chemostratigraphic data from framvaren fjord and cariaco basin sediment cores which exhibit long term trends toward lower mo s toc ratios following development of water column stratification and deepwater anoxia mo burial fluxes peak in weakly sulfidic environments such as saanich inlet owing to a combination of greater mo aq availability and enhanced mo transport to the sediment water interface via fe mn redox cycling and are lower in strongly sulfidic environments such as the black sea and framvaren fjord these observations demonstrate that at timescales associated with deepwater renewal in anoxic silled basins decreased sedimentary mo concentrations and burial fluxes are associated with lower benthic redox potentials i e more sulfidic conditions these conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction e g silled basins and are not valid for low oxygen facies in open marine settings such as continent margin upwelling systems

thermotectonic history of the southeastern brazilian margin evidence from apatite fission track data of the offshore santos basin and continental basement abstract the santos basin is the largest offshore sedimentary basin in the southeastern brazilian margin and originated by breakup of west gondwana in the early cretaceous we carried out a new thermochronological study by apatite fission track analysis from borehole samples of the santos basin and its continental basement to constrain the tectonic history of the southeastern brazilian margin apatite fission track central ages of the basement and borehole samples vary from 21 0 1 8 to 157 0 35 0 ma and from 6 5 1 1 to 208 0 11 0 ma respectively from thermal modeling the basement samples reached the maximum paleotemperatures during the final breakup of south america and africa the onshore basement and offshore basin record an early thermotectonic event during the late cretaceous linked to the uplift and denudation of the serra do mar and serra da mantiqueira maturation of the organic matter in the offshore basin is related with the progressive increase of the geothermal gradient due to burial the thermal modeling indicates that the oil generation window started at 55 25 ma the basement samples experienced the final cooling during the cenozoic with an estimated amount of denudation linked to the sedimentary influx in the offshore basin a rapid cooling during the neogene becomes evident and it is linked to the reactivation along precambrian shear zones and change of the paraiba do sul drainage system

ocean stagnation and end permian anoxia ocean stagnation has been invoked to explain the widespread occurrence of organic carbon rich laminated sediments interpreted to have been deposited under anoxic bottom waters at the time of the end permian mass extinction however to a first approximation stagnation would severely reduce the upwelling supply of nutrients to the photic zone reducing productivity moreover it is not obvious that ocean stagnation can be achieved numerical experiments performed with a three dimensional global ocean model linked to a biogeochemical model of phosphate and oxygen cycling indicate that a low equator to pole temperature gradient could have produced weak oceanic circulation and widespread anoxia in the late permian ocean we find that polar warming and tropical cooling of sea surface temperatures cause anoxia throughout the deep ocean as a result of both lower dissolved oxygen in bottom source waters and increased nutrient utilization buildup of quantities of h2s and co2 in the late permian ocean sufficient to directly cause a mass extinction however would have required large increases in the oceanic nutrient inventory

archaean crustal development in the lewisian complex of northwest scotland the lewisian complex of northwest scotland is typical of many archaean terrains and has a well documented history starting 2 700 myr ago1 6 here we present new isotopic data that extend this history back to 3 300 myr and provide some insight into how the earliest continental crust may have formed the lewisian is dominated by tonalite trondhjemite and granodiorite ttg rocks which require a mafic lithospheric source rather than being direct mantle melts7 11 but the mafic and ultramafic rocks in the high grade granulite facies part of this terrain show little evidence of a significantly older crustal history12 13 and precursor material to the ttg lithologies has not yet been identified here we show that older amphibolite material has survived at a lower metamorphic grade coexisting amphibolite minerals yield indistinguishable 207pb 206pb and 147sm 143nd ages of 3 310 27 myr and 3 298 73 myr respectively these data are consistent with an origin for much of the lewisian terrain by the re melting of preexisting lithosphere with an isotopic signature similar to that of the amphibolites studied here

new shrimp u pb zircon ages and chime monazite ages from south harris granulites lewisian complex nw scotland implications for two stages of zircon formation during palaeoproterozoic uht metamorphism abstract sensitive high resolution ion microprobe shrimp u pb zircon age dating was applied to several types of granulites and gneisses from the lewisian complex in south harris nw scotland to clarify the timing of palaeoproterozoic tectono thermal events for comparison monazite was dated using the chemical th u total pb isochron method chime the obtained u pb zircon age data were evaluated under the constrains of the modal amount of constituent minerals and whole rock compositions of analysed samples the zircon internal texture based on th u ratios and the monazite chime ages zircons in three samples characterised by zoned overgrowth rims sector zoned grains and structureless high u cores yielded concordant ages of approximately 1900 ma 1909 3 ma 1899 10 and 1888 15 ma respectively the age of 1899 10 ma obtained from melanocratic granulite garnet and kyanite sillimanite 76 91 vol is interpreted to represent the timing of zircon crystallisation prior to the formation of restitic garnet and kyanite sillimanite therefore the age of 1900 ma is proposed to represent the timing of early metamorphism close to the thermal peak m1 in the leucocratic gneiss sample 93921 7 the age population at 1868 35 ma obtained from low u and low th overgrowth rims represents a second stage of zircon growth from crystallising anatectic melt m3 whereas the high u cores represent the timing of zircon growth and recrystallisation prior to anatexis monazites from two leucocratic gneisses yield a relatively narrow range of chime ages 1858 7 ma and 1840 7 ma these ages probably represent the timing of zircon and monazite crystallisation from partial melt and resetting of the u th pb system these results suggest that the timing of zircon formation was controlled by metamorphic processes such as restite formation and crystallisation from partial melt

petrologic and geochemical evidence for the formation of organic rich siliceous rocks of the late permian dalong formation lower yangtze region southern china abstract black shales are economically and scientifically important because they are commonly associated with high quality hydrocarbon source rocks and metal accumulations formed during periods of abrupt environmental change organic rich siliceous rocks of the late permian dalong formation developed extensively within middle upper yangtze region western yangtze platform however the formation mechanism of its formation in the lower yangtze region eastern yangtze platform remains a subject of debate due to a lack of outcrop exposure petrologic mineralogic and geochemical data are presented for three sections of the dalong formation within the lower yangtze region geochemical ratios including v v ni v cr ni co u th and v sc suggest that the dalong formation was deposited in an oxygen poor environment the elevated concentration of mo 100 ppm further suggests that free h2s existed in the bottom water which was favorable for the accumulation of large amounts of organic matter mo toc ratios reveal that water flow in the lower yangtze basin was moderately to strongly restricted during deposition of the dalong formation flow within the upper middle parts of the yangtze basin was also restricted suggesting that northern marginal basins on the yangtze platform formed part of a single ocean during the late permian i e changhsingian cherts of the dalong formation reflect high primary productivity similar to that of modern day upwelling zones which increased the amount of organic matter these results combined with previous work suggest that the long term oxygen poor conditions during deposition of the dalong formation were caused by restricted water circulation in a unique paleogeographic environment characterized by low latitude and submarine paleo highlands such settings of low latitude characterized by tropical climate favored the growth of siliceous zooplankton in the zones with submarine paleo highlands having created topographic barriers to water circulation restricted bottom water flow and oceanic anoxia during the late permian were likely the main controls on the accumulation of organic rich rocks elevated nutrient levels caused by frequent oceanic upwelling along the western margin of the south china craton and global volcanism at this time facilitated siliceous organism productivity

proterozoic anticlockwise p t path of the lewisian complex of south harris outer hebrides nw scotland the leverburgh belt and south harris igneous complex in south harris northwest scotland experienced high pressure granulite facies metamorphism during the palaeoproterozoic the metamorphic history has been determined from the following mineral textures and compositions observed in samples of pelitic quartzofeldspathic and mafic gneisses especially in pelitic gneisses from the leverburgh belt 1 some coarse grained garnet in the pelitic gneiss includes biotite and quartz in the inner core sillimanite in the outer core and is overgrown by kyanite at the rims 2 garnet in the pelitic gneiss shows a progressive increase in grossular content from outer core to rims 3 the alvi aliv ratio of clinopyroxene from mafic gneiss increases from core to rim 4 retrograde reaction coronas of cordierite and hercynite cordierite are formed between garnet and kyanite and orthopyroxene cordierite and orthopyroxene plagioclase reaction coronas develop between garnet and quartz 5 a p t path is deduced from inclusion assemblages in garnet and from staurolite breakdown reactions to produce garnet sillimanite and garnet sillimanite hercynite with increasing temperature and 6 in sheared and foliated rocks hydrous minerals such as biotite muscovite and hornblende form a foliation modifying pre existing textures the inferred metamorphic history of the leverburgh belt is divided into four stages as follows m1 prograde metamorphism with increasing temperature m2 prograde metamorphism with increasing pressure m3 retrograde decompressional metamorphism with decreasing pressure and temperature and m4 retrograde metamorphism accompanied by shearing peak p t conditions of the m2 stage are 800 30 c 13 14 kbar pressure increasing from m1 to m2 suggests thrusting of continental crust over the south harris belt during continent continent collision the inferred p t path and tectonic history of the south harris belt are different from those of the lewisian of the mainland

regional scale grenvillian age uht metamorphism in the mollendo camana block basement of the peruvian andes the mollendo camana block mcb is a 50 150 km precambrian inlier of the andean belt that outcrops along the pacific coast of southern peru it consists of stromatic migmatites of paleoproter ozoic heritage intensely metamorphosed during the grenville event c 1 ga u pb and u th pb ages on zircon and monazite in the migmatites aluminous mesosomes fmas and quartzofeldspathic leucosomes kfmash contain various amounts of k feldspar kfs orthopyroxene xmg opx 0 86 plagioclase pl sillimanite sil exceptionally kyanite ky ilmenite ilm magnetite mag quartz qtz and minor amounts of garnet xmg grt 0 60 sapphirine xmg spr 0 87 cordierite xmg crd 0 92 and biotite xmg bt 0 83 the ubiquitous peak mineral assemblage is opx sil kfs qtz grt in most of the mcb which together with the high al content of orthopyroxene 10 al2o3 and the local coexistence of sapphirine quartz attest to regional uht metamorphism 900 c at pressures in excess of 1 0 gpa fluid absent melting of biotite is responsible for the massive production of orthopyroxene that proceeded until exhaustion of biotite and most of the garnet in the southern part of the mcb mollendo cocachacra areas in this area a first stage of decompression from 1 1 1 2 to 0 8 0 9 gpa at temperatures in excess of 950 c is marked by the breakdown of sil opx to spr opx crd assemblages according to several bivariant fmas reactions high t decompression is also shown by mg rich garnet being replaced by crd spr and crd opx bearing symplectites and reacting with quartz to produce low al opx sil symplectites in quartz rich migmatites neither osumilite nor spinel quartz assemblages being formed isobaric cooling at about 0 9 gpa probably followed the initial decompression and proceeded with massive precipitation of melts towards the os invariant point as demonstrated by bt qtz pl symplectites in quartz rich migmatites melt opx sil bt grt kfs qtz finally opx rims around secondary biotite attest to late fluid absent melting compatible with a second stage of decompression below 900 c the two stages of decompression are interpreted as due to rapid tectonic denudation whereas the regional extent of uht metamorphism in the area probably results from large scale penetration of hot asthenospheric mantle at the base of an over thickened crust

the stability of sapphirine quartz and hypersthene sillimanite quartz assemblages an experimental investigation in the system feo mgo al 2 o 3 sio 2 under h 2 o and co 2 conditions phase relations and mineral chemistry involving the phases garnet gt spinel sp hypersthene hy sapphirine sa cordierite cd sillimanite sil and quartz qz have been experimentally determined in the system fmas feo mgo al2o2 sio2 under low fo2 and for various h2o co2 conditions several compositions were studied with 100 mg mg fe ratio ranging from 64 to 87 with excess quartz and sillimanite our data do not show any differences in gt cd stability and composition as a function of h2o co2 and h2o co2 ch4 content in good agreement with a previous experimental study at lower temperature aranovich and podlesskii 1983 at 1 000 c and 11 kbar under co2 saturated conditions cordierite grew from a crystalline mix unseeded with cordierite thus under water absent conditions cordierite will have a high p stability field in the presence of co2 if water has a pressure stabilizing effect on cordierite then our results would indicate that the effects of h2o and co2 are of the same magnitude at high temperature our data support the theoretical p t grid proposed by hensen 1986 for high t metapelites and are largely consistent with the high temperature experimental data of hensen and green 1973 the univariant boundary gt cd hy sil qz which marks the disappearance of hy sil qz assemblages has a negative dp dt slope above 1 000 c and a positive one below this temperature extrapolation of our data to iron free systems shows that the high p breakdown limit of mg cordierite has a negative slope in the range 1 025 1 300 c and probably positive below 1 000 c this indicates a maximum of stability for mg cordierite at around 1 000 c and 13 kbar because of the curvature of the univariant reactions en sil py qz mg cd en sil qz and gt cd hy sil qz the iron free invariant point involving the phases py en cd sil and qz probably does not exist sapphirine qz bearing assemblages are stable only at temperatures above 1 050 c at 1 075 c the joint gt sa is stable up to 11 kbar at higher pressure garnet sapphirine and quartz react according to the reaction gt sa qz hy sil reequilibrated sapphirines are more aluminous than the theoretical endmember mg2al4sio10 due to alal mgsi substitutions 100 al2o3 al2o3 feo mgo in experimental sapphirines ranges from 50 5 to 52 2 sapphirine in the assemblage sa cd sil qz shows a decrease in al content with decreasing temperature and pressure such that the alumina isopleths for sapphirine have a slight negative dp dt slope a similar decrease in al content of sapphirine with temperature is also observed in sa sil qz assemblages

polymetamorphic evolution of the lewisian complex nw scotland as recorded by u pb isotopic compositions of zircon titanite and rutile u pb isotopic relations in zircon and titanite of granulite and amphibolite gneisses in the lewisian complex and bordering laxford front reveal complex discordance patterns indicating multiple late archean and early proterozoic crystallization overgrowth and pb loss events the earliest stages in the evolution of the complex remain poorly resolved zircon ages of 2710 ma date high grade metamorphism and magmatism probably related to tectonic and magmatic accretion in a continental arc setting a distinct event at 2490 2480 ma possibly initiated by metamorphism and deformation at high grade conditions caused amphibolitization of the granulites and emplacement of granitic pegmatites this event can be correlated with development of inverian shear zones and formation of granitoid layers along the laxford front the emplacement of a younger generation of granitoid sheets during the laxfordian event fromed hydrothermal titanite at 1754 ma in gneisses south of the laxford bridge and partially reset older titanite at scourie growth of secondary titanite and rutile also occurred during subsequent low grade metamorphism at 1690 1670 ma

new evidence for protolith ages of lewisian granulites northwest scotland the granulite facies scourian rocks in the central region of the lewisian complex of northwest scotland play an important role in the understanding of the development of mid archean high grade gneiss complexes whereas the later largely proterozoic history of the scourian has been clarified in a detailed u pb study unraveling the archean ages of the protoliths using conventional isotope techniques has proved impossible in order to advance our understanding of the deep crustal processes responsible for these granulites it is essential to establish the age s of accretion of the protoliths and their subsequent tectonothermal history a combined cathodoluminescence cl and sensitive high mass resolution ion microprobe shrimp single zircon study has revealed a hitherto unrecognized morphological complexity in zircons from the type localities of the granulites use of cl allowed identification of relict oscillatory igneous zoning metamorphic overgrowths and irregular areas of recrystallization from the shrimp data an age of ca 2960 ma is inferred for the gneiss protoliths which were altered considerably during an important metamorphic event ca 2490 ma

p t grids for silica undersaturated granulites in the systems mas n 4 and fmas n 3 tools for the derivation of p t paths of metamorphism abstract considering the minerals cordierite cd sapphirine sa hypersthene hy garnet ga spinel sp sillimanite si and corundum co in the system feo mgo al2o3 sio2 fmas the stable invariant points are co ga cd and sa constraints imposed by experimental data for the system mas indicate that under low ph2o conditions the invariant points occur at high temperature 900 c and intermediate pressure 7 10 kbar this temperature is higher than that commonly advocated for granulite facies metamorphism in granulites fe mg exchange geothermometers may yield temperatures of 100 150 c below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high temperature assemblages and by al rich cores in orthopyroxene application of the fmas grid to some well documented granulite occurrences introduces important constraints on their p t histories rocks of different bulk compositions occurring in close proximity in the field may record distinct segments of their p t paths this applies particularly to rocks with evidence for reaction in the form of coronas symplectites and zoned minerals consideration of curved reaction boundaries and xms isopleths may explain apparently contradictory results for the stability of cordierite obtained from low temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other

u pb isotopic evidence for the accretion of different crustal blocks to form the lewisian complex of northwest scotland single zircon and titanite u pb shrimp data presented for tonalite trondhjemite granodiorite ttg suite gneisses and an ultramafic rock from the northern and central regions of the lewisian complex of northwest scotland show that protolith ages of tonalitic gneisses in the northern region 2800 2840 ma are significantly younger than those in the central region 2960 3030 ma further evidence of a major 2490 2480 ma metamorphic event in the central region is documented by a metamorphic zircon associated with a granulite facies ultramafic body a dioritic gneiss from the northern region has also been dated at c 2680 ma the northern region therefore does not comprise reworked central region rocks and consequently the old models for the evolution of the lewisian which were based upon this concept need replacing it is instead proposed that two distinct crustal blocks now the northern and central regions were tectonically juxtaposed along a boundary corresponding to the laxford front juxtaposition would appear to have occurred in proterozoic times as it must have postdated the 2490 2480 ma inverian metamorphism recorded only in the central region and the emplacement of granite sheets restricted to the northern side of the boundary the first recorded event common to both regions is resetting of titanite ages associated with c 1750 ma laxfordian amphibolite facies metamorphism zircon inheritance in rocks of both regions is scarce within one zircon from the northern region a c 3550 ma core was found this represents the oldest known material from the region

orthopyroxene sillimanite quartz assemblages distribution petrology quantitative p t x constraints and p t paths granulite facies magnesian metapelites commonly preserve a wide array of mineral assemblages and reaction textures that are useful for deciphering the metamorphic evolution of a terrane quantitative pressure temperature and bulk composition constraints on the development and preservation of characteristic peak granulite facies mineral assemblages such as orthopyroxene sillimanite quartz are assessed with reference to calculated phase diagrams in nckfmash and its chemical subsystems peak assemblages form mainly in high variance fields and most mineral assemblage changes reflect multivariant equilibria the rarity of orthopyroxene sillimanite quartz bearing assemblages in granulite facies rocks reflects the need for bulk rock xmg of greater than approximately 0 60 0 65 with pressures and temperatures exceeding c 8 kbar and 850 c respectively cordierite coronas mantling peak minerals such as orthopyroxene sillimanite and quartz have historically been used to infer isothermal decompression p t paths in ultrahigh temperature granulite facies terranes however a potentially wide range of p t paths from a given peak metamorphic condition facilitate retrograde cordierite growth after orthopyroxene sillimanite quartz indicating that an individual mineral reaction texture is unable to uniquely define a p t vector therefore the interpretation of p t paths in high grade rocks as isothermal decompression or isobaric cooling may be overly simplistic integration of quantitative data from different mineral reaction textures in rocks with varying bulk composition will provide the strongest constraints on a p t path and in turn on tectonic models derived from these paths

a preliminary study on the characterization and controlling factors of porosity and pore structure of the permian shales in lower yangtze region eastern china abstract permian organic rich shales occur throughout the lower yangtze region in eastern china and are potentially a future target for shale gas exploration due to the lack of data the gas potential of these permian shales is difficult to evaluate in this study a total of 22 permian shale samples were collected from two wells and their geochemical and rock property porosity characteristics were investigated the results show that the samples have a total organic carbon toc content of 0 6 17 4 wt derived from mainly gas prone kerogen with a vitrinite reflectance ro value ranging from 2 3 to 2 7 total porosity of the samples ranges between 1 and 5 5 the micropore and mesopore surface areas range between 5 41 35 69 m2 g and 3 59 11 23 m2 g respectively micropore and mesopore volumes range between 0 22 1 44 cm3 100 g and 0 67 1 76 cm3 100 g respectively total porosity and pore structure data of these samples show a correlation with toc for samples with toc 12 wt negative correlation relationships were observed toc content strongly impacts gas storage capacity of the permian shales in the lower yangtze region and the preliminary results of the present study have important implications for the shale gas play evaluation in this area

surinamite a high temperature metamorphic beryllosilicate from lewisian sapphirine bearing kyanite orthopyroxene quartz potassium feldspar gneiss at south harris n w scotland the sapphirine like mineral surinamite mg fe 2 3 al fe 3 3 o albesi 3 o 15 occurs at south harris as tiny grains enclosed in kyanite or as tabular grains up to 1 mm long mostly surrounded by si rich cordierite a few surinamite grains enclose orthopyroxene sillimanite and si rich sapphirine ion microprobe analyses gave 3 52 to 3 81 wt beo 0 766 to 0 824 be atoms per formula unit and 2 to 13 ppm b in surinamite excess si suggests the presence of significant beo in cordierite and sapphirine given the anti clockwise p t time path inferred for the south harris rock we suggest that surinamite formed at first by the continuous reaction besial 2 in sapphirine sillimanite orthopyroxene surinamite quartz and subsequently by the discontinuous reaction be depleted sapphirine quartz surinamite orthopyroxene kyanite with increase of p to 12 kbar at 850 900 c surinamite reacted with orthopyroxene kyanite and quartz to form beryllian cordierite during subsequent decrease in p and t the high silica content and peraluminous composition of the surinamite bearing gneiss are consistent with a metasedimentary origin this rock is markedly depleted in th 0 13 ppm u 0 11 ppm y 0 94 ppm and rare earth elements e g ce 7 7 ppm its bulk be content 9 ppm is not excessive the appearance of a discrete be phase in be poor rocks could be due to the absence of potential carriers of be namely muscovite and primary cordierite at high t and low water activity moreover surinamite is indicative of a distinctive metamorphic history in which high temperature rocks recrystallized at higher pressures or are isobarically cooled and consequently scarcity of be in metamorphic systems is not the only factor controlling surinamite formation

two stages of sapphirine formation during prograde and retrograde metamorphism in the palaeoproterozoic lewisian complex in south harris nw scotland

metamorphic evolution of aluminous granulites from labwor hills uganda sapphirine cordierite quartz and spinel cordierite quartz form relic assemblages of probable archaean age in fe rich aluminous metapelites from labwor hills uganda and reflect an unusually high temperature metamorphism 1 000 c at pressures in the vicinity of 7 9 kbars and a o2 near the magnetite hematite buffer subsequent reaction textures include the replacement of spinel and cordierite by sillimanite and hypersthene and formation of sapphirine hypersthene k feldspar quartz symplectites which are interpreted as pseudomorphs after osumilite a petrogenetic grid appropriate to these assemblages suggests these reaction textures may be due to cooling at constant or increasing pressure and constant a o2 or decreasing a o2 at constant temperature and pressure the former interpretation is supported by the coexistence of ilmenohematite and magnetite during the development of the reaction textures and by the comparatively low al2o3 contents of secondary hypersthene this pressure temperature path implies that 1 metamorphism occurred at deep levels within normal thickness crust probably less than 40 45 km thick due to an extreme thermal perturbation induced either by emplacement of mantle derived magmas or by thinning of the subcontinental lithosphere in an extensional tectonic regime 2 the excavation and surface exposure of the granulites is due to a subsequent postgranulite facies metamorphism crustal thickening most probably involving their incorporation into an allochthonous upper crustal thrust sheet during the formation of the mozambique foldbelt

the stability of osumilite in metapelitic granulites abstract a petrogenetic grid and related diagrams derived from kfmash system experiments demonstrate that osumilite is stable in relatively magnesian bulk rock compositions xmg 0 6 at temperatures in excess of 875 c and pressures less than 11 kbar the experiments involving the dehydration melting of biotite in synthetic metapelites were conducted in the range 850 1000 c both the mineral assemblages and phase compositions reported from well documented natural examples of osumilite bearing rocks are reproduced by the experiments at p t conditions similar to those previously estimated for these occurrences peak metamorphic p t conditions can be reliably inferred from distinctive osumilite bearing assemblages identified in the phase diagrams thereby avoiding the problems of diffusional re equilibration that often prohibits conventional geothermobarometry from recovering peak conditions integration of the experimental data with recent independent experiments after correcting the latter for an underestimated friction correction allows extension of the petrogenetic grid to higher temperatures the extended grid is applied to assess and refine the metamorphic history of the napier complex east antarctica the high p stability limit for osumilite in the napier complex is 9 10 kbar the prograde p t t path is not necessarily anticlockwise and isobaric cooling in the scott and tula mountains occurred respectively at pressures greater and less than reactions in the range 8 9 kbar the stability range for osumilite predicted by the kfmash system petrogenetic grid overlaps many more metamorphic terranes than osumilite is found in whilst osumilite is not distinctive in thin section and is prone to retrogression it is possible that carbon dioxide present in the natural system stabilizes cordierite at the expense of osumilite

reaction textures in a suite of spinel granulites from the eastern ghats belt india evidence for polymetamorphism a partial petrogenetic grid in the system kfmash and the roles of zno and fe2o3 spinel granulites with or without sapphirine occur as lenses in garnetiferous quartzofeldspathic gneisses leptynites near gokavaram in the eastern ghats belt india spinel granulites are mineralogically heterogeneous and six mineral associations occur in closely spaced domains these are i spinel quartz cordierite ii spinel quartz cordierite garnet orthopyroxene sillimanite iii spinel cordierite orthopyroxene sillimanite iv spinel quartz sapphirine sillimanite garnet v spinel quartz sapphirine garnet and iv rhombohedral fe ti oxide cordierite orthopyroxene sillimanite common to all the associations are a porphyroblastic garnet containing an internal schistosify defined by biotite sillimanite and quartz perthite and plagioclase spinel contains variable amounts of exsolved magnetite and is distinctly zn rich in the sapphirine absent associations x mg in the coexisting phases decreases in the order cordierite biotite sapphirine orthopyroxene spinel garnet fe ti oxides textural criteria and compositional characteristics of the phases document several retrograde mineral reactions which occurred subsequent to prograde dehydration melting reactions involving biotite sillimanite quartz plagioclase and spinel the following retrograde mineral reactions are deduced 1 spinel quartz cordierite 2 spinel quartz garnet sillimanite 3 garnet quartz cordierite orthopyroxene 4 garnet quartz sillimanite cordierite 5 spinel cordierite orthopyroxene sillimanite 6 spinel sillimanite quartz sapphirine 7 spinel sapphirine quartz garnet sillimanite and 8 spinel quartz sapphirine garnet a partial petrogenetic grid for the system feo mgo al 2 o 3 sio 2 k 2 o h 2 o at high fo 2 has been constructed and the effects of zno and fe 2 o 3 on this grid have been explored combining available experimental and natural occurrence data the high fo 2 invariant points in the partial grid have been located in p t space geothermobarometric data and consideration of the deduced mineral reactions in the petrogenetic grid show that the spinel granulites evolved through an anticlockwise p t trajectory reaching peak metamorphic conditions 9 kbar and 950 c followed by near isobaric cooling dt dp 150 c kbar this was superimposed by an event of near isothermal decompression dt dp 15 c kbar the studied spinel granulites therefore preserve relic prograde mineral associations and reaction textures despite being metamorphosed at very high temperatures and bear evidence of polymetamorphism

petrology of sapphirine granulite and associated sodic gneisses from the indian head range newfoundland migmatitic granulites from the indian head range ihr are dominated by granoblastic opx bearing quartz dioritic gneiss with subordinate garnet orthopyroxene biotite albite quartz microcline cordierite sillimanite gneiss and comparatively biotite rich sapphirine cordierite orthopyroxene albite microcline sillimanite corundum gneiss the latter contains at least two generations of sapphirine and cordierite one apparently predating migmatization the other associated with or following this event mineral thermobarometers yield temperatures up to 800 c at 8 kbar the ihr granulites have very high 18o values 10 8 to 14 5 which most likely indicate a sedimentary precursor their ree and hfse contents resemble the trace element signature of post archean australian shale paas lithological analogues from other grenvillian inliers in western newfoundland have similar oxygen isotopic and immobile element signatures they too are interpreted as metasedimentary rocks the ihr rocks however have unusually sodic compositions e g na2o cao and na2o k2o 2 1 22 5 and 0 93 13 8 respectively and contain albite or sodic oligoclase despite their high metamorphic grade the geochemical data indicate that these rocks were albitized prior to high grade metamorphism no counterpart of this event is recorded in granulites from the other inliers indicating that the ihr may be highly allochthonous with respect to these other grenvillian terranes