oxygen diffusion in amphiboles the kinetics of oxygen isotope self diffusion in natural samples of hornblende tremolite and richterite have been measured samples were run under hydrothermal conditions using 18o enriched water profiles of 18o 16o 18o vs depth into the crystal were obtained using an ion microprobe the depths of sputtered holes were measured using an optical interferometer at 1000 bars 100 mpa water pressure the activation energies q and pre exponential factors d0 for diffusion parallel to c are d0 cm2 sec q kcal gm atom t c hornblende 1 20 1 10 741 6 650 800 tremolite 2 30 2 10 8 39 5 650 800 richterite 3 5 2 10 4 57 2 650 800 the diffusion coefficient d for hornblende at 800 c and 1000 bars water pressure measured parallel to the c crystallographic direction is at least ten times greater than that parallel to the a or b directions an increase in water pressure from 200 to 2000 bars increases d by a factor of 2 7 for diffusion parallel to c at 800 c the d value for hornblende at 800 c is about 0 01 that for quartz and 0 001 that for anorthite as a result closure temperatures for oxygen exchange in natural primary amphiboles are significantly higher than for quartz or feldspars it is unlikely that amphiboles will exchange oxygen isotopes by diffusion under most crustal conditions

intercalibration of 40ar39ar dating standards a number of standards used for k ar and 40ar39ar dating have been intercalibrated multiple splits 15 40 mg each of mmhb 1 sb 3 biotite lp 6 biotite 40 60 ga1550 biotite fish canyon tuff 3 biotite taylor creek rhyolite sanidine bern 4 muscovite bern 4 biotite and pennsylvania state university orthoclase 1a were irradiated in capsules fused and analyzed in replicate analyses most of these standards proved to be homogeneous in 40ar 39ark ratio age k ca and 36ar contents mmhb 1 is inhomogeneous in age at the 15 mg level indicating it is unsuitable for use as an interlaboratory standard using material taken straight from the bottle the orthoclase sample is also inhomogeneous at the 30 mg level quoted relative to an age of 162 9 ma for the sb 3 biotite standard total fusion ages are lp 6 biotite 40 60 128 1 ma ga1550 biotite 97 8 ma fish canyon tuff 3 biotite 27 95 ma taylor creek rhyolite sanidine 28 0 ma bern 4 muscovite 18 51 ma bern 4 biotite 17 21 ma these ages are in good agreement with the absolute k ar ages of these minerals different preparations of biotite and sanidine standards from a welded tuff such as the fish canyon tuff may not be of identical ages tests on submilligram splits reveals that sb 3 biotite ga1550 biotite fish canyon tuff 3 biotite and taylor creek rhyolite sanidine are suitable for use as monitors in laser work the sanidine specimen appears to be the most suitable for use as the primary monitor for 40ar39ar dating

tectonothermal evolution of the mayuan assemblage in the cathaysia block implications for neoproterozoic collision related assembly of the south china craton the mayuan assemblage in the cathaysia block southeast china consists of felsic paragneiss pelitic schist greenschist amphibolite marble calcsilicate and quartzite that underwent three episodes of deformation d1 d3 and four episodes of metamorphism m1 m4 in the early neoproterozoic the m1 assemblage consists of mineral inclusions defining an early foliation s1 within porphyroblasts represented by chlorite muscovite biotite plagioclase quartz in pelitic schist and actinolite chlorite epidote albite quartz enclosed in amphibolite m2 coincides with the development of the regional schistosity s2 and represents the formation of the porphyroblasts and growth of matrix minerals resulting in development of prograde metamorphic zones chlorite biotite garnet staurolite and kyanite zones m3 is simultaneous with the third phase of deformation d3 and produced sillimanite bearing mineral assemblages in pelitic schist and hornblende bearing assemblages in amphibo lite the last metamorphic episode m4 gave rise to the retrogressive assemblage chlorite muscovite in pelitic rock and actinolite chlorite epidote in amphibolite the sequence of mineral assemblages and history of metamorphic reactions built from the petrogenetic grid of pelites suggest a near isothermal decompression clockwise p t path for the mayuan pelitic schists using the tweequ software program the garnet biotite thermometer and garnet muscovite biotite plagioclase barometer yield p t conditions for m1 of 5 5 to 6 0 kb and 450 to 500 c and conditions for the garnet staurolite and kyanite zones of m2 of 6 0 to 7 0 kb and 550 to 600 c 6 0 to 7 5 kb and 600 c and 11 0 to 11 5 kb and 600 c the p t conditions of m3 were estimated at 570 to 625 c and 4 0 to 4 5 kb using the muscovite biotite thermometer and hornblende plagioclase geothermo barometer the garnet chlorite thermometer yields temperatures of 300 to 400 c for m4 but the pressures of m4 cannot be quantitatively estimated because of the lack of a suitable geobarometer these p t estimates also define an near isothermal decompression clockwise p t path which is involved in initial crustal thickening followed by rapid exhumation and final cooling and is related to amalgamation of the cathaysia and yangtze blocks to form the south china craton

where was south china in the rodinia supercontinent evidence from u pb geochronology and hf isotopes of detrital zircons abstract u pb geochronology and lu hf isotopic studies on detrital zircons from late neoproterozoic sediments of the cathaysia block south china show abundant grenville age 1 0 ga and neoarchaean populations and minor eoarchaean 3 8 ga paleo to mesoarchaean 3 3 3 0 ga mesoproterozoic 1 7 1 4 ga and pan african 0 70 0 54 ga populations this is the first report of 3 8 ga material in the crust of the cathaysia block the presence of euhedral 1 0 ga zircons indicates that a grenville orogenic belt was within or very close to the southern cathaysia block however the provenance containing the 3 8 ga 3 3 3 0 ga 2 5 ga 1 0 ga and 0 70 0 54 ga zircon populations cannot be found in the south china block the combination of zircon age distributions and the rounded strongly abraded shapes of the archean zircons suggesting long transport is consistent with derivation from other continental terranes previously linked with the southern part of the south china block comparison with the tectono magmatic history and compositions of rocks in western laurentia eastern australia and eastern india east antarctica western australia we suggest that these late neoproterozoic sediments may have originated mainly from eastern india east antarctica this indicates that the south china block was linked with the eastern india east antarctica continents in the late neoproterozoic rather than being located between the western laurentia and eastern australia continental blocks u pb and hf isotope studies on detrital zircons provide an important independent constraint on the palaeogeography of the south china block in the context of the breakup of rodinia and the subsequent assembly of gondwana in late neoproterozoic time and insights into the relationship between the cathaysia and yangtze blocks

equilibrium oxygen hydrogen and carbon isotope fractionation factors applicable to geologic systems as demonstrated by the chapters in this short course stable isotope techniques are an important tool in almost every branch of the earth sciences central to many of these applications is a quantitative understanding of equilibrium isotope partitioning between substances indeed it was harold urey s 1947 thermodynamically based estimate of the temperature dependence of 18o 16o fractionation between calcium carbonate and water and a recognition of how this information might be used to determine the temperatures of ancient oceans that launched the science of stable isotope geochemistry the approach pioneered by urey has since been used to estimate temperatures for a wide range of geological processes e g emiliani 1955 anderson et al 1971 clayton 1986 valley this volume in addition to their geothermometric applications equilibrium fractionation data are also important in the study of fluid rock interactions including those associated with diagenetic hydrothermal and metamorphic processes baumgartner and valley this volume shanks this volume finally a knowledge of equilibrium fractionation is a necessary first step in evaluating isotopic disequilibrium a widespread phenomenon that is increasingly being used to study temporal relationships in geological systems cole and chakraborty this volume in the fifty four years since the publication of urey s paper equilibrium fractionation data have been reported for many minerals and fluids of geological interest these data were derived from 1 theoretical calculations following the methods developed by urey 1947 and bigeleisen and mayer 1947 2 direct laboratory experiments 3 semi empirical bond strength models and 4 measurement of fractionations in natural samples each of these methods has its advantages and disadvantages however the availability of a variety of methods for calibrating fractionation factors has led to a plethora of calibrations not all of which are in agreement in this chapter we evaluate the major methods for determining fractionation factors

calcic garnets as a geochronological and petrogenetic tool applicable to a wide variety of rocks abstract this contribution presents u pb geochronological data for ca fe ti garnets from compositionally diverse alkaline and carbonatitic intrusive rocks ranging from neoarchean to permian triassic cinder lake and eden lake in manitoba canada belaya zima and odikhincha in siberia russia afrikanda in the kola peninsula russia obtained using isotope dilution thermal ionization mass spectrometry id tims and their trace element compositions measured by laser ablation inductively coupled plasma mass spectrometry la icpms the studied garnets yield u pb concordant or subconcordant ages obtained with a precision of

terrestrial deposition processes of quaternary gibbsite nodules in the yongjiang basin southeastern margin of tibet and implication for the genesis of ancient karst bauxite abstract the terrestrial deposition process for karst bauxite is difficult to constrain due to its allochthonous origin multiple controls and the difficulty in geodynamic reconstruction this ambiguity has been clarified from investigation of the provenance and geologic geographic conditions for the formation of quaternary gibbsite nodules in the yongjiang basin this karst basin located at the southeastern propagation front of the tibet plateau was affected by the se asia monsoon climate developed since the late oligocene and a later quaternary glaciation a thick blanket of quaternary laterite is located on paleozoic carbonate bedrock and covers an area about 1000 km2 along the east flowing river systems in the basin the laterite profiles are well layered with a layer of iron rich gibbsite nodules overlying on a basal clay zone and irregularly covered by a thin clayey topsoil gibbsite nodules from most localities yield detrital zircons with age peaks at ca 1000 ma 450 ma and 250 ma corresponding to those of the mesozoic clastic and magmatic rocks exposed on the western and southern surrounding highlands nevertheless those from the pingtianshan area produce an exclusive spike at ca 100 ma consistent with the emplacement age of nearby granites apatite fission track modeling for the mesozoic clastic rocks and darongshan granites shows a rapid cooling since the early miocene and that for the pingtianshan cretaceous granite reveals fast exhumation since the late miocene these data indicate differential erosion reshaped the topography and consequently a low relief karst basin was developed allowing a diverse material supply from the surrounding and local highlands via sheet flow and east flowing rivers under a monsoon influenced subtropical climate aluminum and iron oxides hydroxides were accumulated as nodules or concretions which were cemented by and coated with fe oxides due to variations in redox potential and alternating consolidation and swelling while silica was leached at greater depth groundwater solutions preferentially removed and brought upward fe under a reducing environment during the wet seasons while an underlying clay layer occurred due to the reduced solubility of silica later this bauxitization process was probably hindered by the quaternary glaciation which could have promoted the formation of the top clay layer this terrestrial deposition process of quaternary gibbsite nodules serves as one analogue for ancient karst bauxite in which the following submergence would improve the quality via further precipitation of alumina hydroxide and leaching of extra fe

pyrochlores from the lueshe carbonatite complex democratic republic of congo a geochemical record of different alteration stages abstract magmatic pyrochlores from the lueshe syenite carbonatite complex from the northeastern part of democratic republic of congo ex zaire are characterized by ta nb ratios in an increasing order from pyroxenite calcite carbonatite sovite silicate xenoliths nodules to syenite substitutions involving nb ta ti and ree have been precisely described hydrothermal alteration of lueshe pyrochlore involves the substitution of na f va vy and ca o va vy va a site vacancy and vy y site vacancy in calcite carbonatite hydrothermal alteration of pyrochlore took place during and after the precipitation of ancylite ce strontianite celestite baryte and fayalite according to a fluid composition of relatively low ph a na a ca 2 and a hf and high a sr 2 and a lree 3 the supergene alteration is characterized by complete leaching of na ca and f and partial incorporation of k ba sr and ce resulting in the formation of kali bario strontio and ceriopyrochlore respectively the na poor pyrochlore may be an intermediate variety corresponding to an alteration stage between the hydrothermal and weathered pyrochlores the ir spectroscopic study has indicated that the weathered pyrochlore is a hydrated variety containing two bands of oh vibration modes at 3413 and 1630 cm 1 during hydrothermal and supergene alterations the cations at b site remain relatively constant the variable chemical compositions of the pyrochlores from the lueshe complex represent geochemical memories of the different alteration conditions including the variation in the oxidation reduction environment

geochronology of early mesozoic diabase units in southwestern china metallogenic and tectonic implications two phases of diabase sill forming magmatism are recorded within the badu anticline where magmas were emplaced into upper palaeozoic carbonates and clastic rocks of the youjiang fold and thrust belt in the sw south china block china zircons from these diabase units yield weighted mean u pb ages of 249 2 2 0 ma and 187 1 3 3 ma and magmatic oxygen fugacity values from 20 to 6 average of 12 equating to fmq 5 and 20 to 10 average of 15 equating to fmq 2 respectively these data indicate that the sills were emplaced during early triassic and early jurassic times the discovery of c 250 ma mafic magmatism in this area was probably related to post flood basalt extension associated with the emeishan mantle plume or rollback of the subducting palaeo tethys slab the c 190 ma diabase sills indicate that the southwestern south china block records early jurassic mafic magmatism and lithospheric extension that was likely associated with a transition from post collisional to within plate tectonic regimes the emplacement of diabase intrusions at depth may have driven hydrothermal systems enabling the mobilization of elements from sedimentary rocks and causing the formation of a giant epigenetic metallogenic domain the results indicate that high oxygen fugacity materials within basement rocks caused crustal contamination of the magmas contributing to the wide range of oxygen fugacity conditions recorded by the au bearing badu diabase in addition data from inherited xenocrystic zircons within the badu diabase and detrital zircons from basement rocks suggest that the neoproterozoic jiangshao suture extends to the south of the badu anticline

the boundary between the simao and yangtze blocks and their locations in gondwana and rodinia constraints from detrital and inherited zircons abstract the simao block which is the northern extension of the indochina block and the yangtze block were amalgamated by the closure of a paleo tethys branch ocean commonly referred to as the ailaoshan ocean the exact location of the suture between these two continental blocks has been contested some researchers believed that it is located within the cenozoic ailaoshan shear zone whereas other researchers thought that it is present 50 km farther to the west the detrital and inherited xenocrystic zircons from paleozoic sedimentary rocks and a triassic granite dyke respectively in the laowangzhai mojiang suspect terrane which occurs between the two suggested locations for the same suture show that neither of these previously suggested locations is accurate the age distribution pattern of detrital and inherited xenocrystic zircons from the laowangzhai mojiang suspect terrane is similar to that of detrital zircons from the simao indochina block but different from that of those from the jianshui area in the western margin of the yangtze block this indicates that the laowangzhai mojiang suspect terrane belongs to the simao indochina block the actual location of the suture between the simao indochina and yangtze blocks is suggested to superimpose the ailaoshan late devonian to early carboniferous ophiolite belt within the suspect terrane the new interpretation is also supported by hf isotopes of detrital zircons from the related continental blocks the age distribution pattern of precambrian detrital zircons from the western part of the yangtze block is different from that of those from australia or tethyan himalaya the northern part of the indian continent indicating that the yangtze block was not an integral part of australia or india in rodinia the age distribution pattern of pre carboniferous detrital zircons from the simao indochina block is remarkably similar to that of those from the tethyan himalaya indicating that the simao indochina block was derived from indian gondwana

petrogenesis of the nanling mountains granites from south china constraints from systematic apatite geochemistry and whole rock geochemical and sr nd isotope compositions abstract the widespread mesozoic granitoids in south china 135 300 km2 were emplaced in three main periods triassic 16 of the total surface area of mesozoic granitoids jurassic 47 and cretaceous 37 though much study has been conducted on the most abundant jurassic nanling mountains nlm granites their rock affinities relative to the triassic darongshan drs and cretaceous fuzhou zhangzhou complex fzc granites which are typical s and i type respectively and the issue of their petrogenetic evolution is still the subject of much debate in this study we discuss the petrogenesis of nlm granites using apatite geochemistry combined with whole rock geochemical and sr nd isotope compositions sixteen apatite samples from six granite batholiths one gabbro and three syenite bodies in the nlm area were analyzed for their major and trace element abundances and compared with those collected from drs n 7 and fzc n 6 granites the apatite geochemistry reveals that na si s mn sr u th concentrations and ree distribution patterns for apatites from drs and fzc granites basically are similar to the s and i granite types of the lachlan fold belt australia whereas those from nlm granites have intermediate properties and cannot be correlated directly with these granite types according to some indications set by the apatite geochemistry e g lower u and higher eu abundances nlm apatites appear to have formed under oxidizing conditions in addition we further found that their ree distribution patterns are closely related to aluminum saturation index asi and nd isotope composition rather than sio2 content or degree of differentiation of the host rock the majority of apatites from nlm granites asi 0 97 1 08 and end t 8 8 to 11 6 display slightly right inclined apatite ree patterns distinguishable from the typical s and i type however those from few granites with asi 1 1 and end t 6 6 and gabbro and syenite are similar to fzc apatites strongly right inclined in light of sr and nd isotope compositions magmas of nlm intrusives except gabbro and syenite and few granites with end t 8 generally do not involve a mantle component instead they fit with a melt derived largely from in situ melting or anatexis of the pre mesozoic mainly caledonian granitic crust with subordinate pre yanshanian mainly indosinian granitic crust we suggest that an application using combined whole rock asi and end t values is as useful as the apatite geochemistry for recognizing possible sources for the nlm granites

nanpanjiang basin a window on the tectonic development of south china during triassic assembly of the southeastern and eastern asia abstract as the largest siliciclastic sink in the south china block the triassic nanpanjiang basin is generally thought of as a foreland basin related to indosinian collision between the south china and indochina blocks our basin analysis spanning an eight years period provides details of lopingian to middle triassic stratigraphic fill and tectonic subsidence of the basin field observation and sedimentological analyses reveal that the basin was not filled by a monotonous turbidite system and suggest instead the development of five primary depositional units constituted by deep water turbidite systems mass transport deposits condensed drapes shallow water isolated carbonate platforms and volcano volcaniclastic rocks the stratigraphic completeness of the basin the coexistence of widespread but localized carbonate platforms within axially filled deep water basin tectonically generated cyclothems of transverse mass transport deposits along intrabasinal faults and basin fill of extrusive rocks of bimodal composition together apparently call in question the claim that crustal shortening prevailed during the late permian to pre norian triassic patterns of basin fill differential tectonic subsidence and very high subsidence rate of the region adjacent to master fault are more closely match the pattern expected for extensional to transtensional basin in a back arc setting the crustal extension in the southwestern south china was not consistent with hypothesized indosinian collision between south china and indochina blocks but more likely developed in response to northwestward subduction of the paleo pacific plate our results combined with regional considerations lead to a new model for the triassic tectonic evolution of south china during triassic assembly of the southeastern and eastern asia

carlin style gold deposits youjiang basin china tectono thermal and structural analogues of the carlin type gold deposits nevada usa the triassic gold deposits of the youjiang basin southern china have been variously correlated to carlin style and orogenic gold deposits or classified as a new intermediate deposit type however in terms of a multi scale mineral system approach they show remarkable similarities to the tertiary carlin type deposits of nevada and distinct contrasts to orogenic gold deposits both the nevada and youjiang deposit groups formed in a continent scale post orogenic extension event on fragmented continental crust underlain by metasomatized lithosphere both form roughly orthogonal deposit trends that subparallel near orthogonal margins of a continental crustal block with deposits controlled by gentle anticlines monoclines or half horsts and extensional faults not tight locked up anticlines and shear zones the mineralogy and ore geochemistry of the two groups are similar with differences consistent with slightly deeper and higher temperature of formation of the older chinese deposits commensurate with deeper erosional levels the youjiang gold deposits should be classified as carlin type rather than carlin like or other terminologies with their lower gold endowment probably related to a more distal thermal and fluid source than the nevada carlin type deposits

subsolidus processes as a key factor on the distribution of nb species in plutonic carbonatites the tchivira case angola abstract the tchivira carbonatites contain several pyrochlore generations rare pyrochlore crystals close to the fluorcalciopyrochlore end member composition occur as primary magmatic crystals and are replaced due to subsolidus hydrothermal processes these produced leaching of na in the a position balanced by f removal from the y position interaction of oxydizing hydrothermal fluids with carbonatites and host rocks produced a sequence of hydrothermal alterations with zonal distribution from the host rock to the centre of the carbonatite intrusion the succession of alteration types distinguished consists of fenitization fluoritization silicification ankeritization and dolomitization in addition veins with vugs infilled by druse quartz carbonates and fluorite are also common distinct pyrochlore compositions are characteristic for each altered unit plumbopyrochlore and ba rich kenopyrochlore occur in the fenites and in the fluoritized silicified areas ba rich kenopyrochlore is also found in the ankeritized areas and sr rich kenopyrochlore appears as rare crystals in the dolomitized areas sr ba and pb pyrochlores and nb rich rutile replace earlier pyrochlore generations and are present as inclusions in quartz and fluorite or as veinlets in fluoritized silicified carbonatite minerals associated with sr ba and pb pyrochlores include hematite strontianite barite and ree fluocarbonates and phosphates indicating oxidising conditions for mineral precipitation nb rich rutile gorceixite ba rich k feldspar and late ba poor k feldspar can also occur in fenites and fluoritized rocks their presence implies that the composition of the altered host rock also affects the composition of the hydrothermal minerals textural evidences indicate that pyrochlore and these minerals are produced by precipitation from hydrothermal fluids and the distribution of the different pyrochlore types suggests that the nature of the hydrothermal fluid controls their composition finally the replacement of pyrochlore crystals by goethite as well as the occurrence of nb rich goethite and fe mn oxyhydroxides indicates that nb can be also transported by supergenic fluids in synthesis the role of subsolidus processes is a key factor for the distribution of nb bearing minerals in carbonatites

the catalao i niobium deposit central brazil resources geology and pyrochlore chemistry abstract the catalao i alkaline carbonatite phoscorite complex contains both fresh rock and residual weathering related niobium mineralization the fresh rock niobium deposit consists of two plug shaped orebodies named mine ii and east area respectively emplaced in carbonatite and phlogopitite together these orebodies contain 29 mt at 1 22 wt nb2o5 measured and indicated in closer detail the orebodies consist of dike swarms of pyrochlore bearing olivine free phoscorite series rocks nelsonite that can be either apatite rich p2 unit or magnetite rich p3 unit dolomite carbonatite dc is intimately related with nelsonite natropyrochlore and calciopyrochlore are the most abundant niobium phases in the fresh rock deposit pyrochlore supergroup chemistry shows a compositional trend from ca na dominant pyrochlores toward ba enriched kenopyrochlore in fresh rock and the dominance of ba rich kenopyrochlore in the residual deposit carbonates associated with ba sr enriched pyrochlore show higher 18osmow than expected for carbonates crystallizing from mantle derived magmas we interpret both the 18osmow and pyrochlore chemistry variations from the original composition as evidence of interaction with low temperature fluids which albeit not responsible for the mineralization modified its magmatic isotopic features the origin of the catalao i niobium deposit is related to carbonatite magmatism but the process that generated such niobium rich rocks is still undetermined and might be related to crystal accumulation and or emplacement of a phosphate iron oxide magma

episodes of dormancy and eruption of the late pleistocene ciomadul volcanic complex eastern carpathians romania constrained by zircon geochronology abstract ciomadul is the youngest volcanic system in the carpathian pannonian region recording eruptive activity from ca 1 ma to 30 ka based on combined zircon u th and u th he geochronology ciomadul volcanism is divided into two main eruptive periods old ciomadul 1 ma 300 ka ocep and young ciomadul eruptive period 160 30 ka ycep ocep activity comprises eruptive epochs 1 3 whereas new ages for eight lava domes and four pyroclastic units belonging to the ycep lead to its further subdivision into two eruptive epochs eruptive epochs 4 and 5 the extrusion of most of the lava domes occurred between 160 and 90 ka eruptive epoch 4 during three eruptive episodes at ca 155 ka 135 ka and 95 ka eruptive episodes 4 1 4 2 and 4 3 respectively along a ne sw lineament which is perpendicular to the regional nw se trend of the c limani gurghiu harghita volcanic chain eruptive epoch 5 occurred after a ca 40 kyr of quiescence at ca 55 30 ka and is mainly characterized by explosive eruptions with a minor lava dome building activity all of the dated pyroclastic outcrops together with the lava dome of piscul pietros belong to the older eruptive episode 5 1 with an eruption age of 55 45 ka the eruption centers of eruptive epoch 5 are located at the junction of the conjugated nw se and ne sw lineaments defined by the older eruptive centers the whole rock geochemistry of all studied samples is fairly homogeneous sio2 63 69 wt k2o 3 4 wt it also overlaps with the composition of the lava domes of the old ciomadul eruptive period implying a monotonous geochemical characteristic for the past 1 myr the eruption rates for the ciomadul volcanism were determined based on the erupted lava dome volume calculations supplemented with the eruption ages the activity peaked during the eruptive epoch 4 160 90 ka having an eruption rate of 0 1 km3 kyr in comparison these values are 0 05 km3 kyr for the ycep 160 30 ka and 0 01 km3 kyr for the overall ciomadul volcanism 1 ma 30 ka based on the geochemical characteristics the quiescence periods and the lifetime of the complex as well as the relatively small amount of erupted material this volcanic system can be placed in a subduction related post collisional geodynamic setting which shows strong chemical similarities to continental arc volcanism the commonly found long repose times between the active phases suggest that the nature of a volcano cannot be understood solely based on the elapsed time since the last eruption instead comprehensive geochronology coupled with the understanding of the magma storage behavior could be a base of hazard assessment for volcanic fields where the last eruptions occurred several 10 s of thousand years ago and therefore they are not considered as potentially active

detrital zircon u pb geochronological and lu hf isotopic constraints on the precambrian magmatic and crustal evolution of the western yangtze block sw china abstract in situ u pb geochronological and lu hf isotopic studies of detrital zircon from a variety of precambrian strata along the western margin of the yangtze block of south china show that the rocks have maximum depositional ages of ca 1000 740 ma all of the strata probably formed in a subduction related tectonic setting the kunyang group heishantou formation and huili group tongan formation have maximum depositional ages of 960 1000 ma and contain zircons whose ages are dominantly paleoproterozoic 2500 2300 1900 1700 and 1650 1500 ma the yanbian group has a zircon population ranging mainly from 860 to 1000 ma different from that of the bikou group with ages from 745 to 890 ma ages of zircons from the lieguliu and guanyinya formations range from 740 to 900 ma the paleoproterozoic detrital zircons commonly have negative or slightly positive hf values from 8 0 to 2 3 whereas most of the 1000 740 ma zircons have moderately to significantly high positive hf values from 0 3 to 13 0 the new detrital zircon u pb and lu hf isotopic dataset suggests four major episodes of magmatism in the region 2500 2300 1900 1700 1650 1500 and 1000 740 ma and the presence of widespread archean basement of the western yangtze block the data also suggest an important period of juvenile magmatism at 1000 740 ma our results favor a model in which continuous subduction 1000 740 ma along the western margin of the yangtze block lasted for 260 million years

geochronology and petrogenesis of the early paleozoic i type granite in the taishan area south china middle lower crustal melting during orogenic collapse abstract the early paleozoic wuyi yunkai orogeny represents the first extensive tectonothermal event in south china since the neoproterozoic break up of the rodinia supercontinent the early paleozoic igneous rocks in the south china block scb are important for understanding the processes responsible for the orogeny the taishan batholith in the inner cathaysia block consists dominantly of granodiorite and hornblende bearing granite zircon u pb dating of four samples gives a consistent crystallization age of ca 436 ma synchronous with widespread late orogenic s type granitic intrusions in the wuyi yunkai orogen the taishan granitoid rocks are metaluminous or peraluminous a cnk 0 94 1 15 and have variable sio 2 61 5 74 9 wt mgo 0 4 2 4 wt fe 2 o 3 1 8 7 4 wt and cao 1 1 5 5 wt with gradually decreasing p 2 o 5 from mafic to felsic components thus the taishan batholith displays the petrological and geochemical characteristics of i type granites additionally all samples have strongly negative e nd t values 11 to 9 suggesting an overall ancient source the early paleozoic i type granitoid rocks in the taishan area were derived from partial melting of ancient middle to lower crust at a high thermal gradient 35 c km and underwent extensive fractional crystallization of hornblende plagioclase and biotite the wuyi yunkai orogenic collapse may have been responsible for middle to lower crustal melting of the cathaysia block the early paleozoic i and s type granitoid magmatism lack significant input of a juvenile component indicating that the processes accompanying orogenic collapse such as asthenosphere upwelling and basaltic underplating contributed heat to trigger widespread early paleozoic felsic magmatism in the scb an intraplate orogenic event instead of continental or arc collision may be the preferred model for the wuyi yunkai orogeny

the origin of magnetite apatite rocks of mushgai khudag complex south mongolia mineral chemistry and studies of melt and fluid inclusions abstract the concentration and redistribution of ore components from a primary melt to hydrothermal fluids are important for understanding ore formation the mushgai khudag complex is a typical example of an intrusion where hydrothermal processes are widespread and where we can observe the redistribution of ore components during hydrothermal processes in this study we use mineralogical melt and fluid inclusion data to trace element characteristics of apatite from the mushgai khudag complex to reconstruct the formation of the magnetite apatite rocks and their magmatic hydrothermal evolution and to clarify the origin of the ree mineralization we conclude that the magnetite apatite rocks crystallized from salt melt with a high content of phosphate and sulfate components at a temperature of approximately 830 850 c the origin of magnetite apatite rocks probably can be explained by the silicate salt immiscibility that occurred at the alkaline syenite crystallization stage further evolution of the salt melt to the brine of the carbonate fluoride chloride sulfate composition was accompanied by the barite celestite and monazite ce formation at the temperature of approximately 500 580 c the dissolution of apatite and the crystallization of gypsum phosphosiderite and monazite ce pseudomorphs after apatite took place at the hydrothermal stage after a reaction with a fluid that evolved from carbonate chloride sulfate at 250 350 c into a predominantly chloride composition at 150 250 c the high activity of the sulfate component and a significant enrichment of the rocks in ree also occurred at the late hydrothermal stage

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

an experimental calibration of the temperature dependence of oxygen isotope fractionation between apatite and calcite at high temperatures 350 800 c the fractionation of oxygen isotopes between apatite and calcite has been measured experimentally using the direct exchange technique in a piston cylinder apparatus at high temperature and pressure 350 800 c 1 1 gpa the fractional approach to equilibrium ranged from 20 to 80 and varies systematically with temperature and run duration isotopic xrd and sem analyses of the run products indicate that the exchange occurs dominantly by recrystallization in the early stages of the experiments the equilibrium fractionation over the temperature range of this study ranges from 5 5 to 2 but the data at 350 and 425 c have relatively large errors for t 500 c an error weighted least squares fit yields 1000 ln ap cc 0 68 1 60 0 26 106t 2 the magnitude of the temperature dependence is intermediate to that for felsic and mafic minerals to calcite mineral pairs involving apatite are potentially useful indicators of temperature cooling rates and equilibrium disequilibrium phenomena in igneous and metamorphic rocks

stable isotope composition of minerals in the belaya zima plutonic complex russia implications for the sources of the parental magma and metasomatizing fluids abstract the belaya zima alkaline carbonatite complex in the east sayan mountains russia is a multiphase concentric intrusion which comprises diverse alkaline silicate rocks and carbonatites melteigites are the earliest products of crystallization and they are followed by ijolites urtites nepheline syenites and finally calcite calcite dolomite and ankerite carbonatites in this paper we present new geochronological data ar ar method chemical and stable isotope h c and o compositions of the main rock forming minerals in carbonatites and the silicate rocks of the complex the ar ar dating of phlogopite from ankerite carbonatites is consistent with the age of syenite emplacement and implies that the silicate rocks and carbonatites are likely to be comagmatic the evolutionary trends of pyroxene fluorapatite amphibole and phlogopite are consistent with fractional crystallization from a single parental magma primary nb mineralization is represented by pyrochlore whereas columbite fe is a replacement product of pyrochlore at the post magmatic hydrothermal stage at that stage interaction of calcite carbonatites with hydrothermal fluids resulted in the formation of hydrothermal paragenesis comprising dolomite ankerite monazite ce ancylite ce minerals of burbankite group and ca ree fluorcarbonates the hydrothermal processes were also responsible for cation leaching from pyrochlore the replacements of phlogopite by tetraferriphlogopite and early ca rich pyroxene by aegirine the stable isotope data suggest that the rocks crystallized from a 18o depleted magma shifts toward higher 18o values relative to the primary compositions are commonly observed in minerals from carbonatites which is attributed to hydrothermal processes

kwangsian crustal anatexis within the eastern south china block geochemical zircon u pb geochronological and hf isotopic fingerprints from the gneissoid granites of wugong and wuyi yunkai domains abstract gneissoid granites were traditionally thought to be the components of the precambrian basement in the eastern south china block but twenty four gneissoid granite samples from the wugong northern wuyi southern wuyi and yunkai domains gave zircon u pb ages of 424 456 ma 410 457 ma 426 438 ma and 415 450 ma respectively the existence of the abundant kwangsian gneissoid granites which have incorrectly been assigned to the precambrian basement indicates that the so called precambrian stratigraphic sequence of the cathaysia block should be termed complex rather than group these gneissoid granites are peraluminous granites with relatively high al 2 o 3 mgo tio 2 feot and cao na 2 o but low cao al 2 o 3 mgo feot and rb sr ratios they exhibit strongly negative ba sr nb p and ti and positive pb anomalies in primitive mantle normalized diagrams and have initial 87 sr 86 sr ratios ranging from 0 70924 to 0 72935 and negative end t values from 6 4 to 11 4 similar to those of paragneissic enclaves and precambrian sedimentary rocks in the eastern south china block the zircons crystallized in the kwangsian gneissoid granites gave ehf t values ranging from 2 4 to 19 4 with the peak at 4 almost all grains clustering at 1 to 12 and hf model ages of 1 1 2 1 ga the synthesis of these geochemical and in situ zircon hf isotopic data indicates that the kwangsian gneissoid granites dominantly originated from proterozoic metapelitic and meta igneous components with insignificant input of juvenile mantle derived magmas a model of two stage crustal anatexis is proposed for the kwangsian granitic magma in the eastern south china block with 1 the formation of 460 430 ma granite through the breakdown of hydrous minerals under the condition of the doubly thickened crust and 2 the generation of the 430 400 ma granite accompanying promoted melting along a path of isothermal decompression due to the increasing thermal weakening for the collapse of the thickened crust the crustal anatexis has probably occurred in an intracontinental tectonic regime that was geodynamically linked to the far field response to the assembly of the australian indian plate with the cathaysia block during middle paleozoic kwangsian time

phlogopite and tetra ferriphlogopite from brazilian carbonatite complexes petrogenetic constraints and implications for mineral chemistry systematics abstract the results of a mineral chemistry study of phlogopite and tetra ferriphlogopite in carbonatites and associated alkaline silicate rocks from the brazilian carbonatite complexes of jacupiranga tapira and catalao are presented a wide textural variety of mica is found ranging from primary magmatic crystals to late stage metasomatic phases primary micas from the carbonatites and from the associated plutonic silicate rocks follow distinct evolution paths with magma differentiation in all three complexes micas from the silicate rocks evolve from phlogopite towards annite although in jacupiranga they are more al rich among the micas crystallising from carbonatite liquids those from jacupiranga are typically al and mg rich while in tapira and catalao they are extremely al poor metasomatic micas bridge the gap between primary micas from silicate rocks and from carbonatites the complete phlogopite tetra ferriphlogopite series is reported from the catalao and tapira complexes this study shows that micas from carbonatite complexes may span a wide compositional range largely overlapping the fields of micas from several types of alkaline ultrapotassic rocks especially regarding ti and al contents the use of phlogopite composition and evolution to discriminate between different types of alkaline rocks should be undertaken with caution

components and episodic growth of precambrian crust in the cathaysia block south china evidence from u pb ages and hf isotopes of zircons in neoproterozoic sediments abstract detailed lam icpms u pb dating and lam mc icpms lu hf isotope analysis were carried out on zircons from nine samples of basement metamorphic rocks in the southern cathaysia block south china the chemical compositions of these metamorphic rocks and zircon morphology indicate that their protoliths were sedimentary zircon u pb dating results show they mainly formed during the late neoproterozoic the precambrian crust of the cathaysia block can be divided into two distinct tectonic domains the wuyishan terrane to the northeast and the nanling yunkai terrane to the southwest the nearly e w boundary between these two terranes is also supported by geophysical petrological and geochemical evidence and the basement rocks of the two terranes are comprised of different components the wuyishan terrane is characterized by dominant paleoproterozoic 1 86 ga and lesser neoarchean magmatism and the southern central part of the terrane suffered strong neoproterozoic and early paleozoic tectonothermal reworking the zircon data suggest that there were five important episodes of juvenile crust generation 3 6 ga 2 8 ga 2 6 2 4 ga 1 85 ga and 0 8 0 7 ga in the wuyishan terrane strong paleoproterozoic 1 85 ga magmatism in northern wuyishan and neoproterozoic 0 8 0 7 ga magmatism in central and southern wuyishan both principally involved the remelting of old crust with minor input of juvenile material in contrast the nanling yunkai terrane contains abundant neoarchean and grenvillian zircons some evidence of mesoproterozoic activity and rare paleo to meso archean and neoproterozoic zircons u pb ages and hf isotopic compositions of the detrital zircons from the late neoproterozoic to early paleozoic meta sedimentary rocks in the nanling yunkai area suggest that the growth of juvenile crust mainly occurred at 3 6 ga 3 3 ga 2 5 2 6 ga 1 6 ga 1 0 ga and 0 8 0 7 ga neoarchaean 2 6 2 5 ga grenvillian 1 0 ga and neoproterozoic 0 8 0 7 ga magmatism mainly involved the recycling of old crust minor remnants of 4 1 ga crust may remain in the terrane this crustal history is distinct from that of the wuyishan terrane and analogous to those recorded for the eastern india east antarctic domain of northern gondwanaland suggesting that they probably were once linked

reappraisal of the ages of neoproterozoic strata in south china no connection with the grenvillian orogeny the jiangnan fold belt separates the yangtze and cathaysia blocks in south china and has long been considered grenvillian in age in order to place south china in central rodinia it consists of deformed early neoproterozoic strata that are unconformably overlain by undeformed late neoproterozoic strata and intruded by undeformed and unmetamorphosed granitic plutons zircons from the early neoproterozoic strata yield u pb ages as young as 830 ma and one granitic pluton has a zircon u pb age of ca 827 ma the 830 ma mafic rocks along the southeastern margin of the yangtze block have arc affinity geochemical characters whereas mafic rocks younger than 830 ma have typical ocean island basalt oib like compositions thus we suggest that the early neoproterozoic strata were deposited on an active continental margin prior to amalgamation of the yangtze and cathaysia blocks at ca 830 ma the overlying late neoproterozoic strata were deposited in the intracontinental rifted nanhua basin at 820 730 ma and probably reflect backarc spreading above the long lived 950 735 ma oceanic subduction zone along the northern and western margin of the yangtze block this model is consistent with the secular tectonic evolution of south china during the neoproterozoic the jiangnan fold belt is therefore not a grenvillian feature as previously suggested and there is no evidence to place south china in central rodinia instead we believe that south china was located in a marginal position relative to this supercontinent

rare earth elements in phoscorites and carbonatites of the devonian kola alkaline province russia examples from kovdor khibina vuoriyarvi and turiy mys complexes abstract the devonian ca 385 360 ma kola alkaline province includes 22 plutonic ultrabasic alkaline complexes some of which also contain carbonatites and rarely phoscorites the latter are composite silicate oxide phosphate carbonate rocks occurring in close space time genetic relations with various carbonatites several carbonatites types are recognized at kola including abundant calcite carbonatites early and late stage with subordinate amounts of late stage dolomite carbonatites and rarely magnesite siderite and rhodochrosite carbonatites in phoscorites and early stage carbonatites the rare earth elements ree are distributed among the major minerals including calcite up to 490 ppm apatite up to 4400 ppm in kovdor and 3 5 wt ree2o3 in khibina and dolomite up to 77 ppm as well as accessory pyrochlore up to 9 1 wt ree2o3 and zirconolite up to 17 8 wt ree2o3 late stage carbonatites at some localities are strongly enriched in ree up to 5 2 wt ree2o3 in khibina and the ree are major components in diverse major and minor minerals such as burbankite carbocernaite ca and ba fluocarbonates ancylite and others the rare earth minerals form two distinct mineral assemblages primary crystallized from a melt or carbohydrothermal fluid and secondary formed during metasomatic replacement stable c o and radiogenic sr nd isotopes data indicate that the ree minerals and their host calcite and or dolomite have crystallized from a melt derived from the same mantle source and are co genetic

the western segment of the suture between the yangtze and cathaysia blocks constraints from inherited and co magmatic zircons from permian s type granitoids in guangxi south china the location of the suture between the yangtze craton and the cathaysia block is uncertain in the west three proposed locations for the western segment of the suture all go through guangxi but are up to 150 km apart inherited zircon crystals from the darongshan permian s type granitoid plutons which are derived from the lower crust and occur north of the southernmost location for the suture yield u pb ages with a major peak at 960 ma the ehf t values of the inherited zircons with ages around the major peak vary from 6 4 to 23 4 the age distribution and hf isotope variations of the inherited zircons are remarkably similar to those of precambrian detrital zircon crystals from the western cathaysia block but strikingly different from those of precambrian detrital zircon crystals from the yangtze craton these results indicate that the western part of the suture is situated to the north of the darongshan granitoid belt

geochronological and geochemical constraints on the petrogenesis of middle paleozoic kwangsian massive granites in the eastern south china block abstract to achieve a better understanding of the kwangsian orogenic event of the eastern south china block this paper documents a set of new zircon u pb geochronological and hf isotopic data and whole rock elemental and sr nd isotopic analytical results for the representative massive granite intrusions across the jiangshan shaoxing fault the studied samples are classified into two groups representing the rocks from the cathaysia block to the east of the jiangshan shaoxing fault group 1 and those from the eastern yangtze block between the anhua luocheng and jiangshan shaoxing faults group 2 the group 1 samples gave the zircon u pb ages of 405 454 ma and e hf t values of 3 6 to 15 2 with hf model ages of 1 6 2 4 ga group 2 yielded the zircon u pb ages of 400 432 ma and e hf t values of 0 2 to 12 7 with hf model ages of 1 3 2 2 ga geochemically the group 1 samples a cnk 1 02 1 43 have relatively lower al 2 o 3 mgo cao p 2 o 5 and e nd t but higher k 2 o na 2 o than those of group 2 a cnk 0 93 1 44 both groups show similar chondrite normalized patterns of rare earth elements with eu eu values of 0 15 0 92 and strongly negative ba sr nb p and ti anomalies in primitive mantle normalized spider diagrams their e nd t values range from 11 1 to 8 0 for group 1 and 8 9 to 5 0 for group 2 generally similar to those of precambrian paragneiss and contemporaneous gneissoid granites in the eastern south china block our geochronological results indicate that the kwangsian massive granites in the eastern south china block were crystallized between 400 ma and 454 ma synchronous to the kwangsian gneissoid granites along the wugong and wuyi baiyun yunkai domains in the eastern south china block the synthesis of these whole rock geochemical and in situ zircon hf isotopic data suggests that both the group 1 and 2 granites across the jiangshan shaoxing fault were predominantly derived from a crustal source with some proportional metapelitic and metaigneous components from the precambrian basement the input of juvenile mantle derived magma is insignificant taking into account other available evidences it is proposed that the jiangshan shaoxing fault might be a pre existing boundary between the yangtze and cathaysia blocks which was reactivated during the middle paleozoic kwangsian orogenic event the massive granitic magmatism was probably resulted from the doubly crustal thickening and subsequent isostatic readjustment in an intracontinental tectonic regime