permian rifting related fayalite syenite in the panxi region sw china abstract within the panxi rift of the permian emeishan large igneous province elip sw china a 20 km 2 quartz bearing fayalite syenitic pluton intrudes the 260 ma baima igneous complex bic the fayalite syenite is dated using shrimp zircon u pb techniques at 252 2 5 ma the panxi fayalite syenite is mainly composed of alkali feldspar ferroan augite fayalite quartz and fe ti oxides and contains 61 8 63 7 wt sio 2 5 0 7 3 wt fe 2 o 3 t and 9 9 12 1 wt total alkalis belonging to an a type variety of ferroan alkalic metaluminous granitoids the fayalite syenite displays positive eu anomalies eu eu 0 9 8 9 and is enriched in lree la yb n 6 6 11 5 the nd values and 87 sr 86 sr initial ratios of the fayalite syenite range from 1 3 to 1 9 and from 0 7035 to 0 7068 respectively the fayalite syenite and the bic had the same mantle source however the 8 myr age gap between them suggests the fayalite syenite formed by remelting of underplated mafic magmas injected into the crust during elip magmatism the ages of the fayalite syenite and bic are coincident with two permian mass extinction events and suggest that elip may have influenced both

intraplate extension prior to continental extrusion along the ailao shan red river shear zone left lateral movement of the ailao shan red river shear zone lends support to the hypothesis of continental extrusion resulting from the collision of india with asia our new observations from northwestern yunnan china and northwestern vietnam on different sides of the shear zone demonstrate that the sinistral offset was 600 km according to correlations of permian triassic flood basalt successions and late paleogene highly potassic mafic magmas we conclude that the shear was propagating on the south china continental margin and does not correspond to a suture between south china and indochina furthermore the highly potassic magmas were emplaced from ca 40 to 30 ma before the shear movement which was caused by the late oligocene to early miocene ca 27 22 ma extrusion activity this suggests that a late eocene to early oligocene intraplate extension possibly induced by delamination of thickened continental lithosphere took place in northwestern yunnan or eastern tibet as a response to the india asia collision this extension and sea floor spreading of the south china sea that began ca 30 ma could have accounted for the initiation of the ailao shan red river shear zone

two magma series and associated ore deposit types in the permian emeishan large igneous province sw china the late middle permian 260 ma emeishan large igneous province in sw china contains two magmatic series one comprising high ti basalts and fe rich gabbroic and syenitic intrusions the other low ti basalts and mafic ultramafic intrusions the fe rich gabbros are spatially and temporally associated with syenites each series is associated with a distinctive type of mineralization the first with giant fe ti v oxide ore deposits such as panzhihua and baima the second with ni cu pge sulfide deposits such as jinbaoshan limahe and zhubu new shrimp zircon u pb isotopic data yielded 263 3 ma for the limahe intrusion 261 2 ma for the zhubu intrusion and 262 2 ma for a syenitic intrusion these new age dates together with previously reported shrimp zircon u pb ages suggest that all these intrusions are contemporaneous with the emeishan flood basalts and formed during a major igneous event at ca 260 ma the oxide bearing intrusions have higher al2o3 feo as total iron and total alkalis na2o k2o but lower mgo than the sulfide bearing intrusions all intrusions are variably enriched in lree relative to hree the oxide bearing intrusions display positive nb and ti anomalies and in certain cases negative zr hf anomalies whereas the sulfide bearing intrusions have obvious negative nb and ti anomalies a feature of crustal contamination individual intrusions have relatively small ranges of nd t values all the intrusions however have nd t values ranging from 3 9 to 4 6 and initial 87sr 86sr ratios from 0 7039 to 0 7105 the syenites have very low mgo 2 wt but highly variable fe2o3 2 5 to 13 wt with initial 87sr 86sr ratios ranging from 0 7039 to 0 7089 magmas from both series could have derived by melting of a heterogeneous mantle plume the high ti series from a fe rich more fertile source and the low ti series from a fe poor more refractory source in addition the low ti series underwent significant crustal contamination the two magma series evolved along different paths that led to distinct mineralization styles

age and duration of the emeishan flood volcanism sw china geochemistry and shrimp zircon u pb dating of silicic ignimbrites post volcanic xuanwei formation and clay tuff at the chaotian section abstract the age and duration of the emeishan basalts sw china remain poorly constrained largely due to the severe thermo tectonic overprinting of the ar ar system and failure to obtain zircon separates from erupted basalts in an attempt to solve this problem geochemical analyses and shrimp zircon u pb dating have been carried out on rare felsic ignimbrite in the uppermost of the emeishan lava succession the xuanwei formation which immediately overlies the emeishan basalts and a clay tuff at the middle late permian boundary at the chaotian section clastic rocks of the lowermost xuanwei formation group 1 in eastern emeishan large igneous province lip have a geochemical affinity to the emeishan felsic volcanic rocks whereas the overlying sediments group 2 are compositionally more akin to mafic components of the emeishan traps this is the reverse of volcanic sequence of the central emeishan lip where the felsic extrusives sit above predominant mafic rocks it is likely that the clastic rocks are water transported sediments resulting from erosion of the volcanic rocks in the center of the emeishan lip this interpretation is further supported by the general agreement between the age of the lowermost xuanwei formation 257 4 ma 260 5 ma and that of the silicic ignimbrite 263 4 ma and the clay tuff at the middle late permian boundary at chaotian 260 5 ma these ages interpreted as the termination age of the emeishan volcanism are indistinguishable within error from the middle late permian boundary age 260 4 0 4 ma and the main stage 259 262 3 ma of the emeishan volcanism inferred from dating of mafic and alkaline intrusions in the emeishan lip all these suggest that the emplacement of the emeishan volcanism took place over a very short interval moreover the temporal link and geochemical analyses suggest that the chaotian clay at the middle late permian boundary was genetically related to the emeishan silicic volcanism this together with the fact at that both the emeishan basalts and the chaotian clay rest on the maokou formation leads us to infer that the emeishan basalt was emplaced at the middle late permian boundary in this sense the emeishan volcanism can be regarded as a boundary event and its age is presumed at 260 ma although more precise dating is required both stratigraphic correlation and chronologic data presented in this paper lend supports to the notion that the emeishan volcanism was one of likely causes of the end guadalupian mass extinction

a new interpretation of the structure of the sept iles intrusive suite canada the layered mafic intrusion at sept iles canada is one of the largest intrusions in the world a new interpretation of its structure is proposed based on a review of its geology and a comparison with the skaergaard intrusion greenland several different magmatic components are recognized hence the name sept iles intrusive suite siis is proposed emplacement of the suite may have been preceded by eruption of flood basalts the first magmas of the suite rose in the crust to accumulate beneath the density filter afforded by the basalts the largest component is the sept iles mafic intrusion simi the lower series of the simi is dominated by leucotroctolites and leucogabbros above it lie the layered series which is largely comprised of gabbro and troctolite both these units are unchanged from earlier interpretations the anorthosites s l gabbros and monzogabbros formerly called the transitional series are now considered to be the upper border series developed by floatation of plagioclase common autoliths in the layered series are parts of the hydrothermally altered upper border series from towards the interior of the intrusion which have foundered and settled through the magma the contamination of the magma that accompanied this event oxidised iron in the magma and led to the precipitation of magnetite around the periphery of the intrusion the subsequent depletion of fe3 and or increase in sio2 cao and p2o5 may have induced apatite saturation and accumulation to form two layers rich in apatite near the base and at top of the layered series granitic magma was developed by fractional crystallisation and was emplaced along the roof of the chamber where it acquired large quantities of xenoliths these were probably derived from the flood basalts their evolved members and fragments of mafic dykes chilled by the granitic magma accumulations of monzonite pillows in this unit testify to another magmatic event and a floor to the granitic magma chamber indicating lateral transport of magma chemically distinct syenites in the upper part of the intrusion are part of the point du criade intrusion a large late composite sill diabase and leucogabbro components show a close link with the simi and all the acidic magmas may have originally formed by differentiation of the main magma in cupolas towards the centre of the intrusion a series of late gabbro intrusions that cut the simi may represent a rejuvenation of magmatism the border zone is a mass of fine grained rocks that occurs along the border of the simi it may be another magmatic component or just the lateral border series of the simi

chloritization of late ordovician k bentonites from the northern baltic palaeobasin influence from source material or diagenetic environment abstract the baltic ordovician silurian sedimentary succession embodies numerous altered volcanic ash beds that are illite smectite dominated only a few beds rich in chlorite smectite are known from the upper ordovician pirgu stage of estonia mixed layer chlorite smectite occurs commonly in low grade metamorphic and hydrothermal environments however chloritic k bentonites of the pirgu stage have never been buried deeply and lack signs of metamorphic overprinting in order to understand the origin of chloritization three distinct beds were sampled in 14 drillcores from estonia and latvia and analysed by means of xrd and sem the principal authigenic assemblage of the bulk samples consists of chlorite smectite corrensite together with illite smectite and k feldspar the actual mineral composition of k bentonites though varies from sample to sample the clay mineral assemblages range from virtually pure chlorite smectite to illite smectite dominated assemblages with minor or no chlorite smectite the proportion of chlorite smectite in k bentonites shows systematic lateral variations the share of chloritic phases is highest in the shallower water part of the palaeobasin and decreases towards the deeper part of the basin such regular lateral variations suggest a possible link between chloritization and the configuration of ancient palaeobasin the present study suggests that the chloritization of primary felsic ashes occurred during early diagenesis and that it was caused by an influx of mg rich water probably from a marine sabkha type environment

reaction pathways and reaction progress for the smectite to chlorite transformation evidence from hydrothermally altered metabasites the transformation from smectite to chlorite has been interpreted as involving either a disequilibrium chlorite smectite mixed layering sequence or an equilibrated discontinuous sequence involving smectite corrensite chlorite here analysis of the smectite to chlorite transition in different geothermal systems leads us to propose that the transformation proceeds via three contrasting reaction pathways involving i a continuous mixed layer chlorite smectite series ii a discontinuous smectite corrensite chlorite series and iii a direct smectite to chlorite transition such contrasting pathways are not in accord with an equilibrium mineral reaction series suggesting that these pathways record kinetically controlled reaction progress in the geothermal systems reviewed the style of reaction pathway and degree of reaction progress is closely correlated with intensity of recrystallization and not to differences in thermal gradients or clay grain size this suggests a kinetic effect linked to variation in fluid rock ratios and or a contrast between advective or diffusive fluid transport the mode of fluid transport provides a means by which the rates of dissolution nucleation growth can control the reaction style and the reaction progress of the smectite to chlorite transition slow rates of growth are linked to the first reaction pathway involving mixed layering while increasing rates of growth relative to nucleation promote the generation of more ordered structures and ultimately lead to the direct smectite to chlorite transition representative of the third pathway

palaeoceanography and biogeography in the early jurassic panthalassa and tethys oceans abstract a first conceptual palaeoceanographic model for the early jurassic panthalassa and tethys oceans is outlined in the present paper the new palaeoceanographic model uses fundamental physic oceanographic principles known from the modern world and a global palaeogeographic reconstruction for the early jurassic to examine the long term response of the panthalassic and tethyan fossil invertebrate faunas to the proposed surface ocean circulation analysis of palaeobiogeographical data ostracods ammonites brachiopods and bivalves has enabled circulation changes to be reconstructed over the studied period in some detail panthalassic circulation pattern shows an almost hemispherical symmetric pattern with the development of the two large subtropical gyres that rotates clockwise in the northern hemisphere and anti clockwise in the southern hemisphere surface circulation in the tethyan ocean is dominated by monsoonal westerly directed equatorial surface currents that reached its western corner and droved them off to the north along the northern side of the tethys ocean during summer and in opposite direction during the winter

characterization and origin of aluminous a type granites from the lachlan fold belt southeastern australia the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are a distinctive group of igneous rocks on the basis of chemical and mineralogical criteria those granites that contain 65 72 sio2 can be distinguished from other types on the basis of higher abundances of fetotal fetotal mg high field strength elements trivalent rare earth elements ga and zn mineralogically they contain fe rich hydrous mafic minerals and primary ilmenite and hence are reduced relative to the ni nio buffer however the extremely felsic a type granites sio2 72 have the same chemical and mineralogical characteristics as felsic fractionated i type granites recent analyses indicate that the lachlan fold belt a type granites have sc f alkali element trace transition element and h2o contents similar to those of other unfractionated i type granites rb sr and nd sm isotopic compositions are highly variable probably reflecting source region heterogeneity the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are distinct from peralkaline rocks in terms of chemical composition petrography and field associations although these rocks have been grouped together as a single type in current classification schemes we propose that the metaluminous to weakly peraluminous a type granites such as those of the lachlan fold belt should be defined as aluminous a type granites and should not be grouped with peralkaline granites the lachlan fold belt aluminous a type granites have relatively high calculated zircon saturation temperatures we suggest that these granites were produced by high temperature partial melting of a felsic infracrustal source

relationships between clay mineralogy hydrothermal metamorphism and topography in a western cascades watershed oregon usa this study investigates variation in clay mineralogy and its relation to hydrothermal metamorphism hillslope processes and topography in the western cascade mountains the study area is the drainage basin of dorena lake a medium sized 686 km2 watershed located near cottage grove or the bohemia mining district is on the southeastern rim of the watershed in a hydrothermally metamorphosed region associated with a set of granodiorite plutons to characterize large scale patterns of clay mineral distribution within the watershed suspended sediments were collected from 43 stream locations samples of several metamorphosed and unaffected volcanic and volcaniclastic rocks were collected to help clarify metamorphic reaction processes one active earthflow was also sampled x ray diffraction methods were used to determine the mineralogy of the clay sized 2 m fraction of the samples clay mineralogy varies systematically across the watershed and the three major stream tributaries carry sediment with distinct mineralogical signatures discrete minerals include kaolinite smectite chlorite and illite interstratified kaolinite smectite and chlorite vermiculite cv are also present the active earthflow and unmetamorphosed rock samples primarily contain smectite in contrast metamorphosed rock samples are composed of some combination of illite interstratified illite smectite cv and chlorite examination of clay mineral distribution reveals the effects of hydrothermal metamorphism in the mining district on clay mineralogy hillslope processes and landscape development compared with most of the watershed the mining district has steeper slopes and higher elevations and lacks smectite almost entirely analyses of metamorphosed bedrock units indicate that smectite originally present in the rocks was converted to nonexpandable clay minerals during metamorphism induration of bedrock and loss of expandable clays resulted in thin soils and steep topography debris slides and flows are the dominant mass movement processes in this area earthflows are noticeably absent probably because thick smectitic soils are lacking elsewhere in the watershed where smectite is abundant slopes are more gentle and both shallow and deep seated mass movements are common the clay mineralogy of bedrock and soils is thus integrally tied to the hillslope processes operating within the watershed and therefore affects the geomorphology of the landscape high relief hydrothermally metamorphosed volcanic centers surrounded by weaker rocks in adjacent lowlands are common features in the western cascade mountains these massifs were likely formed by differential erosion processes similar to those operating in the dorena lake watershed

tethyan oceanic currents and climate gradients 300 m y ago we reconstruct the oceanic circulation pattern of the tethys ocean 300 m y ago by placing late carboniferous early permian climate sensitive biotic associations from gondwana and laurasia on a pangea paleogeography constrained by selected paleomagnetic data warm climate fossils and facies from iran located at that time along the gondwanan margin of arabia are compatible with the existence in the tethys ocean of a warm subtropical surface current gyre whereas cold surface currents swept the glaciated gondwanan margin at higher southern latitudes redistributing cold biota toward the tropics this tethyan surface current system and the associated narrow zonal barrier show similarities to recent glacial climate patterns when placed on a large scale paleogeographic reconstruction of pangea of the b type it neatly explains the otherwise problematic observation that the carboniferous permian biota of iran and northern arabia is dominated by warm euramerican and or russian taxa that are strikingly different from typical cold gondwanan associations

clay minerals and their paleoenvironmental interpretation in the baoji loess section southern loess plateau china abstract clay minerals from the loess paleosol section at baoji in north central china were studied by x ray diffraction major components calculated by comparing major peak heights were estimated as follows illite 55 75 kaolinite 10 25 mixed layer chlorite vermiculite 10 20 chlorite

interstratification and other reaction microstructures in the chlorite berthierine series transmission electron microscopy tem results show there is a series of periodically and nonperiodically interstratified structures composed of berthierine and chlorite layers in low temperature chlorite that is one of the alteration products of granulite facies archean ironstone from the eastern border of the beartooth mountains montana an antiphase domain structure suggests that the interstratified structures are intermediate transformation products formed by reaction from berthierine serpentine structure to chlorite periodically interstratified structures consisting of chlorite c and serpentine like s or berthierine layers include cs ccs cccs ccccs and ccccccs the layer sequences in interstratified chlorite berthierine are indicative of the reaction mechanisms that produce the interstratified structures e g crystallization from solution or solid state transformation the reaction from berthierine to chlorite is crystallographically much like a cell preserved phase transformation even though it is highly reconstructive berthierine can be considered a polymorph of the fe rich chlorite mineral chamosite with berthierine as the los temperature phase interstratification and integrowths in the chlorite berthierine series may be common phenomena in low temperature layer silicates resembling chlorite although such relations are difficult to recognize from chemical analyses or powder x ray diffraction patterns they can be observed readily with tem method

clay mineral diagenesis and quartz cementation in mudstones the effects of smectite to illite reaction on rock properties abstract the late cretaceous to early tertiary sequence of the voring and more basins from the norwegian sea has been examined with respect to mineralogy based on 319 cutting samples from five wells a clear relationship between mineralogy and well log data is demonstrated a significant change with respect to velocity porosity and density occurs within the depth interval corresponding to 80 90 c at shallow depths temperatures

abundant fe ti oxide inclusions in olivine from the panzhihua and hongge layered intrusions sw china evidence for early saturation of fe ti oxides in ferrobasaltic magma abundant fe ti oxide inclusions in cumulus olivine fo77 81 from the panzhihua and hongge intrusions emeishan large igneous province sw china document the first evidence for early crystallization of fe ti oxides in ferrobasaltic systems in nature the intrusions also contain significant stratiform fe ti v oxide ores the oxide inclusions are sub rounded or irregular range from 5 to 50 m in diameter and are dominated by either titanomagnetite or ilmenite the fact that the inclusions are either titanomagnetite or ilmenite dominant suggests that they are trapped crystals instead of immiscible oxide melt formed during growth of the host olivine the absence of other silicate phases in the inclusion bearing olivine is difficult to reconcile with a possible xenocrystic origin of the oxide inclusions these oxide inclusions are thus interpreted to be cumulus minerals crystallized together and trapped in olivine from the same parental magma in addition to fe ti oxides some inclusions contain amphibole biotite fluorapatite that might have formed by reaction of trapped hydrous liquid with the host olivine numerical modeling of high ti emeishan basalts using the melts program successfully simulates early crystallization of olivine fo81 and fe ti spinel in the presence of a moderate amount of h2o 1 5 wt under pressure and fo2 conditions generally pertinent to the panzhihua and hongge intrusions the modal mineralogy of the oxide inclusions is in good agreement with the bulk compositions of the ore as inferred from whole rock data in a given intrusion this is consistent with the interpretation that the stratiform oxide ores in the intrusions formed by accumulation of fe ti oxide crystals that appeared on the liquidus with olivine and clinopyroxene

zircon u pb and hf isotope constraints on crustal melting associated with the emeishan mantle plume abstract shrimp zircon u pb dates combined with in situ hf isotopic data provide new constraints on the petrogenesis and protolith of peralkaline metaluminous and peraluminous intrusions and rhyolitic tuffs in the emeishan large igneous province with significant bearing on crustal melting associated with mantle plumes syenite and a type granitic intrusions from huili miyi and taihe in the center of this large igneous province yield u pb dates at 260 ma consistent with the ages obtained for mafic layered intrusions in the same province zircon from these rocks exhibits a wide range of initial hf isotope ratios ehf t 1 4 to 13 4 with corresponding tdm1 of 400 900 ma the highest ehf t value is only marginally lower than that of depleted mantle reservoir at 260 ma suggesting that their source is primarily juvenile crust added during emeishan volcanism with incorporation of variable amounts of neoproterozoic crust the trigger of crustal melting is most likely related to advective heating associated with magmatic underplating in contrast the 255 251 ma peraluminous granites from ailanghe and 238 ma rhyolitic tuff from binchuan have negative initial ehf values of 1 3 to 4 4 and of 7 7 to 14 respectively hf isotopic model ages and presence of inherited zircons indicate their derivation from mesoproterozoic and paleoproterozoic crust respectively given the time lag relative to the plume impact 260 ma and insignificant mantle contribution to 255 238 ma magmatism conductive heating is suggested as the trigger of crustal melting that resulted in formation of delayed felsic magmas the involvement of older crust in younger felsic magmas is consistent with upward heat transfer to the lithosphere during plume impregnation if the age of crust is inversely stratified i e changes from paleoproterozoic to mesoproterozoic to neoproterozoic to permian with increasing depth such crust may have resulted from episodic downward crustal growth during the evolution of the western yangtze craton

a temporal link between the emeishan large igneous province sw china and the end guadalupian mass extinction abstract previous studies have suggested that there were two mass extinction events in the late permian one that occurred at the permo triassic p t boundary 251 ma and a second smaller mass extinction that occurred 5 8 myr earlier at the end of the guadalupian many workers have argued that there is a causal relationship between large scale volcanic activity and mass extinctions the major mass extinction event at the p t boundary coincides with the outpouring of huge quantities of lava that formed the siberian flood basalt province in russia courtillot et al earth planet sci lett 166 1999 177 195 and wignall earth sci rev 53 2001 1 33 suggested that the earlier late permian mass extinction coincided with the eruption of the lavas that formed the emeishan flood basalt efb province in sw china however the age of eruption of the efb lavas is poorly constrained using the sensitive high resolution ion microprobe to analyze zircons we have established the age of the xinjie intrusion believed to be a feeder to the main phase of efb volcanism to be 259 3 ma hence the formation of the efb is coincident with a proposed extinction event at 256 259 ma this result supports a temporal link between the emeishan large igneous province and the end guadalupian mass extinction

comparative mineral chemistry and textures of safod fault gouge and damage zone rocks abstract creep in the san andreas fault observatory at depth safod drillhole is localized to two foliated gouges the central deforming zone cdz and southwest deforming zone sdz the gouges consist of porphyroclasts of serpentinite and sedimentary rock dispersed in a foliated matrix of mg smectite clays that formed as a result of shearing enhanced reactions between the serpentinite and quartzofeldspathic rocks the cdz takes up most of the creep and exhibits differences in mineralogy and texture from the sdz that are attributable to its higher shearing rate in addition a 0 2 m wide sector of the cdz at its northeastern margin ne cdz is identical to the sdz and may represent a gradient in creep rate across the cdz the sdz and ne cdz have lower clay contents and larger porphyroclasts than most of the cdz and they contain veinlets and strain fringes of calcite in the gouge matrix not seen elsewhere in the cdz matrix clays in the sdz and ne cdz are saponite and corrensite whereas the rest of the cdz lacks corrensite saponite is younger than corrensite reflecting clay crystallization under declining temperatures and clays in the more actively deforming portions of the cdz have better equilibrated to the lower temperature conditions

mineral compositional constraints on petrogenesis and oxide ore genesis of the late permian panzhihua layered gabbroic intrusion sw china a systematic study of mineral compositions has been carried out on stratigraphic sections through the panzhihua intrusion a layered gabbroic sill of late permian age that hosts significant magmatic fe ti v oxide deposits in the emeishan large igneous province sw china the intrusion which is 19 km long and 2 km thick is divided into a marginal zone mgz lower zone lz middle zone sub zones mza and mzb and upper zone uz from the base upwards compositions of the most primitive cumulus olivine fo81 and plagioclase an73 found in the intrusion suggest that the parental magma was similar to the mg rich end member of the coeval high ti basalts in the region two reversals in terms of mineral compositions are found in the intrusion one in the lower mza and the other at the transition between the mzb and the uz the first reversal is marked by a gradual increase in the an content of plagioclase an60 to an70 over a 100 m vertical interval the other is characterized by abrupt changes in the compositions of olivine fo28 to fo70 clinopyroxene mg 51 to 75 and plagioclase an24 to an51 over a 20 m vertical interval these suggest that the panzhihua intrusion opened at least twice for recharge of more primitive magmas strong depletion of incompatible trace elements such as zr in the cumulate sequence without corresponding enrichment in any border series is suggested to be evidence for significant loss of residual liquid at a late stage during solidification of the intrusion compared to other well studied layered intrusions the panzhihua intrusion contains fe ti oxides crystallized at a high temperature consistent with evidence obtained in earlier studies early appearance of liquidus fe ti oxides in the panzhihua magma is most likely a combined effect of a rather high degree of initial oxidation and elevated volatile fugacity

emeishan large igneous province sw china in recent years there have been major advances in our understanding of the emeishan large igneous province lip of sw china following publication of a number of lip focused investigations of the terrain and associated rocks this paper reviews the current state of knowledge the volcanic and upper intrusive portion of the province is relatively small 0 3 106 km3 even when offset fragments eroded sections and buried portions are included in the volume calculation the most reliable radiometric age dates zircon u pb shrimp from an associated layered intrusive body several kilometers in area indicate generation at 259 ma consistent with the end guadalupian end middle permian stratigraphic age in addition several ar ar dating studies have been carried out mainly on the volcanic rocks with a number of reported dates 253 251 ma late permian but a consensus is emerging that these ages are problematic because they are in conflict with the stratigraphic data possibly due to a monitor standard miscalibration the ar ar investigations have also yielded a large number of secondary ages which are clustered at 175 142 98 and 42 ma and these are inferred to record sub regional tectonic events that affected the western yangtze block as east asia was assembled and later deformed by india s collision indentation into asia magnetostratigraphic data and field observations suggest that the bulk of the volcanic sequence formed within 1 2 my the geochemistry of the volcanic rocks and bio lithostratigraphic studies of the underlying maokou formation suggests a mantle plume generated the province the basalts can be classified into low and high ti groups with different parental magmas the low ti basalts are confined to the western part of the province and are overlain by the high ti basalts the low ti magmas formed at shallow mantle depths in the spinel garnet field transition zone 60 80 km whereas the high ti magmas formed by low degrees of partial melting within the deeper garnet stability field this observation suggests a deepening of the melting column as the lithosphere thickened due to under plating and a transition from peak basalt generation to the waning stage outstanding issues which might focus future studies are also outlined

lacustrine clay mineral assemblages as a proxy for land use and climate changes over the last 4 kyr the amik lake case study southern turkey abstract lake sediments are sensitive to landscape changes and most of these changes seem to be modulated by land use anthropogenic factors coupled to palaeoenvironmental palaeoclimatic changes in its detrital fraction the lacustrine sediments record the history of soil erosion within its catchment via the inputs of clays and others detrital products within a mediterranean context the study investigates the upper sediments infilling the central part of the amik basin in southern turkey this tectonic basin was occupied and exploited by modern human at least since 6000 7000 bce we focus on the clay mineralogy x ray diffraction on oriented aggregates and magnetic susceptibility measurements bartington of the sedimentary record in the area over the last 4000 years to assess environmental changes in relation with the different land uses and or weathering during the successive bronze iron roman islamic ottoman and modern civilizations the clay fraction of amik lake sediments comprises smectite kaolinite illite chlorite and chlorite smectite mixed layers that are the inherited clay phases a relative change in abundance and crystallinity and chemistry of illite attests that environmental conditions evolved in the amik plain from the bronze to modern age in relation with climates and or land use changes the history of the amik lake reveals different soil erosion episode the most intense erosion phase occurred during the bronze iron ages as indicated by the clay and magnetic susceptibility proxies the roman period was an exceptional period with soil erosion products arriving from the watershed probably due the water channelization a reduction of soil erosion occurred during the post roman period until nowadays significant pedogenesis transformations are evidenced especially during the islamic ottoman periods suggesting intense chemical weathering conditions related to climate change

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

geology and geochemistry of paleosols developed on the hekpoort basalt pretoria group south africa the hekpoort paleosols comprise a regional paleoweathering horizon developed on 2 224 0 021 ga basaltic andesite lavas at the top of the hekpoort formation of the pretoria group transvaal supergroup south africa in five separate profiles from outcrops along road cuts near waterval onder and the daspoort tunnel and in three drill cores from the bank break area bb3 bb8 and bb14 the top of the paleosol is a sericite rich zone the sericite zone grades downward into a chlorite rich zone in core bb8 and in the road cut at the daspoort tunnel we sampled the underlying or parent basaltic andesite into which the chlorite zone grades we did not obtain samples of the parent material at waterval onder and in cores bb3 and bb14 but chemical analyses indicate that the chlorite and sericite zones in these profiles derive from underlying lavas similar to the ones we sampled in core bb8 and at the daspoort tunnel the presence of apparent rip up clasts of the paleosol in the overlying ironstones of the strubenkop formation in the cores from bank break makes it very unlikely that most of the alteration was a result of interactions with hydrothermal fluids desiccation cracks at the top of the paleosol that were filled with sand during the deposition of the overlying sediments at waterval onder point to a subaerial weathering origin very little if any al ti zr v or cr moved a discernible distance during weathering of any of the five profiles the vertical distribution of fe mg mn ni and co indicates that these elements were largely removed from the top of the soil during weathering the overall abundance of these elements in each of the profiles indicates that a significant fraction of the complement lost from the top subsequently reprecipitated in the lower portion of the soil as constituents of an fe 2 rich smectite the loss of fe from the top of the soil during weathering of the hekpoort paleosols indicates that atmospheric p o 2 was less than 8 x 10 4 atm about 2 22 ga fe 2 rich smectite should only precipitate during soil formation if atmospheric p co 2 is less than or equal to 2 x 10 2 atm rye kuo and holland 1995 ca and na were largely lost during weathering some na was apparently added to the sericite zone in cores bb3 bb8 and bb14 after weathering all five profiles are enriched in k and rb and most are enriched in ba the distribution of these elements indicates that they all were added during post weathering hydrothermal metasomatism rb sr analysis of the paleosol at the daspoort tunnel indicates that metasomatism last affected that profile 1 925 0 032 ga macfarlane and holland 1991

the role of fe ti oxide crystallization in the formation of a type granitoids with implications for the daly gap an example from the permian baima igneous complex sw china abstract the baima igneous complex bic is a member of the permian emeishan large igneous province sw china and is composed of a layered gabbroic unit and an isotropic syenitic unit the gabbroic unit consists of four distinct zones 1 lower cumulate zone 2 massive fe ti v oxide ore zone 3 olivine gabbro zone and 4 upper gabbro zone shrimp zircon u pb dating results yield an age of 261 2 ma for the gabbroic unit contemporaneous with the spatially associated syenitic unit the gabbroic rocks show slightly lree enriched patterns with la yb n values between 1 6 and 12 1 and positive eu anomalies eu eu 1 1 3 6 in contrast the syenites display stronger enrichments of lree la ybn 6 8 51 3 and negative eu anomalies eu eu 0 3 1 1 rocks of the syenitic unit are chemically similar to the ferroan alkalic a type granitoids k2o na2o 0 58 0 80 feot feot mgo 0 80 0 91 and have a within plate geochemical signature both units have similar trace element ratios zr nb th u ni co cu ni hf ta and zn ga resembling an ocean island basalt source the end t values for the gabbroic unit end t 1 6 4 2 and syenitic unit end t 2 5 3 2 are within the range of the high ti emeishan flood basalts the geochemical and geochronological data indicate that the gabbroic and syenitic units are comagmatic and cogenetic geochemical modeling suggests that the two units originated by fractional crystallization of a common parental magma that resembles the high ti emeishan flood basalts the formation of the two units by fractional crystallization was likely responsible for the enrichment of fe ti v oxide minerals in the gabbroic unit and thus the development of the daly gap the results of this study indicate that silica saturated peralkaline a type granitoids can be formed by fractionation of mantle derived mafic magmas

low grade metamorphism around the down dip limit of seismogenic subduction zones example from an ancient accretionary complex in the shimanto belt japan abstract reactions involving clay minerals during low grade metamorphism at the depth of an ancient accretionary complex in the shimanto belt kyushu japan were studied by integrated transmission electron microscopy energy dispersive x ray spectroscopy and x ray diffraction analyses of the bulk rock and clay fraction the analyzed metasediment the kitagawa unit contain an incipient sub horizontal slaty cleavage illite crystallinity data and mica b dimensions indicate that the conditions of metamorphic deformation were anchizone epizone grade and intermediate pressure cleavage formation was linked to two reactions involving clay minerals 1 the recrystallization of 1m dominant matrix mica inherited from the original sedimentary fabric into thick defect free 2m1 packets along cleavage planes and 2 the formation of chlorite from 7 a berthierine balanced equations among the clay phases based on compositional data and their relative abundance suggest that the decomposition of matrix mica resulted in the formation of paragenetic mica and chlorite along the cleavage planes without significant elemental outflux although a modal increase in phyllosilicates is not indicated by the data the growth of chlorite and mica along cleavage planes may have a large influence on the rheological properties of a decollement and may be related to the occurrence of the seismic aseismic transition at 350 c

clay alteration associated with proterozoic unconformity type uranium deposits in the east alligator rivers uranium field northern territory australia clay minerals associated with the unconformity type uranium deposits of the east alligator rivers uranium field have been used to determine the spatial distribution of hydrothermal alteration and the conditions of uranium deposition within the altered basement underlying the kombolgie unconformity alteration develops a similar pattern at regional and smaller scales with a close relationship between the degree of alteration and the amount of structural discontinuities observed on both sides of the unconformity e g faults fractures breccias these discontinuities were mechanically active during the hydrothermal activity the hydrothermal clay minerals partly to totally replace diagenetic dickite and illite in the kombolgie sandstones and metamorphic phengite and chlorite in the basement rocks the time space alteration sequence established in this study can be summarized as follows 1 early crystallization of illite illite smectite i s mixed layer minerals lree sr bearing aluminum phosphate sulfates aps in sandstones above the unconformity illite i s mixed layer minerals sudoite aps on both sides of the unconformity and in fault gouges and illite i s mixed layer minerals chlorite apatite deeper in the basement and 2 late precipitation of chlorite in the open fracture network mainly centered below the unconformity the main stage of uranium deposition was associated with chloritization in the basement rocks smaller particle size lath shaped or flaky morphology common interstratification with expandable layers lower crystallinity and mg rich chemical compositions can be used to distinguish the hydrothermal clays from the other diagenetic and metamorphic illite and or chlorite hydrothermal illite can be distinguished from diagenetic and metamorphic illite phengite by its poor crystallinity along the c axis interstratification with minor amounts of smectite layers and a 1m polytype in which trans vacant octahedral sites predominate 1 mt chemically the hydrothermal illite differs from diagenetic illite by a lower interlayer charge and a higher mg content the hydrothermal chlorite is distinguished from the metamorphic chlorite by its smaller particle size spherulitic habit poor crystallinity along the c axis and common association with corrensite and or chlorite corrensite mixed layers most of the hydrothermal chlorite is mg rich and has a structural formula close to that of clinochlore xfe 0 20 near the uranium deposits the spherules of hydrothermal chlorite are chemically zoned from xfe 0 10 to xfe 0 40 which is interpreted as the result of a change in the oxidation state of the hydrothermal solution during the chlorite crystallization the space time alteration sequence established for clay minerals and the observed transition of aps to apatite are interpreted as the result of an increasing degree of interaction of the basement rocks with infiltrating acidic and oxidizing sub basinal brines in turn these fluids tend to be neutralized and reduced by their ongoing interaction with ferrous iron bearing phyllosilicates fe chlorite and biotite and iron sulfides of metamorphic origin the ore deposits are surrounded by zones in which the degree of fluid rock interaction was very high and chloritization is most abundant the octahedral mg content of the associated hydrothermal illite from the sandstone can be used as a guide to indicate the possible presence of uranium orebodies in the basement rocks tens to hundreds of meters below the crystal structures of the hydrothermal illite and chlorite suggest that these minerals crystallized at temperatures approximatively 30 to 50 c lower than those expected at the peak diagenesis of the kombolgie sandstones near the unconformity

climate changes caused by degassing of sediments during the emplacement of large igneous provinces most mass extinctions during the last 500 m y coincide with eruptions of large igneous provinces lips the cretaceous tertiary extinction was synchronous with the deccan flood volcanism the permian triassic extinction with the eruption of the enormous siberian traps and the end guadalupian extinction with the emeishan volcanic province the causal link remains disputed however and many lips apparently had no significant impact on the biosphere here we show that a key control on the destructive consequences of lip emplacement is the type of sedimentary rock in basins beneath the flood basalts contact metamorphism around intrusions in dolomite evaporite coal or organic rich shale generates large quantities of greenhouse and toxic gases co 2 ch 4 so 2 which subsequently vent to the atmosphere and cause global warming and mass extinctions the release of sediment derived gases had a far greater impact on the environment than the emission of magmatic gases

geologic geochemical and geophysical consequences of plume involvement in the emeishan flood basalt province prevolcanic kilometer scale lithospheric doming in the emeishan large igneous province southwest china allows us to evaluate the spatial and temporal consequences of uplift on the paleogeography geology geochemistry and geophysics of the region systematic spatial variations are observed across the domal structure in the distribution and thickness of clastic and carbonate sediments the extent of erosion thickness and chemistry of volcanic rocks and the crust mantle structure these features which are best explained by a mantle plume may be used to track older plume sites in the geologic record

origin of fe ti oxide ores in mafic intrusions evidence from the panzhihua intrusion sw china economicconcentrationsof fe ti oxidesoccuras massive conform able lenses or layers in the lower part of the panzhihua intrusion emeishan large igneous province sw china mineral chemistry textures and quilfequilibria indicate that oxides in rocks of theintrusion were subjected to extensive subsolidus re equilibration andexsolution the primary oxide reconstructed from compositions oftitanomagnetite in the oresand associated intergrowths isan alumi nous titanomagnetite usp

hornblende alteration and fluid inclusions in kardla impact crater estonia evidence for impact induced hydrothermal activity the well preserved krdla impact crater on hiiumaa island estonia is a 4 km diameter structure formed in a shallow ordovician sea about 455 ma ago into a target composed of thin 150 m unconsolidated sedimentary layer above a crystalline basement composed of migmatite granites amphibolites and gneisses the fractured and crushed amphibolites in the crater area are strongly altered and replaced with secondary chloritic minerals the most intensive chloritization is found in permeable breccias and heavily shattered basement around and above the central uplift alteration is believed to have resulted from convective flow of hydrothermal fluids through the central areas of the crater chloritic mineral associations suggest formation temperatures of 100 300 c in agreement with the most frequent quartz fluid inclusion homogenization temperatures of 150 300 c in allochthonous breccia the rather low salinity of fluids in krdla crater 13 wt nacleq suggests that the hydrothermal system was recharged either by infiltration of meteoric waters from the cater rim walls raised above sea level after the impact or by invasion of sea water through the disturbed sedimentary cover and fractured crystalline basement the well developed hydrothermal system in krdla crater shows that the thermal history of the shock heated and uplifted rocks in the central crater area rather than cooling of impact melt or suevite sheets controlled the distribution and intensity of the impact induced hydrothermal processes

mineralogy and geochemistry of fe ti oxide and apatite nelsonite deposits and evaluation of the liquid immiscibility hypothesis modal mineralogy determined for 32 fe ti oxide and apatite rocks from localities in virginia new york quebec norway and sweden largely supports the 2 1 oxide apatite ratio suggested as a eutectic mixture by philpotts 1967 a fairly consistent suite of accessory minerals including biotite clinoamphibole spinel zircon and sulfides is present some silicate accessory minerals tend to form glomeroporphyritic intergrowths in an equigranular matrix of nelsonite suggesting that the accessories are xenocrysts that have been trapped in an oxide apatite liquid this liquid may represent an extreme case of partitioning of high charge density cations into a low silica phosphorus enriched immiscible melt temperature and oxygen fugacity estimates for oxide pairs in nelsonites range from 600 degrees to 1 000 degrees c and 10 super 20 to 10 super 11 atm f sub o 2 and bracket an equilibration path that is close to the fmq buffer curve several stages of exsolution are evident in fe ti oxides of some deposits nelsonites contain fluorapatite that is enriched in light lanthanides the occurrence of nelsonite dikes in roseland anorthosite and associated rocks of the roseland piney river district of virginia is explained by the infilling of fractures with nelsonite liquid that has settled out of an overlying ferrodiorite pluton

corrensite and mixed layer chlorite corrensite in metabasalt from northern taiwan tem aem empa xrd and optical studies many chloritic minerals in low grade metamorphic or hydrothermally altered mafic rocks exhibit abnormal optical properties expand slightly upon glycolation expandable chlorite and or have excess alvi relative to aliv as well as significant ca k and na contents chloritic minerals with these properties fill vesicles and interstitial void space in low grade metabasalt from northern taiwan and have been studied with a combination of tem aem empa xrd and optical microscopy the chloritic minerals include corrensite which is an ordered 1 1 mixed layer chlorite smectite and expandable chlorite which is shown to be a mixed layer chlorite corrensite corrensite and some mixed layer chlorite corrensite occur as rims of vesicles and other cavities while later formed mixed layer chlorite corrensite occupies the vesicle cores the tem observations show that the mixed layer chlorite corrensite has ca 20 and the corrensite has ca 50 expandable smectite like layers consistent with xrd observations and with their abnormal optical properties the aem analyses show that high si and ca contents high alvi aliv and low fevi fe mg vi ratios of chlorites are correlated with interstratification of corrensite or smectite like layers in chlorite the aem analyses obtained from 200 500 a thick packets of nearly pure corrensite or chlorite layers always show that corrensite has low aliv siiv and low fevi fe mg vi while chlorite has high aliv siiv and high fevi fe mg vi this implies that the trioctahedral smectite like component of corrensite has significantly lower aliv siiv and fevi fe mg vi the ratios of fevi fe mg vi and aliv siiv thus decrease in the order chlorite corrensite smectite the proportions of corrensite or smectite like layers relative to chlorite layers in low grade rocks are inferred to be controlled principally by fe mg ratio in the fluid or the bulk rock and by temperature compositional variations of chlorites in low grade rocks which appear to correlate with temperature or metamorphic grade more likely reflect variable proportions of mixed layered components the assemblages of trioctahedral phyllosilicates tend to occur as intergrown discrete phases such as chlorite corrensite corrensite smectite or chlorite corrensite smectite a model for the corrensite crystal structure suggests that corrensite should be treated as a unique phase rather than as a 1 1 ordered mixed layer chlorite smectite

the nature and origin of titaniferous magnetite rich layers in the upper zone of the bushveld complex a review and synthesis the occurrence and geologic relationships of ti magnetite layers indicate that their genesis is intimately related to fractional crystallization processes that were responsible for the formation of their silicate host rocks the exact mechanism by which these oxide rich layers form is however still debated most recent models recognize that precipitation of copious quantities of ti magnetite is triggered by episodic increases in f sub o 2 but the process whereby this occurs is not well understood the nature and occurrence of ti magnetite rich layers in the bushveld complex is reviewed and the requirements of any satisfactory genetic model are outlined the formation and subsequent textural evolution of ti magnetite layers is believed to have taken place as follows 1 favorable conditions for the precipitation of large quantities of ti magnetite were created by a lengthy period of fractional crystallization that resulted in the concentration of large amounts of fe ti and v in the residual magma 2 large scale in situ bottom crystallization of plagioclase resulted in a marked increase in the total fe content and density of the immediately surrounding melt which collected as a dense layer on the bottom of the magma chamber this dense fe ti v enriched liquid did not mix with the overlying magma and formed a stagnant layer from which copious amounts of ti magnetite may have crystallized 3 crystallization of ti magnetite is controlled by the fe 2 o 3 feo ratio of the liquid and is a function of f sub o 2 temperature and the f sub h 2 o f sub h 2 ratio the fe 2 o 3 content of the liquid will also increase during the crystallization of ilmenite and fe super 2 bearing silicates clinopy roxene pigeonitc and fayalitic olivine the h 2 o content of the residual liquid will also be increased by the fractionation of anhydrous silicate phases 4 a complex interplay of these factors resulted in the precipitation of relatively large amounts of ti magnetite required for the development of ore rich layers the bulk of the ti magnetite formed by in situ bottom crystallization the nucleation and growth of magnetite within the stagnant layer and gravitative settling of these crystals over a short distance on the order of meters may have augmented the growth of the layers 5 variations in the relative proportions of coexisting oxide and silicate phases may be related to differences in concentrations and diffusion rates of ions in the liquid immediately above the crystallizing layer this resulted in differences in the rates of nucleation and growth of individual phases 6 precipitation of abundant magnetite lowered the density of the stagnant layer until it reached that of the overlying magma the two liquids then mixed thus terminating the cycle of magnetite layer formation and marking a return to silicate dominated fractionation 7 the ore rich layers originally consisted of ti magnetite crystals and variable amounts of silicate crystals which contained a certain amount of interstitial residual liquid densification of these layers and expulsion of significant amounts of this liquid were accomplished by annealing at high subsolidus temperatures possibly augmented by the addition of suitable material from interprecipitate liquids 8 variations in f sub o 2 during subsolidus cooling resulted in the development of a wide range of ulvoespinel and ilmenite microintergrowths

phase relations among smectite r1 illite smectite and illite a variety of smectitic and illitic clays were studied by tem and aem following ex pansion by l r white resin to define phase relations for clay minerals undergoing di genesis and low grade metamorphism samples included a prograde shale sequence from the gulf coast a hydrothermal bentonite from zempleni hungary and shales from the nankai trough japan michigan and the welsh sedimentary basin all samples were dominated by various proportions of only three kinds of clay minerals smectite having no 10 alayers r1 illite smectite i s 50 illite and illite with only small proportions of smectite like interlayers mixed layer phases with intermediate ratios of i s were observed only as minor components of other layer sequences lattice fringe images show that the common layer spacing of r1 i s is 21 aand that it has 0 7 0 8 k pfu its properties are not an average of those of smectite and illite which is consistent with the uniqueness of the r1 i s structure a prograde sequence of clay mineral transitions in the studied samples can be charac terized by five stages with different combinations of the three major phases i e smectite r1 i s and illite corresponding to different grades the sequence from low to high grade is 1 pure smectite 2 smectite with small proportions of discrete r1 i s and illite 3 r1 i s with small proportions of smectite and illite 4 illite with some r1 i s and smectite and 5 illite common exceptions to this scheme are inferred to be caused by the inherent metastability of all phases in occurrences that are rate or path dependent

mineralogical geochemical and isotopic characteristics of hydrothermal alteration processes in the active submarine felsic hosted pacmanus field manus basin papua new guinea during odp leg 193 4 sites were drilled in the active pacmanus hydrothermal field on the crest of the felsic pual ridge to examine the vertical and lateral variations in mineralization and alteration patterns we present new data on clay mineral assemblages clay and whole rock chemistry and clay mineral strontium and oxygen isotopic compositions of altered rocks from a site of diffuse low temperature venting snowcap site 1188 and a site of high temperature venting roman ruins site 1189 in order to investigate the water rock reactions and associated elemental exchanges the volcanic succession at snowcap has been hydrothermally altered producing five alteration zones 1 chlorite illite cristobalite plagioclase alteration apparently overprinted locally by pyrophyllite bleaching at temperatures of 260 310degreesc 2 chlorite mixed layer clay alteration at temperatures of 230degreesc 3 chlorite and illite alteration 4 illite and chlorite illite mixed layer alteration at temperatures of 250 260degreesc and 5 illite chlorite alteration at 290 300degreesc felsic rocks recovered from two holes 1189a and 1189b at roman ruins although very close together show differing alteration features hole 1189a is characterized by a uniform chlorite illite alteration formed at similar to250degreesc overprinted by quartz veining at 350degreesc in contrast four alteration zones occur in hole 1189b 1 illite chlorite alteration formed at similar to300degreesc 2 chlorite illite alteration at 235degreesc 3 chlorite illite and mixed layer clay alteration and 4 chlorite illite alteration at 220degreesc mass balance calculations indicate that the chloritization illitization and bleaching silica pyrophyllite assemblages alteration stages are accompanied by different chemical changes relative to a calculated pristine precursor lava the element cr appears to have a general enrichment in the altered samples from pacmanus the clay concentrate data show that cr and cu are predominantly present in the pyrophyllites illite shows a significant enrichment for cs and cu relative to the bulk altered samples considerations of mineral stability allow us to place some constraints on fluid chemistry hydrothermal fluid ph for the chloritization and illitization was neutral to slightly acidic and relatively acidic for the pyrophyllite alteration in general the fluids especially from roman ruins and at intermediate depths below snowcap show only a small proportion of seawater mixing 10 fluids in shallow and deep parts of the snowcap holes in contrast show stronger seawater influence copyright c 2004 elsevier ltd

permian peralkaline peraluminous and metaluminous a type granites in the panxi district sw china their relationship to the emeishan mantle plume abstract numerous permian granitic intrusions plutons in the panxi region sw china are spatially and temporally associated with the emeishan large igneous province elip granites from the 260 ma panzhihua and taihe plutons are peralkaline asi 0 76 1 12 na k al 0 78 1 2 and have high fe values 0 92 0 99 with negative eu anomalies eu eu 0 22 0 98 contrasted with the peralkaline plutons syenites from the 260 ma woshui pluton are metaluminous asi 0 84 1 0 na k al 0 91 1 01 and have moderately high fe 0 79 0 81 and positive eu anomalies eu eu 0 82 1 91 both the peralkaline and metaluminous rocks have mantle isotopic signatures end t 1 3 3 2 and were derived from the elip mantle plume source the peralkaline rocks were likely derived by fractional crystallization of the temporally spatially and chemically associated layered mafic intrusions which host giant magmatic fe ti v oxide deposits the metaluminous granitic rocks although spatially related to the peralkaline granites and layered gabbros were likely generated by partial melting of underplated mafic rocks granites from the 251 ma ailanghe pluton are peraluminous asi 0 97 1 29 and have high fe 0 82 0 91 values and negative eu anomalies eu eu 0 10 0 37 their crustal isotopic signature end t 5 7 to 6 7 indicates possible derivation from melting of yangtze block basement rocks the ailanghe pluton is contemporaneous with other a type granites in the region supporting the view that elip magmatism was sporadic for at least 9 million years the coexistence of three chemically exclusive a type granites in the elip suggests that a variety of a type granites can be generated in the same tectonic setting

reduced rapakivi type granites the tholeiite connection reduced rapakivi type granites are the most iron enriched and reduced i e least oxidized of the anorogenic granite association the low oxygen fugacity and chemical composition of these granites severely limit their sources in this paper we argue that reduced rapakivi type granites and their eruptive equivalents high potassium fayalite rhyolites are derived from mafic sources because tholeiitic magmas and their derivatives have the required low oxygen fugacity reduced rapakivi type granites are produced either by extreme differentiation of basaltic melts or by partial melting of underplated basalts and their differentiated equivalents they form in extensional environments where the asthenosphere is present at shallow depths we envision three stages in the origin of these rocks 1 tholeiitic melts are emplaced at the base of the crust 2 continued introduction of heat partially remelts these tholeiitic rocks and 3 the hot dry melts so produced migrate into the middle crust to produce rapakivi batholiths or erupt as rhyolites partial melting of felsic continental crust may accompany the intrusion of rapakivi type magmas thereby producing the other metaluminous and peraluminous granite compositions of the anorogenic suite

the smectite to chlorite transition in a fossil seamount hydrothermal system the basement complex of la palma canary islands the hydrothermal metamorphism of a sequence of pliocene aged seamount extrusive and volcanoclastic rocks on la palma includes a relatively complete low p t facies series encompassing the zeolite prehnite pumpellyite and greenschist facies the observed mineral zonations imply metamorphic gradients of 200 300 c km 1 the transition from smectite to chlorite in the la palma seamount series is characterized by discontinuous steps between discrete smectite corrensite and chlorite which occur ubiquitously as vesicles and to a much lesser extent vein in fillings trioctahedral smectites mg fe mg 0 4 0 75 occur with palagonite and na ca zeolites such as analcime and a thompsonite natrolite solid solution corrensite mg fe mg 0 5 0 65 first appears at stratigraphic depths closely corresponding to the disappearance of analcime and first appearance of pumpellyite discrete chlorite mg fe mg 0 4 0 6 becomes the dominant layer silicate mineral coincident with the appearance of epidote and andraditic garnet within the stratigraphic section there is some overlap in the distribution of the three discrete layer silicate phases although random interstratifications of these phases have not been observed although smectite occurs as both low and high charge forms the la palma corrensite is a compositionally restricted 1 1 mixture of low charge trioctahedral smectite and chlorite electron microprobe analyses of coarse grained corrensite yield structural formulae close to ideal values based on 50 negative charge recalculations calcium average 0 20 cations formula unit is the dominant interlayer cation with lesser mg k and na the absence of randomly interlayered chlorite smectite in the la palma seamount series may reflect high time integrated fluid fluxes through the seamount sequence this is consistent with the ubiquity of high variance metamorphic mineral assemblages and the general absence of relict igneous minerals in these samples

sedimentary talc in neoproterozoic carbonate successions abstract mineralogical petrographic and sedimentological observations document early diagenetic talc in carbonate dominated successions deposited on two early neoproterozoic 800 700 million years old platform margins in the akademikerbreen group svalbard talc occurs as nodules that pre date microspar cements that fill molar tooth structures and primary porosity in stromatolitic carbonates in the upper fifteenmile group of the ogilvie mountains nw canada the talc is present as nodules coated grains rip up clasts and massive beds that are several meters thick to gain insight into the chemistry required to form early diagenetic talc we conducted precipitation experiments at 25 c with low so4 synthetic seawater solutions at varying ph mg2 and sio2 aq our experiments reveal a sharp and reproducible ph boundary at 8 7 only above which does poorly crystalline mg silicate precipitate increasing mg2 and or sio2 aq alone is insufficient to produce the material the strong ph control can be explained by mg silica complexing activated by the deprotonation of silicic acid above 8 6 8 7 ft ir tem and xrd of the synthetic precipitates reveal a talc like 2 1 trioctahedral structure with short range stacking order hydrothermal experiments simulating burial diagenesis show that dehydration of the precipitate drives a transition to kerolite hydrated talc and eventually to talc this formation pathway imparts extensive layer stacking disorder to the synthetic talc end product that is identical to neoproterozoic occurrences early diagenetic talc in neoproterozoic carbonate platform successions appears to reflect a unique combination of low al concentrations and by inference low siliciclastic input near modern marine salinity and mg2 elevated sio2 aq and ph 8 7 because the talc occurs in close association with microbially influenced sediments we suggest that soluble species requirements were most easily met through microbial influences on pore water chemistry specifically ph and alkalinity increases driven by anaerobic fe respiration

formation of the late permian panzhihua plutonic hypabyssal volcanic igneous complex implications for the genesis of fe ti oxide deposits and a type granites of sw china the late permian 260 ma emeishan large igneous province of sw china contains numerous magmatic fe ti oxide deposits the fe ti oxide deposits occur in the lower parts of evolved layered gabbroic intrusions which are spatially and temporally associated with a type granitic rocks the 260 ma panzhihua layered gabbroic intrusion hosts one of the largest magmatic fe ti oxide deposits in china and is coeval with a peralkaline a type granitic pluton the granite has intruded the overlying emeishan flood basalts and fed at least one dyke which erupted onto the surface producing columnar jointed trachytes the presence of syenodiorite between the layered gabbro and granite is evidence for compositional evolution from mafic to intermediate to felsic rocks the syenodiorites have intermediate to felsic composition with sio2 61 to 65 wt mgo 0 27 to 0 6 wt and cao 1 0 to 2 5 wt as compared to the granite sio2 65 to 72 wt mgo 0 1 to 0 4 wt cao 1 eu eu 1 1 to 2 6 in the gabbroic intrusion to 1 in the syenodiorite eu eu 0 75 to 0 83 granites and trachytes eu eu 0 55 0 87 previously published end t values from clinopyroxenes end t 1 1 to 3 2 of the gabbroic intrusion match the whole rock values of the syenodiorite end t 2 1 to 2 5 granite and trachyte end t 2 2 to 2 9 suggesting that all rock types originated from the same mantle source melts and trace element modeling confirm that all rock types can be generated by fractional crystallization of high ti emeishan basalt the jump in sio2 from the gabbro to the syenodiorite is attributed to the en masse crystallization of the fe ti oxides the geological and geochemical data indicate that fractional crystallization of a common parental magma produced the layered gabbroic intrusion and fe ti oxide deposit the syenodiorite granites and trachyte of the panzhihua region which thus form a genetically related plutonic hypabyssal volcanic complex other granite gabbro complexes in the region likely formed in a similar manner

a polygenetic palaeosol from the silurian wenlock of southwest ireland a problematic palaeosol unit caps a thick basalt at the top of the clogher head formation dunquin group late wenlock silurian in the dingle peninsula southwest ireland the main part of the unit consists of an altered calc alkaline basalt with a fine clay assemblage of chlorite corrensite and smectite probably resulting from weathering during wenlock times the uppermost part of the unit consists of detrital silicates which were admixed by bioturbation and well defined burrow fills are present the underlying weathered basalt was possibly truncated before the detrital horizon was deposited and finally the whole unit underwent a phase of surface water gleying a catenary relationship existed locally with the pseudo gley developed lower in the landscape while contemporaneous basalts in topographically higher better drained areas largely escaped the effects of gleying these now exhibit reddened tops the complex history and the possiblity of the incompleteness of the profile makes accurate assessment of the soil climate function difficult however the occurrence of calcrete bearing palaeosols in the overlying mill cove formation supports the view that the basalt was weathered under a sub humid or semi arid climate considering the alteration of the dunquin group to actinolite prehnite grade the preservation of the smectite is highly unusual there is no evidence for its origin due to more recent weathering or interstratal neoformation it is thus interpreted as a relict product of the original weathering phase